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 2024-11-25.

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% 6% 2 6% 4 4 5% 4 6% 8% 49%	105957
	Night	4 6% 2 6% 4 4 4 4 6% 8% 51%	102265
Transparency	Day	30% 6% 13% 26% 19% 7%	105706
	Night	29% 5% 14% 28% 18% 6%	102106
Seeing	Day	9% 10% 32% 33% 13% 3	15888
	Night	7% 6% 27% 44% 14% 2	25697

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 5% 4 8% 5% 5% 6% 4 8% 8% 46%	2538
	11th to 20th	Day	1 5% 2 7% 6% 6% 6% 5% 8% 10% 45%	2671
	21st to end	Day	1 5% 2 5% 3 4 4 3 5% 8% 58%	3146
February	1st to 10th	Day	2 8% 3 9% 7% 5% 5% 4 6% 10% 41%	1710
	11th to 20th	Day	1 9% 4 9% 5% 5% 6% 4 6% 9% 43%	1665
	21st to end	Day	2 7% 4 8% 5% 5% 5% 5% 8% 8% 43%	1441
March	1st to 10th	Day	1 5% 3 6% 4 3 5% 4 7% 11% 50%	3427
	11th to 20th	Day	3 8% 3 6% 5% 4 4 3 6% 7% 51%	3490
	21st to end	Day	7% 11% 3 8% 5% 6% 7% 5% 8% 11% 29%	3825
April	1st to 10th	Day	6% 10% 2 7% 5% 5% 5% 5% 7% 8% 40%	2270
	11th to 20th	Day	6% 9% 4 8% 6% 6% 5% 4 6% 7% 37%	2296
	21st to end	Day	11% 11% 3 7% 6% 5% 5% 5% 6% 8% 35%	2470
May	1st to 10th	Day	11% 13% 4 8% 5% 5% 5% 4 6% 6% 31%	4192
	11th to 20th	Day	9% 6% 2 5% 4 4 5% 5% 6% 8% 47%	4388
	21st to end	Day	12% 9% 2 7% 5% 5% 6% 6% 6% 8% 34%	4825
June	1st to 10th	Day	8% 6% 2 6% 4 5% 6% 6% 8% 9% 41%	2660
	11th to 20th	Day	7% 5% 2 5% 4 5% 5% 5% 8% 9% 45%	2677
	21st to end	Day	3 3 1 3 3 3 5% 5% 8% 8% 58%	2850
July	1st to 10th	Day	2 2 1 4 4 5% 7% 5% 7% 8% 56%	4554
	11th to 20th	Day	2 2 1 3 3 5% 6% 6% 8% 8% 56%	4430
	21st to end	Day	2 3 1 4 4 5% 6% 5% 8% 8% 54%	4620
August	1st to 10th	Day	6% 4 2 6% 5% 5% 5% 5% 7% 8% 47%	4092
	11th to 20th	Day	3 2 1 5% 4 5% 6% 5% 7% 8% 54%	4028
	21st to end	Day	3 3 1 3 3 3 5% 3 5% 6% 65%	4371
September	1st to 10th	Day	5% 2 1 4 3 3 4 3 5% 7% 63%	2280
	11th to 20th	Day	7% 3 1 4 3 4 3 3 4 6% 61%	2199
	21st to end	Day	7% 5% 1 4 3 3 4 3 4 6% 59%	2047
October	1st to 10th	Day	6% 5% 2 4 3 3 3 2 4 5% 66%	3170
	11th to 20th	Day	7% 5% 2 4 3 3 3 2 4 6% 62%	3150
	21st to end	Day	4 7% 2 6% 4 4 4 4 6% 8% 52%	3438
November	1st to 10th	Day	2 7% 3 7% 5% 5% 4 3 6% 9% 47%	1645
	11th to 20th	Day	3 6% 3 6% 5% 4 4 4 7% 9% 49%	1600
	21st to end	Day	3 9% 3 6% 4 3 3 4 6% 9% 49%	1552
December	1st to 10th	Day	2 5% 4 7% 6% 5% 5% 5% 6% 8% 49%	1520
	11th to 20th	Day	2 5% 1 6% 4 3 4 5% 8% 11% 50%	1926
	21st to end	Day	3 6% 2 6% 4 4 4 3 6% 9% 52%	2794
January	1st to 10th	Night	1 6% 3 7% 6% 5% 5% 4 8% 9% 46%	4417
	11th to 20th	Night	1 4 4 11% 7% 5% 5% 4 7% 8% 44%	4330
	21st to end	Night	2 6% 3 8% 3 4 4 3 6% 8% 52%	4521
February	1st to 10th	Night	2 7% 3 10% 7% 5% 5% 4 7% 8% 42%	2470
	11th to 20th	Night	3 9% 4 9% 6% 4 5% 3 6% 8% 42%	2402
	21st to end	Night	1 7% 3 8% 6% 6% 5% 4 6% 8% 46%	2011
March	1st to 10th	Night	2 7% 3 7% 3 4 4 4 6% 9% 52%	3444
	11th to 20th	Night	3 6% 2 6% 4 4 5% 4 8% 12% 47%	3490
	21st to end	Night	6% 9% 4 9% 6% 5% 5% 4 7% 11% 33%	3828
April	1st to 10th	Night	7% 10% 4 6% 4 4 5% 5% 6% 7% 43%	1910
	11th to 20th	Night	8% 10% 3 9% 6% 5% 5% 4 5% 7% 37%	1829
	21st to end	Night	9% 11% 3 8% 5% 4 3 4 6% 7% 41%	1710
May	1st to 10th	Night	13% 11% 4 7% 3 3 4 3 4 7% 39%	2778
	11th to 20th	Night	7% 5% 2 4 4 4 5% 3 6% 8% 52%	2517
	21st to end	Night	12% 7% 2 6% 5% 4 5% 4 7% 10% 38%	2762
June	1st to 10th	Night	8% 5% 2 6% 5% 5% 4 5% 7% 9% 44%	1520
	11th to 20th	Night	7% 4 2 5% 3 4 4 3 6% 9% 53%	1493
	21st to end	Night	3 3 1 4 3 4 3 4 6% 8% 62%	1330
July	1st to 10th	Night	2 1 1 3 3 3 4 4 7% 8% 64%	2402
	11th to 20th	Night	1 1 1 3 3 4 4 5% 7% 11% 60%	2540
	21st to end	Night	2 2 1 3 2 3 4 3 7% 9% 65%	2940
August	1st to 10th	Night	6% 4 2 5% 4 4 4 4 6% 8% 54%	2847
	11th to 20th	Night	3 3 1 4 4 4 5% 4 7% 9% 54%	2811
	21st to end	Night	3 2 1 3 2 2 3 3 5% 6% 69%	3296
September	1st to 10th	Night	5% 3 1 3 3 3 3 3 4 7% 65%	1900
	11th to 20th	Night	6% 4 2 4 3 2 3 3 5% 8% 60%	1976
	21st to end	Night	10% 4 2 3 2 3 3 3 4 6% 61%	2132
October	1st to 10th	Night	6% 4 1 3 2 2 3 2 4 6% 68%	3800
	11th to 20th	Night	7% 6% 2 6% 3 3 3 2 4 6% 57%	3778
	21st to end	Night	5% 8% 3 6% 4 4 4 3 5% 6% 54%	4120
November	1st to 10th	Night	4 6% 2 7% 4 4 4 4 6% 9% 49%	2634
	11th to 20th	Night	3 8% 3 7% 4 5% 4 4 6% 8% 49%	2800
	21st to end	Night	4 8% 2 6% 4 4 5% 4 6% 8% 50%	2723
December	1st to 10th	Night	2 5% 3 8% 6% 5% 5% 4 7% 8% 48%	2660
	11th to 20th	Night	2 6% 2 6% 4 3 5% 5% 7% 11% 49%	3271
	21st to end	Night	2 6% 2 6% 5% 5% 4 5% 7% 8% 50%	4873

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	32% 5% 14% 32% 16% 1	2539
	11th to 20th	Day	31% 7% 19% 27% 14% 2	2671
	21st to end	Day	24% 5% 22% 31% 14% 4	3146
February	1st to 10th	Day	37% 5% 19% 27% 11% 1	1710
	11th to 20th	Day	36% 6% 17% 24% 15% 2	1674
	21st to end	Day	34% 7% 16% 23% 17% 4	1441
March	1st to 10th	Day	23% 7% 14% 33% 17% 6%	3427
	11th to 20th	Day	31% 5% 20% 32% 11% 1	3490
	21st to end	Day	45% 10% 18% 22% 4 1	3825
April	1st to 10th	Day	39% 6% 16% 26% 10% 2	2270
	11th to 20th	Day	43% 6% 15% 27% 8% 1	2299
	21st to end	Day	46% 6% 13% 26% 9% 1	2470
May	1st to 10th	Day	50% 6% 10% 22% 7% 5%	4192
	11th to 20th	Day	32% 7% 15% 29% 13% 5%	4392
	21st to end	Day	43% 8% 14% 24% 10% 1	4825
June	1st to 10th	Day	35% 6% 19% 24% 13% 2	2660
	11th to 20th	Day	32% 7% 15% 21% 20% 6%	2677
	21st to end	Day	21% 4 14% 25% 22% 14%	2850
July	1st to 10th	Day	21% 7% 9% 25% 30% 8%	4485
	11th to 20th	Day	19% 5% 6% 21% 30% 19%	4355
	21st to end	Day	23% 5% 6% 18% 31% 17%	4474
August	1st to 10th	Day	31% 5% 9% 22% 19% 14%	4078
	11th to 20th	Day	25% 6% 8% 26% 25% 11%	4067
	21st to end	Day	19% 4 5% 22% 34% 16%	4370
September	1st to 10th	Day	21% 4 7% 22% 31% 15%	2280
	11th to 20th	Day	25% 4 9% 27% 28% 8%	2199
	21st to end	Day	27% 3 8% 29% 27% 7%	2047
October	1st to 10th	Day	23% 3 8% 17% 32% 17%	3170
	11th to 20th	Day	24% 3 10% 32% 22% 8%	3150
	21st to end	Day	29% 5% 10% 36% 17% 4	3438
November	1st to 10th	Day	34% 5% 12% 27% 19% 3	1645
	11th to 20th	Day	30% 5% 17% 27% 18% 3	1600
	21st to end	Day	31% 4 15% 30% 18% 2	1552
December	1st to 10th	Day	30% 5% 18% 27% 15% 4	1520
	11th to 20th	Day	25% 5% 24% 30% 14% 2	1924
	21st to end	Day	28% 4 10% 30% 19% 9%	2794
January	1st to 10th	Night	29% 7% 18% 28% 17% 1	4417
	11th to 20th	Night	34% 6% 21% 23% 13% 3	4330
	21st to end	Night	29% 4 21% 32% 12% 2	4521
February	1st to 10th	Night	37% 5% 18% 29% 9% 2	2470
	11th to 20th	Night	37% 5% 17% 25% 13% 2	2403
	21st to end	Night	34% 4 16% 25% 17% 3	2011
March	1st to 10th	Night	26% 6% 12% 30% 19% 8%	3444
	11th to 20th	Night	26% 6% 21% 33% 12% 2	3490
	21st to end	Night	40% 8% 21% 25% 5% 1	3828
April	1st to 10th	Night	36% 7% 18% 26% 10% 3	1910
	11th to 20th	Night	43% 6% 15% 26% 9% 1	1829
	21st to end	Night	41% 5% 13% 32% 9%	1710
May	1st to 10th	Night	45% 5% 11% 23% 12% 5%	2778
	11th to 20th	Night	28% 6% 16% 36% 12% 2	2518
	21st to end	Night	36% 11% 15% 28% 10%	2762
June	1st to 10th	Night	33% 8% 19% 23% 15% 2	1520
	11th to 20th	Night	28% 5% 18% 25% 17% 7%	1493
	21st to end	Night	20% 4 13% 31% 22% 11%	1330
July	1st to 10th	Night	15% 5% 10% 33% 31% 5%	2362
	11th to 20th	Night	14% 6% 6% 26% 35% 13%	2492
	21st to end	Night	14% 4 6% 23% 36% 17%	2848
August	1st to 10th	Night	26% 4 11% 23% 24% 12%	2838
	11th to 20th	Night	22% 5% 9% 30% 22% 12%	2838
	21st to end	Night	16% 3 6% 25% 32% 19%	3296
September	1st to 10th	Night	20% 3 8% 23% 31% 15%	1900
	11th to 20th	Night	22% 5% 10% 28% 26% 9%	1976
	21st to end	Night	25% 3 7% 29% 28% 7%	2132
October	1st to 10th	Night	20% 3 8% 24% 30% 15%	3800
	11th to 20th	Night	30% 3 8% 31% 24% 4	3778
	21st to end	Night	31% 4 10% 29% 18% 8%	4120
November	1st to 10th	Night	30% 6% 14% 29% 17% 3	2634
	11th to 20th	Night	30% 4 17% 30% 16% 3	2800
	21st to end	Night	31% 4 16% 29% 18% 3	2723
December	1st to 10th	Night	29% 5% 21% 26% 14% 4	2660
	11th to 20th	Night	27% 5% 21% 34% 13% 1	3272
	21st to end	Night	29% 5% 11% 30% 16% 9%	4873

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	7% 11% 30% 34% 15% 3	270
	11th to 20th	Day	2 7% 39% 36% 10% 6%	270
	21st to end	Day	4 14% 35% 35% 10% 2	297
February	1st to 10th	Day	6% 18% 46% 27% 3	270
	11th to 20th	Day	6% 18% 43% 25% 8%	270
	21st to end	Day	9% 12% 38% 28% 11% 1	225
March	1st to 10th	Day	6% 18% 42% 30% 4	443
	11th to 20th	Day	8% 17% 38% 26% 9% 1	460
	21st to end	Day	13% 16% 33% 29% 8% 1	506
April	1st to 10th	Day	14% 17% 39% 25% 5%	460
	11th to 20th	Day	16% 18% 37% 25% 4	452
	21st to end	Day	19% 13% 29% 27% 10% 1	460
May	1st to 10th	Day	15% 14% 29% 26% 14% 2	506
	11th to 20th	Day	19% 8% 24% 31% 15% 4	675
	21st to end	Day	17% 9% 22% 31% 18% 4	734
June	1st to 10th	Day	9% 9% 26% 34% 19% 4	680
	11th to 20th	Day	10% 14% 31% 32% 12% 2	680
	21st to end	Day	5% 7% 30% 36% 19% 3	680
July	1st to 10th	Day	3 6% 28% 35% 23% 5%	671
	11th to 20th	Day	3 5% 28% 44% 17% 2	668
	21st to end	Day	3 7% 31% 36% 20% 3	632
August	1st to 10th	Day	10% 5% 32% 37% 13% 3	487
	11th to 20th	Day	5% 4 30% 39% 18% 4	477
	21st to end	Day	5% 8% 32% 36% 16% 3	539
September	1st to 10th	Day	6% 6% 35% 36% 14% 4	490
	11th to 20th	Day	10% 9% 33% 29% 15% 4	490
	21st to end	Day	10% 11% 31% 31% 14% 2	447
October	1st to 10th	Day	12% 7% 26% 35% 16% 3	300
	11th to 20th	Day	11% 13% 26% 29% 15% 6%	300
	21st to end	Day	10% 13% 36% 29% 9% 3	330
November	1st to 10th	Day	8% 12% 38% 27% 10% 4	300
	11th to 20th	Day	9% 15% 32% 33% 9% 2	300
	21st to end	Day	12% 10% 36% 36% 6%	282
December	1st to 10th	Day	4 10% 39% 34% 9% 3	270
	11th to 20th	Day	9% 9% 34% 40% 7%	270
	21st to end	Day	6% 7% 37% 37% 10% 1	297
January	1st to 10th	Night	3 9% 30% 45% 12% 1	950
	11th to 20th	Night	3 9% 36% 36% 11% 4	889
	21st to end	Night	5% 7% 33% 44% 10% 2	836
February	1st to 10th	Night	4 8% 38% 40% 8% 1	760
	11th to 20th	Night	6% 8% 36% 38% 10% 1	744
	21st to end	Night	4 8% 34% 41% 11% 1	628
March	1st to 10th	Night	4 8% 34% 43% 10% 1	760
	11th to 20th	Night	6% 6% 31% 45% 11% 1	760
	21st to end	Night	9% 7% 29% 44% 10% 1	836
April	1st to 10th	Night	11% 7% 32% 41% 9%	580
	11th to 20th	Night	13% 7% 33% 39% 7% 1	563
	21st to end	Night	15% 5% 25% 41% 15% 1	570
May	1st to 10th	Night	17% 5% 21% 38% 18% 1	570
	11th to 20th	Night	13% 4 17% 43% 21% 2	561
	21st to end	Night	16% 5% 18% 40% 19% 2	617
June	1st to 10th	Night	8% 6% 21% 42% 22% 2	570
	11th to 20th	Night	8% 7% 29% 41% 15%	543
	21st to end	Night	3 3 15% 53% 25%	380
July	1st to 10th	Night	2 2 14% 52% 28% 1	469
	11th to 20th	Night	2 2 20% 54% 21%	552
	21st to end	Night	3 2 17% 51% 25% 2	624
August	1st to 10th	Night	4 2 13% 54% 25% 1	567
	11th to 20th	Night	5% 2 12% 55% 25% 1	564
	21st to end	Night	5% 3 19% 45% 25% 2	627
September	1st to 10th	Night	7% 2 20% 51% 19% 2	570
	11th to 20th	Night	6% 5% 19% 49% 18% 4	646
	21st to end	Night	11% 3 21% 43% 19% 3	760
October	1st to 10th	Night	14% 3 21% 44% 16% 3	760
	11th to 20th	Night	9% 5% 26% 41% 15% 3	760
	21st to end	Night	8% 4 33% 43% 10% 2	836
November	1st to 10th	Night	7% 5% 32% 45% 10% 2	927
	11th to 20th	Night	6% 9% 37% 42% 5% 1	1000
	21st to end	Night	8% 6% 28% 47% 10%	973
December	1st to 10th	Night	4 8% 32% 44% 10% 1	950
	11th to 20th	Night	5% 6% 33% 46% 9% 1	950
	21st to end	Night	6% 5% 33% 45% 8% 2	1045

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