West-wind jet

The weather of the Baltic Sea is mainly determined by a westerly air flow at the surface and in the upper air. This west-wind jet stream is particularly marked in the upper air over central and northern Europe and thus over the Baltic Sea. The jet results from the general circulation of the atmosphere over the Northern Hemisphere. 

Over the European continent, the North-Atlantic cyclones weaken during their eastward propagation, but often they cause the weather to be quite variable. That means, a very diverse but usually heavy cloud cover with occasional precipitation prevails over the Baltic Sea. It is generally moist, cool, and rather windy. The global west-wind jet carries moist and cool air masses from the North Atlantic and the North Sea to central and northern Europe (Nehring et al., 1990). This leads to a predominantly maritime influence on the Baltic Sea region. This... 

In the winter, the weather of the north-eastern and northern parts is increasingly affected by the continent because this region is generally located at the northern or polar side of the west-wind jet. Continental cool air moves frequently from the intense Siberian winter high into the Baltic region. In particular in winter, the southern part of the Baltic remains very often in the center or south of the west-wind jet and thus on its warm side. As a consequence, winters in the northern part are often severe, while they remain usually mild in the southwestern Baltic. 

These gales arise predominantly in the winter when thermally different air masses collide over the North Atlantic and Europe. In the winter time the west-wind jet—the strong wind field in 500 hPa (about 5 km) altitude—is frequently located over central Europe, often directly over northern and central Germany (Fig. 4.2). 

This means that particularly during the winter months central Europe remains more and more frequently directly beneath the west-wind jet of the upper atmosphere, and thus in a strong, but mild and moist air flow from the North Atlantic at the ground. This relatively pronounced warming in winter and spring is reflected by the apparent warming of the months from January to April by about 0.2-0.4 C. 

The tropical regions of both of the hemispheres troposphere exhibit easterly flow called the trade winds. Finally the jet stream -sometimes described as a river of air - flows at mid-latitude of both hemispheres with velocities of 25 to 50 m/sec from west to east, often carrying material completely around the Earth at its altitude close to the tropopause. It is in this flow that balloonists attempt to circle the globe. 

Jet stream—High speed winds that circulate around the Earth at altitudes of 7-12 mi (12-20 km) and affect weather patterns at the surface. Two important jet streams are the subtropical jet, flowing from west to east, and the weaker tropical jet, which flows from east to west. 

Figure 1-6 illustrates the mean zonal wind field for the northern hemisphere. The situation in the southern hemisphere mirrors that of the northern, although not quantitatively. In midlatitudes the dominant wind direction is from west to east, with maximum velocities in the vicinity of the subtropical jet stream near 30° latitude. The location of the polar jet stream associated with the polar front is more diffuse, and it does not show up in the averaged wind field. The westerlies encircle the globe in a wave-like... 

Fig. 1-6. Mean zonal circulation in the northern hemisphere, 0-20 km. Distribution of wind velocities (in units of m/s) was taken from Labitzke (1980). W, Mean winds from the west E, mean winds from the east the heavy lines indicate the approximate location of the polar front, the broken lines the tropopause. The maximum wind speed coincides approximately with the subtropical jet stream. The location of the polar jet fluctuates considerably and does not show up in the average. The center is to illustrate wind directions near the earth surface (trade winds and westerlies) cyclones (C) and anticyclones (A) imbedded in the westerlies are only sketched the frontal systems associated with cyclones cannot be shown in this extremely simplified diagram.

El Nino refers to a sequence of changes in the ocean and atmospheric circulation across the Pacific Ocean. The water around the equator is unusually hot every two to seven years. Trade winds normally blowing east to west across the equatorial latitudes, piling warm water into the western Pacific. A huge mass of heavy thunderstorms usually forms in the area and produce vast currents of rising air that displace heat poleward. This helps create the strong mid-latitude jet streams. The world s climate patterns are disrupted by this change in location of the massive cluster of thunderstorms. The West coast of America experienced a wet winter. Sacramento, California recorded 103 days of rain. 

As we get to the top of the troposphere (the tropopause), winds get stronger, without change of pattern. Of course, this description covers average flow only, but even winds on individual days show the same dominant west-east flow however, the strongest winds may occur at different latitudes and may be strongly concentrated, especially above surface fronts (jet streams). Wavelike perturbations may have much larger amplitudes, with occasional closed circulation patterns even in middle latitudes. 

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