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Chart version of thinner sensitivity ratings
Weather data for Easthampton MA.  Forecast values begin August 3, 2017
Estimated Apple Sensitivity to Chemical Thinners
     Other factors may outweigh the weather effects use for these estimates.  Other influences include: fruit diameter, fruit exposure to temperatures >=75 F, pollination, this year's cropload, last year's cropload, warm February-early April temperatures, frost and prolonged cool and cloudy weather during bloom, previous fall weather, foliage condition last summer)
     Ratings below are for the influence of weather on fruit sensitivity to chemical thinning agents.  Ratings assume fruit are less than 13mm diameter.  Dates in bottom rows are beyond 18mm diameter for most cultivars, and are intended only for late cultivars.
  Look at your fruit! 
  
 The rating for each day accounts for conditions on that day AND the two following days, i.e. each rating is for a three day period starting on the listed day.  The rated value for a day indicates relative efficacy of a thinner applied on that day based on weather conditions in the 72 hours following application.  For evening applications, use the rating for the next calendar day.
Rough guess McIntosh King fruit diameter dates: 
12mm - May 23, Tuesday         15mm - May 27, Saturday          18mm - May 31, Wednesday
Date after which fruit have exposure to 2 days of daily high temperature of at least 75 F:  Wednesday,  May 17
     Very warm night temperatures (average > 69F = 19 heat units in table) can lead to "EXCESSIVE THINNING RISK", especially if combined with cloudy weather, or if combined with use of NAA.

Warm nights + Cloudy days  ----->  Increased sensitivity, maximum thinning.
Warm nights + Sunny days   -----> 
Intermediate senstivity, moderate thinning.
Cool nights + Cloudy days    ----->  Reduced sensitivity
Cool nights + Sunny days     -----> 
Reduced sensitivity, minimum thinning. 
THINNER APPLICATION DATE
Estimates assume morning application.
For afternoon application use rating for next day.
Average
% CLOUD SHADING
on application day and the next two days.


< 20% = "sunny blue sky" 
> 40% = "cloudy"
Cloudy weather on 3 days after application increases thinning.
Average
NIGHT HEAT UNITS

= degrees above 50 F for 7pm--7am
for 3 nights after application. 
Example:  If average night time temperature is 55F, then value
= 5. 
   
Values above 10 for three nights after application increases sensitivity to thinning.
SUMMARY
I
nfluence of weather on sensitivity of fruit to thinning agents for 3 days following a morning application

Ratings assume that fruit
are within diameter range
of susceptibility for thinning.
McIntosh
95% Petal Fall
Sat, May 13   
  60% 0 Reduced sensitivity
 Sun, May 14      60% 1 Reduced sensitivity
McIntosh 100% Petal Fall,  Mon, May 15      62% 4 Intermediate
 Tue, May 16    sunny  4% 9 Increased sensitivity
 Wed, May 17    sunny  4% 20 Increased sensitivity
McIntosh Fruit Set,  Thu, May 18    sunny  6% 20 Intermediate-- sensitivity may be less because of fruit exposure to 2+ days with high temps. > 75F
 Fri, May 19    sunny  10% 8 Reduced sensitivity-- sensitivity may be less because of fruit exposure to 2+ days with high temps. > 75F
 Sat, May 20    sunny  8% 2 Reduced sensitivity-- sensitivity may be less because of fruit exposure to 2+ days with high temps. > 75F
 Sun, May 21    sunny  16% 5 Reduced sensitivity-- sensitivity may be less because of fruit exposure to 2+ days with high temps. > 75F
 Mon, May 22      66% 2 Reduced sensitivity-- sensitivity may be less because of fruit exposure to 2+ days with high temps. > 75F
 Tue, May 23    sunny  12% 5 Reduced sensitivity -- fruit >12mm probably less sensitive to thinning due to size and exposure to 2+ days with high temps. > 75F
 Wed, May 24    29% 6 Intermediate -- fruit >12mm probably less sensitive to thinning due to size and exposure to 2+ days with high temps. > 75F
 Thu, May 25      65% 1 Intermediate -- fruit >12mm probably less sensitive to thinning due to size and exposure to 2+ days with high temps. > 75F
 Fri, May 26      70% 5 Intermediate -- fruit >12mm probably less sensitive to thinning due to size and exposure to 2+ days with high temps. > 75F
 Sat, May 27    38% 8 Intermediate -- fruit >15mm may have minimal sensitivity due to size and exposure to 2+ days with high temps. > 75F
 Sun, May 28      52% 6 Intermediate -- fruit >15mm may have minimal sensitivity due to size and exposure to 2+ days with high temps. > 75F
 Mon, May 29      69% 2 Intermediate -- fruit >15mm may have minimal sensitivity due to size and exposure to 2+ days with high temps. > 75F
 Tue, May 30      68% 6 Intermediate -- fruit >15mm may have minimal sensitivity due to size and exposure to 2+ days with high temps. > 75F
 Wed, May 31      53% 6 Intermediate -- fruit > 18mm are probably beyond predictable chemical thinning due to size and exposure to 2+ days with high temps. > 75F
 Thu, June 1    25% 6 Reduced sensitivity -- fruit > 18mm are probably beyond predictable chemical thinning due to size and exposure to 2+ days with high temps. > 75F
 Fri, June 2    26% 3 Reduced sensitivity -- fruit > 18mm are probably beyond predictable chemical thinning due to size and exposure to 2+ days with high temps. > 75F
 Sat, June 3    40% 4 Intermediate -- fruit > 18mm are probably beyond predictable chemical thinning due to size and exposure to 2+ days with high temps. > 75F
 Sun, June 4    26% 5 Reduced sensitivity -- fruit > 18mm are probably beyond predictable chemical thinning due to size and exposure to 2+ days with high temps. > 75F
 Mon, June 5      66% 3 Reduced sensitivity -- fruit > 18mm are probably beyond predictable chemical thinning due to size and exposure to 2+ days with high temps. > 75F
 Tue, June 6      70% 0 Reduced sensitivity -- fruit > 18mm are probably beyond predictable chemical thinning due to size and exposure to 2+ days with high temps. > 75F
 Wed, June 7    25% 4 Reduced sensitivity -- fruit > 18mm are probably beyond predictable chemical thinning due to size and exposure to 2+ days with high temps. > 75F