High Meadows Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the High Meadows Observatory clear sky chart from 2005-11-06 to 2026-07-16.

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	3 6% 3 7% 5% 5% 5% 4 6% 7% 51%	146571
	Night	2 6% 2 6% 5% 5% 5% 4 6% 7% 52%	143688
Transparency	Day	28% 7% 11% 20% 21% 13%	141319
	Night	27% 7% 12% 22% 21% 10%	139947
Seeing	Day	9% 17% 33% 27% 11% 2	17399
	Night	7% 8% 26% 38% 19% 2	27740

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	2 9% 5% 14% 9% 7% 7% 5% 6% 7% 31%	5318
	11th to 20th	Day	1 7% 3 13% 7% 5% 4 3 5% 5% 47%	5340
	21st to end	Day	1 8% 5% 8% 5% 4 4 3 5% 7% 50%	5844
February	1st to 10th	Day	4 12% 6% 13% 7% 6% 5% 4 5% 5% 33%	1890
	11th to 20th	Day	3 14% 5% 13% 6% 5% 6% 4 6% 7% 30%	1898
	21st to end	Day	2 13% 5% 12% 8% 6% 6% 4 6% 7% 32%	1730
March	1st to 10th	Day	1 11% 5% 14% 6% 6% 5% 4 5% 6% 36%	4967
	11th to 20th	Day	1 6% 3 9% 5% 5% 7% 6% 9% 8% 41%	5110
	21st to end	Day	2 13% 7% 11% 7% 7% 7% 5% 7% 6% 29%	5607
April	1st to 10th	Day	4 11% 5% 8% 4 4 5% 5% 7% 8% 39%	2345
	11th to 20th	Day	5% 11% 4 8% 5% 5% 6% 4 6% 7% 39%	2635
	21st to end	Day	4 10% 3 7% 5% 5% 5% 4 7% 7% 41%	2730
May	1st to 10th	Day	5% 9% 2 6% 5% 5% 5% 4 6% 7% 47%	7626
	11th to 20th	Day	3 5% 2 6% 4 4 5% 4 7% 8% 51%	7596
	21st to end	Day	3 4 2 6% 4 5% 6% 5% 7% 8% 51%	8370
June	1st to 10th	Day	3 3 1 5% 4 4 5% 5% 6% 9% 56%	2887
	11th to 20th	Day	3 3 1 4 3 4 4 3 4 6% 65%	2940
	21st to end	Day	1 2 1 3 3 3 4 3 5% 7% 69%	2827
July	1st to 10th	Day	2 1 1 3 3 3 4 3 5% 6% 71%	6009
	11th to 20th	Day	2 2 1 3 4 4 5% 5% 8% 9% 58%	5946
	21st to end	Day	2 2 1 3 3 3 3 4 5% 7% 67%	6401
August	1st to 10th	Day	2 2 1 2 3 3 4 3 5% 8% 67%	5779
	11th to 20th	Day	1 1 1 2 2 3 4 4 6% 7% 68%	5798
	21st to end	Day	2 3 1 4 4 4 5% 4 7% 8% 58%	6388
September	1st to 10th	Day	3 2 1 3 3 4 4 3 6% 6% 65%	2281
	11th to 20th	Day	4 3 1 4 3 4 5% 4 6% 6% 61%	2200
	21st to end	Day	4 4 1 3 3 3 4 3 4 6% 66%	2200
October	1st to 10th	Day	3 4 1 4 3 3 3 3 4 5% 67%	3412
	11th to 20th	Day	6% 5% 2 5% 3 3 4 3 5% 6% 57%	3250
	21st to end	Day	3 7% 2 5% 3 5% 4 4 4 6% 57%	3548
November	1st to 10th	Day	4 8% 2 8% 5% 4 4 4 6% 6% 47%	1862
	11th to 20th	Day	6% 10% 3 7% 6% 5% 5% 3 6% 7% 44%	1890
	21st to end	Day	3 9% 4 8% 6% 4 5% 3 6% 7% 44%	1696
December	1st to 10th	Day	2 6% 2 6% 5% 4 5% 3 4 7% 56%	2700
	11th to 20th	Day	2 7% 3 10% 6% 6% 8% 6% 10% 9% 33%	3106
	21st to end	Day	2 9% 4 13% 8% 7% 7% 5% 7% 6% 32%	4445
January	1st to 10th	Night	1 8% 4 14% 7% 7% 6% 5% 7% 8% 32%	7605
	11th to 20th	Night	1 6% 3 9% 7% 7% 5% 3 5% 5% 49%	7650
	21st to end	Night	1 7% 3 7% 5% 5% 5% 4 6% 9% 49%	8370
February	1st to 10th	Night	3 10% 4 9% 7% 6% 6% 5% 6% 7% 37%	2730
	11th to 20th	Night	2 10% 4 9% 7% 7% 6% 5% 7% 6% 38%	2604
	21st to end	Night	2 9% 4 10% 6% 5% 5% 5% 7% 8% 40%	2076
March	1st to 10th	Night	2 9% 3 11% 8% 7% 7% 5% 6% 8% 36%	5146
	11th to 20th	Night	1 7% 3 7% 6% 6% 7% 6% 8% 9% 42%	5110
	21st to end	Night	2 9% 4 9% 6% 6% 6% 4 7% 7% 41%	5610
April	1st to 10th	Night	4 11% 3 5% 4 4 4 4 5% 7% 48%	2273
	11th to 20th	Night	3 8% 2 5% 5% 5% 5% 4 7% 7% 48%	1929
	21st to end	Night	5% 7% 2 6% 4 5% 6% 5% 6% 8% 47%	1890
May	1st to 10th	Night	3 7% 2 6% 5% 4 4 4 6% 8% 52%	5321
	11th to 20th	Night	3 4 2 4 3 3 3 4 5% 8% 62%	5296
	21st to end	Night	4 4 1 4 3 3 4 4 6% 9% 57%	5837
June	1st to 10th	Night	3 4 2 4 4 4 5% 3 5% 7% 59%	1722
	11th to 20th	Night	3 2 1 3 2 3 4 3 5% 7% 67%	1680
	21st to end	Night	2 1 1 3 3 3 3 3 4 7% 69%	1783
July	1st to 10th	Night	1 2 1 4 3 3 4 3 4 4 71%	4190
	11th to 20th	Night	1 3 1 4 4 4 5% 4 7% 9% 58%	4145
	21st to end	Night	2 2 1 4 3 3 4 4 6% 6% 66%	4472
August	1st to 10th	Night	2 1 1 2 2 4 4 3 6% 8% 69%	4033
	11th to 20th	Night	2 2 1 3 3 4 4 4 5% 6% 66%	4051
	21st to end	Night	2 2 1 4 4 4 5% 4 7% 7% 59%	4587
September	1st to 10th	Night	3 3 1 4 3 3 3 4 5% 5% 67%	2115
	11th to 20th	Night	4 2 1 3 3 4 4 3 5% 7% 65%	2200
	21st to end	Night	4 3 1 4 3 2 2 2 4 5% 71%	2200
October	1st to 10th	Night	3 3 1 2 2 3 3 3 4 4 72%	3782
	11th to 20th	Night	4 4 2 4 3 4 5% 4 7% 6% 57%	3898
	21st to end	Night	3 5% 2 5% 3 3 5% 4 7% 9% 54%	4252
November	1st to 10th	Night	3 6% 2 6% 4 4 4 5% 7% 8% 51%	2655
	11th to 20th	Night	4 7% 2 6% 4 5% 5% 4 7% 8% 48%	2730
	21st to end	Night	3 8% 4 7% 5% 5% 5% 4 5% 8% 46%	2898
December	1st to 10th	Night	1 5% 3 6% 5% 3 4 3 6% 8% 56%	4710
	11th to 20th	Night	3 7% 3 7% 5% 5% 6% 4 8% 10% 42%	5321
	21st to end	Night	2 9% 3 12% 8% 7% 6% 5% 7% 9% 31%	6817

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	43% 13% 17% 17% 10% 1	5321
	11th to 20th	Day	37% 7% 10% 26% 14% 6%	5340
	21st to end	Day	31% 7% 18% 20% 22% 2	5844
February	1st to 10th	Day	50% 6% 13% 19% 11% 2	1890
	11th to 20th	Day	49% 8% 14% 17% 11% 1	1907
	21st to end	Day	47% 8% 14% 20% 10% 2	1730
March	1st to 10th	Day	40% 11% 11% 21% 12% 6%	4967
	11th to 20th	Day	28% 11% 18% 29% 13% 1	5110
	21st to end	Day	47% 11% 18% 17% 7% 1	5607
April	1st to 10th	Day	39% 8% 14% 21% 15% 4	2345
	11th to 20th	Day	41% 7% 13% 21% 17% 2	2638
	21st to end	Day	37% 7% 11% 24% 19% 1	2730
May	1st to 10th	Day	30% 8% 11% 23% 22% 6%	7626
	11th to 20th	Day	23% 8% 10% 24% 29% 6%	7599
	21st to end	Day	24% 7% 10% 25% 30% 4	8371
June	1st to 10th	Day	22% 6% 10% 22% 31% 9%	2887
	11th to 20th	Day	19% 4 7% 19% 28% 22%	2940
	21st to end	Day	14% 4 5% 13% 31% 33%	2827
July	1st to 10th	Day	13% 4 5% 7% 17% 55%	5946
	11th to 20th	Day	15% 6% 7% 12% 26% 34%	5877
	21st to end	Day	13% 4 6% 10% 24% 43%	6217
August	1st to 10th	Day	17% 5% 7% 20% 24% 26%	4208
	11th to 20th	Day	14% 7% 7% 16% 33% 24%	4197
	21st to end	Day	14% 4 7% 15% 29% 30%	4607
September	1st to 10th	Day	17% 4 7% 17% 28% 26%	2281
	11th to 20th	Day	20% 6% 8% 18% 28% 19%	2200
	21st to end	Day	19% 4 8% 21% 34% 14%	2200
October	1st to 10th	Day	20% 4 6% 15% 35% 20%	3412
	11th to 20th	Day	25% 5% 8% 30% 26% 7%	3250
	21st to end	Day	26% 6% 11% 27% 26% 4	3548
November	1st to 10th	Day	32% 7% 14% 25% 17% 5%	1862
	11th to 20th	Day	36% 6% 14% 26% 16% 1	1890
	21st to end	Day	37% 7% 13% 25% 14% 3	1696
December	1st to 10th	Day	25% 7% 15% 38% 13% 2	2700
	11th to 20th	Day	35% 13% 18% 21% 12% 1	3104
	21st to end	Day	44% 9% 13% 17% 15% 1	4445
January	1st to 10th	Night	41% 13% 18% 17% 9% 1	7605
	11th to 20th	Night	35% 7% 10% 31% 14% 3	7650
	21st to end	Night	29% 9% 16% 20% 22% 3	8370
February	1st to 10th	Night	42% 10% 16% 21% 10% 2	2730
	11th to 20th	Night	41% 7% 14% 21% 16% 1	2605
	21st to end	Night	38% 7% 16% 23% 13% 3	2076
March	1st to 10th	Night	38% 10% 13% 22% 13% 5%	5146
	11th to 20th	Night	28% 11% 17% 27% 15% 1	5110
	21st to end	Night	35% 10% 18% 22% 13% 1	5610
April	1st to 10th	Night	34% 7% 15% 23% 17% 5%	2273
	11th to 20th	Night	30% 7% 16% 24% 21% 3	1929
	21st to end	Night	31% 8% 15% 26% 19% 1	1890
May	1st to 10th	Night	25% 8% 12% 23% 29% 3	5319
	11th to 20th	Night	18% 6% 9% 25% 31% 9%	5297
	21st to end	Night	20% 6% 10% 28% 34% 2	5838
June	1st to 10th	Night	22% 5% 10% 23% 32% 8%	1722
	11th to 20th	Night	16% 4 8% 19% 30% 23%	1680
	21st to end	Night	14% 3 5% 12% 33% 33%	1783
July	1st to 10th	Night	16% 3 4 8% 16% 54%	4145
	11th to 20th	Night	19% 4 6% 11% 26% 34%	4091
	21st to end	Night	16% 4 6% 10% 27% 37%	4344
August	1st to 10th	Night	17% 6% 7% 16% 27% 26%	2928
	11th to 20th	Night	19% 6% 8% 15% 28% 24%	2928
	21st to end	Night	12% 4 8% 12% 32% 31%	3299
September	1st to 10th	Night	17% 4 7% 18% 25% 28%	2115
	11th to 20th	Night	18% 5% 8% 21% 29% 20%	2200
	21st to end	Night	17% 4 6% 21% 35% 16%	2200
October	1st to 10th	Night	15% 3 7% 17% 36% 21%	3782
	11th to 20th	Night	22% 7% 8% 31% 24% 8%	3898
	21st to end	Night	23% 5% 12% 29% 27% 3	4252
November	1st to 10th	Night	27% 7% 13% 27% 21% 5%	2655
	11th to 20th	Night	30% 7% 16% 27% 17% 2	2730
	21st to end	Night	34% 5% 14% 27% 16% 4	2898
December	1st to 10th	Night	23% 7% 14% 36% 19% 2	4710
	11th to 20th	Night	31% 10% 22% 26% 11% 1	5322
	21st to end	Night	42% 11% 16% 19% 11% 1	6817

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	14% 19% 33% 20% 12% 2	330
	11th to 20th	Day	8% 15% 38% 26% 10% 2	330
	21st to end	Day	6% 15% 36% 33% 9% 2	363
February	1st to 10th	Day	16% 23% 34% 21% 5% 1	330
	11th to 20th	Day	14% 23% 39% 19% 4 1	383
	21st to end	Day	12% 26% 38% 19% 4 1	446
March	1st to 10th	Day	14% 25% 37% 19% 5% 1	540
	11th to 20th	Day	13% 25% 36% 19% 6% 1	540
	21st to end	Day	16% 30% 32% 19% 3	594
April	1st to 10th	Day	15% 24% 30% 23% 8% 1	540
	11th to 20th	Day	18% 28% 32% 17% 4 1	532
	21st to end	Day	15% 28% 29% 22% 6% 1	540
May	1st to 10th	Day	14% 20% 36% 20% 8% 1	543
	11th to 20th	Day	13% 22% 30% 23% 10% 2	568
	21st to end	Day	12% 17% 31% 24% 12% 3	617
June	1st to 10th	Day	6% 16% 32% 29% 15% 2	727
	11th to 20th	Day	7% 16% 33% 29% 13% 1	780
	21st to end	Day	4 15% 36% 31% 12% 2	667
July	1st to 10th	Day	2 10% 36% 38% 11% 2	566
	11th to 20th	Day	2 8% 38% 35% 14% 3	545
	21st to end	Day	3 5% 34% 38% 16% 3	575
August	1st to 10th	Day	5% 10% 35% 32% 15% 2	527
	11th to 20th	Day	3 8% 34% 37% 15% 3	517
	21st to end	Day	4 10% 30% 36% 15% 5%	583
September	1st to 10th	Day	5% 10% 29% 34% 17% 6%	530
	11th to 20th	Day	7% 17% 31% 29% 13% 4	530
	21st to end	Day	6% 15% 25% 33% 17% 4	530
October	1st to 10th	Day	8% 17% 28% 27% 16% 5%	420
	11th to 20th	Day	10% 17% 28% 23% 16% 5%	330
	21st to end	Day	9% 17% 34% 28% 10% 1	363
November	1st to 10th	Day	14% 17% 27% 28% 11% 3	330
	11th to 20th	Day	12% 15% 34% 27% 8% 5%	330
	21st to end	Day	9% 13% 31% 30% 15% 3	330
December	1st to 10th	Day	8% 17% 33% 26% 12% 5%	330
	11th to 20th	Day	9% 18% 39% 25% 8% 1	330
	21st to end	Day	15% 21% 33% 21% 9% 1	363
January	1st to 10th	Night	9% 9% 32% 32% 15% 3	840
	11th to 20th	Night	8% 10% 29% 37% 14% 3	840
	21st to end	Night	7% 8% 33% 38% 12% 2	924
February	1st to 10th	Night	9% 10% 32% 36% 10% 2	840
	11th to 20th	Night	6% 11% 37% 34% 11% 1	824
	21st to end	Night	7% 10% 35% 38% 10% 1	692
March	1st to 10th	Night	7% 12% 35% 34% 10% 2	840
	11th to 20th	Night	6% 10% 31% 36% 15% 2	840
	21st to end	Night	8% 14% 30% 38% 9% 1	924
April	1st to 10th	Night	12% 12% 27% 36% 12% 1	803
	11th to 20th	Night	8% 10% 34% 35% 11%	623
	21st to end	Night	9% 9% 26% 38% 17% 2	630
May	1st to 10th	Night	10% 6% 27% 41% 16% 1	630
	11th to 20th	Night	8% 7% 22% 39% 23% 2	621
	21st to end	Night	8% 6% 24% 35% 24% 4	683
June	1st to 10th	Night	6% 8% 18% 43% 23% 2	630
	11th to 20th	Night	5% 9% 22% 45% 18%	630
	21st to end	Night	3 5% 20% 40% 31% 1	630
July	1st to 10th	Night	3 3 16% 42% 33% 3	615
	11th to 20th	Night	4 3 15% 48% 28% 3	597
	21st to end	Night	2 1 11% 39% 44% 1	657
August	1st to 10th	Night	4 2 17% 46% 31% 1	597
	11th to 20th	Night	3 3 16% 43% 33% 2	594
	21st to end	Night	3 3 15% 43% 34% 2	660
September	1st to 10th	Night	3 4 15% 42% 32% 4	715
	11th to 20th	Night	5% 6% 19% 39% 27% 5%	800
	21st to end	Night	6% 5% 17% 37% 28% 7%	800
October	1st to 10th	Night	7% 6% 23% 40% 21% 3	800
	11th to 20th	Night	8% 5% 23% 39% 21% 4	800
	21st to end	Night	8% 6% 26% 38% 18% 3	880
November	1st to 10th	Night	8% 9% 24% 38% 18% 2	820
	11th to 20th	Night	9% 5% 34% 37% 12% 3	840
	21st to end	Night	7% 11% 28% 38% 13% 3	1008
December	1st to 10th	Night	7% 10% 31% 37% 14% 1	1050
	11th to 20th	Night	11% 9% 34% 35% 10% 1	1050
	21st to end	Night	13% 14% 32% 32% 8% 1	1013

Page generated at 2026-07-13 01:14:50. Version ac07

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