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This section of the web site outlines some of the major weather variables and how they are calculated by the Davis Wireless Vantage Pro 2 Plus Weather Station installed at the Gungahlin Weather Centre. Each heading includes a brief discussion of the weather condition and a listing of the various ways in which the unit displays or stores that condition.

Vantage Pro uses the Integrated Sensor Suite's (ISS) temperature sensor to measure the outside air temperature. A second temperature sensor in the console measures the inside air temperature.

The Wireless Vantage Pro 2 Plus Weather Station at Gungahlin calculates apparent temperature readings; wind chill and heat index:

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 form 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 comfy warm air surrounding the body away. The faster the wind blows, the faster the heat is carried away and the colder you feel. At above 30°C wind movement has no effect on apparent temperature, so wind chill is the same as the outside temperature.

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 the humidity is high (ie. the air is saturated with water vapour) the apparent temperature "feels" higher than the actual air temperature, because perspiration evaporates more slowly.

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 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 amount of water vapour in the air, but a ratio of the air's water vapour content to it's capacity. Vantage Pro calculates and displays this relative humidity.

It is important to realise that relative humidity changes with temperature, pressure and water vapour content. For example, 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 warmed so that it has a capacity for 20g of water vapour, the relative humidity drops to 30% even though the water vapour content does not change.

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 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 vapour content; low Dew Point indicates low vapour content. In addition, a high Dew Point indicates a better chance of rain and severe thunderstorms.

You can even use Dew Point to predict 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 is not likely to get colder than the Dew Point anytime during the night.

Vantage Pro provides four separate registers for tracking rainfall totals: "rain storm", "daily rain", "monthly rain" and "yearly rain". The Vantage Pro also calculates the rate of rainfall by measuring the interval of time between each 0.254mm rainfall increment.

The weight of 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 it is 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.

In reality, the Vantage Pro 2 Plus Weather Station measures atmospheric pressure, but this is converted to barometric pressure because the altitude of the Gungahlin Weather Centre (634m) has been entered into the station. This essentially means that the Vantage Pro has stored the necessary 'offset' value to consistently translate atmospheric pressure into 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.

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 ofsolar radiation hitting the solar radiation sensor at any given time, expressed in Watts/sq. meter (W/m2). Solar radiation requires the solar radiation sensor.

UV (Ultra Violet) 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, cataracts, and can suppress the immune system. The Vantage Pro2 helps analyze the changing levels of UV radiation and can advise of situations where exposure is particularly unacceptable. UV radiation requires the UV radiation sensor. The Vantage Pro2 displays UV readings in two scales: MEDs and UV Index:

MED (Minimum Erythemal Dose) is defined as the amount of sunlight exposure necessary to induce a barely perceptible redness of the skin within 24 hours after sun exposure. In other words, exposure to 1 MED will result in a reddening of the skin. Because different skin types burn at different rates, 1 MED for persons with very dark skin is different from 1 MED for persons with very light skin. Both the U.S. Environmental Protection Agency (EPA) and Environment Canada have
developed skin type categories correlating characteristics of skin with rates of sunburn.

* UV Index
Vantage Pro 2 can also display UV Index, an intensity measurement first defined by Environment Canada and since been adopted by the World Meteorological Organization.

UV Index assigns a number between 0 and 16 to the current UV intensity. The US EPA categorizes the Index values as shown in table A-3. The lower the number, the lower the danger of sunburn. The Index value published by the U.S. National Weather Service is a forecast of the next day’s noontime UV intensity. The index values displayed by the Vantage Pro 2 are real-time measurements.

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

The Vantage Pro 2 uses air temperature, relative humidity, average wind speed, and solar radiation data to estimate ET, which is calculated once an hour on the hour. ET requires the optional solar radiation sensor.
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