Tuesday, July 17, 2012

Why are higher elevations cooler?

Hi, it's Rebecca.  Last week I decided to let followers of my webpage and  readers of this blog suggest  a topic they wanted covered.  I'm going to pick three , one will cover why it gets cooler at higher altitudes. The second will go into a bit on solar and space weather. The third will cover bombogenisus .

Gary Sluzky, Asked:  "I live in the mountains, about 1,875' of elevation. I've always known it's cooler in the mountains during the summer, and colder in the winter, than Albany and NYC. Generally, we run about 7° - 8° cooler than Albany and 12° - 18° cooler than NYC. I realize that we are north of NYC by about 100 miles and that alone makes a difference, but we are also about 50 miles south of Albany, yet, we still run cooler. Obviously it's an elevation thing, my question is why?"

There are several reasons why this is occurring.

Longitude: As Gary implied, the farther  you get away from the equator the cooler the average temperatures get. This has to do with the angle of the solar radiation (radiant energy emitted by the sun)  moving through the atmosphere).  The sun is overhead at the equator, as you go farther and farther north the angle slants more and more.  This has a impact on the absorption and scattering of sunshine when passing through the atmosphere.  

Altitude: The reason, it get cooler the higher you go in elevation, has to do with air pressure. As you may remember from your high school earth science, At sea level, the pressure is around 14.7 pounds per square inch ; at 2000 feet the pressure is around 13.7 pounds per square inch,  and at 5000 feet the pressure is 12.2.  The change in pressure results in a change in temperature. At sea level the air molecules are close together (this means they collide with each other a lot), which puts them in a higher energy level. Whereas, at higher altitudes the pressure is lower, so the volume is higher ( the same number of molecules in a larger space) with all the extra room; they don't collide as often resulting in a lower energy level. So each square inch has a much lower temperature than sea level air.

Rural vs. Urban:

Rural areas on average are much cooler than urban areas. The difference between urban and rural temperatures is most prevalent when the winds are light, the dewpoint is moderate and the skies are clear. There is a lot more water in an rural environment. The trees and other vegetation release water vapor through pores on their surface.  This process is part of the water cycle it is called transpiration. Transpiration is a process similar to evaporation. As the water evaporates it cools the air.  Urban areas are mostly concrete and asphalt. Because of this rainfall runs off into storm drains. Whereas, rural areas are mostly grassland and forest, therefore the rainfall soaks into the soil. Also, in rural areas solar energy is absorbed into the environment much more efficiently than it is in urban areas. The solar energy helps evaporate even more water vapor. All of these things combine to keep the rural areas much cooler than their urban counterparts.   

Urban Heat Island Effect:

The urban environmental issues I just mentioned lead to something called urban hear island effect. All the concrete, roads and buildings absorb sunlight and trap heat. As a result, cities create their own, warmer microclimates. Cities are heat traps, because of the buildings and all the roads. most of the solar radiation is absorbed. The buildings are so close together that they reabsorb heat energy  giving off by their neighbors.   

Cities do have an impact on the local-scale weather. There is evidence that the urban environment aided in the development of a tornado that move through Atlanta in 2008. The study indicates a connection between the intensity of the 2008 urban Atlanta tornado and the heat island effect suggests that the hot, dry urban conditions may have led to a larger discrepancy with the surrounding atmospheric conditions, enhancing stability and thus intensifying the storm as it approached the city.

There are several reason why  cities have an impact on local weather. However, the two main ones are:  extra heat over the cities aids in the  formation of thermal updrafts, and the increase of atmospheric particulates caused by pollution.   which leads to more clouds than would have occurred if the city had not been there.

Here are a couple of graphics that show the heat island effect.

I hope this answers your question Gary, If anyone has an question, feel free to ask. The next blog post will be on solar and space weather. The one after that will be on bombogenisus and what the heck the word means.


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