Humidity
The amount of water vapour in the air changes from day to day and from place to place. The amount of water vapour that the air can hold depends on the temperature of the air. Warm air holds more water vapour than cold air. The amount of water vapour in the air is called its humidity. The humidity of the air is measured with a hygrometer.
A hygrometer has two thermometers – the bulb of one is kept wet, while the other is dry.
How to read a hygrometer:
- Read the dry bulb thermometer
- Read the wet bulb thermometer
- Subtract the two readings
- Look up this value on the table of humidity
Atmospheric pressure
Instruments that measure atmospheric temperature are called barometers. There are two types of barometers, an aneroid barometer and a mercury barometer.
Mercury Barometer
The mercury barometer is a long glass tube, with one end dipped
in a dish of liquid mercury, and the other end sealed. The mercury rises up whenever pressure is applied. The space at the top of the tube is left completely empty and acts as a vacuum.
Mercury is used in the barometer, because it has such a high density.
Aneroid barometers
Many people use aneroid barometers in their homes as they are more durable and easier to read than the mercury barometer. They are not as accurate as the mercury barometer.
In the middle of the barometer is a sealed metal box with flexible sides. Most of the air has been removed from this box. The pressure in the atmosphere causes the sides of the box to be pushed in and causes the spring to move the pointer around the scale.
Atmospheric pressure and the weather
High atmospheric pressure brings with it dry, warm weather, and clear, cloudless skies. Low atmospheric pressure brings wet and windy weather.
Weather instruments:
Anemometer:
measures wind speedBarometer: measures air pressure
Stevenson Screen: measures air temperature
Hygrometer: measures humidity i.e. the amount of water vapour in the air
Rain gauge: measures rainfall in millimetres.
A Stevenson Screen
An Anemometer
Clouds
A cloud forms when moist air rises up and cools. The air cools for two reasons, the upper air is cooler and the atmospheric pressure is lower. The water droplets cool and condense to form a cloud. They join up to form larger drops , which then fall as rain or snow.
Cirrus Clouds
Nimbostratus Clouds
Cumulus Clouds