Grundy Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Grundy Observatory clear sky chart from 2005-11-05 to 2017-04-26.

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
40%
14722
20%
45636
12%
14685
50%
14685
26%
28651

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%
718
20%
2491
10%
716
41%
716
19%
1576
January16th to endNight
25%
768
23%
2496
8%
768
43%
768
23%
1558
February1st to 15thNight
23%
704
18%
2287
6%
701
38%
701
20%
1453
February16th to endNight
22%
631
23%
1901
7%
631
41%
631
23%
1217
March1st to 15thNight
28%
720
21%
2049
8%
720
42%
720
26%
1325
March16th to endNight
30%
768
21%
2060
10%
768
40%
768
28%
1375
April1st to 15thNight
29%
718
19%
1796
6%
718
36%
718
30%
1193
April16th to endNight
42%
704
18%
1760
8%
704
45%
704
31%
1182
May1st to 15thNight
43%
465
18%
1455
11%
465
49%
465
24%
957
May16th to endNight
56%
342
16%
1369
16%
342
60%
342
23%
805
June1st to 15thNight
54%
330
18%
1320
19%
330
63%
330
23%
764
June16th to endNight
62%
330
17%
1320
22%
330
66%
330
27%
743
July1st to 15thNight
57%
310
13%
1240
17%
310
59%
310
19%
778
July16th to endNight
72%
426
15%
1377
15%
397
75%
397
31%
782
August1st to 15thNight
73%
499
20%
1479
17%
496
73%
496
33%
886
August16th to endNight
65%
700
19%
1750
11%
700
66%
700
37%
1109
September1st to 15thNight
56%
660
26%
1650
14%
660
61%
660
36%
1052
September16th to endNight
59%
657
25%
1796
19%
657
62%
657
32%
1162
October1st to 15thNight
47%
660
23%
1980
21%
660
54%
660
27%
1251
October16th to endNight
39%
704
26%
2112
15%
704
53%
704
33%
1241
November1st to 15thNight
41%
700
23%
2277
15%
700
53%
700
28%
1373
November16th to endNight
34%
720
20%
2463
13%
720
45%
720
22%
1512
December1st to 15thNight
28%
720
17%
2520
7%
720
40%
720
18%
1651
December16th to endNight
24%
768
18%
2688
5%
768
41%
768
20%
1706

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
14%5%245%3445%7%40%
46029
Night
15%5%2442445%7%42%
45814
Cloud CoverDay
13%6%245%3445%7%40%
46039
Night
14%5%2442445%7%42%
45819
TransparencyDay
41%412%23%15%2
45725
Night
40%412%23%16%2
45636
SeeingDay
17%6%21%25%19%9%
5115
Night
18%6%27%33%13%1
14725

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
15%7%35%43445%6%38%
1446
16th to endDay
13%8%35%5%2445%6%38%
1728
February1st to 15thDay
14%7%35%4345%6%7%37%
1575
16th to endDay
15%7%345%34447%39%
1572
March1st to 15thDay
15%5%24436%45%6%39%
1897
16th to endDay
18%6%35%5%23247%39%
2163
April1st to 15thDay
17%6%24423446%42%
2152
16th to endDay
16%42342445%6%44%
2112
May1st to 15thDay
16%5%25%4246%6%7%36%
2175
16th to endDay
14%6%24425%5%7%9%38%
2394
June1st to 15thDay
12%434426%5%6%10%38%
2310
16th to endDay
10%425%5%346%5%9%41%
2310
July1st to 15thDay
9%4246%36%7%7%9%39%
2310
16th to endDay
7%315%5%46%6%7%9%39%
2379
August1st to 15thDay
10%5%25%5%35%5%6%9%39%
2129
16th to endDay
11%5%2443445%8%44%
2100
September1st to 15thDay
14%5%23434346%47%
1980
16th to endDay
15%5%25%433446%43%
1833
October1st to 15thDay
18%5%14322345%46%
1650
16th to endDay
17%5%23423448%43%
1760
November1st to 15thDay
14%6%24422347%46%
1581
16th to endDay
17%5%345%24346%42%
1497
December1st to 15thDay
17%5%246%25%45%7%37%
1440
16th to endDay
17%6%35%434447%37%
1536
January1st to 15thNight
16%6%35%5%3445%7%38%
2491
16th to endNight
15%7%25%423347%42%
2496
February1st to 15thNight
14%7%35%5%24446%40%
2285
16th to endNight
14%6%245%245%45%43%
1901
March1st to 15thNight
16%5%245%25%346%43%
2037
16th to endNight
17%5%34434346%43%
2060
April1st to 15thNight
17%5%25%43435%8%40%
1796
16th to endNight
17%425%423446%44%
1760
May1st to 15thNight
18%5%34425%447%39%
1455
16th to endNight
12%425%5%2445%9%43%
1367
June1st to 15thNight
13%5%1342446%9%43%
1320
16th to endNight
10%424425%6%6%10%42%
1320
July1st to 15thNight
8%5%26%7%45%5%7%7%39%
1320
16th to endNight
7%4145%35%5%6%10%45%
1480
August1st to 15thNight
8%4145%345%7%8%45%
1490
16th to endNight
11%32442445%6%48%
1750
September1st to 15thNight
13%5%2432346%6%47%
1650
16th to endNight
13%5%245%34346%45%
1796
October1st to 15thNight
18%6%25%323235%45%
1980
16th to endNight
16%413323347%49%
2112
November1st to 15thNight
13%5%25%42343449%
2277
16th to endNight
15%5%245%35%45%7%40%
2463
December1st to 15thNight
18%5%25%5%35%5%5%6%35%
2520
16th to endNight
17%5%25%43445%7%38%
2688

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%6%36%5%35%45%6%38%
1446
16th to endDay
13%8%36%5%3435%7%38%
1728
February1st to 15thDay
14%8%35%5%245%6%7%37%
1575
16th to endDay
14%8%35%5%34446%39%
1572
March1st to 15thDay
14%5%25%5%36%45%6%40%
1908
16th to endDay
18%6%25%5%24347%39%
2162
April1st to 15thDay
16%6%2442345%6%42%
2152
16th to endDay
15%42442446%6%44%
2112
May1st to 15thDay
16%6%245%35%5%6%8%36%
2175
16th to endDay
13%5%34425%5%6%9%38%
2394
June1st to 15thDay
12%4245%35%5%7%10%37%
2310
16th to endDay
10%425%5%445%5%9%41%
2310
July1st to 15thDay
8%425%6%36%5%7%9%38%
2310
16th to endDay
7%4246%36%7%7%9%39%
2379
August1st to 15thDay
9%5%25%6%35%5%6%8%39%
2129
16th to endDay
11%5%245%3446%8%43%
2100
September1st to 15thDay
13%6%245%24347%46%
1980
16th to endDay
13%6%245%2445%6%43%
1833
October1st to 15thDay
17%6%14323346%46%
1650
16th to endDay
17%5%2432345%7%43%
1760
November1st to 15thDay
13%7%24423347%46%
1581
16th to endDay
17%6%35%424436%42%
1497
December1st to 15thDay
16%5%35%425%45%6%38%
1440
16th to endDay
17%6%34435%447%38%
1536
January1st to 15thNight
15%6%35%5%3445%7%38%
2491
16th to endNight
15%7%245%23447%42%
2496
February1st to 15thNight
14%7%35%5%24446%40%
2278
16th to endNight
14%6%345%245%4444%
1901
March1st to 15thNight
16%5%24425%436%43%
2049
16th to endNight
17%5%25%424345%43%
2060
April1st to 15thNight
16%5%245%2446%8%40%
1796
16th to endNight
15%6%24423447%44%
1760
May1st to 15thNight
17%5%34425%447%39%
1455
16th to endNight
12%43443445%8%43%
1367
June1st to 15thNight
13%5%14325%45%9%42%
1320
16th to endNight
9%415%326%6%6%10%42%
1320
July1st to 15thNight
8%5%36%6%45%6%6%7%39%
1320
16th to endNight
6%423436%6%6%11%43%
1480
August1st to 15thNight
7%5%24435%5%6%9%45%
1490
16th to endNight
11%425%4245%5%6%48%
1750
September1st to 15thNight
13%5%2442345%6%47%
1650
16th to endNight
13%5%24434446%45%
1796
October1st to 15thNight
18%6%24422336%45%
1980
16th to endNight
16%423313347%49%
2112
November1st to 15thNight
12%5%35%5%2343449%
2277
16th to endNight
15%5%245%2445%7%40%
2463
December1st to 15thNight
18%6%25%5%445%5%6%35%
2520
16th to endNight
17%6%25%42446%7%38%
2688

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
44%314%21%12%3
1447
16th to endDay
45%312%23%12%2
1728
February1st to 15thDay
44%312%25%11%2
1577
16th to endDay
45%311%25%12%1
1572
March1st to 15thDay
44%312%24%13%1
1908
16th to endDay
45%310%20%17%2
2163
April1st to 15thDay
41%310%23%16%3
2152
16th to endDay
39%48%20%21%5%
2112
May1st to 15thDay
43%5%11%22%14%1
2175
16th to endDay
40%6%14%22%13%1
2398
June1st to 15thDay
39%7%13%26%11%
2310
16th to endDay
37%6%16%23%14%2
2310
July1st to 15thDay
38%7%11%23%15%2
2161
16th to endDay
36%6%14%25%15%1
2219
August1st to 15thDay
40%5%12%22%17%1
2116
16th to endDay
36%5%10%26%18%3
2100
September1st to 15thDay
38%410%21%21%4
1980
16th to endDay
40%47%21%23%2
1833
October1st to 15thDay
39%312%25%17%2
1650
16th to endDay
40%310%28%16%1
1760
November1st to 15thDay
38%313%22%17%4
1581
16th to endDay
43%310%27%13%1
1497
December1st to 15thDay
45%416%21%10%1
1440
16th to endDay
45%414%22%11%1
1536
January1st to 15thNight
44%411%22%14%3
2491
16th to endNight
42%312%26%13%1
2496
February1st to 15thNight
44%313%23%13%1
2287
16th to endNight
42%39%24%17%2
1901
March1st to 15thNight
42%312%25%15%1
2049
16th to endNight
42%311%22%17%2
2060
April1st to 15thNight
41%311%22%17%3
1796
16th to endNight
39%5%8%20%20%5%
1760
May1st to 15thNight
43%5%11%19%17%2
1455
16th to endNight
38%6%16%17%16%3
1369
June1st to 15thNight
37%6%14%23%17%
1320
16th to endNight
34%6%13%26%18%1
1320
July1st to 15thNight
43%6%11%20%14%3
1240
16th to endNight
31%7%18%22%18%2
1377
August1st to 15thNight
34%5%14%26%17%1
1479
16th to endNight
35%412%24%20%3
1750
September1st to 15thNight
35%411%21%23%3
1650
16th to endNight
39%46%21%23%4
1796
October1st to 15thNight
41%39%26%16%2
1980
16th to endNight
35%210%29%19%1
2112
November1st to 15thNight
39%39%24%17%6%
2277
16th to endNight
42%311%25%15%2
2463
December1st to 15thNight
47%412%23%11%
2520
16th to endNight
45%316%22%11%
2688

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
30%15%33%15%31
63
16th to endDay
21%14%36%19%7%
96
February1st to 15thDay
23%17%44%11%3
90
16th to endDay
17%12%41%26%2
231
March1st to 15thDay
17%13%29%30%8%
270
16th to endDay
21%9%32%26%7%2
288
April1st to 15thDay
20%10%28%22%13%4
270
16th to endDay
21%422%22%20%7%
254
May1st to 15thDay
25%10%20%17%17%8%
228
16th to endDay
19%313%23%23%16%
267
June1st to 15thDay
17%5%20%23%20%11%
255
16th to endDay
15%119%29%24%9%
255
July1st to 15thDay
11%213%25%32%13%
245
16th to endDay
8%18%32%35%13%
270
August1st to 15thDay
12%19%28%30%18%
253
16th to endDay
11%1429%28%25%
268
September1st to 15thDay
13%317%15%28%21%
255
16th to endDay
22%11%25%22%17%
254
October1st to 15thDay
23%215%27%23%7%
255
16th to endDay
14%5%31%33%13%1
272
November1st to 15thDay
16%5%27%26%20%2
235
16th to endDay
19%8%32%26%11%
117
December1st to 15thDay
21%16%33%13%8%6%
60
16th to endDay
26%7%40%20%4
64
January1st to 15thNight
20%10%35%28%5%
720
16th to endNight
21%9%37%29%1
768
February1st to 15thNight
19%12%36%29%1
704
16th to endNight
17%14%41%23%3
631
March1st to 15thNight
20%9%36%28%5%
720
16th to endNight
20%7%34%29%7%1
768
April1st to 15thNight
20%7%37%27%6%
718
16th to endNight
19%5%26%35%12%
704
May1st to 15thNight
21%423%36%12%
465
16th to endNight
12%218%40%23%2
343
June1st to 15thNight
15%119%40%23%
330
16th to endNight
11%113%52%20%
330
July1st to 15thNight
12%214%42%27%1
310
16th to endNight
8%29%46%29%3
426
August1st to 15thNight
12%15%49%28%2
499
16th to endNight
14%19%38%32%4
700
September1st to 15thNight
15%315%34%27%3
660
16th to endNight
16%113%35%29%3
657
October1st to 15thNight
22%419%32%19%2
660
16th to endNight
18%6%30%31%12%
704
November1st to 15thNight
15%5%29%34%14%1
700
16th to endNight
18%11%29%29%10%
720
December1st to 15thNight
21%8%34%26%7%
720
16th to endNight
20%9%38%27%3
768
Page generated at 2017-04-27 17:48:52. Version ac04

Copyright 2017 A. Danko


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