# What is the relationship between air temperature and its density

### Pressure/Density/Temperature relationship - ATP Forum Together they create air currents. The Earth is warm and heats up air. The air becomes less dense and rises. In the upper atmosphere the heat. How does the density of a substance change as the pressure of an object, you can calculate its density using the following equation: The density (r) of an object depends on temperature and pressure. You may be aware that the density of water is approximately 1, kg/m3 and the density of air is. We will examine how air temperature, number density, and pressure change in the Air in contact with the warm ground is heated from below, therefore, the air compressed, i.e., squeeze a gas together and its number density increases.

Pressure is merely force per area, so if the force increases but the box stays the same size, the pressure has increased. Air density can decrease with temperature if pressure is also decreasing. If pressure is constant, this cannot happen they would be inversely related.

Any time you specify a relation between any two of pressure, density or temperature you must hold the third constant or specify its behavior.

For example, hot air rises, but why then is it cold on top of a mountain. The answer is that hot air is less dense than the cold air surrounding it for a constant pressure, and being less dense it rises. With a mountain, the pressure is decreasing, and we likewise find in the atmosphere that temperature decreases with decreasing pressure. On a hot day what tends to happen is that the surface, which is being warmed by the sun, heats the lowest level of the atmosphere, reducing its density it is at the same pressure as its surroundings and its T rises.

This will eventually drive convection and mix this warmer air vertically. Given enough time, this will reduce the mass in the column of air and therefore reduce the pressure at the surface.

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These are called "heat lows" and you can see them forming in the desert areas and they play roles in sea breeze formation and the monsoons. To address the expanded question: The point in the FAA written is best understood by forgetting that we fly at constant altitudes -- we don't. In level flight we fly on constant pressure surfaces which we then translate to an altitude. In any given column of atmosphere, if it is warmer than standard a given pressure surface will be higher and when colder than standard the pressure surface will be lower.

To illustrate, let's consider you are flying at ft or roughly mb. Everywhere on this pressure surface will indicate ft on our altimeter for its current setting. If we go somewhere hot, this pressure surface rises, and so we climb though we think we are level with this pressure surface but because the pressure has not changed, we still indicate ft.

• Density of air
• Vertical Structure of the Atmosphere

However, we are higher than ft in reality. This follows into your next question. Aneroid wafers detect pressure changes and your altimeter displays an altitude not corrected for temperature. This is why your true altitude can vary with temperature for a constant indicated altitude. Weigh an empty bag, then fill it with air, it now weighs more. In addition gases, like air, are easily compressed, i. In other words, we say gases are compressible because they can easily be squeezed into a smaller volume. Solids and liquids on the other hand are not easily compressed.

The weight of all of the air above a given point in the atmosphere squeezes air molecules closer together, which causes their numbers in a given volume to increase increase in number density. The more air above a level and hence the more weight of air above a levelthe greater the squeezing effect or compression. Since air density is the number of air molecules in a given space volumeair density is typically greatest at the surface or sea level where it is squeezed by the weight of the entire atmosphere above and decreases as we move up in the atmosphere because the weight of air above becomes less and hence there is less of a squeezing effect See Figure Z.

Pressure Atmospheric air pressure results from the Earth's gravitational pull on the overlying air. Without gravity holding the atmosphere just above the ground surface, air molecules would spread out, and the gas pressure would be close to zero.

The weight of the atmosphere acts as a force upon the underlying surface of the Earth. The amount of force excerted over an area of surface is called atmospheric pressure or air pressure. Near sea level, the average air pressure is about In this class we will use the unit millibars mb to specify air pressure. At sea level the average air pressure is mb. Another way to think of this is that the total weight of all the air above sea levels weighs enough to cause mb of air pressure. Since the air a gas is a fluid, the pressure force acts in all directions, not just downward. The pressure force pushing downward due to the weight of the air is the same as the pressure force acting sideways and even upward.

## What is the relationship between air temperature and air density in the atmosphere?

If you are having trouble understanding this, make an analogy with another fluid liquid water. Consider a deep swimming pool full of water. The water pressure anywhere in the pool depends on the weight of the water above that is the deeper you dive downward in the pool, the stronger the water pressure. The pressure force is not just downward though, it pushes in on your body from all directions. The average air pressure at sea level mb or sometimes called one atmosphere of pressure is caused by the weight of all the air above sea level. In the same way water pressure is caused by the weight of water above you.

At a depth of 32 feet 9. Thus, the entire column of air from sea level to outer space weighs as much as a 32 foot column of water. Of course diving deeper than 32 feet downward into water means you will encouter an increasing water pressure enough to crush you if you go too deep. Typical change in air pressure with altitude. Note how rapidly air pressure falls with increasing altitude. In the atmosphere, the air pressure at any point is caused by the weight per area of the air above that point.

### Explain the relationship between air temperature and air density

As we climb in elevation, fewer air molecules are above us less weight of air above us ; hence, atmospheric pressure always decreases as you move upward in the atmosphere See Figure B. Another way to look at it is that the air pressure at any point in the atmosphere is exactly enough to support the weight of the column of air above it.

A balance exists between the gravitational force pushing air downward and the upward directed pressure force. This balance is called hydrostatic balance see figure. Earlier we made an analogy between diving down in water and moving downward in the atmosphere. In both cases, the fluid presure increases as you move down because there is more and more weight of fluid above you. A big difference between water and air, though, is that air is compressible and water is not.

This affects the rate of pressure changes as one moves up or down in the fluid as shown in Figure C, which compares the rate of vertical pressure changes between water and air.

The relationship between the density and the temperature and pressure

Because air is compressed by its own weight, much of the mass of the atmosphere is squeezed into the troposphere where the air is most dense higher number densitywhile only a small portion of the mass of the atmosphere remains above the stratosphere where air is less dense lower number density. Since air pressure is directly related to the weight of air above a given point, a ratio of air pressure is equivalent to a ratio of weight.

Thus, at a location where the air pressure is mb, roughly half the weight of the atmosphere is above you and the other half is below you. A typical mb height is about meters or 5. Thus, half the weight of the atmosphere is compressed into the vertical column from sea level up to about 5.

This happens because the number density is greatest just above the ground surface and decreases as you move upward. Note in the figure above how rapidly air pressure drops as you move up above the surface where number density is largest, but that the rate of pressure drop slows down as you move to higher altitudes where the number density is much smaller. This is characteristic of an exponential decrease in air pressure with increasing height. On average in Earth's atmosphere, the air pressure approximately drops in half for every 5. 