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 2024-03-28.

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	13% 9% 4 7% 6% 3 5% 5% 5% 7% 36%	78674
	Night	14% 9% 4 7% 5% 3 4 4 5% 7% 37%	81373
Transparency	Day	46% 7% 12% 22% 12% 1	78375
	Night	47% 6% 11% 21% 13% 2	81193
Seeing	Day	16% 11% 29% 25% 13% 5%	14918
	Night	17% 10% 32% 29% 11% 1	24987

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	11% 14% 9% 12% 8% 3 5% 4 4 5% 26%	2527
	11th to 20th	Day	11% 14% 6% 14% 6% 3 5% 9% 7% 5% 19%	2530
	21st to end	Day	17% 17% 11% 10% 5% 3 4 3 4 6% 19%	2858
February	1st to 10th	Day	12% 13% 5% 9% 7% 4 5% 5% 6% 6% 28%	1710
	11th to 20th	Day	14% 9% 4 7% 7% 4 5% 4 5% 6% 36%	1839
	21st to end	Day	12% 13% 5% 9% 7% 3 4 5% 4 7% 30%	1570
March	1st to 10th	Day	14% 9% 5% 9% 6% 3 5% 4 5% 6% 34%	1900
	11th to 20th	Day	12% 8% 4 7% 5% 4 4 4 6% 8% 38%	1967
	21st to end	Day	18% 12% 5% 7% 7% 4 4 4 5% 6% 27%	2460
April	1st to 10th	Day	16% 11% 5% 8% 6% 3 4 4 5% 6% 31%	2160
	11th to 20th	Day	15% 10% 3 6% 6% 4 5% 5% 6% 7% 33%	2134
	21st to end	Day	20% 8% 4 6% 6% 3 5% 5% 5% 6% 32%	2250
May	1st to 10th	Day	17% 8% 3 7% 7% 3 5% 5% 6% 7% 31%	2342
	11th to 20th	Day	18% 7% 3 6% 5% 3 5% 5% 6% 8% 35%	2479
	21st to end	Day	14% 6% 3 6% 6% 3 6% 5% 5% 8% 38%	2725
June	1st to 10th	Day	10% 4 2 6% 5% 3 5% 5% 7% 8% 45%	2520
	11th to 20th	Day	11% 6% 2 4 4 3 4 4 6% 10% 44%	2520
	21st to end	Day	11% 5% 3 6% 6% 4 5% 6% 6% 10% 40%	2520
July	1st to 10th	Day	7% 4 2 4 5% 3 4 5% 8% 10% 48%	2520
	11th to 20th	Day	8% 4 2 5% 5% 4 5% 5% 7% 10% 45%	2520
	21st to end	Day	7% 4 2 4 6% 4 5% 6% 6% 9% 47%	2743
August	1st to 10th	Day	8% 4 2 5% 5% 4 6% 7% 7% 9% 43%	2505
	11th to 20th	Day	9% 5% 3 6% 5% 4 5% 5% 6% 9% 43%	2131
	21st to end	Day	9% 5% 2 4 5% 3 5% 5% 6% 8% 47%	2376
September	1st to 10th	Day	13% 5% 2 5% 5% 3 4 4 6% 8% 45%	2160
	11th to 20th	Day	12% 4 2 3 4 3 3 3 5% 8% 52%	2066
	21st to end	Day	11% 6% 3 6% 6% 4 3 4 6% 8% 43%	1980
October	1st to 10th	Day	14% 7% 3 5% 5% 3 4 4 4 6% 45%	1895
	11th to 20th	Day	20% 9% 3 7% 6% 3 4 4 5% 7% 33%	1800
	21st to end	Day	19% 10% 3 8% 6% 3 3 4 4 5% 36%	1782
November	1st to 10th	Day	12% 8% 4 5% 5% 3 4 4 5% 5% 45%	1672
	11th to 20th	Day	14% 11% 5% 8% 6% 3 4 5% 5% 6% 32%	1710
	21st to end	Day	16% 11% 4 7% 6% 3 4 3 3 6% 37%	1582
December	1st to 10th	Day	14% 12% 6% 8% 7% 4 6% 5% 4 7% 29%	1520
	11th to 20th	Day	13% 13% 6% 8% 7% 4 5% 6% 6% 7% 27%	1918
	21st to end	Day	19% 20% 10% 12% 7% 3 4 3 3 4 15%	2783
January	1st to 10th	Night	12% 13% 8% 10% 7% 3 4 3 4 5% 31%	4428
	11th to 20th	Night	13% 14% 7% 13% 8% 3 4 6% 7% 5% 19%	4440
	21st to end	Night	18% 16% 9% 11% 6% 3 3 4 4 5% 22%	4806
February	1st to 10th	Night	13% 13% 5% 8% 6% 5% 5% 4 4 6% 32%	2470
	11th to 20th	Night	13% 8% 4 6% 6% 3 5% 6% 4 7% 38%	2225
	21st to end	Night	18% 11% 4 6% 4 3 4 4 5% 6% 35%	1884
March	1st to 10th	Night	16% 10% 4 7% 4 3 4 4 5% 7% 36%	2280
	11th to 20th	Night	11% 7% 3 5% 6% 3 5% 6% 5% 8% 41%	2228
	21st to end	Night	19% 10% 5% 7% 5% 4 5% 4 4 5% 32%	2050
April	1st to 10th	Night	16% 7% 4 8% 5% 3 5% 5% 5% 6% 37%	1800
	11th to 20th	Night	19% 8% 4 7% 5% 3 3 4 4 6% 37%	1775
	21st to end	Night	18% 7% 3 6% 5% 3 4 5% 4 7% 38%	1710
May	1st to 10th	Night	14% 8% 3 6% 6% 4 5% 5% 6% 9% 35%	1615
	11th to 20th	Night	16% 8% 3 6% 4 3 4 4 5% 8% 38%	1416
	21st to end	Night	11% 6% 3 6% 6% 3 5% 4 6% 9% 41%	1551
June	1st to 10th	Night	8% 5% 2 4 5% 4 4 6% 7% 8% 48%	1440
	11th to 20th	Night	11% 4 3 4 4 3 5% 5% 6% 9% 46%	1440
	21st to end	Night	8% 5% 3 6% 5% 3 5% 4 6% 9% 46%	1440
July	1st to 10th	Night	7% 3 2 3 4 3 4 5% 6% 11% 50%	1440
	11th to 20th	Night	8% 4 2 4 4 3 4 5% 6% 8% 52%	1440
	21st to end	Night	6% 4 1 4 4 3 4 6% 7% 9% 52%	1575
August	1st to 10th	Night	5% 4 2 5% 4 3 4 6% 8% 8% 51%	1432
	11th to 20th	Night	8% 4 2 4 4 3 3 5% 6% 8% 53%	1767
	21st to end	Night	8% 4 3 3 4 2 4 4 6% 8% 53%	1980
September	1st to 10th	Night	9% 5% 2 5% 6% 2 4 3 5% 6% 54%	1800
	11th to 20th	Night	12% 4 2 4 4 2 3 4 3 7% 54%	1875
	21st to end	Night	12% 6% 3 5% 5% 3 4 3 4 7% 48%	1980
October	1st to 10th	Night	13% 5% 3 5% 4 3 3 4 5% 7% 49%	2060
	11th to 20th	Night	17% 10% 2 5% 4 2 4 2 4 6% 43%	2160
	21st to end	Night	19% 9% 2 5% 5% 2 4 4 5% 7% 37%	2574
November	1st to 10th	Night	11% 8% 3 6% 4 3 4 3 5% 6% 48%	2406
	11th to 20th	Night	13% 10% 5% 7% 6% 3 5% 4 4 6% 37%	2470
	21st to end	Night	16% 9% 4 7% 5% 3 4 4 4 6% 37%	2593
December	1st to 10th	Night	15% 11% 4 8% 7% 4 5% 4 4 7% 31%	2660
	11th to 20th	Night	14% 12% 5% 9% 6% 4 5% 4 6% 6% 30%	3279
	21st to end	Night	21% 16% 7% 11% 6% 3 4 4 4 5% 18%	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	61% 5% 11% 16% 6%	2528
	11th to 20th	Day	61% 7% 12% 13% 7%	2530
	21st to end	Day	67% 4 9% 15% 6%	2858
February	1st to 10th	Day	55% 6% 14% 15% 9% 1	1710
	11th to 20th	Day	48% 5% 10% 23% 11% 2	1849
	21st to end	Day	54% 5% 10% 19% 11% 2	1570
March	1st to 10th	Day	51% 7% 12% 21% 8% 2	1900
	11th to 20th	Day	45% 6% 12% 23% 12% 1	1967
	21st to end	Day	57% 6% 12% 17% 6% 1	2460
April	1st to 10th	Day	54% 6% 11% 18% 10%	2160
	11th to 20th	Day	49% 7% 10% 20% 12% 2	2134
	21st to end	Day	51% 6% 13% 17% 12% 1	2250
May	1st to 10th	Day	50% 7% 14% 23% 6%	2342
	11th to 20th	Day	47% 7% 13% 22% 10%	2483
	21st to end	Day	44% 8% 12% 20% 14% 1	2725
June	1st to 10th	Day	35% 7% 14% 26% 16% 1	2520
	11th to 20th	Day	35% 7% 13% 25% 18% 2	2520
	21st to end	Day	40% 8% 14% 22% 15% 2	2520
July	1st to 10th	Day	30% 8% 15% 32% 13% 3	2451
	11th to 20th	Day	32% 9% 16% 24% 17% 1	2445
	21st to end	Day	31% 8% 16% 26% 16% 3	2591
August	1st to 10th	Day	33% 10% 16% 28% 11% 2	2490
	11th to 20th	Day	36% 8% 17% 26% 12% 1	2131
	21st to end	Day	33% 8% 12% 25% 19% 3	2376
September	1st to 10th	Day	36% 8% 11% 25% 17% 2	2160
	11th to 20th	Day	31% 6% 11% 33% 16% 2	2066
	21st to end	Day	39% 7% 10% 29% 15% 1	1980
October	1st to 10th	Day	40% 5% 11% 28% 15% 1	1895
	11th to 20th	Day	50% 5% 13% 20% 11% 1	1800
	21st to end	Day	51% 5% 10% 23% 11% 1	1782
November	1st to 10th	Day	42% 4 13% 23% 16% 2	1672
	11th to 20th	Day	52% 6% 12% 20% 9% 1	1710
	21st to end	Day	50% 5% 14% 24% 8%	1582
December	1st to 10th	Day	55% 6% 15% 16% 8% 1	1520
	11th to 20th	Day	54% 6% 15% 16% 7% 2	1915
	21st to end	Day	75% 4 7% 9% 5%	2783
January	1st to 10th	Night	56% 5% 10% 19% 9% 1	4428
	11th to 20th	Night	64% 5% 9% 13% 8%	4440
	21st to end	Night	66% 5% 10% 13% 6% 1	4806
February	1st to 10th	Night	53% 5% 10% 18% 12% 2	2470
	11th to 20th	Night	46% 6% 12% 21% 15% 1	2226
	21st to end	Night	50% 5% 10% 20% 11% 4	1884
March	1st to 10th	Night	49% 5% 14% 21% 9% 2	2280
	11th to 20th	Night	41% 6% 13% 22% 15% 2	2228
	21st to end	Night	55% 6% 9% 19% 10% 1	2050
April	1st to 10th	Night	47% 7% 11% 20% 14% 1	1800
	11th to 20th	Night	49% 5% 6% 20% 17% 4	1775
	21st to end	Night	46% 5% 13% 20% 14% 2	1710
May	1st to 10th	Night	46% 8% 13% 21% 12% 1	1615
	11th to 20th	Night	44% 6% 12% 23% 13% 2	1417
	21st to end	Night	39% 7% 14% 22% 16% 2	1552
June	1st to 10th	Night	32% 7% 14% 27% 19% 2	1440
	11th to 20th	Night	34% 8% 9% 24% 23% 2	1440
	21st to end	Night	34% 8% 16% 23% 17% 2	1440
July	1st to 10th	Night	26% 8% 17% 29% 17% 4	1400
	11th to 20th	Night	29% 6% 12% 31% 19% 3	1392
	21st to end	Night	25% 8% 17% 28% 17% 5%	1487
August	1st to 10th	Night	26% 10% 15% 30% 16% 2	1423
	11th to 20th	Night	28% 9% 14% 30% 18% 1	1767
	21st to end	Night	28% 6% 13% 29% 20% 4	1980
September	1st to 10th	Night	32% 6% 11% 25% 23% 3	1800
	11th to 20th	Night	31% 5% 9% 34% 19% 2	1875
	21st to end	Night	37% 4 9% 31% 17% 1	1980
October	1st to 10th	Night	36% 6% 9% 29% 19% 2	2060
	11th to 20th	Night	43% 4 12% 22% 18% 1	2160
	21st to end	Night	47% 5% 12% 24% 12% 1	2574
November	1st to 10th	Night	38% 5% 10% 27% 18% 3	2406
	11th to 20th	Night	48% 5% 13% 21% 11% 1	2470
	21st to end	Night	49% 5% 13% 21% 11% 1	2593
December	1st to 10th	Night	53% 6% 11% 19% 9% 1	2660
	11th to 20th	Night	54% 6% 12% 18% 9% 1	3281
	21st to end	Night	67% 5% 9% 13% 5%	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	17% 19% 31% 27% 6% 1	270
	11th to 20th	Day	20% 18% 37% 23% 2 1	270
	21st to end	Day	20% 13% 39% 21% 6% 1	297
February	1st to 10th	Day	13% 19% 45% 17% 6%	270
	11th to 20th	Day	19% 25% 34% 17% 5%	270
	21st to end	Day	22% 20% 42% 14% 3	225
March	1st to 10th	Day	17% 18% 44% 16% 4	270
	11th to 20th	Day	21% 23% 32% 17% 5% 1	275
	21st to end	Day	25% 13% 32% 20% 7% 2	490
April	1st to 10th	Day	24% 14% 32% 27% 4	420
	11th to 20th	Day	19% 21% 26% 22% 9% 3	412
	21st to end	Day	23% 14% 29% 22% 12% 1	420
May	1st to 10th	Day	21% 12% 31% 27% 7% 2	425
	11th to 20th	Day	23% 10% 28% 25% 11% 3	625
	21st to end	Day	18% 8% 28% 24% 17% 6%	679
June	1st to 10th	Day	10% 7% 30% 31% 16% 6%	630
	11th to 20th	Day	13% 7% 31% 27% 17% 6%	630
	21st to end	Day	12% 9% 27% 31% 15% 4	630
July	1st to 10th	Day	8% 2 20% 31% 27% 13%	621
	11th to 20th	Day	9% 6% 25% 33% 19% 9%	618
	21st to end	Day	8% 3 26% 37% 19% 7%	684
August	1st to 10th	Day	9% 2 21% 32% 25% 11%	624
	11th to 20th	Day	9% 5% 18% 26% 26% 15%	439
	21st to end	Day	10% 4 20% 33% 23% 10%	495
September	1st to 10th	Day	13% 8% 22% 27% 22% 7%	450
	11th to 20th	Day	11% 4 19% 33% 23% 11%	450
	21st to end	Day	14% 12% 25% 24% 15% 9%	450
October	1st to 10th	Day	16% 9% 29% 24% 17% 5%	365
	11th to 20th	Day	25% 18% 30% 19% 7%	270
	21st to end	Day	29% 15% 32% 17% 6% 1	297
November	1st to 10th	Day	15% 16% 38% 23% 7% 1	270
	11th to 20th	Day	20% 16% 39% 17% 9%	270
	21st to end	Day	23% 12% 40% 21% 3	270
December	1st to 10th	Day	16% 22% 38% 17% 6% 1	270
	11th to 20th	Day	19% 17% 36% 21% 6%	270
	21st to end	Day	24% 22% 37% 15% 2	297
January	1st to 10th	Night	18% 16% 38% 23% 5%	950
	11th to 20th	Night	20% 16% 35% 21% 6% 1	950
	21st to end	Night	19% 12% 38% 26% 6%	1045
February	1st to 10th	Night	20% 12% 38% 24% 6%	950
	11th to 20th	Night	19% 15% 38% 25% 4	750
	21st to end	Night	24% 13% 39% 20% 3	628
March	1st to 10th	Night	21% 13% 35% 23% 7% 1	760
	11th to 20th	Night	15% 12% 37% 27% 8%	713
	21st to end	Night	24% 8% 30% 29% 7% 1	615
April	1st to 10th	Night	21% 11% 37% 24% 6%	540
	11th to 20th	Night	22% 11% 34% 25% 7% 1	533
	21st to end	Night	21% 8% 33% 28% 10% 1	540
May	1st to 10th	Night	19% 8% 38% 28% 7% 1	540
	11th to 20th	Night	18% 6% 32% 33% 11%	531
	21st to end	Night	14% 4 27% 35% 20% 1	584
June	1st to 10th	Night	10% 4 28% 42% 15% 1	540
	11th to 20th	Night	13% 4 32% 38% 13% 1	540
	21st to end	Night	9% 5% 27% 41% 17% 1	540
July	1st to 10th	Night	9% 1 17% 40% 31% 2	525
	11th to 20th	Night	8% 3 23% 41% 23% 2	522
	21st to end	Night	6% 3 18% 47% 24% 2	591
August	1st to 10th	Night	6% 1 15% 50% 24% 4	537
	11th to 20th	Night	8% 3 16% 39% 29% 5%	534
	21st to end	Night	9% 3 17% 40% 28% 3	594
September	1st to 10th	Night	10% 5% 22% 34% 24% 5%	540
	11th to 20th	Night	13% 4 21% 33% 26% 4	615
	21st to end	Night	14% 9% 24% 30% 20% 3	720
October	1st to 10th	Night	15% 7% 29% 32% 16% 2	720
	11th to 20th	Night	23% 12% 30% 26% 8% 1	720
	21st to end	Night	24% 12% 33% 24% 7%	792
November	1st to 10th	Night	15% 11% 39% 24% 10% 1	740
	11th to 20th	Night	19% 16% 35% 24% 6%	760
	21st to end	Night	21% 13% 38% 22% 7%	883
December	1st to 10th	Night	22% 13% 39% 21% 5%	950
	11th to 20th	Night	22% 13% 38% 21% 5%	950
	21st to end	Night	28% 15% 36% 20% 2	1045

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