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Understanding Climate

Physical Properties of Air

Hot air expands, and rises; cooled air contracts – gets denser – and sinks; and the ability of the air to hold water depends on its temperature. A given volume of air at 20°C (68°F) can hold twice the amount of water vapor than at 10°C (50°F). The relationship of how much water a given mass of air actually holds compared to the amount it can hold is its relative humidity.

When air holds as much water vapor as it can for a given temperature (100% relative humidity), it is said to be saturated. If saturated air is warmed, it can hold more water (relative humidity drops), which is why warm air is used to dry objects--it absorbs moisture. On the other hand, cooling saturated air (said to be at its dew point) forces water out (condensation). This is why a container of a cold beverage sweats: it cools the air next to it and moisture from the air condenses on the outside of the can.

Air warmed by ocean currents picks up a lot of moisture. As the heated air rises, it expands, which is measured at the surface as low air pressure. Expanding air cools, which forces it to lose its moisture as rain or snow. The opposite is true for sinking air. Such air compresses and warms. In a zone of high pressure like this, moisture is absorbed by the air from its surroundings.

AIRS water vapor — This image represents the total precipitable water vapor for May 2009 as observed by JPL's Atmospheric Infrared Sounder on NASA's Aqua satellite. This image represents the total precipitable water vapor for May 2009 as observed by the Atmospheric Infrared Sounder (AIRS) on NASA's Aqua satellite. It shows the total amount of water vapor present in the atmospheric column above each point of Earth's surface. If all the water vapor in the column were forced to fall as rain, the depth of the resulting puddle on the surface at that point is equal to the value shown on the map. Fifty millimeters is about 2 inches.

Physical Properties of Water

Water is an enormously efficient heat-sink. Solar heat absorbed by bodies of water during the day, or in the summer, is released at night, or in winter. Sites on islands or coasts benefit from the moderating effect of the ocean and have "maritime" climates (like San Francisco). Sites away from the coast lack this temperature buffering and have extreme "continental" climates (like Wichita).

Like the heated air in a hot-air balloon, heated water expands. Solar heat absorbed at the equator causes water to expand. Such heated water raises the normal level of the sea surface, and such changed ocean topography can be measured. The heat in the water is carried to higher latitudes by ocean currents where it is released into the atmosphere. Water chilled by colder temperatures at high latitudes contracts (thus gets denser), sinks, (lowers the local topography) and returns to the equator via the global ocean water circulation conveyor belt to complete the cycle.