Montauk Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Montauk Observatory clear sky chart from 2005-11-05 to 2017-11-30.

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
38%
15315
18%
47731
10%
15284
46%
15284
22%
31604

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
21%
720
17%
2506
7%
716
35%
716
16%
1654
January16th to endNight
19%
768
19%
2670
6%
768
34%
768
16%
1721
February1st to 15thNight
16%
704
17%
2150
4%
701
31%
701
18%
1463
February16th to endNight
22%
631
22%
1893
8%
631
41%
631
21%
1245
March1st to 15thNight
29%
720
17%
2160
9%
720
39%
720
21%
1454
March16th to endNight
31%
768
23%
2015
8%
768
42%
768
28%
1330
April1st to 15thNight
38%
718
18%
1796
6%
718
43%
718
28%
1262
April16th to endNight
41%
639
17%
1719
8%
639
45%
639
26%
1189
May1st to 15thNight
40%
540
16%
1620
9%
540
47%
540
21%
1142
May16th to endNight
49%
419
12%
1542
12%
419
52%
419
16%
1054
June1st to 15thNight
49%
360
14%
1440
16%
360
55%
360
17%
952
June16th to endNight
66%
360
13%
1440
17%
360
69%
360
23%
841
July1st to 15thNight
60%
340
12%
1360
18%
340
61%
340
21%
804
July16th to endNight
63%
376
16%
1418
18%
354
66%
354
22%
848
August1st to 15thNight
67%
535
18%
1606
13%
533
68%
533
30%
1009
August16th to endNight
67%
764
21%
1910
13%
764
68%
764
36%
1230
September1st to 15thNight
54%
720
21%
1842
14%
720
57%
720
31%
1236
September16th to endNight
52%
717
22%
1980
17%
717
57%
717
29%
1303
October1st to 15thNight
42%
720
22%
2024
15%
720
49%
720
25%
1355
October16th to endNight
31%
768
18%
2460
9%
768
42%
768
22%
1606
November1st to 15thNight
35%
760
18%
2472
11%
760
46%
760
19%
1648
November16th to endNight
30%
780
19%
2533
11%
780
41%
780
20%
1657
December1st to 15thNight
25%
720
13%
2487
6%
720
34%
720
14%
1742
December16th to endNight
18%
768
17%
2688
5%
768
34%
768
15%
1859

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
15%5%2442445%7%42%
48681
Night
17%6%25%42445%7%39%
47901
Cloud CoverDay
15%5%2442445%7%42%
48691
Night
17%6%25%42445%7%39%
47906
TransparencyDay
40%6%13%22%15%1
48369
Night
44%6%12%21%14%1
47731
SeeingDay
19%5%21%29%19%5%
6172
Night
20%7%26%28%14%2
15316

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
17%7%236%24447%38%
1432
16th to endDay
15%8%25%42445%6%39%
1554
February1st to 15thDay
16%5%25%6%2446%6%37%
1708
16th to endDay
16%6%145%25%45%6%41%
1580
March1st to 15thDay
18%5%25%43345%8%39%
1790
16th to endDay
16%7%25%324445%44%
2205
April1st to 15thDay
19%5%2332335%7%43%
2152
16th to endDay
16%6%2332445%7%44%
2241
May1st to 15thDay
22%7%34312346%39%
2332
16th to endDay
15%6%24424448%42%
2582
June1st to 15thDay
19%5%24424347%40%
2520
16th to endDay
11%5%243245%6%10%44%
2520
July1st to 15thDay
10%5%1442447%9%44%
2520
16th to endDay
12%4244245%6%8%42%
2654
August1st to 15thDay
12%4133235%5%9%47%
2329
16th to endDay
10%424424447%49%
2292
September1st to 15thDay
12%424323447%50%
2118
16th to endDay
14%6%1442345%7%46%
1980
October1st to 15thDay
18%6%1442345%6%43%
1935
16th to endDay
15%42443445%7%44%
1762
November1st to 15thDay
20%6%14314436%43%
1716
16th to endDay
15%6%334245%5%7%41%
1753
December1st to 15thDay
18%7%35%425%447%35%
1470
16th to endDay
22%8%36%32335%5%33%
1536
January1st to 15thNight
18%6%25%5%23447%38%
2506
16th to endNight
18%8%25%423347%38%
2670
February1st to 15thNight
16%8%36%5%23447%37%
2149
16th to endNight
17%6%24425%45%7%40%
1893
March1st to 15thNight
17%7%245%2445%6%40%
2147
16th to endNight
16%7%24423345%45%
2015
April1st to 15thNight
17%7%25%42445%5%40%
1796
16th to endNight
19%7%35%413338%38%
1719
May1st to 15thNight
18%8%35%5%34447%34%
1620
16th to endNight
19%6%36%5%25%45%7%33%
1540
June1st to 15thNight
21%7%245%24347%37%
1440
16th to endNight
13%325%4245%5%9%43%
1440
July1st to 15thNight
11%5%345%345%6%9%38%
1440
16th to endNight
10%5%25%5%36%6%7%9%37%
1514
August1st to 15thNight
8%415%6%35%6%6%8%43%
1616
16th to endNight
11%425%32445%8%47%
1910
September1st to 15thNight
15%6%235%3445%6%42%
1842
16th to endNight
14%6%35%5%23335%45%
1980
October1st to 15thNight
17%6%35%424345%43%
2024
16th to endNight
18%5%26%423445%41%
2460
November1st to 15thNight
19%6%26%42434441%
2472
16th to endNight
17%6%3442445%6%40%
2533
December1st to 15thNight
20%7%36%5%34447%32%
2487
16th to endNight
20%7%36%5%3445%7%33%
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
17%7%246%2446%7%37%
1432
16th to endDay
16%7%25%425%447%38%
1554
February1st to 15thDay
16%6%35%5%3445%7%36%
1708
16th to endDay
16%6%1442445%7%41%
1580
March1st to 15thDay
18%5%145%2445%8%39%
1800
16th to endDay
16%7%24323445%44%
2205
April1st to 15thDay
19%5%13314347%43%
2152
16th to endDay
16%6%2332445%6%44%
2241
May1st to 15thDay
22%6%34323336%39%
2332
16th to endDay
15%6%24423448%42%
2582
June1st to 15thDay
19%5%24425%347%40%
2520
16th to endDay
11%5%144245%7%9%44%
2520
July1st to 15thDay
10%5%2443446%9%44%
2520
16th to endDay
12%424425%46%8%43%
2654
August1st to 15thDay
12%41442346%9%47%
2329
16th to endDay
10%42342445%7%48%
2292
September1st to 15thDay
12%42431335%6%50%
2118
16th to endDay
14%6%14324446%46%
1980
October1st to 15thDay
18%6%1442335%6%42%
1935
16th to endDay
15%42443445%6%44%
1762
November1st to 15thDay
21%6%24323336%43%
1716
16th to endDay
15%6%244245%47%40%
1753
December1st to 15thDay
18%7%35%42445%7%36%
1470
16th to endDay
22%8%46%32336%6%33%
1536
January1st to 15thNight
18%7%245%24347%37%
2506
16th to endNight
18%8%24423447%38%
2670
February1st to 15thNight
15%8%36%5%33447%37%
2142
16th to endNight
17%5%24425%45%8%39%
1893
March1st to 15thNight
17%8%2442445%5%39%
2160
16th to endNight
16%7%24323345%45%
2014
April1st to 15thNight
17%7%35%424446%40%
1796
16th to endNight
19%7%35%413348%38%
1719
May1st to 15thNight
18%8%45%5%25%447%34%
1620
16th to endNight
19%7%25%5%25%45%8%33%
1540
June1st to 15thNight
21%6%14424347%37%
1440
16th to endNight
13%425%4245%5%8%43%
1440
July1st to 15thNight
12%5%245%345%6%9%38%
1440
16th to endNight
10%5%25%5%36%6%7%9%37%
1514
August1st to 15thNight
8%415%6%35%6%6%8%43%
1616
16th to endNight
11%425%32345%8%47%
1910
September1st to 15thNight
15%5%2343445%6%42%
1842
16th to endNight
15%6%25%5%33325%45%
1980
October1st to 15thNight
17%6%35%423335%43%
2024
16th to endNight
18%5%36%423446%41%
2460
November1st to 15thNight
19%5%25%5%2434441%
2472
16th to endNight
17%6%2443445%7%39%
2533
December1st to 15thNight
19%8%35%5%34447%32%
2487
16th to endNight
20%7%36%5%3445%7%33%
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
45%7%14%20%10%1
1433
16th to endDay
45%5%11%24%12%1
1554
February1st to 15thDay
45%6%14%20%12%
1711
16th to endDay
41%5%12%25%13%1
1580
March1st to 15thDay
43%5%14%23%10%2
1800
16th to endDay
40%412%21%17%3
2205
April1st to 15thDay
40%5%11%21%17%3
2152
16th to endDay
39%48%23%19%3
2241
May1st to 15thDay
46%5%10%21%14%2
2332
16th to endDay
39%7%16%21%12%2
2586
June1st to 15thDay
43%6%12%22%13%1
2520
16th to endDay
34%6%15%25%15%1
2520
July1st to 15thDay
35%7%14%24%17%1
2371
16th to endDay
36%7%13%25%13%3
2487
August1st to 15thDay
33%7%17%23%17%1
2315
16th to endDay
33%6%10%27%19%1
2292
September1st to 15thDay
32%7%14%22%22%1
2118
16th to endDay
37%6%7%23%23%1
1980
October1st to 15thDay
40%5%12%20%17%2
1935
16th to endDay
38%6%12%24%16%1
1762
November1st to 15thDay
44%414%19%15%1
1716
16th to endDay
41%6%13%24%12%1
1753
December1st to 15thDay
47%5%17%17%11%
1470
16th to endDay
50%5%14%17%10%
1536
January1st to 15thNight
46%5%13%21%11%1
2506
16th to endNight
45%5%11%23%13%1
2670
February1st to 15thNight
46%5%13%22%10%1
2150
16th to endNight
41%6%11%23%14%2
1893
March1st to 15thNight
45%5%13%19%13%2
2160
16th to endNight
40%5%10%20%18%5%
2015
April1st to 15thNight
43%5%11%20%15%3
1796
16th to endNight
45%5%9%18%16%4
1719
May1st to 15thNight
49%5%9%18%13%3
1620
16th to endNight
48%7%13%17%10%2
1542
June1st to 15thNight
47%7%10%18%14%1
1440
16th to endNight
36%7%12%25%16%1
1440
July1st to 15thNight
39%7%14%22%15%
1360
16th to endNight
37%8%15%24%11%2
1418
August1st to 15thNight
35%8%14%24%14%1
1606
16th to endNight
33%7%11%27%18%1
1910
September1st to 15thNight
40%5%9%24%18%
1842
16th to endNight
41%5%7%20%23%2
1980
October1st to 15thNight
43%49%22%16%2
2024
16th to endNight
44%5%14%21%14%
2460
November1st to 15thNight
46%5%10%19%15%2
2472
16th to endNight
44%5%13%23%11%2
2533
December1st to 15thNight
51%6%14%17%10%
2487
16th to endNight
51%5%13%20%9%
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
16%11%40%23%8%
60
16th to endDay
19%8%46%22%2
67
February1st to 15thDay
23%11%37%22%4
224
16th to endDay
14%10%38%26%9%
239
March1st to 15thDay
22%11%30%27%6%
270
16th to endDay
19%8%36%25%8%1
288
April1st to 15thDay
22%5%25%33%11%
270
16th to endDay
22%220%37%13%2
270
May1st to 15thDay
26%5%23%27%14%2
288
16th to endDay
19%214%33%22%6%
331
June1st to 15thDay
22%116%28%25%5%
315
16th to endDay
17%12%42%21%5%
315
July1st to 15thDay
14%17%35%33%6%
305
16th to endDay
14%9%33%33%8%
326
August1st to 15thDay
13%17%32%35%10%
313
16th to endDay
11%17%30%33%15%
332
September1st to 15thDay
14%219%28%25%9%
315
16th to endDay
21%115%25%22%12%
314
October1st to 15thDay
21%417%25%24%7%
315
16th to endDay
15%10%27%28%16%1
296
November1st to 15thDay
24%6%25%26%16%1
280
16th to endDay
16%14%27%28%12%
285
December1st to 15thDay
20%15%30%23%10%1
90
16th to endDay
35%6%39%18%
64
January1st to 15thNight
22%15%36%20%4
720
16th to endNight
23%15%36%22%2
768
February1st to 15thNight
20%14%43%19%2
704
16th to endNight
20%13%39%21%5%
631
March1st to 15thNight
22%10%33%27%6%
720
16th to endNight
20%6%35%26%10%1
768
April1st to 15thNight
20%430%33%9%
718
16th to endNight
23%325%30%15%1
639
May1st to 15thNight
20%226%30%16%3
540
16th to endNight
22%115%35%21%3
420
June1st to 15thNight
23%114%30%26%2
360
16th to endNight
15%11%40%29%2
360
July1st to 15thNight
12%112%41%29%2
340
16th to endNight
13%19%35%35%4
376
August1st to 15thNight
11%10%42%31%3
535
16th to endNight
12%18%31%35%9%
764
September1st to 15thNight
19%115%33%23%6%
720
16th to endNight
19%115%30%25%7%
717
October1st to 15thNight
21%6%22%28%16%4
720
16th to endNight
23%9%32%22%11%1
768
November1st to 15thNight
21%9%27%25%13%2
760
16th to endNight
21%13%28%25%9%
780
December1st to 15thNight
25%13%29%23%6%1
720
16th to endNight
23%14%37%21%2
768
Page generated at 2017-12-01 12:13:35. Version ac04

Copyright 2017 A. Danko


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