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Collins Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Collins Observatory clear sky chart from 2005-11-05 to 2018-01-20.

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%
15325
16%
48524
7%
15293
38%
15293
19%
32885

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%
780
17%
2716
6%
776
35%
776
17%
1774
January16th to endNight
17%
788
16%
2756
3%
788
33%
788
14%
1848
February1st to 15thNight
19%
704
13%
2268
6%
701
31%
701
13%
1619
February16th to endNight
18%
631
17%
1893
5%
631
33%
631
18%
1329
March1st to 15thNight
18%
720
15%
2115
5%
720
31%
720
16%
1530
March16th to endNight
26%
768
18%
2090
6%
768
36%
768
22%
1414
April1st to 15thNight
20%
697
16%
1775
3%
697
29%
697
19%
1306
April16th to endNight
23%
540
15%
1620
5%
540
31%
540
17%
1144
May1st to 15thNight
25%
540
13%
1620
7%
540
32%
540
16%
1175
May16th to endNight
25%
420
11%
1542
8%
419
33%
419
13%
1102
June1st to 15thNight
30%
360
9%
1278
7%
360
37%
360
13%
938
June16th to endNight
38%
360
10%
1286
9%
360
44%
360
21%
858
July1st to 15thNight
44%
340
9%
1360
12%
340
49%
340
19%
879
July16th to endNight
46%
376
12%
1418
11%
354
52%
354
21%
884
August1st to 15thNight
44%
518
12%
1589
7%
516
48%
516
22%
1067
August16th to endNight
46%
612
20%
1758
12%
612
50%
612
32%
1092
September1st to 15thNight
40%
720
18%
1896
9%
720
46%
720
33%
1222
September16th to endNight
41%
717
27%
1980
16%
717
51%
717
33%
1200
October1st to 15thNight
36%
720
19%
2049
10%
720
45%
720
25%
1346
October16th to endNight
22%
768
15%
2475
6%
768
33%
768
18%
1695
November1st to 15thNight
30%
760
19%
2472
11%
760
42%
760
20%
1631
November16th to endNight
26%
800
19%
2623
9%
800
40%
800
18%
1726
December1st to 15thNight
19%
825
15%
2906
4%
825
32%
825
15%
2027
December16th to endNight
17%
861
17%
3039
4%
861
32%
861
15%
2079

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
18%6%25%43445%6%36%
48995
Night
18%7%35%5%34446%37%
48694
TransparencyDay
46%5%11%20%12%1
48683
Night
46%5%11%21%12%1
48524
SeeingDay
24%12%30%21%9%2
6540
Night
22%11%32%25%7%
15325

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%7%36%43345%6%37%
1552
16th to endDay
16%10%345%24446%37%
1572
February1st to 15thDay
15%10%36%5%35%446%33%
1589
16th to endDay
14%10%37%5%4445%6%32%
1580
March1st to 15thDay
19%8%35%5%3435%434%
1835
16th to endDay
17%7%34323346%41%
2133
April1st to 15thDay
19%8%35%5%2335%5%36%
2173
16th to endDay
21%6%245%2345%6%37%
2340
May1st to 15thDay
25%7%35%424346%30%
2332
16th to endDay
22%6%245%25%45%7%32%
2582
June1st to 15thDay
27%7%34434447%28%
2679
16th to endDay
21%6%34425%5%5%7%32%
2670
July1st to 15thDay
13%5%246%35%6%6%7%36%
2520
16th to endDay
14%5%35%6%45%5%6%8%34%
2654
August1st to 15thDay
14%5%245%35%5%6%9%37%
2348
16th to endDay
13%42332445%8%46%
2441
September1st to 15thDay
13%5%24425%45%5%45%
2063
16th to endDay
14%414323346%51%
1980
October1st to 15thDay
19%5%24424345%41%
1911
16th to endDay
21%6%26%423445%38%
1746
November1st to 15thDay
20%5%245%24447%37%
1716
16th to endDay
21%6%24423225%43%
1664
December1st to 15thDay
23%8%26%323346%33%
1381
16th to endDay
23%10%35%5%3434431%
1534
January1st to 15thNight
15%8%35%435%446%38%
2716
16th to endNight
14%9%36%5%3445%6%35%
2756
February1st to 15thNight
13%8%48%7%46%5%5%6%31%
2267
16th to endNight
15%10%45%5%2444437%
1893
March1st to 15thNight
19%10%45%5%25%43434%
2102
16th to endNight
17%7%34423345%42%
2090
April1st to 15thNight
20%8%36%5%2344434%
1775
16th to endNight
24%6%25%5%3435%5%34%
1620
May1st to 15thNight
22%7%345%34447%31%
1620
16th to endNight
22%8%35%5%34447%28%
1540
June1st to 15thNight
24%7%35%5%3445%7%27%
1278
16th to endNight
20%6%14423449%36%
1286
July1st to 15thNight
15%6%35%5%35%45%9%35%
1440
16th to endNight
14%425%5%25%46%9%37%
1514
August1st to 15thNight
12%6%25%5%36%6%5%7%38%
1599
16th to endNight
12%42442335%7%48%
1758
September1st to 15thNight
15%5%14424346%48%
1896
16th to endNight
14%413323246%51%
1980
October1st to 15thNight
19%5%2433435%6%41%
2049
16th to endNight
22%7%25%5%23446%36%
2475
November1st to 15thNight
19%6%25%424446%39%
2472
16th to endNight
18%6%245%33446%40%
2623
December1st to 15thNight
23%8%35%5%33345%34%
2906
16th to endNight
21%10%34423346%34%
3039

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
46%5%13%18%14%2
1553
16th to endDay
47%413%22%11%1
1572
February1st to 15thDay
52%5%11%19%11%1
1592
16th to endDay
51%5%10%17%11%3
1580
March1st to 15thDay
52%411%17%12%2
1845
16th to endDay
43%49%19%17%4
2133
April1st to 15thDay
48%5%7%19%16%3
2173
16th to endDay
47%5%9%20%12%3
2340
May1st to 15thDay
53%5%8%18%12%2
2332
16th to endDay
50%7%12%18%11%1
2586
June1st to 15thDay
55%6%10%18%8%
2679
16th to endDay
50%7%11%17%12%
2670
July1st to 15thDay
41%9%13%24%10%
2371
16th to endDay
45%8%13%21%9%1
2487
August1st to 15thDay
43%8%14%25%8%
2334
16th to endDay
34%6%15%24%18%1
2441
September1st to 15thDay
39%6%11%25%16%1
2063
16th to endDay
33%49%29%22%1
1980
October1st to 15thDay
43%5%10%25%14%1
1911
16th to endDay
48%410%23%11%
1746
November1st to 15thDay
45%5%13%19%14%1
1716
16th to endDay
45%414%20%14%1
1664
December1st to 15thDay
51%5%12%21%9%
1381
16th to endDay
55%49%16%13%1
1534
January1st to 15thNight
45%5%12%22%11%1
2716
16th to endNight
48%5%10%20%13%1
2756
February1st to 15thNight
52%5%11%18%10%1
2268
16th to endNight
49%5%10%18%13%2
1893
March1st to 15thNight
53%48%18%11%2
2115
16th to endNight
44%411%17%18%4
2090
April1st to 15thNight
51%5%8%17%15%2
1775
16th to endNight
51%5%10%16%14%3
1620
May1st to 15thNight
51%7%8%17%13%1
1620
16th to endNight
54%6%12%16%9%1
1542
June1st to 15thNight
56%5%11%16%7%1
1278
16th to endNight
44%7%14%20%11%1
1286
July1st to 15thNight
43%8%14%22%10%
1360
16th to endNight
41%8%18%23%8%
1418
August1st to 15thNight
42%8%15%23%9%
1589
16th to endNight
34%6%15%27%14%
1758
September1st to 15thNight
36%5%11%29%15%1
1896
16th to endNight
33%49%33%17%1
1980
October1st to 15thNight
42%5%12%25%14%1
2049
16th to endNight
48%5%11%23%10%
2475
November1st to 15thNight
45%5%12%22%13%1
2472
16th to endNight
45%5%12%21%14%1
2623
December1st to 15thNight
52%49%22%11%
2906
16th to endNight
51%410%19%12%1
3039

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
22%22%28%20%6%
90
16th to endDay
23%20%35%17%2
73
February1st to 15thDay
28%14%32%20%4
224
16th to endDay
18%19%33%21%7%
239
March1st to 15thDay
31%16%32%15%4
270
16th to endDay
26%18%28%15%6%3
288
April1st to 15thDay
30%14%32%13%7%1
270
16th to endDay
24%20%32%12%7%2
270
May1st to 15thDay
38%11%26%16%5%2
288
16th to endDay
25%12%37%17%5%
331
June1st to 15thDay
30%10%27%21%10%
474
16th to endDay
22%8%29%28%9%1
465
July1st to 15thDay
18%8%34%24%12%2
305
16th to endDay
14%6%36%23%13%4
326
August1st to 15thDay
16%7%30%28%14%2
313
16th to endDay
16%9%25%26%16%5%
332
September1st to 15thDay
18%7%28%27%12%5%
315
16th to endDay
16%8%27%19%15%10%
314
October1st to 15thDay
22%14%24%21%13%4
298
16th to endDay
29%11%30%20%7%1
288
November1st to 15thDay
23%11%28%22%10%2
280
16th to endDay
28%13%26%22%7%1
285
December1st to 15thDay
27%13%39%13%6%
106
16th to endDay
38%20%28%12%
96
January1st to 15thNight
21%14%37%23%2
780
16th to endNight
20%16%37%21%3
788
February1st to 15thNight
20%16%35%25%2
704
16th to endNight
20%14%38%20%4
631
March1st to 15thNight
26%14%36%18%3
720
16th to endNight
21%16%30%24%6%
768
April1st to 15thNight
25%16%33%19%4
697
16th to endNight
27%12%32%22%4
540
May1st to 15thNight
25%11%32%25%5%
540
16th to endNight
25%9%33%27%4
420
June1st to 15thNight
27%8%27%30%6%
360
16th to endNight
20%233%35%8%
360
July1st to 15thNight
18%228%36%13%
340
16th to endNight
15%227%39%15%
376
August1st to 15thNight
14%429%37%13%
518
16th to endNight
15%428%34%16%1
612
September1st to 15thNight
17%433%30%12%1
720
16th to endNight
16%6%30%27%15%2
717
October1st to 15thNight
22%8%27%27%12%1
720
16th to endNight
26%13%33%20%6%
768
November1st to 15thNight
23%10%30%26%8%1
760
16th to endNight
22%14%32%22%8%
800
December1st to 15thNight
28%12%35%20%3
825
16th to endNight
29%12%36%18%2
861
Page generated at 2018-01-21 12:19:32. Version ac04

Copyright 2018 A. Danko


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