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 2024-10-25.

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% 3 8% 5% 5% 5% 4 6% 8% 33%	100874
	Night	15% 10% 4 8% 5% 4 4 4 5% 6% 35%	97911
Transparency	Day	47% 7% 13% 20% 11% 1	100675
	Night	48% 7% 12% 21% 12% 1	97782
Seeing	Day	18% 13% 29% 23% 13% 5%	15371
	Night	19% 10% 29% 29% 12% 1	25150

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% 17% 7% 11% 6% 6% 4 3 5% 5% 21%	2527
	11th to 20th	Day	15% 18% 7% 13% 6% 9% 6% 4 5% 5% 12%	2533
	21st to end	Day	27% 20% 7% 12% 5% 4 3 3 4 3 12%	3124
February	1st to 10th	Day	15% 15% 4 12% 7% 6% 5% 5% 5% 5% 21%	1710
	11th to 20th	Day	13% 14% 4 12% 8% 5% 6% 3 5% 7% 24%	1788
	21st to end	Day	16% 14% 5% 10% 5% 4 3 4 5% 7% 26%	1570
March	1st to 10th	Day	18% 15% 5% 9% 6% 4 4 3 4 6% 26%	3210
	11th to 20th	Day	13% 13% 5% 9% 5% 5% 5% 3 5% 7% 31%	3480
	21st to end	Day	19% 12% 4 10% 5% 5% 4 3 5% 6% 26%	3815
April	1st to 10th	Day	15% 14% 4 6% 5% 5% 5% 5% 5% 7% 30%	2194
	11th to 20th	Day	18% 9% 3 8% 5% 5% 4 4 5% 7% 34%	2413
	21st to end	Day	19% 9% 3 7% 5% 4 4 4 5% 6% 34%	2470
May	1st to 10th	Day	18% 8% 3 8% 5% 4 5% 4 5% 7% 34%	4082
	11th to 20th	Day	16% 8% 3 7% 5% 5% 5% 4 7% 8% 33%	4081
	21st to end	Day	10% 6% 3 6% 5% 5% 4 4 6% 8% 45%	4476
June	1st to 10th	Day	10% 6% 2 8% 6% 5% 5% 4 6% 8% 40%	2652
	11th to 20th	Day	11% 6% 2 7% 5% 5% 5% 4 7% 9% 40%	2848
	21st to end	Day	9% 7% 3 7% 5% 5% 6% 5% 7% 10% 36%	2660
July	1st to 10th	Day	10% 6% 2 7% 5% 5% 6% 4 7% 9% 38%	4440
	11th to 20th	Day	7% 4 2 6% 4 5% 5% 5% 8% 11% 43%	4440
	21st to end	Day	6% 5% 2 6% 6% 6% 6% 5% 8% 10% 40%	4498
August	1st to 10th	Day	11% 6% 2 6% 5% 5% 5% 5% 6% 9% 40%	4104
	11th to 20th	Day	9% 5% 3 7% 6% 5% 6% 6% 8% 10% 36%	4044
	21st to end	Day	8% 6% 2 7% 5% 6% 6% 5% 8% 9% 38%	4502
September	1st to 10th	Day	10% 7% 2 7% 5% 5% 5% 4 6% 8% 39%	2138
	11th to 20th	Day	8% 5% 2 5% 4 5% 5% 5% 6% 8% 47%	2090
	21st to end	Day	15% 7% 2 7% 4 4 5% 3 7% 8% 37%	2090
October	1st to 10th	Day	12% 7% 2 7% 4 4 4 4 6% 7% 42%	2032
	11th to 20th	Day	17% 9% 3 7% 6% 5% 5% 4 5% 7% 33%	1900
	21st to end	Day	19% 10% 3 8% 5% 5% 4 3 5% 7% 31%	1895
November	1st to 10th	Day	11% 8% 3 8% 6% 5% 4 4 5% 8% 39%	1672
	11th to 20th	Day	14% 12% 3 8% 5% 5% 5% 3 6% 7% 32%	1710
	21st to end	Day	19% 11% 4 9% 5% 5% 4 3 5% 7% 27%	1611
December	1st to 10th	Day	15% 15% 6% 12% 7% 6% 5% 4 4 5% 21%	1520
	11th to 20th	Day	16% 16% 5% 11% 5% 4 5% 3 6% 6% 22%	1804
	21st to end	Day	27% 20% 7% 13% 5% 4 3 3 4 5% 9%	2751
January	1st to 10th	Night	16% 17% 6% 10% 6% 4 4 3 4 5% 25%	4428
	11th to 20th	Night	15% 19% 7% 14% 5% 4 6% 5% 5% 4 15%	4434
	21st to end	Night	25% 18% 7% 11% 5% 4 3 3 3 4 17%	4532
February	1st to 10th	Night	17% 15% 4 11% 7% 5% 5% 4 5% 4 23%	2470
	11th to 20th	Night	14% 13% 5% 10% 7% 5% 5% 4 5% 5% 27%	2274
	21st to end	Night	19% 12% 4 9% 5% 4 5% 3 4 6% 29%	1884
March	1st to 10th	Night	16% 14% 5% 8% 4 4 4 2 5% 6% 34%	3642
	11th to 20th	Night	10% 8% 2 7% 5% 4 5% 4 7% 8% 41%	3480
	21st to end	Night	17% 8% 3 8% 5% 5% 5% 3 5% 6% 36%	3817
April	1st to 10th	Night	16% 11% 3 7% 5% 3 4 4 6% 6% 36%	2001
	11th to 20th	Night	18% 8% 2 7% 4 3 5% 4 7% 7% 36%	1716
	21st to end	Night	17% 8% 3 6% 4 4 5% 4 5% 6% 38%	1710
May	1st to 10th	Night	18% 9% 2 7% 5% 4 4 3 5% 7% 36%	2813
	11th to 20th	Night	15% 7% 2 8% 4 5% 5% 3 5% 6% 40%	2812
	21st to end	Night	10% 6% 2 6% 4 4 4 3 5% 8% 47%	3079
June	1st to 10th	Night	12% 6% 2 4 4 3 5% 4 6% 9% 46%	1509
	11th to 20th	Night	8% 5% 2 8% 5% 4 5% 4 6% 9% 44%	1330
	21st to end	Night	9% 6% 1 6% 4 4 5% 4 6% 7% 48%	1508
July	1st to 10th	Night	8% 4 1 5% 3 3 5% 4 7% 10% 48%	2530
	11th to 20th	Night	9% 3 2 5% 4 4 3 5% 6% 9% 51%	2530
	21st to end	Night	5% 4 2 6% 5% 5% 6% 5% 7% 9% 47%	3054
August	1st to 10th	Night	11% 6% 2 6% 4 4 4 3 6% 8% 45%	2825
	11th to 20th	Night	8% 6% 3 6% 4 4 5% 3 5% 7% 49%	2785
	21st to end	Night	8% 5% 2 6% 4 5% 5% 4 6% 8% 48%	3151
September	1st to 10th	Night	10% 5% 2 5% 4 5% 5% 3 6% 7% 48%	2023
	11th to 20th	Night	9% 5% 1 4 4 3 4 4 5% 6% 55%	2090
	21st to end	Night	15% 7% 2 6% 4 4 4 4 5% 7% 43%	2090
October	1st to 10th	Night	13% 7% 3 6% 4 3 4 3 5% 7% 46%	2147
	11th to 20th	Night	18% 9% 3 7% 4 4 4 3 5% 6% 36%	2280
	21st to end	Night	20% 9% 3 7% 5% 4 4 3 5% 6% 33%	2566
November	1st to 10th	Night	13% 7% 2 6% 4 4 4 4 5% 6% 44%	2406
	11th to 20th	Night	16% 11% 4 8% 5% 4 5% 4 6% 7% 31%	2470
	21st to end	Night	21% 13% 3 8% 4 4 3 3 5% 6% 29%	2565
December	1st to 10th	Night	17% 15% 6% 10% 6% 5% 5% 4 5% 5% 22%	2660
	11th to 20th	Night	15% 14% 5% 11% 6% 5% 5% 4 5% 6% 24%	3388
	21st to end	Night	28% 22% 6% 10% 4 4 3 3 4 4 12%	4912

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% 7% 12% 11% 6%	2528
	11th to 20th	Day	72% 7% 11% 7% 3	2533
	21st to end	Day	76% 5% 9% 6% 3 1	3124
February	1st to 10th	Day	62% 7% 11% 13% 7%	1710
	11th to 20th	Day	59% 7% 9% 16% 7% 1	1798
	21st to end	Day	57% 6% 12% 16% 7% 1	1570
March	1st to 10th	Day	59% 6% 12% 19% 5% 1	3210
	11th to 20th	Day	50% 7% 15% 19% 8% 2	3480
	21st to end	Day	56% 7% 11% 16% 8% 1	3815
April	1st to 10th	Day	51% 8% 11% 18% 12% 1	2194
	11th to 20th	Day	49% 6% 11% 19% 13% 2	2413
	21st to end	Day	49% 6% 13% 17% 13% 1	2470
May	1st to 10th	Day	46% 8% 11% 23% 10% 2	4120
	11th to 20th	Day	44% 10% 14% 22% 9% 1	4085
	21st to end	Day	36% 7% 12% 19% 21% 6%	4489
June	1st to 10th	Day	39% 7% 11% 26% 13% 3	2652
	11th to 20th	Day	38% 7% 13% 22% 17% 3	2848
	21st to end	Day	39% 9% 13% 21% 16% 2	2660
July	1st to 10th	Day	40% 6% 14% 22% 17% 3	4371
	11th to 20th	Day	30% 7% 14% 32% 16% 1	4365
	21st to end	Day	35% 8% 16% 18% 20% 3	4356
August	1st to 10th	Day	37% 8% 14% 27% 14% 1	4091
	11th to 20th	Day	39% 9% 11% 30% 10% 1	4083
	21st to end	Day	37% 8% 14% 27% 13% 1	4501
September	1st to 10th	Day	39% 8% 15% 22% 14% 2	2138
	11th to 20th	Day	31% 8% 13% 31% 15% 1	2090
	21st to end	Day	42% 7% 14% 24% 13%	2090
October	1st to 10th	Day	38% 7% 12% 26% 14% 2	2032
	11th to 20th	Day	48% 6% 13% 21% 11%	1900
	21st to end	Day	52% 6% 13% 19% 9% 1	1895
November	1st to 10th	Day	43% 7% 13% 23% 13% 1	1672
	11th to 20th	Day	51% 6% 14% 19% 9% 1	1710
	21st to end	Day	55% 7% 12% 18% 7%	1611
December	1st to 10th	Day	64% 7% 12% 13% 4	1520
	11th to 20th	Day	60% 7% 13% 15% 5%	1800
	21st to end	Day	78% 5% 9% 5% 3	2751
January	1st to 10th	Night	60% 6% 12% 15% 6% 1	4428
	11th to 20th	Night	69% 7% 11% 9% 4	4434
	21st to end	Night	72% 5% 9% 10% 3	4532
February	1st to 10th	Night	62% 6% 12% 13% 6% 1	2470
	11th to 20th	Night	57% 5% 10% 18% 9% 1	2275
	21st to end	Night	55% 6% 12% 18% 8% 1	1884
March	1st to 10th	Night	51% 5% 12% 24% 7%	3642
	11th to 20th	Night	37% 8% 16% 20% 18% 1	3480
	21st to end	Night	47% 7% 11% 23% 11% 1	3817
April	1st to 10th	Night	48% 6% 10% 21% 13% 2	2001
	11th to 20th	Night	45% 7% 10% 20% 17% 2	1716
	21st to end	Night	45% 6% 11% 19% 16% 3	1710
May	1st to 10th	Night	46% 8% 9% 21% 14% 2	2840
	11th to 20th	Night	42% 9% 10% 26% 11% 1	2813
	21st to end	Night	32% 8% 12% 21% 22% 5%	3087
June	1st to 10th	Night	34% 7% 13% 26% 17% 3	1509
	11th to 20th	Night	36% 7% 12% 25% 17% 2	1330
	21st to end	Night	33% 7% 12% 26% 20% 1	1508
July	1st to 10th	Night	29% 7% 12% 32% 15% 6%	2490
	11th to 20th	Night	28% 7% 14% 32% 18% 1	2482
	21st to end	Night	30% 9% 17% 20% 21% 3	2957
August	1st to 10th	Night	36% 7% 12% 27% 17% 1	2814
	11th to 20th	Night	33% 7% 11% 34% 13% 2	2812
	21st to end	Night	33% 7% 13% 31% 15% 1	3151
September	1st to 10th	Night	33% 8% 13% 26% 17% 2	2023
	11th to 20th	Night	28% 7% 11% 32% 21% 2	2090
	21st to end	Night	41% 6% 12% 26% 13% 1	2090
October	1st to 10th	Night	37% 5% 12% 25% 19% 1	2147
	11th to 20th	Night	47% 6% 12% 19% 14% 1	2280
	21st to end	Night	51% 5% 12% 21% 10% 1	2566
November	1st to 10th	Night	40% 7% 12% 25% 15% 1	2406
	11th to 20th	Night	50% 6% 14% 20% 9% 1	2470
	21st to end	Night	55% 6% 13% 20% 6%	2565
December	1st to 10th	Night	63% 7% 11% 13% 6%	2660
	11th to 20th	Night	58% 7% 14% 14% 6% 1	3391
	21st to end	Night	76% 5% 10% 7% 2	4912

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% 22% 33% 15% 6% 1	270
	11th to 20th	Day	24% 19% 37% 16% 3 1	270
	21st to end	Day	29% 14% 34% 18% 5%	297
February	1st to 10th	Day	23% 21% 37% 16% 3 1	270
	11th to 20th	Day	24% 19% 36% 19% 2	270
	21st to end	Day	24% 26% 39% 8% 3	225
March	1st to 10th	Day	26% 21% 31% 19% 4	321
	11th to 20th	Day	23% 13% 36% 21% 5% 1	460
	21st to end	Day	27% 17% 32% 17% 6% 1	506
April	1st to 10th	Day	19% 19% 32% 21% 8% 1	460
	11th to 20th	Day	27% 17% 29% 15% 8% 3	452
	21st to end	Day	27% 15% 30% 18% 8% 3	460
May	1st to 10th	Day	24% 16% 29% 20% 8% 4	463
	11th to 20th	Day	19% 11% 30% 22% 13% 5%	487
	21st to end	Day	13% 8% 25% 29% 18% 7%	529
June	1st to 10th	Day	13% 10% 30% 31% 13% 3	634
	11th to 20th	Day	14% 9% 30% 31% 13% 3	680
	21st to end	Day	12% 11% 32% 27% 14% 4	680
July	1st to 10th	Day	9% 6% 25% 30% 21% 9%	671
	11th to 20th	Day	10% 6% 28% 31% 18% 7%	668
	21st to end	Day	8% 4 27% 28% 24% 8%	551
August	1st to 10th	Day	8% 5% 24% 30% 21% 12%	486
	11th to 20th	Day	13% 3 23% 30% 20% 11%	479
	21st to end	Day	11% 5% 21% 29% 23% 10%	539
September	1st to 10th	Day	12% 7% 24% 26% 20% 10%	490
	11th to 20th	Day	8% 4 19% 26% 29% 12%	490
	21st to end	Day	18% 11% 25% 22% 17% 8%	490
October	1st to 10th	Day	13% 12% 28% 23% 17% 7%	490
	11th to 20th	Day	18% 18% 31% 24% 7% 2	490
	21st to end	Day	27% 17% 34% 17% 4 1	382
November	1st to 10th	Day	15% 20% 33% 21% 9% 1	270
	11th to 20th	Day	22% 23% 33% 17% 5% 1	270
	21st to end	Day	27% 19% 34% 14% 6%	223
December	1st to 10th	Day	22% 22% 34% 14% 6% 1	180
	11th to 20th	Day	26% 24% 36% 10% 3 1	180
	21st to end	Day	29% 24% 31% 14% 2	288
January	1st to 10th	Night	23% 17% 35% 20% 5% 1	950
	11th to 20th	Night	27% 14% 32% 22% 4 1	944
	21st to end	Night	20% 14% 34% 27% 5%	836
February	1st to 10th	Night	25% 12% 37% 23% 3	760
	11th to 20th	Night	23% 14% 33% 26% 3	743
	21st to end	Night	25% 15% 37% 21% 2	628
March	1st to 10th	Night	20% 12% 34% 28% 6%	760
	11th to 20th	Night	18% 16% 36% 24% 5% 1	760
	21st to end	Night	23% 11% 30% 28% 7%	836
April	1st to 10th	Night	21% 14% 33% 28% 4	671
	11th to 20th	Night	21% 10% 35% 25% 8% 1	563
	21st to end	Night	20% 9% 33% 29% 8%	570
May	1st to 10th	Night	19% 10% 30% 31% 10% 1	570
	11th to 20th	Night	21% 5% 27% 33% 13% 1	561
	21st to end	Night	10% 4 26% 39% 19% 2	617
June	1st to 10th	Night	13% 3 27% 44% 13%	518
	11th to 20th	Night	9% 6% 25% 47% 13%	380
	21st to end	Night	10% 5% 24% 40% 19% 1	558
July	1st to 10th	Night	8% 3 21% 39% 26% 4	555
	11th to 20th	Night	9% 3 22% 39% 23% 4	552
	21st to end	Night	6% 2 19% 47% 24% 2	624
August	1st to 10th	Night	8% 4 19% 40% 27% 3	567
	11th to 20th	Night	9% 2 20% 36% 30% 3	564
	21st to end	Night	10% 3 18% 37% 30% 3	627
September	1st to 10th	Night	13% 6% 19% 33% 25% 5%	693
	11th to 20th	Night	9% 4 19% 34% 30% 4	760
	21st to end	Night	17% 7% 22% 30% 21% 3	760
October	1st to 10th	Night	17% 8% 26% 30% 17% 3	760
	11th to 20th	Night	23% 13% 28% 24% 10% 1	760
	21st to end	Night	24% 14% 30% 26% 5%	812
November	1st to 10th	Night	17% 12% 31% 26% 12% 1	740
	11th to 20th	Night	22% 16% 32% 22% 7% 1	760
	21st to end	Night	27% 13% 33% 20% 6%	760
December	1st to 10th	Night	26% 15% 34% 19% 6%	760
	11th to 20th	Night	24% 15% 36% 20% 4	826
	21st to end	Night	31% 12% 33% 20% 3	1045

Page generated at 2024-10-22 20:21:00. Version ac07

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