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 2025-06-10.

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	16% 8% 3 8% 5% 5% 5% 4 6% 7% 34%	119382
	Night	15% 8% 3 8% 5% 4 5% 4 5% 7% 36%	118829
Transparency	Day	47% 7% 13% 20% 12% 2	122921
	Night	46% 7% 13% 20% 12% 2	123438
Seeing	Day	23% 19% 30% 18% 8% 2	15559
	Night	21% 16% 29% 26% 8% 1	24590

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	10% 8% 4 14% 8% 7% 6% 5% 6% 5% 29%	3597
	11th to 20th	Day	10% 10% 4 14% 8% 6% 4 3 5% 4 31%	3570
	21st to end	Day	12% 10% 5% 9% 5% 5% 5% 4 6% 6% 35%	3960
February	1st to 10th	Day	12% 10% 4 9% 7% 6% 5% 4 6% 7% 29%	1800
	11th to 20th	Day	9% 9% 4 10% 6% 6% 5% 4 6% 7% 33%	1948
	21st to end	Day	16% 10% 4 9% 6% 4 5% 4 5% 5% 32%	1650
March	1st to 10th	Day	16% 12% 4 10% 5% 5% 5% 3 5% 4 30%	4476
	11th to 20th	Day	14% 8% 3 8% 5% 4 5% 4 6% 6% 36%	4645
	21st to end	Day	21% 9% 2 7% 5% 4 4 3 4 6% 34%	5970
April	1st to 10th	Day	17% 11% 3 7% 5% 5% 4 3 5% 6% 35%	2400
	11th to 20th	Day	17% 8% 2 6% 5% 4 5% 4 6% 7% 36%	2374
	21st to end	Day	19% 8% 3 8% 5% 5% 4 4 5% 6% 32%	2567
May	1st to 10th	Day	30% 10% 3 8% 5% 4 4 3 5% 5% 23%	5886
	11th to 20th	Day	18% 10% 3 9% 5% 5% 4 4 6% 7% 28%	6290
	21st to end	Day	18% 7% 2 7% 4 4 5% 5% 7% 8% 32%	6949
June	1st to 10th	Day	17% 7% 3 7% 5% 6% 5% 5% 7% 8% 31%	2786
	11th to 20th	Day	16% 6% 2 5% 5% 4 5% 4 7% 7% 37%	2660
	21st to end	Day	17% 7% 3 7% 6% 5% 5% 5% 6% 7% 32%	2660
July	1st to 10th	Day	11% 6% 2 7% 5% 5% 5% 5% 7% 10% 37%	4440
	11th to 20th	Day	9% 5% 2 6% 5% 5% 6% 5% 9% 11% 37%	4440
	21st to end	Day	15% 6% 2 7% 5% 6% 5% 4 6% 7% 37%	4814
August	1st to 10th	Day	13% 5% 3 7% 5% 5% 6% 5% 6% 9% 36%	4334
	11th to 20th	Day	13% 6% 2 8% 5% 5% 6% 5% 7% 9% 34%	3890
	21st to end	Day	8% 4 2 6% 4 5% 5% 4 7% 11% 43%	4180
September	1st to 10th	Day	12% 5% 2 6% 4 4 5% 4 6% 8% 45%	2280
	11th to 20th	Day	12% 5% 2 6% 4 4 4 3 6% 6% 49%	2144
	21st to end	Day	16% 5% 2 6% 4 4 4 3 6% 8% 42%	2090
October	1st to 10th	Day	21% 7% 2 8% 4 5% 4 3 5% 5% 36%	2057
	11th to 20th	Day	14% 7% 2 6% 4 5% 4 4 6% 7% 40%	1842
	21st to end	Day	21% 8% 3 8% 5% 4 4 4 5% 6% 33%	1881
November	1st to 10th	Day	11% 7% 2 6% 3 4 4 3 6% 8% 43%	1761
	11th to 20th	Day	15% 8% 3 6% 5% 4 5% 4 5% 6% 41%	1800
	21st to end	Day	18% 8% 3 7% 4 4 3 4 7% 7% 35%	1695
December	1st to 10th	Day	14% 11% 5% 9% 6% 6% 5% 3 5% 7% 30%	2600
	11th to 20th	Day	19% 9% 3 6% 5% 4 4 4 5% 5% 35%	2986
	21st to end	Day	17% 10% 4 8% 5% 5% 5% 3 4 5% 33%	3960
January	1st to 10th	Night	11% 6% 3 12% 8% 6% 5% 4 6% 6% 32%	6349
	11th to 20th	Night	9% 10% 4 13% 7% 6% 6% 4 5% 6% 29%	6315
	21st to end	Night	13% 9% 3 8% 5% 6% 6% 5% 6% 7% 33%	6996
February	1st to 10th	Night	11% 10% 4 9% 6% 5% 5% 5% 6% 7% 32%	2598
	11th to 20th	Night	9% 8% 3 8% 6% 6% 5% 6% 7% 7% 34%	2338
	21st to end	Night	14% 8% 3 7% 4 5% 5% 4 8% 6% 36%	1980
March	1st to 10th	Night	19% 10% 3 7% 4 4 4 4 5% 7% 32%	5364
	11th to 20th	Night	13% 10% 3 7% 5% 5% 5% 4 5% 7% 37%	5296
	21st to end	Night	19% 11% 3 7% 4 4 4 3 5% 5% 34%	4962
April	1st to 10th	Night	18% 7% 2 6% 5% 4 4 4 6% 7% 36%	2000
	11th to 20th	Night	17% 8% 3 6% 4 4 4 4 5% 6% 37%	1975
	21st to end	Night	19% 8% 2 6% 5% 3 4 3 5% 7% 38%	1831
May	1st to 10th	Night	32% 8% 3 6% 4 4 4 3 5% 7% 24%	4054
	11th to 20th	Night	14% 10% 4 10% 6% 5% 5% 4 6% 7% 29%	3633
	21st to end	Night	19% 6% 3 7% 5% 4 5% 4 5% 7% 33%	3926
June	1st to 10th	Night	19% 5% 2 5% 5% 4 5% 3 6% 8% 39%	1598
	11th to 20th	Night	18% 6% 2 6% 3 3 4 4 6% 8% 41%	1520
	21st to end	Night	13% 7% 2 6% 5% 4 5% 5% 7% 6% 38%	1520
July	1st to 10th	Night	9% 4 3 7% 6% 5% 5% 5% 7% 8% 42%	2520
	11th to 20th	Night	13% 5% 2 5% 4 4 4 4 6% 8% 44%	2520
	21st to end	Night	14% 7% 2 6% 4 3 5% 4 6% 7% 42%	2739
August	1st to 10th	Night	18% 7% 3 7% 4 4 4 4 5% 7% 36%	2565
	11th to 20th	Night	14% 7% 2 7% 5% 4 5% 4 4 8% 41%	2934
	21st to end	Night	9% 4 2 4 3 3 4 3 6% 9% 54%	3476
September	1st to 10th	Night	11% 6% 2 5% 3 3 4 4 5% 8% 51%	1900
	11th to 20th	Night	12% 5% 1 4 3 3 4 3 4 6% 53%	2035
	21st to end	Night	15% 6% 2 4 4 4 4 3 6% 7% 46%	2090
October	1st to 10th	Night	20% 7% 3 6% 4 4 4 4 5% 6% 38%	2120
	11th to 20th	Night	14% 6% 2 6% 4 4 6% 5% 5% 7% 41%	2337
	21st to end	Night	21% 8% 2 6% 4 4 5% 4 6% 6% 34%	2717
November	1st to 10th	Night	12% 7% 2 6% 5% 5% 4 3 5% 6% 44%	2537
	11th to 20th	Night	15% 7% 2 6% 5% 4 4 4 5% 7% 41%	2600
	21st to end	Night	13% 8% 3 6% 5% 4 5% 4 6% 7% 39%	2700
December	1st to 10th	Night	12% 11% 4 10% 6% 5% 4 4 6% 7% 31%	4580
	11th to 20th	Night	21% 9% 4 8% 5% 4 4 4 4 5% 34%	5208
	21st to end	Night	18% 13% 4 9% 4 4 4 3 5% 5% 32%	6996

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	53% 6% 11% 23% 7%	3598
	11th to 20th	Day	52% 7% 10% 16% 12% 2	3600
	21st to end	Day	49% 5% 8% 20% 15% 3	3960
February	1st to 10th	Day	52% 8% 13% 17% 9% 1	1800
	11th to 20th	Day	48% 9% 11% 19% 10% 2	1957
	21st to end	Day	52% 6% 12% 17% 12% 2	1650
March	1st to 10th	Day	56% 5% 9% 12% 11% 7%	4479
	11th to 20th	Day	44% 8% 16% 20% 9% 3	4645
	21st to end	Day	50% 4 8% 15% 18% 5%	5970
April	1st to 10th	Day	50% 6% 10% 19% 12% 3	2400
	11th to 20th	Day	45% 9% 11% 18% 15% 2	2374
	21st to end	Day	51% 7% 9% 17% 13% 3	2567
May	1st to 10th	Day	62% 7% 10% 15% 6% 1	5897
	11th to 20th	Day	53% 7% 14% 15% 9% 1	6294
	21st to end	Day	45% 8% 15% 22% 9% 1	6949
June	1st to 10th	Day	49% 8% 13% 18% 10% 1	2786
	11th to 20th	Day	44% 8% 14% 18% 14% 2	2660
	21st to end	Day	49% 10% 13% 17% 10% 1	2660
July	1st to 10th	Day	40% 7% 17% 21% 16%	4370
	11th to 20th	Day	37% 7% 20% 25% 11%	4365
	21st to end	Day	43% 6% 16% 22% 12% 1	4661
August	1st to 10th	Day	44% 7% 18% 20% 11%	4320
	11th to 20th	Day	44% 7% 17% 26% 6% 1	3929
	21st to end	Day	33% 6% 20% 27% 13% 1	4180
September	1st to 10th	Day	36% 7% 14% 27% 15% 1	2280
	11th to 20th	Day	35% 6% 12% 26% 19% 1	2144
	21st to end	Day	40% 6% 14% 25% 14% 2	2090
October	1st to 10th	Day	46% 5% 10% 22% 16% 1	3421
	11th to 20th	Day	39% 5% 13% 28% 14% 1	3032
	21st to end	Day	38% 6% 9% 31% 13% 3	3081
November	1st to 10th	Day	39% 8% 15% 23% 15% 1	1761
	11th to 20th	Day	44% 8% 12% 18% 17% 2	1800
	21st to end	Day	48% 8% 16% 20% 7% 1	1695
December	1st to 10th	Day	54% 5% 15% 21% 5%	2600
	11th to 20th	Day	51% 5% 11% 18% 13% 2	2986
	21st to end	Day	52% 6% 9% 17% 14% 2	3960
January	1st to 10th	Night	50% 7% 13% 21% 9% 1	6349
	11th to 20th	Night	53% 7% 12% 16% 10% 2	6360
	21st to end	Night	49% 6% 10% 15% 15% 4	6996
February	1st to 10th	Night	50% 8% 14% 18% 10% 1	2598
	11th to 20th	Night	45% 9% 14% 19% 12% 2	2340
	21st to end	Night	45% 8% 13% 22% 9% 3	1980
March	1st to 10th	Night	51% 4 11% 16% 14% 4	5374
	11th to 20th	Night	45% 7% 18% 18% 10% 2	5297
	21st to end	Night	51% 5% 8% 14% 20% 2	4962
April	1st to 10th	Night	45% 7% 12% 21% 14% 1	2000
	11th to 20th	Night	45% 7% 11% 21% 14% 1	1975
	21st to end	Night	45% 7% 11% 18% 15% 3	1831
May	1st to 10th	Night	59% 8% 12% 13% 7% 1	4060
	11th to 20th	Night	51% 9% 14% 17% 8% 1	3634
	21st to end	Night	49% 8% 15% 17% 10% 1	3927
June	1st to 10th	Night	43% 8% 15% 22% 12%	1598
	11th to 20th	Night	41% 9% 17% 20% 12% 1	1520
	21st to end	Night	42% 9% 16% 21% 11% 1	1520
July	1st to 10th	Night	39% 7% 18% 23% 14% 1	2480
	11th to 20th	Night	37% 7% 17% 25% 14%	2472
	21st to end	Night	41% 6% 12% 28% 12% 1	2651
August	1st to 10th	Night	47% 8% 15% 21% 9%	2555
	11th to 20th	Night	42% 7% 19% 22% 9%	2961
	21st to end	Night	27% 4 22% 33% 13%	3476
September	1st to 10th	Night	32% 8% 14% 29% 16%	1900
	11th to 20th	Night	32% 7% 12% 29% 19% 1	2035
	21st to end	Night	37% 6% 12% 27% 15% 2	2090
October	1st to 10th	Night	44% 6% 14% 22% 13% 1	3536
	11th to 20th	Night	32% 6% 14% 28% 18% 2	3873
	21st to end	Night	41% 5% 10% 32% 11% 1	4467
November	1st to 10th	Night	40% 8% 13% 22% 14% 3	2537
	11th to 20th	Night	42% 7% 12% 21% 15% 3	2600
	21st to end	Night	44% 7% 17% 21% 9% 1	2700
December	1st to 10th	Night	52% 5% 14% 22% 6%	4580
	11th to 20th	Night	53% 6% 10% 14% 15% 1	5208
	21st to end	Night	54% 6% 10% 17% 12% 2	6996

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% 27% 26% 16% 3	200
	11th to 20th	Day	29% 27% 28% 15% 2 1	200
	21st to end	Day	22% 28% 35% 11% 4	220
February	1st to 10th	Day	29% 35% 27% 9% 1	315
	11th to 20th	Day	33% 26% 27% 13% 2	496
	21st to end	Day	28% 27% 29% 14% 3	414
March	1st to 10th	Day	29% 21% 33% 14% 2	500
	11th to 20th	Day	28% 29% 28% 12% 3 1	500
	21st to end	Day	33% 22% 27% 10% 5% 3	550
April	1st to 10th	Day	36% 21% 25% 12% 6% 1	500
	11th to 20th	Day	34% 23% 24% 12% 5% 2	492
	21st to end	Day	29% 22% 29% 14% 5% 1	500
May	1st to 10th	Day	30% 24% 26% 12% 7% 2	503
	11th to 20th	Day	21% 19% 35% 18% 4 2	527
	21st to end	Day	20% 13% 33% 24% 8% 1	573
June	1st to 10th	Day	21% 16% 35% 19% 8% 1	526
	11th to 20th	Day	18% 17% 33% 19% 10% 2	490
	21st to end	Day	19% 19% 35% 17% 7% 3	490
July	1st to 10th	Day	15% 12% 32% 25% 13% 3	485
	11th to 20th	Day	14% 7% 39% 26% 12% 2	484
	21st to end	Day	13% 11% 40% 23% 11% 2	532
August	1st to 10th	Day	11% 13% 30% 28% 14% 4	486
	11th to 20th	Day	14% 10% 33% 22% 15% 6%	479
	21st to end	Day	12% 14% 28% 25% 14% 7%	539
September	1st to 10th	Day	16% 12% 26% 23% 16% 7%	490
	11th to 20th	Day	15% 13% 26% 18% 17% 10%	490
	21st to end	Day	16% 14% 29% 22% 12% 7%	490
October	1st to 10th	Day	23% 17% 24% 20% 12% 4	490
	11th to 20th	Day	21% 17% 27% 23% 10% 2	459
	21st to end	Day	31% 16% 26% 19% 7% 1	429
November	1st to 10th	Day	23% 18% 25% 22% 11% 1	395
	11th to 20th	Day	29% 22% 25% 17% 6% 1	400
	21st to end	Day	32% 21% 27% 16% 4	295
December	1st to 10th	Day	24% 29% 27% 15% 6% 1	200
	11th to 20th	Day	39% 22% 29% 8% 4	200
	21st to end	Day	29% 20% 30% 17% 4	220
January	1st to 10th	Night	21% 21% 36% 20% 2	800
	11th to 20th	Night	25% 19% 32% 22% 3	800
	21st to end	Night	17% 21% 35% 22% 4	880
February	1st to 10th	Night	22% 21% 37% 18% 2	800
	11th to 20th	Night	21% 24% 35% 18% 2	783
	21st to end	Night	24% 22% 32% 20% 2	660
March	1st to 10th	Night	26% 19% 33% 21% 2	800
	11th to 20th	Night	24% 22% 29% 21% 4	800
	21st to end	Night	25% 20% 29% 21% 6%	880
April	1st to 10th	Night	26% 19% 30% 21% 4	800
	11th to 20th	Night	23% 23% 28% 20% 7%	791
	21st to end	Night	24% 18% 29% 24% 5%	631
May	1st to 10th	Night	27% 15% 28% 23% 7%	600
	11th to 20th	Night	16% 16% 31% 30% 6%	426
	21st to end	Night	18% 9% 30% 31% 12% 1	433
June	1st to 10th	Night	18% 9% 29% 32% 11%	400
	11th to 20th	Night	17% 11% 25% 34% 13%	380
	21st to end	Night	14% 7% 30% 38% 10% 1	380
July	1st to 10th	Night	12% 9% 21% 43% 14% 1	370
	11th to 20th	Night	12% 4 24% 39% 19% 2	368
	21st to end	Night	12% 5% 27% 38% 17% 1	416
August	1st to 10th	Night	11% 4 21% 45% 18% 1	411
	11th to 20th	Night	13% 7% 27% 33% 18% 3	649
	21st to end	Night	15% 6% 21% 35% 21% 2	836
September	1st to 10th	Night	13% 7% 28% 33% 18% 1	760
	11th to 20th	Night	14% 9% 23% 29% 19% 5%	760
	21st to end	Night	17% 9% 25% 31% 15% 3	760
October	1st to 10th	Night	23% 14% 19% 29% 14% 1	760
	11th to 20th	Night	18% 14% 27% 30% 11% 1	760
	21st to end	Night	26% 16% 28% 24% 6%	836
November	1st to 10th	Night	18% 18% 28% 28% 8% 1	780
	11th to 20th	Night	20% 19% 30% 23% 7% 2	800
	21st to end	Night	23% 19% 29% 22% 6% 1	800
December	1st to 10th	Night	23% 22% 33% 18% 4	800
	11th to 20th	Night	27% 21% 29% 21% 3	800
	21st to end	Night	27% 21% 30% 20% 3	880

Page generated at 2025-06-07 10:28:15. Version ac07

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