|SMALL SCALE WEATHER INFLUENCES
METEOROLOGIST JEFF HABY
Every forecast region has meso and micro-scale features that impact the local weather. The first example is
topography. Air flowing over an elevated region will cool while the
Relative Humidity increases
(upslope flow). Air descending over
will warm while RH decreases (downslope flow). An elevated region also causes
convergence and can act as a
trigger mechanism for precipitation. Examples
include upslope snow and mountain convection. Here is an example of when the air on elevated
terrain may be higher in temperature than surrounding low lying areas: suppose a cold air mass settles over the
forecast region. The cold dense air wants to stay in low-lying areas. If the cold air mass is shallow, it will be
warmer at the higher elevation regions.
The second example is nearness to a water source. Evaporation from a lake
in the summer,
even small ones will cool the air above the lake surface more than locations that are further inland (especially
downwind from the lake). This effect
is most pronounced when the RH is low. Evaporation increases as RH decreases,
especially in warm air. Even though evaporation is a cooling process, the lake may warm the surrounding areas. How?
If the lake waters are much warmer than the surrounding air, conduction and convection of heat from the warmer lake
surface will warm the air above it. When forecasting it is important to compare lake temperature to air temperature,
determine how this effect will compare on day versus night, keep track of the wind speed, and keep track of the RH.
Wind speed and direction will determine where the warmed or cooled air from the lake will go. Large lakes can effect
the stability of the atmosphere. A Lake, or
ocean for that matter, which is cooler than the surrounding air will
promote stability above the lake. The cooler water surface causes the air above the lake to be cooler. Cooler air
under warmer air causes stability. If the lake is warmer than the surrounding land, that promotes instability.
Warm / moist air will want to rise into cooler and drier air. Warm air is less dense than cold air;
Moist air is less dense than dry air. Other
local effects include: cloud cover, amount of vegetation / urbanization,
outflow boundaries, and irrigation.