progressing at a rate of 25 to 30 miles per hour (mph). However, extreme cold fronts have been recorded moving at
speeds of up to 60 mph. A typical cold front moves in a manner opposite that of a warm front. Because it is so dense, it
stays close to the ground and acts like a snowplow, sliding under the warmer air and forcing the warmer less dense air
aloft. [Figure 4-5] The rapidly ascending air causes the temperature to decrease suddenly, forcing the creation of clouds.
The type of clouds that form depends on the stability of the warmer air mass. A cold front in the Northern Hemisphere is
normally oriented in a northeast to southwest manner and can extend for several hundred miles, encompassing a large area
of land. Prior to the passage of a typical cold front, cirriform or towering cumulus clouds are present, and cumulonimbus
clouds are possible. Rain showers and haze are possible due to the rapid development of clouds. The wind from the south-
southwest helps to replace the warm temperatures with the relative colder air. A high dew point and falling barometric
pressure are indicative of imminent cold front passage.
St. Louis Indianapolis
200 miles
Columbus
400 miles
Pittsburgh
600 miles
COLD AIR
WARM AIR
NIMBOSTRATUS
ALTOSTRATUS
CIRROSTRATUS
CIRRUS
Figure 4-5. A cold front underrunning warm, moist, stable air. Clouds are stratified and precipitation continuous. Precipitation
induces stratus in the cold air.
As the cold front passes, towering cumulus or cumulonimbus clouds continue to dominate the sky. [Figure 4-6] Depending
on the intensity of the cold front, heavy rain showers form and might be accompanied by lightning, thunder, and/or hail.
More severe cold fronts can also produce tornadoes. During cold front passage, the visibility may be poor, with winds
variable and gusty, and the temperature and dew point drop rapidly. A quickly falling barometric pressure bottoms out
during frontal passage, then begins a gradual increase. After frontal passage, the towering cumulus and cumulonimbus
clouds begin to dissipate to cumulus clouds, with a corresponding decrease in the precipitation. Good visibility eventually
prevails with the winds from the west-northwest. Temperatures remain cooler and the barometric pressure continues to rise.
St. Louis Indianapolis
200 miles
Columbus
400 miles
Pittsburgh
600 miles
WARM AIR
COLD AIR
CUMULONIMBUS
Figure 4-6. A cold front underrunning warm, moist, unstable air. Clouds are cumuliform with possible showers or thunderstorms near
the surface position of the front. Convective clouds often develop in the warm air ahead of the front. The warm, wet ground behind the
front generates low-level convection and fair-weather cumulus in the cold air
Fast-Moving Cold Front
Fast-moving cold fronts are pushed by intense pressure systems far behind the actual front. [ Figure 4-7] The friction
between the ground and the cold front retards the movement of the front and creates a steeper frontal surface. This results
in a very narrow band of weather concentrated along the leading edge of the front. If the warm air being overtaken by
the cold front is relatively stable, overcast skies and rain may occur for some distance ahead of the front. If the warm air
is unstable, scattered thunderstorms and rain showers may form. A continuous line of thunderstorms, or squall line, may
form along or ahead of the front. Squall lines present a serious hazard to pilots as squall type thunderstorms are intense and
move quickly. Behind a fast-moving cold front, the skies usually clear rapidly and the front leaves behind gusty, turbulent
winds and colder temperatures.
St. Louis Indianapolis
200 miles
Columbus
400 miles
Pittsburgh
600 miles
WARM UNSTABLE AIR
WARM UNSTABLE AIR
COLD AIR
Figure 4-7. A fast-moving cold front underrunning warm, moist, unstable air. Showers and thunderstorms develop along the surface
position of the front.
Warm Front
A warm front is actually the trailing edge of a retreating mass of cold air. A warm front occurs when a warm mass of air
advances and replaces a body of colder air. Warm fronts move slowly, typically 10 to 25 mph. The slope of the advancing
front slides over the top of the cooler air and gradually pushes it out of the area. Warm fronts contain warm air that often
has very high humidity. As the warm air is lifted, the temperature drops and condensation occurs. Prior to the passage of a
warm front, cirriform or stratiform clouds, along with fog, can be expected to form along the frontal boundary. [Figure 4-8]
In the summer months, cumulonimbus clouds (thunderstorms) are likely to develop. Light to moderate precipitation is
probable, usually in the form of rain, sleet, snow, or drizzle, punctuated by poor visibility. The wind blows from the south-
southeast, and the outside temperature is cool or cold, with increasing dew point. Finally, as the warm front approaches,
the barometric pressure continues to fall until the front passes completely.
St. Louis Indianapolis
200 miles
Columbus
400 miles
Pittsburgh
600 miles
WARM STABLE AIR
WARM STABLE AIR
Figure 4-8. A warm front with overrunning moist, stable air. Clouds are stratiform and widespread over the shallow front.
Precipitation is continuous and induces widespread stratus in the cold air.
During the passage of a warm front, stratiform clouds are visible and drizzle may be falling. The visibility is generally poor,
but improves with variable winds. The temperature rises steadily from the inflow of relatively warmer air. Usually, the
dew point remains steady and the pressure levels off. After the passage of a warm front, stratocumulus clouds predominate
and rain showers are possible. The visibility eventually improves, but hazy conditions may exist for a short period after
passage. The wind generally blows from the south-southwest. With warming temperatures, the dew point rises and then
levels off. There is generally a slight rise in barometric pressure, followed by a decrease in barometric pressure.
Stationary Front
When an air mass boundary is neither advancing nor retreating along the surface, the front is called a stationary front.
Although there is no movement of the surface position of a true stationary front, an uplift of air may occur along the frontal
boundary. If the uplifted air is stable and saturated, stratiform clouds may occur. Intermittent drizzle may occur, and if lifted
above the freezing level, icing conditions and frozen precipitation will exist. If the uplifted air is conditionally unstable and
saturation occurs, predominately cumuliform clouds will form, possibly generating thunderstorm activity.
Occluded Front
An occluded front occurs when a fast-moving cold front catches up with a slow-moving warm front. As the occluded
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 & 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 in-flight 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.
• Expands
• Decreases density
• Rises
Air Heated
• Flows toward Earth
• Flows back to equator
Air Cooled
Figure 4-9. General circulation theory.
The speed of the Earth’s rotation causes the general flow to break up into three distinct cells in each hemisphere.
[Figure 4-10] In the Northern Hemisphere, the warm air at the equator rises upward from the surface, travels northward,
and is deflected eastward by the rotation of the Earth. By the time it has traveled one-third of the distance from the equator
to the North Pole, it is no longer moving northward, but eastward. This air cools and sinks in a belt-like area at about 30°
latitude, creating an area of high pressure as it sinks toward the surface. Then, it flows southward along the surface back
toward the equator. Coriolis force bends the flow to the right, thus creating the northeasterly trade winds that prevail from
30° latitude to the equator. Similar forces create circulation cells that encircle the Earth between 30° and 60° latitude, and
between 60° and the poles. This circulation pattern results in the prevailing westerly winds in the conterminous United
States.
Figure 4-10. Three cell circulation pattern caused by the rotation of the Earth.
Circulation patterns are further complicated by seasonal changes, differences between the surfaces of continents and oceans,
and other factors such as frictional forces caused by the topography of the Earth’s surface which modify the movement of
the air in the atmosphere. For example, within 2,000 feet of the ground, the friction between the surface and the atmosphere
slows the moving air. The wind is diverted from its path because the frictional force reduces the Coriolis force. Thus, the
wind direction at the surface varies somewhat from the wind direction just a few thousand feet above the Earth.
Coriolis Force
The Coriolis force is not perceptible to humans as they walk around because humans move slowly and travel relatively
short distances compared to the size and rotation rate of the Earth. However, the Coriolis force significantly affects bodies
that move over great distances, such as an air mass or body of water.
The Coriolis force deflects air to the right in the Northern Hemisphere, causing it to follow a curved path instead of a
straight line. The amount of deflection differs depending on the latitude. It is greatest at the poles, and diminishes to zero
at the equator. The magnitude of Coriolis force also differs with the speed of the moving body—the faster the speed, the
greater the deviation. In the Northern Hemisphere, the rotation of the Earth deflects moving air to the right and changes the
general circulation pattern of the air.
Pertinent facts about the Coriolis force:
• The Coriolis force deflection is perpendicular to the flow of air.
• The Coriolis force will deflect air to the right in the Northern Hemisphere, and to the left in the Southern Hemisphere.
• The Coriolis force is strongest at the Poles and decreases to zero at the Equator.
• The Coriolis force is zero with calm winds and increases in magnitude as wind speed increases.
• Coriolis force, in combination with other forces involved, will determine the different circulation patterns over the
Earth.
Pressure Gradient
Pressure gradient is the difference in pressure between high and low pressure areas. It is the rate of change in pressure in
a direction perpendicular, or across the isobars. Wind speed is directly proportional to the pressure gradient. This means
the strongest winds are in the areas where the pressure gradient is the greatest. Since pressure applied to a fluid is exerted
equally in all directions throughout the fluid, a pressure gradient exists in the horizontal (along the surface), as well as in
the vertical (with altitude) plane in the atmosphere. [Figure 4-11]
Isobars
• The wider the pressure gradient,
the weaker the wind.
Weak or Flat Pressure Gradient
• The closer the spacing of isobars,
the stronger the pressure gradient.
• The stronger the pressure gradient,
the stronger the wind.
Strong or Steep Pressure Gradient
Figure 4-11. Principles of pressure gradients.
The horizontal pressure gradient is steep or strong when the isobars determining the pressure gradient are close together. It
is flat or weak when the isobars are far apart. If isobars are considered as depicting atmospheric topography, a high pressure
system represents a hill of air, and a low pressure system represents a valley of air. The vertical pressure gradient always
indicates a decrease in pressure with altitude, but the rate of pressure decrease (gradient) varies directly with changes in air
density with altitude. The vertical cross section through a high and a low depicts the surface pressure gradient.
The pressure gradient force is a force that tries to equalize pressure differences. This is the force that causes high pressure
to push air toward low pressure. Thus, air would flow from high to low pressure if the pressure gradient force was the only
force acting on it.
Surface Friction
Friction is the third component that determines the flow of wind. Because the surface of the Earth is rough, it not only
slows the wind down, it also causes the diverging winds from highs and converging winds near lows. Since the Coriolis
force varies with the speed of the wind, a reduction in the wind speed by friction means a reduction of the Coriolis force.
This results in a momentary disruption of the balance. When the new balance (including friction) is reached, the air flows
at an angle across the isobars from high pressure to low pressure. This angle varies from 10° over the ocean to more than
45° over rugged terrain. Frictional effects on the air are greatest near the ground, but the effects are also carried aloft by
turbulence. Surface friction is effective in slowing the wind up to an average altitude of 2,000 feet above the ground. Above
this level, the effect of friction decreases rapidly and may be considered negligible. Air above 2,000 feet above the ground
normally flows parallel to the isobars. [Figure 4-12 and Figure 4-13]
HIGH
LOW
Surface WindGradient Wind 1,000 Feet
Gradient Wind 2,000 Feet
Gradient Wind 3,000 Feet
Figure 4-12. Examples of variations of wind direction with height.
HIGH
LOW
Increases with velocity
Coriolis Force
Depends on spacing of isobars
Pressure Gradient Force
Depends on curvature of isobars
Centrifugal Force
Gradient wind
Pressure
Gradient Force
Centrifugal &
Coriolis Force
Equal and opposite
Figure 4-13. Gradient winds
Wind Patterns
Since air always seeks out lower pressure, it flows from areas of high pressure into those of low pressure. In the Northern
Hemisphere, this flow of air from areas of high to low pressure is deflected to the right and produces a clockwise circulation
around an area of high pressure known as anticyclonic circulation. The opposite is true of low pressure areas; the air flows
toward a low and is deflected to create a counter-clockwise or cyclonic circulation.
High pressure systems are generally areas of dry, stable, descending air. Good weather is typically associated with high
pressure systems for this reason. Conversely, air flows into a low pressure area to replace rising air. This air tends to be
unstable, and usually brings increasing cloudiness and precipitation. Thus, bad weather is commonly associated with areas
of low pressure.
Convective Currents
Convection currents refer to the upward moving portion of a convection circulation, such as a thermal or the updraft in
cumulus clouds. The uneven heating of the air, due to different surfaces radiating heat in varying amounts, create small
areas of local circulation. For example, plowed ground, rocks, sand, and barren land give off a large amount of heat, while
water, trees, and other areas of vegetation tend to absorb and retain heat. Convective currents cause the bumpy, turbulent
air sometimes experienced when flying at lower altitudes during warmer weather. On a low altitude flight over varying
surfaces, updrafts are likely to occur over pavement or barren places, and downdrafts often occur over water or expansive
areas of vegetation like a group of trees. Typically, these turbulent conditions can be avoided by flying at higher altitudes.
Convective currents are particularly noticeable in areas with a land mass directly adjacent to a large body of water, such
as an ocean, large lake, or other appreciable area of water. [ Figure 4-14] During the day, land heats faster than water, so
the air over the land becomes warmer and less dense. It rises and is replaced by cooler, denser air flowing in from over
the water. This causes an onshore wind, called a sea breeze. Conversely, at night land cools faster than water, as does the
corresponding air. In this case, the warmer air over the water rises and is replaced by the cooler, denser air from the land,
creating an offshore wind called a land breeze. This reverses the local wind circulation pattern. Convective currents can
occur anywhere there is an uneven heating of the Earth’s surface.