Winds blowing across mountains and valleys cause the moving air to alternately ascend and descend. If relief is sufficiently great, the resulting expansional cooling and compressional warming of air affects the development and dissipation of clouds and precipitation.

For example, a mountain range that is oriented perpendicular to the prevailing wind flow forms a barrier that results in a cloudier and wetter climate on one side of the range than on the other side (see Figure 12-4). As air is forced to rise along the windward slope, it expands and cools, which increases its relative humidity. With sufficient cooling, clouds and precipitation develop at and above the LCL. Conversely, on the mountain’s leeward slope, air descends and warms, which reduces its relative humidity, and tends to dissipate clouds and precipitation. In this way, mountain ranges induce two contrasting climatic zones: a moist climate on the windward slope and a dry climate on the leeward slope. Dry conditions often extend hundreds of miles to the lee of a prominent mountain range in a region known as the rain shadow.

The air parcel begins with a temperature of 15°C, dewpoint of 10°C, and a relative humidity of 80 percent at 2,000 ft. As the parcel is lifted on the windward slope, the temperature cools at the dry adiabatic lapse rate of 3°C per 1,000 ft, and the dewpoint cools at a rate of 0.5°C per 1,000 ft until it becomes saturated at the LCL at 4,000 ft. Then, the air parcel’s temperature and dewpoint both cool at the moist adiabatic lapse rate of 2°C per 1,000 ft until the parcel reaches the summit at 12,000 ft. At that altitude, the parcel’s temperature is -7°C, the dewpoint is -7°C, and the relative humidity is 100 percent. As the air parcel descends the leeward slope, the temperature increases at a rate of 3°C per 1,000 ft while the dewpoint increases 0.5°C per 1,000 ft. The air parcel ends with a temperature of 23°C, dewpoint of -2°C, and a relative humidity of 33 percent at 2,000 ft, much warmer and drier than at the beginning.

Orographic effects are especially apparent from west to east across the Pacific Northwest, where the north–south Cascade Range intercepts the prevailing flow of humid air from the Pacific Ocean. Exceptionally cloudy, rainy weather prevails western slopes, whereas semiarid weather characterizes the eastern slopes and areas farther east.

In the Northern Hemisphere, the surface wind spirals clockwise and outward from high pressure, and counterclockwise and inward into low pressure due to frictional force. The end result is that winds diverge away from surface high pressure, causing the air to sink, compress, and warm, which favors the dissipation of clouds and precipitation. Conversely, winds converge into surface low pressure, causing the air to rise, expand, and cool, which favors the formation of clouds and precipitation given sufficient moisture (see Figure 12-5).

Frontal lift (see Figure 12-6) occurs when the cold, denser air wedges under the warm, less dense air, plowing it upward, and/or the warmer air rides up and over the colder air in a process called overrunning. Clouds and precipitation will form given sufficient lift and moisture content of the warm air.

Air near the ground can warm at different rates depending on the insular properties of the ground with which it is in contact. A newly plowed field will warm faster than an adjacent lake. These temperature differences result in different densities, allowing the warm air to become buoyant. The denser cool air will tend to push (i.e., lift) the less dense warm air aloft. On a grand scale, the tendency of air to rise due to heating, and how high it will rise, is referred to as stability and is covered in Chapter 13, Atmospheric Stability.

There are four basic cloud forms (appearances) observed in the Earth’s atmosphere (see Table 12-2). See Appendix A, Cloud Types, for cloud types.

By convention, the part of the atmosphere in which clouds are usually present has been divided into three levels: high, middle, and low (see Table 12-3). Each level is defined by the range of heights at which the cloud of a certain type occurs most frequently. The levels overlap, and their limits vary with latitude. The approximate heights of the limits are included in Table 12-3.