what is Stevenson screen?
A Stevenson screen or instrument shelter is a shelter or an enclosure to shield meteorological instruments against precipitation, and direct heat radiation from outside sources, while still allowing air to circulate freely around them.
A Stevenson screen is a prominent feature on most weather stations. It is used to house weather instruments, for example, the six’s thermometer, hygrometer, barometer, thermograph, and hydrometer.
what is a weather station?
A weather station is a facility, either on land or sea, with instruments and equipment for measuring atmospheric conditions to provide information for weather forecasts and to study the weather and climate.
Most instruments used in measuring weather elements are found in a weather station.
This is a place where weather events are recorded for example weather station inside Dar es salaam International Airport. It is situated in an open space away from buildings and trees.
Trees can affect weather phenomena for example by acting as windbreaks thus affecting the wind speed and direction readings.
Buildings act as thermal sinks i.e they store heat energy thus distorting heat readings.
factors to be taken into account when setting a weather station
- the site should be free from flooding
- the site should be an open place where there is little obstruction of weather elements
- it should be an accessible place to simplify the recording process
- the area where the weather station is located should be fairly level or gently sloping ground (5 degrees) so that it is easy to position the weather instrument
- the place should provide a wide view of the surrounding landscape and the sky
- the place should have security
Instruments and items found in a weather station are:
1. Stevenson screen.
2. Six’s thermometer.
5. Sunshine recorder.
7. Rain gauge.
8. Evaporation dish.
The following are some of the prominent features of the Stevenson screen.
- It is painted white in order to reflect excessive heat.
- It is 1.2 meters above the ground to avoid ground weather conditions from affecting the readings yielded by the housed instruments.
- It has louvered sides to allow for the free circulation of air.
- It has a double roof to reduce excessive heat from solar radiation.
- It has metal legs to avoid attacks by termites. Sometimes the legs are made of treated wood for the same reason.
- it is made of wood which is bad conductor of heat
importance of Stevenson screen
- provide shade conditions for accurate temperature recording
- ensure the safety of thermometers because they are delicate
recording and calculating weather condition
temperature is the degree of hotness of object or place
it is measured using three types of thermometer namely:
- maximum thermometer
- minimum thermometer
- six’s thermometer
it is used to measure the highest temperature reached in a day
it uses mercury
how maximum thermometer works
- the temperature rises causing the mercury to expand
- mercury pushes the index up
- when the temperature falls mercury contracts
- the maximum temperature is read from the scale at the lower end of the index
- the thermometer is reset by shaking it to force the mercury back into the bulb
this is used to record the lowest temperature reached in the day. it uses alcohol
how minimum thermometer works
- temperature falls causing the alcohol to contract
- alcohol pulls the index down
- when the temperature rises alcohol expands and rises in the tube
- the index remains where it was pulled
- the minimum temperature reading is obtained from the scale at the lower end of the index
diurnal or daily temperature range
this is the difference between the maximum and minimum temperature for any one day
mean daily temperature
this is the average of the maximum and minimum daily temperature
mean monthly temperature
this is the sum of the mean daily temperature in the month divided by the number of days in that month
mean monthly minimum temperature
this is the sum of daily minimum temperatures divided by the number of days in that month
mean monthly maximum temperature
this is the sum of the daily maximum temperature divided by the number of days in that month.
mean annual temperature
sum of mean monthly temperature divided by 12
mean annual temperature range
this is the difference between the highest and lowest mean monthly temperature in the year
amount of rainfall in the day is measured by the instrument called a rain gauge
rain gauge should be made of impenetrable material which can not absorb water
how rain gauge works
- it is taken to an open space to prevent water from dropping into the funnel
- it is sunk in the ground to prevent evaporation
- the funnel top is left 30 cm above the ground to prevent splashes of the water and the runoff
- after 24 hours water is emptied into a measuring cylinder
- the reading of the amount of rainfall is got from the measuring cylinder in millimeters
- the figure represents the millimeters of water falling on each square millimeter of the ground
- a rain gauge can be used to measure snowfall by melting it before the reading is taken.
monthly rainfall total
the sum of the rainfall recorded in the month
annual rainfall total
the sum of monthly rainfall totals for 12 month
mean monthly rainfall
the sum of the rainfall totals for the particular month over several years divided by the number of years of observation
mean annual rainfall
the sum of mean monthly rainfall for 12 months of the year
duration of sunshine is measured using Campell stokes sunshine recorder
how compell stokes works
- the spherical lens focuses light on the sensitive paper
- the paper burns when the sun is shining
- the total hours of sunshine is got by adding all the burnt sections from the calibration on the side of sensitive paper
- the sensitive paper is changed every day
humidity is the condition of the atmosphere with reference to its water content
it is measured with the hygrometer or psychrometer which consists of a wet and dry bulb thermometer kept on the Stevenson screen.
wet bulb thermometer is the thermometer covered with muslin bag immersed in water while dry bulb thermometer has no muslin
how hygrometer works
- when the air is dry there is a lot of evaporation from the muslin
- evaporation cools the bulb of the thermometer resulting in the low temperature reading
- when humidity is high there is little evaporation from the muslin
- the wet bulb thermometer is cooled art the slower rate and both thermometer shows almost the same temperature reading.
- the difference in the reading between the two thermometer is used to determine relative humidity
interpretation of hygrometer reading
- when the two reading are the same, relative humidity is 100% that is the air is saturated
- if the difference is small the humidity is high
- if the difference is big it means humidity is very low
it is the actual amount of water vapor a given volume of air can hold
it is expressed in g/m3
this is the mass of water vapour in the given mass of air. it is expressed in g/km
this is the ratio between the absolute humidity and the maximum amount of water vapour the air can hold expressed in percentage
relative humidity = absolute humidity/maximum amount of water the air can hold at the same temperature
For example if the 22 centigrade contain s 10g/m3 and the given air can hold a maximum of 20g/m3 what is its relative humidity
wind direction is determined using the wind vane
how wind vane works
- as the wind blows the arrow swings
- the arrow points in the opposite drection of the wind flow
- the direction is read from the cardinal compass points
- the arrow will point in the direction from which the wind is blowing
- for example if it points to south then the wind is blowing from south towards north
the wind sock
is used to indicate the general direction of the wind flow
windsock is not kept in the weather station because it does not give the accurate direction of wind flow
they are seen near airstrip for the benefit of the pilots
how wind sock works
when the wind blows the sock stretches out in the direction the wind is blown
wind speed or velocity
this is measured by using the anemometer
how anemometer works
- when the wind blows the hemispherical cups rotates
- the number of rotation is obtained from the meter on the lower part of the anemometer
- the units for measuring wind speed is called knot
the amount of cloud cover is observed using eyes
it is given in oktas
okta = is approximately 1/8 of the sky is covered with clouds
this is the force exerted by gases in the atmosphere on some areas or bodies on the earth’s surface
atmospheric pressure is measured using barometers of three types namely: mercury, aneroid, and Fortin barometers
how mercury barometer works
- air exerts pressure on the mercury in the beaker
- the height of the mercury in the tube is proportional to atmospheric pressure
- the reading are taken in mmHg
- it is 760mmHg at the sea level
advantages of mercury barometer
- it is accurate
disadvantages of the mercury barometer
- it is cumbersome to carry around
- it can be damaged quite easily while being carried around
this is used to measure changes in the atmospheric pressure
how aneroid barometer works
- has airtght compartment (vacuum)
- compartment expands when pressure decreases
- it collapses when pressure increases
- the movement is transmitted by lever to a pointer on a dial
- the reading are in kg/cm3
the rate and amount of evaporation is measured using piche and tank evaporimeter
how piche evaporimeter works
- when there is lot of sunshine water evaporates from the blotting paper
- the level of water in the glass tube reduces
- the rate and amount of evaporation is got by looking at the scale on the outside of the glass tube
- the units are in mm
how tank evaporimeter works
- the tank with water is put in the open
- water evaporate when there is a lot of sunshine
- water in the tank reduces
- the rate and amount of evaporation is obtained from calibration in the inner side of the tank in mm