Yale Student Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Yale Student Observatory clear sky chart from 2005-11-05 to 2018-03-21.

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%
15723
19%
49202
8%
15691
39%
15691
19%
31966

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
16%
780
21%
2716
8%
776
34%
776
15%
1682
January16th to endNight
16%
832
21%
2873
5%
832
35%
832
15%
1783
February1st to 15thNight
15%
764
18%
2375
5%
761
31%
761
15%
1590
February16th to endNight
16%
683
23%
2049
7%
683
36%
683
18%
1348
March1st to 15thNight
16%
780
17%
2340
5%
780
30%
780
15%
1581
March16th to endNight
19%
792
24%
2071
7%
792
34%
792
22%
1363
April1st to 15thNight
19%
718
17%
1796
4%
718
27%
718
20%
1274
April16th to endNight
23%
657
17%
1737
3%
657
32%
657
20%
1193
May1st to 15thNight
24%
540
16%
1620
6%
540
33%
540
15%
1145
May16th to endNight
31%
377
13%
1499
9%
376
39%
376
15%
961
June1st to 15thNight
33%
360
14%
1440
11%
360
44%
360
16%
951
June16th to endNight
43%
360
14%
1440
13%
360
52%
360
22%
834
July1st to 15thNight
45%
340
11%
1360
15%
340
49%
340
19%
859
July16th to endNight
47%
376
15%
1418
14%
354
56%
354
20%
859
August1st to 15thNight
48%
523
17%
1594
10%
521
53%
521
25%
996
August16th to endNight
53%
764
18%
1910
9%
764
56%
764
31%
1273
September1st to 15thNight
41%
720
22%
1822
11%
720
46%
720
30%
1190
September16th to endNight
44%
717
24%
1980
16%
717
52%
717
27%
1263
October1st to 15thNight
32%
720
21%
2058
12%
720
42%
720
23%
1353
October16th to endNight
25%
768
20%
2442
7%
768
40%
768
20%
1567
November1st to 15thNight
26%
760
20%
2472
9%
760
41%
760
20%
1562
November16th to endNight
22%
780
23%
2548
10%
780
38%
780
18%
1604
December1st to 15thNight
21%
780
16%
2730
6%
780
34%
780
13%
1806
December16th to endNight
13%
832
19%
2912
4%
832
30%
832
12%
1929

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
16%5%2442445%7%41%
49656
Night
17%6%24424446%42%
49372
Cloud CoverDay
16%5%2442445%7%42%
49666
Night
17%6%24424446%42%
49377
TransparencyDay
41%6%12%21%15%2
49344
Night
42%5%11%21%15%2
49202
SeeingDay
18%19%27%21%9%2
6414
Night
18%23%26%22%7%
15723

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%5%2432345%444%
1552
16th to endDay
16%6%25%323446%42%
1703
February1st to 15thDay
16%6%245%3445%7%38%
1816
16th to endDay
18%6%2442346%7%39%
1710
March1st to 15thDay
18%7%25%435%45%7%36%
1940
16th to endDay
17%6%25%424435%42%
2279
April1st to 15thDay
20%6%24324346%41%
2152
16th to endDay
17%424424346%44%
2223
May1st to 15thDay
23%6%2432335%7%35%
2338
16th to endDay
17%5%25%42345%8%40%
2618
June1st to 15thDay
22%5%245%245%5%7%34%
2520
16th to endDay
12%5%245%345%6%10%39%
2520
July1st to 15thDay
12%5%25%5%36%47%8%38%
2520
16th to endDay
12%5%245%25%5%6%8%39%
2654
August1st to 15thDay
13%424435%5%6%8%42%
2340
16th to endDay
11%5%25%5%2345%8%45%
2292
September1st to 15thDay
12%414444447%46%
2137
16th to endDay
14%424424346%47%
1980
October1st to 15thDay
17%6%24423446%42%
1902
16th to endDay
17%4135%24447%43%
1781
November1st to 15thDay
19%6%23323336%43%
1716
16th to endDay
17%5%23323347%45%
1739
December1st to 15thDay
19%5%245%33446%40%
1560
16th to endDay
20%8%24433446%37%
1664
January1st to 15thNight
19%6%14323346%44%
2716
16th to endNight
17%8%24323336%43%
2873
February1st to 15thNight
16%8%45%423446%39%
2374
16th to endNight
19%6%23423336%42%
2049
March1st to 15thNight
21%6%2342445%6%39%
2327
16th to endNight
16%6%33323345%46%
2071
April1st to 15thNight
18%6%25%424447%39%
1796
16th to endNight
20%5%35%423335%42%
1737
May1st to 15thNight
21%7%245%34446%35%
1620
16th to endNight
20%5%25%334448%37%
1497
June1st to 15thNight
22%5%2442345%6%37%
1440
16th to endNight
13%424424449%45%
1440
July1st to 15thNight
13%5%15%435%5%7%9%38%
1440
16th to endNight
11%4245%36%6%7%8%38%
1514
August1st to 15thNight
11%424425%6%6%7%43%
1604
16th to endNight
12%325%43445%8%44%
1910
September1st to 15thNight
15%423424347%47%
1822
16th to endNight
14%5%2442433448%
1980
October1st to 15thNight
16%5%25%423335%45%
2058
16th to endNight
19%5%2432435%5%43%
2442
November1st to 15thNight
16%6%34323445%44%
2472
16th to endNight
19%5%23424346%43%
2548
December1st to 15thNight
20%6%24325%35%5%38%
2730
16th to endNight
21%6%24424446%38%
2912

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%5%1432346%444%
1552
16th to endDay
16%6%26%32345%5%42%
1703
February1st to 15thDay
17%6%25%435%447%39%
1816
16th to endDay
18%5%1442345%7%40%
1710
March1st to 15thDay
18%6%25%43445%7%37%
1950
16th to endDay
17%7%25%424445%42%
2279
April1st to 15thDay
19%6%23424347%41%
2152
16th to endDay
17%5%14424446%44%
2223
May1st to 15thDay
22%7%2432345%6%36%
2338
16th to endDay
16%425%42445%8%40%
2618
June1st to 15thDay
22%5%2442445%8%35%
2520
16th to endDay
11%5%244345%5%10%41%
2520
July1st to 15thDay
11%5%25%5%35%5%6%10%39%
2520
16th to endDay
12%5%245%25%5%6%8%41%
2654
August1st to 15thDay
13%3244346%6%8%43%
2340
16th to endDay
11%4245%2345%8%46%
2292
September1st to 15thDay
12%314434446%48%
2137
16th to endDay
15%414424346%47%
1980
October1st to 15thDay
17%6%24424446%43%
1902
16th to endDay
16%4134245%47%44%
1781
November1st to 15thDay
19%7%24413336%44%
1716
16th to endDay
17%423324347%46%
1739
December1st to 15thDay
19%5%245%3435%6%40%
1560
16th to endDay
21%7%2442335%6%37%
1664
January1st to 15thNight
19%5%24323346%44%
2716
16th to endNight
17%9%24313236%43%
2873
February1st to 15thNight
16%8%35%424445%40%
2367
16th to endNight
19%6%23433345%42%
2049
March1st to 15thNight
21%6%23324445%39%
2340
16th to endNight
16%6%33323345%46%
2070
April1st to 15thNight
18%6%25%423446%39%
1796
16th to endNight
20%5%245%23345%42%
1737
May1st to 15thNight
20%7%25%43435%6%35%
1620
16th to endNight
20%6%24434449%36%
1497
June1st to 15thNight
22%5%2442435%7%37%
1440
16th to endNight
13%324434349%46%
1440
July1st to 15thNight
13%425%425%5%7%8%39%
1440
16th to endNight
11%425%5%35%6%7%9%38%
1514
August1st to 15thNight
10%4244345%7%7%43%
1604
16th to endNight
12%425%5%2445%8%43%
1910
September1st to 15thNight
15%5%13423447%46%
1822
16th to endNight
14%6%24325%235%48%
1980
October1st to 15thNight
16%6%25%5%23335%44%
2058
16th to endNight
19%5%24424346%42%
2442
November1st to 15thNight
17%6%2432335%445%
2472
16th to endNight
19%5%23424346%43%
2548
December1st to 15thNight
21%6%24325%35%5%37%
2730
16th to endNight
20%7%24434445%39%
2912

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
40%5%13%22%13%4
1553
16th to endDay
42%5%11%23%14%2
1703
February1st to 15thDay
45%6%12%20%13%1
1819
16th to endDay
42%6%11%22%14%1
1710
March1st to 15thDay
47%414%21%9%2
1950
16th to endDay
44%410%18%18%3
2279
April1st to 15thDay
43%5%10%21%16%3
2152
16th to endDay
40%5%9%22%18%3
2223
May1st to 15thDay
48%5%9%19%14%2
2338
16th to endDay
41%8%14%20%13%2
2622
June1st to 15thDay
47%7%12%20%10%1
2520
16th to endDay
37%9%14%23%13%1
2520
July1st to 15thDay
39%8%11%22%16%1
2371
16th to endDay
37%8%15%22%13%2
2487
August1st to 15thDay
36%9%14%22%16%
2326
16th to endDay
36%7%9%25%19%1
2292
September1st to 15thDay
36%7%12%21%20%1
2137
16th to endDay
36%6%7%21%24%2
1980
October1st to 15thDay
42%411%22%17%2
1902
16th to endDay
39%6%10%24%16%2
1781
November1st to 15thDay
43%413%18%17%2
1716
16th to endDay
38%412%26%15%2
1739
December1st to 15thDay
44%416%18%15%1
1560
16th to endDay
47%5%14%18%12%
1664
January1st to 15thNight
42%411%23%15%3
2716
16th to endNight
42%411%22%16%2
2873
February1st to 15thNight
45%6%11%21%12%2
2375
16th to endNight
44%49%22%16%2
2049
March1st to 15thNight
46%5%12%19%13%3
2340
16th to endNight
40%49%21%19%4
2071
April1st to 15thNight
45%5%10%20%15%2
1796
16th to endNight
45%38%20%17%3
1737
May1st to 15thNight
49%6%9%16%15%2
1620
16th to endNight
45%6%17%16%12%1
1499
June1st to 15thNight
47%6%11%19%13%2
1440
16th to endNight
36%6%14%25%15%
1440
July1st to 15thNight
41%8%12%21%14%1
1360
16th to endNight
38%8%16%22%11%2
1418
August1st to 15thNight
35%9%13%24%15%2
1594
16th to endNight
38%6%12%25%17%
1910
September1st to 15thNight
37%5%9%26%18%1
1822
16th to endNight
39%46%21%26%1
1980
October1st to 15thNight
42%5%10%22%16%3
2058
16th to endNight
42%5%13%22%15%
2442
November1st to 15thNight
42%411%20%18%3
2472
16th to endNight
42%410%25%13%2
2548
December1st to 15thNight
47%5%14%19%13%
2730
16th to endNight
47%5%12%19%13%1
2912

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%34%31%16%1
90
16th to endDay
24%19%41%13%
105
February1st to 15thDay
24%28%33%12%
242
16th to endDay
19%26%32%16%3
291
March1st to 15thDay
22%27%32%15%2
330
16th to endDay
20%29%33%11%31
311
April1st to 15thDay
22%25%29%15%6%
270
16th to endDay
20%19%29%20%7%2
270
May1st to 15thDay
28%26%21%14%6%1
288
16th to endDay
20%17%26%24%9%1
331
June1st to 15thDay
20%17%25%22%12%2
315
16th to endDay
15%15%30%25%12%1
315
July1st to 15thDay
16%9%23%33%14%2
305
16th to endDay
11%12%26%31%14%3
326
August1st to 15thDay
13%7%22%33%16%5%
313
16th to endDay
12%8%24%30%15%9%
332
September1st to 15thDay
12%13%27%22%14%7%
315
16th to endDay
17%10%26%20%16%8%
314
October1st to 15thDay
20%11%21%28%12%4
315
16th to endDay
17%20%29%21%8%1
301
November1st to 15thDay
21%20%23%21%10%2
280
16th to endDay
15%30%24%20%8%
269
December1st to 15thDay
23%22%28%17%7%
90
16th to endDay
28%25%39%6%1
96
January1st to 15thNight
18%32%28%16%3
780
16th to endNight
19%29%30%18%1
832
February1st to 15thNight
18%28%32%18%1
764
16th to endNight
21%27%31%16%2
683
March1st to 15thNight
24%27%27%17%2
780
16th to endNight
18%27%31%17%3
792
April1st to 15thNight
19%27%31%18%3
718
16th to endNight
18%26%27%21%5%
657
May1st to 15thNight
20%24%27%18%8%
540
16th to endNight
20%19%21%30%7%
377
June1st to 15thNight
20%19%22%28%8%
360
16th to endNight
14%18%20%35%10%
360
July1st to 15thNight
12%14%20%35%15%1
340
16th to endNight
11%17%15%38%14%1
376
August1st to 15thNight
14%10%20%39%14%
523
16th to endNight
13%10%16%37%19%3
764
September1st to 15thNight
15%12%24%29%15%2
720
16th to endNight
17%12%19%26%21%3
717
October1st to 15thNight
19%15%26%25%12%1
720
16th to endNight
21%19%32%19%7%
768
November1st to 15thNight
17%25%26%19%8%1
760
16th to endNight
17%28%27%19%5%
780
December1st to 15thNight
24%27%23%18%5%
780
16th to endNight
23%31%28%14%2
832
Page generated at 2018-03-22 11:34:27. Version ac04

Copyright 2018 A. Danko


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