Ahwahnee Hills Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Ahwahnee Hills Observatory clear sky chart from 2005-11-05 to 2018-01-12.

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
75%
15839
31%
48347
29%
15805
77%
15805
47%
24388

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%
760
21%
2649
22%
760
70%
760
34%
1452
January16th to endNight
65%
768
26%
2496
28%
768
68%
768
33%
1430
February1st to 15thNight
64%
699
26%
2176
27%
697
67%
697
34%
1263
February16th to endNight
60%
632
24%
1895
25%
632
63%
632
32%
1158
March1st to 15thNight
63%
720
19%
2121
20%
720
65%
720
34%
1270
March16th to endNight
65%
768
23%
2013
18%
768
69%
768
41%
1193
April1st to 15thNight
65%
719
23%
1799
14%
719
67%
719
42%
1093
April16th to endNight
68%
720
26%
1800
14%
720
70%
720
50%
1010
May1st to 15thNight
73%
620
29%
1700
21%
620
75%
620
58%
832
May16th to endNight
76%
408
31%
1531
33%
408
79%
408
49%
647
June1st to 15thNight
83%
360
31%
1440
38%
360
86%
360
55%
535
June16th to endNight
93%
360
40%
1440
53%
360
94%
360
79%
427
July1st to 15thNight
95%
340
36%
1360
48%
340
95%
340
72%
427
July16th to endNight
89%
465
39%
1498
41%
436
89%
436
61%
605
August1st to 15thNight
91%
660
37%
1734
27%
657
91%
657
78%
765
August16th to endNight
95%
764
45%
1910
29%
764
95%
764
82%
855
September1st to 15thNight
93%
720
43%
1800
32%
720
94%
720
78%
820
September16th to endNight
88%
716
46%
1949
43%
716
90%
716
75%
849
October1st to 15thNight
81%
720
33%
2115
35%
720
83%
720
57%
987
October16th to endNight
82%
768
41%
2313
44%
768
85%
768
58%
1062
November1st to 15thNight
76%
760
30%
2470
35%
760
78%
760
47%
1194
November16th to endNight
73%
780
31%
2535
34%
780
75%
780
41%
1309
December1st to 15thNight
67%
780
25%
2691
28%
780
70%
780
33%
1483
December16th to endNight
57%
832
24%
2912
24%
832
62%
832
27%
1722

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
8%412212335%63%
48983
Night
9%312212235%64%
48536
TransparencyDay
23%37%19%25%20%
48662
Night
22%39%20%25%18%
48357
SeeingDay
10%8%28%34%13%4
6197
Night
10%17%33%37%9%
15850

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
16%5%12223446%49%
1529
16th to endDay
18%7%12324335%46%
1728
February1st to 15thDay
15%7%13213345%49%
1676
16th to endDay
20%8%3431323443%
1579
March1st to 15thDay
15%8%2342445%8%40%
1836
16th to endDay
15%6%24423446%43%
2210
April1st to 15thDay
14%6%24414336%47%
2149
16th to endDay
11%5%13323347%54%
2160
May1st to 15thDay
6%313213246%65%
2250
16th to endDay
7%314223447%58%
2579
June1st to 15thDay
3313312336%68%
2520
16th to endDay
1111111385%
2520
July1st to 15thDay
11111112481%
2520
16th to endDay
11111223379%
2565
August1st to 15thDay
111112286%
2214
16th to endDay
11111222382%
2292
September1st to 15thDay
11111122282%
2160
16th to endDay
21111133478%
2010
October1st to 15thDay
6%312113245%67%
1842
16th to endDay
5%22223345%66%
1910
November1st to 15thDay
9%422324245%58%
1719
16th to endDay
10%414423246%56%
1755
December1st to 15thDay
16%5%14413346%50%
1596
16th to endDay
17%6%24424446%43%
1664
January1st to 15thNight
14%413312347%52%
2649
16th to endNight
16%5%1221335%7%50%
2496
February1st to 15thNight
14%41332345%8%48%
2174
16th to endNight
19%6%12213346%47%
1895
March1st to 15thNight
15%413324337%51%
2121
16th to endNight
13%412213247%55%
2013
April1st to 15thNight
12%42222335%7%53%
1799
16th to endNight
8%313213236%64%
1800
May1st to 15thNight
4212112246%70%
1700
16th to endNight
5%212113246%69%
1530
June1st to 15thNight
3111112235%74%
1440
16th to endNight
1111388%
1440
July1st to 15thNight
11112384%
1440
16th to endNight
221121212378%
1601
August1st to 15thNight
1221111284%
1743
16th to endNight
11112385%
1910
September1st to 15thNight
111112285%
1800
16th to endNight
1111112383%
1949
October1st to 15thNight
5%21112345%70%
2115
16th to endNight
6%11212335%72%
2313
November1st to 15thNight
9%312211246%64%
2470
16th to endNight
9%413323346%57%
2535
December1st to 15thNight
13%413323347%53%
2691
16th to endNight
19%423312447%46%
2912

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
34%410%21%19%9%
1529
16th to endDay
40%410%16%16%12%
1728
February1st to 15thDay
37%49%25%17%6%
1676
16th to endDay
44%47%26%16%1
1579
March1st to 15thDay
42%411%18%16%6%
1836
16th to endDay
38%6%14%21%15%2
2210
April1st to 15thDay
37%410%24%19%4
2149
16th to endDay
30%49%27%23%4
2160
May1st to 15thDay
21%37%26%33%7%
2250
16th to endDay
24%5%8%30%25%6%
2583
June1st to 15thDay
17%48%20%28%20%
2520
16th to endDay
7%1211%38%39%
2520
July1st to 15thDay
7%226%25%55%
2371
16th to endDay
11%2310%22%49%
2405
August1st to 15thDay
6%2110%29%49%
2198
16th to endDay
8%2210%39%38%
2292
September1st to 15thDay
9%2212%33%38%
2160
16th to endDay
10%2324%37%21%
2010
October1st to 15thDay
19%27%19%27%23%
1842
16th to endDay
19%35%23%27%20%
1910
November1st to 15thDay
28%310%23%22%10%
1719
16th to endDay
29%210%21%27%8%
1755
December1st to 15thDay
36%48%20%19%9%
1596
16th to endDay
41%410%18%17%7%
1664
January1st to 15thNight
32%312%22%21%8%
2649
16th to endNight
34%312%15%19%14%
2496
February1st to 15thNight
34%39%27%17%7%
2185
16th to endNight
38%313%26%15%3
1895
March1st to 15thNight
34%414%19%21%5%
2121
16th to endNight
29%416%26%18%4
2013
April1st to 15thNight
30%416%27%16%4
1799
16th to endNight
22%414%27%25%5%
1800
May1st to 15thNight
16%39%28%31%10%
1701
16th to endNight
17%38%32%30%7%
1531
June1st to 15thNight
13%211%21%29%21%
1440
16th to endNight
5%312%36%41%
1440
July1st to 15thNight
6%126%27%55%
1360
16th to endNight
12%318%22%51%
1498
August1st to 15thNight
8%1211%26%50%
1734
16th to endNight
6%1110%37%42%
1910
September1st to 15thNight
7%2211%33%42%
1800
16th to endNight
9%1220%43%22%
1949
October1st to 15thNight
16%26%23%28%21%
2115
16th to endNight
16%25%25%28%21%
2313
November1st to 15thNight
22%311%25%25%12%
2470
16th to endNight
26%39%24%27%9%
2535
December1st to 15thNight
31%411%21%19%11%
2691
16th to endNight
38%411%18%18%8%
2912

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
45%118%19%10%4
91
16th to endDay
26%322%27%14%4
256
February1st to 15thDay
24%6%24%21%18%4
253
16th to endDay
23%12%28%21%12%
238
March1st to 15thDay
18%12%28%22%15%2
270
16th to endDay
19%14%27%26%10%1
288
April1st to 15thDay
22%18%30%22%5%
269
16th to endDay
12%21%36%21%7%
270
May1st to 15thDay
9%16%34%29%5%4
288
16th to endDay
10%15%25%34%10%3
330
June1st to 15thDay
7%13%35%30%9%4
315
16th to endDay
15%27%42%12%9%
315
July1st to 15thDay
22%52%12%9%
305
16th to endDay
127%49%10%9%
327
August1st to 15thDay
3333%43%9%6%
315
16th to endDay
2223%48%14%9%
332
September1st to 15thDay
3330%44%15%3
315
16th to endDay
46%32%39%14%1
315
October1st to 15thDay
5%10%33%34%13%2
315
16th to endDay
7%5%27%33%24%3
326
November1st to 15thDay
8%5%17%28%29%11%
135
16th to endDay
14%6%20%31%21%5%
135
December1st to 15thDay
13%29%26%24%6%
98
16th to endDay
15%416%28%21%13%
96
January1st to 15thNight
15%18%39%24%10%
764
16th to endNight
19%19%35%25%8%
768
February1st to 15thNight
18%18%36%26%8%
704
16th to endNight
21%29%34%25%6%
632
March1st to 15thNight
17%211%33%28%6%
720
16th to endNight
14%210%36%30%4
768
April1st to 15thNight
14%212%37%27%4
719
16th to endNight
8%314%35%31%6%
720
May1st to 15thNight
5%113%40%31%5%
621
16th to endNight
7%112%31%41%6%
409
June1st to 15thNight
417%34%47%4
360
16th to endNight
1233%58%3
360
July1st to 15thNight
1122%67%7%
340
16th to endNight
317%61%16%
465
August1st to 15thNight
1227%56%10%
660
16th to endNight
1121%61%13%
764
September1st to 15thNight
230%52%14%
720
16th to endNight
25%30%50%10%
716
October1st to 15thNight
5%17%36%38%10%
720
16th to endNight
7%5%30%38%17%
768
November1st to 15thNight
11%16%35%32%13%
760
16th to endNight
11%18%37%31%10%
780
December1st to 15thNight
15%27%40%26%6%
780
16th to endNight
22%112%37%21%5%
832
Page generated at 2018-01-12 19:01:27. Version ac04

Copyright 2018 A. Danko


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