Greeley Hill Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Greeley Hill Observatory clear sky chart from 2005-11-05 to 2017-12-02.

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
73%
15644
30%
47795
28%
15607
75%
15607
45%
24587

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
67%
716
20%
2482
22%
716
69%
716
33%
1387
January16th to endNight
64%
768
24%
2496
28%
768
66%
768
30%
1496
February1st to 15thNight
63%
700
25%
2205
27%
697
65%
697
31%
1308
February16th to endNight
59%
632
23%
1895
24%
632
61%
632
30%
1180
March1st to 15thNight
59%
720
18%
2100
19%
720
60%
720
31%
1305
March16th to endNight
62%
768
21%
2028
17%
768
65%
768
38%
1237
April1st to 15thNight
57%
719
22%
1799
13%
719
60%
719
37%
1139
April16th to endNight
63%
720
25%
1800
13%
720
67%
720
48%
1032
May1st to 15thNight
68%
630
29%
1710
20%
630
72%
630
59%
824
May16th to endNight
72%
375
31%
1496
34%
374
76%
374
46%
622
June1st to 15thNight
79%
360
30%
1440
36%
360
83%
360
54%
544
June16th to endNight
90%
360
38%
1440
50%
360
92%
360
73%
448
July1st to 15thNight
94%
340
36%
1360
47%
340
95%
340
74%
419
July16th to endNight
89%
480
39%
1512
40%
450
89%
450
64%
596
August1st to 15thNight
91%
632
38%
1705
27%
629
91%
629
78%
726
August16th to endNight
93%
764
44%
1910
29%
764
93%
764
83%
853
September1st to 15thNight
93%
720
42%
1800
31%
720
93%
720
81%
811
September16th to endNight
87%
716
46%
1933
42%
716
89%
716
74%
855
October1st to 15thNight
79%
720
31%
2144
34%
720
82%
720
55%
1011
October16th to endNight
81%
768
41%
2304
44%
768
84%
768
57%
1060
November1st to 15thNight
75%
760
30%
2464
33%
760
77%
760
45%
1232
November16th to endNight
70%
780
28%
2536
30%
780
72%
780
37%
1373
December1st to 15thNight
65%
728
22%
2548
24%
728
68%
728
29%
1477
December16th to endNight
54%
768
22%
2688
22%
768
59%
768
25%
1652

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
9%312212235%64%
48641
Night
10%312212335%63%
47986
TransparencyDay
22%38%18%24%21%
48322
Night
24%39%19%24%18%
47805
SeeingDay
11%8%28%33%13%4
6127
Night
11%19%36%33%8%
15653

By Month

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
17%413212346%52%
1453
16th to endDay
19%6%13423335%46%
1728
February1st to 15thDay
18%5%24313345%48%
1646
16th to endDay
22%6%32324236%40%
1579
March1st to 15thDay
16%7%234245%5%7%39%
1857
16th to endDay
16%5%23324347%44%
2196
April1st to 15thDay
17%5%14323436%48%
2149
16th to endDay
11%413312345%58%
2160
May1st to 15thDay
6%32212235%67%
2235
16th to endDay
7%213324345%60%
2615
June1st to 15thDay
4213212235%70%
2520
16th to endDay
11112286%
2520
July1st to 15thDay
11113385%
2520
16th to endDay
11111222281%
2550
August1st to 15thDay
111112287%
2244
16th to endDay
111122384%
2292
September1st to 15thDay
111112384%
2160
16th to endDay
22111222479%
2025
October1st to 15thDay
6%211212345%69%
1814
16th to endDay
7%22312335%65%
1920
November1st to 15thDay
10%412413345%58%
1722
16th to endDay
13%41332435%7%51%
1752
December1st to 15thDay
18%6%24313235%47%
1448
16th to endDay
21%7%23323346%41%
1536
January1st to 15thNight
15%41321335%7%50%
2482
16th to endNight
17%5%13323345%49%
2496
February1st to 15thNight
16%5%24323346%48%
2203
16th to endNight
21%5%14313335%46%
1895
March1st to 15thNight
19%413323335%48%
2100
16th to endNight
16%312223347%50%
2028
April1st to 15thNight
14%413314446%51%
1799
16th to endNight
9%31222343462%
1800
May1st to 15thNight
5%2112236%71%
1710
16th to endNight
6%213312236%67%
1496
June1st to 15thNight
421112235%73%
1440
16th to endNight
1111111287%
1440
July1st to 15thNight
111111285%
1440
16th to endNight
211111122280%
1616
August1st to 15thNight
11111112284%
1714
16th to endNight
11123384%
1910
September1st to 15thNight
1111122286%
1800
16th to endNight
21111122381%
1933
October1st to 15thNight
5%21222223470%
2144
16th to endNight
6%11212235%70%
2304
November1st to 15thNight
10%312213245%62%
2464
16th to endNight
12%313224446%55%
2536
December1st to 15thNight
16%423314446%49%
2548
16th to endNight
21%5%23223246%44%
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
33%413%20%20%7%
1453
16th to endDay
41%5%9%16%16%10%
1728
February1st to 15thDay
37%311%25%17%4
1646
16th to endDay
43%410%24%16%
1579
March1st to 15thDay
40%6%11%19%17%5%
1857
16th to endDay
37%6%16%21%16%2
2196
April1st to 15thDay
36%411%24%17%5%
2149
16th to endDay
26%5%11%26%23%5%
2160
May1st to 15thDay
19%28%25%33%9%
2235
16th to endDay
22%5%10%26%26%8%
2619
June1st to 15thDay
17%39%19%27%22%
2520
16th to endDay
6%311%32%44%
2520
July1st to 15thDay
6%27%22%60%
2371
16th to endDay
10%139%20%53%
2392
August1st to 15thDay
6%29%27%53%
2228
16th to endDay
6%129%35%44%
2292
September1st to 15thDay
7%1312%32%43%
2160
16th to endDay
10%1422%37%23%
2025
October1st to 15thDay
17%27%20%25%26%
1814
16th to endDay
20%27%22%27%19%
1920
November1st to 15thDay
28%411%24%23%7%
1722
16th to endDay
31%411%21%25%4
1752
December1st to 15thDay
40%410%22%17%4
1448
16th to endDay
42%5%11%19%15%5%
1536
January1st to 15thNight
32%5%13%21%20%7%
2482
16th to endNight
37%410%15%19%12%
2496
February1st to 15thNight
35%410%25%16%7%
2214
16th to endNight
39%312%25%15%3
1895
March1st to 15thNight
37%413%19%19%4
2100
16th to endNight
31%5%15%22%19%5%
2028
April1st to 15thNight
32%6%14%24%17%3
1799
16th to endNight
23%5%14%24%24%7%
1800
May1st to 15thNight
15%39%26%33%11%
1711
16th to endNight
18%310%30%28%9%
1496
June1st to 15thNight
13%312%19%24%25%
1440
16th to endNight
7%1310%32%45%
1440
July1st to 15thNight
6%125%25%59%
1360
16th to endNight
12%136%18%57%
1512
August1st to 15thNight
8%210%26%51%
1705
16th to endNight
6%139%32%46%
1910
September1st to 15thNight
6%1410%31%45%
1800
16th to endNight
9%1319%42%23%
1933
October1st to 15thNight
16%38%20%26%23%
2144
16th to endNight
16%25%24%29%21%
2304
November1st to 15thNight
24%311%26%22%12%
2464
16th to endNight
29%49%23%25%7%
2536
December1st to 15thNight
36%410%23%17%6%
2548
16th to endNight
41%412%18%16%7%
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
40%322%22%8%2
81
16th to endDay
25%5%17%29%17%3
256
February1st to 15thDay
24%7%20%24%18%4
249
16th to endDay
25%12%25%23%13%
238
March1st to 15thDay
20%12%27%21%13%3
270
16th to endDay
18%13%32%23%12%
288
April1st to 15thDay
24%15%32%18%6%1
269
16th to endDay
16%18%32%23%8%
270
May1st to 15thDay
11%14%36%28%6%2
288
16th to endDay
9%16%24%33%10%4
330
June1st to 15thDay
415%39%29%7%3
315
16th to endDay
15%30%42%12%6%
315
July1st to 15thDay
125%50%13%8%
305
16th to endDay
226%49%11%8%
327
August1st to 15thDay
3332%43%10%7%
315
16th to endDay
2123%48%14%9%
332
September1st to 15thDay
1329%45%15%3
315
16th to endDay
37%29%38%18%2
315
October1st to 15thDay
5%11%35%28%15%3
315
16th to endDay
10%5%23%32%22%5%
336
November1st to 15thDay
10%218%24%35%8%
138
16th to endDay
11%6%23%29%25%2
134
December1st to 15thDay
30%427%25%11%
62
16th to endDay
26%312%29%23%4
64
January1st to 15thNight
16%19%35%26%10%
720
16th to endNight
21%17%35%25%9%
768
February1st to 15thNight
17%110%35%27%7%
704
16th to endNight
21%310%35%22%6%
632
March1st to 15thNight
17%312%33%26%6%
720
16th to endNight
16%213%36%26%4
768
April1st to 15thNight
15%415%36%25%2
719
16th to endNight
8%417%37%25%6%
720
May1st to 15thNight
6%317%40%27%5%
631
16th to endNight
7%212%36%37%2
375
June1st to 15thNight
6%10%40%38%3
360
16th to endNight
16%42%48%1
360
July1st to 15thNight
1134%59%4
340
16th to endNight
2227%55%11%
480
August1st to 15thNight
1337%46%10%
632
16th to endNight
331%56%8%
764
September1st to 15thNight
332%50%11%
720
16th to endNight
26%34%43%12%
716
October1st to 15thNight
6%8%40%33%10%
720
16th to endNight
7%7%30%34%19%
768
November1st to 15thNight
12%5%38%31%10%
760
16th to endNight
13%9%36%28%10%
780
December1st to 15thNight
18%17%42%22%7%
728
16th to endNight
23%111%36%21%4
768
Page generated at 2017-12-03 08:16:40. Version ac04

Copyright 2017 A. Danko


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