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Weather Definitions

Apparent Temperatures
Apparent temperatures use additional weather data to calculate what a human body perceives the temperature to be in those conditions.

Wind Chill
Wind Chill takes into account how the speed of the wind affects our perception of the air temperature. Our bodies warm the surrounding air molecules by transferring heat from the skin. If there's no air movement, this insulating layer of warm air molecules stays next to the body and offers some protection from cooler air molecules. However, wind sweeps that warm air surrounding the body away. The faster the wind blows, the faster heat is carried away and the colder you feel.

Heat Index
The Heat Index uses 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 more saturated with water vapour) the apparent temperature "feels" higher than the actual air temperature, because perspiration evaporates more slowly. Heat Index is only significant when the air temperature is above 14°C (57°F). The Heat Index is not calculated above 52°C (135°F).

Temperature/Humidity/Sun/Wind (THSW) Index
The THSW Index uses humidity and temperature like the Heat Index, but also includes the heating effects of sunshine and the cooling effects of wind (like Wind Chill) to calculate an apparent temperature of what it "feels" like out in the sun.

Humidity itself simply refers to the amount of water vapour in the air. However, the amount of water vapour that the air can contain varies with 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 amount of water vapour in the air, but a ratio of the air's water vapour content to its capacity. When we use the term humidity, we mean relative humidity.

It is important to realize that relative humidity changes with temperature, pressure, and water vapour content. A parcel of air with a capacity for 10g of water vapour which contains 4g of water vapour, the relative humidity would be 40%. Adding 2g more water vapour (for a total of 6g) would change the humidity to 60%. If that same parcel of air is then warmed so that it has a capacity for 20g of water vapour, the relative humidity drops to 30% even though water vapour content does not change.

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 to absorb 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 vapour 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 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 vapour content, unlike relative humidity, which takes the air's temperature into account. High dew point indicates high water vapour content; low dew point indicates low water vapour content. In addition a high dew point indicates a better chance of rain and severe thunderstorms. You can also use dew point to predict the minimum overnight temperature. Provided no new fronts are expected overnight and the afternoon Relative Humidity ≥50%, the afternoon's dew point gives you an idea of what minimum temperature to expect overnight, since the air cannot get colder than the dew point anytime.

Solar Radiation
What we call "current solar radiation" is technically known as Global Solar Radiation, a measure of the intensity of the sun's radiation reaching a horizontal surface. This irradiance includes both the direct component from the sun and the reflected component from the rest of the sky. The solar radiation reading gives a measure of the amount of solar radiation hitting the solar radiation sensor at any given time, expressed in Watts/square meter (W/m²).

Evapotranspiration (ET)
Evapotranspiration (ET) is a measurement of the amount of water vapour returned to the air in a given area. It combines the amount of water vapour returned through evaporation (from wet vegetation surfaces and the stoma of leaves) with the amount of water vapour returned through transpiration (exhaling of moisture through plant skin) to arrive at a total. Effectively, ET is the opposite of rainfall, and it is expressed in the same units of measure (inches, millimeters).

Weather station software uses air temperature, relative humidity, average wind speed, and solar radiation data to estimate ET.

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 mountain top. To compensate for this difference and facilitate comparison between locations with different altitudes, atmospheric pressure is generally adjusted to the equivalent sea-level pressure. This adjusted pressure is known as barometric pressure.

Barometric pressure also changes with local weather conditions, making barometric pressure an extremely 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, however, 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.

How to convert sea level to station barometric pressure and visa-versa
Atmospheric pressure changes by approximatly 1hPa (1mbar) for every 10 meters of altitude change (decrease for increase in altitude and visa-versa). So for example, if the barometric pressure at sea level was 1000hPa, then at an altitude of 10 meters, the pressure would be 999hPa. Click on the link below for a detailed description and equations.

Barometric Pressure Conversion.pdf (8kb)

Some further good information is provided here

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