Detroit
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Detroit clear sky chart from 2005-11-06 to 2025-09-04.

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	14% 9% 3 8% 5% 5% 5% 4 6% 7% 33%	125489
	Night	14% 11% 4 8% 5% 4 4 4 5% 6% 34%	123307
Transparency	Day	46% 6% 13% 21% 12% 1	129073
	Night	48% 6% 12% 21% 13% 1	127793
Seeing	Day	16% 9% 28% 27% 15% 6%	16463
	Night	18% 8% 28% 30% 13% 2	26131

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% 14% 7% 17% 7% 5% 4 3 4 5% 23%	3617
	11th to 20th	Day	14% 15% 6% 18% 8% 8% 7% 4 5% 4 11%	3681
	21st to end	Day	22% 16% 7% 13% 6% 4 5% 3 5% 5% 14%	4466
February	1st to 10th	Day	15% 15% 5% 12% 8% 6% 4 4 6% 5% 21%	1800
	11th to 20th	Day	14% 13% 5% 13% 7% 6% 6% 3 5% 6% 23%	1876
	21st to end	Day	17% 13% 4 11% 6% 5% 4 4 6% 6% 26%	1650
March	1st to 10th	Day	16% 13% 5% 10% 5% 4 4 4 4 5% 29%	4633
	11th to 20th	Day	12% 11% 4 9% 5% 5% 4 3 5% 7% 34%	4980
	21st to end	Day	17% 15% 3 10% 5% 5% 4 3 4 6% 28%	5465
April	1st to 10th	Day	16% 13% 3 6% 5% 5% 5% 3 5% 7% 31%	2297
	11th to 20th	Day	17% 9% 3 7% 5% 4 5% 4 5% 7% 34%	2552
	21st to end	Day	18% 9% 2 7% 4 5% 5% 3 5% 6% 35%	2600
May	1st to 10th	Day	19% 11% 3 8% 4 4 4 3 5% 6% 33%	5862
	11th to 20th	Day	18% 7% 3 6% 5% 5% 5% 4 6% 8% 33%	5861
	21st to end	Day	15% 6% 3 7% 4 4 4 3 6% 7% 41%	6433
June	1st to 10th	Day	10% 6% 2 7% 6% 5% 5% 5% 6% 8% 40%	2838
	11th to 20th	Day	10% 6% 3 6% 5% 5% 6% 4 7% 8% 40%	3000
	21st to end	Day	9% 6% 2 6% 5% 5% 6% 4 8% 10% 37%	2862
July	1st to 10th	Day	9% 5% 2 6% 5% 5% 5% 5% 7% 10% 42%	6360
	11th to 20th	Day	7% 5% 2 5% 5% 5% 6% 4 7% 11% 43%	6360
	21st to end	Day	7% 5% 2 6% 5% 5% 5% 4 7% 10% 43%	6424
August	1st to 10th	Day	11% 6% 2 5% 4 5% 5% 4 7% 9% 41%	4234
	11th to 20th	Day	9% 5% 3 7% 5% 6% 6% 6% 9% 10% 35%	4174
	21st to end	Day	8% 5% 2 7% 5% 5% 6% 5% 7% 10% 39%	4658
September	1st to 10th	Day	10% 6% 3 7% 5% 5% 5% 4 6% 8% 42%	2164
	11th to 20th	Day	9% 5% 2 5% 4 4 4 4 7% 9% 47%	2090
	21st to end	Day	16% 7% 3 7% 4 4 4 4 6% 8% 38%	2090
October	1st to 10th	Day	12% 5% 3 7% 6% 4 4 3 6% 8% 42%	2024
	11th to 20th	Day	17% 8% 3 7% 5% 5% 4 4 6% 7% 34%	1900
	21st to end	Day	19% 10% 3 7% 6% 5% 5% 4 5% 7% 31%	1962
November	1st to 10th	Day	11% 7% 4 7% 5% 4 5% 4 5% 8% 39%	1762
	11th to 20th	Day	13% 12% 4 8% 5% 5% 5% 4 6% 7% 31%	1800
	21st to end	Day	17% 13% 4 9% 5% 6% 4 3 5% 7% 27%	1667
December	1st to 10th	Day	13% 13% 5% 13% 7% 6% 5% 4 6% 6% 22%	2600
	11th to 20th	Day	20% 19% 5% 9% 5% 4 4 4 5% 6% 19%	2880
	21st to end	Day	26% 19% 7% 12% 6% 4 3 2 3 4 12%	3867
January	1st to 10th	Night	13% 15% 6% 11% 6% 5% 5% 3 5% 6% 26%	6348
	11th to 20th	Night	12% 17% 8% 14% 8% 5% 6% 4 5% 6% 16%	6291
	21st to end	Night	18% 16% 7% 10% 5% 4 4 3 4 5% 24%	6490
February	1st to 10th	Night	16% 16% 4 11% 8% 5% 4 4 4 5% 23%	2600
	11th to 20th	Night	15% 13% 5% 9% 7% 5% 5% 4 5% 6% 26%	2407
	21st to end	Night	18% 12% 4 9% 4 4 5% 4 5% 5% 29%	1980
March	1st to 10th	Night	15% 12% 4 8% 5% 4 4 3 5% 6% 35%	5218
	11th to 20th	Night	9% 8% 3 7% 5% 4 5% 4 6% 8% 41%	4980
	21st to end	Night	16% 9% 3 9% 5% 4 4 3 4 6% 37%	5467
April	1st to 10th	Night	17% 12% 2 6% 5% 4 4 4 5% 7% 35%	2120
	11th to 20th	Night	17% 8% 3 7% 4 4 4 4 6% 8% 36%	1797
	21st to end	Night	16% 8% 2 7% 4 4 4 4 5% 7% 39%	1800
May	1st to 10th	Night	20% 10% 3 9% 5% 4 5% 3 4 6% 32%	4033
	11th to 20th	Night	16% 8% 3 7% 4 4 4 4 5% 7% 38%	4032
	21st to end	Night	13% 7% 2 6% 4 4 5% 3 6% 7% 43%	4421
June	1st to 10th	Night	12% 6% 1 5% 5% 3 4 4 5% 8% 46%	1552
	11th to 20th	Night	10% 5% 2 7% 5% 4 5% 4 6% 9% 43%	1400
	21st to end	Night	9% 5% 2 6% 4 5% 4 3 6% 8% 49%	1528
July	1st to 10th	Night	7% 4 2 5% 4 4 5% 4 6% 10% 48%	3618
	11th to 20th	Night	9% 4 2 5% 4 4 4 4 6% 9% 47%	3617
	21st to end	Night	7% 5% 2 5% 4 4 5% 4 6% 9% 48%	4422
August	1st to 10th	Night	12% 7% 2 6% 4 4 4 3 5% 8% 46%	2915
	11th to 20th	Night	8% 6% 2 6% 4 4 5% 3 6% 7% 49%	2875
	21st to end	Night	7% 5% 2 6% 5% 5% 5% 4 6% 8% 48%	3223
September	1st to 10th	Night	9% 6% 3 6% 5% 4 4 4 5% 6% 49%	2061
	11th to 20th	Night	9% 5% 2 5% 3 3 4 3 5% 7% 55%	2090
	21st to end	Night	15% 7% 2 6% 4 4 4 4 5% 6% 42%	2090
October	1st to 10th	Night	13% 6% 2 6% 4 4 3 3 5% 7% 47%	2156
	11th to 20th	Night	17% 9% 3 8% 4 4 4 3 5% 6% 38%	2280
	21st to end	Night	20% 8% 3 6% 6% 5% 4 4 4 6% 34%	2631
November	1st to 10th	Night	12% 7% 2 7% 4 4 5% 3 5% 6% 43%	2536
	11th to 20th	Night	16% 10% 3 9% 5% 4 5% 5% 6% 7% 31%	2600
	21st to end	Night	20% 13% 4 8% 5% 4 4 3 5% 7% 28%	2728
December	1st to 10th	Night	13% 13% 6% 13% 6% 6% 5% 4 6% 6% 24%	4580
	11th to 20th	Night	16% 17% 6% 12% 6% 5% 5% 3 4 6% 21%	5315
	21st to end	Night	27% 19% 6% 10% 5% 3 3 3 4 4 15%	7106

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	61% 6% 11% 14% 7% 1	3618
	11th to 20th	Day	71% 5% 10% 7% 6%	3681
	21st to end	Day	70% 4 12% 8% 5%	4466
February	1st to 10th	Day	61% 6% 11% 14% 7% 1	1800
	11th to 20th	Day	59% 6% 10% 17% 7% 1	1885
	21st to end	Day	56% 6% 13% 17% 7% 1	1650
March	1st to 10th	Day	55% 5% 15% 16% 8% 1	4633
	11th to 20th	Day	47% 6% 16% 22% 8% 1	4980
	21st to end	Day	56% 4 13% 15% 11% 1	5465
April	1st to 10th	Day	50% 7% 13% 18% 12% 1	2297
	11th to 20th	Day	47% 6% 12% 20% 13% 2	2552
	21st to end	Day	47% 6% 13% 20% 12% 2	2600
May	1st to 10th	Day	49% 6% 11% 19% 12% 3	5897
	11th to 20th	Day	46% 8% 12% 21% 11% 2	5865
	21st to end	Day	40% 6% 11% 21% 17% 5%	6447
June	1st to 10th	Day	38% 8% 12% 27% 14% 2	2838
	11th to 20th	Day	39% 7% 15% 22% 16% 3	3000
	21st to end	Day	36% 9% 15% 22% 17% 1	2862
July	1st to 10th	Day	36% 6% 13% 26% 17% 2	6291
	11th to 20th	Day	31% 7% 15% 28% 17% 1	6285
	21st to end	Day	33% 7% 15% 23% 20% 2	6240
August	1st to 10th	Day	36% 7% 14% 28% 14% 1	4221
	11th to 20th	Day	38% 8% 12% 31% 9% 1	4213
	21st to end	Day	36% 7% 15% 28% 13% 1	4657
September	1st to 10th	Day	37% 6% 16% 24% 15% 2	2164
	11th to 20th	Day	30% 7% 15% 32% 15% 1	2090
	21st to end	Day	43% 5% 15% 24% 12%	2090
October	1st to 10th	Day	29% 5% 14% 31% 19% 1	3362
	11th to 20th	Day	39% 4 13% 26% 17%	3140
	21st to end	Day	44% 5% 14% 23% 13% 1	3212
November	1st to 10th	Day	43% 7% 12% 24% 12% 1	1762
	11th to 20th	Day	49% 6% 15% 19% 9% 1	1800
	21st to end	Day	57% 6% 13% 17% 7% 1	1667
December	1st to 10th	Day	59% 8% 16% 12% 4	2600
	11th to 20th	Day	64% 6% 11% 13% 6%	2876
	21st to end	Day	76% 4 7% 8% 4	3867
January	1st to 10th	Night	57% 5% 11% 19% 7% 1	6348
	11th to 20th	Night	65% 5% 12% 11% 6%	6291
	21st to end	Night	63% 5% 11% 12% 9%	6490
February	1st to 10th	Night	62% 5% 11% 14% 6% 1	2600
	11th to 20th	Night	57% 5% 11% 18% 8% 1	2409
	21st to end	Night	55% 5% 13% 17% 8% 1	1980
March	1st to 10th	Night	49% 4 14% 21% 10% 1	5218
	11th to 20th	Night	38% 7% 15% 25% 15% 1	4980
	21st to end	Night	47% 6% 11% 19% 16% 1	5467
April	1st to 10th	Night	48% 4 11% 22% 13% 2	2120
	11th to 20th	Night	45% 6% 12% 19% 17% 2	1797
	21st to end	Night	43% 6% 11% 20% 17% 3	1800
May	1st to 10th	Night	53% 5% 9% 15% 15% 3	4060
	11th to 20th	Night	45% 7% 10% 26% 10% 3	4033
	21st to end	Night	38% 7% 12% 23% 17% 4	4429
June	1st to 10th	Night	35% 6% 12% 26% 19% 3	1552
	11th to 20th	Night	36% 8% 14% 25% 16% 2	1400
	21st to end	Night	33% 6% 13% 26% 21% 1	1528
July	1st to 10th	Night	30% 6% 15% 29% 15% 5%	3580
	11th to 20th	Night	32% 7% 14% 29% 16% 2	3572
	21st to end	Night	31% 7% 15% 22% 24% 2	4293
August	1st to 10th	Night	36% 6% 13% 26% 18% 1	2904
	11th to 20th	Night	34% 7% 10% 34% 14% 1	2902
	21st to end	Night	33% 8% 12% 32% 15% 1	3223
September	1st to 10th	Night	33% 8% 12% 27% 17% 2	2061
	11th to 20th	Night	28% 8% 10% 31% 21% 2	2090
	21st to end	Night	41% 6% 12% 25% 14% 1	2090
October	1st to 10th	Night	26% 4 10% 33% 27% 1	3598
	11th to 20th	Night	41% 4 12% 23% 19% 1	3779
	21st to end	Night	41% 5% 11% 28% 14% 1	4331
November	1st to 10th	Night	40% 6% 11% 26% 15% 1	2536
	11th to 20th	Night	50% 6% 13% 20% 11% 1	2600
	21st to end	Night	56% 6% 13% 19% 6%	2728
December	1st to 10th	Night	59% 6% 17% 14% 4	4580
	11th to 20th	Night	62% 6% 12% 12% 6% 1	5318
	21st to end	Night	72% 5% 9% 8% 5%	7106

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	19% 16% 39% 19% 7% 1	300
	11th to 20th	Day	19% 14% 43% 19% 4 1	300
	21st to end	Day	23% 13% 34% 27% 4	330
February	1st to 10th	Day	22% 14% 41% 20% 2 1	300
	11th to 20th	Day	19% 18% 35% 26% 2	300
	21st to end	Day	20% 17% 45% 14% 4	249
March	1st to 10th	Day	20% 15% 36% 23% 5%	362
	11th to 20th	Day	19% 12% 33% 28% 7% 1	500
	21st to end	Day	23% 14% 32% 23% 8% 1	550
April	1st to 10th	Day	19% 12% 33% 28% 9% 1	500
	11th to 20th	Day	22% 15% 30% 19% 9% 4	492
	21st to end	Day	22% 8% 33% 24% 11% 2	500
May	1st to 10th	Day	21% 10% 29% 27% 10% 4	503
	11th to 20th	Day	19% 7% 26% 26% 16% 6%	527
	21st to end	Day	14% 5% 23% 29% 21% 8%	573
June	1st to 10th	Day	11% 6% 28% 34% 16% 5%	663
	11th to 20th	Day	11% 5% 28% 34% 18% 3	730
	21st to end	Day	9% 5% 29% 33% 18% 5%	730
July	1st to 10th	Day	9% 6% 19% 32% 25% 11%	721
	11th to 20th	Day	11% 3 23% 30% 23% 9%	718
	21st to end	Day	8% 3 18% 33% 26% 11%	576
August	1st to 10th	Day	8% 4 15% 35% 23% 15%	526
	11th to 20th	Day	11% 2 14% 35% 26% 11%	519
	21st to end	Day	10% 4 17% 31% 25% 14%	583
September	1st to 10th	Day	11% 3 22% 26% 24% 13%	502
	11th to 20th	Day	9% 2 16% 27% 32% 14%	490
	21st to end	Day	19% 7% 21% 24% 19% 9%	490
October	1st to 10th	Day	11% 8% 27% 24% 20% 10%	490
	11th to 20th	Day	18% 14% 28% 28% 9% 3	490
	21st to end	Day	25% 11% 33% 24% 6% 1	411
November	1st to 10th	Day	15% 12% 37% 21% 11% 3	300
	11th to 20th	Day	18% 20% 32% 23% 6% 1	300
	21st to end	Day	21% 13% 40% 21% 5%	233
December	1st to 10th	Day	20% 18% 35% 20% 7% 2	200
	11th to 20th	Day	27% 14% 40% 17% 2 1	200
	21st to end	Day	26% 17% 34% 19% 4	305
January	1st to 10th	Night	22% 13% 37% 22% 6% 1	1000
	11th to 20th	Night	25% 12% 34% 24% 4	970
	21st to end	Night	19% 12% 35% 28% 6%	880
February	1st to 10th	Night	23% 12% 36% 26% 4	800
	11th to 20th	Night	20% 11% 35% 29% 5%	783
	21st to end	Night	24% 12% 38% 23% 3	660
March	1st to 10th	Night	21% 10% 34% 29% 7%	800
	11th to 20th	Night	17% 12% 36% 29% 6% 1	800
	21st to end	Night	21% 11% 30% 30% 8% 1	880
April	1st to 10th	Night	20% 12% 32% 30% 6%	720
	11th to 20th	Night	19% 10% 33% 28% 9% 1	593
	21st to end	Night	20% 7% 31% 33% 8%	600
May	1st to 10th	Night	20% 5% 30% 32% 11% 1	600
	11th to 20th	Night	20% 3 26% 34% 15% 2	591
	21st to end	Night	12% 3 21% 40% 21% 3	650
June	1st to 10th	Night	13% 3 24% 44% 16% 1	490
	11th to 20th	Night	12% 3 20% 48% 17% 1	400
	21st to end	Night	9% 3 22% 44% 20% 2	528
July	1st to 10th	Night	8% 3 15% 41% 29% 5%	585
	11th to 20th	Night	9% 3 16% 41% 27% 5%	582
	21st to end	Night	6% 2 13% 49% 26% 4	657
August	1st to 10th	Night	8% 2 15% 41% 30% 5%	597
	11th to 20th	Night	10% 3 14% 36% 31% 5%	594
	21st to end	Night	9% 3 16% 37% 31% 5%	660
September	1st to 10th	Night	12% 6% 15% 34% 26% 6%	710
	11th to 20th	Night	9% 4 16% 33% 31% 5%	760
	21st to end	Night	17% 7% 20% 31% 21% 5%	760
October	1st to 10th	Night	14% 6% 26% 31% 19% 4	760
	11th to 20th	Night	21% 12% 29% 25% 12% 1	760
	21st to end	Night	25% 10% 30% 28% 6%	836
November	1st to 10th	Night	16% 10% 33% 25% 15% 1	780
	11th to 20th	Night	22% 13% 35% 22% 8% 1	800
	21st to end	Night	26% 11% 35% 22% 6%	800
December	1st to 10th	Night	24% 14% 35% 21% 5% 1	800
	11th to 20th	Night	23% 12% 36% 23% 4	845
	21st to end	Night	29% 9% 36% 22% 3	1100

Page generated at 2025-09-01 01:51:05. Version ac07

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