The barometer shows high or rising air pressure. That tells the meteorologist that fair weather is on its way. Air that is warm and damp weighs less than dry air. Then the barometer shows low or falling air pressure. A drop in air pressure usually signals foul weather ahead.
Meteorologists use instruments especially made to measure barometric pressure.
They are called barometers. They are important tools for forecasting the weather. Describe the different arrangement of air molecules in high and low air pressure masses. Compare the temperature of high and low pressure masses. Tell how a barometer works.