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 2024-01-16.

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% 9% 4 7% 6% 3 5% 5% 5% 7% 34%	74896
	Night	15% 10% 4 7% 6% 3 4 4 5% 6% 34%	74918
Transparency	Day	49% 5% 12% 21% 12% 1	74602
	Night	51% 5% 11% 20% 12% 1	74733
Seeing	Day	18% 10% 28% 23% 14% 6%	14589
	Night	19% 9% 29% 30% 12% 1	24563

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	15% 14% 8% 11% 10% 5% 5% 5% 5% 5% 17%	1517
	11th to 20th	Day	15% 17% 7% 11% 10% 6% 5% 4 4 5% 15%	1480
	21st to end	Day	16% 19% 9% 11% 9% 3 5% 3 3 5% 16%	1651
February	1st to 10th	Day	14% 13% 6% 12% 10% 4 6% 5% 5% 5% 19%	1621
	11th to 20th	Day	15% 11% 7% 12% 10% 4 6% 4 3 5% 25%	1748
	21st to end	Day	17% 14% 6% 11% 6% 3 4 4 5% 6% 24%	1480
March	1st to 10th	Day	14% 11% 5% 8% 7% 3 5% 5% 6% 7% 29%	1800
	11th to 20th	Day	13% 12% 5% 8% 6% 2 4 5% 5% 8% 32%	1971
	21st to end	Day	18% 12% 4 8% 6% 4 4 3 4 6% 30%	2182
April	1st to 10th	Day	19% 12% 3 7% 7% 3 5% 5% 5% 5% 30%	2160
	11th to 20th	Day	17% 10% 3 6% 6% 3 5% 5% 5% 6% 35%	2235
	21st to end	Day	17% 8% 3 8% 6% 3 4 4 4 8% 34%	2340
May	1st to 10th	Day	16% 8% 4 7% 6% 3 5% 5% 5% 7% 33%	2340
	11th to 20th	Day	19% 7% 3 6% 5% 3 5% 4 5% 7% 36%	2301
	21st to end	Day	13% 6% 2 6% 6% 3 4 4 6% 10% 40%	2624
June	1st to 10th	Day	10% 5% 2 6% 5% 4 5% 6% 5% 8% 43%	2520
	11th to 20th	Day	11% 5% 2 6% 6% 4 5% 5% 6% 8% 43%	2520
	21st to end	Day	9% 5% 3 5% 6% 4 5% 6% 7% 9% 41%	2520
July	1st to 10th	Day	7% 4 2 4 4 3 5% 5% 9% 9% 48%	2520
	11th to 20th	Day	9% 5% 3 5% 5% 3 6% 5% 6% 9% 45%	2520
	21st to end	Day	6% 5% 2 6% 5% 3 6% 6% 7% 9% 44%	2681
August	1st to 10th	Day	9% 4 2 6% 7% 4 6% 5% 6% 9% 42%	2327
	11th to 20th	Day	11% 5% 2 6% 6% 4 5% 6% 6% 9% 40%	2303
	21st to end	Day	10% 5% 3 5% 5% 4 5% 5% 7% 9% 43%	2481
September	1st to 10th	Day	11% 5% 3 7% 6% 4 5% 5% 6% 7% 40%	2082
	11th to 20th	Day	11% 5% 3 5% 5% 3 4 5% 6% 8% 46%	1980
	21st to end	Day	13% 7% 3 7% 6% 3 5% 5% 6% 8% 39%	1980
October	1st to 10th	Day	14% 6% 3 6% 6% 4 4 4 5% 7% 40%	1976
	11th to 20th	Day	19% 9% 3 7% 7% 4 5% 5% 5% 7% 30%	1784
	21st to end	Day	21% 11% 4 7% 6% 3 5% 4 5% 5% 28%	1782
November	1st to 10th	Day	13% 10% 3 7% 5% 4 4 5% 5% 6% 39%	1672
	11th to 20th	Day	15% 14% 5% 8% 7% 3 4 5% 5% 7% 27%	1710
	21st to end	Day	19% 13% 6% 8% 5% 3 4 4 4 5% 28%	1710
December	1st to 10th	Day	14% 15% 10% 13% 7% 4 5% 4 4 5% 20%	1677
	11th to 20th	Day	14% 17% 7% 10% 6% 4 5% 5% 6% 6% 20%	1918
	21st to end	Day	23% 22% 8% 10% 7% 3 4 4 3 4 11%	2783
January	1st to 10th	Night	17% 13% 7% 11% 8% 4 4 5% 5% 5% 21%	2648
	11th to 20th	Night	18% 17% 7% 10% 8% 4 4 4 4 5% 19%	2591
	21st to end	Night	13% 14% 11% 11% 8% 5% 6% 4 4 5% 19%	2704
February	1st to 10th	Night	16% 16% 5% 10% 9% 4 6% 4 4 5% 20%	2336
	11th to 20th	Night	15% 12% 5% 9% 10% 4 5% 5% 5% 5% 25%	2098
	21st to end	Night	18% 11% 5% 9% 6% 3 4 5% 4 6% 28%	1776
March	1st to 10th	Night	15% 10% 5% 7% 6% 4 4 3 5% 7% 34%	2160
	11th to 20th	Night	12% 10% 4 6% 5% 3 5% 5% 5% 6% 39%	1988
	21st to end	Night	20% 8% 4 8% 5% 3 4 4 5% 6% 33%	2178
April	1st to 10th	Night	15% 10% 4 6% 5% 3 4 6% 5% 7% 35%	1813
	11th to 20th	Night	19% 8% 3 6% 6% 2 4 4 5% 6% 37%	1674
	21st to end	Night	17% 7% 3 6% 5% 3 4 4 5% 7% 38%	1620
May	1st to 10th	Night	16% 8% 3 7% 6% 4 4 4 5% 8% 33%	1620
	11th to 20th	Night	20% 7% 2 4 5% 2 3 4 5% 7% 41%	1592
	21st to end	Night	11% 5% 2 5% 5% 3 5% 5% 6% 10% 42%	1648
June	1st to 10th	Night	12% 4 2 4 5% 3 5% 5% 5% 9% 46%	1440
	11th to 20th	Night	11% 5% 2 4 6% 3 4 4 5% 8% 47%	1440
	21st to end	Night	8% 5% 2 6% 4 3 4 4 6% 10% 46%	1440
July	1st to 10th	Night	7% 4 2 4 4 3 4 5% 6% 10% 52%	1440
	11th to 20th	Night	9% 5% 2 3 4 3 5% 4 6% 8% 51%	1440
	21st to end	Night	6% 3 3 5% 4 3 4 5% 6% 9% 52%	1634
August	1st to 10th	Night	7% 4 2 6% 4 3 4 5% 6% 12% 48%	1611
	11th to 20th	Night	9% 3 1 4 6% 3 5% 5% 7% 8% 49%	1598
	21st to end	Night	10% 6% 2 5% 5% 3 4 4 5% 6% 50%	1872
September	1st to 10th	Night	9% 6% 2 5% 5% 3 5% 5% 6% 7% 48%	1859
	11th to 20th	Night	11% 4 2 5% 3 3 3 3 5% 6% 54%	1980
	21st to end	Night	14% 5% 3 5% 5% 3 5% 5% 5% 7% 44%	1980
October	1st to 10th	Night	16% 7% 2 5% 5% 3 3 4 5% 6% 44%	1984
	11th to 20th	Night	19% 11% 3 7% 6% 4 4 3 5% 5% 32%	2173
	21st to end	Night	22% 10% 3 6% 6% 3 4 4 5% 7% 31%	2574
November	1st to 10th	Night	14% 9% 3 6% 5% 3 4 4 4 6% 41%	2406
	11th to 20th	Night	14% 13% 5% 7% 6% 4 6% 5% 6% 7% 28%	2470
	21st to end	Night	18% 14% 5% 8% 6% 3 4 4 5% 6% 28%	2470
December	1st to 10th	Night	16% 17% 8% 11% 7% 3 5% 5% 4 5% 19%	2498
	11th to 20th	Night	14% 15% 7% 12% 8% 4 5% 4 5% 5% 21%	3279
	21st to end	Night	27% 22% 6% 9% 5% 3 4 3 4 4 12%	4884

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% 5% 10% 13% 4	1518
	11th to 20th	Day	72% 5% 11% 8% 4	1480
	21st to end	Day	73% 3 8% 12% 4 1	1651
February	1st to 10th	Day	66% 5% 10% 13% 6%	1621
	11th to 20th	Day	65% 2 8% 16% 8% 1	1758
	21st to end	Day	63% 4 10% 15% 7% 1	1480
March	1st to 10th	Day	55% 5% 12% 19% 8% 1	1800
	11th to 20th	Day	51% 4 11% 19% 14% 2	1971
	21st to end	Day	56% 3 10% 20% 9% 1	2182
April	1st to 10th	Day	56% 5% 10% 18% 11%	2160
	11th to 20th	Day	49% 5% 11% 19% 14% 2	2235
	21st to end	Day	50% 5% 12% 17% 14% 2	2340
May	1st to 10th	Day	50% 7% 11% 21% 10% 2	2340
	11th to 20th	Day	48% 5% 10% 21% 14% 2	2305
	21st to end	Day	40% 6% 14% 26% 13% 1	2624
June	1st to 10th	Day	37% 6% 12% 27% 16% 2	2520
	11th to 20th	Day	39% 6% 12% 24% 18% 2	2520
	21st to end	Day	38% 7% 13% 23% 16% 2	2520
July	1st to 10th	Day	30% 7% 12% 29% 18% 5%	2451
	11th to 20th	Day	35% 5% 12% 27% 19% 2	2445
	21st to end	Day	34% 8% 12% 26% 18% 3	2532
August	1st to 10th	Day	37% 7% 14% 27% 13% 2	2314
	11th to 20th	Day	39% 7% 14% 25% 13% 2	2303
	21st to end	Day	36% 6% 12% 27% 17% 2	2481
September	1st to 10th	Day	41% 7% 13% 23% 14% 2	2082
	11th to 20th	Day	36% 4 13% 31% 16% 1	1980
	21st to end	Day	42% 6% 13% 24% 15%	1980
October	1st to 10th	Day	43% 5% 10% 26% 14% 2	1976
	11th to 20th	Day	53% 5% 13% 19% 9%	1784
	21st to end	Day	57% 4 13% 17% 8% 1	1782
November	1st to 10th	Day	45% 6% 12% 22% 14% 1	1672
	11th to 20th	Day	57% 4 12% 19% 7% 1	1710
	21st to end	Day	58% 4 14% 17% 6% 1	1710
December	1st to 10th	Day	68% 3 12% 14% 3	1677
	11th to 20th	Day	64% 5% 13% 15% 3	1915
	21st to end	Day	78% 4 8% 6% 3	2783
January	1st to 10th	Night	65% 4 10% 16% 5%	2648
	11th to 20th	Night	68% 5% 12% 10% 6%	2591
	21st to end	Night	67% 4 11% 12% 5% 1	2704
February	1st to 10th	Night	66% 5% 9% 13% 5% 1	2336
	11th to 20th	Night	60% 4 9% 17% 9% 1	2099
	21st to end	Night	57% 4 13% 17% 8% 2	1776
March	1st to 10th	Night	51% 4 11% 22% 10% 1	2160
	11th to 20th	Night	46% 5% 11% 22% 15% 2	1988
	21st to end	Night	53% 5% 8% 22% 10% 2	2178
April	1st to 10th	Night	48% 5% 11% 20% 15% 1	1813
	11th to 20th	Night	48% 5% 9% 19% 17% 2	1674
	21st to end	Night	46% 4 13% 19% 15% 3	1620
May	1st to 10th	Night	50% 4 11% 22% 11% 3	1620
	11th to 20th	Night	44% 4 8% 22% 18% 3	1593
	21st to end	Night	37% 6% 14% 26% 14% 3	1649
June	1st to 10th	Night	35% 5% 11% 29% 17% 3	1440
	11th to 20th	Night	36% 6% 14% 25% 18% 3	1440
	21st to end	Night	32% 6% 15% 28% 18% 2	1440
July	1st to 10th	Night	27% 7% 11% 31% 19% 4	1400
	11th to 20th	Night	31% 5% 12% 28% 21% 3	1392
	21st to end	Night	27% 7% 15% 26% 21% 3	1543
August	1st to 10th	Night	30% 6% 16% 31% 16% 1	1600
	11th to 20th	Night	31% 7% 13% 30% 16% 3	1598
	21st to end	Night	33% 5% 11% 26% 22% 2	1872
September	1st to 10th	Night	35% 7% 10% 29% 18% 2	1859
	11th to 20th	Night	32% 4 11% 31% 20% 2	1980
	21st to end	Night	39% 6% 12% 27% 15% 1	1980
October	1st to 10th	Night	41% 5% 10% 26% 17% 1	1984
	11th to 20th	Night	53% 5% 11% 19% 11% 1	2173
	21st to end	Night	54% 4 11% 21% 9% 1	2574
November	1st to 10th	Night	45% 5% 11% 25% 14% 1	2406
	11th to 20th	Night	54% 5% 13% 20% 7% 1	2470
	21st to end	Night	57% 4 13% 19% 6%	2470
December	1st to 10th	Night	67% 5% 11% 12% 5%	2498
	11th to 20th	Night	65% 5% 11% 13% 6% 1	3281
	21st to end	Night	77% 4 10% 7% 3	4884

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	21% 14% 37% 19% 6% 2	270
	11th to 20th	Day	22% 14% 44% 15% 5% 2	255
	21st to end	Day	28% 18% 30% 18% 6%	264
February	1st to 10th	Day	25% 21% 37% 14% 3	240
	11th to 20th	Day	26% 17% 41% 14% 2	240
	21st to end	Day	26% 27% 34% 11% 3	198
March	1st to 10th	Day	23% 18% 43% 13% 4	240
	11th to 20th	Day	22% 14% 33% 22% 9%	411
	21st to end	Day	27% 13% 31% 21% 6% 2	462
April	1st to 10th	Day	23% 13% 32% 23% 8% 1	420
	11th to 20th	Day	26% 14% 30% 16% 8% 5%	412
	21st to end	Day	22% 12% 31% 20% 13% 2	420
May	1st to 10th	Day	24% 13% 29% 23% 9% 3	423
	11th to 20th	Day	20% 9% 27% 23% 16% 6%	447
	21st to end	Day	17% 6% 21% 27% 20% 8%	578
June	1st to 10th	Day	13% 5% 29% 30% 17% 5%	630
	11th to 20th	Day	14% 6% 28% 30% 18% 5%	630
	21st to end	Day	11% 8% 28% 30% 18% 6%	630
July	1st to 10th	Day	9% 3 19% 30% 24% 15%	621
	11th to 20th	Day	10% 6% 25% 28% 20% 11%	618
	21st to end	Day	8% 4 24% 31% 24% 9%	622
August	1st to 10th	Day	8% 2 20% 30% 18% 22%	446
	11th to 20th	Day	13% 4 16% 27% 25% 14%	439
	21st to end	Day	13% 3 19% 27% 25% 13%	495
September	1st to 10th	Day	14% 5% 21% 25% 23% 13%	450
	11th to 20th	Day	12% 3 21% 22% 27% 15%	450
	21st to end	Day	16% 11% 20% 24% 18% 11%	450
October	1st to 10th	Day	20% 7% 26% 22% 17% 8%	450
	11th to 20th	Day	21% 17% 28% 26% 8% 1	434
	21st to end	Day	30% 16% 34% 14% 5% 1	297
November	1st to 10th	Day	19% 15% 35% 23% 7% 1	270
	11th to 20th	Day	24% 20% 32% 18% 4 1	270
	21st to end	Day	27% 17% 38% 12% 6%	270
December	1st to 10th	Day	23% 24% 29% 17% 6% 1	270
	11th to 20th	Day	23% 18% 40% 14% 4	270
	21st to end	Day	23% 25% 34% 16% 2	297
January	1st to 10th	Night	22% 14% 35% 22% 6% 1	950
	11th to 20th	Night	26% 13% 35% 22% 4 1	926
	21st to end	Night	19% 11% 35% 29% 6%	922
February	1st to 10th	Night	25% 13% 36% 24% 2	720
	11th to 20th	Night	23% 14% 34% 25% 4	703
	21st to end	Night	26% 14% 37% 20% 3	592
March	1st to 10th	Night	21% 11% 36% 25% 7% 1	720
	11th to 20th	Night	18% 13% 35% 26% 6% 1	720
	21st to end	Night	23% 11% 30% 28% 8% 1	792
April	1st to 10th	Night	19% 11% 37% 27% 5% 1	553
	11th to 20th	Night	22% 9% 35% 24% 9% 1	533
	21st to end	Night	23% 7% 33% 29% 7% 1	540
May	1st to 10th	Night	20% 6% 31% 33% 9% 1	540
	11th to 20th	Night	23% 5% 29% 30% 12% 1	531
	21st to end	Night	13% 3 25% 36% 21% 2	584
June	1st to 10th	Night	12% 2 24% 44% 16% 1	540
	11th to 20th	Night	13% 4 24% 39% 19% 1	540
	21st to end	Night	11% 2 23% 41% 22% 1	540
July	1st to 10th	Night	8% 2 16% 41% 30% 3	525
	11th to 20th	Night	11% 2 17% 41% 26% 2	522
	21st to end	Night	6% 3 19% 48% 23% 2	591
August	1st to 10th	Night	8% 1 14% 46% 28% 2	537
	11th to 20th	Night	10% 1 16% 40% 29% 5%	534
	21st to end	Night	12% 2 16% 37% 29% 4	594
September	1st to 10th	Night	12% 4 18% 38% 24% 4	599
	11th to 20th	Night	13% 4 16% 36% 27% 4	720
	21st to end	Night	16% 7% 22% 29% 21% 5%	720
October	1st to 10th	Night	18% 6% 23% 33% 17% 2	720
	11th to 20th	Night	23% 13% 28% 28% 7% 1	720
	21st to end	Night	27% 10% 33% 24% 6%	792
November	1st to 10th	Night	19% 11% 29% 30% 10% 1	740
	11th to 20th	Night	21% 14% 35% 22% 7% 1	760
	21st to end	Night	25% 11% 33% 26% 4	760
December	1st to 10th	Night	26% 12% 36% 20% 6%	788
	11th to 20th	Night	23% 13% 37% 22% 4	950
	21st to end	Night	32% 11% 36% 19% 2	1045

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