UNIT 7—LESSON 3
Identify the procedures used in
locating fronts on a surface chart analysis.
OUTLINE Location of fronts Stationary and nonfrontal troughs Satellite cloud pictures Surface frontal analysis thumb rules FRONTAL ANALYSIS The location of fronts is determined by past
Location of fronts
Stationary and nonfrontal troughs
Satellite cloud pictures
Surface frontal analysis thumb rules
The location of fronts is determined by pasthistory, air-mass analysis, satellite data, and weather observations. Since fronts and their ac-companying weather move across the earth in established directions with somewhat definite speeds, we are able to track their movement from chart to chart.
When a front is drawn on the weather map, the analyst considers many factors, some of which may not be apparent on the map. Location of the various observation stations and local effects play a great part in weighing the elements which are used in locating fronts. A front may be well de-fined in the isobaric pattern and easy to locate through the surface weather changes peculiar to it. On the other hand, the front may be weak or indistinct and therefore difficult to locate from surface parameters. Consideration of frontal structures from upper air information is helpful in locating all types of fronts. These features of fronts on upper air charts are discussed in a later lesson.
When a front approaches and passes a land station, a sequence of events maybe anticipated, but at sea, the movement of the ship combines with the movement of the front to produce changes which are not so easy to foresee. For example, when a ship and front travel toward one another, the front passes quickly because of the combined speeds of both. If a ship is mov-ing in about the same direction as a front, the change of wind and associated weather conditions may take place slowly. In exceptional cases, a fast ship may overtake a front and pass through it, thus reversing the sequence of changes. Varia-tions in the type of weather encountered with frontal passages must also be taken into account, because the models available only represent typical conditions. Other factors such as strength of the front and nature of the terrain over which it passed or is passing, and subsequent modifications to the air mass, speed of the front, and type of front must be taken into con-sideration. In this lesson, some of the rules and aids in locating fronts on the surface weather map are discussed.
Learning Objective: Identify the pro-ceduresused in locating fronts on a surface chart analysis.
LOCATION OF FRONTS
When locating fronts, you must study air-massesbecause they are separated by fronts. Factors that determine the various types of fronts on a weather chart are presented here.
Cold fronts are normally located in well-definedpressure troughs. In most cases, a careful analysis of the isobars outlines the pressure trough that contains the front. Without a satellite pic-ture, this is frequently the only possible means of locating fronts over ocean areas or regions of scanty surface reports. More precise pinpointing of a cold front’s location within a trough is dependent on your analysis of the other elements of weather observations. These elements provide prefrontal, frontal, or postfrontal indications and are as follows:
1. Pressure tendencies. Prefrontal tendencies show steady or unsteady fails while isallobars (lines of equal pressure change) of falling pressure usually form an elongated pattern approxi-mately parallel to the front. Postfrontal tenden-cies generally show a steady rise. Stations behind the front show a tendency characteristic of /, k, or /. The first two indicate that the front passed the station during the 3-hour period before map time.
2. Wind. Prefrontal winds are normally from the south or southwest in the Northern Hemisphere, veering to parallel the front. At fron-tal passage, the wind generally shifts abruptly to the northwest. Gusty winds frequently occur at the front and after passage.
3. Cloud forms. Prefrontal 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 hun-dred miles with the slow-moving cold front. Rapid clearing is associated with the passage of a fast-moving cold front. Well back in the cold air in both types of cold fronts, the only clouds nor-mally found are fair weather cumulus.
4. Precipitation. Showers and sometimes thunderstorms occur with the cold front passage. Postfrontal precipitation continues for some hours with slow-moving cold fronts, while rapid clearing occurs with fast-moving cold fronts.
5. Temperatures. Prefrontal temperatures are those of the warmer air mass. They will be relatively high. After passage of slow-moving cold fronts, temperatures decrease rapidly. The temperature change accompanying the passage of fast-moving cold fronts is more gradual; the colder temperatures are usually observed some distance (as far as 50 to 100 miles) behind the front. The cold front, therefore, should be located on the warm air side of the transition zone be-tween the two air masses and along a line of cyclonic wind shear (trough).
6. Dew point. A drop in the dew point is observed with the passage of either type of cold front.
7. Visibility and ceiling. With the approach and passage of a slow-moving cold front, the visibility and ceiling decrease and remain low after the passage until well within the cold air. Fast-moving cold fronts are preceded by regions of poor visibility and low ceilings due to shower activity. After their passage, the ceiling rapidly becomes unlimited and the visibility unrestricted.