-->

Station Observations

The station still uses traditional meteorological equipment housed in a Stevenson's Screen:

In addition a grass minimum thermometer is in place on the lawn and a standard rainguage located to ensure maximum catch.

A good quality aneroid barometer and whirling hygrometer are available.

All other observations are done by eye. The range of data record each day is shown in the table below. Instrument based observations are made at 0800, 1700 and 2000 hours each day, GMT or BST. Because of work commitments it is impossible to do morning observations at 0900 GMT/UTC (1000 BST) in the summer!

 

 

Observation Method/Procedure
Maximum and Minimum Temperatures Records taken at 0800 each day and thrown back ot the previous 24 hours. All temperature are recorded to one decimal place of a degree, using the Celsius scale.

 

For the purposes of returns to the Climatological Link the minimum temperature is attributed to the current day. This is the official Met Office standard, but I have never applied it for my own personal records because it can result in very misleading temperatures being listed for specific days, e.g. a very low minimum temperature, say -10oC, on one morning may coincide with the time of the morning reading; assuming the day warmed up dramatically and a warm front went through, by the following morning the temperature may have risen to 10oC; the whole of the next day may be very mild, but a minimum of -10oC will be recorded for it using the offical method!
Mean Temperature Both values are calculated from the maximum and minimum temperatures.

 

For the technically minded, this is handled online via JavaScript and verified using PHP during upload to the database. The same is true for all calculated values handled by the database.
Diurnal Range of Temperature
Grass Minimum Temperature Recorded from the minimum thermometer at 0800 and thrown back to the previous day.
Apparent Temperature Calculated value taking into account Wind Chill (Using the recently revised NOAA formula). This states the equivalent temperature in calm conditions that would result from the same heat loss from exposed human skin.
Pressure Readings are taken 12 hours apart, at 0800 and 1200 and recorded in millibars (mb)
Dry and Wet Bulb Temperature Readings are taken from both thermometers at 0800 and 1700. The values recorded are used to calculate the percentage Relative Humidity
Precipitation Measured in millimetres (mm) at 0800 and thrown back to the previous day. Any snow, hail or ice in the rainguage is thawed out before measuring is done.
Visibility This is assessed at the time of the morning observation and record again each time there is a significant change in value. It is recorded in Beaufort Visibility Code The occurrence of fog (visibility <=3) and mist (visibility<=4) is recorded, as well as good (>=7) or excellent (=9) visibility using boolean values.
Wind Direction Recorded using the 16 compass directions, plus calm
Wind Force Recorded using the Beaufort Wind Scale
Cloud Cover Recorded at 0800 and 1700 as a percentage. In the monthly reports it is also sometimes expressed as oktas
Snow Boolean values are used to record whether snow was falling at any time during the 24 hours of the calendar day, or lying at 0900 UTC.
Hail Boolean values are used to record whether any hail fell at any time during the 24 hours of the calendar day. Further boolean values are used to record the average size of the hailstones, which are split into two categories: less than 5mm diameter (measured across the smallest axis), and 5mm or more in diameter.
Thunder Boolean values are used to record whether any thunder was heard at any time during the 24 hours of the calendar day.
Weather Details of the weather are recorded as a text string using a modified form of Beaufort Code. You can download a full explanation of the code used here. Details of cloud cover and types are also indicated. You can find a full account of the cloud codes here

Weather Home

PJS Web Design 2010

Valid XHTML 1.0 TransitionalStreamline.net - the best web hosting deal available in the UK Valid CSS!

Boolean: Values that can only be true or false, e.g. thunder, hail, etc. Either it did thunder on a particular day (true) or it did not (false).
Crepuscular Rays: Rays of sunlight shining through gaps in clouds. They are most common near sunset when the sun is just above the horizon. Normally they radiate downwards towards the surface. With very dense cumulus clouds upward radiation visible against the blue of the sky can sometimes be seen.
Glaciation: where the supercooled water droplets that form clouds are converted to ice crystals, causing the edge of the cloud to have a fibrous appearance. High in the troposphere where the air is very clean, lacking particles of dust, salt, etc. water does not freeze at 0o Celsius - temperatures below -30o are required.
Temperature Inversion: a base of a layer in the atmosphere above which the temperature increases with height. Temperature inversions are common at ground level after cold clear calm nights, and help to explain why radiation fog often fills valleys and not nearby hills
Oktas: A unit of measurement for recording cloud cover. One okta is one eighth of the sky. The sky is sectioned into areas between the 8 main compass points: N, NE, E, SE, S, SW, W and NW - if a section is more than half covered it is recorded as 1 okta.
Relative Humidityis the percentage of the possible moisture content that can be held in vapour form (invisible) in a mass of air for a specific temperature. The higher the temperature the more moisture can be stored. When relative humidity reaches 100% saturation occurs and cloud or fog can form.
Snow Falling: Snow or sleet (rain mixed with wet snow) must be observed to fall at some point during the calendar day .
Snow Lying: This is recorded if more than or equal to 50% of an exposed level grass surface is coverd by snow at 0900 UTC/GMT on the day of observation. It excludes locations where drifting may have occurred.
Tropopause: the atmospheric boundary between the troposphere (the lowest layer of the atmosphere) and the stratosphere. The tropopause marks to upper limit of normal cloud formation, as beyond it temperature does not decrease with height so convection cannot take place. The tropopause is at about 17km altitude neat he equator but falls to below 11km at the poles, and during winter it is much lower than this.
Virga are those lineated trails that can occur beneath certain types of cloud. They are caused by falling precipitation, usually snow or ice crystals. When they fall into the dry air below the cloud they eventually disappear due to evaporation or sublimation.
Code Distance Description
X 20m Dense Fog
E 20m Dense Fog
0 40m Thick Fog
1 100m Thick Fog
2 200m Fog
3 400m Moderate Fog
4 1km Very Poor/Mist
5 2km Poor
6 4km Moderate
7 10km Good
8 20km Very Good
9 40km Excellent
Wind Chill represents the extra heat lost from exposed skin because of the strength of the wind. Often wind chill is expressed as an apparent temperature, which is the temperature still air would have to be to cause the same heat loss. Wind Chill can also be expressed more directly as the number of kilocalories of heat lost per square metre of exposed skin per hour.