Boring Explanations

Wind speed
Measured by anemometer sited about 6ft above the and direction ground. Instrument is sheltered from the west round to north. Exposed from north clockwise to south west. Met Office wind is usually measured 30ft above ground level, but for agricultural purposes (evapotranspiration readings) 3-6ft is the more usual height.

Temperature
This should be pretty accurate. Measurement is about 4ft above ground level, so ground frost will be experienced before readings reach zero.

Wind chill
Takes into account how windspeed affects our perception of temperature. This accentuates both cold and high temperatures (fan ovens!).

Heat Index
Takes into account the relative humidity to calculate how hot the air feels. Low humidity has a cooling effect, high humidity restricts perspiration and has an apparent heating effect. Below about 18c its effect is negligible.

THW Index
Temp/heat and wind index combines all three of the above produce a more accurate apparent temperature. This is how the temperature will feel when you are out of the sun.

THSW Index
Same as the THW index, but this includes the effect of the sun, so this is how the temperature would feel in the sun. At night the two indices will obviously be the same.

Humidity
This is actually 'relative humidity'.The amount of water vapour in the air compared with the amount that could be held in air of that temperature. Warm air can hold more moisture than cold. Therefore if air is warmed, but its humidity stays the same, its relative humidity will fall. Air with 100% relative humidity is saturated and forms fog/cloud. This is of value to farmers and gardeners as less evaporation takes place when humidity is high.

Dew Point
This is the temperature at which the air would become saturated (100% rel humidity). This helps predict low cloud levels and fog. High dew point signifies moist air. An approximate cloudbase calculation allows 400ft for every 1 degree difference between temperature and dewpoint.

Rainfall
Measured by 'tipping bucket' principle. It takes 0.2mm to tip the bucket, so exact time of rainfall is inaccurate unless rainfall rates are high.

Storm Rain
This is used to see how much rain a particular weather system dumps on us. Measurement starts when rain starts and ends when there has been 24 hrs without rain (or, more accurately, 24 hrs without the rainfall catchment bucket tipping).

Barometric Pressure
High pressure generally indicates settled weather and low pressure, unsettled. More important in weather forecasting is the fact that rising pressure indicates improving weather and falling pressure, deteriorating.

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