Crawford Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Crawford Observatory clear sky chart from 2005-11-05 to 2017-09-20.

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
50%
15299
24%
47002
17%
15256
58%
15256
29%
28724

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
27%
720
25%
2327
11%
716
43%
716
23%
1420
January16th to endNight
27%
768
26%
2322
11%
768
42%
768
24%
1504
February1st to 15thNight
32%
704
25%
2112
11%
701
45%
701
26%
1312
February16th to endNight
29%
631
25%
1893
12%
631
40%
631
26%
1202
March1st to 15thNight
31%
720
21%
2142
11%
720
40%
720
26%
1341
March16th to endNight
41%
731
24%
2075
13%
731
49%
731
32%
1244
April1st to 15thNight
42%
539
25%
1797
18%
539
54%
539
30%
1032
April16th to endNight
47%
540
27%
1800
20%
540
59%
540
34%
990
May1st to 15thNight
53%
540
23%
1632
16%
540
61%
540
31%
1011
May16th to endNight
67%
561
22%
1682
18%
561
71%
561
33%
1001
June1st to 15thNight
75%
540
15%
1620
13%
540
75%
540
30%
1020
June16th to endNight
78%
540
19%
1620
17%
540
79%
540
34%
963
July1st to 15thNight
75%
510
13%
1530
12%
510
76%
510
29%
978
July16th to endNight
73%
564
16%
1614
16%
531
75%
531
30%
1026
August1st to 15thNight
77%
539
17%
1788
21%
536
77%
536
29%
1108
August16th to endNight
70%
573
19%
1910
23%
573
71%
573
23%
1274
September1st to 15thNight
65%
630
20%
1890
20%
630
66%
630
25%
1260
September16th to endNight
64%
677
27%
1870
21%
677
67%
677
35%
1130
October1st to 15thNight
59%
660
34%
1815
25%
660
65%
660
42%
1033
October16th to endNight
52%
704
33%
2111
27%
704
60%
704
36%
1197
November1st to 15thNight
52%
700
29%
2276
25%
700
61%
700
36%
1209
November16th to endNight
41%
720
28%
2340
19%
720
53%
720
29%
1343
December1st to 15thNight
38%
720
23%
2340
17%
720
47%
720
26%
1450
December16th to endNight
31%
768
20%
2496
14%
768
40%
768
20%
1676

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
11%5%25%5%35%5%5%7%41%
47826
Night
10%5%2442445%7%46%
47203
TransparencyDay
40%7%11%18%17%5%
47530
Night
35%6%10%19%20%7%
47004
SeeingDay
12%6%18%23%24%13%
5211
Night
13%7%22%28%22%5%
15301

By Month

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
18%6%34413345%44%
1611
16th to endDay
15%6%345%2344445%
1899
February1st to 15thDay
14%7%26%424335%43%
1750
16th to endDay
18%7%34423335%42%
1580
March1st to 15thDay
15%7%25%5%2325%5%42%
1818
16th to endDay
13%5%25%6%3445%7%42%
2157
April1st to 15thDay
11%5%345%345%5%6%45%
2151
16th to endDay
8%425%435%45%8%47%
2160
May1st to 15thDay
10%425%5%35%5%6%8%43%
2319
16th to endDay
7%5%25%5%35%5%5%8%43%
2431
June1st to 15thDay
7%426%6%47%7%8%9%34%
2340
16th to endDay
5%324436%6%8%12%41%
2340
July1st to 15thDay
8%426%7%5%7%7%9%10%30%
2340
16th to endDay
5%426%7%48%7%8%12%31%
2441
August1st to 15thDay
6%415%6%49%8%8%9%31%
2160
16th to endDay
11%5%36%7%47%7%8%8%29%
2292
September1st to 15thDay
11%6%36%7%36%5%5%7%36%
2055
16th to endDay
11%6%246%345%5%6%43%
1859
October1st to 15thDay
10%41441445%7%51%
1815
16th to endDay
9%423323235%58%
1758
November1st to 15thDay
12%5%24313235%54%
1582
16th to endDay
13%5%23423346%49%
1620
December1st to 15thDay
17%7%245%2435%5%41%
1620
16th to endDay
19%10%47%5%2323436%
1728
January1st to 15thNight
14%6%24324346%46%
2327
16th to endNight
16%7%34434345%42%
2322
February1st to 15thNight
14%5%25%423346%46%
2103
16th to endNight
15%435%5%24345%46%
1893
March1st to 15thNight
15%6%24323447%45%
2142
16th to endNight
10%5%233235%6%7%48%
2075
April1st to 15thNight
9%42442445%8%49%
1797
16th to endNight
8%31342445%6%55%
1800
May1st to 15thNight
7%3246%45%6%48%47%
1631
16th to endNight
43245%36%5%5%11%47%
1682
June1st to 15thNight
33246%37%6%8%12%40%
1620
16th to endNight
3215%435%6%9%13%43%
1620
July1st to 15thNight
5%425%5%36%6%8%11%39%
1620
16th to endNight
4336%6%36%5%7%12%39%
1723
August1st to 15thNight
5%4145%45%6%8%13%41%
1800
16th to endNight
8%5%25%6%38%5%7%9%37%
1910
September1st to 15thNight
9%5%146%36%6%6%8%41%
1890
16th to endNight
8%4245%2345%7%51%
1870
October1st to 15thNight
8%413422346%56%
1815
16th to endNight
10%5%1442332358%
2111
November1st to 15thNight
11%412323336%57%
2276
16th to endNight
10%424423445%52%
2340
December1st to 15thNight
13%7%24423446%45%
2340
16th to endNight
19%8%34334446%37%
2496

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%46%14%25%5%
1612
16th to endDay
41%35%18%23%7%
1899
February1st to 15thDay
43%36%21%20%4
1752
16th to endDay
45%35%15%23%6%
1580
March1st to 15thDay
44%39%17%19%5%
1818
16th to endDay
41%5%10%19%19%3
2158
April1st to 15thDay
38%6%9%20%19%7%
2151
16th to endDay
33%6%10%20%25%3
2160
May1st to 15thDay
37%7%12%18%20%4
2319
16th to endDay
37%7%13%23%14%3
2435
June1st to 15thDay
41%11%16%19%10%1
2340
16th to endDay
32%11%17%21%14%3
2340
July1st to 15thDay
41%14%18%16%7%1
2201
16th to endDay
41%12%17%18%9%
2288
August1st to 15thDay
41%12%16%17%9%2
2148
16th to endDay
47%11%13%16%9%2
2292
September1st to 15thDay
45%8%11%16%13%4
2055
16th to endDay
39%6%11%18%18%5%
1859
October1st to 15thDay
32%5%10%17%23%10%
1815
16th to endDay
30%3416%32%11%
1758
November1st to 15thDay
34%26%17%28%10%
1582
16th to endDay
36%46%16%26%9%
1620
December1st to 15thDay
44%48%19%16%6%
1620
16th to endDay
53%36%17%12%5%
1728
January1st to 15thNight
39%46%18%24%5%
2327
16th to endNight
43%35%17%21%8%
2322
February1st to 15thNight
39%35%20%19%10%
2112
16th to endNight
40%4418%23%7%
1893
March1st to 15thNight
38%5%9%18%19%7%
2142
16th to endNight
32%5%9%24%23%4
2075
April1st to 15thNight
33%5%9%21%20%9%
1797
16th to endNight
29%5%10%23%22%8%
1800
May1st to 15thNight
33%7%9%18%23%5%
1632
16th to endNight
29%9%14%25%15%5%
1684
June1st to 15thNight
31%12%18%21%14%1
1620
16th to endNight
28%10%17%23%16%3
1620
July1st to 15thNight
32%12%21%21%9%2
1530
16th to endNight
33%11%22%20%11%
1614
August1st to 15thNight
31%12%17%22%12%3
1788
16th to endNight
39%11%14%19%12%2
1910
September1st to 15thNight
37%12%11%19%14%5%
1890
16th to endNight
31%6%10%22%21%7%
1870
October1st to 15thNight
28%39%19%23%14%
1815
16th to endNight
30%4411%32%16%
2111
November1st to 15thNight
29%37%16%28%14%
2276
16th to endNight
34%46%17%24%12%
2340
December1st to 15thNight
40%49%19%21%5%
2340
16th to endNight
48%46%17%16%6%
2496

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
15%20%40%14%6%3
90
16th to endDay
17%23%46%7%31
96
February1st to 15thDay
17%27%34%18%1
90
16th to endDay
24%14%41%18%
81
March1st to 15thDay
16%25%38%12%5%
108
16th to endDay
18%9%30%23%16%1
288
April1st to 15thDay
17%12%28%20%19%1
270
16th to endDay
10%7%27%29%20%5%
270
May1st to 15thDay
12%9%21%27%19%9%
288
16th to endDay
10%5%20%27%20%16%
331
June1st to 15thDay
10%112%21%30%22%
315
16th to endDay
9%19%20%33%26%
315
July1st to 15thDay
6%1420%36%30%
305
16th to endDay
6%5%15%38%33%
326
August1st to 15thDay
9%18%23%33%23%
313
16th to endDay
12%426%35%21%
332
September1st to 15thDay
10%410%22%33%18%
315
16th to endDay
11%115%31%27%11%
270
October1st to 15thDay
15%416%27%31%4
255
16th to endDay
13%6%17%30%22%8%
187
November1st to 15thDay
23%12%24%24%13%2
90
16th to endDay
7%16%41%20%12%2
90
December1st to 15thDay
27%14%34%13%8%1
90
16th to endDay
21%25%26%21%5%
96
January1st to 15thNight
20%15%32%23%7%
720
16th to endNight
21%13%35%25%5%
768
February1st to 15thNight
18%13%32%30%5%
704
16th to endNight
17%13%35%27%4
631
March1st to 15thNight
19%14%33%27%6%
720
16th to endNight
13%11%32%27%15%
731
April1st to 15thNight
11%9%33%27%16%1
539
16th to endNight
10%8%29%32%17%1
540
May1st to 15thNight
7%427%30%26%2
540
16th to endNight
6%116%36%32%7%
563
June1st to 15thNight
5%7%36%40%9%
540
16th to endNight
45%26%45%16%
540
July1st to 15thNight
6%7%22%46%17%
510
16th to endNight
5%7%22%43%20%
564
August1st to 15thNight
7%5%22%50%12%
539
16th to endNight
11%6%28%39%13%
573
September1st to 15thNight
12%111%23%38%11%
630
16th to endNight
10%113%36%33%4
677
October1st to 15thNight
11%221%32%27%3
660
16th to endNight
13%6%22%29%24%3
704
November1st to 15thNight
13%7%23%30%22%3
700
16th to endNight
13%13%27%31%12%
720
December1st to 15thNight
18%10%29%29%11%
720
16th to endNight
24%13%27%26%7%
768
Page generated at 2017-09-21 06:18:11. Version ac04

Copyright 2017 A. Danko


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