thermal tongues oriented nearly parallel to the gradient
3. On this axis, locate the point with the greatest
temperature gradient within 50 to 100 miles to the right,
and normal to the flow.
4. From this reference point, a rectangle is
constructed with its left side along the axis of the thermal
tongue by locating comer points on the axis 25 miles
upstream and 125 miles downstream from the reference
point. The rectangle is 150 miles long and 50 miles
5. This is the forecast area for possible tornado or
funnel cloud development.
There are three distinct tornadic types over the
United States. They are the Great Plains type, the Gulf
Coast type, and West Coast type.
Great Plains Type
This type of tornado will generally form on the
squall line in advance of a fast moving cold front, hence
its prediction involves timely forecasting of the squall
line formation along, or in advance of, a cold front,
upper cold front, or trough. Conditions must favor a
downrush of air from aloft.
Gulf Coast Type
In contrast to the air mass type (Great Plains type),
tornadoes also form in equatorial type air masses that
are moist to great heights. Such storms are most
common on the coasts of the Gulf of Mexico and
produce the waterspouts often reported over Florida.
Tornadoes are triggered in this air mass primarily by
lifting at the intersection of a thunderstorm line with a
warm front, and less frequently by frontal and prefrontal
West Coast Type
Tornadoes also form in relatively cold moist air.
This air mass tornado is the Pacific or West Coast type.
It is responsible for waterspouts on the West Coast.
Tornadoes in this type of air mass are normally in a
rather extensive cloudy area with scattered rain showers
and isolated thunderstorms. Clouds are mostly
Favorable situations for tornado
development in this air mass type include the rear of
Maritime Polar (mP) cold frontswell cooled air
behind squall lines.
Waterspouts fall into two classes-tornadoes over
water and fair weather waterspouts. The fair weather
waterspout is comparable to a dust devil. It may rotate
in either direction, whereas the other type of waterspout
rotates cyclonically. In general, waterspouts are not as
strong as tornadoes, in spite of the large moisture source
and the reduced frictiion of the underlying surface. The
water surface beneath a waterspout is either raised or
lowered, depending on whether it is affected more by
the atmospheric pressure reduction or the wind force.
There is less inflow and upflow of air in a waterspout
than in a tornado. The waterspout does not lift a
significant amount of water from the surface. Ships
passing through waterspouts have mostly encountered
LEARNING OBJECTIVES: Be familiar with
the effects of air-mass stability on fog
formation. Identify the procedures used in the
forecasting of fog.
favorable for the formation of the various types
of fog. Calculate fog parameters by using the
Skew T Log P Diagram.
Fog and stratus clouds are hazardous conditions for
both aircraft and ship operations. You will frequently
be called upon to forecast formation, lifting, or
dissipation of these phenomena. To provide the best
information available, we will discuss the various
factors that influence the formation and dissipation of
fog and stratus.
EFFECT OF AIR MASS STABILITY
Fog and stratus are typical phenomena of a warm
air mass. Since a warm air mass is warmer than the
underlying surface, it is stable, especially in the lower