Weather Fundamentals

Weather Almanac, (2004)

Seasonal variation of temperature

Temperature variation with latitude

The shape of the Earth causes a geographical variation in the angle of incident solar radiation. Since the Earth is essentially spherical, the Sun is closer to being overhead in equatorial regions than at higher latitudes. Equatorial regions, therefore, receive the most radiant energy and are warmest. Slanting rays of the Sun at higher latitudes deliver less energy over a given area with the least being received at the poles. Thus, temperature varies with latitude from the warm equator to the cold poles.

Temperature variations with topography

Not related to movement or shape of the Earth are temperature variations induced by water and terrain. Water absorbs and radiates energy with less temperature change than does land. Large, deep water bodies tend to minimize temperature changes, while continents favor large changes. Wet soil, such as in swamps and marshes, is almost as effective as water in suppressing temperature changes. Thick vegetation tends to control temperature changes since it contains some water and also insulates against heat transfer between the ground and the atmosphere. Arid, barren surfaces permit the greatest temperature changes.

These topographical influences are both diurnal and seasonal. For example, the difference between a daily maximum and minimum may be 10°F or less over water, near a shoreline, or over a swamp or marsh, while a difference of 50°F or more is common over rocky or sandy deserts. In the Northern Hemisphere in July, temperatures are warmer over continents than over oceans; in January they are colder over continents than over oceans. The opposite is true in the Southern Hemisphere, but not as pronounced because of more water surface in the Southern Hemisphere.

To compare land and water effect on seasonal temperature variation, consider northern Asia and southern California near San Diego. In the deep continental interior of northern Asia, July average temperature is about 50°F, and January average, about -30°F. Seasonal range is about 80°F. Near San Diego, due to the proximity of the Pacific Ocean, July average is about 70°F, and January average, about 50°F. Seasonal variation is only about 20°F.

Prevailing wind is also a factor in temperature controls. In an area where prevailing winds are from large water bodies, temperature changes are rather small. Most islands enjoy fairly constant temperatures. On the other hand, temperature changes are more pronounced where prevailing wind is from dry, barren regions.

The air slowly transfers heat from the surface upward. Thus, temperature changes aloft are more gradual than at the surface.

Temperature variation with altitude

Temperature normally decreases with increasing altitude throughout the troposphere. This decrease of temperature with altitude is defined as lapse rate. The average decrease of temperature—average lapse rate—in the troposphere is 3.6°F per 1,000 ft. But since this is an average, the exact value seldom exists. In fact, temperature sometimes increases with height through a layer. An increase in temperature with altitude is defined as an inversion, i.e., lapse rate is inverted.