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 2024-12-06.

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% 3 8% 5% 5% 5% 4 6% 8% 34%	101185
	Night	15% 10% 4 8% 5% 4 4 3 5% 7% 35%	98526
Transparency	Day	47% 7% 12% 21% 13% 1	104812
	Night	47% 6% 11% 22% 13% 1	103039
Seeing	Day	17% 11% 28% 24% 14% 6%	15609
	Night	19% 9% 29% 29% 13% 1	25612

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% 18% 5% 10% 6% 5% 5% 3 5% 5% 21%	2527
	11th to 20th	Day	16% 17% 6% 12% 7% 7% 7% 4 6% 6% 12%	2530
	21st to end	Day	27% 20% 7% 12% 6% 3 3 2 3 4 13%	3042
February	1st to 10th	Day	14% 14% 5% 11% 8% 6% 5% 4 5% 6% 21%	1710
	11th to 20th	Day	14% 12% 4 12% 8% 5% 4 4 4 6% 25%	1817
	21st to end	Day	15% 16% 4 11% 5% 4 4 4 5% 6% 26%	1570
March	1st to 10th	Day	17% 15% 4 9% 6% 4 4 3 4 6% 25%	3174
	11th to 20th	Day	13% 10% 4 9% 6% 5% 4 3 5% 7% 33%	3480
	21st to end	Day	19% 11% 4 9% 5% 5% 5% 4 5% 5% 27%	3815
April	1st to 10th	Day	15% 13% 3 7% 6% 4 4 4 6% 6% 31%	2228
	11th to 20th	Day	16% 9% 3 7% 5% 5% 4 4 6% 7% 35%	2390
	21st to end	Day	18% 10% 3 7% 5% 4 4 3 5% 7% 35%	2470
May	1st to 10th	Day	18% 8% 3 6% 5% 4 4 4 5% 7% 35%	4082
	11th to 20th	Day	16% 8% 3 7% 5% 5% 5% 4 6% 8% 34%	4081
	21st to end	Day	10% 6% 2 6% 5% 4 4 3 5% 8% 45%	4476
June	1st to 10th	Day	9% 7% 2 6% 6% 4 5% 5% 6% 9% 41%	2660
	11th to 20th	Day	10% 6% 2 6% 5% 4 5% 4 7% 9% 40%	2660
	21st to end	Day	9% 6% 2 7% 5% 5% 6% 5% 8% 10% 38%	2660
July	1st to 10th	Day	10% 6% 2 7% 5% 5% 5% 4 7% 9% 39%	4440
	11th to 20th	Day	6% 5% 2 5% 5% 4 5% 4 7% 11% 47%	4440
	21st to end	Day	6% 6% 2 7% 6% 6% 6% 5% 7% 9% 42%	4562
August	1st to 10th	Day	11% 5% 2 6% 4 5% 5% 4 6% 9% 44%	4104
	11th to 20th	Day	9% 6% 2 7% 5% 5% 6% 5% 7% 10% 38%	4044
	21st to end	Day	8% 5% 2 6% 5% 5% 6% 5% 7% 10% 40%	4440
September	1st to 10th	Day	11% 6% 2 6% 5% 6% 5% 3 6% 10% 40%	2160
	11th to 20th	Day	8% 6% 2 5% 4 4 4 5% 6% 9% 47%	2090
	21st to end	Day	14% 6% 2 8% 5% 5% 4 4 7% 7% 38%	2090
October	1st to 10th	Day	12% 5% 3 6% 5% 4 4 4 6% 7% 43%	2054
	11th to 20th	Day	18% 9% 3 7% 4 6% 5% 4 5% 7% 34%	1900
	21st to end	Day	19% 9% 3 9% 5% 4 4 4 5% 7% 31%	1921
November	1st to 10th	Day	11% 9% 3 8% 6% 4 5% 4 5% 6% 39%	1762
	11th to 20th	Day	15% 13% 4 9% 5% 5% 4 3 5% 6% 29%	1800
	21st to end	Day	18% 12% 4 10% 6% 5% 5% 3 5% 6% 28%	1795
December	1st to 10th	Day	14% 15% 6% 14% 6% 4 6% 4 5% 6% 21%	1560
	11th to 20th	Day	15% 15% 6% 10% 6% 5% 4 4 6% 7% 22%	1868
	21st to end	Day	25% 21% 6% 12% 6% 4 4 2 3 5% 11%	2783
January	1st to 10th	Night	16% 16% 7% 11% 6% 4 4 3 4 6% 24%	4428
	11th to 20th	Night	15% 17% 7% 14% 6% 5% 6% 5% 5% 6% 15%	4440
	21st to end	Night	24% 17% 8% 12% 5% 3 3 3 3 4 17%	4614
February	1st to 10th	Night	16% 16% 5% 10% 7% 5% 5% 3 5% 5% 24%	2470
	11th to 20th	Night	14% 13% 5% 10% 6% 5% 4 3 5% 6% 28%	2247
	21st to end	Night	19% 11% 4 11% 5% 5% 4 3 4 6% 30%	1884
March	1st to 10th	Night	17% 12% 3 8% 4 4 4 3 4 6% 35%	3679
	11th to 20th	Night	10% 7% 3 6% 4 4 5% 4 7% 8% 44%	3480
	21st to end	Night	17% 9% 3 8% 5% 4 5% 3 5% 6% 35%	3817
April	1st to 10th	Night	15% 10% 2 6% 5% 4 4 4 6% 6% 37%	1968
	11th to 20th	Night	18% 8% 2 6% 4 4 5% 4 6% 7% 37%	1739
	21st to end	Night	17% 9% 3 6% 5% 4 5% 3 5% 6% 39%	1710
May	1st to 10th	Night	17% 9% 3 7% 4 5% 4 3 6% 7% 34%	2813
	11th to 20th	Night	16% 7% 2 7% 5% 4 4 3 5% 7% 40%	2812
	21st to end	Night	10% 5% 2 6% 3 4 4 3 5% 9% 48%	3062
June	1st to 10th	Night	11% 4 1 4 5% 5% 4 3 5% 10% 46%	1520
	11th to 20th	Night	10% 5% 2 8% 4 4 5% 4 6% 9% 43%	1520
	21st to end	Night	9% 6% 2 6% 4 4 4 3 7% 8% 48%	1520
July	1st to 10th	Night	9% 4 1 5% 4 4 5% 4 6% 10% 48%	2530
	11th to 20th	Night	8% 4 2 4 3 4 3 4 6% 8% 55%	2530
	21st to end	Night	5% 4 2 6% 5% 5% 5% 5% 7% 10% 47%	2991
August	1st to 10th	Night	11% 6% 3 5% 5% 4 4 4 5% 7% 47%	2825
	11th to 20th	Night	8% 6% 2 6% 4 4 5% 4 6% 8% 49%	2785
	21st to end	Night	8% 6% 2 7% 5% 4 5% 3 6% 7% 48%	3213
September	1st to 10th	Night	10% 5% 2 5% 5% 3 4 4 6% 6% 50%	2000
	11th to 20th	Night	9% 6% 2 4 3 4 4 3 5% 8% 53%	2090
	21st to end	Night	14% 7% 2 6% 4 5% 5% 3 5% 6% 43%	2090
October	1st to 10th	Night	12% 6% 3 7% 4 3 4 3 5% 6% 47%	2124
	11th to 20th	Night	17% 9% 3 7% 4 4 4 4 4 5% 37%	2280
	21st to end	Night	20% 8% 3 7% 4 4 4 4 4 6% 35%	2674
November	1st to 10th	Night	13% 8% 3 5% 5% 4 4 4 5% 6% 44%	2536
	11th to 20th	Night	15% 11% 4 8% 6% 5% 4 3 6% 6% 32%	2600
	21st to end	Night	19% 14% 5% 8% 5% 4 5% 3 5% 6% 28%	2600
December	1st to 10th	Night	17% 14% 6% 11% 6% 5% 4 4 4 7% 22%	2735
	11th to 20th	Night	15% 14% 5% 10% 6% 5% 5% 3 5% 7% 25%	3316
	21st to end	Night	27% 21% 5% 11% 5% 4 4 3 5% 4 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	63% 8% 12% 13% 6%	2528
	11th to 20th	Day	70% 6% 12% 8% 5%	2530
	21st to end	Day	76% 5% 9% 7% 3 1	3042
February	1st to 10th	Day	62% 7% 9% 14% 8%	1710
	11th to 20th	Day	59% 6% 10% 16% 8% 1	1827
	21st to end	Day	58% 6% 12% 16% 7% 2	1570
March	1st to 10th	Day	59% 6% 11% 17% 6% 1	3174
	11th to 20th	Day	49% 5% 15% 20% 8% 2	3480
	21st to end	Day	58% 6% 11% 15% 9% 1	3815
April	1st to 10th	Day	52% 6% 11% 18% 12%	2228
	11th to 20th	Day	48% 6% 11% 20% 13% 2	2390
	21st to end	Day	49% 5% 13% 17% 14% 1	2470
May	1st to 10th	Day	47% 6% 12% 23% 12% 1	4120
	11th to 20th	Day	45% 8% 12% 23% 10% 1	4085
	21st to end	Day	36% 7% 11% 19% 23% 5%	4489
June	1st to 10th	Day	38% 7% 12% 26% 14% 3	2660
	11th to 20th	Day	38% 9% 12% 21% 18% 2	2660
	21st to end	Day	37% 9% 14% 21% 16% 2	2660
July	1st to 10th	Day	40% 6% 13% 21% 17% 2	4371
	11th to 20th	Day	29% 7% 12% 32% 18% 1	4365
	21st to end	Day	36% 9% 13% 19% 22% 2	4417
August	1st to 10th	Day	36% 6% 13% 27% 17% 1	4091
	11th to 20th	Day	38% 8% 11% 30% 11% 1	4083
	21st to end	Day	37% 7% 13% 29% 13% 2	4439
September	1st to 10th	Day	38% 8% 15% 21% 16% 2	2160
	11th to 20th	Day	32% 8% 10% 32% 17% 1	2090
	21st to end	Day	42% 7% 12% 24% 15%	2090
October	1st to 10th	Day	30% 5% 11% 31% 22% 1	3418
	11th to 20th	Day	40% 4 11% 25% 20%	3140
	21st to end	Day	44% 5% 13% 24% 13% 1	3145
November	1st to 10th	Day	44% 6% 12% 25% 13% 1	1762
	11th to 20th	Day	55% 5% 11% 19% 9% 2	1800
	21st to end	Day	57% 6% 12% 17% 7% 1	1795
December	1st to 10th	Day	62% 7% 12% 14% 5%	1560
	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	61% 6% 11% 16% 6% 1	4428
	11th to 20th	Night	67% 6% 11% 11% 4	4440
	21st to end	Night	71% 5% 9% 10% 3 1	4614
February	1st to 10th	Night	62% 6% 11% 14% 7% 1	2470
	11th to 20th	Night	56% 6% 9% 19% 9% 1	2248
	21st to end	Night	57% 4 11% 18% 9% 2	1884
March	1st to 10th	Night	50% 5% 12% 25% 8%	3679
	11th to 20th	Night	36% 7% 15% 21% 21% 1	3480
	21st to end	Night	49% 6% 12% 22% 10% 1	3817
April	1st to 10th	Night	47% 7% 10% 21% 14% 2	1968
	11th to 20th	Night	44% 8% 10% 19% 18% 1	1739
	21st to end	Night	46% 6% 12% 17% 16% 3	1710
May	1st to 10th	Night	47% 7% 10% 21% 14% 2	2840
	11th to 20th	Night	43% 7% 9% 27% 13% 1	2813
	21st to end	Night	32% 7% 13% 22% 22% 4	3070
June	1st to 10th	Night	33% 7% 11% 29% 18% 3	1520
	11th to 20th	Night	37% 8% 12% 24% 17% 2	1520
	21st to end	Night	33% 7% 13% 27% 20% 1	1520
July	1st to 10th	Night	31% 6% 12% 30% 15% 6%	2490
	11th to 20th	Night	26% 6% 13% 30% 22% 2	2482
	21st to end	Night	30% 8% 18% 18% 23% 2	2896
August	1st to 10th	Night	35% 7% 12% 28% 17% 1	2814
	11th to 20th	Night	33% 6% 11% 36% 12% 2	2812
	21st to end	Night	35% 6% 11% 31% 16% 1	3213
September	1st to 10th	Night	33% 7% 11% 28% 19% 2	2000
	11th to 20th	Night	30% 6% 9% 30% 23% 2	2090
	21st to end	Night	40% 6% 11% 27% 15% 1	2090
October	1st to 10th	Night	26% 4 8% 33% 27% 1	3540
	11th to 20th	Night	41% 4 10% 24% 20%	3779
	21st to end	Night	38% 4 11% 32% 15% 1	4400
November	1st to 10th	Night	40% 7% 11% 25% 16% 1	2536
	11th to 20th	Night	50% 6% 13% 20% 10% 1	2600
	21st to end	Night	56% 6% 12% 18% 7%	2600
December	1st to 10th	Night	63% 5% 11% 15% 6%	2735
	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% 15% 42% 16% 2 1	270
	21st to end	Day	29% 15% 31% 19% 5% 1	297
February	1st to 10th	Day	23% 18% 39% 14% 3 1	270
	11th to 20th	Day	24% 17% 37% 19% 3	270
	21st to end	Day	25% 24% 36% 13% 1	225
March	1st to 10th	Day	24% 21% 36% 15% 3	298
	11th to 20th	Day	22% 12% 34% 23% 7% 1	460
	21st to end	Day	26% 15% 32% 20% 6% 1	506
April	1st to 10th	Day	20% 15% 34% 21% 9% 1	460
	11th to 20th	Day	22% 19% 31% 15% 7% 4	452
	21st to end	Day	24% 13% 30% 19% 12% 2	460
May	1st to 10th	Day	22% 12% 30% 21% 11% 4	463
	11th to 20th	Day	20% 10% 27% 22% 14% 7%	487
	21st to end	Day	13% 7% 22% 26% 20% 10%	529
June	1st to 10th	Day	11% 8% 28% 31% 18% 5%	680
	11th to 20th	Day	12% 9% 29% 31% 15% 5%	680
	21st to end	Day	11% 9% 28% 31% 15% 6%	680
July	1st to 10th	Day	10% 4 21% 28% 25% 11%	671
	11th to 20th	Day	11% 6% 22% 30% 21% 9%	668
	21st to end	Day	8% 4 23% 31% 23% 12%	589
August	1st to 10th	Day	10% 2 19% 29% 24% 16%	486
	11th to 20th	Day	13% 5% 19% 29% 24% 11%	479
	21st to end	Day	10% 3 19% 30% 26% 12%	539
September	1st to 10th	Day	12% 6% 22% 24% 23% 12%	490
	11th to 20th	Day	9% 4 17% 26% 30% 15%	490
	21st to end	Day	18% 9% 23% 22% 20% 9%	490
October	1st to 10th	Day	15% 9% 28% 23% 17% 8%	490
	11th to 20th	Day	18% 15% 29% 26% 10% 1	490
	21st to end	Day	27% 13% 36% 18% 6%	370
November	1st to 10th	Day	14% 19% 32% 24% 9% 2	300
	11th to 20th	Day	24% 19% 32% 18% 6% 2	300
	21st to end	Day	24% 15% 41% 15% 4	297
December	1st to 10th	Day	22% 19% 34% 17% 6% 2	190
	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	25% 13% 35% 21% 5% 1	950
	21st to end	Night	19% 12% 35% 27% 6% 1	879
February	1st to 10th	Night	23% 12% 35% 25% 4	760
	11th to 20th	Night	22% 14% 34% 27% 3	743
	21st to end	Night	24% 12% 39% 22% 3	628
March	1st to 10th	Night	22% 11% 35% 26% 7%	760
	11th to 20th	Night	18% 16% 36% 25% 6% 1	760
	21st to end	Night	23% 10% 31% 28% 7%	836
April	1st to 10th	Night	20% 14% 33% 28% 4 1	638
	11th to 20th	Night	22% 9% 36% 24% 7% 1	563
	21st to end	Night	22% 7% 32% 30% 8%	570
May	1st to 10th	Night	19% 8% 30% 32% 10% 1	570
	11th to 20th	Night	23% 2 28% 33% 14% 1	561
	21st to end	Night	13% 3 25% 34% 23% 2	617
June	1st to 10th	Night	13% 2 24% 44% 16% 1	570
	11th to 20th	Night	11% 5% 23% 41% 20%	570
	21st to end	Night	11% 4 21% 40% 22% 2	570
July	1st to 10th	Night	8% 1 17% 40% 29% 4	555
	11th to 20th	Night	9% 3 18% 39% 26% 4	552
	21st to end	Night	6% 1 17% 47% 27% 2	624
August	1st to 10th	Night	8% 2 17% 42% 28% 3	567
	11th to 20th	Night	9% 3 18% 36% 31% 4	564
	21st to end	Night	10% 2 16% 37% 31% 4	627
September	1st to 10th	Night	12% 5% 19% 32% 28% 5%	670
	11th to 20th	Night	10% 4 17% 32% 32% 5%	760
	21st to end	Night	17% 6% 20% 31% 22% 4	760
October	1st to 10th	Night	14% 7% 25% 33% 18% 3	760
	11th to 20th	Night	22% 13% 28% 27% 10% 1	760
	21st to end	Night	24% 11% 33% 26% 6%	836
November	1st to 10th	Night	18% 12% 30% 27% 12% 1	780
	11th to 20th	Night	22% 14% 36% 21% 7% 1	800
	21st to end	Night	26% 12% 34% 23% 6%	800
December	1st to 10th	Night	24% 14% 35% 22% 5%	783
	11th to 20th	Night	23% 13% 38% 21% 5%	874
	21st to end	Night	32% 12% 33% 21% 2	1045

Page generated at 2024-12-03 06:45:12. Version ac07

Copyright © 2024 Attilla Danko.