Stamford Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Stamford Observatory clear sky chart from 2005-11-05 to 2017-09-16.

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%
14992
19%
46826
8%
14961
40%
14961
19%
29871

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
17%
720
21%
2506
8%
716
34%
716
15%
1543
January16th to endNight
14%
768
21%
2622
5%
768
35%
768
16%
1607
February1st to 15thNight
15%
704
18%
2210
3%
701
31%
701
16%
1426
February16th to endNight
17%
631
25%
1893
7%
631
39%
631
17%
1197
March1st to 15thNight
19%
720
19%
2160
7%
720
34%
720
16%
1411
March16th to endNight
19%
768
25%
1995
7%
768
34%
768
21%
1302
April1st to 15thNight
20%
718
17%
1796
4%
718
27%
718
21%
1241
April16th to endNight
23%
681
18%
1761
3%
681
32%
681
21%
1201
May1st to 15thNight
23%
519
17%
1599
5%
519
33%
519
16%
1089
May16th to endNight
33%
374
15%
1497
11%
374
42%
374
15%
952
June1st to 15thNight
34%
360
14%
1440
12%
360
45%
360
16%
920
June16th to endNight
46%
360
14%
1440
14%
360
55%
360
21%
828
July1st to 15thNight
48%
340
12%
1360
15%
340
53%
340
19%
823
July16th to endNight
49%
376
15%
1418
15%
354
59%
354
21%
835
August1st to 15thNight
47%
539
18%
1610
11%
537
53%
537
27%
987
August16th to endNight
56%
764
18%
1910
8%
764
59%
764
31%
1275
September1st to 15thNight
41%
720
23%
1806
11%
720
47%
720
31%
1177
September16th to endNight
45%
658
25%
1818
15%
658
53%
658
29%
1146
October1st to 15thNight
30%
660
20%
1909
12%
660
40%
660
23%
1240
October16th to endNight
23%
704
19%
2218
6%
704
38%
704
20%
1403
November1st to 15thNight
25%
700
22%
2277
10%
700
42%
700
21%
1400
November16th to endNight
22%
720
21%
2373
8%
720
37%
720
17%
1488
December1st to 15thNight
20%
720
16%
2520
6%
720
34%
720
12%
1625
December16th to endNight
13%
768
19%
2688
4%
768
32%
768
12%
1755

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%40%
47924
Night
17%5%24424446%42%
46996
Cloud CoverDay
16%5%2442445%7%41%
47934
Night
17%5%24424446%42%
47001
TransparencyDay
41%6%12%22%15%2
47612
Night
41%412%22%15%2
46826
SeeingDay
17%17%27%23%11%3
5804
Night
17%24%25%22%8%
14993

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
16%6%145%35%446%41%
1432
16th to endDay
14%7%35%424448%40%
1599
February1st to 15thDay
16%6%26%5%2435%6%39%
1651
16th to endDay
17%5%25%4245%46%41%
1580
March1st to 15thDay
17%6%25%5%3446%7%37%
1790
16th to endDay
19%6%24423346%41%
2229
April1st to 15thDay
19%7%33323346%40%
2152
16th to endDay
17%5%23433446%44%
2198
May1st to 15thDay
22%7%243145%5%6%35%
2361
16th to endDay
17%5%25%425%45%9%37%
2618
June1st to 15thDay
21%5%35%42445%8%35%
2520
16th to endDay
12%5%246%35%5%6%9%37%
2520
July1st to 15thDay
11%6%245%35%5%6%8%38%
2520
16th to endDay
11%5%345%35%6%6%8%39%
2654
August1st to 15thDay
12%5%15%6%345%6%9%39%
2326
16th to endDay
12%424435%45%8%43%
2292
September1st to 15thDay
13%5%14425%46%6%45%
2154
16th to endDay
15%424424446%46%
1815
October1st to 15thDay
18%7%24433347%42%
1719
16th to endDay
17%415%42445%6%43%
1653
November1st to 15thDay
19%5%23213347%45%
1581
16th to endDay
17%5%23324446%44%
1584
December1st to 15thDay
17%6%25%43345%7%39%
1440
16th to endDay
21%8%24323346%38%
1536
January1st to 15thNight
18%5%24423346%43%
2506
16th to endNight
16%7%2442335%6%43%
2622
February1st to 15thNight
15%6%35%5%24346%41%
2209
16th to endNight
17%5%24423346%44%
1893
March1st to 15thNight
19%6%14323446%41%
2147
16th to endNight
16%6%34323346%45%
1995
April1st to 15thNight
19%6%2442445%6%40%
1796
16th to endNight
18%5%26%5%12345%42%
1761
May1st to 15thNight
20%6%25%5%2435%7%37%
1599
16th to endNight
19%6%245%33446%39%
1495
June1st to 15thNight
20%5%23424347%39%
1440
16th to endNight
10%5%243245%48%46%
1440
July1st to 15thNight
13%425%5%35%5%6%9%38%
1440
16th to endNight
10%4345%36%5%7%9%39%
1514
August1st to 15thNight
11%423435%45%7%46%
1620
16th to endNight
13%3145%346%5%7%43%
1910
September1st to 15thNight
14%414424446%48%
1806
16th to endNight
13%5%35%323335%50%
1818
October1st to 15thNight
16%6%3442433445%
1909
16th to endNight
18%5%15%324446%42%
2218
November1st to 15thNight
17%6%24323335%46%
2277
16th to endNight
19%5%24423446%41%
2373
December1st to 15thNight
20%6%24424446%38%
2520
16th to endNight
20%6%25%42435%6%38%
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
16%6%145%25%5%46%41%
1432
16th to endDay
15%6%25%5%2445%7%41%
1599
February1st to 15thDay
16%6%25%5%34446%41%
1651
16th to endDay
17%5%25%4245%47%41%
1580
March1st to 15thDay
17%6%25%435%45%7%37%
1800
16th to endDay
18%6%245%22446%42%
2229
April1st to 15thDay
19%6%24323447%40%
2152
16th to endDay
16%5%23433346%44%
2198
May1st to 15thDay
22%6%2431445%6%36%
2361
16th to endDay
17%5%15%43446%8%38%
2618
June1st to 15thDay
20%5%3442345%9%35%
2520
16th to endDay
11%5%25%5%345%6%9%39%
2520
July1st to 15thDay
11%5%245%35%6%5%9%40%
2520
16th to endDay
11%5%25%5%25%5%6%9%40%
2654
August1st to 15thDay
11%425%5%35%5%6%8%40%
2326
16th to endDay
12%4243345%5%7%45%
2292
September1st to 15thDay
13%4244245%5%6%46%
2154
16th to endDay
14%425%33445%6%46%
1815
October1st to 15thDay
18%5%24423437%43%
1719
16th to endDay
17%315%42445%5%43%
1653
November1st to 15thDay
19%6%23213346%45%
1581
16th to endDay
17%5%2332435%6%44%
1584
December1st to 15thDay
17%6%25%42345%7%40%
1440
16th to endDay
21%8%24324446%38%
1536
January1st to 15thNight
18%5%24323446%44%
2506
16th to endNight
16%8%34323446%43%
2622
February1st to 15thNight
15%7%25%5%33345%41%
2202
16th to endNight
18%424423345%44%
1893
March1st to 15thNight
19%6%24323446%42%
2160
16th to endNight
16%7%34323336%45%
1994
April1st to 15thNight
19%6%25%423446%40%
1796
16th to endNight
18%5%35%5%13346%42%
1761
May1st to 15thNight
20%6%25%5%2345%7%36%
1599
16th to endNight
19%6%2443445%6%39%
1495
June1st to 15thNight
20%5%1443345%7%39%
1440
16th to endNight
10%5%344245%5%8%46%
1440
July1st to 15thNight
13%416%4245%7%9%38%
1440
16th to endNight
11%3245%36%48%9%39%
1514
August1st to 15thNight
11%3244345%5%6%46%
1620
16th to endNight
13%3235%345%6%7%43%
1910
September1st to 15thNight
14%423433445%48%
1806
16th to endNight
13%5%25%423335%49%
1818
October1st to 15thNight
17%7%25%423245%45%
1909
16th to endNight
19%5%15%424346%42%
2218
November1st to 15thNight
17%6%24323335%47%
2277
16th to endNight
18%5%24424346%41%
2373
December1st to 15thNight
20%6%25%42435%7%38%
2520
16th to endNight
21%6%2442445%6%38%
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
43%5%12%22%13%3
1433
16th to endDay
42%412%24%13%2
1599
February1st to 15thDay
44%5%12%20%14%2
1654
16th to endDay
42%310%25%15%1
1580
March1st to 15thDay
45%414%22%10%2
1800
16th to endDay
43%49%20%19%3
2229
April1st to 15thDay
44%49%21%16%3
2152
16th to endDay
40%49%21%20%3
2198
May1st to 15thDay
46%6%9%20%14%1
2361
16th to endDay
42%8%14%19%13%1
2622
June1st to 15thDay
46%6%12%21%11%1
2520
16th to endDay
39%8%13%23%14%1
2520
July1st to 15thDay
40%7%10%22%16%1
2371
16th to endDay
38%7%14%22%13%2
2487
August1st to 15thDay
38%8%14%22%16%
2312
16th to endDay
36%6%10%24%19%1
2292
September1st to 15thDay
37%6%12%22%20%1
2154
16th to endDay
38%5%8%21%24%2
1815
October1st to 15thDay
41%410%22%17%2
1719
16th to endDay
40%411%24%16%2
1653
November1st to 15thDay
40%413%20%18%2
1581
16th to endDay
40%311%25%15%2
1584
December1st to 15thDay
42%6%16%19%14%1
1440
16th to endDay
47%413%20%11%1
1536
January1st to 15thNight
41%412%24%13%3
2506
16th to endNight
41%311%25%15%2
2622
February1st to 15thNight
43%413%22%13%2
2210
16th to endNight
41%39%24%18%2
1893
March1st to 15thNight
43%412%21%14%3
2160
16th to endNight
40%310%22%18%4
1995
April1st to 15thNight
44%411%20%16%2
1796
16th to endNight
44%39%19%20%3
1761
May1st to 15thNight
48%410%19%14%2
1599
16th to endNight
44%6%15%18%13%1
1497
June1st to 15thNight
45%5%12%20%14%1
1440
16th to endNight
35%7%14%26%15%
1440
July1st to 15thNight
39%8%14%21%14%1
1360
16th to endNight
37%7%16%23%12%2
1418
August1st to 15thNight
35%6%12%26%16%1
1610
16th to endNight
37%7%12%23%17%1
1910
September1st to 15thNight
36%5%10%27%18%1
1806
16th to endNight
37%46%23%25%2
1818
October1st to 15thNight
42%410%23%15%3
1909
16th to endNight
42%315%22%15%
2218
November1st to 15thNight
40%310%22%18%3
2277
16th to endNight
43%311%25%12%2
2373
December1st to 15thNight
45%414%19%14%
2520
16th to endNight
46%413%20%13%1
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%30%33%15%3
60
16th to endDay
18%20%44%16%1
75
February1st to 15thDay
20%32%33%12%1
167
16th to endDay
17%28%35%15%3
239
March1st to 15thDay
19%31%30%14%4
270
16th to endDay
18%27%31%17%32
288
April1st to 15thDay
21%22%30%15%8%1
270
16th to endDay
20%17%33%17%8%2
270
May1st to 15thDay
28%23%23%15%6%1
288
16th to endDay
18%14%25%27%11%2
331
June1st to 15thDay
19%16%24%23%14%1
315
16th to endDay
15%12%27%30%11%1
315
July1st to 15thDay
15%9%21%35%15%3
305
16th to endDay
11%11%23%28%23%2
326
August1st to 15thDay
14%7%18%34%18%6%
313
16th to endDay
9%8%17%34%19%10%
332
September1st to 15thDay
12%10%30%22%15%7%
315
16th to endDay
15%9%26%24%12%12%
254
October1st to 15thDay
22%11%22%25%15%2
255
16th to endDay
16%23%30%21%8%
253
November1st to 15thDay
21%21%24%21%8%2
235
16th to endDay
17%28%26%20%6%
204
December1st to 15thDay
20%25%36%10%8%
60
16th to endDay
32%14%45%7%
64
January1st to 15thNight
17%33%28%17%3
720
16th to endNight
17%32%31%16%1
768
February1st to 15thNight
17%29%33%18%1
704
16th to endNight
18%33%28%17%2
631
March1st to 15thNight
23%30%25%19%2
720
16th to endNight
17%30%29%16%5%
768
April1st to 15thNight
18%28%30%17%4
718
16th to endNight
18%25%28%21%5%1
681
May1st to 15thNight
20%26%26%19%6%
519
16th to endNight
21%20%18%29%10%
375
June1st to 15thNight
16%20%22%29%10%
360
16th to endNight
10%17%20%41%10%
360
July1st to 15thNight
12%13%17%37%17%
340
16th to endNight
11%18%14%35%18%2
376
August1st to 15thNight
14%10%18%36%19%
539
16th to endNight
12%10%14%33%26%3
764
September1st to 15thNight
14%13%24%28%16%1
720
16th to endNight
16%12%20%27%19%4
658
October1st to 15thNight
18%16%28%23%11%1
660
16th to endNight
20%20%32%17%7%
704
November1st to 15thNight
17%25%27%20%8%1
700
16th to endNight
17%30%24%18%8%
720
December1st to 15thNight
23%30%22%16%5%
720
16th to endNight
22%34%26%13%2
768
Page generated at 2017-09-16 17:07:00. Version ac04

Copyright 2017 A. Danko


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