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 2005-11-05 to 2024-10-14.

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	3 7% 3 7% 5% 4 5% 4 6% 7% 49%	100810
	Night	3 7% 3 7% 5% 5% 5% 4 6% 7% 49%	97990
Transparency	Day	30% 7% 11% 19% 19% 13%	100575
	Night	31% 7% 13% 21% 19% 10%	97814
Seeing	Day	9% 17% 33% 27% 11% 2	15057
	Night	7% 8% 25% 38% 19% 2	25365

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 10% 5% 16% 10% 6% 6% 4 5% 6% 29%	2858
	11th to 20th	Day	2 12% 6% 17% 10% 7% 6% 3 5% 4 27%	2860
	21st to end	Day	2 14% 8% 12% 5% 4 4 3 4 6% 39%	3146
February	1st to 10th	Day	3 13% 6% 13% 7% 6% 5% 4 5% 5% 32%	1710
	11th to 20th	Day	4 15% 5% 12% 6% 5% 5% 3 5% 7% 33%	1712
	21st to end	Day	2 14% 5% 13% 7% 7% 6% 4 6% 7% 28%	1570
March	1st to 10th	Day	1 10% 5% 15% 7% 6% 6% 4 6% 7% 31%	3375
	11th to 20th	Day	2 8% 4 8% 5% 5% 6% 4 7% 8% 44%	3490
	21st to end	Day	3 17% 8% 12% 7% 7% 7% 5% 6% 6% 22%	3825
April	1st to 10th	Day	4 12% 5% 9% 4 4 6% 5% 7% 7% 37%	2121
	11th to 20th	Day	6% 12% 4 8% 6% 6% 6% 4 6% 6% 38%	2381
	21st to end	Day	4 11% 4 7% 5% 5% 5% 4 7% 7% 41%	2470
May	1st to 10th	Day	6% 13% 4 8% 6% 6% 5% 4 6% 6% 35%	4110
	11th to 20th	Day	4 5% 2 5% 3 4 4 4 6% 9% 55%	4071
	21st to end	Day	5% 6% 2 6% 5% 5% 6% 4 6% 8% 48%	4477
June	1st to 10th	Day	3 3 1 5% 4 5% 5% 5% 6% 9% 54%	2612
	11th to 20th	Day	4 4 1 4 3 4 4 3 4 6% 63%	2660
	21st to end	Day	1 2 1 3 3 3 4 3 5% 6% 69%	2558
July	1st to 10th	Day	1 1 2 2 2 3 3 4 6% 76%	4110
	11th to 20th	Day	2 2 1 3 4 4 6% 5% 8% 10% 56%	4110
	21st to end	Day	3 3 1 3 3 3 4 4 6% 8% 62%	4454
August	1st to 10th	Day	2 2 1 3 3 4 5% 4 6% 8% 61%	4092
	11th to 20th	Day	1 2 1 3 3 3 5% 4 6% 8% 65%	4028
	21st to end	Day	2 2 1 3 2 2 3 2 5% 8% 69%	4467
September	1st to 10th	Day	3 2 1 3 2 3 4 3 5% 6% 67%	2167
	11th to 20th	Day	4 3 1 4 3 3 4 4 5% 6% 61%	2090
	21st to end	Day	4 4 1 3 2 3 3 2 4 6% 67%	2090
October	1st to 10th	Day	6% 6% 2 5% 4 4 4 3 5% 5% 55%	1986
	11th to 20th	Day	6% 6% 2 5% 3 3 3 3 5% 6% 59%	1830
	21st to end	Day	3 7% 2 7% 4 4 4 3 5% 7% 54%	1980
November	1st to 10th	Day	4 9% 3 8% 5% 4 5% 4 6% 6% 45%	1682
	11th to 20th	Day	6% 10% 3 6% 5% 5% 4 3 6% 7% 45%	1710
	21st to end	Day	3 9% 4 9% 7% 5% 6% 3 6% 6% 43%	1535
December	1st to 10th	Day	4 10% 3 9% 7% 6% 6% 4 5% 7% 39%	1520
	11th to 20th	Day	3 11% 3 12% 7% 7% 7% 4 7% 8% 30%	1926
	21st to end	Day	2 10% 4 12% 7% 6% 5% 4 6% 7% 37%	3027
January	1st to 10th	Night	2 9% 5% 15% 7% 7% 5% 4 5% 7% 33%	4097
	11th to 20th	Night	2 11% 4 13% 10% 10% 7% 4 5% 6% 29%	4110
	21st to end	Night	2 12% 5% 11% 6% 6% 5% 4 6% 7% 36%	4521
February	1st to 10th	Night	3 10% 4 10% 7% 6% 6% 6% 7% 7% 35%	2470
	11th to 20th	Night	2 10% 4 9% 7% 7% 5% 4 6% 6% 40%	2350
	21st to end	Night	2 10% 4 11% 7% 6% 5% 5% 7% 8% 38%	1884
March	1st to 10th	Night	2 9% 3 11% 7% 7% 7% 5% 7% 8% 34%	3498
	11th to 20th	Night	1 7% 2 6% 4 5% 5% 4 7% 9% 48%	3490
	21st to end	Night	3 13% 6% 11% 7% 6% 5% 4 6% 5% 34%	3828
April	1st to 10th	Night	5% 12% 3 5% 4 4 4 4 5% 6% 46%	2057
	11th to 20th	Night	4 8% 2 6% 5% 5% 5% 4 6% 7% 47%	1743
	21st to end	Night	5% 8% 3 6% 4 4 5% 4 6% 8% 47%	1710
May	1st to 10th	Night	4 10% 3 8% 7% 6% 5% 4 7% 8% 39%	2860
	11th to 20th	Night	3 4 1 4 4 3 4 4 5% 8% 60%	2834
	21st to end	Night	5% 4 2 5% 4 4 4 4 7% 9% 52%	3110
June	1st to 10th	Night	4 4 2 4 5% 4 5% 4 5% 7% 56%	1558
	11th to 20th	Night	3 2 1 3 3 4 4 4 5% 7% 65%	1520
	21st to end	Night	2 1 1 3 3 3 3 3 4 8% 69%	1613
July	1st to 10th	Night	2 1 1 3 3 3 3 3 3 5% 74%	2860
	11th to 20th	Night	2 3 1 4 4 4 5% 4 8% 10% 56%	2860
	21st to end	Night	2 2 1 5% 4 4 5% 4 7% 6% 59%	3109
August	1st to 10th	Night	2 2 1 3 3 5% 5% 3 6% 9% 61%	2847
	11th to 20th	Night	2 2 1 4 4 4 5% 4 6% 7% 62%	2811
	21st to end	Night	1 2 1 3 3 2 3 3 5% 7% 71%	3198
September	1st to 10th	Night	3 3 1 4 3 3 3 4 5% 5% 67%	2009
	11th to 20th	Night	4 2 1 3 2 3 4 3 5% 7% 66%	2090
	21st to end	Night	4 3 1 4 3 2 2 2 4 4 71%	2090
October	1st to 10th	Night	6% 5% 1 4 3 4 4 4 5% 6% 61%	2194
	11th to 20th	Night	5% 5% 2 4 3 3 3 3 6% 6% 60%	2208
	21st to end	Night	4 6% 2 4 3 3 4 4 5% 6% 59%	2376
November	1st to 10th	Night	4 7% 3 6% 4 3 4 5% 6% 8% 50%	2395
	11th to 20th	Night	4 8% 2 6% 4 4 5% 4 7% 8% 49%	2470
	21st to end	Night	3 9% 4 7% 5% 6% 5% 4 5% 7% 46%	2622
December	1st to 10th	Night	2 9% 4 8% 7% 5% 5% 4 6% 8% 43%	2660
	11th to 20th	Night	4 10% 3 8% 6% 5% 7% 5% 7% 9% 36%	3271
	21st to end	Night	3 11% 4 12% 8% 6% 6% 4 6% 7% 33%	4667

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	49% 11% 14% 16% 9% 1	2859
	11th to 20th	Day	55% 9% 14% 14% 6% 1	2860
	21st to end	Day	45% 10% 20% 16% 9% 2	3146
February	1st to 10th	Day	52% 6% 13% 18% 10% 1	1710
	11th to 20th	Day	48% 7% 14% 18% 12% 1	1721
	21st to end	Day	50% 8% 14% 18% 9% 1	1570
March	1st to 10th	Day	42% 13% 13% 20% 10% 2	3375
	11th to 20th	Day	30% 10% 18% 30% 10% 1	3490
	21st to end	Day	54% 10% 15% 14% 5% 1	3825
April	1st to 10th	Day	42% 7% 14% 19% 13% 4	2121
	11th to 20th	Day	43% 7% 13% 20% 16% 1	2384
	21st to end	Day	38% 7% 11% 25% 18% 1	2470
May	1st to 10th	Day	40% 8% 13% 20% 15% 4	4110
	11th to 20th	Day	22% 6% 11% 32% 26% 3	4074
	21st to end	Day	28% 7% 11% 28% 23% 3	4477
June	1st to 10th	Day	24% 6% 11% 23% 27% 9%	2612
	11th to 20th	Day	21% 5% 7% 20% 28% 18%	2660
	21st to end	Day	14% 4 5% 13% 30% 34%	2558
July	1st to 10th	Day	11% 2 4 8% 16% 59%	4046
	11th to 20th	Day	16% 7% 8% 13% 22% 34%	4041
	21st to end	Day	15% 5% 8% 11% 25% 35%	4314
August	1st to 10th	Day	17% 6% 7% 20% 24% 26%	4078
	11th to 20th	Day	14% 7% 7% 15% 33% 24%	4067
	21st to end	Day	13% 4 7% 16% 29% 31%	4466
September	1st to 10th	Day	17% 4 7% 16% 28% 28%	2167
	11th to 20th	Day	20% 6% 8% 19% 28% 20%	2090
	21st to end	Day	19% 3 7% 21% 35% 14%	2090
October	1st to 10th	Day	30% 4 9% 21% 26% 10%	1986
	11th to 20th	Day	26% 3 9% 30% 24% 7%	1830
	21st to end	Day	28% 5% 16% 26% 21% 3	1980
November	1st to 10th	Day	34% 8% 15% 23% 17% 4	1682
	11th to 20th	Day	36% 5% 14% 26% 17% 1	1710
	21st to end	Day	39% 7% 13% 24% 13% 4	1535
December	1st to 10th	Day	40% 9% 19% 17% 10% 4	1520
	11th to 20th	Day	44% 10% 15% 19% 11% 1	1924
	21st to end	Day	42% 7% 15% 15% 18% 2	3027
January	1st to 10th	Night	47% 9% 14% 20% 9% 1	4097
	11th to 20th	Night	54% 11% 12% 15% 6% 2	4110
	21st to end	Night	43% 13% 20% 16% 7% 2	4521
February	1st to 10th	Night	43% 10% 16% 20% 10% 1	2470
	11th to 20th	Night	41% 6% 14% 21% 16% 1	2351
	21st to end	Night	41% 7% 16% 21% 12% 2	1884
March	1st to 10th	Night	37% 11% 17% 22% 13%	3498
	11th to 20th	Night	25% 10% 19% 32% 13% 1	3490
	21st to end	Night	45% 11% 12% 21% 10% 1	3828
April	1st to 10th	Night	35% 7% 15% 22% 15% 5%	2057
	11th to 20th	Night	31% 7% 16% 24% 20% 1	1743
	21st to end	Night	32% 7% 15% 25% 19% 1	1710
May	1st to 10th	Night	34% 10% 14% 20% 20% 2	2860
	11th to 20th	Night	19% 9% 12% 31% 23% 6%	2835
	21st to end	Night	24% 6% 13% 32% 24% 1	3110
June	1st to 10th	Night	24% 6% 10% 24% 28% 7%	1558
	11th to 20th	Night	17% 5% 8% 20% 31% 19%	1520
	21st to end	Night	14% 3 5% 12% 31% 35%	1613
July	1st to 10th	Night	13% 3 5% 8% 13% 58%	2815
	11th to 20th	Night	19% 5% 6% 13% 26% 31%	2806
	21st to end	Night	21% 5% 8% 12% 25% 29%	3011
August	1st to 10th	Night	18% 6% 7% 17% 27% 25%	2838
	11th to 20th	Night	19% 7% 8% 15% 28% 24%	2838
	21st to end	Night	12% 4 8% 12% 32% 32%	3198
September	1st to 10th	Night	17% 4 7% 17% 25% 30%	2009
	11th to 20th	Night	18% 5% 8% 21% 28% 21%	2090
	21st to end	Night	17% 4 6% 21% 36% 17%	2090
October	1st to 10th	Night	24% 4 10% 23% 28% 10%	2194
	11th to 20th	Night	25% 5% 8% 29% 26% 7%	2208
	21st to end	Night	24% 6% 13% 29% 24% 4	2376
November	1st to 10th	Night	28% 7% 13% 27% 20% 5%	2395
	11th to 20th	Night	31% 7% 16% 26% 18% 3	2470
	21st to end	Night	36% 5% 14% 26% 16% 4	2622
December	1st to 10th	Night	37% 9% 17% 23% 12% 3	2660
	11th to 20th	Night	39% 11% 18% 20% 12% 1	3272
	21st to end	Night	44% 9% 16% 17% 12% 2	4667

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	16% 17% 29% 21% 14% 2	270
	11th to 20th	Day	10% 15% 40% 24% 10% 2	270
	21st to end	Day	7% 15% 34% 32% 10% 2	297
February	1st to 10th	Day	14% 25% 35% 21% 4	270
	11th to 20th	Day	14% 21% 38% 20% 5% 2	317
	21st to end	Day	12% 27% 39% 19% 2 1	382
March	1st to 10th	Day	14% 25% 37% 18% 5%	460
	11th to 20th	Day	11% 23% 38% 21% 7% 1	460
	21st to end	Day	17% 30% 32% 19% 2	506
April	1st to 10th	Day	17% 23% 30% 23% 7% 1	460
	11th to 20th	Day	19% 27% 32% 16% 5% 1	452
	21st to end	Day	15% 27% 28% 23% 6% 1	460
May	1st to 10th	Day	16% 22% 36% 19% 7% 1	463
	11th to 20th	Day	11% 22% 30% 24% 11% 3	488
	21st to end	Day	12% 18% 30% 25% 12% 3	529
June	1st to 10th	Day	5% 15% 32% 29% 16% 3	632
	11th to 20th	Day	8% 16% 32% 28% 14% 1	680
	21st to end	Day	4 13% 35% 33% 13% 2	578
July	1st to 10th	Day	2 11% 35% 38% 11% 2	486
	11th to 20th	Day	2 8% 39% 35% 13% 3	484
	21st to end	Day	3 5% 33% 39% 16% 3	531
August	1st to 10th	Day	6% 9% 35% 33% 15% 3	487
	11th to 20th	Day	2 8% 35% 37% 14% 3	477
	21st to end	Day	4 11% 30% 35% 15% 5%	539
September	1st to 10th	Day	5% 10% 29% 33% 17% 6%	490
	11th to 20th	Day	7% 17% 31% 29% 13% 3	490
	21st to end	Day	7% 15% 25% 33% 16% 4	490
October	1st to 10th	Day	8% 16% 27% 27% 17% 5%	386
	11th to 20th	Day	9% 16% 27% 25% 18% 6%	279
	21st to end	Day	8% 16% 37% 28% 9% 1	297
November	1st to 10th	Day	15% 17% 27% 27% 10% 4	270
	11th to 20th	Day	11% 14% 33% 28% 9% 5%	270
	21st to end	Day	9% 13% 29% 31% 16% 3	270
December	1st to 10th	Day	9% 16% 33% 28% 10% 4	270
	11th to 20th	Day	8% 18% 42% 24% 7% 1	270
	21st to end	Day	14% 19% 36% 22% 8% 1	297
January	1st to 10th	Night	10% 8% 30% 33% 17% 3	760
	11th to 20th	Night	9% 11% 27% 37% 14% 3	760
	21st to end	Night	7% 9% 30% 38% 13% 3	836
February	1st to 10th	Night	9% 10% 33% 36% 11% 2	760
	11th to 20th	Night	6% 10% 35% 36% 13% 1	744
	21st to end	Night	7% 10% 33% 38% 11% 1	628
March	1st to 10th	Night	8% 12% 34% 35% 10% 2	760
	11th to 20th	Night	6% 9% 29% 38% 16% 2	760
	21st to end	Night	8% 15% 29% 37% 10% 1	836
April	1st to 10th	Night	13% 13% 27% 35% 11% 1	727
	11th to 20th	Night	9% 11% 34% 35% 12%	563
	21st to end	Night	9% 9% 25% 37% 18% 2	570
May	1st to 10th	Night	10% 6% 28% 39% 16% 1	570
	11th to 20th	Night	8% 6% 21% 39% 24% 2	561
	21st to end	Night	8% 6% 24% 35% 24% 4	617
June	1st to 10th	Night	7% 8% 18% 42% 24% 2	570
	11th to 20th	Night	5% 9% 22% 44% 19%	570
	21st to end	Night	2 4 19% 41% 32% 2	570
July	1st to 10th	Night	3 4 16% 38% 36% 3	555
	11th to 20th	Night	4 3 15% 47% 28% 3	552
	21st to end	Night	3 1 11% 38% 46% 1	624
August	1st to 10th	Night	4 2 15% 46% 32% 1	567
	11th to 20th	Night	2 3 16% 43% 33% 2	564
	21st to end	Night	3 3 16% 43% 33% 2	627
September	1st to 10th	Night	4 4 15% 42% 32% 5%	679
	11th to 20th	Night	5% 5% 18% 38% 28% 5%	760
	21st to end	Night	6% 5% 17% 37% 28% 7%	760
October	1st to 10th	Night	8% 5% 21% 40% 22% 3	760
	11th to 20th	Night	7% 5% 22% 40% 22% 4	734
	21st to end	Night	8% 6% 26% 37% 19% 3	792
November	1st to 10th	Night	9% 10% 22% 37% 20% 2	740
	11th to 20th	Night	9% 6% 31% 38% 13% 4	760
	21st to end	Night	7% 11% 27% 38% 14% 3	912
December	1st to 10th	Night	7% 9% 31% 39% 13% 1	950
	11th to 20th	Night	11% 9% 33% 35% 10% 1	950
	21st to end	Night	13% 11% 32% 34% 8% 1	917

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