Global winds Jet Streams
In Friday's lecture, we discussed small scale localized winds. Today we will discuss winds that exist on the synoptic or planetary scale.
Planetary winds
Due to the unequal heating properties of land versus water, horizontal temperature and pressure gradients set up on large scales across the planet. Large scale average winds set up in specific locations. For instance, in January, the waters of the North Pacific Ocean south of Alaska are much warmer than the land regions of Alaska and Siberia to the north and west. As a result, a region of lower pressure sets up permanently over this part of the Pacific and is called the Aleutian Low. This semi-permanent feature helps to describe the general wind circulation in this part of the world.
At different latitudes, different pressure patterns tend to set up. For instance, at the Equator, a somewhat permanent area of low pressure develops. However, in the vicinity of 30 N and 30 S latitude, there is a somewhat permanent area of higher pressure.
A permanent feature of this part of the world is a circulation called Hadley Circulation. Air from the 30 N/S region migrates toward the lower pressure that has set up over the Equatorial regions. Since air is approaching the Equatorial areas from the north and south, convergence, thus vertical motion and convective development, will occur. However air is sinking in the 30 N/S region so a somewhat permanent area of high pressure is present providing tranquil, calm weather.
This region of converging air is also called the Intertropical Convergence Zone (ITCZ). The regions in the 30 N/S vicinity under the influence of a semi-permanent high are sometimes referred to as the Horse Latitudes.
The Bermuda High
During the summer months in the Northern Hemisphere, a semi-permanent high often sets up in the western Atlantic east of the Carolinas in the vicinity of Bermuda, hence the name Bermuda High. The Bermuda High has a huge influence on the weather of the eastern United States.
Several consecutive days of hot, sticky, humid weather often set up over the eastern United States during the Bermuda High. These types of days are often referred to as the Dog Days. This name actually comes about because the time of year that they occur (July and August) is when the star Sirius which is also called the Dog Star is visible.
The Siberian High
During the winter, a dome of extremely cold and dry air sets up over the eastern part of the former Soviet Union in Siberia. Occasionally, chunks of cold air break off and migrate southward into Canada and the United States. This often causes several days to weeks of extremely cold weather. This phenomena is sometimes referred to as the Siberian Express.
Jet Streams
A jet stream can be thought of as a fast moving current of air in the upper atmosphere. The jet stream occurs as a result of horizontal gradients of temperature and pressure. Winds in the jet stream blow at speeds of upwards of 100 mph. The jet stream can provide energy to developing storm systems.
Storms that form just to the north of the jet stream gain energy from positive (counterclockwise) rotation as evidenced in the picture below.
