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 2017-09-10.

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%
15347
30%
46789
29%
15313
77%
15313
47%
23625

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
68%
716
21%
2494
23%
716
71%
716
35%
1335
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%
700
43%
1750
32%
700
94%
700
79%
795
September16th to endNight
88%
656
44%
1786
41%
656
90%
656
75%
779
October1st to 15thNight
80%
660
34%
1939
36%
660
82%
660
55%
926
October16th to endNight
82%
704
39%
2120
42%
704
85%
704
57%
980
November1st to 15thNight
77%
700
32%
2275
36%
700
79%
700
48%
1088
November16th to endNight
73%
720
31%
2340
33%
720
75%
720
42%
1200
December1st to 15thNight
66%
720
24%
2484
26%
720
68%
720
30%
1415
December16th to endNight
56%
768
24%
2688
23%
768
61%
768
26%
1597

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%
47832
Night
9%312212235%64%
46978
TransparencyDay
23%37%19%25%20%
47511
Night
22%39%20%25%17%
46799
SeeingDay
11%8%28%34%12%4
5816
Night
10%17%33%37%9%
15358

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%12123346%52%
1441
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
11111122283%
2097
16th to endDay
21111133478%
1843
October1st to 15thDay
6%31211335%5%64%
1689
16th to endDay
6%22223345%66%
1751
November1st to 15thDay
10%422323245%58%
1584
16th to endDay
10%424423246%56%
1620
December1st to 15thDay
17%5%14413345%47%
1473
16th to endDay
18%7%24413346%43%
1536
January1st to 15thNight
13%313212347%54%
2494
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
111112186%
1750
16th to endNight
1111212383%
1786
October1st to 15thNight
5%22113345%68%
1939
16th to endNight
6%11112335%72%
2120
November1st to 15thNight
9%212211246%65%
2275
16th to endNight
8%413323346%59%
2340
December1st to 15thNight
14%413323347%50%
2484
16th to endNight
20%413312447%46%
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%310%21%20%9%
1441
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%2211%34%40%
2097
16th to endDay
10%2322%37%23%
1843
October1st to 15thDay
20%27%21%28%18%
1689
16th to endDay
20%35%24%29%17%
1751
November1st to 15thDay
27%310%22%23%10%
1584
16th to endDay
30%210%20%27%8%
1620
December1st to 15thDay
39%5%8%21%20%5%
1473
16th to endDay
42%411%19%15%7%
1536
January1st to 15thNight
30%312%22%22%8%
2494
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%2110%34%44%
1750
16th to endNight
9%1220%42%23%
1786
October1st to 15thNight
17%37%25%30%15%
1939
16th to endNight
16%26%25%29%19%
2120
November1st to 15thNight
21%311%25%25%13%
2275
16th to endNight
25%39%23%27%9%
2340
December1st to 15thNight
34%5%11%22%19%6%
2484
16th to endNight
39%412%18%17%8%
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
47%120%17%8%4
69
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
3328%45%15%3
294
16th to endDay
5%7%28%42%14%1
255
October1st to 15thDay
6%10%33%36%11%1
255
16th to endDay
7%5%31%34%19%1
263
November1st to 15thDay
8%47%24%36%17%
90
16th to endDay
13%8%24%35%15%2
90
December1st to 15thDay
18%35%27%18%
65
16th to endDay
20%414%35%15%9%
64
January1st to 15thNight
14%18%39%24%11%
720
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
1229%52%14%
700
16th to endNight
25%28%52%11%
656
October1st to 15thNight
6%17%35%38%10%
660
16th to endNight
7%5%32%37%16%
704
November1st to 15thNight
11%5%34%33%14%
700
16th to endNight
10%18%37%31%10%
720
December1st to 15thNight
16%27%41%25%6%
720
16th to endNight
23%112%37%20%5%
768
Page generated at 2017-09-11 16:35:23. Version ac04

Copyright 2017 A. Danko


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