Low –level Jet

[flynhgh31]

Low –level Jet

During a morning flight, you may have noticed that the winds were quiet on the surface, but screaming just a few hundred feet above the surface. You have probably noticed this phenomenon particularly on morning flights in the late spring to early summertime, when the frequency of what is referred to as the low-level jet reaches a maximum. The low-level jet often peaks in intensity at night right above the surface.

There are several complex explanations as to why the low level jet forms, but the primary reason is caused by fluctuations in the wind that are created when the low levels of the atmosphere are no longer influenced by the daytime surface heating. The low-level jet forms because the daytime mixing caused by surface heating is not sustained overnight.


The low-level jet generally peaks in intensity between 400 to 750 meters (1300 to 2500 feet) above there surface, but its effects can be found much closer to the surface. This is a problematic area in meteorology, because many of the instruments that we use do not measure winds aloft. Instrumented balloons, which are launched twice a day from various sites across the US, record wind, temperature and moisture information at various heights through the atmosphere. Because the cost of each one of these balloons is around $ 500.00, the network where each of these launches occur is sparse. Another tool located in the Midwest is in the wind profiler. As mentioned in a previous article, this instrument does not take a measurement below one-half of a kilometer above ground level, which can be above the maximum peak in the low-level jet.

To aid with this sampling problem in Central Iowa, the Central Iowa National Weather Association has considered instrumenting a 2000 foot radio tower located just north of Des Moines. The idea was to place various weather instruments at various heights on the tower that would record various weather information including temperature, dew point and wind speed and direction. This would likely be a costly endeavor as each instrument pack would cost around $800.00. The sustainability of the instruments due to damage from birds and icing became an issue, and this project has been placed on hold indefinitely. However, wind data at these levels would be extremely helpful for balloonists and meteorologists alike in determining the presence and strength of the low-level jet and other meteorological features.