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The Challenge: Predict the Weather

What are ridges and troughs?

Ridges and troughs are often mentioned on the weather forecast. A ridge is an elongated area of relatively high pressure extending from the center of a high-pressure region. A trough is an elongated area of relatively low pressure extending from the center of a region of low pressure.

Air in a high pressure area compresses and warms as it descends. This warming inhibits the formation of clouds, meaning the sky is normally sunny in high-pressure areas. But haze and fog still might form. The opposite occurs within an area of low atmospheric pressure.

Weather forecasters measure air pressure with a barometer. Barometers measure the current air pressure at a particular location in "inches of mercury" or in "millibars" (mb). A measurement of 29.92 inches of mercury is equivalent to 1013.25 millibars, a more modern unit or measure of pressure.

Weather map