UPPER AIR FLOW

The height of the atmosphere The height that the atmosphere reaches a certain pressure can tell us information about the air at a certain location. This information is often put into a map called a constant pressure chart. A common constant pressure chart that is used by meteorologists is the 500mb. This chart represents the height at which the atmosphere reaches 500mb.

The height of the atmosphere Con’t Where cooler air exists aloft, this is often associated with more dense air, and lower heights that reach the 500mb level. Where warmer air exists aloft, less dense air, and greater heights of the 500mb level.

The Jet Streams There are two well known jet streams that exist over N. America and affect the weather we experience. Both are narrow bands of high speed winds at an elevation between 25,000 – 40,000 ft., having widths of miles and wind speeds of mph. 1. The mid-latitude jet stream(Polar Jet Stream) – This high speed wind band is the result of large temperature differences on the surface(especially in the winter). The large temp. differences produce steep pressure gradients aloft.

Mid- Latitude Jet Stream Jet streams are located where there is large temperature ranges in short distances. The mid- latitude jet stream occurs along the polar front. The polar jet stream generally runs west to east across N. America, but sometimes makes dips and runs north and south for a distance. The Polar jet stream travels at higher velocity during winter, because there is greater temperature variance. This jet stream migrates to the south in the winter and to the north in the summer.

2. Sub-Tropical Jet Stream The Sub-Tropical jet is only present in the winter, because during the summer there is lack of temperature contrast in the tropics. West to east flow is centered at about 25 degrees latitude. During certain conditions this jet stream will migrate to the north and merge with the polar jet.

Waves in the Westerlies The polar jet stream is the core of the prevailing westerly wind flow over N. America. Within this core, waves often form. These waves have been termed Rosby waves after C.G. Rosby, who first studied them.

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Energy Transfer and the Westerlies The earth’s wind systems are constantly trying to distribute heat equally between the equator and the poles. This heat distribution is difficult when the general flow is from west to east. This type of flow is termed a zonal flow and will cause conditions to be more mild during winter months for positions to the south of the polar jet.

Energy Transfer and the Westerlies Con’t When large temperature differences between areas start to develop, the polar front starts to meander to a north/south or meridional flow. During times of meridional flow, steep pressure gradients often organize into rotating cyclonic systems(low pressure systems) – more on this to come. The polar jet supports these systems by providing divergence aloft. During a meridional flow time, energy is transformed from equator to poles and eventually, the pressure gradient weakens and a flatter flow aloft is returned with less intense weather on the ground.