Mother Nature's Tricks

Have you ever been caught in an un-forecast downpour during a picnic? Or have you flown in an area of anticipated updraft, yet all you find are downdrafts?

Occasionally the wind defies all common sense reasoning and visualization. When this occurs it is usually due to one or a combination of the following:

• subsidence
• inversion
• terrain modification
• valley breeze
• mountain breeze

Circulation

(This discussion is limited to the northern hemisphere)

A quick review of some basic weather phenomena helps make the point. Circulation refers simply to the movement of air about the earth's surface. The sun heats the Earth's surface unevenly. The most direct rays strike near the equator, heating the equatorial regions more than the polar regions. The equatorial region re-radiates to space less heat than is received from the sun, while the reverse is true at the poles.

Yet the equator does not continue to get hotter and hotter, nor does the polar region get colder. The only explanation is that heat is transferred from one latitude to another by the actual transport of air.

Warm air forced aloft at the equator begins to move north at high elevation. Coriolis force turns it to the right (east). This turning develops a strong band of winds, "prevailing westerlies," at about 30º north latitude.

Similarly, cool air from the poles begins a low-elevation journey toward the equator. It is also deflected to its right by Coriolis force creating a belt of low-level "polar easterlies." The result is to create an temporary impasse that disrupts simple, convective transfer.

The atmosphere seeks stability and in an attempt to reach equilibrium, huge masses of air overturn in the middle latitudes. Cold air masses break through the barriers, plunging southward. The result is a mid-latitude bank of migratory storms with ever-changing weather.

Air Mass

The large air masses are high pressure areas. In the northern hemisphere, high pressure areas circulate in a clockwise direction. The high pressure system depicted on weather maps should be visualized as a mountain of air. The mountain is composed of isobars or lines of equal pressure. Consider the isobars as topographic in nature. If they are far apart, the high pressure area has a shallow topography. When close together, there is a very steep slope to the mountain of air.

Where isobars are close together it indicates the air is squeezed into a smaller, more confined area with a steep slope creating a rapid flow of air and strong surface winds.

Between the high pressure areas will be areas of low pressure where the air flows counter-clockwise. Visualize the low pressure area as a valley between air masses.

None of the pressure areas are stagnant. The earth's atmosphere is in a constant state of imbalance, but there is always a tendency to regain a state of balance.

Wind

Three forces act on wind. The pressure gradient force drives the wind. Pressure gradient is the decrease of pressure with distance and is in the direction of greatest decrease, thus, pressure gradient is from higher to lower pressure and perpendicular to the isobars. If pressure gradient was the only force acting on the wind, wind would always blow perpendicular to the isobars.

Rotation of the earth generates a force that deflects from a straight path any mass moving relative to the earth's surface. Coriolis force is zero at the equator and increases with latitude to a maximum at the poles. It is at a right angle to wind direction and is directly proportional to wind speed. Air in motion, due to pressure gradient, blows straight across the isobars from higher to lower pressure. When the air is in motion, Coriolis force begins to act at right angles to the wind, turning it to the right. Coriolis force continues to deflect the wind until is is blowing parallel to the isobars. Coriolis force and pressure gradient force balance, and above surface friction (about 2,000 feet), causes the wind to blow parallel to the isobars.

The winds at the earth's surface do not blow parallel to the isobars. Instead, they cross the isobars at an angle from higher to lower pressure.

Frictional force always acts opposite to wind direction. As friction slows the wind speed, Coriolis force decreases; however, friction has no effect on pressure gradient force. Pressure gradient and Coriolis forces are no longer in balance. Above 2,000 feet AGL the wind blows parallel to isobars. Below that altitude, friction causes the surface wind to blow 45º inward toward a low pressure area and 45º outward from a high pressure area.

Subsidence

Variations in temperature and humidity create a contrast in pressure and density. The pressure differences drive a complex system of air currents in a never-ending attempt to attain equilibrium.

Suppose an air mass (high pressure area) arrives over the plateau area of the upper Arkansas River Valley near Leadville, Colorado. The down flow, sinking are may be a stronger force than the prevailing winds aloft. The pilot departing Aspen and flying up the Roaring Fork River toward Independence Pass will be hard pressed to find an updraft in the face of this down flow. Yet it's always been there before. This pilot may be an accident waiting to happen.

According to Aviation Space Environment Medicine, 232 airplanes crashed within 50 nautical miles of Aspen, CO, between 1964 and 1987. A total of 202 people died and 69 were seriously injured. This points out the need for better training in mountain flying.