Ash Weather

Glossary

Wind Speed

Both wind speed and direction are measured. Wind speed in measured in mph to an accuracy of 1 mph. A ten minute average wind speed is also calculated, together with dominant direction.

Outside Temperature

Outside temperature (in the shade) is measured to an accuracy of 0.1C

Apparent Temperature Measures

• Wind chill. Wind chill takes into account how the speed of the wind affects our perception of temperature. Our bodies warm the surrounding air molecules by transferring heat from the skin. If there is no air movement, this insulating layer stays next to the body and offers some protection from cooler air molecules. However, wind sweeps the comfy warm air surrounding the body away. The faster the wind blows, the faster heat is carried away and the colder you feel. Above 91F (33C) wind movement has no apparent effect on temperature & hence the wind chill is the same as outside temperature.

• 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 actual air temperature, since perspiration evaporates rapidly to cool the body. However, when the humidity is high the apparent temperature feels higher than the air temperature, because perspiration evaporates more slowly.

Humidity

Humidity itself refers to the amount of water vapour in the air. However, the amount of water vapour that the air can contain varies with the air temperature and pressure. Relative humidity takes into account these factors and offers a humidity reading which reflects the amount of water vapour in the air as a percentage of the amount the air is capable of holding. Relative humidity, therefore, is not actually a measure of the water vapour in the air, but a ratio of the air's water vapour content to its capacity. Relative humidity is an important factor in determining the amount of evaporation from plants and wet surfaces since warm air with low humidity has a large capacity for extra water vapour.

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 late-afternoon when the air begins to turn cooler, fog is likely during the night.

Rainfall

Rainfall is tracked as five measurements:

• Daily rain. The amount of rain since midnight.

• Monthly rain. The amount of rain since the 1st of the month.

• Yearly rain. The amount of rain since 1st January.

• Rain storm. The amount of rain of the last rain event. (It takes 24 hours without rain to end a rain storm).

• Rain rate. The amount of rain per hour.

Barometric Pressure

The weight of the air that makes up our atmosphere exerts a pressure on the surface of the earth. High pressure zones are generally associated with fair weather while low pressure zones are generally associated with poor weather. For forecasting purposes, however, the absolute atmospheric pressure is generally less important than the change in barometric pressure. Rising pressure generally indicates improving weather while falling pressure indicates deteriorating weather conditions.

Solar Radiation

Solar radiation is a measure of the amount of solar radiation hitting the ground at anytime.

UV Radiation

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 sunburn, skin cancer, skin aging, and cataracts, and can suppress the immune systems. The level of UV radiation is measured in UV Index.

The higher the UV Index the greater the health risks - although this also depends on skin type.

EvapoTranspiration (ET)

ET is a measurement of the amount of water vapour returned to the air for a given area.