UM-Dearborn Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the UM-Dearborn Observatory clear sky chart from 2005-11-06 to 2025-04-26.

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% 4 9% 5% 5% 5% 4 6% 7% 32%	113152
	Night	14% 11% 4 8% 5% 4 4 3 5% 6% 34%	115521
Transparency	Day	48% 7% 13% 20% 11% 1	116768
	Night	48% 6% 12% 21% 12% 1	120033
Seeing	Day	18% 13% 30% 23% 12% 4	15940
	Night	19% 10% 29% 28% 11% 1	25894

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% 15% 7% 16% 6% 5% 4 3 4 5% 24%	3617
	11th to 20th	Day	14% 16% 6% 18% 8% 8% 7% 4 4 5% 10%	3636
	21st to end	Day	22% 17% 7% 13% 6% 4 5% 3 5% 4 14%	4466
February	1st to 10th	Day	15% 15% 4 13% 7% 5% 5% 5% 5% 5% 21%	1800
	11th to 20th	Day	13% 14% 4 12% 8% 5% 6% 3 4 6% 23%	1885
	21st to end	Day	16% 14% 5% 10% 5% 4 3 4 5% 7% 26%	1650
March	1st to 10th	Day	16% 12% 6% 10% 6% 4 4 3 4 5% 29%	4612
	11th to 20th	Day	12% 12% 4 9% 5% 4 5% 3 5% 7% 34%	4980
	21st to end	Day	18% 14% 4 10% 5% 4 4 3 4 6% 27%	5465
April	1st to 10th	Day	16% 14% 3 6% 5% 5% 5% 5% 5% 7% 30%	2310
	11th to 20th	Day	17% 9% 3 8% 5% 5% 4 4 5% 7% 34%	2542
	21st to end	Day	19% 9% 3 7% 5% 4 4 4 5% 6% 35%	2535
May	1st to 10th	Day	18% 8% 3 8% 5% 4 5% 4 5% 7% 34%	4082
	11th to 20th	Day	16% 8% 3 7% 5% 5% 5% 4 7% 8% 33%	4081
	21st to end	Day	10% 6% 3 6% 5% 5% 4 4 6% 8% 45%	4476
June	1st to 10th	Day	10% 6% 2 8% 6% 5% 5% 4 6% 8% 40%	2652
	11th to 20th	Day	11% 6% 2 7% 5% 5% 5% 4 7% 9% 40%	2848
	21st to end	Day	9% 7% 3 7% 5% 5% 6% 5% 7% 10% 36%	2660
July	1st to 10th	Day	10% 6% 2 7% 5% 5% 6% 4 7% 9% 38%	4440
	11th to 20th	Day	7% 4 2 6% 4 5% 5% 5% 8% 11% 43%	4440
	21st to end	Day	6% 5% 2 6% 6% 6% 6% 5% 8% 10% 40%	4498
August	1st to 10th	Day	11% 6% 2 6% 5% 5% 5% 5% 6% 9% 40%	4104
	11th to 20th	Day	9% 5% 3 7% 6% 5% 6% 6% 8% 10% 36%	4044
	21st to end	Day	8% 6% 2 7% 5% 6% 6% 5% 8% 9% 38%	4502
September	1st to 10th	Day	10% 7% 2 7% 5% 5% 5% 4 6% 8% 39%	2138
	11th to 20th	Day	8% 5% 2 5% 4 5% 5% 5% 6% 8% 47%	2090
	21st to end	Day	15% 7% 2 7% 4 4 5% 3 7% 8% 37%	2090
October	1st to 10th	Day	12% 7% 2 7% 4 4 4 4 6% 7% 42%	2032
	11th to 20th	Day	17% 9% 3 7% 6% 5% 5% 4 5% 7% 33%	1900
	21st to end	Day	19% 10% 3 8% 5% 5% 4 3 5% 7% 31%	1952
November	1st to 10th	Day	11% 8% 3 8% 6% 5% 4 3 5% 8% 39%	1762
	11th to 20th	Day	15% 12% 4 8% 6% 5% 5% 3 6% 7% 31%	1800
	21st to end	Day	18% 12% 4 10% 5% 5% 4 3 5% 7% 26%	1695
December	1st to 10th	Day	13% 13% 6% 15% 7% 6% 5% 4 5% 6% 21%	2600
	11th to 20th	Day	21% 19% 5% 10% 5% 4 5% 3 5% 5% 18%	2832
	21st to end	Day	27% 20% 6% 13% 5% 4 3 3 4 4 12%	3936
January	1st to 10th	Night	14% 15% 6% 12% 6% 4 4 3 5% 5% 25%	6348
	11th to 20th	Night	12% 17% 7% 15% 7% 5% 6% 5% 5% 5% 15%	6340
	21st to end	Night	19% 16% 7% 10% 5% 4 4 3 4 5% 23%	6490
February	1st to 10th	Night	17% 15% 4 11% 7% 5% 5% 4 4 5% 23%	2600
	11th to 20th	Night	15% 13% 5% 10% 7% 5% 5% 4 5% 5% 27%	2397
	21st to end	Night	19% 12% 4 9% 5% 4 5% 3 5% 6% 28%	1980
March	1st to 10th	Night	15% 12% 5% 8% 4 4 4 2 5% 6% 34%	5240
	11th to 20th	Night	10% 8% 3 6% 4 4 5% 4 7% 8% 41%	4980
	21st to end	Night	16% 10% 3 8% 4 4 4 3 5% 6% 36%	5467
April	1st to 10th	Night	17% 11% 3 7% 5% 4 4 4 6% 6% 35%	2106
	11th to 20th	Night	17% 8% 2 7% 4 4 4 4 6% 7% 37%	1807
	21st to end	Night	17% 8% 3 6% 4 4 5% 4 5% 6% 38%	1761
May	1st to 10th	Night	18% 9% 2 7% 5% 4 4 3 5% 7% 36%	2813
	11th to 20th	Night	15% 7% 2 8% 4 5% 5% 3 5% 6% 40%	2812
	21st to end	Night	10% 6% 2 6% 4 4 4 3 5% 8% 47%	3079
June	1st to 10th	Night	12% 6% 2 4 4 3 5% 4 6% 9% 46%	1509
	11th to 20th	Night	8% 5% 2 8% 5% 4 5% 4 6% 9% 44%	1330
	21st to end	Night	9% 6% 1 6% 4 4 5% 4 6% 7% 48%	1508
July	1st to 10th	Night	8% 4 1 5% 3 3 5% 4 7% 10% 48%	2530
	11th to 20th	Night	9% 3 2 5% 4 4 3 5% 6% 9% 51%	2530
	21st to end	Night	5% 4 2 6% 5% 5% 6% 5% 7% 9% 47%	3054
August	1st to 10th	Night	11% 6% 2 6% 4 4 4 3 6% 8% 45%	2825
	11th to 20th	Night	8% 6% 3 6% 4 4 5% 3 5% 7% 49%	2785
	21st to end	Night	8% 5% 2 6% 4 5% 5% 4 6% 8% 48%	3151
September	1st to 10th	Night	10% 5% 2 5% 4 5% 5% 3 6% 7% 48%	2023
	11th to 20th	Night	9% 5% 1 4 4 3 4 4 5% 6% 55%	2090
	21st to end	Night	15% 7% 2 6% 4 4 4 4 5% 7% 43%	2090
October	1st to 10th	Night	13% 7% 3 6% 4 3 4 3 5% 7% 46%	2147
	11th to 20th	Night	18% 9% 3 7% 4 4 4 3 5% 6% 36%	2280
	21st to end	Night	20% 9% 3 7% 5% 4 5% 3 5% 6% 34%	2645
November	1st to 10th	Night	13% 7% 2 6% 4 4 4 4 5% 6% 44%	2536
	11th to 20th	Night	16% 11% 4 8% 5% 4 5% 4 6% 7% 30%	2600
	21st to end	Night	20% 13% 3 9% 5% 4 3 3 5% 6% 28%	2700
December	1st to 10th	Night	12% 13% 6% 14% 6% 6% 5% 4 5% 5% 24%	4580
	11th to 20th	Night	16% 17% 7% 12% 6% 5% 4 3 4 5% 19%	5364
	21st to end	Night	27% 20% 7% 9% 4 4 4 3 4 4 15%	7024

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	62% 6% 12% 13% 6% 1	3618
	11th to 20th	Day	74% 6% 9% 7% 4 1	3636
	21st to end	Day	71% 5% 10% 8% 5%	4466
February	1st to 10th	Day	63% 7% 11% 13% 7%	1800
	11th to 20th	Day	60% 7% 9% 16% 6% 1	1895
	21st to end	Day	57% 6% 12% 17% 7% 1	1650
March	1st to 10th	Day	57% 5% 13% 17% 8% 1	4612
	11th to 20th	Day	47% 6% 16% 22% 8% 1	4980
	21st to end	Day	57% 6% 10% 16% 10% 1	5465
April	1st to 10th	Day	51% 7% 11% 18% 12% 1	2310
	11th to 20th	Day	48% 6% 12% 20% 12% 2	2542
	21st to end	Day	48% 6% 14% 17% 13% 1	2535
May	1st to 10th	Day	46% 8% 11% 23% 10% 2	4120
	11th to 20th	Day	44% 10% 14% 22% 9% 1	4085
	21st to end	Day	36% 7% 12% 19% 21% 6%	4489
June	1st to 10th	Day	39% 7% 11% 26% 13% 3	2652
	11th to 20th	Day	38% 7% 13% 22% 17% 3	2848
	21st to end	Day	39% 9% 13% 21% 16% 2	2660
July	1st to 10th	Day	40% 6% 14% 22% 17% 3	4371
	11th to 20th	Day	30% 7% 14% 32% 16% 1	4365
	21st to end	Day	35% 8% 16% 18% 20% 3	4356
August	1st to 10th	Day	37% 8% 14% 27% 14% 1	4091
	11th to 20th	Day	39% 9% 11% 30% 10% 1	4083
	21st to end	Day	37% 8% 14% 27% 13% 1	4501
September	1st to 10th	Day	39% 8% 15% 22% 14% 2	2138
	11th to 20th	Day	31% 8% 13% 31% 15% 1	2090
	21st to end	Day	42% 7% 14% 24% 13%	2090
October	1st to 10th	Day	30% 6% 13% 31% 20% 1	3370
	11th to 20th	Day	38% 5% 13% 26% 17%	3140
	21st to end	Day	43% 5% 15% 23% 13% 1	3189
November	1st to 10th	Day	43% 7% 13% 24% 12% 1	1762
	11th to 20th	Day	52% 6% 14% 18% 9% 1	1800
	21st to end	Day	57% 6% 12% 18% 7%	1695
December	1st to 10th	Day	63% 6% 16% 11% 4	2600
	11th to 20th	Day	66% 5% 11% 13% 4	2828
	21st to end	Day	77% 4 7% 8% 4	3936
January	1st to 10th	Night	59% 6% 10% 18% 6% 1	6348
	11th to 20th	Night	67% 7% 10% 10% 5%	6340
	21st to end	Night	64% 5% 11% 11% 9%	6490
February	1st to 10th	Night	62% 6% 11% 13% 6% 1	2600
	11th to 20th	Night	58% 5% 10% 18% 8% 1	2398
	21st to end	Night	55% 6% 13% 18% 8% 1	1980
March	1st to 10th	Night	50% 5% 14% 21% 9% 1	5240
	11th to 20th	Night	38% 6% 15% 25% 14% 1	4980
	21st to end	Night	48% 6% 10% 20% 14% 1	5467
April	1st to 10th	Night	49% 6% 10% 21% 12% 2	2106
	11th to 20th	Night	44% 7% 11% 19% 17% 2	1807
	21st to end	Night	45% 6% 12% 19% 16% 3	1761
May	1st to 10th	Night	46% 8% 9% 21% 14% 2	2840
	11th to 20th	Night	42% 9% 10% 26% 11% 1	2813
	21st to end	Night	32% 8% 12% 21% 22% 5%	3087
June	1st to 10th	Night	34% 7% 13% 26% 17% 3	1509
	11th to 20th	Night	36% 7% 12% 25% 17% 2	1330
	21st to end	Night	33% 7% 12% 26% 20% 1	1508
July	1st to 10th	Night	29% 7% 12% 32% 15% 6%	2490
	11th to 20th	Night	28% 7% 14% 32% 18% 1	2482
	21st to end	Night	30% 9% 17% 20% 21% 3	2957
August	1st to 10th	Night	36% 7% 12% 27% 17% 1	2814
	11th to 20th	Night	33% 7% 11% 34% 13% 2	2812
	21st to end	Night	33% 7% 13% 31% 15% 1	3151
September	1st to 10th	Night	33% 8% 13% 26% 17% 2	2023
	11th to 20th	Night	28% 7% 11% 32% 21% 2	2090
	21st to end	Night	41% 6% 12% 26% 13% 1	2090
October	1st to 10th	Night	26% 4 10% 33% 27% 1	3576
	11th to 20th	Night	41% 5% 12% 24% 18%	3779
	21st to end	Night	40% 4 12% 29% 14% 1	4358
November	1st to 10th	Night	41% 6% 12% 26% 15% 1	2536
	11th to 20th	Night	50% 6% 13% 20% 10% 1	2600
	21st to end	Night	56% 6% 13% 19% 6%	2700
December	1st to 10th	Night	60% 6% 16% 13% 4	4580
	11th to 20th	Night	65% 6% 12% 12% 5% 1	5367
	21st to end	Night	73% 5% 10% 7% 5%	7024

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	23% 21% 36% 14% 5% 1	300
	11th to 20th	Day	23% 20% 38% 15% 2 1	300
	21st to end	Day	27% 17% 35% 17% 5%	330
February	1st to 10th	Day	24% 21% 37% 15% 3 1	300
	11th to 20th	Day	24% 20% 36% 18% 2	300
	21st to end	Day	24% 26% 38% 8% 3	249
March	1st to 10th	Day	25% 21% 32% 18% 3	354
	11th to 20th	Day	24% 14% 34% 21% 5% 1	500
	21st to end	Day	27% 17% 32% 17% 6% 1	550
April	1st to 10th	Day	22% 19% 31% 20% 7% 1	500
	11th to 20th	Day	27% 18% 30% 14% 8% 3	492
	21st to end	Day	26% 14% 31% 18% 8% 3	480
May	1st to 10th	Day	24% 16% 29% 20% 8% 4	463
	11th to 20th	Day	19% 11% 30% 22% 13% 5%	487
	21st to end	Day	13% 8% 25% 29% 18% 7%	529
June	1st to 10th	Day	13% 10% 30% 31% 13% 3	634
	11th to 20th	Day	14% 9% 30% 31% 13% 3	680
	21st to end	Day	12% 11% 32% 27% 14% 4	680
July	1st to 10th	Day	9% 6% 25% 30% 21% 9%	671
	11th to 20th	Day	10% 6% 28% 31% 18% 7%	668
	21st to end	Day	8% 4 27% 28% 24% 8%	551
August	1st to 10th	Day	8% 5% 24% 30% 21% 12%	486
	11th to 20th	Day	13% 3 23% 30% 20% 11%	479
	21st to end	Day	11% 5% 21% 29% 23% 10%	539
September	1st to 10th	Day	12% 7% 24% 26% 20% 10%	490
	11th to 20th	Day	8% 4 19% 26% 29% 12%	490
	21st to end	Day	18% 11% 25% 22% 17% 8%	490
October	1st to 10th	Day	13% 12% 28% 23% 17% 7%	490
	11th to 20th	Day	18% 18% 31% 24% 7% 2	490
	21st to end	Day	27% 17% 34% 16% 5% 1	401
November	1st to 10th	Day	16% 21% 34% 20% 8% 1	300
	11th to 20th	Day	23% 23% 33% 16% 5% 1	300
	21st to end	Day	27% 18% 35% 14% 5%	247
December	1st to 10th	Day	23% 22% 33% 16% 6% 1	200
	11th to 20th	Day	28% 23% 36% 10% 3 1	200
	21st to end	Day	29% 23% 32% 13% 3	320
January	1st to 10th	Night	22% 17% 36% 20% 5% 1	1000
	11th to 20th	Night	26% 15% 33% 22% 4 1	993
	21st to end	Night	21% 15% 34% 26% 4	880
February	1st to 10th	Night	25% 13% 37% 23% 3	800
	11th to 20th	Night	23% 14% 33% 26% 3	783
	21st to end	Night	25% 15% 38% 20% 2	660
March	1st to 10th	Night	20% 13% 34% 27% 6%	800
	11th to 20th	Night	19% 16% 36% 24% 5% 1	800
	21st to end	Night	23% 12% 30% 27% 7%	880
April	1st to 10th	Night	22% 14% 32% 27% 4	706
	11th to 20th	Night	22% 11% 34% 25% 8% 1	593
	21st to end	Night	20% 10% 33% 29% 7%	587
May	1st to 10th	Night	19% 10% 30% 31% 10% 1	570
	11th to 20th	Night	21% 5% 27% 33% 13% 1	561
	21st to end	Night	10% 4 26% 39% 19% 2	617
June	1st to 10th	Night	13% 3 27% 44% 13%	518
	11th to 20th	Night	9% 6% 25% 47% 13%	380
	21st to end	Night	10% 5% 24% 40% 19% 1	558
July	1st to 10th	Night	8% 3 21% 39% 26% 4	555
	11th to 20th	Night	9% 3 22% 39% 23% 4	552
	21st to end	Night	6% 2 19% 47% 24% 2	624
August	1st to 10th	Night	8% 4 19% 40% 27% 3	567
	11th to 20th	Night	9% 2 20% 36% 30% 3	564
	21st to end	Night	10% 3 18% 37% 30% 3	627
September	1st to 10th	Night	13% 6% 19% 33% 25% 5%	693
	11th to 20th	Night	9% 4 19% 34% 30% 4	760
	21st to end	Night	17% 7% 22% 30% 21% 3	760
October	1st to 10th	Night	17% 8% 26% 30% 17% 3	760
	11th to 20th	Night	23% 13% 28% 24% 10% 1	760
	21st to end	Night	24% 14% 31% 26% 6%	836
November	1st to 10th	Night	18% 12% 31% 26% 11% 1	780
	11th to 20th	Night	22% 16% 32% 22% 7% 1	800
	21st to end	Night	27% 13% 34% 20% 6%	800
December	1st to 10th	Night	26% 15% 33% 19% 6%	800
	11th to 20th	Night	25% 15% 36% 20% 4	870
	21st to end	Night	31% 12% 33% 20% 3	1100

Page generated at 2025-04-23 09:25:06. Version ac07

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