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General characteristics of fronts

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GENERAL CHARACTERISTICS OF FRONTS

All fronts have certain characteristics that are common and usually predictable for that type of front. Cold frontal weather differs from warm frontal weather, and not every cold front has the same weather associated with it. The weather, in-tensity of the weather, and the movement of fronts are, to a large degree, associated with the slope of the front.

Frontal Slope

When we speak of the slope of a front, we are speaking basically of the steepness of the frontal surface, using a vertical dimension and a horizon-tal dimension. The vertical dimension used is nor-mally 1 mile. A slope of 1:50 (1 mile vertically for every 50 miles horizontally) would be considered a steep slope, and a slope of 1:300 a gradual slope. Factors favoring a steep slope are a large wind velocity difference between air masses, small temperature difference, and high latitude. The frontal slope therefore depends on the latitude of the front, the wind speed, and the temperature difference between the air masses. Because cold air tends to underrun warm air, the steeper the slope, the more intense the lifting and vertical motion of the warm air and, therefore, the more intense the weather.

Clouds and Weather 

Cloud decks are usually in the warm air mass because of the upward vertical movement of the warm air. Clouds forming in a cold air mass are caused by the evaporation of moisture from precipitation from the overlying warm air mass and\or by vertical lifting. Convergence at the front results in a lifting of both types of air. The stability of air masses determines the cloud and weather structure at the fronts as well as the weather in advance of the fronts.

Frontal Intensity

No completely acceptable set of criteria is in existence as to the determination of frontal inten-sity, as it depends upon a number of variables. Some of the criteria that may be helpful in delineating frontal intensity are discussed in the following paragraphs.

TURBULENCE. Except when turbulence or gustiness may result, weather phenomena are not taken into account when specifying frontal inten-sity, because a front is not defined in terms of weather. A front may be intense in terms of discontinuity of density across it, but may be accompanied by no weather phenomena other than strong winds and a drop in temperature. A front that would otherwise be classified as weak is considered moderate if turbulence and gustiness are prevalent along it, and an otherwise moderate front is classified as strong if sufficient turbulence and gustiness exist. The term gustiness for this purpose includes convective phenomena such as thunderstorms and strong winds.

TEMPERATURE GRADIENT. Tempera-ture gradient, rather than true difference of temperature across the frontal surface, is used in defining the frontal intensity. Temperature gradient, when determining frontal intensity, is defined as the difference between the repre-sentative warm air immediately adjacent to the front and the representative surface temperature 100 miles from the front on the cold air side. A suggested set of criteria based on the horizontal temperature gradient has been devised. A weak front is one where the temperature gra-dient is less than 10F per 100 miles; a moderate front is where the temperature gradient is 10F to 20F per 100 miles; and a strong front is where the gradient is over 20F per 100 miles. The 850-mb level temperatures may be used in lieu of the surface temperatures if representative surface temperatures are not available and the ter-rain elevation is not over 3,000 feet. Over much of the western section of the United States, the 700-mb level temperatures can be used in lieu of the surface temperatures.

Speed

The speed of the movement of frontal systems is an important determining factor of weather conditions. Rapidly moving fronts usually cause more severe weather than slower moving fronts. For example, fast-moving cold fronts often cause severe prefrontal squall lines that are extremely hazardous to flying. The fast-moving front does have the advantage of moving across the area rapidly, permitting the particular locality to en-joy a quick return of good weather. Slow-moving fronts, on the other hand, may cause extended periods of unfavorable weather. A stationary front that may bring bad weather can disrupt flight operations for several days in succession. The specific characteristics of each of the types of fronts is discussed in lessons 3 through 6.

Wind Component

The speed of a front is controlled by a resultant component of wind behind a front. The wind component normal to a front is determined by the angle at which the geostrophic winds blow toward the front, resulting in a perpendicular force applied to the back of the front. For exam-ple, the component of the wind normal to a front that has a geostrophic wind with a perpendicular flow of 30 knots behind the front has a 30-knot component. However, a 30-knot geostrophic wind blowing at a 45 angle to the front has only a 15-knot component that is normal to or perpen-dicular to the front. The greater the angle of the wind to the front, the greater the wind component normal to that front. The smaller the angle, the less the wind component normal to the front.

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