Published Tuesday, December 2, 2003 in the San Jose Mercury News
A. The Earth's energy balance is indeed remarkable. The vapor you
refer to it is called ``evaporation fog'' or ``steam fog.'' As the sun's
energy strikes water droplets that are on the roof, some evaporate into
water vapor just above the surface of the roof. However, if the air is cold
enough, they will almost instantaneously condense into tiny cloud droplets,
and this is what you are seeing.
Q. If there is no wind, how fast does a typical raindrop fall? Same for a typical snowflake? Daryl Adams - San Jose
The terminal velocity of a falling raindrop through still air depends on
its size. An average raindrop is about 2 millimeters in diameter and has a
maximum fall rate of about 14.5 miles per hour or 21 feet per second. A
large raindrop, 5 mm in diameter, falls at 20 mph (29 feet/second), but
drops of this size tend to fall apart into smaller drops. Drizzle, which has
a diameter of 0.5 mm, has a fall rate of 4.5 mph (7 feet/second).
Snowfall rates are just a variable. Heavy wet snow can ``zip'' down at as much as 9 mph (13 feet/second), while a small light flake will seemingly float down at 1.5 mph or just over 2 feet per second.
Q. I am wondering if a tornado would show up on GOES (a weather satellite). If so, what would it look like? Alissa Hicks - Novato
A. There are a number of limitations that prevent tornadoes from being visible from space. First, the best resolution of the GOES (Geostationary Operational Environmental Satellite) is just slightly more than a half-mile, while the diameter of a tornado averages about 100 yards. Tornadoes also occur under massive cloud shields that would prevent seeing them from space, even with higher resolution imagery.
Jan Null, founder of Golden Gate Weather Services, is a retired lead forecaster with the National Weather Service. Send questions to him c/o WeatherCorner, San Jose Mercury News, 750 Ridder Park Drive, San Jose, Calif. 95190. You also can telephone questions at (510) 657-2246, fax them to (510) 315-3015 or e-mail email@example.com, or fill out a form online at http://ggweather.com/questions.htm. Please indicate in your e-mail what city you live in.