A friend of mine was travelling from Regina to Toronto yesterday.  He called from Saskatchewan to tell me that the plane coming in from Toronto was going to be late arriving because of weather.  It’s not winter, we’re not dealing with icy runways, snow clearing or deicing.  There were thunderstorm in the vicinity, but no rain was reported at the airport. The meteorologist in me had to get to the bottom of this. I checked the charts and found a very well defined low level jet or LLJ.

The “regular” jet stream is a fast flowing, narrow current of air about 11 kilometers above the surface of the Earth.   It forms at the boundaries of adjacent air masses with significant differences in temperature. When the Jet Stream is south of us, cold air is allowed to drain down from the North; when it pulls north of our region, we are treated to a nice warm-up.

The low level stream is below the upper level jet. As the name implies, it’s a fast moving ribbon of air in the low levels of the atmosphere.  It can draw Gulf moisture and warmer temperatures to the North at speeds ranging from 50 to 130 km/h. There are two types of low-level jets: the nocturnal low level jet and the mid-latitude cyclone induced low level jet. Yesterday, aircraft were dealing with the latter!

This low level jet is caused by the cooling of high elevation air relative to the air at the same height - but further east. This causes a pressure gradient to flow from the warmer air toward the cooler western air. When you add the Coirolis effect, wich rotates windflow in a counterclockwise fashion in the northern hemisphere, your LLJ flows from the south. 

The wind shear associated with the low level jet is of particular interest if you’re flying. LLJ can cause severe wind shear and turbulence during take-off and landing. It's been known to snap the wings off small plane 

While I understand that waiting in an airport is no fun when all you want to do is get home to your loved ones, arriving safely is the ultimate goal!

 

 

Chief Meteorologist

Cindy Day