frontal intensity, is defined as the difference between
the representative 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 gradient is less than 100F
per 100 miles; a moderate front is where the
temperature gradient is 10 0F to 20 0F per 100 miles;
and a strong front is where the gradient is over 20 0F per
The 850-mb level temperatures may be used in lieu
of the surface temperatures if representative surface
temperatures are not available and the terrain 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.
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 enjoy 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.
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 example, 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 450 angle to the front has only a
15-knot component that is normal to or perpendicular
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.
What is the definition of a frontal surface?
Where is the frontal zone located?
What is the difference between a stable wave
and an unstable wave?
Where does frontogenesis occur?
Where is the polar front normally found
during the winter?
THE COLD FRONT
slow-moving cold fronts, fast-moving cold
fronts, secondary cold fronts, and cold fronts
A cold front is the leading edge of a wedge of cold
air that is under running warm air. Cold fronts usually
move faster and have a steeper slope than other types of
fronts. Cold fronts that move very rapidly have very
steep slopes in the lower levels and narrow bands of
clouds that are predominant along or just ahead of the
front. Slower moving cold fronts have less steep slopes,
and their cloud systems may extend far to the rear of the
surface position of the fronts. Both fast-moving and
slow-moving cold fronts may be associated with either
stability or instability and either moist or dry air
Certain weather characteristics and conditions are
typical of cold fronts. In general, the temperature and
humidity decrease, the pressure rises, and in the
Northern Hemisphere the wind shifts (usually from
southwest to northwest) with the passage of a cold
front. The distribution and type of cloudiness and the
intensity and distribution of precipitation depend
primarily on the vertical motion within the warm air
mass. This vertical motion is in part dependent upon the
speed of that cold front.
SLOW-MOVING COLD FRONTS (ACTIVE
With the slow-moving cold front, there is a general
upward motion of warm air along the entire frontal
surface and pronounced lifting along the lower portion
of the front. The average slope of the front is