Arrived back on the job on this sleety, rainy morning to an emailed question from MarylandWeather.com reader George Brauer. He writes: "Could you explain in layman's terms Barometric pressure and Dew point and what it does to the weather."
I'll try, with help from Walter Lyon's book, "The Handy Weather Answer Book."
First, barometric pressure:
This is simply the pressure exerted at the Earth's surface by the weight of all the air directly above us. Although we live at the bottom of this ocean of air, we're rarely directly aware of how it presses on us. But we can sense it when we ride up or down in an elevator in a tall building. As we zip higher into that ocean of air, there is less and less air above us, so the air pressure - or barometric pressure - goes down. We feel it as pressure - actually, reduced pressure - on our eardrums, and we swallow or yawn to get them to "pop." What we're really doing is struggling to equalize the air pressure on the inside and the outside of our eardrums. It happens again when we descend, as the volume of air above us, and the pressure it exerts on us, go up.
The first instrument for measuring atmospheric pressure was invented in 1644 by Evangelista Torricelli, a student of Galileo. Scientists learned that barometric readings rise and fall with the passage of weather systems. It's usually measured in inches (or millimeters) of mercury, a reference to instruments like Torricelli's that used a column of mercury in a glass tube as a measuring device. Most modern barometers have other kinds of sensors, but we still mostly use "inches of mercury" as our common reference.
Stormy weather is usually associated with low atmospheric pressure - that is, less air weighing down on us. Clear, sunny (or starry) weather is associated with high atmospheric pressure. So, in the days before electronic communications and weather satellites, when the barometer began to fall, weather prognosticators learned to expect increasing clouds and perhaps precipitation or stormy weather in the next few days. Likewise, when the barometer was rising, they could predict that the bad weather would clear. From most perspectives, the higher the reading, the nicer the weather. The lower the pressure, the more dangerous the storm.
The average barometric reading at sea level is 29.92 inches of mercury. (It's lower at locations with elevations above sea level, since there is less air overhead weighing down on the instruments.) The highest reading ever noted was 32.01 inches of mercury at a spot in Siberia. Very cold locations tend to have higher barometric readings because cold air is denser than warm air, and therefore it weighs more.) The lowest reading ever was 25.63 inches, in the eye of Typhoon Tip, which struck the Philippines in October 1979.
Got it? Okay. Now dew point.
The dew point is expressed as a temperature reading, and it indicates the temperature at which dew will form, wetting the grass and cars. What's really happening is this: The atmosphere always contains some amount of water vapor, or moisture. How much it can hold depends on the temperature. Warmer air can hold a lot more water vapor than colder air. So, as the temperature drops overnight, it will sometimes reach its "dew point" the temperature at which it can no longer hold all its water vapor, causing some of it to condense on exposed surfaces as "dew." The same thing happens in summertime, when we carry a glass of iced tea outside. Because the temperature of the glass is cooled by the ice inside it, the cold glass then cools the air adjacent to it below its dew point. Baltimore summertime humidity being what it is, the chilled air around the glass can't hold all its moisture, and dew forms on the glass. We say it "sweats," and when you take a drink, the moisture rolls down the surface of the glass and into your lap.
In a daily forecast, a high dew point - one close to the real, or ambient temperature - suggests a chance of dew, or fog, or rain, since it's possible a small drop in temperature could cause the moisture in the air to condense and make things wetter. On the other hand, a low dew point, or a wide gap between the dew point at the ambient temperature, suggests the air is dry, and unlikely to produce dew or fog or rain.
Bottom line: high, rising barometer and low dewpoint suggest sunny, dry weather; low, or falling barometer and high dewpoint predict cloudier, wetter weather.
For a case in point, just take a look at the two-day weather data for Sunday and Monday at Baltimore (BWI). In the temperature columns, you can see the dew point ("Dwpt") readings rising closer and closer to the "Air" temperature. At the same time, in the column labeled "Weather," the description of actual conditions at BWI gets wetter and wetter, until it is foggy and misty and rainy. You can also see that the wind shifts to the East, bringing in more Atlantic moisture, which also explains why the moisture in the air, and therefore the dew point, were rising.