Winds result due to differences in air pressure. Pressure, which is higher at one place, sets up a force which pushes from a higher pressure zone towards a lower pressure area. The pressure difference is directly proportional to the force. The distance between a high pressure zone and a low pressure area is vital in determining the acceleration attained by a fast moving air mass. Pressure gradient force is the term used by a meteorologist to refer to the force that starts a wind. High and low pressure amounts are relative. Movement of moisture and heat around the world is facilitated by wind which also brings about much of the weather changes. Wind always blows from high pressure to low pressure areas though the strength varies from time to time.
A jet stream is a current of rapidly moving air at high altitudes above the earth that generally flows from west to east over the mid-latitudes. These winds can be several hundred miles wide and one to two miles in depth. A stream of air must be over 56 miles per hour in order to be considered a jet stream. Jet streams were discovered during World War II by Pilots who noticed that flying to the United Kingdom was quicker than flying to the United States of America. Research has shown that in high-flying airplane areas, the areas around the airplane are favorable for precipitation and storm development. Air motion in these areas tends to enhance an upward motion depending on the atmospheric conditions. At the earth’s surface, air pressure is lower. Rising air is related to density, air temperature and height above the earth’s surface. As air rises, it cools and the humidity in it begins to condense into tiny drops of water, or if cold enough, into tiny ice crystals. With enough water or ice, rain or snow begins to fall. This explains the reason why sunny, windy or calm weather conditions are associated with low pressure whereas rainy or wet conditions are associated with high pressure.
A jet stream results when air masses of different densities meet under the tropopause where winds are strongest. This results in an increase in the wind force due to the resultant pressure created by different air mass densities. The winds are deflected by the earth’s rotation, a phenomenon known as the Coriolis Effect, as they attempt to flow from the nearby stratosphere, where it is warm, down into the cooler troposphere along the boundaries of two air masses.
The earth's atmosphere contains two major jet streams to the north and south of the equator. However, the Southern Hemisphere jet streams show less variability compared to the northern hemisphere jet streams. Zonal flow, a wind flow pattern referring to jet streams blowing from east to west, establishes a small temperature contrast between the north and the south, with the effect of keeping the weather calm. Large temperature contrasts supply energy required for storms.
Another wind flow pattern, meridional flow, brought about by jet streams blowing towards the south and then back to the north brings about stormy weather in places like the United States of America’s California. This is due to the fact that it brings cold air from the north and warm air from the south as a result of the large temperature contrast. Curvature effects, which are strongest in an amplified meridional flow and minimal in a zonal flow, have significant implications for the surface weather. The anti-cyclonic curvature mostly brings about sunny weather as it favors formation of surface-high pressure areas. On the other hand, cyclonic curvature brings about clouds and precipitation as it helps in the development of low pressure zones. Curvature effects are affected by the position of a jet streak among other factors. Throughout the northern hemisphere, the anti-cyclonic curvature induces a clockwise circulation while the cyclonic curvature tends to favor a counter-clockwise circulation. This pattern reverses in the southern hemisphere. Other wind flow patterns include Pineapple Express, which is a southwesterly wind, that brings heavy rains and flooding in the Pacific Northwest during winter.
Separation of counterclockwise wind circulation from the main belt of a jet stream blowing from east to west results in a cutoff low. Its occurrence is a result of a deep trough combining with a strong downstream ridge, forcing the bottom portion of the trough to separate from the main belt of the jet stream. If there is nothing to steer a cutoff low, which is usually slow moving, it can spin for a couple of days. The land below experiences dreary weather with periods of rain and snow. Cutoff lows are associated with low pressure areas at the earth’s surface. The location of jet streams keeps shifting north during warm weather and south during cold weather throughout the year. In winter, jet streams are stronger due to the large contrast between colliding arctic and tropical air masses. However, they are weaker in summer due to lesser temperature differences between the air masses. The polar and subtropical jet streams are two of the most common jet streams, both of which are responsible for the various weather patterns and events depending on the time of the year.
The polar jet stream is formed due to air movements. These movements are associated with the cold front that separates the colder and drier air masses from the north and the warmer and moister air from the south. The polar jet stream plays a role in the development of storms and also tends to steer the storms. Steering refers to the reflection of very strong air masses, such as jet streams. The subtropical jet stream, which tends to develop during winter season, is formed by air movements in the tropical and subtropical regions and is associated with strong temperature gradients in the mid-troposphere. Normally, it is accompanied by periods of heavy convectional rainfall. Being weaker, it does not lead to the occurrence of migrating cyclones. This jet stream helps develop and steer storms and disturbances. Often, the jet stream contains jet streaks, which are portions of wind that have a higher speed than the surrounding winds, brought about by a strong temperature gradient. This creates a high pressure gradient over a horizontal distance. Since pressure increases with height, strong winds will occur at upper levels. The acceleration and deceleration points in a jet stream are crucial to weather forecasters in determining where a rain-bearing depression will form. In case such depressions occur, meteorologists need to know whether it will develop into a full-blown storm, which may cause structural damage. Jet streaks can play a very significant role in precipitation and storm formation depending on their strength and position.
Weather patterns around the world are always changing. These changes are brought about by the shifting jet streams which also contribute enormously to severe weather events like floods and drought. Therefore, jet streams are important in forecasting the weather, based on their strength and position. Research has established that in areas where jet streams are not present, the weather is relatively uniform and less violent. Precipitation, which is normally accompanied by storms, usually increases in a given area when a polar jet stream moves farther south. Similarly, a dry season sets out in a given area when the polar jet stream moves further north. Therefore, an understanding of jet streams and the continued tracking of their movement by meteorologists is necessary in order to monitor the weather around the globe.