Geography: Stevenson Screen

Disadvantages of a Sunshine recorder

- Measurement of burn length is very subjective - It is a manual instrument with no logger output. - The card must be changed daily - It needs an open site which is safely accessible on a daily basis - The cards are expensive and not readily available

How to calculate relative humidity

1. Record the dry bulb temperature 2. Record the wet bulb temperature 3. Calculate the depression by subtracting the wet bulb temperature from the dry bulb temperature 4. Use the relative humidity table to find where the depression and the dry bulb temperature intersect 5. Write the answer as a percentage

How does a sunshine recorder work?

A glass sphere focuses the sun's direct radiation on a graduated card which is marked in hours and the length of the burn trace on the card corresponds to the duration of sunshine. As the sun moves in the sky the concentrated rays burn a spot along the length rof the card, which is marked in-hours. When the sun is obscured (by cloud's) the trace is interrupted. The total length of the burnt spot corresponds with the total duration of continuous sunshine. At the end of the day the card is removed, the reading recorded in hours:of sunshine) and a new card inserted.

How does a hygrometer work?

Dry bulb is a normal mercury thermometer and it measures actual air temperature Wet bulb is same but bulb is covered with a fine cloth which is connected to a reservoir of water. Water evaporates from the cloth & cools temperature so it reads a few degrees lower than air temp. Both wet & dry bulb temperatures read

wind vane

Instrument used to measure wind direction

How does a six's thermometer work?

Max thermometer contains mercury & min contains alcohol As temp. rises, mercury expands & pushes up a metal index and when it cools, mercury contracts and index is left in place at highest temp As temp. falls, alcohol contracts & pulls metal index with it; but as alcohol expands, it flows past index, leaving it at lowest temp Both indexes read from bottom once every 24hrs

Sunshine recorder

Records the amount of time in a day the sun is shining

why does the stevenson screen have slats/louvres

The screen has slats to allow air to circulate freely around the instruments

why should a stevenson be away from buildings, hills, trees etc

To avoid any heat from buildings or any shadows from buildings.

Why should the stevenson screen be on a grass surface

To avoid reflection from white surfaces or absorption from dark surfaces, Stevenson screens are usually placed on grass. Grass has an albedo of 23%

What is a stevenson screen?

a small weather station that is used to measure local weather conditions

What is albedo?

ability to reflect incoming sunlight

Which way does the door of the stevenson screen always face

it always faces away from the sun. This means that in the nouthern hemisphere it should face north

what will you find outside the stevenson screen

rain gauge an anemometer a wind vane

what does it mean when one says the air is saturated

relative humidity will be 100% and the two thermometers will show the same reading. This means the chances of rain are very high.

why should a stevenson screen be at least 1.2 metres high

so that it does not receive heat released from the ground

What will a stevenson screen include in it

thermometer (max/min) or Six's thermometer hygrometer (wet/dry bulb thermometer) barometer

Why should the stevenson screen be in an open area

this will ensure that air is allowed to circulate and that the screen is not in the shade (if a tree is 10 metres, then it should be 20 metres away)

Why does the stevenson screen have double roofing

to protect the instruments from precipitation. Rain could affect temperature or humidity readings

Why is the Stevenson screen painted white?

to reflect incoming solar radiation

What is an aneroid barometer?

An instrument that measures air pressure without using a liquid in order to determine short term weather changes