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Temperature, Pressure, Altitude, Boiling
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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