High pressure area

A high pressure area (also called a high or high-pressure) is a region where the atmospheric pressure is greater than surrounding areas. In the northern hemisphere high pressure areas move clockwise, whereas they move counter-clockwise in the southern hemisphere. In some countries, these regions may be referred to as "anticyclones". Highs are frequently associated with light winds and subsidence. Subsidence will generally evaporate most cloud droplets after less than 500 meters by adiabatic heating. Thus, high pressure typically brings clear skies. During the day, since no clouds are present to reflect sunlight or reflected inki, there is more incoming shortwave solar radiation and temperatures are higher in the summer and lower in the winter. At night, the absence of clouds means that outgoing longwave radiation (i.e. heat energy from the surface) is not absorbed, giving cooler diurnal low temperatures in all seasons.

Climatologically, high pressure forms at the Horse Latitudes as a result of air which has been uplifted at the equator. As the hot air rises it cools, losing moisture; it is then transported poleward where it descends, creating the high pressure area. This is also known as Hadley Cell circulation. Many of the world's deserts are associated with these climatological high pressure systems. Surface high pressure systems tend to be larger in area and have weaker surface winds than low pressure systems, because of the addition of surface friction to the pressure gradient and coriolis effect that drives the circulation.

Some high pressure areas persist long enough that they acquire names. The land-based Siberian High often remains quasi-stationary for more than a month during the most frigid time of the year, making it unique in that regard. In the United States the Azores High, also known as the Bermuda High, brings fair weather over much of the North Atlantic Ocean. Along its southerly periphery, the clockwise circulation often impels easterly waves across the ocean towards North America during the hurricane season.

High Pressure and Connections to Droughts

Ridges of high pressure remaining over one particular place too long are extremely notorious for causing droughts. That's because these pressure ridges tend to often get upgraded to caps; thunderstorm or rain development is highly restricted, often prevented altogether. If these patterns continue for very prolonged periods of time across one particular region and refuse to let up, drought situations are possible because of the domination and unusual prevalence for high pressure ridges moving over the same area, one after another for a longer than normal period of time.

The Dust Bowl Droughts of the 1930s were likely to have been caused by excessive high pressure ridges running straight across exactly the same vicinity and certain regional portions of the United States. Excessive heat, dust storms and incredible dry spells decimated many states during certain years. Making matters worse, the disaster happened at exactly the same time as the famous Great Depression. Record breaking heat simmered many regions across the United States with 100 to 110 degree fahrenheit readings constantly happening in many places over the United States.

The 1980s decade was extremely full of droughts blasting portions of the US. In 1980 drought problems hammered the Central and Northeastern United States, accompanied by excessive heat conditions which killed multitudes of people across the United States.

Extremely unusual heat and very excessive heat spells during 1983 contributed to between 500-711 mortalities, damage totals in hundreds of millions-even billions-across the United States. The Upper Midwest, Ohio Valley Region along with portions of states bordering the Great Lakes and the metropolitan areas of St. Louis, Missouri and East St. Louis had seen many people perishing because of extended and unusually severe spells of heat. Droughts enveloped across the Upper Midwest, the Ohio Valley and certain Northeastern US Regions next to Baltimore, Louisville, Kentucky, Philadelphia, Cincinnati and Danville, Illinois.

But nothing would really compare to severe conditions connected with the US Drought of 1988-The drought enveloped portions of the Eastern United States, Middle Atlantic States, the Southeast United States around Georgia, South and North Carolina, multiple regions around the Midwestern United States, the Western United States and the Rocky Mountain States area. The Drought of 1988 decimated multiple farms and growing regions across the United States. The unusual abundance of high pressure systems which dominated the US in 1988 were instigators of the extensive drought plaguing the United States.

The associating heat waves created temperatures of 90 degrees or higher; hundred degree readings were common and prevalent during the Drought Spells of 1988. The official numbers of mortalities connected with the heat and dryness during 1988 was between 4000 to 17000 people (estimates related to the death toll are varying). In addition, damage totaled $60 billion (1988 USD).

For further information: Droughts in the United States.

See also

*Anticyclone
*Atmospheric circulation
*Block (meteorology)
*Low pressure area
*North American High
*Droughts in the United States