On a normal early-October afternoon, Chicagoans can expect daytime temperatures in the upper 60s — that’s the climatological expectation — but, as humorist Mark Twain once quipped, “Climate is what we expect; weather is what we get.” And the weather Chicagoans are likely to get in upcoming days includes temperatures as much as 15 degrees below normal along with clouds and considerably more rain than we want. Despite the overall chilly pattern, Tuesday’s temperatures spike briefly to near 70 degrees. As of 9 p.m., Thursday’s rain totaled 0.49 inches at Midway, nearly equaling September’s full-month total of 0.54 inches.
Suburban temperatures dipped into the middle and upper 30s early Thursday morning. Those readings, in combination with calm air and a clear sky, were sufficient to produce the autumn season’s first frost in the metropolitan area. Weather observers in Arlington Heights and Mundelein observed patchy light frost on grass and roof tops. The early-morning minimum temperature at Mundelein was 36 degrees, as reported by Phil Rider.
Why is it that clouds are flat at the bottom and have irregular shapes on top?
You have described cumulus clouds, the “dab of cotton” clouds that populate the sky on warm afternoons. When warm air ascends in rising currents, its pressure falls and its temperature drops at a steady rate of 5.4 degrees per 1,000 feet of vertical climb.
Rising air, if the process continues long enough, eventually ascends to a critical height and chills to a critical temperature at which its load of water vapor suddenly begins to condense. That defines the flat bottom of a cumulus cloud.
However, ongoing condensation at the cloud top, rather than being limited by specific air pressure and temperature values, occurs in irregular surges determined by constantly varying factors like available moisture and updraft strength.
The fall colors have reached their peak in
south central Alaska. I took these photos in the Anchorage area–from
Girdwood to the southeast on the Turnagain Arm to Palmer an hour north
of Anchorage off the Glenn Highway. Autumn colors here are always
spectacular but this year’s have been especially vivid and are quite
remarkable because the peak colors are occurring simultaneously across
the entire area. It’s not unusual for colors in the Palmer area an hour
north of Anchorage to be a week ahead of those near Girdwood–an hour’s
drive south. That’s not the case this year (in 2009). Also fascinating
to watch is the snow at the tops of the mountains in the area. It’s
been proceeding to lower elevations qite steadily. The past week has
seen mid to upper 40s by day and 30s at night. Snow usually reaches the
valley floor here the a few weeks into October.
In marked contrast to the relatively quiescent weather regime that dominated weather across much of North America during September, the October atmosphere has become strongly energized. It’s part of the annual cycle of the seasons during which the atmosphere makes a turbulent transition from summer to winter. And now, computer models indicate that we can expect a parade of vigorous storm systems in the next few weeks. The first system brings rain and plenty of it to Chicago and the Midwest from Thursday into Saturday. That’s good news for Wisconsin, which has been struggling through several months of far subnormal precipitation that has built into a drought situation across much of the state. This system’s rainfall is likely to double Chicago’s full September total.
And here we go again
A second vigorous storm system heads into the Midwest quickly on the heels of the first. And like the first rainy spell, it too will be attended by abundant moisture and copious rainfall. Preliminary estimates suggest this storm has the potential to deliver one to four inches of rain to the Midwest — on top of one to four inches from the previous system.
What exactly does it mean when the forecast says there is a 50 percent chance it will rain for a given area? Does it mean there is a 50 chance it will rain in that entire area, or that 50 percent of that area will definitely get rain
James Shaw, Northrop Grumman Corp.
Neither of your statements is precisely correct. In weather forecasts, the probability of rain is a so-called “point probability,” the chance of measurable precipitation (at least 0.01 inch) at a specific location, such as at your residence. Regarding a 50-percent forecast, the forecaster may believe the entire area will experience rain if it rains, but his confidence that rain will actually arrive is only 50 percent. Or he may believe scattered showers will definitely occur, but affect only 50 percent of the area. Regardless of the weather situation, the meaning for you is always the same: a 50 percent chance of rain on your head