Dryden
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Dryden 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
31%
16273
11%
46672
6%
16226
36%
16226
18%
34324

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
23%
900
7%
2685
5%
895
26%
895
10%
2212
January16th to endNight
21%
960
8%
2766
4%
960
25%
960
10%
2303
February1st to 15thNight
31%
879
13%
2288
6%
875
35%
875
18%
1729
February16th to endNight
30%
780
12%
2042
4%
780
35%
780
17%
1570
March1st to 15thNight
29%
629
15%
2069
9%
629
36%
629
16%
1444
March16th to endNight
32%
576
17%
2070
12%
576
41%
576
17%
1392
April1st to 15thNight
39%
539
17%
1792
15%
539
47%
539
25%
1111
April16th to endNight
36%
540
17%
1572
10%
540
43%
540
24%
1060
May1st to 15thNight
27%
540
13%
1440
5%
540
32%
540
20%
1066
May16th to endNight
37%
563
9%
1444
5%
561
39%
561
27%
1008
June1st to 15thNight
38%
540
4%
1260
3%
540
39%
540
30%
879
June16th to endNight
47%
540
3%
1260
3%
540
48%
540
31%
880
July1st to 15thNight
50%
510
6%
1190
5%
510
52%
510
41%
758
July16th to endNight
52%
564
13%
1314
6%
531
54%
531
37%
832
August1st to 15thNight
51%
539
17%
1482
9%
536
54%
536
35%
898
August16th to endNight
46%
573
18%
1754
14%
573
52%
573
32%
1042
September1st to 15thNight
45%
530
14%
1767
12%
530
51%
530
27%
1095
September16th to endNight
35%
547
14%
1809
12%
547
39%
547
20%
1195
October1st to 15thNight
28%
660
15%
1980
8%
660
34%
660
17%
1423
October16th to endNight
26%
729
9%
2227
4%
729
30%
729
14%
1698
November1st to 15thNight
21%
875
8%
2451
4%
875
25%
875
12%
1979
November16th to endNight
20%
900
9%
2520
4%
900
24%
900
10%
2080
December1st to 15thNight
20%
900
6%
2610
2%
900
23%
900
8%
2203
December16th to endNight
17%
960
8%
2880
4%
960
21%
960
6%
2467

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%345%5%6%29%
48012
Night
15%11%47%6%35%45%5%29%
46824
Cloud CoverDay
16%10%47%6%35%5%5%6%29%
48023
Night
15%11%47%6%35%45%5%29%
46828
Cloud CoverDay
16%10%47%6%35%5%5%6%29%
48021
Night
14%11%47%6%35%5%45%29%
46828
TransparencyDay
54%312%20%7%
47682
Night
55%311%20%7%
46672
SeeingDay
21%9%31%23%10%3
6255
Night
20%8%29%28%11%1
16274

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
12%14%6%10%7%5%7%8%5%5%16%
1253
16th to endDay
11%13%6%12%10%5%5%9%6%6%11%
1456
February1st to 15thDay
8%11%48%7%5%8%8%6%6%23%
1575
16th to endDay
9%11%7%11%7%447%6%5%23%
1430
March1st to 15thDay
11%14%6%7%5%345%45%31%
1891
16th to endDay
14%10%38%7%445%5%5%30%
2154
April1st to 15thDay
12%10%46%5%34446%37%
2154
16th to endDay
17%10%35%424435%36%
2388
May1st to 15thDay
20%12%47%5%2445%5%28%
2511
16th to endDay
22%7%35%5%25%446%32%
2669
June1st to 15thDay
20%7%35%5%345%5%8%30%
2700
16th to endDay
19%6%26%6%35%6%6%8%27%
2700
July1st to 15thDay
15%5%25%5%345%5%7%39%
2700
16th to endDay
13%5%25%4445%5%9%38%
2764
August1st to 15thDay
12%5%26%5%36%5%6%10%35%
2466
16th to endDay
15%6%35%433447%40%
2447
September1st to 15thDay
16%9%35%42445%5%37%
2112
16th to endDay
23%10%245%24336%33%
1810
October1st to 15thDay
19%15%5%7%6%34335%24%
1650
16th to endDay
24%17%46%5%24335%20%
1644
November1st to 15thDay
21%16%7%6%7%2344421%
1407
16th to endDay
16%15%7%13%9%345%5%5%13%
1440
December1st to 15thDay
10%16%9%13%9%6%7%5%45%12%
1347
16th to endDay
15%19%9%12%6%5%5%445%10%
1344
January1st to 15thNight
11%13%7%11%8%6%8%7%6%414%
2685
16th to endNight
12%15%6%11%8%5%6%8%6%414%
2766
February1st to 15thNight
8%11%5%9%6%49%7%46%26%
2285
16th to endNight
7%12%7%10%7%37%5%5%5%25%
2042
March1st to 15thNight
11%12%6%6%6%35%5%45%33%
2057
16th to endNight
16%12%36%5%24345%34%
2070
April1st to 15thNight
12%9%35%42345%6%41%
1792
16th to endNight
14%9%45%5%24345%39%
1572
May1st to 15thNight
22%9%45%5%2445%5%30%
1440
16th to endNight
20%6%15%424347%37%
1442
June1st to 15thNight
21%5%2333445%6%39%
1260
16th to endNight
16%5%13426%45%9%39%
1260
July1st to 15thNight
10%32442446%9%46%
1260
16th to endNight
12%313434347%50%
1404
August1st to 15thNight
10%415%4345%5%7%48%
1491
16th to endNight
13%425%323337%49%
1754
September1st to 15thNight
15%6%2443345%7%42%
1767
16th to endNight
20%8%24433436%38%
1809
October1st to 15thNight
20%11%35%434446%31%
1980
16th to endNight
21%14%36%5%24446%25%
2227
November1st to 15thNight
18%16%6%8%5%34435%21%
2451
16th to endNight
18%14%6%10%8%35%44418%
2520
December1st to 15thNight
11%13%7%13%11%5%8%5%5%413%
2610
16th to endNight
16%17%7%13%11%45%44311%
2880

Percentage Cloud Cover SW (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
12%14%6%10%8%5%7%8%5%5%16%
1253
16th to endDay
11%14%7%12%10%5%6%9%6%6%11%
1456
February1st to 15thDay
9%11%49%7%5%8%8%6%6%23%
1575
16th to endDay
9%11%7%11%7%45%7%5%5%24%
1430
March1st to 15thDay
12%13%6%7%5%245%4431%
1902
16th to endDay
15%10%38%7%345%46%29%
2154
April1st to 15thDay
12%9%46%5%2435%6%37%
2154
16th to endDay
17%10%36%424445%36%
2388
May1st to 15thDay
19%11%36%6%2445%5%28%
2511
16th to endDay
22%7%35%5%24446%32%
2669
June1st to 15thDay
19%7%35%5%25%6%5%8%29%
2700
16th to endDay
19%6%36%6%36%6%6%8%28%
2700
July1st to 15thDay
15%5%245%35%5%5%7%39%
2700
16th to endDay
13%5%25%5%35%5%6%9%38%
2764
August1st to 15thDay
12%5%36%5%35%5%6%9%35%
2466
16th to endDay
14%6%35%434447%40%
2447
September1st to 15thDay
16%8%35%42445%6%37%
2112
16th to endDay
22%10%245%24346%33%
1810
October1st to 15thDay
19%15%48%6%34435%24%
1650
16th to endDay
24%17%46%6%33335%20%
1644
November1st to 15thDay
21%15%8%7%6%2344421%
1407
16th to endDay
17%14%7%13%10%3445%5%13%
1440
December1st to 15thDay
10%17%8%13%9%6%6%5%45%11%
1347
16th to endDay
14%19%9%11%6%46%5%45%10%
1344
January1st to 15thNight
11%12%7%12%8%6%8%7%5%414%
2685
16th to endNight
11%15%7%11%8%5%6%8%6%414%
2766
February1st to 15thNight
8%11%49%6%49%7%47%26%
2278
16th to endNight
7%13%6%10%7%37%5%5%5%25%
2042
March1st to 15thNight
11%12%6%6%5%345%45%33%
2069
16th to endNight
16%12%36%5%24345%34%
2070
April1st to 15thNight
12%9%35%5%2335%6%41%
1792
16th to endNight
14%9%45%5%24335%39%
1572
May1st to 15thNight
22%10%45%43445%5%30%
1439
16th to endNight
20%7%15%424346%37%
1442
June1st to 15thNight
20%6%2342435%6%39%
1260
16th to endNight
16%4145%16%46%8%39%
1260
July1st to 15thNight
10%3245%2446%9%46%
1260
16th to endNight
12%313434346%50%
1404
August1st to 15thNight
9%425%4345%5%7%48%
1491
16th to endNight
13%424324347%48%
1754
September1st to 15thNight
15%6%2443345%7%42%
1767
16th to endNight
20%7%25%433446%38%
1809
October1st to 15thNight
20%11%35%434446%30%
1980
16th to endNight
21%13%37%5%24445%25%
2227
November1st to 15thNight
19%16%6%8%5%35%435%21%
2451
16th to endNight
18%14%7%10%8%45%44418%
2520
December1st to 15thNight
11%13%7%12%11%5%8%5%5%413%
2610
16th to endNight
16%17%7%13%10%45%44311%
2880

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
12%14%6%11%7%5%7%7%5%5%15%
1253
16th to endDay
11%13%7%11%9%5%6%8%6%6%11%
1456
February1st to 15thDay
8%12%39%7%48%9%6%6%23%
1575
16th to endDay
9%12%6%10%8%45%7%6%5%24%
1430
March1st to 15thDay
11%14%6%7%6%345%46%30%
1902
16th to endDay
15%9%47%7%45%5%5%5%30%
2152
April1st to 15thDay
12%10%46%5%2435%6%37%
2154
16th to endDay
16%10%36%424435%37%
2388
May1st to 15thDay
19%11%47%6%245%45%27%
2511
16th to endDay
22%7%35%435%45%6%32%
2669
June1st to 15thDay
19%7%36%5%35%5%5%8%29%
2700
16th to endDay
19%6%36%5%45%6%6%8%27%
2700
July1st to 15thDay
14%5%35%5%35%5%5%7%38%
2700
16th to endDay
13%6%245%35%5%5%8%39%
2764
August1st to 15thDay
11%5%36%5%36%5%6%9%35%
2466
16th to endDay
14%6%26%434448%40%
2447
September1st to 15thDay
16%9%35%5%245%5%6%36%
2112
16th to endDay
23%8%345%24336%33%
1810
October1st to 15thDay
19%15%5%8%6%34335%24%
1650
16th to endDay
24%17%47%6%3434420%
1644
November1st to 15thDay
21%15%8%7%6%3344420%
1407
16th to endDay
17%14%8%13%9%445%46%12%
1440
December1st to 15thDay
9%17%9%13%9%7%7%5%45%11%
1347
16th to endDay
15%17%10%12%6%5%5%5%45%10%
1344
January1st to 15thNight
10%13%7%11%8%6%8%7%6%414%
2685
16th to endNight
12%14%7%11%8%5%6%8%6%414%
2766
February1st to 15thNight
8%10%5%9%6%49%7%46%26%
2278
16th to endNight
7%13%6%10%8%36%5%6%5%25%
2042
March1st to 15thNight
11%12%5%6%5%35%5%46%33%
2069
16th to endNight
16%11%46%6%24345%34%
2070
April1st to 15thNight
12%9%35%43335%6%42%
1792
16th to endNight
14%9%46%5%24346%39%
1572
May1st to 15thNight
21%10%46%4245%5%5%30%
1439
16th to endNight
20%6%25%424347%37%
1442
June1st to 15thNight
20%7%2433435%6%38%
1260
16th to endNight
15%425%416%5%5%9%38%
1260
July1st to 15thNight
10%3245%245%6%8%46%
1260
16th to endNight
12%314424447%50%
1404
August1st to 15thNight
9%5%15%4345%5%7%47%
1491
16th to endNight
13%424323447%48%
1754
September1st to 15thNight
15%7%2442345%8%41%
1767
16th to endNight
20%7%345%23436%38%
1809
October1st to 15thNight
20%12%35%5%34445%31%
1980
16th to endNight
20%14%47%5%34346%25%
2227
November1st to 15thNight
19%16%6%7%6%34445%21%
2451
16th to endNight
18%14%6%10%8%5%5%43417%
2520
December1st to 15thNight
11%13%7%13%11%6%8%5%4412%
2610
16th to endNight
15%17%7%13%11%45%44311%
2880

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
68%38%15%3
1254
16th to endDay
70%38%11%5%
1456
February1st to 15thDay
58%312%16%8%
1576
16th to endDay
59%39%21%5%
1430
March1st to 15thDay
54%413%17%8%1
1902
16th to endDay
55%312%18%8%2
2154
April1st to 15thDay
47%39%22%14%2
2154
16th to endDay
50%18%24%12%2
2388
May1st to 15thDay
56%38%22%8%
2511
16th to endDay
52%311%20%10%1
2673
June1st to 15thDay
52%312%21%9%
2700
16th to endDay
52%416%19%6%
2700
July1st to 15thDay
42%312%31%9%
2541
16th to endDay
42%416%26%9%
2591
August1st to 15thDay
43%5%17%27%5%
2451
16th to endDay
42%317%27%8%
2447
September1st to 15thDay
47%315%27%6%
2112
16th to endDay
51%317%22%4
1810
October1st to 15thDay
62%313%16%3
1650
16th to endDay
66%213%15%1
1644
November1st to 15thDay
66%313%12%4
1407
16th to endDay
71%49%10%3
1440
December1st to 15thDay
74%49%11%1
1347
16th to endDay
75%28%9%4
1344
January1st to 15thNight
69%38%15%2
2685
16th to endNight
69%47%14%3
2766
February1st to 15thNight
56%312%18%8%
2288
16th to endNight
58%310%21%5%
2042
March1st to 15thNight
53%312%17%11%1
2069
16th to endNight
52%210%21%9%3
2070
April1st to 15thNight
42%210%25%17%1
1792
16th to endNight
47%28%23%15%2
1572
May1st to 15thNight
54%26%24%10%
1440
16th to endNight
46%311%21%14%2
1444
June1st to 15thNight
45%211%23%15%
1260
16th to endNight
41%5%18%24%8%1
1260
July1st to 15thNight
32%417%33%11%
1190
16th to endNight
34%413%33%12%1
1314
August1st to 15thNight
34%416%33%10%
1482
16th to endNight
35%319%29%11%
1754
September1st to 15thNight
40%317%29%9%
1767
16th to endNight
46%416%26%5%
1809
October1st to 15thNight
53%313%23%5%
1980
16th to endNight
60%313%19%2
2227
November1st to 15thNight
66%311%14%3
2451
16th to endNight
68%311%11%4
2520
December1st to 15thNight
72%310%10%2
2610
16th to endNight
76%27%10%2
2880

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
11%15%37%26%8%
90
16th to endDay
28%6%34%19%8%3
96
February1st to 15thDay
27%14%33%16%6%1
90
16th to endDay
22%17%39%16%22
81
March1st to 15thDay
20%6%31%32%8%1
90
16th to endDay
16%10%35%23%8%5%
138
April1st to 15thDay
15%9%32%28%11%2
273
16th to endDay
25%9%32%21%9%1
450
May1st to 15thDay
29%13%29%18%5%3
468
16th to endDay
25%11%29%21%8%3
517
June1st to 15thDay
24%12%31%19%8%4
495
16th to endDay
23%8%32%18%10%5%
495
July1st to 15thDay
15%7%32%27%11%5%
475
16th to endDay
15%6%29%26%14%6%
515
August1st to 15thDay
13%5%28%28%17%6%
493
16th to endDay
19%7%25%28%16%2
487
September1st to 15thDay
17%6%32%25%15%2
299
16th to endDay
31%13%26%21%5%2
151
October1st to 15thDay
23%5%35%23%10%2
90
16th to endDay
36%6%41%11%4
96
November1st to 15thDay
32%6%40%11%10%
90
16th to endDay
40%11%24%14%7%2
90
December1st to 15thDay
20%13%36%21%6%2
90
16th to endDay
20%10%45%15%25%
96
January1st to 15thNight
17%10%33%28%7%1
900
16th to endNight
19%10%35%27%6%
960
February1st to 15thNight
13%8%32%32%11%1
880
16th to endNight
13%8%33%27%11%5%
780
March1st to 15thNight
16%13%31%25%11%1
629
16th to endNight
23%9%24%27%11%3
576
April1st to 15thNight
17%8%28%33%11%
539
16th to endNight
18%6%29%34%10%
540
May1st to 15thNight
27%8%30%25%7%1
540
16th to endNight
23%6%24%32%11%1
563
June1st to 15thNight
26%5%24%30%13%
540
16th to endNight
19%5%20%35%17%1
540
July1st to 15thNight
10%5%28%35%19%1
510
16th to endNight
12%424%30%24%3
564
August1st to 15thNight
12%325%30%25%3
539
16th to endNight
15%5%27%32%17%2
573
September1st to 15thNight
19%5%25%30%17%2
530
16th to endNight
24%7%24%26%14%1
547
October1st to 15thNight
29%8%28%26%6%
660
16th to endNight
30%6%27%23%9%1
729
November1st to 15thNight
28%11%28%22%7%1
875
16th to endNight
28%9%30%23%5%1
900
December1st to 15thNight
17%9%35%27%7%2
900
16th to endNight
23%9%29%24%10%1
960
Page generated at 2017-09-13 06:39:20. Version ac04

Copyright 2017 A. Danko


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