Emily Johnston was looking out the window of her home in Westminster last night and she spotted the full moon. Or at least it looked like a full moon.

Then she checked her calendar. It told her the full moon isn't until today, Tuesday, the 22nd.

"So define 'FULL MOON.' I don't mean dropping yer drawers, either," she said.

It sure did look full last night, didn't it? It was a gorgeous night, with a brilliant moon, bright Mars overhead, lots of stars. If it hadn't been so darned cold, it would have been a perfect night for stargazing.

The moon is officially "full" when it is precisely opposite the sun as seen from Earth - sitting along a straight line drawn from the sun, through the Earth, to the moon. At that moment the side facing Earth is fully illuminated. That's where it was at 8:34 a.m. today, although it had already set for Marylanders. By the time the moon rises this evening at around 5:30 p.m., it will be nine hours past "full."  Last evening, it was about 12 hours or so short of "full."

But it doesn't really matter. It was big, bright and beautiful. BTW, the first full moon of the year is called the Moon After Yule, or the Old Moon.

Not much chance we'll see the moon tonight.

Posted by Frank Roylance at 10:00 AM | Permalink | Comments (0)
                    

Here's an odd one. "Dee" writes from a Baltimore County Public Library: "On the afternoon of Sept. 12, I will be flying from BWI to Salt Lake City. I want to sit where the sun will not be in my eyes. The location of sunsets in the continental U.S., in different seasons, has always puzzled me, and probably always will. Is there a rule of thumb?"

Let’s see, Dee. You could change your reservation and fly after sunset (7:19 on the 12^th). You could fly east, around the planet, and face the east for the entire flight and miss all the sunsets. You could, of course, pull the shade. Problem solved.

But, if you're like me, you probably like to look out the window at the countryside drifting by. So, here are some thoughts.

You'll be flying almost exactly due west. That means the passenger windows will face either north or south.

Sunsets (and sunrises) during late summer and fall are moving slightly farther south each evening, from their farthest north at the summer solstice in June, to their farthest south at the winter solstice in December.

At the time of the Autumnal Equinox (Sept. 23 this year), the sun rises due east and sets due west. If you were to fly on that date, the sunset would be in the pilots' eyes, but passengers looking north or south wouldn't have a problem. (This assumes a flight plan taking you straight west. Course changes, turns to avoid thunderstorms and maneuvers during climb and descent mean all bets are off. You're on your own.)

But you're flying on the 12th, or 11 days before the equinox. That means sunset will occur a bit north of due west. If you're on the right side of the airplane, you might get some sun in your eyes just at sunset.

On the other hand, you'll probably spend several hours on the plane before sunset. If sunlight is bothersome to you, that presents another problem.

Hypothetically, if you were flying west along the equator, on the equinox (Sept. 23), the sun's path during the afternoon would be from directly overhead at noon, to due west at sunset. You'd never see the sun from a passenger's seat. Eleven days before the equinox, however, the sun, at the equator, would be slightly north of straight up. So, the right side of the plane would be sunnier throughout the flight.

But you'll be flying at a latitude of, roughly, 40 degrees north. That means the sun's path through the sky will be along a path well south of straight up. That means the seats on the left (south-facing) side of the plane will be sunny throughout the flight, until shortly before sunset, when sol will cross in front of the plane and touch down at the horizon slightly north of due west.

The bottom line: Sit anywhere on the right side of the plane until perhaps 30 minutes before sunset. Then, when somebody gets up from a left-hand window seat to use the rest room, steal their seat. Or, rather than raise a ruckus and get yourself locked up, you could a) make your reservation for a right-hand seat, and then pull the shade at sunset; or b) if it's only the sunset that offends you, get a left-hand seat, enjoy the afternoon sunshine and let the sunset fall on those on the right side of the airplane.

Posted by Frank Roylance at 1:09 PM | Permalink | Comments (2)
                    

The WeatherBlog continues to receive quite a number of notes and queries regarding what looks to some to be a skewed - or at least curious - lack of precipitation on the official National Weather Service instruments at BWI-Marshall Airport. Writers insist that their own observations - some anecdotal, some recorded on home rain gauges - are often/usually higher than those at the airport.

We have posted some of these notes from readers. And, we have passed along at least one theory, suggesting that - particularly when wet weather systems move in from the Atlantic as they did during the heavy rains here at the end of June - the rising terrain from the coastal plain (where BWI sits) to the Piedmont forces the wet air to rise. That cools the air temperature and causes some of the moisture to condense, increasing total rainfall on the higher inland terrain.

Here's another theory, from Jack Wennerstrom, of Randallstown. He draws on the same sort of physics, but sees it from another perspective:

"Most parts of western and northern Baltimore County sit higher than Baltimore City. The difference can be dramatic: at Soldier's Delight, a nearby natural area, the Visitor Center is at 726 feet above sea level, as is a nearby hill. For evidence, walk just to the west of the center, at Red Dog Lodge, and gaze westward, and downward, into Carroll County. In the long-ago days when all these hills were clear-cut for timber, from here you could see the Blue Ridge to the west and the Chesapeake Bay to the east.

"Due to uplift and the relative erosion-resistance of the hard surface rock ... this area and others are noticeably higher than the coastal plain ... that they abut. Indeed, they form a series of hills and low ridges that roughly constitute the so-called 'fall-line' on a southwest-northeast axis paralleling Route 1.

"My point is that this line of hills, which come to surround Baltimore west and north, produce a kind of 'mini-rain shadow' effect, whereby west-to-east approaching rain clouds tend to drop their rain on the windward (western) sides of the ridges (as well as the crests), leaving less or little for the lee sides (Baltimore's coastal plain), a phenomenon well-known and more dramatic in lofty western Maryland.

"The result? I believe our annual rainfall here in Randallstown, and surrounding Pikesville, Granite, and Owings Mills is, on average, four to ten inches more than at BWI, which sits on the coastal plain lee, or 'mini-rain'shadow.'  I base my estimate on 20 years of observation, taking notice of official rainfalls here and in these nearby communities. There is a mitigating factor in that rainstorms from the northeast or south, which are much rarer, are not affected by this rain-shadow factor - otherwise the difference would be even higher. Many is the time that I have noted rainfalls of 2, 3 or more inches here in the Randallstown, owings Mills, Pikesville region, while little or none fell at BWI. It is very rarely the other way around.

"So ... the BWI rainfall stats are misleading to a large percentage of the Baltimore-area residents. I know of no one else who holds this theory or has even mentioned the possibility of its existence."

Readers?  What do you think?

Posted by Frank Roylance at 11:28 AM | Permalink | Comments (5)
                    

Henry E. Temple, 80, of Baltimore, whose daughter describes him as "a big ol' weather geek from way back," believes there's a more accurate way to compare temperatures for two time periods than the one used by the National Weather Service. Here's his question:

"When comparing the same two calendar time periods from different years to determine which was the hotter time period, I believe that the area under the sawtooth of highs and lows starting from a baseline would yield a more accurate measurement than averaging high and low readings. I am curious as to your opinion on this assumption. Thank you.  - Henry E. Temple, Baltimore City

I'm no mathematician, Mr. Temple, but I agree with you. Simply averaging the daily high and low, or the month's high and low, ignores the dimension of time.

For example, imagine a day during a prolonged heat wave. When the new day begins at midnight, the temperature at BWI is still 83 degrees. It remains way too hot all through the wee hours. Fans and air conditioners are running all night. And when the sun comes up, it only gets hotter. The mercury climbs quickly to 90 degrees by 9:30 a.m., and by 2 p.m. it's 100 degrees. The heat persists into the evening. (We've had days just like this in Baltimore; you can look it up.)

Then around 11 p.m., a cold front pushes into the region. Thunderstorms break out, rain cools the pavement and a new air mass replaces the hot air. Temperatures at BWI drop from 80 degrees at 11 p.m. to 65 degrees by the time the day expires at midnight. Average temperature for the date: 82.5 degrees (100 degrees + 65 degrees divided by 2= 82.5 degrees).

Now consider another scenario. Same heat wave. Same high temperature of 100 degrees at 2 p.m. But this time the cold front moves in at 2:30 p.m. instead of 11 p.m. By 4 p.m., the temperature has dropped to 70 degrees, and by 11:59 p.m., it's 65 degrees at the airport. Average temperature for the day is 82.5 degrees (100 degrees + 65 degrees divided by 2= 82.5 degrees).

On the weather service record books, and for all eternity, the days will look identical. The high was 100 degrees, the low 65 degrees, and the daily average temperature was 82.5 degrees. But arguably, for anyone who had to be outdoors in it, the first scenario produced a much "hotter" more miserable day than the second. There were more hours with higher temperatures. Perhaps we need a new term - maybe "degree-minutes," or "heat-hours."

It's not an alien concept to the science. For example, hurricane researchers have a measurement for hurricanes that captures the element of time. In addition to a storm's maximum wind speed, and the maximum category it attains on the Saffir-Simpson Scale of Hurricane Intensity, scientists record "storm days" (the number of days a storm spins at tropical storm strength); "hurricane days" (the number of days a storm packs hurricane-strength winds); and "intense hurricane days" (days at Category 3 or higher).

That capacity to include time in the measurements allows a more precise comparison of storms and storm seasons. A storm that blows hard enough and long enough to be a Category 1 hurricane for 8 days, is clearly more of a menace than a tropical storm that reaches hurricane strength for only one day. Yet both are remembered and recorded as Category 1 storms.

And you're right, Mr. Temple, the way to capture that element of time in the temperature data would be to measure the area under the daily temperature curve, above a standard baseline. Makes sense, but it also involves a more involved and sophisticated calculation. The weather service folks and their computers are surely capable of it. But as far as I'm aware, nobody does it. And implementing it would mean a break from the way things have always been done, and a break from more than a century of continuous, comparable data - a valuable resource. So, while such a measure could be added to the database, I suspect it would never replace the old system of simple averaging.

Good question. Thanks. Comments always welcome.

Posted by Frank Roylance at 11:15 AM | Permalink | Comments (1)
                    

Damon Costantini writes to ask whether Maryland has more extreme weather than "nice" weather. Here's his note:

"Do we have more days over the course of a year in Maryland when it is considered 'very nice out' as opposed to extremes of heat or coldness?

"What I mean is, it seems like there are less times in Maryland when we say 'wow it is really nice out ' (ie, about 70-75 with a cool breeze) than there are times when we say 'ugh, it's brutal out' (ie, 100 degrees and humid or 25 degrees and freezing).

"Using an assumed general consensus on what 'nice out' is as compared to 'brutal out', do we have more brutal extremes than we have 'nice' days in Maryland? Or does it just seem that way because we possibly notice the extremes moreso than we may notice when it is nice out? "

Damon, I'd say without any hesitation whatsoever that we notice and remember the extremes more than the "nice" days. I suspect your question was prompted by the current heat wave. And reader traffic on this blog invariably peaks when we're facing a snowstorm, as we were in mid-February, or when we're enduring some other misery such as last month's rains. When the weather is benign, only the poets among us take notice.

I'm going to climb way out on a limb here, Damon and guess that you're a Maryland native. Anybody who has grown up or lived nearly anywhere else will agree with me that Maryland has the longest, nicest spring and fall of nearly anyplace in the country.  Winters here are generally short and mild. I've lived and worked in New Hampshire and upstate New York, and I know what "mud season" and "lake effect snows" are. We have neither here. I've had to take my car battery inside for the night to ensure the car started in the morning; I've watched the snow piles melting in May, and witnessed the leaves turning in late August as a fragile New England summer rushes away.

Sure, we can have bouts of cold weather and ice in Maryland. Some of us can remember the bay freezing over, or record snows collapsing the roof of the railroad museum. And we know what summer heat and humidity mean.

But this is neither Plattsburgh nor New Orleans. Extreme events, while memorable, are brief and few. Mostly Maryland enjoys long seasons of gorgeous weather in spring and fall, and short, mild winters. Summers are sultry, but we adjust and learn to stay out of the mid-day sun. Sometime late in August, we almost always get a break in the heat and look forward to months of fine weather. We don't turn the AC on in earnest until June, and resist the furnace switch until  November.

Weather records show we routinely see some highs in the 60s - sometimes 70s - in every winter month. Summer days in the upper 90s and 100s are pretty uncommon. Using your criteria for "brutal" - that is, 100 and above or 25 and below - here is the tally for last year:

2005 Summer days of 100 degrees or more:  None

2005-2006 Winter days that never rose above 25 degrees: One.

On the other hand:

2005-2006 Winter days that reached 60 or more: 11

That's my response. Readers? Any comments?

Posted by Frank Roylance at 5:25 PM | Permalink | Comments (2)
                    

David Gerstman writes to Ask Mr. WeatherBlogger about rainbows:

"On Sunday we were headed back from the Catskills; south on 81 through Pennsylvania. Shortly after we passed Hazelton a storm passed us - traveling eastward. After we passed through the storm we could see the setting sun in the west and the overcast sky to the east.

"In similar circumstances at home those would be conditions to create a rainbow. But on Sunday we didn't see one. Why not? Other than sun shining on a passing rainstorm, what other conditions are necessary for a rainbow? - David."

Sunshine and rainfall are both necessary to produce a rainbow, David, but they aren’t, by themselves, sufficient. It’s all about angles, and French mathematician Rene Descartes figured it all out in 1637. Here's what I learned after poking into your question for a while:

Rainbows occur when sunlight strikes the spherical drops of rain as they fall from clouds. The light enters each drop and sort of ricochets around inside the droplet. It gets refracted, or bent, as it passes from air to water, reflected off the rear inside surface of the drop, then refracted again as it passes from water back into the air and back toward the observer. It emerges at an angle 42 degrees from the original path of the sunlight.

In addition, the refraction splits the white light into its various constituent frequencies, or colors, each of which bends at a slightly different angle – between 42 and 40 degrees. That’s what produces the concentric bands of color. We see just one color from each droplet, depending on our angle of view.

The size of the droplets, too, affects the colors we see. The smallest drops produce very white bows, including "fog bows." (Airline passengers can sometimes see such fully circular fog bows or "cloud bows" when they look out the window toward a cloud that’s in the opposite direction from the sun.) The largest droplets yield very red and yellow rainbows, weak in greens and blues.

Sometimes, some of the sunlight knocks around inside the droplet a second time, emerging at a slightly different angle, and we see a double rainbow.

What we actually perceive is the combined effect of light refracted and reflected from every droplet in the shower, and of the angles at which it returns to our eyes.

But to see any rainbow, the observer has to be exactly between the sun and the rain – facing what scientists call the "anti-solar point," or the spot in the sky directly opposite the sun.

The ‘bow" is actually the top portion of a circle with an angular radius from the observer of 42 degrees around that point. The Earth blocks the lower portion of the circle. At sunset, with the sun at the horizon, the top of the arc would be 42 degrees above the horizon.

I’m not sure why you didn’t see a rainbow on your trip south on I-81. There are a couple of possible answers.

If you’re not standing (or driving) directly between the sun and the rain shower, you will see nothing, or perhaps only a partial bow. Or, I would guess that if high clouds float between the sun and the shower, observers on the ground might be able to see both, but the direct sunlight may not reach the falling droplets. So, no rainbow.

That’s why rainbows (and consequently pots of gold) are so rare. And delightful.

You can read more about rainbows here. And here's a link to rainbow photos.

Posted by Frank Roylance at 10:30 AM | Permalink | Comments (1)
                    

Jim Anthony, of Mt. Airy, asks: "Why is the barometer reading for the Baltimore area NOT listed in the papers, and very seldom shown on TV ???"

I know, Jim. I like a good barometer, too. I inherited a handsome brass one from my grandparents. And I have an electronic one as part of the wireless weather station on the WeatherDeck in Cockeysville. It's a fascinating instrument. It measures changes in atmospheric pressure that can signal a frontal passage, or the end of the worst part of a storm, or the approach of better, or worse weather.

I can't answer for the TV stations. We do still include it on the MarylandWeather.com main page. As near as we can tell, The Sun dropped its barometer readings from the printed weather page sometime in the late 1990s or early 2000s. You're the first reader we can recall who's raised a question about it. That suggests something about how many people relied on it.

It was probably dropped to make more room for other data. Our weather page - actually a half-page on the back of the Maryland section - is crammed with information compiled by a commercial service. It includes national and international temperatures and forecasts for 89 cities, national and Maryland weather maps, daily, monthly and annual weather data, lunar phases, sun and moon rise and set times, ultraviolet, air quality and pollen readings, marine forecasts and tide tables and wave heights and water temperatures on the bay and at the beaches. Oh, and there's a five-day forecast and some guy chewing on his spectacles.

That's a lot to pile onto half a page. I'm looking at a copy of The Sun from July 9, 1952, and if you subscribed then you got a daily half-column containing yesterday's temperatures for 32 U.S. cities, a one-paragraph, two-day forecast for Baltimore, hourly temperatures for the day before, tide, sunset and sunrise times, a pollen count from Hopkins, cumulative precipitation data for the day, month and year, and - yes - barometer readings taken at 8:30 a.m., 2:30 p.m. and 8:30 p.m.

There was also a hand-drawn national weather map on another page, showing isobars, or lines of equal barometric readings. (It ran adjacent to the passenger, mail and freight stats for the day at Friendship Airport; they indicated 362 people flew out that day on 54 flights, and a ton - 2,067 pounds - of freight arrived. Boy, have those numbers changed! But I digress.)

Even with all the information on today's weather page, we could probably shoehorn in a barometer reading. The more important question seems to be "why bother?"

Barometer readings and isobars are indirect indicators, and arguably less accessible and useful to the vast majority of readers, on the vast majority of days, than explicit local reports and forecasts. I would argue that relatively few readers fully understand what atmospheric pressure is, or what changes suggest about the weather to come.

So, they're gone, along with the daily passenger counts at BWI.

But we aim to serve our readers, so we're investigating whether and how we could restore barometric readings to the Weather Page - maybe the previous day's high and low, like the temperature. If anyone else out there, besides Jim, would like to see them return, leave a comment here and I will be sure our "deciders" see it.

Posted by Frank Roylance at 6:00 PM | Permalink | Comments (5)
                    

Sun Weather Page reader Emily Johnston writes with a question about why her rain gauges don't agree with each other:

"I have 4 cheapy rain gauges, two of which agree; one is hard to read, so I'm guessing; and the last is totally out of line with the readings of the others. How can I calibrate these (or at least one of them) without some sort of fancy equipment?  Is there a rule of thumb relating the cross-section of the collector with inches of rainfall? Or should I just add up all the numbers and divide by 4? (I really don't want to do that.)  - Emily Johnston, Westminster, MD"

Well, Emily, I think you've answered your own question. Your problem is that you are using at least two "cheapy" rain gauges. Like anything else, you get what you pay for. Some rain gauges are made with some care and precision - which costs money - while others are made cheaply and consequently without such precision. You might as well use a jelly jar and a plastic ruler.

I would say the odds are that the two gauges that agree with each other are somewhere close to accurate. The one that's hard to read is badly made; a scientific instrument has to be readable. And the fourth, which seems to be "totally out of line" is most likely just badly calibrated.

When you lift the veil of mystery - and all the fancy technology - the truth is that measuring rainfall is pretty simple. Real meteorologists assure me that an inch of rain really means an inch of rain. So it doesn't really matter whether your gauge is a beer glass or a kiddie pool. So long as the sides are vertical and the bottom is flat, if the clouds drop an inch of rain on the yard, you should get an inch of rain everywhere.

Professional instruments, of course, are usually far more complicated. The "standard" gauge used by the weather service - invented over a century ago - is a cylinder 20 centimeters (almost 8 inches) wide at the mouth. It's actually a funnel that sends the raindrops into an inner cylinder that is 50 cm tall. Its cross-sectional area is exactly one-tenth that of the mouth of the collecting funnel. But it's not as arcane as it sounds. By concentrating the collection area in the inner tube, forecasters can stretch or exaggerate the vertical dimension of the accumulated rain. That lets them measure the depth more precisely with a specially calibrated scale - usually to the nearest 1/100th of an inch.

The gauge on my WeatherDeck is a "tipping bucket" instrument. The rain from the funnel-like collector drips into a little bucket inside the contraption. When the bucket fills and gets heavy enough with the accumulated rain water, it tips like a seesaw, dumps its water and brings the second bucket under the funnel. The cycle repeats as long as it rains. Each tipping action sends a wireless electronic signal, which registers as 0.01 inch of rain on the console in the house. A hundred tips equals an inch of rain.

But there's no need for most of us to invest in that kind of instrument. There are well-made rain gauges on the market that are simple plastic cylinders with carefully calibrated measurements on the outside. An inch of rain fills the cylinder to the one-inch mark.

Your options would seem to be these: Fill the one you can't read with water and stick a flower in it. Throw the outlier away. And use one or both of the two that agree. We'll assume that if two inexpensive gauges agree, there's at least a chance that someone at the factories got it right.

Or, if you need a rain gauge for some serious purpose, throw them all away and invest in a good instrument. One place to start looking would be Ambient Weather, which offers gauges from a variety of manufacturers on line. Prices seem to range from about $10 to $35.  But look around. And this time, don't go "cheapy."

Posted by Frank Roylance at 12:17 PM | Permalink | Comments (0)
                    

I guess everybody has a recollection in their head about a snowstorm that struck when they were young that just had to be about the worst ever. Snow to your hips; blinding whiteouts; roads clogged for a week. Mine may be the Blizzard of 1947, which until Sunday was the record snowstorm for New York City.

I don't remember it myself. But it was always a legend in my family, because it struck at the end of the very snowy December in New Jersey into which I was born.

For my kids, it's probably the 22.8-inch storm that struck Baltimore on Feb. 11-12, 1983 - precisely 23 years before this past weekend's snow. They were 8 and 5 then, and when we tried to walk three blocks to the 7-Eleven for milk, they bogged down in the drifts on our unplowed street. They still talk about it.

For Sun reader "Walter," it was the Blizzard of 1966. He wrote us yesterday to ask why his storm wasn't on the list of the top 10 biggest snowfalls on Baltimore's record books:

"Re: top 10 snowstorms for Baltimore...Wasn't the "blizzard of 66" also one of the top storms? I remember we were out of school the whole week,and there were drifts well over 4-5 feet..I would have thought we had gotten between 15 and 20 or more inches then.. Can you elaborate as to where that storm fits in? Thanks! "

The Blizzard of '66 was a ferocious storm, as the following account by the National Weather Service attests. Lots of snow, high winds and very cold temperatures. And it came at the end of a very snowy month, which added to its impact.

But the storm itself dropped only 12 inches of new snow on the official weather station at BWI. It now ranks No. 13. This week's storm, at 13.1 inches, is No. 10.

It is a perfect example of why there was a need for an index, like the Northeast Snowfall Impact Scale we wrote about in yesterday's story, to measure the real impact of these storms. They are very complex, dropping dramatically greater snows in very localized areas, while letting other locations - including official weather stations - off relatively easy. The NESIS system, while it may not account for high winds and low temperatures, at least it provides a way to capture the extent of deep snows across a broad region, and its population, and to objectively compare that from storm to storm.

Here's how the NWS recalls the 1966 blizzard:

"January 30-31, 1966: A blizzard struck Maryland and the Northeast US. It began following morning lows of subzero in some portions of the state. Temperatures remained in the single digits as the wind and snow increased. Gusts of 50 to 60 mph caused white-out conditions over portions of western Maryland and into the Baltimore and Washington areas.

"Hagerstown reported 15 inches of snow on top of 12 inches already on the ground and some drifts as high as 20 feet. One to two feet of snow covered a large part of Virginia and Maryland. Washington had 14 inches (added to a previous snow, the depth on the ground came to 20 inches). Drifts were up to 10 feet deep in some areas.

"Baltimore had 12 inches, Conowingo Dam had 11 inches and Bel Air had 17 inches. Easton recorded 25 inches on the ground by February 2 and a January monthly snowfall total of almost 27 inches. Baltimore recorded over 21 inches for the month. Intense blowing and drifting snow continued and kept roads closed for several more days crippling transportation lines and causing a food shortage and rationing. Baltimore and Washington airports were closed for two to three days."

To read more about Baltimore's biggest snowfalls, click here.

Posted by Frank Roylance at 10:46 AM | Permalink | Comments (2) | TrackBacks (3)
                    

Well, that was quite a night. Rain, wind, snow, ice, even thunder and lightning.  And this morning, slippery roads and school delays. And almost none of it was in yesterday's forecasts.

But my editor, Mike Himowitz, knew it was coming. Yesterday, as senior editors were calling for a Wednesday story on all the mild weather in January, he warned them that such a story would upset the gods and bring snow and ice and other wintry woes. We write about a weather trend, and it comes to a crashing end.

Sure enough, the brass woke up this morning with a warm-weather story on the front page, and ice and a dusting of snow on the ground, and realized Mike was right.

The wild night was enough to stir some comments and queries from normally quiet WeatherBlog readers. Here are two:

From Robert Loskot - "Snow on green leaves, as my sainted mother always used to say.  Here in Upper Crossroads in Harford County, we not only had about a half dozen displays of staccato-like, vicious lightning with thunder that shook the dishes in the cabinets, but we received a burst of snow that turned the yard a dusty white with the grass tufts showing through.  It was an impressive 20 minutes, reminding me of that bygone commercial: It's not nice to fool Mother Nature."

And this from David Gerstman -  "Last night about 10 PM two of our sons came upstairs excitedly telling us that there was a "thunder snow" going on. We were of course aware of the thunder as it had been quite loud. And it was unusual to hear at this time of the year. But the idea of there being a thunder storm in which it was snowing is not something I ever remember.

"How (in)frequent are such occurrences? Since I suspect that the Weather service doesn't actually track "thundersnows" let me ask what unique conditions (if any) must occur for there to be a "thundersnow."

Obviously, thundersnow is very rare. Meteorologist David Schultz has studied the phenomenon and estimates that only seven in 10,000 recorded thunderstorms produce snow.  But there are a lot of thunderstorms, and a lot of snowstorms. So there are plenty of opportunities for thunderstorms to produce snow, or snowstorms to produce thunder. I can recall several severe winter storms in Baltimore since I moved here 25 years ago that have been spiked by claps of snow-muffled thunder.

But the question this morning is why the cold front that ripped through the region last night produced lightning (which makes the thunder we heard) and why it wasn't forecast more than a few minutes before it occurred. So, I called the NWS forecast office in Sterling, Va. and asked meteorologist Brian Guyer what happened last night.

He said lightning (and therefore thunder) requires strong vertical motion in the clouds. That motion strips electrons from water and ice particles and creates an electrical difference between the ground (positive) and the base of the clouds (negative). When that difference is strong enough, a spark - a lightning bolt - jumps the gap.

In a severe winter storm, that electrical gap is created by the strong vertical motion of falling snow and ice. That's true "thundersnow."

But last night's event, despite the date, was not a typical winter storm, Guyer said. "It was just associated with a cold front." 

A cold air mass was driving south and east out of Canada, barging into the warmer air ahead of it that's made January here so mild. And because cold air is more dense, and therefore heavier than warm air, it drove under the warmer air like a shovel, and forced it to rise.

That vertical motion along the fast-moving moving front produced the electrical charge that gave us our lightning, and thunder. It's  different from the kind of thunderstorms we see in mid-summer. In those, the sun heats air at the surface and starts it rising by convection, forming towering thunderclouds that produce the familiar lightning and thunder.

"But in the spring and fall you do see storms that form along cold fronts," Guyer said. But "much of this month has been like March" in Maryland, he said. So last night's storm was really a springtime, cold-front thunderstorm in late January. Any snow that came with it was no more than flurries triggered by the cold air and moisture behind the front.

No such storms were predicted in yesterday's forecasts, so I asked Guyer whether they took forecasters as much by surprise as the rest of us.

"I wouldn't say it was a surprise," he said. "It was more that there were not strong indications in the models or in any (balloon) soundings that showed we would have as much instability as we saw at the surface ... The amount of instability that did come to the surface was more than we anticipated. And it was very brief. It moved through very quickly."

Sounds like a surprise to me.

Posted by Frank Roylance at 10:49 AM | Permalink | Comments (0) | TrackBacks (2)
                    

M.J. Dunlap writes with the following question about tides in the Chesapeake Bay:

"Out of curiosity, can you explain why the water levels in the Bay are so low?  Is it because of the severe winds?  In the Chesapeake Beach/ Deale area the water looks to be about 4-5 feet below the normal lows.  In some places, like Rose Haven, it looks like the Bay receded 100'.  I can't remember seeing it like this in the last 5 years I've lived down here. Thanks."

I don't have a ready answer for her. We're a week past the full moon and another week short of the new moon. The winds are out of the southwest now, so that shouldn't be pushing water out of the bay the way a northwest wind would.  I can't even find any dramatically low tides on the area tide gauges.

So, I'm stumped. Has anyone else out there noted very low tides in recent days? Can anyone explain them? If so, leave a comment and enlighten us all.

Posted by Frank Roylance at 2:34 PM | Permalink | Comments (0) | TrackBacks (5)
                    

Roseanne Lantz writes to ask this question:

"Does it look like we might be getting any snow in the next few weeks?"

Looking for a little break from work? Or school? Jeepers, Rosie, we just had a pair of long weekends. Some of us even made it an 11-day holiday. Now you want a snow break? Or, maybe you dread snow, and want to sleep easier for a coupla weeks. OK. Either way, it's tough for the pros to forecast with much accuracy beyond a week or so. But here's the best I can find:

The coast is clear for the next week. Today's storms are well to our north. Tomorrow's are well to our south. There's no precipitation in the forecast for our region except for a chance for showers by next Tuesday. Anyway, the daytime highs will be in the 40s - too warm for snow. And overnight lows will barely dip below freezing. That's all 3 or 4 degrees above normal for this time of year.

Beyond next Wednesday, it's harder to say. Here's the 8-14-day forecast map for precipitation. Looks like the above-normal precip is to our north, with below-normal to our south. We're near normal. And here's the map for temperatures. Again, it looks like we're near-normal or above. Not the best scenario if you want snow.

Finally, if you want to go by averages, we've had 6.5 inches so far this season - a half-inch in November and 6 inches in December. That's well above average for the season to date. We're still well short of the seasonal average of 18 inches for BWI, but there's still a long way to go 'til Spring.

Hope that helps.

Posted by Frank Roylance at 2:09 PM | Permalink | Comments (2) | TrackBacks (2)
                    

Rick Dimont writes with this question:

I am looking at some of your charts and just want to be clear on definitions. When it says "wind direction" is that the direction the wind is headed or from where it is coming from; i.e. wind is out of the SSE means it's headed NNW.  Is that what wind direction means?

Good question. It can be confusing. The wind direction designations mean that the wind is COMING FROM the cited direction. "North winds at 24 mph" means the wind is from the north at that speed. Similarly, "westerly" winds are blowing out of the west. Ditto for "easterlies," such as the "easterly" trade winds." And just to confuse matters, weather vane arrows point to the direction from which the wind is coming, not where it is going.

Early weather observers once assumed that storms were coming at them from the same direction as the winds. But as communications improved, they began to realize that storms sometimes reached City B after they struck City A, even though the wind was blowing from the direction of City B .

As we have all learned during the past couple of hurricane seasons, the winds around cyclonic storms blow in a counter-clockwise direction (at least in the Northern Hemisphere). As hurricanes approached the Gulf Coast from the south, communities on the east side of the storm's center got their strongest winds from the south, off the Gulf. But towns west of the storm center were getting pounded from the north - inland. In the absence of modern satellites and communications, they might well have concluded the storm itself was coming at them from the north.

Posted by Frank Roylance at 10:51 AM | Permalink | Comments (0)
                    

The WeatherBlog has received this question from Sally Ryerson, at the Saint James Academy:

Dear Sally,

Good question. Actually, everyplace on the Earth's surface is subject to tidal action. Even land. The combined gravity of the sun and moon are always pulling on the Earth's surface and tugging it out of shape like a rubber ball. But it's the oceans where we see the action most clearly.

Bodies of fresh water are mostly too small to be visibly affected in the way the oceans are. But they do actually rise and fall twice a day as the Earth's crust beneath them rises and falls. The Great Lakes are an exception. They're so big that a small tidal effect - a couple of inches - has been measured in the lake waters. But it is swamped by other influences, such as wind and atmospheric pressure changes.

Here is the best discussion I've found on the Great Lakes tides. It's from the National Oceanic and Atmospheric Administration. Just scroll down to the fourth item.

Posted by Frank Roylance at 10:30 AM | Permalink | Comments (0)
                    

MarylandWeather.com reader Paul Snyder asks: "The papers and newscasters say we are in a drought, but I notice Maryland is +1" above average rain fall for the year and the local reservoirs are full. What is the official definition of a drought?"

Officially-designated drought conditions are determined by a complex formula that takes into account measurements of soil moisture, streamflow, precipitation and the health of vegetation as measured by satellite imagery.

The data is fed into a computer, which crunches the numbers and spits out a drought "index," and the results are mapped to produce a weekly "drought monitor" map. That's what we were going by last week when The Sun published a story about the deepening drought, and the prospects for relief from the remnants of Tropical Storm Tammy.

Ask your note suggests, it is possible for some of the indicators, such as water levels in reservoirs, to be close to normal, while others are sufficiently low to push the region into the drought categories. In our case, the September drought was considered an "agricultural" drought, primarily affecting farming interests. It was not regarded as a "hydrological drought," because water supplies had not yet been seriously depleted.

The drought monitor calculations are done weekly. The most recent was dated Oct. 4, which showed most of Maryland in moderate drought. Those numbers will be updated shortly, and the results will be updated for Oct. 11, and will likely show that the drought has ended, thanks to the record-breaking rains from Tammy. The seasonal outlook, thanks to Tammy, looks brighter.

Posted by Frank Roylance at 10:57 AM | Permalink | Comments (0)
                    

Heather Moyer writes from Columbia:

"I love reading the Baltimore Sun weather blog. My question is this: is the state's rainfall amount below normal? I ask because it seems as if many of the trees and lawns are turning brown and looking stressed - at least where I live. Thanks!"

Thanks for writing, Heather. Yes, it's been very dry lately. In fact, until nearly a quarter-inch fell today, we had recorded no rain at all at Baltimore-Washington International Airport since Aug. 28 - more than two weeks ago. We're now about 1.5 inches short for the month. It's the first significantly dry month since May. I think we're all hoping for a little blast from Hurricane Ophelia. The forecast says we may get some.

Posted by Frank Roylance at 6:15 PM | Permalink | Comments (0)
                    

Joyce writes to ask: "Could you explain the pressure to me? I have vestibular neurocity (a form of vertigo). When the pressure drops there is a real increase in my symptoms of light headedness and nausea. Storms send me to bed for days. For fun info, can you tell me the best and worst states to live in? I thank you so much!"

Thanks for the query, Joyce. You can think of atmospheric, or "barometric" pressure as the weight of all the air above your head pressing down on the Earth's surface - including people like you and me walking around on it. It's much like the pressure of water on the bodies of scuba divers as they descend to the bottom of a pool or a lake. The weight of the air pressing down on us at sea level amounts to about 14.7 pounds per square inch, or roughly a ton per square foot.

Fortunately, the pressure in our bodies pushing out is about the same as that pushing in, so we aren't crushed. But we can feel the change of pressure in our ears when we ride up or down on the elevators in tall buildings, or when we drive up or down mountain roads. The higher we get, the lower the atmospheric pressure.

When we're going down, there is more air pressing on our eardrums, and we feel a need to "clear" or "pop" our ears by swallowing or yawning. What we're actually doing is stretching our throats, allowing air at the increased pressure to enter our eustachian tubes, which lead from high in the back of our throats into our inner ears. That equalizes the pressure on the inside and the outside of our eardrums.

The same thing happens in reverse when we go up the elevator. The atmospheric pressure on the outside drops, and the pressure of air in our inner ears becomes higher than that of air on the outside, and it presses against our eardrums. When we yawn or swallow, the air escapes and the pressure is relieved.

Weather systems are much slower than elevators, but they have a similar effect. Where atmospheric pressure is low, the weather becomes stormy. Clear, sunny weather comes with high pressure systems. Scientists have long noted the link between changes in atmospheric pressure and physical symptoms, such as arthritis and Meniere's disease. So you're probably not imagining it, especially since our inner ear is where our balance control mechanisms are located. When something affects those balance centers, we can experience vertigo, nausea and similar symptoms. 

If you think your symptoms get worse is stormy weather, it might make sense to live somewhere where the weather is clear and dry, and atmospheric pressure typically is high, perhaps in the Southwest. High-altitude states like Colorado would appear to be out, however, as would stormy places like Seattle. Far-northern places like Canada or Alaska might work. Cold air is dense air, which is also heavier. Some of the highest atmospheric pressures ever recorded have been in far northern regions. Ideally, though, I guess what you're looking for is a relatively comfortable coastal desert. Maybe Lima, Peru or Casablanca, in Morocco.

Posted by Frank Roylance at 11:39 AM | Permalink | Comments (2)
                    

Dorothy Crumb writes from upstate New York to ask:

Can you tell me where the rain comes from in a hurricane? If it picks it up from the ocean, why isn't it salty? Thanks, Dorothy W. Crumb, Pompey, New York (Near Syracuse)

Dear Ms. Crumb,
The answer is that the rain does indeed come from the ocean. But as the seawater evaporates under the hot tropical sun, and moves up into the atmosphere as water vapor, it leaves its salts behind.

It's just like distilling water by boiling it, capturing the steam and condensing it again as a liquid. The process leaves most everything that isn't water behind. And the water you condense and capture is fresh.

That's how people have obtained salt for centuries - by evaporating seawater and scraping up all the salt that's left behind when the water is gone.

So, the tropical sun heats the ocean, turning some of the seawater into water vapor and separating it from its salts. The water vapor rises inside the hurricane's thunderstorms, cools and condenses, and falls again as rain - freshwater rain.

Thanks for asking.

For more than you ever wanted to know about evaporation's role in the water cycle, click here.

Posted by at 5:37 PM | Permalink | Comments (1)