full text of the classic FAA guide
WIND, PRESSURE SYSTEMS, AND WEATHER
We already have shown that wind speed is proportional to the spacing of isobars or contours on a weather map. However, with the same spacing, wind speed at the surface will be less than aloft because of surface friction.
You also can determine wind direction from a weather map. If you face along an isobar or contour with lower pressure on your left, wind will be blowing in the direction you are facing. On a surface map, wind will cross the isobar at an angle toward lower pressure; on an upper air chart, it will be parallel to the contour.
Wind blows counterclockwise (Northern Hemisphere) around a low and clockwise around a high. At the surface where winds cross the isobars at an angle, you can see a transport of air from high to low pressure. Although winds are virtually parallel to contours on an upper air chart, there still is a slow transport of air from high to low pressure.
At the surface when air converges into a low, it cannot go outward against the pressure gradient, nor can it go downward into the ground; it must go upward.* Therefore, a low or trough is an area of rising air.
* You may recall that earlier we said air “piles up” in the vicinity of 30° latitude increasing pressure and forming the subtropical high pressure belt. Why, then, does not air flowing into a low or trough increase pressure and fill the system? Dynamic forces maintain the low or trough; and these forces differ from the forces that maintain the subtropical high.
Rising air is conducive to cloudiness and precipitation; thus we have the general association of low pressure—bad weather. Reasons for the inclement weather are developed in later chapters.
By similar reasoning, air moving out of a high or ridge depletes the quantity of air. Highs and ridges, therefore, are areas of descending air. Descending air favors dissipation of cloudiness; hence the association, high pressure-good weather.
Many times weather is more closely associated with an upper air pattern than with features shown by the surface map. Although features on the two charts are related, they seldom are identical. A weak surface system often loses its identity in the upper air pattern, while another system may be more evident on the upper air chart than on the surface map.
Widespread cloudiness and precipitation often develop in advance of an upper trough or low. A line of showers and thunderstorms is not uncommon with a trough aloft even though the surface pressure pattern shows little or no cause for the development.
On the other hand, downward motion in a high or ridge places a “cap” on convection, preventing any upward motion. Air may become stagnant in a high, trap moisture and contamination in low levels, and restrict ceiling and visibility. Low stratus, fog, haze, and smoke are not uncommon in high pressure areas. However, a high or ridge aloft with moderate surface winds most often produces good flying weather.
Highs and lows tend to lean from the surface into the upper atmosphere. Due to this slope, winds aloft often blow across the associated surface systems. Upper winds tend to steer surface systems in the general direction of the upper wind flow.
An intense, cold, low pressure vortex leans less than does a weaker system. The intense low becomes oriented almost vertically and is clearly evident on both surface and upper air charts. Upper winds encircle the surface low and do not blow across it. Thus, the storm moves very slowly and usually causes an extensive and persistent area of clouds, precipitation, strong winds, and generally adverse flying weather. The term cold low sometimes used by the weatherman describes such a system.
A contrasting analogy to the cold low is the thermal low. A dry, sunny region becomes quite warm from intense surface heating thus generating a surface low pressure area. The warm air is carried to high levels by convection, but cloudiness is scant because of lack of moisture. Since in warm air, pressure decreases slowly with altitude, the warm surface low is not evident at upper levels. Unlike the cold low, the thermal low is relatively shallow with weak pressure gradients and no well defined cyclonic circulation. It generally supports good flying weather. However, during the heat of the day, one must be alert for high density altitude and convective turbulence.
We have cited three exceptions to the low pressure—bad weather, high pressure—good weather rule: (1) cloudiness and precipitation with an upper air trough or low not evident on the surface chart; (2) the contaminated high; and (3) the thermal low. As this book progresses, you can further relate weather systems more specifically to flight operations.