Ypsilanti
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Ypsilanti clear sky chart from 2005-11-06 to 2025-07-21.

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 8% 5% 5% 5% 4 6% 7% 33%	119329
	Night	15% 10% 4 8% 5% 4 4 4 5% 6% 34%	119875
Transparency	Day	47% 8% 12% 20% 12% 1	122942
	Night	47% 7% 12% 20% 12% 1	124400
Seeing	Day	17% 11% 29% 24% 14% 5%	16433
	Night	19% 9% 29% 29% 12% 1	26432

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% 16% 7% 16% 5% 5% 4 4 5% 5% 23%	3617
	11th to 20th	Day	14% 15% 6% 17% 8% 7% 7% 4 5% 6% 10%	3620
	21st to end	Day	22% 17% 6% 13% 5% 4 4 3 4 5% 16%	4396
February	1st to 10th	Day	14% 15% 5% 10% 8% 6% 5% 4 6% 5% 22%	1800
	11th to 20th	Day	15% 12% 5% 13% 8% 6% 5% 4 4 6% 24%	1910
	21st to end	Day	15% 16% 4 10% 6% 4 4 3 5% 6% 27%	1650
March	1st to 10th	Day	16% 13% 5% 10% 5% 5% 4 3 4 6% 29%	4567
	11th to 20th	Day	12% 10% 4 8% 6% 4 4 3 5% 8% 36%	4980
	21st to end	Day	18% 14% 4 9% 5% 5% 4 4 5% 6% 27%	5465
April	1st to 10th	Day	17% 11% 4 8% 5% 5% 4 4 5% 7% 31%	2336
	11th to 20th	Day	16% 9% 3 8% 4 5% 4 4 5% 7% 35%	2522
	21st to end	Day	18% 9% 3 7% 4 4 4 2 5% 8% 35%	2600
May	1st to 10th	Day	19% 9% 3 7% 5% 4 4 4 5% 6% 34%	5862
	11th to 20th	Day	17% 8% 3 7% 5% 5% 5% 4 6% 9% 33%	5861
	21st to end	Day	14% 6% 2 7% 5% 4 4 3 5% 8% 42%	6433
June	1st to 10th	Day	10% 6% 3 6% 6% 5% 4 5% 7% 8% 41%	2786
	11th to 20th	Day	11% 6% 2 6% 5% 4 6% 5% 8% 10% 38%	2800
	21st to end	Day	9% 5% 2 7% 5% 5% 6% 5% 8% 9% 39%	2800
July	1st to 10th	Day	11% 6% 2 7% 5% 6% 5% 4 7% 9% 40%	4580
	11th to 20th	Day	7% 4 2 5% 5% 5% 4 4 7% 10% 46%	4580
	21st to end	Day	6% 5% 3 7% 5% 5% 6% 5% 7% 9% 42%	4540
August	1st to 10th	Day	11% 5% 2 6% 4 4 5% 4 6% 8% 44%	4104
	11th to 20th	Day	9% 6% 3 7% 6% 5% 6% 5% 8% 9% 37%	4044
	21st to end	Day	8% 5% 3 6% 5% 5% 6% 5% 7% 10% 40%	4446
September	1st to 10th	Day	11% 5% 2 6% 5% 5% 5% 4 5% 8% 42%	2155
	11th to 20th	Day	8% 5% 2 5% 4 4 4 4 6% 10% 47%	2090
	21st to end	Day	15% 6% 3 6% 5% 4 5% 4 6% 9% 38%	2090
October	1st to 10th	Day	12% 6% 3 6% 4 4 4 4 5% 8% 43%	2049
	11th to 20th	Day	17% 8% 3 7% 5% 5% 5% 4 5% 6% 35%	1900
	21st to end	Day	18% 10% 3 8% 5% 4 5% 4 5% 7% 32%	1929
November	1st to 10th	Day	12% 8% 3 8% 6% 4 4 3 6% 7% 39%	1762
	11th to 20th	Day	15% 13% 3 9% 6% 5% 6% 3 5% 6% 29%	1800
	21st to end	Day	18% 12% 5% 9% 7% 4 5% 4 5% 5% 28%	1775
December	1st to 10th	Day	13% 13% 7% 13% 7% 5% 4 4 6% 7% 21%	2600
	11th to 20th	Day	19% 20% 5% 8% 6% 5% 4 4 5% 5% 18%	2898
	21st to end	Day	26% 21% 7% 11% 5% 4 4 3 3 4 13%	3982
January	1st to 10th	Night	13% 15% 6% 13% 6% 4 4 3 5% 5% 25%	6348
	11th to 20th	Night	12% 16% 7% 15% 7% 5% 5% 4 6% 6% 17%	6360
	21st to end	Night	18% 15% 7% 11% 5% 4 4 3 4 5% 24%	6559
February	1st to 10th	Night	16% 15% 4 11% 7% 5% 5% 4 4 5% 24%	2600
	11th to 20th	Night	14% 13% 5% 10% 6% 5% 5% 3 5% 7% 27%	2373
	21st to end	Night	19% 10% 4 10% 6% 4 4 3 5% 6% 30%	1980
March	1st to 10th	Night	16% 10% 4 8% 4 4 4 4 5% 6% 36%	5285
	11th to 20th	Night	10% 7% 2 6% 4 4 5% 4 6% 8% 43%	4980
	21st to end	Night	17% 9% 3 7% 5% 4 4 4 4 6% 36%	5467
April	1st to 10th	Night	16% 10% 3 7% 5% 4 4 4 5% 6% 37%	2080
	11th to 20th	Night	17% 8% 2 6% 4 4 5% 4 6% 6% 38%	1827
	21st to end	Night	17% 8% 2 6% 5% 4 5% 4 5% 6% 39%	1800
May	1st to 10th	Night	20% 11% 3 7% 4 4 4 3 4 7% 31%	4033
	11th to 20th	Night	19% 8% 2 6% 4 4 4 3 4 6% 38%	4032
	21st to end	Night	14% 5% 2 6% 4 4 4 3 5% 7% 45%	4421
June	1st to 10th	Night	12% 4 1 5% 3 5% 4 4 6% 8% 47%	1610
	11th to 20th	Night	10% 4 3 7% 4 5% 4 5% 6% 8% 44%	1600
	21st to end	Night	9% 6% 3 5% 4 3 3 4 7% 8% 48%	1600
July	1st to 10th	Night	9% 3 2 4 4 5% 4 4 6% 10% 49%	2610
	11th to 20th	Night	8% 4 2 4 3 3 3 4 6% 9% 54%	2610
	21st to end	Night	5% 4 2 6% 5% 5% 5% 5% 7% 10% 47%	3014
August	1st to 10th	Night	11% 5% 2 6% 4 3 4 4 5% 8% 47%	2825
	11th to 20th	Night	8% 6% 2 6% 4 4 5% 4 5% 7% 49%	2785
	21st to end	Night	8% 5% 2 7% 4 5% 4 3 6% 7% 48%	3206
September	1st to 10th	Night	9% 6% 2 5% 3 4 4 3 6% 7% 50%	2005
	11th to 20th	Night	8% 6% 2 4 3 4 4 2 5% 8% 54%	2090
	21st to end	Night	14% 6% 2 7% 4 4 5% 4 5% 6% 43%	2090
October	1st to 10th	Night	12% 7% 3 7% 4 3 3 3 4 6% 47%	2129
	11th to 20th	Night	16% 8% 3 8% 5% 4 4 4 5% 6% 37%	2280
	21st to end	Night	20% 8% 3 7% 5% 4 5% 3 4 6% 35%	2665
November	1st to 10th	Night	13% 8% 2 5% 5% 4 4 3 5% 7% 43%	2536
	11th to 20th	Night	15% 11% 3 8% 6% 5% 4 3 6% 7% 32%	2600
	21st to end	Night	19% 14% 5% 7% 5% 4 4 3 5% 5% 28%	2619
December	1st to 10th	Night	13% 13% 6% 13% 6% 5% 5% 4 5% 6% 25%	4580
	11th to 20th	Night	16% 18% 6% 12% 6% 4 5% 3 5% 5% 20%	5280
	21st to end	Night	28% 20% 5% 10% 4 4 4 3 4 4 15%	6996

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	60% 8% 11% 15% 6%	3618
	11th to 20th	Day	69% 9% 10% 7% 5%	3620
	21st to end	Day	69% 6% 10% 10% 5%	4396
February	1st to 10th	Day	61% 8% 10% 13% 7%	1800
	11th to 20th	Day	58% 8% 10% 16% 7% 1	1920
	21st to end	Day	56% 7% 13% 15% 7% 2	1650
March	1st to 10th	Day	55% 6% 12% 16% 9% 1	4567
	11th to 20th	Day	44% 8% 17% 22% 8% 2	4980
	21st to end	Day	56% 7% 10% 15% 11% 1	5465
April	1st to 10th	Day	52% 7% 11% 18% 11% 1	2336
	11th to 20th	Day	47% 8% 11% 20% 12% 2	2522
	21st to end	Day	48% 7% 13% 18% 14% 1	2600
May	1st to 10th	Day	48% 7% 11% 18% 13% 2	5897
	11th to 20th	Day	44% 9% 13% 20% 12% 1	5865
	21st to end	Day	39% 7% 10% 22% 18% 4	6447
June	1st to 10th	Day	37% 9% 12% 25% 14% 3	2786
	11th to 20th	Day	38% 9% 14% 21% 17% 2	2800
	21st to end	Day	37% 10% 14% 21% 16% 2	2800
July	1st to 10th	Day	39% 8% 14% 21% 16% 2	4511
	11th to 20th	Day	29% 8% 14% 30% 18% 1	4505
	21st to end	Day	35% 10% 13% 18% 21% 2	4396
August	1st to 10th	Day	35% 7% 14% 27% 16% 1	4091
	11th to 20th	Day	38% 10% 12% 28% 11% 1	4083
	21st to end	Day	36% 8% 13% 28% 12% 2	4445
September	1st to 10th	Day	37% 9% 16% 20% 15% 2	2155
	11th to 20th	Day	31% 8% 12% 31% 16% 1	2090
	21st to end	Day	41% 8% 13% 23% 14%	2090
October	1st to 10th	Day	29% 5% 12% 31% 22% 1	3400
	11th to 20th	Day	39% 6% 11% 25% 19%	3140
	21st to end	Day	43% 6% 14% 24% 13% 1	3153
November	1st to 10th	Day	43% 7% 12% 24% 13% 1	1762
	11th to 20th	Day	54% 6% 12% 18% 9% 2	1800
	21st to end	Day	56% 7% 13% 17% 7%	1775
December	1st to 10th	Day	60% 9% 15% 11% 4	2600
	11th to 20th	Day	66% 7% 11% 13% 4	2895
	21st to end	Day	75% 5% 7% 8% 4	3982
January	1st to 10th	Night	58% 7% 10% 17% 7% 1	6348
	11th to 20th	Night	65% 7% 12% 10% 6%	6360
	21st to end	Night	62% 6% 12% 11% 9%	6559
February	1st to 10th	Night	61% 7% 11% 13% 6% 1	2600
	11th to 20th	Night	57% 6% 10% 18% 8% 1	2374
	21st to end	Night	54% 6% 12% 18% 8% 1	1980
March	1st to 10th	Night	47% 5% 14% 22% 11% 1	5285
	11th to 20th	Night	36% 7% 14% 27% 15% 1	4980
	21st to end	Night	47% 7% 12% 19% 14% 2	5467
April	1st to 10th	Night	47% 7% 11% 20% 13% 2	2080
	11th to 20th	Night	43% 8% 11% 18% 18% 1	1827
	21st to end	Night	45% 7% 12% 17% 16% 3	1800
May	1st to 10th	Night	51% 7% 10% 16% 13% 3	4060
	11th to 20th	Night	45% 8% 11% 24% 10% 2	4033
	21st to end	Night	35% 7% 13% 23% 18% 3	4429
June	1st to 10th	Night	33% 7% 11% 27% 19% 2	1610
	11th to 20th	Night	36% 9% 12% 25% 16% 2	1600
	21st to end	Night	32% 8% 14% 25% 21% 1	1600
July	1st to 10th	Night	30% 7% 13% 30% 15% 6%	2570
	11th to 20th	Night	26% 6% 15% 30% 21% 2	2562
	21st to end	Night	30% 9% 18% 18% 22% 2	2918
August	1st to 10th	Night	35% 7% 13% 27% 17% 1	2814
	11th to 20th	Night	31% 9% 12% 35% 12% 2	2812
	21st to end	Night	34% 7% 12% 30% 16% 1	3206
September	1st to 10th	Night	32% 9% 12% 27% 19% 2	2005
	11th to 20th	Night	29% 7% 10% 30% 22% 2	2090
	21st to end	Night	41% 6% 11% 27% 14% 1	2090
October	1st to 10th	Night	26% 5% 10% 32% 27% 1	3558
	11th to 20th	Night	40% 5% 11% 24% 19%	3779
	21st to end	Night	38% 4 10% 32% 15% 1	4391
November	1st to 10th	Night	40% 7% 12% 25% 15% 1	2536
	11th to 20th	Night	49% 6% 14% 20% 9% 1	2600
	21st to end	Night	56% 7% 13% 18% 6%	2619
December	1st to 10th	Night	58% 7% 16% 15% 4	4580
	11th to 20th	Night	64% 7% 11% 13% 5% 1	5282
	21st to end	Night	72% 6% 9% 7% 6%	6996

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% 15% 37% 18% 5% 1	300
	11th to 20th	Day	21% 16% 45% 14% 3 1	300
	21st to end	Day	29% 14% 34% 18% 4 1	330
February	1st to 10th	Day	24% 21% 38% 14% 3 1	300
	11th to 20th	Day	25% 18% 38% 17% 2	300
	21st to end	Day	26% 24% 35% 14% 1	249
March	1st to 10th	Day	24% 21% 36% 16% 3	335
	11th to 20th	Day	23% 14% 31% 24% 7% 1	500
	21st to end	Day	24% 16% 32% 21% 5% 1	550
April	1st to 10th	Day	21% 16% 34% 20% 8% 1	500
	11th to 20th	Day	22% 20% 32% 15% 8% 4	492
	21st to end	Day	24% 13% 31% 18% 11% 2	500
May	1st to 10th	Day	23% 15% 29% 20% 10% 4	503
	11th to 20th	Day	20% 10% 26% 24% 14% 6%	527
	21st to end	Day	14% 9% 23% 27% 19% 8%	573
June	1st to 10th	Day	11% 9% 28% 32% 16% 4	716
	11th to 20th	Day	13% 8% 29% 30% 16% 4	730
	21st to end	Day	12% 8% 29% 31% 15% 5%	730
July	1st to 10th	Day	10% 4 23% 28% 25% 10%	721
	11th to 20th	Day	10% 6% 25% 30% 21% 8%	718
	21st to end	Day	8% 4 24% 31% 22% 11%	580
August	1st to 10th	Day	7% 3 21% 27% 27% 14%	486
	11th to 20th	Day	12% 5% 19% 30% 23% 12%	479
	21st to end	Day	11% 2 18% 29% 28% 11%	539
September	1st to 10th	Day	11% 6% 23% 25% 22% 13%	490
	11th to 20th	Day	8% 4 16% 27% 29% 16%	490
	21st to end	Day	17% 10% 22% 23% 20% 8%	490
October	1st to 10th	Day	16% 8% 27% 23% 18% 8%	490
	11th to 20th	Day	18% 16% 28% 28% 9% 2	490
	21st to end	Day	25% 14% 34% 18% 7% 1	378
November	1st to 10th	Day	14% 19% 34% 23% 9% 2	300
	11th to 20th	Day	23% 17% 33% 19% 6% 2	300
	21st to end	Day	24% 17% 40% 14% 5%	287
December	1st to 10th	Day	18% 20% 39% 16% 7% 2	200
	11th to 20th	Day	25% 16% 40% 14% 3 1	230
	21st to end	Day	29% 20% 32% 17% 2	330
January	1st to 10th	Night	22% 14% 36% 22% 5% 1	1000
	11th to 20th	Night	25% 14% 34% 22% 5% 1	1000
	21st to end	Night	19% 14% 35% 26% 5%	910
February	1st to 10th	Night	23% 13% 36% 24% 3	800
	11th to 20th	Night	23% 14% 33% 26% 4	783
	21st to end	Night	24% 13% 40% 21% 2	660
March	1st to 10th	Night	21% 12% 35% 26% 6%	800
	11th to 20th	Night	17% 17% 36% 24% 6% 1	800
	21st to end	Night	23% 11% 31% 28% 7%	880
April	1st to 10th	Night	20% 14% 34% 28% 4 1	680
	11th to 20th	Night	21% 10% 35% 25% 7% 1	593
	21st to end	Night	22% 8% 31% 31% 8%	600
May	1st to 10th	Night	19% 9% 31% 30% 10% 1	600
	11th to 20th	Night	23% 4 26% 34% 13% 1	591
	21st to end	Night	12% 4 24% 36% 22% 2	650
June	1st to 10th	Night	13% 3 24% 45% 14% 1	600
	11th to 20th	Night	12% 4 21% 44% 18%	600
	21st to end	Night	10% 3 22% 43% 21% 1	600
July	1st to 10th	Night	8% 2 18% 40% 28% 4	585
	11th to 20th	Night	9% 3 20% 39% 26% 4	582
	21st to end	Night	5% 2 17% 47% 25% 3	624
August	1st to 10th	Night	8% 2 17% 42% 28% 2	567
	11th to 20th	Night	9% 2 17% 37% 31% 3	564
	21st to end	Night	9% 2 16% 38% 31% 4	627
September	1st to 10th	Night	12% 5% 18% 32% 27% 5%	675
	11th to 20th	Night	10% 3 16% 33% 32% 5%	760
	21st to end	Night	17% 6% 21% 30% 23% 4	760
October	1st to 10th	Night	14% 7% 27% 31% 18% 3	760
	11th to 20th	Night	21% 12% 30% 26% 10% 1	760
	21st to end	Night	24% 10% 33% 27% 6%	836
November	1st to 10th	Night	18% 11% 31% 27% 12% 1	780
	11th to 20th	Night	22% 13% 36% 22% 7% 1	800
	21st to end	Night	25% 14% 33% 22% 6%	800
December	1st to 10th	Night	23% 15% 35% 22% 5%	800
	11th to 20th	Night	23% 14% 37% 22% 4	905
	21st to end	Night	31% 12% 33% 21% 3	1100

Page generated at 2025-07-18 01:10:01. Version ac07

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