Understanding your forecast

Temperature
Precipitation
Humidity
Cloud
Air
Storms
Wind
Wind speed terms

What causes weather? And what do the terms in Environment Canada's weather bulletins mean? This fact sheet should help you get the most out of Environment Canada's weather information .

The public weather bulletin often includes a report, a forecast and an outlook. The report describes the current weather for a specific location. The forecast predicts the coming weather for today and tomorrow. The extended forecast or outlook indicates general weather trends for up to three days beyond the forecast.

Temperature

In summer, hot or very warm means more than 7 Celsius above normal; warm, 4 to 7 Celsius above normal; cool, 4 to 7 Celsius below normal; and very cool, more than 7 Celsius below normal. In winter, very mild means more than 7 Celsius above normal; mild, 4 to 7 Celsius above normal; cold, 4 to 7 Celsius below normal; and bitterly cold or very cold, more than 7 Celsius below normal. Normal refers to a long-term average, usually over 30 years.

Precipitation

Precipitation is condensation of atmospheric water vapour deposited on the earth's surface. It occurs when airborne water droplets become too heavy for the air to support any longer. The moisture can fall as rain, snow, ice pellets or hail, depending on the air temperature and currents.

In Environment Canada reports, the term "rain", used alone, means liquid precipitation of significant duration and extent. Rain showers stop and start suddenly and vary widely in intensity, and are gone in less than an hour. Intermittent rain stops and starts repeatedly, although not as abruptly or as frequently as showers. In drizzle, the droplets are fine and minute--much smaller than in rain--and appear to float in the air. Freezing rain or freezing drizzle freezes on impact and forms a coat of ice on the ground and on the objects they strike. They occur when the air temperature is below zero Celsius near the ground but above zero Celsius higher up. A thunderstorm produces heavy rain, high winds, lightning and even hail or tornadoes.

Snow is composed of millions of star-shaped hexagonal ice crystals. When the term snow is used without qualification, it means precipitation of significant duration and extent. A flurry or snow shower is a snowfall that suddenly stops and starts and changes rapidly in intensity; the accumulation and extent of the snow are limited. A snow squall brings strong winds, flurries and poor visibility. Blowing snow is lifted by the wind from the earth's surface to a height of 2 m or more, whereas drifting snow is blown to a height of less than 2 m. A blizzard is a severe storm that lasts three or more hours, and brings low temperatures, strong winds and poor visibility due to blowing snow.

Snow pellets are brittle and easily crushed; when they fall on hard ground, they bounce and often break up. They always occur in showers and are often accompanied by snow flakes or rain drops, when the surface temperature is around zero Celsius.

Snow grains are minute, white and opaque grains of ice. When they hit hard ground, they do not bounce or shatter. They usually fall in very small quantities, and never in the form of a shower.

Ice pellets are tiny, translucent frozen raindrops or snowflakes, or snow encased in ice, which bounce and make a sound on contact with the ground. Ice crystals are tiny sprinkles that sparkle in the sunshine like diamond dust and hang in the air. Hail is precipitation in the form of lumps of ice, larger than ice pellets, usually the size of peas or cherries but sometimes as large as oranges. Often associated with thunderstorms, hail forms when drafts carry raindrops upward into extremely cold regions of the atmosphere. There, the drops freeze and merge into lumps of ice. When the lumps become too heavy to be supported by the updraft, they fall to the ground at high speeds. Hail causes hundreds of millions of dollars in damage in Canada each year; crops are particularly vulnerable.

The probability of precipitation is a subjective estimate of the chance of precipitation. The forecaster bases the probability on current weather patterns, including wind and humidity, as well as the effect of terrain and long-term weather statistics. The probability does not predict when, where or how much precipitation will occur. For example, a 60 per cent probability of snow does not mean that it will snow during 60 per cent of the day. Instead, it means that there is a 60 per cent chance of snow falling over some or all of the region included in the forecast. Probabilities are given in 10 per cent increments ranging from zero to 100. A probability of zero means no chance of rain or snow. A 100 per cent probability means precipitation is a certainty.

Humidity

Humidity is the amount of moisture in the air. Relative humidity is the ratio of water vapour in the air at a given temperature to the maximum amount which could exist at that temperature. On hot summer days, the higher the relative humidity, the greater the discomfort, since perspiration evaporates less readily and the body feels more hot and sticky. The humidex is the scale describing how hot, humid weather feels to the average person. The humidex combines the temperature and humidity into one number to reflect the perceived temperature. Because the humidex takes into account the two most important factors that affect summer comfort, it can be a better measure of how stifling the air feels than either temperature or humidity alone.

Dew: As the surface of the earth cools at night, warm moist air near the ground is chilled and water vapour in the air condenses into droplets on the grass and other objects. Dew is particularly heavy on clear nights. When a blanket of cloud insulates the earth, the cooling rate is slower.

Frost is water vapour which deposits directly as a solid on a surface colder than the surrounding air and which has a temperature below freezing. It is not frozen dew. A killing frost is a frost severe enough to destroy annual plants and new growth on trees (in the spring) or to end the growing season (in the fall).

The dew point is the temperature to which air must be cooled to become saturated by the water vapour already present in the air. The warmer the air, the more moisture it can hold.

See the main humidity brochure

See humidex table

Cloud

A cloud is a visible cluster of tiny water and/or ice particles in the atmosphere. Fog is cloud at ground level, and occurs when air is cooled to its dew point and below, or when atmospheric moisture increases through evaporation from water that is warmer than the air. Mist is microscopic water droplets suspended in the air; it casts a thin greyish veil over the landscape but reduces visibility to a lesser extent than fog. Haze consists of fine particles of dust and pollution suspended in the atmosphere, and is distinguished from fog by its bluish or yellowish tinge.

A clear sky is virtually cloud-free. Sunny or a few clouds means that less than half the sky has clouds. Cloudy means that clouds cover more than 60 per cent of the sky. Mainly sunny means sunny with some cloudy periods. Mainly cloudy means cloudy with some sunny periods. Overcast means grey and dull skies, with extensive cloud cover.

Air

Air masses and fronts An air mass is a large, horizontal body of air with a uniform distribution of moisture and temperature throughout. A front is the boundary between two different air masses. A cold front is the leading edge of an advancing cold air mass and moves in such a way that the cold air replaces the warm air. A warm front is the trailing edge of a retreating cold air mass and moves in such a way that the warmer air replaces the colder air. A wave, in meteorology, is the intersection of warm and cold fronts.

The atmosphere is the mass of air held close to the earth by gravity. Atmospheric pressure, also known as air pressure or barometric pressure, is the force exerted by the weight of the atmosphere. A drop in pressure usually predicts foul weather, while a rise in pressure usually means fair weather ahead. In the northern hemisphere, a high is an area of high atmospheric pressure with a closed, clockwise movement of air, while a low is an area of low atmospheric pressure with a closed counterclockwise circulation of air. A ridge is an elongated area of relatively high pressure extending from the centre of a high pressure region. A trough is an elongated area of relatively low pressure extending from the centre of a region of low pressure.

Storms

A tornado appears as a violent funnel-shaped wind vortex in the lower atmosphere with upward spiralling winds of high speeds. The tornado usually appears from a bulge in the base of a thunderstorm cloud. It has a typical width of tens to hundreds of metres and a lifespan of minutes to hours. In extent, it is one of the smallest of all storms, but in violence, it is among the world's most severe.

A hurricane is a huge cyclonic storm that develops over the tropics. With wind speeds of 64 knots (118 km/h) or greater, a hurricane can span several thousand kilometres and last several days. Atlantic Canada is generally affected by four or five hurricanes or tropical storms each year, normally between May and November, with the peak from August to October. (A tropical storm has cyclonic winds of between 34 and 63 knots). Pacific hurricanes, called typhoons, or their remnants affect Canada's west coast once every two to three years. In other regions of Canada, remnants of typhoons or hurricanes may have indirect effects on weather.

Wind

Wind is the horizontal movement of air relative to the earth's surface and is caused by variations in temperature and pressure (for instance, air rises as it warms and a cool breeze moves in to take the place of the rising air.) The wind direction is the direction from which the wind is blowing (for example, a north wind comes from the north and blows toward the south.) Prevailing winds are the wind direction most frequently observed during a given period. A squall is a strong, sudden wind which generally lasts a few minutes then quickly decreases in speed. A draft is a small gusty air current that moves upward or downward abruptly; hence the terms updraft and downdraft. A gust is a sudden, brief increase in wind speed that generally lasts less than 20 seconds. A chinook is a warm, dry, gusty wind that occasionally occurs to the leeward side of a mountain range, particularly the Rocky Mountains.

Jet streams are undulating bands of strong, high-altitude winds, associated with cold fronts. They have an average altitude of 10 km and may occasionally exceed 400 km/h. Pilots often seek out a jet stream to speed their jet planes along.

Wind Chill expresses the combined effect of wind and air temperature on body heat loss. It feels colder, for example, at -15 Celsius with a strong wind that at the same temperature with no wind.