Walnut Street Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Walnut Street Observatory clear sky chart from 2005-11-06 to 2026-02-01.

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	16% 8% 3 8% 5% 5% 5% 4 6% 7% 34%	127744
	Night	15% 9% 3 7% 5% 4 4 3 5% 7% 36%	124281
Transparency	Day	46% 7% 12% 21% 12% 2	131302
	Night	46% 7% 12% 21% 13% 2	128810
Seeing	Day	23% 18% 28% 19% 9% 3	16336
	Night	20% 15% 28% 26% 10% 1	25745

By Month

Percentage Cloud Cover NE (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% 5% 11% 5% 5% 5% 4 5% 6% 29%	3677
	11th to 20th	Day	12% 13% 6% 15% 6% 5% 5% 3 5% 6% 23%	3809
	21st to end	Day	16% 13% 4 9% 5% 5% 4 3 4 6% 32%	4565
February	1st to 10th	Day	15% 11% 4 9% 6% 6% 5% 4 5% 6% 28%	1800
	11th to 20th	Day	15% 10% 4 9% 7% 5% 4 4 5% 6% 31%	1823
	21st to end	Day	18% 11% 3 9% 5% 5% 4 4 4 6% 31%	1650
March	1st to 10th	Day	20% 10% 3 8% 5% 5% 4 3 4 5% 35%	4630
	11th to 20th	Day	12% 9% 3 7% 5% 5% 5% 3 6% 7% 40%	4980
	21st to end	Day	15% 8% 3 6% 5% 4 5% 3 6% 6% 39%	5689
April	1st to 10th	Day	18% 11% 3 7% 6% 5% 5% 3 4 6% 33%	2400
	11th to 20th	Day	16% 10% 3 6% 5% 5% 4 3 5% 7% 37%	2374
	21st to end	Day	16% 7% 2 8% 4 4 4 3 7% 7% 38%	2400
May	1st to 10th	Day	26% 7% 2 7% 4 5% 4 4 5% 7% 27%	6016
	11th to 20th	Day	22% 8% 3 8% 6% 4 5% 3 5% 7% 28%	6318
	21st to end	Day	20% 6% 2 6% 4 5% 5% 5% 7% 8% 32%	6949
June	1st to 10th	Day	13% 6% 3 7% 6% 5% 5% 4 7% 8% 35%	2800
	11th to 20th	Day	15% 6% 2 6% 5% 5% 5% 4 7% 8% 35%	2800
	21st to end	Day	12% 7% 2 6% 5% 4 5% 4 7% 10% 39%	2800
July	1st to 10th	Day	7% 6% 2 9% 6% 6% 6% 4 7% 8% 38%	6360
	11th to 20th	Day	8% 7% 3 8% 6% 6% 7% 5% 8% 11% 32%	6360
	21st to end	Day	11% 7% 3 7% 5% 5% 5% 4 6% 9% 38%	6745
August	1st to 10th	Day	19% 8% 3 7% 5% 5% 5% 4 6% 7% 31%	4234
	11th to 20th	Day	15% 6% 3 6% 5% 5% 6% 5% 6% 8% 34%	3987
	21st to end	Day	14% 6% 2 6% 4 4 4 3 5% 9% 43%	4312
September	1st to 10th	Day	15% 6% 2 6% 4 4 4 4 6% 6% 43%	2400
	11th to 20th	Day	12% 6% 2 5% 4 3 4 3 4 7% 50%	2400
	21st to end	Day	20% 8% 3 6% 5% 4 5% 3 5% 6% 36%	2133
October	1st to 10th	Day	17% 6% 2 5% 4 4 4 3 6% 8% 41%	2000
	11th to 20th	Day	17% 7% 3 6% 4 4 5% 4 5% 8% 38%	2000
	21st to end	Day	19% 8% 3 6% 4 4 5% 3 5% 7% 36%	2165
November	1st to 10th	Day	12% 9% 3 6% 3 4 4 3 5% 7% 46%	1851
	11th to 20th	Day	14% 10% 3 8% 4 5% 4 4 5% 7% 35%	1843
	21st to end	Day	18% 10% 3 6% 5% 5% 4 4 5% 7% 34%	1680
December	1st to 10th	Day	14% 9% 5% 10% 6% 6% 6% 4 5% 6% 30%	2680
	11th to 20th	Day	20% 12% 3 8% 4 3 4 2 5% 6% 32%	3066
	21st to end	Day	16% 11% 5% 12% 7% 5% 5% 3 6% 5% 25%	4048
January	1st to 10th	Night	11% 11% 4 12% 6% 6% 5% 4 5% 6% 30%	6489
	11th to 20th	Night	12% 11% 6% 11% 7% 5% 5% 4 5% 6% 28%	6291
	21st to end	Night	16% 9% 3 9% 5% 6% 5% 4 5% 6% 32%	6633
February	1st to 10th	Night	15% 12% 3 7% 6% 5% 4 3 6% 6% 34%	2601
	11th to 20th	Night	15% 11% 4 8% 6% 5% 5% 4 5% 6% 31%	2463
	21st to end	Night	17% 10% 3 9% 6% 4 4 4 6% 6% 31%	1980
March	1st to 10th	Night	17% 11% 4 9% 5% 4 3 3 4 5% 35%	5208
	11th to 20th	Night	10% 9% 3 7% 4 4 4 3 5% 6% 44%	4979
	21st to end	Night	15% 10% 3 7% 5% 4 4 3 4 5% 39%	5243
April	1st to 10th	Night	20% 10% 3 7% 6% 5% 4 3 5% 6% 32%	2000
	11th to 20th	Night	16% 7% 3 7% 5% 4 4 3 5% 7% 38%	1975
	21st to end	Night	17% 7% 2 6% 3 4 4 4 5% 6% 42%	2000
May	1st to 10th	Night	28% 9% 2 6% 4 4 4 3 5% 7% 27%	3952
	11th to 20th	Night	22% 8% 3 6% 4 5% 4 4 5% 6% 33%	3575
	21st to end	Night	20% 6% 3 6% 5% 4 5% 3 5% 8% 34%	3926
June	1st to 10th	Night	13% 7% 2 7% 5% 4 4 4 5% 8% 42%	1600
	11th to 20th	Night	14% 6% 2 6% 5% 3 5% 4 7% 10% 39%	1600
	21st to end	Night	12% 5% 2 5% 4 4 5% 4 7% 10% 41%	1600
July	1st to 10th	Night	10% 7% 3 8% 6% 6% 4 4 6% 9% 38%	3600
	11th to 20th	Night	12% 6% 3 8% 5% 5% 6% 5% 7% 9% 33%	3600
	21st to end	Night	12% 6% 3 6% 4 5% 4 3 6% 7% 43%	4102
August	1st to 10th	Night	18% 8% 3 7% 4 5% 4 4 6% 8% 34%	2915
	11th to 20th	Night	14% 8% 3 6% 4 4 4 4 5% 7% 41%	3029
	21st to end	Night	11% 5% 2 6% 3 4 5% 3 6% 8% 46%	3586
September	1st to 10th	Night	14% 6% 2 5% 4 5% 4 3 5% 6% 47%	2000
	11th to 20th	Night	11% 6% 2 5% 4 3 4 3 5% 7% 51%	2000
	21st to end	Night	17% 7% 3 6% 4 3 4 3 5% 7% 40%	2262
October	1st to 10th	Night	19% 7% 2 5% 3 3 4 2 4 7% 44%	2400
	11th to 20th	Night	15% 7% 2 6% 4 4 4 4 5% 6% 44%	2400
	21st to end	Night	20% 7% 3 6% 4 3 4 3 5% 7% 39%	2675
November	1st to 10th	Night	10% 7% 3 7% 4 4 4 3 6% 6% 47%	2667
	11th to 20th	Night	15% 10% 3 7% 5% 4 4 4 6% 6% 37%	2772
	21st to end	Night	14% 12% 3 7% 6% 4 4 4 5% 7% 34%	2940
December	1st to 10th	Night	14% 11% 5% 8% 5% 4 4 4 5% 7% 33%	4720
	11th to 20th	Night	21% 12% 3 7% 4 4 4 4 5% 5% 32%	5348
	21st to end	Night	19% 13% 5% 9% 5% 4 3 3 4 5% 29%	7150

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	53% 6% 11% 21% 8% 2	3678
	11th to 20th	Day	61% 5% 9% 15% 9% 2	3840
	21st to end	Day	54% 6% 10% 19% 11%	4565
February	1st to 10th	Day	55% 7% 12% 16% 10% 1	1800
	11th to 20th	Day	52% 7% 9% 23% 8% 1	1832
	21st to end	Day	54% 4 10% 21% 8% 2	1650
March	1st to 10th	Day	50% 6% 10% 20% 9% 5%	4633
	11th to 20th	Day	42% 6% 15% 25% 9% 4	4980
	21st to end	Day	44% 7% 11% 15% 20% 3	5689
April	1st to 10th	Day	52% 5% 9% 19% 13% 3	2400
	11th to 20th	Day	47% 6% 11% 21% 13% 3	2374
	21st to end	Day	44% 6% 10% 21% 15% 4	2400
May	1st to 10th	Day	55% 8% 11% 17% 8% 1	6025
	11th to 20th	Day	54% 7% 10% 18% 9% 1	6322
	21st to end	Day	45% 9% 12% 21% 12% 1	6949
June	1st to 10th	Day	43% 9% 12% 22% 13% 1	2800
	11th to 20th	Day	45% 8% 13% 21% 12% 2	2800
	21st to end	Day	39% 8% 14% 25% 12% 1	2800
July	1st to 10th	Day	42% 10% 12% 24% 13%	6290
	11th to 20th	Day	41% 11% 16% 21% 10% 1	6285
	21st to end	Day	41% 8% 12% 20% 19% 1	6551
August	1st to 10th	Day	50% 8% 13% 19% 9%	4221
	11th to 20th	Day	43% 9% 14% 26% 8%	4026
	21st to end	Day	37% 7% 12% 32% 12% 1	4312
September	1st to 10th	Day	40% 7% 12% 22% 17% 2	2400
	11th to 20th	Day	33% 6% 11% 25% 22% 2	2400
	21st to end	Day	49% 6% 9% 17% 17% 1	2133
October	1st to 10th	Day	38% 6% 10% 23% 18% 5%	3260
	11th to 20th	Day	36% 6% 9% 27% 18% 4	3240
	21st to end	Day	32% 5% 15% 28% 18% 2	3479
November	1st to 10th	Day	39% 6% 13% 23% 17% 2	1851
	11th to 20th	Day	48% 6% 11% 21% 12% 2	1843
	21st to end	Day	48% 6% 13% 24% 8% 1	1680
December	1st to 10th	Day	53% 7% 12% 21% 6% 1	2680
	11th to 20th	Day	52% 5% 15% 19% 8%	3066
	21st to end	Day	58% 7% 12% 14% 9%	4048
January	1st to 10th	Night	50% 8% 11% 19% 10% 2	6489
	11th to 20th	Night	54% 8% 12% 15% 8% 4	6335
	21st to end	Night	50% 7% 10% 17% 14% 2	6633
February	1st to 10th	Night	50% 7% 13% 22% 8% 1	2601
	11th to 20th	Night	53% 5% 11% 18% 11% 2	2465
	21st to end	Night	50% 6% 10% 19% 12% 2	1980
March	1st to 10th	Night	51% 5% 11% 19% 11% 4	5218
	11th to 20th	Night	36% 7% 18% 25% 9% 4	4980
	21st to end	Night	46% 5% 8% 17% 20% 4	5243
April	1st to 10th	Night	52% 5% 10% 19% 12% 2	2000
	11th to 20th	Night	44% 7% 11% 20% 16% 1	1975
	21st to end	Night	42% 6% 10% 21% 17% 3	2000
May	1st to 10th	Night	56% 6% 10% 18% 8% 2	3959
	11th to 20th	Night	51% 9% 12% 15% 12% 1	3576
	21st to end	Night	47% 9% 11% 20% 11% 2	3927
June	1st to 10th	Night	40% 7% 11% 25% 16% 1	1600
	11th to 20th	Night	39% 9% 15% 24% 12% 1	1600
	21st to end	Night	35% 9% 14% 28% 13% 1	1600
July	1st to 10th	Night	42% 8% 13% 21% 15% 1	3560
	11th to 20th	Night	44% 10% 15% 21% 10% 1	3552
	21st to end	Night	36% 9% 12% 24% 19% 1	3982
August	1st to 10th	Night	47% 9% 15% 20% 8% 1	2904
	11th to 20th	Night	41% 7% 14% 23% 14%	3056
	21st to end	Night	33% 7% 12% 32% 15% 1	3586
September	1st to 10th	Night	38% 7% 11% 26% 16% 3	2000
	11th to 20th	Night	33% 6% 11% 27% 22% 2	2000
	21st to end	Night	43% 6% 10% 22% 18% 1	2262
October	1st to 10th	Night	43% 6% 10% 21% 19% 1	3920
	11th to 20th	Night	33% 6% 11% 27% 20% 4	3899
	21st to end	Night	32% 6% 15% 28% 19% 1	4311
November	1st to 10th	Night	37% 6% 13% 23% 18% 3	2667
	11th to 20th	Night	47% 5% 11% 21% 13% 3	2772
	21st to end	Night	48% 6% 13% 22% 10% 1	2940
December	1st to 10th	Night	50% 5% 14% 25% 5%	4720
	11th to 20th	Night	53% 5% 13% 19% 10% 1	5348
	21st to end	Night	56% 5% 11% 17% 10% 1	7150

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	34% 25% 28% 10% 1 1	220
	11th to 20th	Day	42% 25% 26% 7% 1	257
	21st to end	Day	27% 33% 26% 10% 3	363
February	1st to 10th	Day	33% 30% 26% 9% 2	300
	11th to 20th	Day	33% 28% 30% 8% 1	370
	21st to end	Day	29% 26% 31% 13% 1	414
March	1st to 10th	Day	33% 21% 30% 14% 3	500
	11th to 20th	Day	29% 26% 29% 12% 3 1	500
	21st to end	Day	29% 26% 26% 13% 4 2	550
April	1st to 10th	Day	33% 23% 27% 12% 4 1	500
	11th to 20th	Day	28% 27% 24% 13% 7% 2	492
	21st to end	Day	27% 20% 32% 13% 6% 2	500
May	1st to 10th	Day	29% 21% 30% 11% 6% 3	503
	11th to 20th	Day	23% 20% 30% 18% 7% 2	527
	21st to end	Day	22% 12% 26% 23% 14% 3	573
June	1st to 10th	Day	19% 14% 29% 26% 9% 3	530
	11th to 20th	Day	19% 17% 27% 25% 11% 2	530
	21st to end	Day	15% 13% 32% 27% 11% 1	530
July	1st to 10th	Day	13% 8% 27% 30% 17% 5%	525
	11th to 20th	Day	9% 8% 31% 30% 18% 4	524
	21st to end	Day	14% 9% 32% 30% 13% 3	576
August	1st to 10th	Day	13% 9% 24% 28% 18% 7%	526
	11th to 20th	Day	14% 9% 24% 29% 17% 7%	519
	21st to end	Day	13% 10% 27% 27% 14% 8%	583
September	1st to 10th	Day	17% 12% 23% 23% 17% 9%	530
	11th to 20th	Day	12% 7% 25% 25% 22% 9%	530
	21st to end	Day	23% 15% 22% 21% 14% 4	530
October	1st to 10th	Day	22% 11% 23% 23% 15% 6%	530
	11th to 20th	Day	22% 20% 27% 19% 10% 2	530
	21st to end	Day	26% 16% 30% 20% 7% 1	564
November	1st to 10th	Day	23% 18% 29% 19% 8% 2	425
	11th to 20th	Day	28% 19% 31% 18% 3	383
	21st to end	Day	33% 25% 31% 9% 2	220
December	1st to 10th	Day	36% 26% 25% 8% 4	220
	11th to 20th	Day	35% 23% 29% 11% 2	220
	21st to end	Day	29% 26% 30% 12% 3	242
January	1st to 10th	Night	21% 25% 36% 15% 3	840
	11th to 20th	Night	23% 23% 31% 20% 2	840
	21st to end	Night	19% 24% 38% 17% 2	924
February	1st to 10th	Night	24% 22% 32% 21% 2	801
	11th to 20th	Night	25% 24% 37% 13% 1	783
	21st to end	Night	26% 22% 35% 15% 3	660
March	1st to 10th	Night	25% 21% 33% 19% 3	800
	11th to 20th	Night	23% 22% 31% 19% 4 1	800
	21st to end	Night	23% 20% 35% 18% 4	880
April	1st to 10th	Night	26% 19% 32% 18% 4	800
	11th to 20th	Night	21% 19% 31% 21% 7%	791
	21st to end	Night	20% 19% 28% 28% 6%	800
May	1st to 10th	Night	26% 14% 26% 24% 10%	564
	11th to 20th	Night	16% 13% 32% 30% 9%	394
	21st to end	Night	14% 12% 24% 36% 14%	433
June	1st to 10th	Night	15% 6% 25% 39% 16%	400
	11th to 20th	Night	14% 8% 27% 39% 13%	400
	21st to end	Night	12% 6% 25% 45% 13% 1	400
July	1st to 10th	Night	13% 5% 22% 42% 17% 2	390
	11th to 20th	Night	10% 4 22% 44% 20% 1	388
	21st to end	Night	12% 5% 21% 42% 19% 2	513
August	1st to 10th	Night	15% 2 15% 42% 24% 2	597
	11th to 20th	Night	13% 4 14% 39% 29% 1	683
	21st to end	Night	13% 5% 18% 36% 25% 2	880
September	1st to 10th	Night	16% 5% 23% 34% 20% 2	800
	11th to 20th	Night	12% 6% 21% 32% 25% 5%	800
	21st to end	Night	19% 10% 21% 33% 15% 2	800
October	1st to 10th	Night	22% 11% 19% 26% 19% 3	800
	11th to 20th	Night	20% 13% 27% 32% 8% 1	800
	21st to end	Night	23% 14% 29% 27% 7%	880
November	1st to 10th	Night	16% 15% 33% 25% 10% 1	820
	11th to 20th	Night	22% 19% 28% 24% 6% 1	840
	21st to end	Night	23% 19% 33% 21% 4	840
December	1st to 10th	Night	26% 22% 31% 18% 4	840
	11th to 20th	Night	24% 20% 32% 21% 4	840
	21st to end	Night	26% 18% 34% 20% 2	924

Page generated at 2026-01-29 01:21:46. Version ac07

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