Conversely, air that sinks always gets warmer. Air always cools when it is lifted, because it moves from higher pressure to lower pressure by going up. By far the most common way air is cooled is by lifting. Those water droplets and/or ice particles are what clouds are made of.Ĭooling of unsaturated air, then, is the key to forming clouds and eventually causing precipitation. some water vapor (gas) will change phase to a liquid (water droplets) or solid (ice particles) form. Any additional cooling of saturated air will cause condensation to occur, i.e. Air containing any amount of water vapor, if cooled, will eventually reach a point where the water vapor content is exactly equal to the capacity of the air to hold water vapor. For any given air temperature, there is a maximum amount of water vapor that can be stored. Lowering the temperature of air reduces its capacity to store water as a gas (water vapor). We use the term "rainfall" here instead of "precipitation" because, although we include frozen precipitation as a minor component, the other major precipitation component, fog drip, is not included.Ĭloud formation leading to precipitation is always associated with air being cooled. However, fog drip, derived from direct interception of cloud droplets by vegetation, is a major source of water in the middle-elevation fog zones of Hawai‘i's mountains. Frozen precipitation is a minor contributor overall. Precipitation in Hawai‘i includes rain, various types of frozen precipitation (such as snow, sleet, hail, and freezing rain), and fog drip. What causes these patterns? Why "Rainfall" instead of "Precipitation"? In general, high mean rainfall is found on the windward mountain slopes, and low rainfall prevails in leeward lowlands and on the upper slopes of the highest mountains. Annual means range from 204 mm (8 inches) near the summit of Mauna Kea to 10,271 mm (404 inches) near Big Bog on the windward slope of Haleakalā, Maui. Hawaiʻi’s rainfall pattern is spectacularly diverse (above).
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