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The yearly sums of (+) values and (-) values describe the yearly precipitation regime at a location, as illustrated for
Missoula in Table 4. The sum of (+) values does not include any month when mean temperature is less than 400 F-the
approximate lower limit for plant growth activity. At lower temperatures the level of soil moisture content is not considered
as a major factor limiting plant growth. Also, in calculating the yearly sum of(+) values the maximum value used
for any month is +40. This value represents a rather humid month-equal to an average monthly precipitation of 4.51
inches occurring on 10 rain days for a month having an average daily mean temperature of 600 F. The highest possible
humidity value for a location is 12 times 40, or +480. The (-) values for all bum months-designated as "B" months-are
totaled, regardless of monthly mean temperature, in calculating their yearly sum.
N Table 4
Missoula, Montana
Month Av. Av. Av, PD Humidity values
* mean monthly rain days, value from Figure 26 f]
temp. prec. > 0.1 in. (+) value () value
(OF.) (Ins.) (No.)
I Jan. 22 1.3 5.9 7.7 (7.4)
Feb. 27 0.9 4.4 4.0 (3-5)
Mar. 36 1.0 5.2 5.2 (4.0)
Apr. 45 1.0 5.2 5.2 +3.0
May 54 2.0 5.9 11.8 +7.9
June 60 2.1 6.7 14.1 +9.0
July 68 1.0 3.7 3.7 -4.0
Aug. 66 0.9 3.7 3.3 -3.9
Sept. 56 1.3 4.4 5.7 +1.4
Oct. 46 1.2 4.4 5.3 +2.9
Nov. 33 1.2 5.9 7.1 (6.2)
Dec. 25 1.3 5.9 7.7 (7.3)
Yearly sum2/ +24.2 -7.9
I No. of"B" mos. 3/ 2
j/ A (+) or (-) value for any month is its PD value less the value of the 50/50 line for the mean temperature of that month.
For example, in this table the PD value for January is 7.7, the 50/50 line value from Figure 26 for 220 mean temperature
is 0.3, and the (+) value for January is 7.4.
;1 Yearly sum of (+) values does not include months with mean temperature less than 40%F.
N/ A "B" month is any one having a (-) value.
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