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 2024-03-28.

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	15% 9% 4 7% 6% 4 5% 5% 6% 7% 32%	78644
	Night	16% 10% 4 7% 6% 3 5% 5% 5% 6% 33%	81353
Transparency	Day	48% 7% 13% 20% 11% 1	78355
	Night	50% 6% 12% 20% 11% 1	81163
Seeing	Day	16% 8% 27% 27% 16% 6%	14432
	Night	18% 8% 28% 31% 13% 2	24337

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% 15% 7% 11% 8% 4 5% 4 4 5% 21%	2527
	11th to 20th	Day	16% 16% 7% 11% 9% 5% 7% 6% 5% 5% 12%	2565
	21st to end	Day	27% 17% 9% 11% 6% 3 3 4 4 4 13%	3124
February	1st to 10th	Day	15% 14% 6% 10% 10% 5% 4 5% 6% 5% 22%	1710
	11th to 20th	Day	14% 12% 5% 11% 8% 5% 6% 4 5% 6% 24%	1779
	21st to end	Day	18% 12% 5% 9% 7% 3 4 4 6% 6% 26%	1570
March	1st to 10th	Day	16% 12% 5% 7% 7% 4 5% 5% 5% 6% 28%	1959
	11th to 20th	Day	15% 11% 5% 8% 6% 3 3 4 5% 7% 33%	2090
	21st to end	Day	18% 13% 5% 9% 7% 4 4 4 4 6% 26%	2255
April	1st to 10th	Day	16% 11% 4 6% 6% 4 5% 5% 5% 7% 31%	2067
	11th to 20th	Day	17% 9% 4 6% 6% 3 4 5% 5% 7% 33%	2294
	21st to end	Day	19% 9% 3 6% 5% 3 5% 4 5% 6% 34%	2340
May	1st to 10th	Day	16% 9% 4 7% 6% 4 5% 5% 5% 7% 33%	2340
	11th to 20th	Day	18% 9% 3 6% 6% 3 5% 5% 5% 7% 33%	2301
	21st to end	Day	12% 5% 3 5% 5% 4 4 4 7% 8% 42%	2531
June	1st to 10th	Day	10% 6% 3 6% 6% 3 5% 6% 6% 8% 41%	2553
	11th to 20th	Day	11% 6% 3 5% 5% 4 5% 5% 6% 8% 41%	2700
	21st to end	Day	9% 6% 3 5% 5% 4 6% 6% 7% 11% 38%	2576
July	1st to 10th	Day	8% 4 2 4 6% 4 6% 6% 6% 8% 46%	2520
	11th to 20th	Day	9% 4 2 5% 6% 4 5% 5% 7% 11% 42%	2520
	21st to end	Day	8% 4 3 6% 6% 4 6% 6% 7% 10% 41%	2547
August	1st to 10th	Day	8% 6% 2 5% 5% 3 6% 6% 8% 10% 41%	2327
	11th to 20th	Day	10% 5% 2 5% 6% 4 7% 7% 8% 10% 37%	2303
	21st to end	Day	10% 5% 3 5% 5% 4 5% 5% 6% 10% 42%	2571
September	1st to 10th	Day	11% 6% 3 6% 6% 4 6% 5% 6% 9% 40%	2017
	11th to 20th	Day	9% 5% 2 5% 5% 3 4 6% 7% 9% 46%	1980
	21st to end	Day	15% 6% 3 5% 5% 3 4 5% 6% 8% 40%	1980
October	1st to 10th	Day	13% 6% 3 6% 7% 3 4 5% 6% 8% 41%	1918
	11th to 20th	Day	17% 7% 3 6% 7% 4 4 5% 6% 7% 33%	1800
	21st to end	Day	20% 9% 3 6% 7% 4 5% 4 4 7% 30%	1859
November	1st to 10th	Day	11% 6% 4 7% 6% 3 5% 4 5% 8% 39%	1672
	11th to 20th	Day	13% 11% 5% 6% 6% 4 5% 5% 5% 7% 32%	1710
	21st to end	Day	18% 11% 4 7% 6% 4 5% 4 4 7% 28%	1584
December	1st to 10th	Day	16% 13% 6% 10% 7% 5% 6% 5% 5% 6% 21%	1520
	11th to 20th	Day	16% 15% 5% 9% 7% 3 5% 5% 6% 7% 22%	1839
	21st to end	Day	27% 18% 8% 10% 8% 3 3 3 4 5% 10%	2696
January	1st to 10th	Night	16% 16% 6% 9% 6% 4 5% 4 4 5% 25%	4428
	11th to 20th	Night	15% 18% 8% 12% 7% 3 6% 6% 5% 5% 15%	4399
	21st to end	Night	24% 17% 8% 10% 6% 3 4 3 3 4 18%	4532
February	1st to 10th	Night	16% 15% 5% 9% 9% 4 5% 4 4 5% 24%	2470
	11th to 20th	Night	15% 12% 6% 9% 7% 4 5% 5% 5% 6% 26%	2284
	21st to end	Night	19% 11% 4 8% 6% 3 5% 5% 5% 5% 30%	1884
March	1st to 10th	Night	16% 10% 4 7% 6% 4 4 4 5% 5% 34%	2219
	11th to 20th	Night	13% 9% 3 5% 6% 3 4 6% 6% 8% 38%	2090
	21st to end	Night	20% 7% 4 7% 6% 3 4 5% 4 6% 34%	2256
April	1st to 10th	Night	15% 10% 3 5% 6% 2 5% 5% 5% 7% 37%	1908
	11th to 20th	Night	18% 7% 3 6% 5% 2 4 5% 6% 8% 36%	1615
	21st to end	Night	17% 8% 3 6% 5% 2 5% 5% 5% 6% 39%	1620
May	1st to 10th	Night	16% 9% 3 6% 6% 4 5% 5% 5% 7% 34%	1620
	11th to 20th	Night	18% 6% 2 5% 6% 4 3 4 5% 7% 39%	1592
	21st to end	Night	10% 5% 3 6% 4 3 4 5% 7% 9% 45%	1744
June	1st to 10th	Night	12% 5% 2 4 4 2 5% 5% 5% 9% 46%	1398
	11th to 20th	Night	10% 4 2 5% 6% 3 5% 5% 6% 9% 44%	1260
	21st to end	Night	10% 4 3 5% 4 3 5% 4 6% 8% 48%	1374
July	1st to 10th	Night	7% 3 3 4 4 3 5% 5% 5% 8% 53%	1440
	11th to 20th	Night	9% 3 3 4 4 3 5% 4 6% 8% 53%	1440
	21st to end	Night	5% 4 2 3 5% 3 5% 6% 7% 9% 50%	1766
August	1st to 10th	Night	8% 5% 2 4 5% 3 4 5% 6% 9% 50%	1611
	11th to 20th	Night	7% 5% 2 4 4 3 5% 5% 6% 8% 50%	1598
	21st to end	Night	9% 4 2 4 4 3 4 5% 6% 9% 51%	1782
September	1st to 10th	Night	10% 5% 3 5% 6% 3 4 5% 5% 7% 47%	1923
	11th to 20th	Night	10% 5% 2 4 3 2 4 4 6% 7% 53%	1980
	21st to end	Night	14% 6% 2 5% 5% 3 4 5% 5% 6% 44%	1980
October	1st to 10th	Night	13% 6% 3 6% 5% 3 3 3 5% 7% 45%	2042
	11th to 20th	Night	17% 9% 3 6% 6% 3 4 4 5% 6% 37%	2160
	21st to end	Night	21% 8% 4 5% 6% 3 5% 4 4 6% 33%	2492
November	1st to 10th	Night	13% 7% 3 6% 5% 3 5% 5% 5% 7% 43%	2406
	11th to 20th	Night	16% 9% 4 7% 6% 3 5% 5% 6% 7% 31%	2470
	21st to end	Night	20% 12% 5% 7% 5% 3 4 4 5% 7% 29%	2591
December	1st to 10th	Night	18% 14% 7% 9% 7% 3 5% 5% 6% 5% 22%	2660
	11th to 20th	Night	15% 13% 5% 9% 6% 4 6% 5% 5% 6% 25%	3353
	21st to end	Night	28% 20% 7% 9% 6% 3 4 3 4 4 12%	4966

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	64% 8% 10% 12% 5%	2528
	11th to 20th	Day	72% 6% 12% 8% 3	2565
	21st to end	Day	75% 5% 9% 7% 3	3124
February	1st to 10th	Day	61% 6% 11% 14% 7% 1	1710
	11th to 20th	Day	58% 6% 10% 17% 8% 1	1788
	21st to end	Day	56% 6% 13% 17% 7% 1	1570
March	1st to 10th	Day	54% 7% 13% 18% 6% 1	1959
	11th to 20th	Day	50% 5% 12% 19% 11% 2	2090
	21st to end	Day	58% 5% 11% 18% 7% 1	2255
April	1st to 10th	Day	50% 7% 13% 18% 12% 1	2067
	11th to 20th	Day	48% 6% 12% 19% 13% 2	2294
	21st to end	Day	49% 6% 14% 18% 12% 2	2340
May	1st to 10th	Day	49% 6% 11% 20% 11% 2	2340
	11th to 20th	Day	49% 6% 11% 18% 13% 2	2305
	21st to end	Day	38% 7% 15% 24% 15% 2	2531
June	1st to 10th	Day	38% 7% 12% 28% 13% 2	2553
	11th to 20th	Day	38% 7% 15% 22% 16% 3	2700
	21st to end	Day	35% 9% 15% 22% 17% 1	2576
July	1st to 10th	Day	33% 7% 11% 26% 20% 4	2451
	11th to 20th	Day	33% 7% 15% 26% 17% 2	2445
	21st to end	Day	34% 7% 15% 24% 17% 2	2405
August	1st to 10th	Day	32% 8% 15% 31% 12% 1	2314
	11th to 20th	Day	37% 9% 15% 25% 12% 1	2303
	21st to end	Day	34% 8% 15% 25% 18% 2	2571
September	1st to 10th	Day	39% 6% 17% 23% 14% 2	2017
	11th to 20th	Day	31% 7% 15% 33% 14% 1	1980
	21st to end	Day	41% 5% 16% 26% 12%	1980
October	1st to 10th	Day	38% 7% 14% 26% 13% 2	1918
	11th to 20th	Day	48% 6% 14% 21% 11% 1	1800
	21st to end	Day	53% 6% 14% 18% 8% 1	1859
November	1st to 10th	Day	42% 7% 13% 24% 12% 1	1672
	11th to 20th	Day	48% 6% 16% 20% 9% 1	1710
	21st to end	Day	55% 6% 13% 17% 7% 1	1584
December	1st to 10th	Day	62% 7% 12% 14% 5%	1520
	11th to 20th	Day	59% 7% 14% 15% 5%	1835
	21st to end	Day	76% 6% 9% 7% 3	2696
January	1st to 10th	Night	60% 6% 12% 16% 6% 1	4428
	11th to 20th	Night	67% 6% 13% 10% 3	4399
	21st to end	Night	71% 5% 9% 12% 3	4532
February	1st to 10th	Night	62% 5% 11% 15% 6% 1	2470
	11th to 20th	Night	57% 5% 10% 19% 9% 1	2286
	21st to end	Night	55% 5% 13% 17% 9% 1	1884
March	1st to 10th	Night	51% 5% 11% 22% 10% 1	2219
	11th to 20th	Night	41% 8% 13% 21% 15% 2	2090
	21st to end	Night	51% 6% 10% 21% 11% 1	2256
April	1st to 10th	Night	46% 5% 11% 23% 14% 1	1908
	11th to 20th	Night	45% 6% 12% 19% 17% 2	1615
	21st to end	Night	44% 6% 11% 19% 16% 4	1620
May	1st to 10th	Night	49% 6% 12% 18% 12% 3	1620
	11th to 20th	Night	46% 6% 9% 19% 17% 3	1593
	21st to end	Night	34% 7% 14% 26% 16% 2	1745
June	1st to 10th	Night	35% 5% 13% 28% 17% 3	1398
	11th to 20th	Night	35% 8% 14% 24% 16% 2	1260
	21st to end	Night	33% 6% 12% 27% 21% 1	1374
July	1st to 10th	Night	30% 6% 11% 28% 21% 4	1400
	11th to 20th	Night	29% 7% 13% 31% 18% 2	1392
	21st to end	Night	27% 8% 18% 24% 19% 4	1668
August	1st to 10th	Night	30% 8% 15% 29% 17% 2	1600
	11th to 20th	Night	31% 7% 15% 31% 14% 3	1598
	21st to end	Night	30% 8% 14% 28% 19% 2	1782
September	1st to 10th	Night	35% 8% 13% 26% 16% 2	1923
	11th to 20th	Night	29% 8% 11% 30% 20% 2	1980
	21st to end	Night	39% 7% 13% 26% 15% 1	1980
October	1st to 10th	Night	39% 6% 12% 24% 19% 1	2042
	11th to 20th	Night	47% 6% 13% 20% 14% 1	2160
	21st to end	Night	52% 6% 11% 21% 10% 1	2492
November	1st to 10th	Night	40% 7% 12% 26% 15% 1	2406
	11th to 20th	Night	50% 6% 13% 20% 10% 1	2470
	21st to end	Night	55% 5% 13% 19% 6%	2591
December	1st to 10th	Night	62% 6% 12% 15% 6%	2660
	11th to 20th	Night	56% 8% 14% 15% 7%	3356
	21st to end	Night	75% 6% 10% 7% 2	4966

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% 16% 37% 19% 7% 1	270
	11th to 20th	Day	19% 13% 41% 20% 5% 1	270
	21st to end	Day	24% 10% 33% 29% 4	297
February	1st to 10th	Day	21% 13% 40% 22% 3 1	270
	11th to 20th	Day	19% 17% 34% 27% 3	270
	21st to end	Day	18% 17% 46% 15% 4	225
March	1st to 10th	Day	21% 15% 34% 25% 5%	329
	11th to 20th	Day	19% 10% 33% 29% 7% 1	460
	21st to end	Day	22% 14% 32% 23% 9% 1	490
April	1st to 10th	Day	16% 11% 31% 30% 10% 1	420
	11th to 20th	Day	23% 15% 28% 20% 9% 5%	412
	21st to end	Day	24% 7% 33% 23% 11% 3	420
May	1st to 10th	Day	21% 8% 28% 27% 11% 4	423
	11th to 20th	Day	20% 7% 26% 25% 16% 6%	447
	21st to end	Day	14% 4 20% 31% 21% 10%	485
June	1st to 10th	Day	11% 4 25% 37% 17% 5%	569
	11th to 20th	Day	12% 4 26% 35% 21% 3	630
	21st to end	Day	10% 4 28% 33% 19% 6%	630
July	1st to 10th	Day	8% 4 18% 31% 28% 12%	621
	11th to 20th	Day	11% 2 21% 31% 24% 10%	618
	21st to end	Day	8% 3 17% 33% 27% 12%	488
August	1st to 10th	Day	9% 3 13% 34% 24% 17%	446
	11th to 20th	Day	12% 2 14% 33% 28% 12%	439
	21st to end	Day	11% 4 16% 28% 26% 15%	495
September	1st to 10th	Day	13% 3 21% 26% 24% 13%	450
	11th to 20th	Day	10% 2 16% 26% 31% 14%	450
	21st to end	Day	17% 7% 21% 24% 20% 10%	450
October	1st to 10th	Day	12% 7% 26% 23% 21% 11%	450
	11th to 20th	Day	18% 14% 29% 27% 9% 2	450
	21st to end	Day	26% 11% 33% 24% 6% 1	374
November	1st to 10th	Day	14% 12% 36% 23% 12% 3	270
	11th to 20th	Day	17% 20% 32% 24% 6% 1	270
	21st to end	Day	23% 12% 39% 22% 4	210
December	1st to 10th	Day	19% 17% 37% 17% 7% 2	180
	11th to 20th	Day	26% 13% 41% 17% 2 1	180
	21st to end	Day	26% 17% 33% 20% 3	274
January	1st to 10th	Night	22% 13% 36% 22% 6% 1	950
	11th to 20th	Night	26% 11% 33% 25% 5%	922
	21st to end	Night	19% 11% 35% 30% 6%	836
February	1st to 10th	Night	24% 11% 36% 26% 4	760
	11th to 20th	Night	20% 10% 35% 30% 5%	743
	21st to end	Night	24% 12% 37% 24% 3	628
March	1st to 10th	Night	21% 9% 33% 29% 7%	760
	11th to 20th	Night	16% 12% 36% 29% 6% 1	760
	21st to end	Night	21% 10% 29% 30% 9% 1	821
April	1st to 10th	Night	19% 11% 32% 32% 6%	648
	11th to 20th	Night	19% 9% 33% 28% 10% 1	533
	21st to end	Night	21% 6% 31% 32% 9%	540
May	1st to 10th	Night	20% 4 30% 33% 11% 1	540
	11th to 20th	Night	19% 3 25% 36% 15% 2	531
	21st to end	Night	11% 2 20% 40% 23% 3	584
June	1st to 10th	Night	13% 2 23% 46% 16% 1	442
	11th to 20th	Night	12% 3 19% 48% 18% 1	360
	21st to end	Night	10% 3 20% 44% 20% 2	474
July	1st to 10th	Night	8% 3 14% 39% 31% 6%	525
	11th to 20th	Night	9% 2 16% 41% 27% 5%	522
	21st to end	Night	6% 2 14% 49% 26% 4	591
August	1st to 10th	Night	9% 2 13% 41% 30% 5%	537
	11th to 20th	Night	10% 3 13% 37% 32% 5%	534
	21st to end	Night	9% 3 16% 36% 31% 5%	594
September	1st to 10th	Night	13% 6% 15% 35% 25% 7%	663
	11th to 20th	Night	10% 4 17% 33% 30% 5%	720
	21st to end	Night	16% 6% 19% 31% 22% 5%	720
October	1st to 10th	Night	15% 6% 26% 31% 19% 4	720
	11th to 20th	Night	21% 12% 30% 25% 11% 1	720
	21st to end	Night	26% 10% 30% 28% 6%	792
November	1st to 10th	Night	15% 10% 33% 26% 15% 1	740
	11th to 20th	Night	21% 13% 35% 21% 8% 1	760
	21st to end	Night	25% 10% 35% 23% 6%	760
December	1st to 10th	Night	24% 13% 36% 21% 6% 1	760
	11th to 20th	Night	23% 12% 37% 24% 4	802
	21st to end	Night	29% 10% 36% 22% 3	1045

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