The Weather Guys

Tides in the ocean are caused by the gravitational force between Earth and the moon. There are also atmospheric tides.

Lunar gravity affects the density of the thermosphere, which is the largest layer of the atmosphere. This is also where many satellites and the International Space Station orbit Earth. This lunar-induced drag is small, but it has to be included in the models used to predict the satellites’ orbits. The moon also affects the pressure at Earth’s surface.

When the moon is overhead, its gravitation pull causes Earth’s atmosphere to bulge toward it. Since the pressure at the surface is determined by the amount of air above you, the pressure, or weight, of the atmosphere on that side of the planet increases.

The fact that air pressure changes were correlated to the position of the moon was first detected in 1847 using ground-based measurements. The change is very small and was detected using careful statistical analysis on a long data record. A correlation with temperature at the surface was found in 1932.

A relatively recent study by atmospheric scientists at the University of Washington found that rain is slightly lighter when the moon is higher in the sky. You wouldn’t notice this difference as the change is only about 1 percent of the total rainfall.

Which is also not enough to impact other aspects of weather. The scientists discovered this by careful statistical analysis of special observation made from satellites over a 15-year period, from 1998 to 2012.

Our string of beautiful days at the end of last week were related, believe it or not, to the presence of the Rocky Mountains hundreds of miles to our west.

Last week, the atmospheric flow at levels just above the mountains’ height was oriented almost directly across the high terrain. In such a case, like water flowing over a pebble in a stream, the air is forced to sink on the downwind — or lee — side of the mountains. As air sinks, it moves to higher pressures and is compressed. Upon being compressed, the air warms. Thus, whenever the flow above mountain height is directed as it was last week, very warm temperatures develop over Nebraska and the Dakotas.

The resulting warm air is then pushed eastward by the winds and is dragged toward the Great Lakes in a process known as “warm air advection.” On Friday that warm air advection was very strong and led to a high temperature for the day of 79 degrees. If not for some wispy high clouds early in the afternoon on Friday, we may well have reached 80 degrees or higher.

The last time Madison was officially 80 degrees or warmer was Sept. 2. The earliest day on which Madison has ever recorded its last 80-degree day of the year was Sept. 2, 1977. Thus, since the mercury stopped at 79 on Friday, we now have a very good chance of tying that unusual record — even though the early fall has been very pleasant.

Just for completeness, the all-time latest 80-degree day in Madison’s history is Oct. 23 so, if we fail in the first quest, perhaps we can succeed in overturning the second.

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.

Over the past weekend southern Wisconsin experienced its first cold snap of the season with widespread morning lows in the lower 30s on Friday and Sunday mornings.

Very often cold snaps in the autumn are very short-lived as this recent example was, affecting usually one or two nights at most.

There are a variety of reasons for this brevity.

First, cold snaps at this time of year require a substantial southerly excursion of cold air from high-latitude Canada to get us cold.

Though the Arctic night creeps ever farther south each day after the autumnal equinox, its southerly progress is slow.

Consequently, cold air production is limited to the very highest latitudes well into November, which means that any cold air that makes it as far south as Madison is not well-connected to a broader reservoir of cold air that would allow the cold to remain.

Second, there is little snow on the ground even in central Canada at this time of year and so the cold air that does migrate southward is modified (warmed) to a greater degree on that migration at this time of year than in the late fall and winter, when snow cover is widespread.

Third, the lack of snow on the ground in Madison itself limits the longevity of cold snaps. That’s even true in the wintertime.

Overnight cooling over a snowfield can reinvigorate the chill of a cold snap and render it longer-lasting as a result.

So, if you were complaining about the cold this weekend, be glad we are still in October and much more likely to experience a quick rebound.