Measuring Temperature: A thermometer measures temperature (max/min). The National Weather
Service keeps the thermometer six feet above ground in a white box thus keeping the sensor in the
Measuring Pressure: A barometer measures pressure. *Pressure (P) - the force exerted on a surface
divided by the area of the surface
Properties of Pressure
Pressure increases as more gas is added
Conclusion - Pressure (P) is directly proportional to moles of gas (n)
Pressure is measured in units that describe force exerted by the gas divided by area
over which the gas is distributed.
Pressure due to a gas is the same in all directions whereas
pressure due to weight is directional
Barometer - apparatus used to measure pressure;
is derived from the Greek "baros" meaning "weight"
Created by Evangelista Torricelli in 1646
Inverted a tube filled with mercury into a dish until the force of the Hg
inside the tube balanced the force of the atmosphere on the
surface of the liquid outside the tube
Aneroid Barometers: (without liquid)
The aneroid barometer responds to variation in air
pressure by the flexing of an evacuated bellows against
the tension of a powerful spring. The movement of the
bellows is amplified by a system of levers and converted
into a rotation by a chain wrapped around a pivoted rod to
which a pointer is attached. The chain is kept in tension by
a hairspring, which also takes the slack out of the lever
joints. Variations in air pressure can give about 200
degrees of movement of the pointer. Aneroid barometers
require no special precautions when moved, and many
were designed as pocket barometers and altimeters for
surveyors and climbers.
Measuring Winds: An anemometer measures wind speed; A wind vane measures wind direction.
Measuring Precipitation: A rain gauge measures precipitation. There are two major types of rain
gauges used in weather observations. One type is a tipping bucket and the other is a weighing rain
gauge. Generally, 10" of snow for 1" of rain can work as a fairly good ration but not always.
Measuring Humidity: A hygrometer measures humidity.
Measuring Temperature, Dewpoint, and Humidity (mobile instead of a fixed station):
A sling psychrometer measures wet bulb and dry bulb from which you can get temperature, dewpoint
and relative humidity.
Definitions of humidity-related terms
Absolute humidity: The mass of water vapor in a given volume of air( i.e., density of
water vapor in a given parcel, usually expressed in grams per cubic meter
Specific humidity: The mass of water vapor in a parcel divided by the total mass of the
air in the parcel (including water vapor)
Relative humidity: The amount of water vapor actually in the air divided by the amount of
water vapor the air can hold. Relative humidity is expressed as a percentage and can be
computed in a variety of ways. One way is to divide the actual vapor pressure by the
saturation vapor pressure and then multiply by 100 to convert to a percent.
Wet bulb temperature: The lowest temperature that can be obtained by evaporating
water into the air at constant pressure. The name comes from the technique of putting a
wet cloth over the bulb of a mercury thermometer and then blowing air over the cloth until
the water evaporates. Since evaporation takes up heat, the thermometer will cool to a
lower temperature than a thermometer with a dry bulb at the same time and place. Wet
bulb temperatures can be used along with the dry bulb temperature to calculate dew point
or relative humidity
Actual vapor pressure: The partial pressure exerted by the water vapor present in a
parcel. Water in a gaseous state (i.e. water vapor) exerts a pressure just like the
atmospheric air. Vapor pressure is also measured in millibars.
Condensation: The phase change of a gas to a liquid. In the atmosphere, the change of
water vapor to liquid water.
Dewpoint: the temperature air would have to be cooled to in order for saturation to occur.
The dewpoint temperature assumes there is no change in air pressure or moisture content
of the air.
Dry bulb temperature: The actual air temperature. See wet bulb temperature below.
Freezing: The phase change of liquid water into ice.
Evaporation: The phase change of liquid water into water vapor.
Melting: The phase change of ice into liquid water.
Mixing ratio: The mass of water vapor in a parcel divided by the mass of the dry air in the parcel (not including
Saturation of air: The condition under which the amount of water vapor in the air is the
maximum possible at the existing temperature and pressure. Condensation or sublimation
will begin if the temperature falls or water vapor is added to the air.
Saturation vapor pressure: The maximum partial pressure that water vapor molecules
would exert if the air were saturated with vapor at a given temperature. Saturation vapor
pressure varies with atmospheric pressure. When a given atmospheric pressure is steady,
then the saturated vapor pressure is directly proportional to the temperature.
Sublimation: In U.S. meteorology, the phase change of water vapor in the air directly into
ice or the chance of ice directly into water vapor. Chemists, and sometimes
meteorologists, refer to the vapor to solid phase change as "deposition."