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 2024-01-15.

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% 9% 4 7% 6% 4 5% 5% 6% 8% 34%	74607
	Night	15% 9% 4 7% 6% 3 4 4 5% 7% 35%	75183
Transparency	Day	48% 7% 11% 20% 12% 1	74317
	Night	48% 6% 11% 20% 12% 1	74994
Seeing	Day	17% 10% 28% 24% 15% 6%	14358
	Night	19% 9% 29% 29% 13% 1	24455

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% 14% 4 10% 8% 5% 5% 5% 5% 6% 21%	1517
	11th to 20th	Day	17% 16% 6% 11% 9% 5% 4 5% 5% 6% 17%	1477
	21st to end	Day	18% 16% 8% 10% 8% 4 4 4 4 5% 18%	1739
February	1st to 10th	Day	15% 13% 7% 10% 9% 4 5% 5% 5% 6% 20%	1620
	11th to 20th	Day	15% 11% 4 11% 10% 4 4 5% 4 6% 24%	1719
	21st to end	Day	16% 15% 6% 10% 5% 3 5% 4 5% 6% 26%	1480
March	1st to 10th	Day	15% 11% 4 7% 8% 4 6% 5% 6% 7% 27%	1826
	11th to 20th	Day	14% 11% 4 8% 7% 3 4 4 5% 7% 34%	1980
	21st to end	Day	20% 11% 4 8% 6% 3 5% 5% 4 6% 28%	2178
April	1st to 10th	Day	16% 12% 4 7% 6% 3 4 5% 5% 6% 31%	2110
	11th to 20th	Day	16% 9% 3 6% 6% 3 4 5% 5% 7% 34%	2262
	21st to end	Day	19% 9% 4 7% 5% 3 5% 4 5% 7% 34%	2340
May	1st to 10th	Day	18% 8% 4 6% 7% 4 5% 5% 5% 8% 31%	2340
	11th to 20th	Day	19% 8% 3 6% 5% 4 5% 4 4 7% 34%	2301
	21st to end	Day	12% 6% 2 5% 5% 4 5% 5% 6% 8% 41%	2531
June	1st to 10th	Day	9% 6% 3 5% 6% 4 5% 6% 6% 9% 41%	2520
	11th to 20th	Day	11% 5% 2 5% 6% 4 6% 5% 7% 9% 40%	2520
	21st to end	Day	9% 5% 2 6% 5% 3 6% 7% 7% 10% 39%	2520
July	1st to 10th	Day	7% 5% 2 4 6% 3 5% 5% 6% 9% 47%	2520
	11th to 20th	Day	8% 4 2 5% 6% 3 6% 6% 6% 10% 44%	2520
	21st to end	Day	8% 5% 2 5% 6% 4 6% 6% 7% 9% 43%	2601
August	1st to 10th	Day	8% 5% 2 6% 6% 4 6% 5% 7% 9% 43%	2327
	11th to 20th	Day	10% 5% 3 7% 6% 5% 6% 6% 7% 8% 38%	2303
	21st to end	Day	10% 5% 2 4 6% 3 5% 6% 7% 9% 42%	2524
September	1st to 10th	Day	12% 5% 2 6% 6% 5% 5% 5% 6% 10% 38%	2047
	11th to 20th	Day	9% 5% 2 4 5% 3 5% 6% 6% 10% 45%	1980
	21st to end	Day	14% 6% 2 6% 6% 4 5% 5% 6% 8% 40%	1980
October	1st to 10th	Day	13% 5% 3 5% 6% 3 5% 5% 6% 7% 42%	1946
	11th to 20th	Day	19% 8% 3 6% 5% 5% 5% 5% 5% 7% 32%	1800
	21st to end	Day	20% 8% 3 8% 6% 3 4 4 5% 7% 31%	1820
November	1st to 10th	Day	11% 8% 3 7% 7% 2 5% 5% 5% 7% 39%	1672
	11th to 20th	Day	15% 12% 4 7% 7% 5% 4 4 5% 6% 31%	1710
	21st to end	Day	19% 11% 4 8% 6% 3 5% 5% 5% 6% 29%	1706
December	1st to 10th	Day	14% 13% 7% 12% 7% 3 6% 5% 4 6% 22%	1520
	11th to 20th	Day	15% 14% 6% 10% 6% 3 4 6% 6% 7% 22%	1868
	21st to end	Day	26% 19% 7% 11% 7% 3 4 3 3 5% 11%	2783
January	1st to 10th	Night	17% 13% 5% 10% 7% 4 4 4 5% 6% 25%	2648
	11th to 20th	Night	18% 15% 6% 10% 8% 4 4 4 4 6% 21%	2589
	21st to end	Night	14% 13% 9% 12% 8% 3 4 5% 5% 5% 23%	2614
February	1st to 10th	Night	17% 15% 6% 9% 8% 4 5% 4 4 5% 23%	2340
	11th to 20th	Night	15% 12% 6% 9% 7% 4 5% 4 5% 6% 28%	2125
	21st to end	Night	19% 10% 5% 9% 6% 3 4 4 4 6% 30%	1776
March	1st to 10th	Night	17% 9% 4 7% 6% 3 4 5% 5% 5% 34%	2134
	11th to 20th	Night	14% 9% 3 6% 5% 3 5% 5% 6% 8% 38%	1980
	21st to end	Night	20% 9% 4 6% 6% 3 4 4 4 6% 34%	2178
April	1st to 10th	Night	15% 9% 3 5% 6% 4 5% 5% 5% 6% 38%	1865
	11th to 20th	Night	18% 8% 2 5% 5% 3 5% 5% 6% 7% 37%	1647
	21st to end	Night	17% 8% 3 5% 5% 2 5% 4 5% 6% 39%	1620
May	1st to 10th	Night	17% 8% 3 7% 6% 4 5% 4 5% 8% 33%	1620
	11th to 20th	Night	19% 6% 2 6% 5% 3 4 5% 5% 7% 39%	1592
	21st to end	Night	11% 5% 3 5% 4 3 5% 5% 5% 9% 46%	1740
June	1st to 10th	Night	11% 4 2 4 5% 4 5% 4 5% 10% 46%	1440
	11th to 20th	Night	10% 5% 2 7% 5% 3 5% 5% 5% 9% 44%	1440
	21st to end	Night	9% 6% 2 5% 4 3 3 4 7% 8% 48%	1440
July	1st to 10th	Night	8% 4 2 4 4 3 4 5% 5% 8% 54%	1440
	11th to 20th	Night	8% 5% 2 4 4 2 3 4 6% 9% 53%	1440
	21st to end	Night	4 4 2 5% 4 3 4 6% 6% 10% 52%	1714
August	1st to 10th	Night	7% 4 2 4 6% 3 4 5% 6% 8% 51%	1611
	11th to 20th	Night	8% 5% 2 4 5% 3 5% 5% 6% 9% 48%	1598
	21st to end	Night	9% 4 1 4 5% 3 5% 4 5% 7% 52%	1829
September	1st to 10th	Night	11% 5% 2 4 5% 3 3 5% 6% 7% 49%	1894
	11th to 20th	Night	10% 5% 2 4 3 3 5% 4 5% 8% 52%	1980
	21st to end	Night	13% 6% 2 5% 5% 3 5% 5% 5% 7% 45%	1980
October	1st to 10th	Night	13% 6% 3 6% 5% 2 4 4 5% 6% 45%	2012
	11th to 20th	Night	18% 9% 4 6% 6% 4 4 5% 4 5% 36%	2160
	21st to end	Night	21% 8% 3 6% 5% 3 5% 5% 4 6% 34%	2533
November	1st to 10th	Night	13% 7% 3 5% 5% 3 4 4 5% 6% 44%	2406
	11th to 20th	Night	16% 10% 4 6% 7% 3 5% 4 6% 6% 32%	2470
	21st to end	Night	19% 12% 5% 7% 5% 3 5% 4 4 6% 30%	2470
December	1st to 10th	Night	17% 14% 6% 9% 7% 4 5% 4 4 7% 22%	2658
	11th to 20th	Night	15% 13% 6% 9% 7% 4 5% 4 5% 7% 25%	3316
	21st to end	Night	28% 20% 6% 9% 6% 3 4 4 4 5% 12%	4884

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% 8% 10% 14% 6%	1518
	11th to 20th	Day	68% 7% 12% 8% 4	1477
	21st to end	Day	68% 5% 8% 12% 6% 1	1739
February	1st to 10th	Day	64% 7% 8% 13% 7%	1620
	11th to 20th	Day	60% 6% 9% 16% 7% 2	1729
	21st to end	Day	59% 6% 12% 15% 7% 2	1480
March	1st to 10th	Day	54% 7% 12% 18% 8% 1	1826
	11th to 20th	Day	50% 5% 10% 19% 12% 3	1980
	21st to end	Day	57% 5% 10% 20% 7% 1	2178
April	1st to 10th	Day	53% 6% 11% 18% 12%	2110
	11th to 20th	Day	48% 7% 11% 19% 13% 2	2262
	21st to end	Day	50% 5% 13% 16% 14% 1	2340
May	1st to 10th	Day	50% 6% 11% 19% 11% 2	2340
	11th to 20th	Day	50% 6% 9% 18% 15% 2	2305
	21st to end	Day	40% 8% 13% 24% 14% 2	2531
June	1st to 10th	Day	38% 7% 12% 26% 14% 3	2520
	11th to 20th	Day	38% 9% 12% 21% 18% 2	2520
	21st to end	Day	37% 9% 14% 22% 16% 3	2520
July	1st to 10th	Day	33% 8% 11% 25% 19% 4	2451
	11th to 20th	Day	34% 8% 13% 25% 19% 2	2445
	21st to end	Day	35% 9% 12% 23% 19% 2	2456
August	1st to 10th	Day	35% 7% 14% 28% 14% 1	2314
	11th to 20th	Day	39% 10% 12% 22% 14% 2	2303
	21st to end	Day	37% 7% 11% 24% 18% 2	2524
September	1st to 10th	Day	40% 8% 15% 20% 15% 2	2047
	11th to 20th	Day	34% 8% 10% 33% 15% 1	1980
	21st to end	Day	40% 7% 12% 25% 15%	1980
October	1st to 10th	Day	40% 7% 11% 26% 15% 2	1946
	11th to 20th	Day	51% 5% 12% 20% 12%	1800
	21st to end	Day	53% 6% 12% 18% 10% 1	1820
November	1st to 10th	Day	44% 6% 12% 24% 14% 1	1672
	11th to 20th	Day	54% 5% 12% 19% 9% 2	1710
	21st to end	Day	56% 6% 12% 18% 7% 1	1706
December	1st to 10th	Day	63% 7% 12% 14% 5%	1520
	11th to 20th	Day	59% 8% 13% 15% 4	1865
	21st to end	Day	78% 5% 8% 6% 3	2783
January	1st to 10th	Night	60% 5% 10% 17% 7% 1	2648
	11th to 20th	Night	65% 6% 12% 12% 6%	2589
	21st to end	Night	63% 6% 12% 13% 6% 1	2614
February	1st to 10th	Night	64% 6% 10% 13% 6% 1	2340
	11th to 20th	Night	57% 6% 9% 17% 9% 1	2126
	21st to end	Night	57% 4 11% 18% 9% 2	1776
March	1st to 10th	Night	50% 6% 10% 22% 11% 1	2134
	11th to 20th	Night	44% 7% 11% 20% 16% 2	1980
	21st to end	Night	52% 6% 9% 22% 10% 2	2178
April	1st to 10th	Night	46% 7% 10% 21% 14% 2	1865
	11th to 20th	Night	44% 7% 10% 19% 18% 1	1647
	21st to end	Night	46% 6% 12% 17% 16% 3	1620
May	1st to 10th	Night	50% 6% 11% 19% 12% 2	1620
	11th to 20th	Night	45% 6% 9% 19% 18% 2	1593
	21st to end	Night	35% 7% 15% 25% 15% 3	1741
June	1st to 10th	Night	33% 7% 10% 29% 18% 3	1440
	11th to 20th	Night	36% 8% 13% 25% 16% 2	1440
	21st to end	Night	33% 7% 12% 27% 20% 1	1440
July	1st to 10th	Night	29% 6% 13% 28% 20% 3	1400
	11th to 20th	Night	29% 7% 12% 29% 21% 3	1392
	21st to end	Night	26% 9% 16% 24% 21% 4	1619
August	1st to 10th	Night	30% 8% 13% 31% 16% 2	1600
	11th to 20th	Night	32% 8% 14% 30% 13% 3	1598
	21st to end	Night	31% 6% 12% 26% 22% 2	1829
September	1st to 10th	Night	34% 7% 11% 28% 18% 2	1894
	11th to 20th	Night	31% 6% 9% 30% 22% 1	1980
	21st to end	Night	38% 6% 11% 28% 15% 1	1980
October	1st to 10th	Night	39% 6% 9% 25% 20% 1	2012
	11th to 20th	Night	49% 6% 11% 19% 14% 1	2160
	21st to end	Night	51% 5% 12% 22% 10% 1	2533
November	1st to 10th	Night	40% 7% 11% 25% 17% 1	2406
	11th to 20th	Night	50% 6% 14% 20% 9% 1	2470
	21st to end	Night	55% 6% 12% 19% 7%	2470
December	1st to 10th	Night	63% 5% 11% 14% 6%	2658
	11th to 20th	Night	59% 6% 13% 14% 7% 1	3318
	21st to end	Night	75% 6% 10% 7% 3	4884

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% 35% 19% 6% 1	270
	11th to 20th	Day	23% 14% 43% 17% 2 1	254
	21st to end	Day	27% 16% 33% 18% 5% 1	264
February	1st to 10th	Day	25% 18% 41% 14% 2	240
	11th to 20th	Day	26% 18% 38% 16% 2	240
	21st to end	Day	27% 24% 36% 12% 1	198
March	1st to 10th	Day	25% 21% 35% 15% 3	266
	11th to 20th	Day	21% 12% 34% 24% 8% 1	420
	21st to end	Day	27% 14% 30% 21% 6% 1	462
April	1st to 10th	Day	21% 14% 32% 22% 10% 1	420
	11th to 20th	Day	22% 19% 31% 15% 8% 5%	412
	21st to end	Day	25% 11% 31% 19% 11% 2	420
May	1st to 10th	Day	23% 12% 30% 20% 11% 4	423
	11th to 20th	Day	21% 11% 28% 22% 12% 7%	447
	21st to end	Day	14% 7% 21% 26% 21% 11%	485
June	1st to 10th	Day	11% 6% 29% 31% 17% 5%	630
	11th to 20th	Day	12% 8% 28% 30% 16% 5%	630
	21st to end	Day	10% 7% 28% 32% 16% 6%	630
July	1st to 10th	Day	8% 3 20% 28% 27% 12%	621
	11th to 20th	Day	12% 6% 22% 30% 21% 9%	618
	21st to end	Day	9% 4 23% 29% 23% 12%	542
August	1st to 10th	Day	10% 1 18% 28% 26% 17%	446
	11th to 20th	Day	13% 4 17% 29% 25% 12%	439
	21st to end	Day	11% 3 18% 29% 27% 12%	495
September	1st to 10th	Day	13% 5% 22% 24% 23% 13%	450
	11th to 20th	Day	10% 4 17% 25% 29% 15%	450
	21st to end	Day	16% 10% 21% 22% 21% 10%	450
October	1st to 10th	Day	15% 8% 28% 23% 17% 9%	450
	11th to 20th	Day	19% 15% 29% 27% 10% 1	450
	21st to end	Day	27% 13% 36% 18% 5%	335
November	1st to 10th	Day	13% 19% 32% 25% 10% 2	270
	11th to 20th	Day	23% 19% 31% 19% 7% 2	270
	21st to end	Day	23% 15% 41% 15% 5%	268
December	1st to 10th	Day	21% 20% 35% 16% 7% 2	180
	11th to 20th	Day	21% 19% 43% 14% 3	216
	21st to end	Day	29% 19% 34% 17% 2	297
January	1st to 10th	Night	23% 14% 36% 21% 6% 1	950
	11th to 20th	Night	26% 13% 35% 21% 5% 1	925
	21st to end	Night	18% 12% 36% 27% 7% 1	832
February	1st to 10th	Night	25% 12% 36% 25% 3	720
	11th to 20th	Night	23% 14% 34% 26% 4	703
	21st to end	Night	25% 11% 39% 22% 3	592
March	1st to 10th	Night	22% 10% 35% 26% 7%	720
	11th to 20th	Night	18% 15% 35% 25% 6% 1	720
	21st to end	Night	23% 10% 31% 28% 8% 1	792
April	1st to 10th	Night	19% 14% 33% 29% 4 1	605
	11th to 20th	Night	22% 9% 35% 25% 8% 1	533
	21st to end	Night	23% 6% 32% 30% 9%	540
May	1st to 10th	Night	19% 8% 31% 31% 10% 1	540
	11th to 20th	Night	23% 2 28% 33% 13% 1	531
	21st to end	Night	13% 2 25% 34% 24% 2	584
June	1st to 10th	Night	13% 2 23% 46% 16% 1	540
	11th to 20th	Night	11% 4 22% 41% 20%	540
	21st to end	Night	11% 3 21% 41% 22% 2	540
July	1st to 10th	Night	8% 2 15% 40% 30% 4	525
	11th to 20th	Night	9% 2 17% 41% 26% 4	522
	21st to end	Night	5% 1 18% 47% 27% 2	591
August	1st to 10th	Night	8% 2 16% 42% 28% 3	537
	11th to 20th	Night	9% 2 17% 37% 32% 4	534
	21st to end	Night	10% 2 16% 37% 31% 4	594
September	1st to 10th	Night	12% 4 18% 33% 28% 5%	634
	11th to 20th	Night	10% 4 17% 32% 31% 5%	720
	21st to end	Night	15% 5% 20% 31% 23% 4	720
October	1st to 10th	Night	14% 6% 25% 33% 18% 3	720
	11th to 20th	Night	23% 13% 28% 27% 9% 1	720
	21st to end	Night	25% 10% 33% 26% 6%	792
November	1st to 10th	Night	18% 11% 30% 27% 13% 1	740
	11th to 20th	Night	22% 13% 36% 21% 7% 1	760
	21st to end	Night	25% 12% 33% 23% 6%	760
December	1st to 10th	Night	24% 14% 35% 22% 6%	760
	11th to 20th	Night	23% 13% 38% 21% 5%	874
	21st to end	Night	32% 12% 33% 21% 2	1045

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