Mt. SAC Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Mt. SAC Observatory clear sky chart from 2005-11-06 to 2025-10-19.

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	5% 3 1 3 2 2 3 2 4 6% 69%	135427
	Night	6% 5% 2 4 3 2 3 2 4 6% 64%	131812
Transparency	Day	18% 4 6% 17% 27% 28%	130513
	Night	22% 4 7% 17% 25% 24%	127698
Seeing	Day	6% 7% 25% 36% 19% 7%	16987
	Night	7% 3 12% 38% 34% 7%	26820

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	5% 3 1 3 2 2 2 2 3 6% 71%	3639
	11th to 20th	Day	6% 2 1 3 2 2 3 2 5% 6% 68%	4232
	21st to end	Day	11% 4 2 5% 3 3 3 3 5% 9% 52%	5016
February	1st to 10th	Day	10% 4 1 3 3 3 3 3 4 5% 61%	2000
	11th to 20th	Day	10% 3 1 3 2 2 3 3 4 6% 61%	1950
	21st to end	Day	9% 4 2 5% 3 3 4 4 5% 6% 56%	1650
March	1st to 10th	Day	12% 6% 2 5% 3 3 4 4 5% 7% 48%	4518
	11th to 20th	Day	10% 4 2 5% 4 3 4 3 5% 7% 54%	4665
	21st to end	Day	10% 4 1 4 3 4 3 4 6% 8% 54%	5964
April	1st to 10th	Day	6% 4 1 4 3 3 3 3 5% 7% 61%	2400
	11th to 20th	Day	5% 4 2 4 4 3 3 3 4 5% 63%	2369
	21st to end	Day	5% 4 2 3 3 2 2 2 4 6% 66%	2400
May	1st to 10th	Day	4 6% 2 4 2 2 3 2 4 5% 65%	5647
	11th to 20th	Day	4 6% 2 5% 3 3 3 3 5% 7% 60%	5841
	21st to end	Day	4 4 2 5% 3 3 3 2 4 5% 65%	6423
June	1st to 10th	Day	4 5% 2 6% 3 2 2 2 5% 7% 63%	2600
	11th to 20th	Day	3 4 1 3 2 1 2 1 3 5% 74%	2747
	21st to end	Day	2 3 1 3 2 2 2 2 3 5% 76%	2800
July	1st to 10th	Day	1 1 1 2 1 1 1 2 3 86%	6319
	11th to 20th	Day	1 2 1 3 3 2 2 2 4 5% 76%	6110
	21st to end	Day	1 2 1 2 1 1 2 2 3 4 81%	6054
August	1st to 10th	Day	1 1 1 1 1 1 1 3 4 86%	5346
	11th to 20th	Day	1 1 1 2 1 1 1 1 2 3 87%	5364
	21st to end	Day	1 1 2 2 2 2 2 3 5% 81%	5984
September	1st to 10th	Day	2 1 1 3 1 1 2 2 3 6% 78%	2400
	11th to 20th	Day	3 2 1 2 2 1 2 1 3 4 80%	2235
	21st to end	Day	2 2 1 2 2 1 2 2 4 4 78%	2200
October	1st to 10th	Day	3 1 1 2 2 2 2 2 3 4 80%	3600
	11th to 20th	Day	3 3 1 3 1 2 2 2 3 5% 74%	3404
	21st to end	Day	3 2 1 3 2 3 4 3 5% 6% 68%	3337
November	1st to 10th	Day	3 3 1 2 2 2 2 2 4 6% 73%	1764
	11th to 20th	Day	5% 2 1 2 2 2 3 2 4 5% 72%	1800
	21st to end	Day	5% 4 1 4 3 4 3 3 4 7% 62%	1800
December	1st to 10th	Day	4 2 1 3 3 2 4 3 4 8% 66%	2950
	11th to 20th	Day	9% 4 1 4 3 3 5% 4 7% 8% 52%	3409
	21st to end	Day	9% 5% 2 6% 4 4 4 3 6% 8% 49%	4490
January	1st to 10th	Night	5% 3 1 4 2 2 3 2 3 4 70%	6286
	11th to 20th	Night	5% 4 1 4 2 2 3 2 4 6% 65%	5724
	21st to end	Night	13% 7% 2 4 3 3 3 4 5% 5% 52%	5984
February	1st to 10th	Night	10% 5% 2 4 4 3 3 3 3 4 61%	2400
	11th to 20th	Night	9% 5% 1 3 3 2 3 3 4 6% 60%	2338
	21st to end	Night	11% 5% 1 4 3 3 4 4 4 5% 57%	1980
March	1st to 10th	Night	14% 7% 2 5% 3 3 3 3 6% 7% 47%	5376
	11th to 20th	Night	8% 6% 2 5% 4 3 4 3 5% 7% 54%	5335
	21st to end	Night	12% 9% 2 4 3 3 3 2 5% 6% 51%	5006
April	1st to 10th	Night	8% 6% 2 4 3 3 3 3 4 6% 59%	2000
	11th to 20th	Night	6% 8% 2 4 2 2 3 2 3 5% 63%	1979
	21st to end	Night	6% 7% 2 5% 3 2 3 2 4 6% 60%	2000
May	1st to 10th	Night	8% 10% 3 5% 3 2 2 1 3 4 59%	4340
	11th to 20th	Night	4 9% 4 6% 4 2 3 2 4 5% 57%	4074
	21st to end	Night	4 9% 3 4 2 2 2 2 3 5% 64%	4474
June	1st to 10th	Night	4 9% 2 6% 3 2 3 3 6% 6% 58%	1800
	11th to 20th	Night	3 5% 2 4 2 2 2 2 4 6% 68%	1645
	21st to end	Night	1 4 2 4 2 2 2 2 5% 6% 70%	1600
July	1st to 10th	Night	2 1 3 2 2 1 1 2 5% 82%	3640
	11th to 20th	Night	1 1 1 4 3 2 3 3 5% 6% 72%	3842
	21st to end	Night	1 2 1 3 2 1 2 2 4 6% 76%	4830
August	1st to 10th	Night	1 1 1 1 1 2 1 3 5% 83%	4486
	11th to 20th	Night	1 2 1 4 2 1 1 1 3 5% 79%	4506
	21st to end	Night	1 2 1 4 2 2 2 1 4 6% 76%	5015
September	1st to 10th	Night	4 3 1 4 2 2 2 1 3 4 74%	2000
	11th to 20th	Night	2 4 1 2 2 1 1 2 3 4 78%	2165
	21st to end	Night	2 3 1 3 2 2 1 2 3 5% 78%	2200
October	1st to 10th	Night	2 2 1 2 2 2 2 2 3 3 80%	3600
	11th to 20th	Night	4 10% 2 2 2 2 3 2 4 6% 64%	3707
	21st to end	Night	3 3 1 4 2 3 3 2 3 5% 73%	4226
November	1st to 10th	Night	4 3 1 3 2 3 2 2 3 6% 70%	2535
	11th to 20th	Night	4 3 2 3 2 2 3 2 4 6% 70%	2600
	21st to end	Night	6% 4 1 4 3 3 3 3 6% 8% 59%	2600
December	1st to 10th	Night	5% 3 1 4 3 2 3 3 4 7% 65%	4240
	11th to 20th	Night	9% 4 2 5% 4 4 5% 4 5% 7% 52%	4811
	21st to end	Night	9% 6% 2 6% 3 3 4 4 7% 10% 45%	6468

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	17% 2 6% 19% 30% 26%	3639
	11th to 20th	Day	18% 4 11% 18% 17% 32%	4232
	21st to end	Day	30% 4 6% 15% 23% 23%	5016
February	1st to 10th	Day	26% 3 6% 14% 24% 26%	2000
	11th to 20th	Day	24% 4 5% 18% 25% 24%	1960
	21st to end	Day	28% 6% 7% 16% 23% 20%	1650
March	1st to 10th	Day	30% 7% 10% 16% 22% 16%	4518
	11th to 20th	Day	28% 6% 10% 26% 20% 10%	4665
	21st to end	Day	25% 6% 12% 29% 17% 10%	5964
April	1st to 10th	Day	22% 6% 10% 25% 22% 15%	2400
	11th to 20th	Day	22% 5% 6% 22% 28% 18%	2372
	21st to end	Day	20% 4 7% 26% 31% 12%	2400
May	1st to 10th	Day	21% 5% 7% 20% 32% 16%	5647
	11th to 20th	Day	21% 6% 7% 22% 35% 9%	5844
	21st to end	Day	19% 5% 6% 25% 34% 10%	6424
June	1st to 10th	Day	23% 5% 8% 21% 29% 14%	2600
	11th to 20th	Day	16% 2 4 14% 32% 31%	2747
	21st to end	Day	15% 2 3 9% 28% 43%	2800
July	1st to 10th	Day	6% 1 2 5% 20% 65%	6251
	11th to 20th	Day	12% 2 5% 10% 37% 34%	6035
	21st to end	Day	9% 2 4 15% 34% 36%	5880
August	1st to 10th	Day	8% 1 2 12% 43% 33%	3890
	11th to 20th	Day	6% 1 2 12% 29% 50%	3880
	21st to end	Day	7% 2 3 9% 29% 51%	4311
September	1st to 10th	Day	11% 3 4 10% 33% 40%	2400
	11th to 20th	Day	12% 2 3 11% 35% 36%	2235
	21st to end	Day	11% 3 3 17% 31% 36%	2200
October	1st to 10th	Day	11% 3 3 11% 18% 55%	3600
	11th to 20th	Day	15% 2 4 14% 35% 31%	3404
	21st to end	Day	16% 2 5% 9% 27% 40%	3337
November	1st to 10th	Day	14% 3 5% 16% 25% 36%	1764
	11th to 20th	Day	16% 4 6% 16% 19% 40%	1800
	21st to end	Day	23% 4 7% 12% 22% 32%	1800
December	1st to 10th	Day	18% 3 5% 18% 21% 35%	2950
	11th to 20th	Day	28% 5% 8% 14% 25% 19%	3408
	21st to end	Day	33% 5% 9% 23% 20% 9%	4490
January	1st to 10th	Night	19% 3 4 15% 32% 26%	6286
	11th to 20th	Night	22% 5% 10% 19% 16% 29%	5724
	21st to end	Night	33% 5% 7% 13% 21% 22%	5984
February	1st to 10th	Night	27% 4 6% 15% 22% 26%	2400
	11th to 20th	Night	25% 5% 7% 19% 20% 24%	2339
	21st to end	Night	29% 5% 6% 15% 21% 23%	1980
March	1st to 10th	Night	32% 7% 11% 18% 20% 12%	5376
	11th to 20th	Night	29% 7% 10% 26% 18% 11%	5335
	21st to end	Night	31% 8% 14% 22% 18% 7%	5006
April	1st to 10th	Night	26% 5% 11% 22% 20% 14%	2000
	11th to 20th	Night	23% 5% 9% 23% 24% 16%	1979
	21st to end	Night	25% 6% 10% 25% 25% 10%	2000
May	1st to 10th	Night	29% 5% 9% 20% 28% 8%	4337
	11th to 20th	Night	27% 7% 11% 26% 24% 5%	4075
	21st to end	Night	21% 7% 10% 30% 23% 9%	4474
June	1st to 10th	Night	28% 6% 11% 21% 24% 11%	1800
	11th to 20th	Night	18% 4 6% 18% 31% 23%	1645
	21st to end	Night	18% 3 4 12% 30% 34%	1600
July	1st to 10th	Night	9% 1 2 10% 17% 60%	3600
	11th to 20th	Night	14% 2 8% 12% 36% 27%	3794
	21st to end	Night	12% 2 6% 18% 33% 28%	4692
August	1st to 10th	Night	10% 2 3 15% 43% 27%	3256
	11th to 20th	Night	8% 2 3 15% 33% 39%	3256
	21st to end	Night	11% 2 4 8% 33% 42%	3608
September	1st to 10th	Night	17% 2 4 10% 32% 35%	2000
	11th to 20th	Night	13% 2 4 13% 33% 35%	2165
	21st to end	Night	13% 3 5% 14% 33% 32%	2200
October	1st to 10th	Night	11% 3 4 11% 19% 52%	3600
	11th to 20th	Night	23% 3 5% 12% 29% 28%	3707
	21st to end	Night	16% 3 3 9% 30% 39%	4226
November	1st to 10th	Night	17% 4 5% 16% 23% 35%	2535
	11th to 20th	Night	17% 4 5% 15% 20% 39%	2600
	21st to end	Night	24% 4 8% 16% 18% 31%	2600
December	1st to 10th	Night	20% 3 6% 14% 23% 34%	4240
	11th to 20th	Night	31% 5% 8% 13% 22% 20%	4811
	21st to end	Night	31% 7% 13% 23% 17% 9%	6468

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	13% 5% 28% 37% 17% 2	200
	11th to 20th	Day	11% 7% 27% 34% 20% 2	393
	21st to end	Day	9% 11% 31% 32% 14% 2	550
February	1st to 10th	Day	13% 9% 30% 31% 13% 4	500
	11th to 20th	Day	13% 9% 33% 33% 10% 2	495
	21st to end	Day	11% 11% 34% 34% 10%	414
March	1st to 10th	Day	14% 14% 31% 28% 12% 2	500
	11th to 20th	Day	14% 13% 29% 30% 13% 2	500
	21st to end	Day	10% 15% 36% 25% 11% 2	550
April	1st to 10th	Day	8% 12% 31% 28% 16% 6%	500
	11th to 20th	Day	5% 15% 37% 27% 13% 3	492
	21st to end	Day	5% 14% 29% 30% 14% 8%	500
May	1st to 10th	Day	10% 12% 31% 29% 15% 3	503
	11th to 20th	Day	7% 11% 30% 31% 16% 5%	528
	21st to end	Day	6% 11% 31% 31% 17% 3	573
June	1st to 10th	Day	3 7% 22% 38% 23% 7%	530
	11th to 20th	Day	4 5% 25% 38% 20% 8%	530
	21st to end	Day	2 3 22% 43% 21% 8%	530
July	1st to 10th	Day	2 2 16% 44% 26% 11%	526
	11th to 20th	Day	1 10% 45% 28% 16%	524
	21st to end	Day	1 2 13% 45% 24% 15%	575
August	1st to 10th	Day	2 1 15% 47% 22% 13%	527
	11th to 20th	Day	2 1 12% 47% 24% 14%	517
	21st to end	Day	2 2 17% 44% 25% 11%	583
September	1st to 10th	Day	2 2 22% 43% 18% 12%	530
	11th to 20th	Day	3 5% 25% 41% 18% 8%	530
	21st to end	Day	2 6% 28% 32% 23% 9%	530
October	1st to 10th	Day	4 8% 25% 34% 23% 6%	530
	11th to 20th	Day	4 8% 26% 35% 22% 5%	523
	21st to end	Day	4 7% 22% 36% 22% 8%	477
November	1st to 10th	Day	5% 5% 17% 39% 29% 6%	300
	11th to 20th	Day	5% 4 20% 41% 24% 6%	300
	21st to end	Day	6% 5% 25% 38% 21% 5%	300
December	1st to 10th	Day	9% 3 14% 45% 24% 5%	300
	11th to 20th	Day	6% 5% 22% 39% 23% 4	300
	21st to end	Day	14% 6% 23% 40% 15% 2	327
January	1st to 10th	Night	10% 4 16% 44% 22% 5%	800
	11th to 20th	Night	9% 3 17% 43% 26% 3	800
	21st to end	Night	13% 3 16% 43% 21% 3	880
February	1st to 10th	Night	11% 4 16% 42% 22% 5%	800
	11th to 20th	Night	10% 3 18% 43% 21% 4	784
	21st to end	Night	13% 5% 16% 44% 19% 3	660
March	1st to 10th	Night	9% 4 19% 48% 19% 3	800
	11th to 20th	Night	11% 3 17% 40% 24% 6%	800
	21st to end	Night	9% 4 19% 43% 22% 2	880
April	1st to 10th	Night	9% 4 14% 42% 26% 4	800
	11th to 20th	Night	7% 6% 16% 41% 27% 3	791
	21st to end	Night	7% 6% 16% 37% 28% 7%	800
May	1st to 10th	Night	8% 6% 14% 37% 31% 4	710
	11th to 20th	Night	7% 4 16% 34% 36% 3	591
	21st to end	Night	6% 6% 16% 39% 30% 3	650
June	1st to 10th	Night	4 3 16% 40% 35% 3	600
	11th to 20th	Night	3 2 6% 37% 47% 5%	445
	21st to end	Night	3 1 3 34% 54% 5%	400
July	1st to 10th	Night	2 1 1 15% 71% 12%	390
	11th to 20th	Night	2 3 21% 62% 12%	473
	21st to end	Night	2 1 4 24% 56% 13%	810
August	1st to 10th	Night	2 1 4 26% 53% 13%	796
	11th to 20th	Night	1 1 2 23% 55% 18%	792
	21st to end	Night	2 1 3 23% 54% 17%	880
September	1st to 10th	Night	5% 2 6% 26% 47% 15%	800
	11th to 20th	Night	3 2 7% 32% 46% 11%	800
	21st to end	Night	3 1 7% 35% 45% 9%	800
October	1st to 10th	Night	4 1 9% 43% 39% 5%	800
	11th to 20th	Night	5% 3 10% 38% 37% 7%	792
	21st to end	Night	4 2 12% 38% 32% 11%	836
November	1st to 10th	Night	5% 1 10% 42% 33% 9%	780
	11th to 20th	Night	4 1 11% 45% 32% 8%	800
	21st to end	Night	7% 3 13% 46% 27% 4	800
December	1st to 10th	Night	9% 3 13% 43% 28% 5%	800
	11th to 20th	Night	13% 3 15% 42% 22% 4	800
	21st to end	Night	10% 3 21% 43% 19% 4	880

Page generated at 2025-10-15 22:50:29. Version ac07

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