Chester
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Chester clear sky chart from 2005-11-06 to 2026-08-15.

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	18% 7% 2 6% 4 4 4 3 5% 7% 40%	143482
	Night	18% 8% 2 6% 4 4 4 3 5% 6% 41%	138417
Transparency	Day	30% 35% 8% 15% 11% 2	147029
	Night	30% 34% 7% 15% 12% 2	142959
Seeing	Day	23% 18% 32% 19% 7% 2	17335
	Night	22% 15% 32% 23% 7% 1	26270

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% 9% 3 6% 4 4 4 3 5% 5% 45%	4661
	11th to 20th	Day	15% 9% 2 6% 3 4 3 3 4 5% 46%	4650
	21st to end	Day	16% 9% 2 4 3 4 4 3 5% 6% 44%	5202
February	1st to 10th	Day	17% 9% 2 5% 5% 5% 5% 4 5% 6% 39%	1996
	11th to 20th	Day	15% 7% 3 6% 5% 5% 5% 3 6% 6% 41%	2048
	21st to end	Day	20% 10% 3 5% 4 4 3 3 4 6% 37%	1730
March	1st to 10th	Day	24% 9% 2 5% 3 3 3 2 4 5% 40%	4576
	11th to 20th	Day	14% 8% 3 5% 4 4 4 4 6% 6% 41%	4756
	21st to end	Day	21% 7% 3 6% 4 4 4 4 6% 7% 34%	6089
April	1st to 10th	Day	20% 8% 2 5% 4 4 4 3 5% 7% 39%	2520
	11th to 20th	Day	20% 7% 2 4 3 4 4 3 4 6% 42%	2494
	21st to end	Day	21% 5% 2 6% 4 4 4 3 5% 6% 40%	2659
May	1st to 10th	Day	27% 9% 2 5% 4 3 4 3 5% 7% 32%	7639
	11th to 20th	Day	20% 8% 3 6% 4 4 4 3 4 7% 38%	7860
	21st to end	Day	24% 7% 2 7% 4 4 4 4 5% 8% 32%	9060
June	1st to 10th	Day	17% 5% 2 5% 5% 5% 5% 4 7% 8% 37%	2940
	11th to 20th	Day	17% 6% 2 5% 4 4 4 4 6% 8% 39%	2940
	21st to end	Day	14% 7% 2 6% 5% 5% 4 4 6% 9% 38%	2940
July	1st to 10th	Day	18% 6% 2 6% 4 4 5% 4 6% 9% 35%	8280
	11th to 20th	Day	12% 5% 2 5% 4 4 4 4 6% 10% 42%	8280
	21st to end	Day	17% 6% 2 6% 4 3 4 3 5% 7% 42%	8993
August	1st to 10th	Day	20% 6% 2 7% 4 4 5% 4 6% 8% 33%	4474
	11th to 20th	Day	13% 6% 2 6% 4 5% 4 4 7% 8% 43%	3969
	21st to end	Day	8% 5% 2 6% 4 5% 6% 5% 7% 10% 43%	4312
September	1st to 10th	Day	15% 6% 2 6% 4 3 4 4 5% 6% 46%	2400
	11th to 20th	Day	12% 6% 2 5% 3 3 4 3 4 7% 52%	2240
	21st to end	Day	20% 7% 2 5% 4 4 4 3 5% 7% 39%	2200
October	1st to 10th	Day	21% 7% 2 5% 3 3 3 3 5% 5% 43%	2200
	11th to 20th	Day	15% 6% 1 5% 3 3 4 3 5% 7% 48%	1961
	21st to end	Day	18% 7% 2 6% 3 3 4 3 5% 6% 43%	1980
November	1st to 10th	Day	13% 6% 3 5% 4 4 4 3 4 5% 48%	1851
	11th to 20th	Day	17% 6% 2 4 3 3 3 3 5% 5% 49%	1890
	21st to end	Day	17% 7% 2 5% 4 4 4 3 4 7% 44%	1890
December	1st to 10th	Day	15% 6% 2 6% 4 4 3 3 4 6% 46%	2688
	11th to 20th	Day	21% 7% 2 5% 3 2 2 2 4 5% 47%	3066
	21st to end	Day	20% 10% 2 6% 4 3 3 3 4 5% 40%	4048
January	1st to 10th	Night	15% 7% 2 5% 3 3 4 3 5% 6% 47%	8234
	11th to 20th	Night	16% 9% 3 7% 4 3 3 3 4 4 44%	8235
	21st to end	Night	16% 8% 2 7% 4 5% 4 3 5% 5% 41%	8960
February	1st to 10th	Night	16% 9% 2 7% 5% 4 4 3 5% 5% 41%	2622
	11th to 20th	Night	18% 8% 2 6% 4 3 3 3 5% 6% 41%	2458
	21st to end	Night	20% 8% 2 6% 4 4 4 3 5% 5% 39%	2076
March	1st to 10th	Night	22% 10% 2 5% 3 3 3 2 4 6% 40%	5484
	11th to 20th	Night	16% 10% 3 6% 4 4 3 3 4 5% 41%	5422
	21st to end	Night	22% 9% 3 6% 4 3 4 3 4 5% 37%	5083
April	1st to 10th	Night	21% 7% 3 6% 5% 4 3 4 5% 6% 37%	2100
	11th to 20th	Night	20% 6% 2 6% 4 4 4 3 5% 6% 40%	2075
	21st to end	Night	22% 6% 2 6% 4 3 3 3 4 4 42%	1959
May	1st to 10th	Night	25% 10% 3 6% 4 4 3 3 4 6% 31%	5259
	11th to 20th	Night	21% 9% 3 6% 4 4 4 4 5% 7% 33%	5056
	21st to end	Night	24% 9% 3 6% 4 4 4 3 5% 6% 32%	5114
June	1st to 10th	Night	19% 7% 2 5% 5% 4 4 4 5% 7% 40%	1680
	11th to 20th	Night	19% 7% 2 5% 4 4 4 4 6% 8% 38%	1680
	21st to end	Night	14% 7% 2 5% 4 4 5% 4 6% 9% 40%	1680
July	1st to 10th	Night	20% 8% 3 7% 4 4 4 4 6% 8% 32%	4680
	11th to 20th	Night	17% 7% 2 6% 4 4 4 4 5% 9% 38%	4680
	21st to end	Night	16% 7% 2 5% 4 4 4 4 5% 8% 41%	5084
August	1st to 10th	Night	24% 7% 3 7% 5% 4 5% 4 6% 7% 29%	2860
	11th to 20th	Night	12% 7% 2 7% 5% 5% 6% 4 5% 7% 40%	3163
	21st to end	Night	9% 5% 1 4 3 3 4 3 5% 7% 55%	3586
September	1st to 10th	Night	14% 6% 3 6% 4 4 5% 4 5% 6% 43%	2000
	11th to 20th	Night	13% 6% 2 5% 3 3 3 2 4 5% 53%	2159
	21st to end	Night	19% 8% 2 6% 4 4 5% 3 4 5% 40%	2200
October	1st to 10th	Night	19% 7% 3 5% 4 3 3 3 4 6% 43%	2200
	11th to 20th	Night	17% 5% 3 5% 3 2 4 3 4 6% 48%	2433
	21st to end	Night	21% 7% 2 5% 3 3 3 3 5% 6% 42%	2860
November	1st to 10th	Night	13% 5% 2 5% 4 3 3 3 5% 6% 49%	2667
	11th to 20th	Night	18% 6% 2 5% 3 3 3 3 4 6% 47%	2730
	21st to end	Night	15% 6% 2 6% 4 3 4 3 4 6% 47%	2730
December	1st to 10th	Night	17% 7% 2 5% 3 3 3 3 4 5% 48%	4710
	11th to 20th	Night	25% 8% 2 5% 3 3 3 3 4 4 42%	5348
	21st to end	Night	19% 9% 3 6% 3 3 3 2 4 5% 43%	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	33% 22% 7% 20% 15% 3	4662
	11th to 20th	Day	31% 21% 6% 18% 15% 8%	4680
	21st to end	Day	33% 23% 8% 20% 15% 1	5202
February	1st to 10th	Day	20% 59% 6% 9% 6% 1	1996
	11th to 20th	Day	20% 58% 3 9% 9% 1	2057
	21st to end	Day	18% 60% 5% 10% 6% 1	1730
March	1st to 10th	Day	36% 29% 6% 9% 10% 9%	4579
	11th to 20th	Day	28% 31% 10% 23% 9% 1	4756
	21st to end	Day	37% 30% 6% 10% 13% 5%	6089
April	1st to 10th	Day	22% 59% 4 7% 6% 2	2520
	11th to 20th	Day	18% 59% 5% 8% 10%	2494
	21st to end	Day	19% 59% 3 10% 7% 1	2659
May	1st to 10th	Day	44% 25% 11% 11% 8% 1	7651
	11th to 20th	Day	38% 24% 11% 17% 9% 1	7864
	21st to end	Day	43% 24% 8% 15% 9%	9061
June	1st to 10th	Day	18% 59% 4 10% 9%	2940
	11th to 20th	Day	17% 60% 6% 10% 6% 1	2940
	21st to end	Day	17% 58% 5% 10% 9% 1	2940
July	1st to 10th	Day	39% 24% 11% 16% 10% 1	8210
	11th to 20th	Day	30% 23% 14% 23% 10% 1	8205
	21st to end	Day	35% 22% 8% 19% 16% 1	8795
August	1st to 10th	Day	35% 36% 10% 13% 5%	4460
	11th to 20th	Day	24% 38% 11% 22% 5%	4007
	21st to end	Day	20% 38% 10% 19% 12% 1	4312
September	1st to 10th	Day	16% 58% 5% 12% 8% 1	2400
	11th to 20th	Day	16% 57% 4 10% 12% 1	2240
	21st to end	Day	22% 57% 4 8% 7% 1	2200
October	1st to 10th	Day	22% 37% 6% 12% 17% 5%	3590
	11th to 20th	Day	16% 36% 4 14% 27% 3	3177
	21st to end	Day	18% 36% 10% 18% 16% 2	3180
November	1st to 10th	Day	19% 54% 7% 9% 10% 1	1851
	11th to 20th	Day	15% 54% 4 11% 14% 3	1890
	21st to end	Day	18% 54% 5% 14% 6% 2	1890
December	1st to 10th	Day	25% 35% 10% 20% 10%	2688
	11th to 20th	Day	28% 30% 9% 13% 16% 4	3066
	21st to end	Day	37% 26% 3 16% 13% 4	4048
January	1st to 10th	Night	31% 21% 8% 23% 15% 2	8234
	11th to 20th	Night	34% 21% 8% 16% 15% 5%	8280
	21st to end	Night	38% 23% 6% 15% 15% 3	8960
February	1st to 10th	Night	19% 59% 5% 11% 5% 1	2622
	11th to 20th	Night	18% 57% 3 9% 11% 2	2460
	21st to end	Night	20% 59% 4 9% 7% 2	2076
March	1st to 10th	Night	36% 29% 7% 12% 10% 7%	5494
	11th to 20th	Night	32% 30% 11% 16% 8% 3	5423
	21st to end	Night	39% 29% 3 6% 19% 4	5083
April	1st to 10th	Night	21% 60% 3 8% 6% 2	2100
	11th to 20th	Night	18% 60% 5% 8% 9% 1	2075
	21st to end	Night	19% 59% 3 10% 8% 1	1959
May	1st to 10th	Night	44% 24% 11% 12% 8%	5265
	11th to 20th	Night	41% 25% 12% 15% 6% 2	5057
	21st to end	Night	45% 24% 8% 17% 6%	5115
June	1st to 10th	Night	18% 59% 3 10% 9% 1	1680
	11th to 20th	Night	18% 60% 5% 11% 6% 1	1680
	21st to end	Night	17% 58% 5% 13% 7% 1	1680
July	1st to 10th	Night	43% 22% 10% 15% 9% 1	4640
	11th to 20th	Night	37% 22% 11% 20% 10% 1	4632
	21st to end	Night	34% 22% 10% 20% 14% 1	4967
August	1st to 10th	Night	40% 37% 8% 9% 6% 1	2850
	11th to 20th	Night	28% 38% 10% 20% 4	3191
	21st to end	Night	13% 37% 13% 22% 14% 1	3586
September	1st to 10th	Night	17% 58% 4 14% 8% 1	2000
	11th to 20th	Night	16% 56% 4 10% 12% 2	2159
	21st to end	Night	23% 58% 3 8% 8% 1	2200
October	1st to 10th	Night	23% 37% 8% 16% 14% 2	3590
	11th to 20th	Night	12% 36% 6% 16% 27% 4	3956
	21st to end	Night	17% 36% 8% 21% 16% 2	4610
November	1st to 10th	Night	19% 53% 5% 11% 11% 1	2667
	11th to 20th	Night	16% 54% 4 9% 12% 4	2730
	21st to end	Night	18% 55% 5% 15% 7% 1	2730
December	1st to 10th	Night	26% 34% 7% 21% 11% 1	4710
	11th to 20th	Night	34% 31% 6% 11% 16% 2	5348
	21st to end	Night	35% 27% 5% 14% 15% 4	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	30% 25% 29% 14% 2	220
	11th to 20th	Day	33% 29% 26% 11% 1	220
	21st to end	Day	27% 30% 30% 8% 4	256
February	1st to 10th	Day	30% 23% 33% 12% 1	436
	11th to 20th	Day	30% 28% 32% 9% 1	536
	21st to end	Day	31% 26% 30% 11% 3	446
March	1st to 10th	Day	29% 24% 33% 13% 2	540
	11th to 20th	Day	27% 24% 34% 11% 2 1	540
	21st to end	Day	33% 24% 29% 10% 3 1	594
April	1st to 10th	Day	32% 26% 26% 11% 4 1	540
	11th to 20th	Day	29% 26% 29% 11% 5%	532
	21st to end	Day	29% 19% 34% 13% 3 1	540
May	1st to 10th	Day	33% 20% 30% 12% 5% 1	543
	11th to 20th	Day	25% 17% 40% 13% 5% 1	567
	21st to end	Day	23% 16% 30% 22% 8% 1	617
June	1st to 10th	Day	19% 14% 34% 22% 9% 1	570
	11th to 20th	Day	20% 18% 33% 21% 7% 1	570
	21st to end	Day	17% 14% 33% 25% 9% 2	570
July	1st to 10th	Day	18% 9% 33% 26% 12% 2	565
	11th to 20th	Day	14% 9% 32% 31% 12% 2	564
	21st to end	Day	15% 11% 38% 27% 7% 2	620
August	1st to 10th	Day	13% 9% 31% 33% 10% 4	566
	11th to 20th	Day	15% 7% 32% 29% 13% 4	535
	21st to end	Day	10% 12% 35% 24% 13% 5%	583
September	1st to 10th	Day	19% 10% 29% 24% 14% 3	530
	11th to 20th	Day	13% 10% 33% 24% 12% 7%	530
	21st to end	Day	22% 12% 35% 20% 8% 2	530
October	1st to 10th	Day	27% 12% 25% 21% 10% 5%	530
	11th to 20th	Day	21% 20% 29% 21% 7% 2	508
	21st to end	Day	25% 18% 30% 19% 7% 2	462
November	1st to 10th	Day	21% 17% 33% 20% 8% 1	425
	11th to 20th	Day	25% 24% 27% 18% 5%	430
	21st to end	Day	26% 22% 27% 20% 5%	430
December	1st to 10th	Day	26% 25% 31% 14% 4	228
	11th to 20th	Day	36% 24% 28% 10% 2	220
	21st to end	Day	30% 24% 27% 14% 4 1	242
January	1st to 10th	Night	22% 23% 36% 16% 3	840
	11th to 20th	Night	29% 20% 32% 16% 2	840
	21st to end	Night	21% 24% 35% 18% 2	924
February	1st to 10th	Night	23% 21% 35% 20% 2	840
	11th to 20th	Night	26% 22% 37% 14% 1	823
	21st to end	Night	26% 22% 33% 16% 3	692
March	1st to 10th	Night	26% 18% 34% 19% 3	840
	11th to 20th	Night	24% 18% 36% 18% 3	840
	21st to end	Night	24% 22% 35% 15% 3	924
April	1st to 10th	Night	25% 20% 34% 18% 3	840
	11th to 20th	Night	23% 17% 36% 21% 3	831
	21st to end	Night	23% 16% 37% 17% 5% 1	699
May	1st to 10th	Night	26% 15% 33% 21% 6%	630
	11th to 20th	Night	22% 14% 39% 20% 5%	560
	21st to end	Night	22% 7% 31% 32% 8% 1	455
June	1st to 10th	Night	22% 6% 29% 33% 10%	420
	11th to 20th	Night	21% 7% 31% 34% 8%	420
	21st to end	Night	16% 5% 32% 37% 9%	420
July	1st to 10th	Night	16% 6% 28% 34% 15%	410
	11th to 20th	Night	15% 4 26% 39% 14% 1	408
	21st to end	Night	17% 5% 27% 40% 11% 1	460
August	1st to 10th	Night	12% 4 29% 39% 15% 1	534
	11th to 20th	Night	14% 4 26% 37% 17% 2	756
	21st to end	Night	14% 6% 26% 35% 18% 2	880
September	1st to 10th	Night	15% 6% 32% 33% 13% 2	800
	11th to 20th	Night	16% 7% 25% 31% 18% 3	800
	21st to end	Night	21% 9% 27% 28% 14% 2	800
October	1st to 10th	Night	21% 11% 29% 24% 13% 3	800
	11th to 20th	Night	21% 14% 32% 23% 10%	800
	21st to end	Night	25% 13% 34% 20% 7%	880
November	1st to 10th	Night	17% 16% 34% 22% 10% 1	820
	11th to 20th	Night	23% 20% 32% 20% 5% 1	840
	21st to end	Night	22% 20% 31% 22% 5%	840
December	1st to 10th	Night	27% 19% 29% 20% 5% 1	840
	11th to 20th	Night	27% 21% 32% 18% 2	840
	21st to end	Night	27% 18% 36% 17% 2	924

Page generated at 2026-08-12 03:37:17. Version ac07

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