|LOWER TROPOSPHERIC ANALYSIS
METEOROLOGIST JEFF HABY
The meteorological processes to look for on the surface and
850 mb chart are
temperature advection, position
fronts, wind speed and direction,
dewpoint advection, intensity of
low and high pressures, and how
climatic controls will influence the weather.
Temperature advection is a function of the wind speed, the temperature gradient
(change in temperature with distance), and the angle the isobars or height contours make with the isotherms. If the
wind is moving colder air toward your region, that is a cold air advection pattern. If the wind is moving warmer
air toward your region, that is a warm air advection pattern.
On the surface chart, fronts will be marked. On the
850 chart, you will have to use wind direction and the thermal gradient to infer frontal position. Frontal speed
is a function of how fast the low pressure and high pressure systems are moving, the strength of the upper winds
(zonal winds more favorable to quick
movement), and the strength of the low level winds.
Wind speed and direction
determines the type of air (from a source region) that will move into your locale. Winds from a warm/moist ocean
will increase the humidity and temperature. Winds
from the interior of a continent will bring drier air into the
region. In the SE U.S., a south wind brings an increase of moisture and temperature, a north wind brings drier air
and cooler temperatures.
Often surface charts will show pressure falls while the 850-mb chart will show height falls.
Pressure falls and height falls indicate if low pressure regions are
deepening and indicate the direction they are
propagating. Lows tend to move toward the region of greatest pressure falls and height falls.
Climatic controls such
as topography, latitude, altitude, continentality, and ocean currents influence the likely weather a location will
receive. It is important to see if the air is moving from a high elevation or low elevation region,
from a higher
latitude or lower latitude, from a dry source or a moisture source. Cool and dry air will tend to be stable while
warm and moist air will tend to be
unstable. Temperature changes due to
daytime and nighttime is only one factor
that changes temperature during the day. Temperature advection, fronts,
rising or sinking air,
latent heat release/absorption, and
cloud cover need to be looked at also. The surface and 850 millibar charts can be found at: