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 2025-05-11.

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	7% 5% 1 3 2 2 3 2 4 5% 65%	117722
	Night	9% 4 1 3 2 2 3 2 4 5% 63%	120087
Transparency	Day	22% 4 7% 18% 26% 24%	117471
	Night	23% 4 8% 20% 24% 20%	119928
Seeing	Day	10% 10% 28% 34% 14% 4	15861
	Night	10% 3 16% 36% 29% 6%	25453

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	11% 8% 2 4 2 2 3 3 4 7% 55%	3639
	11th to 20th	Day	11% 5% 2 5% 3 4 3 3 3 4 55%	3913
	21st to end	Day	14% 6% 2 6% 4 4 4 3 5% 6% 47%	4510
February	1st to 10th	Day	17% 9% 1 4 3 3 3 3 4 5% 48%	1876
	11th to 20th	Day	12% 10% 2 4 4 3 4 3 5% 6% 48%	1950
	21st to end	Day	16% 9% 4 7% 4 3 4 3 3 5% 43%	1650
March	1st to 10th	Day	13% 15% 3 7% 4 4 4 4 6% 7% 34%	4518
	11th to 20th	Day	15% 7% 2 4 3 3 4 3 4 7% 47%	4912
	21st to end	Day	16% 10% 3 6% 4 3 4 3 5% 6% 40%	5852
April	1st to 10th	Day	12% 7% 2 5% 3 2 4 3 4 7% 52%	2400
	11th to 20th	Day	11% 6% 2 5% 4 3 3 3 5% 7% 52%	2369
	21st to end	Day	8% 6% 1 4 3 4 4 3 5% 6% 55%	2400
May	1st to 10th	Day	6% 3 1 3 2 2 2 2 4 5% 71%	4032
	11th to 20th	Day	3 2 1 2 2 1 2 2 3 5% 76%	4129
	21st to end	Day	4 3 1 3 2 2 2 2 3 5% 75%	4813
June	1st to 10th	Day	4 3 1 4 3 2 3 3 5% 7% 67%	2660
	11th to 20th	Day	1 1 1 2 2 1 2 1 2 5% 82%	2660
	21st to end	Day	2 2 1 1 2 1 2 3 2 84%	2660
July	1st to 10th	Day	1 1 1 1 1 2 2 91%	4420
	11th to 20th	Day	1 1 1 1 1 2 1 2 3 3 84%	4420
	21st to end	Day	1 1 1 1 1 1 2 2 2 3 86%	4552
August	1st to 10th	Day	1 2 1 2 1 1 2 1 2 3 84%	3918
	11th to 20th	Day	1 1 1 1 2 1 2 2 90%	3724
	21st to end	Day	1 1 1 1 1 1 1 1 2 2 88%	4180
September	1st to 10th	Day	2 1 1 1 1 2 1 2 2 87%	2280
	11th to 20th	Day	2 1 1 2 1 2 2 2 3 4 79%	2234
	21st to end	Day	2 2 1 1 1 1 2 2 3 4 80%	2090
October	1st to 10th	Day	3 3 1 2 2 2 2 2 4 4 76%	3283
	11th to 20th	Day	3 2 1 2 1 2 2 2 3 3 78%	3161
	21st to end	Day	4 2 1 3 3 3 3 3 6% 6% 65%	3420
November	1st to 10th	Day	9% 5% 1 4 3 3 4 2 6% 6% 57%	1764
	11th to 20th	Day	8% 4 2 3 3 2 3 2 4 7% 61%	1800
	21st to end	Day	10% 5% 1 4 4 3 3 2 3 6% 58%	1800
December	1st to 10th	Day	9% 3 1 3 3 3 4 3 6% 6% 60%	2701
	11th to 20th	Day	22% 7% 2 6% 3 3 3 3 4 7% 40%	3028
	21st to end	Day	16% 8% 3 6% 3 4 4 3 6% 8% 39%	4004
January	1st to 10th	Night	10% 6% 2 4 3 3 3 4 6% 8% 53%	6306
	11th to 20th	Night	14% 6% 2 3 2 3 3 3 4 5% 56%	6056
	21st to end	Night	13% 7% 2 5% 3 3 3 3 4 6% 50%	6468
February	1st to 10th	Night	16% 7% 2 4 3 2 3 3 5% 6% 49%	2521
	11th to 20th	Night	13% 7% 2 3 3 2 3 3 4 7% 53%	2338
	21st to end	Night	14% 9% 2 4 2 2 3 2 4 6% 50%	1980
March	1st to 10th	Night	15% 10% 2 5% 4 4 4 3 5% 7% 41%	5376
	11th to 20th	Night	13% 8% 2 5% 3 3 3 3 4 5% 51%	5073
	21st to end	Night	19% 7% 2 4 3 2 3 3 4 7% 47%	5121
April	1st to 10th	Night	12% 6% 2 3 2 3 2 3 5% 6% 58%	2000
	11th to 20th	Night	7% 4 1 4 3 4 3 3 4 5% 60%	1979
	21st to end	Night	6% 5% 1 3 2 2 3 3 4 7% 64%	2000
May	1st to 10th	Night	4 2 1 2 2 1 2 2 4 5% 76%	3163
	11th to 20th	Night	3 1 1 2 1 2 1 2 4 6% 78%	2774
	21st to end	Night	3 2 1 2 1 1 2 1 3 4 81%	2762
June	1st to 10th	Night	4 2 1 2 2 2 2 2 5% 5% 72%	1520
	11th to 20th	Night	1 1 1 1 2 2 2 2 5% 85%	1520
	21st to end	Night	1 1 1 1 1 1 2 2 3 86%	1520
July	1st to 10th	Night	1 1 1 2 2 93%	2540
	11th to 20th	Night	3 2 1 2 2 2 2 2 2 3 79%	2540
	21st to end	Night	1 2 1 1 2 1 2 1 3 3 83%	3004
August	1st to 10th	Night	1 1 1 2 2 2 2 1 3 4 81%	3025
	11th to 20th	Night	1 1 1 1 1 2 2 90%	3126
	21st to end	Night	3 1 1 1 1 1 1 1 2 89%	3498
September	1st to 10th	Night	1 1 1 1 1 1 1 1 2 90%	1900
	11th to 20th	Night	2 2 1 2 1 1 2 1 3 3 82%	1944
	21st to end	Night	2 1 1 1 1 1 2 1 2 3 83%	2090
October	1st to 10th	Night	2 2 1 1 1 1 2 2 2 3 82%	3693
	11th to 20th	Night	3 1 1 1 1 2 2 2 3 4 81%	3777
	21st to end	Night	5% 3 1 2 1 2 2 2 3 5% 75%	4146
November	1st to 10th	Night	9% 4 1 3 2 2 2 2 5% 7% 63%	2535
	11th to 20th	Night	7% 4 1 4 2 2 3 2 5% 7% 62%	2600
	21st to end	Night	10% 5% 1 3 3 3 3 3 5% 6% 59%	2600
December	1st to 10th	Night	8% 3 1 4 3 3 3 3 4 6% 63%	4464
	11th to 20th	Night	21% 6% 2 4 3 3 3 4 7% 8% 40%	5176
	21st to end	Night	14% 7% 3 8% 6% 5% 6% 4 6% 8% 35%	6952

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	29% 3 17% 26% 19% 5%	3639
	11th to 20th	Day	33% 5% 6% 15% 24% 17%	3913
	21st to end	Day	36% 6% 9% 17% 24% 9%	4510
February	1st to 10th	Day	39% 3 7% 25% 17% 9%	1876
	11th to 20th	Day	35% 5% 9% 23% 21% 7%	1960
	21st to end	Day	42% 6% 7% 23% 19% 3	1650
March	1st to 10th	Day	45% 8% 8% 16% 17% 6%	4518
	11th to 20th	Day	34% 6% 15% 29% 9% 7%	4912
	21st to end	Day	38% 11% 12% 22% 15% 3	5852
April	1st to 10th	Day	31% 5% 11% 26% 21% 5%	2400
	11th to 20th	Day	31% 5% 10% 26% 23% 5%	2372
	21st to end	Day	27% 7% 9% 31% 24% 3	2400
May	1st to 10th	Day	16% 5% 7% 30% 31% 11%	4032
	11th to 20th	Day	12% 3 5% 30% 41% 9%	4132
	21st to end	Day	14% 4 4 33% 40% 5%	4814
June	1st to 10th	Day	17% 5% 9% 22% 32% 15%	2660
	11th to 20th	Day	8% 3 4 19% 42% 25%	2660
	21st to end	Day	8% 2 2 9% 34% 44%	2660
July	1st to 10th	Day	4 1 1 4 16% 75%	4351
	11th to 20th	Day	7% 2 2 6% 18% 65%	4345
	21st to end	Day	6% 2 2 7% 28% 55%	4408
August	1st to 10th	Day	8% 2 2 6% 29% 54%	3903
	11th to 20th	Day	4 1 1 5% 25% 63%	3760
	21st to end	Day	6% 1 3 6% 31% 53%	4179
September	1st to 10th	Day	7% 1 2 7% 29% 52%	2280
	11th to 20th	Day	10% 3 2 21% 38% 25%	2234
	21st to end	Day	9% 2 3 18% 37% 30%	2090
October	1st to 10th	Day	13% 3 3 11% 20% 50%	3283
	11th to 20th	Day	12% 2 3 20% 42% 19%	3161
	21st to end	Day	18% 3 5% 16% 40% 18%	3420
November	1st to 10th	Day	26% 5% 8% 21% 23% 16%	1764
	11th to 20th	Day	24% 3 10% 24% 26% 14%	1800
	21st to end	Day	29% 3 8% 17% 27% 16%	1800
December	1st to 10th	Day	25% 3 6% 26% 26% 13%	2701
	11th to 20th	Day	44% 5% 9% 15% 21% 6%	3028
	21st to end	Day	42% 6% 13% 25% 10% 4	4004
January	1st to 10th	Night	28% 5% 16% 27% 19% 5%	6306
	11th to 20th	Night	31% 4 7% 18% 28% 11%	6056
	21st to end	Night	34% 5% 8% 20% 21% 11%	6468
February	1st to 10th	Night	36% 3 8% 22% 20% 10%	2521
	11th to 20th	Night	30% 6% 10% 25% 22% 7%	2339
	21st to end	Night	37% 3 10% 25% 18% 7%	1980
March	1st to 10th	Night	40% 5% 9% 19% 20% 6%	5376
	11th to 20th	Night	35% 5% 11% 30% 12% 7%	5073
	21st to end	Night	37% 5% 19% 20% 15% 4	5121
April	1st to 10th	Night	28% 4 16% 27% 19% 6%	2000
	11th to 20th	Night	25% 6% 11% 29% 20% 8%	1979
	21st to end	Night	21% 5% 11% 35% 25% 3	2000
May	1st to 10th	Night	12% 4 8% 35% 31% 10%	3163
	11th to 20th	Night	9% 4 5% 32% 41% 10%	2775
	21st to end	Night	10% 3 6% 36% 40% 5%	2762
June	1st to 10th	Night	14% 3 11% 24% 33% 14%	1520
	11th to 20th	Night	5% 3 5% 22% 40% 25%	1520
	21st to end	Night	7% 1 3 11% 32% 47%	1520
July	1st to 10th	Night	2 1 1 3 17% 76%	2500
	11th to 20th	Night	11% 2 2 5% 18% 62%	2492
	21st to end	Night	7% 3 1 7% 24% 58%	2910
August	1st to 10th	Night	10% 1 1 9% 23% 55%	3016
	11th to 20th	Night	4 1 2 6% 19% 68%	3156
	21st to end	Night	7% 1 1 6% 30% 56%	3498
September	1st to 10th	Night	5% 2 1 8% 29% 56%	1900
	11th to 20th	Night	9% 3 3 16% 43% 26%	1944
	21st to end	Night	9% 2 2 19% 39% 29%	2090
October	1st to 10th	Night	10% 2 3 11% 24% 50%	3693
	11th to 20th	Night	10% 2 4 23% 41% 19%	3777
	21st to end	Night	15% 2 3 20% 37% 23%	4146
November	1st to 10th	Night	22% 3 9% 23% 24% 18%	2535
	11th to 20th	Night	21% 3 9% 23% 28% 15%	2600
	21st to end	Night	27% 3 7% 20% 29% 14%	2600
December	1st to 10th	Night	24% 3 7% 29% 24% 13%	4464
	11th to 20th	Night	40% 5% 12% 17% 21% 5%	5176
	21st to end	Night	44% 7% 12% 23% 10% 5%	6952

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	26% 12% 24% 24% 12% 3	200
	11th to 20th	Day	19% 8% 25% 30% 13% 5%	317
	21st to end	Day	19% 9% 26% 28% 13% 5%	440
February	1st to 10th	Day	25% 13% 24% 22% 13% 3	438
	11th to 20th	Day	18% 15% 32% 23% 10% 2	495
	21st to end	Day	15% 14% 35% 27% 7% 1	414
March	1st to 10th	Day	23% 13% 33% 23% 7% 1	500
	11th to 20th	Day	22% 16% 29% 21% 11% 1	500
	21st to end	Day	19% 19% 28% 26% 7%	550
April	1st to 10th	Day	16% 16% 32% 24% 11% 1	500
	11th to 20th	Day	14% 19% 32% 26% 7% 1	492
	21st to end	Day	8% 18% 30% 28% 13% 3	500
May	1st to 10th	Day	8% 18% 33% 29% 9% 3	503
	11th to 20th	Day	9% 13% 32% 31% 12% 3	489
	21st to end	Day	11% 11% 26% 36% 12% 5%	529
June	1st to 10th	Day	5% 11% 28% 36% 16% 5%	490
	11th to 20th	Day	3 9% 32% 37% 13% 6%	490
	21st to end	Day	2 7% 30% 39% 16% 7%	490
July	1st to 10th	Day	1 2 24% 50% 15% 9%	486
	11th to 20th	Day	1 1 19% 52% 15% 12%	484
	21st to end	Day	1 2 23% 49% 15% 9%	531
August	1st to 10th	Day	3 4 31% 42% 13% 7%	487
	11th to 20th	Day	2 6% 24% 45% 16% 7%	477
	21st to end	Day	2 3 22% 49% 18% 6%	539
September	1st to 10th	Day	2 4 26% 47% 18% 3	490
	11th to 20th	Day	5% 7% 32% 37% 17% 1	490
	21st to end	Day	3 8% 26% 46% 16% 1	490
October	1st to 10th	Day	4 8% 31% 37% 18% 2	490
	11th to 20th	Day	3 10% 30% 38% 18% 2	490
	21st to end	Day	6% 7% 28% 38% 20% 2	523
November	1st to 10th	Day	9% 9% 28% 28% 22% 5%	300
	11th to 20th	Day	7% 9% 29% 30% 18% 6%	300
	21st to end	Day	14% 7% 26% 29% 18% 5%	300
December	1st to 10th	Day	15% 3 29% 26% 22% 4	227
	11th to 20th	Day	16% 9% 23% 33% 14% 6%	200
	21st to end	Day	21% 9% 27% 23% 14% 6%	220
January	1st to 10th	Night	18% 5% 20% 33% 17% 8%	800
	11th to 20th	Night	13% 5% 22% 35% 20% 5%	800
	21st to end	Night	18% 5% 19% 34% 20% 5%	880
February	1st to 10th	Night	21% 5% 17% 35% 18% 5%	800
	11th to 20th	Night	17% 6% 20% 28% 22% 7%	784
	21st to end	Night	18% 5% 21% 38% 16% 3	660
March	1st to 10th	Night	21% 6% 20% 33% 17% 4	800
	11th to 20th	Night	17% 7% 17% 28% 26% 5%	800
	21st to end	Night	17% 5% 18% 39% 19% 2	880
April	1st to 10th	Night	13% 5% 20% 38% 22% 3	800
	11th to 20th	Night	9% 6% 22% 37% 21% 4	791
	21st to end	Night	8% 3 20% 38% 25% 6%	800
May	1st to 10th	Night	6% 5% 21% 39% 25% 5%	735
	11th to 20th	Night	6% 2 17% 41% 32% 2	526
	21st to end	Night	6% 1 18% 36% 34% 6%	411
June	1st to 10th	Night	4 3 11% 38% 38% 5%	380
	11th to 20th	Night	1 1 15% 41% 39% 4	380
	21st to end	Night	1 12% 37% 47% 4	380
July	1st to 10th	Night	7% 39% 49% 5%	370
	11th to 20th	Night	4 5% 38% 44% 8%	368
	21st to end	Night	2 8% 28% 48% 13%	558
August	1st to 10th	Night	2 1 9% 36% 43% 9%	666
	11th to 20th	Night	1 9% 35% 47% 8%	752
	21st to end	Night	2 5% 32% 51% 10%	836
September	1st to 10th	Night	1 1 7% 37% 45% 9%	760
	11th to 20th	Night	3 1 10% 39% 39% 8%	760
	21st to end	Night	3 1 12% 36% 41% 8%	760
October	1st to 10th	Night	5% 1 12% 47% 29% 7%	760
	11th to 20th	Night	5% 2 13% 39% 31% 9%	760
	21st to end	Night	6% 2 16% 33% 30% 14%	836
November	1st to 10th	Night	11% 2 13% 35% 27% 12%	780
	11th to 20th	Night	9% 4 19% 36% 25% 7%	800
	21st to end	Night	12% 3 19% 38% 22% 7%	800
December	1st to 10th	Night	16% 4 16% 39% 21% 5%	800
	11th to 20th	Night	21% 2 20% 34% 19% 5%	800
	21st to end	Night	21% 5% 22% 35% 15% 4	880

Page generated at 2025-05-08 06:33:57. Version ac07

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