MSU Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the MSU Observatory clear sky chart from 2005-11-06 to 2025-11-03.

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	14% 10% 4 9% 5% 5% 5% 4 5% 7% 34%	126485
	Night	14% 11% 4 9% 5% 4 4 3 5% 6% 33%	123706
Transparency	Day	49% 4 11% 22% 13% 1	130038
	Night	51% 4 11% 21% 12% 1	128223
Seeing	Day	18% 11% 28% 24% 14% 6%	17131
	Night	19% 9% 29% 30% 12% 1	26935

By Month

Percentage Cloud Cover SE (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% 16% 7% 17% 7% 5% 4 3 4 5% 20%	3617
	11th to 20th	Day	12% 16% 6% 19% 10% 11% 5% 3 4 4 9%	3620
	21st to end	Day	21% 19% 6% 13% 6% 4 6% 2 3 4 15%	4140
February	1st to 10th	Day	13% 14% 5% 13% 9% 6% 5% 4 5% 6% 21%	1802
	11th to 20th	Day	14% 13% 6% 14% 9% 6% 4 3 3 4 24%	1948
	21st to end	Day	16% 15% 5% 12% 5% 4 4 4 5% 5% 25%	1650
March	1st to 10th	Day	14% 12% 4 10% 6% 4 4 3 5% 6% 32%	4480
	11th to 20th	Day	11% 11% 4 8% 6% 4 4 4 5% 8% 35%	4971
	21st to end	Day	18% 15% 4 9% 5% 4 4 3 4 6% 28%	5482
April	1st to 10th	Day	18% 13% 3 8% 6% 4 4 4 5% 5% 30%	2400
	11th to 20th	Day	16% 10% 3 7% 5% 5% 4 4 5% 6% 36%	2487
	21st to end	Day	16% 9% 3 8% 5% 4 4 4 5% 8% 35%	2600
May	1st to 10th	Day	20% 9% 3 7% 4 4 4 4 4 6% 35%	5862
	11th to 20th	Day	15% 7% 3 8% 4 5% 5% 4 6% 8% 35%	5861
	21st to end	Day	15% 7% 2 6% 4 4 4 3 5% 8% 42%	6680
June	1st to 10th	Day	10% 6% 2 7% 5% 5% 5% 5% 5% 8% 43%	2800
	11th to 20th	Day	11% 5% 2 7% 5% 5% 4 4 6% 8% 42%	2800
	21st to end	Day	9% 5% 2 6% 6% 6% 5% 4 7% 9% 41%	2800
July	1st to 10th	Day	10% 5% 2 7% 5% 5% 6% 4 8% 10% 38%	6360
	11th to 20th	Day	8% 5% 2 6% 4 5% 5% 4 6% 9% 47%	6360
	21st to end	Day	8% 6% 2 6% 4 4 5% 4 7% 9% 46%	6754
August	1st to 10th	Day	12% 5% 2 6% 5% 5% 5% 4 6% 9% 42%	4234
	11th to 20th	Day	11% 6% 2 6% 4 5% 4 4 6% 8% 43%	4174
	21st to end	Day	8% 5% 3 6% 5% 5% 6% 5% 7% 9% 41%	4493
September	1st to 10th	Day	10% 5% 2 7% 4 4 5% 4 7% 7% 42%	2312
	11th to 20th	Day	10% 5% 2 5% 4 4 4 4 6% 8% 49%	2200
	21st to end	Day	14% 8% 2 8% 5% 4 5% 4 6% 7% 38%	2200
October	1st to 10th	Day	13% 6% 3 7% 5% 5% 4 4 5% 6% 43%	2195
	11th to 20th	Day	18% 9% 3 8% 5% 5% 5% 4 5% 7% 32%	1982
	21st to end	Day	19% 11% 3 8% 5% 4 5% 3 6% 5% 29%	1980
November	1st to 10th	Day	12% 11% 3 8% 4 4 4 4 5% 6% 39%	1780
	11th to 20th	Day	15% 15% 4 9% 6% 4 4 4 6% 6% 26%	1800
	21st to end	Day	19% 14% 5% 10% 5% 4 4 3 3 6% 27%	1800
December	1st to 10th	Day	11% 16% 10% 15% 6% 5% 5% 3 5% 7% 18%	2871
	11th to 20th	Day	17% 22% 8% 11% 6% 4 4 3 4 4 16%	3008
	21st to end	Day	24% 21% 7% 12% 5% 4 4 3 3 5% 12%	3982
January	1st to 10th	Night	13% 16% 7% 16% 7% 4 5% 3 4 5% 20%	6348
	11th to 20th	Night	13% 15% 7% 17% 8% 8% 5% 4 5% 5% 13%	6360
	21st to end	Night	18% 17% 8% 11% 5% 6% 5% 3 4 5% 18%	6816
February	1st to 10th	Night	15% 16% 5% 12% 8% 5% 4 3 5% 5% 22%	2595
	11th to 20th	Night	14% 13% 5% 11% 9% 5% 5% 4 4 5% 25%	2338
	21st to end	Night	17% 12% 4 10% 5% 4 4 4 5% 6% 28%	1980
March	1st to 10th	Night	14% 10% 5% 8% 4 4 4 2 5% 7% 37%	5373
	11th to 20th	Night	10% 9% 3 6% 4 4 5% 3 5% 8% 44%	4988
	21st to end	Night	17% 11% 4 9% 4 4 3 3 5% 6% 34%	5467
April	1st to 10th	Night	16% 11% 3 7% 4 4 4 4 5% 7% 35%	2013
	11th to 20th	Night	18% 9% 3 7% 5% 4 4 3 5% 6% 37%	1862
	21st to end	Night	16% 8% 2 8% 5% 5% 4 4 5% 6% 38%	1800
May	1st to 10th	Night	19% 9% 3 8% 5% 4 4 4 5% 7% 32%	4033
	11th to 20th	Night	18% 9% 3 7% 4 3 3 3 4 5% 41%	4032
	21st to end	Night	15% 7% 2 6% 4 4 4 3 5% 7% 44%	4169
June	1st to 10th	Night	12% 5% 2 5% 4 4 4 4 6% 9% 46%	1600
	11th to 20th	Night	11% 5% 2 5% 5% 4 4 3 6% 8% 45%	1600
	21st to end	Night	9% 5% 2 6% 4 4 4 4 7% 10% 46%	1600
July	1st to 10th	Night	8% 4 2 6% 4 4 5% 4 7% 10% 47%	3618
	11th to 20th	Night	9% 5% 2 5% 4 4 4 4 5% 9% 51%	3617
	21st to end	Night	9% 5% 2 6% 4 4 4 4 5% 8% 48%	4108
August	1st to 10th	Night	10% 5% 2 6% 4 3 4 3 6% 10% 48%	2915
	11th to 20th	Night	12% 6% 2 5% 4 4 4 3 6% 8% 47%	2875
	21st to end	Night	9% 6% 3 7% 4 4 4 4 6% 7% 46%	3388
September	1st to 10th	Night	8% 6% 1 6% 4 4 5% 4 6% 7% 49%	2067
	11th to 20th	Night	10% 4 2 5% 3 4 3 3 5% 6% 57%	2200
	21st to end	Night	14% 6% 3 6% 4 4 4 4 4 7% 43%	2200
October	1st to 10th	Night	14% 7% 2 6% 4 4 3 3 5% 6% 47%	2205
	11th to 20th	Night	18% 11% 3 7% 5% 5% 4 2 5% 5% 34%	2415
	21st to end	Night	20% 10% 3 7% 4 4 5% 3 5% 7% 33%	2860
November	1st to 10th	Night	14% 10% 3 7% 4 3 4 3 5% 5% 41%	2563
	11th to 20th	Night	14% 14% 4 9% 5% 5% 6% 4 6% 7% 27%	2600
	21st to end	Night	18% 16% 4 9% 5% 4 4 3 5% 6% 27%	2600
December	1st to 10th	Night	12% 17% 8% 15% 5% 4 4 3 4 6% 21%	4306
	11th to 20th	Night	15% 21% 6% 14% 8% 4 4 3 4 4 16%	5199
	21st to end	Night	25% 22% 6% 10% 4 4 3 3 4 4 13%	6996

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	68% 3 11% 14% 4	3618
	11th to 20th	Day	78% 5% 8% 6% 3	3620
	21st to end	Day	75% 2 10% 9% 3	4140
February	1st to 10th	Day	63% 5% 10% 15% 7%	1802
	11th to 20th	Day	66% 2 7% 16% 9% 1	1958
	21st to end	Day	62% 4 11% 16% 7% 1	1650
March	1st to 10th	Day	54% 4 11% 19% 10% 2	4480
	11th to 20th	Day	48% 4 15% 23% 9% 1	4971
	21st to end	Day	59% 3 9% 16% 12% 2	5482
April	1st to 10th	Day	56% 5% 9% 19% 11% 1	2400
	11th to 20th	Day	48% 5% 10% 21% 14% 2	2487
	21st to end	Day	48% 5% 12% 19% 14% 2	2600
May	1st to 10th	Day	49% 5% 10% 19% 15% 1	5897
	11th to 20th	Day	47% 5% 13% 21% 12% 2	5865
	21st to end	Day	42% 4 9% 24% 19% 2	6694
June	1st to 10th	Day	37% 6% 12% 27% 16% 2	2800
	11th to 20th	Day	39% 5% 13% 25% 17% 1	2800
	21st to end	Day	38% 7% 13% 23% 17% 2	2800
July	1st to 10th	Day	40% 4 12% 27% 14% 2	6291
	11th to 20th	Day	34% 3 12% 30% 19% 1	6285
	21st to end	Day	34% 4 15% 25% 21% 1	6560
August	1st to 10th	Day	39% 5% 12% 27% 17% 1	4221
	11th to 20th	Day	38% 5% 11% 33% 12% 1	4213
	21st to end	Day	38% 4 13% 31% 12% 1	4492
September	1st to 10th	Day	39% 6% 12% 27% 14% 2	2312
	11th to 20th	Day	33% 4 13% 30% 18% 1	2200
	21st to end	Day	44% 5% 12% 23% 16%	2200
October	1st to 10th	Day	31% 3 9% 35% 21% 1	3572
	11th to 20th	Day	42% 3 11% 27% 17%	3210
	21st to end	Day	48% 3 12% 25% 11% 1	3180
November	1st to 10th	Day	45% 5% 12% 23% 13% 1	1780
	11th to 20th	Day	58% 4 12% 19% 7% 1	1800
	21st to end	Day	59% 4 13% 17% 6% 1	1800
December	1st to 10th	Day	67% 3 18% 10% 2	2871
	11th to 20th	Day	72% 3 9% 13% 2	3005
	21st to end	Day	77% 3 6% 9% 4	3982
January	1st to 10th	Night	67% 3 8% 15% 5% 1	6348
	11th to 20th	Night	73% 3 9% 9% 6%	6360
	21st to end	Night	70% 3 10% 12% 4	6816
February	1st to 10th	Night	64% 5% 9% 14% 7% 1	2595
	11th to 20th	Night	60% 4 9% 18% 9% 1	2339
	21st to end	Night	56% 4 13% 17% 8% 2	1980
March	1st to 10th	Night	48% 4 13% 25% 10% 1	5373
	11th to 20th	Night	39% 4 13% 27% 16% 1	4988
	21st to end	Night	52% 3 11% 19% 13% 1	5467
April	1st to 10th	Night	48% 5% 10% 20% 15% 1	2013
	11th to 20th	Night	47% 5% 9% 19% 17% 2	1862
	21st to end	Night	46% 4 13% 19% 15% 3	1800
May	1st to 10th	Night	52% 4 10% 16% 14% 3	4060
	11th to 20th	Night	46% 4 9% 26% 13% 3	4033
	21st to end	Night	40% 4 9% 24% 18% 4	4177
June	1st to 10th	Night	35% 5% 10% 28% 19% 3	1600
	11th to 20th	Night	37% 5% 13% 26% 17% 3	1600
	21st to end	Night	32% 6% 15% 27% 18% 1	1600
July	1st to 10th	Night	33% 4 12% 30% 16% 5%	3580
	11th to 20th	Night	32% 3 14% 30% 20% 2	3572
	21st to end	Night	35% 4 13% 22% 24% 2	3989
August	1st to 10th	Night	33% 4 14% 28% 19% 1	2904
	11th to 20th	Night	35% 5% 10% 35% 14% 1	2902
	21st to end	Night	37% 4 11% 31% 15% 1	3388
September	1st to 10th	Night	33% 7% 10% 30% 18% 2	2067
	11th to 20th	Night	29% 4 11% 32% 22% 2	2200
	21st to end	Night	40% 5% 12% 26% 16% 1	2200
October	1st to 10th	Night	27% 3 9% 36% 24% 1	3608
	11th to 20th	Night	45% 3 10% 22% 19% 1	3926
	21st to end	Night	41% 3 10% 31% 14% 1	4610
November	1st to 10th	Night	45% 5% 10% 25% 14% 1	2563
	11th to 20th	Night	55% 4 13% 19% 8% 1	2600
	21st to end	Night	59% 4 12% 19% 6% 1	2600
December	1st to 10th	Night	66% 3 14% 14% 3	4306
	11th to 20th	Night	71% 3 9% 12% 5%	5201
	21st to end	Night	76% 3 8% 8% 5%	6996

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	20% 14% 39% 20% 5% 2	300
	11th to 20th	Day	20% 17% 44% 14% 4 1	300
	21st to end	Day	29% 18% 29% 18% 5%	330
February	1st to 10th	Day	24% 21% 36% 15% 3 1	300
	11th to 20th	Day	25% 17% 40% 16% 2	300
	21st to end	Day	26% 25% 35% 11% 3	249
March	1st to 10th	Day	23% 18% 42% 14% 4	300
	11th to 20th	Day	23% 14% 34% 21% 8%	491
	21st to end	Day	27% 14% 32% 21% 5% 1	550
April	1st to 10th	Day	23% 15% 33% 21% 7% 1	500
	11th to 20th	Day	26% 15% 31% 16% 8% 4	492
	21st to end	Day	20% 12% 30% 23% 13% 2	500
May	1st to 10th	Day	23% 14% 29% 22% 9% 2	503
	11th to 20th	Day	20% 9% 27% 23% 16% 5%	527
	21st to end	Day	18% 8% 22% 26% 18% 7%	677
June	1st to 10th	Day	13% 8% 29% 29% 16% 5%	730
	11th to 20th	Day	14% 6% 29% 30% 17% 5%	730
	21st to end	Day	11% 8% 28% 31% 17% 5%	730
July	1st to 10th	Day	9% 4 20% 31% 22% 13%	721
	11th to 20th	Day	10% 6% 25% 29% 20% 11%	718
	21st to end	Day	8% 4 23% 32% 24% 9%	724
August	1st to 10th	Day	9% 2 20% 29% 20% 20%	526
	11th to 20th	Day	13% 4 17% 27% 25% 13%	519
	21st to end	Day	12% 3 19% 28% 25% 13%	583
September	1st to 10th	Day	13% 6% 22% 24% 22% 14%	530
	11th to 20th	Day	10% 2 19% 23% 28% 18%	530
	21st to end	Day	18% 10% 19% 25% 18% 10%	530
October	1st to 10th	Day	19% 7% 26% 23% 18% 7%	530
	11th to 20th	Day	20% 15% 29% 27% 9% 1	512
	21st to end	Day	27% 15% 36% 16% 5% 1	363
November	1st to 10th	Day	19% 15% 35% 23% 7% 1	306
	11th to 20th	Day	25% 19% 33% 17% 4 1	300
	21st to end	Day	26% 18% 38% 13% 6%	300
December	1st to 10th	Day	22% 24% 29% 17% 7% 1	300
	11th to 20th	Day	24% 20% 38% 14% 3	300
	21st to end	Day	24% 24% 33% 16% 2 1	330
January	1st to 10th	Night	22% 14% 35% 23% 6% 1	1000
	11th to 20th	Night	25% 14% 35% 22% 4 1	1000
	21st to end	Night	21% 11% 35% 27% 5%	1024
February	1st to 10th	Night	24% 13% 36% 24% 2	800
	11th to 20th	Night	23% 14% 35% 25% 4	783
	21st to end	Night	24% 15% 39% 20% 3	660
March	1st to 10th	Night	21% 11% 37% 24% 6% 1	800
	11th to 20th	Night	18% 14% 36% 26% 6% 1	800
	21st to end	Night	23% 11% 31% 27% 8%	880
April	1st to 10th	Night	20% 11% 37% 25% 5% 1	613
	11th to 20th	Night	22% 11% 35% 23% 8% 1	593
	21st to end	Night	22% 8% 34% 29% 7% 1	600
May	1st to 10th	Night	21% 7% 31% 32% 9% 1	600
	11th to 20th	Night	22% 6% 29% 31% 12% 1	591
	21st to end	Night	14% 3 26% 36% 20% 2	650
June	1st to 10th	Night	13% 3 26% 42% 15% 1	600
	11th to 20th	Night	13% 4 24% 40% 19% 1	600
	21st to end	Night	11% 2 24% 41% 20% 2	600
July	1st to 10th	Night	8% 2 17% 43% 28% 3	585
	11th to 20th	Night	11% 2 17% 42% 26% 2	582
	21st to end	Night	7% 2 18% 48% 23% 2	657
August	1st to 10th	Night	8% 1 15% 46% 28% 2	597
	11th to 20th	Night	10% 1 17% 39% 28% 5%	594
	21st to end	Night	12% 2 15% 39% 28% 3	660
September	1st to 10th	Night	11% 4 19% 37% 24% 4	667
	11th to 20th	Night	12% 3 15% 36% 30% 4	800
	21st to end	Night	17% 6% 22% 30% 21% 5%	800
October	1st to 10th	Night	17% 6% 24% 35% 17% 2	800
	11th to 20th	Night	22% 12% 28% 29% 8% 1	800
	21st to end	Night	25% 10% 34% 25% 7%	880
November	1st to 10th	Night	19% 11% 30% 30% 10% 1	789
	11th to 20th	Night	21% 14% 36% 21% 7% 1	800
	21st to end	Night	25% 12% 33% 26% 4	800
December	1st to 10th	Night	26% 12% 36% 20% 7%	830
	11th to 20th	Night	24% 14% 37% 22% 4	1000
	21st to end	Night	32% 11% 36% 19% 2	1100

Page generated at 2025-10-31 02:02:37. Version ac07

Copyright © 2025 Attilla Danko.