front approaches, warm front weather prevails, but is  immediately followed by cold front weather. There are two  types of occluded fronts that can occur, and the temperatures  of the colliding frontal systems play a large part in defining  the type of front and the resulting weather. A cold front  occlusion occurs when a fast-moving cold front is colder  than the air ahead of the slow-moving warm front. When  this occurs, the cold air replaces the cool air and forces the  warm front aloft into the atmosphere. Typically, the cold  front occlusion creates a mixture of weather found in both  warm and cold fronts, if the air is relatively stable. A warm  front occlusion occurs when the air ahead of the warm front  is colder than the air of the cold front. When this is the case,  the cold front rides up and over the warm front. If the air  forced aloft by the warm front occlusion is unstable, the  weather will be more severe than the weather found in a cold  front occlusion. Embedded thunderstorms, rain, and fog are  likely to occur. Pressure and temperature changes produce two kinds of  motion in the atmosphere—vertical movement of ascending  and descending currents, and horizontal movement in the  form of wind. Both types of motion in the atmosphere are  important as they affect the takeoff, landing, and inflight  operations. More important, however, is that these motions  in the atmosphere, otherwise called atmospheric circulation,  cause weather changes. Understanding wind and current circulation patterns is  important for a balloon pilot because balloons are maneuvered  solely through interaction with the different layers of wind  and current. By using knowledge of the Coriolis force,  pressure gradient force, and surface friction, it is possible to  predict with a high degree of accuracy the potential track over  the countryside and land at a predetermined point. This skill  is the mark of a competent, safety conscious balloon pilot.  Three forces cause the wind to move as it does: the Coriolis  force, the pressure gradient force, and surface friction. All  three forces work together at the same time. The air mass boundaries indicated on a surface weather map  are called surface fronts. A surface front is the position of a  front at the Earth’s surface. The weather map shows only the  location of fronts on the surface, but these fronts also have  vertical extent. For example, the colder, heavier air mass  tends to flow under the warmer air mass. The underrunning  mass produces the lifting action of warm air over cold air,  causing clouds and associated frontal weather. As defined earlier, atmospheric circulation is the movement  of air around the surface of the Earth caused by the uneven  heating of the Earth’s surface that upsets the equilibrium  of the atmosphere, creates changes in air movement, and  affects atmospheric pressure. Because the Earth has a curved  surface that rotates on a tilted axis while orbiting the sun, the  equatorial regions of the Earth receive a greater amount of  heat from the sun than the polar regions. The amount of sun  heating the Earth depends upon the time of day, time of year,  and the latitude of the specific region. All of these factors  affect the length of time and the angle at which sunlight  strikes the surface.  The vertical boundary between the warm and cold air masses  is a frontal surface, and slopes upward over the colder air  mass. The frontal surface lifts the warmer air mass and  produces frontal cloud systems. The slope of the frontal  surface varies with the speed of the moving cold air mass,  and the roughness of the underlying terrain. Under normal  conditions, the angle of inclination (slope ratio) between the  frontal surface and the Earth’s surface is greater with cold  fronts than with warm fronts. The approximate height of  the frontal surface over any station is determined from the  analysis of upper air observations. In general atmospheric circulation theory, areas of low  pressure exist over the equatorial regions, and areas of high  pressure exist over the polar regions due to a difference in  temperature. Solar heating causes air to become less dense  and rise in equatorial areas. The resulting low pressure allows  the high pressure air at the poles to flow along the planet’s  surface toward the equator. As the warm air flows toward the  poles, it cools, becoming more dense, and sinks back toward  the surface.  [Figure 4-9]  This pattern of air circulation is  correct in theory, but the circulation of air is modified by  other forces.  Frontal passage (FROPA) affects ballooning activities  because it can generate precipitation, wind shifts, significant  changes in temperature, and many other conditions hazardous  to ballooning. Balloon pilots usually do not fly in the face  of an approaching front; in fact, many have a rule that they  do not fly within 18 to 24 hours prior to frontal passage,  particularly if the approaching front has any significant  strength associated with it. The FSS often can advise of the  time a cold front will pass a given reporting station, which  assists in flight planning. 4-10 Surface Fronts Atmospheric Circulation Winds and Currents 