January is one of the cloudier months of the year in Green Bay so it shouldn’t be much of a surprise to see the lack of sun we’ve experienced the last several days. So what’s the reasoning behind all the cloud cover lately and what needs to be done for these stubborn clouds to finally move out? Let’s take a look at the science behind this stretch of cloudy weather.
To better understand what is creating the cloud cover we will use what is called a Sket T/Log P diagram. Essentially, this is a plot that displays data gathered from a weather balloon. These plots show information such as temperature, humidity, pressure, and use a GPS unit to determine wind speed and direction. All of this data is then displayed on a chart for meteorologists to get an idea of the current state of the atmosphere in their area. Here’s what one of those plots look like.
This may look like a bunch of lines, which it is, but there is some vital information on this display that is critical for meteorologists. We won’t go into too much detail on this, but here are a few important things we’ll focus on. First, the bottom of the plot represents the ground and the height increases to the top. Essentially we are looking at a vertical profile of the atmosphere. The blue horizontal lines represent a pressure level measured in millibars while the blue slanted (or skewed) lines indicate temperature. We’ve highlighted the freezing, or 0°C line. To the left of that line temperatures are below freezing and to the right temperatures are above freezing. The green squiggly line you see plotted represents the dew point temperature and the red line is the air temperature.
A general rule of thumb is that for clouds to begin to form the air temperature (red) and dew point temperature (green) need to be within 5°C of each other. When we look at this plot we can see that when both of these lines get very close or are touching that there is cloud cover. This is evident near the surface or towards the bottom of the display. As we move up the plot towards the middle and upper portions you can see the lines distance themselves indicating drier air and no cloud cover.
Let’s zoom in on the lower portions of the diagram to further investigate the reasoning behind the clouds.
As previously mentioned, the closer the red and green lines are to each other the higher the humidity and likelihood for clouds to form. We can see this is true for the bottom half of this plot or about the first 5,000 feet above the surface. Once we get above 5,000′ the lines get further apart indicating drier air.
Typically air cools as you increase height above the surface, but there are situations where the air can actually get warmer with height. This is called an inversion. We have a perfect example of that on this plot and is helping to contribute to the stubborn cloud cover we have been experiencing.
Starting from the bottom of the plot and moving upward you see the red line slants to the top left, this is indicating temperatures cooling the higher up into the atmosphere we go. Something happens though around 4,000 feet. Notice how the red line then switches direction and moves to the right! This is the inversion or the air getting warmer with height. This is often times talked about during severe weather season as a “cap” or “lid” preventing air from the surface to rise. This is very much a similar situation. The air below this inversion is essentially trapped below the shallow layer of warm air above it.
The inversion limits the amount of mixing the air can do to help lower the humidity and clear out the skies. That, combined with added moisture from melting snow recently, is leading to higher humidity near the surface and producing the clouds that we have seen.
So how do we get rid of an inversion? Well, there are a few ways, but what’s most likely to happen this time of the year is for a change of air mass to occur due to a passing storm system such as a cold front which could happen later this weekend. This would help mix up our air currently in place and eventually lead to some drier conditions as well. Until then, our air mass will pretty much stay the same and this will continue to lead to a good amount of cloud cover across the area. Good news is that as we move through the next few months we will be inching closer to a sunnier time of year!