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Name: Lisa E.
Status: Educator
Grade: 9-12
Location: NE
Country: United States
Date: March 2009


Question:
I teach 10th grade earth science. In the text book it says that temperature and pressure have an inverse relationship when it comes to altitude. It says that as air goes up in altitude the pressure decreases and the temperature increases. Why it take longer to boil water at high altitudes. why does air rise , cool and expand? Why is it really cold at higher altitudes?



Replies:
If the textbook states "as air goes up in altitude the pressure decreases and the temperature increases" it is wrong. Temperature does not normally increase with altitude in the bottom portion of Earth's atmosphere known as the troposphere.

Air rises on its own as a result of being heated. When the sun heats the earth, the earth absorbs and converts the sun's shortwave radiation into long wave radiation. The earth's long wave radiation is principally responsible for heating the lower atmosphere. As the air close to the surface heats, it expands. Because it expands, it becomes less dense. Because it is less dense, it rises. As it rises, it cools. Eventually it cools enough that clouds may form. The temperature at which clouds form (condensation occurs) is referred to as the dew point. The dew point temperature varies throughout the year but is generally higher in the summer than the winter.

Steven A. Miller


Wow. Good questions. There are two ways to look at these questions and the atmosphere in general. We can look at air as “big bag of gasses” or we can look at how individual air molecules behave. Some things are easier to understand from one point of view, others from a different point of view.

Everything depends upon Earth's gravity pulling the air (or its molecules) downward towards the surface. This gives air weight and causes the air pressure we are all familiar with. We also need to know that the “hotter” a gas is, the faster its molecules are moving. One, last principle. Molecules can lose or gain energy (heat)through colliding with other molecules in a process called conduction.

WHY DOES HOT AIR RISE?

Easy explanation: a bag of air.

We can say that warm air is less dense ("lighter") than cold. The same way wood floats on water because it is less dense, the hot air is forced up by the air pressure around and below it and up it goes. This explains why hot air balloons rise.

WHY DOES AIR COOL AS IT RISES?

Easy explanation: air as a gas

As air rises, air pressure drops and the gas expands. To expand, the gas needs energy which it robs from the surrounding matter (be it solid, liquid or gas). This cools things off. This explains why the gas coming out of a spray can feels so cold. It is expanding rapidly so it absorbs a lot of heat from its surroundings.

WHY IS IT COLD AT ELEVATION EVEN IF THE BOOK SAYS THE TEMPERATURE GOES UP?

At the atomic level:

Heat is transmitted when a molecule with high energy hits one with lower energy. Some of the energy goes from high speed to molecules to the lower speed one. Thus, heat is transferred from hot to cold areas. Warm air molecules hit your skin, dump energy and rebound. The skin molecule vibrates faster and you feel warm. The speed of the gas atoms depends upon temperature (the higher the temperature the faster they go) but the amount of heat transferred to you or a thermometer depends upon both the speed of the molecules AND the number that hit you.

At very high elevations (above the troposphere) the molecules travel very fast so are very “hot” (have a high temperature), but there are so few hitting you that it would feel very cold. At those elevations, it is radiation from the sun that warms you, not the air. In fact, in space where there is no air to move the heat around, you would roast in the sun and freeze in the shadow.

Robert Avakian


Lisa,

As you can see by some web sites that I list below, pressure certainly decreases with height, but temperature does not always increase with height (in fact this only occurs in certain layers of the atmosphere, particularly the Stratosphere and lower Exosphere, or in a temperature inversion in the lower part of the Troposphere).

Pressure is a measure of the mass of air above the surface. As gravity acts on the air, it has weight, just as we do.

The greater the altitude, the less mass of air is above you and therefore the lower the pressure.

Temperature decreases with height (particularly in the Troposphere, Mesosphere, and upper Exosphere) in general. The ideal gas law is in effect, PV=RT, where P=pressure, V=volume, R=gas constant, T=temperature. Keeping the volume of air the same, as that volume of air rises, the pressure decreases, and therefore the temperature of the volume must decrease also.

However, parcels of rising air do not maintain the same volume. The Sun heats the ground, which then heats the air just above it. The heating of the ground and the air above it is not evenly distributed, so some of the ground and air above it is heated more than in other areas. The warmer air is warmer than surrounding air (making it less dense) and therefore it rises. As a parcel of air rises, it expands in the continually reducing pressure. Therefore, the volume of the air parcel increases as the pressure decreases, but resulting in less of a temperature decrease than if a constant volume of air was maintained in the parcel.

Air temperature tends to decrease with height as it is further removed from the source of heating, the warming of the Earth's surface by the Sun. That is why its colder at greater altitudes, such as on top of a mountain, than it is in a valley nearby.

However, there are other sources of heating of the atmosphere, including chemicals (such as ozone) and aerosols that persist at certain levels of the atmosphere. Those chemicals and aerosols absorb the Sun's energy before it gets to the ground, thereby raising the temperature at that level. Therefore, the atmosphere does not decrease in temperature uniformly with height. See the figures at

http://www.windows.ucar.edu/tour/link=/earth/images/profile_jpg_image.html,

and http://www.eoearth.org/article/Atmosphere_layers,

and http://earthguide.ucsd.edu/earthguide/diagrams/atmosphere/index.html.

What these figures do not show is the decrease in temperature that occurs in the upper part of the Exosphere. The temperature in free space is close to zero degrees K.

Since we live in a dynamic, changing atmosphere, especially the lowest layer (Troposphere), weather systems move air of different temperatures and mass around, resulting in changes in pressure and temperature. Temperature sometimes increases with height from the surface upward a ways when a temperature inversion exists (resulting from cooling of the surface more rapidly than cooling of air above, as can happen on many nights). So, temperature does not always decrease with height even in the lowest part of the atmosphere, but over the extent of the whole atmosphere (surface to free space) it is generally true.

At greater heights there is less pressure and therefore less mass per unit volume of air. This decreased pressure results in a fluid (such as water) also expanding and having less mass per unit volume. The number of water molecules are fewer and further apart in decreased pressure. Therefore it requires a lower temperature for water to boil at a higher altitude, and thus less time (not more), assuming that the amount of heat being used is the same as at sea level.

David R. Cook
Meteorologist
Argonne National Laboratory


Lisa,

The textbook you are reading may be misleading - it depends on the layer of the atmosphere. In the troposphere the temperature decreases with increasing altitude. The troposphere is the lowest layer of the atmosphere that extends to around 11 km. You might have noticed that snow is common on the tops of mountains where at lower elevations there is an absence of snow; the tops of mountains are still well within the troposphere and it's cold up there. All of the weather happens in the troposphere.

Above the troposphere, in the stratosphere, however, we do find an increase in temperature with an increase in altitude. There are few molecules in the stratosphere so there is not a lot of heat up there, yet there is a high average molecular speed (that is, a high temperature, yet still cold).

Atmospheric pressure always decreases with increasing altitude. Air pressure (like water pressure) is a measure of the weight of the air (or water) above. Think about a pool: the deeper one dives, the greater the pressure because of the overlying weight of the water above. As one ascends (climbs up a mountain) there is less atmosphere above, so the pressure decreases with increasing altitude. That is why we pressurize airplanes. You might imagine some movies when a door opens in a plane at high altitude and everything gets blown out the door - the cabin pressure is greater than the ambient (outdoor) pressure. This is why the inside of airplane doors are larger than the outside -- it creates a safer seal.

Water does not take longer to boil at high altitudes (high altitudes are associated with lower pressure). Boiling is the temperature at which a liquid changes state to a vapor. Lower pressures allow boiling to occur at lower temperatures. Water can reach the boiling point more quickly at higher altitudes (lower pressures) because it does not have to be heated as much as it would at lower altitudes. At high altitudes, however, the water will boil below 100 degrees C. It takes longer to boil a potato at high altitudes because the water is not as hot, yet it is boiling. Have you ever used a pressure cooker? A pressure cooker boils water under pressure, thus it boils at a temperature above 100 degrees C so a potato will be done more quickly.

Not all air rises. Cold air, and air in atmospheric high pressure centers, are associated with descending air. Warm air, and air in atmospheric low pressure centers, are associated with rising air. Hot air rises because it has a lower density than cold air. As air rises the ambient atmospheric air pressure decreases, which allows the air to expand into a larger volume, so there are fewer interactions between the molecules in the air and the air temperature drops. This process is known as adiabatic cooling. It is similar to what happens in a lava lamp.

Let me know if you want some more information or some graphs that depict the relationships between volume, pressure, temperature, and altitude.

You did not ask about relative humidity and why clouds and fog form. It is closely related to the topics addressed above.

Les
Leslie Kanat, Ph.D.
Professor of Geology
Department of Environmental Sciences


Lisa,

Pressure is a function of the amount of force applied to a certain area over time. Since gas particles are applying the force, then the number of gas particles hitting a certain area will have an effect on the pressure measured.

Remembering that gravity has an effect on gas particles, we can expect that there would be fewer gas particles at higher altitudes since gravity tends to pull gas particles downward.

The fewer the gas particles, the fewer hits on a certain area over time, the lower the pressure.

Boiling point can be defined as the temperature at which the pressure of the liquid turning into gas is equal to the pressure of the atmosphere applied on the liquid. If the atmospheric pressure is lower (because of altitude) than it does not require as much heat or rise in temperature to reach the boiling point. The boiling point at high altitudes is lower. Everything else being equal, it should not take as long to reach the boiling point of water - however, what you are cooking is now being cooked at a lower temperature and so will take longer to get completely cooked.

Greg (Roberto Gregorius)



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