Ann Arbor
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Ann Arbor clear sky chart from 2005-11-06 to 2026-03-18.

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% 10% 3 9% 5% 5% 5% 4 6% 7% 33%	130379
	Night	14% 10% 4 9% 5% 4 4 3 5% 6% 34%	131227
Transparency	Day	48% 6% 11% 21% 13% 1	133962
	Night	48% 6% 11% 21% 13% 1	135715
Seeing	Day	18% 12% 28% 23% 14% 6%	17271
	Night	19% 10% 29% 29% 12% 1	27402

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% 16% 6% 16% 6% 5% 5% 4 5% 5% 20%	4691
	11th to 20th	Day	13% 14% 7% 17% 10% 8% 6% 3 5% 5% 11%	4710
	21st to end	Day	18% 15% 6% 12% 8% 6% 5% 4 5% 5% 15%	5646
February	1st to 10th	Day	14% 14% 5% 12% 8% 6% 5% 4 5% 6% 21%	1890
	11th to 20th	Day	14% 13% 5% 13% 8% 5% 4 4 4 6% 25%	2015
	21st to end	Day	14% 15% 5% 11% 5% 4 4 4 5% 6% 26%	1730
March	1st to 10th	Day	15% 12% 4 11% 6% 5% 4 3 4 5% 30%	4663
	11th to 20th	Day	12% 11% 4 8% 5% 5% 4 3 5% 7% 36%	5057
	21st to end	Day	19% 13% 4 9% 5% 5% 4 4 5% 6% 27%	5465
April	1st to 10th	Day	16% 14% 3 7% 6% 4 4 4 5% 6% 31%	2345
	11th to 20th	Day	16% 9% 3 8% 5% 5% 4 4 6% 7% 34%	2517
	21st to end	Day	17% 9% 3 7% 5% 4 4 3 5% 7% 36%	2600
May	1st to 10th	Day	19% 9% 3 7% 5% 4 4 4 5% 6% 34%	5862
	11th to 20th	Day	17% 8% 2 7% 5% 5% 5% 4 6% 8% 33%	5861
	21st to end	Day	14% 7% 3 6% 5% 4 4 3 6% 8% 41%	6433
June	1st to 10th	Day	9% 7% 2 5% 6% 4 5% 5% 6% 9% 41%	2800
	11th to 20th	Day	11% 6% 2 7% 5% 5% 6% 4 8% 9% 39%	2800
	21st to end	Day	9% 6% 2 7% 5% 5% 6% 5% 8% 10% 39%	2800
July	1st to 10th	Day	9% 5% 2 6% 5% 5% 5% 4 7% 11% 40%	6360
	11th to 20th	Day	6% 5% 2 6% 5% 4 5% 4 7% 11% 45%	6360
	21st to end	Day	7% 6% 2 6% 5% 5% 5% 4 7% 9% 45%	6562
August	1st to 10th	Day	11% 5% 2 6% 4 5% 5% 4 6% 9% 44%	4234
	11th to 20th	Day	9% 6% 2 7% 5% 5% 6% 5% 7% 10% 38%	4174
	21st to end	Day	8% 5% 2 6% 5% 5% 6% 5% 7% 10% 40%	4580
September	1st to 10th	Day	11% 5% 2 6% 5% 5% 5% 3 6% 10% 41%	2273
	11th to 20th	Day	8% 5% 2 5% 4 4 4 5% 5% 9% 48%	2200
	21st to end	Day	14% 7% 2 8% 5% 5% 4 4 6% 7% 39%	2200
October	1st to 10th	Day	12% 5% 3 6% 5% 4 4 4 6% 7% 44%	2162
	11th to 20th	Day	17% 8% 3 7% 4 6% 5% 4 5% 7% 34%	2000
	21st to end	Day	19% 9% 3 9% 5% 4 4 4 5% 7% 32%	2022
November	1st to 10th	Day	11% 9% 3 8% 6% 4 5% 4 5% 6% 39%	1852
	11th to 20th	Day	15% 13% 4 9% 5% 6% 4 3 5% 6% 30%	1890
	21st to end	Day	18% 12% 4 10% 6% 5% 5% 3 4 6% 27%	1884
December	1st to 10th	Day	12% 14% 6% 16% 6% 4 5% 4 5% 6% 20%	2680
	11th to 20th	Day	18% 20% 6% 10% 6% 5% 4 4 5% 5% 18%	2991
	21st to end	Day	26% 20% 7% 12% 5% 4 4 2 3 4 14%	4070
January	1st to 10th	Night	15% 15% 7% 13% 6% 5% 4 3 4 5% 24%	8232
	11th to 20th	Night	10% 15% 7% 15% 7% 7% 6% 4 6% 6% 17%	8280
	21st to end	Night	15% 14% 7% 11% 5% 5% 5% 4 5% 6% 23%	8539
February	1st to 10th	Night	15% 15% 5% 10% 7% 5% 5% 3 5% 5% 25%	2730
	11th to 20th	Night	14% 13% 5% 10% 6% 5% 4 3 5% 6% 28%	2489
	21st to end	Night	17% 11% 4 11% 5% 5% 4 3 4 6% 30%	2076
March	1st to 10th	Night	16% 11% 3 8% 4 4 4 4 5% 7% 35%	5410
	11th to 20th	Night	10% 7% 3 6% 4 4 5% 4 7% 8% 43%	5058
	21st to end	Night	17% 10% 3 7% 4 4 4 3 5% 6% 36%	5467
April	1st to 10th	Night	16% 11% 2 6% 5% 4 4 4 5% 6% 37%	2071
	11th to 20th	Night	18% 8% 2 6% 4 4 5% 4 6% 7% 37%	1832
	21st to end	Night	16% 8% 3 6% 5% 4 5% 3 5% 5% 39%	1800
May	1st to 10th	Night	20% 11% 3 8% 5% 4 4 3 5% 6% 32%	4033
	11th to 20th	Night	19% 8% 2 7% 4 4 4 3 4 6% 38%	4032
	21st to end	Night	14% 5% 2 6% 4 4 4 3 5% 8% 45%	4404
June	1st to 10th	Night	12% 4 1 5% 5% 4 4 3 5% 10% 47%	1600
	11th to 20th	Night	10% 5% 2 8% 4 4 5% 5% 6% 9% 43%	1600
	21st to end	Night	9% 6% 2 6% 4 4 4 3 7% 8% 48%	1600
July	1st to 10th	Night	7% 4 2 5% 5% 4 5% 4 6% 10% 48%	3618
	11th to 20th	Night	7% 5% 2 5% 4 4 4 4 6% 9% 50%	3617
	21st to end	Night	8% 5% 2 6% 4 4 4 4 6% 8% 48%	4292
August	1st to 10th	Night	11% 6% 3 5% 4 4 4 4 5% 8% 48%	2915
	11th to 20th	Night	8% 6% 2 6% 4 4 5% 4 6% 8% 49%	2875
	21st to end	Night	8% 5% 2 7% 5% 5% 5% 3 6% 7% 48%	3315
September	1st to 10th	Night	10% 5% 2 4 4 4 4 4 6% 6% 51%	2106
	11th to 20th	Night	9% 5% 2 4 3 4 4 3 5% 8% 55%	2200
	21st to end	Night	14% 6% 2 6% 4 5% 5% 3 5% 7% 43%	2200
October	1st to 10th	Night	12% 6% 3 7% 4 3 4 3 5% 6% 48%	2236
	11th to 20th	Night	17% 9% 3 7% 4 5% 4 4 4 5% 38%	2400
	21st to end	Night	19% 8% 3 7% 4 4 4 4 4 6% 36%	2815
November	1st to 10th	Night	13% 8% 3 5% 5% 4 4 4 5% 6% 44%	2666
	11th to 20th	Night	14% 11% 4 8% 6% 5% 4 3 6% 7% 32%	2730
	21st to end	Night	19% 14% 4 8% 5% 4 5% 3 5% 6% 28%	2730
December	1st to 10th	Night	13% 12% 5% 14% 6% 5% 5% 4 5% 7% 25%	4718
	11th to 20th	Night	16% 17% 6% 12% 6% 5% 4 3 5% 6% 20%	5391
	21st to end	Night	26% 21% 5% 10% 5% 4 4 3 4 4 14%	7150

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	65% 6% 10% 14% 5%	4692
	11th to 20th	Day	74% 5% 7% 7% 7%	4710
	21st to end	Day	69% 5% 9% 12% 6%	5646
February	1st to 10th	Day	62% 7% 8% 15% 8%	1890
	11th to 20th	Day	60% 5% 10% 16% 8% 1	2025
	21st to end	Day	58% 6% 12% 16% 7% 2	1730
March	1st to 10th	Day	56% 5% 12% 17% 9% 2	4663
	11th to 20th	Day	47% 5% 16% 22% 9% 2	5057
	21st to end	Day	58% 5% 10% 15% 12% 1	5465
April	1st to 10th	Day	53% 6% 11% 18% 12%	2345
	11th to 20th	Day	48% 6% 11% 21% 13% 2	2517
	21st to end	Day	48% 5% 13% 19% 14% 1	2600
May	1st to 10th	Day	50% 5% 10% 19% 14% 2	5897
	11th to 20th	Day	46% 8% 11% 21% 12% 1	5865
	21st to end	Day	40% 6% 9% 22% 18% 4	6447
June	1st to 10th	Day	38% 7% 12% 26% 15% 3	2800
	11th to 20th	Day	39% 9% 12% 22% 17% 2	2800
	21st to end	Day	37% 9% 14% 22% 16% 2	2800
July	1st to 10th	Day	37% 6% 12% 27% 16% 2	6291
	11th to 20th	Day	31% 7% 13% 29% 18% 1	6285
	21st to end	Day	34% 8% 13% 23% 21% 1	6375
August	1st to 10th	Day	36% 6% 14% 27% 17% 1	4221
	11th to 20th	Day	38% 8% 11% 31% 11% 1	4213
	21st to end	Day	37% 6% 13% 29% 13% 2	4579
September	1st to 10th	Day	37% 8% 14% 23% 16% 2	2273
	11th to 20th	Day	31% 7% 10% 33% 17% 1	2200
	21st to end	Day	42% 6% 11% 24% 16%	2200
October	1st to 10th	Day	30% 5% 10% 32% 22% 1	3526
	11th to 20th	Day	40% 4 11% 25% 20%	3240
	21st to end	Day	44% 5% 13% 24% 14% 1	3246
November	1st to 10th	Day	44% 6% 12% 25% 13% 1	1852
	11th to 20th	Day	55% 4 11% 19% 8% 2	1890
	21st to end	Day	58% 6% 11% 18% 7% 1	1884
December	1st to 10th	Day	62% 7% 14% 12% 5%	2680
	11th to 20th	Day	67% 6% 9% 13% 4	2988
	21st to end	Day	76% 4 7% 8% 5%	4070
January	1st to 10th	Night	62% 5% 9% 16% 8% 1	8233
	11th to 20th	Night	65% 5% 10% 12% 7%	8280
	21st to end	Night	62% 5% 10% 15% 9%	8539
February	1st to 10th	Night	61% 6% 10% 15% 7% 1	2730
	11th to 20th	Night	56% 6% 10% 19% 9% 1	2490
	21st to end	Night	56% 4 11% 18% 9% 1	2076
March	1st to 10th	Night	48% 5% 12% 23% 11% 1	5410
	11th to 20th	Night	37% 6% 14% 26% 16% 1	5058
	21st to end	Night	49% 5% 11% 20% 14% 2	5467
April	1st to 10th	Night	47% 6% 10% 21% 14% 2	2071
	11th to 20th	Night	44% 7% 10% 19% 18% 1	1832
	21st to end	Night	46% 6% 11% 18% 17% 3	1800
May	1st to 10th	Night	52% 6% 9% 16% 13% 3	4060
	11th to 20th	Night	46% 6% 9% 26% 11% 2	4033
	21st to end	Night	37% 6% 12% 24% 19% 3	4412
June	1st to 10th	Night	33% 7% 10% 28% 20% 2	1600
	11th to 20th	Night	37% 7% 12% 25% 16% 2	1600
	21st to end	Night	32% 7% 13% 27% 21% 1	1600
July	1st to 10th	Night	31% 6% 14% 29% 16% 4	3580
	11th to 20th	Night	30% 6% 14% 28% 20% 2	3572
	21st to end	Night	32% 7% 14% 21% 25% 2	4166
August	1st to 10th	Night	35% 7% 12% 28% 17% 1	2904
	11th to 20th	Night	32% 6% 11% 37% 12% 2	2902
	21st to end	Night	34% 6% 11% 31% 16% 1	3315
September	1st to 10th	Night	32% 7% 10% 30% 18% 2	2106
	11th to 20th	Night	28% 5% 9% 32% 23% 2	2200
	21st to end	Night	40% 6% 11% 27% 15% 1	2200
October	1st to 10th	Night	26% 4 8% 33% 28% 1	3652
	11th to 20th	Night	41% 4 10% 24% 20% 1	3899
	21st to end	Night	38% 4 10% 32% 15% 1	4541
November	1st to 10th	Night	41% 6% 11% 25% 17% 1	2666
	11th to 20th	Night	49% 5% 13% 21% 10% 2	2730
	21st to end	Night	57% 6% 12% 18% 7%	2730
December	1st to 10th	Night	59% 5% 15% 16% 4	4718
	11th to 20th	Night	65% 5% 11% 13% 6% 1	5393
	21st to end	Night	73% 5% 9% 8% 6%	7150

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% 16% 37% 17% 5% 1	330
	11th to 20th	Day	23% 20% 40% 13% 2 1	330
	21st to end	Day	29% 17% 32% 17% 4 1	363
February	1st to 10th	Day	24% 19% 38% 15% 3 1	330
	11th to 20th	Day	25% 19% 36% 16% 3	330
	21st to end	Day	25% 25% 36% 13% 1	273
March	1st to 10th	Day	25% 23% 35% 13% 3	361
	11th to 20th	Day	24% 14% 33% 21% 6% 1	528
	21st to end	Day	27% 15% 32% 20% 5% 1	550
April	1st to 10th	Day	23% 16% 33% 20% 8% 1	500
	11th to 20th	Day	23% 20% 31% 15% 7% 4	492
	21st to end	Day	24% 13% 31% 19% 11% 2	500
May	1st to 10th	Day	23% 13% 30% 20% 10% 3	503
	11th to 20th	Day	21% 11% 28% 21% 13% 6%	527
	21st to end	Day	16% 8% 22% 25% 19% 10%	573
June	1st to 10th	Day	12% 9% 28% 30% 16% 4	730
	11th to 20th	Day	12% 8% 30% 30% 15% 5%	730
	21st to end	Day	11% 9% 29% 32% 15% 5%	730
July	1st to 10th	Day	10% 5% 21% 29% 25% 11%	721
	11th to 20th	Day	11% 7% 23% 30% 21% 9%	718
	21st to end	Day	8% 4 23% 31% 22% 11%	636
August	1st to 10th	Day	9% 2 18% 30% 25% 15%	526
	11th to 20th	Day	12% 5% 18% 29% 24% 11%	519
	21st to end	Day	10% 3 19% 30% 25% 12%	583
September	1st to 10th	Day	12% 6% 22% 23% 23% 13%	530
	11th to 20th	Day	8% 4 16% 26% 30% 16%	530
	21st to end	Day	18% 9% 22% 22% 19% 9%	530
October	1st to 10th	Day	14% 9% 28% 24% 17% 8%	530
	11th to 20th	Day	18% 14% 31% 27% 10% 1	530
	21st to end	Day	27% 14% 36% 17% 6%	405
November	1st to 10th	Day	17% 19% 32% 22% 8% 2	330
	11th to 20th	Day	25% 20% 30% 17% 6% 2	330
	21st to end	Day	26% 15% 40% 14% 4	326
December	1st to 10th	Day	20% 20% 35% 17% 7% 1	220
	11th to 20th	Day	27% 19% 37% 13% 3	264
	21st to end	Day	32% 19% 33% 14% 2	363
January	1st to 10th	Night	23% 15% 36% 20% 5% 1	1050
	11th to 20th	Night	25% 15% 35% 20% 5% 1	1050
	21st to end	Night	20% 14% 34% 25% 6% 1	971
February	1st to 10th	Night	23% 13% 35% 25% 4	840
	11th to 20th	Night	22% 16% 33% 26% 3	823
	21st to end	Night	24% 13% 40% 20% 3	692
March	1st to 10th	Night	22% 13% 35% 25% 6%	840
	11th to 20th	Night	20% 16% 35% 23% 5% 1	829
	21st to end	Night	24% 10% 31% 27% 7%	880
April	1st to 10th	Night	21% 14% 33% 27% 4 1	671
	11th to 20th	Night	22% 10% 36% 24% 8% 1	593
	21st to end	Night	22% 9% 32% 30% 8%	600
May	1st to 10th	Night	20% 8% 31% 31% 10% 1	600
	11th to 20th	Night	23% 3 28% 32% 13% 1	591
	21st to end	Night	13% 4 26% 33% 21% 2	650
June	1st to 10th	Night	13% 3 24% 43% 16% 1	600
	11th to 20th	Night	12% 5% 23% 41% 19%	600
	21st to end	Night	10% 5% 22% 40% 22% 2	600
July	1st to 10th	Night	9% 2 17% 41% 28% 4	585
	11th to 20th	Night	9% 3 19% 39% 26% 4	582
	21st to end	Night	6% 2 17% 47% 27% 2	657
August	1st to 10th	Night	8% 2 17% 43% 28% 3	597
	11th to 20th	Night	9% 3 18% 37% 30% 4	594
	21st to end	Night	10% 3 16% 37% 30% 4	660
September	1st to 10th	Night	11% 5% 19% 31% 28% 5%	706
	11th to 20th	Night	9% 4 16% 32% 34% 5%	800
	21st to end	Night	17% 6% 21% 31% 22% 4	800
October	1st to 10th	Night	14% 7% 25% 33% 18% 3	800
	11th to 20th	Night	22% 13% 28% 27% 10% 1	800
	21st to end	Night	24% 11% 33% 26% 6%	880
November	1st to 10th	Night	18% 13% 30% 27% 11% 1	820
	11th to 20th	Night	22% 15% 35% 21% 7% 1	840
	21st to end	Night	26% 13% 34% 22% 5%	840
December	1st to 10th	Night	23% 15% 34% 22% 5%	840
	11th to 20th	Night	24% 14% 37% 20% 4	966
	21st to end	Night	32% 13% 32% 20% 2	1155

Page generated at 2026-03-15 02:23:56. Version ac07

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