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 2024-03-01.

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	8% 5% 2 3 3 2 3 3 4 5% 62%	78615
	Night	10% 4 2 3 3 2 3 3 4 6% 61%	80820
Transparency	Day	25% 4 7% 19% 25% 20%	78327
	Night	25% 4 8% 21% 24% 18%	80630
Seeing	Day	10% 9% 28% 35% 14% 4	14259
	Night	10% 3 14% 36% 30% 7%	23868

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 3 2 3 4 5% 6% 46%	2539
	11th to 20th	Day	16% 8% 3 7% 6% 4 5% 5% 5% 6% 36%	2729
	21st to end	Day	19% 7% 2 5% 5% 3 4 5% 5% 7% 37%	3146
February	1st to 10th	Day	16% 9% 2 4 3 2 3 3 4 6% 49%	1782
	11th to 20th	Day	12% 10% 2 3 4 2 3 4 5% 6% 49%	1850
	21st to end	Day	17% 9% 4 6% 4 2 3 4 3 5% 44%	1570
March	1st to 10th	Day	18% 11% 3 5% 5% 3 4 4 5% 7% 35%	1801
	11th to 20th	Day	16% 9% 3 4 5% 3 3 4 5% 7% 41%	1944
	21st to end	Day	15% 7% 3 5% 5% 3 3 5% 5% 6% 44%	2324
April	1st to 10th	Day	13% 6% 3 4 4 2 4 4 4 7% 51%	2160
	11th to 20th	Day	12% 6% 2 4 4 2 3 3 4 7% 52%	2129
	21st to end	Day	9% 6% 1 4 3 3 4 4 4 7% 56%	2160
May	1st to 10th	Day	6% 4 2 4 3 2 3 3 4 6% 63%	2217
	11th to 20th	Day	6% 4 2 4 3 1 3 3 4 6% 64%	2331
	21st to end	Day	7% 4 2 4 3 2 3 4 4 7% 60%	2723
June	1st to 10th	Day	4 3 1 3 3 2 3 3 4 7% 67%	2520
	11th to 20th	Day	1 1 2 2 1 2 2 2 5% 82%	2520
	21st to end	Day	2 2 1 1 1 2 2 2 3 84%	2520
July	1st to 10th	Day	1 1 1 1 1 2 2 3 4 85%	2520
	11th to 20th	Day	2 1 1 1 2 1 2 2 3 3 82%	2520
	21st to end	Day	1 2 1 2 2 2 2 3 3 5% 77%	2604
August	1st to 10th	Day	2 2 1 1 1 1 2 2 3 3 83%	2233
	11th to 20th	Day	1 1 1 1 1 3 2 2 3 85%	2122
	21st to end	Day	1 1 1 1 1 1 1 2 2 3 86%	2376
September	1st to 10th	Day	2 1 1 1 1 1 2 2 2 87%	2160
	11th to 20th	Day	2 1 1 1 2 1 2 3 3 4 80%	2117
	21st to end	Day	3 2 1 1 1 1 2 3 3 4 79%	1980
October	1st to 10th	Day	5% 4 1 2 2 2 3 2 4 5% 70%	1864
	11th to 20th	Day	3 2 1 2 2 2 4 3 4 5% 72%	1800
	21st to end	Day	6% 2 2 2 2 2 3 4 6% 70%	1965
November	1st to 10th	Day	10% 5% 2 3 4 2 4 3 5% 6% 56%	1674
	11th to 20th	Day	8% 3 2 2 3 2 4 3 5% 7% 62%	1710
	21st to end	Day	9% 5% 2 4 4 2 3 3 3 6% 59%	1710
December	1st to 10th	Day	15% 5% 1 3 5% 2 3 4 5% 6% 52%	1573
	11th to 20th	Day	25% 8% 2 5% 5% 2 4 3 3 7% 35%	1928
	21st to end	Day	13% 8% 3 6% 5% 3 5% 5% 5% 8% 39%	2794
January	1st to 10th	Night	13% 7% 3 4 4 2 3 4 5% 6% 48%	4406
	11th to 20th	Night	19% 8% 3 4 4 3 4 5% 5% 7% 38%	4234
	21st to end	Night	18% 8% 2 5% 4 3 4 5% 5% 8% 38%	4521
February	1st to 10th	Night	16% 6% 2 4 3 2 3 4 5% 6% 50%	2395
	11th to 20th	Night	13% 6% 2 3 3 2 3 4 4 7% 54%	2218
	21st to end	Night	16% 8% 3 3 3 2 3 3 4 6% 50%	1884
March	1st to 10th	Night	17% 9% 2 4 4 3 3 3 4 6% 45%	2165
	11th to 20th	Night	14% 7% 2 4 3 2 3 4 4 8% 47%	2014
	21st to end	Night	15% 5% 2 3 3 1 4 3 4 6% 55%	2030
April	1st to 10th	Night	12% 5% 2 2 2 2 3 3 5% 7% 57%	1800
	11th to 20th	Night	8% 4 2 3 3 3 4 3 4 5% 61%	1779
	21st to end	Night	7% 4 1 3 2 1 3 3 4 7% 65%	1800
May	1st to 10th	Night	5% 3 1 2 3 1 3 3 5% 7% 67%	1742
	11th to 20th	Night	5% 2 1 3 2 1 2 3 4 7% 71%	1558
	21st to end	Night	5% 2 1 3 1 1 3 3 4 5% 72%	1552
June	1st to 10th	Night	4 2 2 2 2 2 3 3 4 5% 72%	1440
	11th to 20th	Night	1 1 1 1 1 2 2 2 5% 85%	1440
	21st to end	Night	1 1 1 1 1 1 2 2 3 86%	1440
July	1st to 10th	Night	1 2 2 2 3 88%	1440
	11th to 20th	Night	3 2 2 2 1 1 2 2 4 81%	1440
	21st to end	Night	2 2 2 2 3 2 3 2 3 5% 74%	1710
August	1st to 10th	Night	1 2 1 2 2 1 1 2 2 3 83%	1704
	11th to 20th	Night	1 1 1 2 1 2 3 3 3 83%	1780
	21st to end	Night	1 1 1 1 1 1 1 2 3 88%	1980
September	1st to 10th	Night	1 1 1 1 1 1 1 1 2 90%	1800
	11th to 20th	Night	2 1 1 2 1 1 1 2 3 3 83%	1841
	21st to end	Night	2 1 1 1 1 1 2 2 2 3 82%	1980
October	1st to 10th	Night	4 3 1 2 2 2 3 3 3 4 73%	2092
	11th to 20th	Night	5% 2 1 1 1 2 2 4 3 5% 75%	2160
	21st to end	Night	5% 1 1 2 2 1 1 2 2 5% 78%	2389
November	1st to 10th	Night	10% 4 1 2 2 2 2 2 4 7% 63%	2405
	11th to 20th	Night	7% 4 1 3 2 1 3 3 5% 7% 63%	2470
	21st to end	Night	9% 4 2 3 3 2 3 3 4 5% 61%	2470
December	1st to 10th	Night	15% 5% 1 4 4 2 2 3 4 7% 54%	2603
	11th to 20th	Night	26% 6% 2 4 3 2 3 4 6% 6% 39%	3276
	21st to end	Night	14% 6% 3 6% 6% 4 5% 6% 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	42% 6% 7% 23% 19% 2	1570
March	1st to 10th	Day	48% 6% 9% 17% 16% 5%	1801
	11th to 20th	Day	41% 6% 10% 21% 17% 6%	1944
	21st to end	Day	38% 8% 13% 24% 14% 4	2324
April	1st to 10th	Day	33% 5% 12% 24% 21% 5%	2160
	11th to 20th	Day	32% 5% 10% 25% 23% 6%	2132
	21st to end	Day	27% 6% 9% 30% 25% 3	2160
May	1st to 10th	Day	23% 5% 9% 27% 30% 6%	2217
	11th to 20th	Day	21% 4 8% 28% 32% 8%	2334
	21st to end	Day	23% 6% 8% 32% 25% 6%	2724
June	1st to 10th	Day	17% 5% 9% 22% 31% 15%	2520
	11th to 20th	Day	8% 3 4 19% 40% 25%	2520
	21st to end	Day	9% 2 2 10% 35% 42%	2520
July	1st to 10th	Day	6% 2 2 7% 28% 56%	2451
	11th to 20th	Day	9% 2 3 9% 29% 48%	2445
	21st to end	Day	10% 4 3 11% 28% 44%	2459
August	1st to 10th	Day	9% 3 3 10% 27% 49%	2217
	11th to 20th	Day	7% 2 2 9% 34% 47%	2122
	21st to end	Day	7% 1 1 9% 37% 44%	2376
September	1st to 10th	Day	8% 1 3 8% 29% 51%	2160
	11th to 20th	Day	10% 3 2 21% 37% 27%	2117
	21st to end	Day	10% 2 3 19% 38% 28%	1980
October	1st to 10th	Day	17% 3 6% 18% 27% 29%	1864
	11th to 20th	Day	15% 3 5% 21% 27% 29%	1800
	21st to end	Day	17% 3 5% 20% 32% 23%	1965
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% 2 11% 25% 18% 5%	1884
March	1st to 10th	Night	40% 5% 12% 18% 21% 5%	2165
	11th to 20th	Night	35% 5% 10% 23% 20% 7%	2014
	21st to end	Night	31% 5% 16% 27% 16% 5%	2030
April	1st to 10th	Night	28% 4 17% 26% 19% 6%	1800
	11th to 20th	Night	25% 5% 11% 28% 22% 8%	1779
	21st to end	Night	21% 5% 12% 35% 25% 3	1800
May	1st to 10th	Night	17% 5% 10% 29% 30% 8%	1742
	11th to 20th	Night	15% 3 7% 33% 33% 9%	1559
	21st to end	Night	16% 4 10% 34% 30% 6%	1552
June	1st to 10th	Night	15% 3 11% 23% 33% 14%	1440
	11th to 20th	Night	5% 3 5% 23% 38% 26%	1440
	21st to end	Night	7% 1 3 11% 34% 44%	1440
July	1st to 10th	Night	4 2 2 6% 30% 57%	1400
	11th to 20th	Night	10% 2 2 8% 27% 50%	1392
	21st to end	Night	13% 4 2 10% 26% 45%	1615
August	1st to 10th	Night	10% 1 2 13% 23% 50%	1695
	11th to 20th	Night	8% 2 3 10% 32% 46%	1780
	21st to end	Night	6% 1 1 8% 36% 48%	1980
September	1st to 10th	Night	5% 2 1 8% 29% 55%	1800
	11th to 20th	Night	8% 3 3 16% 43% 27%	1841
	21st to end	Night	9% 2 3 19% 40% 27%	1980
October	1st to 10th	Night	16% 3 5% 19% 30% 27%	2092
	11th to 20th	Night	13% 3 6% 20% 30% 28%	2160
	21st to end	Night	12% 2 4 24% 32% 25%	2389
November	1st to 10th	Night	23% 3 9% 22% 24% 19%	2405
	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% 35% 26% 8% 1	382
March	1st to 10th	Day	24% 13% 32% 23% 8% 1	421
	11th to 20th	Day	21% 15% 28% 21% 13% 2	420
	21st to end	Day	20% 18% 28% 27% 7%	462
April	1st to 10th	Day	17% 17% 32% 24% 10%	420
	11th to 20th	Day	15% 19% 33% 25% 6% 1	412
	21st to end	Day	8% 19% 30% 28% 13% 2	420
May	1st to 10th	Day	8% 17% 34% 29% 9% 3	423
	11th to 20th	Day	10% 14% 32% 30% 11% 3	448
	21st to end	Day	11% 12% 26% 35% 11% 5%	485
June	1st to 10th	Day	5% 12% 28% 34% 16% 5%	450
	11th to 20th	Day	4 10% 32% 37% 13% 6%	450
	21st to end	Day	2 7% 29% 39% 16% 8%	450
July	1st to 10th	Day	1 2 24% 50% 16% 8%	446
	11th to 20th	Day	1 1 19% 52% 16% 11%	444
	21st to end	Day	1 2 22% 50% 16% 9%	487
August	1st to 10th	Day	3 4 31% 42% 13% 6%	447
	11th to 20th	Day	2 4 23% 47% 17% 7%	437
	21st to end	Day	2 2 21% 49% 19% 6%	495
September	1st to 10th	Day	3 4 25% 47% 18% 3	450
	11th to 20th	Day	4 6% 32% 38% 17% 1	450
	21st to end	Day	3 8% 25% 47% 16% 1	450
October	1st to 10th	Day	5% 8% 30% 37% 18% 2	450
	11th to 20th	Day	3 9% 29% 38% 18% 3	450
	21st to end	Day	6% 6% 26% 38% 21% 3	480
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% 38% 16% 3	628
March	1st to 10th	Night	21% 5% 17% 34% 19% 4	722
	11th to 20th	Night	17% 5% 14% 29% 28% 6%	720
	21st to end	Night	16% 5% 18% 39% 20% 3	792
April	1st to 10th	Night	14% 4 18% 38% 24% 3	720
	11th to 20th	Night	10% 5% 22% 35% 23% 5%	711
	21st to end	Night	8% 3 19% 38% 26% 6%	720
May	1st to 10th	Night	6% 4 19% 39% 26% 6%	662
	11th to 20th	Night	6% 2 17% 40% 33% 2	496
	21st to end	Night	6% 1 17% 34% 35% 6%	389
June	1st to 10th	Night	5% 3 10% 37% 40% 5%	360
	11th to 20th	Night	1 1 14% 40% 41% 4	360
	21st to end	Night	1 11% 34% 49% 5%	360
July	1st to 10th	Night	6% 37% 51% 5%	350
	11th to 20th	Night	3 5% 37% 46% 9%	348
	21st to end	Night	2 7% 27% 50% 14%	528
August	1st to 10th	Night	2 1 8% 34% 46% 9%	630
	11th to 20th	Night	1 7% 34% 49% 9%	712
	21st to end	Night	2 5% 31% 52% 10%	792
September	1st to 10th	Night	1 1 7% 35% 47% 10%	720
	11th to 20th	Night	3 1 9% 39% 41% 9%	720
	21st to end	Night	3 1 10% 35% 43% 8%	720
October	1st to 10th	Night	5% 1 12% 45% 30% 7%	720
	11th to 20th	Night	6% 1 12% 39% 33% 9%	720
	21st to end	Night	6% 2 13% 33% 32% 14%	792
November	1st to 10th	Night	11% 2 11% 35% 28% 12%	740
	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 2024-02-27 03:30:17. Version ac07

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