Nick Drew Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Nick Drew Observatory clear sky chart from 2005-11-05 to 2018-03-21.

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
43%
16648
21%
49215
14%
16602
50%
16602
27%
31306

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
26%
974
18%
2716
8%
970
34%
970
22%
1871
January16th to endNight
25%
1040
19%
2720
7%
1040
34%
1040
23%
1926
February1st to 15thNight
25%
884
14%
2412
4%
881
32%
881
19%
1741
February16th to endNight
25%
683
19%
2049
6%
683
34%
683
21%
1367
March1st to 15thNight
26%
741
18%
2301
8%
741
34%
741
21%
1546
March16th to endNight
32%
594
15%
2082
10%
594
39%
594
18%
1418
April1st to 15thNight
34%
539
21%
1797
13%
539
47%
539
26%
1107
April16th to endNight
41%
540
22%
1800
16%
540
51%
540
25%
1129
May1st to 15thNight
44%
540
17%
1654
13%
539
49%
539
24%
1101
May16th to endNight
56%
563
21%
1497
14%
561
60%
561
31%
985
June1st to 15thNight
66%
540
22%
1440
17%
540
70%
540
43%
848
June16th to endNight
65%
540
22%
1440
15%
540
68%
540
41%
845
July1st to 15thNight
73%
510
20%
1360
17%
510
74%
510
41%
821
July16th to endNight
75%
564
23%
1457
20%
531
77%
531
43%
829
August1st to 15thNight
76%
539
24%
1694
26%
536
77%
536
38%
926
August16th to endNight
73%
573
25%
1910
28%
573
76%
573
36%
1037
September1st to 15thNight
65%
540
27%
1800
29%
540
68%
540
33%
1014
September16th to endNight
62%
678
33%
1941
26%
678
67%
678
37%
1120
October1st to 15thNight
54%
720
28%
2131
21%
720
60%
720
36%
1230
October16th to endNight
39%
768
27%
2343
19%
768
48%
768
31%
1375
November1st to 15thNight
39%
760
27%
2471
17%
760
52%
760
31%
1416
November16th to endNight
33%
803
25%
2558
15%
803
44%
803
26%
1570
December1st to 15thNight
29%
975
16%
2730
8%
975
36%
975
21%
1925
December16th to endNight
22%
1040
16%
2912
5%
1040
31%
1040
17%
2159

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
12%6%35%5%3445%7%41%
49647
Night
13%6%25%424446%44%
49394
TransparencyDay
40%6%11%23%14%2
49329
Night
39%5%10%24%17%3
49217
SeeingDay
13%6%23%26%20%9%
6445
Night
16%6%27%31%15%1
16649

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
14%10%48%5%34345%34%
1552
16th to endDay
12%8%46%5%2335%6%40%
1853
February1st to 15thDay
15%9%46%4245%5%6%35%
1780
16th to endDay
15%10%46%5%34435%36%
1710
March1st to 15thDay
17%7%46%43444438%
2001
16th to endDay
17%8%46%6%45%45%7%30%
2266
April1st to 15thDay
15%5%34425%45%6%42%
2151
16th to endDay
17%6%35%5%34446%38%
2160
May1st to 15thDay
19%435%5%3446%7%36%
2295
16th to endDay
10%6%25%5%35%6%5%8%40%
2617
June1st to 15thDay
8%314445%5%5%9%45%
2520
16th to endDay
6%3245%35%6%6%10%44%
2520
July1st to 15thDay
8%325%436%6%7%9%42%
2520
16th to endDay
5%214445%6%6%11%47%
2610
August1st to 15thDay
8%4245%36%5%6%9%42%
2256
16th to endDay
7%5%15%435%6%7%8%43%
2292
September1st to 15thDay
11%5%34424348%46%
2160
16th to endDay
8%424425%45%8%49%
2007
October1st to 15thDay
13%5%15%5%24336%47%
1827
16th to endDay
13%41442335%5%51%
1880
November1st to 15thDay
13%5%23423338%49%
1717
16th to endDay
16%8%245%23336%44%
1729
December1st to 15thDay
17%9%45%5%23446%36%
1560
16th to endDay
19%10%46%5%24345%33%
1664
January1st to 15thNight
15%7%36%5%34445%38%
2716
16th to endNight
12%10%5%6%5%34345%37%
2720
February1st to 15thNight
14%9%47%425%45%6%34%
2403
16th to endNight
16%8%36%5%23335%41%
2049
March1st to 15thNight
18%8%25%5%2434440%
2301
16th to endNight
17%8%36%43445%7%34%
2082
April1st to 15thNight
13%6%34423446%46%
1797
16th to endNight
16%6%24424436%44%
1800
May1st to 15thNight
16%6%25%5%23447%40%
1654
16th to endNight
7%435%435%5%5%7%45%
1496
June1st to 15thNight
6%313425%46%8%52%
1440
16th to endNight
7%4134235%6%11%49%
1440
July1st to 15thNight
8%31333445%9%50%
1440
16th to endNight
33245%345%6%10%51%
1556
August1st to 15thNight
5%3134245%6%8%53%
1704
16th to endNight
5%32443446%8%53%
1910
September1st to 15thNight
9%42441446%7%51%
1800
16th to endNight
8%414423446%54%
1941
October1st to 15thNight
12%413323447%51%
2131
16th to endNight
15%5%24323345%51%
2343
November1st to 15thNight
11%6%23433336%51%
2471
16th to endNight
16%6%24313335%46%
2558
December1st to 15thNight
20%9%26%424346%35%
2730
16th to endNight
20%11%36%5%24445%31%
2912

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
52%48%20%12%1
1553
16th to endDay
43%413%27%9%
1853
February1st to 15thDay
48%412%23%9%
1782
16th to endDay
49%411%21%11%1
1710
March1st to 15thDay
48%5%9%23%11%1
2001
16th to endDay
53%6%13%17%7%
2268
April1st to 15thDay
41%5%9%25%16%1
2151
16th to endDay
46%5%10%23%12%1
2160
May1st to 15thDay
44%7%13%24%9%
2295
16th to endDay
40%7%13%19%15%2
2621
June1st to 15thDay
32%8%14%23%17%2
2520
16th to endDay
32%8%15%21%18%3
2520
July1st to 15thDay
35%8%11%26%14%4
2371
16th to endDay
28%8%15%27%17%1
2446
August1st to 15thDay
35%9%11%25%17%
2242
16th to endDay
33%9%13%20%18%5%
2292
September1st to 15thDay
36%48%24%20%4
2160
16th to endDay
32%6%8%28%21%2
2007
October1st to 15thDay
37%5%9%24%19%3
1827
16th to endDay
33%410%29%19%3
1880
November1st to 15thDay
35%310%24%19%5%
1717
16th to endDay
42%49%28%13%1
1729
December1st to 15thDay
48%5%12%21%10%
1560
16th to endDay
52%411%20%9%1
1664
January1st to 15thNight
47%48%24%13%1
2716
16th to endNight
47%5%11%23%11%
2720
February1st to 15thNight
49%6%10%19%13%1
2412
16th to endNight
46%412%22%14%
2049
March1st to 15thNight
46%5%8%22%14%2
2301
16th to endNight
49%5%12%21%10%
2082
April1st to 15thNight
39%5%9%25%17%2
1797
16th to endNight
41%5%8%26%16%3
1800
May1st to 15thNight
43%6%10%22%15%1
1655
16th to endNight
36%8%12%21%16%4
1498
June1st to 15thNight
27%5%13%29%21%1
1440
16th to endNight
27%7%14%24%20%6%
1440
July1st to 15thNight
29%6%11%30%16%6%
1360
16th to endNight
25%6%16%31%18%1
1457
August1st to 15thNight
26%7%13%27%23%2
1694
16th to endNight
27%6%10%24%24%5%
1910
September1st to 15thNight
31%5%7%25%21%8%
1800
16th to endNight
30%5%7%25%27%4
1941
October1st to 15thNight
32%310%25%21%6%
2131
16th to endNight
35%47%26%21%4
2343
November1st to 15thNight
34%310%22%21%7%
2471
16th to endNight
39%48%27%19%1
2558
December1st to 15thNight
50%410%21%12%
2730
16th to endNight
54%5%10%18%9%1
2912

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
28%11%37%13%8%
135
16th to endDay
9%18%44%23%3
144
February1st to 15thDay
23%11%40%20%3
135
16th to endDay
27%18%30%20%2
120
March1st to 15thDay
32%9%37%14%5%
135
16th to endDay
20%13%33%24%7%
208
April1st to 15thDay
16%11%32%22%15%1
270
16th to endDay
20%5%29%25%14%4
270
May1st to 15thDay
17%7%26%28%13%5%
306
16th to endDay
12%5%28%30%14%7%
517
June1st to 15thDay
10%220%29%25%11%
495
16th to endDay
6%323%37%20%7%
495
July1st to 15thDay
11%113%34%25%13%
475
16th to endDay
5%111%29%33%18%
515
August1st to 15thDay
12%8%27%31%19%
409
16th to endDay
7%111%25%37%17%
332
September1st to 15thDay
13%312%19%35%16%
315
16th to endDay
9%5%11%24%30%18%
314
October1st to 15thDay
13%423%35%18%3
162
16th to endDay
13%9%34%28%12%1
144
November1st to 15thDay
14%428%22%18%10%
135
16th to endDay
25%11%32%22%7%
135
December1st to 15thDay
11%20%35%19%8%5%
135
16th to endDay
25%11%49%10%2
144
January1st to 15thNight
20%12%37%24%5%
975
16th to endNight
19%12%37%27%3
1040
February1st to 15thNight
19%13%35%28%2
884
16th to endNight
21%10%37%29%1
683
March1st to 15thNight
24%9%36%25%3
741
16th to endNight
22%7%31%29%9%
594
April1st to 15thNight
17%8%35%27%9%
539
16th to endNight
19%430%31%12%
540
May1st to 15thNight
19%424%35%13%1
540
16th to endNight
9%124%40%21%1
563
June1st to 15thNight
7%18%44%26%1
540
16th to endNight
10%16%44%25%2
540
July1st to 15thNight
10%19%41%34%3
510
16th to endNight
5%8%37%41%6%
564
August1st to 15thNight
7%6%40%39%5%
539
16th to endNight
6%9%35%40%6%
573
September1st to 15thNight
10%212%35%32%7%
540
16th to endNight
10%315%30%33%6%
678
October1st to 15thNight
13%323%30%23%4
720
16th to endNight
18%6%32%30%10%1
768
November1st to 15thNight
15%6%34%29%13%1
760
16th to endNight
20%9%32%27%8%
803
December1st to 15thNight
25%10%30%26%6%
975
16th to endNight
25%10%35%25%2
1040
Page generated at 2018-03-22 10:34:22. Version ac04

Copyright 2018 A. Danko


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