|LOCATING FRONTAL BOUNDARIES
METEOROLOGIST JEFF HABY
WIND: A front is a zone of
confluence. Air streams approach each other
from different directions. Winds
at the front and as one moves in the direction of the front, the winds shift cyclonically (counterclockwise). With
time, wind (and other surface parameters) is beneficial for identifying times of frontal passage (FROPA). For strong
fronts, the wind shift is quickly followed by a rapid temperature change. For weaker fronts, over oceans or for the
southern extension of deep penetrating cold fronts such as those that move into the Gulf of Mexico, the
zone may be spread out evenly or is located far behind the wind shift. Mesoscale processes such as mountains and coasts
also influence fronts.
PRESSURE: Fronts are located in
pressure troughs. The pressure trough
is created by rising air
at and near the frontal boundary. Due to the gradient wind from to the presence of
friction, the winds at the surface
will blow across the isobars
at approximately 30 degrees angle towards low pressure. Thus, the winds are observed
to converge at areas of low pressure and diverge at areas of high pressure at the surface.
TEMPERATURE: The front
is located on the warm side of the
temperature gradient. It is not necessarily located in the region temperatures
fall at the greatest rate. The temperature gradient is usually observed to be greater along cold fronts than along
Dewpoint temperatures are usually lower in
continental air and in colder air.
Fronts represent the change in one airmass from another. Often one airmass will be more moist or
drier than the
other will. Dewpoint temperatures are of primary importance in locating
drylines and are observed to decrease rapidly
behind the dryline.
PRESSURE TENDENCY: Pressure falls are observed ahead of fronts and
pressure rises behind fronts.
This is because
upward vertical motion on the synoptic scale lies along the frontal boundary. The surface low
usually moves from the area of greatest pressure rises toward the region of greatest pressure falls. Pressure
tendencies can be adjusted for the diurnal pressure changes caused by tides in the upper atmosphere. The
atmosphere has tides just as the ocean does (the atmospheric tide just isn't visible to the human eye). These
tides vary with location and season and have a greater amplitude in low latitudes.
OTHER INDICATORS: weather and clouds, satellite