Friday, April 17, 2015

Let's Talk Air and Temperature!

Hello and welcome back! This week we are going to be looking at answering this question:

“I thought heat rose, so why’s it often colder up on the hill where I live rather than down in the valley below?”

Yes that notion is true that heat does rise but it still does not help to answer the question. 

To get a better idea of how air and temperature work, let’s look at the vertical structure of the atmosphere!

The atmosphere contains billions of air molecules. How dense the air is depends on the number of air molecules in a given volume while the air pressure depends open the pressure exerted by the mass of air above a given point in the atmosphere. Both air density and air pressure decrease with increasing altitude. In other words, these components decrease with height. 

Here are two images to see this concept in a different way:

Image from: Essentials of Meteorology: An Invitation to the Atmosphere
This idea works the same when it comes to temperature in the layer of atmosphere which all weather occurs.
Image from: Essentials of Meteorology: An Invitation to the Atmosphere

There are four layer of the atmosphere: the troposphere, the stratosphere, the mesosphere, and the thermosphere. In each layer, the temperature change with height. In the troposphere and mesosphere, temperatures decrease with height while temperatures in the stratosphere and thermosphere increase with height. This figure to the right shows the layers of the atmosphere and the changes in temperature (the red line).

The rate at which temperature decreases is a called a lapse rate with the average lapse rate being 3.6°F (6.5°C) per 1 km. 

So how does this all relate to the question? Well there is a change in height between a hill and a valley. This goes along with the idea that the air pressure and density on the hill will be less than in the valley because the hill is at a higher altitude. This same idea goes with the differences in temperature between the hill and the valley. Temperatures decrease with altitude (or height) therefore, making the temperature of the hill colder than in the valley. 

Another way to look at this is through talking about air parcels. As an air parcel rises, the air pressure is lower. Therefore, the force exerted on the air parcel decreases, allowing it to expand and cool. And the opposite, when an air parcel sinks, air pressure increases. Therefore, the force exerted on the air parcel is greater causing it to compress and warm. The temperature of a parcel is based on the amount of molecular collisions. When the air parcels are in a smaller space, they collide more causing a warmer temperature and when the parcel is expanded, collisions are less causing a lower temperature. 

Image from: Essentials of Meteorology: An Invitation to the Atmosphere
This image shows this process through the use of a mountain. At the bottom of the mountain, the air parcel is warm and compressed. As it goes up in altitude, the air parcel expands and cools so by the top of the mountain the parcel is cold. It shows this process with a sinking air parcel in that it becomes smaller and increases in temperature as the air parcel reaches the surface.

These various elements show why the temperature at the top of the hill is colder than the valley below. 

I hope you enjoyed this week’s posting! Make sure to comment down below with any weather-related you want answered by me. You can also ask your questions on Twitter using the hashtag #WeatherwithHeather. See you all next week!

*Special thanks to Brett Rathbun who provided me with these images for this week’s posting.

Works Cited
Ahrens, C. Donald. Essentials of Meteorology: An Invitation to the Atmosphere. 6th ed. Andover: Cengage Learning, 2012. Print.

Creative Commons License
What's the Weather Heather? by Heather Janssen is licensed under a Creative Commons Attribution-NoDerivatives 4.0 International License.

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