Why so dry at BWI?
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?