Local Weather: What a Difference a (Mountain, Lake, Ocean, City) Makes

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Total U.S. precipitation For 2013.

Total U.S. precipitation For 2013. Image courtesy of NOAA

Tip O’Neill, former Speaker of the U.S. House of Representatives, once famously said, “All politics is local.

He could just as well have been talking about the weather.

Anyone who lives near a seashore or on a mountain can vouch for the fact that the terrain and the proximity to water are important in day-to-day — or day-to-night — weather.

Local Weather: Continental or Marine Climate

The proximity to water has a huge effect on local weather.

Miami, Florida, at the tip of a peninsula, receives more rainfall than any other large American city (average 62 inches per year).

At the same latitude and on the Gulf of Mexico, Brownsville, Texas receives less than half as much (27 inches). And Torreon, Mexico gets barely one-eighth the amount (9 inches), though it is at the same latitude.

Clearly proximity to water on all sides makes a location wetter than one with water on one side. And considerable distance from any water source leads to downright desert-like conditions.

Local Weather: Over the Mountain

There is also a vast difference in rainfall between the windward and lee sides of a mountain. Winds encountering a mountain are forced upwards and the water vapor is squeezed out as precipitation; there’s nothing left to precipitate on the lee side. On Henderson Island, British Columbia, Henderson Lake receives 250 inches of rain annually. On the other side of the mountain, Victoria gets about 25 inches.

The Hot and Cold of Local Weather: Land and Sea

The temperature of the air at the surface of the earth tends to mirror the temperature of that surface. Where the wind blows off an ocean, the air temperature is close to the water temperature. Anyone who has gone on a whale watch out of Boston can vouch for this. It can be 90 degrees in the city, but out on Stellwagen Bank (where the whales are) the air temperature will be very close to the water temperature — normally in the low 60s in the summer.

Land absorbs the sun’s heat much more efficiently than water, so away from the ocean, daytime temperatures can get very hot , especially in summer. On the other hand, the land also emits microwave radiation much better than water. In Kingman Arizona, the average difference between the daily high and low temperatures is 26 degrees; in New York, near the water, the variation from day to night is about 14 degrees.

Average temperatures in the United States for 2013.

Average temperatures in the United States for 2013. Image courtesy of NOAA

The Hot and Cold of Local Weather: Mountains

In the lower atmosphere, the temperature decreases with height at an average of about four degrees per thousand feet; you’ll feel the effect strongly, even on a modest-sized mountain.

The average annual temperature on top of Mount Washington, New Hampshire at about 6200 feet is 27 degrees. In nearby Boston, at sea level, the annual average temperature is 51 degrees.

On top of 29,000 foot Mount Everest, the temperature never rises above freezing, averages below zero even in the ‘warmest’ month, and averages minus 33 in the winter. Temperatures as low as minus 76 degrees have been recorded.

Local Weather: Heat Islands

Modern cities, with their landscape of roads and buildings, produce and hold heat better than the fields and forests they replaced. Asphalt absorbs solar radiation better than trees, and grasslands emit more microwave radiation than buildings. The result is that a city becomes a heat island; cities are several, and in some cases many, degrees warmer than the surrounding countryside.

For example, the average minimum temperature in Boston in January is 23 degrees; in the nearby suburb of Norwood the average is 18 degrees.

If Tip O’Neill had been a meteorologist, he would have had it pretty much right. Not all weather is local, but a lot of it is.

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