Most barometers use mercury in a glass column, like a thermometer, to measure the change in air pressure. If a high pressure system is on its way, often you can expect cooler temperatures and clear skies. If a low pressure system is coming, then look for warmer weather, storms and rain. Air pressure changes with altitude. When you move to a higher place, say a tall mountain, air pressure decreases because there are fewer air molecules as you move higher in the sky.
Relative humidity is the amount of moisture the air can hold before it rains.
The most it can hold is 100 percent. Humidity is measured by a psychrometer, which indicates the amount of water in the air at any one temperature.