Shaun Tanner has been a meteorologist at Weather Underground since 2004.
By: Shaun Tanner , 7:58 PM GMT on December 09, 2011
Hello everybody, and I apologize for the long delay.
Now that we have all learned about various things, including the greenhouse effect and how it is essential for life on Earth, we can move on to some actual meteorology.
Have you ever wondered how and why the atmosphere gets warm during the day? Well, the obvious answer is the Sun. But, the exact way the atmosphere warms via the Sun comes in a roundabout fashion. You see, remember from the greenhouse effect lesson, the atmosphere is actually quite transparent to solar radiation. Thus, it passes right though many of the gases that make up the atmosphere. Instead, the way the lower part of the atmosphere receives warmth is by conduction. As a review, conduction is the form of heat transfer by molecules. So when cool molecules rub against warm molecules, that warmth is transferred from the warm to the cool molecules. During the day, the warmth of the Sun is transferred directly to the top layer of the surface, where it is absorbed. As the air molecules in the lowest layer of the atmosphere rub against these warm surface molecules, the heat is transferred into the atmosphere. Then, these newly-warm molecules rub against air molecules just above then and transfer that heat. In this way, heat is transferred from the surface well into the troposphere. It would not be hard to imagine then, that the warmest part of the atmosphere during the day is most likely going to be the lowest part of the atmosphere. Thus, during the day, temperature decreases with height. This is the normal state of things.
You may have noticed a beehive-looking thing sitting near-ish to the tarmac at your local airport. If you haven't noticed it before, take a look for one the next time you are on a plane. These white boxes are not beehives, rather they are the official NWS weather stations that are responsible for reporting weather to not only the NWS, but also to your favorite weather website, Weather Underground. The boxes are painted white and are 5.5 feet off the ground. Why these two things? Well, the boxes are pained white so the solar radiation hitting the box will be reflected instead of absorbed. If any solar radiation were to be absorbed, then it would give false readings on the thermometer. The box and thermometer is 5.5 feet off the ground because the warmest part of the atmosphere is actually below that. And 5.5 feet is actually around the region where we perceive heat. Thus, that is where we would like to have the most temperature readings.
Have you ever noticed that the air temperature during calm, Summer days can be actually quite warm compared to windy, Summer days? Why?
For that, I bring in a blender:
Note the blender in the picture is not turned on. Not only that, but it has a variety of ingredients just ready to be blended together into a delicious smoothie. Also note the warmer ingredients are on the bottom and the cooler ingredients, such as ice, is on the top. In other words, this setup mimics the lower part of the atmosphere during the day. Warmer on the bottom, colder on the top. Given no other forcing, the state of things will remain this way.
But, let's turn on the blender! The role of the blender is to act like wind in the environment. When the blender turns on, it mixes all of the ingredients in the blender together. If you were to take the temperature of the bottom of the blender and compare it to the temperature of the smoothie at the top of the blender, you will see there is very little difference. In other words, the smoothie is isothermal. Isothermal simply means the "same temperature". "Iso" means same, or individual, and "thermal" means temperature.
In the atmosphere, when winds picks up, it acts the same way as the blender does. It creates an isothermal atmosphere where the temperature near the surface is not much different than temperatures a couple hundred feet in the air. This is why calm, Summer days can be much warmer than windy, Summer days at the surface.
Last thing for the day. When is the hottest part of the day? If you said noon, then you are most likely incorrect. If you said later in the afternoon, such as 3-5 p.m., then give yourself a pat on the back. That time should raise a red flag in your head. Why is the hottest part of the day AFTER the time of maximum solar heating? Talk about weird!
To answer this question, you have to know one simple fact. The temperate of ANYTHING will rise as long as the incoming energy (absorption) is greater than the outgoing energy (emission). Right now, you are emitting radiation (outgoing energy). You are also absorbing radiation (incoming energy) from a variety of sources such as the Sun, a heater, a loved one, etc. If incoming energy is equal to outgoing energy, otherwise known as equilibrium, then temperature will stabilize. If outgoing energy is greater than incoming energy, then temperature will get colder. So forth.
Thus, the surface temperature will continue to rise as long as incoming solar energy is greater than outgoing surface energy. Maximum solar heating occurs around noon each day, but that is NOT the time at which incoming is balanced by outgoing energy. These two things are not balanced until around 3-5 p.m., making it the hottest part of the day. After that time, outgoing energy will be greater than incoming energy, and temperature will decrease.
Still confused? Let's talk money. The balance (temperature) in your bank account will continue to increase as long as the amount of money deposited (incoming solar energy) is greater than the amount of money withdrawn (outgoing Earth energy). The moment withdrawals exceed the amount of money deposited, your balance will decrease.
Thus, it simply does not matter THE RATE at which you deposit money or THE RATE at which it is withdrawn. The only thing that matters is when withdrawals exceed deposits. Does that make more sense?
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