The Art of Mixing

Looks like the next couple of days are going to be reasonably warm in Sioux Falls. But, it's not going to be nearly as warm as it could be. The reason for this is poor atmospheric mixing. The ability for the air to mix with itself at different levels of the atmosphere is a key part in how much the temperature can warm during the day. The more mixing that occurs, usually the warmer it will get. On a typical day, the atmosphere is able to mix the bottom 1000-2000 feet. This means that the air 2000 feet above the surface can eventually find its way to the surface where you and I are standing. On windy days, air 7000 or 8000 feet above the ground can actually find its way to the surface. This is where wind gusts come from. When faster winds at the upper levels of the atmosphere are mixed down to the surface. Now, the reason why you get so much more mixing on a windy day is because the winds are traveling faster which in turn, they can travel greater distances. Well, to get this kind of mixing even on a windy day, you need a unidirectional wind. That means that you need your winds to be coming from the same direction. If you have wind coming in from opposite directions, then they just cancel each other out and you get very little mixing to occur.

So, in explaining all of that, we get to what's going to be happening over the next couple of days. Well, we will be seeing some breezy conditions, however, the winds will not be unidirectional. This will inhibit our mixing causing temperatures to stay cooler then their potential. The pictures below are a profile look at the atmosphere. The wind barbs are on the right and are circled. The first picture is a forecast for today at 4pm. The barbs are all facing the same direction which means more mixing and warmer temperatures.

The second picture is a forecast for tomorrow at . The barbs near the surface change drastically with height. This is not a good scenario for mixing and that's why temperatures will be cooler at 11 degrees.So, even though temperatures are going to be warmer then they have been, they aren't going to be as warm as they could be.

~KDLT Meteorologist Cody Matz