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 2024-11-28.

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	16% 9% 3 7% 5% 5% 5% 4 6% 7% 35%	101619
	Night	16% 9% 3 7% 5% 4 4 3 5% 7% 36%	97919
Transparency	Day	46% 7% 12% 22% 12% 2	105194
	Night	45% 7% 12% 21% 13% 2	102433
Seeing	Day	22% 17% 28% 20% 10% 3	14900
	Night	20% 15% 29% 26% 10% 1	24212

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	14% 14% 5% 8% 5% 5% 5% 3 4 5% 34%	2517
	11th to 20th	Day	13% 14% 7% 15% 5% 5% 5% 3 4 5% 22%	2620
	21st to end	Day	20% 16% 5% 10% 5% 4 4 3 4 6% 24%	3124
February	1st to 10th	Day	16% 11% 4 8% 6% 6% 5% 4 5% 6% 28%	1710
	11th to 20th	Day	14% 10% 3 9% 7% 5% 5% 4 5% 6% 32%	1729
	21st to end	Day	18% 12% 3 9% 5% 5% 4 4 4 6% 31%	1570
March	1st to 10th	Day	24% 11% 3 8% 5% 4 4 3 4 5% 30%	3228
	11th to 20th	Day	11% 9% 3 8% 6% 5% 4 4 6% 7% 39%	3480
	21st to end	Day	19% 10% 3 6% 4 3 4 3 4 6% 39%	3969
April	1st to 10th	Day	17% 11% 2 7% 6% 5% 4 3 4 6% 34%	2280
	11th to 20th	Day	16% 10% 3 6% 5% 5% 4 3 5% 7% 37%	2254
	21st to end	Day	16% 6% 2 8% 4 4 4 3 7% 7% 37%	2280
May	1st to 10th	Day	28% 6% 2 6% 4 4 4 3 5% 6% 30%	4194
	11th to 20th	Day	22% 9% 3 9% 6% 4 5% 4 5% 7% 27%	4398
	21st to end	Day	12% 6% 2 5% 4 4 5% 4 7% 9% 42%	4837
June	1st to 10th	Day	13% 6% 3 7% 6% 5% 5% 4 7% 9% 35%	2660
	11th to 20th	Day	14% 6% 3 7% 5% 5% 5% 4 7% 9% 36%	2660
	21st to end	Day	12% 7% 2 6% 5% 5% 5% 5% 7% 10% 39%	2660
July	1st to 10th	Day	8% 6% 2 8% 6% 6% 6% 5% 7% 9% 36%	4440
	11th to 20th	Day	8% 6% 2 6% 5% 5% 6% 4 7% 12% 39%	4440
	21st to end	Day	11% 8% 4 8% 6% 6% 6% 5% 6% 8% 33%	4689
August	1st to 10th	Day	19% 8% 3 7% 5% 5% 5% 4 6% 7% 30%	4104
	11th to 20th	Day	15% 6% 3 6% 5% 5% 6% 5% 6% 8% 35%	3863
	21st to end	Day	14% 6% 2 6% 4 4 4 3 5% 9% 43%	4180
September	1st to 10th	Day	15% 7% 2 6% 4 4 4 4 6% 6% 43%	2280
	11th to 20th	Day	12% 6% 2 5% 4 3 4 3 4 7% 49%	2280
	21st to end	Day	20% 8% 3 6% 5% 4 4 3 5% 6% 37%	2026
October	1st to 10th	Day	18% 6% 2 5% 4 4 4 3 6% 8% 40%	1900
	11th to 20th	Day	17% 7% 3 6% 4 4 5% 3 5% 7% 39%	1900
	21st to end	Day	20% 8% 3 6% 4 4 4 3 5% 7% 36%	2057
November	1st to 10th	Day	12% 9% 3 6% 3 3 4 3 4 6% 46%	1761
	11th to 20th	Day	15% 10% 3 8% 4 5% 4 4 5% 7% 36%	1755
	21st to end	Day	18% 9% 3 6% 5% 4 4 4 5% 7% 35%	1576
December	1st to 10th	Day	19% 11% 5% 9% 6% 6% 4 4 4 6% 27%	1520
	11th to 20th	Day	20% 12% 3 7% 4 3 4 2 6% 7% 31%	1906
	21st to end	Day	19% 12% 5% 12% 8% 6% 5% 3 5% 4 21%	2772
January	1st to 10th	Night	14% 13% 4 9% 5% 5% 4 4 4 6% 32%	4429
	11th to 20th	Night	15% 11% 6% 11% 6% 5% 5% 3 5% 6% 27%	4335
	21st to end	Night	22% 13% 4 10% 5% 4 4 3 4 5% 25%	4532
February	1st to 10th	Night	15% 11% 3 7% 6% 5% 4 3 6% 6% 34%	2470
	11th to 20th	Night	14% 11% 4 8% 6% 5% 5% 4 5% 6% 31%	2337
	21st to end	Night	18% 10% 3 8% 6% 4 4 4 6% 6% 31%	1884
March	1st to 10th	Night	22% 13% 4 7% 4 4 3 3 4 5% 32%	3610
	11th to 20th	Night	11% 11% 3 7% 5% 5% 4 4 5% 6% 39%	3479
	21st to end	Night	14% 10% 3 7% 5% 4 4 3 3 5% 42%	3663
April	1st to 10th	Night	19% 10% 3 7% 5% 5% 4 3 6% 6% 32%	1900
	11th to 20th	Night	16% 7% 3 7% 5% 4 4 3 5% 7% 39%	1875
	21st to end	Night	17% 7% 2 6% 3 4 4 4 5% 6% 42%	1900
May	1st to 10th	Night	28% 8% 2 6% 4 4 4 3 4 7% 30%	2760
	11th to 20th	Night	22% 9% 3 6% 5% 5% 4 4 5% 7% 31%	2495
	21st to end	Night	11% 6% 3 7% 5% 5% 5% 4 5% 10% 39%	2738
June	1st to 10th	Night	13% 6% 2 6% 5% 4 4 4 5% 8% 43%	1520
	11th to 20th	Night	12% 5% 2 6% 5% 3 5% 4 7% 11% 39%	1520
	21st to end	Night	12% 5% 2 5% 4 4 5% 4 7% 10% 41%	1520
July	1st to 10th	Night	9% 6% 3 8% 6% 6% 5% 4 7% 8% 40%	2520
	11th to 20th	Night	8% 5% 2 7% 5% 5% 7% 5% 8% 10% 39%	2520
	21st to end	Night	14% 7% 2 6% 5% 5% 4 4 6% 8% 39%	2859
August	1st to 10th	Night	19% 8% 3 7% 4 4 4 4 6% 8% 33%	2825
	11th to 20th	Night	14% 8% 4 6% 4 4 4 4 5% 7% 41%	2934
	21st to end	Night	11% 5% 2 6% 3 4 5% 3 6% 8% 46%	3476
September	1st to 10th	Night	14% 6% 2 5% 4 5% 4 3 5% 6% 46%	1900
	11th to 20th	Night	12% 6% 2 5% 4 3 4 3 5% 7% 51%	1900
	21st to end	Night	17% 7% 3 5% 4 3 4 3 5% 6% 42%	2149
October	1st to 10th	Night	19% 8% 2 5% 3 3 4 2 5% 7% 42%	2280
	11th to 20th	Night	15% 7% 2 6% 4 4 4 4 5% 6% 44%	2280
	21st to end	Night	20% 7% 3 6% 4 4 4 3 5% 7% 38%	2541
November	1st to 10th	Night	10% 7% 3 7% 4 4 4 3 6% 6% 48%	2537
	11th to 20th	Night	15% 10% 3 6% 5% 4 4 4 6% 6% 37%	2640
	21st to end	Night	14% 12% 3 7% 5% 4 4 4 5% 7% 34%	2759
December	1st to 10th	Night	18% 11% 4 8% 5% 5% 4 4 6% 6% 28%	2660
	11th to 20th	Night	17% 12% 3 8% 5% 5% 5% 4 6% 6% 30%	3288
	21st to end	Night	20% 12% 5% 9% 6% 5% 4 4 4 6% 26%	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	51% 5% 14% 22% 7% 2	2518
	11th to 20th	Day	64% 5% 10% 14% 6% 1	2620
	21st to end	Day	61% 6% 10% 16% 6% 1	3124
February	1st to 10th	Day	55% 6% 13% 16% 10% 1	1710
	11th to 20th	Day	51% 7% 9% 23% 8% 1	1738
	21st to end	Day	55% 4 10% 21% 8% 2	1570
March	1st to 10th	Day	55% 6% 10% 22% 6% 1	3231
	11th to 20th	Day	42% 7% 15% 24% 10% 2	3480
	21st to end	Day	47% 5% 10% 18% 18% 2	3969
April	1st to 10th	Day	51% 5% 9% 19% 13% 3	2280
	11th to 20th	Day	46% 6% 11% 21% 13% 3	2254
	21st to end	Day	45% 6% 10% 20% 15% 4	2280
May	1st to 10th	Day	54% 6% 10% 20% 9% 2	4206
	11th to 20th	Day	56% 8% 12% 16% 7% 1	4402
	21st to end	Day	35% 9% 13% 25% 16% 2	4837
June	1st to 10th	Day	43% 9% 12% 22% 13% 1	2660
	11th to 20th	Day	44% 8% 13% 22% 12% 2	2660
	21st to end	Day	39% 8% 14% 26% 12%	2660
July	1st to 10th	Day	42% 10% 14% 23% 11% 1	4370
	11th to 20th	Day	36% 9% 17% 24% 12% 1	4365
	21st to end	Day	47% 8% 11% 17% 15% 1	4540
August	1st to 10th	Day	51% 8% 13% 19% 9%	4091
	11th to 20th	Day	43% 9% 14% 26% 8%	3902
	21st to end	Day	37% 7% 12% 31% 12% 1	4180
September	1st to 10th	Day	41% 7% 11% 21% 17% 2	2280
	11th to 20th	Day	33% 6% 11% 24% 23% 2	2280
	21st to end	Day	48% 6% 9% 18% 17% 1	2026
October	1st to 10th	Day	39% 6% 10% 22% 18% 4	3160
	11th to 20th	Day	36% 6% 9% 27% 18% 4	3140
	21st to end	Day	31% 5% 15% 28% 18% 2	3371
November	1st to 10th	Day	39% 6% 13% 23% 17% 2	1761
	11th to 20th	Day	48% 5% 11% 22% 12% 2	1755
	21st to end	Day	48% 6% 13% 24% 8% 1	1576
December	1st to 10th	Day	59% 6% 13% 15% 7% 1	1520
	11th to 20th	Day	52% 5% 17% 20% 5%	1906
	21st to end	Day	63% 8% 9% 14% 5%	2772
January	1st to 10th	Night	49% 8% 13% 22% 8% 2	4429
	11th to 20th	Night	54% 8% 12% 17% 7% 1	4335
	21st to end	Night	58% 7% 9% 17% 7% 1	4532
February	1st to 10th	Night	49% 7% 13% 21% 9% 1	2470
	11th to 20th	Night	52% 5% 11% 18% 11% 2	2339
	21st to end	Night	50% 6% 10% 20% 11% 3	1884
March	1st to 10th	Night	55% 5% 9% 20% 9% 1	3620
	11th to 20th	Night	40% 8% 16% 26% 10% 1	3480
	21st to end	Night	45% 4 8% 18% 19% 5%	3663
April	1st to 10th	Night	51% 5% 10% 20% 12% 2	1900
	11th to 20th	Night	43% 7% 11% 21% 16% 1	1875
	21st to end	Night	43% 6% 10% 21% 17% 3	1900
May	1st to 10th	Night	55% 6% 9% 21% 8% 2	2767
	11th to 20th	Night	52% 9% 13% 15% 9% 1	2496
	21st to end	Night	39% 11% 14% 19% 14% 3	2739
June	1st to 10th	Night	39% 8% 12% 25% 16% 1	1520
	11th to 20th	Night	37% 9% 15% 25% 12% 1	1520
	21st to end	Night	35% 9% 14% 28% 12% 1	1520
July	1st to 10th	Night	41% 9% 14% 18% 16% 2	2480
	11th to 20th	Night	37% 9% 17% 23% 13% 1	2472
	21st to end	Night	41% 9% 13% 17% 18% 1	2768
August	1st to 10th	Night	47% 9% 15% 20% 8%	2814
	11th to 20th	Night	42% 7% 14% 22% 14%	2961
	21st to end	Night	33% 7% 12% 32% 15% 1	3476
September	1st to 10th	Night	38% 7% 11% 25% 16% 3	1900
	11th to 20th	Night	33% 6% 11% 26% 22% 2	1900
	21st to end	Night	42% 6% 10% 22% 19% 1	2149
October	1st to 10th	Night	44% 6% 10% 20% 19% 1	3800
	11th to 20th	Night	33% 6% 11% 27% 20% 4	3779
	21st to end	Night	32% 6% 15% 28% 19% 1	4177
November	1st to 10th	Night	37% 6% 13% 23% 19% 2	2537
	11th to 20th	Night	46% 5% 11% 22% 13% 3	2640
	21st to end	Night	47% 7% 13% 22% 10% 1	2759
December	1st to 10th	Night	56% 7% 12% 18% 7% 1	2660
	11th to 20th	Night	52% 7% 13% 18% 9% 1	3288
	21st to end	Night	57% 6% 13% 19% 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	32% 24% 28% 12% 2 1	180
	11th to 20th	Day	37% 25% 28% 8% 1	209
	21st to end	Day	24% 32% 29% 11% 4	297
February	1st to 10th	Day	31% 29% 27% 10% 2	270
	11th to 20th	Day	31% 29% 32% 8% 1	336
	21st to end	Day	30% 25% 30% 13% 1	382
March	1st to 10th	Day	32% 20% 30% 14% 3	460
	11th to 20th	Day	29% 27% 28% 12% 3 1	460
	21st to end	Day	29% 26% 25% 13% 5% 2	506
April	1st to 10th	Day	30% 23% 28% 13% 4 1	460
	11th to 20th	Day	25% 27% 25% 13% 8% 2	452
	21st to end	Day	27% 20% 32% 13% 6% 2	460
May	1st to 10th	Day	29% 21% 29% 11% 7% 3	463
	11th to 20th	Day	22% 19% 31% 18% 8% 2	487
	21st to end	Day	20% 12% 26% 23% 15% 3	529
June	1st to 10th	Day	18% 15% 29% 26% 10% 2	490
	11th to 20th	Day	17% 17% 27% 26% 12% 2	490
	21st to end	Day	15% 14% 33% 26% 11% 1	490
July	1st to 10th	Day	13% 9% 26% 30% 18% 5%	485
	11th to 20th	Day	9% 8% 31% 30% 19% 3	484
	21st to end	Day	14% 9% 32% 29% 13% 3	532
August	1st to 10th	Day	14% 9% 24% 28% 18% 7%	486
	11th to 20th	Day	14% 9% 24% 30% 16% 7%	479
	21st to end	Day	13% 9% 27% 28% 14% 9%	539
September	1st to 10th	Day	17% 12% 22% 22% 17% 9%	490
	11th to 20th	Day	13% 7% 26% 24% 20% 10%	490
	21st to end	Day	24% 15% 22% 21% 14% 4	490
October	1st to 10th	Day	23% 12% 23% 23% 14% 6%	490
	11th to 20th	Day	21% 19% 28% 20% 10% 2	490
	21st to end	Day	27% 15% 30% 20% 8% 1	522
November	1st to 10th	Day	22% 16% 31% 20% 9% 2	395
	11th to 20th	Day	27% 19% 33% 18% 3	355
	21st to end	Day	32% 22% 34% 10% 3	194
December	1st to 10th	Day	36% 25% 26% 9% 4 1	180
	11th to 20th	Day	33% 23% 30% 12% 2	180
	21st to end	Day	29% 26% 30% 12% 3	198
January	1st to 10th	Night	21% 25% 37% 15% 3	760
	11th to 20th	Night	22% 23% 32% 22% 2	760
	21st to end	Night	18% 23% 38% 18% 2	836
February	1st to 10th	Night	23% 22% 31% 22% 2	760
	11th to 20th	Night	23% 24% 38% 13% 1	743
	21st to end	Night	27% 22% 33% 15% 3	628
March	1st to 10th	Night	24% 20% 34% 19% 3	760
	11th to 20th	Night	24% 22% 31% 19% 4 1	760
	21st to end	Night	23% 19% 35% 18% 5%	836
April	1st to 10th	Night	25% 19% 33% 19% 4	760
	11th to 20th	Night	20% 19% 31% 22% 7%	751
	21st to end	Night	20% 19% 28% 27% 6%	760
May	1st to 10th	Night	26% 14% 26% 25% 10%	536
	11th to 20th	Night	16% 13% 33% 29% 9%	374
	21st to end	Night	13% 11% 24% 37% 15%	411
June	1st to 10th	Night	15% 6% 25% 38% 17%	380
	11th to 20th	Night	12% 8% 27% 40% 13%	380
	21st to end	Night	12% 5% 25% 44% 13%	380
July	1st to 10th	Night	13% 4 22% 40% 18% 2	370
	11th to 20th	Night	10% 4 22% 44% 20% 1	368
	21st to end	Night	12% 5% 21% 42% 19% 2	487
August	1st to 10th	Night	15% 2 15% 42% 23% 2	567
	11th to 20th	Night	13% 4 14% 39% 29% 1	648
	21st to end	Night	13% 5% 18% 36% 26% 2	836
September	1st to 10th	Night	17% 5% 23% 33% 20% 2	760
	11th to 20th	Night	13% 6% 21% 30% 26% 5%	760
	21st to end	Night	20% 10% 21% 32% 15% 2	760
October	1st to 10th	Night	23% 11% 19% 26% 18% 3	760
	11th to 20th	Night	19% 12% 27% 32% 9% 1	760
	21st to end	Night	23% 14% 29% 26% 7%	836
November	1st to 10th	Night	15% 16% 34% 25% 10% 1	780
	11th to 20th	Night	21% 19% 29% 24% 6% 1	800
	21st to end	Night	22% 18% 33% 22% 5%	789
December	1st to 10th	Night	26% 21% 30% 17% 5%	760
	11th to 20th	Night	23% 20% 33% 21% 4	760
	21st to end	Night	25% 18% 35% 20% 2	836

Page generated at 2024-11-25 00:22:16. Version ac07

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