Wind

The anemometer measures wind speed and direction, and is part of the Integrated Sensor Suite (ISS). The console also calculates a 10-minute average wind speed and 10-minute dominant wind direction.

Temperature

The ISS houses the outside temperature sensor in a vented and shielded enclosure that minimizes the solar radiation induced temperature error.

Apparent Temperatures

Vantage Pro calculates three apparent temperature readings: wind chill and Heat Index and THW INdex. Apparent temperatures use additional weather data to calculate what a human body perceives the temperature to be in those conditions.

Temperature Humidity Wind (THW) Index

The THW Index uses humidity, temperature and wind to calculate an apparent temperature that incorporates the cooling effects of wind on our perception of temperature.

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 comfy 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 Appendix A: Weather Data + saturated with water vapor) the apparent temperature “feels” higher than the actual air temperature, because perspiration evaporates more slowly. Note: Vantage Pro measures Heat Index only when the air temperature is above 57° F (14° C), because it's insignificant at lower temperatures. (Below 57°, Heat Index = the air temperature.) The Heat Index is not calculated above 135° F (52° C).

Humidity

Humidity itself simply refers to the amount of water vapor in the air. However, the amount of water vapor 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 vapor 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 vapor in the air, but a ratio of the air's water vapor content to its capacity. When we use the term humidity in the manual and on the screen, we mean relative humidity. It is important to realize that relative humidity changes with temperature, pressure, and water vapor content. A parcel of air with a capacity for 10 g of water vapor which contains 4 g of water vapor, the relative humidity would be 40%. Adding 2 g more water vapor (for a total of 6 g) would change the humidity to 60%. If that same parcel of air is then warmed so that it has a capacity for 20 g of water vapor, the relative humidity drops to 30% even though water vapor 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 vapor.

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 vapor 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 vapor content, unlike relative humidity, which takes the air's temperature into account. High dew point indicates high water vapor content; low dew point indicates low water vapor content. In addition a high dew point indicates a better chance of rain and severe thunderstorms. Rain +/ 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.

Rain

Vantage Pro incorporates a tipping-bucket rain collector in the ISS that measures 0.01” for each tip of the bucket. Your station also logs rain data in inch units. If you select millimeters for the rain measurement unit, your station still logs rain data in inches but will convert the logged totals from inches to millimetres at the time it is displayed. Converting the logged rain totals reduces possible conversion losses to a minimum. Four separate variables track rain totals: “rain storm”, “daily rain”, “monthly rain”, and “yearly rain”. Rain rate calculations are based on the interval of time between each bucket tip, which is each 0.01” rainfall increment.

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 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 measures atmospheric pressure. When you enter your location's altitude in Setup Mode, the Vantage Pro stores 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.

Source: Vatage Pro Console Manual by Davis