On this sounding the upper winds that show the
greatest variation above the surface layer are those
between the 800- to 650-mb layers. This indication
coincides closely with the frontal indications of the
temperature (T) and dew point (Td) curves (see fig.
4-30). Since the wind veers with height through the
layer, the front would be warm. The vertical wind shift
through a frontal zone depends on the direction of the
slope. In cold fronts the wind backs with height, while
in warm fronts the wind veers with height. At the
surface the wind always veers across the front, and the
isobars have a sharp cyclonic bend or trough that points
toward higher pressure. Sometimes the associated
pressure trough is not coincident with the front; in such
cases there may not be an appreciable wind shift across
the frontonly a speed discontinuity.
One of the important characteristics of all fronts is
that on both sides of a front the pressure is higher than
at the front. This is true even though one of the air
masses is relatively warm and the other is relatively
cold. Fronts are associated with troughs of low
pressure. (A trough is an elongated area of relatively
low pressure.) A trough may have U-shaped or
V-shaped isobars. How the pressure changes with the
passage of a front is of prime importance when you are
determining frontal passage and future movement.
Friction causes the air (wind) near the ground to
drift across the isobars toward lower pressure. This
causes a drift of air toward the front from both sides.
Since the air cannot disappear into the ground, it must
move upward. Hence, there is always a net movement
of air upward in the region of a front. This is an
important characteristic of fronts, since the lifting of the
air causes condensation, clouds, and weather.
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, intensity of the
weather, and the movement of fronts are, to a large
degree, associated with the slope of the front.
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 horizontal
dimension. The vertical dimension used is normally 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
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 under run 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.
No completely acceptable set of criteria is in
existence as to the determination of frontal intensity, 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 intensity, 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.
gradient, rather than true difference of temperature
across the frontal surface, is used in defining the frontal
intensity. Temperature gradient, when determining