The hiker's pocket altimeter and the altimeter in an aircraft are merely barometers measuring atmospheric pressure and indicating the equivalent height according to average weather conditions. Pressure, which always decreases with altitude, is usually measured in millibars, as shown at the right side of Table 1.1, or in inches (or millimeters) of mercury. Some equivalent pressures and heights for average atmospheric conditions in the California-Nevada region are listed in Table 1.1. The physiological effects of increasing altitude and decreasing pressure are mainly caused by the reduced amount of oxygen and the greater effort one has to exert to get enough oxygen into the lungs. Most hikers have to become acclimated for a day or two before they can be comfortable above 10,000 ft.
At most times and places, air temperature also decreases with height because the atmosphere is mainly heated from below. The atmosphere does not absorb much of the direct radiation from the sun, but the earth's surface does and reradiates the energy at a longer wavelength, which the atmosphere absorbs mainly through two of its variable gases: water vapor and carbon dioxide. The average lapse rate (generally, the decrease of temperature with altitude) is approximately 3.6°F per 1,000 ft (6.5°C per km). At midday in summer with strong thermal activity, the lapse rate on the slopes approaches the adiabatic value of about 5.4°F per 1,000 ft (10°C per km) of ascent. Thus, on days when the temperature is 90°F (32°C), for example, in the Owens Valley, it can be 45° to 55°F (7° to 13°C) on Mt. Whitney or White Mountain Peak (both above 14,000 ft or 4,000 m). On clear nights, on the other hand, cooler air with its greater density sinks and collects in the valleys, forming inversions in which the temperature increases with height in the lowest few hundred feet (» 100 m) above the ground.
Topography influences local weather and climate in many ways, examples of which will be noted throughout the following sections of this chapter.