Our WAP site is available at www.upminsterweather.co.uk/wap

Weather Terms

For those of you who are not familiar with some of the terms used to describe the weather I will endeavour to give you a brief outline of their meaning.

Let’s start with "Wind Chill" what does this term mean?
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 is no air movement the insulating layer of warm air molecules stays next to body and offers some protection from the cooler air molecules.

However, wind sweeps away that comfy warm air surrounding the exposed parts of our body. The faster the wind blows, the faster heat is carried away and the colder you feel. Bye the way above 91F or 32C wind movement has no effect on the apparent temperature, so wind chill is then the same as the outside temperature.

Another term you will have heard of is "Humidity"
Humidity itself simply refers to the amount of water vapour in the air. However the amount of water vapour that the air can contain varies with the temperature and pressure. Relative Humidity takes in to account those factors and offers a humidity reading which reflects the amount of water vapour in the air as a percentage of the amount the air is capable of holding. Relative Humidity, therefore, is not actually a measure of water vapour in the air, but a ratio of the air’s water vapour content compared to how much water vapour the air could hold at a given temperature and pressure.

It sounds a bit complicated, basically the air can hold more water vapour when it’s warm than when it’s cold. If you were for example in your living room and the relative humidity was 60% and the temperature was 20C. If you were then to close the door and put an electric fire on to heat the room up to 25C then the relative humidity would be seen to decrease to 45%. Conversely if you cool the room the relative humidity would increase. Don't forget the total amount of water vapour in the room has not changed.

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 vapour.
The Relative Humidity can also have a bearing on your "well being". For example have you ever noticed that if you are suffering from a blocked up nose or a touch of bronchitis if you go outside you can breath easier, your nose clears and your bronchitis eases. This is because the air from outside the house usually contains more water vapour than the air in your living room. In other words the relative humidity (RH) outside is greater than the air inside. Having the central heating too high is one of the main causes of low humidity. Recommended ways to increase the humidity are:- Open a window, turn the heating down a bit or buy a humidifier. Try to aim for an RH of not less than 50%.

"Dew Point"
Dew Point is the temperature to which air must be cooled for saturation (ie 100% relative humidity) to occur, providing there is no change in water content.
The dew point is an important measure 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 then fog is likely to form during the night. Dew point is also a good indicator of the air’s actual water vapour content unlike relative humidity, which takes the air’s temperature into account.
High dew point indicates high water vapour content and low dew point indicates low water vapour content.
Now that you have read about relative humidity you will be able understand the explanation to the term "Heat Index" as this uses both temperature and the relative humidity to determine how hot the air actually "feels".
When the humidity is low, the apparent temperature will be lower than the air temperature, since perspiration evaporates rapidly to cool the body. However when the humidity is high (ie the air is saturated with water vapour) the apparent temperature "feels" high than the actual temperature, because perspiration evaporates more slowly.

My new weather station has two additional sensors which measure Solar and UV (Ultra Violet) radiation.

"Solar Radiation"
What we call "current solar radiation" is technically know as Global Solar radiation, it’s 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 solar radiation hitting the sensor at any given time and is expressed in Watts per square meter.

"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 and serious health problems, such as sunburn, skin cancer, skin aging, and cataracts. UV rays also suppress the immune system.
The UV sensor on my weather station monitors the changing levels of UV radiation and can advise of situations where exposure is particularly unacceptable.
My Davis weather station displays UV readings in two scales:
MED’s and UV Index.

MED stands for Minimum Erythemal Dose. This is defined as the amount of sunlight exposure necessary to induce a barely perceptible redness of the skin within 24 hours after sun exposure. In other words, an expose of 1MED will result in a reddening of the skin. Because different types of skin burn at different rates, 1 MED for persons with very dark skin is different from 1 MED for persons with very light skin.
To overcome this problem a table of skin type categories correlating characteristics of skin with rates of sunburn has been developed.

These tables will be put on to the site at a later date.
Basically the higher the number of MEDS the greater the chance of sunburn.
On a day with sunshine the number of MED’s gradually builds up over the period of one day’s sunshine to give a total dose.

"Heat Index"
Heat Index is an intensity measurement which has been adopted by many of the world’s Meteorological Organizations. What it does is to assign a number between 0 and 16 to the current UV intensity. The lower the value the lower the risk of sunburn for example:- 0 - 2 minimal exposure category, 5 - 6 moderate and 7 - 9 high and 10 + very high.

The index value shown on this equipment is a real time measurement.