Back To CourseEarth Science 102: Weather and Climate
13 chapters | 127 lessons
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Julie has taught high school Zoology, Biology, Physical Science and Chem Tech. She has a Bachelor of Science in Biology and a Master of Education.
You've surely heard of the Three Little Pigs fairy tale, right? But you probably didn't hear about the Three Little Pigs and the Wind Belts. Don't worry, though; I'll tell you the story. But before I start, I should fill you in on wind belts, or wind generated by the earth's rotation and unequal heating from the sun. We'll get into the details of that definition when we examine the three main wind belts: the polar easterlies, the tropical easterlies, and the prevailing westerlies. Oh yeah, all of our piggies inhabit the Northern Hemisphere, so even though we'll mention the Southern Hemisphere, the focus will be on the Northern Hemisphere.
Before we get into the details, there are some things you should know about wind that will help you on this fairy tale. Wind blows from high-pressure areas to low-pressure areas. Cold, dense air causes high pressure, and warm, less-dense air causes low pressure. This is because cold air has more molecules packed into a certain volume, and the more molecules that are present, the greater the pressure exerted. Okay, onto our story!
Once upon a time there were three little pigs: each one lived in a different part of the world. Let's start with pig number one who lived in an igloo at the North Pole. Technically he lived in the 60-90 degree north latitude part of the world.
Now this little piggy had to deal with the polar easterlies, because of where he lived. The polar easterlies are winds that are found between 60 and 90 degrees north and south latitude that blow from the poles and are deflected towards the west.
Although our little piggy lives near the North Pole, you may have gathered from that definition that the polar easterlies can be found near both poles. Before the polar easterlies try to blow our little piggy's igloo down, let's look at how they form.
The air at the poles is cold and dense, creating high pressure. This high-pressure air heads to the equator, towards lower-pressure air, but since the earth is rotating, these winds are deflected in what is called the Coriolis Effect, or the deflection of wind due to the rotation of the earth. It can get a little tricky to remember which way the winds are deflected, so let me give you some rules. Remember, wind travels from high to low pressures, so as they do, here is the deflection:
Wind is deflected to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.
So, based on our rules, and since the polar easterlies are blowing away from the poles, we can assume that the winds are blowing towards the right, or west, in the Northern Hemisphere. You may be wondering why they are called polar easterlies then, right? Well, winds are named based on where they start, so since they start in poles, and due to the Coriolis effect, blow from the east, they are called polar easterlies.
Uh oh, and here come those polar easterlies! The little piggy is taking shelter but can't help but notice the winds are cold and dry, and, thankfully for our polar piggy, not very strong. So all is well in the north.
Now to our second little pig who lives near the equator in a house made from palm trees and is worried about the tropical easterlies, or winds that blow from the northeast in the Northern Hemisphere and from the southeast in the Southern Hemisphere and are located between 0 and 30 degrees north and south latitude. Their more familiar name is the trade winds. Before these winds flow over the palm hut, let's see how they are formed.
Remember, the sun heats the equatorial region more than the polar regions, so warm air from the equator rises and flows north and south. This air eventually cools and sinks at a latitude of about 30 degrees (again, north and south of the equator) and some of it returns to the equator. This air moving back to the equator is the tropical easterly. And remember our rules from before. As wind blows from high pressure to low pressure it will be deflected to the right, or west, in the Northern Hemisphere, so as the air heads back to the equator, it is going to be deflected westward. So, like the polar easterlies, this wind originated from the east so it's called an easterly.
And here it comes! Look out, piggy number two! Although the tropical piggy has taken shelter, he notices these winds are steady and warm, and actually more like a breeze, so they don't knock down his hut!
Now the third little piggy lived in between polar pig and tropical pig at latitudes between 30 and 60 degrees, so he was worried about the prevailing westerlies, which are winds located 30 to 60 degrees north and south of the equator that blow eastward towards the poles.
You might notice these are a change from our first two winds as they blow eastward, so they're called westerlies (since they blow from the west). Remember, they are blowing towards the poles, so they are deflected to the right, which causes them to blow eastward.
Now, before our mid-latitude piggy has to deal with this wind, let's see how the prevailing westerlies form.
Hot air from the equator rises and heads towards the poles, but as it gets further away from the equator, it cools and sinks around 30 degrees north and south latitude. Now, remember, some of this air goes back to the equator and some of it continues on to the poles. The air that continues towards the poles causes the prevailing westerlies. Now the prevailing westerlies can be quite strong, so let's hope mid-latitude piggy can make it.
Here comes the wind! Oh no, it's blowing his house down! The prevailing westerlies are powerful and strong, so run, little piggy, run! It looks like he's made it! Wow, that was close!
Thankfully our little piggies survived the three wind belts! Before we wish them a 'happily ever after,' let's take a moment to review our story. Wind belts are formed due to unequal heating of the earth and the earth's rotation. The first wind belt in our story, the polar easterlies, lies between 60 and 90 degrees north and south latitude and blows from the poles. And don't forget our rules regarding the Coriolis effect: winds are deflected to the right in the Northern Hemisphere and to the left in the Southern Hemisphere, and this is as they blow from high to low pressure.
Next we looked at the tropical easterlies, which are winds that are located between 0 and 30 degrees and blow towards the equator.
Finally, we looked at the prevailing westerlies which are winds that are located between 30 and 60 degrees and blow towards the poles.
And now we're ready: the first two little piggies lived happily ever after and the third pig built a stronger house to withstand the prevailing westerlies. So, they all lived happily ever after.
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Back To CourseEarth Science 102: Weather and Climate
13 chapters | 127 lessons