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Weather and Climate Controls

Climate, which consists of observations of meteorological parameters (i.e., weather) over a specified period of time, is shaped by many environmental factors. Although weather conditions at any particular time can be ephemeral and deviate from expectations, over a long period of time, certain characteristics are associated with any given location on Earth. The longer the period of record, the more likely that climate regimes will be identifiable with recognizable temperature and precipitation characteristics. This entry discusses seasonal changes in climate as well as major controls and localized controls on weather and climate.

Seasonal Changes in Climate

The seasons are among the most well-known environmental parameters that affect long-term climate and shorter periods of time associated with weather. Seasonal changes in climate for a given locale are imprinted on the daily lives of the biology in these locales, in everything from the smallest plants to the local human inhabitants. For example, anyone reading this passage may be able to picture the environmental conditions in their own hometown during each of the four astronomical seasons and may also imagine the likely climate conditions present during these seasons. These “memory maps” of climate changes due to the seasons can be found in anyone inhabiting a particular area for a long enough period; in fact, with enough living experience, nearly all biological life forms appear to incorporate temperature and/or moisture into their evolutionary adaptations, associated with particular sites on Earth.

It is important to note that the effect of seasons on weather and climate is solely due to changes in solar insolation (incoming sunlight) during the year, and this is reflected in the latitude of an individual location above or below the equator—all places along a given latitude will receive these changes in incoming solar energy. The general patterns of incoming energy are reflected in the seasonality, as discussed above. Although we have four seasons in the United States, other regions of the world do not have such distinct divisions in yearly climate. For example, areas located within 23.5° of the equator have very minor changes in temperature throughout the year—these latitudes are commonly known as the tropics, as they lie between the maximum latitudinal extents of vertical solar declination. We refer to the annual range in temperatures in the tropics as very minor. In areas far to the north, such as Alaska and above the Arctic Circle, the annual range can be very great—these areas can receive lengthy periods of dim to no sunlight during winter, contrasted with lengthy periods of daylight through summer. Therefore, latitude plays a significant role in seasonal-scale climate trends. However, many other geographic parameters control the actual weather that occurs at any selected location, and these other controls on weather are the focus of this entry.

Major Controls on Weather and Climate

A general discussion of the controls on climate typically begins with the assessment of the thermal properties of land or soil versus those of water, typically referring to the oceans and their enormous heat storage capacity. Land absorbs the solar energy that is received during the day and rapidly heats up due to a low specific heat (a physical property referring to the energy a substance must absorb to raise the temperature of a 1-gram mass of water by Io C). The reverse is also true: At night, when the sun is no longer providing incoming energy to the surface of Earth, the land cools down rapidly due to the same characteristic of low specific heat. The opposite is true of water, which has a very high specific heat. It takes a long time, therefore, for water to heat up and cool down. This is reflected in the long lag times between peak solar insolation and peak water temperatures. For example, if one were to visit one of the Great Lakes (such as Lake Huron or Superior) in May, one might find the water to be too cold to swim even on a 100° F day. However, visiting in August on a day with a maximum temperature of 85° F, one would likely experience water temperatures 10° to 15° higher. This is because even though the sun reaches it seasonal declination zenith on the summer solstice (June 21), it takes about another 30 days for this peak in incoming energy to translate into a peak in water temperatures. The end result in these differences in specific heat between land and water is a greater range in temperatures in locations that are far removed from large bodies of water (called continental climates), with a smaller annual range of temperatures at locations located near such bodies of water due to the moderating effects of proximity to these features. Furthermore, these properties are reflected in the differences in climates between locations in the Northern Hemisphere versus those in the Southern Hemisphere. In general, Northern Hemisphere climates are more variable and have a higher annual temperature range than Southern Hemisphere climates. Once these more rapid swings in temperature over large land masses are considered, it is logical that the majority of severe weather events such as tornadoes, mesoscale convective complexes, and derechos occur in the Northern Hemisphere.

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