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Middlebury College Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Middlebury College Observatory clear sky chart from 2005-11-05 to 2017-02-18.

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
27%
14117
11%
44882
7%
14086
33%
14086
14%
33290

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
15%
720
7%
2506
5%
716
18%
716
6%
2041
January16th to endNight
16%
768
8%
2688
4%
768
23%
768
8%
2103
February1st to 15thNight
12%
704
8%
2258
2%
701
20%
701
7%
1851
February16th to endNight
18%
588
9%
1762
5%
588
24%
588
9%
1400
March1st to 15thNight
21%
660
12%
1980
7%
660
27%
660
13%
1515
March16th to endNight
24%
704
16%
1836
6%
704
31%
704
16%
1356
April1st to 15thNight
20%
658
14%
1646
4%
658
25%
658
15%
1267
April16th to endNight
26%
575
14%
1565
5%
575
32%
575
18%
1125
May1st to 15thNight
29%
415
16%
1405
11%
415
37%
415
17%
959
May16th to endNight
35%
342
11%
1369
11%
342
42%
342
15%
921
June1st to 15thNight
30%
330
9%
1320
8%
330
40%
330
12%
955
June16th to endNight
48%
330
8%
1320
11%
330
53%
330
18%
860
July1st to 15thNight
52%
310
11%
1240
15%
310
56%
310
20%
772
July16th to endNight
54%
348
11%
1306
14%
326
57%
326
21%
803
August1st to 15thNight
49%
376
15%
1357
14%
374
53%
374
20%
871
August16th to endNight
52%
700
15%
1750
8%
700
53%
700
31%
1161
September1st to 15thNight
46%
660
15%
1657
9%
660
48%
660
28%
1143
September16th to endNight
45%
657
20%
1815
13%
657
50%
657
30%
1142
October1st to 15thNight
30%
660
15%
1914
10%
660
36%
660
18%
1399
October16th to endNight
22%
704
8%
2258
4%
704
27%
704
10%
1776
November1st to 15thNight
23%
700
13%
2277
9%
700
29%
700
13%
1698
November16th to endNight
22%
720
12%
2445
9%
720
28%
720
11%
1832
December1st to 15thNight
15%
720
5%
2520
2%
720
18%
720
6%
2092
December16th to endNight
11%
768
6%
2688
2%
768
16%
768
6%
2248

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
17%8%36%5%3445%6%31%
45288
Night
18%10%47%5%34446%30%
45052
Cloud CoverDay
18%9%46%5%3445%6%30%
45298
Night
19%11%47%5%34445%29%
45057
TransparencyDay
53%7%11%17%9%
44976
Night
56%6%11%16%8%
44882
SeeingDay
23%15%27%20%10%3
4841
Night
24%15%27%22%8%
14118

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
18%12%6%10%7%36%5%5%6%17%
1432
16th to endDay
13%12%47%8%37%45%6%25%
1536
February1st to 15thDay
13%12%6%12%7%45%5%45%23%
1599
16th to endDay
14%13%7%10%7%33345%24%
1470
March1st to 15thDay
17%12%5%8%5%25%46%5%25%
1640
16th to endDay
16%10%5%7%5%23235%35%
2035
April1st to 15thDay
21%10%45%5%23345%33%
1972
16th to endDay
18%8%35%5%23446%35%
2065
May1st to 15thDay
22%7%36%42345%5%33%
2224
16th to endDay
17%6%36%5%2445%7%35%
2394
June1st to 15thDay
24%6%36%5%345%5%7%27%
2310
16th to endDay
16%6%36%6%36%5%6%8%31%
2310
July1st to 15thDay
12%5%35%6%35%5%6%9%36%
2310
16th to endDay
13%425%5%36%6%7%10%33%
2458
August1st to 15thDay
13%6%26%6%36%6%7%8%34%
2250
16th to endDay
12%425%5%345%6%9%38%
2100
September1st to 15thDay
16%5%24423446%42%
1972
16th to endDay
13%5%1442445%6%46%
1815
October1st to 15thDay
22%8%36%6%34345%32%
1713
16th to endDay
24%10%35%6%34347%25%
1614
November1st to 15thDay
17%10%36%43445%6%32%
1581
16th to endDay
18%9%47%435%35%6%30%
1512
December1st to 15thDay
22%14%411%5%2434420%
1440
16th to endDay
25%17%6%7%6%3434317%
1536
January1st to 15thNight
16%16%5%11%7%45%435%19%
2506
16th to endNight
13%13%7%9%6%45%445%24%
2688
February1st to 15thNight
13%14%6%11%9%5%6%345%19%
2257
16th to endNight
15%14%5%10%7%35%34424%
1762
March1st to 15thNight
20%11%6%8%5%2423430%
1967
16th to endNight
16%10%5%7%5%23435%34%
1836
April1st to 15thNight
21%11%46%6%23336%30%
1646
16th to endNight
21%8%345%23436%37%
1565
May1st to 15thNight
19%8%35%5%2345%6%35%
1405
16th to endNight
18%7%26%43445%8%34%
1367
June1st to 15thNight
25%6%35%5%34346%30%
1320
16th to endNight
16%6%24425%46%9%37%
1320
July1st to 15thNight
12%415%5%36%5%7%9%38%
1320
16th to endNight
10%445%5%245%6%8%42%
1402
August1st to 15thNight
12%5%345%25%5%6%7%40%
1367
16th to endNight
11%5%25%5%2446%8%42%
1750
September1st to 15thNight
18%425%42325%6%44%
1657
16th to endNight
14%413424446%48%
1815
October1st to 15thNight
21%8%36%5%2435%5%33%
1914
16th to endNight
26%11%37%5%3343426%
2258
November1st to 15thNight
20%10%36%5%43336%30%
2277
16th to endNight
19%11%47%5%34345%28%
2445
December1st to 15thNight
22%16%6%9%5%2433419%
2520
16th to endNight
22%17%6%8%6%3433418%
2688

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
18%13%6%9%8%36%5%5%417%
1432
16th to endDay
13%13%5%9%6%36%5%5%6%24%
1536
February1st to 15thDay
12%13%6%10%9%47%24422%
1599
16th to endDay
15%14%7%13%6%24345%23%
1470
March1st to 15thDay
17%13%6%9%4345%5%5%24%
1650
16th to endDay
17%12%5%7%42333434%
2035
April1st to 15thDay
22%11%46%323335%32%
1972
16th to endDay
19%10%25%5%33445%34%
2065
May1st to 15thDay
23%8%36%5%23445%32%
2224
16th to endDay
19%7%35%43445%7%33%
2394
June1st to 15thDay
26%8%35%5%345%5%7%25%
2310
16th to endDay
19%7%35%6%35%5%5%7%29%
2310
July1st to 15thDay
13%6%35%6%245%6%9%34%
2310
16th to endDay
15%6%245%46%6%7%8%33%
2458
August1st to 15thDay
15%6%35%6%345%6%8%32%
2250
16th to endDay
14%5%25%6%3446%8%38%
2100
September1st to 15thDay
17%7%24424347%41%
1972
16th to endDay
15%5%25%32345%6%46%
1815
October1st to 15thDay
23%9%47%5%3333431%
1713
16th to endDay
26%12%35%5%33445%24%
1614
November1st to 15thDay
17%11%46%5%2435%6%32%
1581
16th to endDay
19%10%39%42445%5%29%
1512
December1st to 15thDay
22%15%5%10%5%22445%20%
1440
16th to endDay
25%18%6%8%5%3333317%
1536
January1st to 15thNight
17%15%7%10%7%45%345%18%
2506
16th to endNight
15%14%7%9%6%36%445%22%
2688
February1st to 15thNight
14%14%6%11%9%5%5%34419%
2250
16th to endNight
15%15%6%10%7%35%33423%
1762
March1st to 15thNight
20%12%7%7%433225%28%
1980
16th to endNight
17%10%5%7%424345%33%
1835
April1st to 15thNight
23%12%5%6%423335%28%
1646
16th to endNight
22%8%35%5%13435%36%
1565
May1st to 15thNight
22%8%34424445%34%
1405
16th to endNight
20%7%245%3445%7%33%
1367
June1st to 15thNight
26%6%36%5%23447%28%
1320
16th to endNight
18%4344345%6%9%35%
1320
July1st to 15thNight
14%4245%35%7%5%8%37%
1320
16th to endNight
11%5%345%246%7%8%40%
1402
August1st to 15thNight
15%6%25%4345%5%7%40%
1367
16th to endNight
12%6%2443445%9%41%
1750
September1st to 15thNight
20%6%23323336%44%
1657
16th to endNight
15%423323446%48%
1815
October1st to 15thNight
22%9%36%5%23345%33%
1914
16th to endNight
27%12%45%5%2433424%
2258
November1st to 15thNight
21%10%47%5%24345%29%
2277
16th to endNight
20%12%48%6%2334428%
2445
December1st to 15thNight
24%15%7%9%5%3443418%
2520
16th to endNight
23%17%6%9%6%2433417%
2688

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
67%5%10%11%4
1433
16th to endDay
58%7%14%11%7%
1536
February1st to 15thDay
65%5%8%14%4
1602
16th to endDay
63%6%8%9%10%
1470
March1st to 15thDay
60%8%10%12%6%
1650
16th to endDay
54%411%14%13%2
2035
April1st to 15thDay
55%49%17%11%2
1972
16th to endDay
50%5%9%18%12%2
2065
May1st to 15thDay
53%6%7%19%12%1
2224
16th to endDay
48%8%11%18%11%1
2398
June1st to 15thDay
58%7%12%13%7%
2310
16th to endDay
51%7%13%16%9%
2310
July1st to 15thDay
42%9%15%21%9%
2161
16th to endDay
45%10%15%20%7%
2291
August1st to 15thDay
47%9%15%20%7%
2236
16th to endDay
42%7%14%23%11%
2100
September1st to 15thDay
42%5%12%27%11%
1972
16th to endDay
37%6%13%26%14%
1815
October1st to 15thDay
56%6%9%19%7%1
1713
16th to endDay
60%6%13%13%5%
1614
November1st to 15thDay
53%6%13%16%8%1
1581
16th to endDay
56%7%12%14%8%1
1512
December1st to 15thDay
65%7%11%10%3
1440
16th to endDay
72%6%8%7%5%
1536
January1st to 15thNight
68%5%11%9%4
2506
16th to endNight
63%5%10%12%6%
2688
February1st to 15thNight
68%5%8%11%5%
2258
16th to endNight
64%5%8%12%7%1
1762
March1st to 15thNight
59%410%17%6%1
1980
16th to endNight
52%5%10%16%12%2
1836
April1st to 15thNight
57%48%16%10%1
1646
16th to endNight
50%5%8%18%14%2
1565
May1st to 15thNight
49%5%9%16%17%1
1405
16th to endNight
48%7%13%16%12%1
1369
June1st to 15thNight
55%6%12%17%7%
1320
16th to endNight
43%9%13%23%9%
1320
July1st to 15thNight
40%8%17%23%10%
1240
16th to endNight
38%9%17%21%11%1
1306
August1st to 15thNight
42%8%14%24%10%
1357
16th to endNight
38%6%17%25%11%
1750
September1st to 15thNight
43%5%11%29%9%
1657
16th to endNight
36%5%13%28%14%1
1815
October1st to 15thNight
53%5%10%18%10%1
1914
16th to endNight
63%5%10%14%5%
2258
November1st to 15thNight
57%6%11%13%8%2
2277
16th to endNight
59%411%15%6%1
2445
December1st to 15thNight
69%6%10%11%1
2520
16th to endNight
70%6%9%9%3
2688

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
18%35%23%18%5%
60
16th to endDay
26%29%29%10%3
64
February1st to 15thDay
25%31%29%9%3
127
16th to endDay
27%17%32%15%6%
195
March1st to 15thDay
24%19%29%22%4
210
16th to endDay
25%21%23%16%8%4
224
April1st to 15thDay
30%17%26%18%6%
210
16th to endDay
22%23%23%14%11%4
210
May1st to 15thDay
32%11%24%15%12%2
228
16th to endDay
20%16%32%22%6%1
267
June1st to 15thDay
30%12%28%20%7%
255
16th to endDay
19%10%36%22%9%
255
July1st to 15thDay
15%9%29%28%15%2
245
16th to endDay
15%9%33%27%11%1
270
August1st to 15thDay
20%9%26%25%13%3
253
16th to endDay
17%7%19%25%20%9%
268
September1st to 15thDay
17%10%19%24%17%9%
255
16th to endDay
16%12%27%17%14%11%
254
October1st to 15thDay
25%14%20%19%16%3
255
16th to endDay
35%15%24%20%3
245
November1st to 15thDay
26%13%26%24%7%
235
16th to endDay
23%25%25%15%7%2
132
December1st to 15thDay
26%26%30%10%6%
60
16th to endDay
46%17%29%6%
64
January1st to 15thNight
23%24%32%17%2
720
16th to endNight
21%21%33%20%2
768
February1st to 15thNight
20%21%38%17%1
704
16th to endNight
22%23%31%19%3
588
March1st to 15thNight
25%19%30%22%2
660
16th to endNight
23%21%26%19%7%
704
April1st to 15thNight
27%20%28%18%4
658
16th to endNight
26%18%25%20%8%
575
May1st to 15thNight
24%11%31%22%9%
415
16th to endNight
20%9%30%27%9%1
343
June1st to 15thNight
27%8%26%24%12%
330
16th to endNight
19%324%38%11%1
330
July1st to 15thNight
14%225%33%21%2
310
16th to endNight
13%320%46%14%
348
August1st to 15thNight
19%419%39%15%1
376
16th to endNight
17%5%19%32%23%2
700
September1st to 15thNight
22%6%20%32%15%2
660
16th to endNight
18%9%24%24%19%3
657
October1st to 15thNight
28%12%23%21%13%1
660
16th to endNight
34%15%22%20%6%
704
November1st to 15thNight
26%15%29%19%7%1
700
16th to endNight
24%23%25%16%8%
720
December1st to 15thNight
31%23%24%17%2
720
16th to endNight
32%18%33%13%2
768
Page generated at 2017-02-18 16:04:33. Version ac04

Copyright 2017 A. Danko


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