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 2005-11-06 to 2026-09-14.

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 Type	Minimum conditions for hour to be usable				Out of hours
	Clear of Clouds	Transparency	Seeing	Sky Brightness
Planetary:	50%	(no limit)	Average	(no limit)	Nighttime Hours
Deep Sky:	70%	Average	(no limit)	5.9	Nighttime Hours
CCD Imaging:	70%	Below Average	Poor	(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

By Month

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 Forecast	Data Points
Cloud Cover	Day	6% 4 1 3 2 2 2 2 3 5% 69%	157125
	Night	8% 4 1 3 2 2 3 2 4 5% 66%	151485
Transparency	Day	19% 4 6% 18% 28% 25%	152150
	Night	22% 3 7% 20% 26% 22%	147452
Seeing	Day	9% 10% 29% 34% 14% 4	17684
	Night	10% 3 17% 37% 27% 6%	27105

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
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	15% 7% 2 4 2 2 3 3 5% 7% 50%	4722
	11th to 20th	Day	9% 4 2 4 3 3 3 2 3 4 62%	5097
	21st to end	Day	11% 5% 2 6% 4 4 4 3 5% 7% 48%	5843
February	1st to 10th	Day	16% 9% 2 5% 2 3 3 3 4 5% 49%	1970
	11th to 20th	Day	12% 11% 2 4 4 3 4 3 5% 6% 47%	2050
	21st to end	Day	16% 9% 4 6% 4 3 4 3 3 5% 43%	1730
March	1st to 10th	Day	13% 14% 3 7% 4 4 4 4 5% 7% 35%	4618
	11th to 20th	Day	15% 7% 2 4 3 3 4 3 4 7% 48%	5020
	21st to end	Day	16% 10% 2 6% 4 3 4 3 5% 7% 40%	5981
April	1st to 10th	Day	11% 7% 2 5% 3 2 4 3 4 7% 52%	2520
	11th to 20th	Day	10% 6% 2 5% 4 3 3 3 5% 7% 51%	2489
	21st to end	Day	8% 6% 2 5% 3 4 4 3 4 6% 55%	2520
May	1st to 10th	Day	6% 4 2 3 2 2 2 2 3 5% 70%	7243
	11th to 20th	Day	2 2 1 2 2 2 2 2 3 4 79%	7704
	21st to end	Day	3 3 1 3 2 2 2 2 3 4 76%	8992
June	1st to 10th	Day	3 3 1 3 2 2 3 3 5% 7% 68%	2940
	11th to 20th	Day	1 1 2 2 1 2 1 2 4 84%	2940
	21st to end	Day	2 2 1 1 2 1 2 2 2 84%	2940
July	1st to 10th	Day	1 1 1 1 1 2 2 92%	8219
	11th to 20th	Day	1 1 1 2 1 2 1 1 2 3 84%	8220
	21st to end	Day	1 1 1 1 2 2 2 3 87%	8477
August	1st to 10th	Day	1 1 1 1 1 1 1 1 2 91%	7237
	11th to 20th	Day	1 1 1 1 1 2 1 2 3 87%	7001
	21st to end	Day	3 2 1 2 1 1 2 2 3 5% 78%	7750
September	1st to 10th	Day	2 1 1 1 1 2 1 2 2 87%	2520
	11th to 20th	Day	3 1 1 2 1 2 2 2 3 4 78%	2388
	21st to end	Day	2 2 1 1 1 2 2 2 3 4 79%	2200
October	1st to 10th	Day	3 3 1 2 2 2 2 2 4 4 76%	3386
	11th to 20th	Day	3 2 1 2 1 2 2 2 3 3 77%	3261
	21st to end	Day	4 2 1 4 3 3 3 3 5% 6% 65%	3529
November	1st to 10th	Day	9% 5% 1 4 3 3 3 2 5% 7% 58%	1854
	11th to 20th	Day	10% 4 2 3 2 2 4 2 4 7% 59%	1890
	21st to end	Day	10% 5% 1 4 4 2 3 2 3 6% 59%	1890
December	1st to 10th	Day	8% 3 1 3 3 3 3 3 5% 6% 60%	2784
	11th to 20th	Day	21% 7% 2 6% 3 3 3 3 4 7% 41%	3108
	21st to end	Day	17% 8% 3 6% 3 4 4 4 6% 8% 39%	4092
January	1st to 10th	Night	15% 6% 2 4 3 2 3 3 5% 7% 48%	8172
	11th to 20th	Night	10% 4 1 3 2 2 3 3 4 6% 61%	7878
	21st to end	Night	11% 8% 2 6% 3 3 3 3 5% 7% 49%	8371
February	1st to 10th	Night	16% 7% 2 4 3 2 3 3 5% 6% 49%	2647
	11th to 20th	Night	14% 7% 2 4 3 2 3 3 4 7% 51%	2458
	21st to end	Night	14% 9% 2 4 3 2 3 2 4 6% 50%	2076
March	1st to 10th	Night	15% 10% 2 5% 4 4 4 3 5% 6% 42%	5496
	11th to 20th	Night	13% 8% 2 5% 3 3 3 3 4 5% 52%	5185
	21st to end	Night	18% 7% 2 4 3 2 3 3 4 7% 47%	5234
April	1st to 10th	Night	12% 6% 2 3 2 3 2 3 5% 6% 57%	2100
	11th to 20th	Night	8% 4 1 4 3 4 4 3 4 6% 59%	2079
	21st to end	Night	7% 5% 1 3 2 2 3 3 4 6% 63%	2100
May	1st to 10th	Night	3 3 1 3 2 2 2 2 4 6% 72%	5723
	11th to 20th	Night	2 1 1 2 1 2 2 2 3 5% 80%	5176
	21st to end	Night	3 1 1 2 1 1 1 1 3 3 83%	5181
June	1st to 10th	Night	3 2 1 2 2 2 2 2 5% 5% 73%	1680
	11th to 20th	Night	1 1 1 2 2 1 2 4 86%	1680
	21st to end	Night	1 1 1 1 1 1 1 2 3 87%	1680
July	1st to 10th	Night	1 1 1 2 94%	4740
	11th to 20th	Night	3 2 1 2 1 1 1 1 2 3 81%	4740
	21st to end	Night	1 1 1 1 1 1 1 2 3 87%	5606
August	1st to 10th	Night	1 1 1 1 1 1 1 2 2 88%	5587
	11th to 20th	Night	1 1 1 1 2 2 3 4 85%	5886
	21st to end	Night	3 2 1 2 1 2 2 2 3 5% 76%	6499
September	1st to 10th	Night	1 1 1 1 1 1 1 1 2 90%	2100
	11th to 20th	Night	3 2 1 2 1 1 2 1 3 3 82%	2077
	21st to end	Night	2 1 1 2 1 2 2 2 2 3 82%	2200
October	1st to 10th	Night	2 2 1 1 1 1 2 2 2 3 82%	3810
	11th to 20th	Night	3 1 1 1 1 2 2 2 3 4 80%	3897
	21st to end	Night	5% 2 1 2 1 2 2 2 3 5% 75%	4279
November	1st to 10th	Night	9% 4 1 2 2 2 2 2 5% 6% 64%	2665
	11th to 20th	Night	9% 4 1 4 2 2 3 2 5% 7% 60%	2730
	21st to end	Night	9% 5% 1 3 3 3 3 3 5% 6% 60%	2730
December	1st to 10th	Night	8% 3 1 4 2 3 3 3 4 6% 64%	4601
	11th to 20th	Night	21% 6% 2 4 3 3 3 4 7% 8% 41%	5316
	21st to end	Night	14% 7% 3 8% 6% 5% 6% 4 6% 8% 35%	7106

Transparency (when cloud cover <= 30%)
Color Meanings: Too cloudy to forecast Poor Below Average Average Above average Transparent
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	34% 3 16% 25% 17% 5%	4723
	11th to 20th	Day	27% 4 5% 12% 26% 26%	5097
	21st to end	Day	34% 5% 8% 19% 26% 8%	5843
February	1st to 10th	Day	38% 3 7% 25% 17% 10%	1970
	11th to 20th	Day	36% 5% 9% 23% 21% 6%	2060
	21st to end	Day	41% 6% 8% 24% 18% 3	1730
March	1st to 10th	Day	44% 8% 9% 16% 18% 6%	4618
	11th to 20th	Day	33% 6% 14% 29% 9% 8%	5020
	21st to end	Day	37% 11% 12% 22% 15% 3	5981
April	1st to 10th	Day	31% 6% 11% 25% 22% 5%	2520
	11th to 20th	Day	31% 6% 10% 26% 22% 5%	2492
	21st to end	Day	28% 7% 9% 30% 24% 3	2520
May	1st to 10th	Day	18% 4 7% 27% 36% 8%	7243
	11th to 20th	Day	9% 3 4 30% 43% 11%	7707
	21st to end	Day	13% 3 3 24% 51% 5%	8994
June	1st to 10th	Day	16% 4 8% 23% 33% 15%	2940
	11th to 20th	Day	7% 3 4 17% 42% 27%	2940
	21st to end	Day	8% 2 2 9% 35% 44%	2940
July	1st to 10th	Day	3 1 1 4 22% 70%	8151
	11th to 20th	Day	8% 2 2 8% 16% 66%	8145
	21st to end	Day	5% 2 2 9% 33% 49%	8288
August	1st to 10th	Day	6% 1 1 6% 31% 55%	5732
	11th to 20th	Day	7% 2 2 16% 30% 44%	5517
	21st to end	Day	11% 2 3 15% 33% 37%	6077
September	1st to 10th	Day	7% 1 3 10% 30% 49%	2520
	11th to 20th	Day	10% 3 2 21% 38% 25%	2388
	21st to end	Day	10% 3 4 19% 37% 29%	2200
October	1st to 10th	Day	13% 3 4 11% 21% 49%	3386
	11th to 20th	Day	13% 2 3 20% 43% 19%	3261
	21st to end	Day	18% 3 5% 16% 40% 18%	3529
November	1st to 10th	Day	25% 5% 9% 21% 24% 16%	1854
	11th to 20th	Day	26% 3 9% 24% 25% 13%	1890
	21st to end	Day	29% 3 8% 18% 28% 16%	1890
December	1st to 10th	Day	24% 3 6% 27% 26% 13%	2784
	11th to 20th	Day	44% 5% 9% 15% 20% 7%	3108
	21st to end	Day	42% 6% 13% 25% 10% 4	4092
January	1st to 10th	Night	34% 5% 16% 25% 16% 4	8172
	11th to 20th	Night	25% 4 6% 17% 30% 19%	7878
	21st to end	Night	34% 5% 7% 22% 21% 10%	8371
February	1st to 10th	Night	36% 3 8% 22% 21% 10%	2647
	11th to 20th	Night	32% 6% 10% 24% 21% 7%	2459
	21st to end	Night	37% 3 11% 25% 18% 6%	2076
March	1st to 10th	Night	39% 5% 9% 20% 21% 6%	5496
	11th to 20th	Night	35% 5% 11% 30% 12% 7%	5185
	21st to end	Night	37% 5% 18% 21% 16% 4	5234
April	1st to 10th	Night	28% 4 16% 27% 20% 6%	2100
	11th to 20th	Night	26% 6% 12% 30% 19% 8%	2079
	21st to end	Night	22% 5% 11% 35% 24% 2	2100
May	1st to 10th	Night	14% 4 9% 32% 33% 8%	5721
	11th to 20th	Night	8% 3 4 29% 45% 11%	5177
	21st to end	Night	9% 2 4 28% 50% 6%	5182
June	1st to 10th	Night	14% 3 10% 25% 33% 15%	1680
	11th to 20th	Night	5% 3 4 21% 40% 28%	1680
	21st to end	Night	6% 1 3 10% 32% 47%	1680
July	1st to 10th	Night	2 1 1 3 20% 73%	4700
	11th to 20th	Night	12% 2 1 7% 14% 64%	4692
	21st to end	Night	5% 2 1 10% 29% 53%	5483
August	1st to 10th	Night	8% 1 1 7% 27% 57%	4421
	11th to 20th	Night	7% 2 2 17% 24% 47%	4636
	21st to end	Night	12% 2 1 12% 33% 40%	5092
September	1st to 10th	Night	5% 2 1 10% 30% 52%	2100
	11th to 20th	Night	10% 3 3 16% 43% 25%	2077
	21st to end	Night	9% 2 3 19% 39% 28%	2200
October	1st to 10th	Night	10% 2 3 12% 25% 49%	3810
	11th to 20th	Night	11% 2 4 23% 42% 19%	3897
	21st to end	Night	15% 2 4 20% 37% 23%	4279
November	1st to 10th	Night	22% 3 9% 24% 25% 18%	2665
	11th to 20th	Night	23% 3 9% 23% 27% 14%	2730
	21st to end	Night	26% 3 7% 21% 30% 13%	2730
December	1st to 10th	Night	23% 3 7% 29% 25% 13%	4601
	11th to 20th	Night	39% 5% 12% 17% 21% 6%	5316
	21st to end	Night	44% 7% 12% 23% 10% 5%	7106

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
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	27% 13% 22% 23% 12% 3	220
	11th to 20th	Day	17% 8% 23% 32% 15% 6%	343
	21st to end	Day	18% 9% 26% 29% 14% 5%	473
February	1st to 10th	Day	24% 12% 25% 23% 14% 3	472
	11th to 20th	Day	20% 15% 31% 22% 9% 2	535
	21st to end	Day	15% 14% 37% 27% 7% 1	446
March	1st to 10th	Day	22% 14% 34% 23% 7% 1	540
	11th to 20th	Day	20% 15% 29% 24% 11% 1	540
	21st to end	Day	19% 18% 29% 26% 8%	594
April	1st to 10th	Day	15% 16% 32% 25% 11% 1	540
	11th to 20th	Day	14% 19% 30% 27% 7% 2	532
	21st to end	Day	8% 19% 30% 27% 12% 3	540
May	1st to 10th	Day	9% 17% 33% 29% 8% 4	543
	11th to 20th	Day	10% 14% 32% 29% 11% 4	568
	21st to end	Day	10% 11% 28% 35% 11% 5%	617
June	1st to 10th	Day	4 10% 28% 37% 15% 5%	570
	11th to 20th	Day	3 10% 33% 36% 12% 5%	570
	21st to end	Day	2 8% 30% 37% 15% 7%	570
July	1st to 10th	Day	1 3 28% 45% 15% 8%	566
	11th to 20th	Day	2 1 20% 51% 14% 12%	564
	21st to end	Day	1 2 24% 48% 15% 10%	619
August	1st to 10th	Day	3 4 29% 43% 13% 8%	567
	11th to 20th	Day	2 6% 23% 46% 15% 8%	557
	21st to end	Day	2 3 23% 48% 18% 6%	627
September	1st to 10th	Day	2 6% 28% 44% 17% 3	570
	11th to 20th	Day	6% 7% 32% 37% 17% 1	543
	21st to end	Day	3 8% 28% 44% 15% 2	530
October	1st to 10th	Day	5% 9% 31% 37% 17% 2	530
	11th to 20th	Day	4 10% 29% 37% 17% 2	530
	21st to end	Day	6% 7% 28% 38% 20% 2	566
November	1st to 10th	Day	9% 8% 28% 28% 22% 5%	330
	11th to 20th	Day	11% 9% 29% 28% 17% 5%	330
	21st to end	Day	13% 7% 27% 30% 18% 5%	330
December	1st to 10th	Day	14% 4 29% 26% 22% 5%	250
	11th to 20th	Day	14% 8% 23% 33% 15% 7%	220
	21st to end	Day	23% 10% 26% 24% 13% 5%	242
January	1st to 10th	Night	19% 5% 21% 32% 16% 8%	840
	11th to 20th	Night	12% 5% 22% 37% 19% 5%	840
	21st to end	Night	17% 5% 21% 33% 19% 5%	924
February	1st to 10th	Night	20% 5% 18% 35% 17% 5%	840
	11th to 20th	Night	17% 7% 21% 27% 21% 7%	824
	21st to end	Night	17% 5% 23% 37% 15% 3	692
March	1st to 10th	Night	20% 6% 21% 33% 16% 4	840
	11th to 20th	Night	16% 7% 17% 30% 25% 5%	840
	21st to end	Night	16% 5% 18% 40% 18% 2	924
April	1st to 10th	Night	13% 5% 20% 39% 21% 2	840
	11th to 20th	Night	9% 6% 22% 38% 21% 4	831
	21st to end	Night	9% 3 21% 38% 24% 6%	840
May	1st to 10th	Night	6% 5% 21% 39% 24% 5%	772
	11th to 20th	Night	5% 3 20% 40% 30% 2	580
	21st to end	Night	6% 1 18% 38% 31% 5%	455
June	1st to 10th	Night	4 3 12% 41% 36% 4	420
	11th to 20th	Night	16% 44% 35% 3	420
	21st to end	Night	1 13% 40% 42% 4	420
July	1st to 10th	Night	11% 40% 45% 5%	410
	11th to 20th	Night	3 7% 40% 41% 8%	408
	21st to end	Night	2 9% 33% 43% 12%	617
August	1st to 10th	Night	1 1 10% 38% 42% 8%	736
	11th to 20th	Night	9% 37% 45% 7%	832
	21st to end	Night	2 6% 35% 47% 9%	924
September	1st to 10th	Night	1 10% 39% 41% 9%	840
	11th to 20th	Night	3 1 11% 40% 37% 8%	812
	21st to end	Night	3 1 13% 36% 40% 7%	800
October	1st to 10th	Night	5% 1 14% 46% 28% 6%	800
	11th to 20th	Night	6% 2 15% 39% 30% 8%	800
	21st to end	Night	5% 2 16% 34% 29% 13%	880
November	1st to 10th	Night	11% 3 14% 36% 26% 11%	820
	11th to 20th	Night	11% 4 20% 35% 24% 6%	840
	21st to end	Night	11% 3 20% 38% 21% 7%	840
December	1st to 10th	Night	15% 4 16% 39% 21% 4	840
	11th to 20th	Night	20% 2 20% 35% 18% 4	840
	21st to end	Night	21% 5% 22% 34% 14% 4	924

Page generated at 2026-09-11 01:11:53. Version ac07

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