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 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	8% 5% 2 5% 3 3 4 3 5% 7% 54%	113978
	Night	8% 5% 2 5% 3 3 3 3 5% 7% 57%	114554
Transparency	Day	29% 5% 11% 26% 23% 6%	117716
	Night	27% 4 11% 27% 24% 7%	118936
Seeing	Day	13% 14% 34% 27% 11% 2	16039
	Night	11% 13% 36% 30% 9% 1	26118

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	10% 8% 2 8% 4 4 4 3 4 6% 47%	4077
	11th to 20th	Day	6% 7% 3 9% 4 4 4 3 4 6% 52%	4080
	21st to end	Day	9% 8% 2 7% 3 3 3 3 4 6% 53%	4488
February	1st to 10th	Day	7% 11% 3 7% 4 4 4 3 4 7% 47%	1885
	11th to 20th	Day	6% 7% 3 7% 3 4 4 3 5% 6% 53%	1949
	21st to end	Day	6% 7% 2 5% 3 3 3 4 5% 6% 58%	1650
March	1st to 10th	Day	7% 7% 2 4 3 3 3 2 4 6% 59%	4670
	11th to 20th	Day	7% 5% 2 3 2 2 3 2 4 8% 60%	4980
	21st to end	Day	7% 5% 2 5% 4 4 4 3 6% 9% 50%	5462
April	1st to 10th	Day	10% 6% 2 6% 3 2 3 3 5% 7% 53%	2239
	11th to 20th	Day	11% 6% 3 5% 4 4 4 3 6% 6% 48%	2334
	21st to end	Day	16% 6% 2 5% 4 4 3 4 5% 7% 46%	2470
May	1st to 10th	Day	15% 6% 2 6% 4 4 4 3 5% 7% 43%	4082
	11th to 20th	Day	11% 5% 2 6% 4 4 4 4 5% 6% 50%	4082
	21st to end	Day	11% 5% 2 6% 5% 4 4 4 6% 7% 47%	4477
June	1st to 10th	Day	5% 4 1 5% 4 4 3 3 5% 10% 56%	2470
	11th to 20th	Day	6% 3 1 4 4 4 4 5% 6% 7% 57%	2470
	21st to end	Day	5% 3 1 4 3 3 4 4 7% 9% 57%	2470
July	1st to 10th	Day	9% 4 2 6% 4 4 4 4 5% 7% 51%	4120
	11th to 20th	Day	4 3 1 4 4 4 4 4 6% 9% 55%	4120
	21st to end	Day	5% 3 2 5% 4 4 5% 4 6% 9% 54%	4465
August	1st to 10th	Day	8% 5% 2 6% 4 4 4 3 5% 7% 52%	4104
	11th to 20th	Day	8% 4 2 5% 3 3 4 4 6% 8% 54%	4041
	21st to end	Day	4 3 1 4 3 3 4 4 6% 8% 60%	4279
September	1st to 10th	Day	10% 3 1 3 3 4 3 3 5% 6% 58%	2119
	11th to 20th	Day	9% 4 1 5% 3 3 3 2 5% 6% 58%	2090
	21st to end	Day	7% 4 2 4 4 3 4 2 5% 6% 60%	2090
October	1st to 10th	Day	12% 3 2 3 3 3 2 2 4 6% 60%	2085
	11th to 20th	Day	9% 4 1 3 3 2 2 2 3 5% 65%	1900
	21st to end	Day	10% 6% 1 4 3 2 3 2 3 5% 61%	2048
November	1st to 10th	Day	7% 4 2 3 3 3 3 2 4 5% 64%	1763
	11th to 20th	Day	7% 6% 2 3 4 2 2 3 4 6% 61%	1800
	21st to end	Day	10% 5% 1 3 3 3 3 2 3 5% 63%	1800
December	1st to 10th	Day	4 4 1 3 2 2 2 2 4 5% 70%	2940
	11th to 20th	Day	5% 5% 2 5% 3 2 2 2 3 5% 65%	3391
	21st to end	Day	13% 10% 3 6% 4 4 4 3 4 6% 42%	4488
January	1st to 10th	Night	11% 7% 3 8% 4 4 4 3 5% 6% 46%	5888
	11th to 20th	Night	4 4 2 5% 4 4 4 3 4 6% 59%	5900
	21st to end	Night	11% 7% 3 6% 3 3 2 2 3 4 56%	6490
February	1st to 10th	Night	9% 8% 4 7% 3 3 4 3 4 5% 49%	2532
	11th to 20th	Night	8% 8% 3 7% 3 3 3 4 6% 6% 49%	2338
	21st to end	Night	7% 5% 3 5% 3 3 3 3 4 5% 59%	1980
March	1st to 10th	Night	5% 4 1 4 3 3 3 3 4 5% 65%	5195
	11th to 20th	Night	6% 4 1 3 3 3 4 3 5% 7% 60%	4980
	21st to end	Night	7% 6% 2 5% 4 4 4 3 5% 6% 54%	5467
April	1st to 10th	Night	9% 6% 2 4 3 3 3 3 6% 8% 54%	2158
	11th to 20th	Night	11% 6% 2 5% 4 3 3 3 5% 6% 51%	1801
	21st to end	Night	14% 5% 2 5% 3 3 3 3 6% 7% 49%	1710
May	1st to 10th	Night	17% 7% 2 4 3 3 3 3 4 6% 49%	2823
	11th to 20th	Night	12% 3 1 4 3 3 4 4 7% 9% 51%	2823
	21st to end	Night	9% 3 1 3 3 3 4 4 6% 10% 53%	3090
June	1st to 10th	Night	5% 4 2 4 2 3 3 3 7% 10% 58%	1710
	11th to 20th	Night	5% 3 1 3 3 4 5% 4 8% 10% 55%	1710
	21st to end	Night	4 2 1 4 3 3 3 4 7% 10% 59%	1710
July	1st to 10th	Night	6% 3 1 5% 4 4 5% 4 6% 10% 51%	2850
	11th to 20th	Night	4 2 1 3 3 3 4 4 7% 11% 58%	2850
	21st to end	Night	5% 3 1 4 4 4 4 5% 6% 10% 54%	3098
August	1st to 10th	Night	5% 3 2 4 3 3 4 4 5% 10% 55%	2836
	11th to 20th	Night	8% 4 1 6% 3 3 4 3 6% 8% 53%	2798
	21st to end	Night	4 3 1 4 2 2 3 3 6% 10% 62%	3384
September	1st to 10th	Night	8% 4 1 3 3 4 3 3 4 6% 60%	2060
	11th to 20th	Night	7% 4 1 5% 3 3 4 3 5% 7% 57%	2090
	21st to end	Night	6% 3 1 4 3 3 3 4 5% 5% 62%	2090
October	1st to 10th	Night	10% 4 1 4 3 2 3 2 4 6% 61%	2090
	11th to 20th	Night	8% 2 1 3 2 2 2 2 3 5% 70%	2266
	21st to end	Night	9% 4 1 4 2 3 2 2 3 5% 63%	2546
November	1st to 10th	Night	8% 3 1 3 3 2 2 2 4 6% 65%	2535
	11th to 20th	Night	6% 5% 1 4 3 3 3 3 4 7% 63%	2600
	21st to end	Night	10% 5% 2 3 2 2 3 3 4 5% 60%	2600
December	1st to 10th	Night	4 4 2 4 3 2 3 2 3 5% 68%	4250
	11th to 20th	Night	6% 5% 2 4 3 3 3 2 4 5% 63%	4816
	21st to end	Night	13% 9% 3 6% 4 3 3 3 4 6% 46%	6490

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	38% 4 11% 23% 19% 5%	4078
	11th to 20th	Day	33% 5% 12% 20% 24% 6%	4080
	21st to end	Day	34% 3 9% 23% 27% 4	4488
February	1st to 10th	Day	38% 5% 12% 23% 16% 6%	1885
	11th to 20th	Day	32% 5% 13% 26% 18% 6%	1959
	21st to end	Day	27% 4 10% 25% 25% 9%	1650
March	1st to 10th	Day	27% 3 8% 17% 28% 17%	4670
	11th to 20th	Day	23% 4 10% 17% 32% 14%	4980
	21st to end	Day	29% 6% 14% 35% 14% 2	5464
April	1st to 10th	Day	30% 5% 11% 25% 22% 7%	2239
	11th to 20th	Day	36% 6% 11% 25% 17% 6%	2334
	21st to end	Day	37% 5% 12% 27% 17% 2	2470
May	1st to 10th	Day	39% 8% 11% 19% 15% 7%	4120
	11th to 20th	Day	32% 8% 13% 29% 17% 1	4086
	21st to end	Day	34% 8% 12% 21% 19% 5%	4489
June	1st to 10th	Day	25% 7% 18% 31% 15% 4	2470
	11th to 20th	Day	25% 7% 14% 28% 21% 6%	2470
	21st to end	Day	23% 7% 13% 25% 24% 8%	2470
July	1st to 10th	Day	31% 6% 11% 23% 24% 5%	4056
	11th to 20th	Day	22% 7% 13% 32% 22% 4	4051
	21st to end	Day	25% 6% 13% 27% 28% 2	4325
August	1st to 10th	Day	31% 5% 13% 26% 21% 3	4090
	11th to 20th	Day	27% 6% 16% 28% 20% 4	4080
	21st to end	Day	21% 5% 13% 34% 21% 6%	4278
September	1st to 10th	Day	27% 6% 9% 26% 23% 10%	2119
	11th to 20th	Day	28% 4 9% 26% 28% 5%	2090
	21st to end	Day	26% 5% 10% 26% 25% 9%	2090
October	1st to 10th	Day	17% 3 6% 20% 41% 13%	3462
	11th to 20th	Day	17% 3 10% 27% 31% 11%	3140
	21st to end	Day	23% 3 9% 24% 32% 10%	3353
November	1st to 10th	Day	24% 4 8% 26% 29% 8%	1763
	11th to 20th	Day	25% 3 9% 27% 27% 9%	1800
	21st to end	Day	27% 3 9% 28% 27% 7%	1800
December	1st to 10th	Day	18% 3 8% 41% 27% 3	2940
	11th to 20th	Day	24% 3 6% 37% 28% 3	3389
	21st to end	Day	42% 5% 8% 29% 15% 3	4488
January	1st to 10th	Night	38% 4 10% 26% 17% 3	5888
	11th to 20th	Night	26% 4 12% 22% 28% 7%	5900
	21st to end	Night	34% 3 8% 21% 28% 6%	6490
February	1st to 10th	Night	37% 5% 11% 23% 17% 6%	2532
	11th to 20th	Night	35% 5% 11% 25% 19% 6%	2338
	21st to end	Night	27% 4 8% 22% 25% 13%	1980
March	1st to 10th	Night	22% 4 6% 22% 29% 17%	5195
	11th to 20th	Night	22% 5% 8% 19% 30% 16%	4980
	21st to end	Night	31% 5% 14% 35% 13% 2	5467
April	1st to 10th	Night	29% 4 11% 23% 24% 8%	2158
	11th to 20th	Night	34% 5% 13% 24% 20% 5%	1801
	21st to end	Night	35% 5% 12% 24% 21% 3	1710
May	1st to 10th	Night	36% 6% 11% 20% 16% 11%	2850
	11th to 20th	Night	28% 8% 17% 31% 15% 1	2824
	21st to end	Night	24% 9% 17% 26% 20% 5%	3099
June	1st to 10th	Night	21% 7% 19% 31% 18% 3	1710
	11th to 20th	Night	23% 6% 14% 31% 21% 5%	1710
	21st to end	Night	20% 5% 14% 30% 22% 8%	1710
July	1st to 10th	Night	28% 5% 14% 23% 25% 4	2805
	11th to 20th	Night	20% 4 12% 31% 28% 5%	2796
	21st to end	Night	24% 7% 15% 28% 24% 2	2999
August	1st to 10th	Night	25% 5% 13% 29% 23% 5%	2826
	11th to 20th	Night	29% 7% 15% 25% 20% 4	2825
	21st to end	Night	19% 5% 14% 39% 18% 5%	3384
September	1st to 10th	Night	25% 5% 9% 24% 25% 11%	2060
	11th to 20th	Night	26% 6% 10% 24% 28% 6%	2090
	21st to end	Night	24% 5% 10% 22% 30% 10%	2090
October	1st to 10th	Night	17% 2 7% 16% 46% 11%	3480
	11th to 20th	Night	14% 3 8% 27% 33% 15%	3765
	21st to end	Night	19% 2 7% 25% 37% 10%	4182
November	1st to 10th	Night	23% 3 9% 26% 27% 12%	2535
	11th to 20th	Night	23% 3 7% 28% 30% 8%	2600
	21st to end	Night	27% 3 8% 27% 25% 10%	2600
December	1st to 10th	Night	21% 3 10% 40% 23% 3	4250
	11th to 20th	Night	24% 2 6% 35% 25% 8%	4817
	21st to end	Night	38% 5% 10% 29% 13% 4	6490

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	12% 26% 38% 16% 7% 1	300
	11th to 20th	Day	19% 24% 39% 16% 2	300
	21st to end	Day	12% 25% 45% 15% 2	330
February	1st to 10th	Day	17% 31% 40% 11% 2	300
	11th to 20th	Day	18% 27% 37% 14% 4	300
	21st to end	Day	15% 31% 40% 11% 2	249
March	1st to 10th	Day	15% 22% 45% 15% 3	300
	11th to 20th	Day	23% 21% 39% 13% 3 1	300
	21st to end	Day	23% 26% 39% 10% 1 1	330
April	1st to 10th	Day	19% 23% 43% 14% 2	339
	11th to 20th	Day	16% 24% 42% 15% 2	532
	21st to end	Day	22% 21% 33% 18% 5% 1	650
May	1st to 10th	Day	15% 17% 42% 20% 6%	653
	11th to 20th	Day	15% 15% 30% 27% 11% 1	675
	21st to end	Day	15% 13% 34% 26% 10% 2	734
June	1st to 10th	Day	7% 7% 31% 35% 15% 4	680
	11th to 20th	Day	7% 8% 40% 32% 13% 1	680
	21st to end	Day	6% 6% 43% 33% 10% 2	680
July	1st to 10th	Day	8% 3 20% 44% 21% 3	671
	11th to 20th	Day	3 3 24% 44% 21% 4	668
	21st to end	Day	8% 1 20% 44% 21% 5%	739
August	1st to 10th	Day	6% 4 23% 44% 21% 2	676
	11th to 20th	Day	10% 2 25% 43% 19% 2	665
	21st to end	Day	6% 3 27% 42% 20% 2	599
September	1st to 10th	Day	11% 3 20% 47% 17% 2	329
	11th to 20th	Day	4 5% 39% 38% 12% 2	300
	21st to end	Day	14% 10% 34% 24% 15% 2	300
October	1st to 10th	Day	13% 15% 32% 25% 12% 2	300
	11th to 20th	Day	10% 23% 38% 23% 5% 1	300
	21st to end	Day	23% 16% 37% 16% 7% 2	330
November	1st to 10th	Day	17% 21% 36% 19% 6% 1	300
	11th to 20th	Day	20% 17% 41% 15% 7% 1	300
	21st to end	Day	18% 22% 34% 18% 7% 2	300
December	1st to 10th	Day	10% 25% 34% 23% 7%	300
	11th to 20th	Day	12% 25% 39% 20% 4	300
	21st to end	Day	20% 30% 35% 14% 2	330
January	1st to 10th	Night	10% 16% 41% 28% 6%	1000
	11th to 20th	Night	10% 18% 38% 27% 6% 1	1000
	21st to end	Night	10% 18% 43% 25% 5%	1100
February	1st to 10th	Night	14% 18% 39% 24% 4	932
	11th to 20th	Night	15% 20% 38% 24% 2	784
	21st to end	Night	12% 20% 39% 26% 4	660
March	1st to 10th	Night	10% 21% 39% 27% 4	725
	11th to 20th	Night	15% 16% 38% 23% 8%	600
	21st to end	Night	17% 16% 42% 22% 3	660
April	1st to 10th	Night	15% 17% 39% 23% 5%	600
	11th to 20th	Night	15% 25% 33% 24% 4	565
	21st to end	Night	18% 16% 35% 24% 6%	570
May	1st to 10th	Night	18% 14% 37% 23% 7%	570
	11th to 20th	Night	14% 9% 33% 32% 10% 1	561
	21st to end	Night	12% 11% 36% 28% 12% 1	617
June	1st to 10th	Night	8% 6% 35% 35% 14% 1	570
	11th to 20th	Night	5% 8% 42% 34% 10%	570
	21st to end	Night	6% 7% 42% 36% 9% 1	570
July	1st to 10th	Night	6% 1 26% 46% 18% 1	555
	11th to 20th	Night	5% 1 32% 44% 17% 2	552
	21st to end	Night	7% 2 27% 41% 22% 1	624
August	1st to 10th	Night	4 2 27% 46% 20% 1	567
	11th to 20th	Night	10% 3 24% 44% 17%	564
	21st to end	Night	5% 3 27% 42% 22% 1	627
September	1st to 10th	Night	10% 3 31% 40% 15% 1	570
	11th to 20th	Night	9% 5% 37% 35% 12% 2	570
	21st to end	Night	8% 8% 35% 35% 14% 1	570
October	1st to 10th	Night	13% 8% 32% 31% 14% 1	570
	11th to 20th	Night	11% 13% 39% 29% 7%	746
	21st to end	Night	12% 15% 37% 26% 9% 1	874
November	1st to 10th	Night	11% 11% 37% 30% 10% 1	975
	11th to 20th	Night	9% 16% 42% 26% 7%	1000
	21st to end	Night	13% 15% 33% 32% 7%	1000
December	1st to 10th	Night	10% 14% 35% 33% 7%	1000
	11th to 20th	Night	15% 16% 40% 25% 3	1000
	21st to end	Night	14% 20% 33% 27% 6%	1100

Page generated at 2025-04-08 08:17:27. Version ac07

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