Middlebury Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Middlebury Observatory clear sky chart from 2005-11-05 to 2019-06-19.

This page is a summary of past forecasts. Even though it's based only on forecasts, some people use these pages as clue to a site's climate. But climate data based on actual observation should be preferred (if you can find it!).

Cloud, Transparency and Seeing forecasts are summarized individually. But because good observing conditions require a combination of minimum cloud cover, transparency and seeing, the individual forecast summaries can be difficult to interpret. The observing conditions section counts the combinations of conditions needed for common types of astronomical observation.

Observing Conditions

The following tables count the occurrence of the combinations of forecast and darkness conditions needed for different observations. Conditions are rated by by what percentage of the hours the observation would likely be attempted are usable.
Observation TypeMinimum conditions for hour to be usableOut of hours
Clear of CloudsTransparencySeeingSky Brightness
Planetary:50%(no limit)Average(no limit)Nighttime Hours
Deep Sky:70%Average(no limit)5.9Nighttime Hours
CCD Imaging:70%Below AveragePoor(no limit)Nighttime Hours

Percentages are of nighttime hours per month, or per year, when a particular observing conditions was met. Deep Sky percentages never exceed 50% because the moon is up half the time. Deep sky percentages are much lower for charts at high latitudes because of fewer dark hours in the summer.

Overall

PlanetaryData
Pts
Deep Sky
Max 60%
Data
Pts
Both Planetary and Deep Sky
Max 60%
Data
Pts
Either Planetary or Deep SkyData
Pts
CCD ImagingData
Pts
Night
28%
17119
13%
53996
8%
17084
35%
17084
16%
38106

By Month

MonthDays of MonthPlanetaryData
Pts
Deep Sky
Max 60%
Data
Pts
Both Planetary and Deep Sky
Max 60%
Data
Pts
Either Planetary or Deep SkyData
Pts
CCD ImagingData
Pts
January1st to 15thNight
11%
837
8%
2923
3%
836
17%
836
6%
2360
January16th to endNight
11%
896
8%
2912
2%
896
18%
896
7%
2316
February1st to 15thNight
8%
824
7%
2677
1%
821
15%
821
6%
2129
February16th to endNight
15%
735
10%
2319
3%
735
23%
735
10%
1774
March1st to 15thNight
18%
840
14%
2332
6%
840
25%
840
13%
1777
March16th to endNight
23%
896
17%
2450
7%
896
31%
896
20%
1748
April1st to 15thNight
20%
838
14%
2096
3%
838
26%
838
18%
1558
April16th to endNight
34%
733
18%
1979
8%
733
39%
733
26%
1330
May1st to 15thNight
40%
532
16%
1792
11%
532
46%
532
19%
1180
May16th to endNight
38%
438
16%
1753
13%
438
48%
438
24%
991
June1st to 15thNight
38%
420
16%
1680
14%
420
50%
420
20%
1019
June16th to endNight
44%
398
11%
1592
11%
398
52%
398
23%
921
July1st to 15thNight
53%
370
17%
1480
21%
370
60%
370
30%
773
July16th to endNight
54%
491
17%
1623
14%
463
60%
463
30%
923
August1st to 15thNight
53%
575
17%
1715
12%
572
57%
572
33%
1022
August16th to endNight
56%
731
21%
1973
12%
731
60%
731
37%
1202
September1st to 15thNight
47%
780
21%
1950
10%
780
52%
780
33%
1271
September16th to endNight
51%
777
22%
2158
17%
777
54%
777
31%
1360
October1st to 15thNight
34%
780
16%
2340
11%
780
42%
780
20%
1631
October16th to endNight
19%
832
11%
2496
4%
832
27%
832
14%
1851
November1st to 15thNight
27%
820
17%
2740
12%
820
35%
820
17%
1871
November16th to endNight
20%
840
10%
2940
6%
840
26%
840
11%
2173
December1st to 15thNight
13%
840
7%
2940
2%
840
19%
840
7%
2316
December16th to endNight
10%
896
5%
3136
1%
896
16%
896
5%
2610

Individual Forecasts

The length of each colored bar in the "occurrence of each Forecast" column is proportional to the number of times the chart forecast that color. The percentages are of the total number of hours per month, or per year, with a particular forecast. The colors as the same as those used in clear sky charts.

Note: Transparency probabilities are only counted for those hours that have 30% or less cloud cover. Seeing probabilities are only counted for those hours that have 80% or less cloud cover. If that's too tricky, try Observing Conditions instead.

Overall

Occurrence of each ForecastData
Points
Cloud CoverDay
14%10%48%7%35%5%5%6%27%
54725
Night
13%9%48%6%35%5%5%7%29%
54173
Cloud CoverDay
14%10%48%7%35%5%5%6%28%
54735
Night
13%9%48%7%35%5%5%7%29%
54178
TransparencyDay
54%411%18%9%
54420
Night
51%5%13%19%9%
53996
SeeingDay
22%14%28%20%9%4
7623
Night
20%14%30%24%9%1
17123

By Month

Percentage Cloud Cover NE (2006-)
Color Meanings:
Overcast 90% covered 80% covered 70% covered 60% covered 50% covered 40% covered 30% covered 20% covered 10% covered Clear
MonthDays in MonthOccurrence of each ForecastData
Points
January1st to 15thDay
11%14%7%13%13%47%5%4413%
1675
16th to endDay
8%13%9%14%13%5%7%5%4412%
2016
February1st to 15thDay
12%15%8%14%10%46%5%4313%
1845
16th to endDay
14%15%6%11%9%4435%5%18%
1726
March1st to 15thDay
14%13%7%12%8%4444421%
2274
16th to endDay
14%13%5%7%8%245%45%28%
2478
April1st to 15thDay
16%13%5%7%6%35%445%27%
2512
16th to endDay
16%11%37%5%23445%34%
2599
May1st to 15thDay
17%8%37%6%345%47%30%
2816
16th to endDay
12%6%36%5%45%45%8%36%
3066
June1st to 15thDay
17%7%37%435%5%5%7%32%
2940
16th to endDay
13%5%25%6%36%6%6%9%34%
2776
July1st to 15thDay
7%5%36%5%45%5%6%9%39%
2730
16th to endDay
11%6%26%7%37%6%7%9%30%
2792
August1st to 15thDay
11%7%36%6%46%6%7%8%32%
2487
16th to endDay
13%5%35%6%45%5%6%8%35%
2568
September1st to 15thDay
17%6%35%6%35%45%6%35%
2340
16th to endDay
14%6%35%5%3446%8%37%
2130
October1st to 15thDay
20%8%36%5%34346%30%
1950
16th to endDay
21%13%49%6%44435%21%
2080
November1st to 15thDay
12%13%48%7%3445%6%28%
1773
16th to endDay
14%17%6%10%6%35%446%20%
1680
December1st to 15thDay
13%20%6%11%8%45%5%45%14%
1680
16th to endDay
14%18%11%13%9%446%449%
1792
January1st to 15thNight
11%12%6%14%10%5%6%5%46%14%
2923
16th to endNight
10%12%7%12%10%5%6%6%6%6%14%
2912
February1st to 15thNight
12%13%6%12%9%47%6%5%6%14%
2675
16th to endNight
11%11%5%13%9%46%5%5%6%20%
2319
March1st to 15thNight
14%11%5%9%8%46%445%25%
2320
16th to endNight
14%8%49%7%34435%34%
2450
April1st to 15thNight
15%9%46%6%35%45%7%30%
2096
16th to endNight
15%6%246%2446%7%38%
1979
May1st to 15thNight
15%6%36%5%35%46%8%33%
1792
16th to endNight
10%6%344345%5%8%44%
1751
June1st to 15thNight
14%426%5%35%5%7%8%38%
1680
16th to endNight
11%5%25%5%245%6%8%41%
1592
July1st to 15thNight
5%4245%335%6%9%49%
1560
16th to endNight
9%313435%6%6%9%45%
1725
August1st to 15thNight
7%4245%35%5%6%9%44%
1726
16th to endNight
9%4245%25%5%7%8%45%
1973
September1st to 15thNight
14%5%35%32345%7%44%
1950
16th to endNight
14%5%24325%47%7%43%
2158
October1st to 15thNight
17%7%35%5%345%5%7%34%
2340
16th to endNight
21%11%47%5%3445%7%25%
2496
November1st to 15thNight
15%9%36%6%35%446%32%
2740
16th to endNight
14%12%6%9%7%45%5%6%6%23%
2940
December1st to 15thNight
14%16%6%9%8%46%5%46%16%
2940
16th to endNight
15%13%9%13%8%46%5%5%411%
3136

Percentage Cloud Cover SE (2006-)
Color Meanings:
Overcast 90% covered 80% covered 70% covered 60% covered 50% covered 40% covered 30% covered 20% covered 10% covered Clear
MonthDays in MonthOccurrence of each ForecastData
Points
January1st to 15thDay
11%14%7%13%13%37%5%45%13%
1675
16th to endDay
8%13%8%14%13%5%7%5%4413%
2016
February1st to 15thDay
12%15%8%14%10%47%5%4413%
1845
16th to endDay
14%15%6%11%9%44345%18%
1726
March1st to 15thDay
14%12%7%12%9%44445%20%
2285
16th to endDay
14%13%5%7%7%245%45%28%
2477
April1st to 15thDay
16%12%5%7%6%35%445%27%
2512
16th to endDay
16%11%37%5%23445%34%
2599
May1st to 15thDay
17%7%37%6%35%45%7%30%
2816
16th to endDay
12%6%36%5%45%5%5%8%36%
3066
June1st to 15thDay
17%7%37%5%35%5%6%6%32%
2940
16th to endDay
13%425%6%35%6%6%9%34%
2776
July1st to 15thDay
7%5%36%6%46%5%6%9%39%
2730
16th to endDay
11%5%36%6%36%6%7%9%30%
2792
August1st to 15thDay
10%7%36%6%46%6%6%8%32%
2487
16th to endDay
13%5%35%6%45%5%6%8%35%
2568
September1st to 15thDay
16%6%35%6%3446%7%34%
2340
16th to endDay
14%6%25%5%35%46%8%37%
2130
October1st to 15thDay
20%8%36%6%35%347%30%
1950
16th to endDay
20%12%5%8%7%45%445%22%
2080
November1st to 15thDay
12%13%48%7%3445%6%28%
1773
16th to endDay
14%16%6%9%7%45%446%20%
1680
December1st to 15thDay
13%19%7%11%8%35%45%415%
1680
16th to endDay
14%18%10%13%9%445%449%
1792
January1st to 15thNight
11%12%6%13%10%5%6%5%46%14%
2923
16th to endNight
9%12%7%12%10%5%6%6%6%6%14%
2912
February1st to 15thNight
12%13%6%12%10%47%6%46%15%
2668
16th to endNight
11%11%5%12%10%46%5%5%6%20%
2319
March1st to 15thNight
14%11%5%9%7%46%445%25%
2332
16th to endNight
14%8%49%7%34435%34%
2450
April1st to 15thNight
15%9%46%7%35%45%6%30%
2096
16th to endNight
15%6%246%25%45%8%38%
1979
May1st to 15thNight
15%6%36%5%35%46%8%33%
1792
16th to endNight
10%6%244345%5%8%44%
1751
June1st to 15thNight
14%426%5%35%5%7%8%38%
1680
16th to endNight
10%5%245%245%6%8%42%
1592
July1st to 15thNight
5%4145%3446%9%49%
1560
16th to endNight
9%413435%6%6%8%46%
1725
August1st to 15thNight
7%425%5%45%5%6%9%45%
1726
16th to endNight
8%424425%5%6%8%46%
1973
September1st to 15thNight
13%5%25%32345%6%45%
1950
16th to endNight
13%5%2433447%7%43%
2158
October1st to 15thNight
17%7%35%6%25%45%7%34%
2340
16th to endNight
21%11%47%5%3445%7%25%
2496
November1st to 15thNight
15%9%36%5%35%45%6%33%
2740
16th to endNight
14%12%5%9%7%35%5%6%6%23%
2940
December1st to 15thNight
14%16%6%9%8%36%5%45%16%
2940
16th to endNight
15%13%9%13%8%46%5%5%411%
3136

Transparency (when cloud cover <= 30%)
Color Meanings:
Too cloudy to forecast Poor Below Average Average Above average Transparent
MonthDays in MonthOccurrence of each ForecastData
Points
January1st to 15thDay
69%49%11%4
1676
16th to endDay
71%311%10%2
2016
February1st to 15thDay
71%5%10%9%3
1847
16th to endDay
66%38%14%5%
1726
March1st to 15thDay
64%310%14%5%
2285
16th to endDay
56%48%20%9%1
2478
April1st to 15thDay
57%310%17%9%
2512
16th to endDay
50%38%19%15%2
2599
May1st to 15thDay
52%410%22%10%
2816
16th to endDay
43%414%23%11%1
3070
June1st to 15thDay
49%5%11%20%12%
2940
16th to endDay
42%5%14%23%12%
2776
July1st to 15thDay
39%5%10%26%16%1
2581
16th to endDay
45%6%14%20%11%1
2631
August1st to 15thDay
44%6%14%21%11%
2474
16th to endDay
44%412%26%10%1
2568
September1st to 15thDay
47%412%23%11%1
2340
16th to endDay
42%413%23%14%
2130
October1st to 15thDay
52%411%18%11%
1950
16th to endDay
62%313%15%4
2080
November1st to 15thDay
53%5%13%17%8%1
1773
16th to endDay
63%312%14%5%
1680
December1st to 15thDay
68%412%12%1
1680
16th to endDay
74%5%10%7%2
1792
January1st to 15thNight
67%5%10%11%5%
2923
16th to endNight
66%412%12%3
2912
February1st to 15thNight
65%5%12%12%3
2677
16th to endNight
61%312%15%6%
2319
March1st to 15thNight
59%311%18%6%
2332
16th to endNight
51%39%20%13%1
2450
April1st to 15thNight
52%5%11%19%11%
2096
16th to endNight
43%411%22%16%2
1979
May1st to 15thNight
46%5%12%23%11%1
1792
16th to endNight
36%5%16%24%14%1
1753
June1st to 15thNight
41%5%12%24%15%
1680
16th to endNight
37%6%15%23%15%2
1592
July1st to 15thNight
30%5%15%24%21%2
1480
16th to endNight
33%5%15%27%14%2
1623
August1st to 15thNight
33%5%18%30%12%
1715
16th to endNight
32%415%31%13%1
1973
September1st to 15thNight
37%415%27%13%1
1950
16th to endNight
36%5%14%26%15%
2158
October1st to 15thNight
47%5%12%19%13%
2340
16th to endNight
55%5%16%16%5%
2496
November1st to 15thNight
48%5%12%20%9%2
2740
16th to endNight
58%415%16%4
2940
December1st to 15thNight
64%5%13%13%2
2940
16th to endNight
71%410%11%2
3136

Seeing (when cloud cover <= 80%)
Color Meanings:
Too cloudy to forecast Bad 1/5 Poor 2/5 Average 3/5 Good 4/5 Excellent 5/5
MonthDays in MonthOccurrence of each ForecastData
Points
January1st to 15thDay
23%27%34%8%4
121
16th to endDay
27%29%33%8%1
192
February1st to 15thDay
36%30%26%6%1
180
16th to endDay
32%27%28%11%
229
March1st to 15thDay
28%22%28%15%5%
390
16th to endDay
25%22%29%16%5%1
416
April1st to 15thDay
33%19%28%13%41
390
16th to endDay
25%14%29%19%7%2
379
May1st to 15thDay
27%14%27%18%8%3
408
16th to endDay
16%14%33%20%10%3
459
June1st to 15thDay
20%11%34%23%8%1
435
16th to endDay
16%8%35%25%12%1
388
July1st to 15thDay
10%5%25%29%21%5%
365
16th to endDay
14%7%28%30%13%5%
390
August1st to 15thDay
11%426%31%16%8%
358
16th to endDay
12%426%27%16%12%
396
September1st to 15thDay
18%5%20%25%17%11%
375
16th to endDay
16%12%20%18%19%12%
374
October1st to 15thDay
25%9%27%20%12%5%
375
16th to endDay
31%19%29%15%4
400
November1st to 15thDay
24%16%30%18%8%1
235
16th to endDay
26%24%35%13%
120
December1st to 15thDay
30%25%23%15%5%
120
16th to endDay
42%22%27%7%
128
January1st to 15thNight
22%24%34%14%4
840
16th to endNight
18%23%36%17%2
896
February1st to 15thNight
24%25%33%13%1
824
16th to endNight
22%23%33%18%2
735
March1st to 15thNight
24%21%30%19%3
840
16th to endNight
19%15%35%21%6%1
896
April1st to 15thNight
23%15%34%22%3
838
16th to endNight
19%10%30%27%10%1
733
May1st to 15thNight
19%9%22%35%11%1
532
16th to endNight
10%8%35%30%13%
439
June1st to 15thNight
16%7%30%32%12%
420
16th to endNight
11%434%37%11%
398
July1st to 15thNight
6%426%41%20%1
370
16th to endNight
9%327%36%20%3
491
August1st to 15thNight
9%226%35%22%3
575
16th to endNight
10%322%34%24%3
731
September1st to 15thNight
15%6%22%31%20%2
780
16th to endNight
17%6%20%26%21%7%
777
October1st to 15thNight
22%9%26%26%12%2
780
16th to endNight
30%15%30%20%3
832
November1st to 15thNight
22%13%31%22%8%1
820
16th to endNight
25%20%27%18%7%
840
December1st to 15thNight
25%21%32%16%3
840
16th to endNight
28%20%32%15%2
896
Page generated at 2019-06-20 08:02:31. Version ac04

Copyright 2019 A. Danko


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