Olin Observatory
Clear Sky Chart
History

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

2017-09-12
.

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
32%
14961
18%
46768
9%
14930
42%
14930
21%
30738

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
18%
720
20%
2506
7%
716
35%
716
16%
1588
January16th to endNight
17%
768
20%
2683
5%
768
36%
768
16%
1702
February1st to 15thNight
15%
704
18%
2154
4%
701
32%
701
18%
1430
February16th to endNight
19%
631
24%
1893
7%
631
41%
631
22%
1226
March1st to 15thNight
23%
720
17%
2160
6%
720
35%
720
19%
1449
March16th to endNight
24%
768
22%
2017
7%
768
36%
768
25%
1353
April1st to 15thNight
26%
718
17%
1796
5%
718
33%
718
25%
1282
April16th to endNight
30%
629
16%
1709
6%
629
37%
629
23%
1191
May1st to 15thNight
30%
540
15%
1620
6%
540
39%
540
20%
1138
May16th to endNight
37%
413
10%
1536
7%
413
42%
413
15%
1054
June1st to 15thNight
41%
360
12%
1440
11%
360
49%
360
16%
958
June16th to endNight
55%
360
12%
1440
14%
360
61%
360
23%
841
July1st to 15thNight
49%
340
11%
1360
15%
340
52%
340
18%
863
July16th to endNight
54%
376
14%
1418
16%
354
57%
354
21%
887
August1st to 15thNight
55%
514
16%
1585
12%
512
58%
512
26%
1002
August16th to endNight
60%
764
20%
1910
12%
764
62%
764
33%
1256
September1st to 15thNight
46%
707
20%
1809
11%
707
50%
707
30%
1213
September16th to endNight
49%
657
22%
1815
16%
657
53%
657
30%
1174
October1st to 15thNight
35%
660
23%
1853
12%
660
46%
660
26%
1234
October16th to endNight
25%
704
16%
2263
6%
704
39%
704
21%
1475
November1st to 15thNight
30%
700
20%
2277
11%
700
43%
700
22%
1467
November16th to endNight
26%
720
20%
2340
9%
720
40%
720
19%
1509
December1st to 15thNight
23%
720
16%
2496
5%
720
36%
720
16%
1650
December16th to endNight
17%
768
19%
2688
5%
768
34%
768
15%
1796

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%5%2442445%7%41%
47895
Night
17%6%24424446%41%
46938
Cloud CoverDay
17%5%24424447%41%
47905
Night
18%6%24424446%41%
46943
TransparencyDay
41%6%13%21%15%2
47583
Night
43%5%12%21%15%2
46768
SeeingDay
20%10%30%24%11%3
5881
Night
21%10%31%26%9%1
14962

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%2432345%5%43%
1432
16th to endDay
16%6%34433347%42%
1539
February1st to 15thDay
17%6%245%34447%38%
1705
16th to endDay
16%5%25%5%245%5%6%40%
1580
March1st to 15thDay
19%6%24324347%39%
1790
16th to endDay
17%8%24423345%43%
2204
April1st to 15thDay
20%5%3432435%6%41%
2152
16th to endDay
18%424324347%43%
2250
May1st to 15thDay
24%7%34423445%35%
2332
16th to endDay
17%6%23424447%41%
2588
June1st to 15thDay
21%6%244245%5%7%34%
2520
16th to endDay
12%5%25%5%245%6%10%39%
2520
July1st to 15thDay
13%5%24425%5%7%9%39%
2520
16th to endDay
14%425%5%25%5%6%8%38%
2654
August1st to 15thDay
13%5%24425%6%6%8%40%
2349
16th to endDay
10%426%425%5%5%7%44%
2292
September1st to 15thDay
14%424424445%45%
2071
16th to endDay
15%414423445%48%
1815
October1st to 15thDay
17%5%2442345%6%42%
1775
16th to endDay
14%415%4335%46%45%
1609
November1st to 15thDay
19%7%23423335%44%
1581
16th to endDay
17%42442445%5%45%
1620
December1st to 15thDay
20%5%25%32335%5%41%
1461
16th to endDay
23%8%25%323235%38%
1536
January1st to 15thNight
19%6%24323336%42%
2506
16th to endNight
17%8%24423346%41%
2683
February1st to 15thNight
15%8%35%5%23346%41%
2153
16th to endNight
17%6%23324445%44%
1893
March1st to 15thNight
19%6%3442445%6%39%
2147
16th to endNight
17%6%24324335%44%
2017
April1st to 15thNight
19%7%24333446%38%
1796
16th to endNight
23%5%24423435%40%
1709
May1st to 15thNight
20%7%3443435%6%34%
1620
16th to endNight
19%5%35%43445%8%34%
1534
June1st to 15thNight
23%6%23424447%36%
1440
16th to endNight
15%5%1342445%10%41%
1440
July1st to 15thNight
15%5%25%5%35%5%5%8%37%
1440
16th to endNight
12%6%35%5%345%7%9%36%
1514
August1st to 15thNight
10%5%35%6%345%7%8%40%
1595
16th to endNight
11%424425%5%6%7%44%
1910
September1st to 15thNight
15%5%145%2445%6%43%
1809
16th to endNight
16%5%25%41444447%
1815
October1st to 15thNight
16%6%245%33445%45%
1853
16th to endNight
18%5%15%5%34446%41%
2263
November1st to 15thNight
18%6%34323335%44%
2277
16th to endNight
19%5%25%424445%42%
2340
December1st to 15thNight
21%6%34424346%39%
2496
16th to endNight
21%6%25%324446%37%
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%24323445%42%
1432
16th to endDay
15%7%34324347%43%
1539
February1st to 15thDay
17%6%2443445%7%37%
1705
16th to endDay
16%5%25%5%2445%6%40%
1580
March1st to 15thDay
20%6%24424347%39%
1800
16th to endDay
16%8%25%324336%43%
2204
April1st to 15thDay
20%5%24323446%42%
2152
16th to endDay
18%424324447%44%
2250
May1st to 15thDay
25%7%24423436%35%
2332
16th to endDay
17%5%23424447%41%
2588
June1st to 15thDay
22%5%24424447%35%
2520
16th to endDay
12%5%244345%5%10%40%
2520
July1st to 15thDay
14%5%2443447%9%40%
2520
16th to endDay
15%4245%345%5%9%39%
2654
August1st to 15thDay
13%415%435%6%5%7%41%
2349
16th to endDay
10%426%4245%5%7%44%
2292
September1st to 15thDay
15%5%24425%445%46%
2071
16th to endDay
16%414324345%47%
1815
October1st to 15thDay
17%6%15%42445%6%42%
1775
16th to endDay
14%4245%235%46%45%
1609
November1st to 15thDay
20%6%23423335%43%
1581
16th to endDay
17%324424445%45%
1620
December1st to 15thDay
20%5%2432335%6%40%
1461
16th to endDay
24%7%24313345%38%
1536
January1st to 15thNight
20%6%24323336%43%
2506
16th to endNight
18%8%24422445%42%
2683
February1st to 15thNight
15%8%35%5%23335%42%
2146
16th to endNight
17%6%2333434444%
1893
March1st to 15thNight
19%7%24423445%39%
2160
16th to endNight
17%7%24324245%44%
2016
April1st to 15thNight
19%7%33434446%38%
1796
16th to endNight
23%43443333439%
1709
May1st to 15thNight
20%8%35%434346%35%
1620
16th to endNight
20%6%25%43445%8%34%
1534
June1st to 15thNight
24%6%33324346%36%
1440
16th to endNight
16%41343345%8%42%
1440
July1st to 15thNight
15%5%25%5%25%46%8%37%
1440
16th to endNight
13%5%26%5%45%46%9%36%
1514
August1st to 15thNight
11%5%25%5%345%7%9%40%
1595
16th to endNight
11%5%24435%46%8%44%
1910
September1st to 15thNight
16%424434447%42%
1809
16th to endNight
16%5%24424345%47%
1815
October1st to 15thNight
16%6%145%24335%45%
1853
16th to endNight
18%5%25%5%33446%41%
2263
November1st to 15thNight
17%6%24324335%44%
2277
16th to endNight
19%5%25%323446%42%
2340
December1st to 15thNight
22%6%24324346%39%
2496
16th to endNight
21%6%25%424346%37%
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
42%6%14%21%12%3
1433
16th to endDay
41%411%25%15%1
1539
February1st to 15thDay
44%6%14%20%13%1
1708
16th to endDay
42%5%12%23%12%2
1580
March1st to 15thDay
44%414%22%10%2
1800
16th to endDay
43%411%19%17%4
2204
April1st to 15thDay
42%5%12%19%16%3
2152
16th to endDay
39%6%11%21%17%3
2250
May1st to 15thDay
49%6%8%19%13%2
2332
16th to endDay
42%7%14%20%12%2
2592
June1st to 15thDay
47%7%12%20%10%
2520
16th to endDay
36%9%15%23%13%1
2520
July1st to 15thDay
37%9%14%22%14%1
2371
16th to endDay
38%7%16%21%12%2
2487
August1st to 15thDay
36%9%17%20%15%
2335
16th to endDay
35%7%11%25%18%1
2292
September1st to 15thDay
36%6%13%19%20%1
2071
16th to endDay
36%48%23%24%1
1815
October1st to 15thDay
40%5%11%22%16%3
1775
16th to endDay
37%6%13%23%16%2
1609
November1st to 15thDay
43%5%14%16%17%2
1581
16th to endDay
39%5%12%25%14%2
1620
December1st to 15thDay
43%5%16%18%14%
1461
16th to endDay
48%415%18%12%
1536
January1st to 15thNight
43%5%12%22%13%2
2506
16th to endNight
44%410%22%15%2
2683
February1st to 15thNight
45%413%22%11%2
2154
16th to endNight
41%410%22%17%2
1893
March1st to 15thNight
44%5%12%20%13%3
2160
16th to endNight
42%49%18%20%5%
2017
April1st to 15thNight
45%5%10%19%15%2
1796
16th to endNight
47%48%18%16%4
1709
May1st to 15thNight
50%411%17%13%3
1620
16th to endNight
47%10%15%15%9%1
1536
June1st to 15thNight
48%6%11%17%14%1
1440
16th to endNight
36%7%15%24%13%1
1440
July1st to 15thNight
42%9%11%22%13%
1360
16th to endNight
41%7%14%22%11%2
1418
August1st to 15thNight
37%8%14%22%14%2
1585
16th to endNight
35%8%11%25%18%1
1910
September1st to 15thNight
39%6%10%23%18%
1809
16th to endNight
39%5%7%19%25%2
1815
October1st to 15thNight
42%410%22%15%4
1853
16th to endNight
42%6%14%19%15%1
2263
November1st to 15thNight
43%411%19%17%3
2277
16th to endNight
43%5%12%23%12%2
2340
December1st to 15thNight
47%5%14%18%13%
2496
16th to endNight
47%5%12%21%12%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%33%25%20%3
60
16th to endDay
23%20%43%13%
65
February1st to 15thDay
23%20%40%12%2
221
16th to endDay
15%22%39%18%3
239
March1st to 15thDay
24%21%29%20%3
270
16th to endDay
23%14%39%15%41
288
April1st to 15thDay
23%15%32%19%8%1
270
16th to endDay
21%10%34%22%7%3
270
May1st to 15thDay
33%12%26%18%6%2
288
16th to endDay
20%6%31%26%12%2
331
June1st to 15thDay
22%9%26%24%15%2
315
16th to endDay
18%8%28%31%10%2
315
July1st to 15thDay
19%425%29%17%2
305
16th to endDay
13%327%35%14%5%
326
August1st to 15thDay
14%225%34%15%7%
313
16th to endDay
11%324%30%21%7%
332
September1st to 15thDay
16%8%25%28%14%6%
299
16th to endDay
17%329%25%14%9%
254
October1st to 15thDay
20%7%27%24%16%4
255
16th to endDay
15%13%35%24%9%1
245
November1st to 15thDay
22%13%28%21%11%1
235
16th to endDay
19%15%31%24%9%
240
December1st to 15thDay
23%23%32%13%7%
81
16th to endDay
34%20%34%10%
64
January1st to 15thNight
22%18%35%19%3
720
16th to endNight
22%20%35%20%1
768
February1st to 15thNight
19%17%41%19%1
704
16th to endNight
21%15%40%20%2
631
March1st to 15thNight
23%14%35%23%3
720
16th to endNight
21%11%37%23%5%
768
April1st to 15thNight
24%8%36%25%4
718
16th to endNight
24%7%32%27%6%1
629
May1st to 15thNight
22%5%33%27%9%
540
16th to endNight
23%327%33%10%1
414
June1st to 15thNight
26%322%32%14%
360
16th to endNight
19%118%44%15%
360
July1st to 15thNight
17%319%39%19%1
340
16th to endNight
13%221%42%19%1
376
August1st to 15thNight
12%119%44%20%1
514
16th to endNight
14%114%39%25%4
764
September1st to 15thNight
18%323%35%15%2
707
16th to endNight
18%220%31%21%4
657
October1st to 15thNight
20%6%32%24%12%2
660
16th to endNight
21%11%37%21%7%
704
November1st to 15thNight
21%11%31%23%11%1
700
16th to endNight
21%15%32%21%8%
720
December1st to 15thNight
26%15%31%20%5%
720
16th to endNight
24%16%37%18%2
768
Page generated at 2017-09-13 07:39:20. Version ac04

Copyright 2017 A. Danko


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