|Wicklow Weather Glossary of Terms|
|Work in progress|
Wind (10min avg)
|10cm Soil Temperature|
|30cm Soil Temperature|
temperature is measured by a sensor
mounted in a radiation shield. Standard exposure is in an
open grass covered area, 1.2m (4ft) above ground. AWS sensors are electronic, and the
radiation shield is a white plastic. Standard mercury
thermometers are housed in a Stevenson's Screen. The
shields and screen needs to be cleaned regularly. The
purpose of the shields is to keep the sensors in the
shade and to expose them to the ambient air.
An air frost occurs when air temp falls below 0.0°C. An Ice Day is when the maximum temperature is below 0.0C during a day. Such a day has never been recorded at the station
Temperatures above 25°C are not often recorded. The close proximity to the sea often means that high summer temperatures are reserved for more inland locations.
|Humidity, or more accurately Relative Humidity is a percentage of the amount of water vapour the air could hold if it were totally saturated. Generally humidity is above 70% with the annual average about 84% which makes it quite a damp climate. Surprising 100% humidity occurs quite infrequently. Typically it rises into the 90s when it is raining, and if it is a long spell of rain it'll get to high 90s. Humidity of below 50% is also quite infrequent. The AWS is usually accurate to within about 3%, though accuracy falls at levels above 90%.|
is the temperature to which air must be cooled for
saturation (100% relative humidity)
to occur, providing there is no change in water content.
It is actually a better indicator of how 'close' or muggy
During Summer dewpoint usually varies between 10 and 16°C At levels of 18°C and above, it is what we often term as very humid, regardless of whether the Air Temperature is 20 or 25°C.
During Winter it is more typical -2 to 8°C. When looking to see if precipitation will be snow, a dewpoint of below -2°C is desirable. It is calculated using the Humidity and Air Temperature readings.
is measured by an anemometer mounted 10m (33ft) high on
top of a pole. Standard mean speed is a 10 minute average.
Gusts are often about 1.7 times the mean speed. A gale is
a mean speed of 39mph (34Knots, 62km/h) and is very
rarely recorded at the station. Gusts above 60mph do not
occur often and usually occur during a storm.
The one minute speed is the average and max gust during the minute prior to the update.
|Wind chill takes into account how the speed of the wind affects our perception of the air temperature. It is calculated using the Air Temperature and mean Windspeed|
|Solar Radiation is technically known as Global Solar Radiation, a measure of the intensity of the suns 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 per square meter (W/m2).|
|UV Index measures the intensity of UV. It was first defined by Environment Canada and since has been adopted by the World Meteorological Organization. UV Index uses a scale of 0 to 16 to rate the current intensity of UV. A more detailed description will follow.|
is a measure of the amount of precipitation and is
usually given in mm or inches. Precipitation would
include hail, snow and dew and frost. Because of the
latter 2, 'rainfall' can be recorded even in the absence
of rain etc although it does not amount to much. The AWS measures precipitation using a
tipping bucket gauge, and self empties every .254mm (0.1
inches). Manaul readings are taken once daily using a
copper funnel gauge.
Rain Rate is calculated by measuring the time between tips. It is a spot rate as opposed to the rainfall during previous hour.
|Air Pressure is 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. High pressure above 1030mb usually means settled weather though can often be cloudy. Pressure below 1000mb usually means unsettled and windy. If pressure is rising or falling rapidly, it can also be an indication of strong winds. The pressure trend (rising, falling, steady etc) is the trend during previous 3 hours. This can result in the current pressure moving in an opposite direction to the trend as a result in the passing of a front or low pressure.|
|EvapoTranspiration (ET) is a measurement of the amount of water vapour returned to the air in a given area from the earths surface. It combines the amount of water vapour through evaporation with the amount of water vapour returned through transpiration (exhaling of moisture from plants). Effectively, ET is the opposite of rainfall and is expressed in the same units. The calculation is done taking account of wind, solar, humidity and temperature.|
|Grass temperature is measured by sensors placed in contact with the tips of short grass, about 5cm (2") above ground. This is measured by the AWS with an electronic probe. A standard Min thermometer placed on pegs is used for manual readings. When temp falls below 0.0°C, then a ground frost is recorded. The temp usually needs to be down to -3°C before it really becomes visible and hence damaging. Quite often the grass temp can be 6°C and more lower than the air temperature just 4ft above.|
|Soil Temperature is a measure of the temperature at the depths below surface that is indicated. Growth generally ceases at temperatures below 6°C|
|AWS is short for Automatic Weather Station|
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