Chester State Park
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Chester State Park clear sky chart from 2005-11-06 to 2025-12-18.

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	13% 6% 2 6% 4 4 5% 4 6% 8% 43%	127170
	Night	14% 6% 2 6% 4 4 4 4 5% 7% 44%	123873
Transparency	Day	38% 7% 12% 19% 18% 6%	130829
	Night	38% 6% 11% 18% 21% 7%	128283
Seeing	Day	17% 9% 25% 25% 17% 7%	16768
	Night	16% 9% 23% 31% 19% 2	26842

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	15% 5% 2 4 3 2 3 2 4 5% 55%	4056
	11th to 20th	Day	12% 6% 2 4 3 2 3 2 3 4 59%	4060
	21st to end	Day	17% 8% 2 7% 4 4 3 3 4 4 44%	4466
February	1st to 10th	Day	15% 7% 2 5% 4 4 4 3 5% 7% 43%	1810
	11th to 20th	Day	22% 6% 1 4 4 3 4 3 4 4 44%	1948
	21st to end	Day	19% 7% 3 6% 3 4 4 2 4 5% 43%	1690
March	1st to 10th	Day	20% 8% 2 6% 4 3 3 3 4 5% 42%	4935
	11th to 20th	Day	9% 5% 2 6% 4 3 4 3 4 6% 54%	4980
	21st to end	Day	16% 7% 2 6% 4 4 3 3 3 5% 47%	5465
April	1st to 10th	Day	12% 4 2 5% 3 4 5% 4 6% 8% 49%	2200
	11th to 20th	Day	12% 6% 1 5% 4 4 4 4 5% 7% 49%	2344
	21st to end	Day	11% 4 2 5% 4 4 4 4 6% 8% 48%	2562
May	1st to 10th	Day	11% 6% 2 6% 5% 5% 6% 5% 7% 11% 37%	5862
	11th to 20th	Day	19% 7% 3 7% 5% 5% 5% 5% 7% 9% 30%	5861
	21st to end	Day	12% 6% 2 6% 4 4 5% 4 6% 8% 43%	6433
June	1st to 10th	Day	9% 5% 2 7% 6% 5% 5% 5% 7% 11% 37%	2600
	11th to 20th	Day	7% 4 2 6% 5% 6% 5% 5% 7% 12% 41%	2600
	21st to end	Day	5% 4 2 5% 6% 7% 7% 5% 8% 10% 40%	2600
July	1st to 10th	Day	10% 6% 3 8% 6% 6% 6% 5% 8% 10% 33%	5900
	11th to 20th	Day	6% 5% 2 7% 6% 6% 7% 6% 9% 13% 33%	5900
	21st to end	Day	9% 7% 3 8% 6% 7% 7% 6% 9% 11% 28%	6424
August	1st to 10th	Day	22% 7% 3 7% 5% 6% 6% 5% 7% 9% 24%	4234
	11th to 20th	Day	11% 6% 2 6% 5% 5% 6% 6% 8% 10% 34%	4174
	21st to end	Day	6% 4 2 5% 4 4 5% 4 7% 9% 50%	4360
September	1st to 10th	Day	8% 5% 2 6% 5% 5% 6% 4 7% 9% 43%	2200
	11th to 20th	Day	12% 7% 2 7% 5% 4 5% 4 6% 8% 40%	2200
	21st to end	Day	17% 7% 3 5% 5% 5% 4 4 5% 6% 41%	2200
October	1st to 10th	Day	14% 5% 2 5% 3 4 3 3 4 5% 52%	2095
	11th to 20th	Day	14% 4 2 4 3 3 3 2 3 5% 59%	2000
	21st to end	Day	14% 5% 1 4 2 3 3 3 4 5% 57%	2200
November	1st to 10th	Day	14% 6% 2 4 3 3 3 3 3 6% 54%	2057
	11th to 20th	Day	13% 6% 1 4 2 2 3 3 3 7% 54%	1942
	21st to end	Day	14% 5% 1 5% 3 3 3 3 5% 6% 51%	1890
December	1st to 10th	Day	15% 6% 2 4 3 4 3 3 4 5% 51%	3020
	11th to 20th	Day	17% 7% 3 7% 3 3 3 3 4 6% 43%	3436
	21st to end	Day	21% 5% 2 5% 3 3 3 3 4 6% 46%	4466
January	1st to 10th	Night	13% 5% 1 5% 3 3 3 2 3 4 57%	5889
	11th to 20th	Night	12% 6% 2 5% 3 3 3 3 5% 5% 53%	5900
	21st to end	Night	16% 9% 2 6% 3 3 3 3 3 5% 47%	6490
February	1st to 10th	Night	15% 6% 2 6% 4 4 4 3 4 5% 47%	2585
	11th to 20th	Night	20% 7% 3 6% 3 3 3 3 4 5% 42%	2338
	21st to end	Night	16% 6% 2 5% 4 3 4 3 5% 8% 44%	1938
March	1st to 10th	Night	21% 7% 3 6% 4 3 3 3 4 6% 41%	4918
	11th to 20th	Night	10% 4 2 4 3 3 4 3 5% 7% 55%	4980
	21st to end	Night	15% 5% 2 5% 4 4 4 3 5% 6% 47%	5467
April	1st to 10th	Night	13% 4 2 5% 4 4 4 4 5% 7% 49%	2200
	11th to 20th	Night	12% 5% 2 4 3 3 4 3 5% 6% 53%	2005
	21st to end	Night	10% 5% 2 6% 4 3 3 4 5% 8% 51%	1854
May	1st to 10th	Night	12% 6% 3 6% 4 5% 5% 3 6% 9% 41%	4033
	11th to 20th	Night	20% 7% 2 6% 4 4 5% 4 7% 9% 32%	4032
	21st to end	Night	12% 6% 2 6% 4 5% 5% 4 7% 10% 40%	4421
June	1st to 10th	Night	11% 6% 2 8% 6% 6% 6% 5% 7% 11% 33%	1800
	11th to 20th	Night	10% 6% 2 6% 6% 4 5% 5% 8% 10% 37%	1800
	21st to end	Night	8% 5% 3 6% 7% 7% 6% 5% 8% 11% 36%	1800
July	1st to 10th	Night	11% 8% 3 8% 6% 6% 6% 5% 8% 10% 28%	4058
	11th to 20th	Night	8% 6% 2 7% 6% 6% 7% 6% 11% 14% 25%	4057
	21st to end	Night	12% 10% 4 10% 7% 6% 6% 6% 7% 9% 23%	4427
August	1st to 10th	Night	23% 8% 2 8% 5% 5% 5% 4 7% 9% 23%	2915
	11th to 20th	Night	12% 6% 3 7% 5% 5% 5% 6% 8% 11% 32%	2875
	21st to end	Night	6% 4 2 6% 4 4 5% 5% 7% 9% 47%	3499
September	1st to 10th	Night	9% 6% 2 7% 5% 4 5% 4 6% 10% 43%	2200
	11th to 20th	Night	13% 6% 2 7% 6% 4 4 4 6% 8% 39%	2200
	21st to end	Night	16% 6% 2 7% 5% 4 5% 3 6% 6% 41%	2200
October	1st to 10th	Night	14% 4 2 4 3 3 3 3 5% 5% 52%	2302
	11th to 20th	Night	13% 6% 2 5% 3 3 3 2 4 6% 54%	2400
	21st to end	Night	13% 5% 2 4 3 3 3 3 3 5% 57%	2640
November	1st to 10th	Night	14% 6% 2 4 3 3 2 2 4 6% 53%	2461
	11th to 20th	Night	13% 6% 2 4 3 3 3 3 4 6% 53%	2675
	21st to end	Night	13% 6% 2 5% 3 4 4 3 5% 6% 50%	2730
December	1st to 10th	Night	16% 5% 2 5% 3 3 3 3 4 5% 53%	4380
	11th to 20th	Night	18% 7% 2 6% 3 3 3 3 4 6% 45%	4914
	21st to end	Night	20% 5% 2 6% 3 3 3 3 5% 5% 45%	6490

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	32% 3 5% 21% 31% 7%	4057
	11th to 20th	Day	32% 3 4 15% 37% 9%	4060
	21st to end	Day	43% 3 4 17% 22% 11%	4466
February	1st to 10th	Day	40% 4 10% 21% 19% 5%	1810
	11th to 20th	Day	44% 4 6% 18% 23% 6%	1958
	21st to end	Day	44% 4 9% 14% 22% 7%	1690
March	1st to 10th	Day	46% 3 6% 13% 19% 12%	4935
	11th to 20th	Day	31% 5% 7% 13% 26% 19%	4980
	21st to end	Day	42% 4 5% 20% 23% 6%	5465
April	1st to 10th	Day	33% 5% 11% 20% 26% 6%	2200
	11th to 20th	Day	34% 6% 10% 20% 20% 10%	2344
	21st to end	Day	32% 7% 12% 23% 21% 4	2562
May	1st to 10th	Day	39% 9% 18% 22% 11% 2	5897
	11th to 20th	Day	48% 9% 14% 17% 12% 1	5865
	21st to end	Day	37% 8% 13% 21% 19% 3	6447
June	1st to 10th	Day	39% 8% 16% 24% 10% 2	2600
	11th to 20th	Day	34% 10% 19% 23% 13% 1	2600
	21st to end	Day	34% 12% 17% 21% 14% 2	2600
July	1st to 10th	Day	42% 11% 16% 22% 8% 1	5835
	11th to 20th	Day	37% 15% 29% 16% 3 1	5831
	21st to end	Day	43% 13% 17% 20% 5% 1	6240
August	1st to 10th	Day	53% 10% 17% 15% 5%	4221
	11th to 20th	Day	38% 13% 17% 19% 12% 1	4213
	21st to end	Day	28% 8% 16% 29% 16% 3	4360
September	1st to 10th	Day	35% 9% 16% 19% 18% 4	2200
	11th to 20th	Day	41% 8% 12% 17% 20% 3	2200
	21st to end	Day	44% 6% 10% 20% 15% 6%	2200
October	1st to 10th	Day	30% 5% 7% 20% 28% 10%	3407
	11th to 20th	Day	20% 2 6% 17% 29% 26%	3240
	21st to end	Day	26% 3 5% 23% 28% 15%	3538
November	1st to 10th	Day	33% 4 7% 17% 29% 9%	2057
	11th to 20th	Day	31% 4 8% 23% 26% 9%	1942
	21st to end	Day	34% 5% 9% 22% 23% 8%	1890
December	1st to 10th	Day	37% 3 6% 18% 27% 9%	3020
	11th to 20th	Day	42% 5% 8% 18% 20% 7%	3433
	21st to end	Day	40% 4 8% 24% 22% 2	4466
January	1st to 10th	Night	31% 3 5% 18% 35% 7%	5889
	11th to 20th	Night	33% 4 6% 15% 35% 7%	5900
	21st to end	Night	41% 3 4 16% 28% 8%	6490
February	1st to 10th	Night	40% 4 9% 20% 20% 7%	2585
	11th to 20th	Night	44% 4 8% 16% 22% 7%	2339
	21st to end	Night	38% 4 10% 17% 21% 9%	1938
March	1st to 10th	Night	43% 5% 8% 12% 25% 7%	4918
	11th to 20th	Night	28% 4 7% 13% 29% 18%	4980
	21st to end	Night	37% 5% 6% 21% 21% 9%	5467
April	1st to 10th	Night	34% 5% 10% 19% 24% 8%	2200
	11th to 20th	Night	32% 4 9% 23% 21% 10%	2005
	21st to end	Night	31% 5% 11% 27% 20% 4	1854
May	1st to 10th	Night	39% 7% 15% 23% 14% 2	4060
	11th to 20th	Night	46% 8% 16% 19% 10% 1	4033
	21st to end	Night	36% 9% 14% 23% 16% 2	4429
June	1st to 10th	Night	43% 9% 16% 20% 11% 2	1800
	11th to 20th	Night	38% 12% 17% 20% 12% 2	1800
	21st to end	Night	39% 11% 16% 17% 13% 3	1800
July	1st to 10th	Night	47% 11% 19% 16% 6% 1	4015
	11th to 20th	Night	41% 18% 26% 10% 3 1	4006
	21st to end	Night	54% 12% 15% 14% 4	4297
August	1st to 10th	Night	55% 10% 18% 14% 4	2904
	11th to 20th	Night	40% 12% 23% 16% 8% 1	2902
	21st to end	Night	29% 10% 13% 26% 20% 3	3499
September	1st to 10th	Night	35% 8% 15% 20% 17% 5%	2200
	11th to 20th	Night	42% 8% 12% 17% 18% 2	2200
	21st to end	Night	42% 7% 12% 18% 16% 6%	2200
October	1st to 10th	Night	31% 5% 9% 20% 28% 7%	3770
	11th to 20th	Night	23% 3 5% 17% 27% 25%	3899
	21st to end	Night	27% 4 5% 20% 27% 17%	4252
November	1st to 10th	Night	33% 4 7% 17% 26% 12%	2461
	11th to 20th	Night	33% 4 7% 17% 27% 12%	2675
	21st to end	Night	34% 4 6% 21% 27% 8%	2730
December	1st to 10th	Night	35% 4 6% 19% 29% 8%	4380
	11th to 20th	Night	41% 5% 11% 15% 19% 9%	4916
	21st to end	Night	41% 3 9% 24% 20% 2	6490

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	22% 23% 38% 16% 1	300
	11th to 20th	Day	27% 21% 33% 15% 4	300
	21st to end	Day	24% 18% 42% 12% 3	330
February	1st to 10th	Day	21% 19% 41% 15% 5%	300
	11th to 20th	Day	39% 14% 38% 8% 1	300
	21st to end	Day	25% 19% 36% 18% 2	289
March	1st to 10th	Day	25% 17% 32% 19% 7% 1	500
	11th to 20th	Day	22% 16% 39% 17% 6% 1	500
	21st to end	Day	24% 17% 35% 18% 5% 1	550
April	1st to 10th	Day	22% 14% 35% 20% 8% 2	500
	11th to 20th	Day	14% 16% 34% 23% 10% 3	492
	21st to end	Day	15% 9% 30% 26% 15% 5%	500
May	1st to 10th	Day	14% 14% 34% 22% 13% 4	503
	11th to 20th	Day	17% 7% 23% 28% 16% 8%	527
	21st to end	Day	16% 6% 18% 27% 19% 12%	573
June	1st to 10th	Day	12% 4 18% 35% 22% 9%	530
	11th to 20th	Day	8% 4 20% 30% 28% 11%	530
	21st to end	Day	8% 3 15% 33% 29% 12%	530
July	1st to 10th	Day	9% 2 11% 26% 35% 18%	525
	11th to 20th	Day	8% 2 9% 31% 34% 16%	524
	21st to end	Day	6% 2 12% 34% 33% 12%	576
August	1st to 10th	Day	19% 5% 14% 25% 24% 13%	526
	11th to 20th	Day	15% 3 11% 27% 31% 13%	519
	21st to end	Day	11% 3 13% 32% 25% 16%	583
September	1st to 10th	Day	9% 4 14% 32% 31% 11%	530
	11th to 20th	Day	15% 4 15% 27% 26% 12%	530
	21st to end	Day	19% 6% 19% 29% 22% 5%	530
October	1st to 10th	Day	15% 5% 24% 29% 20% 7%	530
	11th to 20th	Day	13% 8% 26% 27% 21% 5%	530
	21st to end	Day	18% 8% 25% 26% 18% 5%	583
November	1st to 10th	Day	20% 9% 24% 28% 16% 3	535
	11th to 20th	Day	16% 15% 37% 23% 8% 2	382
	21st to end	Day	13% 14% 36% 25% 11% 2	330
December	1st to 10th	Day	27% 15% 37% 17% 4	330
	11th to 20th	Day	23% 12% 36% 19% 8% 2	321
	21st to end	Day	28% 14% 33% 15% 7% 3	330
January	1st to 10th	Night	23% 16% 30% 25% 6%	800
	11th to 20th	Night	20% 14% 32% 28% 6%	800
	21st to end	Night	20% 16% 32% 29% 3	880
February	1st to 10th	Night	19% 14% 33% 30% 4	800
	11th to 20th	Night	25% 15% 36% 22% 1	783
	21st to end	Night	19% 16% 34% 27% 4	660
March	1st to 10th	Night	21% 13% 32% 29% 6%	800
	11th to 20th	Night	19% 13% 36% 25% 8%	800
	21st to end	Night	18% 14% 35% 26% 6%	880
April	1st to 10th	Night	15% 16% 31% 28% 9%	800
	11th to 20th	Night	15% 13% 28% 31% 11% 1	791
	21st to end	Night	13% 6% 30% 35% 16%	654
May	1st to 10th	Night	11% 8% 29% 38% 16%	600
	11th to 20th	Night	15% 6% 24% 34% 20% 1	591
	21st to end	Night	12% 4 15% 34% 32% 3	650
June	1st to 10th	Night	13% 3 16% 39% 27% 3	600
	11th to 20th	Night	12% 3 17% 31% 30% 7%	600
	21st to end	Night	9% 2 14% 36% 34% 6%	600
July	1st to 10th	Night	10% 3 9% 33% 37% 8%	585
	11th to 20th	Night	11% 2 6% 31% 41% 10%	582
	21st to end	Night	9% 4 10% 30% 39% 7%	657
August	1st to 10th	Night	16% 4 10% 27% 36% 7%	597
	11th to 20th	Night	12% 3 10% 30% 39% 6%	594
	21st to end	Night	12% 2 10% 30% 40% 7%	809
September	1st to 10th	Night	11% 3 12% 31% 38% 6%	800
	11th to 20th	Night	15% 3 12% 31% 35% 5%	800
	21st to end	Night	19% 6% 13% 33% 26% 4	800
October	1st to 10th	Night	16% 4 18% 29% 28% 5%	800
	11th to 20th	Night	17% 8% 23% 29% 21% 3	800
	21st to end	Night	15% 9% 21% 32% 21% 2	880
November	1st to 10th	Night	16% 7% 23% 34% 17% 3	820
	11th to 20th	Night	15% 13% 27% 34% 11% 1	840
	21st to end	Night	15% 12% 26% 34% 13% 1	840
December	1st to 10th	Night	23% 12% 26% 32% 7%	840
	11th to 20th	Night	20% 10% 25% 33% 11% 1	829
	21st to end	Night	26% 14% 22% 30% 8%	880

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