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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 anti-cyclonic circulation. The opposite is true of low pressure
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102
8102410201016HIGHLOWIncreases with velocityCoriolis ForceDepends on spacing of isobarsPressure Gradient ForceDepends on curvature of isobarsCentrifugal ForceGradient windPressureGradient ForceCentrifugal &
Coriolis ForceEqual and opposite
Figure 4-13. Gradient winds.
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. 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. [Figure 4-14]
Convection currents close to the ground can affect a pilot’s ability to control the balloon. On final approach, for example, the rising air from terrain devoid of vegetation sometimes produces a ballooning effect that can cause a pilot to overshoot the intended landing spot. On the other hand, an approach over a large body of water or an area of thick vegetation tends to create a sinking effect that can cause an unwary pilot to land short of the intended landing spot. This could prove particularly hazardous to a balloon landing in a small, confined area, as the “undershoot” of the approach could potentially put the balloon into the trees or power lines.
The Jet Stream
The jet stream refers to relatively strong winds concentrated in a narrow stream in the atmosphere. These winds are normally horizontal, high altitude winds. The position and orientation of jet streams vary from day to day. General weather patterns (hot/cold, wet/dry) are related closely to the position, strength, and orientation of the jet stream (or jet streams). A jet stream at low levels is known as a low level jet stream. Since it is of interest primarily to high level flight, further discussion is not necessary.
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Warm
CoolReturn flowLand breezeWarmCoolReturn flowSea breeze
Figure 4-14. Land-sea breezes.
Local and Small-Scale Winds Gradient Winds
Pressure gradients initiate the movement of air and as soon as the air acquires velocity, the Coriolis force deflects it to the right in the Northern Hemisphere. As the speed of the air along the isobars increases, the Coriolis force becomes equal and opposite to the pressure gradient force. After a period of time, the air moves directly parallel to the curved isobars if there is no frictional drag with the surface. The air no longer moves toward lower pressure because the pressure gradient force is completely neutralized by the Coriolis force and the centrifugal force.Orographic Winds
The term “orographic” has multiple meanings, when placed in the context of weather phenomena. In a general sense, according to the American Meteorological Society, wind flows that are caused, affected, or influenced by mountains may be said to be orographic winds flows. The term has come to mean any winds that are affected by terrain, not just mountains; this definition is probably the most frequently used, when discussing balloon flight.
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Balloon Flying Handbook(46)
