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 2026-10-14.

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% 7% 3 7% 5% 5% 5% 4 6% 7% 35%	149379
	Night	14% 8% 3 7% 5% 5% 5% 4 6% 7% 37%	142364
Transparency	Day	45% 6% 14% 21% 12% 2	152875
	Night	45% 6% 14% 21% 12% 2	146944
Seeing	Day	23% 18% 30% 18% 8% 2	17341
	Night	20% 16% 29% 26% 8% 1	26223

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	8% 8% 4 15% 8% 8% 6% 5% 6% 5% 27%	4661
	11th to 20th	Day	9% 10% 4 14% 8% 6% 5% 3 5% 5% 31%	4650
	21st to end	Day	9% 10% 4 8% 5% 5% 5% 4 6% 7% 38%	5124
February	1st to 10th	Day	12% 10% 4 10% 6% 6% 5% 4 6% 7% 30%	1891
	11th to 20th	Day	9% 9% 4 10% 6% 6% 5% 4 6% 7% 33%	2048
	21st to end	Day	15% 12% 4 9% 6% 4 5% 4 5% 5% 32%	1730
March	1st to 10th	Day	16% 12% 4 10% 5% 5% 5% 3 5% 4 30%	4576
	11th to 20th	Day	14% 8% 3 8% 5% 4 5% 4 6% 6% 36%	4747
	21st to end	Day	21% 9% 2 7% 5% 4 4 3 4 6% 34%	6102
April	1st to 10th	Day	17% 11% 3 7% 5% 5% 4 3 5% 6% 35%	2520
	11th to 20th	Day	16% 8% 2 6% 5% 5% 5% 4 6% 7% 36%	2494
	21st to end	Day	19% 8% 3 8% 5% 5% 5% 4 5% 6% 33%	2695
May	1st to 10th	Day	26% 9% 3 7% 5% 5% 4 4 6% 6% 26%	7639
	11th to 20th	Day	18% 9% 3 8% 5% 5% 4 4 5% 7% 33%	8180
	21st to end	Day	19% 8% 3 7% 5% 5% 5% 4 7% 8% 30%	9060
June	1st to 10th	Day	17% 6% 2 7% 6% 6% 5% 4 7% 8% 32%	2940
	11th to 20th	Day	16% 6% 2 5% 5% 4 5% 4 7% 8% 36%	2940
	21st to end	Day	16% 7% 3 7% 6% 5% 5% 4 6% 7% 33%	2940
July	1st to 10th	Day	12% 6% 2 7% 4 5% 6% 5% 7% 10% 36%	8280
	11th to 20th	Day	7% 4 2 6% 4 5% 5% 5% 8% 10% 44%	8280
	21st to end	Day	15% 5% 2 6% 4 4 5% 4 6% 8% 42%	8993
August	1st to 10th	Day	12% 6% 3 8% 5% 6% 6% 5% 7% 9% 35%	6349
	11th to 20th	Day	12% 6% 2 6% 4 4 5% 5% 7% 10% 39%	5723
	21st to end	Day	10% 4 2 6% 4 5% 5% 5% 8% 11% 40%	6080
September	1st to 10th	Day	12% 5% 2 6% 4 4 4 4 6% 9% 45%	2520
	11th to 20th	Day	12% 5% 1 6% 3 4 4 3 6% 7% 49%	2370
	21st to end	Day	16% 5% 2 6% 4 4 4 3 6% 8% 42%	2310
October	1st to 10th	Day	19% 7% 2 7% 4 4 4 3 5% 5% 39%	2273
	11th to 20th	Day	15% 7% 2 6% 4 4 4 4 6% 8% 40%	1969
	21st to end	Day	21% 8% 3 8% 5% 4 4 4 4 6% 33%	1980
November	1st to 10th	Day	11% 7% 2 7% 3 4 4 3 6% 8% 43%	1851
	11th to 20th	Day	15% 8% 3 6% 5% 4 5% 4 5% 6% 40%	1890
	21st to end	Day	17% 8% 3 7% 4 4 3 5% 7% 7% 34%	1780
December	1st to 10th	Day	13% 11% 4 9% 6% 6% 5% 3 5% 6% 31%	2680
	11th to 20th	Day	19% 9% 3 7% 5% 4 4 4 5% 5% 35%	3066
	21st to end	Day	17% 10% 4 9% 5% 5% 5% 3 4 5% 32%	4048
January	1st to 10th	Night	10% 7% 4 13% 8% 6% 5% 4 6% 6% 30%	8234
	11th to 20th	Night	10% 10% 4 12% 8% 7% 6% 4 5% 6% 27%	8235
	21st to end	Night	10% 9% 3 7% 5% 6% 6% 5% 6% 7% 36%	9057
February	1st to 10th	Night	11% 10% 4 9% 6% 5% 5% 5% 6% 7% 33%	2727
	11th to 20th	Night	9% 8% 3 8% 6% 6% 5% 6% 7% 7% 35%	2458
	21st to end	Night	14% 8% 3 7% 4 5% 5% 4 7% 6% 36%	2076
March	1st to 10th	Night	19% 10% 3 7% 4 4 4 4 5% 7% 32%	5484
	11th to 20th	Night	13% 10% 3 7% 5% 5% 5% 4 5% 7% 37%	5414
	21st to end	Night	19% 12% 3 7% 4 4 4 3 5% 5% 33%	5072
April	1st to 10th	Night	18% 7% 2 6% 5% 4 4 4 6% 7% 36%	2100
	11th to 20th	Night	17% 9% 3 6% 4 4 4 4 5% 6% 37%	2075
	21st to end	Night	18% 7% 2 6% 5% 3 4 3 5% 7% 39%	1923
May	1st to 10th	Night	28% 8% 3 6% 4 4 4 4 6% 7% 26%	5259
	11th to 20th	Night	14% 9% 3 9% 6% 5% 5% 4 7% 8% 32%	4723
	21st to end	Night	19% 6% 2 6% 5% 4 5% 4 6% 8% 33%	5114
June	1st to 10th	Night	18% 5% 3 5% 4 4 4 3 6% 8% 40%	1680
	11th to 20th	Night	17% 6% 2 6% 3 3 4 5% 6% 8% 40%	1680
	21st to end	Night	13% 7% 2 6% 5% 5% 5% 5% 6% 7% 39%	1680
July	1st to 10th	Night	11% 5% 2 6% 5% 5% 5% 4 6% 9% 42%	4680
	11th to 20th	Night	9% 5% 2 5% 4 5% 5% 4 7% 9% 46%	4680
	21st to end	Night	13% 5% 2 5% 3 3 4 3 5% 7% 48%	5084
August	1st to 10th	Night	14% 6% 3 8% 5% 5% 5% 4 6% 8% 37%	3755
	11th to 20th	Night	13% 7% 2 7% 4 4 5% 4 5% 8% 41%	4319
	21st to end	Night	10% 4 2 5% 3 4 5% 4 7% 10% 48%	5045
September	1st to 10th	Night	11% 6% 2 5% 3 2 4 3 5% 7% 50%	2100
	11th to 20th	Night	12% 5% 2 4 3 3 4 3 4 6% 53%	2249
	21st to end	Night	17% 6% 2 4 4 4 4 3 6% 7% 45%	2310
October	1st to 10th	Night	18% 6% 2 5% 4 4 4 3 5% 6% 42%	2344
	11th to 20th	Night	15% 5% 2 6% 4 4 5% 5% 5% 8% 40%	2497
	21st to end	Night	21% 8% 2 6% 4 5% 5% 4 6% 6% 34%	2860
November	1st to 10th	Night	13% 7% 2 6% 5% 5% 4 3 5% 6% 44%	2667
	11th to 20th	Night	15% 7% 2 6% 5% 4 4 4 5% 7% 41%	2730
	21st to end	Night	13% 9% 3 6% 5% 4 5% 5% 6% 7% 38%	2835
December	1st to 10th	Night	12% 11% 4 10% 6% 5% 4 4 6% 7% 31%	4720
	11th to 20th	Night	20% 9% 4 8% 5% 4 4 4 5% 5% 34%	5348
	21st to end	Night	18% 13% 4 9% 4 4 4 3 5% 5% 32%	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	54% 5% 12% 22% 7%	4662
	11th to 20th	Day	53% 6% 10% 16% 13% 2	4680
	21st to end	Day	46% 4 10% 24% 14% 2	5124
February	1st to 10th	Day	51% 8% 13% 17% 10% 1	1891
	11th to 20th	Day	48% 9% 11% 20% 10% 2	2057
	21st to end	Day	53% 6% 12% 16% 12% 2	1730
March	1st to 10th	Day	56% 5% 10% 12% 11% 7%	4579
	11th to 20th	Day	44% 8% 16% 20% 9% 3	4747
	21st to end	Day	50% 4 8% 15% 18% 5%	6102
April	1st to 10th	Day	50% 6% 10% 19% 12% 3	2520
	11th to 20th	Day	45% 9% 11% 18% 15% 2	2494
	21st to end	Day	51% 7% 9% 17% 13% 3	2695
May	1st to 10th	Day	57% 6% 11% 17% 9% 1	7651
	11th to 20th	Day	50% 6% 15% 18% 10% 1	8184
	21st to end	Day	48% 7% 13% 23% 9% 1	9061
June	1st to 10th	Day	48% 8% 13% 19% 11% 1	2940
	11th to 20th	Day	44% 7% 15% 18% 15% 2	2940
	21st to end	Day	48% 9% 13% 18% 11% 1	2940
July	1st to 10th	Day	42% 5% 18% 23% 13%	8210
	11th to 20th	Day	33% 5% 17% 31% 14% 1	8205
	21st to end	Day	39% 4 16% 25% 14% 1	8795
August	1st to 10th	Day	44% 6% 21% 16% 11% 1	6335
	11th to 20th	Day	39% 5% 20% 29% 6% 1	5762
	21st to end	Day	34% 6% 21% 26% 12% 1	6080
September	1st to 10th	Day	36% 7% 14% 28% 15% 1	2520
	11th to 20th	Day	34% 6% 12% 27% 20% 2	2370
	21st to end	Day	40% 5% 13% 25% 14% 2	2310
October	1st to 10th	Day	44% 5% 10% 22% 17% 2	3637
	11th to 20th	Day	40% 5% 13% 28% 14% 1	3159
	21st to end	Day	38% 6% 9% 31% 13% 3	3180
November	1st to 10th	Day	40% 7% 15% 23% 15% 1	1851
	11th to 20th	Day	44% 8% 12% 18% 17% 2	1890
	21st to end	Day	49% 8% 16% 20% 7% 1	1780
December	1st to 10th	Day	53% 5% 15% 20% 6%	2680
	11th to 20th	Day	50% 5% 11% 18% 13% 2	3066
	21st to end	Day	52% 6% 9% 17% 14% 2	4048
January	1st to 10th	Night	51% 6% 13% 19% 9% 1	8234
	11th to 20th	Night	55% 6% 12% 14% 10% 3	8280
	21st to end	Night	47% 5% 9% 18% 16% 4	9057
February	1st to 10th	Night	49% 8% 13% 19% 11% 1	2727
	11th to 20th	Night	44% 8% 14% 20% 12% 2	2460
	21st to end	Night	45% 8% 13% 22% 9% 3	2076
March	1st to 10th	Night	51% 4 11% 16% 14% 4	5494
	11th to 20th	Night	45% 7% 18% 18% 10% 2	5415
	21st to end	Night	51% 5% 8% 14% 20% 2	5072
April	1st to 10th	Night	46% 7% 12% 20% 14% 1	2100
	11th to 20th	Night	45% 7% 11% 21% 14% 1	2075
	21st to end	Night	45% 7% 11% 18% 16% 3	1923
May	1st to 10th	Night	56% 6% 14% 14% 10% 1	5265
	11th to 20th	Night	48% 7% 15% 21% 8% 1	4724
	21st to end	Night	47% 6% 13% 23% 9% 1	5115
June	1st to 10th	Night	41% 8% 15% 24% 12% 1	1680
	11th to 20th	Night	42% 9% 16% 21% 12% 1	1680
	21st to end	Night	41% 9% 16% 23% 10% 1	1680
July	1st to 10th	Night	39% 5% 20% 24% 12%	4640
	11th to 20th	Night	34% 5% 16% 27% 17% 1	4632
	21st to end	Night	36% 4 16% 31% 13% 1	4967
August	1st to 10th	Night	45% 7% 18% 19% 9% 2	3745
	11th to 20th	Night	41% 5% 20% 25% 8%	4346
	21st to end	Night	32% 4 23% 29% 12%	5045
September	1st to 10th	Night	34% 7% 14% 28% 16%	2100
	11th to 20th	Night	31% 6% 13% 29% 20% 1	2249
	21st to end	Night	39% 6% 12% 26% 15% 2	2310
October	1st to 10th	Night	42% 6% 14% 23% 14% 1	3760
	11th to 20th	Night	33% 6% 14% 28% 18% 2	4033
	21st to end	Night	41% 5% 10% 33% 10% 1	4610
November	1st to 10th	Night	40% 8% 13% 23% 14% 2	2667
	11th to 20th	Night	42% 7% 13% 22% 15% 2	2730
	21st to end	Night	45% 7% 17% 21% 9% 1	2835
December	1st to 10th	Night	52% 5% 14% 22% 6%	4720
	11th to 20th	Night	53% 6% 10% 15% 15% 1	5348
	21st to end	Night	54% 6% 10% 17% 11% 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	28% 28% 24% 17% 3	220
	11th to 20th	Day	30% 25% 29% 14% 1 1	220
	21st to end	Day	23% 29% 34% 11% 4	242
February	1st to 10th	Day	27% 34% 28% 10% 2	346
	11th to 20th	Day	31% 24% 28% 15% 2	536
	21st to end	Day	28% 27% 28% 14% 3	446
March	1st to 10th	Day	29% 20% 34% 15% 2	540
	11th to 20th	Day	30% 27% 27% 12% 3 1	540
	21st to end	Day	34% 22% 27% 10% 4 2	594
April	1st to 10th	Day	37% 21% 25% 12% 5% 1	540
	11th to 20th	Day	34% 23% 25% 12% 5% 2	532
	21st to end	Day	29% 21% 30% 14% 5% 1	540
May	1st to 10th	Day	30% 24% 27% 11% 6% 1	543
	11th to 20th	Day	21% 20% 35% 17% 4 2	567
	21st to end	Day	21% 14% 34% 23% 7% 1	617
June	1st to 10th	Day	20% 16% 34% 20% 9% 1	570
	11th to 20th	Day	20% 17% 34% 18% 9% 1	570
	21st to end	Day	18% 18% 34% 19% 8% 3	570
July	1st to 10th	Day	15% 12% 32% 25% 14% 3	565
	11th to 20th	Day	13% 9% 38% 25% 13% 2	564
	21st to end	Day	14% 11% 37% 23% 12% 3	620
August	1st to 10th	Day	11% 11% 29% 30% 14% 4	566
	11th to 20th	Day	14% 10% 32% 22% 15% 7%	559
	21st to end	Day	12% 14% 28% 25% 14% 7%	627
September	1st to 10th	Day	16% 12% 28% 22% 15% 7%	570
	11th to 20th	Day	15% 12% 27% 19% 17% 10%	570
	21st to end	Day	18% 13% 29% 22% 12% 6%	570
October	1st to 10th	Day	22% 16% 25% 20% 13% 4	570
	11th to 20th	Day	23% 17% 27% 22% 10% 2	508
	21st to end	Day	30% 16% 27% 19% 7% 1	462
November	1st to 10th	Day	25% 19% 24% 21% 10% 1	425
	11th to 20th	Day	29% 21% 27% 17% 6% 1	430
	21st to end	Day	32% 21% 28% 16% 3	320
December	1st to 10th	Day	25% 27% 28% 14% 5%	220
	11th to 20th	Day	38% 21% 29% 8% 4	220
	21st to end	Day	31% 19% 30% 17% 4	242
January	1st to 10th	Night	21% 20% 36% 20% 2	840
	11th to 20th	Night	25% 19% 33% 21% 3	840
	21st to end	Night	18% 21% 35% 22% 4	924
February	1st to 10th	Night	21% 20% 37% 19% 2	840
	11th to 20th	Night	21% 23% 35% 19% 2	823
	21st to end	Night	24% 22% 32% 20% 2	692
March	1st to 10th	Night	26% 18% 34% 20% 2	840
	11th to 20th	Night	25% 22% 29% 21% 4	840
	21st to end	Night	24% 20% 30% 20% 5%	924
April	1st to 10th	Night	26% 19% 30% 21% 4	840
	11th to 20th	Night	23% 22% 29% 20% 7%	831
	21st to end	Night	23% 18% 29% 25% 5%	663
May	1st to 10th	Night	27% 15% 29% 23% 7%	630
	11th to 20th	Night	16% 15% 33% 31% 6%	448
	21st to end	Night	18% 8% 31% 30% 11% 1	455
June	1st to 10th	Night	18% 9% 28% 33% 12%	420
	11th to 20th	Night	17% 11% 25% 34% 12%	420
	21st to end	Night	14% 6% 29% 40% 10% 1	420
July	1st to 10th	Night	12% 8% 20% 46% 13% 1	410
	11th to 20th	Night	12% 5% 24% 40% 19% 1	408
	21st to end	Night	12% 4 27% 39% 17% 2	460
August	1st to 10th	Night	11% 5% 20% 45% 19% 1	455
	11th to 20th	Night	13% 6% 27% 34% 18% 3	719
	21st to end	Night	15% 6% 22% 36% 20% 2	924
September	1st to 10th	Night	14% 7% 27% 34% 18% 1	840
	11th to 20th	Night	14% 9% 22% 31% 20% 5%	840
	21st to end	Night	19% 8% 24% 31% 15% 3	840
October	1st to 10th	Night	22% 13% 20% 29% 15% 1	840
	11th to 20th	Night	19% 13% 26% 30% 10% 1	813
	21st to end	Night	25% 15% 28% 26% 6%	880
November	1st to 10th	Night	19% 18% 28% 27% 8%	820
	11th to 20th	Night	20% 19% 30% 22% 6% 1	840
	21st to end	Night	23% 19% 29% 23% 5% 1	840
December	1st to 10th	Night	23% 21% 33% 19% 4	840
	11th to 20th	Night	27% 20% 29% 21% 3	840
	21st to end	Night	27% 21% 30% 19% 2	924

Page generated at 2026-10-11 05:06:31. Version ac07

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