CMS Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the CMS Observatory clear sky chart from 2005-11-06 to 2026-01-17.

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 5% 2 6% 4 5% 5% 4 7% 8% 49%	137082
	Night	4 6% 2 6% 4 4 4 4 6% 8% 51%	132107
Transparency	Day	30% 6% 13% 27% 19% 7%	131847
	Night	28% 5% 14% 29% 18% 5%	128346
Seeing	Day	9% 11% 33% 32% 13% 3	17431
	Night	7% 6% 28% 44% 14% 2	27310

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	1 8% 4 9% 5% 6% 6% 4 8% 8% 41%	3718
	11th to 20th	Day	5% 4 12% 6% 5% 5% 4 7% 9% 43%	3889
	21st to end	Day	1 4 2 6% 4 4 5% 3 5% 7% 59%	4510
February	1st to 10th	Day	2 7% 3 9% 7% 5% 5% 4 6% 11% 40%	1800
	11th to 20th	Day	1 9% 4 9% 5% 5% 6% 4 6% 9% 42%	1755
	21st to end	Day	2 7% 3 8% 5% 5% 5% 5% 8% 8% 44%	1515
March	1st to 10th	Day	2 7% 3 6% 3 3 5% 4 6% 10% 50%	4923
	11th to 20th	Day	3 7% 3 6% 5% 5% 5% 4 6% 8% 49%	5000
	21st to end	Day	6% 9% 3 7% 5% 6% 7% 5% 8% 10% 33%	5486
April	1st to 10th	Day	6% 10% 2 7% 5% 5% 5% 5% 7% 8% 41%	2390
	11th to 20th	Day	7% 9% 4 8% 6% 5% 5% 4 6% 7% 38%	2419
	21st to end	Day	11% 11% 3 8% 6% 5% 5% 5% 6% 8% 34%	2600
May	1st to 10th	Day	11% 10% 3 7% 5% 4 4 4 6% 7% 40%	5990
	11th to 20th	Day	9% 6% 2 6% 4 5% 6% 5% 7% 9% 42%	6298
	21st to end	Day	9% 7% 2 6% 5% 5% 6% 5% 7% 8% 41%	6925
June	1st to 10th	Day	8% 6% 2 6% 4 5% 6% 5% 8% 9% 41%	2800
	11th to 20th	Day	6% 5% 2 5% 4 5% 5% 5% 8% 9% 45%	2818
	21st to end	Day	3 3 1 3 3 3 5% 5% 8% 8% 58%	3000
July	1st to 10th	Day	2 2 1 5% 4 5% 7% 5% 7% 8% 55%	6515
	11th to 20th	Day	3 2 1 3 3 5% 6% 5% 8% 8% 56%	6340
	21st to end	Day	2 3 1 5% 5% 6% 6% 6% 8% 8% 50%	6637
August	1st to 10th	Day	4 3 2 5% 4 5% 5% 5% 8% 9% 50%	5779
	11th to 20th	Day	2 2 1 5% 4 5% 6% 6% 7% 8% 53%	5798
	21st to end	Day	4 4 1 5% 3 3 4 3 5% 7% 60%	6253
September	1st to 10th	Day	5% 2 1 4 3 3 4 3 5% 7% 63%	2400
	11th to 20th	Day	7% 3 1 4 3 4 3 4 5% 6% 60%	2315
	21st to end	Day	7% 5% 1 4 3 3 4 3 4 6% 59%	2155
October	1st to 10th	Day	6% 5% 1 4 3 3 3 2 4 5% 65%	3270
	11th to 20th	Day	7% 5% 2 4 3 3 3 2 4 6% 62%	3250
	21st to end	Day	4 7% 2 6% 4 4 4 4 6% 8% 53%	3548
November	1st to 10th	Day	2 7% 3 7% 5% 4 5% 4 6% 9% 48%	1728
	11th to 20th	Day	3 6% 3 6% 5% 4 4 4 7% 9% 49%	1680
	21st to end	Day	3 8% 3 7% 4 4 3 4 6% 9% 49%	1680
December	1st to 10th	Day	1 4 2 7% 5% 4 4 4 5% 7% 59%	2700
	11th to 20th	Day	1 4 2 6% 4 4 5% 5% 8% 13% 49%	3106
	21st to end	Day	3 7% 3 8% 6% 5% 5% 4 6% 8% 44%	4092
January	1st to 10th	Night	1 7% 3 7% 5% 5% 5% 4 8% 9% 45%	6467
	11th to 20th	Night	1 5% 4 12% 6% 5% 5% 4 6% 7% 44%	6269
	21st to end	Night	1 5% 3 8% 3 4 4 3 5% 7% 58%	6468
February	1st to 10th	Night	2 7% 3 10% 7% 6% 5% 4 7% 8% 41%	2600
	11th to 20th	Night	2 9% 4 9% 6% 5% 5% 4 6% 8% 41%	2532
	21st to end	Night	1 6% 3 7% 6% 6% 5% 4 6% 8% 47%	2113
March	1st to 10th	Night	4 9% 3 6% 3 4 4 3 5% 7% 52%	4967
	11th to 20th	Night	4 6% 2 6% 4 3 4 4 7% 11% 50%	5000
	21st to end	Night	6% 9% 3 8% 5% 5% 5% 4 7% 11% 37%	5489
April	1st to 10th	Night	7% 9% 4 6% 4 4 5% 5% 6% 7% 44%	2010
	11th to 20th	Night	7% 10% 3 9% 6% 5% 5% 4 5% 7% 38%	1926
	21st to end	Night	9% 11% 3 8% 5% 4 3 4 6% 7% 41%	1800
May	1st to 10th	Night	12% 9% 3 6% 3 3 3 3 4 7% 48%	3976
	11th to 20th	Night	9% 7% 2 6% 4 4 5% 4 6% 9% 44%	3617
	21st to end	Night	9% 6% 2 5% 5% 5% 5% 5% 8% 10% 41%	3972
June	1st to 10th	Night	8% 5% 2 6% 5% 5% 4 5% 7% 9% 44%	1600
	11th to 20th	Night	7% 4 2 5% 3 4 5% 3 6% 9% 53%	1571
	21st to end	Night	3 3 1 4 3 4 3 4 6% 8% 62%	1400
July	1st to 10th	Night	2 2 1 3 3 3 4 4 7% 8% 62%	3446
	11th to 20th	Night	3 2 1 3 3 4 4 5% 9% 10% 57%	3640
	21st to end	Night	2 2 1 4 3 3 4 3 6% 9% 63%	4233
August	1st to 10th	Night	4 3 2 4 4 4 4 4 6% 9% 56%	4033
	11th to 20th	Night	2 2 1 3 3 3 5% 4 8% 8% 59%	4051
	21st to end	Night	4 5% 2 4 3 3 3 3 6% 7% 59%	4724
September	1st to 10th	Night	5% 3 1 3 3 3 3 3 4 7% 65%	2000
	11th to 20th	Night	6% 4 2 4 3 2 3 3 6% 8% 59%	2080
	21st to end	Night	10% 4 2 3 2 3 3 3 4 6% 61%	2244
October	1st to 10th	Night	6% 4 1 3 2 2 3 2 4 6% 67%	3920
	11th to 20th	Night	8% 6% 2 6% 3 3 3 2 4 6% 56%	3898
	21st to end	Night	5% 7% 3 6% 3 4 4 3 5% 6% 54%	4252
November	1st to 10th	Night	4 6% 2 7% 4 4 4 4 6% 9% 50%	2770
	11th to 20th	Night	3 8% 3 7% 4 5% 4 4 6% 8% 49%	2940
	21st to end	Night	4 8% 2 6% 4 4 5% 4 6% 7% 51%	2940
December	1st to 10th	Night	1 4 2 6% 4 4 4 3 5% 7% 59%	4710
	11th to 20th	Night	1 5% 2 8% 4 4 5% 4 6% 9% 51%	5321
	21st to end	Night	2 6% 2 7% 5% 5% 5% 4 7% 9% 47%	7128

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	36% 6% 15% 28% 15% 1	3719
	11th to 20th	Day	35% 5% 17% 25% 16% 1	3889
	21st to end	Day	23% 4 18% 32% 19% 4	4510
February	1st to 10th	Day	37% 5% 19% 26% 12% 1	1800
	11th to 20th	Day	36% 7% 17% 23% 15% 3	1764
	21st to end	Day	33% 7% 16% 23% 17% 5%	1515
March	1st to 10th	Day	26% 6% 12% 31% 17% 7%	4923
	11th to 20th	Day	30% 5% 15% 29% 18% 2	5000
	21st to end	Day	40% 10% 17% 25% 6% 1	5486
April	1st to 10th	Day	38% 6% 15% 27% 11% 2	2390
	11th to 20th	Day	43% 6% 14% 27% 9% 1	2422
	21st to end	Day	47% 6% 13% 25% 9% 1	2600
May	1st to 10th	Day	42% 6% 11% 27% 12% 3	5990
	11th to 20th	Day	35% 7% 16% 25% 13% 3	6302
	21st to end	Day	36% 8% 14% 31% 10% 1	6926
June	1st to 10th	Day	35% 6% 19% 25% 13% 2	2800
	11th to 20th	Day	31% 7% 15% 22% 20% 5%	2818
	21st to end	Day	21% 4 14% 26% 23% 13%	3000
July	1st to 10th	Day	23% 6% 8% 25% 25% 12%	6447
	11th to 20th	Day	20% 5% 8% 23% 29% 16%	6265
	21st to end	Day	25% 6% 10% 19% 26% 14%	6446
August	1st to 10th	Day	31% 5% 9% 23% 19% 14%	4208
	11th to 20th	Day	24% 6% 8% 26% 25% 11%	4197
	21st to end	Day	19% 4 6% 22% 34% 15%	4508
September	1st to 10th	Day	21% 4 7% 24% 31% 14%	2400
	11th to 20th	Day	25% 4 9% 27% 28% 8%	2315
	21st to end	Day	27% 3 8% 28% 27% 7%	2155
October	1st to 10th	Day	23% 3 8% 18% 31% 17%	3270
	11th to 20th	Day	24% 4 10% 32% 22% 8%	3250
	21st to end	Day	28% 5% 9% 36% 17% 3	3548
November	1st to 10th	Day	33% 6% 12% 28% 18% 3	1728
	11th to 20th	Day	30% 5% 16% 28% 18% 3	1680
	21st to end	Day	32% 4 16% 29% 18% 2	1680
December	1st to 10th	Day	23% 4 15% 34% 20% 4	2700
	11th to 20th	Day	24% 5% 26% 32% 12% 1	3104
	21st to end	Day	35% 4 9% 29% 17% 6%	4092
January	1st to 10th	Night	30% 6% 18% 31% 14% 1	6467
	11th to 20th	Night	35% 5% 17% 27% 13% 3	6269
	21st to end	Night	25% 4 17% 33% 19% 3	6468
February	1st to 10th	Night	37% 5% 17% 28% 9% 3	2600
	11th to 20th	Night	38% 5% 16% 25% 13% 2	2533
	21st to end	Night	32% 4 16% 26% 17% 4	2113
March	1st to 10th	Night	30% 5% 9% 28% 21% 7%	4967
	11th to 20th	Night	26% 5% 15% 34% 19% 1	5000
	21st to end	Night	37% 7% 20% 27% 8% 1	5489
April	1st to 10th	Night	36% 7% 17% 27% 11% 2	2010
	11th to 20th	Night	43% 6% 14% 27% 10% 1	1926
	21st to end	Night	41% 5% 14% 31% 9%	1800
May	1st to 10th	Night	37% 4 13% 26% 17% 3	3973
	11th to 20th	Night	34% 6% 16% 29% 13% 2	3618
	21st to end	Night	33% 10% 17% 32% 9%	3972
June	1st to 10th	Night	33% 8% 19% 23% 15% 2	1600
	11th to 20th	Night	28% 5% 18% 26% 17% 7%	1571
	21st to end	Night	20% 4 13% 31% 22% 10%	1400
July	1st to 10th	Night	16% 5% 9% 33% 28% 9%	3406
	11th to 20th	Night	17% 5% 7% 28% 32% 11%	3592
	21st to end	Night	16% 4 9% 25% 31% 15%	4112
August	1st to 10th	Night	26% 4 11% 23% 23% 12%	2928
	11th to 20th	Night	22% 5% 9% 30% 22% 12%	2928
	21st to end	Night	17% 3 7% 24% 31% 18%	3400
September	1st to 10th	Night	19% 3 8% 25% 30% 15%	2000
	11th to 20th	Night	23% 5% 10% 29% 25% 9%	2080
	21st to end	Night	25% 3 7% 29% 28% 7%	2244
October	1st to 10th	Night	20% 3 8% 24% 30% 15%	3920
	11th to 20th	Night	30% 3 8% 31% 24% 4	3898
	21st to end	Night	30% 4 10% 30% 18% 8%	4252
November	1st to 10th	Night	29% 6% 14% 30% 17% 4	2770
	11th to 20th	Night	31% 4 17% 30% 15% 3	2940
	21st to end	Night	30% 4 17% 30% 17% 3	2940
December	1st to 10th	Night	22% 4 18% 34% 19% 3	4710
	11th to 20th	Night	27% 5% 19% 35% 13% 1	5322
	21st to end	Night	30% 6% 11% 32% 15% 6%	7128

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	8% 12% 32% 33% 13% 3	330
	11th to 20th	Day	3 9% 39% 35% 9% 5%	318
	21st to end	Day	5% 14% 34% 35% 11% 2	330
February	1st to 10th	Day	5% 18% 46% 27% 3	300
	11th to 20th	Day	5% 17% 44% 26% 8%	300
	21st to end	Day	11% 13% 38% 26% 11% 1	249
March	1st to 10th	Day	7% 17% 40% 30% 5%	482
	11th to 20th	Day	9% 18% 38% 26% 8% 1	500
	21st to end	Day	14% 17% 33% 29% 7% 1	550
April	1st to 10th	Day	14% 17% 39% 25% 5%	500
	11th to 20th	Day	16% 17% 37% 25% 4	492
	21st to end	Day	19% 13% 29% 27% 11% 1	500
May	1st to 10th	Day	16% 13% 29% 25% 15% 2	548
	11th to 20th	Day	19% 8% 24% 31% 15% 4	725
	21st to end	Day	16% 9% 23% 31% 18% 4	789
June	1st to 10th	Day	9% 9% 26% 34% 18% 4	730
	11th to 20th	Day	9% 13% 32% 32% 12% 2	730
	21st to end	Day	5% 8% 31% 36% 18% 3	730
July	1st to 10th	Day	3 6% 29% 35% 23% 5%	721
	11th to 20th	Day	3 5% 29% 43% 17% 2	718
	21st to end	Day	2 7% 33% 36% 19% 3	681
August	1st to 10th	Day	9% 5% 33% 36% 13% 4	527
	11th to 20th	Day	5% 5% 31% 38% 17% 4	517
	21st to end	Day	5% 8% 33% 35% 16% 3	583
September	1st to 10th	Day	5% 6% 34% 36% 14% 4	530
	11th to 20th	Day	10% 9% 33% 29% 15% 5%	530
	21st to end	Day	10% 11% 30% 31% 16% 3	485
October	1st to 10th	Day	12% 8% 26% 35% 15% 3	330
	11th to 20th	Day	11% 14% 26% 29% 14% 5%	330
	21st to end	Day	9% 14% 36% 29% 9% 3	363
November	1st to 10th	Day	8% 13% 39% 25% 10% 4	330
	11th to 20th	Day	9% 15% 32% 33% 9% 2	330
	21st to end	Day	10% 11% 38% 34% 6% 1	330
December	1st to 10th	Day	6% 12% 39% 32% 9% 3	330
	11th to 20th	Day	11% 11% 36% 36% 6%	330
	21st to end	Day	7% 9% 39% 35% 9% 1	363
January	1st to 10th	Night	3 9% 32% 44% 11% 1	1050
	11th to 20th	Night	3 9% 38% 35% 11% 4	966
	21st to end	Night	5% 6% 34% 44% 10% 2	880
February	1st to 10th	Night	5% 8% 39% 40% 8% 1	800
	11th to 20th	Night	6% 8% 35% 39% 11% 1	784
	21st to end	Night	4 10% 34% 40% 11% 1	660
March	1st to 10th	Night	5% 8% 34% 42% 10% 1	800
	11th to 20th	Night	6% 7% 32% 44% 11% 1	800
	21st to end	Night	9% 8% 31% 43% 10% 1	880
April	1st to 10th	Night	10% 7% 33% 40% 10%	610
	11th to 20th	Night	13% 7% 34% 38% 7% 1	593
	21st to end	Night	14% 5% 25% 41% 14% 1	600
May	1st to 10th	Night	17% 5% 21% 39% 18% 1	600
	11th to 20th	Night	13% 4 18% 43% 20% 2	591
	21st to end	Night	15% 5% 18% 41% 19% 2	650
June	1st to 10th	Night	8% 6% 21% 42% 21% 2	600
	11th to 20th	Night	8% 7% 29% 42% 14%	571
	21st to end	Night	4 3 16% 53% 24%	400
July	1st to 10th	Night	2 2 15% 53% 27% 1	495
	11th to 20th	Night	3 2 20% 54% 20%	582
	21st to end	Night	3 2 18% 52% 24% 2	657
August	1st to 10th	Night	5% 3 14% 54% 24% 1	597
	11th to 20th	Night	5% 2 12% 56% 24% 1	594
	21st to end	Night	5% 3 20% 45% 24% 2	660
September	1st to 10th	Night	7% 2 19% 51% 20% 2	600
	11th to 20th	Night	6% 5% 19% 49% 19% 3	680
	21st to end	Night	10% 3 21% 43% 19% 3	800
October	1st to 10th	Night	14% 3 21% 44% 16% 3	800
	11th to 20th	Night	9% 6% 27% 40% 15% 3	800
	21st to end	Night	7% 4 34% 42% 10% 2	880
November	1st to 10th	Night	6% 5% 33% 44% 10% 2	975
	11th to 20th	Night	5% 9% 37% 42% 5% 1	1050
	21st to end	Night	8% 6% 29% 46% 10%	1050
December	1st to 10th	Night	4 9% 33% 43% 10% 1	1050
	11th to 20th	Night	5% 7% 34% 44% 9% 1	1050
	21st to end	Night	6% 5% 36% 43% 8% 2	1155

Page generated at 2026-01-13 23:14:27. Version ac07

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