How Temperature Affects Atmospheric Pressure and Why It Increases With Altitude

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The atmosphere, which is the blanket of gas surrounding the Earth, is divided into five layers. It is thickest near the bottom (also called the surface), and thins out with rising altitude until it reaches space.

Layers of the Atmosphere

The lowest is called the troposphere, contains the Earth’s weather systems.

Right above it is the stratosphere where airplanes fly and where the ozone layer is located.

Moving up towards space are the remaining three layers – the mesosphere, the thermosphere and the exosphere.

Even though the atmosphere is invisible to man, the atmosphere actually has a mass that is pulled to Earth by gravity. In other words, air bears weight just as human beings do. The atmosphere exerts pressure which is strongest near the Earth’s surface (sea level), and weaker at greater heights.

Atmospheric pressure is the amount of force exerted over a surface area, caused by the weight of air molecules above it. A elevation increases, fewer air molecules are present. Therefore, atmospheric pressure always decreases with increasing height. A column of air, 1 square inch in cross section, measured from sea level to the top of the atmosphere would weight approximately 14.7 pounds per square inch. The standard value for atmospheric pressure at sea level is: 29.92 inches or 760 millimeters of mercury.

When molecules bump into an object, they create a force on that object. Pressure is a measure of that force applied per unit area. Atmospheric pressure, then, which is also called barometric pressure or air pressure, is a measure of the force per unit area of the air above. It is a measure of the weight of the air over a given location. At higher altitudes there is always less air above than there is below. Thus atmospheric pressure decreases with altitude. Another way of expressing this is that air pressure is inversely related to altitude. At sea level, for example, the measure of atmospheric pressure is approximately 1,014 mb (millibars), whereas at the top of Mt. Everest the atmospheric pressure drops to about 265 mb.

Air density is a related term that refers to the density of air molecules in the atmosphere. Since the gravitational pull of the atmosphere to Earth is strongest near Earth’s surface, the density of air molecules there is greater as well. Therefore air density is directly related to atmospheric pressure. That is, the higher the density, the greater the pressure, and vice versa.

These concepts are easier to understand when applied to everyday situations. For example, the fact that air pressure and air density decrease with altitude explains why mountain climbers become more winded the higher they climb. Since human respiration depends on air pressure, and since air is thinner or less dense at greater altitudes, climbers often need bottled oxygen to help them breathe. The same principles apply to airplanes. Airplane cabins are purposely pressurized to enable people to breathe normally at high altitudes. This is also why in movies, when a plane door opens, a great gust is seen, which sends everything in the plane flying out the door. This occurs because the pressure in the plane is greater than the pressure outside. So too, airplane doors are constructed such that the inside of the door is bigger than the outside. This design creates a stronger and safer seal.

Finally, another real-life demonstration of the physics underlying the concept of atmospheric pressure versus altitude or height is a diver in a body of water. Since atmospheric pressure is a measure of the weight of the air above a given locale, the deeper a diver dives, the more pressure the diver experiences since the weight of the water above is greater.

Atmospheric pressure can be expressed in a variety of terms and units. Meteorologists, or weather forecasters, use the bar and the millibar (mb) to describe air pressure. The weather stations commonly refer to terms such as ‘high pressure’ and ‘low pressure weather systems.’ When atmospheric pressure is measured by a mercury barometer, it is reported in inches of mercury (Hg), or, using the English system of units, in pounds per square inch. Finally, the metric system employs the pascal (Pa).

How Does Temperature Affect Atmospheric Air Pressure?

The laws of physics indicate that when the air’s volume is held constant, an increase in temperature, which is a measure of molecular activity, produces a corresponding increase in pressure, which is a measure of molecular force. If air is confined in a closed container, heating the container will increase its internal pressure. Conversely, compressing a given volume of air causes its temperature to increase whereas expanding the same volume of air causes its temperature to decrease. These are the laws that form the basis of refrigeration and air conditioning technology.

On the other hand, the Earth’s atmosphere is not a constant volume. An increase of temperature in a high pressure area warmed by the sun causes the air to expand both horizontally and vertically, which results in only a minor increase in pressure. This increase is nonetheless measurable as the atmospheric volume does not respond immediately to the increase in temperature. In temperate latitudes, air pressure is actually greater during the late summer and early autumn than it is during the winter. This is because high pressure areas are warmer during the summer and build up more air above them, resulting in greater weight and higher pressure. Another example is the core of an extratropical cyclone, which is a low pressure zone and is colder than the surrounding air.

So why do the highest pressure zones occur in the coldest locations, such as Siberia? The problem in defining the relationship between temperature and atmospheric pressure is that a host of complicating factors, from the Earth’s rotation to the dynamics of upper air movements, exert competing influences on global weather patterns and create the impression that low temperatures indicate high atmospheric pressure.

At higher elevations in the atmosphere, the atmospheric pressure decreases. The temperature also decreases, not as a result of the decrease in pressure but because of the increased distance from the Earth, which is the primary source of heat in the atmosphere. “Adiabatic cooling” is a phenomenon that occurs when wind blows the air up into higher elevations where the decrease in air pressure causes the air to expand. The resulting decrease in density causes the air temperature to drop. Conversely, air blown down undergoes compression because of the higher pressure occurring at lower elevations and becomes warmer.

As the sun heats the ground or the ocean, the surrounding air becomes less dense and begins to rise, lowering the air pressure close to the Earth’s surface. This is called a “thermal low” and is indicated on weather maps by an “L.” At the same time, land heats up faster than water, causing the air over land to begin rising sooner than the air over a body of water. As the warm air rising over the land lowers the air pressure, cooler air from over the water flows in to replace it, creating a sea breeze. In this case, changes in atmospheric pressure are not caused by changes in temperature but by the effect of temperature on the air’s density.

The large low and high pressure centers that move across the Earth’s surface as storms and clear sky are formed indirectly as a result of unequal heating of the Earth by the sun. Due to the Earth’s shape, the tropical areas that flank the equator are more exposed to the sun than the regions farther north and south. As the air in this region begins to rise and flow to the north and south, cooler air flows in to replace it, resulting in the tropical trade winds. As the warmer air distances itself from the tropical region, it starts to come down, creating areas of high pressure.

Resources about Atmospheric Pressure

NASA.gov information about Atmospheric Pressure
Teaching Materials and lesson plans about atmospheric pressure:
What Is Atmospheric Pressure?
Audience: Educators and Students
Grades: K-8
Year: 2002

Resource by F.Rosseti

I am a teacher in Michigan. I grew up in Florida and have lived here in MI for close to 15 years. I enjoy writing and skiing in my spare time.

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