Yerkes Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Yerkes Observatory clear sky chart from 2005-11-06 to 2025-09-04.

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% 9% 3 8% 5% 4 4 4 6% 8% 37%	126237
	Night	13% 10% 4 8% 5% 4 4 3 5% 6% 38%	122734
Transparency	Day	43% 6% 12% 24% 13% 1	129733
	Night	45% 6% 11% 22% 14% 2	127300
Seeing	Day	16% 11% 30% 25% 13% 5%	17036
	Night	17% 10% 32% 29% 11% 1	26824

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	9% 12% 6% 15% 7% 6% 5% 4 5% 6% 24%	3617
	11th to 20th	Day	8% 11% 7% 12% 4 4 4 6% 7% 6% 30%	3620
	21st to end	Day	13% 14% 8% 9% 3 3 3 2 4 7% 32%	4099
February	1st to 10th	Day	12% 14% 5% 11% 5% 6% 5% 4 5% 6% 27%	1800
	11th to 20th	Day	13% 10% 4 8% 6% 5% 4 3 5% 6% 36%	1939
	21st to end	Day	12% 13% 4 11% 5% 5% 4 4 5% 7% 30%	1650
March	1st to 10th	Day	14% 8% 3 7% 4 3 3 3 5% 6% 43%	4480
	11th to 20th	Day	11% 6% 3 8% 5% 5% 5% 4 6% 7% 42%	4708
	21st to end	Day	19% 13% 5% 12% 6% 5% 4 3 4 5% 24%	5967
April	1st to 10th	Day	16% 13% 4 8% 6% 5% 4 3 5% 6% 31%	2400
	11th to 20th	Day	15% 10% 3 7% 6% 5% 4 4 6% 7% 33%	2374
	21st to end	Day	19% 8% 4 8% 5% 4 5% 4 5% 6% 33%	2500
May	1st to 10th	Day	14% 9% 3 7% 5% 4 4 4 6% 7% 38%	5878
	11th to 20th	Day	13% 6% 2 6% 4 4 5% 4 6% 8% 41%	6319
	21st to end	Day	15% 6% 2 6% 4 5% 5% 4 6% 9% 37%	6935
June	1st to 10th	Day	10% 5% 2 6% 5% 4 5% 4 7% 8% 45%	2800
	11th to 20th	Day	10% 6% 2 6% 4 5% 5% 4 7% 10% 43%	2800
	21st to end	Day	10% 6% 3 7% 5% 5% 5% 4 6% 9% 40%	2800
July	1st to 10th	Day	10% 7% 3 6% 5% 5% 5% 4 8% 9% 38%	6360
	11th to 20th	Day	7% 6% 2 6% 5% 5% 5% 4 7% 10% 43%	6360
	21st to end	Day	9% 6% 2 7% 5% 5% 5% 4 6% 9% 41%	6924
August	1st to 10th	Day	10% 5% 2 6% 4 5% 5% 5% 7% 9% 42%	4547
	11th to 20th	Day	11% 6% 2 5% 4 4 5% 4 6% 9% 44%	3855
	21st to end	Day	8% 6% 2 6% 4 4 4 4 6% 9% 47%	4312
September	1st to 10th	Day	12% 5% 2 6% 4 4 3 4 5% 8% 47%	2316
	11th to 20th	Day	12% 4 2 4 3 4 3 3 6% 8% 52%	2180
	21st to end	Day	12% 6% 3 7% 5% 4 3 4 6% 7% 43%	2090
October	1st to 10th	Day	13% 7% 3 5% 4 4 4 3 5% 6% 47%	2000
	11th to 20th	Day	18% 9% 2 8% 5% 4 4 3 6% 7% 34%	1899
	21st to end	Day	18% 10% 2 9% 5% 4 3 3 5% 5% 37%	1881
November	1st to 10th	Day	12% 9% 3 6% 4 4 3 3 5% 5% 44%	1762
	11th to 20th	Day	14% 13% 4 10% 6% 4 4 4 5% 6% 31%	1800
	21st to end	Day	16% 13% 4 9% 5% 4 3 2 4 5% 35%	1665
December	1st to 10th	Day	8% 8% 4 8% 6% 5% 5% 4 5% 9% 38%	2610
	11th to 20th	Day	11% 15% 6% 11% 6% 5% 5% 4 7% 7% 24%	3008
	21st to end	Day	20% 21% 9% 14% 5% 4 3 2 4 5% 13%	3982
January	1st to 10th	Night	9% 12% 6% 14% 6% 4 4 2 4 5% 32%	6348
	11th to 20th	Night	11% 13% 6% 14% 7% 6% 5% 5% 6% 5% 22%	6360
	21st to end	Night	14% 14% 7% 11% 5% 4 3 3 4 5% 31%	6876
February	1st to 10th	Night	12% 14% 4 9% 6% 6% 4 3 4 5% 32%	2600
	11th to 20th	Night	12% 10% 3 7% 6% 4 5% 4 4 7% 37%	2345
	21st to end	Night	17% 12% 3 7% 4 4 4 3 5% 6% 36%	1980
March	1st to 10th	Night	15% 10% 3 7% 3 4 4 2 4 5% 43%	5373
	11th to 20th	Night	8% 7% 2 6% 4 4 4 3 5% 8% 47%	5294
	21st to end	Night	20% 12% 6% 10% 5% 5% 4 3 4 4 28%	4962
April	1st to 10th	Night	16% 8% 3 8% 4 4 4 4 5% 6% 37%	2000
	11th to 20th	Night	18% 9% 4 8% 4 4 3 3 5% 6% 38%	1975
	21st to end	Night	17% 8% 3 6% 4 5% 4 3 5% 7% 38%	1899
May	1st to 10th	Night	16% 7% 3 6% 4 4 4 4 5% 7% 38%	4002
	11th to 20th	Night	11% 6% 2 6% 4 4 5% 4 5% 8% 43%	3596
	21st to end	Night	15% 8% 2 7% 5% 4 4 3 5% 6% 40%	3942
June	1st to 10th	Night	8% 5% 2 5% 5% 5% 4 4 7% 8% 47%	1600
	11th to 20th	Night	10% 5% 2 6% 4 5% 5% 3 7% 9% 45%	1600
	21st to end	Night	8% 6% 2 8% 4 4 4 4 7% 9% 44%	1600
July	1st to 10th	Night	11% 5% 3 7% 5% 4 5% 4 6% 9% 42%	3618
	11th to 20th	Night	6% 5% 2 6% 4 4 4 4 6% 8% 52%	3617
	21st to end	Night	8% 5% 2 6% 4 4 5% 4 6% 8% 47%	3947
August	1st to 10th	Night	7% 5% 2 6% 4 4 4 4 7% 8% 51%	2610
	11th to 20th	Night	9% 7% 2 6% 4 4 3 4 6% 8% 48%	3191
	21st to end	Night	8% 5% 2 5% 3 3 5% 4 6% 9% 51%	3586
September	1st to 10th	Night	9% 4 2 6% 4 3 3 3 5% 5% 56%	1930
	11th to 20th	Night	12% 4 2 5% 3 3 4 3 4 7% 54%	1980
	21st to end	Night	11% 7% 2 6% 4 4 4 2 4 6% 48%	2090
October	1st to 10th	Night	12% 5% 3 5% 3 3 3 3 4 7% 50%	2175
	11th to 20th	Night	17% 10% 2 5% 4 3 4 2 4 6% 44%	2280
	21st to end	Night	19% 9% 2 6% 4 4 4 3 5% 7% 37%	2717
November	1st to 10th	Night	12% 8% 3 6% 4 4 3 3 5% 6% 47%	2536
	11th to 20th	Night	14% 11% 4 8% 5% 4 4 4 4 6% 36%	2600
	21st to end	Night	16% 11% 4 9% 5% 4 4 3 5% 6% 35%	2730
December	1st to 10th	Night	9% 9% 3 7% 5% 5% 5% 4 5% 7% 41%	4580
	11th to 20th	Night	15% 15% 5% 10% 5% 5% 4 3 5% 5% 27%	5199
	21st to end	Night	19% 19% 6% 13% 5% 4 4 3 4 5% 17%	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	58% 5% 12% 18% 6%	3618
	11th to 20th	Day	52% 5% 12% 11% 17% 3	3620
	21st to end	Day	53% 4 12% 19% 13%	4099
February	1st to 10th	Day	55% 6% 13% 15% 10% 1	1800
	11th to 20th	Day	49% 5% 10% 23% 10% 2	1949
	21st to end	Day	54% 5% 10% 19% 11% 2	1650
March	1st to 10th	Day	42% 5% 10% 27% 12% 4	4480
	11th to 20th	Day	40% 5% 12% 30% 12% 1	4708
	21st to end	Day	60% 7% 11% 13% 8% 1	5969
April	1st to 10th	Day	54% 6% 10% 19% 10%	2400
	11th to 20th	Day	48% 7% 10% 21% 12% 2	2374
	21st to end	Day	50% 6% 13% 18% 12% 1	2500
May	1st to 10th	Day	43% 7% 11% 23% 15% 1	5913
	11th to 20th	Day	39% 8% 11% 26% 15% 1	6323
	21st to end	Day	41% 7% 12% 25% 13% 2	6949
June	1st to 10th	Day	35% 7% 14% 27% 16% 1	2800
	11th to 20th	Day	35% 7% 13% 25% 17% 2	2800
	21st to end	Day	40% 8% 13% 22% 15% 3	2800
July	1st to 10th	Day	39% 7% 13% 28% 11% 1	6291
	11th to 20th	Day	34% 8% 15% 28% 15% 1	6285
	21st to end	Day	37% 8% 15% 22% 17% 2	6728
August	1st to 10th	Day	35% 9% 15% 28% 11% 1	4532
	11th to 20th	Day	34% 8% 14% 32% 12% 1	3891
	21st to end	Day	33% 7% 13% 25% 20% 2	4311
September	1st to 10th	Day	35% 8% 11% 28% 17% 2	2316
	11th to 20th	Day	30% 6% 11% 33% 18% 2	2180
	21st to end	Day	38% 7% 10% 29% 16% 1	2090
October	1st to 10th	Day	25% 3 7% 39% 25% 1	3325
	11th to 20th	Day	34% 4 11% 27% 23% 1	3127
	21st to end	Day	43% 4 9% 28% 13% 3	3081
November	1st to 10th	Day	43% 4 13% 23% 15% 1	1762
	11th to 20th	Day	53% 6% 12% 19% 9% 1	1800
	21st to end	Day	51% 5% 13% 23% 7%	1665
December	1st to 10th	Day	42% 6% 21% 24% 6% 1	2610
	11th to 20th	Day	57% 5% 12% 17% 7% 1	3005
	21st to end	Day	75% 3 6% 9% 6%	3982
January	1st to 10th	Night	54% 5% 9% 21% 12% 1	6348
	11th to 20th	Night	61% 5% 9% 10% 15% 1	6360
	21st to end	Night	56% 5% 13% 15% 11% 1	6876
February	1st to 10th	Night	54% 5% 11% 17% 12% 2	2600
	11th to 20th	Night	47% 6% 11% 21% 14% 1	2346
	21st to end	Night	49% 5% 11% 20% 11% 4	1980
March	1st to 10th	Night	45% 4 13% 23% 11% 5%	5373
	11th to 20th	Night	34% 5% 12% 32% 13% 3	5294
	21st to end	Night	59% 5% 8% 16% 10% 1	4962
April	1st to 10th	Night	46% 7% 10% 21% 15% 1	2000
	11th to 20th	Night	48% 5% 6% 21% 17% 3	1975
	21st to end	Night	45% 5% 13% 21% 14% 2	1899
May	1st to 10th	Night	44% 6% 13% 17% 18% 2	4029
	11th to 20th	Night	37% 8% 15% 22% 14% 4	3597
	21st to end	Night	43% 7% 11% 21% 15% 4	3950
June	1st to 10th	Night	32% 7% 13% 27% 19% 2	1600
	11th to 20th	Night	34% 8% 10% 24% 22% 2	1600
	21st to end	Night	35% 8% 15% 22% 18% 2	1600
July	1st to 10th	Night	38% 7% 13% 28% 11% 3	3580
	11th to 20th	Night	28% 6% 15% 31% 18% 1	3572
	21st to end	Night	30% 8% 15% 24% 21% 2	3832
August	1st to 10th	Night	29% 8% 14% 29% 18% 2	2601
	11th to 20th	Night	32% 8% 12% 32% 15%	3221
	21st to end	Night	29% 6% 13% 30% 18% 3	3586
September	1st to 10th	Night	30% 6% 11% 26% 23% 3	1930
	11th to 20th	Night	31% 5% 9% 33% 20% 2	1980
	21st to end	Night	37% 4 9% 31% 17% 1	2090
October	1st to 10th	Night	24% 5% 7% 37% 26% 1	3630
	11th to 20th	Night	35% 4 10% 25% 24% 3	3779
	21st to end	Night	42% 4 9% 26% 17% 3	4467
November	1st to 10th	Night	39% 5% 10% 27% 17% 3	2536
	11th to 20th	Night	49% 5% 12% 21% 11% 2	2600
	21st to end	Night	50% 5% 13% 21% 10% 1	2730
December	1st to 10th	Night	41% 6% 16% 26% 9% 2	4580
	11th to 20th	Night	58% 6% 10% 15% 10% 1	5201
	21st to end	Night	69% 5% 8% 12% 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	16% 19% 33% 26% 5% 1	300
	11th to 20th	Day	18% 20% 37% 22% 2 1	300
	21st to end	Day	19% 16% 39% 19% 5% 1	330
February	1st to 10th	Day	14% 20% 45% 16% 5%	300
	11th to 20th	Day	18% 25% 35% 17% 4	300
	21st to end	Day	22% 20% 41% 13% 2	249
March	1st to 10th	Day	17% 18% 45% 15% 4	300
	11th to 20th	Day	22% 23% 33% 15% 5% 1	305
	21st to end	Day	24% 14% 34% 20% 6% 2	550
April	1st to 10th	Day	24% 16% 33% 24% 3	500
	11th to 20th	Day	20% 23% 25% 21% 9% 2	492
	21st to end	Day	23% 13% 31% 21% 11% 1	500
May	1st to 10th	Day	19% 13% 32% 26% 7% 3	506
	11th to 20th	Day	22% 10% 29% 25% 11% 3	725
	21st to end	Day	18% 9% 29% 23% 15% 5%	789
June	1st to 10th	Day	10% 8% 31% 30% 15% 5%	730
	11th to 20th	Day	12% 7% 34% 26% 15% 5%	730
	21st to end	Day	12% 10% 29% 31% 15% 4	730
July	1st to 10th	Day	9% 2 21% 32% 25% 11%	721
	11th to 20th	Day	9% 7% 25% 32% 19% 9%	718
	21st to end	Day	8% 3 25% 36% 19% 8%	794
August	1st to 10th	Day	9% 3 22% 32% 24% 10%	723
	11th to 20th	Day	11% 4 19% 27% 25% 14%	519
	21st to end	Day	9% 4 20% 33% 25% 9%	583
September	1st to 10th	Day	12% 8% 23% 26% 22% 9%	502
	11th to 20th	Day	10% 3 19% 32% 24% 11%	490
	21st to end	Day	15% 12% 26% 24% 15% 8%	490
October	1st to 10th	Day	16% 9% 30% 24% 17% 5%	400
	11th to 20th	Day	24% 17% 30% 21% 7%	300
	21st to end	Day	28% 15% 34% 18% 5% 1	330
November	1st to 10th	Day	16% 17% 38% 22% 7% 1	300
	11th to 20th	Day	21% 18% 37% 16% 8%	300
	21st to end	Day	24% 12% 41% 20% 3	300
December	1st to 10th	Day	15% 22% 38% 18% 6% 1	300
	11th to 20th	Day	19% 19% 36% 20% 6%	300
	21st to end	Day	25% 21% 35% 15% 3 1	330
January	1st to 10th	Night	17% 17% 38% 23% 5%	1000
	11th to 20th	Night	20% 17% 36% 21% 6% 1	1000
	21st to end	Night	19% 12% 38% 25% 5%	1100
February	1st to 10th	Night	20% 13% 39% 23% 5%	1000
	11th to 20th	Night	19% 15% 38% 24% 3	790
	21st to end	Night	23% 13% 40% 20% 3	660
March	1st to 10th	Night	21% 13% 36% 23% 7% 1	800
	11th to 20th	Night	15% 13% 36% 27% 8%	750
	21st to end	Night	24% 9% 31% 28% 7% 1	660
April	1st to 10th	Night	21% 11% 38% 25% 6%	600
	11th to 20th	Night	22% 13% 34% 25% 6% 1	593
	21st to end	Night	21% 9% 33% 28% 9% 1	600
May	1st to 10th	Night	19% 9% 37% 28% 7% 1	600
	11th to 20th	Night	17% 6% 32% 33% 12%	591
	21st to end	Night	15% 5% 27% 35% 18%	650
June	1st to 10th	Night	10% 4 28% 43% 14% 1	600
	11th to 20th	Night	12% 5% 33% 38% 13% 1	600
	21st to end	Night	9% 6% 28% 41% 16% 1	600
July	1st to 10th	Night	9% 2 18% 40% 29% 2	585
	11th to 20th	Night	9% 3 22% 40% 23% 3	582
	21st to end	Night	6% 3 18% 47% 24% 2	657
August	1st to 10th	Night	6% 1 16% 49% 23% 4	597
	11th to 20th	Night	9% 3 16% 39% 28% 5%	594
	21st to end	Night	8% 3 18% 41% 28% 3	660
September	1st to 10th	Night	9% 4 22% 34% 25% 6%	579
	11th to 20th	Night	12% 4 20% 34% 26% 4	650
	21st to end	Night	13% 9% 25% 31% 19% 3	760
October	1st to 10th	Night	14% 7% 29% 32% 16% 2	760
	11th to 20th	Night	22% 12% 30% 27% 9% 1	760
	21st to end	Night	23% 12% 33% 24% 8%	836
November	1st to 10th	Night	16% 11% 38% 24% 10% 1	780
	11th to 20th	Night	20% 16% 35% 23% 6%	800
	21st to end	Night	21% 14% 38% 21% 6%	930
December	1st to 10th	Night	21% 14% 39% 21% 5%	1000
	11th to 20th	Night	22% 14% 38% 21% 5%	1000
	21st to end	Night	28% 14% 35% 20% 2	1100

Page generated at 2025-09-01 00:51:04. Version ac07

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