Mountain Skies Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Mountain Skies Observatory clear sky chart from 2005-11-06 to 2024-11-02.

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	4 6% 2 6% 4 4 5% 4 6% 7% 51%	100862
	Night	4 6% 2 6% 5% 4 5% 4 6% 7% 50%	98362
Transparency	Day	29% 7% 12% 18% 20% 15%	100627
	Night	31% 7% 12% 20% 18% 12%	98186
Seeing	Day	7% 6% 19% 32% 27% 8%	14753
	Night	8% 4 16% 41% 27% 3	25294

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	4 10% 5% 14% 9% 7% 7% 6% 8% 6% 24%	2858
	11th to 20th	Day	5% 14% 7% 13% 7% 6% 6% 5% 6% 7% 25%	2860
	21st to end	Day	5% 12% 5% 10% 7% 6% 5% 6% 6% 7% 31%	3146
February	1st to 10th	Day	5% 10% 3 10% 5% 7% 7% 6% 7% 5% 35%	1710
	11th to 20th	Day	4 10% 3 10% 7% 6% 5% 5% 6% 7% 36%	1749
	21st to end	Day	5% 11% 4 9% 6% 6% 6% 4 6% 7% 36%	1570
March	1st to 10th	Day	3 9% 3 9% 6% 8% 6% 4 6% 7% 39%	3347
	11th to 20th	Day	4 9% 3 6% 5% 4 5% 4 7% 8% 45%	3490
	21st to end	Day	6% 14% 4 11% 8% 7% 6% 5% 7% 8% 26%	3825
April	1st to 10th	Day	5% 8% 3 7% 5% 6% 5% 4 6% 7% 43%	2096
	11th to 20th	Day	5% 7% 3 7% 5% 5% 5% 5% 6% 6% 46%	2405
	21st to end	Day	5% 6% 3 6% 5% 5% 5% 4 5% 8% 49%	2470
May	1st to 10th	Day	6% 9% 2 7% 5% 6% 5% 5% 6% 8% 41%	4110
	11th to 20th	Day	4 3 1 3 3 4 4 4 6% 9% 59%	4071
	21st to end	Day	4 4 2 5% 4 4 5% 5% 7% 8% 52%	4477
June	1st to 10th	Day	2 3 1 5% 3 3 4 4 7% 10% 57%	2470
	11th to 20th	Day	4 4 1 3 3 3 4 3 4 6% 65%	2470
	21st to end	Day	2 1 1 2 3 3 3 3 4 6% 73%	2470
July	1st to 10th	Day	1 1 2 2 2 3 2 4 5% 77%	4110
	11th to 20th	Day	2 2 1 3 3 4 5% 5% 6% 9% 59%	4110
	21st to end	Day	2 2 1 3 3 3 4 4 5% 7% 67%	4454
August	1st to 10th	Day	2 2 1 3 3 3 4 4 6% 8% 64%	4092
	11th to 20th	Day	2 1 1 3 3 3 4 4 6% 6% 67%	4028
	21st to end	Day	2 2 1 3 2 2 3 3 5% 6% 71%	4495
September	1st to 10th	Day	3 2 1 2 2 3 4 3 5% 6% 70%	2126
	11th to 20th	Day	4 2 1 3 3 3 4 3 6% 6% 65%	2090
	21st to end	Day	5% 2 1 3 2 3 2 2 4 6% 70%	2090
October	1st to 10th	Day	6% 5% 1 5% 3 4 5% 3 6% 6% 57%	2008
	11th to 20th	Day	5% 4 2 5% 3 3 3 4 4 6% 62%	1900
	21st to end	Day	4 4 1 5% 3 3 5% 4 6% 8% 55%	2081
November	1st to 10th	Day	7% 5% 2 6% 4 4 5% 5% 6% 7% 48%	1683
	11th to 20th	Day	6% 7% 3 6% 4 4 4 4 7% 7% 48%	1710
	21st to end	Day	3 7% 4 9% 6% 5% 6% 6% 6% 8% 40%	1635
December	1st to 10th	Day	6% 8% 3 9% 6% 6% 6% 4 6% 9% 38%	1520
	11th to 20th	Day	6% 10% 3 8% 7% 6% 7% 6% 8% 8% 31%	1990
	21st to end	Day	5% 10% 3 11% 6% 6% 7% 6% 8% 9% 30%	3146
January	1st to 10th	Night	5% 9% 3 11% 8% 7% 6% 5% 6% 8% 31%	4097
	11th to 20th	Night	5% 15% 5% 12% 6% 6% 6% 5% 7% 8% 23%	4110
	21st to end	Night	4 11% 4 10% 7% 7% 7% 7% 8% 8% 27%	4521
February	1st to 10th	Night	4 8% 4 7% 6% 6% 7% 6% 8% 10% 35%	2470
	11th to 20th	Night	4 9% 3 8% 5% 6% 6% 5% 5% 8% 42%	2316
	21st to end	Night	3 10% 3 8% 5% 5% 6% 4 6% 9% 40%	1884
March	1st to 10th	Night	3 7% 3 9% 6% 8% 8% 6% 7% 8% 36%	3526
	11th to 20th	Night	3 6% 3 6% 4 4 5% 5% 6% 7% 52%	3490
	21st to end	Night	6% 10% 4 9% 7% 5% 5% 4 6% 7% 38%	3828
April	1st to 10th	Night	7% 8% 2 6% 5% 4 4 3 5% 7% 50%	2083
	11th to 20th	Night	4 6% 2 5% 5% 4 6% 5% 6% 8% 49%	1720
	21st to end	Night	7% 5% 2 5% 4 4 5% 4 6% 6% 51%	1710
May	1st to 10th	Night	5% 7% 2 6% 5% 5% 5% 5% 6% 8% 45%	2860
	11th to 20th	Night	3 4 1 4 3 3 4 3 5% 7% 62%	2834
	21st to end	Night	5% 5% 2 5% 4 3 4 3 5% 9% 56%	3110
June	1st to 10th	Night	4 4 2 3 4 5% 4 3 6% 7% 58%	1710
	11th to 20th	Night	3 3 1 3 2 3 4 3 4 7% 67%	1710
	21st to end	Night	1 2 1 3 3 3 4 3 5% 8% 67%	1710
July	1st to 10th	Night	1 1 1 3 3 3 3 2 4 5% 73%	2860
	11th to 20th	Night	2 3 1 4 3 5% 6% 4 7% 9% 56%	2860
	21st to end	Night	2 3 1 4 4 4 4 4 6% 6% 61%	3109
August	1st to 10th	Night	2 1 1 4 3 3 4 4 7% 9% 62%	2847
	11th to 20th	Night	2 2 1 5% 3 3 5% 4 6% 7% 64%	2811
	21st to end	Night	3 2 1 3 2 3 3 3 4 6% 70%	3172
September	1st to 10th	Night	3 3 1 4 2 3 2 3 4 6% 68%	2050
	11th to 20th	Night	3 3 1 4 3 3 3 3 5% 7% 66%	2090
	21st to end	Night	5% 2 1 3 2 2 3 2 3 5% 72%	2090
October	1st to 10th	Night	5% 4 2 5% 3 3 4 3 4 6% 62%	2171
	11th to 20th	Night	4 3 1 4 3 3 4 3 4 6% 64%	2280
	21st to end	Night	4 4 1 5% 3 3 3 4 5% 7% 62%	2515
November	1st to 10th	Night	6% 6% 2 6% 4 3 4 4 5% 8% 51%	2418
	11th to 20th	Night	5% 5% 2 5% 3 4 5% 4 7% 8% 53%	2470
	21st to end	Night	5% 7% 2 6% 5% 4 5% 5% 6% 8% 46%	2519
December	1st to 10th	Night	5% 7% 3 8% 6% 5% 5% 4 6% 8% 45%	2660
	11th to 20th	Night	7% 10% 3 8% 5% 5% 6% 5% 7% 9% 34%	3230
	21st to end	Night	5% 10% 4 10% 6% 7% 8% 5% 8% 9% 28%	4521

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	50% 15% 16% 15% 5%	2859
	11th to 20th	Day	56% 11% 16% 12% 4 1	2860
	21st to end	Day	48% 14% 18% 13% 6% 1	3146
February	1st to 10th	Day	49% 9% 14% 16% 10% 1	1710
	11th to 20th	Day	44% 9% 14% 21% 11% 1	1758
	21st to end	Day	44% 8% 16% 20% 11% 1	1570
March	1st to 10th	Day	36% 11% 14% 26% 11% 2	3347
	11th to 20th	Day	32% 7% 15% 32% 12% 2	3490
	21st to end	Day	49% 10% 16% 16% 8% 2	3825
April	1st to 10th	Day	36% 8% 14% 19% 16% 6%	2096
	11th to 20th	Day	34% 6% 13% 22% 20% 5%	2408
	21st to end	Day	30% 7% 12% 25% 23% 3	2470
May	1st to 10th	Day	33% 6% 14% 20% 20% 7%	4110
	11th to 20th	Day	19% 5% 13% 27% 31% 5%	4074
	21st to end	Day	25% 7% 13% 22% 28% 6%	4477
June	1st to 10th	Day	21% 5% 9% 19% 32% 13%	2470
	11th to 20th	Day	19% 4 8% 15% 29% 25%	2470
	21st to end	Day	14% 2 6% 10% 27% 42%	2470
July	1st to 10th	Day	9% 2 5% 7% 14% 62%	4046
	11th to 20th	Day	18% 3 9% 10% 24% 36%	4041
	21st to end	Day	14% 4 8% 10% 23% 41%	4314
August	1st to 10th	Day	17% 4 8% 17% 27% 28%	4078
	11th to 20th	Day	16% 4 7% 13% 32% 28%	4067
	21st to end	Day	14% 3 6% 14% 28% 34%	4494
September	1st to 10th	Day	14% 3 8% 14% 28% 33%	2126
	11th to 20th	Day	16% 5% 9% 18% 27% 24%	2090
	21st to end	Day	16% 4 7% 19% 31% 22%	2090
October	1st to 10th	Day	26% 6% 10% 18% 29% 11%	2008
	11th to 20th	Day	24% 4 9% 28% 29% 7%	1900
	21st to end	Day	25% 7% 15% 28% 22% 3	2081
November	1st to 10th	Day	34% 7% 13% 23% 19% 4	1683
	11th to 20th	Day	35% 8% 13% 28% 16% 1	1710
	21st to end	Day	39% 9% 16% 21% 13% 2	1635
December	1st to 10th	Day	44% 9% 18% 20% 8% 1	1520
	11th to 20th	Day	48% 13% 13% 19% 6%	1988
	21st to end	Day	45% 11% 18% 14% 11%	3146
January	1st to 10th	Night	44% 13% 17% 21% 6%	4097
	11th to 20th	Night	55% 13% 14% 12% 4 1	4110
	21st to end	Night	46% 14% 19% 15% 6% 1	4521
February	1st to 10th	Night	42% 12% 18% 19% 8% 1	2470
	11th to 20th	Night	40% 6% 14% 24% 14% 1	2317
	21st to end	Night	40% 7% 14% 25% 12% 2	1884
March	1st to 10th	Night	41% 8% 14% 26% 10% 1	3526
	11th to 20th	Night	28% 6% 13% 35% 16% 2	3490
	21st to end	Night	41% 11% 12% 20% 14% 2	3828
April	1st to 10th	Night	34% 5% 12% 24% 18% 7%	2083
	11th to 20th	Night	30% 7% 10% 24% 25% 4	1720
	21st to end	Night	31% 7% 13% 24% 23% 3	1710
May	1st to 10th	Night	31% 6% 12% 20% 25% 5%	2860
	11th to 20th	Night	20% 5% 8% 29% 29% 9%	2835
	21st to end	Night	25% 6% 11% 25% 29% 4	3110
June	1st to 10th	Night	23% 5% 10% 20% 29% 14%	1710
	11th to 20th	Night	17% 4 8% 16% 31% 24%	1710
	21st to end	Night	15% 3 6% 9% 26% 41%	1710
July	1st to 10th	Night	15% 2 5% 7% 10% 61%	2815
	11th to 20th	Night	23% 3 9% 9% 24% 31%	2806
	21st to end	Night	23% 4 8% 11% 22% 33%	3011
August	1st to 10th	Night	20% 5% 9% 14% 25% 27%	2838
	11th to 20th	Night	19% 6% 7% 15% 26% 27%	2838
	21st to end	Night	16% 3 7% 9% 29% 35%	3172
September	1st to 10th	Night	17% 3 8% 13% 25% 34%	2050
	11th to 20th	Night	18% 5% 8% 17% 27% 26%	2090
	21st to end	Night	18% 3 4 16% 33% 26%	2090
October	1st to 10th	Night	22% 5% 8% 21% 32% 12%	2171
	11th to 20th	Night	22% 5% 8% 26% 31% 8%	2280
	21st to end	Night	21% 6% 12% 30% 25% 5%	2515
November	1st to 10th	Night	29% 6% 11% 27% 22% 5%	2418
	11th to 20th	Night	28% 8% 15% 30% 16% 3	2470
	21st to end	Night	33% 9% 14% 27% 14% 3	2519
December	1st to 10th	Night	37% 8% 18% 24% 10% 2	2660
	11th to 20th	Night	45% 11% 16% 20% 8%	3231
	21st to end	Night	48% 9% 18% 16% 8%	4521

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% 9% 17% 31% 18% 9%	270
	11th to 20th	Day	10% 7% 21% 35% 21% 7%	270
	21st to end	Day	12% 5% 17% 37% 23% 7%	297
February	1st to 10th	Day	13% 11% 23% 34% 14% 4	270
	11th to 20th	Day	10% 8% 24% 34% 20% 3	354
	21st to end	Day	8% 9% 27% 38% 16% 1	382
March	1st to 10th	Day	12% 9% 23% 36% 15% 5%	460
	11th to 20th	Day	8% 8% 28% 30% 21% 4	460
	21st to end	Day	15% 12% 27% 30% 14% 1	506
April	1st to 10th	Day	11% 10% 26% 29% 19% 4	460
	11th to 20th	Day	11% 12% 29% 29% 17% 3	452
	21st to end	Day	11% 12% 24% 29% 19% 4	460
May	1st to 10th	Day	8% 10% 27% 34% 16% 6%	463
	11th to 20th	Day	8% 6% 23% 32% 24% 8%	488
	21st to end	Day	7% 7% 21% 25% 29% 11%	529
June	1st to 10th	Day	3 7% 18% 29% 34% 9%	490
	11th to 20th	Day	6% 6% 21% 34% 25% 7%	490
	21st to end	Day	2 3 22% 34% 31% 7%	490
July	1st to 10th	Day	1 2 15% 36% 34% 11%	486
	11th to 20th	Day	4 2 12% 34% 37% 11%	484
	21st to end	Day	3 9% 31% 44% 13%	531
August	1st to 10th	Day	5% 2 12% 35% 37% 10%	487
	11th to 20th	Day	3 3 14% 31% 36% 14%	477
	21st to end	Day	4 3 13% 29% 40% 10%	539
September	1st to 10th	Day	3 3 12% 31% 34% 17%	490
	11th to 20th	Day	4 6% 18% 28% 31% 13%	490
	21st to end	Day	6% 5% 16% 24% 36% 14%	490
October	1st to 10th	Day	5% 5% 20% 25% 27% 17%	408
	11th to 20th	Day	7% 5% 15% 31% 28% 14%	300
	21st to end	Day	7% 6% 20% 35% 26% 6%	330
November	1st to 10th	Day	13% 8% 14% 30% 24% 11%	273
	11th to 20th	Day	8% 7% 14% 38% 23% 10%	270
	21st to end	Day	10% 5% 16% 31% 28% 9%	270
December	1st to 10th	Day	9% 8% 13% 36% 24% 10%	270
	11th to 20th	Day	11% 7% 13% 44% 21% 4	270
	21st to end	Day	15% 8% 19% 35% 21% 2	297
January	1st to 10th	Night	13% 5% 14% 41% 22% 5%	760
	11th to 20th	Night	12% 5% 17% 36% 25% 5%	760
	21st to end	Night	12% 5% 14% 41% 23% 4	836
February	1st to 10th	Night	10% 6% 19% 47% 15% 4	760
	11th to 20th	Night	11% 4 18% 46% 19% 2	744
	21st to end	Night	10% 5% 21% 45% 18% 1	628
March	1st to 10th	Night	9% 7% 20% 45% 17% 2	760
	11th to 20th	Night	9% 4 18% 44% 24% 2	760
	21st to end	Night	11% 6% 22% 44% 15% 2	836
April	1st to 10th	Night	13% 8% 20% 40% 18% 2	753
	11th to 20th	Night	6% 9% 23% 47% 14% 1	563
	21st to end	Night	9% 5% 21% 40% 23% 2	570
May	1st to 10th	Night	8% 6% 23% 41% 20% 1	570
	11th to 20th	Night	6% 4 15% 41% 31% 3	561
	21st to end	Night	8% 6% 17% 36% 29% 4	617
June	1st to 10th	Night	5% 6% 16% 40% 31% 2	570
	11th to 20th	Night	4 6% 21% 42% 26% 1	570
	21st to end	Night	2 5% 14% 43% 34% 2	570
July	1st to 10th	Night	3 1 13% 37% 41% 5%	555
	11th to 20th	Night	4 2 9% 43% 39% 4	552
	21st to end	Night	5% 1 6% 32% 53% 3	624
August	1st to 10th	Night	4 1 11% 41% 41% 2	567
	11th to 20th	Night	3 1 9% 44% 39% 4	564
	21st to end	Night	3 2 10% 39% 44% 2	627
September	1st to 10th	Night	5% 2 9% 38% 40% 6%	720
	11th to 20th	Night	4 2 14% 37% 36% 7%	760
	21st to end	Night	7% 3 11% 32% 38% 8%	760
October	1st to 10th	Night	7% 3 15% 39% 29% 6%	760
	11th to 20th	Night	6% 3 14% 40% 31% 5%	760
	21st to end	Night	6% 4 17% 42% 28% 4	836
November	1st to 10th	Night	11% 4 16% 35% 29% 4	744
	11th to 20th	Night	7% 4 16% 48% 20% 5%	760
	21st to end	Night	9% 4 16% 41% 24% 6%	809
December	1st to 10th	Night	11% 4 15% 45% 21% 3	950
	11th to 20th	Night	16% 3 16% 44% 18% 2	922
	21st to end	Night	15% 6% 18% 44% 16% 2	836

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