York Factory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the York Factory clear sky chart from 2005-11-06 to 2026-07-01.

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	12% 11% 4 11% 6% 6% 6% 5% 7% 7% 25%	139210
	Night	9% 12% 5% 13% 7% 8% 8% 6% 7% 7% 19%	133493
Transparency	Day	57% 4 12% 20% 6%	142411
	Night	63% 4 10% 17% 6%	138388
Seeing	Day	17% 4 19% 31% 22% 7%	18105
	Night	17% 4 22% 34% 20% 4	27050

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	4 9% 5% 19% 12% 9% 9% 6% 10% 7% 12%	3527
	11th to 20th	Day	4 9% 5% 14% 10% 13% 11% 8% 10% 7% 9%	3729
	21st to end	Day	6% 9% 5% 13% 8% 7% 12% 10% 11% 7% 13%	4532
February	1st to 10th	Day	2 10% 4 15% 10% 9% 8% 8% 9% 8% 16%	1538
	11th to 20th	Day	3 10% 4 10% 8% 8% 10% 7% 12% 10% 16%	1797
	21st to end	Day	3 10% 4 9% 7% 10% 8% 8% 13% 9% 19%	1557
March	1st to 10th	Day	5% 10% 5% 11% 6% 7% 8% 8% 10% 10% 19%	4463
	11th to 20th	Day	7% 11% 4 14% 11% 10% 8% 7% 7% 5% 16%	5077
	21st to end	Day	4 9% 4 9% 9% 9% 11% 10% 9% 8% 19%	5866
April	1st to 10th	Day	8% 11% 4 12% 8% 10% 8% 6% 6% 6% 22%	2546
	11th to 20th	Day	10% 12% 4 10% 8% 8% 7% 5% 5% 6% 25%	2773
	21st to end	Day	7% 9% 3 12% 10% 8% 6% 4 6% 7% 28%	2940
May	1st to 10th	Day	16% 20% 8% 17% 6% 4 3 3 4 4 14%	8406
	11th to 20th	Day	19% 11% 7% 16% 5% 4 4 3 5% 6% 22%	9255
	21st to end	Day	10% 9% 4 10% 5% 4 4 4 6% 8% 37%	10338
June	1st to 10th	Day	16% 12% 5% 9% 5% 4 4 4 5% 8% 29%	3360
	11th to 20th	Day	17% 7% 2 6% 5% 5% 5% 4 6% 8% 36%	3414
	21st to end	Day	13% 7% 3 8% 5% 5% 5% 4 7% 7% 35%	3570
July	1st to 10th	Day	10% 7% 3 7% 6% 5% 5% 4 6% 8% 38%	7268
	11th to 20th	Day	11% 6% 2 6% 5% 4 5% 4 7% 9% 40%	7260
	21st to end	Day	10% 5% 3 6% 5% 6% 6% 5% 7% 10% 36%	7595
August	1st to 10th	Day	15% 6% 2 6% 4 4 5% 4 7% 9% 40%	4835
	11th to 20th	Day	15% 7% 3 7% 4 4 4 3 5% 7% 40%	4496
	21st to end	Day	22% 8% 2 8% 4 4 4 4 6% 8% 31%	4784
September	1st to 10th	Day	22% 11% 4 9% 4 5% 5% 3 5% 7% 23%	2435
	11th to 20th	Day	22% 12% 4 9% 5% 5% 5% 3 5% 7% 23%	2400
	21st to end	Day	18% 16% 3 8% 5% 3 5% 3 5% 7% 26%	2095
October	1st to 10th	Day	20% 19% 4 10% 5% 4 4 3 4 5% 21%	2000
	11th to 20th	Day	27% 18% 6% 10% 4 5% 4 3 4 5% 15%	1903
	21st to end	Day	20% 21% 8% 13% 5% 4 3 2 4 4 15%	1767
November	1st to 10th	Day	14% 19% 8% 14% 8% 6% 6% 4 4 4 13%	1640
	11th to 20th	Day	12% 19% 7% 14% 8% 6% 5% 4 5% 6% 13%	1390
	21st to end	Day	8% 21% 9% 16% 7% 6% 8% 5% 6% 5% 9%	1260
December	1st to 10th	Day	4 16% 6% 22% 10% 6% 6% 5% 9% 8% 8%	2020
	11th to 20th	Day	8% 14% 7% 17% 8% 7% 6% 5% 9% 7% 12%	2316
	21st to end	Day	9% 20% 7% 16% 7% 7% 7% 5% 5% 5% 12%	3058
January	1st to 10th	Night	4 11% 6% 17% 11% 10% 9% 8% 9% 7% 9%	9398
	11th to 20th	Night	3 9% 5% 19% 12% 15% 11% 8% 8% 5% 6%	9183
	21st to end	Night	6% 11% 5% 13% 9% 9% 12% 9% 10% 7% 11%	9682
February	1st to 10th	Night	4 10% 5% 12% 10% 10% 11% 9% 10% 7% 13%	3082
	11th to 20th	Night	3 10% 4 10% 8% 10% 10% 8% 12% 8% 17%	2706
	21st to end	Night	4 10% 5% 10% 6% 6% 8% 10% 13% 10% 17%	2249
March	1st to 10th	Night	3 8% 4 12% 7% 8% 9% 10% 10% 8% 20%	5601
	11th to 20th	Night	8% 11% 4 12% 9% 10% 8% 7% 7% 6% 16%	5147
	21st to end	Night	4 8% 4 9% 9% 10% 11% 9% 8% 8% 18%	5307
April	1st to 10th	Night	7% 12% 4 10% 8% 7% 9% 5% 6% 6% 25%	2071
	11th to 20th	Night	9% 11% 3 11% 7% 7% 7% 4 6% 7% 29%	1798
	21st to end	Night	8% 10% 4 10% 8% 7% 5% 4 6% 6% 33%	1680
May	1st to 10th	Night	16% 17% 6% 17% 7% 5% 4 3 3 4 18%	4521
	11th to 20th	Night	16% 11% 6% 16% 5% 4 4 3 4 5% 25%	3636
	21st to end	Night	10% 7% 3 9% 4 4 4 4 6% 8% 41%	3870
June	1st to 10th	Night	18% 10% 4 8% 4 4 5% 4 5% 6% 33%	1260
	11th to 20th	Night	15% 7% 2 8% 6% 5% 6% 4 7% 7% 33%	1198
	21st to end	Night	10% 10% 3 8% 6% 6% 6% 4 6% 8% 33%	1050
July	1st to 10th	Night	12% 8% 3 8% 5% 6% 6% 4 7% 8% 31%	2707
	11th to 20th	Night	11% 7% 3 7% 4 5% 6% 5% 6% 9% 37%	2720
	21st to end	Night	10% 5% 3 7% 5% 6% 6% 4 6% 9% 38%	3277
August	1st to 10th	Night	15% 6% 2 6% 4 5% 5% 4 6% 7% 38%	2324
	11th to 20th	Night	12% 6% 2 6% 4 4 5% 4 6% 7% 42%	2563
	21st to end	Night	20% 7% 2 5% 4 4 5% 3 5% 8% 37%	3122
September	1st to 10th	Night	20% 11% 3 8% 6% 5% 4 3 6% 7% 26%	1965
	11th to 20th	Night	21% 10% 2 7% 6% 5% 6% 4 6% 7% 26%	2000
	21st to end	Night	19% 16% 5% 7% 4 4 4 3 4 6% 29%	2280
October	1st to 10th	Night	23% 17% 5% 9% 4 4 4 4 4 5% 21%	2400
	11th to 20th	Night	24% 21% 5% 9% 5% 4 3 2 4 5% 18%	2497
	21st to end	Night	20% 22% 7% 11% 6% 5% 4 2 4 5% 16%	3070
November	1st to 10th	Night	14% 20% 8% 15% 7% 6% 5% 4 5% 5% 13%	2877
	11th to 20th	Night	11% 17% 7% 13% 8% 8% 7% 5% 5% 5% 14%	3229
	21st to end	Night	9% 20% 7% 16% 8% 6% 6% 5% 6% 6% 11%	3360
December	1st to 10th	Night	3 14% 7% 21% 9% 8% 8% 7% 10% 7% 7%	5390
	11th to 20th	Night	6% 14% 7% 15% 10% 10% 9% 8% 8% 6% 8%	6111
	21st to end	Night	9% 16% 8% 16% 7% 7% 8% 5% 6% 5% 14%	8162

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	68% 3 8% 17% 4	3528
	11th to 20th	Day	70% 4 8% 16% 2	3754
	21st to end	Day	64% 3 9% 18% 6%	4532
February	1st to 10th	Day	59% 5% 11% 20% 4	1538
	11th to 20th	Day	55% 6% 14% 20% 6%	1806
	21st to end	Day	52% 6% 9% 20% 13%	1557
March	1st to 10th	Day	56% 4 8% 20% 11% 2	4465
	11th to 20th	Day	67% 3 6% 13% 11%	5077
	21st to end	Day	59% 4 7% 17% 12% 1	5866
April	1st to 10th	Day	60% 6% 7% 19% 7%	2546
	11th to 20th	Day	57% 6% 9% 19% 7% 1	2773
	21st to end	Day	55% 6% 12% 19% 7% 1	2940
May	1st to 10th	Day	75% 3 7% 12% 4	8415
	11th to 20th	Day	64% 3 8% 18% 7%	9259
	21st to end	Day	46% 3 11% 24% 14% 2	10339
June	1st to 10th	Day	54% 5% 11% 23% 7%	3360
	11th to 20th	Day	45% 6% 11% 27% 9% 1	3414
	21st to end	Day	45% 7% 11% 28% 8%	3570
July	1st to 10th	Day	45% 4 15% 29% 8%	7190
	11th to 20th	Day	40% 3 21% 32% 4	7173
	21st to end	Day	42% 4 22% 29% 4	7377
August	1st to 10th	Day	42% 5% 22% 28% 3	4818
	11th to 20th	Day	44% 5% 15% 25% 11%	4538
	21st to end	Day	53% 5% 17% 23% 3	4783
September	1st to 10th	Day	59% 8% 17% 14% 2	2435
	11th to 20th	Day	63% 6% 14% 15% 2	2400
	21st to end	Day	57% 7% 15% 17% 4	2095
October	1st to 10th	Day	67% 4 6% 21% 2	3260
	11th to 20th	Day	66% 4 8% 18% 3	3080
	21st to end	Day	72% 4 10% 10% 4	2839
November	1st to 10th	Day	76% 8% 7% 7% 2	1640
	11th to 20th	Day	74% 7% 9% 10% 1	1390
	21st to end	Day	73% 7% 9% 9% 1	1260
December	1st to 10th	Day	73% 4 11% 10% 2	2020
	11th to 20th	Day	71% 4 7% 16% 2	2316
	21st to end	Day	74% 5% 8% 11% 3	3058
January	1st to 10th	Night	71% 4 10% 11% 5%	9398
	11th to 20th	Night	77% 3 9% 9% 2	9233
	21st to end	Night	67% 3 9% 16% 6%	9682
February	1st to 10th	Night	61% 6% 11% 17% 5%	3082
	11th to 20th	Night	56% 6% 13% 18% 6%	2707
	21st to end	Night	51% 6% 11% 20% 11%	2249
March	1st to 10th	Night	55% 5% 10% 17% 9% 3	5612
	11th to 20th	Night	66% 4 6% 15% 10%	5148
	21st to end	Night	59% 2 7% 16% 15% 1	5307
April	1st to 10th	Night	56% 5% 8% 21% 10%	2071
	11th to 20th	Night	55% 4 9% 21% 11% 1	1798
	21st to end	Night	51% 6% 11% 23% 8%	1680
May	1st to 10th	Night	72% 2 6% 14% 5%	4528
	11th to 20th	Night	63% 2 7% 17% 10%	3637
	21st to end	Night	41% 3 10% 24% 16% 5%	3870
June	1st to 10th	Night	51% 5% 8% 25% 10%	1260
	11th to 20th	Night	48% 5% 10% 22% 14% 1	1198
	21st to end	Night	47% 7% 10% 25% 9% 1	1050
July	1st to 10th	Night	50% 4 10% 31% 5%	2677
	11th to 20th	Night	43% 3 18% 30% 5%	2684
	21st to end	Night	43% 4 21% 27% 5%	3181
August	1st to 10th	Night	43% 5% 23% 26% 3	2317
	11th to 20th	Night	41% 6% 14% 28% 12%	2587
	21st to end	Night	47% 4 19% 24% 6%	3122
September	1st to 10th	Night	58% 8% 15% 17% 2	1965
	11th to 20th	Night	56% 9% 17% 16% 3	2000
	21st to end	Night	58% 7% 14% 16% 5%	2280
October	1st to 10th	Night	65% 4 8% 19% 4	3920
	11th to 20th	Night	63% 3 11% 17% 5%	4059
	21st to end	Night	67% 4 7% 17% 5%	4958
November	1st to 10th	Night	75% 6% 9% 9% 1	2877
	11th to 20th	Night	73% 8% 9% 9% 1	3229
	21st to end	Night	72% 7% 11% 9% 1	3360
December	1st to 10th	Night	74% 4 9% 11% 2	5390
	11th to 20th	Night	71% 5% 10% 12% 2	6110
	21st to end	Night	70% 5% 8% 14% 2	8162

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	6% 1 31% 36% 21% 5%	220
	11th to 20th	Day	6% 3 25% 40% 21% 4	259
	21st to end	Day	11% 4 18% 34% 27% 6%	363
February	1st to 10th	Day	7% 5% 19% 40% 24% 4	330
	11th to 20th	Day	7% 2 21% 39% 24% 6%	330
	21st to end	Day	6% 3 18% 35% 28% 10%	273
March	1st to 10th	Day	8% 2 18% 34% 24% 15%	330
	11th to 20th	Day	7% 4 21% 32% 24% 12%	330
	21st to end	Day	7% 4 19% 34% 27% 10%	489
April	1st to 10th	Day	10% 3 17% 26% 31% 12%	566
	11th to 20th	Day	13% 4 22% 33% 21% 7%	740
	21st to end	Day	11% 5% 20% 32% 23% 9%	750
May	1st to 10th	Day	18% 5% 20% 30% 20% 6%	753
	11th to 20th	Day	18% 6% 25% 29% 17% 5%	775
	21st to end	Day	16% 4 23% 32% 21% 4	844
June	1st to 10th	Day	20% 5% 20% 29% 21% 5%	780
	11th to 20th	Day	19% 4 17% 30% 23% 7%	780
	21st to end	Day	16% 3 15% 33% 24% 10%	780
July	1st to 10th	Day	20% 3 14% 32% 25% 6%	722
	11th to 20th	Day	16% 4 13% 33% 27% 7%	718
	21st to end	Day	17% 4 15% 30% 23% 10%	794
August	1st to 10th	Day	21% 3 14% 29% 25% 8%	726
	11th to 20th	Day	20% 4 14% 27% 24% 12%	715
	21st to end	Day	26% 4 14% 27% 21% 7%	803
September	1st to 10th	Day	28% 5% 21% 27% 15% 4	565
	11th to 20th	Day	24% 6% 21% 26% 18% 4	530
	21st to end	Day	26% 4 23% 29% 14% 4	370
October	1st to 10th	Day	19% 7% 28% 28% 16% 3	330
	11th to 20th	Day	32% 5% 22% 22% 13% 5%	330
	21st to end	Day	23% 7% 28% 23% 16% 2	363
November	1st to 10th	Day	18% 8% 28% 29% 13% 5%	325
	11th to 20th	Day	20% 6% 29% 32% 10% 2	220
	21st to end	Day	20% 12% 19% 25% 16% 8%	220
December	1st to 10th	Day	13% 5% 25% 36% 20% 1	220
	11th to 20th	Day	9% 6% 29% 33% 20% 3	220
	21st to end	Day	8% 6% 22% 33% 24% 6%	242
January	1st to 10th	Night	12% 4 24% 36% 19% 6%	1050
	11th to 20th	Night	9% 4 23% 38% 21% 4	1050
	21st to end	Night	10% 5% 22% 36% 23% 4	1155
February	1st to 10th	Night	8% 4 23% 37% 25% 3	1050
	11th to 20th	Night	7% 3 18% 42% 24% 6%	1029
	21st to end	Night	8% 3 18% 41% 22% 8%	865
March	1st to 10th	Night	10% 3 21% 34% 28% 4	898
	11th to 20th	Night	10% 5% 21% 38% 23% 3	662
	21st to end	Night	8% 4 21% 34% 26% 6%	693
April	1st to 10th	Night	12% 3 18% 36% 26% 5%	630
	11th to 20th	Night	14% 3 19% 36% 23% 4	623
	21st to end	Night	10% 6% 21% 37% 22% 3	630
May	1st to 10th	Night	25% 4 19% 31% 18% 3	630
	11th to 20th	Night	19% 5% 23% 36% 15% 2	463
	21st to end	Night	20% 3 24% 35% 17% 1	455
June	1st to 10th	Night	19% 4 20% 37% 17% 3	420
	11th to 20th	Night	18% 4 18% 36% 23% 1	358
	21st to end	Night	13% 1 17% 42% 23% 3	210
July	1st to 10th	Night	19% 2 15% 38% 24% 2	377
	11th to 20th	Night	17% 4 13% 37% 26% 4	388
	21st to end	Night	19% 3 12% 31% 29% 7%	569
August	1st to 10th	Night	20% 3 12% 32% 29% 5%	597
	11th to 20th	Night	20% 3 14% 34% 26% 4	594
	21st to end	Night	26% 4 13% 35% 19% 3	660
September	1st to 10th	Night	27% 3 22% 33% 14% 2	600
	11th to 20th	Night	28% 5% 22% 28% 17% 2	600
	21st to end	Night	28% 6% 24% 29% 12% 2	724
October	1st to 10th	Night	34% 7% 23% 26% 9% 1	800
	11th to 20th	Night	35% 6% 18% 26% 12% 3	800
	21st to end	Night	31% 6% 26% 25% 10% 2	1090
November	1st to 10th	Night	22% 8% 29% 27% 12% 2	1025
	11th to 20th	Night	19% 5% 30% 31% 13% 2	1050
	21st to end	Night	17% 9% 24% 33% 12% 5%	1050
December	1st to 10th	Night	11% 4 30% 36% 17% 2	1050
	11th to 20th	Night	11% 4 24% 34% 23% 4	1050
	21st to end	Night	12% 3 23% 35% 20% 6%	1155

Page generated at 2026-06-28 01:10:47. Version ac07

Copyright © 2026 Attilla Danko.