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 2018-03-21.

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%
16582
34%
49135
31%
16537
80%
16537
49%
24660

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
70%
776
29%
2638
30%
776
75%
776
43%
1293
January16th to endNight
67%
832
35%
2609
33%
832
73%
832
44%
1334
February1st to 15thNight
74%
756
36%
2283
36%
756
78%
756
46%
1163
February16th to endNight
63%
684
29%
2051
29%
684
67%
684
38%
1158
March1st to 15thNight
73%
780
27%
2340
29%
780
76%
780
47%
1196
March16th to endNight
73%
792
29%
2052
24%
792
76%
792
47%
1165
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%
720
37%
1806
30%
720
87%
720
61%
936
September16th to endNight
89%
716
43%
1980
43%
716
90%
716
64%
914
October1st to 15thNight
83%
720
36%
2034
38%
720
84%
720
56%
994
October16th to endNight
82%
768
42%
2304
46%
768
83%
768
56%
1079
November1st to 15thNight
80%
760
34%
2461
37%
760
84%
760
54%
1101
November16th to endNight
76%
780
37%
2535
39%
780
80%
780
51%
1162
December1st to 15thNight
68%
780
35%
2535
33%
780
75%
780
44%
1247
December16th to endNight
61%
832
33%
2837
31%
832
70%
832
40%
1399

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%
49692
Night
5%312213346%65%
49337
TransparencyDay
21%26%19%27%21%
49384
Night
22%26%18%26%24%
49145
SeeingDay
5%16%24%41%20%
6621
Night
7%26%23%38%21%
16597

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%13323336%59%
1629
16th to endDay
10%412213245%59%
1964
February1st to 15thDay
7%322212336%62%
1901
16th to endDay
10%314323236%57%
1709
March1st to 15thDay
6%31322345%7%58%
1950
16th to endDay
432332446%10%55%
2296
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%71%
2154
16th to endDay
1211212246%74%
1980
October1st to 15thDay
4212213246%69%
1923
16th to endDay
2213212345%69%
1920
November1st to 15thDay
3313213336%66%
1726
16th to endDay
4213213347%64%
1755
December1st to 15thDay
5%31331445%9%57%
1755
16th to endDay
8%314313348%56%
1735
January1st to 15thNight
6%413323347%58%
2638
16th to endNight
9%323212235%61%
2609
February1st to 15thNight
6%31421235%6%62%
2281
16th to endNight
12%323323335%57%
2051
March1st to 15thNight
5%41321446%7%57%
2340
16th to endNight
441333345%7%58%
2052
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%
1806
16th to endNight
111112235%77%
1980
October1st to 15thNight
3212213335%70%
2034
16th to endNight
4312212225%72%
2304
November1st to 15thNight
3212212335%70%
2461
16th to endNight
4312213346%66%
2535
December1st to 15thNight
6%223323245%62%
2535
16th to endNight
10%312213436%60%
2837

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
27%17%16%22%24%
1629
16th to endDay
26%26%19%18%26%
1964
February1st to 15thDay
23%2416%27%24%
1901
16th to endDay
28%27%23%26%11%
1709
March1st to 15thDay
23%310%22%21%18%
1950
16th to endDay
26%412%24%18%14%
2296
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%28%
2154
16th to endDay
18%2217%32%27%
1980
October1st to 15thDay
18%15%22%25%25%
1923
16th to endDay
16%25%18%23%32%
1920
November1st to 15thDay
20%26%17%22%29%
1726
16th to endDay
19%37%15%24%29%
1755
December1st to 15thDay
24%38%16%22%24%
1755
16th to endDay
26%38%18%22%21%
1735
January1st to 15thNight
25%17%16%22%25%
2638
16th to endNight
25%27%16%17%30%
2609
February1st to 15thNight
24%25%17%24%26%
2292
16th to endNight
32%25%17%25%17%
2051
March1st to 15thNight
23%39%21%24%18%
2340
16th to endNight
26%311%23%22%13%
2052
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%31%
1806
16th to endNight
14%1115%33%33%
1980
October1st to 15thNight
19%2418%29%26%
2034
16th to endNight
18%2413%25%36%
2304
November1st to 15thNight
17%26%18%21%34%
2461
16th to endNight
19%25%17%22%31%
2535
December1st to 15thNight
25%17%16%21%28%
2535
16th to endNight
25%27%17%24%21%
2837

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
17%6%40%27%7%
167
16th to endDay
9%29%33%34%10%
326
February1st to 15thDay
8%35%33%32%17%
326
16th to endDay
11%113%34%28%9%
290
March1st to 15thDay
6%212%33%34%10%
330
16th to endDay
7%210%29%39%10%
311
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%21%
315
16th to endDay
2220%54%20%
315
October1st to 15thDay
226%29%46%12%
315
16th to endDay
26%27%38%23%
336
November1st to 15thDay
5%230%39%20%
142
16th to endDay
317%36%37%14%
135
December1st to 15thDay
4211%31%43%6%
135
16th to endDay
5%14%32%36%11%
112
January1st to 15thNight
9%110%36%30%12%
780
16th to endNight
10%110%30%35%10%
832
February1st to 15thNight
7%26%31%37%14%
764
16th to endNight
14%112%30%32%8%
684
March1st to 15thNight
7%28%35%35%11%
780
16th to endNight
7%210%30%36%12%
792
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%10%43%38%
720
16th to endNight
31217%47%28%
716
October1st to 15thNight
5%15%23%46%18%
720
16th to endNight
7%1421%42%23%
768
November1st to 15thNight
416%31%37%18%
760
16th to endNight
6%18%28%37%16%
780
December1st to 15thNight
8%211%28%38%10%
780
16th to endNight
11%315%30%30%8%
832
Page generated at 2018-03-22 08:34:24. Version ac04

Copyright 2018 A. Danko


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