York
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the York clear sky chart from 2005-11-06 to 2026-08-15.

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	9% 5% 2 5% 3 3 4 3 5% 7% 54%	143516
	Night	8% 5% 2 5% 3 3 4 3 5% 7% 56%	138768
Transparency	Day	29% 5% 11% 27% 23% 6%	147213
	Night	27% 4 11% 27% 24% 7%	143129
Seeing	Day	13% 14% 34% 27% 10% 2	18034
	Night	11% 13% 36% 30% 9% 1	27835

By Month

Percentage Cloud Cover SE (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	9% 7% 2 7% 4 3 4 3 5% 7% 49%	5289
	11th to 20th	Day	4 6% 3 7% 3 3 4 3 4 6% 56%	5310
	21st to end	Day	10% 8% 2 7% 3 3 3 3 3 6% 53%	5812
February	1st to 10th	Day	7% 10% 3 7% 4 4 4 3 4 7% 48%	1979
	11th to 20th	Day	6% 7% 2 7% 3 4 4 3 5% 6% 53%	2049
	21st to end	Day	6% 6% 2 5% 2 3 3 4 5% 6% 59%	1730
March	1st to 10th	Day	7% 7% 2 4 3 3 3 2 4 6% 59%	4774
	11th to 20th	Day	7% 5% 2 3 2 2 3 2 4 8% 61%	5090
	21st to end	Day	7% 5% 2 5% 4 4 4 3 6% 9% 51%	5583
April	1st to 10th	Day	9% 5% 2 6% 3 2 3 3 5% 8% 53%	2351
	11th to 20th	Day	11% 6% 3 5% 4 4 4 3 6% 6% 48%	2582
	21st to end	Day	16% 6% 2 5% 4 3 3 4 5% 6% 45%	2730
May	1st to 10th	Day	15% 6% 2 5% 4 4 4 3 5% 6% 46%	7617
	11th to 20th	Day	7% 3 1 4 3 3 4 3 5% 7% 59%	7627
	21st to end	Day	19% 7% 2 6% 4 4 4 3 5% 5% 41%	8392
June	1st to 10th	Day	6% 5% 2 5% 4 4 3 3 5% 10% 55%	2730
	11th to 20th	Day	6% 3 1 4 3 4 4 5% 6% 7% 56%	2730
	21st to end	Day	6% 3 1 5% 3 3 4 4 7% 9% 55%	2730
July	1st to 10th	Day	6% 3 2 5% 3 4 4 4 6% 9% 53%	7680
	11th to 20th	Day	6% 3 1 4 3 4 4 3 5% 9% 59%	7680
	21st to end	Day	5% 3 2 4 4 4 5% 4 6% 9% 55%	8337
August	1st to 10th	Day	7% 5% 2 5% 4 4 4 3 5% 7% 52%	4364
	11th to 20th	Day	8% 4 2 5% 3 3 4 4 6% 9% 55%	4224
	21st to end	Day	5% 3 1 4 3 3 4 4 6% 8% 59%	4414
September	1st to 10th	Day	10% 3 1 3 3 4 3 3 5% 6% 58%	2230
	11th to 20th	Day	8% 4 1 5% 4 3 3 2 5% 7% 57%	2200
	21st to end	Day	7% 5% 2 5% 4 3 4 3 5% 6% 59%	2200
October	1st to 10th	Day	12% 3 2 3 3 3 2 2 5% 6% 60%	2195
	11th to 20th	Day	9% 4 1 3 3 2 2 2 3 5% 65%	2000
	21st to end	Day	11% 6% 1 4 3 2 3 2 3 5% 59%	2156
November	1st to 10th	Day	7% 4 2 3 3 3 3 2 3 5% 65%	1853
	11th to 20th	Day	7% 6% 2 3 4 2 2 3 4 6% 61%	1890
	21st to end	Day	10% 5% 1 3 3 3 3 2 3 5% 62%	1890
December	1st to 10th	Day	4 4 1 3 2 2 2 2 4 5% 70%	3030
	11th to 20th	Day	5% 5% 2 5% 3 2 2 2 3 5% 65%	3481
	21st to end	Day	12% 10% 3 6% 4 4 4 3 4 6% 43%	4587
January	1st to 10th	Night	11% 7% 3 7% 4 4 4 3 4 6% 48%	7634
	11th to 20th	Night	4 4 2 5% 4 4 4 3 4 6% 61%	7680
	21st to end	Night	12% 8% 3 6% 3 3 3 2 4 4 53%	8402
February	1st to 10th	Night	9% 8% 4 7% 3 3 4 3 4 6% 50%	2658
	11th to 20th	Night	8% 8% 3 7% 3 3 3 4 6% 6% 49%	2458
	21st to end	Night	7% 4 3 5% 3 3 3 3 4 5% 59%	2076
March	1st to 10th	Night	5% 5% 1 4 3 3 3 3 4 5% 64%	5311
	11th to 20th	Night	5% 4 1 3 3 3 4 3 5% 7% 61%	5090
	21st to end	Night	7% 6% 2 5% 4 4 4 3 5% 6% 54%	5588
April	1st to 10th	Night	9% 6% 2 4 3 3 3 3 6% 8% 54%	2266
	11th to 20th	Night	11% 6% 2 5% 4 3 3 3 5% 6% 52%	1987
	21st to end	Night	15% 6% 2 5% 3 3 3 3 6% 7% 48%	1890
May	1st to 10th	Night	15% 5% 2 4 3 4 4 3 5% 6% 48%	5266
	11th to 20th	Night	8% 3 1 3 3 3 4 3 6% 7% 59%	5265
	21st to end	Night	20% 5% 2 4 3 3 3 4 5% 7% 46%	5793
June	1st to 10th	Night	6% 4 2 4 3 3 4 3 7% 9% 56%	1890
	11th to 20th	Night	5% 3 1 3 3 3 5% 4 8% 10% 55%	1890
	21st to end	Night	5% 3 1 4 3 3 4 4 7% 10% 57%	1890
July	1st to 10th	Night	5% 3 1 4 3 4 5% 5% 7% 11% 51%	5308
	11th to 20th	Night	5% 3 1 4 3 3 4 4 6% 10% 56%	5307
	21st to end	Night	5% 3 1 5% 4 4 4 4 6% 9% 53%	5770
August	1st to 10th	Night	5% 3 2 4 4 3 4 4 5% 10% 55%	3016
	11th to 20th	Night	7% 4 1 6% 3 3 4 3 6% 8% 54%	2924
	21st to end	Night	4 3 1 4 2 2 3 3 6% 10% 61%	3491
September	1st to 10th	Night	8% 4 1 3 3 4 3 3 4 6% 60%	2169
	11th to 20th	Night	7% 4 1 5% 3 3 4 4 6% 7% 57%	2200
	21st to end	Night	6% 4 2 4 3 3 3 4 5% 5% 61%	2200
October	1st to 10th	Night	10% 4 1 4 3 2 3 2 4 6% 61%	2200
	11th to 20th	Night	8% 2 1 3 2 2 2 2 3 5% 70%	2385
	21st to end	Night	10% 4 1 4 2 3 2 2 3 5% 63%	2680
November	1st to 10th	Night	8% 3 1 3 3 2 3 2 4 6% 66%	2665
	11th to 20th	Night	6% 5% 1 3 2 2 3 3 4 7% 63%	2730
	21st to end	Night	11% 6% 1 3 2 2 3 3 4 5% 60%	2730
December	1st to 10th	Night	4 4 2 4 3 2 3 2 3 5% 67%	4380
	11th to 20th	Night	6% 5% 2 4 3 3 3 2 4 5% 63%	4946
	21st to end	Night	13% 8% 3 6% 4 3 3 3 4 6% 47%	6633

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	36% 4 9% 27% 21% 4	5290
	11th to 20th	Day	29% 4 10% 26% 23% 8%	5310
	21st to end	Day	35% 3 8% 23% 27% 3	5812
February	1st to 10th	Day	37% 4 11% 25% 17% 6%	1979
	11th to 20th	Day	32% 5% 14% 26% 18% 6%	2059
	21st to end	Day	26% 4 10% 25% 27% 9%	1730
March	1st to 10th	Day	28% 3 8% 17% 28% 17%	4774
	11th to 20th	Day	23% 4 10% 17% 32% 13%	5090
	21st to end	Day	29% 6% 14% 35% 14% 2	5585
April	1st to 10th	Day	30% 4 11% 26% 22% 7%	2351
	11th to 20th	Day	35% 5% 11% 24% 17% 7%	2582
	21st to end	Day	39% 5% 12% 26% 17% 1	2730
May	1st to 10th	Day	39% 6% 10% 22% 20% 4	7655
	11th to 20th	Day	24% 5% 11% 25% 27% 8%	7631
	21st to end	Day	44% 6% 10% 19% 17% 4	8405
June	1st to 10th	Day	26% 7% 17% 30% 16% 4	2730
	11th to 20th	Day	25% 6% 14% 27% 22% 6%	2730
	21st to end	Day	24% 7% 13% 25% 23% 8%	2730
July	1st to 10th	Day	26% 6% 12% 26% 26% 4	7616
	11th to 20th	Day	22% 5% 13% 30% 21% 8%	7611
	21st to end	Day	25% 5% 12% 36% 20% 2	8155
August	1st to 10th	Day	31% 5% 13% 26% 21% 4	4350
	11th to 20th	Day	27% 6% 16% 28% 20% 4	4263
	21st to end	Day	22% 5% 13% 33% 21% 6%	4413
September	1st to 10th	Day	28% 6% 9% 26% 22% 10%	2230
	11th to 20th	Day	28% 4 10% 27% 27% 5%	2200
	21st to end	Day	26% 5% 9% 26% 25% 9%	2200
October	1st to 10th	Day	17% 3 6% 20% 42% 12%	3572
	11th to 20th	Day	18% 3 10% 28% 31% 10%	3240
	21st to end	Day	24% 3 8% 23% 32% 10%	3461
November	1st to 10th	Day	23% 3 8% 27% 30% 8%	1853
	11th to 20th	Day	26% 3 9% 27% 27% 9%	1890
	21st to end	Day	27% 3 10% 28% 26% 6%	1890
December	1st to 10th	Day	19% 3 7% 41% 27% 3	3030
	11th to 20th	Day	23% 3 6% 37% 28% 3	3479
	21st to end	Day	41% 5% 8% 28% 15% 3	4587
January	1st to 10th	Night	37% 3 9% 26% 21% 3	7635
	11th to 20th	Night	25% 4 11% 26% 27% 8%	7680
	21st to end	Night	37% 3 8% 21% 26% 6%	8402
February	1st to 10th	Night	36% 4 11% 24% 18% 6%	2658
	11th to 20th	Night	35% 5% 11% 25% 18% 6%	2458
	21st to end	Night	27% 4 8% 23% 26% 13%	2076
March	1st to 10th	Night	22% 4 6% 21% 29% 17%	5311
	11th to 20th	Night	22% 5% 7% 19% 30% 16%	5090
	21st to end	Night	31% 5% 14% 35% 13% 2	5588
April	1st to 10th	Night	29% 4 12% 23% 24% 8%	2266
	11th to 20th	Night	33% 5% 12% 24% 20% 6%	1987
	21st to end	Night	36% 5% 12% 24% 20% 3	1890
May	1st to 10th	Night	36% 5% 11% 21% 21% 6%	5293
	11th to 20th	Night	24% 6% 12% 28% 21% 10%	5266
	21st to end	Night	37% 7% 13% 23% 16% 4	5805
June	1st to 10th	Night	23% 7% 19% 29% 18% 4	1890
	11th to 20th	Night	23% 6% 14% 31% 21% 5%	1890
	21st to end	Night	22% 5% 14% 29% 22% 8%	1890
July	1st to 10th	Night	26% 5% 17% 28% 22% 2	5265
	11th to 20th	Night	23% 4 14% 29% 22% 9%	5256
	21st to end	Night	27% 5% 11% 36% 20% 1	5641
August	1st to 10th	Night	24% 5% 13% 29% 23% 5%	3006
	11th to 20th	Night	28% 7% 15% 26% 21% 4	2951
	21st to end	Night	20% 5% 14% 38% 18% 5%	3491
September	1st to 10th	Night	25% 5% 11% 24% 25% 11%	2169
	11th to 20th	Night	26% 6% 10% 24% 28% 6%	2200
	21st to end	Night	24% 5% 9% 23% 29% 9%	2200
October	1st to 10th	Night	17% 2 7% 16% 46% 11%	3590
	11th to 20th	Night	14% 3 8% 27% 33% 15%	3884
	21st to end	Night	20% 2 6% 25% 37% 10%	4316
November	1st to 10th	Night	22% 3 9% 27% 28% 11%	2665
	11th to 20th	Night	23% 3 7% 28% 31% 8%	2730
	21st to end	Night	28% 3 9% 27% 24% 9%	2730
December	1st to 10th	Night	22% 3 9% 39% 23% 3	4380
	11th to 20th	Night	24% 2 6% 35% 25% 7%	4947
	21st to end	Night	37% 5% 10% 29% 14% 4	6633

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	14% 25% 39% 15% 7% 1	330
	11th to 20th	Day	19% 25% 39% 15% 2	330
	21st to end	Day	13% 24% 46% 15% 2	363
February	1st to 10th	Day	16% 30% 41% 11% 2	330
	11th to 20th	Day	19% 26% 35% 15% 4	330
	21st to end	Day	14% 30% 43% 10% 2	273
March	1st to 10th	Day	15% 23% 45% 15% 3	330
	11th to 20th	Day	24% 21% 38% 14% 3 1	330
	21st to end	Day	21% 26% 41% 10% 1 1	363
April	1st to 10th	Day	19% 23% 43% 13% 2	371
	11th to 20th	Day	16% 26% 41% 14% 2	620
	21st to end	Day	23% 22% 33% 16% 4 1	750
May	1st to 10th	Day	15% 16% 42% 21% 5%	753
	11th to 20th	Day	15% 17% 31% 26% 10% 1	775
	21st to end	Day	18% 12% 33% 26% 9% 2	844
June	1st to 10th	Day	8% 8% 33% 33% 14% 4	780
	11th to 20th	Day	7% 9% 40% 32% 12% 1	780
	21st to end	Day	7% 7% 43% 32% 9% 2	780
July	1st to 10th	Day	8% 4 22% 43% 20% 3	771
	11th to 20th	Day	4 3 24% 43% 22% 4	768
	21st to end	Day	8% 2 22% 44% 20% 5%	849
August	1st to 10th	Day	6% 4 26% 42% 19% 2	776
	11th to 20th	Day	9% 2 26% 42% 19% 2	736
	21st to end	Day	7% 3 26% 41% 20% 2	646
September	1st to 10th	Day	12% 4 21% 46% 16% 2	360
	11th to 20th	Day	5% 5% 39% 38% 12% 2	330
	21st to end	Day	14% 10% 35% 25% 15% 2	330
October	1st to 10th	Day	12% 15% 35% 25% 11% 2	330
	11th to 20th	Day	12% 22% 39% 22% 5% 1	330
	21st to end	Day	24% 15% 37% 16% 6% 1	363
November	1st to 10th	Day	16% 21% 38% 18% 5% 1	330
	11th to 20th	Day	18% 16% 42% 16% 7% 1	330
	21st to end	Day	19% 21% 35% 17% 7% 2	330
December	1st to 10th	Day	10% 26% 35% 22% 7%	330
	11th to 20th	Day	13% 25% 37% 20% 5%	330
	21st to end	Day	19% 28% 36% 15% 3	363
January	1st to 10th	Night	10% 16% 41% 27% 6%	1050
	11th to 20th	Night	10% 19% 38% 27% 5%	1050
	21st to end	Night	10% 17% 43% 24% 5%	1155
February	1st to 10th	Night	14% 18% 39% 25% 4	978
	11th to 20th	Night	15% 21% 38% 24% 2	824
	21st to end	Night	12% 20% 39% 26% 4	692
March	1st to 10th	Night	11% 21% 39% 26% 4	761
	11th to 20th	Night	15% 17% 39% 22% 7%	630
	21st to end	Night	17% 16% 42% 21% 3	693
April	1st to 10th	Night	14% 17% 40% 23% 5%	630
	11th to 20th	Night	16% 25% 34% 22% 3	623
	21st to end	Night	20% 18% 34% 22% 6%	630
May	1st to 10th	Night	17% 14% 36% 24% 8%	630
	11th to 20th	Night	14% 10% 34% 31% 10% 1	621
	21st to end	Night	15% 11% 35% 27% 11% 1	683
June	1st to 10th	Night	8% 7% 35% 36% 13% 1	630
	11th to 20th	Night	6% 10% 41% 33% 10%	630
	21st to end	Night	7% 7% 43% 35% 8%	630
July	1st to 10th	Night	6% 2 28% 45% 17% 1	615
	11th to 20th	Night	5% 1 32% 44% 17% 1	612
	21st to end	Night	7% 3 28% 40% 21% 1	690
August	1st to 10th	Night	4 3 31% 43% 18% 1	627
	11th to 20th	Night	10% 3 26% 44% 17%	606
	21st to end	Night	6% 3 26% 42% 22% 1	660
September	1st to 10th	Night	10% 4 31% 40% 15% 1	600
	11th to 20th	Night	9% 5% 38% 35% 12% 2	600
	21st to end	Night	8% 8% 34% 36% 13% 2	600
October	1st to 10th	Night	13% 8% 34% 30% 13% 1	600
	11th to 20th	Night	11% 14% 39% 28% 7%	785
	21st to end	Night	14% 15% 36% 25% 9% 1	920
November	1st to 10th	Night	11% 12% 37% 29% 10% 1	1025
	11th to 20th	Night	9% 16% 42% 27% 6%	1050
	21st to end	Night	14% 15% 33% 31% 6%	1050
December	1st to 10th	Night	11% 15% 36% 32% 6%	1050
	11th to 20th	Night	15% 16% 41% 25% 3	1050
	21st to end	Night	14% 20% 34% 27% 6%	1155

Page generated at 2026-08-12 02:37:18. Version ac07

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