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 2025-04-11.

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%	113089
	Night	18% 8% 2 6% 4 4 4 3 5% 6% 41%	115192
Transparency	Day	26% 43% 7% 13% 10% 2	116682
	Night	27% 40% 6% 13% 11% 2	119763
Seeing	Day	23% 18% 32% 19% 7% 2	15542
	Night	22% 15% 32% 23% 7% 1	24689

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	13% 8% 2 5% 4 4 4 3 4 5% 47%	3597
	11th to 20th	Day	14% 9% 2 6% 3 4 3 3 5% 5% 45%	3570
	21st to end	Day	19% 9% 3 5% 4 4 4 3 4 4 41%	4024
February	1st to 10th	Day	17% 9% 2 5% 5% 5% 5% 4 5% 6% 38%	1901
	11th to 20th	Day	15% 7% 2 6% 5% 5% 5% 3 6% 6% 41%	1948
	21st to end	Day	20% 9% 3 5% 4 4 4 3 4 7% 37%	1650
March	1st to 10th	Day	24% 9% 1 5% 3 3 3 2 4 5% 41%	4476
	11th to 20th	Day	14% 8% 3 5% 4 4 4 4 6% 6% 41%	4653
	21st to end	Day	22% 7% 3 6% 4 4 4 4 6% 7% 35%	5958
April	1st to 10th	Day	20% 8% 2 6% 4 4 4 3 5% 7% 38%	2400
	11th to 20th	Day	21% 7% 2 4 3 4 4 3 4 6% 42%	2254
	21st to end	Day	21% 6% 1 6% 4 4 4 3 5% 6% 40%	2405
May	1st to 10th	Day	28% 10% 3 5% 4 3 3 3 5% 7% 30%	4106
	11th to 20th	Day	24% 8% 3 6% 4 4 4 3 5% 7% 33%	4204
	21st to end	Day	16% 5% 2 5% 3 3 4 3 5% 8% 45%	4837
June	1st to 10th	Day	18% 6% 2 6% 5% 5% 5% 4 7% 8% 36%	2660
	11th to 20th	Day	17% 6% 2 5% 4 4 4 4 6% 8% 39%	2660
	21st to end	Day	13% 7% 2 6% 5% 5% 4 4 6% 9% 38%	2660
July	1st to 10th	Day	16% 6% 2 7% 5% 5% 5% 4 6% 8% 35%	4440
	11th to 20th	Day	15% 5% 2 5% 4 4 5% 4 6% 9% 39%	4440
	21st to end	Day	19% 8% 3 7% 5% 4 5% 4 6% 7% 33%	4814
August	1st to 10th	Day	21% 6% 3 7% 4 4 5% 4 6% 8% 32%	4208
	11th to 20th	Day	13% 6% 2 6% 4 5% 4 4 7% 8% 43%	3797
	21st to end	Day	8% 5% 3 6% 4 5% 6% 5% 7% 10% 43%	4180
September	1st to 10th	Day	15% 6% 3 6% 4 3 4 4 5% 6% 46%	2280
	11th to 20th	Day	13% 6% 2 5% 3 3 3 3 4 7% 51%	2128
	21st to end	Day	20% 8% 2 5% 3 4 4 3 5% 6% 40%	2090
October	1st to 10th	Day	22% 7% 2 5% 4 3 3 3 5% 5% 41%	2090
	11th to 20th	Day	14% 6% 2 5% 3 3 4 3 5% 7% 49%	1863
	21st to end	Day	18% 7% 2 6% 3 3 4 4 5% 6% 43%	1881
November	1st to 10th	Day	14% 6% 3 5% 4 4 4 3 4 5% 49%	1761
	11th to 20th	Day	18% 6% 2 4 3 3 3 3 5% 5% 49%	1800
	21st to end	Day	17% 6% 2 5% 4 4 4 3 4 7% 45%	1800
December	1st to 10th	Day	15% 6% 2 6% 4 4 3 3 4 6% 46%	2608
	11th to 20th	Day	21% 8% 2 5% 3 3 2 2 4 5% 47%	2986
	21st to end	Day	20% 10% 2 6% 4 3 3 3 4 5% 40%	3960
January	1st to 10th	Night	14% 6% 2 4 3 3 4 3 5% 6% 49%	6349
	11th to 20th	Night	15% 8% 3 7% 4 3 3 3 4 4 46%	6315
	21st to end	Night	18% 9% 3 7% 4 5% 4 4 5% 5% 37%	6927
February	1st to 10th	Night	17% 9% 2 6% 5% 3 4 3 5% 5% 41%	2497
	11th to 20th	Night	18% 8% 2 6% 4 3 4 3 5% 6% 41%	2338
	21st to end	Night	20% 8% 2 6% 4 4 4 3 5% 5% 40%	1980
March	1st to 10th	Night	22% 10% 2 5% 3 3 3 2 4 6% 40%	5364
	11th to 20th	Night	16% 10% 3 6% 4 4 3 3 4 5% 41%	5305
	21st to end	Night	22% 9% 3 6% 4 3 4 3 4 5% 37%	4972
April	1st to 10th	Night	21% 7% 3 6% 5% 4 3 4 5% 6% 37%	2000
	11th to 20th	Night	21% 6% 2 6% 4 4 4 3 5% 6% 40%	1882
	21st to end	Night	22% 6% 2 6% 4 3 3 3 4 4 41%	1773
May	1st to 10th	Night	25% 11% 4 8% 4 5% 4 3 4 6% 28%	2834
	11th to 20th	Night	25% 8% 3 8% 4 4 5% 4 5% 6% 29%	2718
	21st to end	Night	18% 7% 2 5% 4 3 3 3 5% 6% 42%	2738
June	1st to 10th	Night	19% 7% 2 5% 5% 4 4 4 5% 7% 39%	1520
	11th to 20th	Night	18% 7% 2 5% 4 4 4 3 6% 9% 39%	1520
	21st to end	Night	15% 6% 3 4 4 4 5% 3 6% 10% 39%	1520
July	1st to 10th	Night	17% 7% 3 8% 5% 4 5% 5% 6% 8% 32%	2520
	11th to 20th	Night	19% 6% 2 5% 4 5% 3 4 6% 10% 36%	2520
	21st to end	Night	20% 10% 3 6% 4 4 4 4 5% 7% 33%	2739
August	1st to 10th	Night	24% 7% 3 8% 5% 4 5% 4 6% 7% 28%	2688
	11th to 20th	Night	12% 7% 2 7% 5% 5% 6% 4 5% 7% 39%	3027
	21st to end	Night	9% 5% 1 4 3 3 4 3 5% 7% 55%	3476
September	1st to 10th	Night	14% 6% 3 6% 4 4 5% 4 5% 6% 42%	1900
	11th to 20th	Night	13% 6% 2 5% 3 3 3 2 4 5% 54%	2051
	21st to end	Night	19% 7% 2 6% 4 4 5% 3 4 5% 40%	2090
October	1st to 10th	Night	20% 7% 3 5% 4 4 4 3 4 6% 41%	2090
	11th to 20th	Night	16% 5% 3 5% 3 2 4 3 4 6% 49%	2311
	21st to end	Night	21% 7% 3 5% 4 3 3 3 5% 6% 41%	2717
November	1st to 10th	Night	13% 5% 2 5% 4 3 3 3 5% 6% 49%	2537
	11th to 20th	Night	19% 6% 2 5% 3 2 3 3 4 6% 47%	2600
	21st to end	Night	16% 6% 2 5% 4 3 4 3 4 6% 46%	2600
December	1st to 10th	Night	17% 8% 2 5% 3 3 3 3 4 5% 48%	4570
	11th to 20th	Night	25% 8% 2 5% 3 3 3 3 4 4 42%	5208
	21st to end	Night	19% 9% 3 7% 3 3 3 2 4 5% 43%	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	29% 27% 4 21% 16% 3	3598
	11th to 20th	Day	28% 27% 6% 18% 14% 8%	3600
	21st to end	Day	36% 29% 6% 15% 13% 1	4024
February	1st to 10th	Day	19% 61% 6% 8% 4	1901
	11th to 20th	Day	19% 61% 2 8% 9% 1	1957
	21st to end	Day	17% 62% 5% 9% 5% 2	1650
March	1st to 10th	Day	36% 30% 6% 9% 9% 9%	4479
	11th to 20th	Day	27% 32% 10% 22% 9% 1	4653
	21st to end	Day	37% 30% 6% 10% 13% 5%	5958
April	1st to 10th	Day	21% 62% 4 6% 6% 1	2400
	11th to 20th	Day	15% 65% 3 7% 9%	2254
	21st to end	Day	18% 65% 3 8% 6% 1	2405
May	1st to 10th	Day	36% 43% 9% 6% 5% 1	4120
	11th to 20th	Day	34% 42% 9% 11% 4	4208
	21st to end	Day	23% 42% 5% 17% 14%	4837
June	1st to 10th	Day	16% 65% 3 9% 6%	2660
	11th to 20th	Day	14% 66% 5% 9% 6% 1	2660
	21st to end	Day	14% 64% 4 9% 8%	2660
July	1st to 10th	Day	32% 38% 7% 12% 10% 1	4370
	11th to 20th	Day	28% 38% 11% 14% 8% 1	4365
	21st to end	Day	35% 37% 7% 10% 8% 1	4661
August	1st to 10th	Day	35% 38% 10% 12% 4	4194
	11th to 20th	Day	24% 39% 10% 22% 5%	3835
	21st to end	Day	20% 39% 10% 18% 11% 1	4180
September	1st to 10th	Day	15% 61% 5% 11% 8% 1	2280
	11th to 20th	Day	15% 60% 3 9% 11% 2	2128
	21st to end	Day	21% 60% 4 8% 7% 1	2090
October	1st to 10th	Day	22% 38% 6% 12% 16% 5%	3480
	11th to 20th	Day	14% 37% 4 13% 28% 4	3079
	21st to end	Day	18% 37% 10% 18% 16% 2	3081
November	1st to 10th	Day	18% 56% 6% 9% 10% 1	1761
	11th to 20th	Day	14% 56% 4 10% 14% 3	1800
	21st to end	Day	16% 57% 5% 14% 6% 2	1800
December	1st to 10th	Day	25% 35% 10% 20% 10%	2608
	11th to 20th	Day	28% 31% 9% 12% 16% 4	2986
	21st to end	Day	37% 27% 3 16% 13% 4	3960
January	1st to 10th	Night	26% 26% 6% 24% 15% 3	6349
	11th to 20th	Night	29% 27% 9% 16% 14% 5%	6360
	21st to end	Night	39% 29% 6% 12% 11% 3	6927
February	1st to 10th	Night	18% 62% 5% 10% 4 1	2497
	11th to 20th	Night	17% 60% 3 7% 10% 2	2340
	21st to end	Night	18% 61% 3 8% 7% 2	1980
March	1st to 10th	Night	36% 29% 7% 11% 10% 7%	5374
	11th to 20th	Night	32% 30% 11% 16% 8% 3	5306
	21st to end	Night	38% 30% 3 6% 19% 4	4972
April	1st to 10th	Night	20% 62% 3 8% 6% 2	2000
	11th to 20th	Night	15% 65% 4 7% 7% 1	1882
	21st to end	Night	16% 65% 3 9% 6% 1	1773
May	1st to 10th	Night	38% 42% 8% 9% 3	2840
	11th to 20th	Night	38% 43% 8% 7% 3	2719
	21st to end	Night	26% 42% 8% 17% 7% 1	2739
June	1st to 10th	Night	16% 65% 2 8% 8% 1	1520
	11th to 20th	Night	14% 66% 4 11% 5%	1520
	21st to end	Night	14% 63% 4 11% 7%	1520
July	1st to 10th	Night	35% 38% 7% 11% 8% 1	2480
	11th to 20th	Night	30% 37% 11% 15% 6%	2472
	21st to end	Night	36% 37% 7% 8% 11% 1	2651
August	1st to 10th	Night	40% 39% 7% 8% 5% 1	2678
	11th to 20th	Night	27% 39% 10% 20% 3	3055
	21st to end	Night	13% 38% 13% 22% 14% 1	3476
September	1st to 10th	Night	16% 60% 4 12% 7% 1	1900
	11th to 20th	Night	15% 59% 4 9% 11% 2	2051
	21st to end	Night	22% 60% 3 7% 8% 1	2090
October	1st to 10th	Night	23% 38% 8% 16% 14% 2	3480
	11th to 20th	Night	11% 37% 5% 16% 27% 4	3834
	21st to end	Night	16% 37% 8% 21% 16% 2	4467
November	1st to 10th	Night	18% 56% 5% 9% 10% 1	2537
	11th to 20th	Night	16% 57% 3 8% 11% 4	2600
	21st to end	Night	17% 57% 5% 14% 6% 2	2600
December	1st to 10th	Night	26% 35% 7% 21% 10% 1	4570
	11th to 20th	Night	34% 31% 6% 10% 16% 2	5208
	21st to end	Night	35% 27% 5% 14% 15% 4	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	31% 25% 29% 14% 2	200
	11th to 20th	Day	32% 29% 27% 12% 1	200
	21st to end	Day	24% 31% 31% 9% 5%	233
February	1st to 10th	Day	32% 23% 32% 12% 1	401
	11th to 20th	Day	30% 28% 32% 9% 1	496
	21st to end	Day	30% 26% 30% 11% 3	414
March	1st to 10th	Day	27% 25% 33% 13% 2	500
	11th to 20th	Day	25% 25% 35% 12% 2 1	500
	21st to end	Day	31% 23% 31% 11% 3 1	550
April	1st to 10th	Day	31% 27% 26% 11% 5%	500
	11th to 20th	Day	29% 24% 29% 13% 5%	452
	21st to end	Day	29% 18% 33% 14% 4 2	460
May	1st to 10th	Day	32% 19% 30% 14% 5% 1	463
	11th to 20th	Day	24% 16% 41% 14% 5% 1	487
	21st to end	Day	20% 14% 31% 24% 9% 1	529
June	1st to 10th	Day	20% 13% 34% 22% 10% 2	490
	11th to 20th	Day	18% 17% 34% 22% 8% 1	490
	21st to end	Day	16% 14% 34% 23% 10% 2	490
July	1st to 10th	Day	18% 8% 32% 26% 14% 2	485
	11th to 20th	Day	14% 7% 31% 32% 13% 3	484
	21st to end	Day	15% 10% 39% 27% 8% 1	532
August	1st to 10th	Day	14% 9% 30% 33% 10% 4	486
	11th to 20th	Day	16% 8% 31% 29% 13% 4	479
	21st to end	Day	11% 11% 34% 25% 14% 5%	539
September	1st to 10th	Day	19% 10% 29% 24% 15% 3	490
	11th to 20th	Day	14% 11% 33% 23% 12% 8%	490
	21st to end	Day	23% 13% 34% 19% 9% 2	490
October	1st to 10th	Day	28% 12% 25% 21% 10% 4	490
	11th to 20th	Day	19% 21% 29% 22% 8% 2	470
	21st to end	Day	24% 18% 30% 19% 7% 2	429
November	1st to 10th	Day	20% 16% 33% 21% 9% 1	395
	11th to 20th	Day	25% 24% 28% 18% 5%	400
	21st to end	Day	24% 22% 29% 20% 6%	400
December	1st to 10th	Day	25% 25% 31% 14% 4	208
	11th to 20th	Day	36% 25% 28% 10% 3	200
	21st to end	Day	26% 25% 28% 15% 5% 1	220
January	1st to 10th	Night	22% 24% 36% 16% 3	800
	11th to 20th	Night	29% 20% 33% 17% 2	800
	21st to end	Night	20% 24% 35% 18% 2	880
February	1st to 10th	Night	23% 21% 35% 20% 1	800
	11th to 20th	Night	26% 22% 38% 13% 1	783
	21st to end	Night	26% 23% 33% 15% 3	660
March	1st to 10th	Night	26% 19% 35% 18% 3	800
	11th to 20th	Night	23% 18% 37% 18% 4 1	800
	21st to end	Night	24% 21% 36% 16% 3	880
April	1st to 10th	Night	25% 20% 34% 18% 3	800
	11th to 20th	Night	23% 16% 36% 22% 3	754
	21st to end	Night	24% 16% 37% 18% 6% 1	633
May	1st to 10th	Night	25% 14% 32% 22% 6%	570
	11th to 20th	Night	23% 14% 38% 20% 5%	506
	21st to end	Night	20% 7% 29% 34% 9% 1	411
June	1st to 10th	Night	23% 6% 29% 32% 9% 1	380
	11th to 20th	Night	20% 6% 29% 36% 9%	380
	21st to end	Night	17% 5% 31% 37% 9%	380
July	1st to 10th	Night	15% 5% 28% 34% 16% 1	370
	11th to 20th	Night	15% 3 26% 40% 15% 1	368
	21st to end	Night	18% 4 27% 40% 10% 1	416
August	1st to 10th	Night	12% 4 30% 39% 14% 1	482
	11th to 20th	Night	15% 4 26% 36% 18% 2	704
	21st to end	Night	14% 5% 25% 35% 18% 2	836
September	1st to 10th	Night	15% 5% 32% 33% 13% 2	760
	11th to 20th	Night	17% 7% 25% 30% 18% 3	760
	21st to end	Night	21% 9% 28% 27% 14% 2	760
October	1st to 10th	Night	21% 11% 29% 24% 12% 2	760
	11th to 20th	Night	19% 14% 33% 24% 10%	760
	21st to end	Night	26% 13% 34% 20% 8%	836
November	1st to 10th	Night	16% 16% 35% 22% 11% 1	780
	11th to 20th	Night	23% 19% 32% 20% 5% 1	800
	21st to end	Night	22% 20% 32% 21% 6%	800
December	1st to 10th	Night	27% 19% 29% 20% 5% 1	800
	11th to 20th	Night	26% 21% 32% 19% 3	800
	21st to end	Night	26% 18% 37% 18% 2	880

Page generated at 2025-04-08 09:17:26. Version ac07

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