Dearborn Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Dearborn Observatory clear sky chart from 2005-11-06 to 2024-11-05.

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	12% 8% 3 7% 5% 5% 4 4 6% 7% 38%	101295
	Night	14% 10% 4 7% 5% 4 4 4 5% 7% 36%	97823
Transparency	Day	24% 47% 7% 14% 8% 1	104865
	Night	26% 46% 6% 13% 8% 1	102394
Seeing	Day	16% 2 15% 33% 26% 8%	15815
	Night	19% 5% 23% 31% 18% 4	25653

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% 12% 7% 10% 6% 6% 5% 4 4 7% 25%	2527
	11th to 20th	Day	12% 13% 6% 14% 4 4 5% 7% 9% 8% 17%	2530
	21st to end	Day	19% 22% 6% 9% 4 4 3 3 4 5% 21%	2812
February	1st to 10th	Day	13% 13% 5% 9% 6% 6% 5% 4 5% 7% 28%	1770
	11th to 20th	Day	13% 10% 4 8% 5% 4 5% 4 6% 5% 34%	1848
	21st to end	Day	12% 12% 4 11% 7% 5% 4 3 6% 6% 30%	1570
March	1st to 10th	Day	14% 11% 3 8% 5% 5% 4 4 4 6% 36%	3120
	11th to 20th	Day	9% 7% 2 7% 5% 5% 5% 5% 6% 8% 40%	3260
	21st to end	Day	19% 11% 6% 11% 6% 5% 5% 4 5% 6% 22%	4165
April	1st to 10th	Day	16% 11% 3 8% 5% 4 4 4 6% 7% 33%	2280
	11th to 20th	Day	15% 8% 2 7% 6% 5% 6% 3 6% 7% 36%	2254
	21st to end	Day	17% 9% 3 7% 5% 5% 4 4 6% 6% 34%	2408
May	1st to 10th	Day	16% 7% 3 6% 5% 5% 5% 4 6% 8% 36%	4082
	11th to 20th	Day	15% 6% 2 5% 4 4 4 3 6% 8% 43%	4261
	21st to end	Day	10% 6% 3 5% 5% 4 4 4 6% 9% 45%	4824
June	1st to 10th	Day	9% 5% 2 6% 4 6% 4 4 6% 8% 45%	2660
	11th to 20th	Day	9% 5% 2 6% 4 4 4 4 6% 9% 45%	2660
	21st to end	Day	9% 7% 2 6% 5% 5% 5% 5% 7% 9% 41%	2660
July	1st to 10th	Day	10% 7% 3 7% 5% 5% 5% 5% 8% 9% 36%	4440
	11th to 20th	Day	6% 4 1 4 3 4 4 4 7% 8% 53%	4440
	21st to end	Day	7% 6% 2 6% 5% 5% 5% 5% 6% 8% 45%	4813
August	1st to 10th	Day	11% 4 2 5% 4 4 5% 4 7% 8% 46%	4300
	11th to 20th	Day	9% 5% 2 5% 4 4 4 4 6% 9% 50%	3802
	21st to end	Day	6% 5% 2 5% 4 4 4 4 6% 8% 50%	4180
September	1st to 10th	Day	11% 6% 2 5% 5% 4 4 3 6% 7% 48%	2280
	11th to 20th	Day	11% 5% 2 4 2 3 4 4 5% 9% 51%	2142
	21st to end	Day	11% 6% 3 7% 5% 4 5% 4 7% 8% 40%	2090
October	1st to 10th	Day	13% 7% 3 7% 4 4 4 3 5% 6% 45%	2066
	11th to 20th	Day	17% 8% 3 8% 4 4 5% 3 5% 7% 35%	1863
	21st to end	Day	18% 9% 3 6% 5% 6% 4 4 4 8% 33%	1881
November	1st to 10th	Day	12% 9% 3 5% 5% 4 4 3 6% 8% 42%	1708
	11th to 20th	Day	14% 12% 3 9% 5% 5% 4 4 5% 7% 31%	1710
	21st to end	Day	18% 11% 3 7% 5% 5% 5% 3 4 5% 34%	1668
December	1st to 10th	Day	15% 15% 5% 10% 6% 6% 4 5% 5% 6% 23%	1520
	11th to 20th	Day	12% 12% 5% 10% 8% 6% 4 5% 7% 6% 27%	1918
	21st to end	Day	19% 20% 6% 13% 6% 5% 4 3 4 6% 16%	2783
January	1st to 10th	Night	15% 15% 6% 10% 5% 4 5% 4 5% 6% 25%	4428
	11th to 20th	Night	16% 17% 6% 13% 6% 4 4 5% 7% 6% 16%	4440
	21st to end	Night	22% 18% 7% 9% 5% 4 3 3 4 4 21%	4852
February	1st to 10th	Night	15% 15% 4 8% 5% 4 4 3 4 5% 32%	2409
	11th to 20th	Night	13% 11% 4 8% 5% 4 5% 4 5% 6% 35%	2218
	21st to end	Night	19% 11% 3 8% 4 4 3 3 4 5% 35%	1884
March	1st to 10th	Night	18% 11% 3 7% 4 3 3 3 4 6% 38%	3733
	11th to 20th	Night	9% 7% 2 7% 4 4 4 3 5% 7% 47%	3718
	21st to end	Night	20% 12% 8% 9% 5% 4 4 3 4 5% 27%	3476
April	1st to 10th	Night	17% 8% 3 6% 5% 4 5% 3 5% 7% 37%	1900
	11th to 20th	Night	19% 9% 2 5% 5% 4 5% 3 4 6% 38%	1875
	21st to end	Night	18% 8% 3 6% 4 4 5% 4 5% 7% 37%	1770
May	1st to 10th	Night	16% 8% 3 7% 6% 4 4 4 5% 8% 34%	2813
	11th to 20th	Night	17% 7% 2 6% 4 4 5% 4 7% 8% 37%	2636
	21st to end	Night	12% 8% 3 6% 4 4 4 3 5% 7% 44%	2743
June	1st to 10th	Night	9% 6% 3 4 4 5% 5% 4 5% 8% 46%	1520
	11th to 20th	Night	10% 7% 2 5% 4 4 5% 5% 6% 9% 44%	1520
	21st to end	Night	9% 7% 3 6% 5% 5% 5% 5% 6% 8% 41%	1520
July	1st to 10th	Night	11% 5% 2 7% 6% 5% 5% 5% 7% 9% 39%	2530
	11th to 20th	Night	5% 4 2 5% 4 4 4 4 6% 9% 53%	2530
	21st to end	Night	5% 4 2 6% 5% 6% 6% 5% 7% 9% 46%	2750
August	1st to 10th	Night	7% 5% 2 6% 5% 4 4 4 7% 8% 47%	2632
	11th to 20th	Night	8% 5% 2 5% 4 4 4 3 6% 7% 51%	3024
	21st to end	Night	8% 4 2 5% 4 4 4 3 6% 9% 50%	3476
September	1st to 10th	Night	10% 6% 2 5% 4 3 4 3 4 7% 51%	1900
	11th to 20th	Night	10% 5% 1 5% 3 3 4 3 5% 9% 53%	2019
	21st to end	Night	13% 7% 2 6% 4 4 5% 3 5% 7% 44%	2090
October	1st to 10th	Night	13% 7% 2 5% 3 4 4 4 5% 6% 49%	2110
	11th to 20th	Night	17% 8% 2 7% 3 4 4 3 4 6% 41%	2316
	21st to end	Night	21% 8% 3 7% 4 4 4 3 5% 7% 36%	2717
November	1st to 10th	Night	12% 8% 3 6% 5% 4 4 3 5% 6% 44%	2470
	11th to 20th	Night	15% 11% 3 8% 4 4 4 4 6% 7% 34%	2470
	21st to end	Night	18% 9% 4 7% 5% 5% 4 4 5% 5% 35%	2511
December	1st to 10th	Night	17% 13% 3 8% 7% 5% 5% 4 6% 7% 24%	2660
	11th to 20th	Night	14% 14% 4 9% 5% 5% 4 4 5% 7% 28%	3279
	21st to end	Night	23% 17% 6% 10% 6% 4 4 3 4 6% 17%	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	41% 38% 8% 10% 3	2528
	11th to 20th	Day	44% 38% 8% 7% 3	2530
	21st to end	Day	44% 39% 3 12% 2	2812
February	1st to 10th	Day	19% 64% 5% 6% 5%	1770
	11th to 20th	Day	19% 63% 2 10% 5% 1	1858
	21st to end	Day	18% 64% 4 9% 3 1	1570
March	1st to 10th	Day	27% 42% 7% 15% 7% 2	3120
	11th to 20th	Day	21% 42% 7% 23% 6%	3260
	21st to end	Day	39% 43% 9% 6% 3	4165
April	1st to 10th	Day	20% 64% 3 9% 3	2280
	11th to 20th	Day	15% 66% 4 8% 5% 2	2254
	21st to end	Day	16% 64% 5% 7% 7% 1	2408
May	1st to 10th	Day	26% 42% 7% 15% 9% 1	4120
	11th to 20th	Day	22% 42% 8% 19% 8%	4265
	21st to end	Day	18% 42% 7% 15% 15% 3	4837
June	1st to 10th	Day	12% 65% 4 10% 9% 1	2660
	11th to 20th	Day	15% 65% 5% 8% 6% 1	2660
	21st to end	Day	13% 64% 5% 12% 5% 1	2660
July	1st to 10th	Day	31% 37% 9% 14% 7% 2	4371
	11th to 20th	Day	15% 37% 13% 24% 11%	4365
	21st to end	Day	23% 37% 11% 12% 16% 1	4662
August	1st to 10th	Day	22% 38% 12% 20% 9%	4285
	11th to 20th	Day	20% 39% 7% 25% 9%	3840
	21st to end	Day	21% 38% 9% 17% 14% 1	4179
September	1st to 10th	Day	15% 60% 6% 11% 7% 1	2280
	11th to 20th	Day	11% 59% 4 15% 8% 2	2142
	21st to end	Day	19% 60% 3 11% 7% 1	2090
October	1st to 10th	Day	14% 36% 4 29% 16% 1	3430
	11th to 20th	Day	19% 37% 7% 20% 16%	3079
	21st to end	Day	28% 37% 6% 19% 8% 3	3081
November	1st to 10th	Day	20% 58% 6% 7% 7% 1	1708
	11th to 20th	Day	22% 59% 4 10% 5% 1	1710
	21st to end	Day	25% 59% 5% 8% 3	1668
December	1st to 10th	Day	27% 59% 4 5% 4	1520
	11th to 20th	Day	27% 48% 10% 11% 3 1	1915
	21st to end	Day	50% 37% 5% 6% 2	2783
January	1st to 10th	Night	41% 38% 7% 9% 3	4428
	11th to 20th	Night	49% 37% 6% 5% 2	4440
	21st to end	Night	44% 39% 5% 9% 3	4852
February	1st to 10th	Night	20% 64% 4 6% 6% 1	2409
	11th to 20th	Night	18% 63% 2 11% 5%	2219
	21st to end	Night	18% 64% 2 8% 5% 2	1884
March	1st to 10th	Night	28% 40% 7% 16% 6% 2	3733
	11th to 20th	Night	20% 41% 6% 18% 14% 1	3718
	21st to end	Night	39% 41% 5% 12% 4 1	3476
April	1st to 10th	Night	19% 64% 3 10% 4 1	1900
	11th to 20th	Night	18% 65% 2 7% 6% 2	1875
	21st to end	Night	15% 64% 3 8% 8% 2	1770
May	1st to 10th	Night	29% 41% 6% 11% 10% 2	2840
	11th to 20th	Night	26% 43% 10% 17% 4	2637
	21st to end	Night	25% 41% 5% 11% 15% 5%	2751
June	1st to 10th	Night	13% 64% 4 11% 8% 1	1520
	11th to 20th	Night	17% 64% 3 9% 7%	1520
	21st to end	Night	14% 64% 5% 9% 7%	1520
July	1st to 10th	Night	31% 37% 11% 12% 5% 4	2490
	11th to 20th	Night	17% 37% 10% 23% 12% 1	2482
	21st to end	Night	21% 37% 10% 11% 17% 3	2662
August	1st to 10th	Night	20% 39% 10% 19% 11% 1	2623
	11th to 20th	Night	20% 38% 8% 21% 13%	3052
	21st to end	Night	21% 38% 8% 20% 11% 1	3476
September	1st to 10th	Night	13% 59% 5% 13% 9% 1	1900
	11th to 20th	Night	11% 59% 2 15% 11% 1	2019
	21st to end	Night	19% 59% 3 11% 7%	2090
October	1st to 10th	Night	12% 36% 5% 29% 17% 1	3526
	11th to 20th	Night	25% 37% 4 15% 18% 1	3839
	21st to end	Night	25% 36% 5% 18% 14% 1	4467
November	1st to 10th	Night	24% 57% 3 9% 6% 2	2470
	11th to 20th	Night	21% 59% 3 11% 6% 1	2470
	21st to end	Night	24% 59% 4 9% 3	2511
December	1st to 10th	Night	26% 59% 4 7% 3 1	2660
	11th to 20th	Night	27% 49% 8% 11% 5%	3281
	21st to end	Night	45% 38% 7% 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	21% 6% 24% 32% 15% 1	270
	11th to 20th	Day	21% 7% 28% 32% 10% 2	270
	21st to end	Day	23% 1 23% 37% 15% 2	297
February	1st to 10th	Day	18% 4 30% 36% 10% 3	270
	11th to 20th	Day	19% 6% 29% 33% 12% 1	270
	21st to end	Day	16% 3 30% 39% 11% 1	225
March	1st to 10th	Day	15% 3 27% 42% 13%	270
	11th to 20th	Day	20% 3 24% 36% 15% 2	313
	21st to end	Day	26% 3 24% 34% 11% 2	506
April	1st to 10th	Day	20% 3 21% 40% 14% 1	460
	11th to 20th	Day	16% 3 20% 41% 15% 6%	452
	21st to end	Day	22% 2 16% 37% 19% 5%	460
May	1st to 10th	Day	22% 1 15% 39% 20% 3	463
	11th to 20th	Day	19% 1 8% 42% 27% 4	589
	21st to end	Day	15% 1 7% 35% 30% 12%	734
June	1st to 10th	Day	12% 1 8% 35% 32% 11%	680
	11th to 20th	Day	12% 1 7% 36% 34% 11%	680
	21st to end	Day	12% 2 8% 37% 34% 8%	680
July	1st to 10th	Day	8% 5% 28% 39% 19%	671
	11th to 20th	Day	11% 1 6% 32% 37% 13%	668
	21st to end	Day	7% 1 7% 32% 43% 11%	739
August	1st to 10th	Day	9% 5% 28% 41% 17%	605
	11th to 20th	Day	11% 2 7% 31% 37% 13%	479
	21st to end	Day	9% 1 5% 26% 43% 15%	539
September	1st to 10th	Day	14% 2 10% 26% 37% 12%	490
	11th to 20th	Day	10% 2 8% 23% 40% 18%	490
	21st to end	Day	15% 3 17% 24% 29% 12%	490
October	1st to 10th	Day	18% 2 13% 30% 28% 9%	466
	11th to 20th	Day	21% 5% 21% 27% 24% 3	300
	21st to end	Day	28% 3 21% 34% 13% 1	330
November	1st to 10th	Day	18% 3 21% 30% 23% 5%	282
	11th to 20th	Day	24% 7% 27% 27% 11% 4	270
	21st to end	Day	27% 3 24% 36% 9%	270
December	1st to 10th	Day	21% 6% 30% 31% 10% 1	270
	11th to 20th	Day	16% 7% 36% 26% 13% 1	270
	21st to end	Day	27% 5% 34% 25% 8%	297
January	1st to 10th	Night	21% 10% 36% 26% 7% 1	950
	11th to 20th	Night	25% 8% 33% 27% 7% 1	950
	21st to end	Night	22% 7% 33% 30% 7% 1	1045
February	1st to 10th	Night	22% 5% 35% 29% 8% 1	889
	11th to 20th	Night	19% 7% 33% 34% 7%	743
	21st to end	Night	25% 6% 32% 31% 6%	628
March	1st to 10th	Night	23% 4 31% 31% 11% 1	760
	11th to 20th	Night	15% 4 27% 40% 11% 2	760
	21st to end	Night	23% 3 26% 32% 14% 1	637
April	1st to 10th	Night	21% 5% 25% 35% 12% 2	570
	11th to 20th	Night	24% 2 21% 35% 17% 2	563
	21st to end	Night	21% 2 21% 36% 17% 2	570
May	1st to 10th	Night	18% 2 15% 44% 18% 3	570
	11th to 20th	Night	20% 1 12% 34% 29% 5%	561
	21st to end	Night	15% 1 9% 33% 33% 9%	617
June	1st to 10th	Night	12% 1 8% 38% 34% 6%	570
	11th to 20th	Night	14% 1 9% 38% 32% 6%	570
	21st to end	Night	11% 1 10% 36% 35% 7%	570
July	1st to 10th	Night	10% 1 3 30% 39% 16%	555
	11th to 20th	Night	9% 1 6% 33% 39% 12%	552
	21st to end	Night	8% 1 6% 36% 41% 8%	624
August	1st to 10th	Night	8% 1 5% 34% 42% 10%	567
	11th to 20th	Night	10% 1 8% 32% 37% 13%	564
	21st to end	Night	11% 1 9% 30% 40% 10%	627
September	1st to 10th	Night	14% 1 11% 30% 34% 10%	570
	11th to 20th	Night	14% 2 10% 28% 34% 13%	689
	21st to end	Night	16% 4 18% 27% 26% 11%	760
October	1st to 10th	Night	19% 2 18% 31% 24% 7%	760
	11th to 20th	Night	23% 5% 26% 28% 16% 2	760
	21st to end	Night	27% 7% 25% 28% 12% 1	836
November	1st to 10th	Night	19% 6% 30% 27% 17% 1	760
	11th to 20th	Night	22% 12% 32% 26% 8% 1	760
	21st to end	Night	22% 10% 34% 26% 7%	801
December	1st to 10th	Night	24% 11% 36% 24% 6%	950
	11th to 20th	Night	23% 9% 35% 26% 7%	950
	21st to end	Night	31% 9% 32% 24% 3	1045

Page generated at 2024-11-03 01:11:17. Version ac07

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