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. 

## Surface Fronts 

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. 

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. 

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. 

## Winds and Currents 

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. 

## Atmospheric Circulation 

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. 

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. 

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. 

