The Weather Guys

One of our readers awoke to some beautiful clouds in the summer sky recently, and those two excellent questions popped into her mind.

Clouds are composed of tiny liquid water droplets (whose diameters are about the width of a human hair) and tiny shards of ice in a variety of shapes.

Whether a cloud is mostly liquid water droplets or ice particles depends, as you might guess, on the temperature of the air in the cloud.

Tiny cloud liquid water droplets can remain in the liquid state to temperatures as low as about -10 degrees Celsius (14 degrees Fahrenheit), and when they do they are known as supercooled liquid water droplets.

These droplets feel the downward force of gravity just like a baseball or a watermelon would. But because the droplets are so small, and therefore have small masses, the gravitational force can easily be balanced by an upward friction force resulting from the interaction of the droplets with the air molecules around them. The droplets remain suspended, and that’s what holds clouds up in the air.

When these droplets grow, they gain mass and eventually the gravitational force overwhelms the friction force and the now-larger droplets fall to the surface.

The fluffy appearance of the tops of some clouds are evidence of convection, when buoyant air parcels within the cloud literally bubble to the top. As the air rises, it encounters environments with lower and lower pressure and cools by expansion.

This cooling increases the relative humidity of the air. Once that relative humidity gets to 100%, condensation of the invisible water vapor begins to produce liquid water droplets. The bottom of clouds often appears flat because the first level at which rising air parcels begin to condense is usually rather uniform over a given region. This level is known as the lifted condensation level — that is, the level at which lifted air parcels first begin to experience condensation.

Steve Ackerman and Jonathan Martin, professors in the UW-Madison department of atmospheric and oceanic sciences, are guests on WHA radio (970 AM) at 11:45 a.m. the last Monday of each month. Send them your questions at stevea @ssec.wisc.edu or jemarti1 @wisc.edu.

The accumulated lack of precipitation we experienced the first part of this year has produced severe drought conditions in southern Lafayette, southern Walworth, Racine and Kenosha counties as of the end of last week.

Moderate drought continues across southern Sauk, southern Columbia, Dodge, Washington, Ozaukee, Iowa, Dane, Jefferson, Waukesha, Milwaukee, Green and Rock counties as well.

Since Aug. 1, 2020, Madison has recorded a deficit of 10.34 inches of precipitation. That translates to only 71.6% of the area’s normal amount of precipitation over that interval.

Most alarming is that the growing season months of April, May and now July have contributed 7.04 inches of that 10.34-inch deficit. As a result, 33% of the region’s top soil and 35% of its subsoil moisture is currently considered “short” or “very short” by the National Weather Service’s U.S. Drought Monitor.

Some crops are beginning to show signs of stress under these conditions.

To make matters worse, there appears to be no relief on the immediate horizon. Any precipitation in the next seven or so days — if it occurs at all — will likely be the result of local thundershowers and will not be associated with the passage of a well-organized, “synoptic-scale” weather system (covering hundreds or thousands of miles) capable of delivering widespread precipitation.

In addition, the monthly climatological drop-off in precipitation from August (4.27 inches is normal) to September (3.13 inches is normal) is the largest such drop-off in the calendar year.

Consequently, a wet August is perhaps our last best hope for returning closer to normal and forestalling a compromised growing season.

Steve Ackerman and Jonathan Martin, professors in the UW-Madison department of atmospheric and oceanic sciences, are guests on WHA radio (970 AM) at 11:45 a.m. the last Monday of each month. Send them your questions at stevea@ssec.wisc.edu or jemarti1@wisc.edu.

We are in the midst of another hot spell here in southern Wisconsin.

Last Friday was the first of several days that were forecast to see high temperatures at or above 90 degrees (we’ve had only six through Friday all season!).

The high on Friday only reached 88 as the sky was a bit more obscured than was forecasted.

The obscuring factor was a combination of high cirrus clouds and smoke from the Western wildfires. The first of those factors, high cirrus clouds, is a common summertime suppressor of the high temperature.

Their exact location, timing and duration are very hard to predict accurately and so they are often the primary reason why a given summer day’s high temperature forecast might be too high.

Despite the difficulty of forecasting high clouds, the numerical models used by the National Center for Environmental Prediction (NCEP) are designed to simulate their development through the routine inclusion of humidity, wind and temperature data in the models.

The other factor conspiring to lessen the heat on these several days, wildfire smoke, is not well accounted for in those same models.

Because such large-scale smoke plumes are only episodic it is much more difficult to confidently account for their presence in these forecast models.

Smoke definitely reduces the amount of sunlight that is absorbed at the ground and so conspires to keep the temperature a bit cooler than it might have been in the absence of the smoke.

So, if the forecasted high temperatures over the weekend and early this week seem to be systematically a bit too high, it might well be a direct result of the wildfires that are raging in the Western states.

Steve Ackerman and Jonathan Martin, professors in the UW-Madison department of atmospheric and oceanic sciences, are guests on WHA radio (970 AM) at 11:45 a.m. the last Monday of each month. Send them your questions at stevea@ssec.wisc.edu or jemarti1@wisc.edu.