Grady
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Grady clear sky chart from 2005-11-05 to 2017-09-24.

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
51%
15351
24%
47052
17%
15307
59%
15307
29%
28439

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
31%
720
25%
2321
11%
716
46%
716
23%
1394
January16th to endNight
26%
768
26%
2304
11%
768
41%
768
24%
1469
February1st to 15thNight
33%
704
26%
2111
12%
700
46%
700
27%
1291
February16th to endNight
29%
631
27%
1893
12%
631
42%
631
28%
1150
March1st to 15thNight
32%
720
24%
2130
12%
720
43%
720
28%
1305
March16th to endNight
42%
753
25%
2097
14%
753
51%
753
32%
1257
April1st to 15thNight
46%
539
27%
1797
20%
539
57%
539
31%
987
April16th to endNight
48%
540
28%
1800
21%
540
61%
540
36%
961
May1st to 15thNight
54%
539
23%
1647
15%
539
62%
539
34%
971
May16th to endNight
68%
561
23%
1682
19%
561
71%
561
36%
990
June1st to 15thNight
71%
540
15%
1620
13%
540
72%
540
27%
1061
June16th to endNight
73%
540
18%
1620
16%
540
73%
540
31%
1002
July1st to 15thNight
76%
510
11%
1530
11%
510
77%
510
26%
1013
July16th to endNight
76%
564
14%
1641
15%
531
76%
531
29%
1042
August1st to 15thNight
73%
539
12%
1788
15%
536
74%
536
23%
1167
August16th to endNight
66%
573
18%
1910
20%
573
66%
573
22%
1259
September1st to 15thNight
66%
645
19%
1905
18%
645
67%
645
27%
1252
September16th to endNight
62%
693
25%
1914
20%
693
65%
693
31%
1229
October1st to 15thNight
59%
660
34%
1815
24%
660
67%
660
44%
998
October16th to endNight
55%
704
35%
2075
28%
704
63%
704
39%
1144
November1st to 15thNight
54%
700
31%
2276
28%
700
64%
700
37%
1201
November16th to endNight
43%
720
29%
2340
21%
720
57%
720
33%
1282
December1st to 15thNight
41%
720
25%
2340
19%
720
51%
720
27%
1379
December16th to endNight
34%
768
20%
2496
13%
768
43%
768
21%
1635

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%40%
47863
Night
10%5%2442445%8%46%
47257
TransparencyDay
40%5%10%18%17%6%
47569
Night
35%5%11%18%19%9%
47055
SeeingDay
12%6%17%23%24%14%
5271
Night
12%7%21%29%23%5%
15354

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
15%7%35%322246%46%
1614
16th to endDay
15%6%24424345%45%
1920
February1st to 15thDay
14%5%15%43445%5%45%
1750
16th to endDay
15%6%35%423346%43%
1580
March1st to 15thDay
12%6%35%5%24347%44%
1830
16th to endDay
13%5%24424436%47%
2135
April1st to 15thDay
12%5%26%425%45%7%44%
2151
16th to endDay
9%3245%33447%51%
2160
May1st to 15thDay
10%425%5%35%5%6%7%42%
2304
16th to endDay
7%425%6%35%6%5%9%42%
2431
June1st to 15thDay
8%426%7%37%7%7%9%32%
2340
16th to endDay
6%326%7%47%7%8%11%34%
2340
July1st to 15thDay
10%5%26%8%5%7%9%9%10%24%
2340
16th to endDay
8%426%7%5%7%9%9%10%26%
2412
August1st to 15thDay
9%6%27%8%48%6%8%11%25%
2160
16th to endDay
12%8%27%7%46%6%6%8%27%
2292
September1st to 15thDay
12%6%25%7%45%6%6%9%32%
2040
16th to endDay
10%6%25%6%35%5%5%7%41%
1905
October1st to 15thDay
10%416%42345%6%49%
1815
16th to endDay
10%313324345%56%
1794
November1st to 15thDay
10%5%24424336%53%
1582
16th to endDay
13%413433346%52%
1620
December1st to 15thDay
13%6%25%424436%45%
1620
16th to endDay
18%9%35%5%24335%36%
1728
January1st to 15thNight
12%7%34323345%46%
2321
16th to endNight
17%7%24313335%45%
2304
February1st to 15thNight
12%6%24424245%48%
2103
16th to endNight
14%6%2332335%6%48%
1893
March1st to 15thNight
11%5%15%424447%46%
2130
16th to endNight
9%5%245%2335%6%50%
2097
April1st to 15thNight
9%4143245%6%9%48%
1797
16th to endNight
7%3143235%5%7%54%
1800
May1st to 15thNight
9%324435%5%6%7%49%
1646
16th to endNight
5%3145%35%6%6%11%46%
1682
June1st to 15thNight
6%5%25%6%46%5%8%12%36%
1620
16th to endNight
5%325%447%5%7%13%40%
1620
July1st to 15thNight
5%325%5%37%8%10%13%34%
1620
16th to endNight
5%325%5%35%6%9%14%37%
1753
August1st to 15thNight
6%425%7%47%8%9%13%31%
1800
16th to endNight
10%6%26%5%36%5%7%10%35%
1910
September1st to 15thNight
9%5%26%5%45%6%7%9%38%
1905
16th to endNight
8%5%35%5%4445%8%43%
1914
October1st to 15thNight
8%31331245%7%56%
1815
16th to endNight
10%314312335%59%
2075
November1st to 15thNight
8%313223345%58%
2276
16th to endNight
11%413323345%55%
2340
December1st to 15thNight
11%6%25%42345%6%47%
2340
16th to endNight
16%7%25%5%34447%38%
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
39%26%16%25%8%
1615
16th to endDay
40%36%16%24%9%
1920
February1st to 15thDay
40%36%17%25%7%
1752
16th to endDay
42%3416%24%9%
1580
March1st to 15thDay
41%37%19%21%6%
1830
16th to endDay
37%410%19%20%6%
2136
April1st to 15thDay
39%36%22%19%8%
2151
16th to endDay
31%410%23%24%5%
2160
May1st to 15thDay
39%5%10%19%20%5%
2304
16th to endDay
36%6%12%24%17%2
2435
June1st to 15thDay
43%9%13%21%10%1
2340
16th to endDay
39%9%16%19%11%2
2340
July1st to 15thDay
47%11%20%13%5%1
2201
16th to endDay
45%11%19%19%4
2261
August1st to 15thDay
46%9%19%16%7%
2148
16th to endDay
51%8%14%12%11%1
2292
September1st to 15thDay
45%8%12%16%12%3
2040
16th to endDay
40%5%10%18%19%4
1905
October1st to 15thDay
34%48%17%25%10%
1815
16th to endDay
29%3416%27%19%
1794
November1st to 15thDay
32%36%16%27%13%
1582
16th to endDay
34%37%15%24%14%
1620
December1st to 15thDay
40%29%19%20%6%
1620
16th to endDay
50%35%19%14%6%
1728
January1st to 15thNight
39%26%16%26%8%
2321
16th to endNight
42%35%13%24%10%
2304
February1st to 15thNight
38%35%17%22%12%
2112
16th to endNight
37%35%17%22%13%
1893
March1st to 15thNight
36%48%19%21%9%
2130
16th to endNight
33%39%19%23%9%
2097
April1st to 15thNight
30%5%9%19%24%10%
1797
16th to endNight
27%5%8%25%22%9%
1800
May1st to 15thNight
32%5%13%17%22%8%
1647
16th to endNight
30%7%16%23%16%6%
1684
June1st to 15thNight
36%10%16%20%14%1
1620
16th to endNight
33%8%18%24%11%3
1620
July1st to 15thNight
33%15%26%16%5%2
1530
16th to endNight
32%12%25%22%6%
1641
August1st to 15thNight
37%12%21%20%5%2
1788
16th to endNight
41%10%17%15%13%3
1910
September1st to 15thNight
39%8%15%19%11%6%
1905
16th to endNight
37%5%11%19%17%7%
1914
October1st to 15thNight
26%39%20%24%15%
1815
16th to endNight
28%3314%29%20%
2075
November1st to 15thNight
27%35%18%26%17%
2276
16th to endNight
31%27%17%24%16%
2340
December1st to 15thNight
36%38%20%23%6%
2340
16th to endNight
45%47%18%18%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
20%16%37%8%13%3
90
16th to endDay
18%25%37%13%41
96
February1st to 15thDay
21%24%30%21%3
90
16th to endDay
12%23%49%12%2
81
March1st to 15thDay
12%27%37%17%4
120
16th to endDay
16%11%28%25%16%2
288
April1st to 15thDay
17%12%25%22%18%2
270
16th to endDay
10%7%27%27%20%7%
270
May1st to 15thDay
10%7%25%23%22%10%
288
16th to endDay
9%317%28%22%18%
331
June1st to 15thDay
10%28%26%32%19%
315
16th to endDay
9%16%23%32%25%
315
July1st to 15thDay
5%320%36%31%
305
16th to endDay
5%6%18%41%27%
326
August1st to 15thDay
16%422%31%24%
313
16th to endDay
12%16%19%32%26%
332
September1st to 15thDay
13%28%26%28%19%
315
16th to endDay
15%18%33%26%13%
287
October1st to 15thDay
10%320%30%30%4
255
16th to endDay
11%427%24%25%7%
218
November1st to 15thDay
20%437%25%10%2
90
16th to endDay
10%12%43%20%12%2
90
December1st to 15thDay
24%13%38%15%5%2
90
16th to endDay
28%13%20%32%5%
96
January1st to 15thNight
18%15%29%27%9%
720
16th to endNight
22%15%32%23%6%
768
February1st to 15thNight
16%13%33%30%6%
704
16th to endNight
17%16%33%26%6%
631
March1st to 15thNight
13%15%34%27%8%
720
16th to endNight
10%12%29%29%16%
753
April1st to 15thNight
10%11%28%29%18%
539
16th to endNight
8%8%29%33%20%
540
May1st to 15thNight
10%226%33%22%4
540
16th to endNight
6%114%35%35%7%
563
June1st to 15thNight
8%8%31%41%8%
540
16th to endNight
5%15%28%43%13%
540
July1st to 15thNight
6%427%43%17%
510
16th to endNight
6%1424%47%15%
564
August1st to 15thNight
8%423%46%17%
539
16th to endNight
12%26%23%40%14%
573
September1st to 15thNight
11%18%28%38%11%
645
16th to endNight
10%112%35%35%5%
693
October1st to 15thNight
10%222%34%27%3
660
16th to endNight
11%7%20%30%24%5%
704
November1st to 15thNight
11%6%23%35%18%4
700
16th to endNight
13%12%27%32%13%
720
December1st to 15thNight
14%12%27%32%12%1
720
16th to endNight
22%11%28%27%8%
768
Page generated at 2017-09-25 08:02:28. Version ac04

Copyright 2017 A. Danko


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