Friday, November 11, 2005
No Frying this Friday ...
A. Camden Walker @ 2:02 AM
Remember: today would feel like a warm day...in January. You'll be sniffling a lot from the cold this morning but MID 50's can be expected at mid-afternoon under periodic clouds in an otherwise sunny sky. The more sun, the more likely we hit 57. However, a brief breeze this afternoon under a cloud's shadow, could make you think it's only in the low-40's. See extreme headgear, at left, you WILL NOT need today. Question: does one break out the big, wooly scarf yet? We dipped below the 40 degree benchmark at National this morning...HOT Snow: maybe you weren't seeing things afterall.
SunTrust clock reads 52ºF. White flakes appear on your jacket arms amid some rain drops, but quickly disappear. You tell your colleagues the black blade of your windshield wiper showed a thin white layer when scraping past your face. But no one believes you. ONLY YOU had the quick eye to catch this rare event.
Review: Snow forms when the atmospheric temperature is at or below freezing (0ºC/32ºF) and there is a minimum amount of moisture in the air. Since snow forms in the atmosphere, not at the surface, snow can fall when surface temps are above freezing. Boring for this discussion--but worth reiterating--is that if the surface temp is at or below freezing, of course the snow will reach the ground.
As a general rule, snow will not form if the "ground" (the lowest 2 meters of the atmosphere) temperature is above 41ºF (5ºC). But, yes, under hard-to-attain atmospheric conditions that can only briefly exist above this critical surface temperature, non-accumulating snow CAN maintain it's crystalline structure long enough to be visible in very mild air.
Why is this temperature arena around 40 degrees such a critical tipping point for how moisture behaves in the atmosphere? Water exists at its most-dense state at the temperature of 39.2ºF and therefore physics dictates it "prefers" this stable state of lowest potential energy. Little internal molecular energy exists for evaporation, but just enough so that freezing is difficult. But when the atmospheric conditions rapidly change, energy gets injected--even into a water molecule that was "happy" at its equilibrated state--thereby making weather more extreme & dynamic!
What is "minimum amount of moisture" that was mentioned above? Basically, prevailing dry air conditions enables snow to form in clouds & then, eventually, reach the ground more easily--remarkably independent of temperature conditions. As you might expect, snowflakes will begin to melt as they reach this warmer temperature between cloud and ground. This melting creates evaporative cooling which cools the air immediately around the snow flake. Just what is this "evaporative cooling"? Leave the bathroom door open during your next shower. The mirror won't be fogged up as all of your steam & water vapor escape out of the bathroom. You become chilled, as water evaporates off of your skin. This same concept produces a small pocket of cold air around each individual snow flake that retards melting. The dryer an air mass into which a snowflake falls, the more opportunity it has to take advantage of evaporative cooling. So the temperature disparity between sub-freezing cloud temperature to above-freezing ground temperature can be WIDE if the column of air between cloud:ground is very dry as the snow begins falling. The snow will cease once the dew point is brought above freezing in the air column--indicating rising moisture levels--preventing evaporation from cooling the air temperature below freezing.
A quick snow-burst can survive for about a minute above 50 degrees if the dew point temperature is very low, such as in the mid-teens! Saturation (100% relative humidity) quickly occurs. The dew point has catapulted due to the presence of moisture falling into the atmospheric layer between cloud and ground. Simultaneously, evaporational cooling quickly plunges the temperature several degrees. But, in the end, 100% relative humidity will be reached at a temperature well above the freezing mark. This effectively turns the snow into rain since the entire lower-atmosphere is saturated and yet still warm (above 32 degrees).
It is ever too cold to snow? No. The converse is never true and is not nearly as complicated as the "preservation" of snow flakes in a warm atmosphere. It can snow even at incredibly cold temperatures as long as there is some source of moisture and some way to lift or cool the air.
Curious what the median "key" temperature is for the heaviest snow? Typically 15°F or slightly warmer. In the Rockies, light winds and temps around 15, have produced snows that use moisture extremely "efficiently" thereby not requiring all that much atmospheric moisture to produce a large, yet fluffy (porous) crystalline matrix.
Stop. GAMMA-time?
What is the third letter in the Greek Alphabet? Well, the Tropical Prediction Center is watching a persistent area of showers just north of Panama. It is a prime area for late-season development. This cluster is predicted to drift northward, near where Beta formed. Only a few hundred miles to its north--around the latitude of Honduras--there are very brisk upper level winds. Check back here at CapitalWeather on Sunday.
VETERAN'S DAY
Given the importance of weather during wartime, it may not be surprising that the U.S. National Weather Bureau began as a part of the Army Signal Corps. Although modern technology allows most militaries to perform well in nearly any kind of weather conditions, current weather information and weather forecasting are still very important, for instance, to U.S. operations in Afghanistan & Iraq.
General Eisenhower famously executed Operation Overlord (D-Day) in 1944 during a small window of opportunity to cross the English Channel through which a Cold Front had just cleared. Eisenhower's decision to proceed is regarded as an inspired one. The only other candidate date--June 18--marked the worst storm in 60 years. Had he waited, D-Day might have been a disaster, as all three meteorology teams incorrectly forecast the storm which swept through the Channel.
Russian winters are famous for "defending" the country. In 1812, Napoleon sacrificed over 70% (372,000+) of his grand army solely to subfreezing weather in an attempt to infiltrate beyond Moscow. During Germany's zealous, eerily similar, eastward empirical expansion of 1941, rain and cold completely halted infiltration beyond Moscow. It is unclear who in Berlin was so severely uninformed, so as to allow for a November campaign into Russian average temps of 28ºF...
During the American Civil War, General Burnside made his failed "Mud March" January 20-24, 1863. Never again during winter would the Union invade Virginia. Teams of mules, supplies, wagons, and weapons disappeared (sank) during their march to Richmond after 30 hours of continuous rain.
When weather ("Tian") is an element which one cannot control, the best strategy will be taking full advantage of the condition when permits. Going against the force of nature may prove rewarding when one overcomes, but it usually spells destruction.
-Sun Tzu contemplates weather in 400 B.C.
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