Vocabulary
Atmosphere
Density
The amount of matter in a given volume or area. Density is also described as a
measure of the compactness of matter. It is determined by dividing the amount of
mass of a substance by its volume. D = m/vol
Units g/mL or g/cm3
Atmosphere
The thin layer of gases that surrounds the Earth. It is held in place by the force of
Earth’s gravity.
Charles’ Law
The law that describes the relationship between the temperature and volume of a gas
under conditions in which the pressure and the amount of gas are constant. Charles’
Law states that the volume of a given amount of dry ideal gas is directly proportional
to the Kelvin temperature. Charles’ Law can be represented by the mathematical
formula:
V1 / T1 = V2 / T2.
Troposphere
Atmospheric
pressure
Barometric
pressure
The layer of the atmosphere that extends from the surface of the Earth to
approximately ten to twelve kilometers above the Earth’s surface. It is the layer in
which most of the Earth’s weather occurs. Air is warmest at the bottom of the
troposphere near ground level. Higher up it gets colder.
The sum of the forces exerted on the surface of the Earth by the molecules that
compose the atmosphere per unit area. Atmospheric pressure can be represented by
the following equation and is measured in units of millibars.
P = F/A
F = the sum of forces exerted by the molecules of the atmosphere
A = area
An indication of the atmospheric pressure in an area as measured in millimeters of
mercury (mm Hg) with a barometer. The pressure in mm Hg can be converted to
millibars using the equation below.
P in mb = P in mm Hg / 0.750062
High pressure
system
When the atmospheric pressure at one area of the Earth is higher than the areas that
surround it.
Low pressure
system
When the atmospheric pressure at one area of the Earth is lower than the areas that
surround it.
Air mass
A large body of air that possesses certain temperature and humidity characteristics.
Humidity
Humidity is the amount of water vapor in the air. Water vapor is the gaseous state of
water and is invisible. Humidity indicates the likelihood of precipitation, dew, or fog.
Front
The boundary between air masses of two different densities.
Cold front
A boundary between a cold air mass and warm air mass. In a cold front, the colder air
mass moves in behind the warmer air mass.
Warm front
A boundary between a cold air mass and warm air mass. In a warm front, the warmer
air mass moves in behind the colder air mass.
Wind
Horizontal movements of air set into motion by pressure differences that exist within
the Earth’s atmosphere.
Pressure
gradient force
(PGF)
The difference in pressure between higher and lower areas of pressure and the
distance between the two areas.
PGF =
𝑎𝑟𝑒𝑎 𝑜𝑓 ℎ𝑖𝑔ℎ𝑒𝑟 𝑝𝑟𝑒𝑠𝑠𝑢𝑟𝑒 (𝑚𝑏)− 𝑎𝑟𝑒𝑎 𝑜𝑓 𝑙𝑜𝑤𝑒𝑟 𝑝𝑟𝑒𝑠𝑠𝑢𝑟𝑒 (𝑚𝑏)
𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒 𝑏𝑒𝑡𝑤𝑒𝑒𝑛 𝑎𝑟𝑒𝑎𝑠 𝑜𝑓 𝑝𝑟𝑒𝑠𝑠𝑢𝑟𝑒 (𝑘𝑚)
Isobar
Lines on a weather or atmospheric pressure map that indicate areas of equal
atmospheric pressure.
Barometer
One of the key instruments used by meteorologists to measure changes in
atmospheric pressure. A mercury barometer is one type of barometer. Changes in
atmospheric pressure measured with a mercury barometer are reported in millimeters
of mercury (mm Hg).
Evaporation
A change of phase in which the liquid phase of a substance is converted to its gaseous
phase. Evaporation at occurs from the surface of a liquid into a gaseous phase that is
not saturated with the evaporating substance. It can happen at temperatures below
the boiling point of the liquid.
Condensation
A change of phase in which the gaseous phase of a substance is converted to its liquid
phase.
Precipitation
Liquid and solid phases of water that fall from the sky. Precipitation includes rain, hail,
snow, freezing rain, and sleet.