What types of weather are associated with high and low pressure systems?
Low pressure systems result in unsettled weather with precipitation or storms, while high pressure brings in settled dryer weather over longer periods.
A low pressure system is a large mass of air that is rising due to warmer land or water below it. The air becomes hot and starts to expand, becoming less dense. When there is also moisture in the air mass, it will weigh less due to the water vapor whose molecules are lighter than air molecules. The end result is wet, less dense air that rises and begins to cool in the upper atmosphere.
We can see this happening from the ground when the air rises enough to form fluffy white cumulus clouds if the air mass is not moving. As the air continues to cool, the water vapor may condense into some form of precipitation.
Low pressure systems tend to result in unsettled weather, and may present clouds, high winds, and precipitation. As the low pressure intensifies, storms or hurricanes can be formed. Low pressure systems rotate in a cyclone which is characterized by motion in a counter-clockwise direction in the northern hemisphere (clockwise in the southern hemisphere). Rotation is initiated by the rotation of the earth.
Images of low and high pressure systems are here:
A high pressure system is colder air moving from the upper layers of the atmosphere towards the earth's surface. The air is becoming more dense as it sinks, and any water is vaporized into the air mass. There is no water to form clouds and the air is stable, fair, and dry. A cold air mass will rotate in an anti-cyclone, or clockwise in the northern hemisphere (counter-clockwise in the south).
High pressure systems are settled, are usually larger than lows, and they last longer, into days or weeks. Lower humidity is associated with high pressure which is usually a relief. But, if the dense air in a high becomes too warm, it may result in a drought.