Stamford Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Stamford Observatory clear sky chart from 2005-11-06 to 2024-01-12.

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
30%
23079
20%
74565
9%
23035
42%
23035
21%
47475

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 10thNight
17%
760
23%
2649
8%
757
37%
757
19%
1565
January11th to 20thNight
19%
725
19%
2535
6%
725
37%
725
16%
1635
January21st to endNight
16%
792
23%
2673
6%
792
37%
792
19%
1624
February1st to 10thNight
16%
720
18%
2308
4%
720
32%
720
17%
1509
February11th to 20thNight
13%
703
20%
2100
4%
700
32%
700
17%
1374
February21st to endNight
16%
592
25%
1776
8%
592
37%
592
17%
1178
March1st to 10thNight
20%
720
22%
2160
8%
720
39%
720
22%
1345
March11th to 20thNight
19%
720
21%
2094
7%
720
35%
720
21%
1371
March21st to endNight
19%
792
23%
1980
6%
792
32%
792
22%
1357
April1st to 10thNight
21%
720
19%
1800
4%
720
30%
720
21%
1257
April11th to 20thNight
25%
711
18%
1775
4%
711
32%
711
26%
1225
April21st to endNight
23%
662
19%
1742
5%
662
33%
662
23%
1212
May1st to 10thNight
25%
540
15%
1620
5%
540
32%
540
17%
1144
May11th to 20thNight
27%
413
19%
1475
10%
413
38%
413
18%
962
May21st to endNight
44%
388
14%
1551
13%
388
50%
388
18%
971
June1st to 10thNight
37%
360
16%
1440
13%
360
49%
360
18%
896
June11th to 20thNight
45%
360
15%
1440
15%
360
53%
360
20%
877
June21st to endNight
53%
360
15%
1440
15%
360
61%
360
24%
837
July1st to 10thNight
52%
350
14%
1400
15%
350
57%
350
21%
849
July11th to 20thNight
56%
348
14%
1392
18%
348
60%
348
21%
840
July21st to endNight
56%
394
18%
1486
19%
372
62%
372
24%
869
August1st to 10thNight
53%
445
17%
1509
14%
443
57%
443
27%
900
August11th to 20thNight
53%
710
15%
1774
7%
710
55%
710
26%
1265
August21st to endNight
53%
792
18%
1980
8%
792
57%
792
33%
1315
September1st to 10thNight
43%
720
22%
1800
10%
720
49%
720
31%
1217
September11th to 20thNight
49%
720
26%
1898
16%
720
55%
720
35%
1172
September21st to endNight
42%
720
25%
1980
15%
720
50%
720
28%
1295
October1st to 10thNight
33%
720
16%
2044
10%
720
39%
720
22%
1379
October11th to 20thNight
34%
720
24%
2160
12%
720
48%
720
28%
1337
October21st to endNight
23%
792
19%
2549
9%
792
36%
792
20%
1665
November1st to 10thNight
32%
740
24%
2407
11%
740
49%
740
27%
1411
November11th to 20thNight
23%
760
26%
2470
10%
760
42%
760
23%
1507
November21st to endNight
26%
760
23%
2522
9%
760
44%
760
22%
1487
December1st to 10thNight
19%
760
19%
2660
6%
760
36%
760
16%
1700
December11th to 20thNight
20%
760
19%
3182
5%
759
35%
759
14%
1944
December21st to endNight
16%
830
14%
4794
5%
817
35%
817
10%
2984

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%6%25%5%345%5%7%40%
74764
Night
17%6%35%43445%7%42%
74932
TransparencyDay
42%6%12%22%16%2
74469
Night
42%5%12%23%16%2
74761
SeeingDay
20%14%30%23%10%2
13874
Night
21%13%31%25%9%1
23086

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 10thDay
15%6%245%35%46%6%43%
1517
11th to 20thDay
17%8%35%5%345%6%8%36%
1448
21st to endDay
14%7%35%5%345%5%8%41%
1678
February1st to 10thDay
19%7%26%436%46%6%37%
1652
11th to 20thDay
16%5%35%6%34447%43%
1748
21st to endDay
21%8%26%4345%46%37%
1480
March1st to 10thDay
15%5%35%6%245%6%7%43%
1797
11th to 20thDay
15%6%35%6%445%7%8%37%
1864
21st to endDay
21%8%25%5%33346%39%
2376
April1st to 10thDay
18%8%4442445%7%39%
2160
11th to 20thDay
18%6%35%5%34447%42%
2134
21st to endDay
21%6%245%3445%6%40%
2215
May1st to 10thDay
23%7%344245%5%7%34%
2340
11th to 20thDay
19%6%25%435%5%5%8%38%
2433
21st to endDay
16%5%25%4345%6%8%42%
2725
June1st to 10thDay
19%5%25%5%345%6%9%36%
2520
11th to 20thDay
16%6%35%5%35%5%6%8%37%
2520
21st to endDay
12%6%35%6%45%5%6%10%39%
2520
July1st to 10thDay
12%6%25%6%35%6%7%9%39%
2520
11th to 20thDay
11%5%25%5%446%7%10%41%
2520
21st to endDay
13%6%35%5%36%6%7%8%38%
2744
August1st to 10thDay
11%6%26%6%36%6%6%11%38%
2399
11th to 20thDay
13%6%25%6%45%5%6%9%39%
2126
21st to endDay
12%5%35%5%35%5%6%9%42%
2376
September1st to 10thDay
13%6%25%435%5%6%7%43%
2160
11th to 20thDay
14%42432345%7%50%
2062
21st to endDay
18%5%25%5%35%5%5%6%41%
1980
October1st to 10thDay
23%6%2443345%6%41%
1913
11th to 20thDay
16%5%25%43346%8%45%
1800
21st to endDay
22%6%144245%5%6%39%
1805
November1st to 10thDay
15%6%2442335%7%50%
1671
11th to 20thDay
19%5%2332445%7%47%
1710
21st to endDay
16%5%344345%5%6%45%
1653
December1st to 10thDay
19%7%25%6%445%6%7%37%
1520
11th to 20thDay
23%5%24323345%45%
1906
21st to endDay
18%10%47%6%35%5%47%32%
2772
January1st to 10thNight
16%6%25%424347%46%
2649
11th to 20thNight
20%7%345%3345%6%40%
2535
21st to endNight
14%8%35%42445%6%44%
2673
February1st to 10thNight
18%8%36%5%34446%41%
2308
11th to 20thNight
18%7%35%5%24446%41%
2098
21st to endNight
20%7%35%5%34445%42%
1776
March1st to 10thNight
17%6%2443345%7%45%
2150
11th to 20thNight
16%6%26%5%3445%7%41%
2093
21st to endNight
17%7%445%34446%42%
1980
April1st to 10thNight
19%7%35%5%345%5%6%40%
1800
11th to 20thNight
19%6%26%5%2345%6%41%
1775
21st to endNight
20%6%36%5%23445%41%
1742
May1st to 10thNight
23%7%36%5%34447%34%
1620
11th to 20thNight
20%8%26%43446%6%37%
1474
21st to endNight
18%7%245%3345%6%42%
1550
June1st to 10thNight
19%5%25%435%448%41%
1440
11th to 20thNight
15%7%35%5%25%45%9%40%
1440
21st to endNight
12%5%34425%5%6%10%44%
1440
July1st to 10thNight
13%5%36%6%45%6%6%8%39%
1440
11th to 20thNight
13%5%36%5%36%47%9%40%
1440
21st to endNight
12%5%34446%5%7%10%41%
1574
August1st to 10thNight
8%6%35%5%36%6%6%8%44%
1519
11th to 20thNight
12%6%26%7%45%7%7%7%37%
1774
21st to endNight
14%5%25%4345%5%8%44%
1980
September1st to 10thNight
15%5%25%5%34446%45%
1800
11th to 20thNight
14%5%2442435%5%52%
1898
21st to endNight
17%6%35%434346%45%
1980
October1st to 10thNight
20%7%35%5%24335%42%
2044
11th to 20thNight
16%6%25%43445%6%47%
2160
21st to endNight
22%7%25%424346%41%
2549
November1st to 10thNight
13%5%25%42445%6%50%
2407
11th to 20thNight
21%6%24323346%47%
2470
21st to endNight
15%5%2442445%7%47%
2522
December1st to 10thNight
20%7%35%5%34347%39%
2660
11th to 20thNight
20%6%2443445%6%42%
3288
21st to endNight
21%9%35%42435%6%37%
4884

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 10thDay
40%5%10%28%14%4
1518
11th to 20thDay
46%6%13%20%13%2
1448
21st to endDay
41%5%11%24%16%3
1678
February1st to 10thDay
47%5%12%20%14%2
1652
11th to 20thDay
41%5%10%24%17%3
1757
21st to endDay
49%410%20%15%3
1480
March1st to 10thDay
40%5%13%23%15%3
1800
11th to 20thDay
42%6%12%23%15%2
1864
21st to endDay
47%47%18%18%5%
2376
April1st to 10thDay
44%5%10%21%16%4
2160
11th to 20thDay
42%5%10%21%19%3
2134
21st to endDay
44%5%9%21%17%4
2215
May1st to 10thDay
48%6%9%21%14%2
2340
11th to 20thDay
43%6%10%20%17%3
2437
21st to endDay
39%8%17%21%14%2
2725
June1st to 10thDay
43%8%13%22%13%1
2520
11th to 20thDay
42%8%12%24%12%1
2520
21st to endDay
39%8%13%24%14%1
2520
July1st to 10thDay
40%7%12%22%17%2
2450
11th to 20thDay
37%7%15%25%14%2
2445
21st to endDay
40%8%12%20%17%2
2591
August1st to 10thDay
38%8%15%25%13%1
2385
11th to 20thDay
39%8%14%24%14%1
2126
21st to endDay
38%7%11%23%19%2
2376
September1st to 10thDay
39%6%13%22%17%2
2160
11th to 20thDay
34%5%11%24%25%2
2062
21st to endDay
43%6%9%21%19%3
1980
October1st to 10thDay
43%7%8%23%17%3
1913
11th to 20thDay
37%5%13%26%17%2
1800
21st to endDay
44%5%10%21%17%3
1805
November1st to 10thDay
36%413%24%20%3
1671
11th to 20thDay
39%410%22%21%3
1710
21st to endDay
39%413%28%13%3
1653
December1st to 10thDay
45%6%15%18%14%1
1520
11th to 20thDay
42%415%20%16%2
1906
21st to endDay
51%6%10%22%9%1
2772
January1st to 10thNight
39%312%25%16%4
2649
11th to 20thNight
45%5%14%24%10%3
2535
21st to endNight
40%410%25%19%3
2673
February1st to 10thNight
46%413%23%13%2
2308
11th to 20thNight
45%410%20%17%5%
2100
21st to endNight
46%47%24%16%4
1776
March1st to 10thNight
39%412%25%16%4
2160
11th to 20thNight
43%410%22%17%4
2094
21st to endNight
44%48%19%19%6%
1980
April1st to 10thNight
46%410%20%17%3
1800
11th to 20thNight
45%311%19%21%1
1775
21st to endNight
46%49%21%15%5%
1742
May1st to 10thNight
51%5%10%18%14%2
1620
11th to 20thNight
47%5%10%18%18%2
1475
21st to endNight
44%6%15%21%12%2
1551
June1st to 10thNight
44%5%10%20%18%2
1440
11th to 20thNight
41%7%15%25%12%1
1440
21st to endNight
35%9%14%26%15%1
1440
July1st to 10thNight
41%6%15%20%15%2
1400
11th to 20thNight
39%8%14%27%11%1
1392
21st to endNight
36%8%17%22%15%2
1486
August1st to 10thNight
36%8%16%24%16%1
1509
11th to 20thNight
43%8%13%24%11%1
1774
21st to endNight
36%8%13%23%19%2
1980
September1st to 10thNight
40%6%10%27%16%1
1800
11th to 20thNight
34%410%24%25%3
1898
21st to endNight
41%48%23%21%3
1980
October1st to 10thNight
45%5%11%22%13%3
2044
11th to 20thNight
38%5%12%29%15%1
2160
21st to endNight
46%312%22%15%1
2549
November1st to 10thNight
36%412%24%20%4
2407
11th to 20thNight
40%39%25%19%4
2470
21st to endNight
38%413%30%13%2
2522
December1st to 10thNight
46%412%22%15%1
2660
11th to 20thNight
43%412%19%18%3
3288
21st to endNight
48%5%11%23%13%1
4884

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 10thDay
25%28%27%17%3
180
11th to 20thDay
17%33%36%10%3
162
21st to endDay
18%26%38%13%5%
218
February1st to 10thDay
27%27%34%10%1
272
11th to 20thDay
23%25%38%13%1
416
21st to endDay
27%23%34%13%2
346
March1st to 10thDay
16%27%37%17%3
420
11th to 20thDay
20%23%38%15%21
420
21st to endDay
25%22%34%14%41
462
April1st to 10thDay
22%25%31%16%6%
420
11th to 20thDay
23%16%34%17%9%1
412
21st to endDay
28%16%36%16%41
420
May1st to 10thDay
29%16%30%17%5%2
423
11th to 20thDay
19%12%39%23%6%1
447
21st to endDay
18%7%28%30%13%3
485
June1st to 10thDay
19%10%27%30%13%2
450
11th to 20thDay
17%10%32%27%11%2
450
21st to endDay
15%6%28%34%14%2
450
July1st to 10thDay
16%6%22%33%18%5%
445
11th to 20thDay
10%5%23%37%22%3
444
21st to endDay
12%6%28%36%16%1
488
August1st to 10thDay
13%7%20%38%17%5%
446
11th to 20thDay
16%419%37%19%5%
439
21st to endDay
10%8%27%32%16%7%
495
September1st to 10thDay
18%6%28%25%18%5%
450
11th to 20thDay
14%7%29%28%13%9%
450
21st to endDay
19%10%33%22%12%4
450
October1st to 10thDay
28%8%27%19%14%4
450
11th to 20thDay
16%18%29%25%11%1
450
21st to endDay
25%16%31%20%7%
408
November1st to 10thDay
17%16%29%27%9%3
365
11th to 20thDay
23%20%33%19%5%
370
21st to endDay
21%19%32%23%5%1
313
December1st to 10thDay
23%23%33%16%5%
180
11th to 20thDay
35%21%31%11%3
180
21st to endDay
24%20%36%16%4
198
January1st to 10thNight
20%21%38%19%3
760
11th to 20thNight
24%19%33%20%3
725
21st to endNight
19%21%39%18%3
792
February1st to 10thNight
23%17%38%20%2
720
11th to 20thNight
24%22%36%17%1
703
21st to endNight
24%21%34%18%2
592
March1st to 10thNight
23%19%35%20%3
720
11th to 20thNight
22%18%38%18%31
720
21st to endNight
21%18%36%19%5%
792
April1st to 10thNight
23%19%34%19%5%
720
11th to 20thNight
23%12%36%24%5%
711
21st to endNight
24%13%34%23%5%1
662
May1st to 10thNight
26%12%32%24%7%
540
11th to 20thNight
23%12%35%24%6%
413
21st to endNight
23%421%37%13%1
389
June1st to 10thNight
19%6%29%33%11%1
360
11th to 20thNight
16%426%40%13%
360
21st to endNight
11%326%48%12%1
360
July1st to 10thNight
14%321%39%21%1
350
11th to 20thNight
12%222%42%20%2
348
21st to endNight
15%420%42%18%1
394
August1st to 10thNight
10%324%42%20%1
445
11th to 20thNight
15%418%37%25%2
710
21st to endNight
15%6%19%35%22%3
792
September1st to 10thNight
17%5%26%34%16%3
720
11th to 20thNight
17%7%22%33%18%4
720
21st to endNight
21%5%27%31%15%2
720
October1st to 10thNight
24%10%28%24%12%3
720
11th to 20thNight
19%12%30%29%10%1
720
21st to endNight
27%12%35%20%7%
792
November1st to 10thNight
14%14%35%26%10%1
740
11th to 20thNight
24%14%35%19%7%
760
21st to endNight
19%19%30%25%6%1
760
December1st to 10thNight
25%18%33%18%5%1
760
11th to 20thNight
24%19%34%20%3
760
21st to endNight
26%18%36%18%3
836
Page generated at 2024-01-08 22:31:24. Version ac07

Copyright 2024 Attilla Danko


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