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Measuring Cloud Cover

During the day the sun is always shining, so the amount of sunshine reaching the ground depends on the amount and duration of any cloud cover. The amount of cloud cover is usually given in units called oktas.

Each okta represents one eighth of the sky covered by cloud. Clear sky /1 okta / 2 oktas / 3 oktas / 4 oktas / 5 oktas / 6 oktas / 7 oktas / Overcast

Using a Cloud Mirror

You can use a cloud mirror to simply measure oktas. Divide a large mirror into a grid with 16 equal squares using a dark crayon. Lay the mirror on the ground somewhere you can see the whole sky.

Count the number of grid squares, or fractions of squares with cloud in them. Divide that number by two to convert sixteenths into oktas.

http://www.bbc.co.uk/weather/weatherwise/activities/weatherstation/cloud_measuring.shtml

Measuring Humidity

There is water vapour in the air at all times. Humidity is the amount of water vapour present in the air and we can measure this using a hygrometer.

Relative humidity is the ratio of the actual amount of moisture in the atmosphere to the amount of moisture the atmosphere can hold. Therefore, a relative humidity of 100% means the air can hold no more water (rain or dew is likely), and a relative humidity of 0% indicates there is no moisture in the atmosphere. Relative humidity is used by meteorologists to help predict the weather, by pathologists to predict disease development on plants, and by agricultural scientists to estimate evapotranspiration

Relative humidity, combined with air temperature, can be used to estimate the actual amount of moisture in the atmosphere, sometimes referred to as precipitable water. Water vapour acts as a green house gas by trapping infrared radiation reflected from the earth. This explains why desert temperatures can become much lower at night, as there is little moisture in the air to trap the heat.

Hygrometer

The best hygrometer is one which has two thermometers - one of which has its bulb wetted continuously by a wet cloth. This is called a Mason's or a 'wet-and dry-bulb' hygrometer. Air passing over this wet bulb evaporates some water and lowers the wet bulb temperature by an amount that depends on the humidity of the air. To demonstrate this cooling, wet your finger and blow in it. The humidity can be found by noting the difference in temperature between the wet and dry bulb and looking up the RH value in a set of tables. (see attached). The relative humidity (RH value) is the amount as a percentage of that required to saturate the air completely.

http://www.uswcl.ars.ag.gov/exper/relhum.htm

Measuring Pressure

Barometers are used for measuring pressure. You can make a simple barometer but a proper one is needed to make correct observations.

The simplest and cheapest is a basic aneroid barometer which has a partial vacuum in a special metal box. This expands and contracts as air pressure changes, moving a pointer around a dial. Measurements are given in millibars (mb) or in hectopascals (hPa), identical in value. You must adjust your barometer to sea-level and it is best to put it inside a screen.

Measuring Rainfall

Rain gauges are required to measure rainfall and should be located in an open area, away from anything that might cause a shadow effect. It is possible to make a simple gauge out of a 2 litre plastics drinks bottle. The instructions are given very clearly at the website

http://weblife.bangor.ac.uk/cyfrif/eng/activities/spring/rain/pupilnotes.htm

Temperature: max and min

Special Max/Min Thermometers

A special maximum and minimum thermometer actually records the highest and lowest temperatures during a particular time period, usually one day. There are two types - the liquid in glass type (known as 'Six's thermometer') has little tiny metal markers inside the tube which are pushed around by the liquid and read against a scale. The bimetallic dial type has pointers which are very easy to read. Both need to be reset after every reading.


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