Weather is the day to day conditions of the atmosphere, whereas climate is what we expect to happen over a large area based on records of at least 30 years
We measure weather with a number of different instruments. Some are located 'outside', others are found in the Stevenson Screen
Instruments placed outside the Stevenson screen
The standard rainguage is made of copper, 12.8cm in diameter and is buried in the ground with 30 cm exposed. This prevents the sun evaporating the water. The rain goes into the funnel and then into the container and it can then be measured (usually every 24 hours) with a measuring cylinder which has a tapered end to allow accurate recordings up to 0.1mm
The weather vane shows the direction. It pivots on a vertical shaft. When the wind blows the arm swings until the pointer faces the wind.
The anemometer allows the wind speed to be measured. It generally consists of three of four cups, which rotate freely; the faster the wind, the faster the cups rotate.
The Stevenson screen is a wooden box about 1.1 m above the ground on a stand. It has double louvers to allow air to circulate; it is painted white to reflect the sun's rays and the door opens to the north to prevent the sun affecting the recordings. The roof is made of double boarding to prevent the sun's heat from reaching the inside of the screen. It is situated in an open space, clear from buildings and trees to allow accurate readings to be taken.
Inside the screen we find the rest of the instruments which are thermometers.
The wet and dry thermometers, (also called hygrometers) allow the calculation of the humidity of the air
The maximum and minimum allow the 24 hour temperatures to be recorded. This is sometimes called a 'Sixs thermometer.
Measuring air pressure
Air preesure is simply the weight that the air exerts, so as you go up the amount of air above you decreases and therefore so does air pressure. Air pressure is measured by a barometer and it is usually measured in millibars.
There are basically two types of barometer: a mercury barometer and an aneroid barometer.
A mercury barometer is an inverted tube which has a vacuum at the top, the open end is placed in mercury bath. The pressure of the air forces the mercury up the tube and the height of the mercury gives the air pressure, this is often quoted as 'inches of mercury'. This varies with the weather. Low air pressure usually suggests changeable weather, whereas high pressure usually means stable weather.
Here we see a small metal cylinder which is a vacuum chamber. As the air pressure increase so the cylinder is compressed more, low air pressure causes the cylinder to expand. This movement is recorded by a series of levers which moves a pointer.
A barograph records changes in air pressure over time.
Each weather station has it's air pressure 'reduced to sea level' and then plotted on a map. Lines are then drawn through points whose pressure is the same. These lines are called isobars and the map is called a synoptic chart.