### Why is it Normally Windier During the Daylight Hours?

Have you ever noticed how the wind tends to be a lot breezier during the daylight hours and very light or calm during the evening or early morning? Essentially this phenomenon has to do with daytime mixing and nocturnal inversions.  The wind is an extremely interesting phenomenon to study and its presence drives weather systems throughout the world.  It can also give us a clue to how strong low or high pressure systems will be and help us determine chances of severe weather, as is the case of investigating wind shear.

The first key to understanding why it gets windier during the daylight hours is to compare winds at different heights in the atmosphere.  Below are two different charts.  The one on the left shows the surface observations in Green Bay (GRB) on August 12, 2009 at 10am.  This particular observation shows us that the wind is from the SSW at only 5 knots (about 6 mph).

Now the graph on the right (above) shows wind speeds at the 500mb height, or around 5700 meters (about 18,000 feet) off the ground in Green Bay, WI.  In this observation, the wind is measured to be 20 knots (about 23 mph) from the North.  Notice the sizable difference in wind speeds vertically above Green Bay. Wind will always be slower at the surface than at points higher above a location.

This difference is largely due to the effects of friction.  A couple of examples can help to explain this.  Imagine you have a shoebox and you place it on a long shag carpet in your living room.  You’ll notice that it is harder to push the shoebox across the carpet than if the box was pushed across vinyl flooring.  This is a great analogy to compare winds at the surface to winds farther off the ground.  Also, think back to a time when you climbed a large hill or mountain.  Did you notice that at the top it was a lot windier than at lower elevations?  This is because trees, buildings, and mountains provide friction that slows the wind down at or near the ground level.

Now that we’ve established that it is windier the farther you go above the surface, we can begin to explain why the winds pick up during the day. The main concept that explains this is daytime heating and mixing.  Below is a graphic that shows that illustrates how this occurs.  In the mornings, most areas see a temperature inversion, where the temperature higher off the ground is warmer than right at the surface.  This is due to the fact that the Earth radiates heat faster than the air does.  Due to this inversion, cooler air, which is more dense, remains at the surface and air cannot rise and displace the warmer air above it.  This is a very stable situation.  As a result, moisture is trapped at the surface allowing for higher Relative Humidities.

As the day goes on, more radiation from the sun reaches the Earth’s surface.  This allows the Earth to quickly re-radiate that heat back to the air just above the surface.  The image below illustrates this next step. The re-radiated heat then warms the air just off of the ground to a higher temperature than the pocket of air off higher off the surface.  Since warm air rises, it begins to move vertically into the atmosphere up to where the wind speeds are faster.

As a result, air must come down to fill the area where the rising surface air once was.  This mixing of air helps to pull and mix the faster winds from higher up in the atmosphere, closer to the ground, and increases the speed of the wind at the surface.  Think about the following example.  Imagine you have a box of packing peanuts and you reach down and scoop a handful from the bottom of the box and put them on the top.  Now, after you pull out the packing peanuts, there is not a hole where the peanuts once were.  Instead other peanuts move in to fill the empty space. The same holds true for the atmosphere in this explanation.

Also, more mixing of the air allows for the moisture spread out vertically and horizontally and to not become as concentrated at the surface.  The warmer temperatures at the surface and faster winds help to lower Relative Humidity values and create drier air that we are more accustomed to during the late morning and afternoon hours.

During the evening (see below), the ground cools off faster than the surrounding air.  As a result, air at the surface no longer rises and becomes cooler than the air farther off the ground again.  The inversion begins to set up again in a process called de-coupling. The inversion prevents the mixing of air and as a result, and winds at the surface die down for the overnight hours.

Understanding why its cooler near Lake Michigan in the summer.

There is another common instance in Southeast Wisconsin that is responsible for increased winds during the daylight hours.  This is the effect of Lake Michigan on producing sea breezes during the day and producing an effect we know as “cooler by the lake”.  This sets up in a similar way to the process described above.

The radiated heat from the Earth causes a small scale low pressure to form (see below) due to the fact that the air is vacating the area (less air, lower pressure). The air congregates over the water and produces a small scale high pressure (more air, higher pressure).  By nature, wind likes to flow from high pressure to low pressure.  As a result, a sea breeze forms, which we know as a lake breeze.  This provides cooler conditions along boundaries of water like Lake Michigan.

The wind is an extremely complex entity but we are very familiar with it on a daily basis.  Now you have a basic understanding of why the wind picks up during the daylight hours and why you can catch a bit of relief from the heat during the summer along the lakefront.

Andrew Winters - NWS Milwaukee/Sullivan Student Volunteer