Some Weather Terms

Black ice
Black ice is a thin layer of new ice on a road or on the water surface. The ice appears dark because it is transparent.

Blizzard
A blizzard is a severe winter storm with high winds, bitter cold, and low visibility because of blowing snow. Across much of southern Canada, Environment Canada calls a severe winter storm a blizzard when the following conditions are expected to last 4 hours or more: sustained wind speeds or gusts of 40 kilometres an hour or more, widespread reduction of visibility to less than 1 kilometre due to snow and/or blowing snow, and temperatures colder than 0C.

Blowing snow
Meteorologists use this term when winds lift snow from the ground to a height of 2 metres or more -- which is high enough to affect the visibility.

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.
Chinooks, also called foehn winds is a word meaning snow eater. These winds are warm, dry and gusty and occur 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
Wind Chill - The wind chill temperature is what the temperature "feels like" to people and animals during cold weather. Wind chill is based on the rate of heat loss from exposed skin caused by wind and cold. As the wind increases, it draws heat from the body, driving down skin temperature and eventually the internal body temperature. Once temperatures drop below 10 °F and the wind is gusting, conditions are ripe for cold-related illnesses. Below -20.5C (-5F) , any wind is a major factor in frostbite and hypothermia.

HUMIDITY
Humidity or relative humidity measures the amount of water vapor in the air relative to the temperature. It is important in weather because humidity affects how humans feel. A hot, humid day feels hotter because we cannot sweat as effectively. A cool, dry day feels colder because moisture evaporates more easily.

HUMIDEX
Definition Humidex - Over the years, several measures have been proposed to relate various combinations of temperature and humidity into a single number to approximate what hot, humid weather feels like to the average person. Of these, humidex is the one most familiar to Canadians. Humidex was introduced into Canada in 1965. The index is a summer analogue of the wind chill factor in that it is an equivalent air temperature. Air of a given temperature and humidity is equated in comfort to air of a higher temperature that has negligible moisture content. Comfort is quite subjective and largely dependent on the age and health of the individual. Weather conditions causing prickly heat in an infant may result in heat cramps in a teenager, heat exhaustion in a middle-aged and heat stroke in a senior. Humidex is also limited as an overall hot-weather comfort index because it does not consider other factors such as pressure, wind speed, precipitation, sunshine or pollen.

HEAT INDEX
The Heat Index (HI) (or apparent temperature) is a measure of relative discomfort due to combined heat and high humidity. It was developed by R.G. Steadman (1979) and is based on physiological studies of evaporative skin cooling for various combinations of ambient temperature and humidity. As temperatures climb above 32c and humidity goes above 40 percent, conditions are ripe for heat-related illnesses.

ADVISORIES
A statement generally provides additional or follow up information to an existing weather condition. An advisory is for less serious conditions that cause significant inconvenience and, if caution is not exercised, could lead to situations that may threaten life and/or property. A watch is used when the risk of a hazardous weather event has increased significantly, but its occurrence, locations, and/or timing is still uncertain. It is intended to provide advance notice of possible inclement weather. A warning is used for conditions posing an immediate threat to life or property. Depending on the type of warning, you should take immediate, appropriate action.

MICROBURST
"Microburst: A small downburst with its outburst, damaging winds extending only 4 km (2.5 miles) or less. In spite of its small horizontal scale, an intense microburst could induce damaging winds as high as 75 m/sec (168 mph)."
Courtesy of NOAA

BAROMETRIC PRESSURE
The air that makes up our atmosphere exerts a pressure on the surface of the earth. This pressure is known as atmospheric pressure. Generally, the more air above an area, the higher the atmospheric pressure. Barometric pressure changes with local weather conditions, making barometric pressure an important and useful weather forecasting tool. High pressure zones are generally associated with fair weather, while low pressure zones are generally associated with poor weather. For forecasting purposes, the absolute barometric pressure value is generally less important than the change in barometric pressure. In general, rising pressure indicates improving weather conditions, while falling pressure indicates deteriorating weather conditions.

RAIN FALL
Terms such as "slight chance" of rain (10-20%), "chance" of rain (30-50%) or rain "likely" (60-70%) are used when there is uncertainty of receiving measurable precipitation anywhere in the forecast area (such as the Greater Los Angeles Area). For instance, if there is only a 30-50 percent chance that rain will fall anywhere in the Leduc area, then the forecast will call for a "chance" of rain.

CLOUD HEIGHT
The cloud height on this site is an estimate of cumulus clouds using a formula based on temperature and dew point. Actual measurements of cloud height are made with a Micropulse Lidar (MPL). This device fires a laser into the sky and measures the backscattered signal. Costs for such a device are beyond the scope of weather hobbyists.

DEW POINT
Dew point is the temperature to which air must be cooled for saturation (100% relative humidity) to occur. The dew point is an important measurement used to predict the formation of dew, frost, and fog. If dew point and temperature are close together in the late afternoon when the air begins to turn colder, fog is likely during the night. Dew point is also a good indicator of the air's actual water vapor content, unlike relative humidity, which takes the air's temperature into account. High dew point indicates high vapor content; low dew point indicates low vapor content. In addition a high dew point indicates a better chance of rain and severe thunderstorms. You can even use dew point to predict the minimum overnight temperature. Provided no fronts or other weather pattern changes are expected overnight, the afternoon's dew point gives you an idea of what minimum temperature to expect overnight.

METAR
An Acroymn for Meteorological Aerodrome Report. It is the primary observation code used in the United States to satisfy requirements for reporting surface meteorological data. Minimum reporting requirements includes wind, visibility, runway visual range, present weather, sky condition, temperature, dew point, and altimeter setting.

WEATHER STATION
Weather Station - A weather station is a facility with instruments and equipment to make weather observations by monitoring atmospheric conditions to study the weather. This weather station has a thermometer for measuring temperature; barometer for measuring changes in air pressure; hygrometer for measuring humidity; anemometer for measuring wind speed and wind direction; and rain gauge for measuring precipitation.

LIGHTNING SAFETY
Lightning Safety - "How far away was that lightning strike? To estimate the distance between you and a lightning flash, use the "Flash to Bang" method: Count the seconds between the flash and the thunder clap. Each second represents about 300 metres. If you can count less than 30 seconds between the lightning strike and the thunder, the storm is less than 10 km away and there is an 80 percent chance the next strike will happen within that 10 km. If you count less than 30 seconds, take shelter, preferably in a house or all-metal automobile (not a convertible top) or in a low-lying area.

HURRICANE FACTS
Hurricane Facts: 1961 was the last time two Category 5 hurricanes formed in the same year. 1960 was the last time two Category 5 hurricanes formed and eventually made U.S. land fall. 2005 has now matched that with Katrina and now Rita.