www.isleofwightweather.co.uk - The Isle of Wight Weather Station
Subscribe to the Isle of Wight Weather RSS Feed Privacy and Cookies Guest Map Guest Book
 

 

This page outlines each of the weather conditions measured or calculated by the Davis Vantage Pro Plus Automatic Weather Station (AWS) and a Davis leaf & soil moisture / temperature station we use for gathering our data. The images generated by our WeatherLink Software are uploaded to the Isleofwightweather website, automatically, every 15 minutes, and the time and date of creation of each image is shown at the bottom of each.

 

WIND

The Davis Vantage Pro Plus measures wind speed which is shown in miles per hour, and the direction from which it is blowing. The ten minute average wind speed is displayed in the weather ticker.

OUTSIDE AIR TEMPERATURE

The Davis Vantage Pro Plus uses the Integrated Sensor Suite's temperature sensor to measure the outside air temperature. A second temperature sensor in the console measures the inside air temperature.

Ground temperature GROUND TEMPERATURE

The Davis Vantage Pro Plus uses a temperature sensor connected to a wireless leaf and soil moisture temperature station to measure the ground temperature. This temperature sensor reads the temperature from bare earth.

Grass temperature GRASS TEMPERATURE

The Davis Vantage Pro Plus uses a temperature sensor connected to a wireless leaf and soil moisture temperature station to measure the grass temperature. This temperature sensor reads the temperature from the grass to verify if a ground frost has occurred overnight.

    APPARENT TEMPERATURE MEASUREMENTS

The Davis Vantage Pro Plus calculates three apparent readings: wind chill, heat index, and the temperature/humidity/sun/wind index (THSW Index).

  • Wind Chill takes into account how the speed of the wind affects our perception of air temperature. Our bodies warm the surrounding air molecules by transferring heat from the skin. If there is 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 cooler you feel. Above 91 F, wind movement has no effect on the apparent temperature, so wind chill is the same as the outside temperature.
  • 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. The heat index is only measured when the air temperature is above 14 c because it is insignificant at lower air temperatures. (Below 14 c, heat index = air temperature). The heat index is not calculated above 52 c.
  • THW (Temperature - Humidity - Wind) Index uses humidity and temperature to calculate an apparent temperature.
  • THSW (Temperature - Humidity - Sun - Wind) Index uses humidity and temperature to calculate an apparent temperature. In addition, THSW incorporates the heating effects of direct solar radiation and the cooling effects of wind on our perception of temperature.
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. It is important to realize that relative humidity changes with temperature, pressure, and water vapor content. For 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 the 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 for 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 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 also takes the air's temperature into account. High dew-point indicates high vapor content, low dew-point indicates low vapor content. In addition a high dew-point indicates a better chance of rain and severe thunderstorms. You can even use dew-point to predict the minimum overnight temperature. Providing no new fronts are expected overnight and the afternoon relative humidity  is more than 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.

    RAINFALL

The Davis Vantage Pro Plus provides four separate registers for tracking rainfall totals: rain storm, daily rain, monthly rain and yearly rain. The Vantage Pro Plus also calculates the rate of rainfall by measuring the interval of time between each .01" or 0.254mm rainfall increment. The Vantage Pro Plus physically measures increments of .01", but the console converts this to the metric display we show. There may occasionally be a skipped reading due to rounding up.

Storm rain is the amount of rain in a continuous period of rain with an amount of leeway for beginning and end. As used here, storm begins when 0.5mm of rain has fallen in a 24-hour period, and continues until 24 hours pass without 0.5mm of rain.

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 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 Davis Vantage Pro Plus measures atmospheric pressure, but then translates this to 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 usually 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

Air Density AIR DENSITY

The Davis Vantage Pro Plus measures the air density by taking account of the air temperature, pressure and how much water vapor is in the air. We know that one cubic foot of air contains around 78% nitrogen molecules, with an atomic weight of 28. Another 21% of the air is oxygen, with the molecules having an atomic weight of 32. The last 1% is a mixture of other gases, which are insignificant. Other molecules are free to move in and out of our cubic foot of air. If water vapor molecules are added to our cubic foot of air, nitrogen and oxygen molecules will leave, the total number of molecules in our cubic foot of air staying the same. As the water molecules have an atomic weight of 18, (lighter than nitrogen and oxygen), the weight of our cubic foot decreases, thus the density decreases. We can only get a calculation for air density in the outside temperature is above freezing point.

SOLAR RADIATION

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 of solar radiation hitting the solar radiation sensor at any given time, expressed in Watts / sq.m (W/m).

Bright sunshine is considered to be occurring when the solar radiation level exceeds 100 W/m, so we can easily work out how many hours of sunshine occur daily from this reading.

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 sun burn, skin cancer, skin aging, and cataracts, and can suppress the immune system. The Davis Vantage Pro Plus can help analyze the changing levels of UV radiation and can advise of situations where exposure is particularly unacceptable.

We can display UV readings in two scales: MEDs and UV Index.

  • MED stands for Minimal Erythemal Dose, defined as the amount of sunlight exposure necessary to induce a barely perceptible redness of the skin within 24 hours after exposure. In other words, exposure to 1 MED will result in a reddening of the skin. As 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.
  • We display UV Index, an intensity measurement first defined by Environment Canada and since adopted by the World Meteorological Organization. UV Index assigns a number between 0 and 16 to the current UV intensity. The lower the number, the lower the danger of sunburn. An index value of 0-2 represents a minimal exposure category whilst 10+ represents a very high category.
COOLING DEGREE DAYS

Cooling degree days (CDD) are used to estimate the amount of air conditioning usage during the warm season. To calculate CDDs, you must first find the mean temperature for the day. This is usually done by taking the high and low temperature for the day, adding them together and dividing by two. If the mean temperature is at or below 18.3C, then the CDD value is zero. If the mean temperature is above 18.3 C, then the CDD amount equals the mean temperature minus 18.3 C. For example, if the mean temperature was 25 C then the CDD amount equals 6.7 C.

We will not get a reading during the autumn and winter, unless we get a particularly hot day.

HEATING DEGREE DAYS

Heating degree days (HDD) are used to estimate the amount of energy required for residential space heating during the cool season. To calculate the HDDs you must first find the mean temperature for the day. This is usually done by taking the high and low temperature for the day, adding them together and dividing by two. If the mean temperature is at or above 18.3 C, then the HDD amount is zero. If the mean temperature is below 18.3 C, then the HDD amount equals 18.3 minus the mean temperature. For example, if the mean temperature was 8.0 C then the HDD amount equals 10.3 C.

We will not get a reading during the spring and summer, unless we get a particularly cold day.

EVAPOTRANSPIRATION (ET)

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 vegetation surfaces and the stoma of leaves) with the amount of water vapor returned through transpiration (exhalings of moisture through plant skin) to arrive at a total. Effectively, ETo is the opposite of rainfall, and is expressed in the same units of measure (Inches or millimeters)

The Davis Vantage Pro Plus uses air temperature, relative humidity, average wind speed, and solar radiation data to estimate ET.

EMC EQUILIBRIUM MOISTURE CONTENT (EMC)

The equilibrium moisture content describes the active behavior of a hygroscopic material.

Hygroscopic materials release water vapor until the surrounding air exhibits a relative humidity corresponding to the equilibrium moisture content of the material. Each material has its own wholly specific equilibrium moisture content.

As water vapor is released, the water content of the material diminishes and the water content of the air rises, i.e. the material dries out. A continual supply of fresh air, which has a relative humidity which is lower than the equilibrium moisture content, would cause the material to release ever more water vapor (shrinkage). The eventual consequence of this would be, for example, that fruit would increasingly dry out and diminish in quality. It must therefore be ensured that air in the system is not too dry, so as to prevent excessive drying-out.

Leaf Wetness LEAF WETNESS

Leaf wetness provides an indication of whether the surface of foliage in the area is wet or dry by indicating how wet the surface sensor is. The leaf wetness reading ranges from 0 (dry) to 15.

Soil Temperature SOIL TEMPERATURE

Soil moisture, as the name suggests, is a measure of the temperature of the soil. The Davis Vantage Pro Plus has a remote station with soil temperature, leaf wetness and soil moisture sensors attached.

The temperature probe is positioned 100mm below ground level, giving information of temperature around the roots of growing plants, and also allowing accurate calculation of the soil moisture levels in conjunction with the soil moisture sensor below.

Soil Moisture SOIL MOISTURE

Soil moisture, as the name suggests, is a measure of the moisture content of the soil. Soil moisture is measured on a scale of 0 to 200 centibars, and can help choose times to water crops. The soil moisture sensor measures the vacuum created by the soil by the lack of moisture. A high reading indicates dryer soil; a lower soil moisture reading means wetter soil.

The moisture level probe is positioned 100mm below ground level, giving information of soil moisture around the roots of growing plants.

 

VP Reception Graph VP RECEPTION

This allows me to remotely monitor the performance on the VP console. It shows how good the reception is between the VP Console, and the remote transmitters.

 

We have a much more detailed report, with data from the previous two days, archived at 5 minute intervals available as a downloadable and printable .txt file, here. At midnight each day, the data file changes to incorporate the previous day. Data included are temperatures, wind speed and direction, wind chill, heat index, pressure, rain rates and totals,  solar radiation and UV data, ET, and heat and cool degree days.



Join the Isle of Wight Weather Station Network mailing list to receive daily forecasts, weather data and alarms.
Enter your name and email address below:
Name:
Email:
Subscribe  Unsubscribe 

UK - WEATHER STATION TOPSITES - UK
LightningRing
Interactive Independant Climate Change Project

 

The Isle Of Wight Weather Station Network cannot be held responsible for the accuracy of this data. We will attempt to keep the data systems running, but occasionally they may go offline for maintenance. Copyright isleofwightweather 2003-2013. All copyrights, trademarks, service marks, and logo's are the respective property of their holders. This site may not be copied without the written consent of isleofwightweather. All text that is intellectual property is copyrighted. Images and our logos are copyrighted and may not be reproduced in any way without the written consent of isleofwightweather. We sell spare parts for milking machines, dairy water heaters and portable milking machines. Specialist dairy engineers and milking machine installers here. dc engineering are the leading Isle of Wight PAT Testers. Also see iww.co.uk, iww.com, iww.info, iww.org, iww.eu, iww.cc and iww.net which are all part of the Isle of Wight Weather Station Network. This site is supported by calf-feeding, and is maintained by Shaun Wheeler. Last updated on 07/04/2013.