The Summer of 2012 in Southeast Lower Michigan will be remembered again for its heat and relative dryness. It seemed as though summer would never end with all three months bringing above normal temperatures; June and July well above. Not only did temperatures average well above normal but the number of days of 90 degrees or better surged to 30 at Detroit (two were 100+) through August; Flint tallied up 28 (also two 100+) and Saginaw with just 22 (one 100). On average Southeast Lower Michigan "feels" 8-12 days of 90 or above.
Summer Temperatures /departures from normal/
For two summer's in a row; Detroit's summer average (or mean) temperature placed it in the top 5 hottest summers. In addition; that 74.8 degree average attained this summer was the second time it averaged that warm...the first being back in 2005. Therefore; 2012 and 2005 are now tied for the hottest summer on record in Detroit and Detroit only. Looking at both Flint and Saginaw paints a different picture (especially at Flint) with the Summer of 2012 being only the 13th warmest summer at Flint and the 4th warmest at Saginaw. Note the big difference in Flint's average temperature /71.7/ and Detroit's /74.8/. That's a solid three degrees /3.1/ difference with Saginaw averaging a bit higher at 72.0 (or 2.8 degrees lower than Detroit). Why the big difference in average temperatures? In my opinion; there's three reasons this summer and leading off in order of precedence:
1 - The good ole' metro Detroit heat island.
And to a lesser extent (and the precedence of 2 and 3 probably could be interchanged);
2 - Normally (negating the heat island) Detroit is warmer than Flint or Saginaw anyway.
3 - Generally it was drier around Detroit and thus; temperatures would be able to warm and rise easier with less moisture in/above the landscape. Now; one might say "given the drier conditions temperatures should also normally fall off easier at night", true but remember reason #1 and the heat island greatest effect on temperatures is at night (and many times, at least twice or more than the added departure on afternoon highs). Therefore; the heat island offsets much of the normal cooling of the nighttime, relatively drier air.
One would have to wonder; given the heat reflected in the 1930s, if at least a few of those hot "dust bowl" summers would have remained nearer the top at Detroit had it NOT been for the urban heat island sprawl. The highest 1930s warmer summer in Detroit has now been relegated down to 8th place with a 1933 summer average temperature of 74.0 degrees (1931 is 15th @ 73.5). I will say however; some of Flint's average summer stats in the 1930s look somewhat suspect (high) with all top five hottest summers in the 1930s. Some of those summers averaged higher than in Detroit (particularly 1934 and 1936): 1933; 74.2/74.0 - 1934; 74.0/72.7 & 1936; 72.7/71.2). Hmmm. F / D F / D F / D
Warmest Summers on Record
Rains came too late (and some areas way too much, too late)
The summer also had notable dry periods; especially in July and was accompanied by intense heat. Driest totals for the entire summer extended from I-69 south across the southern two thirds of the region. The rains re-appeared in August; most noteworthy around the Saginaw region which got deluged with over eight inches /8.03"/ on the 9-10th leading up to a whopping 9.43" for August...wettest August on record at Saginaw!
Summer Rainfall /percent of normal/
Fall Color Time
What does all this mean for the annual fall foliage display? Actually not as much as one might think. Fall leaf color is basically caused by lack of sunlight and to a lesser extent is influenced by the late summer weather. However; drier than normal weather for the entire summer into early fall tends to accelerate the leaf changing process, causing the leaves to fall prematurely. Likewise, a wet late summer into September and October will tend to produce fewer vivid colors and the leaves may also fall earlier due to the rain, wind and storms.
The prime weather conditions which are conducive for brilliant fall colors are warm, sun dominant days and cool, crisp nights but without frosts or freezes; such as high temperatures in the 60s and 70s with lows in the upper 30s to lower 50s. These sharp, daily temperature swings and more importantly, the decrease in sunlight, play vital roles in the development of the leaf color. This combination of weather and lack of sunlight, creates a blocking effect on the sugars which are manufactured in the leaves and keeps them from reaching the root system. Eventually, these sugars convert to pigments that produce the vivid and brilliant colors seen on many trees in the fall. Evidently, the green chlorophyll in the leaves begins to fade during the shorter fall days with subsequently, less sunlight. Thus, the other color pigments already in the tree leaves are exposed, come out and produce the fall color splendor. The yellow color seen in some leaves is created by the xanthophyll pigment, while the orange-red color is caused by the carotene pigment and the red-purple color can be attributed to the anthocyanin pigment.
While color peak may vary season to season across Southeast Lower Michigan, generally the maximum leaf color occurs during the second and third week in October. This appears to be a bit ahead of schedule especially if drier than normal weather continues dominates; excepting the Saginaw Valley region. While there's been a notable increase in rainfall the past month; dry conditions still prevail.