thermal tongues oriented nearly parallel to the gradient flow. 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 wide. 5. This is the forecast area for possible tornado or funnel  cloud  development. TORNADO TYPES 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 squall lines. 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 stratocumulus. Favorable  situations  for  tornado development  in  this  air  mass  type  include  the  rear  of Maritime  Polar  (mP)  cold  fronts—well  cooled  air behind  squall  lines. WATERSPOUTS 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 fresh  water. FORECASTING FOG STRATUS AND LEARNING  OBJECTIVES:  Be  familiar  with the   effects   of   air-mass   stability   on   fog formation. Identify the procedures used in the forecasting   of   fog. Recognize   conditions 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 ON FOG 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 layers. 5-18