Frontal Systems, Continued
Cold fronts are normally located in well-deemed pressure troughs
whenever there is a marked temperature contrast between two air
masses. In most cases, a careful analysis of the isobars indicates the
correct position of the pressure trough that contains the front. This
method of isobaric analysis is frequently the only possible means of
locating fronts over ocean areas or regions of scanty surface reports.
Other indications of cold fronts can be classified as prefrontal, frontal, or
postfrontal as follows:
1. Pressure tendencies. In advance of cold fronts, the tendency
characteristic is usually indicated by a steady or unsteady fall. The
isobars of falling pressure in advance of the front usually form an
elongated pattern approximately parallel to the front. After passage of
the front, the tendency generally shows a steady rise.
2. Wind. With the approach of the front, the wind is normally from
the south or southwest in the Northern Hemisphere, veering to parallel
the front. At the passage, the wind generally shifts abruptly to the
northwest. Very gusty winds frequently occur at the frontal passage and
usually after passage.
3. Cloud forms. In advance of cold fronts, the cloud types are typical
of the warm air. Towering cumulus, cumulonimbus, stratocumulus, and
nimbostratus are associated with the passage. After passage, these cloud
forms may prevail for several hundred miles with the slow-moving cold
front. Very rapid clearing conditions are associated with the
fast-moving cold front after passage. Well back in the cold air in both
types of cold fronts, the only clouds normally found are fair-weather
4. Precipitation. Showers and sometimes thunderstorms occur as a
cold front passes. Continuous precipitation is observed for some hours
after passage of a slow-moving cold front. Showers and thunderstorm
activity of short duration will occur with the passage of a fast-moving
cold front, followed by very rapid clearing conditions.