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You can learn how temperatures influence short range, local weather forecasts. |
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Temperature |
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During the day, the earth is heated by the sun. If skies are clear, more heat reaches the earth's surface (as in the diagram at right). This leads to warmer temperatures. |
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However, if skies are cloudy, some of the sun's rays are reflected off the cloud droplets back into space. Therefore, less of the sun's energy is able to reach the earth's surface, which causes the earth to heat up more slowly. This leads to cooler temperatures. |
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Only some heat reaches surface. |
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Some heat escapes into space. |
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At night cloud cover has the opposite effect. If skies are clear, heat emitted from the earth's surface freely escapes into space, resulting in colder temperatures. |
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Clouds absorb some heat, radiate it back to Earth. |
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However, if clouds are present, some of the heat emitted from the earth's surface is trapped by the clouds and reemitted back towards the earth. As a result, temperatures decrease more slowly than if the skies were clear |
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· When forecasting daytime temperatures, if cloudy skies are expected, forecast lower temperatures than you would predict if clear skies were expected.
· When forecasting nighttime temperatures, if cloudy skies are expected, forecast warmer temperatures than you would predict if clear skies were expected.
If a city is expected to be located west of a high pressure center then warmer temperatures are likely. However, if the city is expected to be in the northerly winds of a high pressure center, then forecast colder temperatures. Cities under the influence of high pressure centers can expect generally fair weather with little or no precipitation.
In contrast, clouds and precipitation generally accompany a low pressure center and winds flow counterclockwise around lows. This means that winds on the back (western) side of the low are generally from a northerly direction and typically mean colder temperatures. On the front (eastern) side of a low, winds are generally from the south and this typically results in warmer temperatures.
If a city is expected to be located west of a low pressure center then colder temperatures are likely. However, if the city is expected to be in the southerly winds of a high pressure center, then forecast warmer temperatures. Cities under the influence of low pressure centers can expect generally cloudy conditions with precipitation.
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Cloud cover reflects some of the sun’s heat back into space |
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This section deals with the temperatures of air masses and how they behave.
An air mass is a very large body of air that has similar properties. Air masses move, so they can be tracked over time. They are clues to the future temperature, moisture level and air pressure of the area into which they are moving.
When one air mass meets another, the air in different masses usually does not mix because the properties of the air are different. A boundary, or front, forms between the two air masses. Stormy weather often occurs along these fronts.
Cold Fronts: A cold front forms where a cold air mass moves into a warm air mass. Cold air contracts, becomes more dense, and thus heavier than warm air, so the cold air pushes underneath the warm air. Cumulus clouds and showers or thunderstorms may be scattered along cold fronts. As the front passes, the wind changes direction. Skies begin to clear, and the temperature usually drops.
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Warm Fronts: A warm front forms where a warm air mass moves into a cold air mass. Because the warm air is less dense, it slides up and over the colder air. At first, cirrus clouds may appear. They may be followed by stratus clouds and some precipitation, either rain or snow. When the front passes, the sky clears and the air pressure rises. Temperatures also rise as warm air replaces cold air.
Stationary Fronts: Even though air masses often move, they sometimes stall, resulting in a stationary front. Precipitation and weak winds usually occur in areas with stationary fronts.
Effects of Cloud Cover on Temperature |
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Diagram of area surrounding a cold front |