What type of weather will you have with high pressure and low pressure?

Answer 1

An important characteristic of the Earth's atmosphere is its pressure as it often determines wind and weather patterns across the globe. By definition, atmospheric or air pressure is the force per unit of area exerted on the Earth's surface by the weight of the air above the surface. The force exerted by an air mass is created by the molecules that make it up and their size, motion, and number present in the air. These are important factors because they determine the temperature and density of the air and thus its pressure.

Molecules are significant for measuring air pressure because if the number of air molecules above a surface increases, there are more molecules to exert pressure on a surface and total atmospheric pressure increases. By contrast, if the number of molecules decreases, so too does the air pressure.

Today, air pressure is measured with a Mercury or aneroid barometer. A mercury barometer measures the height of a mercury column in a vertical glass tube. As air pressure changes, the height of the mercury column does as well- it drops when pressure falls and rises when it increases. An aneroid barometer uses a coil of tubing with most of the air removed. The coil then bends inward when pressure rises and bows out when pressure drops. Using instruments such as these, scientists have set the standard of normal sea level pressure at about 1013.2 millibars (force per square meter of surface area).

Low and High PressureAir 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.

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.

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.