Webb Schools Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Webb Schools Observatory clear sky chart from 2005-11-05 to 2017-10-06.

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
76%
15968
34%
46932
30%
15923
79%
15923
52%
23109

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%
718
29%
2439
29%
716
73%
716
41%
1213
January16th to endNight
62%
768
34%
2304
28%
768
71%
768
40%
1253
February1st to 15thNight
69%
696
35%
2101
34%
696
74%
696
48%
1063
February16th to endNight
62%
632
29%
1895
30%
632
66%
632
37%
1077
March1st to 15thNight
69%
720
31%
2160
30%
720
75%
720
48%
1076
March16th to endNight
71%
768
30%
2015
24%
768
74%
768
51%
1096
April1st to 15thNight
64%
719
28%
1799
18%
719
68%
719
49%
1023
April16th to endNight
72%
720
32%
1800
18%
720
75%
720
55%
962
May1st to 15thNight
72%
598
30%
1678
21%
598
73%
598
50%
893
May16th to endNight
75%
561
30%
1685
24%
561
76%
561
53%
837
June1st to 15thNight
79%
513
30%
1593
27%
513
80%
513
54%
771
June16th to endNight
96%
360
39%
1440
48%
360
96%
360
63%
499
July1st to 15thNight
94%
340
35%
1360
42%
340
94%
340
58%
497
July16th to endNight
87%
642
32%
1665
25%
603
88%
603
59%
818
August1st to 15thNight
91%
717
36%
1790
24%
713
91%
713
71%
853
August16th to endNight
91%
764
43%
1910
30%
764
91%
764
72%
910
September1st to 15thNight
88%
720
38%
1842
30%
720
89%
720
68%
889
September16th to endNight
88%
716
47%
1980
44%
716
91%
716
74%
845
October1st to 15thNight
79%
684
37%
1881
34%
684
82%
684
59%
918
October16th to endNight
77%
704
39%
2144
41%
704
80%
704
53%
1028
November1st to 15thNight
79%
700
36%
2275
38%
700
84%
700
56%
993
November16th to endNight
75%
720
37%
2340
39%
720
79%
720
51%
1090
December1st to 15thNight
67%
720
33%
2340
32%
720
73%
720
41%
1207
December16th to endNight
60%
768
33%
2496
29%
768
68%
768
39%
1298

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
5%212213336%67%
48230
Night
4212213235%68%
47134
TransparencyDay
18%25%17%28%27%
47922
Night
20%25%17%26%27%
46944
SeeingDay
6%7%28%37%15%5%
5999
Night
6%29%35%37%8%
15981

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
8%313213235%62%
1479
16th to endDay
12%41332333456%
1920
February1st to 15thDay
8%313213345%60%
1751
16th to endDay
14%5%24423336%49%
1579
March1st to 15thDay
6%31332436%8%56%
1800
16th to endDay
5%31343445%8%54%
2205
April1st to 15thDay
6%323424338%56%
2149
16th to endDay
5%311323237%65%
2160
May1st to 15thDay
4422223346%64%
2271
16th to endDay
4322213336%64%
2429
June1st to 15thDay
4212113236%68%
2361
16th to endDay
11111385%
2520
July1st to 15thDay
211112335%75%
2519
16th to endDay
2111112346%72%
2384
August1st to 15thDay
11122483%
2160
16th to endDay
1111223480%
2292
September1st to 15thDay
1111122481%
2118
16th to endDay
11111122380%
1980
October1st to 15thDay
6%212112225%69%
1876
16th to endDay
431221233371%
1725
November1st to 15thDay
4312213236%67%
1584
16th to endDay
6%313323235%63%
1620
December1st to 15thDay
7%32332336%10%55%
1620
16th to endDay
12%5%23222336%55%
1728
January1st to 15thNight
7%313322345%61%
2439
16th to endNight
10%423223236%58%
2304
February1st to 15thNight
7%323313235%63%
2101
16th to endNight
13%424324345%51%
1895
March1st to 15thNight
4422224347%61%
2160
16th to endNight
5%223423347%60%
2015
April1st to 15thNight
6%323314346%60%
1799
16th to endNight
3312223236%68%
1800
May1st to 15thNight
3312323346%65%
1678
16th to endNight
2212123247%69%
1684
June1st to 15thNight
112113336%73%
1593
16th to endNight
1112387%
1440
July1st to 15thNight
111111125%80%
1440
16th to endNight
2112212246%72%
1778
August1st to 15thNight
1111135%81%
1800
16th to endNight
111122384%
1910
September1st to 15thNight
1111122380%
1842
16th to endNight
111112384%
1980
October1st to 15thNight
321321222472%
1881
16th to endNight
331232213472%
2144
November1st to 15thNight
331221222472%
2275
16th to endNight
5%312213246%64%
2340
December1st to 15thNight
7%323424347%56%
2340
16th to endNight
10%413213336%58%
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
23%35%13%24%29%
1479
16th to endDay
28%26%14%19%28%
1920
February1st to 15thDay
23%25%15%25%27%
1751
16th to endDay
33%2418%28%11%
1579
March1st to 15thDay
22%38%22%21%22%
1800
16th to endDay
23%211%25%21%15%
2205
April1st to 15thDay
24%38%26%23%13%
2149
16th to endDay
18%27%24%28%18%
2160
May1st to 15thDay
22%27%19%28%18%
2271
16th to endDay
22%35%28%29%11%
2432
June1st to 15thDay
21%25%15%34%20%
2361
16th to endDay
10%126%33%46%
2520
July1st to 15thDay
12%3211%26%43%
2370
16th to endDay
13%26%21%32%23%
2236
August1st to 15thDay
9%212%37%38%
2146
16th to endDay
9%2110%40%35%
2292
September1st to 15thDay
10%2212%33%39%
2118
16th to endDay
9%115%34%38%
1980
October1st to 15thDay
18%1418%28%27%
1876
16th to endDay
17%2412%21%41%
1725
November1st to 15thDay
16%15%14%25%35%
1584
16th to endDay
22%2413%23%34%
1620
December1st to 15thDay
23%39%18%20%25%
1620
16th to endDay
29%17%17%20%22%
1728
January1st to 15thNight
25%25%12%23%30%
2439
16th to endNight
28%37%14%15%31%
2304
February1st to 15thNight
23%26%17%23%27%
2112
16th to endNight
32%27%17%24%15%
1895
March1st to 15thNight
22%27%22%24%20%
2160
16th to endNight
22%310%24%23%16%
2015
April1st to 15thNight
27%19%26%20%14%
1799
16th to endNight
19%37%25%26%17%
1800
May1st to 15thNight
22%27%23%26%16%
1679
16th to endNight
20%28%31%26%11%
1685
June1st to 15thNight
18%37%18%34%17%
1593
16th to endNight
10%228%32%44%
1440
July1st to 15thNight
12%2213%29%40%
1360
16th to endNight
16%36%20%30%22%
1665
August1st to 15thNight
11%2112%40%32%
1790
16th to endNight
10%210%39%35%
1910
September1st to 15thNight
13%138%35%37%
1842
16th to endNight
9%115%34%38%
1980
October1st to 15thNight
16%2318%32%25%
1881
16th to endNight
19%2211%21%41%
2144
November1st to 15thNight
16%1415%24%36%
2275
16th to endNight
20%15%17%18%36%
2340
December1st to 15thNight
27%27%16%20%25%
2340
16th to endNight
28%28%13%26%21%
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
21%228%35%11%
78
16th to endDay
14%9%30%29%13%1
288
February1st to 15thDay
11%7%30%34%12%3
266
16th to endDay
15%12%31%28%10%
238
March1st to 15thDay
7%14%28%26%18%4
270
16th to endDay
7%11%39%26%13%1
288
April1st to 15thDay
7%18%37%21%9%4
269
16th to endDay
220%40%26%6%3
270
May1st to 15thDay
8%11%35%30%10%3
288
16th to endDay
7%10%37%28%13%2
330
June1st to 15thDay
5%10%26%38%14%4
315
16th to endDay
325%45%16%8%
315
July1st to 15thDay
2119%46%22%7%
305
16th to endDay
1113%52%21%10%
327
August1st to 15thDay
2120%53%16%6%
315
16th to endDay
1217%53%16%9%
332
September1st to 15thDay
2228%47%13%5%
315
16th to endDay
35%30%40%13%7%
315
October1st to 15thDay
410%34%31%16%2
278
16th to endDay
8%5%27%35%19%3
231
November1st to 15thDay
2414%28%34%15%
90
16th to endDay
35%35%38%14%2
90
December1st to 15thDay
2314%44%32%3
90
16th to endDay
13%5%21%37%20%1
96
January1st to 15thNight
8%313%42%25%6%
720
16th to endNight
13%312%39%26%5%
768
February1st to 15thNight
9%29%43%28%6%
704
16th to endNight
15%111%41%24%4
632
March1st to 15thNight
6%114%46%24%6%
720
16th to endNight
6%312%40%32%4
768
April1st to 15thNight
9%5%12%42%26%3
719
16th to endNight
5%413%40%29%7%
720
May1st to 15thNight
6%312%36%34%6%
599
16th to endNight
4312%38%33%7%
563
June1st to 15thNight
2212%38%39%4
513
16th to endNight
31%61%5%
360
July1st to 15thNight
212%69%14%
340
16th to endNight
2221%58%14%
642
August1st to 15thNight
1221%58%14%
717
16th to endNight
1217%57%19%
764
September1st to 15thNight
3324%50%17%
720
16th to endNight
21429%47%13%
716
October1st to 15thNight
417%39%39%7%
684
16th to endNight
6%18%34%37%12%
704
November1st to 15thNight
39%40%35%10%
700
16th to endNight
7%9%38%33%10%
720
December1st to 15thNight
9%210%41%29%6%
720
16th to endNight
13%215%41%22%5%
768
Page generated at 2017-10-07 11:07:43. Version ac04

Copyright 2017 A. Danko


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