Differential Heating

Land surfaces warm and cool much quicker than water bodies, leading to differential heating of the Earth's surface. Water has a higher specific heat (capacity to hold energy) and circulates energy to greater depth than does land, and so water bodies such as oceans, seas, and large lakes can absorb more energy yet heat more slowly than land surfaces. Since energy is more evenly distributed throughout the water column, water bodies also release energy and cool more slowly than do land surfaces. Nearly three quarters of Earth's surface is covered with water, and so oceans and other large water bodies represent immense reservoirs for energy storage and release, strongly influencing climates worldwide. For example, differential heating results in the development of a series of regional thermal high and low pressure systems that reverse positions during daily and seasonal cycles. Wind and precipitation patterns reflect these alternating pressure patterns, and areas where these patterns persist for several months may experience a monsoon climate.

Water bodies and land surfaces transfer energy to depth by distinct processes. The land surface is relatively immobile, and so energy is transferred by conduction from one particle to the next. Conduction is a relatively slow process, resulting in most energy on land being stored near the surface. Energy is rapidly transferred and stored to great depths by convection in water via turbulence. Water is also relatively transparent, while land surfaces are opaque, and incoming solar radiation (“insolation”) penetrates deep into the water column but is absorbed at the surface on land. With more energy stored near the surface, land has much greater daily and seasonal temperature fluctuations.

The specific heat of water is also much higher than those of typical land surfaces, meaning it requires more energy to heat water than to heat land by equivalent amounts. Because of the high specific heat of water relative to the land surface, the temperature change is less even though water-bodies typically absorb and release more energy than does the land surface. During the daytime and summer, when insolation is greatest, water bodies absorb and store vast amounts of energy [Page 741]with only moderate increases in temperature. At night and during winter, a portion of this stored energy is released with only minor cooling of the water body.

During periods of maximum insolation, more energy is transferred to the atmosphere by evaporation over water bodies than over land surfaces. As water molecules change from a liquid to a gaseous phase, energy is used to break the molecular bonds attracting individual water particles, causing the water temperature to decrease as energy is transported to the atmosphere as latent heat. Therefore, during periods of high insolation, increased evaporation diminishes temperature increases of water bodies and further intensifies land-water temperature contrasts.

Differential heating results in temperature and precipitation patterns for coastal areas distinct from locations in continental interiors, particularly in the midlatitudes. Water heats and cools much slower than does land, and, consequently, coastal areas experience only moderate annual temperature variations. In addition, rainfall is typically more evenly distributed throughout the year in coastal areas because of the proximity to extensive moistures sources. In the continental interior, temperatures fluctuate considerably in response to seasonal variations in insolation rates, and most precipitation is delivered as convective summer storms. For example, San Francisco, California, along the west coast of the United States, and Kansas City, Missouri, in the Central United States, are at similar latitudes and have similar mean annual temperatures. However, Kansas City has a humid continental climate, with cold, dry winters and hot summers with increased precipitation, while San Francisco experiences a relatively Mediterranean climate, with similar temperatures year-round, moist winters, and dry summers.

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