CSM Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the CSM Observatory clear sky chart from 2005-11-06 to 2025-10-04.

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% 4 2 5% 4 4 4 3 5% 8% 55%	134938
	Night	7% 6% 2 6% 4 5% 5% 4 5% 7% 49%	131933
Transparency	Day	23% 7% 11% 19% 24% 17%	129860
	Night	28% 7% 12% 18% 20% 14%	127998
Seeing	Day	9% 12% 28% 32% 16% 4	16646
	Night	11% 7% 17% 32% 27% 6%	25956

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	11% 6% 2 7% 3 4 5% 4 7% 10% 42%	3939
	11th to 20th	Day	9% 6% 3 8% 5% 5% 3 3 3 5% 50%	4100
	21st to end	Day	15% 9% 3 7% 5% 5% 4 3 6% 6% 37%	4510
February	1st to 10th	Day	13% 8% 3 6% 5% 4 4 4 4 6% 43%	1800
	11th to 20th	Day	11% 8% 4 6% 5% 5% 4 4 6% 8% 40%	1836
	21st to end	Day	10% 6% 3 7% 5% 4 3 4 6% 6% 46%	1650
March	1st to 10th	Day	9% 8% 4 8% 6% 6% 6% 5% 7% 8% 32%	4905
	11th to 20th	Day	8% 5% 2 6% 5% 5% 4 4 6% 9% 46%	5000
	21st to end	Day	8% 6% 3 6% 6% 6% 6% 5% 8% 11% 35%	5546
April	1st to 10th	Day	10% 6% 2 7% 4 3 5% 4 6% 7% 45%	2400
	11th to 20th	Day	8% 5% 2 6% 4 3 4 3 5% 7% 53%	2369
	21st to end	Day	4 5% 2 4 3 3 4 3 6% 10% 55%	2400
May	1st to 10th	Day	3 3 1 3 2 2 2 2 4 6% 73%	5646
	11th to 20th	Day	2 3 1 3 2 3 2 2 5% 7% 71%	5917
	21st to end	Day	2 6% 2 4 3 3 3 3 6% 10% 60%	6902
June	1st to 10th	Day	4 5% 2 4 3 3 4 3 6% 9% 57%	2800
	11th to 20th	Day	1 3 1 3 2 2 3 3 5% 9% 70%	2800
	21st to end	Day	2 3 1 2 3 3 3 3 4 8% 66%	2800
July	1st to 10th	Day	2 1 2 2 2 2 2 4 7% 77%	6277
	11th to 20th	Day	1 1 1 2 2 2 4 3 5% 10% 69%	5880
	21st to end	Day	3 2 7% 5% 5% 7% 4 6% 10% 52%	6402
August	1st to 10th	Day	1 2 1 3 2 3 4 4 6% 11% 62%	5777
	11th to 20th	Day	1 2 1 4 3 4 4 3 4 9% 67%	5536
	21st to end	Day	2 1 4 3 3 3 3 6% 10% 64%	5984
September	1st to 10th	Day	1 3 1 4 3 2 3 3 3 8% 69%	2400
	11th to 20th	Day	2 6% 2 6% 3 4 4 3 6% 10% 55%	2400
	21st to end	Day	4 2 1 3 2 2 3 2 4 8% 70%	2114
October	1st to 10th	Day	2 3 1 4 3 2 3 3 4 7% 70%	3211
	11th to 20th	Day	3 3 2 5% 4 5% 4 3 4 7% 60%	3161
	21st to end	Day	4 4 2 5% 4 4 4 4 6% 8% 57%	3448
November	1st to 10th	Day	6% 5% 2 5% 3 4 4 4 5% 9% 53%	1944
	11th to 20th	Day	7% 7% 2 7% 5% 5% 5% 3 4 8% 47%	1800
	21st to end	Day	10% 5% 2 5% 4 3 4 4 5% 8% 50%	1632
December	1st to 10th	Day	6% 4 2 5% 4 4 4 4 6% 10% 53%	2620
	11th to 20th	Day	15% 8% 3 9% 5% 6% 7% 4 7% 8% 28%	3028
	21st to end	Day	16% 10% 4 9% 6% 5% 5% 4 6% 8% 27%	4004
January	1st to 10th	Night	9% 7% 3 6% 4 4 5% 4 6% 9% 43%	6016
	11th to 20th	Night	11% 8% 4 7% 5% 5% 5% 3 4 4 45%	5880
	21st to end	Night	13% 11% 4 8% 4 4 4 3 5% 6% 39%	6468
February	1st to 10th	Night	13% 8% 2 7% 4 4 4 3 5% 6% 46%	2600
	11th to 20th	Night	10% 7% 2 8% 5% 5% 4 3 6% 8% 41%	2449
	21st to end	Night	9% 7% 2 5% 4 3 4 3 6% 7% 48%	1980
March	1st to 10th	Night	11% 9% 3 9% 6% 5% 6% 4 6% 7% 34%	4984
	11th to 20th	Night	11% 8% 3 7% 4 4 3 3 4 7% 48%	5000
	21st to end	Night	11% 10% 4 8% 5% 5% 5% 3 5% 7% 38%	5429
April	1st to 10th	Night	7% 7% 3 6% 5% 4 4 4 7% 8% 46%	2000
	11th to 20th	Night	7% 6% 2 5% 4 4 3 4 5% 8% 51%	1979
	21st to end	Night	3 5% 1 4 4 3 4 4 5% 9% 57%	2000
May	1st to 10th	Night	3 4 2 4 3 2 2 2 4 6% 69%	4340
	11th to 20th	Night	1 3 1 4 5% 5% 5% 3 6% 7% 60%	3992
	21st to end	Night	2 4 2 5% 4 3 5% 2 4 5% 64%	3972
June	1st to 10th	Night	3 5% 2 6% 3 3 5% 4 7% 10% 52%	1600
	11th to 20th	Night	1 2 1 3 2 3 3 3 5% 9% 67%	1600
	21st to end	Night	1 4 2 5% 3 3 3 3 5% 8% 64%	1600
July	1st to 10th	Night	2 1 2 2 3 7% 3 5% 7% 68%	3679
	11th to 20th	Night	1 2 1 5% 3 5% 13% 6% 8% 8% 49%	4100
	21st to end	Night	6% 4 11% 9% 12% 10% 3 5% 7% 34%	4472
August	1st to 10th	Night	1 3 1 5% 4 7% 10% 5% 6% 9% 48%	4035
	11th to 20th	Night	4 1 5% 4 7% 6% 4 5% 8% 56%	4323
	21st to end	Night	1 2 2 6% 4 5% 8% 5% 6% 8% 52%	5015
September	1st to 10th	Night	1 3 1 4 4 4 5% 3 5% 9% 62%	2000
	11th to 20th	Night	3 5% 2 6% 4 5% 5% 3 5% 9% 52%	2000
	21st to end	Night	2 2 2 4 4 3 3 2 3 8% 67%	2281
October	1st to 10th	Night	3 3 1 3 3 3 4 3 5% 6% 67%	3836
	11th to 20th	Night	4 4 2 5% 5% 6% 7% 4 4 6% 53%	3777
	21st to end	Night	5% 4 1 4 3 3 4 4 5% 7% 59%	4121
November	1st to 10th	Night	6% 6% 2 4 4 4 4 3 5% 8% 53%	2352
	11th to 20th	Night	7% 7% 2 6% 5% 4 4 3 6% 8% 48%	2600
	21st to end	Night	7% 5% 2 5% 4 4 4 3 5% 8% 53%	2765
December	1st to 10th	Night	6% 5% 2 6% 4 3 4 3 4 7% 56%	4560
	11th to 20th	Night	17% 9% 3 9% 5% 4 5% 4 7% 9% 27%	5176
	21st to end	Night	17% 9% 3 9% 6% 5% 5% 4 6% 6% 29%	6952

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	35% 7% 18% 21% 16% 4	3939
	11th to 20th	Day	36% 7% 8% 13% 22% 16%	4100
	21st to end	Day	44% 7% 6% 12% 13% 17%	4510
February	1st to 10th	Day	40% 6% 8% 18% 18% 9%	1800
	11th to 20th	Day	39% 9% 11% 16% 19% 6%	1845
	21st to end	Day	39% 6% 8% 24% 17% 6%	1650
March	1st to 10th	Day	43% 11% 9% 18% 17% 2	4905
	11th to 20th	Day	31% 7% 16% 28% 10% 7%	5000
	21st to end	Day	36% 12% 18% 22% 10% 2	5546
April	1st to 10th	Day	36% 9% 13% 21% 18% 4	2400
	11th to 20th	Day	29% 8% 15% 23% 19% 5%	2372
	21st to end	Day	24% 9% 14% 26% 22% 4	2400
May	1st to 10th	Day	13% 6% 9% 27% 32% 14%	5646
	11th to 20th	Day	12% 5% 10% 31% 34% 8%	5920
	21st to end	Day	17% 8% 11% 27% 34% 3	6904
June	1st to 10th	Day	23% 11% 14% 23% 23% 7%	2800
	11th to 20th	Day	11% 7% 11% 21% 35% 14%	2800
	21st to end	Day	13% 6% 12% 12% 29% 28%	2800
July	1st to 10th	Day	6% 4 7% 8% 20% 56%	6209
	11th to 20th	Day	7% 4 8% 5% 18% 57%	5811
	21st to end	Day	11% 5% 13% 18% 27% 26%	6217
August	1st to 10th	Day	14% 5% 10% 11% 28% 31%	4206
	11th to 20th	Day	9% 5% 9% 9% 42% 27%	4007
	21st to end	Day	12% 5% 11% 12% 33% 28%	4311
September	1st to 10th	Day	13% 5% 9% 11% 27% 37%	2400
	11th to 20th	Day	21% 8% 11% 20% 25% 15%	2400
	21st to end	Day	14% 5% 7% 21% 32% 22%	2114
October	1st to 10th	Day	16% 3 5% 12% 27% 37%	3211
	11th to 20th	Day	18% 7% 10% 21% 28% 16%	3161
	21st to end	Day	21% 4 11% 24% 26% 14%	3448
November	1st to 10th	Day	28% 6% 10% 24% 20% 11%	1944
	11th to 20th	Day	34% 7% 10% 23% 20% 7%	1800
	21st to end	Day	32% 4 12% 17% 27% 9%	1632
December	1st to 10th	Day	26% 6% 8% 24% 30% 6%	2620
	11th to 20th	Day	47% 8% 13% 14% 14% 4	3028
	21st to end	Day	51% 9% 12% 17% 10% 2	4004
January	1st to 10th	Night	33% 9% 20% 21% 13% 3	6016
	11th to 20th	Night	38% 8% 10% 13% 20% 11%	5880
	21st to end	Night	45% 6% 7% 11% 17% 15%	6468
February	1st to 10th	Night	38% 6% 10% 18% 18% 11%	2600
	11th to 20th	Night	39% 8% 11% 17% 19% 6%	2451
	21st to end	Night	33% 7% 13% 27% 15% 5%	1980
March	1st to 10th	Night	43% 10% 11% 20% 13% 2	4984
	11th to 20th	Night	36% 6% 12% 24% 15% 7%	5000
	21st to end	Night	42% 10% 15% 20% 10% 2	5429
April	1st to 10th	Night	35% 8% 14% 23% 16% 4	2000
	11th to 20th	Night	31% 9% 12% 24% 17% 8%	1979
	21st to end	Night	23% 9% 14% 29% 20% 6%	2000
May	1st to 10th	Night	18% 5% 8% 26% 28% 14%	4337
	11th to 20th	Night	18% 6% 15% 26% 24% 10%	3993
	21st to end	Night	18% 7% 11% 27% 35% 2	3972
June	1st to 10th	Night	23% 11% 14% 21% 23% 7%	1600
	11th to 20th	Night	11% 9% 13% 19% 33% 16%	1600
	21st to end	Night	15% 7% 11% 13% 27% 28%	1600
July	1st to 10th	Night	5% 4 7% 10% 20% 54%	3639
	11th to 20th	Night	7% 5% 8% 11% 26% 44%	4046
	21st to end	Night	16% 8% 22% 14% 22% 19%	4345
August	1st to 10th	Night	17% 7% 13% 15% 27% 22%	2929
	11th to 20th	Night	12% 6% 13% 9% 36% 24%	3122
	21st to end	Night	13% 8% 15% 11% 29% 25%	3608
September	1st to 10th	Night	13% 8% 10% 11% 23% 35%	2000
	11th to 20th	Night	22% 7% 16% 18% 23% 13%	2000
	21st to end	Night	15% 6% 9% 20% 27% 23%	2281
October	1st to 10th	Night	16% 5% 6% 11% 23% 40%	3836
	11th to 20th	Night	20% 6% 15% 19% 22% 18%	3777
	21st to end	Night	22% 5% 8% 22% 24% 19%	4121
November	1st to 10th	Night	27% 8% 12% 23% 19% 12%	2352
	11th to 20th	Night	31% 8% 12% 24% 17% 8%	2600
	21st to end	Night	29% 6% 11% 20% 22% 13%	2765
December	1st to 10th	Night	28% 6% 7% 23% 27% 9%	4560
	11th to 20th	Night	48% 6% 12% 15% 16% 3	5176
	21st to end	Night	51% 9% 14% 15% 8% 3	6952

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	24% 11% 28% 21% 15%	331
	11th to 20th	Day	16% 12% 25% 30% 16% 2	400
	21st to end	Day	14% 12% 28% 27% 18% 2	440
February	1st to 10th	Day	17% 16% 25% 25% 15% 2	400
	11th to 20th	Day	16% 19% 30% 25% 10% 1	436
	21st to end	Day	12% 16% 31% 28% 12% 1	414
March	1st to 10th	Day	19% 21% 30% 20% 9% 1	500
	11th to 20th	Day	17% 18% 28% 21% 12% 3	500
	21st to end	Day	13% 21% 32% 24% 8% 1	550
April	1st to 10th	Day	15% 18% 31% 23% 11% 2	500
	11th to 20th	Day	11% 20% 32% 24% 9% 3	492
	21st to end	Day	6% 21% 32% 25% 10% 5%	500
May	1st to 10th	Day	7% 18% 35% 28% 10% 2	503
	11th to 20th	Day	9% 16% 38% 25% 11% 2	528
	21st to end	Day	8% 17% 33% 27% 13% 3	573
June	1st to 10th	Day	7% 12% 33% 33% 13% 3	530
	11th to 20th	Day	2 13% 34% 37% 11% 2	530
	21st to end	Day	2 7% 32% 42% 14% 2	530
July	1st to 10th	Day	5% 4 30% 46% 14% 2	526
	11th to 20th	Day	2 2 24% 50% 20% 2	524
	21st to end	Day	3 2 26% 49% 16% 4	575
August	1st to 10th	Day	4 6% 28% 43% 14% 4	527
	11th to 20th	Day	3 3 31% 41% 19% 3	517
	21st to end	Day	3 3 25% 44% 20% 5%	583
September	1st to 10th	Day	4 4 23% 40% 22% 6%	530
	11th to 20th	Day	4 12% 26% 36% 15% 6%	530
	21st to end	Day	4 9% 25% 31% 26% 5%	530
October	1st to 10th	Day	3 9% 28% 30% 20% 10%	503
	11th to 20th	Day	5% 10% 25% 28% 25% 7%	490
	21st to end	Day	9% 9% 23% 29% 23% 7%	539
November	1st to 10th	Day	8% 12% 23% 25% 25% 6%	480
	11th to 20th	Day	9% 11% 17% 31% 26% 6%	300
	21st to end	Day	11% 8% 27% 29% 23% 2	215
December	1st to 10th	Day	15% 13% 19% 25% 24% 5%	200
	11th to 20th	Day	14% 15% 22% 31% 18% 2	200
	21st to end	Day	19% 16% 27% 25% 12%	220
January	1st to 10th	Night	18% 8% 17% 31% 22% 5%	800
	11th to 20th	Night	15% 7% 16% 31% 25% 6%	800
	21st to end	Night	18% 7% 13% 34% 24% 4	880
February	1st to 10th	Night	18% 8% 16% 32% 23% 4	800
	11th to 20th	Night	13% 12% 19% 31% 21% 4	784
	21st to end	Night	12% 13% 20% 34% 19% 3	660
March	1st to 10th	Night	17% 14% 20% 27% 19% 3	800
	11th to 20th	Night	17% 11% 17% 30% 22% 4	800
	21st to end	Night	15% 11% 24% 29% 18% 2	880
April	1st to 10th	Night	11% 11% 25% 32% 18% 3	800
	11th to 20th	Night	12% 14% 22% 28% 19% 4	791
	21st to end	Night	7% 11% 27% 29% 21% 5%	800
May	1st to 10th	Night	6% 8% 25% 35% 22% 4	710
	11th to 20th	Night	6% 6% 22% 33% 30% 4	562
	21st to end	Night	9% 11% 23% 35% 18% 4	433
June	1st to 10th	Night	6% 6% 24% 37% 25% 3	400
	11th to 20th	Night	1 6% 22% 44% 24% 3	400
	21st to end	Night	5% 4 15% 39% 34% 4	400
July	1st to 10th	Night	3 3 16% 36% 36% 6%	403
	11th to 20th	Night	3 1 11% 38% 37% 10%	582
	21st to end	Night	6% 4 13% 33% 36% 8%	657
August	1st to 10th	Night	9% 4 15% 33% 32% 7%	597
	11th to 20th	Night	5% 2 17% 36% 32% 6%	709
	21st to end	Night	3 2 14% 36% 37% 8%	880
September	1st to 10th	Night	3 3 13% 32% 38% 11%	800
	11th to 20th	Night	8% 5% 17% 32% 32% 7%	800
	21st to end	Night	2 4 16% 30% 38% 10%	800
October	1st to 10th	Night	7% 3 14% 32% 33% 10%	772
	11th to 20th	Night	7% 5% 13% 28% 37% 10%	760
	21st to end	Night	8% 5% 12% 29% 33% 12%	836
November	1st to 10th	Night	9% 5% 14% 28% 35% 8%	780
	11th to 20th	Night	10% 5% 13% 34% 30% 7%	800
	21st to end	Night	11% 7% 12% 39% 26% 5%	800
December	1st to 10th	Night	16% 6% 16% 32% 25% 6%	800
	11th to 20th	Night	24% 8% 14% 31% 21% 4	800
	21st to end	Night	20% 11% 19% 29% 19% 1	880

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