By Ken Meehan
Published: Oct. 2, 2017 at 7:15 PM EDT
High pressure and low pressure have two distinct types of weather associated with them...typically. High pressure often means dry weather with sunshine. Low pressure often means clouds and precipitation.
High pressure is associated with sinking air. Air pressure is higher because it is pushing DOWN on the ground.
When air sinks from high in the atmosphere to the lower levels it warms up and dries out. This is because air at the top of the atmosphere is less dense. If we take that air lower to where the density is greater, it gets compressed. This acts to warm up the air (moving the molecules faster). The faster moving air molecules mean that the water molecules are unlikely to stick together, due to high momentum. This means the humidity is lower. This is associated with clear skies.
Low pressure is the opposite. Air is RISING UP in the atmosphere. The process of lifting air means that less pressure is exerted on the ground. That is why the air pressure is low.
When the air begins where the density is high (low levels) and we take it up in the atmosphere where the density is low, the air expands. In the process it gives away it energy to the surrounding air. The air gets cooler and the humidity increases. The air gets cooler because it gives away energy to the surrounding cooler air as it ascends.
The humidity goes up because the water molecules are also moving slower. They are more likely to stick together as they slow down. Water has a positive charge on one end and a negative charge on the other. This is called a 'dipole'. With the relatively slow motion of the air molecules, those made of water can stick together, positive to negative charge. This bond cannot be overcome due to the slow moving nature of the colder air. When many water molecules stick together, the result is clouds. When this process continues for long periods, the result becomes precipitation.
