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 2024-01-14.

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	15% 8% 3 6% 6% 4 5% 5% 5% 7% 35%	74722
	Night	15% 8% 3 6% 6% 3 5% 5% 5% 7% 37%	75031
Transparency	Day	47% 7% 12% 20% 12% 2	74432
	Night	45% 7% 12% 21% 12% 2	74856
Seeing	Day	22% 17% 28% 20% 10% 3	13702
	Night	20% 15% 29% 26% 10% 1	23127

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	12% 12% 7% 7% 6% 4 6% 5% 4 5% 32%	1517
	11th to 20th	Day	14% 10% 5% 11% 7% 5% 5% 5% 6% 6% 24%	1522
	21st to end	Day	13% 10% 5% 10% 8% 4 5% 5% 5% 7% 28%	1782
February	1st to 10th	Day	16% 10% 5% 8% 7% 4 6% 6% 5% 7% 26%	1620
	11th to 20th	Day	15% 10% 3 8% 8% 4 5% 5% 5% 6% 31%	1636
	21st to end	Day	19% 11% 4 8% 5% 3 5% 4 4 6% 30%	1480
March	1st to 10th	Day	13% 10% 5% 7% 5% 4 5% 4 5% 6% 36%	1853
	11th to 20th	Day	16% 9% 4 6% 7% 4 5% 4 5% 7% 34%	1980
	21st to end	Day	19% 9% 4 6% 6% 3 4 4 4 6% 35%	2263
April	1st to 10th	Day	16% 10% 3 6% 6% 4 5% 4 5% 6% 35%	2160
	11th to 20th	Day	16% 9% 3 5% 6% 3 5% 4 5% 7% 38%	2134
	21st to end	Day	17% 6% 3 7% 5% 3 4 4 7% 7% 37%	2160
May	1st to 10th	Day	20% 6% 3 6% 6% 4 5% 5% 5% 6% 33%	2384
	11th to 20th	Day	18% 7% 3 7% 6% 3 5% 5% 5% 7% 33%	2478
	21st to end	Day	15% 7% 3 5% 5% 3 4 6% 6% 7% 39%	2725
June	1st to 10th	Day	14% 5% 3 7% 6% 4 5% 5% 6% 8% 36%	2520
	11th to 20th	Day	15% 6% 3 6% 6% 4 6% 4 6% 9% 35%	2520
	21st to end	Day	12% 6% 2 5% 5% 4 5% 6% 6% 10% 39%	2520
July	1st to 10th	Day	11% 5% 2 6% 6% 4 6% 6% 6% 9% 38%	2520
	11th to 20th	Day	8% 6% 2 5% 6% 3 6% 5% 7% 11% 42%	2520
	21st to end	Day	11% 6% 3 6% 6% 4 7% 6% 6% 9% 36%	2672
August	1st to 10th	Day	11% 6% 3 6% 7% 4 6% 7% 7% 9% 34%	2327
	11th to 20th	Day	13% 6% 3 5% 6% 4 5% 7% 6% 8% 36%	2206
	21st to end	Day	14% 6% 2 6% 6% 3 4 5% 6% 10% 37%	2376
September	1st to 10th	Day	16% 6% 3 5% 5% 3 4 5% 6% 6% 41%	2160
	11th to 20th	Day	13% 6% 3 4 4 3 4 4 4 7% 49%	2160
	21st to end	Day	18% 7% 3 5% 5% 3 5% 4 5% 6% 39%	1921
October	1st to 10th	Day	18% 6% 2 5% 5% 3 4 4 6% 8% 39%	1800
	11th to 20th	Day	18% 6% 3 6% 5% 3 5% 5% 5% 7% 37%	1800
	21st to end	Day	21% 8% 3 5% 5% 3 4 4 5% 7% 34%	1949
November	1st to 10th	Day	13% 8% 3 6% 4 3 4 4 4 7% 45%	1671
	11th to 20th	Day	15% 8% 4 7% 5% 4 4 5% 5% 7% 36%	1668
	21st to end	Day	18% 8% 4 5% 5% 3 5% 5% 5% 7% 35%	1520
December	1st to 10th	Day	19% 11% 5% 7% 7% 4 6% 4 4 6% 27%	1520
	11th to 20th	Day	21% 11% 3 6% 4 3 4 3 5% 7% 32%	1906
	21st to end	Day	19% 12% 5% 11% 9% 4 5% 4 5% 4 21%	2772
January	1st to 10th	Night	13% 11% 4 8% 6% 4 5% 5% 4 7% 32%	2649
	11th to 20th	Night	16% 10% 4 7% 7% 3 6% 5% 5% 7% 28%	2511
	21st to end	Night	12% 10% 5% 7% 6% 4 6% 6% 6% 7% 32%	2574
February	1st to 10th	Night	16% 11% 4 6% 7% 4 5% 4 5% 6% 32%	2340
	11th to 20th	Night	15% 11% 5% 7% 6% 3 5% 5% 5% 6% 31%	2210
	21st to end	Night	18% 9% 3 8% 7% 3 5% 4 5% 6% 31%	1776
March	1st to 10th	Night	15% 8% 4 6% 5% 3 4 5% 5% 7% 37%	2092
	11th to 20th	Night	16% 8% 3 5% 5% 3 4 4 5% 7% 39%	1979
	21st to end	Night	15% 10% 4 7% 6% 3 4 4 4 6% 37%	2093
April	1st to 10th	Night	18% 9% 3 7% 6% 4 5% 4 6% 6% 33%	1800
	11th to 20th	Night	15% 6% 3 6% 6% 3 4 4 5% 8% 40%	1775
	21st to end	Night	18% 7% 2 5% 4 3 4 4 5% 6% 42%	1800
May	1st to 10th	Night	21% 8% 3 5% 5% 3 4 4 5% 6% 35%	1589
	11th to 20th	Night	15% 6% 2 5% 5% 4 5% 6% 6% 7% 40%	1415
	21st to end	Night	11% 6% 3 6% 6% 4 5% 5% 5% 9% 40%	1550
June	1st to 10th	Night	14% 5% 2 6% 5% 3 4 5% 5% 8% 43%	1440
	11th to 20th	Night	13% 5% 2 6% 6% 2 5% 5% 7% 10% 39%	1440
	21st to end	Night	12% 5% 2 4 4 3 5% 5% 7% 10% 43%	1440
July	1st to 10th	Night	10% 6% 3 6% 6% 3 5% 5% 7% 8% 41%	1440
	11th to 20th	Night	8% 4 3 5% 4 4 5% 6% 8% 9% 45%	1440
	21st to end	Night	11% 5% 2 5% 5% 4 5% 5% 7% 10% 41%	1641
August	1st to 10th	Night	10% 5% 2 5% 5% 4 5% 5% 8% 9% 41%	1611
	11th to 20th	Night	11% 8% 3 5% 6% 3 5% 6% 6% 8% 39%	1677
	21st to end	Night	11% 6% 3 6% 5% 3 5% 4 6% 9% 43%	1980
September	1st to 10th	Night	15% 5% 3 4 5% 4 5% 4 5% 6% 45%	1800
	11th to 20th	Night	12% 6% 2 4 4 2 4 4 5% 7% 50%	1800
	21st to end	Night	15% 6% 3 5% 4 2 5% 4 5% 7% 44%	2035
October	1st to 10th	Night	19% 7% 3 5% 4 2 3 3 4 7% 42%	2160
	11th to 20th	Night	15% 7% 3 6% 5% 3 4 5% 5% 5% 43%	2160
	21st to end	Night	21% 7% 3 5% 5% 3 4 4 5% 6% 36%	2407
November	1st to 10th	Night	10% 7% 3 6% 5% 3 4 4 6% 6% 47%	2407
	11th to 20th	Night	16% 9% 3 5% 6% 3 4 5% 5% 6% 37%	2508
	21st to end	Night	14% 10% 4 6% 6% 3 4 5% 5% 8% 35%	2660
December	1st to 10th	Night	19% 10% 5% 7% 6% 3 5% 5% 5% 7% 28%	2660
	11th to 20th	Night	18% 11% 3 7% 6% 3 6% 5% 5% 6% 30%	3288
	21st to end	Night	20% 12% 5% 8% 7% 3 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	53% 5% 12% 20% 8% 1	1518
	11th to 20th	Day	59% 6% 11% 13% 8% 1	1522
	21st to end	Day	55% 6% 11% 17% 10% 1	1782
February	1st to 10th	Day	56% 6% 13% 14% 9% 1	1620
	11th to 20th	Day	52% 7% 9% 22% 8% 1	1645
	21st to end	Day	55% 4 10% 21% 8% 2	1480
March	1st to 10th	Day	48% 6% 12% 23% 9% 2	1856
	11th to 20th	Day	50% 6% 10% 20% 12% 2	1980
	21st to end	Day	52% 5% 9% 17% 14% 3	2263
April	1st to 10th	Day	50% 5% 9% 19% 13% 3	2160
	11th to 20th	Day	46% 6% 10% 21% 14% 3	2134
	21st to end	Day	45% 7% 10% 20% 14% 4	2160
May	1st to 10th	Day	50% 7% 9% 18% 13% 3	2384
	11th to 20th	Day	51% 7% 10% 18% 12% 2	2482
	21st to end	Day	42% 9% 14% 20% 12% 2	2725
June	1st to 10th	Day	44% 9% 12% 21% 13% 1	2520
	11th to 20th	Day	45% 8% 13% 21% 12% 2	2520
	21st to end	Day	39% 9% 14% 26% 12%	2520
July	1st to 10th	Day	42% 10% 13% 21% 13% 1	2450
	11th to 20th	Day	35% 9% 17% 25% 13% 1	2445
	21st to end	Day	42% 9% 13% 20% 15% 1	2523
August	1st to 10th	Day	43% 10% 14% 20% 12%	2314
	11th to 20th	Day	43% 10% 15% 20% 11% 1	2206
	21st to end	Day	41% 8% 12% 22% 15% 2	2376
September	1st to 10th	Day	43% 8% 12% 20% 16% 2	2160
	11th to 20th	Day	34% 6% 11% 24% 22% 2	2160
	21st to end	Day	46% 6% 10% 19% 18% 2	1921
October	1st to 10th	Day	43% 7% 14% 21% 14% 2	1800
	11th to 20th	Day	45% 7% 11% 24% 12% 1	1800
	21st to end	Day	49% 7% 13% 20% 10% 2	1949
November	1st to 10th	Day	40% 6% 13% 22% 17% 2	1671
	11th to 20th	Day	48% 5% 11% 21% 12% 2	1668
	21st to end	Day	47% 6% 13% 25% 8% 1	1520
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	51% 6% 11% 21% 10% 2	2649
	11th to 20th	Night	53% 8% 14% 17% 6% 2	2511
	21st to end	Night	50% 5% 12% 20% 12% 1	2574
February	1st to 10th	Night	51% 7% 13% 20% 8% 1	2340
	11th to 20th	Night	53% 5% 11% 17% 11% 2	2212
	21st to end	Night	51% 6% 9% 19% 12% 2	1776
March	1st to 10th	Night	45% 6% 13% 23% 10% 3	2102
	11th to 20th	Night	44% 7% 11% 23% 14% 2	1980
	21st to end	Night	49% 5% 10% 17% 16% 3	2093
April	1st to 10th	Night	50% 5% 10% 20% 12% 2	1800
	11th to 20th	Night	42% 7% 12% 21% 17% 2	1775
	21st to end	Night	44% 6% 10% 21% 16% 3	1800
May	1st to 10th	Night	49% 7% 9% 21% 11% 3	1589
	11th to 20th	Night	43% 8% 11% 19% 16% 2	1416
	21st to end	Night	41% 10% 15% 17% 13% 3	1551
June	1st to 10th	Night	39% 8% 11% 25% 16% 1	1440
	11th to 20th	Night	39% 9% 15% 23% 12% 1	1440
	21st to end	Night	35% 9% 14% 29% 13% 1	1440
July	1st to 10th	Night	40% 9% 13% 22% 13% 2	1400
	11th to 20th	Night	33% 10% 17% 25% 14% 1	1392
	21st to end	Night	35% 11% 17% 20% 16% 1	1550
August	1st to 10th	Night	36% 10% 16% 24% 13% 1	1600
	11th to 20th	Night	41% 9% 15% 19% 14% 1	1677
	21st to end	Night	37% 9% 13% 21% 19% 2	1980
September	1st to 10th	Night	40% 7% 12% 24% 14% 3	1800
	11th to 20th	Night	33% 6% 11% 26% 21% 2	1800
	21st to end	Night	40% 6% 10% 24% 20% 1	2035
October	1st to 10th	Night	43% 6% 13% 23% 15% 1	2160
	11th to 20th	Night	41% 7% 13% 25% 11% 2	2160
	21st to end	Night	46% 7% 11% 22% 12% 1	2407
November	1st to 10th	Night	38% 6% 14% 22% 18% 3	2407
	11th to 20th	Night	46% 5% 12% 21% 13% 3	2508
	21st to end	Night	46% 7% 13% 23% 10% 1	2660
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% 26% 28% 9% 1	192
	21st to end	Day	24% 33% 28% 11% 4	264
February	1st to 10th	Day	34% 33% 24% 8% 2	240
	11th to 20th	Day	30% 30% 33% 7%	303
	21st to end	Day	30% 26% 30% 12% 1	346
March	1st to 10th	Day	31% 21% 31% 14% 3	420
	11th to 20th	Day	28% 26% 29% 12% 3 1	420
	21st to end	Day	29% 26% 25% 13% 5% 2	462
April	1st to 10th	Day	29% 23% 29% 13% 4 1	420
	11th to 20th	Day	24% 26% 25% 14% 8% 2	412
	21st to end	Day	28% 20% 33% 12% 5% 2	420
May	1st to 10th	Day	27% 22% 30% 11% 7% 3	423
	11th to 20th	Day	22% 20% 31% 18% 7% 2	447
	21st to end	Day	21% 12% 25% 23% 15% 4	485
June	1st to 10th	Day	18% 14% 30% 26% 9% 2	450
	11th to 20th	Day	18% 18% 26% 25% 11% 2	450
	21st to end	Day	14% 13% 33% 26% 11% 2	450
July	1st to 10th	Day	13% 9% 25% 30% 18% 6%	445
	11th to 20th	Day	9% 7% 30% 31% 20% 3	444
	21st to end	Day	14% 9% 32% 29% 13% 3	488
August	1st to 10th	Day	13% 9% 24% 28% 20% 7%	446
	11th to 20th	Day	14% 7% 25% 29% 18% 7%	439
	21st to end	Day	14% 9% 26% 27% 14% 9%	495
September	1st to 10th	Day	18% 12% 22% 21% 18% 9%	450
	11th to 20th	Day	14% 8% 27% 23% 19% 9%	450
	21st to end	Day	21% 14% 23% 22% 15% 5%	450
October	1st to 10th	Day	23% 12% 22% 23% 15% 6%	450
	11th to 20th	Day	21% 19% 28% 20% 10% 1	450
	21st to end	Day	28% 15% 30% 19% 7% 1	480
November	1st to 10th	Day	23% 15% 30% 20% 10% 2	365
	11th to 20th	Day	27% 19% 32% 18% 4	328
	21st to end	Day	29% 23% 36% 10% 2	180
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% 21% 32% 22% 2	736
	21st to end	Night	17% 23% 39% 19% 2	792
February	1st to 10th	Night	24% 23% 31% 20% 2	720
	11th to 20th	Night	24% 24% 37% 14% 1	703
	21st to end	Night	27% 22% 34% 15% 2	592
March	1st to 10th	Night	23% 20% 34% 19% 3	720
	11th to 20th	Night	24% 21% 31% 19% 5% 1	720
	21st to end	Night	22% 18% 36% 18% 5% 1	792
April	1st to 10th	Night	25% 20% 32% 19% 5%	720
	11th to 20th	Night	19% 19% 32% 23% 8%	711
	21st to end	Night	21% 18% 29% 26% 6%	720
May	1st to 10th	Night	25% 14% 25% 25% 10%	509
	11th to 20th	Night	15% 13% 34% 29% 9%	354
	21st to end	Night	13% 11% 23% 37% 15% 1	389
June	1st to 10th	Night	15% 5% 26% 38% 17%	360
	11th to 20th	Night	13% 8% 26% 39% 14%	360
	21st to end	Night	12% 4 25% 46% 13%	360
July	1st to 10th	Night	13% 4 23% 39% 19% 2	350
	11th to 20th	Night	10% 4 22% 43% 20% 1	348
	21st to end	Night	12% 4 21% 42% 19% 2	461
August	1st to 10th	Night	14% 2 15% 42% 24% 2	537
	11th to 20th	Night	13% 4 14% 39% 29% 1	613
	21st to end	Night	13% 5% 18% 36% 26% 2	792
September	1st to 10th	Night	17% 5% 23% 33% 20% 2	720
	11th to 20th	Night	13% 6% 23% 29% 24% 5%	720
	21st to end	Night	18% 9% 22% 34% 16% 2	720
October	1st to 10th	Night	23% 10% 19% 26% 18% 3	720
	11th to 20th	Night	20% 12% 27% 32% 8% 1	720
	21st to end	Night	25% 15% 29% 26% 6%	792
November	1st to 10th	Night	15% 15% 33% 25% 11% 1	740
	11th to 20th	Night	21% 19% 29% 24% 6% 1	760
	21st to end	Night	21% 18% 34% 23% 5%	760
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 2024-01-11 13:31:58. Version ac07

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