Jet stream is a word I am sure you have heard the Weather Authority team use quite often, but what exactly is it? In this episode of Weather Word of the Week we are going to explore what it is and how it changes our weather patterns.
What are Jet Streams and Why Do They Exist?
Located at about 30,000 ft AGL (above ground level), these jet stream winds are created by an extreme north to south global temperature difference. The tropics are very warm, because that area absorbs most of the sun’s direct rays. The poles in contrast are very cold due to the limited amount of solar energy that reaches them.
The United States is impacted by two different jet streams. The primary jet stream that we focus on is the polar jet. This particular jet stream is created from the temperature difference between the cold Canadian air and the warm air across the continental United States.
The secondary jet stream is the subtropical jet. It is created by the difference in temperature between the warm continental air of the U.S. and the hot tropical air of the equator. It is strongest during the summertime, and can often stick us in southwest flow here across Arkansas.
An important fact to realize is the Earth HATES temperature imbalances! It is constantly trying to wipe them out by sending warm air toward the poles and cold air toward the equator.
This temperature imbalance drives the jet stream to form all sorts of different shapes to help the earth accomplish this task. There are two primary shapes to the jet stream that we talk about in Meteorology.
Types of Jet Stream Patterns
Zonal: This type of jet stream pattern usually features a west to east wind flow near the US. Under the influence of a zonal jet stream, weather patterns move through the flow quickly. Temperatures usually moderate and stay about the same without much change. Unless directly in the path of the jet stream, most locations experience quiet weather.
Meridional: This type of jet stream pattern has large ripples in a north-south oriented (meridional) direction. The upward punches of warm air are ridges and the downward punches of cold air are troughs. Think of it this way… when the jet stream rides high, the weather will be nice and dry. When the jet stream dips low, thunderstorms come and go. Click on the links for more information on troughs or ridges.
Weather patterns become MUCH more active when the jet stream buckles. Depending on the size of the trough or ridge, the weather can become more extreme underneath either. Temperatures also change dramatically across an area under a meridional jet stream.
What Causes the Jet Stream to Shift Positions?
The short answer is the global temperature imbalance. Remember the Earth can’t stand to have massive temperature differences. A zonal pattern traps the cold air to the north and the warm air to the south. The longer the period of time that we are under a zonal jet stream the greater the temperature differences become. Eventually the temperature imbalance overcomes the zonal jet stream and the jet stream snaps and buckles.
The size of the “buckle” depends on how large the temperature difference is. In the Fall and Spring the buckle is greater due to the active production of both cold and warm air masses. In the summer, the production of warm air greatly overshadows the production of cold air, and shoves the jet stream far into Canada. In the winter, the production of cold air overtakes the production of warm air, and allows the jet stream to dive deep across the southern United States.
The clash of air masses created from a meridional jet stream can enhance severe weather potential, especially across our area. We see two peaks of severe weather here across Arkansas. We have one in the Spring and secondary one in the Fall. The severe weather potential is higher in the Spring due to the greater abundance of moisture and warmer temperatures. The severe potential in the Fall is there, but often lacks the depth and richness of the moisture that the Spring has.
Nevertheless, if you hear our weather team mention a meridional jet stream pattern is in the forecast, you can expect the weather to change in the coming days.
That change often means that it is time to break out the rain gear and keep your umbrella close.