- CLIMATE is a year-round AVERAGE of the weather conditions that prevail in a particular area.
Important climate variables include temperature, rainfall, humidity, winds, and seasonal variations in all these variables.
- Temperature:
The average annual temperature is an important component of climate.
Other things being equal, average annual temperatures are higher near the Equator and progressively lower toward the poles.
Annual temperatures are also affected by ocean currents and by winds:
Warm ocean currents and prevailing warm winds will raise the
annual average temperature higher than it might be otherwise.
The best example of this is that the warm-water Gulf Stream (and
winds warmed by the Gulf Stream) cause Europe to enjoy warmer average
temperatures (and more cloud cover) than other regions at comparable latitude.
(Canada and Siberia at the same latitude are MUCH colder.)
Seasonal temperature variations are generally greater at higher latitudes. Maximum monthly temperatures are comparable at various latitudes,
but minimum monthly temperatures vary greatly, with the lowest values near the poles.
In large continents, regions near a coastline often have less seasonal variation because the surrounding water stays cooler in summer and warmer than the land in winter.
Continental interiors often reach the coldest winter temperatures at a given latitude.
- Humidity and Rainfall:
The Annual average rainfall (or snowfall) follows a fairly complex pattern:
- Rainfall is highest close to the equator. Rainfall (and prevailing humidity) is lowest at latitudes
near Latitude 30o N or S.
(The global wind circulation responsible for this pattern is explained in Topic 64..)
- Rainfall is also higher where prevailing winds or seasonal winds blow onto a continent from a nearby ocean.
At a more local level, the same phenomenon
across a large lake can produce "Lake Effect" rain or snow in places like Upper New York State.
- Monsoon rains are seasonally heavy rains (in places like India) driven by strong seasonal winds blowing off the oceans.
- Rainfall is substantially lower in the shadow of large mountains or mountain chains (see Topic 65 for an explanation of Rain shadows).
- Humidity generally follows patterns of rainfall: areas of higher rainfall have higher humidity, even when it is not raining. This is especially true in the tropics.
- Prevailing winds:
The worldwide pattern of prevailing winds (shown on page 14 of this reference chart) is as follows:
- Two bands of "Trade winds", blowing from East to West between the Equator and 30o N or S Latitude, converging toward the Equator;
- Bands of Prevaling Westerlies, blowing from West to East between latitudes 30o and 60o N or S (including most of the United States); and
- Bands of Polar Easterlies, blowing from East to West between latitudes 60o and the poles.
- THE REASONS for these patterns are explained in Topic 64.
- Wind variations:
Some winds are seasonal, driven by changes in atmospheric pressure.
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