Chestnut Branch Park
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Chestnut Branch Park clear sky chart from 2005-11-06 to 2026-07-31.

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% 6% 2 6% 4 4 4 4 6% 7% 41%	138287
	Night	15% 7% 2 6% 4 4 4 3 5% 7% 43%	134737
Transparency	Day	40% 6% 12% 23% 17% 3	141816
	Night	40% 5% 11% 23% 17% 4	139295
Seeing	Day	19% 14% 29% 23% 12% 3	17251
	Night	18% 11% 32% 29% 10% 1	26487

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% 6% 2 6% 4 4 4 3 5% 6% 46%	4661
	11th to 20th	Day	15% 7% 2 7% 5% 4 2 2 3 5% 47%	4809
	21st to end	Day	16% 6% 2 5% 4 4 5% 3 5% 6% 44%	5780
February	1st to 10th	Day	14% 8% 2 7% 4 5% 4 4 6% 6% 41%	1890
	11th to 20th	Day	17% 7% 2 6% 5% 4 4 3 5% 6% 40%	2013
	21st to end	Day	17% 10% 3 6% 4 4 4 3 5% 6% 39%	1730
March	1st to 10th	Day	20% 6% 2 5% 3 3 3 3 4 6% 46%	4576
	11th to 20th	Day	11% 6% 2 6% 4 4 4 4 5% 6% 47%	5085
	21st to end	Day	18% 5% 2 6% 4 4 4 3 5% 6% 43%	5930
April	1st to 10th	Day	16% 8% 2 5% 4 4 4 4 6% 7% 42%	2520
	11th to 20th	Day	14% 6% 2 6% 4 4 5% 4 5% 7% 43%	2494
	21st to end	Day	16% 6% 2 5% 5% 4 4 4 6% 6% 41%	2520
May	1st to 10th	Day	22% 7% 2 6% 4 4 4 3 5% 8% 34%	7457
	11th to 20th	Day	19% 7% 3 6% 4 4 4 3 5% 7% 39%	8221
	21st to end	Day	24% 6% 2 6% 4 4 4 3 5% 7% 36%	9060
June	1st to 10th	Day	12% 6% 2 6% 5% 4 5% 5% 7% 8% 41%	2940
	11th to 20th	Day	12% 6% 2 6% 5% 5% 5% 5% 7% 8% 40%	2940
	21st to end	Day	10% 4 2 6% 5% 4 6% 4 7% 9% 42%	2940
July	1st to 10th	Day	8% 5% 2 6% 5% 6% 6% 5% 7% 11% 38%	6500
	11th to 20th	Day	10% 6% 3 8% 6% 5% 7% 5% 8% 11% 31%	6500
	21st to end	Day	10% 6% 2 7% 5% 5% 5% 4 7% 10% 38%	6933
August	1st to 10th	Day	17% 6% 2 7% 5% 6% 5% 4 7% 8% 34%	4212
	11th to 20th	Day	12% 6% 2 6% 4 4 5% 5% 6% 9% 41%	3850
	21st to end	Day	9% 5% 2 5% 4 4 5% 4 6% 9% 48%	4312
September	1st to 10th	Day	13% 6% 2 6% 5% 4 4 4 6% 6% 45%	2400
	11th to 20th	Day	12% 4 1 5% 4 3 5% 3 5% 7% 51%	2364
	21st to end	Day	18% 8% 3 8% 5% 4 5% 3 5% 6% 38%	2200
October	1st to 10th	Day	17% 6% 2 5% 3 3 4 3 5% 6% 46%	2010
	11th to 20th	Day	16% 5% 2 4 3 3 4 2 5% 6% 50%	2000
	21st to end	Day	16% 6% 2 6% 3 3 3 3 4 6% 48%	2131
November	1st to 10th	Day	13% 5% 2 4 4 3 3 2 4 7% 53%	1851
	11th to 20th	Day	14% 6% 1 4 3 4 4 4 5% 7% 47%	1890
	21st to end	Day	15% 5% 2 5% 5% 3 4 3 5% 7% 47%	1774
December	1st to 10th	Day	14% 6% 2 6% 2 4 4 4 6% 7% 45%	2680
	11th to 20th	Day	19% 10% 2 5% 3 3 3 3 3 5% 44%	3066
	21st to end	Day	15% 6% 2 5% 3 5% 5% 4 6% 8% 40%	4048
January	1st to 10th	Night	12% 8% 2 6% 4 4 4 3 5% 6% 46%	8234
	11th to 20th	Night	16% 7% 2 6% 3 3 3 3 3 5% 49%	8073
	21st to end	Night	15% 7% 2 7% 4 3 3 3 4 6% 45%	8401
February	1st to 10th	Night	16% 8% 3 6% 4 4 3 3 5% 7% 41%	2730
	11th to 20th	Night	19% 8% 2 6% 4 4 3 3 5% 6% 41%	2491
	21st to end	Night	17% 8% 3 6% 4 4 4 3 5% 6% 42%	2076
March	1st to 10th	Night	18% 8% 2 6% 3 3 3 2 4 6% 45%	5484
	11th to 20th	Night	11% 8% 3 5% 4 4 3 3 4 5% 51%	5089
	21st to end	Night	16% 8% 2 6% 4 4 5% 3 4 5% 43%	5242
April	1st to 10th	Night	17% 7% 2 7% 4 4 4 4 5% 6% 40%	2100
	11th to 20th	Night	15% 6% 3 6% 4 4 4 4 5% 7% 42%	2075
	21st to end	Night	17% 5% 2 5% 4 4 4 3 5% 7% 44%	2100
May	1st to 10th	Night	18% 8% 3 7% 4 5% 5% 4 6% 7% 33%	5483
	11th to 20th	Night	19% 7% 3 6% 4 4 4 4 6% 8% 35%	4639
	21st to end	Night	26% 7% 3 7% 4 4 4 3 4 6% 33%	5114
June	1st to 10th	Night	13% 6% 1 5% 4 4 4 3 6% 9% 44%	1680
	11th to 20th	Night	10% 7% 2 7% 6% 4 4 4 6% 9% 40%	1680
	21st to end	Night	9% 6% 2 5% 4 5% 5% 4 8% 11% 41%	1680
July	1st to 10th	Night	9% 6% 3 8% 5% 6% 7% 5% 6% 9% 37%	3680
	11th to 20th	Night	12% 7% 3 8% 5% 5% 5% 5% 7% 10% 32%	3680
	21st to end	Night	12% 6% 2 6% 5% 4 5% 4 6% 8% 43%	4135
August	1st to 10th	Night	20% 7% 2 6% 4 4 5% 5% 6% 8% 34%	2915
	11th to 20th	Night	13% 7% 2 7% 4 4 4 3 6% 7% 42%	3188
	21st to end	Night	10% 5% 2 6% 4 4 4 3 5% 7% 49%	3586
September	1st to 10th	Night	12% 6% 2 7% 5% 3 4 3 5% 6% 47%	2000
	11th to 20th	Night	11% 5% 2 5% 3 3 3 3 5% 7% 54%	2035
	21st to end	Night	19% 7% 2 6% 5% 3 4 3 5% 7% 39%	2200
October	1st to 10th	Night	18% 6% 2 6% 3 3 4 3 5% 6% 45%	2385
	11th to 20th	Night	16% 5% 1 5% 3 3 3 2 5% 6% 51%	2400
	21st to end	Night	16% 6% 2 4 4 2 4 3 5% 6% 48%	2707
November	1st to 10th	Night	11% 5% 2 5% 3 3 4 3 5% 6% 53%	2667
	11th to 20th	Night	16% 6% 2 5% 4 3 3 3 4 5% 49%	2730
	21st to end	Night	12% 6% 2 6% 4 4 4 4 6% 6% 45%	2840
December	1st to 10th	Night	13% 7% 2 6% 3 3 4 3 5% 6% 48%	4720
	11th to 20th	Night	22% 7% 2 5% 3 3 3 3 5% 7% 41%	5348
	21st to end	Night	18% 7% 3 6% 4 4 4 4 5% 6% 40%	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	40% 3 10% 26% 19% 2	4662
	11th to 20th	Day	42% 2 7% 23% 20% 6%	4839
	21st to end	Day	40% 4 13% 28% 13% 2	5780
February	1st to 10th	Day	42% 6% 13% 25% 12% 2	1890
	11th to 20th	Day	44% 6% 10% 23% 15% 3	2022
	21st to end	Day	46% 5% 9% 22% 14% 4	1730
March	1st to 10th	Day	40% 4 9% 24% 17% 7%	4579
	11th to 20th	Day	36% 5% 9% 29% 16% 5%	5085
	21st to end	Day	41% 4 10% 17% 23% 5%	5930
April	1st to 10th	Day	41% 6% 11% 20% 16% 6%	2520
	11th to 20th	Day	39% 6% 12% 23% 16% 4	2494
	21st to end	Day	42% 5% 11% 19% 19% 4	2520
May	1st to 10th	Day	49% 5% 13% 18% 15% 1	7466
	11th to 20th	Day	45% 5% 11% 23% 14% 2	8225
	21st to end	Day	48% 5% 9% 21% 14% 2	9061
June	1st to 10th	Day	38% 8% 13% 23% 17% 2	2940
	11th to 20th	Day	38% 10% 13% 24% 13% 3	2940
	21st to end	Day	36% 8% 14% 23% 17% 2	2940
July	1st to 10th	Day	38% 10% 16% 17% 17% 2	6430
	11th to 20th	Day	43% 9% 17% 20% 10% 1	6425
	21st to end	Day	39% 7% 11% 21% 21% 1	6737
August	1st to 10th	Day	46% 8% 13% 23% 10% 1	4199
	11th to 20th	Day	38% 7% 13% 27% 14% 1	3886
	21st to end	Day	32% 7% 11% 27% 20% 2	4312
September	1st to 10th	Day	38% 6% 10% 20% 22% 3	2400
	11th to 20th	Day	33% 5% 11% 23% 24% 4	2364
	21st to end	Day	48% 5% 9% 18% 19% 2	2200
October	1st to 10th	Day	35% 4 9% 17% 29% 5%	3270
	11th to 20th	Day	26% 3 11% 23% 29% 9%	3240
	21st to end	Day	25% 3 11% 28% 29% 5%	3421
November	1st to 10th	Day	33% 3 13% 26% 21% 4	1851
	11th to 20th	Day	36% 6% 10% 24% 22% 3	1890
	21st to end	Day	38% 4 11% 31% 14% 2	1774
December	1st to 10th	Day	37% 4 17% 25% 14% 2	2680
	11th to 20th	Day	44% 3 11% 30% 10% 3	3066
	21st to end	Day	40% 5% 12% 23% 17% 2	4048
January	1st to 10th	Night	39% 4 11% 24% 17% 4	8234
	11th to 20th	Night	39% 3 9% 22% 19% 8%	8118
	21st to end	Night	41% 3 8% 26% 20% 3	8401
February	1st to 10th	Night	43% 4 13% 24% 14% 2	2730
	11th to 20th	Night	45% 4 9% 20% 17% 5%	2493
	21st to end	Night	43% 4 10% 22% 15% 6%	2076
March	1st to 10th	Night	42% 3 9% 24% 13% 9%	5494
	11th to 20th	Night	35% 5% 13% 26% 16% 5%	5090
	21st to end	Night	43% 4 8% 16% 24% 4	5242
April	1st to 10th	Night	43% 5% 9% 20% 18% 4	2100
	11th to 20th	Night	41% 5% 10% 20% 19% 4	2075
	21st to end	Night	41% 5% 9% 21% 20% 5%	2100
May	1st to 10th	Night	48% 7% 11% 19% 13% 2	5490
	11th to 20th	Night	47% 6% 11% 23% 12% 1	4640
	21st to end	Night	52% 6% 9% 19% 13% 2	5115
June	1st to 10th	Night	37% 7% 11% 23% 21% 2	1680
	11th to 20th	Night	40% 8% 13% 24% 13% 2	1680
	21st to end	Night	35% 9% 15% 24% 15% 2	1680
July	1st to 10th	Night	43% 8% 16% 20% 12% 1	3640
	11th to 20th	Night	45% 9% 19% 19% 8% 1	3632
	21st to end	Night	38% 7% 13% 23% 19% 1	4016
August	1st to 10th	Night	48% 8% 18% 18% 9%	2904
	11th to 20th	Night	39% 7% 12% 27% 14% 1	3218
	21st to end	Night	33% 5% 10% 29% 20% 2	3586
September	1st to 10th	Night	38% 6% 12% 20% 21% 4	2000
	11th to 20th	Night	31% 6% 10% 24% 26% 4	2035
	21st to end	Night	44% 6% 8% 20% 19% 2	2200
October	1st to 10th	Night	39% 4 10% 22% 21% 3	3905
	11th to 20th	Night	25% 3 11% 26% 29% 6%	3899
	21st to end	Night	26% 3 11% 31% 25% 5%	4367
November	1st to 10th	Night	32% 5% 12% 25% 21% 6%	2667
	11th to 20th	Night	39% 5% 9% 21% 21% 6%	2730
	21st to end	Night	37% 5% 12% 26% 18% 2	2840
December	1st to 10th	Night	37% 4 12% 30% 15% 2	4720
	11th to 20th	Night	44% 3 10% 23% 15% 4	5348
	21st to end	Night	44% 5% 11% 20% 17% 2	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	26% 25% 30% 15% 3 1	220
	11th to 20th	Day	27% 33% 26% 12% 1	250
	21st to end	Day	23% 29% 27% 16% 4 1	363
February	1st to 10th	Day	24% 24% 34% 14% 5%	330
	11th to 20th	Day	30% 23% 33% 12% 2	501
	21st to end	Day	26% 20% 36% 15% 3	446
March	1st to 10th	Day	25% 22% 31% 17% 5%	540
	11th to 20th	Day	22% 25% 32% 15% 5% 1	540
	21st to end	Day	25% 22% 30% 19% 4 1	594
April	1st to 10th	Day	25% 20% 34% 13% 7% 1	540
	11th to 20th	Day	20% 19% 33% 19% 8% 1	532
	21st to end	Day	22% 18% 31% 21% 6% 1	540
May	1st to 10th	Day	26% 21% 28% 15% 8% 1	543
	11th to 20th	Day	21% 13% 31% 23% 9% 2	567
	21st to end	Day	22% 10% 23% 25% 17% 4	617
June	1st to 10th	Day	13% 12% 26% 32% 14% 4	570
	11th to 20th	Day	16% 11% 29% 28% 14% 2	570
	21st to end	Day	12% 9% 29% 30% 16% 4	570
July	1st to 10th	Day	11% 5% 21% 33% 22% 7%	565
	11th to 20th	Day	7% 3 24% 34% 25% 7%	564
	21st to end	Day	11% 5% 25% 34% 19% 6%	616
August	1st to 10th	Day	12% 5% 26% 29% 21% 7%	526
	11th to 20th	Day	13% 4 18% 34% 23% 7%	519
	21st to end	Day	10% 6% 28% 28% 18% 9%	583
September	1st to 10th	Day	16% 9% 22% 27% 17% 8%	530
	11th to 20th	Day	13% 5% 26% 30% 17% 10%	530
	21st to end	Day	24% 11% 23% 25% 12% 5%	530
October	1st to 10th	Day	22% 7% 25% 24% 13% 8%	530
	11th to 20th	Day	19% 15% 27% 22% 15% 3	530
	21st to end	Day	20% 11% 35% 20% 10% 3	544
November	1st to 10th	Day	16% 13% 33% 25% 11% 2	425
	11th to 20th	Day	20% 18% 35% 20% 7%	430
	21st to end	Day	20% 18% 31% 24% 6% 1	314
December	1st to 10th	Day	23% 24% 36% 10% 6%	220
	11th to 20th	Day	29% 21% 34% 10% 6%	220
	21st to end	Day	28% 19% 30% 14% 7% 1	242
January	1st to 10th	Night	20% 15% 41% 20% 3	840
	11th to 20th	Night	23% 17% 34% 23% 3	840
	21st to end	Night	19% 16% 40% 24% 2	924
February	1st to 10th	Night	20% 13% 40% 24% 2	840
	11th to 20th	Night	26% 17% 38% 18% 1	823
	21st to end	Night	22% 17% 40% 18% 3	692
March	1st to 10th	Night	21% 15% 36% 24% 4	840
	11th to 20th	Night	20% 17% 38% 19% 5% 1	840
	21st to end	Night	21% 12% 42% 19% 5%	924
April	1st to 10th	Night	20% 19% 37% 18% 5%	840
	11th to 20th	Night	17% 14% 37% 27% 4	831
	21st to end	Night	20% 13% 33% 30% 5%	840
May	1st to 10th	Night	20% 12% 38% 24% 6%	712
	11th to 20th	Night	18% 8% 38% 30% 6%	414
	21st to end	Night	21% 5% 24% 36% 13% 1	455
June	1st to 10th	Night	15% 5% 30% 40% 10%	420
	11th to 20th	Night	12% 6% 27% 43% 11%	420
	21st to end	Night	10% 5% 24% 50% 11%	420
July	1st to 10th	Night	11% 3 23% 44% 18% 1	410
	11th to 20th	Night	7% 4 19% 49% 20% 2	408
	21st to end	Night	9% 3 25% 41% 21% 1	512
August	1st to 10th	Night	12% 2 16% 44% 24% 2	597
	11th to 20th	Night	14% 3 18% 40% 23% 2	781
	21st to end	Night	14% 3 20% 38% 23% 3	880
September	1st to 10th	Night	14% 5% 24% 36% 18% 2	800
	11th to 20th	Night	13% 3 19% 36% 24% 4	800
	21st to end	Night	22% 6% 24% 30% 18% 1	800
October	1st to 10th	Night	20% 6% 27% 27% 17% 4	800
	11th to 20th	Night	19% 10% 27% 32% 12% 1	800
	21st to end	Night	20% 10% 34% 27% 9%	880
November	1st to 10th	Night	15% 8% 36% 29% 11%	820
	11th to 20th	Night	19% 13% 34% 26% 8%	840
	21st to end	Night	15% 15% 36% 26% 7% 1	840
December	1st to 10th	Night	23% 13% 34% 22% 6% 1	840
	11th to 20th	Night	19% 14% 36% 26% 5%	840
	21st to end	Night	21% 14% 35% 28% 2	924

Page generated at 2026-07-28 03:25:26. Version ac07

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