A high air pressure system is associated with clear skies and often cool, dense air. A low air pressure system is associated with rain and may indicate violent storms such as thunderstorms, hurricanes, or tornadoes. Isobars are lines drawn on a weather map which connect weather stations with the same atmospheric pressure. Isobars may form closed or open curves. The center of the pressure system is labeled either HIGH (H) or LOW (L).

Recall that a thunderstorm develops in towering cumulus clouds when a cold front meets a warm front. Thunderstorms produce strong winds, rain, and thunder and lightning.

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