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 2026-05-02.

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	20% 10% 3 8% 5% 5% 5% 4 5% 7% 28%	132676
	Night	20% 10% 3 9% 5% 5% 4 4 5% 6% 29%	129946
Transparency	Day	53% 6% 12% 18% 10% 1	136263
	Night	54% 6% 11% 19% 10% 1	134448
Seeing	Day	26% 16% 31% 19% 6% 1	19294
	Night	26% 15% 30% 22% 7% 1	26860

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	14% 13% 5% 13% 9% 7% 5% 4 4 6% 20%	4690
	11th to 20th	Day	17% 14% 6% 15% 8% 7% 6% 4 5% 5% 13%	4946
	21st to end	Day	15% 9% 3 10% 8% 6% 6% 4 6% 7% 26%	5781
February	1st to 10th	Day	19% 11% 5% 11% 8% 5% 5% 4 5% 7% 20%	1890
	11th to 20th	Day	16% 10% 3 12% 7% 6% 6% 5% 6% 6% 24%	1843
	21st to end	Day	13% 11% 4 10% 6% 5% 5% 3 5% 7% 32%	1589
March	1st to 10th	Day	23% 11% 4 11% 6% 5% 5% 3 3 4 24%	5040
	11th to 20th	Day	17% 14% 4 7% 5% 5% 4 3 5% 6% 30%	5090
	21st to end	Day	27% 10% 3 11% 6% 5% 4 3 5% 5% 21%	5587
April	1st to 10th	Day	24% 12% 3 9% 5% 3 4 3 4 5% 27%	2496
	11th to 20th	Day	21% 9% 3 7% 5% 4 4 4 5% 7% 32%	2578
	21st to end	Day	22% 9% 3 6% 5% 5% 4 3 5% 7% 32%	2730
May	1st to 10th	Day	27% 12% 3 9% 6% 5% 5% 4 5% 6% 19%	5981
	11th to 20th	Day	24% 9% 3 8% 5% 4 5% 4 5% 6% 27%	6318
	21st to end	Day	19% 10% 3 7% 4 4 4 4 6% 8% 30%	6948
June	1st to 10th	Day	23% 10% 3 7% 6% 4 5% 4 6% 7% 26%	2800
	11th to 20th	Day	21% 9% 3 6% 5% 3 4 3 5% 6% 35%	2890
	21st to end	Day	26% 9% 3 8% 6% 5% 4 3 5% 6% 24%	3000
July	1st to 10th	Day	21% 10% 3 8% 5% 5% 5% 4 7% 9% 25%	6618
	11th to 20th	Day	16% 11% 3 7% 4 4 4 4 6% 9% 31%	6360
	21st to end	Day	15% 6% 2 6% 4 5% 4 4 6% 9% 37%	6622
August	1st to 10th	Day	21% 10% 2 7% 5% 4 5% 4 5% 8% 28%	4235
	11th to 20th	Day	15% 8% 3 7% 4 4 5% 4 6% 9% 34%	4175
	21st to end	Day	9% 5% 2 6% 4 4 5% 4 6% 9% 46%	4493
September	1st to 10th	Day	15% 7% 2 6% 4 4 4 3 5% 8% 42%	2400
	11th to 20th	Day	17% 6% 2 6% 4 5% 4 3 5% 6% 44%	2305
	21st to end	Day	18% 7% 2 6% 4 4 4 3 4 6% 42%	2147
October	1st to 10th	Day	17% 6% 2 7% 4 4 4 4 5% 8% 39%	2000
	11th to 20th	Day	19% 8% 3 6% 5% 4 4 4 7% 8% 33%	2000
	21st to end	Day	20% 9% 3 7% 4 4 4 4 5% 6% 33%	2200
November	1st to 10th	Day	20% 9% 3 9% 4 5% 5% 3 4 7% 30%	1728
	11th to 20th	Day	22% 11% 3 9% 5% 5% 4 4 6% 7% 24%	1680
	21st to end	Day	21% 11% 4 9% 7% 5% 4 3 5% 6% 26%	1680
December	1st to 10th	Day	23% 10% 4 10% 5% 4 5% 4 6% 7% 22%	2690
	11th to 20th	Day	30% 9% 2 7% 4 5% 4 4 5% 5% 25%	3076
	21st to end	Day	22% 11% 4 10% 5% 4 3 3 4 7% 27%	4070
January	1st to 10th	Night	14% 12% 5% 16% 8% 6% 6% 4 5% 6% 18%	8234
	11th to 20th	Night	22% 12% 6% 14% 7% 5% 5% 4 5% 5% 15%	7954
	21st to end	Night	13% 11% 4 11% 8% 6% 6% 5% 7% 8% 21%	8400
February	1st to 10th	Night	20% 12% 4 10% 6% 6% 5% 6% 6% 5% 21%	2730
	11th to 20th	Night	16% 10% 4 9% 8% 5% 6% 5% 6% 7% 24%	2663
	21st to end	Night	13% 11% 4 9% 6% 5% 5% 5% 5% 6% 29%	2212
March	1st to 10th	Night	25% 12% 4 8% 6% 5% 4 4 4 5% 23%	5020
	11th to 20th	Night	19% 13% 5% 8% 5% 4 4 4 5% 6% 27%	5089
	21st to end	Night	31% 10% 3 9% 4 3 3 2 4 5% 26%	5587
April	1st to 10th	Night	25% 12% 3 6% 4 4 5% 4 4 5% 29%	2123
	11th to 20th	Night	25% 7% 2 7% 5% 4 4 3 5% 7% 32%	1989
	21st to end	Night	24% 9% 3 6% 5% 4 3 3 4 6% 33%	1890
May	1st to 10th	Night	28% 12% 3 8% 5% 4 4 3 4 5% 24%	3986
	11th to 20th	Night	27% 11% 3 6% 4 4 4 3 4 5% 29%	3596
	21st to end	Night	18% 13% 4 8% 5% 5% 4 3 4 6% 30%	3948
June	1st to 10th	Night	26% 10% 3 7% 3 4 3 4 4 7% 29%	1600
	11th to 20th	Night	21% 9% 3 7% 4 4 4 3 5% 6% 35%	1522
	21st to end	Night	28% 11% 2 6% 5% 4 4 4 5% 5% 26%	1400
July	1st to 10th	Night	19% 11% 4 9% 6% 5% 5% 4 5% 8% 25%	3295
	11th to 20th	Night	20% 10% 3 7% 4 4 4 4 5% 7% 32%	3620
	21st to end	Night	16% 8% 2 6% 4 4 4 3 5% 8% 40%	4224
August	1st to 10th	Night	19% 9% 3 7% 5% 4 4 3 4 7% 35%	2915
	11th to 20th	Night	18% 8% 3 6% 4 4 3 3 6% 8% 37%	2872
	21st to end	Night	10% 6% 2 5% 3 4 4 3 6% 7% 51%	3404
September	1st to 10th	Night	15% 7% 2 5% 4 3 3 3 5% 7% 45%	2000
	11th to 20th	Night	18% 6% 2 4 3 3 4 3 5% 6% 46%	2092
	21st to end	Night	19% 8% 3 6% 4 3 3 3 5% 7% 39%	2251
October	1st to 10th	Night	17% 6% 2 6% 4 3 3 3 5% 6% 44%	2400
	11th to 20th	Night	20% 6% 2 6% 4 4 4 3 5% 7% 38%	2400
	21st to end	Night	24% 8% 2 4 4 4 4 3 5% 7% 35%	2640
November	1st to 10th	Night	21% 8% 2 6% 5% 4 4 3 6% 7% 35%	2787
	11th to 20th	Night	20% 11% 4 9% 5% 4 4 4 5% 7% 28%	2940
	21st to end	Night	21% 9% 3 8% 5% 5% 5% 4 6% 6% 28%	2940
December	1st to 10th	Night	16% 10% 4 10% 5% 5% 5% 5% 6% 8% 26%	4720
	11th to 20th	Night	26% 10% 3 7% 4 4 4 3 5% 6% 28%	5353
	21st to end	Night	24% 10% 3 9% 5% 4 4 3 5% 5% 29%	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	63% 6% 13% 13% 5%	4691
	11th to 20th	Day	68% 6% 10% 11% 4	4977
	21st to end	Day	54% 6% 10% 15% 13% 2	5781
February	1st to 10th	Day	62% 5% 10% 15% 7% 1	1890
	11th to 20th	Day	59% 7% 11% 16% 8% 1	1851
	21st to end	Day	51% 5% 12% 19% 10% 3	1589
March	1st to 10th	Day	64% 4 9% 13% 8% 3	5044
	11th to 20th	Day	52% 7% 13% 20% 7% 1	5090
	21st to end	Day	64% 4 6% 15% 10% 1	5587
April	1st to 10th	Day	59% 3 8% 14% 12% 3	2496
	11th to 20th	Day	50% 6% 9% 21% 12% 1	2578
	21st to end	Day	51% 5% 12% 18% 12% 3	2730
May	1st to 10th	Day	64% 7% 11% 12% 6% 1	5992
	11th to 20th	Day	56% 7% 11% 19% 6% 1	6322
	21st to end	Day	50% 7% 15% 18% 9% 1	6948
June	1st to 10th	Day	56% 6% 11% 19% 8% 1	2800
	11th to 20th	Day	48% 6% 12% 19% 12% 2	2890
	21st to end	Day	58% 7% 9% 15% 9% 1	3000
July	1st to 10th	Day	54% 7% 14% 13% 9% 1	6547
	11th to 20th	Day	48% 8% 12% 23% 9%	6284
	21st to end	Day	41% 10% 16% 23% 10%	6434
August	1st to 10th	Day	53% 8% 12% 18% 9% 1	4222
	11th to 20th	Day	43% 7% 17% 25% 7%	4214
	21st to end	Day	32% 6% 14% 28% 19% 1	4493
September	1st to 10th	Day	40% 5% 15% 24% 14% 2	2400
	11th to 20th	Day	40% 5% 11% 25% 17% 1	2305
	21st to end	Day	42% 6% 8% 25% 18% 1	2147
October	1st to 10th	Day	41% 6% 12% 26% 13% 1	3260
	11th to 20th	Day	39% 6% 10% 26% 18%	3240
	21st to end	Day	44% 5% 15% 21% 12% 3	3538
November	1st to 10th	Day	53% 5% 11% 20% 9% 1	1728
	11th to 20th	Day	57% 7% 12% 17% 6% 1	1680
	21st to end	Day	59% 5% 12% 17% 6%	1680
December	1st to 10th	Day	60% 6% 12% 20% 3	2690
	11th to 20th	Day	60% 5% 11% 15% 10%	3075
	21st to end	Day	57% 5% 15% 14% 9%	4070
January	1st to 10th	Night	63% 6% 11% 15% 4	8234
	11th to 20th	Night	67% 6% 10% 12% 4	7998
	21st to end	Night	56% 6% 12% 15% 10% 1	8400
February	1st to 10th	Night	62% 6% 9% 14% 8% 1	2730
	11th to 20th	Night	56% 6% 12% 18% 7% 1	2666
	21st to end	Night	52% 7% 11% 20% 8% 1	2212
March	1st to 10th	Night	63% 4 7% 14% 11% 2	5029
	11th to 20th	Night	57% 6% 12% 17% 8% 1	5090
	21st to end	Night	62% 3 7% 17% 10% 1	5587
April	1st to 10th	Night	57% 5% 9% 14% 12% 3	2123
	11th to 20th	Night	52% 4 9% 19% 14% 1	1989
	21st to end	Night	52% 4 11% 18% 12% 3	1890
May	1st to 10th	Night	63% 5% 10% 13% 8% 1	3992
	11th to 20th	Night	57% 5% 9% 20% 7% 2	3597
	21st to end	Night	54% 6% 11% 20% 8% 1	3949
June	1st to 10th	Night	55% 6% 10% 20% 8%	1600
	11th to 20th	Night	50% 6% 10% 22% 12% 1	1522
	21st to end	Night	60% 6% 9% 14% 10% 1	1400
July	1st to 10th	Night	58% 6% 11% 16% 8% 1	3258
	11th to 20th	Night	50% 7% 11% 18% 13% 1	3572
	21st to end	Night	43% 6% 12% 24% 13% 1	4099
August	1st to 10th	Night	50% 7% 13% 21% 9%	2903
	11th to 20th	Night	44% 7% 17% 23% 7%	2899
	21st to end	Night	31% 7% 15% 28% 19%	3403
September	1st to 10th	Night	39% 6% 11% 27% 16% 2	2000
	11th to 20th	Night	39% 5% 8% 31% 16% 1	2092
	21st to end	Night	45% 6% 7% 25% 15% 1	2251
October	1st to 10th	Night	43% 5% 12% 26% 14% 1	3920
	11th to 20th	Night	36% 5% 11% 28% 20%	3899
	21st to end	Night	39% 4 14% 25% 17% 1	4252
November	1st to 10th	Night	49% 5% 13% 20% 12% 1	2787
	11th to 20th	Night	55% 5% 12% 19% 9% 1	2940
	21st to end	Night	54% 8% 15% 16% 6% 1	2940
December	1st to 10th	Night	54% 6% 13% 23% 4	4720
	11th to 20th	Night	57% 4 10% 17% 10% 1	5355
	21st to end	Night	57% 5% 11% 18% 9% 1	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% 31% 12% 2	430
	11th to 20th	Day	31% 22% 29% 13% 4	430
	21st to end	Day	22% 24% 36% 16% 2	473
February	1st to 10th	Day	29% 22% 30% 15% 3	430
	11th to 20th	Day	28% 25% 30% 15% 2	426
	21st to end	Day	23% 23% 33% 17% 4 1	363
March	1st to 10th	Day	26% 25% 35% 12% 1	540
	11th to 20th	Day	27% 26% 32% 11% 4 1	540
	21st to end	Day	31% 23% 30% 11% 4 1	594
April	1st to 10th	Day	34% 22% 30% 11% 2	540
	11th to 20th	Day	27% 23% 34% 13% 3 1	532
	21st to end	Day	25% 19% 33% 16% 6% 1	540
May	1st to 10th	Day	30% 20% 31% 14% 3 1	537
	11th to 20th	Day	23% 16% 31% 23% 6% 1	724
	21st to end	Day	29% 14% 30% 21% 5% 1	790
June	1st to 10th	Day	29% 14% 29% 20% 7%	730
	11th to 20th	Day	24% 13% 33% 21% 7% 1	730
	21st to end	Day	32% 9% 28% 21% 9% 1	730
July	1st to 10th	Day	23% 8% 30% 28% 9% 1	720
	11th to 20th	Day	21% 7% 33% 26% 11% 1	718
	21st to end	Day	23% 9% 31% 29% 8%	669
August	1st to 10th	Day	19% 7% 31% 32% 9% 2	526
	11th to 20th	Day	19% 7% 32% 28% 12% 3	519
	21st to end	Day	17% 11% 33% 24% 12% 4	583
September	1st to 10th	Day	17% 11% 35% 27% 8% 2	530
	11th to 20th	Day	22% 12% 29% 21% 12% 4	530
	21st to end	Day	21% 10% 35% 23% 9% 2	500
October	1st to 10th	Day	20% 10% 34% 23% 10% 2	420
	11th to 20th	Day	24% 15% 32% 18% 8% 2	420
	21st to end	Day	26% 17% 34% 16% 6% 1	462
November	1st to 10th	Day	25% 16% 34% 19% 5% 1	425
	11th to 20th	Day	32% 22% 26% 14% 6%	430
	21st to end	Day	31% 18% 34% 13% 3	430
December	1st to 10th	Day	33% 20% 31% 13% 3	430
	11th to 20th	Day	35% 26% 27% 11% 1	430
	21st to end	Day	32% 23% 29% 13% 3	473
January	1st to 10th	Night	28% 21% 34% 15% 2	950
	11th to 20th	Night	31% 18% 31% 17% 3	898
	21st to end	Night	24% 24% 33% 15% 3	924
February	1st to 10th	Night	29% 19% 33% 17% 2	840
	11th to 20th	Night	26% 23% 35% 14% 3	823
	21st to end	Night	21% 19% 36% 19% 5%	692
March	1st to 10th	Night	25% 20% 37% 15% 3	840
	11th to 20th	Night	24% 18% 31% 20% 6%	840
	21st to end	Night	27% 20% 31% 16% 5%	924
April	1st to 10th	Night	35% 15% 29% 19% 3	653
	11th to 20th	Night	27% 16% 34% 18% 4 1	624
	21st to end	Night	28% 13% 30% 22% 7% 1	630
May	1st to 10th	Night	30% 13% 29% 21% 6% 1	604
	11th to 20th	Night	27% 10% 27% 27% 8% 1	592
	21st to end	Night	23% 8% 32% 29% 7% 1	649
June	1st to 10th	Night	31% 8% 28% 26% 7%	600
	11th to 20th	Night	23% 9% 29% 30% 8% 1	522
	21st to end	Night	32% 5% 19% 32% 10% 1	400
July	1st to 10th	Night	20% 6% 27% 35% 12%	445
	11th to 20th	Night	20% 7% 23% 34% 16% 1	582
	21st to end	Night	21% 7% 21% 37% 13%	656
August	1st to 10th	Night	18% 5% 22% 35% 19% 1	598
	11th to 20th	Night	21% 5% 21% 33% 17% 2	592
	21st to end	Night	17% 6% 28% 32% 15% 2	660
September	1st to 10th	Night	20% 6% 28% 32% 13% 1	600
	11th to 20th	Night	20% 9% 26% 27% 16% 2	692
	21st to end	Night	24% 9% 29% 26% 10% 2	800
October	1st to 10th	Night	20% 10% 33% 27% 10% 1	800
	11th to 20th	Night	23% 14% 33% 19% 10% 1	800
	21st to end	Night	28% 17% 30% 19% 5%	880
November	1st to 10th	Night	27% 16% 31% 19% 5% 2	905
	11th to 20th	Night	28% 20% 27% 18% 6%	950
	21st to end	Night	27% 22% 29% 17% 5%	950
December	1st to 10th	Night	30% 19% 32% 15% 4	950
	11th to 20th	Night	32% 24% 29% 12% 3	950
	21st to end	Night	29% 22% 29% 18% 2	1045

Page generated at 2026-04-29 03:21:42. Version ac07

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