Weather Terms and Definitions

Climate: The average and expected weather conditions of an area over a long period of time--usually for 30 years or more.

Climatologist: A person who studies the climate and its effect on the environment. Climatologists analyze long-term weather patterns and make long-range weather predictions. Today, a professional climatologist usually has a degree in climatology, geography, or meteorology.

Cold front: The boundary between an advancing cold air mass and a relatively warmer air mass. Generally characterized by steady precipitation followed by showers.

Disturbance/Impulse: An unsettled area of weather located in the middle or upper part of the atmosphere. Clouds and precipitation are often present.

Fair: Less than 4/10ths opaque cloud cover, no precipitation, and no extremes in temperature, visibility or wind.

Front: The transition zone between two distinct air masses. The basic types are cold, warm, and occluded fronts. Stormy weather is often found along a front.

High: The center of a high-pressure area, which is an air mass with barometric pressure exceeding atmospheric pressure. Usually brings dry, fair weather, and is accompanied by outward wind flow.

Hydrologist: A person who studies the properties and distribution of water. Hydrologists analyze the earth's water supply and predict surface runoff. Today, a professional hydrologist usually has a degree in hydrology, engineering, or meteorology.

Inversion: Warmer air above colder air, a reversal of the normal situation.

Jet stream: Strong narrow bands of winds in the atmosphere. These winds often steer fronts and low-pressure systems.

Low: The center of a low-pressure area, which is an air mass with barometric pressure less than atmospheric pressure. Usually accompanied by cyclonic and inward wind flow, and often produces cloudy, stormy weather..

Meteorologist: A person who studies the atmosphere and its weather. Meteorologist analyze atmospheric phenomena and make short and medium-range weather forecasts. Today, a professional meteorologist usually has a degree in meteorology or physics.

Microburst: A strong local downdraft from a thunderstorm with peak gusts lasting two to five minutes.

Orographic uplift: Air forced vertically by hills and mountains. It can create clouds and rain.

Precipitation: Any water, whether liquid or solid that falls from the atmosphere to the ground. It includes rain, snow, hail and sleet.

Precipitation Probability: The stated probability (or percent change) of precipitation is a specific forecast giving the chance for precipitation at any point in the forecast area. For example: if a 40% probability is forecast for a specific location, measurable precipitation (above a "trace") should be received at that location four times out of 10 such forecasts; or there are four chances out of 10 that it will rain at a particular point during the period of the forecast.

Words used to qualify such probability forecasts--known as "expression of uncertainty"--are as follows:

Ridge: An elongated area of high pressure in the atmosphere.

Showers: Precipitation that starts and stops suddenly, or that occurs for only a short period of time.

Trace: A precipitation amount too small to measure. Less than one hundredth of an inch of precipitation.

Trough: An elongated area of low pressure.

Virga: Streaks or wisps of precipitation falling from a cloud but evaporating before reaching the ground.

Weather: The condition of the atmosphere at any given time and place with respect to temperature, pressure, winds, humidity, cloudiness, precipitation, etc.

Williams Wave: A "wave" that forms on a cold front because of developing low pressure in Nevada. The wave moves eastward along the front, causing the front to stall and intensify. The front then produces heavy precipitation as it moves through Utah. This meteorological phenomena is called a "Williams Wave" after meteorologist Phil Williams who discovered that this type of storm can bring very heavy snow to Salt Lake City. A Williams Wave can bring four to six inches of snow in less than twelve hours to the valleys of the Wasatch Front, and one to two feet of snow within 24 hours to the higher Wasatch Mountains. Depending on the position of the jet stream and storm front, central and southern Utah can also experience Williams Waves.

Much of the information for this section originally appeared in the copyrighted book Utah's Weather and Climate, edited by Dan Pope and Clayton Brough, in 1996. UCCW Directors have received permission from the copyright owners of this book to reproduce such information on its website and to revise and updated it where appropriate.