The Weather Guys

The first two days of this month had high temperatures of 87 and 84, respectively, and daily average temperatures (the average of the daily high and daily low) that were 15 and 19 degrees above normal.

In the next 12 days, the daily average temperature has been just shy of 6 degrees below normal. In fact, during that nearly two week stretch, nine days have had overnight lows in the 30s, including a streak of eight straight days from May 7 through Friday. The morning low on Tuesday was 30 degrees — notably cold and memorable for most of us, but only the 61st coldest morning in the first half of May in Madison’s history, far behind the all-time lowest of 19 degrees recorded on May 1, 1978.

However, the streak of cold mornings we have just endured is much more unusual. In fact, only four eight-day streaks of early May mornings with a low temperature at or below 39 degrees have occurred in Madison history — in 1971, 1976, 1989 and this month.

During this unusual spell of persistent cold nights, we have also been falling behind in precipitation. For the month of May, we are already 0.92 inches below normal and we are only halfway through the month. Coupled with our unusually dry April (and dry winter months of December-March) we are currently running 5.21 inches below normal in precipitation since December 1.

That is a substantial deficit which will require excessive late spring and summer rains to remedy. So, with the near-certain approach of warmer weather, attention to our accrued water deficit will become more urgent.

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.

Last week’s snow showers garnered their fair share of attention.

The showers were intermittent, a snowy period often followed by a break in the clouds through which an interval of sunshine prevailed. On both days this showery activity ended near or just before sundown.

The conditions that foster such behavior are interesting but not all that uncommon. The way in which the temperature of the air varies with increasing height above the ground determines the atmospheric stability. It turns out that when the difference between the surface temperature and the temperature some distance above the ground is large, the atmosphere is very susceptible to the development of showers.

Our showery days last week were characterized by air temperatures at about 3 miles above the ground that were near minus 35 degrees Celsius, while the surface temperature was about 5 degrees Celsius. That 40-degree temperature difference rendered the stratification quite unstable — especially to the ascent of saturated air.

Consequently, any initial movement upward of air bubbles near the ground resulted in those air bubbles rising freely to much higher elevation. As air rises it expands into the lower pressure environment and the expansion forces cooling which increases the relative humidity of the air. Eventually such freely rising bubbles of air produce clouds and precipitation.

The scattered nature of the snow showers resulted from the fact that the regions of ascending air just described are surrounded by sinking air on their sides. The sinking forces the air to warm and dry out — thus producing the cloud-free regions between the showers. In such cloud-free regions, the surface temperature can rise in response to sunlight, which initiates another round of ascent and cloud production.

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.