WIND: A front is a zone of confluence. Air streams approach each other from different directions. Winds converge 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 baroclinic 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 warm fronts. DEWPOINT TEMPERATURES: 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 images, radar, 1000-500mb-thickness field. |