Why Does It Rain? Understanding the Dew Point and Air Movements

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Why does it rain? Could our planet survive without rain? Probably not. Image by Alvimann.

Why does it rain? The simple answer is: It rains because, at any given temperature, the atmosphere can only hold so much water vapor.

The Ubiquitous Water Molecule

Water is everywhere on planet Earth. An average adult is about 60% water, but the Earth’s surface has us beat: Approximately 70% of the planet is awash, and oceans account for most of the water.

It is not surprising that some water finds its way into the atmosphere as gaseous water vapor. Meteorologists pay homage to this water vapor by solemnly announcing the value of something called the ‘humidity.’ Normally they mean ‘relative humidity,’ which is a poor measure of water content. Pay no heed to this bogus value and find out what the dew point is. This will tell you how much water is in the air.

What is the Dew Point?

The warmer air is, the more water vapor it can hold. At 68 degrees Fahrenheit, the atmosphere can accommodate four times as much water vapor as it can at 32 degrees. At a tropical 86 degrees, it can hold twice as much as it could hold at 68 degrees. That, to put it simply, is why it rains.

The dew point is the temperature at which the air can exactly hold the amount of moisture present. If the temperature falls below this value, some of the water vapor must condense. So all we have to do is figure out when the temperature will fall below the dew point and we can forecast when it wil rain. Except it isn’t so simple.

Motion in the Atmosphere

We are familiar with movement of air; we call it wind. But our experience with wind is movement horizontally. There can also be a vertical compnent of air movement, but it is small relative to the horizontal motion. Unfortunately it is the vertical movement of air that makes it rain.

One of the properties of the lower atmosphere (below about eight miles) is that the higher you go, the colder it gets. The average decrease of temperature with height is about fifteen degrees per mile. Rain is entirely the result of air being lifted until it cools to the dew point. Moisture begins to condense, and clouds, consisting of tiny water droplets, form. If the clouds get large enough, the little drops form bigger drops and it rains.

Where Does Air Rise?

One place where air obviously rises is where it is forced upward by the topography. Any place where the prevailing wind blows over a mountain will create a rainy  area on the windward side of the mountain. The rainiest place in the world is in the foothills of the Indian Himalayas, where the monsoon wind blows in from the Indian Ocean and up the slope. In Hawaii, the airport of Kona, on the lee side of a mountain, is not even covered, as it almost never rains. Yet in other parts of the island chain there are rain forests.

But it also rains over relatively flat land. This occurs because there is a small vertical component of air motion. Since air cools so rapidly as it rises, a vertical motion of a fraction of a mile per hour can cause a lot of rain. The conditions for air to rise occur in the mid-latitudes near warm fronts, where warm air rides up and over cold, or near cold fronts, where cold air wedges beneath warm. The sun’s heat can also cause air at the surface to heat up and rise, bringing on daytime thunderstorms.

Water Content in the Air

The dew point determines how much rain will fall when air is lifted. A combination of proximity to a large body of water and the prevailing wind direction often determines the amount of precipitation a given area gets. Consider the following three cities, all within a degree of 43 degrees north latitude. Crescent City, California, on the Pacific Ocean and in the zone of westerly wind, gets 67 inches of rain per year. Boston, Massachusetts, on the Atlantic Ocean but within the zone of westerly winds, gets a moderate 44 inches per year. Chimbay, Uzbekistan, far from any ocean, gets about 4 inches of rain per year. Steady west winds off the Pacific bring a lot of rain to Crescent City; occasional winds from the Atlantic ocean and Gulf of Mexico bring moderate amounts of rain to Boston; Chimbay is cut off from any source of water vapor and gets almost no rain. As with everything else, with rain – it’s location, location, location.

Resources

GES DISC. Atmospheric Water Vapor. NASA. Accessed November 8, 2013.

USGS. The water in you. Accessed November 8, 2013.

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