Atmospheric pressure at the Earth's surface is one of the keys to weather, which is one reason weather maps feature H's and L's, representing areas of high and low air pressure. High and low are pressure are important because they affect the weather.
As the name says, a "high" is an area where they air's pressure is higher than the pressure of the surrounding air. A "low' is where it's lower. Meteorologists don't have any particular number that divides high from low pressure; it's the relative differences that count.
The pressure is high at the surface where air is slowly descending - much to slowly to feel. And, this is going on over a large area, maybe a few hundred square miles. As air descends, it warms, which inhibits the formation of clouds. This is why high pressure is generally - but not quite always - associated with good weather.
The air that descends in high pressure areas, has to get to high altitudes in some way, and that way is by rising in areas where the pressure at the surface is low.
As air rises it cools. As the air cools, the humidity in it begins to condense into tiny drops of water
, or if it's cold enough, into tiny ice crystals. If there is enough water, or ice, rain or snow begin to fall. This is why low pressure is associated with bad weather.
The air descending in high pressure flows out in a clockwise spiral in the Northern Hemisphere. Air flowing into an area of low pressure to rise, makes a counterclockwise spiral on the way in.
In general, falling air pressure means that clouds and precipitation are likely. Rising air pressure signals that clear weather is likely.
Using winds and a barometer to make forecasts
from the National Weather Service
Here are some general statements of wind-barometer indications that are generally applicable to all parts of the country (USA):
* "When the wind sets in form points between south and southeast and the barometer falls steadily, a storm is approaching from the west or northwest, and its center will pass near or north of the observer within 12 to 24 hours, with wind shifting to the northwest by way of south and southwest.
* "When the wind sets in from points between east and northeast and the barometer falls steadily, a storm is approaching from the south or southwest, and its center will pass near or to the south of the observer within 12 to 24 hours, with winds shifting to northwest by way of north. The rapidity of the storm's approach and its intensity will be indicated by the rate and amount of the fall in the barometer.
* "As a rule
, winds from the east quadrants and falling barometric pressure indicate foul weather, and winds shifting to the west quadrants indicate clearing and fair weather, but again there are exceptions and in some parts
of the country these rules do not apply."
The following table generally summarizes wind and barometer indications in the United States. The amateur forecaster should modify the table in accordance with his or her own observations. The following show the wind direction, the barometer reduced to sea level and the character of the weather indicated:
* SW to NW, 30.10 to 30.20 and steady - Fair with slight temperature change for 1 to 2 days.
* SW to NW, 30.10 to 30.20 and rising rapidly - Fair, followed within 2 days by rain.
* SW to NW, 30.20 and above and stationary - Continued fair, with no decided temperature change.
* SW to NW, 30.20 and above and falling slowly - Slowly rising temperature and fair for 2 days.
* S to SE, 30.10 to 30.20 and falling slowly - Rain within 24 hours.
* S to SE, 30.10 to 30.20 and falling rapidly - Wind increasing in force, with rain within 12 to 24 hours.
* SE to NE, 30.10 to 30.20 and falling slowly - Rain in 12 to 18 hours.
* SE to NE, 30.10 to 30.20 and falling rapidly - Increasing wind, and rain within 12 hours.
* E to NE, 30.10 and above and falling slowly - In summer, with light winds, rain may not fall for several days. In winter, rain within 24 hours.
* E to NE, 30.10 and above and falling rapidly - In summer, rain probably within 12 to 24 hours. In winter, rain or snow, with increasing winds, will often set in when the barometer begins to fall and the wind sets in from the NE.
* SE to NE, 30.00 or below and falling slowly - Rain will continue 1 to 2 days. SE to NE, 30.00 or below and falling rapidly - Rain, with high wind, followed, within 36 hours by clearing, and in winter by colder.
* S to SW, 30.00 or below and rising slowly - Clearing within a few hours, and fair for several days.
* S to E, 29.80 or below and falling rapidly - Severe storm imminent, followed within 24 hours, by clearing, and in winter by colder.
* E to N, 29.80 or below and falling rapidly - Severe northeast gale and heavy precipitation; in winter, heavy snow, followed by a cold wave.
* Going to W, 29.80 or below and rising rapidly - Clearing and colder.
E. & O. E.