This page made possible by the sponsorship of: Patrick Manley.

Dee-Top Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Dee-Top Observatory clear sky chart from 2005-11-06 to 2024-01-16.

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%
22991
14%
74770
8%
22947
35%
22947
15%
53241

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
12%
760
11%
2649
4%
757
20%
757
8%
2026
January11th to 20thNight
14%
741
9%
2591
4%
741
21%
741
9%
2023
January21st to endNight
13%
792
12%
2583
4%
792
22%
792
9%
1984
February1st to 10thNight
12%
720
10%
2340
3%
720
21%
720
10%
1791
February11th to 20thNight
10%
703
9%
2200
2%
700
19%
700
8%
1722
February21st to endNight
12%
592
15%
1776
5%
592
23%
592
11%
1336
March1st to 10thNight
16%
720
14%
2143
5%
720
27%
720
13%
1592
March11th to 20thNight
17%
720
15%
1980
6%
720
25%
720
14%
1478
March21st to endNight
18%
792
17%
2066
5%
792
27%
792
17%
1512
April1st to 10thNight
17%
720
13%
1800
3%
720
23%
720
15%
1380
April11th to 20thNight
21%
711
15%
1775
4%
711
26%
711
20%
1293
April21st to endNight
23%
720
15%
1800
3%
720
29%
720
20%
1303
May1st to 10thNight
29%
454
16%
1534
9%
454
37%
454
14%
1103
May11th to 20thNight
33%
354
18%
1416
13%
354
43%
354
16%
915
May21st to endNight
40%
388
15%
1551
15%
388
48%
388
19%
958
June1st to 10thNight
42%
360
15%
1440
15%
360
51%
360
20%
873
June11th to 20thNight
43%
360
14%
1440
13%
360
52%
360
20%
891
June21st to endNight
48%
360
14%
1440
17%
360
54%
360
22%
849
July1st to 10thNight
54%
350
13%
1400
17%
350
58%
350
22%
841
July11th to 20thNight
55%
348
17%
1392
21%
348
59%
348
24%
800
July21st to endNight
51%
394
16%
1486
17%
372
56%
372
22%
881
August1st to 10thNight
56%
519
15%
1583
11%
517
59%
517
28%
1001
August11th to 20thNight
55%
619
14%
1683
10%
619
57%
619
23%
1184
August21st to endNight
52%
792
19%
1980
10%
792
54%
792
31%
1333
September1st to 10thNight
44%
720
17%
1800
8%
720
48%
720
28%
1235
September11th to 20thNight
50%
720
26%
1807
16%
720
55%
720
34%
1158
September21st to endNight
41%
720
21%
2016
15%
720
47%
720
25%
1348
October1st to 10thNight
36%
720
15%
2160
11%
720
42%
720
20%
1491
October11th to 20thNight
28%
720
16%
2160
7%
720
39%
720
20%
1487
October21st to endNight
18%
792
13%
2456
7%
792
27%
792
13%
1859
November1st to 10thNight
27%
740
19%
2407
10%
740
39%
740
19%
1590
November11th to 20thNight
21%
760
17%
2512
10%
760
31%
760
14%
1759
November21st to endNight
21%
760
13%
2660
7%
760
30%
760
13%
1892
December1st to 10thNight
15%
760
9%
2660
4%
760
20%
760
8%
2099
December11th to 20thNight
15%
760
9%
3244
3%
759
21%
759
7%
2441
December21st to endNight
11%
830
6%
4840
3%
817
20%
817
4%
3813

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%10%47%6%35%5%5%7%31%
74759
Night
16%10%48%6%35%45%7%32%
75029
TransparencyDay
53%6%11%19%10%1
74464
Night
52%6%12%19%10%1
74858
SeeingDay
25%17%28%18%9%3
13659
Night
23%17%28%23%9%1
22998

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
12%13%9%11%7%46%447%23%
1517
11th to 20thDay
16%14%6%14%9%5%5%5%45%17%
1480
21st to endDay
10%14%6%14%9%5%5%446%23%
1770
February1st to 10thDay
16%15%7%10%10%45%5%5%5%18%
1620
11th to 20thDay
14%13%7%11%8%46%5%5%5%23%
1646
21st to endDay
19%13%5%11%8%24445%23%
1480
March1st to 10thDay
14%13%7%11%6%34345%29%
1810
11th to 20thDay
16%14%5%9%6%34445%29%
1980
21st to endDay
19%13%48%6%25%45%5%30%
2290
April1st to 10thDay
17%13%5%8%5%3445%6%32%
2160
11th to 20thDay
16%9%47%6%35%45%6%34%
2134
21st to endDay
19%13%46%5%34346%33%
2160
May1st to 10thDay
20%10%26%6%35%5%5%6%31%
2441
11th to 20thDay
19%8%36%6%35%5%5%7%33%
2478
21st to endDay
15%6%36%6%45%5%5%8%38%
2725
June1st to 10thDay
18%6%36%6%35%6%6%9%33%
2520
11th to 20thDay
16%6%36%6%45%6%6%10%32%
2520
21st to endDay
14%7%36%6%35%5%6%9%36%
2520
July1st to 10thDay
11%6%36%6%46%6%7%9%35%
2520
11th to 20thDay
11%6%26%6%46%6%7%9%38%
2520
21st to endDay
13%6%36%6%46%6%7%9%34%
2744
August1st to 10thDay
13%5%26%6%45%6%7%9%36%
2341
11th to 20thDay
15%7%36%6%36%6%6%8%33%
2198
21st to endDay
14%8%36%6%45%6%6%8%34%
2376
September1st to 10thDay
18%7%35%5%345%5%7%38%
2160
11th to 20thDay
15%7%35%5%345%5%7%43%
2152
21st to endDay
17%8%36%5%35%5%6%7%36%
1942
October1st to 10thDay
18%8%47%5%35%5%5%7%33%
1800
11th to 20thDay
16%11%46%6%3446%6%33%
1800
21st to endDay
23%13%37%6%4445%6%25%
1898
November1st to 10thDay
14%11%46%5%3445%6%38%
1671
11th to 20thDay
16%15%5%9%6%3344431%
1668
21st to endDay
17%15%47%7%4445%6%25%
1520
December1st to 10thDay
18%16%7%10%6%45%5%4420%
1520
11th to 20thDay
22%15%9%10%5%23335%25%
1906
21st to endDay
23%18%8%10%7%35%43415%
2772
January1st to 10thNight
14%13%6%13%7%46%46%6%22%
2649
11th to 20thNight
16%13%6%12%8%46%45%6%21%
2591
21st to endNight
11%11%7%10%11%5%5%5%5%6%22%
2583
February1st to 10thNight
16%12%6%11%8%46%5%46%23%
2340
11th to 20thNight
14%13%6%11%9%37%5%5%6%22%
2198
21st to endNight
20%11%410%8%34446%27%
1776
March1st to 10thNight
15%11%6%11%6%345%46%29%
2133
11th to 20thNight
16%11%5%7%8%245%45%31%
1979
21st to endNight
17%13%47%5%245%5%5%33%
2066
April1st to 10thNight
18%12%5%7%6%35%5%46%30%
1800
11th to 20thNight
19%9%37%5%2445%7%35%
1775
21st to endNight
18%10%46%6%3435%7%35%
1800
May1st to 10thNight
22%11%45%5%34445%32%
1534
11th to 20thNight
14%6%46%5%36%5%7%7%36%
1415
21st to endNight
13%6%36%6%25%46%9%41%
1550
June1st to 10thNight
17%6%36%5%2336%10%39%
1440
11th to 20thNight
15%6%35%5%25%5%5%8%41%
1440
21st to endNight
13%335%5%35%5%6%9%43%
1440
July1st to 10thNight
10%5%36%6%35%5%6%9%43%
1440
11th to 20thNight
11%4335%236%8%10%44%
1440
21st to endNight
12%6%26%5%345%6%8%42%
1574
August1st to 10thNight
9%6%35%5%46%6%6%9%42%
1593
11th to 20thNight
13%7%36%7%46%5%6%6%38%
1683
21st to endNight
13%6%345%345%6%7%44%
1980
September1st to 10thNight
15%6%35%5%2445%7%43%
1800
11th to 20thNight
16%424323447%51%
1807
21st to endNight
16%7%36%5%35%45%7%41%
2016
October1st to 10thNight
18%8%35%6%35%45%6%37%
2160
11th to 20thNight
19%9%36%5%3445%6%37%
2160
21st to endNight
24%11%46%6%34347%27%
2456
November1st to 10thNight
12%12%37%5%3445%6%39%
2407
11th to 20thNight
19%11%46%5%3446%7%32%
2512
21st to endNight
15%12%5%7%6%35%5%6%7%29%
2660
December1st to 10thNight
18%15%7%10%7%3445%6%21%
2660
11th to 20thNight
20%13%6%10%6%345%45%24%
3288
21st to endNight
25%14%8%9%5%34445%19%
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
62%5%9%17%6%1
1518
11th to 20thDay
68%6%9%9%6%2
1480
21st to endDay
62%5%11%14%7%1
1770
February1st to 10thDay
67%6%8%12%6%1
1620
11th to 20thDay
60%6%7%19%7%
1655
21st to endDay
62%5%9%16%8%1
1480
March1st to 10thDay
56%6%12%17%7%1
1813
11th to 20thDay
58%5%8%17%10%2
1980
21st to endDay
57%47%15%13%2
2290
April1st to 10thDay
54%48%22%9%3
2160
11th to 20thDay
50%5%10%19%13%2
2134
21st to endDay
52%5%9%19%11%4
2160
May1st to 10thDay
53%6%9%19%10%2
2441
11th to 20thDay
50%6%9%21%11%2
2482
21st to endDay
45%7%14%21%11%1
2725
June1st to 10thDay
49%7%12%20%12%
2520
11th to 20thDay
47%8%13%20%10%1
2520
21st to endDay
45%7%12%25%10%1
2520
July1st to 10thDay
44%8%14%22%11%1
2450
11th to 20thDay
40%8%15%25%12%
2445
21st to endDay
44%8%13%21%12%1
2591
August1st to 10thDay
45%8%14%22%11%
2327
11th to 20thDay
48%7%14%23%9%
2198
21st to endDay
45%8%10%23%12%2
2376
September1st to 10thDay
46%6%12%23%12%1
2160
11th to 20thDay
40%6%11%22%21%1
2152
21st to endDay
47%5%11%22%13%1
1942
October1st to 10thDay
49%6%11%20%12%2
1800
11th to 20thDay
50%6%10%23%10%1
1800
21st to endDay
60%412%14%8%2
1898
November1st to 10thDay
47%5%13%20%13%2
1671
11th to 20thDay
57%410%18%9%2
1668
21st to endDay
58%6%14%16%5%1
1520
December1st to 10thDay
65%6%13%10%5%1
1520
11th to 20thDay
64%413%15%4
1906
21st to endDay
72%5%7%11%4
2772
January1st to 10thNight
61%7%10%14%7%1
2649
11th to 20thNight
63%6%12%13%5%1
2591
21st to endNight
62%5%10%14%8%1
2583
February1st to 10thNight
61%5%12%14%8%
2340
11th to 20thNight
62%7%10%14%6%1
2200
21st to endNight
59%410%16%9%1
1776
March1st to 10thNight
56%5%11%18%9%1
2143
11th to 20thNight
54%5%10%19%10%1
1980
21st to endNight
51%6%9%17%15%2
2066
April1st to 10thNight
57%5%9%18%11%2
1800
11th to 20thNight
49%5%10%19%16%
1775
21st to endNight
49%6%11%20%12%3
1800
May1st to 10thNight
54%6%7%19%12%2
1534
11th to 20thNight
45%7%10%22%13%2
1416
21st to endNight
41%7%16%23%11%2
1551
June1st to 10thNight
43%5%13%24%15%1
1440
11th to 20thNight
41%7%16%24%10%1
1440
21st to endNight
36%9%15%26%13%1
1440
July1st to 10thNight
36%10%15%23%14%2
1400
11th to 20thNight
34%9%17%27%12%
1392
21st to endNight
38%7%16%22%14%2
1486
August1st to 10thNight
37%8%16%27%11%1
1583
11th to 20thNight
44%9%14%23%10%
1683
21st to endNight
39%7%12%25%16%1
1980
September1st to 10thNight
41%6%13%25%13%1
1800
11th to 20thNight
35%5%11%27%20%1
1807
21st to endNight
43%5%11%23%16%2
2016
October1st to 10thNight
47%6%12%21%12%1
2160
11th to 20thNight
47%5%14%22%10%2
2160
21st to endNight
58%6%11%16%8%1
2456
November1st to 10thNight
45%6%12%20%15%2
2407
11th to 20thNight
51%6%12%18%11%2
2512
21st to endNight
53%6%15%19%6%
2660
December1st to 10thNight
64%7%11%14%4
2660
11th to 20thNight
61%6%12%15%6%
3288
21st to endNight
67%6%9%14%4
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
32%25%31%11%11
180
11th to 20thDay
42%25%26%5%11
170
21st to endDay
25%32%31%8%4
260
February1st to 10thDay
35%35%24%5%
240
11th to 20thDay
38%30%27%41
313
21st to endDay
37%26%26%10%1
346
March1st to 10thDay
35%20%31%12%3
420
11th to 20thDay
31%27%25%13%3
420
21st to endDay
34%25%25%9%5%2
462
April1st to 10thDay
35%21%29%10%41
420
11th to 20thDay
27%26%28%10%8%3
412
21st to endDay
31%21%30%11%5%2
420
May1st to 10thDay
34%17%27%11%8%3
423
11th to 20thDay
21%17%33%18%7%3
447
21st to endDay
18%13%30%23%11%5%
485
June1st to 10thDay
21%13%32%22%10%2
450
11th to 20thDay
20%15%25%24%13%2
450
21st to endDay
17%14%35%23%9%2
450
July1st to 10thDay
15%7%27%30%16%5%
445
11th to 20thDay
11%10%26%30%18%5%
444
21st to endDay
15%9%36%27%10%3
488
August1st to 10thDay
14%9%25%26%19%7%
446
11th to 20thDay
18%7%25%26%18%5%
439
21st to endDay
16%10%27%24%14%9%
495
September1st to 10thDay
20%12%24%20%18%7%
450
11th to 20thDay
17%9%28%21%16%10%
450
21st to endDay
23%11%29%20%12%5%
450
October1st to 10thDay
23%13%24%22%13%5%
450
11th to 20thDay
26%17%31%19%6%1
450
21st to endDay
35%18%25%17%5%
453
November1st to 10thDay
28%18%22%20%9%2
365
11th to 20thDay
34%23%22%13%6%
328
21st to endDay
36%23%33%8%1
180
December1st to 10thDay
33%31%26%5%42
180
11th to 20thDay
37%25%25%12%1
180
21st to endDay
34%29%26%9%21
198
January1st to 10thNight
25%27%32%13%3
760
11th to 20thNight
27%23%29%18%3
741
21st to endNight
19%27%35%17%2
792
February1st to 10thNight
28%22%33%16%1
720
11th to 20thNight
28%28%32%12%1
703
21st to endNight
32%22%31%12%2
592
March1st to 10thNight
27%25%29%17%3
720
11th to 20thNight
28%23%29%17%4
720
21st to endNight
26%22%31%15%6%1
792
April1st to 10thNight
28%23%28%18%3
720
11th to 20thNight
23%20%31%19%7%
711
21st to endNight
26%17%30%22%5%
720
May1st to 10thNight
30%13%24%24%9%
454
11th to 20thNight
17%12%34%29%7%1
354
21st to endNight
15%8%29%34%13%1
389
June1st to 10thNight
16%8%26%37%13%1
360
11th to 20thNight
18%9%24%36%13%1
360
21st to endNight
14%5%26%45%10%
360
July1st to 10thNight
11%421%42%21%1
350
11th to 20thNight
13%122%43%20%1
348
21st to endNight
13%6%22%42%16%1
394
August1st to 10thNight
12%322%38%23%2
519
11th to 20thNight
16%5%13%39%23%4
619
21st to endNight
15%6%19%34%24%2
792
September1st to 10thNight
18%7%22%34%16%3
720
11th to 20thNight
16%7%21%29%22%5%
720
21st to endNight
19%9%25%29%16%2
720
October1st to 10thNight
23%11%24%24%16%2
720
11th to 20thNight
25%13%29%26%7%1
720
21st to endNight
32%16%29%19%5%
792
November1st to 10thNight
21%16%32%23%8%
740
11th to 20thNight
28%20%28%18%5%1
760
21st to endNight
23%22%31%20%4
760
December1st to 10thNight
29%23%30%15%4
760
11th to 20thNight
27%24%28%19%2
760
21st to endNight
28%25%32%13%3
836
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