Grasswood Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Grasswood Observatory 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
77%
15820
33%
46784
30%
15775
80%
15775
49%
23524

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
71%
716
30%
2434
30%
716
76%
716
45%
1170
January16th to endNight
66%
768
34%
2409
32%
768
72%
768
42%
1254
February1st to 15thNight
72%
696
36%
2103
35%
696
77%
696
45%
1081
February16th to endNight
63%
632
27%
1895
28%
632
66%
632
36%
1095
March1st to 15thNight
74%
720
29%
2160
31%
720
77%
720
50%
1074
March16th to endNight
74%
768
29%
1986
23%
768
76%
768
47%
1123
April1st to 15thNight
73%
719
31%
1799
21%
719
75%
719
50%
1007
April16th to endNight
78%
720
32%
1800
20%
720
79%
720
56%
973
May1st to 15thNight
77%
659
28%
1739
18%
659
78%
659
49%
961
May16th to endNight
81%
561
28%
1685
23%
561
82%
561
52%
841
June1st to 15thNight
84%
413
26%
1493
29%
413
84%
413
38%
758
June16th to endNight
88%
360
33%
1440
40%
360
88%
360
41%
640
July1st to 15thNight
90%
340
33%
1360
41%
340
90%
340
50%
549
July16th to endNight
83%
633
29%
1654
23%
592
84%
592
50%
863
August1st to 15thNight
84%
717
33%
1790
22%
713
84%
713
54%
963
August16th to endNight
89%
764
42%
1910
28%
764
89%
764
63%
964
September1st to 15thNight
87%
706
38%
1770
30%
706
87%
706
61%
914
September16th to endNight
89%
656
41%
1815
41%
656
89%
656
63%
847
October1st to 15thNight
82%
660
37%
1864
38%
660
83%
660
56%
915
October16th to endNight
81%
704
41%
2112
43%
704
83%
704
56%
991
November1st to 15thNight
81%
700
36%
2267
38%
700
85%
700
56%
994
November16th to endNight
76%
720
36%
2340
38%
720
80%
720
52%
1064
December1st to 15thNight
67%
720
35%
2340
33%
720
75%
720
42%
1173
December16th to endNight
60%
768
32%
2619
29%
768
68%
768
39%
1310

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
4312213347%65%
47856
Night
5%312213346%65%
46986
TransparencyDay
22%26%19%27%21%
47548
Night
22%26%18%26%24%
46794
SeeingDay
5%15%24%41%20%
5964
Night
7%26%23%38%21%
15835

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
6%5%13313335%61%
1503
16th to endDay
11%412213245%59%
1813
February1st to 15thDay
7%322212337%60%
1751
16th to endDay
10%314423246%54%
1579
March1st to 15thDay
5%31322345%7%60%
1800
16th to endDay
432332445%10%56%
2235
April1st to 15thDay
5%413313347%59%
2149
16th to endDay
3212212347%68%
2160
May1st to 15thDay
4312213348%63%
2208
16th to endDay
24122123410%65%
2429
June1st to 15thDay
45%23312347%60%
2463
16th to endDay
1212112346%72%
2520
July1st to 15thDay
231221335%7%65%
2520
16th to endDay
221221335%6%69%
2394
August1st to 15thDay
222212246%72%
2160
16th to endDay
11112246%76%
2292
September1st to 15thDay
111112249%72%
2109
16th to endDay
121212246%73%
1815
October1st to 15thDay
5%212213246%67%
1763
16th to endDay
221321235%5%68%
1760
November1st to 15thDay
3312213235%69%
1591
16th to endDay
4312213346%65%
1620
December1st to 15thDay
6%413314449%55%
1620
16th to endDay
9%314313338%56%
1602
January1st to 15thNight
6%413323447%59%
2434
16th to endNight
10%323212235%60%
2409
February1st to 15thNight
6%31421235%6%61%
2101
16th to endNight
13%323423335%54%
1895
March1st to 15thNight
441321346%7%60%
2160
16th to endNight
441333345%7%58%
1986
April1st to 15thNight
6%5%21213246%64%
1799
16th to endNight
3312123346%67%
1800
May1st to 15thNight
5%312222437%63%
1739
16th to endNight
2312213337%67%
1684
June1st to 15thNight
132321445%7%62%
1493
16th to endNight
1212112347%70%
1440
July1st to 15thNight
122212337%70%
1440
16th to endNight
221222235%8%65%
1769
August1st to 15thNight
2212222347%68%
1800
16th to endNight
112212346%73%
1910
September1st to 15thNight
3211212336%72%
1770
16th to endNight
111112246%76%
1815
October1st to 15thNight
3212213335%70%
1864
16th to endNight
4312112225%72%
2112
November1st to 15thNight
321221223472%
2267
16th to endNight
4312213345%67%
2340
December1st to 15thNight
7%323323245%61%
2340
16th to endNight
10%312213336%59%
2619

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
26%16%15%22%26%
1503
16th to endDay
27%27%18%18%26%
1813
February1st to 15thDay
24%25%17%27%22%
1751
16th to endDay
30%27%23%26%10%
1579
March1st to 15thDay
21%310%23%22%19%
1800
16th to endDay
26%411%24%18%14%
2235
April1st to 15thDay
23%211%28%23%11%
2149
16th to endDay
17%311%23%28%16%
2160
May1st to 15thDay
22%210%23%24%16%
2208
16th to endDay
21%210%30%27%8%
2432
June1st to 15thDay
27%37%18%30%13%
2463
16th to endDay
19%2211%33%29%
2520
July1st to 15thDay
24%3212%28%28%
2371
16th to endDay
20%27%21%29%18%
2246
August1st to 15thDay
20%117%35%23%
2146
16th to endDay
13%3115%40%25%
2292
September1st to 15thDay
17%2315%31%29%
2109
16th to endDay
19%2117%31%27%
1815
October1st to 15thDay
19%16%23%25%23%
1763
16th to endDay
17%35%19%23%31%
1760
November1st to 15thDay
19%25%16%24%31%
1591
16th to endDay
19%38%16%23%29%
1620
December1st to 15thDay
26%38%17%23%20%
1620
16th to endDay
27%39%19%20%20%
1602
January1st to 15thNight
24%17%15%23%27%
2434
16th to endNight
26%27%15%16%30%
2409
February1st to 15thNight
24%25%17%23%25%
2112
16th to endNight
34%26%17%24%15%
1895
March1st to 15thNight
21%39%21%25%19%
2160
16th to endNight
26%311%22%22%14%
1986
April1st to 15thNight
24%17%28%25%12%
1799
16th to endNight
20%29%23%24%19%
1800
May1st to 15thNight
24%37%23%25%14%
1740
16th to endNight
20%29%27%28%10%
1685
June1st to 15thNight
28%28%17%31%11%
1493
16th to endNight
22%3310%32%27%
1440
July1st to 15thNight
17%2313%27%35%
1360
16th to endNight
22%36%23%25%18%
1654
August1st to 15thNight
21%3112%35%25%
1790
16th to endNight
15%212%42%26%
1910
September1st to 15thNight
17%239%34%32%
1770
16th to endNight
14%2115%32%33%
1815
October1st to 15thNight
19%1419%29%25%
1864
16th to endNight
18%2414%26%35%
2112
November1st to 15thNight
16%25%17%21%37%
2267
16th to endNight
19%25%18%22%31%
2340
December1st to 15thNight
26%17%16%21%25%
2340
16th to endNight
26%28%17%23%20%
2619

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
19%8%45%20%6%
131
16th to endDay
11%210%36%31%8%
271
February1st to 15thDay
10%35%33%34%10%
266
16th to endDay
14%212%34%26%10%
238
March1st to 15thDay
4212%29%37%12%
270
16th to endDay
6%310%29%40%11%
288
April1st to 15thDay
6%28%35%35%11%
269
16th to endDay
1314%34%32%13%
270
May1st to 15thDay
227%29%39%19%
288
16th to endDay
5%2426%43%16%
330
June1st to 15thDay
7%2228%39%19%
315
16th to endDay
19%52%35%
315
July1st to 15thDay
418%47%37%
305
16th to endDay
216%41%47%
327
August1st to 15thDay
2110%53%30%
315
16th to endDay
110%47%39%
332
September1st to 15thDay
21115%56%22%
300
16th to endDay
2122%52%21%
255
October1st to 15thDay
328%30%43%12%
255
16th to endDay
36%27%40%20%
272
November1st to 15thDay
41425%37%27%
97
16th to endDay
227%44%37%5%
90
December1st to 15thDay
6%410%31%47%
90
16th to endDay
8%16%36%34%5%
75
January1st to 15thNight
9%111%36%29%12%
720
16th to endNight
11%110%30%35%10%
768
February1st to 15thNight
8%27%30%39%11%
704
16th to endNight
15%111%30%33%8%
632
March1st to 15thNight
6%28%34%35%11%
720
16th to endNight
6%210%29%37%13%
768
April1st to 15thNight
9%47%31%37%9%
719
16th to endNight
5%37%29%38%14%
720
May1st to 15thNight
9%26%24%40%17%
660
16th to endNight
436%24%39%21%
563
June1st to 15thNight
43221%43%25%
413
16th to endNight
4219%53%28%
360
July1st to 15thNight
31239%53%
340
16th to endNight
5%22135%52%
633
August1st to 15thNight
5%216%44%39%
717
16th to endNight
311439%49%
764
September1st to 15thNight
5%9%44%38%
706
16th to endNight
31216%47%29%
656
October1st to 15thNight
5%15%24%44%19%
660
16th to endNight
7%421%43%22%
704
November1st to 15thNight
416%29%38%19%
700
16th to endNight
6%19%29%37%16%
720
December1st to 15thNight
9%210%29%38%10%
720
16th to endNight
12%216%30%30%6%
768
Page generated at 2017-09-13 04:39:04. Version ac04

Copyright 2017 A. Danko


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