Fog May Prevent Warm Up This Weekend
November 29, 2012 8:26 PM
As we have been talking about all week long, temperatures are expected to rise significantly over the weekend thanks to southwesterly winds ushering in much warmer air off of the western Plains. However, low-level moisture in the form of fog could prevent that from happening for some of us. The picture on your left is what fog can leave behind this time of year making for some picturesque sites, but fog can also lead to freezing drizzle and icy roadways, which is not welcomed by most. In many cases, the fog dissipates pretty quickly in the morning as the sun comes up and helps temperatures rise allow moisture levels to drop and the fog to fade away. But during the winter, it often takes longer because the sun angle is lower and there is typically more low level moisture, so what otherwise would be a warm day could turn into a chilly and foggy mess.
Lets take this weekend for example. Temperatures 5000 feet above the surface, which are not affected by the daily heating and cooling of the surface, will be rising over the next couple of days. Normally, this would allow temperatures at the surface to warm further as well. But if you have fog (clouds) preventing the sun from warming the surface, then it doesn’t matter how warm your temperatures get above the surface because it won’t make a difference. However, it does show just how warm things *could* become. Below are 2 pictures of temperatures at 5000 feet above the surface. The first one is for Thursday morning and the second is for Saturday afternoon.
Notice, there isn’t a lot of difference between the two but Saturday is still warmer. Temperatures do warm in the north by quite a bit, but the south doesn’t experience a lot of warming however temperatures are warmer nonetheless. So why was Thursday so cool and what makes Saturday and Sunday different?? Well, it comes down to the mixing factor. If you watch our broadcast enough, I’m sure you have heard me talk about mixing. This is something that helps warm the atmosphere. I’m not going to go into the details of how at the moment, so you will have to trust me. But with northerly winds at the surface as that cold front moved through on Thursday, it caused temperatures to plummet in the north with the south staying warmer on the other side of the front. Because of the stalling front, the south had a different wind direction and were able to mix the air more efficiently with higher altitudes. The combination of that and more sunshine helped temperatures warm into the 50’s in Valentine and Winner. Fog works in a very similar way as a cold front, preventing this mixing from taking place at the surface and therefore keeping temperatures cooler through the day.
Fog forms in many different ways, but the same is true for all types of fog; your temperature and your dewpoint must be very close together. This gives you a high relative humidity (fancy wording for saying that there is a lot of moisture in the air) which could eventually lead to cloud formation or what we would call fog. The most common types of fog are radiation fog and advection fog. Radiation fog is formed by the cooling of land after sunset in calm conditions with clear skies. This type of fog occurs the most, but also typically dissapates quickly in the morning. Advection fog, is formed when moist air passes over a cool surface by wind and is cooled. This is most common around here when there is snow on the ground and warmer air masses move in. We usually experience plenty of foggy days in the Spring because of this type of fog. But we could experience both types of fog Friday and Saturday morning as a warmer air mass moves into the region.
The picture below is an image I have taken from one of our forecasting models. I don’t expect you to know what any of this means but there are a few things I want to point out.
First off, on the right is a cross section of the atmosphere that has a red and green line that travel up and down. The red line is the forecasted temperature in the atmosphere for 8am Friday and the green line is the forecasted dewpoint in the atmosphere for 8am Friday. Now notice the white dashed lines that go from left to right. These lines are elevation. They count up from 1 at the bottom and go up to 13 near the top. This is elevation in thousands of feet. So at the bottom you have 1000 feet above the surface, followed by 2000, then 3000 and so on… So this picture ultimately shows you what your temperature and dewpoint are doing with height. You can see at the surface, or the bottom of the picture, both lines quickly veer to the left and get very close together. This shows that the temperature and dewpoint cool rapidly near the surface and become very similar temperatures. Your temperature and your dewpoint need to be within 4 degrees of each other in order for fog to form. From the looks of this profile, I’d say they are nearly the same so that means that fog could form in this environment.
Now lets look to the left. Look at the two columns that have been circled in blue. When the computer model calculates that there are conditions that fog could form, the bars become longer and longer pointing down. So this would be a perfect example of good conditions for fog to form.
The smaller image on the right is the opposite. This shows that conditions are not favorable for fog to form, so both columns are now pointing upwards.
The conditions that fog forms in are typically easy to predict. Clear skies, calm winds, and lots of low level moisture are prime conditions for fog formation. But fog is one of the hardest weather phenomena to forecast for. It is next to impossible to figure out exactly where fog forms, how dense it will become, how widespread it will be, and how long it will take to dissipate. There are nights that fog formation looks certain and absolutely northing happens. Then there are other nights where fog is quickly forming and you have no idea why or how. So it can be a very finicky creature. Not to mention, forecasting around it is even more impossible. Take Saturday for example. I showed you what 5000-foot temperatures are going to be like. Well, with sunshine all day, temperatures could soar into the 50’s and 60’s with no problems. But the fog suddenly becomes the problem. So lets say you have fog on Saturday through 11am. Well then you just lost a little more then 3 hours of warming. Since there are far fewer hours of daylight in the winter, it could literally mean the difference between 60 and 40. So now you see why forecasting around fog can be much harder then forecasting the fog itself, because the nature of fog is so unpredictable. If you want it warm this weekend, keep your fingers crossed that the fog doesn’t ruin it.