Quite simply, humidity is the amount of water vapour contained in the atmosphere. The moisture content of the air surrounding us is continuously changing, depending on air pressure and temperature, but can have a major impact on the comfort of humans. We lose heat to cool the body so with high humidity there is little cooling and vice versa.
Absolute humidity is the water content of air whereas relative humidity, expressed as a percentage, measures the current absolute humidity relative to the maximum for that temperature.
Warm air can contain more moisture than cold air, the difference in humidity for temperatures between 0°C and 20°C is fourfold, which is why the most intense downpours are experienced in the summer months. During the course of several hot days the humidity can fall to under 40% but in contrast a foggy morning can give a reading of 100% – completely saturated air. Very low humidity will dry out property and gardens very quickly but necessitate frequent watering and the same applies to humans in keeping refreshed with liquids. High humidity on the other hand, can be uncomfortable, particularly at higher temperatures.
Traditionally, relative humidity is calculated using two separate glass thermometers, called the dry bulb and wet bulb thermometers, contained within a screen. The wet bulb thermometer has a sleeve that covers the bulb area and drops into a container containing water, preferably distilled, to avoid contamination. The moisture evaporating from the wet sleeve takes with it a little warmth and thus depresses the temperature registered by the bulb. If you blow on a wet finger you will understand the cooling effect. The difference between the two readings is used to consult a table from which the relative humidity is calculated.
A modern equivalent is called a hygrometer, which has a self-contained wet and dry bulb unit, manufactured from plastic, that can be installed in a weather screen.
There are modest electronic units that measure several items of weather related data including humidity, some with a data logger function in order that results can be saved over a limited time period. Most of these weather instruments invariably incorporate a display monitor.
The more advanced automatic weather stations, which contain a variety of equipment and thus are more expensive, utilise the incorporated sensors such as temperature and air pressure to automatically produce a continuous figure for relative humidity. There is therefore no need to take daily readings and produce a figure for relative humidity using tables.
A useful addition to any weather station is a means of measuring the amount of sunshine occurring each day, which is recorded in hours.
A long-standing sunshine recorder, which is still in use at some weather stations but recently superseded by automatic equipment, is the Campbell-Stokes sunshine recorder. This unit consists of a spherical glass lens mounted on a stand. Attached to the stand, but behind the sphere, is a specially produced piece of paper that has a graduated grid of hours printed on it. The sphere concentrates the rays of the sun, which at a particular strength starts to burn a trace on the paper. The paper is changed daily, at which point the length of burn, in hours, is calculated from the traces on the paper. Any covering of the sphere, however little from matter such as dust, dew and snow, can affect the daily total.
The Campbell-Stokes recorder is not particularly precise and is very expensive to purchase. Most official weather stations have been installing electronic sunshine recorders over the last few years that do not need daily maintenance and can save information to a data logger that are more suitable for remote stations. These recorders use photo sensitive devices that are often included in the more expensive automatic weather stations or can be added at a later date. As discussed later in chapter 10, the station must be placed in a horizontal position with a clear view to the horizon, particularly when the sun is low in the sky. The recommended height for such stations is 1 to 2 metres above the ground on a secure stand.
There are two types of electronic sensors that measure sunshine. The type which produces data comparable to the World Meteorological Organization use direct solar radiation which is measured from a small area around the solar disk and perpendicular to the incoming beam using a sensitive narrow-aperture sensor known as a pyrheliometer. The threshold for such instruments is 120 Watts/m2. The sunshine sensors in a weather station such as the Davis VP2 measure global solar radiation from the all-sky solar radiation incident on a horizontal surface, measured by a pyrometer with a threshold of 100 Watts/m2. The latter usually exceeds 100 Watts/m2 near local noon in mid-latitudes in most months of the year, even under cloudy skies. The data from the latter is not comparable to the WMO type of instrument.
Unfortunately, the data from the Campbell-Stokes recorder cannot be compared directly with the electronic recorders. As a result, for some long-standing stations, the record for continuous and directly comparable data is broken.
Ultra Violet Radiation
At times during the summer we are warned to be aware of the effect of ultra violet radiation on our skin. Exposure to UV rays can cause health problems from sunburn and skin cancer. This is one aspect of the energy emanating from the sun that can be measured. An ultra violet sensor is often incorporated into the upmarket automatic weather stations but with others it can be added at a later date. Many of these stations have the facility to capture data over a period of time, which can be downloaded to a computer at a later date. It is important that the sensor is mounted horizontally with a clear view to the horizon for best results.
Solar Radiation and Solar Energy
Solar radiation is the electromagnetic output from the sun and is expressed in Watts/m2. This radiation is variable and either comes directly from the sun or reflections from the sky. With much talk of harnessing solar energy from solar panels, this aspect of weather recording has been very relevant to many. As with ultraviolet radiation, a solar radiation sensor, often using a silicon photo diode, is often included in the more expensive automatic weather stations or can be added later with the ability to record information to a data logger for downloading at a later date. To obtain best results, as with the UV sensor, it needs to be mounted horizontally and with the clearest view to the horizon that can be obtained in all directions.
Evaporation is a measurement of the amount of water vapour returned to the air in a given area. An important set of data for many, such as those involved with gardening, landscaping or with water resources, is the amount of evaporation on a daily basis. During the warmer, drier months of the year the rate of evaporation from the ground and surfaces of water such as rivers and ponds is quite considerable. Another component of moisture loss, which adds to the total figure, is the amount that occurs from plant life called transpiration. It is not uncommon for the amount of evaporation to total 5 or 6mm on a summer’s day. When there is consistently hot and dry weather the total for a month can easily exceed 100mm, which is often more than is received from precipitation during a winter month.
Particular high-end automatic weather stations use air temperature, relative humidity, average wind speed and solar radiation to estimate what is often referred to as evapotranspiration, which combines both the loss from ground, water surfaces and plant life. The total is expressed in mm and calculated once an hour.
When a severe winter is experienced, the Meteorological Office often refers to a wind chill factor, which is the perceived decrease in air temperature felt by the body, on exposed skin, due to the flow of air. As the wind strength builds it has an increasingly cooling effect, especially on humans. Therefore, the total effect of sub zero air temperatures is enhanced by the effect of the wind, referred to as wind chill. Upmarket automatic weather stations use data from wind strength, air temperature and humidity to produce a figure that is comparable to the air temperature in still conditions. With many weather stations this is displayed on a monitor and can be saved by a data logger for downloading to a computer at a later date.