Chapter 8, Atmospheric Pressure and Altimetry 8-1
8 Atmospheric Pressure and Altimetry
8.1 Introduction
Atmospheric pressure is one of the most basic variables used to describe the state of the atmosphere and is
commonly reported in weather observations. Unlike temperature and relative humidity, changes in
atmospheric pressure are not as readily sensed by people. However, variations of pressure across the Earth
are associated with pressure centers (either high -pressure centers or low -pressure centers) that cause the
wind to blow and can bring important weather changes. Density, which is directly related to pressure, is a
property of the atmosphere, which can be used by pilots to help determine how their aircraft will perform
at various altitudes.
This chapter discusses atmospheric pressure, how it is measured, and how it varies across the Earth. This
chapter also covers the altimeter, which is a pressure sensor used by pilots to determine altitude. Finally,
density will be discussed, along with its relationship to density altitude.
Chapter 8, Atmospheric Pressure and Altimetry 8-2
8.2 Atmospheric Pressure
The atoms and molecules that make up the various layers in the atmosphere are always moving in random
directions. Despite their tiny size, when they strike a surface, they exert pressure.
Each molecule is too small to feel and only exerts a tiny bit of pressure. However, when add ing up all the
pressures from the large number of molecules that strike a surface each moment, the total pressure is
considerable. This is air pressure. As the density of the air increases, the number of strikes per unit of time
and area also increases.
Since molecules move in all directions, they even exert air pressure upwards as they smash into objects
from underneath. Air pressure is exerted in all directions.
Atmospheric pressure is the force per unit area exerted by the weight of the atmosphere. Since air is not
solid, it cannot be weighed with conventional scales. Yet, three centuries ago, Evangelista Torricelli proved
he could weigh the atmosphere by balancing it against a column of mercury. He actually measured pressure,
converting it directly to weight.
Air is composed of matter and thus, has weight due to
the pull of Earth’s gravity.
Figure 8-1. Air Has Weight
8.2.1 Barometer
The instrument Torricelli designed to measure pressure was called a barometer. The aneroid barometer is
the type most commonly used by meteorologists and the aviation community.
Essential features of an aneroid barometer (see Figure 8-2) are a flexible metal cell and the registering
mechanism. Air is taken out of the cell to create a partial vacuum. The cell contracts or expands as pressure
changes. One end of the cell is fixed, while the other end moves the registering mechanism. The co upling
mechanism magnifies the movement of the cell driving an indicator hand along a scale graduated in
pressure units.
Chapter 8, Atmospheric Pressure and Altimetry 8-3
Figure 8-2. Aneroid Barometer
8.2.2 Atmospheric Pressure Units
Atmospheric pressure is expressed in many ways throughout the world (see Table 8-1). Meteorologists
worldwide have long measured atmospheric pressure in millibars (mb or mbar), which denote pressure as
a force per square centimeter. However, after the introduction of the Intern ational System of Units (SI)
in 1960, the hectopascal (hPa) was adopted by most countries and is used in the METAR/SPECI code first
developed in 1968. Many meteorologists prefer to use the term they learned during their education and
work experience. Therefore, some continue to use the term “millibars,” while others use “hectopascal”
(which are equivalent). The unit inch of mercury (inHg or Hg) is still used in the United States for altimetry.
Table 8-1. Units of Pressure
Units of Pressure Standard Atmosphere
Value at Sea Level Common Use
Hectopascals (hPa) 1013.2 hPa METAR/SPECI
Millibars (mb or mbar) 1013.2 mb U.S. Weather Charts
Inches of mercury (inHg or Hg) 29.92 inHg U.S. Aviation
Pounds per square inch (psi) 14.7 psi U.S. Engineering
8.2.3 Station Pressure
The pressure measured at an airport is called station pressure, or the actual pressure at field elevation.
Pressure is lower at higher altitudes. Therefore, airports with higher field elevations usually have lower
pressure than airports w ith lower field elevations. For instance, station pressure at Denver is less than at
New Orleans (see Figure 8-3).
Chapter 8, Atmospheric Pressure and Altimetry 8-4
Figure 8-3. Station Pressure
The next few sections will examine some factors that influence pressure.
8.2.4 Pressure Variation
Atmospheric pressure varies with altitude and the temperature of the air, as well as with other minor
influences, such as water vapor.
8.2.4.1 Pressure Changes with Altitude
As a person moves upward through the atmosphere, the weight of the air above the person decreases. If a
person carries a barometer, then they can measure a decrease in pressure as the weight of the air above them
decreases. Figure 8-4 shows the pressure decrease with height in the standard atmosphere.
The standard altitudes in Figure 8-4 are based on standard temperatures. In the real atmosphere,
temperatures are seldom standard, so temperature ’s effects on pressure will be explored in the following
section.
Chapter 8, Atmospheric Pressure and Altimetry 8-5
Figure 8-4. Air Pressure in the Standard Atmosphere
Chapter 8, Atmospheric Pressure and Altimetry 8-6
8.2.4.2 Temperature’s Effects on Pressure
Like most substances, air expands as it becomes warmer and contracts as it cools. Figure 8-5 shows three
columns of air: one colder than standard, one with standard temperature, and one warmer than standard.
Pressure is equal at the bottom and top of each column. Vertical expansion of the warm column has made
it taller than the column at standard temperature. Contraction of the cold column has made it shorter than
the standard column. Since the total pressure decrease is the same in each column, the rate of decrease of
pressure with height in warm air is less than standard, while the rate of decrease in pressure with height in
cold air is greater than standard.
Figure 8-5. Temperature’s Effect on Pressure
8.2.5 Sea Level Pressure
Since pressure varies greatly with altitude, people cannot readily compare station pressures between stations
at different altitudes. To make them comparable, they are adjusted to some common level. Mean sea level
(MSL) is the most useful common reference. In Figure 8-6, pressure measured at a station at a 5,000 -ft
elevation is 25 inHg; pressure increases about 1 inHg for each 1,000 ft, or a total of 5 inHg. Sea level
pressure is approximately 25 + 5, or 30 inHg.
Figure 8-6. Reduction of Station Pressure to Sea Level
Chapter 8, Atmospheric Pressure and Altimetry 8-7
Sea level pressure is typically displayed on surface weather charts. Pressure continually changes across the
Earth, so a sequence of surface charts must be viewed to follow these changing pressures.
8.3 Density
Density is the ratio of any quantity to the volume or area it occupies. Atmospheric density is defined as
ratio of the mass (or weight) of the air to the volume occupied by it, usually expressed in kilograms per
cubic meter (see Figure 8-7).
Figure 8-7. Density is Mass (Weight) per Volume
8.3.1 Volume’s Effects on Density
The density of an air parcel varies inversely with its volume. Assuming equal mass, an air parcel with a
higher density has a smaller volume than an air parcel with a lower density (see Figure 8-8).
The shorter parcel (i.e., the parcel with the smaller volume) has a higher density than the taller parcel, which
contains the larger volume. This is due to the fact that the air molecules within the shorter parcel must be
compressed within the smaller volume.
Chapter 8, Atmospheric Pressure and Altimetry 8-8
Figure 8-8. Volume’s Effects on Density
8.3.2 Changes in Density
In general, the density of an air parcel can be changed by changing its mass, pressure, or temperature.
Boyle’s law says that the density of an ideal gas (ρ, the Greek letter rho) is given by:
𝜌 = 𝑀𝑃
𝑅𝑇
Where M is the molar mass, P is the pressure, R is the universal gas constant, and T is the absolute
temperature.
8.3.3 Density’s Effects on Pressure
Density is directly related to pressure. Assuming constant mass and temperature, an air parcel with a higher
pressure is denser than an air parcel with a lower pressure.
As previously discussed, air pressure decreases with height in the atmosphere. Therefore, the density also
decreases with height (see Figure 8-9). In the atmosphere, pressure has the greatest effect on density in the
vertical direction.