The jetstream (also known as "jet stream", "upper level winds", or simply, "the jet") is a band of fast moving air found at high altitudes. It is usually over mid-latitudes, but fluctuates to the north and south on a short time scale.
Let's learn some terms associated with the jet. They will be used throughout this page.
mb or millibar - a measure of pressure equal to 1 hPa.
pressure surface (for example, the 250 mb pressure surface) - a surface with varying height such that each point on the surface has the given pressure.
Real life example
The following picture shows the intensity of the jetstream at the 250 mb surface as of the morning of September 27, 2005. Click to enlarge
The following picture is a cross section of the atmosphere at the exact same time. It is a vertical section along the 180 degree longitude line from 30 north latitude to 60 north latitude (which appeared to be the strongest flow in the previous map). The scale on the y-axis is the pressure (which is proportional to vertical height). Plotted in color is the velocity of the jet. Plotted with lines is the temperature. Click to enlarge
This is an average vertical cross section for the Northern Hemisphere, and the month of September. The North Pole is on the left, and the equator is at the right.
Click to enlarge
As we have seen in the previous section, there is a north-south temperature gradient. It is warm at the equator and cool at the pole (at the ground). This gradient is reversed at the very top.
The temperature gradient induces a north-south pressure gradient. This pressure gradient is strongest in the middle layers of the atmosphere, and relatively weak at the top and bottom of the atmosphere. We would expect the strongest flow, then, to be in the middle layers, and flow from south to north (high pressure to low pressure), but this is not the case, because we live on a rotating Earth.
Click to enlarge
The rotation of the Earth leads to the so-called Coriolis effect, deflecting air parcels in the Northern Hemisphere to the right. This effect exactly balances the presure gradient effect. Therefore the jet flows from west to east (instead of south to north), in general.