Climate Zones/'Cooling' at night

Lee Poulsen
Fri, 28 Oct 2005 14:48:40 PDT
On Oct 28, 2005, at 12:14 PM, Susan Hayek wrote:

>> Even
>> tomatoes stop producing new fruits during the peak hottest summer
>> months in Austin--because the nights never cool down enough to let 
>> them
>> form. (They've bred a few special heat tolerant varieties that can do
>> it, and cherry tomatoes seem to already have that ability.) In
>> California, even in areas where the days are hotter in summer than in
>> Austin, the nights are still cooler and tomatoes fruit all summer 
>> long.
> **I'm confused on the tomato thing. My parents lived in the
> California Central Valley for years (from 1952 on), Bakersfied, Taft,
> Fresno and Kerman (south portions of the Central Valley). Days  can
> be well over 100F and nights may cool to 95-97F at 2 am. When I went
> over to help them I did the gardening after midnight.
> They always had the best tomatoes, the huge ones (and the best roses).
> So when we talk of nights 'cooling' how do we define 'cool'? Do we
> mean certain amount of degrees less than afternoon temps? Or do we
> mean a set temperature?

First off, I believe the night time low temperature point above which 
tomato fruits start having difficulty forming is around 70 or 71°F. If 
it doesn't regularly drop below that temperature for long periods of 
time, then no fruit form and 30 days later or however long it takes to 
ripen, there won't be any fruit to ripen. Here is a PDF file from 
Alabama that describes this facet of tomato growing and the new 'heat 
set' tomato varieties that have been developed. 

Here are the monthly average maximum and minimum temperatures for the 
warmest months of the year for Bakersfield and Fresno, and for the 
triangle of Dallas, San Antonio, and Houston (as well as Austin) in 
Texas, which covers a much larger area than the south Central Valley of 
California. Remember than each number is the average of approx. 30 low 
or high temperatures (for each day in a given month) times 30 years 
worth of data. I.e., each value is the average of about 900 individual 
high or low temperatures measured during that month of the year. So of 
course sometimes the low will be higher than this and sometimes the low 
will be lower than this value. So it gives you an idea of about what 
you can expect the low temperature will be on any given day of a given 
month in that locality.

Looking at the California lows (especially in July and August) and 
comparing them with the Texas lows (for June, July, and August), even 
though I'm sure there are days in the summer in Bakersfield or Fresno 
where the low temperature didn't drop as low as the average, enough did 
that the average, even in Bakersfield, even in July, is still below the 
70-71°F critical point. Another thing to keep in mind is that the 
summertime temperatures in Texas are remarkably consistent day after 
day year after year. They virtually never get any kind of cool front 
all summer long. Morning after morning for weeks on end, the 
temperature always drops to 72, 73, 74 or 75°F and no lower--not from 
some time in June until sometime in September does it get any lower.

May   Jun   Jul   Aug   Sep   Oct

Bakersfield, California
84.6  92.3  98.4  96.4  90.0  80.6
57.2  63.9  69.4  68.4  63.5  54.7

Fresno, California
84.2  92.7  98.6  96.6  90.0  79.7
53.6  60.3  64.9  63.7  58.6  50.5


May   Jun   Jul   Aug   Sep   Oct

Austin, Texas
84.6  91.0  95.0  95.4  90.5  82.0
66.4  71.4  73.8  73.8  69.8  59.9

Dallas, Texas
82.8  91.8  96.4  96.1  87.8  78.4
62.6  70.0  73.9  73.6  66.7  55.8

Houston, Texas
84.7  89.8  92.3  92.3  88.0  81.3
66.9  72.5  74.5  74.5  71.1  61.2

San Antonio, Texas
85.3  91.8  95.0  95.2  89.2  81.7
65.7  72.5  74.8  74.5  69.1  58.6

--Lee Poulsen
Pasadena area, California, USDA Zone 10a

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