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 2024-11-04.

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% 7% 37%	101378
	Night	13% 9% 4 8% 4 4 4 3 5% 6% 38%	97718
Transparency	Day	43% 7% 12% 23% 13% 1	104919
	Night	44% 6% 11% 23% 14% 2	102306
Seeing	Day	16% 11% 29% 25% 13% 5%	15865
	Night	17% 10% 32% 29% 11% 1	25702

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% 15% 8% 13% 7% 5% 4 3 3 5% 26%	2527
	11th to 20th	Day	10% 14% 6% 16% 5% 5% 5% 8% 8% 5% 19%	2530
	21st to end	Day	17% 18% 11% 11% 4 4 3 2 4 6% 19%	2858
February	1st to 10th	Day	11% 14% 5% 11% 5% 6% 5% 4 6% 6% 27%	1710
	11th to 20th	Day	13% 10% 3 7% 6% 5% 4 3 6% 6% 36%	1839
	21st to end	Day	12% 14% 4 11% 5% 5% 4 4 5% 6% 30%	1570
March	1st to 10th	Day	13% 9% 4 8% 5% 4 4 3 5% 6% 40%	3120
	11th to 20th	Day	11% 6% 3 8% 5% 5% 5% 4 7% 6% 41%	3292
	21st to end	Day	19% 13% 5% 11% 5% 4 4 3 5% 6% 24%	4165
April	1st to 10th	Day	16% 13% 4 8% 6% 4 4 3 5% 6% 31%	2280
	11th to 20th	Day	15% 10% 2 7% 6% 5% 4 4 6% 7% 33%	2254
	21st to end	Day	19% 8% 4 7% 5% 4 5% 4 5% 6% 33%	2375
May	1st to 10th	Day	15% 8% 3 7% 5% 4 5% 4 6% 7% 37%	4098
	11th to 20th	Day	14% 7% 2 6% 4 4 5% 3 5% 7% 44%	4399
	21st to end	Day	12% 6% 2 7% 4 5% 5% 5% 6% 9% 39%	4824
June	1st to 10th	Day	10% 5% 2 6% 5% 4 5% 4 7% 8% 45%	2660
	11th to 20th	Day	10% 6% 2 5% 3 5% 5% 4 6% 10% 44%	2660
	21st to end	Day	10% 6% 3 7% 5% 5% 5% 4 6% 10% 40%	2660
July	1st to 10th	Day	11% 8% 3 7% 5% 5% 5% 5% 8% 9% 34%	4440
	11th to 20th	Day	5% 4 1 5% 4 5% 5% 5% 7% 11% 47%	4440
	21st to end	Day	7% 6% 2 7% 5% 5% 5% 4 6% 9% 43%	4813
August	1st to 10th	Day	10% 6% 2 6% 4 5% 5% 5% 7% 8% 42%	4407
	11th to 20th	Day	10% 5% 2 6% 4 4 5% 4 6% 9% 44%	3735
	21st to end	Day	9% 7% 2 6% 4 4 4 4 6% 8% 47%	4180
September	1st to 10th	Day	12% 5% 2 6% 4 4 4 4 5% 8% 47%	2280
	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% 45%	1699
	11th to 20th	Day	13% 13% 4 9% 6% 3 5% 5% 5% 6% 32%	1710
	21st to end	Day	16% 12% 4 8% 5% 4 3 3 4 6% 36%	1582
December	1st to 10th	Day	13% 13% 5% 9% 6% 5% 5% 4 4 7% 28%	1520
	11th to 20th	Day	12% 13% 5% 9% 6% 5% 5% 4 6% 6% 27%	1918
	21st to end	Day	19% 20% 9% 14% 6% 4 4 3 4 4 14%	2783
January	1st to 10th	Night	12% 14% 7% 11% 6% 4 4 2 4 5% 31%	4428
	11th to 20th	Night	13% 15% 7% 15% 6% 4 3 6% 7% 5% 19%	4440
	21st to end	Night	18% 17% 8% 12% 5% 4 4 3 4 5% 21%	4806
February	1st to 10th	Night	12% 14% 4 9% 6% 6% 4 3 4 5% 32%	2470
	11th to 20th	Night	13% 9% 3 7% 6% 4 5% 4 4 7% 38%	2225
	21st to end	Night	18% 12% 3 7% 4 4 4 3 5% 6% 35%	1884
March	1st to 10th	Night	17% 11% 3 6% 3 4 4 3 4 6% 41%	3733
	11th to 20th	Night	9% 6% 2 6% 5% 4 5% 4 6% 8% 45%	3696
	21st to end	Night	22% 12% 5% 10% 4 4 4 3 3 4 28%	3466
April	1st to 10th	Night	16% 8% 3 8% 4 5% 4 4 4 6% 37%	1900
	11th to 20th	Night	18% 9% 4 8% 4 4 3 4 4 6% 38%	1875
	21st to end	Night	18% 7% 3 6% 4 5% 4 3 4 7% 38%	1804
May	1st to 10th	Night	15% 7% 3 6% 5% 5% 5% 4 6% 7% 37%	2795
	11th to 20th	Night	13% 7% 2 6% 4 4 5% 5% 6% 8% 40%	2506
	21st to end	Night	11% 8% 2 7% 5% 4 4 3 6% 7% 43%	2743
June	1st to 10th	Night	8% 5% 2 5% 5% 5% 4 5% 7% 8% 46%	1520
	11th to 20th	Night	10% 4 2 5% 4 5% 5% 3 6% 9% 46%	1520
	21st to end	Night	8% 6% 2 7% 4 4 4 4 7% 8% 44%	1520
July	1st to 10th	Night	14% 6% 3 7% 5% 5% 5% 4 6% 8% 39%	2530
	11th to 20th	Night	5% 4 1 5% 4 4 4 4 7% 8% 55%	2530
	21st to end	Night	6% 5% 2 6% 5% 4 5% 5% 7% 8% 47%	2750
August	1st to 10th	Night	8% 5% 2 6% 4 4 4 4 7% 8% 51%	2530
	11th to 20th	Night	8% 7% 2 5% 4 4 3 4 6% 8% 48%	3091
	21st to end	Night	8% 5% 2 5% 3 4 5% 4 6% 9% 50%	3476
September	1st to 10th	Night	9% 4 2 6% 4 3 3 3 5% 5% 55%	1900
	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% 3 4 3 3 4 6% 48%	2452
	11th to 20th	Night	13% 11% 4 8% 5% 4 4 4 4 6% 36%	2470
	21st to end	Night	16% 10% 3 8% 5% 4 4 3 5% 6% 37%	2593
December	1st to 10th	Night	15% 12% 4 9% 5% 5% 4 3 4 7% 31%	2660
	11th to 20th	Night	14% 13% 5% 10% 5% 5% 4 3 6% 5% 30%	3279
	21st to end	Night	21% 17% 6% 12% 5% 4 4 3 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	44% 7% 11% 25% 11% 3	3120
	11th to 20th	Day	40% 6% 10% 30% 12% 1	3292
	21st to end	Day	60% 7% 12% 14% 5% 1	4165
April	1st to 10th	Day	54% 6% 10% 19% 10%	2280
	11th to 20th	Day	48% 7% 10% 21% 12% 2	2254
	21st to end	Day	50% 6% 13% 17% 13% 1	2375
May	1st to 10th	Day	44% 7% 11% 23% 14%	4136
	11th to 20th	Day	39% 7% 11% 29% 14%	4403
	21st to end	Day	39% 8% 13% 20% 17% 3	4837
June	1st to 10th	Day	35% 7% 14% 26% 16% 2	2660
	11th to 20th	Day	34% 7% 13% 25% 18% 2	2660
	21st to end	Day	40% 8% 13% 21% 14% 2	2660
July	1st to 10th	Day	43% 7% 14% 25% 10% 2	4371
	11th to 20th	Day	27% 9% 17% 28% 18% 1	4365
	21st to end	Day	37% 8% 15% 18% 20% 2	4662
August	1st to 10th	Day	36% 9% 15% 28% 11% 1	4392
	11th to 20th	Day	33% 8% 14% 32% 12% 1	3771
	21st to end	Day	34% 7% 13% 25% 20% 2	4179
September	1st to 10th	Day	35% 8% 11% 27% 17% 2	2280
	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	42% 5% 13% 23% 16% 2	1699
	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	45% 5% 13% 23% 9% 4	3733
	11th to 20th	Night	36% 6% 12% 28% 17% 2	3696
	21st to end	Night	60% 6% 7% 17% 9% 1	3466
April	1st to 10th	Night	47% 7% 10% 20% 14% 1	1900
	11th to 20th	Night	48% 5% 6% 20% 17% 3	1875
	21st to end	Night	45% 6% 13% 21% 14% 2	1804
May	1st to 10th	Night	43% 7% 13% 18% 16% 2	2822
	11th to 20th	Night	39% 8% 12% 28% 11% 1	2507
	21st to end	Night	38% 8% 12% 17% 19% 6%	2751
June	1st to 10th	Night	32% 7% 14% 26% 18% 2	1520
	11th to 20th	Night	33% 8% 9% 25% 23% 3	1520
	21st to end	Night	35% 8% 16% 22% 18% 2	1520
July	1st to 10th	Night	42% 7% 15% 20% 12% 4	2490
	11th to 20th	Night	24% 6% 15% 35% 17% 2	2482
	21st to end	Night	30% 9% 16% 20% 22% 3	2662
August	1st to 10th	Night	29% 8% 14% 29% 18% 1	2521
	11th to 20th	Night	31% 8% 12% 32% 16%	3121
	21st to end	Night	29% 6% 13% 30% 18% 3	3476
September	1st to 10th	Night	31% 6% 11% 26% 24% 3	1900
	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	2452
	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% 33% 21% 7% 2	506
April	1st to 10th	Day	24% 15% 32% 25% 3	460
	11th to 20th	Day	20% 22% 25% 22% 9% 2	452
	21st to end	Day	23% 14% 29% 22% 11% 1	460
May	1st to 10th	Day	20% 13% 31% 27% 7% 2	466
	11th to 20th	Day	22% 10% 29% 25% 11% 3	675
	21st to end	Day	18% 9% 28% 24% 16% 5%	734
June	1st to 10th	Day	10% 8% 31% 30% 15% 6%	680
	11th to 20th	Day	13% 7% 33% 27% 16% 5%	680
	21st to end	Day	12% 10% 29% 30% 15% 4	680
July	1st to 10th	Day	8% 2 21% 31% 26% 12%	671
	11th to 20th	Day	9% 6% 25% 32% 19% 9%	668
	21st to end	Day	8% 3 25% 37% 20% 8%	739
August	1st to 10th	Day	9% 3 22% 32% 23% 10%	673
	11th to 20th	Day	10% 4 20% 27% 25% 14%	479
	21st to end	Day	10% 4 19% 33% 25% 10%	539
September	1st to 10th	Day	12% 8% 23% 27% 22% 7%	490
	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	15% 15% 38% 23% 7% 1	279
	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% 9% 30% 29% 7% 1	627
April	1st to 10th	Night	21% 11% 38% 24% 6%	570
	11th to 20th	Night	21% 13% 34% 24% 7% 1	563
	21st to end	Night	21% 9% 33% 28% 9% 1	570
May	1st to 10th	Night	19% 8% 38% 28% 6% 1	570
	11th to 20th	Night	17% 6% 32% 34% 11%	561
	21st to end	Night	14% 5% 27% 35% 19%	617
June	1st to 10th	Night	10% 4 28% 42% 15% 1	570
	11th to 20th	Night	12% 4 33% 38% 12% 1	570
	21st to end	Night	9% 5% 27% 41% 16% 1	570
July	1st to 10th	Night	10% 1 18% 39% 30% 2	555
	11th to 20th	Night	8% 3 23% 40% 23% 3	552
	21st to end	Night	6% 3 18% 47% 24% 2	624
August	1st to 10th	Night	6% 1 16% 49% 23% 4	567
	11th to 20th	Night	8% 3 16% 39% 28% 5%	564
	21st to end	Night	8% 3 17% 41% 28% 3	627
September	1st to 10th	Night	9% 5% 22% 34% 25% 5%	570
	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% 39% 24% 10% 1	755
	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

Page generated at 2024-11-01 21:47:44. Version ac07

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