Air - Atmospheric Climate Variables
This page describes some of the 50 Essential Climate Variables identified by the Global Climate Observing System (GCOS) for worldwide monitoring. Panels of experts at GCOS helped identify which climate observations should be made on an ongoing basis, and agreed upon principles and guidelines for the best ways to make them.
Surface Air Variables
Air Temperature
Air temperature sensor
Image source: NOAA
Surface air temperature is the temperature of the air around us, generally measured at a height of around two meters (about 6 and a half feet) above the surface. Thermometers, shielded from direct solar energy, are used to measure surface air temperature. The most common type of thermometer is the liquid-in-glass thermometer. More precise thermometers measure air temperature by checking how much electricity can pass through a sample of pure metal.
More information:
- Measuring surface air temperature at U.S. Climate Reference Network stations
- Q&A on Monitoring U.S. and Global Temperatures
Links to Air Temperature Data:
- Automated Surface Observing Systems monitor weather conditions at many airports. Choose a state and click any station on the map, and then click its 4-letter ID code. Click the Decoded radio button after Format for human-readable headings about the measurements.
- NOAA Climate at a Glance provides access to authoritative climate information via maps, graphs, and data tables.
Precipitation
Rain Gauge
Image source: NASA
Precipitation is water in liquid or solid form that falls to Earth's surface from clouds. It can be in the form of drizzle, snow, ice, freezing rain, or hail. Rain gauges are the most common instrument used to measure rainfall. A rain gauge is an open-at-the-top container that is calibrated to measure the depth of liquid caught. In the United States, the depth of precipitation is reported in inches. Satellite instruments can also detect and estimate precipitation amounts.
More information: Precipitation
Links to data:
- Automated Surface Observing Systems monitor weather conditions at many airports. Choose a state and click any station on the map, and then click its 4-letter ID code. Click the Decoded radio button after Format for human-readable headings about the measurements.
- CoCoRaHS (Community Collaborative Rain, Hail, and Snow Network) Daily rain maps
- NASA's Global Precipitation Measurement Mission homepage
Solar Radiation
Solar pyranometer and other instruments at NREL.
Source: Max D/Flickr
Solar radiation is energy from the sun. Solar radiation in a range of wavelengths is the main source energy for the Earth-Atmosphere system. On the ground, an instrument called a solar pyranometer measures the amount of incoming solar radiation that reaches Earth. Instruments on satellites measure solar radiation at the top of the atmosphere.
More information: Radiation Budget Products
Links to data:
- Automated Surface Observing Systems monitor weather conditions at many airports. Choose a state and click any station on the map, and then click its 4-letter ID code. Click the Decoded radio button after Format for human-readable headings about the measurements
- Solar Maps from NREL (National Renewable Energy Laboratory)
Air Pressure
Barometer
Source: Wikimedia
Air pressure is the weight-per-unit area of the column of air above it. As gas molecules are always moving in every direction, air pressure is the same in all directions. Barometers measure air pressure. The most common type of barometer is a sealed flexible container of air. When the air pressure outside the container changes, the container responds by contracting or expanding. This change is registered by a needle or digital readout. These values are expressed in millibars. A millibar is a unit of pressure commonly used in aviation and meteorology. One millibar is equal to 100 Newtons per square meter. One thousand millibars is the average pressure at sea level. Changes in atmospheric pressure can indicate a change in weather.
More information: Air Pressure
Links to data:
- Automated Surface Observing Systems monitor weather conditions at many airports. Choose a state and click any station on the map, and then click its 4-letter ID code. Click the Decoded radio button after Format for human-readable headings about the measurements.
- Read how scientists have used past records of air pressure to "reconstruct" historical weather events.
Wind
Anemometer, top left, as part of a weather station.
Source: MagicFoundry/Flickr
Wind is air in motion relative to the Earth's surface. It is a vector quantity, meaning it is described in terms of both speed and direction of motion. Winds are most commonly described in only their horizontal direction. Anemometers are used to measure wind speed. Wind vanes and windsocks measure wind direction. Wind directions refer to where the wind is coming from; for example, a north wind is coming from the north and blowing towards the south.
More information: Origin of Wind
Links to data:
- Automated Surface Observing Systems monitor weather conditions at many airports. Choose a state and click any station on the map, and then click its 4-letter ID code. Click the Decoded radio button after Format for human-readable headings about the measurements.
- Vizualization of Global Winds
Water Vapor
Water vapor instrument
Image source: NOAA
Water vapor is water in the atmosphere in its vapor (gaseous) form. Water vapor is the raw material from which clouds form. Though it is invisible to the human eye, this gas absorbs and emits infrared radiation, which traps heat energy near Earth's surface. Half of the water vapor in the atmosphere is found within two kilometers of the Earth's surface. Absolute humidity is the measure of the amount of water vapor in air. Relative humidity tells how much water vapor is in the air relative to the amount it has the potential to hold at a given temperature and pressure. The instrument used to measure water vapor content in the air is called a hygrometer. The simplest type of hygrometer is made from human hair, which swells and lengthens as it absorbs water vapor from the air.
More information: Humidity
Links to data:
- Automated Surface Observing Systems monitor weather conditions at many airports. Choose a state and click any station on the map, and then click its 4-letter ID code. Click the Decoded radio button after Format for human-readable headings about the measurements.
- Middle atmosphere water vapor data
Upper Air Variables
Cloud Properties
MODIS instrument on Aqua satellite
Image source: NOAA
Climate scientists observe and monitor the properties of clouds because the type of clouds in the sky are an indicator of the atmospheric processes occurring where they formed. Cloud properties include ice crystal size and type, temperature, and thickness. Instruments on satellites are commonly used to observe cloud properties.
More information: Clouds
Links to data:
- Cloud data from NASA's MODIS Instruments
- Search NASA EarthData for Clouds
- Cloud Appreciation Society
Composition
Scientist with air sample flasks.
Source: NOAA/CIRES
The composition of the atmosphere refers to the mix of all the gases in the air. The relative amounts of major atmospheric gases have changed dramatically since Earth's early history, but they have been relatively stable for thousands of years. Atmospheric gases are well mixed up to an altitude of 80 km (50 mi). The relative concentrations of several trace gases in the atmosphere determine the atmosphere's ability to trap solar radiation. In order to make precise measurements of atmospheric composition, scientists gather air in flasks, and analyze it in a laboratory.
More information:
- What's in the Air (13-minute video)
- Explore the steps in collecting and analyzing samples of the atmosphere
- Climate forcing and atmospheric composition
Links to data: