Weather Glossary


Humidity measures the amount of water vapor (water in gas form) in the atmosphere. 
Most of the water vapor in the air comes from evaporation of water from the sea and land. Evaporation is the name of the process for liquid water becoming water vapor. The opposite process, water vapor coming liquid water, is called condensation.
Water vapor in the atmosphere has many important effects on weather. It is the source of all condensation, producing clouds and fogs, and rain. Water vapor helps warm the air by absorbing energy from the sun and is source of energy for storms.

Relative Humidity

Relative humidity measures the water vapor (water in gas form) content of the atmosphere relative to the content at saturation. Saturated air has a relative humidity of 100%. Near the Earth’s surface, relative humidity is usually above 30%.

The water vapor saturation point is very sensitive to temperature. At 86°F (30°C), a volume of air can contain up to 3% water vapor before the relative humidity reaches 100%. At 68°F (20°C), the same volume of air can contain less than 2% water vapor before the relative humidity reaches 100%. Relative humidity can reach 100% in three different ways in the atmosphere. The most common way is when air rising cools by rising up a mountain side or mixing with a cooler mass of air. Two air masses at different temperatures, each with a relative humidity less than 100%, mixed together can have a relative humidity of 100%. Relative humidity can also reach 100% by evaporation of more water into the air.

Water vapor content in the atmosphere is driven by evaporation and condensation. When the relative humidity is below 100%, evaporation occurs more often than condensation. When relative humidity exceeds 100%, when a rising mass of air cools, for example, condensation occurs more easily than evaporation. In the atmosphere, condensation occurs on tiny particles in the air such as smog, dust or ice crystals.  



Evaporation is the name of the process for liquid water becoming water vapor. The opposite process, water vapor coming liquid water, is called condensation.  



Condensation is the name of the process for water vapor, or water in gas form, becoming liquid water. The opposite process, liquid water becoming water vapor is called evaporation.


Wind Chill Temperature

The wind chill temperature measures the cooling power of wind on bare skin. It is an indication of how cold animals and people feel outside. The wind chill temperature is always equal to (in still air) or below the air temperature. Wind chill temperature has no effect on inanimate objects such as radiators and water pipes. A water pipe will be at air temperature (unless it has hot or cold water flowing through it).

Metabolism maintains our internal temperature at around 98.6 °F. We are most comfortable when the air temperature is between 60 and 80 °F. In this environment our skin temperature is about 90 °F because heat from our body escapes into the environment cooling our skin. If wind is blowing past our skin, more heat escapes into the environment reducing the skin temperature further and we feel colder. The wind chill temperature is a measure of how much colder we feel when the wind is blowing. It is the temperature, at some standard wind speed (typically walking speed), that would produce the same amount heat loss through the skin as a given air temperature and wind speed. For example, if the air temperature is 40 °F and there is a 5 mph wind blowing into your face, the air temperature would feel more like 36 °F on a windless day. If the wind increased to 25 mph, the air temperature would feel more similar to 29 °F on a windless day.


Dew Point Temperature

The dew point is the temperature where objects, such as a blade of grass or window pane, will become wet with dew from the air. When the dew point falls below the freezing point of water (32 °F, 0 °C), the dew point is known as the frost point.

Dew is the water droplets that formed on cold surfaces exposed to the air. This process is driven by the humidity of the air, the temperature of the air and the temperature of the surface. When a surface is at, or below, the dew point temperature, droplets of dew can appear.

Dew often occurs on clear still nights and can form on most surfaces such as plants, decks, grass and windows. Dew can harm plants because it may stimulate the growth of harmful fungi. It can also be a nuisance to astronomers where dew can hinder viewing or damage expensive equipment. Dew may be a hazard in the morning making pathways or decks slippery.

On clear nights outdoor surfaces will radiate energy into space and can become cooler than the air. Microscopic droplets of water vapor from the air are continually condensing onto surfaces then evaporating away. It becomes harder for the water condensed on the surface to evaporate when the surface temperature cools, because the cold surface cools the nearby air. As the air temperature drops, the relative humidity increases. As long as the humidity near the surface stays below 100%, any condensing water will very quickly evaporate again. If the surface is cold enough, the dew point temperature, the local relative humidity can reach 100% or more. Then, water will condense on the surface much more quickly than it evaporates and water droplets will build up on the surface. 


Barometric Pressure or Atmospheric Pressure

Barometric, or atmospheric, pressure measures the weight of air in the atmosphere above as a pressure. The term barometric pressure comes from the instrument once commonly used for measuring atmospheric pressure: a barometer.

Atmospheric pressure is greatest at sea level, when the column of air above is largest, and decreases with rising altitude. Pressure is commonly measured in millimeters of mercury (mm-Hg), hecto-Pascals (hPa) or inches of mercury. At sea level, the average pressure is about 760 mm-Hg, 1013 hPa or 30 in-Hg.

Changes in atmospheric pressure are one of the most commonly used ways to forecast changes in the weather because weather patterns are carried around in regions of high and low pressure. A slowly rising atmospheric pressure, over a week or two, typically indicates settled weather that will last a long time. A sudden drop in atmospheric pressure over a few hours often forecasts an approaching storm, which will not last long, with heavy rain and strong winds. Pioneering meteorologist Vice-Admiral Robert Fitzroy once noted: “long foretold, long last; short notice, soon past”.

By carefully watching the pressure on a barometer, you can forecast local weather using these simple guidelines:

  • Decreasing barometric pressure indicates storms, rain and windy weather.
  • Rising barometric pressure indicates fair, dry, and colder weather.
  • Slow, regular and moderate falls in pressure suggest a low pressure area is passing some distance away. Marked changes in the weather are unlikely.
  • Small rapid decreases in pressure indicate a nearby change in weather. They are usually followed by short lasting wind and showers.
  • A quick drop in pressure over a short time indicates a storm is likely in 5 to 6 hours.
  • Large, slow and sustained decreasing pressure forecasts a long period of poor weather. The weather will be more pronounced if the pressure started rising before it began to drop.
  • A rapid rise in pressure, during fair weather and average, or above average pressure, indicates a low pressure cell is approaching. The pressure will soon decrease forecasting poorer weather.
  • Quickly rising pressure, when the pressure is low, indicates a short period of fair weather is likely.
  • A large, slow and sustained rise in pressure forecasts a long period of good weather is on its way.

Air Mass

An air mass is a large volume of air, which may cover many hundreds or thousands of square miles. They are shown on weather maps as zones of high or low pressure.

The temperature, atmospheric or barometric pressure and water vapor content are fairly uniform across an air-mass, though all three change with altitude.

There are about 50 air-masses scattered across the surface of the planet at any one time dominating our weather. Air masses are separated by narrow transition zones called fronts. Sometimes these transition zones are subtle, revealed only by a shift in wind direction. Other times the transitions are tumultuous, with conflicts between hot and cold air masses producing heavy rain, thunderstorms or snow.


High Pressure Air Mass

Zones of high pressure, or anticyclones, are formed by a sinking column of air at the centre of an air mass. They typically provide fine weather, with no rain and clear skies and can remain in place for many weeks. In the summer-time, high pressure zones provide hot, sunny days. In the winter, high pressure zones produce cooler weather, fog and frosts.

The high pressure air forms when descending air reaches the surface. If the air is descending more quickly than it can flow away, the falling mass of air accumulates. The larger mass of air increases the local atmospheric pressure.

Clouds usually do not form in regions of high pressure because the descending mass of air gets warmer as it sinks. This is because warming the air decreases its relative humidity making it less likely relative humidity will reach 100%. Clouds will not form until relative humidity reaches 100%.


Low Pressure Air Mass

Zones of low pressure are formed by a rising column of air at the centre of an air mass. They typically produce cloudy, wet weather.

The low pressure air forms when air is ascending more quickly than it can be replaced by air flowing in from surrounding areas. This flow reduces the mass of air in an area reducing the local barometric pressure.

Low pressure zones are usually accompanied by cloudy wet weather. Air in a low pressure zone cools as it rises because the gas expands as altitude increases. Warm air has a lower relative humidity (the ratio of water content to saturation point) than cooler air. So, as the air rises and cools, humidity will increase. If the air temperature reaches the dew point temperature, the point when humidity is 100%, droplets of water will begin to form on tiny particles in the air such as smog, dust or ice crystals. These are called nucleation sites. When droplets of water condense in the air over a large enough area they can be seen from the ground as clouds. If there is a large supply of water vapor, some of it comes back to earth as rain.