Earth Science: Atmosphere – Weather
All living organisms on this planet are influenced by the weather. Humans rely heavily on accurate forecasts of temperature, air pressure, humidity, and the presence or absence of wind or clouds.
Due to the very complex processes happening in the lower part of the atmosphere, it is almost impossible to predict the weather more than a few days into the future as there are too many variable and uncertain factors affecting the likelihood of rain, storm, or sunshine.
Earth Science: Atmosphere – High- and Low-Pressure Weather Fronts
The sun acts as an engine of the weather system. Its energy moves enormous air masses which travel as currents throughout the lower atmosphere. The daily change in weather usually results from events at the boundaries.
The air may seem weightless, but surprisingly it has a considerable weight. This weight can be measured as air pressure, using the unit hectopascal (hPa). Air pressure is created due to the uneven warming of the Earth’s surface. Warm air masses rise up and leave behind a low-pressure area near the ground, while high-pressure areas develop where air masses descend after cooling. Air masses from high-pressure areas move into low-pressure areas in order to equalize differences in pressure.
In temperate zones, the weather is determined by the interaction of cold and warm air masses. When air masses of different temperatures collide, a front forms. If lighter, warmer air slides above the cold air, a warm front is created. A cold front is created by cold air pushing underneath the warm air like a wedge. Warm air rising results in cloud formations and precipitation. In a warm front, these processes happen evenly and relatively slowly while a cold front shows rapid formations of cumulus clouds and precipitation in the form of showers. Thunderstorms may also occur.
The process where a cold and warm front merges is called occlusion. At this location, a cold front moving along catches up with a warm front. Cold masses of air move underneath the warm air and push it up from the Earth’s surface. The warm air that is pushed up cools, resulting in reduced wind and precipitation. The air circulation in temperate zones keeps moving due to the air pressure gradient between the subtropical high-pressure belts and the subpolar low-pressure areas.
Differences in air pressure are balanced by strong westerly winds. Hot tropical air and cold air from the Poles create a boundary called the polar I where major air turbulences result. These are called cyclones and can reach a diameter of 620 miles (1,000 km)
JET STREAMS
Jet streams are created where very cold air collides with very warm air masses resulting in an extreme pressure drop and an exceptionally fast wind. Jet streams may reach speeds of up to 370 mph (600 km/h) at a height of 5.6 to 7.5 miles (9 to 12 km).
They can be up to several miles wide and over a thousand miles long. Often jet streams can be recognized by cirrus clouds that consist of ice crystals.