Glossary

Barometric Pressure

The weight of 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. This, in turn, means that atmospheric pressure changes with altitude. For example, atmospheric pressure is greater at sea-level than on a mountaintop. To compensate for this difference in pressure at different elevations, and to facilitate comparison between locations with different altitudes, meteorologists adjust atmospheric pressure so that it reflects what the pressure would be if measured at sea-level. This adjusted pressure is known as barometric 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.

Dew Point

Dew-point is the temperature to which air must be cooled for saturation (100% relative humidity) to occur, providing there is no change in water content. 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 after noon 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 vapour content, unlike relative humidity, which takes the air’s temperature into account. High dew-point indicates high vapour content; low dew-point indicates low vapour content. In addition a high dew-point indicates a better chance of rain and severe thunder storms. You can even use dew-point to predict the minimum overnight temperature. Provided no new fronts are expected overnight and the afternoon Relative Humidity ³ 50%, the afternoon’s dew-point gives you an idea of what minimum temperature to expect overnight, since the air is not likely to get colder than the dew-point anytime during the night.

Fog

A dense mass of small water droplets or particles in the lower atmosphere.

Mist

Similar to fog, but visibility remains more than a kilometre.

Frost

Deposit of soft white crystals or frozen dew drops on objects near the ground; formed when surface temperature falls below freezing point.

Heat Index

The Heat Index uses the temperature and the relative humidity to determine how hot the air actually "feels." When humidity is low, the apparent temperature will be lower than the air temperature, since perspiration evaporates rapidly to cool the body. However, when humidity is high (i.e., the air is saturated with water vapour) the apparent temperature "feels" higher than the actual air temperature, because perspiration evaporates more slowly.

Humidity

The ratio of the actual amount of water vapour in the air to the maximum amount of water vapour the air could hold at that temperature. More precisely called relative humidity.

UV Index & UV Dose

Energy from the sun reaches the earth as visible, infrared, and ultraviolet (UV) rays. Exposure to UV rays can cause numerous health problems, such as sun burn, skin cancer, skin aging, and cataracts, and can suppress the immune system.

Caution: The UV readings do not take into account UV reflected off snow, sand, or water, which can significantly increase the amount of UV to which you are exposed. Nor does the information take into account the dangers of pro longed exposure to UV radiation. The readings do not suggest that any amount of exposure is safe or healthful. Do not use the information to determine the amount of UV radiation to which you expose yourself. Scientific evidence suggests that UV exposure should be avoided and that even low UV doses can be harmful.

UV Index

The UV Index measures the intensity of UV. It was first defined by Environment Canada and since has been adopted by the World Meteorological Organization. UV Index uses a scale of 0 to 16 to rate the current intensity of UV.

The US EPA categorizes the UV Index values as shown below in Table 1.

Table 1 – UV Index and Exposure Category

Index Values	Exposure Category
0-2	Low
3-5	Moderate
6-7	High
8-10	Very High
11+	Extreme

Wind Chill

Wind chill takes into account how the speed of the wind affects our perception of air temperature. Your body warms the surrounding air molecules by transferring heat from your skin. If there’s no air movement, this insulating layer of warm air molecules stays next to your body and offers some protection from cooler air molecules. Wind disperses this layer of warm air, causing the air temperature to "feel" colder. The faster the wind blows, the quicker the layer of warm air is dispersed, and the colder you feel. Above 76.7ºF (24.8ºC), wind movement has no effect on the apparent temperature.

Fire Legend (FDI)

FIRE ALERT STAGES	BLUE	GREEN	YELLOW	ORANGE	RED
FIRE DANGER INDEX	0-19	20-44	45-59	60-74	75-100
FIRE BEHAVIOUR	SAFE	MODERATE	DANGEROUS	VERY DANGEROUS	EXTREMELY DANGEROUS
FLAME LENGTHS	0-1	1-1,2	1,2-1,8	1,8-2,4	2,4 +
GIRE CONTROL GUIDE	Fires are not likely to start. If started they spread very slowly or may go out without aid from suppression forces. There is little flaming combustion and intensity is low under all conditions. Control is readily achieved and little or no mopping up is required.	Ignition may take place near prolonged heat sources (campfires etc.) spread is slow in forests, moderate in open areas. These are light surface fires, with low flames. Control is readily achieved by direct manual attack methods and with minimum forces, difficulty may be experienced on exposed, dry slopes and some light mopping up will be necessary.	Flaming matches, etc. may start fires. Mature grassland and forest litter will burn readily; spread is moderate in forests, fast in open areas. Fires burn on the surface with moderate flame. Control is not difficult but direct and indirect attack with fire truck and labour should be used. Like to moderate mopping up will be necessary.	Ignition can occur readily, spread may be fast in the forests though not for sustained periods. Grass fires could outstrip forces with a spread of approx. 7km/hour. Fires may be very hot with local crowning and "short to medium range" spotting. Control will be very difficult requiring indirect attack methods with major assistance necessary. Mopping up may require an extended effort.	Ignition can occur from sparks. Rate of spread will be extremely fast for extended periods. Fires will be extremely hot with a dangerous heat effect on people within 10m of fire and there may be extensive crowning, fire whirls and "long range" spotting. Control may not be possible by frontal attack during the day and fire fighters should limit their efforts to containing lateral spread - until weather changes. Damage potential total and mopping up operations may be very extensive and difficult. Full assistance necessary throughout.
	Low fire hazard. Controlled burning operations can normally be executed with a reasonable degree of safety.	Although controlled burning operations can be done without creating a fire hazard, care must be taken when burning on exposed, dry slopes. Keep a constant watch for unexpected wind speed and direction changes.	Controlled burning is not recommended when the F.D.I. exceeds 45. Aircraft should be called in at the early stages of a fire.	No controlled burning of any nature should take place. Careful note should be taken of any sign of smoke anywhere - especially on the up-wind side of any plantation. Any fire that occurs, should be attacked with the maximum force all available aircraft at the time.	All personnel and equipment should be removed from field. Fire teams, labour and equipment are to be placed on full stand-by. At he first sign of smoke, every possible measure should be taken in order to bring the fire under control in the shortest possible time. All available aircraft are to be called for without delay.