Dearborn Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Dearborn Observatory clear sky chart from 2005-11-06 to 2025-09-19.

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	12% 8% 3 7% 5% 5% 5% 4 6% 8% 37%	126215
	Night	14% 10% 4 8% 5% 4 4 4 5% 7% 37%	123060
Transparency	Day	28% 39% 8% 16% 9% 1	129740
	Night	31% 38% 7% 14% 9% 1	127609
Seeing	Day	16% 2 15% 33% 26% 8%	17037
	Night	19% 5% 23% 31% 18% 4	26815

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% 10% 6% 12% 6% 6% 5% 5% 5% 8% 25%	3617
	11th to 20th	Day	10% 11% 6% 12% 5% 4 5% 6% 7% 7% 28%	3620
	21st to end	Day	16% 17% 5% 7% 3 3 3 3 5% 6% 32%	4025
February	1st to 10th	Day	13% 13% 5% 9% 6% 6% 6% 4 5% 7% 27%	1864
	11th to 20th	Day	13% 11% 4 9% 5% 4 5% 4 5% 5% 34%	1948
	21st to end	Day	12% 12% 4 10% 7% 5% 4 3 6% 6% 31%	1650
March	1st to 10th	Day	15% 10% 3 7% 4 4 3 3 4 5% 41%	4480
	11th to 20th	Day	8% 7% 2 6% 5% 5% 5% 4 6% 9% 42%	4648
	21st to end	Day	18% 12% 7% 11% 6% 6% 5% 4 5% 5% 21%	5967
April	1st to 10th	Day	16% 11% 3 8% 5% 4 4 4 6% 7% 32%	2400
	11th to 20th	Day	14% 8% 2 7% 6% 5% 6% 3 6% 7% 36%	2374
	21st to end	Day	16% 9% 3 7% 5% 5% 5% 4 6% 6% 34%	2535
May	1st to 10th	Day	17% 8% 3 6% 4 4 5% 4 6% 9% 35%	5862
	11th to 20th	Day	14% 6% 3 6% 5% 4 4 3 6% 8% 40%	6125
	21st to end	Day	14% 7% 2 5% 4 4 4 4 6% 9% 42%	6935
June	1st to 10th	Day	9% 5% 2 6% 4 5% 4 4 6% 8% 46%	2800
	11th to 20th	Day	10% 5% 2 6% 4 4 4 4 6% 10% 45%	2800
	21st to end	Day	9% 6% 2 6% 5% 5% 5% 5% 7% 9% 42%	2800
July	1st to 10th	Day	9% 6% 3 7% 5% 5% 5% 5% 8% 9% 38%	6360
	11th to 20th	Day	8% 5% 2 5% 4 5% 5% 4 7% 9% 46%	6360
	21st to end	Day	9% 7% 3 6% 4 5% 5% 5% 7% 9% 42%	6924
August	1st to 10th	Day	11% 4 2 5% 4 4 5% 4 7% 8% 46%	4436
	11th to 20th	Day	9% 5% 2 5% 4 4 4 4 6% 9% 49%	3924
	21st to end	Day	6% 5% 2 5% 4 4 4 4 6% 8% 50%	4312
September	1st to 10th	Day	11% 6% 2 5% 4 4 4 3 6% 7% 48%	2400
	11th to 20th	Day	11% 5% 2 4 2 3 3 4 5% 9% 52%	2232
	21st to end	Day	11% 6% 3 7% 5% 4 5% 4 7% 8% 40%	2090
October	1st to 10th	Day	13% 7% 3 7% 4 4 4 3 5% 6% 45%	2066
	11th to 20th	Day	17% 8% 3 8% 4 4 5% 3 5% 7% 35%	1863
	21st to end	Day	18% 9% 3 6% 5% 6% 4 4 4 8% 33%	1881
November	1st to 10th	Day	12% 9% 3 5% 5% 4 4 3 6% 8% 41%	1762
	11th to 20th	Day	14% 12% 3 10% 5% 5% 4 4 5% 7% 31%	1800
	21st to end	Day	18% 11% 3 9% 5% 5% 5% 3 4 5% 34%	1755
December	1st to 10th	Day	9% 10% 4 8% 6% 6% 5% 4 5% 8% 34%	2610
	11th to 20th	Day	12% 14% 6% 12% 7% 6% 5% 5% 5% 6% 23%	3008
	21st to end	Day	22% 20% 7% 11% 6% 5% 4 3 4 5% 14%	3982
January	1st to 10th	Night	13% 12% 5% 12% 6% 5% 4 4 5% 6% 27%	6348
	11th to 20th	Night	13% 15% 6% 12% 6% 6% 5% 4 7% 6% 21%	6360
	21st to end	Night	17% 15% 6% 8% 4 3 3 3 4 5% 32%	6950
February	1st to 10th	Night	15% 15% 4 8% 5% 4 4 3 4 5% 32%	2535
	11th to 20th	Night	13% 11% 4 8% 5% 4 5% 4 5% 6% 35%	2338
	21st to end	Night	18% 11% 3 8% 4 4 3 3 4 5% 36%	1980
March	1st to 10th	Night	16% 10% 3 8% 4 4 4 3 4 6% 40%	5373
	11th to 20th	Night	9% 7% 2 6% 4 4 4 3 5% 8% 47%	5316
	21st to end	Night	17% 12% 7% 9% 5% 5% 4 3 4 5% 28%	4972
April	1st to 10th	Night	17% 8% 3 6% 5% 4 5% 3 5% 7% 37%	2000
	11th to 20th	Night	19% 9% 2 5% 5% 4 5% 3 5% 6% 38%	1975
	21st to end	Night	17% 8% 3 7% 4 4 5% 4 5% 7% 37%	1863
May	1st to 10th	Night	19% 8% 3 7% 5% 4 4 4 5% 7% 34%	4033
	11th to 20th	Night	14% 7% 4 8% 4 3 4 3 6% 7% 40%	3778
	21st to end	Night	17% 8% 3 7% 4 3 4 3 4 6% 42%	3942
June	1st to 10th	Night	8% 7% 3 4 5% 5% 5% 4 5% 8% 47%	1600
	11th to 20th	Night	10% 7% 2 5% 4 4 5% 5% 5% 9% 43%	1600
	21st to end	Night	9% 7% 2 6% 5% 5% 5% 5% 6% 8% 42%	1600
July	1st to 10th	Night	10% 5% 3 7% 5% 5% 5% 4 7% 9% 40%	3618
	11th to 20th	Night	6% 5% 2 6% 4 4 4 4 6% 9% 50%	3617
	21st to end	Night	9% 5% 2 6% 4 5% 5% 5% 6% 8% 45%	3947
August	1st to 10th	Night	7% 5% 2 6% 5% 4 5% 4 7% 8% 47%	2716
	11th to 20th	Night	8% 5% 2 5% 4 4 4 3 6% 7% 51%	3121
	21st to end	Night	8% 4 2 5% 4 4 4 3 6% 9% 51%	3586
September	1st to 10th	Night	10% 6% 2 5% 4 3 4 3 5% 8% 50%	2000
	11th to 20th	Night	10% 5% 1 5% 3 3 4 3 5% 9% 53%	2104
	21st to end	Night	13% 7% 2 6% 4 4 5% 3 5% 7% 44%	2090
October	1st to 10th	Night	13% 7% 2 5% 3 4 4 4 5% 6% 49%	2110
	11th to 20th	Night	17% 8% 2 7% 3 4 4 3 4 6% 41%	2316
	21st to end	Night	21% 8% 3 7% 4 4 4 3 5% 7% 36%	2717
November	1st to 10th	Night	12% 8% 2 6% 4 4 4 4 5% 6% 44%	2536
	11th to 20th	Night	15% 11% 3 8% 4 4 4 4 6% 7% 33%	2600
	21st to end	Night	17% 10% 4 8% 5% 5% 4 4 5% 5% 34%	2644
December	1st to 10th	Night	11% 11% 3 8% 6% 5% 5% 4 5% 10% 34%	4580
	11th to 20th	Night	15% 14% 4 10% 6% 5% 5% 4 5% 6% 26%	5199
	21st to end	Night	23% 18% 6% 10% 6% 4 4 3 4 5% 16%	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	47% 29% 9% 11% 4	3618
	11th to 20th	Day	42% 28% 13% 7% 9% 1	3620
	21st to end	Day	39% 27% 9% 18% 8%	4025
February	1st to 10th	Day	21% 61% 5% 7% 5%	1864
	11th to 20th	Day	21% 60% 2 11% 5% 1	1958
	21st to end	Day	20% 62% 4 10% 4 1	1650
March	1st to 10th	Day	30% 30% 7% 19% 10% 4	4480
	11th to 20th	Day	25% 30% 10% 27% 7%	4648
	21st to end	Day	47% 31% 8% 8% 5% 1	5969
April	1st to 10th	Day	22% 61% 3 9% 4	2400
	11th to 20th	Day	16% 63% 5% 9% 6% 1	2374
	21st to end	Day	17% 61% 6% 8% 7% 1	2535
May	1st to 10th	Day	32% 31% 9% 16% 10% 2	5897
	11th to 20th	Day	30% 31% 7% 20% 10% 1	6129
	21st to end	Day	27% 30% 8% 21% 12% 2	6949
June	1st to 10th	Day	12% 61% 5% 11% 10% 1	2800
	11th to 20th	Day	16% 62% 6% 9% 6% 1	2800
	21st to end	Day	13% 62% 6% 12% 6% 1	2800
July	1st to 10th	Day	31% 28% 10% 20% 10% 1	6291
	11th to 20th	Day	24% 28% 12% 23% 12%	6285
	21st to end	Day	29% 27% 12% 17% 14% 1	6728
August	1st to 10th	Day	22% 37% 12% 20% 9%	4421
	11th to 20th	Day	20% 37% 8% 25% 9%	3962
	21st to end	Day	21% 37% 10% 18% 14% 1	4311
September	1st to 10th	Day	15% 57% 6% 13% 7% 1	2400
	11th to 20th	Day	11% 57% 4 16% 9% 2	2232
	21st to end	Day	19% 60% 3 11% 7% 1	2090
October	1st to 10th	Day	14% 36% 4 29% 16% 1	3430
	11th to 20th	Day	19% 37% 7% 20% 16%	3079
	21st to end	Day	28% 37% 6% 19% 8% 3	3081
November	1st to 10th	Day	22% 56% 7% 8% 7% 1	1762
	11th to 20th	Day	25% 56% 4 9% 5% 1	1800
	21st to end	Day	28% 56% 5% 8% 3	1755
December	1st to 10th	Day	32% 37% 12% 15% 3	2610
	11th to 20th	Day	43% 32% 9% 11% 4	3005
	21st to end	Day	58% 27% 5% 8% 2	3982
January	1st to 10th	Night	47% 29% 7% 13% 5%	6348
	11th to 20th	Night	51% 28% 10% 4 7%	6360
	21st to end	Night	41% 28% 12% 12% 7%	6950
February	1st to 10th	Night	22% 61% 4 6% 6% 1	2535
	11th to 20th	Night	20% 60% 2 12% 6%	2339
	21st to end	Night	18% 61% 4 9% 5% 2	1980
March	1st to 10th	Night	33% 28% 8% 19% 9% 2	5373
	11th to 20th	Night	25% 30% 8% 25% 12% 1	5316
	21st to end	Night	43% 30% 6% 13% 7%	4972
April	1st to 10th	Night	21% 61% 3 11% 4 1	2000
	11th to 20th	Night	20% 62% 3 7% 6% 2	1975
	21st to end	Night	17% 62% 3 10% 8% 2	1863
May	1st to 10th	Night	36% 30% 8% 10% 13% 3	4060
	11th to 20th	Night	32% 30% 10% 17% 9% 3	3779
	21st to end	Night	34% 29% 5% 17% 13% 3	3950
June	1st to 10th	Night	14% 61% 4 12% 9% 1	1600
	11th to 20th	Night	19% 61% 3 10% 7%	1600
	21st to end	Night	15% 61% 5% 11% 8%	1600
July	1st to 10th	Night	32% 27% 10% 21% 6% 3	3580
	11th to 20th	Night	24% 27% 11% 23% 14% 1	3572
	21st to end	Night	27% 27% 10% 19% 16% 2	3832
August	1st to 10th	Night	20% 38% 11% 19% 11% 1	2707
	11th to 20th	Night	21% 37% 8% 21% 13%	3149
	21st to end	Night	21% 37% 8% 21% 11% 1	3586
September	1st to 10th	Night	13% 57% 6% 15% 9% 1	2000
	11th to 20th	Night	12% 57% 2 17% 12% 1	2104
	21st to end	Night	19% 59% 3 11% 7%	2090
October	1st to 10th	Night	12% 36% 5% 29% 17% 1	3526
	11th to 20th	Night	25% 37% 4 15% 18% 1	3839
	21st to end	Night	25% 36% 5% 18% 14% 1	4467
November	1st to 10th	Night	24% 55% 3 10% 6% 2	2536
	11th to 20th	Night	23% 56% 3 11% 6% 1	2600
	21st to end	Night	28% 56% 4 9% 3	2644
December	1st to 10th	Night	33% 37% 9% 15% 4 1	4580
	11th to 20th	Night	42% 33% 9% 10% 5%	5201
	21st to end	Night	54% 28% 7% 8% 3	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	21% 7% 25% 33% 14% 1	300
	11th to 20th	Day	19% 8% 31% 31% 9% 2	300
	21st to end	Day	22% 1 27% 35% 14% 2	330
February	1st to 10th	Day	18% 4 29% 37% 9% 2	300
	11th to 20th	Day	19% 7% 30% 33% 11% 1	300
	21st to end	Day	16% 2 32% 39% 10% 1	249
March	1st to 10th	Day	16% 3 26% 42% 12%	300
	11th to 20th	Day	19% 3 26% 35% 14% 2	345
	21st to end	Day	24% 3 25% 35% 10% 2	550
April	1st to 10th	Day	20% 4 22% 40% 14% 1	500
	11th to 20th	Day	15% 3 22% 40% 15% 5%	492
	21st to end	Day	21% 2 17% 38% 19% 4	500
May	1st to 10th	Day	22% 1 16% 38% 20% 3	503
	11th to 20th	Day	18% 1 8% 41% 27% 5%	635
	21st to end	Day	16% 1 7% 36% 29% 12%	789
June	1st to 10th	Day	12% 1 8% 36% 32% 11%	730
	11th to 20th	Day	13% 1 6% 35% 34% 10%	730
	21st to end	Day	11% 2 7% 37% 36% 7%	730
July	1st to 10th	Day	8% 1 5% 27% 40% 18%	721
	11th to 20th	Day	11% 1 6% 32% 37% 13%	718
	21st to end	Day	8% 1 7% 32% 43% 11%	794
August	1st to 10th	Day	8% 4 28% 42% 17%	651
	11th to 20th	Day	11% 2 7% 29% 38% 13%	519
	21st to end	Day	9% 1 5% 27% 43% 15%	583
September	1st to 10th	Day	14% 2 10% 25% 36% 13%	530
	11th to 20th	Day	10% 2 7% 22% 40% 20%	522
	21st to end	Day	15% 3 17% 24% 29% 12%	490
October	1st to 10th	Day	18% 2 13% 30% 28% 9%	466
	11th to 20th	Day	21% 5% 21% 27% 24% 3	300
	21st to end	Day	28% 3 21% 34% 13% 1	330
November	1st to 10th	Day	17% 3 23% 30% 23% 4	300
	11th to 20th	Day	24% 7% 28% 25% 12% 4	300
	21st to end	Day	27% 4 24% 36% 8%	300
December	1st to 10th	Day	19% 6% 32% 30% 11% 1	300
	11th to 20th	Day	16% 8% 37% 27% 12% 1	300
	21st to end	Day	29% 5% 33% 25% 8% 1	330
January	1st to 10th	Night	20% 10% 36% 27% 7% 1	1000
	11th to 20th	Night	24% 8% 34% 27% 7% 1	1000
	21st to end	Night	22% 8% 34% 29% 7% 1	1100
February	1st to 10th	Night	22% 6% 36% 28% 8% 1	935
	11th to 20th	Night	20% 7% 33% 33% 7%	783
	21st to end	Night	24% 5% 32% 33% 6%	660
March	1st to 10th	Night	22% 4 31% 32% 11% 1	800
	11th to 20th	Night	15% 4 27% 40% 12% 2	800
	21st to end	Night	22% 4 26% 33% 14% 1	670
April	1st to 10th	Night	21% 5% 26% 35% 12% 2	600
	11th to 20th	Night	23% 2 22% 35% 16% 2	593
	21st to end	Night	20% 3 21% 37% 18% 2	600
May	1st to 10th	Night	19% 3 15% 44% 18% 3	600
	11th to 20th	Night	19% 1 13% 33% 29% 5%	591
	21st to end	Night	16% 1 9% 34% 32% 8%	650
June	1st to 10th	Night	12% 1 9% 39% 34% 6%	600
	11th to 20th	Night	14% 1 9% 38% 33% 6%	600
	21st to end	Night	10% 2 9% 36% 37% 7%	600
July	1st to 10th	Night	10% 1 3 30% 40% 16%	585
	11th to 20th	Night	10% 1 6% 33% 39% 12%	582
	21st to end	Night	8% 1 6% 35% 42% 7%	657
August	1st to 10th	Night	8% 1 5% 34% 42% 11%	597
	11th to 20th	Night	10% 1 8% 31% 37% 13%	594
	21st to end	Night	11% 1 9% 31% 38% 10%	660
September	1st to 10th	Night	13% 1 13% 29% 34% 10%	600
	11th to 20th	Night	14% 2 9% 27% 34% 15%	718
	21st to end	Night	16% 4 18% 27% 26% 11%	760
October	1st to 10th	Night	19% 2 18% 31% 24% 7%	760
	11th to 20th	Night	23% 5% 26% 28% 16% 2	760
	21st to end	Night	27% 7% 25% 28% 12% 1	836
November	1st to 10th	Night	19% 6% 31% 27% 16% 1	780
	11th to 20th	Night	22% 12% 32% 25% 8% 1	800
	21st to end	Night	22% 12% 33% 26% 7%	844
December	1st to 10th	Night	23% 11% 36% 24% 6%	1000
	11th to 20th	Night	23% 9% 36% 26% 6%	1000
	21st to end	Night	31% 8% 32% 25% 3	1100

Page generated at 2025-09-16 00:49:11. Version ac07

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