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 2024-11-06.

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% 3 8% 5% 5% 5% 4 5% 7% 34%	101227
	Night	15% 11% 4 8% 5% 4 4 3 5% 6% 34%	97861
Transparency	Day	48% 5% 12% 22% 13% 1	104827
	Night	49% 4 11% 22% 13% 1	102401
Seeing	Day	18% 11% 28% 24% 14% 6%	15760
	Night	19% 9% 29% 30% 12% 1	25599

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	16% 18% 7% 12% 6% 5% 4 4 4 5% 19%	2527
	11th to 20th	Day	15% 19% 6% 14% 9% 11% 4 3 4 4 10%	2530
	21st to end	Day	27% 22% 7% 11% 6% 3 3 2 3 4 13%	2889
February	1st to 10th	Day	13% 14% 5% 13% 9% 6% 5% 4 5% 6% 20%	1711
	11th to 20th	Day	14% 13% 6% 13% 8% 6% 4 3 3 5% 25%	1848
	21st to end	Day	16% 15% 5% 12% 5% 4 4 4 5% 5% 25%	1570
March	1st to 10th	Day	16% 16% 4 8% 5% 4 4 3 5% 6% 29%	3120
	11th to 20th	Day	12% 11% 4 9% 7% 5% 5% 4 5% 7% 30%	3471
	21st to end	Day	19% 13% 4 9% 5% 4 4 3 4 5% 29%	3832
April	1st to 10th	Day	18% 13% 3 8% 6% 4 4 4 5% 5% 30%	2280
	11th to 20th	Day	17% 10% 2 7% 5% 5% 4 4 5% 6% 36%	2361
	21st to end	Day	17% 9% 3 9% 5% 4 4 3 5% 7% 34%	2470
May	1st to 10th	Day	17% 8% 3 7% 4 4 5% 4 5% 6% 36%	4082
	11th to 20th	Day	16% 7% 2 6% 4 4 5% 4 6% 8% 38%	4081
	21st to end	Day	11% 6% 3 6% 4 4 4 3 5% 8% 45%	4653
June	1st to 10th	Day	10% 6% 2 7% 5% 5% 5% 5% 5% 8% 43%	2660
	11th to 20th	Day	11% 5% 2 7% 5% 5% 4 4 6% 8% 43%	2660
	21st to end	Day	9% 5% 2 6% 6% 6% 5% 4 7% 9% 40%	2660
July	1st to 10th	Day	11% 6% 3 8% 5% 5% 6% 4 8% 8% 37%	4440
	11th to 20th	Day	7% 4 1 5% 4 4 5% 4 6% 10% 49%	4440
	21st to end	Day	6% 5% 2 6% 5% 5% 6% 5% 7% 9% 44%	4698
August	1st to 10th	Day	12% 5% 2 6% 5% 5% 5% 4 6% 9% 42%	4104
	11th to 20th	Day	11% 6% 2 6% 4 5% 4 3 6% 8% 44%	4044
	21st to end	Day	8% 5% 3 6% 5% 5% 6% 5% 7% 9% 41%	4355
September	1st to 10th	Day	11% 5% 2 8% 5% 4 5% 4 7% 7% 42%	2197
	11th to 20th	Day	10% 5% 2 5% 4 4 4 4 6% 8% 48%	2090
	21st to end	Day	13% 8% 2 8% 5% 4 5% 4 6% 7% 38%	2090
October	1st to 10th	Day	14% 6% 3 7% 5% 5% 4 4 5% 7% 42%	2085
	11th to 20th	Day	18% 9% 3 8% 5% 5% 5% 4 5% 7% 32%	1883
	21st to end	Day	20% 12% 4 8% 5% 4 5% 3 6% 5% 28%	1881
November	1st to 10th	Day	12% 11% 3 7% 4 4 4 4 5% 6% 38%	1717
	11th to 20th	Day	14% 15% 4 9% 6% 4 4 4 6% 6% 27%	1710
	21st to end	Day	19% 14% 5% 9% 5% 4 4 3 4 6% 28%	1710
December	1st to 10th	Day	14% 17% 8% 14% 7% 4 5% 3 4 5% 19%	1677
	11th to 20th	Day	14% 19% 6% 10% 6% 5% 5% 4 6% 6% 20%	1918
	21st to end	Day	23% 22% 8% 12% 5% 4 4 3 3 4 11%	2783
January	1st to 10th	Night	16% 18% 7% 12% 6% 4 4 3 4 5% 20%	4428
	11th to 20th	Night	15% 17% 8% 16% 7% 8% 4 3 4 4 13%	4440
	21st to end	Night	23% 18% 8% 12% 5% 4 4 3 3 4 14%	4760
February	1st to 10th	Night	15% 17% 5% 11% 8% 5% 4 3 5% 5% 22%	2466
	11th to 20th	Night	14% 13% 5% 10% 9% 5% 5% 4 5% 5% 26%	2218
	21st to end	Night	17% 12% 4 11% 5% 4 4 4 5% 6% 28%	1884
March	1st to 10th	Night	15% 11% 4 8% 5% 4 4 2 5% 7% 36%	3733
	11th to 20th	Night	9% 9% 2 6% 4 4 5% 3 5% 7% 45%	3488
	21st to end	Night	17% 10% 4 10% 5% 5% 4 3 5% 6% 32%	3817
April	1st to 10th	Night	16% 11% 3 7% 4 4 4 4 5% 7% 35%	1913
	11th to 20th	Night	19% 9% 2 7% 5% 4 4 3 5% 6% 36%	1768
	21st to end	Night	17% 8% 2 7% 5% 5% 4 4 5% 6% 38%	1710
May	1st to 10th	Night	15% 9% 2 7% 5% 4 4 4 6% 9% 35%	2813
	11th to 20th	Night	17% 7% 2 6% 4 3 3 3 4 6% 43%	2812
	21st to end	Night	11% 6% 2 6% 3 4 5% 4 6% 8% 46%	2905
June	1st to 10th	Night	12% 5% 2 5% 4 4 4 4 6% 9% 45%	1520
	11th to 20th	Night	11% 5% 2 5% 5% 4 4 3 6% 8% 46%	1520
	21st to end	Night	9% 6% 2 6% 4 4 4 4 7% 10% 45%	1520
July	1st to 10th	Night	9% 4 2 6% 4 5% 5% 4 7% 10% 45%	2530
	11th to 20th	Night	9% 5% 1 4 3 3 4 3 5% 7% 55%	2530
	21st to end	Night	6% 4 2 7% 5% 5% 5% 4 6% 9% 47%	2859
August	1st to 10th	Night	10% 5% 2 6% 4 3 4 3 6% 9% 47%	2825
	11th to 20th	Night	12% 6% 2 5% 4 4 4 3 6% 8% 47%	2785
	21st to end	Night	10% 6% 3 7% 4 4 4 4 6% 7% 46%	3284
September	1st to 10th	Night	8% 6% 2 6% 4 4 5% 4 6% 7% 49%	1963
	11th to 20th	Night	10% 4 2 5% 3 4 3 2 5% 6% 56%	2090
	21st to end	Night	14% 6% 3 6% 4 4 5% 4 4 7% 42%	2090
October	1st to 10th	Night	15% 7% 2 6% 4 3 3 3 5% 6% 46%	2095
	11th to 20th	Night	19% 11% 3 7% 5% 5% 4 2 5% 5% 33%	2294
	21st to end	Night	21% 10% 3 7% 4 4 5% 3 5% 7% 32%	2717
November	1st to 10th	Night	14% 10% 3 7% 4 4 4 3 4 5% 41%	2483
	11th to 20th	Night	14% 14% 4 9% 5% 5% 6% 4 6% 7% 28%	2470
	21st to end	Night	18% 15% 4 8% 5% 4 4 3 5% 6% 28%	2470
December	1st to 10th	Night	16% 18% 7% 12% 6% 5% 5% 4 4 5% 19%	2498
	11th to 20th	Night	14% 16% 6% 14% 7% 5% 5% 3 5% 4 21%	3279
	21st to end	Night	26% 22% 6% 10% 4 4 3 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% 4 11% 14% 3	2528
	11th to 20th	Day	77% 5% 9% 7% 2	2530
	21st to end	Day	78% 3 9% 8% 3	2889
February	1st to 10th	Day	64% 5% 10% 14% 7%	1711
	11th to 20th	Day	64% 2 7% 16% 9% 1	1858
	21st to end	Day	62% 4 11% 16% 6% 2	1570
March	1st to 10th	Day	56% 5% 11% 20% 8% 1	3120
	11th to 20th	Day	52% 5% 12% 21% 9% 1	3471
	21st to end	Day	58% 4 11% 16% 9% 2	3832
April	1st to 10th	Day	55% 5% 10% 19% 11%	2280
	11th to 20th	Day	48% 5% 10% 20% 14% 2	2361
	21st to end	Day	49% 5% 12% 18% 14% 2	2470
May	1st to 10th	Day	47% 7% 10% 22% 13% 1	4120
	11th to 20th	Day	43% 6% 14% 25% 10% 1	4085
	21st to end	Day	37% 5% 11% 21% 23% 3	4666
June	1st to 10th	Day	37% 6% 12% 27% 16% 2	2660
	11th to 20th	Day	38% 6% 12% 25% 18% 1	2660
	21st to end	Day	39% 7% 13% 23% 16% 2	2660
July	1st to 10th	Day	43% 5% 13% 22% 15% 3	4371
	11th to 20th	Day	30% 4 13% 34% 18% 1	4365
	21st to end	Day	35% 6% 15% 21% 21% 2	4549
August	1st to 10th	Day	39% 5% 12% 26% 17% 1	4091
	11th to 20th	Day	38% 5% 11% 33% 13% 1	4083
	21st to end	Day	38% 4 13% 30% 13% 2	4354
September	1st to 10th	Day	40% 6% 12% 25% 15% 2	2197
	11th to 20th	Day	34% 4 12% 31% 18% 1	2090
	21st to end	Day	44% 6% 12% 23% 15%	2090
October	1st to 10th	Day	32% 3 9% 34% 21% 1	3462
	11th to 20th	Day	42% 3 11% 27% 17%	3111
	21st to end	Day	49% 3 12% 25% 10% 1	3081
November	1st to 10th	Day	46% 6% 12% 22% 13% 1	1717
	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	67% 4 10% 13% 5% 1	4428
	11th to 20th	Night	74% 4 9% 8% 5%	4440
	21st to end	Night	74% 4 8% 11% 3	4760
February	1st to 10th	Night	64% 6% 9% 14% 7% 1	2466
	11th to 20th	Night	59% 4 9% 18% 9% 1	2219
	21st to end	Night	56% 4 13% 17% 8% 2	1884
March	1st to 10th	Night	48% 5% 13% 26% 7% 1	3733
	11th to 20th	Night	38% 6% 14% 22% 20% 1	3488
	21st to end	Night	53% 4 12% 21% 9% 1	3817
April	1st to 10th	Night	48% 5% 11% 21% 15% 1	1913
	11th to 20th	Night	48% 5% 9% 19% 17% 2	1768
	21st to end	Night	46% 4 13% 19% 15% 3	1710
May	1st to 10th	Night	46% 6% 10% 22% 13% 3	2840
	11th to 20th	Night	42% 5% 9% 27% 15% 2	2813
	21st to end	Night	36% 6% 11% 23% 20% 5%	2913
June	1st to 10th	Night	35% 6% 11% 28% 18% 3	1520
	11th to 20th	Night	36% 5% 13% 25% 18% 3	1520
	21st to end	Night	33% 6% 15% 27% 18% 1	1520
July	1st to 10th	Night	34% 5% 11% 30% 14% 7%	2490
	11th to 20th	Night	29% 3 12% 32% 22% 2	2482
	21st to end	Night	33% 6% 17% 18% 24% 2	2768
August	1st to 10th	Night	34% 4 14% 29% 18% 1	2814
	11th to 20th	Night	35% 5% 10% 35% 14% 1	2812
	21st to end	Night	38% 4 11% 31% 15% 1	3284
September	1st to 10th	Night	33% 7% 10% 29% 19% 2	1963
	11th to 20th	Night	30% 4 10% 31% 22% 2	2090
	21st to end	Night	41% 5% 12% 27% 15% 1	2090
October	1st to 10th	Night	27% 3 9% 37% 24% 1	3498
	11th to 20th	Night	46% 3 10% 22% 19%	3805
	21st to end	Night	41% 3 11% 31% 14% 1	4467
November	1st to 10th	Night	45% 5% 11% 25% 14% 1	2483
	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	20% 14% 44% 15% 4 1	270
	21st to end	Day	30% 16% 29% 20% 5%	297
February	1st to 10th	Day	23% 21% 36% 16% 3 1	270
	11th to 20th	Day	24% 17% 40% 17% 2	270
	21st to end	Day	24% 26% 35% 12% 3	225
March	1st to 10th	Day	23% 17% 42% 14% 4	270
	11th to 20th	Day	22% 14% 33% 21% 8%	451
	21st to end	Day	27% 13% 32% 21% 6% 1	506
April	1st to 10th	Day	22% 15% 33% 23% 7% 1	460
	11th to 20th	Day	26% 15% 31% 16% 8% 5%	452
	21st to end	Day	21% 12% 32% 22% 13% 2	460
May	1st to 10th	Day	22% 14% 29% 22% 9% 2	463
	11th to 20th	Day	19% 8% 27% 23% 17% 6%	487
	21st to end	Day	17% 6% 22% 28% 20% 8%	628
June	1st to 10th	Day	13% 7% 29% 29% 17% 5%	680
	11th to 20th	Day	13% 6% 29% 30% 17% 5%	680
	21st to end	Day	11% 8% 29% 29% 17% 6%	680
July	1st to 10th	Day	10% 4 20% 30% 23% 14%	671
	11th to 20th	Day	10% 6% 25% 29% 20% 11%	668
	21st to end	Day	8% 4 23% 32% 24% 9%	673
August	1st to 10th	Day	9% 2 20% 29% 18% 21%	486
	11th to 20th	Day	13% 5% 17% 28% 25% 13%	479
	21st to end	Day	12% 3 19% 28% 24% 13%	539
September	1st to 10th	Day	13% 5% 22% 24% 23% 13%	490
	11th to 20th	Day	11% 2 20% 23% 28% 17%	490
	21st to end	Day	18% 10% 20% 24% 18% 10%	490
October	1st to 10th	Day	20% 8% 26% 23% 17% 7%	490
	11th to 20th	Day	20% 16% 28% 26% 9% 1	473
	21st to end	Day	29% 15% 35% 15% 5% 1	330
November	1st to 10th	Day	19% 15% 34% 23% 7% 1	285
	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	25% 13% 34% 22% 4 1	950
	21st to end	Night	20% 10% 35% 28% 6%	973
February	1st to 10th	Night	24% 13% 36% 25% 3	760
	11th to 20th	Night	22% 14% 34% 25% 4	743
	21st to end	Night	25% 15% 37% 20% 3	628
March	1st to 10th	Night	21% 11% 36% 25% 7% 1	760
	11th to 20th	Night	17% 14% 36% 26% 6% 1	760
	21st to end	Night	22% 11% 31% 28% 8%	836
April	1st to 10th	Night	19% 11% 38% 26% 5% 1	583
	11th to 20th	Night	23% 10% 35% 23% 8% 1	563
	21st to end	Night	22% 7% 34% 29% 7% 1	570
May	1st to 10th	Night	19% 6% 32% 33% 9% 1	570
	11th to 20th	Night	22% 5% 29% 31% 12% 1	561
	21st to end	Night	13% 3 25% 36% 21% 2	617
June	1st to 10th	Night	12% 3 25% 43% 16% 1	570
	11th to 20th	Night	13% 4 24% 40% 19% 1	570
	21st to end	Night	11% 2 24% 41% 21% 1	570
July	1st to 10th	Night	8% 2 17% 42% 28% 3	555
	11th to 20th	Night	11% 2 17% 41% 27% 3	552
	21st to end	Night	6% 2 18% 48% 23% 2	624
August	1st to 10th	Night	8% 1 15% 46% 28% 2	567
	11th to 20th	Night	10% 1 17% 40% 28% 5%	564
	21st to end	Night	12% 2 16% 38% 28% 3	627
September	1st to 10th	Night	11% 4 19% 37% 24% 4	633
	11th to 20th	Night	12% 4 16% 36% 29% 4	760
	21st to end	Night	17% 6% 22% 30% 20% 5%	760
October	1st to 10th	Night	17% 6% 24% 34% 17% 2	760
	11th to 20th	Night	22% 12% 28% 28% 8% 1	760
	21st to end	Night	25% 10% 33% 24% 7%	836
November	1st to 10th	Night	20% 11% 29% 29% 10% 1	764
	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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