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 2024-11-01.

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% 4 1 3 2 2 3 2 4 5% 66%	101055
	Night	9% 4 1 3 2 2 3 2 4 5% 64%	98102
Transparency	Day	21% 4 6% 18% 25% 25%	100804
	Night	23% 4 7% 21% 23% 22%	97943
Seeing	Day	10% 9% 28% 35% 14% 4	15237
	Night	10% 3 15% 36% 29% 7%	24692

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	14% 10% 2 4 2 3 3 3 5% 6% 46%	2539
	11th to 20th	Day	16% 8% 3 8% 5% 6% 5% 4 5% 6% 36%	2729
	21st to end	Day	19% 8% 2 6% 4 4 4 4 5% 8% 37%	3146
February	1st to 10th	Day	16% 9% 1 4 2 3 3 3 4 5% 49%	1782
	11th to 20th	Day	12% 11% 2 4 3 3 4 3 5% 6% 49%	1850
	21st to end	Day	17% 9% 4 7% 4 3 3 3 3 4 43%	1570
March	1st to 10th	Day	14% 15% 3 7% 5% 5% 4 4 6% 7% 29%	3138
	11th to 20th	Day	11% 6% 2 4 3 3 3 3 4 6% 56%	3428
	21st to end	Day	17% 13% 3 7% 5% 3 4 4 5% 5% 35%	4087
April	1st to 10th	Day	12% 7% 2 5% 3 2 4 3 4 7% 52%	2280
	11th to 20th	Day	11% 6% 2 5% 4 3 3 3 5% 7% 52%	2249
	21st to end	Day	9% 6% 1 4 3 4 4 3 5% 6% 56%	2280
May	1st to 10th	Day	6% 3 2 3 2 2 2 2 3 5% 71%	3909
	11th to 20th	Day	3 2 1 2 2 1 2 2 3 5% 76%	4127
	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	5% 4 1 2 2 2 2 2 4 4 71%	1967
	11th to 20th	Day	3 2 1 2 2 3 3 3 4 4 73%	1900
	21st to end	Day	6% 2 3 2 2 2 3 4 6% 69%	2074
November	1st to 10th	Day	9% 5% 1 4 3 3 4 2 5% 6% 56%	1674
	11th to 20th	Day	8% 4 2 3 2 2 3 2 5% 7% 62%	1710
	21st to end	Day	9% 5% 1 5% 4 3 3 2 3 6% 59%	1710
December	1st to 10th	Day	15% 5% 1 4 4 3 3 3 5% 6% 52%	1573
	11th to 20th	Day	25% 9% 2 6% 4 3 3 2 4 6% 35%	1928
	21st to end	Day	13% 9% 3 7% 4 4 5% 4 6% 8% 39%	2794
January	1st to 10th	Night	13% 7% 3 5% 3 3 3 4 5% 6% 48%	4406
	11th to 20th	Night	19% 8% 3 4 3 4 4 4 6% 7% 38%	4234
	21st to end	Night	18% 8% 2 5% 4 3 4 4 5% 8% 38%	4521
February	1st to 10th	Night	16% 7% 2 4 4 2 3 3 5% 6% 49%	2395
	11th to 20th	Night	13% 7% 1 3 2 2 3 3 4 7% 54%	2218
	21st to end	Night	15% 9% 2 4 2 2 3 2 4 6% 50%	1884
March	1st to 10th	Night	18% 10% 2 5% 4 4 4 3 5% 6% 39%	3736
	11th to 20th	Night	12% 6% 1 4 2 2 2 2 3 6% 60%	3537
	21st to end	Night	20% 7% 2 4 3 3 4 2 4 6% 44%	3564
April	1st to 10th	Night	12% 6% 1 3 2 3 2 3 5% 6% 57%	1900
	11th to 20th	Night	8% 4 1 4 3 4 3 3 4 5% 61%	1879
	21st to end	Night	6% 4 1 3 2 2 3 3 4 7% 65%	1900
May	1st to 10th	Night	4 2 1 2 2 1 2 2 4 5% 76%	3066
	11th to 20th	Night	3 1 1 2 1 2 1 2 4 6% 78%	2770
	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	4 3 1 2 2 2 3 2 3 4 74%	2209
	11th to 20th	Night	4 2 1 2 1 2 2 3 4 5% 75%	2280
	21st to end	Night	5% 2 1 2 1 1 2 2 2 5% 78%	2522
November	1st to 10th	Night	10% 5% 1 2 2 2 2 2 5% 7% 63%	2411
	11th to 20th	Night	7% 4 1 3 2 2 3 2 5% 7% 63%	2470
	21st to end	Night	9% 5% 1 3 3 3 3 3 5% 5% 60%	2470
December	1st to 10th	Night	14% 5% 1 5% 3 3 2 3 4 6% 54%	2603
	11th to 20th	Night	25% 7% 2 4 3 3 3 3 6% 6% 39%	3276
	21st to end	Night	14% 7% 3 7% 5% 5% 5% 4 6% 8% 38%	4862

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	38% 4 15% 26% 15% 3	2539
	11th to 20th	Day	47% 7% 9% 16% 12% 9%	2729
	21st to end	Day	44% 7% 12% 21% 12% 4	3146
February	1st to 10th	Day	38% 3 7% 26% 17% 9%	1782
	11th to 20th	Day	35% 6% 8% 23% 22% 7%	1860
	21st to end	Day	43% 6% 7% 23% 19% 2	1570
March	1st to 10th	Day	47% 10% 10% 16% 12% 5%	3138
	11th to 20th	Day	27% 7% 11% 33% 11% 11%	3428
	21st to end	Day	42% 13% 10% 21% 12% 2	4087
April	1st to 10th	Day	32% 5% 11% 25% 21% 5%	2280
	11th to 20th	Day	31% 5% 10% 26% 22% 6%	2252
	21st to end	Day	26% 7% 9% 31% 25% 3	2280
May	1st to 10th	Day	16% 5% 7% 30% 31% 11%	3909
	11th to 20th	Day	12% 3 5% 30% 40% 9%	4130
	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	17% 3 5% 17% 27% 31%	1967
	11th to 20th	Day	15% 3 5% 21% 29% 28%	1900
	21st to end	Day	17% 3 5% 20% 33% 22%	2074
November	1st to 10th	Day	27% 5% 8% 20% 23% 17%	1674
	11th to 20th	Day	23% 3 10% 23% 26% 15%	1710
	21st to end	Day	29% 3 8% 17% 27% 16%	1710
December	1st to 10th	Day	33% 4 9% 22% 20% 12%	1573
	11th to 20th	Day	52% 5% 6% 14% 14% 9%	1928
	21st to end	Day	41% 6% 9% 26% 12% 5%	2794
January	1st to 10th	Night	35% 5% 14% 27% 16% 3	4406
	11th to 20th	Night	44% 6% 10% 19% 13% 8%	4234
	21st to end	Night	42% 6% 11% 22% 13% 6%	4521
February	1st to 10th	Night	36% 3 8% 23% 20% 10%	2395
	11th to 20th	Night	29% 6% 10% 25% 22% 8%	2219
	21st to end	Night	38% 3 10% 25% 18% 5%	1884
March	1st to 10th	Night	42% 7% 10% 22% 15% 5%	3736
	11th to 20th	Night	27% 4 9% 35% 15% 10%	3537
	21st to end	Night	40% 6% 14% 24% 13% 3	3564
April	1st to 10th	Night	28% 4 16% 26% 19% 6%	1900
	11th to 20th	Night	25% 6% 11% 29% 21% 8%	1879
	21st to end	Night	20% 5% 11% 35% 25% 3	1900
May	1st to 10th	Night	12% 5% 8% 35% 31% 10%	3066
	11th to 20th	Night	9% 4 5% 32% 40% 10%	2771
	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	15% 3 5% 18% 29% 30%	2209
	11th to 20th	Night	13% 3 6% 20% 31% 27%	2280
	21st to end	Night	13% 2 4 24% 32% 25%	2522
November	1st to 10th	Night	23% 3 9% 22% 24% 18%	2411
	11th to 20th	Night	21% 3 9% 24% 28% 16%	2470
	21st to end	Night	26% 3 7% 21% 29% 14%	2470
December	1st to 10th	Night	33% 4 9% 21% 20% 13%	2603
	11th to 20th	Night	46% 5% 9% 15% 17% 7%	3276
	21st to end	Night	41% 7% 11% 22% 13% 6%	4862

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	28% 12% 22% 23% 11% 3	180
	11th to 20th	Day	21% 7% 24% 30% 14% 5%	291
	21st to end	Day	20% 8% 25% 29% 14% 5%	407
February	1st to 10th	Day	24% 11% 24% 24% 14% 3	404
	11th to 20th	Day	18% 14% 31% 25% 11% 2	455
	21st to end	Day	16% 15% 34% 26% 8% 1	382
March	1st to 10th	Day	23% 13% 32% 23% 7% 1	460
	11th to 20th	Day	20% 15% 28% 23% 12% 2	460
	21st to end	Day	20% 19% 29% 26% 6%	506
April	1st to 10th	Day	16% 17% 32% 24% 11%	460
	11th to 20th	Day	15% 19% 32% 27% 7% 1	452
	21st to end	Day	8% 18% 30% 29% 13% 2	460
May	1st to 10th	Day	8% 18% 35% 28% 9% 3	463
	11th to 20th	Day	9% 13% 32% 31% 12% 3	488
	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% 37% 20% 2	523
November	1st to 10th	Day	10% 8% 26% 27% 23% 6%	270
	11th to 20th	Day	7% 6% 27% 33% 20% 7%	270
	21st to end	Day	12% 6% 26% 31% 20% 6%	270
December	1st to 10th	Day	17% 2 29% 27% 21% 4	205
	11th to 20th	Day	15% 9% 24% 33% 13% 6%	180
	21st to end	Day	21% 8% 26% 24% 15% 7%	198
January	1st to 10th	Night	19% 4 18% 33% 18% 9%	760
	11th to 20th	Night	13% 4 19% 37% 21% 6%	760
	21st to end	Night	18% 5% 17% 33% 21% 6%	836
February	1st to 10th	Night	20% 4 15% 37% 19% 5%	760
	11th to 20th	Night	17% 5% 18% 29% 23% 7%	744
	21st to end	Night	19% 4 19% 39% 16% 3	628
March	1st to 10th	Night	21% 6% 18% 34% 18% 4	760
	11th to 20th	Night	17% 6% 15% 29% 27% 5%	760
	21st to end	Night	17% 5% 18% 38% 19% 2	836
April	1st to 10th	Night	13% 4 19% 38% 23% 3	760
	11th to 20th	Night	9% 6% 21% 36% 22% 5%	751
	21st to end	Night	8% 3 20% 38% 26% 6%	760
May	1st to 10th	Night	6% 5% 20% 39% 25% 6%	698
	11th to 20th	Night	6% 2 17% 41% 32% 2	524
	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 15% 33% 30% 14%	836
November	1st to 10th	Night	11% 2 11% 35% 28% 12%	742
	11th to 20th	Night	9% 2 17% 38% 26% 7%	760
	21st to end	Night	11% 1 18% 39% 23% 7%	760
December	1st to 10th	Night	17% 4 15% 38% 21% 5%	760
	11th to 20th	Night	21% 2 19% 35% 19% 5%	760
	21st to end	Night	21% 4 20% 36% 16% 4	836

Page generated at 2024-10-29 08:20:51. Version ac07

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