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 2026-08-30.

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	13% 9% 3 9% 5% 5% 5% 4 6% 8% 33%	143033
	Night	14% 11% 4 8% 5% 4 5% 4 5% 6% 34%	139282
Transparency	Day	47% 6% 12% 21% 12% 1	146605
	Night	48% 6% 12% 21% 13% 1	143771
Seeing	Day	18% 13% 30% 23% 12% 4	17841
	Night	19% 11% 29% 28% 11% 1	27586

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	13% 15% 6% 16% 7% 5% 4 3 5% 4 21%	4691
	11th to 20th	Day	12% 16% 7% 18% 9% 9% 6% 3 4 4 11%	4726
	21st to end	Day	18% 15% 6% 12% 8% 6% 6% 4 5% 5% 15%	5780
February	1st to 10th	Day	15% 14% 4 13% 7% 6% 5% 5% 5% 5% 21%	1890
	11th to 20th	Day	13% 14% 5% 12% 8% 5% 6% 3 5% 6% 23%	1982
	21st to end	Day	15% 14% 5% 11% 5% 5% 3 4 5% 7% 25%	1730
March	1st to 10th	Day	16% 12% 6% 10% 6% 4 4 3 4 5% 29%	4715
	11th to 20th	Day	13% 12% 4 9% 5% 5% 5% 3 5% 7% 33%	5090
	21st to end	Day	18% 14% 4 10% 6% 4 4 3 4 6% 27%	5586
April	1st to 10th	Day	16% 14% 3 6% 5% 5% 5% 5% 5% 7% 30%	2425
	11th to 20th	Day	17% 9% 3 8% 5% 5% 4 4 5% 7% 34%	2670
	21st to end	Day	18% 9% 3 7% 5% 4 4 4 5% 6% 35%	2730
May	1st to 10th	Day	17% 9% 3 8% 5% 5% 5% 4 5% 7% 33%	7616
	11th to 20th	Day	14% 8% 3 7% 5% 5% 6% 4 7% 9% 33%	7626
	21st to end	Day	14% 6% 2 6% 4 4 4 3 5% 7% 44%	8391
June	1st to 10th	Day	10% 6% 2 7% 6% 5% 5% 5% 6% 8% 41%	2932
	11th to 20th	Day	11% 6% 2 7% 5% 5% 6% 4 7% 9% 38%	3148
	21st to end	Day	9% 7% 3 7% 5% 5% 5% 5% 7% 10% 37%	2940
July	1st to 10th	Day	10% 6% 2 6% 5% 5% 5% 4 7% 10% 41%	8280
	11th to 20th	Day	5% 4 2 6% 4 5% 5% 4 8% 12% 46%	8280
	21st to end	Day	7% 5% 2 6% 5% 5% 5% 4 7% 10% 43%	8400
August	1st to 10th	Day	10% 6% 2 6% 5% 5% 5% 5% 6% 9% 41%	4364
	11th to 20th	Day	9% 5% 3 7% 6% 5% 6% 6% 8% 10% 35%	4304
	21st to end	Day	8% 5% 2 7% 5% 6% 6% 5% 8% 9% 39%	4761
September	1st to 10th	Day	10% 7% 2 6% 5% 5% 5% 4 6% 8% 41%	2250
	11th to 20th	Day	8% 5% 2 5% 4 5% 5% 5% 6% 9% 48%	2200
	21st to end	Day	15% 7% 2 8% 4 4 5% 4 7% 8% 37%	2200
October	1st to 10th	Day	11% 7% 2 7% 4 4 4 4 6% 7% 43%	2139
	11th to 20th	Day	16% 8% 3 7% 6% 5% 5% 4 5% 7% 34%	2000
	21st to end	Day	18% 10% 3 8% 5% 5% 4 3 5% 7% 31%	2054
November	1st to 10th	Day	12% 8% 3 8% 6% 5% 4 3 5% 7% 39%	1852
	11th to 20th	Day	14% 12% 4 8% 5% 5% 4 3 6% 7% 31%	1890
	21st to end	Day	18% 12% 5% 10% 6% 5% 4 3 5% 6% 26%	1780
December	1st to 10th	Day	13% 13% 6% 15% 7% 6% 5% 4 5% 6% 20%	2680
	11th to 20th	Day	20% 19% 5% 10% 5% 4 5% 3 6% 5% 18%	2908
	21st to end	Day	27% 19% 6% 13% 5% 4 3 3 4 4 12%	4023
January	1st to 10th	Night	14% 16% 6% 13% 6% 4 4 3 4 5% 24%	8232
	11th to 20th	Night	10% 17% 7% 15% 8% 6% 6% 5% 5% 5% 16%	8246
	21st to end	Night	15% 15% 7% 11% 6% 5% 5% 4 5% 5% 23%	8401
February	1st to 10th	Night	16% 15% 4 11% 7% 5% 5% 4 4 5% 24%	2730
	11th to 20th	Night	15% 13% 5% 10% 7% 5% 5% 4 5% 5% 27%	2520
	21st to end	Night	18% 13% 4 9% 5% 4 4 3 5% 6% 28%	2076
March	1st to 10th	Night	15% 12% 5% 8% 4 4 4 2 5% 6% 34%	5357
	11th to 20th	Night	10% 8% 3 7% 5% 4 5% 4 6% 8% 40%	5090
	21st to end	Night	16% 10% 3 8% 5% 4 4 3 5% 6% 36%	5588
April	1st to 10th	Night	17% 11% 3 7% 5% 4 4 4 5% 6% 35%	2211
	11th to 20th	Night	17% 8% 2 7% 4 4 5% 4 6% 7% 36%	1899
	21st to end	Night	17% 8% 3 6% 4 4 5% 3 5% 6% 39%	1890
May	1st to 10th	Night	19% 10% 3 8% 5% 4 5% 3 5% 6% 32%	5237
	11th to 20th	Night	15% 7% 3 7% 4 4 5% 3 5% 6% 40%	5234
	21st to end	Night	14% 6% 2 6% 4 4 4 3 6% 7% 43%	5761
June	1st to 10th	Night	12% 6% 2 4 3 3 4 4 6% 8% 47%	1667
	11th to 20th	Night	9% 5% 2 7% 5% 4 5% 4 6% 9% 44%	1470
	21st to end	Night	9% 5% 1 6% 4 4 5% 3 6% 7% 49%	1667
July	1st to 10th	Night	7% 5% 2 6% 4 4 5% 4 7% 9% 47%	4708
	11th to 20th	Night	7% 4 2 5% 4 5% 4 5% 7% 9% 49%	4707
	21st to end	Night	8% 5% 2 6% 4 4 4 4 5% 8% 49%	5677
August	1st to 10th	Night	11% 6% 2 6% 4 4 4 3 6% 8% 46%	3005
	11th to 20th	Night	7% 7% 3 6% 4 4 5% 3 5% 7% 48%	2965
	21st to end	Night	7% 5% 2 6% 4 5% 5% 4 6% 8% 48%	3332
September	1st to 10th	Night	9% 5% 2 5% 4 5% 5% 3 5% 7% 48%	2130
	11th to 20th	Night	8% 5% 1 4 4 3 4 4 5% 6% 57%	2200
	21st to end	Night	15% 7% 2 6% 4 4 4 4 5% 7% 43%	2200
October	1st to 10th	Night	13% 6% 3 6% 4 3 4 3 5% 7% 47%	2260
	11th to 20th	Night	17% 9% 3 7% 4 5% 4 3 5% 6% 37%	2400
	21st to end	Night	19% 9% 3 8% 5% 4 5% 3 5% 6% 35%	2784
November	1st to 10th	Night	13% 8% 2 6% 4 5% 4 4 5% 6% 43%	2666
	11th to 20th	Night	15% 11% 4 8% 5% 4 5% 4 7% 7% 31%	2730
	21st to end	Night	20% 14% 4 9% 5% 4 3 3 5% 6% 28%	2835
December	1st to 10th	Night	12% 13% 6% 14% 6% 5% 5% 4 5% 5% 24%	4720
	11th to 20th	Night	16% 17% 7% 12% 6% 5% 4 3 4 6% 20%	5508
	21st to end	Night	27% 20% 7% 10% 4 4 4 3 4 4 15%	7179

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% 6% 12% 13% 5% 1	4692
	11th to 20th	Day	75% 5% 8% 7% 5%	4726
	21st to end	Day	69% 5% 10% 11% 5%	5780
February	1st to 10th	Day	62% 7% 10% 13% 7%	1890
	11th to 20th	Day	60% 7% 9% 16% 7% 1	1992
	21st to end	Day	57% 6% 12% 17% 7% 1	1730
March	1st to 10th	Day	56% 5% 13% 17% 8% 1	4715
	11th to 20th	Day	48% 6% 16% 21% 8% 1	5090
	21st to end	Day	57% 6% 10% 16% 10% 1	5586
April	1st to 10th	Day	51% 7% 11% 18% 12% 1	2425
	11th to 20th	Day	49% 6% 11% 19% 13% 2	2670
	21st to end	Day	48% 6% 13% 19% 12% 1	2730
May	1st to 10th	Day	48% 6% 11% 22% 10% 2	7651
	11th to 20th	Day	45% 8% 13% 21% 12% 1	7630
	21st to end	Day	39% 5% 9% 20% 22% 5%	8405
June	1st to 10th	Day	38% 7% 11% 26% 15% 3	2932
	11th to 20th	Day	39% 7% 13% 22% 16% 3	3148
	21st to end	Day	38% 8% 13% 22% 16% 2	2940
July	1st to 10th	Day	37% 6% 13% 26% 17% 1	8211
	11th to 20th	Day	28% 6% 13% 32% 20% 1	8205
	21st to end	Day	33% 7% 14% 24% 21% 2	8216
August	1st to 10th	Day	36% 7% 14% 26% 15% 1	4351
	11th to 20th	Day	39% 10% 11% 29% 10% 1	4343
	21st to end	Day	36% 8% 15% 28% 12% 1	4760
September	1st to 10th	Day	38% 8% 15% 23% 14% 2	2250
	11th to 20th	Day	30% 8% 14% 32% 15% 1	2200
	21st to end	Day	42% 7% 14% 24% 13%	2200
October	1st to 10th	Day	30% 6% 12% 31% 20% 1	3477
	11th to 20th	Day	38% 5% 13% 27% 17%	3240
	21st to end	Day	43% 5% 15% 23% 13% 1	3291
November	1st to 10th	Day	43% 7% 13% 24% 12% 1	1852
	11th to 20th	Day	52% 5% 14% 19% 9% 1	1890
	21st to end	Day	58% 6% 11% 18% 6%	1780
December	1st to 10th	Day	63% 6% 16% 11% 4	2680
	11th to 20th	Day	66% 5% 11% 13% 4	2904
	21st to end	Day	77% 4 7% 7% 4	4023
January	1st to 10th	Night	61% 5% 11% 15% 6% 1	8233
	11th to 20th	Night	67% 6% 10% 11% 6%	8246
	21st to end	Night	62% 5% 10% 15% 8%	8401
February	1st to 10th	Night	61% 6% 11% 15% 6% 1	2730
	11th to 20th	Night	58% 5% 10% 18% 8% 1	2521
	21st to end	Night	55% 6% 13% 17% 8% 1	2076
March	1st to 10th	Night	50% 5% 14% 21% 9% 1	5357
	11th to 20th	Night	39% 6% 15% 25% 14% 1	5090
	21st to end	Night	48% 6% 11% 20% 14% 1	5588
April	1st to 10th	Night	50% 5% 10% 20% 12% 2	2211
	11th to 20th	Night	45% 7% 11% 19% 17% 2	1899
	21st to end	Night	44% 6% 12% 20% 16% 3	1890
May	1st to 10th	Night	51% 7% 9% 16% 14% 2	5264
	11th to 20th	Night	44% 6% 10% 26% 13% 2	5235
	21st to end	Night	38% 6% 11% 21% 19% 4	5771
June	1st to 10th	Night	34% 6% 12% 25% 20% 3	1667
	11th to 20th	Night	36% 7% 13% 25% 17% 2	1470
	21st to end	Night	33% 7% 12% 26% 20% 1	1667
July	1st to 10th	Night	31% 7% 13% 31% 15% 4	4670
	11th to 20th	Night	29% 6% 12% 31% 20% 1	4662
	21st to end	Night	32% 6% 12% 22% 26% 3	5549
August	1st to 10th	Night	35% 7% 13% 26% 18% 1	2994
	11th to 20th	Night	34% 7% 11% 34% 12% 2	2992
	21st to end	Night	32% 7% 14% 32% 14% 1	3332
September	1st to 10th	Night	33% 7% 12% 28% 17% 2	2130
	11th to 20th	Night	27% 7% 11% 33% 21% 2	2200
	21st to end	Night	40% 6% 13% 26% 14% 1	2200
October	1st to 10th	Night	26% 4 10% 33% 27% 1	3689
	11th to 20th	Night	41% 5% 11% 23% 19%	3899
	21st to end	Night	39% 4 12% 29% 15% 1	4497
November	1st to 10th	Night	41% 6% 12% 25% 15% 1	2666
	11th to 20th	Night	49% 6% 14% 21% 10% 1	2730
	21st to end	Night	57% 6% 12% 19% 6%	2835
December	1st to 10th	Night	60% 6% 16% 14% 4	4720
	11th to 20th	Night	65% 6% 12% 12% 6% 1	5511
	21st to end	Night	73% 5% 9% 8% 5%	7179

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	24% 20% 38% 13% 5% 1	330
	11th to 20th	Day	24% 22% 37% 14% 2 1	330
	21st to end	Day	28% 17% 33% 17% 5%	363
February	1st to 10th	Day	22% 20% 38% 15% 3 1	330
	11th to 20th	Day	24% 20% 36% 18% 2	330
	21st to end	Day	22% 26% 40% 8% 3	273
March	1st to 10th	Day	26% 21% 33% 17% 3	387
	11th to 20th	Day	26% 15% 34% 20% 5% 1	540
	21st to end	Day	27% 18% 32% 17% 5% 1	594
April	1st to 10th	Day	24% 19% 31% 19% 7% 1	540
	11th to 20th	Day	28% 18% 30% 14% 8% 3	532
	21st to end	Day	26% 14% 31% 17% 8% 3	540
May	1st to 10th	Day	24% 17% 31% 19% 6% 3	543
	11th to 20th	Day	19% 13% 31% 22% 11% 4	567
	21st to end	Day	15% 9% 26% 27% 16% 6%	617
June	1st to 10th	Day	13% 10% 32% 30% 12% 3	730
	11th to 20th	Day	15% 10% 32% 29% 12% 2	780
	21st to end	Day	11% 11% 32% 29% 13% 4	780
July	1st to 10th	Day	9% 6% 25% 31% 20% 8%	771
	11th to 20th	Day	10% 7% 28% 31% 17% 7%	768
	21st to end	Day	8% 5% 27% 29% 23% 8%	641
August	1st to 10th	Day	8% 5% 23% 32% 19% 12%	566
	11th to 20th	Day	13% 4 24% 30% 20% 9%	559
	21st to end	Day	10% 5% 23% 30% 23% 9%	619
September	1st to 10th	Day	12% 7% 23% 26% 20% 11%	530
	11th to 20th	Day	8% 4 18% 26% 31% 13%	530
	21st to end	Day	18% 10% 24% 22% 17% 8%	530
October	1st to 10th	Day	13% 11% 28% 23% 18% 7%	530
	11th to 20th	Day	18% 16% 33% 24% 6% 2	530
	21st to end	Day	27% 18% 34% 16% 5% 1	437
November	1st to 10th	Day	18% 19% 35% 20% 8% 1	330
	11th to 20th	Day	23% 24% 32% 16% 4 1	330
	21st to end	Day	28% 19% 33% 15% 5%	272
December	1st to 10th	Day	24% 20% 32% 17% 5% 1	220
	11th to 20th	Day	29% 23% 35% 9% 3	220
	21st to end	Day	30% 22% 32% 13% 3	352
January	1st to 10th	Night	23% 17% 36% 20% 5% 1	1050
	11th to 20th	Night	27% 15% 33% 21% 4 1	1042
	21st to end	Night	21% 15% 34% 25% 4	924
February	1st to 10th	Night	25% 13% 36% 23% 3	840
	11th to 20th	Night	23% 14% 34% 26% 3	823
	21st to end	Night	25% 15% 39% 19% 2	692
March	1st to 10th	Night	20% 14% 34% 27% 5%	840
	11th to 20th	Night	20% 16% 35% 23% 5% 1	840
	21st to end	Night	23% 12% 31% 27% 6%	924
April	1st to 10th	Night	23% 14% 32% 26% 4	741
	11th to 20th	Night	22% 12% 34% 24% 8% 1	623
	21st to end	Night	20% 10% 32% 29% 8%	630
May	1st to 10th	Night	20% 11% 31% 29% 9% 1	630
	11th to 20th	Night	21% 6% 29% 31% 12% 1	621
	21st to end	Night	11% 5% 27% 39% 18% 1	683
June	1st to 10th	Night	13% 4 27% 43% 13%	572
	11th to 20th	Night	10% 7% 26% 46% 11%	420
	21st to end	Night	11% 5% 24% 41% 19% 1	617
July	1st to 10th	Night	8% 3 21% 40% 25% 4	615
	11th to 20th	Night	9% 4 22% 39% 23% 4	612
	21st to end	Night	6% 3 19% 46% 23% 2	690
August	1st to 10th	Night	9% 4 19% 40% 27% 3	627
	11th to 20th	Night	9% 3 20% 37% 29% 3	624
	21st to end	Night	9% 3 19% 37% 29% 3	687
September	1st to 10th	Night	12% 6% 19% 32% 25% 5%	730
	11th to 20th	Night	9% 4 18% 35% 31% 4	800
	21st to end	Night	17% 7% 22% 30% 21% 4	800
October	1st to 10th	Night	16% 7% 26% 31% 17% 3	800
	11th to 20th	Night	23% 13% 28% 24% 10% 1	800
	21st to end	Night	23% 14% 31% 26% 5%	880
November	1st to 10th	Night	18% 13% 31% 26% 11% 1	820
	11th to 20th	Night	22% 16% 32% 23% 7% 1	840
	21st to end	Night	28% 13% 34% 19% 6%	840
December	1st to 10th	Night	26% 16% 33% 20% 5%	840
	11th to 20th	Night	24% 16% 36% 20% 4	914
	21st to end	Night	31% 13% 33% 20% 3	1155

Page generated at 2026-08-27 04:07:13. Version ac07

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