WEATHER - HIGH & LOW PRESSURE

If you are a regular viewer of weather broadcasts, chances are you’ve heard the following from your local TV meteorologist: 
“plenty of sunshine is in store today as high pressure is in control over the area.” Or: “expect rain to spread into the area as a low pressure system approaches.” 
It is well established that high pressure is generally associated with nice weather, while low pressure is generally associated with cloudy, rainy, or snowy weather. But have you ever wondered why?
In order to understand the types of weather conditions generally associated with high and low pressure systems, we must think “vertically.” 

• The motion of air in the atmosphere above our heads plays a large part in the weather we experience here at earth’s surface. 
•  Basically, air cools as it rises, which can cause water vapor in the air to condense into liquid water droplets, sometimes forming clouds and precipitation. 
• On the other hand, sinking air is associated with warming and drying conditions. 
•  So the first important point to keep in mind is:
• rising air = moistening 
• sinking air = drying.

So what does this have to do with high and low pressure? Well, as you may have guessed, 

• high pressure is associated with sinking air 
• low pressure is associated with rising air. 

 But why? The answer has to do with the typical air flow around high and low pressure. Physically, it seems to make sense to have air flow from high pressure to low pressure. For reasons I won’t get into in this post, the airflow (due to the Earth’s rotation and friction) is directed slightly inward toward the low pressure center, and slightly outward away from the high pressure center:

The slightly inward moving air in low pressure causes air to converge and since it can’t move downward due to the surface, the air is forced upward, leading to condensation and precipitation as discussed earlier. 
The opposite occurs with high pressure. 

• Air is moving away from the high pressure center at the surface (or “diverging”) so as a result, air from above must sink to take its place. The surface flow is accompanied by the opposite behavior at upper levels of the atmosphere, as depicted in this schematic diagram:

Now there is much more to it than just high pressure = nice weather and low pressure = bad weather (otherwise I would be out of a job!), but hopefully after reading this, you have a better understanding of why meteorologists talk about pressure systems.


REVIEW - Low and High Pressure

Air pressure is not uniform across the Earth however. The normal range of the Earth's air pressure is from 980 millibars (mb) to 1050 mb. These differences are the result of low and high air pressure systems which are caused by unequal heating across the Earth's surface and the pressure gradient force.






High Pressure System: This is a region of relatively higher pressure. 

Conversely, a high pressure system, or "high," is an area where the atmospheric pressure is greater than that of the surrounding area. In some places highs are referred to as anticyclones. These move clockwise in the northern hemisphere and counterclockwise in the southern due to the Coriolis Effect.

High pressure areas are normally caused by a phenomenon called subsidence, meaning that as the air in the high cools it becomes denser and moves toward the ground. Pressure increases here because more air fills the space left from the low. Subsidence also evaporates most of the atmosphere's water vapor so high pressure systems are usually associated with clear skies and calm weather.

Unlike areas of low pressure, the absence of clouds means that areas prone to high pressure experience extremes in diurnal and seasonal temperatures since there are no clouds to block incoming solar radiation or trap outgoing long wave radiation at night. Thus such areas have higher high temperatures and lower lows.

• There will often be sinking air and thus stable weather. 
• Sunny weather will often occur under high pressure. 
• High pressure is colored in blue. The choice of the color blue has nothing to do with temperature. 
• High pressures can bring with warm or cold weather depending on the type of high pressure it is.

Low Pressure System: This is a region of relatively lower pressure. 

A low pressure system, or "low," is an area where the atmospheric pressure is lower than that of the area surrounding it. Lows are usually associated with high winds, warm air, and atmospheric lifting. Because of this, lows normally produce clouds, precipitation, and other bad weather such as tropical storms and cyclones.

In addition, areas prone to low pressure do not have extreme diurnal (day vs. night) nor extreme seasonal temperatures because the clouds present over such areas reflect incoming solar radiation back into the atmosphere so they cannot warm as much during the day (or in the summer) and at night they act as a blanket, trapping heat below.

• There will be rising air and thus often there will also be clouds and precipitation. 
• Low pressure is colored in red. The choice of the color red has nothing to do with temperature. 
• While it is typically cooler under a low due to the the cooling from cloud cover as well as rain or snow, temperatures can vary between being mild to being very cold.