Weather Terms used at Ullapool Weather station - Ardmair Point 

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Dewpoint is the temperature to which air must be cooled for saturation (100% relative humidity) to occur. The dewpoint is an important measurement used to predict the formation of dew, frost, and fog. If dewpoint and temperature are close together in the late afternoon when the air begins to turn colder, fog is likely during the night. Dewpoint is also a good indicator of the airs actual water vapor content, unlike relative humidity, which takes the airs temperature into account. High dewpoint indicates high vapor content; low dewpoint indicates low vapor content. In addition a high dewpoint indicates a better chance of rain and severe thunderstorms. You can even use dewpoint to predict the minimum overnight temperature. Provided no fronts or other weather pattern changes are expected overnight , the afternoons dewpoint gives you an idea of what minimum temperature to expect overnight, since the air is not likely to get colder than the dewpoint anytime during the night.

Barometric Pressure

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 in pressure at different elevations, and to facilitate comparison between locations with different altitudes, meteorologists adjust atmospheric pressure so that it reflects what the pressure would be if measured at sea-level. This adjusted pressure is known as barometric pressure.

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

Pressure Trend

"Rising Rapidly" - pressure increases more than 2 mb (>0.06") over past three hours
"Rising Slowly" - pressure increases more than 1 mb but less than 2 mb (> 0.02" but < 0.06")
over past three hours
"Steady" - pressure changes less than 1 mb (< 0.02")
over past three hours
"Falling Slowly" - pressure falls more than 1 mb but less than 2 mb (> 0.02" but < 0.06")
over past three hours
"Falling Rapidly" - pressure decreases more than 2 mb (>0.06")
over past three hours

Current Wind

Current wind is the average direction and speed for the last 2 minutes. 


Wind gust represents the highest wind speed observed in the last 10 minutes.  Gusts are displayed if the gust speed is above 10 mph and at least 5 mph higher than the current (2 minute average) wind speed.

Solar Energy

What we call "Solar Energy" is technically known as global solar radiation, a measure of the intensity of the suns radiation reaching a horizontal surface. The Vantage Pro solar sensor measures both the direct sunlight component and the diffuse (reflected & scattered light) component from the rest of the sky. The solar radiation reading gives a measure in Watts per square meter (W/m2).  High Solar Energy is the peak solar radiation measured during the calendar day.

By comparing the observed solar energy to the maximum possible for a given time of the day and year a probable sky condition can be determined.  Maximum possible solar energy is derived by computing the deviation from the known solar constant (1366 W/m2) as a function of the cosine of the sun's elevation above the local horizon.  Clear sky attenuation is applied to this value, as a cosine function of the sun's elevation.  The most intense possible radiation is always at solar noon and tapers to zero at sunrise and sunset.  Percent possible intensity is displayed after the radiation value.
UV Index

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, and cataracts, and can suppress the immune system. The Vantage Pro can help analyze the changing levels of UV radiation and can advise of situations where exposure is particularly unacceptable.  The UV sensor detects radiation primarily in the UV-B portion of the radiation spectrum (280-360 nm).

The 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. The US EPA categorizes the UV Index values as shown below.

UV Index Exposure Category
0.0-0.9 No Sunburn Risk
1.0-2.9 Low Sunburn Risk
3.0-5.9 Moderate Sunburn Risk
6.0-7.9 High Sunburn Risk
8.0-10.9 Very High Sunburn Risk
11.0+ Extreme Sunburn Risk

Caution: Be aware that the Vantage Pros UV readings do not take into account UV reflected off snow, sand, or water, which can significantly increase the amount of UV to which you are exposed. Additionally, the sensor measures UV rays striking a surface horizontal to the ground and does not account for more direct sun exposure that portions of your body may experience while outside.   These index values represent burn risk over a short duration (< 1 hour) and DO NOT account for the dangers of prolonged exposure to UV radiation. The readings do not suggest that any amount of exposure is safe or healthful. 

For more information on interpreting UV readings Click Here

Heat Index 

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

Note:  We use the Steadman (1998) formula to calculate Heat Index. 


THW Index (Temperature, Heat, Wind 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 temperature is how cold people and animals feel when outside.  Wind chill quantifies the rate of heat loss from exposed skin caused by wind and cold.  As the wind increases, it draws heat from the body, driving down skin temperature and eventually the internal body temperature. Therefore, the wind makes it FEEL much colder. If the temperature is 0F and the wind is blowing at 15 mph, the wind chill is -19F. At this wind chill temperature, exposed skin can freeze in 30 minutes.

The only effect wind chill has on inanimate objects, such as car radiators and water pipes, is to shorten the amount of time for the object to cool.  The inanimate object cannot cool below the actual air temperature.  For example, if the temperature outside is -5F and the wind chill temperature is -31F, then your car's radiator will not drop lower than -5F

Note: 10-minute average wind speed is used to calculate wind chill

 Precipitation Rate

The precipitation rate is calculated by measuring the time interval between each measured precipitation increment of .01".   When there is precipitation within the archive period, the highest measured value is reported. When precipitation ends, the rain rate will slowly decay based on the elapse time since the last measured precipitation. If no precipitation is measured for 15 minutes the precipitation rate is reset to zero. 


ET is an estimate of the amount of water vapor returned to the air in a given area by vegetation.  It combines the evaporation from the leaves with the amount of water 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).  The Vantage Pro uses air temperature, relative humidity, average wind speed, and solar radiation data to estimate ET.  (ET is calculated once an hour on the hour.)

The ET calculations use a K factor of 1.00, representative of a well-watered grassy area that is uniformly clipped to a few inches in height.

Wet Bulb Temperature

The wet bulb temperature is a measure of the amount of moisture, in the form of invisible water vapor contained in the air. As the name implies it is measured by a standard thermometer whose bulb is covered by a muslin sleeve that has been moistened by pure water.  This is the temperature air can be cooled to by evaporating water into it and therefore the wet bulb temperature provides a good estimate of how far temperatures will fall if it starts raining or snowing. 

The principle of the wet bulb thermometer is as follows; water evaporates from the muslin cover passing into the air in the form of invisible water vapor. In so doing it absorbs heat from the thermometer  bulb. The thermometer therefore indicates a lower temperature than that of the dry bulb thermometer. The difference between the readings of the dry and the wet thermometers is called the depression of the wet bulb.

If the air contains nearly all the moisture it can possibly hold, evaporation from the muslin will be slight and the depression of the wet bulb will be small. However, if the air is very dry, containing little moisture, evaporation will be quite rapid and the depression of the wet bulb will be quite large. In hot dry desert climates depressions of over 25C have been observed, but at sea the depression is seldom more than 5C. If the air contains all the moisture it can possibly hold, there is no evaporation from the muslin, and the dry and wet bulb thermometers will read the same. When this condition exists the air is said to be saturated.

Provided that the wet bulb is adequately moistened and given proper ventilation, its reading will always be equal to or less than that of the dry bulb when the air temperature is above freezing. Under certain conditions when the air temperature is below freezing and there is ice on the wet bulb, its reading may be slightly higher than the dry bulb. This is called a negative depression and usually occurs with fog or precipitation. Negative depressions are rare.

Location of Sun

The sun's position in the sky at any given time is indicated by two number, an elevation angle and an azimuth angle.  The elevation angle represents the angular height of the sun above the local horizon. In non-tropical regions, the noon sun reaches a much higher elevation in summer than in winter. Solar elevation is calculated throughout the day and at night a negative elevation angle results. The azimuth angle positions the sun relative to due north as it would be measured by a compass.  An azimuth angle between 0 (due north)  and 90  (due east) indicates the sun is in the northeastern sky; an azimuth angle between 90  and 180  (due south) means the sun is in the southeastern sky; azimuth angles between 180  and 270 (due west) result when the sun is positioned in the southwestern sky and azimuths between 270 and 360 (also due north) occur when the sun is in the northwestern sky.