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 2025-01-07.

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% 9% 3 8% 5% 5% 4 4 5% 7% 29%	105611
	Night	20% 10% 3 8% 5% 4 4 4 5% 6% 30%	103512
Transparency	Day	52% 6% 13% 19% 10% 1	109211
	Night	53% 6% 12% 19% 10% 1	108002
Seeing	Day	26% 16% 31% 19% 6% 1	17502
	Night	26% 15% 30% 22% 7% 1	25075

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	16% 11% 5% 11% 7% 6% 5% 4 5% 6% 26%	2574
	11th to 20th	Day	16% 12% 4 15% 8% 6% 6% 4 5% 6% 17%	2667
	21st to end	Day	19% 9% 3 9% 6% 5% 5% 4 6% 7% 26%	3124
February	1st to 10th	Day	21% 12% 5% 9% 7% 5% 5% 4 5% 6% 21%	1710
	11th to 20th	Day	17% 11% 3 12% 7% 6% 6% 4 6% 5% 23%	1663
	21st to end	Day	13% 11% 4 10% 6% 4 5% 3 5% 6% 32%	1441
March	1st to 10th	Day	25% 12% 3 8% 5% 5% 5% 3 4 5% 26%	3430
	11th to 20th	Day	16% 12% 4 9% 7% 6% 6% 4 5% 6% 26%	3480
	21st to end	Day	30% 9% 4 11% 6% 5% 3 2 3 4 22%	3816
April	1st to 10th	Day	24% 13% 4 9% 5% 3 3 3 4 5% 26%	2258
	11th to 20th	Day	20% 9% 2 7% 5% 4 4 3 5% 7% 33%	2330
	21st to end	Day	22% 9% 3 6% 5% 5% 4 3 5% 7% 32%	2470
May	1st to 10th	Day	23% 11% 3 9% 6% 5% 5% 4 6% 6% 22%	4160
	11th to 20th	Day	22% 10% 4 8% 5% 4 4 4 5% 6% 29%	4398
	21st to end	Day	19% 8% 2 6% 5% 4 5% 4 7% 8% 32%	4836
June	1st to 10th	Day	23% 10% 3 8% 6% 4 4 4 6% 7% 26%	2660
	11th to 20th	Day	21% 9% 3 7% 5% 3 4 3 5% 6% 35%	2745
	21st to end	Day	26% 10% 3 8% 6% 4 5% 3 5% 6% 24%	2850
July	1st to 10th	Day	21% 9% 3 7% 5% 5% 5% 4 6% 7% 28%	4614
	11th to 20th	Day	18% 11% 3 7% 4 5% 4 4 6% 8% 30%	4440
	21st to end	Day	20% 8% 3 6% 4 5% 4 4 6% 8% 32%	4608
August	1st to 10th	Day	22% 10% 2 8% 5% 4 5% 4 5% 8% 27%	4105
	11th to 20th	Day	16% 8% 3 7% 4 4 5% 4 6% 9% 33%	4045
	21st to end	Day	10% 5% 2 5% 4 4 5% 4 6% 9% 47%	4355
September	1st to 10th	Day	16% 6% 2 6% 4 4 4 3 5% 8% 42%	2280
	11th to 20th	Day	18% 6% 2 6% 4 5% 4 3 5% 6% 43%	2190
	21st to end	Day	18% 7% 2 6% 4 4 3 3 5% 6% 41%	2040
October	1st to 10th	Day	17% 6% 2 7% 4 4 4 4 6% 8% 38%	1900
	11th to 20th	Day	19% 7% 2 6% 5% 3 4 4 7% 8% 34%	1900
	21st to end	Day	20% 9% 4 7% 4 4 4 4 4 6% 33%	2090
November	1st to 10th	Day	20% 10% 3 9% 4 5% 5% 3 4 7% 31%	1644
	11th to 20th	Day	22% 10% 3 8% 6% 5% 4 4 6% 7% 25%	1600
	21st to end	Day	21% 11% 4 10% 7% 5% 4 3 5% 5% 26%	1600
December	1st to 10th	Day	23% 10% 4 10% 5% 4 5% 4 6% 7% 21%	2610
	11th to 20th	Day	30% 9% 2 6% 4 5% 4 4 5% 5% 25%	2996
	21st to end	Day	22% 11% 4 10% 5% 4 3 3 4 7% 27%	3982
January	1st to 10th	Night	15% 10% 4 14% 6% 6% 6% 4 6% 6% 23%	4514
	11th to 20th	Night	19% 11% 5% 14% 8% 6% 5% 4 5% 6% 17%	4298
	21st to end	Night	18% 12% 4 10% 6% 5% 5% 4 7% 7% 22%	4532
February	1st to 10th	Night	20% 13% 4 8% 6% 6% 5% 6% 6% 5% 21%	2470
	11th to 20th	Night	17% 10% 4 9% 8% 5% 6% 5% 6% 8% 23%	2403
	21st to end	Night	13% 11% 4 9% 6% 5% 5% 5% 5% 6% 29%	2008
March	1st to 10th	Night	23% 11% 3 6% 6% 6% 5% 4 5% 5% 25%	3410
	11th to 20th	Night	18% 13% 4 10% 6% 5% 5% 5% 6% 6% 23%	3479
	21st to end	Night	32% 9% 4 11% 4 3 3 2 3 4 25%	3816
April	1st to 10th	Night	25% 11% 3 6% 4 4 5% 4 4 5% 29%	1921
	11th to 20th	Night	23% 7% 2 7% 5% 4 3 3 4 7% 33%	1797
	21st to end	Night	25% 8% 3 6% 5% 4 3 4 4 6% 32%	1710
May	1st to 10th	Night	20% 12% 4 9% 5% 4 4 3 5% 6% 27%	2782
	11th to 20th	Night	25% 10% 3 7% 4 5% 5% 3 4 6% 28%	2506
	21st to end	Night	20% 11% 4 8% 5% 5% 4 3 5% 6% 29%	2749
June	1st to 10th	Night	27% 10% 3 7% 3 4 3 4 5% 7% 28%	1520
	11th to 20th	Night	21% 9% 3 7% 4 4 4 3 5% 6% 34%	1446
	21st to end	Night	29% 11% 2 6% 4 4 3 4 5% 5% 26%	1330
July	1st to 10th	Night	20% 9% 4 8% 6% 5% 5% 4 6% 7% 26%	2303
	11th to 20th	Night	19% 10% 3 8% 5% 4 4 4 6% 8% 29%	2530
	21st to end	Night	19% 9% 2 5% 4 4 5% 3 5% 7% 37%	2948
August	1st to 10th	Night	19% 9% 3 7% 5% 4 5% 3 4 7% 34%	2825
	11th to 20th	Night	18% 8% 3 7% 4 4 3 3 6% 8% 36%	2782
	21st to end	Night	10% 6% 2 4 3 4 4 3 6% 7% 51%	3300
September	1st to 10th	Night	15% 7% 2 5% 4 3 4 3 5% 7% 45%	1900
	11th to 20th	Night	18% 6% 2 5% 3 3 4 3 5% 6% 45%	1987
	21st to end	Night	20% 9% 3 6% 4 3 3 3 5% 7% 38%	2138
October	1st to 10th	Night	18% 6% 2 7% 4 3 3 3 6% 7% 42%	2280
	11th to 20th	Night	20% 5% 2 5% 4 4 4 3 5% 8% 39%	2280
	21st to end	Night	23% 8% 2 4 4 4 4 3 5% 7% 35%	2508
November	1st to 10th	Night	20% 8% 2 6% 5% 4 4 3 6% 7% 35%	2651
	11th to 20th	Night	20% 10% 4 8% 5% 4 4 4 5% 7% 29%	2800
	21st to end	Night	21% 9% 3 8% 5% 5% 5% 4 6% 6% 28%	2800
December	1st to 10th	Night	16% 10% 4 10% 5% 5% 5% 5% 6% 8% 26%	4580
	11th to 20th	Night	26% 10% 3 7% 4 4 4 3 5% 6% 28%	5213
	21st to end	Night	25% 10% 3 8% 5% 4 4 3 4 5% 29%	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	57% 6% 17% 15% 5% 1	2575
	11th to 20th	Day	63% 6% 13% 14% 4	2667
	21st to end	Day	55% 6% 12% 17% 9%	3124
February	1st to 10th	Day	62% 5% 11% 14% 7% 1	1710
	11th to 20th	Day	60% 6% 10% 15% 7% 1	1671
	21st to end	Day	52% 5% 12% 19% 9% 3	1441
March	1st to 10th	Day	62% 3 9% 17% 7% 2	3434
	11th to 20th	Day	55% 7% 13% 17% 8% 1	3480
	21st to end	Day	67% 3 6% 13% 10% 1	3816
April	1st to 10th	Day	60% 3 8% 14% 11% 3	2258
	11th to 20th	Day	49% 6% 10% 22% 12% 1	2330
	21st to end	Day	51% 5% 12% 17% 13% 3	2470
May	1st to 10th	Day	59% 7% 14% 14% 6% 1	4174
	11th to 20th	Day	55% 6% 13% 18% 7%	4402
	21st to end	Day	45% 9% 18% 18% 9% 1	4836
June	1st to 10th	Day	56% 6% 11% 18% 7%	2660
	11th to 20th	Day	49% 6% 13% 19% 12% 1	2745
	21st to end	Day	59% 7% 9% 14% 9% 1	2850
July	1st to 10th	Day	53% 7% 12% 14% 12% 2	4543
	11th to 20th	Day	49% 9% 12% 21% 8% 1	4364
	21st to end	Day	48% 8% 15% 19% 9% 1	4461
August	1st to 10th	Day	54% 8% 12% 17% 8% 1	4092
	11th to 20th	Day	44% 7% 17% 24% 7%	4084
	21st to end	Day	32% 6% 14% 27% 19% 1	4355
September	1st to 10th	Day	39% 5% 15% 24% 14% 2	2280
	11th to 20th	Day	41% 5% 11% 25% 17% 2	2190
	21st to end	Day	43% 6% 8% 25% 17% 1	2040
October	1st to 10th	Day	42% 6% 12% 26% 13% 1	3160
	11th to 20th	Day	37% 6% 11% 27% 19%	3140
	21st to end	Day	44% 5% 16% 21% 12% 3	3428
November	1st to 10th	Day	53% 5% 11% 20% 9% 1	1644
	11th to 20th	Day	56% 7% 13% 17% 7% 1	1600
	21st to end	Day	60% 5% 12% 17% 6%	1600
December	1st to 10th	Day	59% 6% 12% 20% 3	2610
	11th to 20th	Day	60% 5% 11% 15% 10%	2995
	21st to end	Day	57% 5% 15% 14% 9%	3982
January	1st to 10th	Night	57% 8% 14% 17% 4	4514
	11th to 20th	Night	65% 7% 12% 11% 5%	4298
	21st to end	Night	59% 6% 13% 16% 6%	4532
February	1st to 10th	Night	62% 6% 10% 14% 7% 1	2470
	11th to 20th	Night	58% 6% 12% 16% 7% 1	2406
	21st to end	Night	53% 6% 12% 20% 8% 2	2008
March	1st to 10th	Night	60% 4 8% 18% 8% 1	3419
	11th to 20th	Night	58% 7% 12% 14% 9% 1	3480
	21st to end	Night	65% 3 7% 14% 9% 1	3816
April	1st to 10th	Night	57% 5% 9% 15% 12% 3	1921
	11th to 20th	Night	51% 4 9% 20% 14% 1	1797
	21st to end	Night	54% 4 10% 17% 11% 3	1710
May	1st to 10th	Night	58% 6% 13% 15% 7% 1	2788
	11th to 20th	Night	56% 6% 11% 19% 8%	2507
	21st to end	Night	54% 7% 12% 19% 7% 1	2750
June	1st to 10th	Night	56% 6% 11% 19% 7%	1520
	11th to 20th	Night	51% 6% 10% 21% 11% 1	1446
	21st to end	Night	61% 6% 10% 14% 9% 1	1330
July	1st to 10th	Night	57% 7% 11% 15% 9% 1	2266
	11th to 20th	Night	52% 8% 13% 17% 10% 1	2482
	21st to end	Night	47% 7% 13% 21% 12% 1	2855
August	1st to 10th	Night	51% 7% 14% 20% 8%	2813
	11th to 20th	Night	45% 8% 17% 23% 7%	2809
	21st to end	Night	31% 7% 15% 27% 19%	3299
September	1st to 10th	Night	39% 6% 11% 27% 16% 2	1900
	11th to 20th	Night	39% 5% 7% 31% 16% 1	1987
	21st to end	Night	46% 6% 7% 24% 15% 1	2138
October	1st to 10th	Night	44% 5% 12% 26% 13% 1	3800
	11th to 20th	Night	35% 5% 11% 28% 20%	3779
	21st to end	Night	39% 4 14% 24% 18% 1	4120
November	1st to 10th	Night	48% 6% 14% 21% 11% 1	2651
	11th to 20th	Night	54% 6% 12% 19% 9% 1	2800
	21st to end	Night	54% 8% 15% 16% 6%	2800
December	1st to 10th	Night	54% 6% 13% 23% 4	4580
	11th to 20th	Night	57% 4 10% 17% 11% 1	5215
	21st to end	Night	57% 5% 11% 18% 9% 1	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% 24% 31% 12% 2	388
	11th to 20th	Day	32% 21% 31% 13% 3	370
	21st to end	Day	22% 23% 35% 16% 3	407
February	1st to 10th	Day	32% 21% 29% 15% 3	370
	11th to 20th	Day	28% 27% 29% 14% 2	366
	21st to end	Day	24% 22% 33% 16% 3 1	313
March	1st to 10th	Day	27% 25% 34% 12% 2	460
	11th to 20th	Day	27% 26% 30% 12% 4 1	460
	21st to end	Day	31% 22% 30% 12% 5% 1	506
April	1st to 10th	Day	34% 22% 29% 12% 3	460
	11th to 20th	Day	25% 23% 34% 13% 4 1	452
	21st to end	Day	25% 18% 32% 17% 6% 1	460
May	1st to 10th	Day	29% 21% 31% 15% 3 1	491
	11th to 20th	Day	23% 16% 31% 23% 6% 1	674
	21st to end	Day	29% 14% 31% 21% 5% 1	735
June	1st to 10th	Day	29% 14% 30% 20% 6%	680
	11th to 20th	Day	24% 13% 33% 22% 8% 1	680
	21st to end	Day	33% 9% 28% 20% 9% 1	680
July	1st to 10th	Day	24% 8% 29% 28% 10% 1	670
	11th to 20th	Day	22% 6% 34% 25% 11% 1	668
	21st to end	Day	24% 9% 30% 28% 8%	621
August	1st to 10th	Day	20% 7% 32% 30% 9% 2	486
	11th to 20th	Day	20% 7% 33% 25% 12% 3	479
	21st to end	Day	18% 9% 32% 25% 12% 4	539
September	1st to 10th	Day	17% 10% 35% 28% 8% 2	490
	11th to 20th	Day	24% 12% 29% 20% 12% 4	490
	21st to end	Day	22% 10% 34% 22% 9% 3	463
October	1st to 10th	Day	21% 10% 34% 23% 10% 2	390
	11th to 20th	Day	25% 16% 31% 17% 8% 3	390
	21st to end	Day	27% 17% 34% 16% 6% 1	429
November	1st to 10th	Day	24% 16% 34% 19% 6% 1	395
	11th to 20th	Day	32% 23% 25% 15% 6%	400
	21st to end	Day	32% 18% 35% 13% 3 1	400
December	1st to 10th	Day	33% 19% 32% 14% 3	400
	11th to 20th	Day	34% 27% 26% 11% 1 1	400
	21st to end	Day	33% 22% 28% 13% 3	440
January	1st to 10th	Night	29% 20% 34% 15% 2	881
	11th to 20th	Night	30% 19% 31% 17% 2	808
	21st to end	Night	25% 22% 33% 16% 3	836
February	1st to 10th	Night	30% 19% 32% 17% 2	760
	11th to 20th	Night	26% 23% 35% 13% 2	743
	21st to end	Night	21% 19% 36% 19% 5%	628
March	1st to 10th	Night	25% 20% 36% 15% 3	760
	11th to 20th	Night	24% 18% 31% 20% 6%	760
	21st to end	Night	26% 21% 30% 17% 6%	836
April	1st to 10th	Night	34% 13% 29% 19% 4	591
	11th to 20th	Night	26% 16% 35% 18% 4 1	564
	21st to end	Night	29% 14% 30% 21% 6% 1	570
May	1st to 10th	Night	29% 13% 30% 21% 6% 1	570
	11th to 20th	Night	27% 10% 27% 27% 8% 1	562
	21st to end	Night	23% 9% 33% 28% 6% 1	616
June	1st to 10th	Night	32% 8% 28% 26% 7%	570
	11th to 20th	Night	23% 9% 29% 30% 9% 1	496
	21st to end	Night	33% 5% 19% 31% 11% 1	380
July	1st to 10th	Night	21% 6% 26% 35% 12%	422
	11th to 20th	Night	20% 8% 23% 34% 15% 1	552
	21st to end	Night	22% 7% 22% 37% 13%	623
August	1st to 10th	Night	18% 5% 22% 35% 18% 1	568
	11th to 20th	Night	22% 5% 21% 33% 16% 2	562
	21st to end	Night	17% 6% 27% 32% 16% 2	627
September	1st to 10th	Night	19% 6% 29% 32% 13% 1	570
	11th to 20th	Night	21% 9% 27% 27% 15% 2	657
	21st to end	Night	24% 10% 29% 26% 10% 2	760
October	1st to 10th	Night	21% 9% 33% 26% 10% 1	760
	11th to 20th	Night	23% 14% 33% 19% 10% 1	760
	21st to end	Night	28% 17% 31% 19% 5%	836
November	1st to 10th	Night	26% 15% 32% 20% 5% 2	857
	11th to 20th	Night	28% 20% 27% 19% 6%	900
	21st to end	Night	28% 22% 28% 17% 5%	900
December	1st to 10th	Night	29% 19% 32% 15% 4	900
	11th to 20th	Night	32% 23% 29% 13% 3	900
	21st to end	Night	30% 20% 29% 18% 2	990

Page generated at 2025-01-04 04:24:04. Version ac07

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