YFOS
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the YFOS clear sky chart from 2005-11-06 to 2025-10-19.

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	16% 8% 3 7% 5% 5% 5% 4 6% 7% 36%	126467
	Night	16% 8% 3 6% 4 4 5% 4 6% 7% 38%	123254
Transparency	Day	46% 4 12% 22% 14% 2	129974
	Night	44% 4 13% 22% 14% 2	127834
Seeing	Day	25% 18% 30% 18% 7% 2	16141
	Night	23% 17% 29% 23% 7% 1	25011

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	10% 7% 3 10% 6% 5% 6% 6% 8% 7% 32%	3597
	11th to 20th	Day	12% 8% 4 9% 5% 4 4 4 6% 7% 38%	3570
	21st to end	Day	12% 12% 4 7% 5% 4 4 3 6% 5% 38%	3960
February	1st to 10th	Day	14% 10% 3 7% 5% 7% 6% 4 6% 7% 31%	1808
	11th to 20th	Day	12% 7% 3 9% 6% 5% 5% 4 6% 8% 35%	1948
	21st to end	Day	16% 12% 3 8% 5% 4 4 4 5% 5% 34%	1650
March	1st to 10th	Day	17% 13% 3 8% 5% 5% 5% 4 5% 5% 31%	4476
	11th to 20th	Day	15% 9% 3 7% 5% 5% 5% 4 6% 7% 36%	4699
	21st to end	Day	21% 11% 3 5% 4 4 4 3 5% 7% 33%	5878
April	1st to 10th	Day	17% 11% 3 7% 6% 5% 4 3 5% 6% 35%	2400
	11th to 20th	Day	16% 7% 2 6% 5% 5% 5% 5% 6% 6% 37%	2374
	21st to end	Day	19% 9% 3 8% 5% 4 5% 4 5% 6% 33%	2595
May	1st to 10th	Day	31% 9% 3 7% 4 5% 4 4 5% 5% 23%	5886
	11th to 20th	Day	19% 9% 3 9% 5% 5% 5% 4 5% 7% 29%	6173
	21st to end	Day	18% 7% 2 7% 4 4 5% 4 6% 8% 33%	6949
June	1st to 10th	Day	18% 7% 2 7% 6% 5% 4 4 6% 8% 32%	2800
	11th to 20th	Day	16% 6% 2 7% 6% 4 5% 3 6% 9% 36%	2800
	21st to end	Day	19% 7% 2 6% 6% 5% 6% 5% 6% 7% 32%	2800
July	1st to 10th	Day	14% 6% 2 7% 5% 5% 6% 5% 7% 10% 34%	6360
	11th to 20th	Day	9% 5% 2 6% 5% 5% 6% 5% 8% 10% 39%	6360
	21st to end	Day	15% 5% 2 5% 4 4 5% 4 6% 8% 42%	6926
August	1st to 10th	Day	14% 5% 2 7% 5% 6% 5% 5% 6% 8% 36%	4404
	11th to 20th	Day	14% 7% 3 7% 5% 5% 5% 5% 7% 8% 35%	4081
	21st to end	Day	9% 3 2 6% 5% 5% 5% 5% 7% 9% 44%	4312
September	1st to 10th	Day	11% 6% 2 5% 4 4 4 4 6% 7% 46%	2400
	11th to 20th	Day	13% 5% 3 5% 4 3 4 3 5% 7% 50%	2230
	21st to end	Day	16% 5% 2 6% 4 4 4 4 6% 7% 43%	2200
October	1st to 10th	Day	20% 7% 3 6% 5% 4 4 3 4 5% 39%	2200
	11th to 20th	Day	15% 7% 2 6% 4 3 5% 4 5% 8% 42%	1898
	21st to end	Day	22% 8% 3 6% 4 4 6% 3 5% 7% 34%	1881
November	1st to 10th	Day	11% 7% 3 6% 4 4 4 3 5% 7% 45%	1761
	11th to 20th	Day	16% 6% 2 7% 4 3 5% 3 5% 5% 44%	1800
	21st to end	Day	18% 9% 2 7% 4 3 4 4 6% 7% 36%	1745
December	1st to 10th	Day	14% 9% 3 7% 4 5% 5% 4 5% 7% 36%	2600
	11th to 20th	Day	20% 9% 3 5% 3 4 4 3 5% 5% 39%	2986
	21st to end	Day	18% 10% 3 8% 5% 5% 4 3 5% 5% 34%	3960
January	1st to 10th	Night	12% 7% 3 7% 4 5% 6% 5% 7% 8% 37%	6349
	11th to 20th	Night	10% 9% 3 10% 6% 5% 6% 5% 6% 8% 33%	6315
	21st to end	Night	14% 9% 3 7% 5% 5% 5% 4 6% 6% 36%	6996
February	1st to 10th	Night	13% 10% 3 7% 4 5% 5% 4 7% 6% 35%	2588
	11th to 20th	Night	11% 9% 3 6% 5% 5% 6% 5% 6% 8% 36%	2338
	21st to end	Night	13% 10% 3 7% 5% 4 5% 5% 6% 6% 36%	1980
March	1st to 10th	Night	19% 10% 2 6% 4 4 5% 4 6% 7% 34%	5364
	11th to 20th	Night	14% 9% 3 7% 5% 4 4 4 5% 7% 36%	5260
	21st to end	Night	23% 9% 2 6% 4 4 4 3 6% 6% 34%	5038
April	1st to 10th	Night	18% 8% 2 6% 4 5% 5% 4 6% 6% 35%	2000
	11th to 20th	Night	17% 9% 3 6% 4 3 5% 3 6% 6% 38%	1975
	21st to end	Night	19% 9% 3 7% 4 4 4 3 5% 7% 37%	1804
May	1st to 10th	Night	32% 10% 3 6% 4 4 3 4 5% 6% 23%	4054
	11th to 20th	Night	16% 10% 3 9% 5% 6% 5% 4 6% 7% 29%	3763
	21st to end	Night	20% 7% 2 7% 5% 4 5% 4 6% 7% 33%	3926
June	1st to 10th	Night	18% 8% 2 6% 6% 4 4 3 5% 8% 36%	1600
	11th to 20th	Night	18% 6% 3 4 4 5% 4 4 6% 8% 38%	1600
	21st to end	Night	16% 6% 2 6% 5% 3 5% 4 6% 7% 39%	1600
July	1st to 10th	Night	11% 5% 2 6% 4 5% 4 4 7% 10% 42%	3600
	11th to 20th	Night	12% 6% 2 6% 5% 4 6% 5% 6% 8% 40%	3600
	21st to end	Night	12% 5% 2 5% 3 3 5% 4 5% 7% 50%	3926
August	1st to 10th	Night	20% 6% 3 7% 4 4 4 3 5% 7% 36%	2713
	11th to 20th	Night	13% 9% 3 7% 4 4 5% 4 6% 8% 39%	2963
	21st to end	Night	9% 3 1 4 3 3 3 4 6% 9% 56%	3586
September	1st to 10th	Night	12% 5% 2 5% 3 3 3 3 5% 7% 52%	2000
	11th to 20th	Night	13% 5% 2 4 2 3 4 3 4 6% 54%	2170
	21st to end	Night	14% 5% 2 5% 4 4 4 4 6% 7% 46%	2200
October	1st to 10th	Night	19% 6% 2 5% 5% 4 4 3 5% 6% 41%	2200
	11th to 20th	Night	16% 6% 2 5% 4 4 5% 4 6% 7% 42%	2454
	21st to end	Night	22% 8% 3 6% 4 4 4 4 5% 5% 34%	2717
November	1st to 10th	Night	13% 7% 2 6% 5% 4 4 3 5% 6% 45%	2537
	11th to 20th	Night	15% 6% 3 5% 5% 4 4 4 5% 5% 43%	2600
	21st to end	Night	14% 8% 2 6% 4 4 4 4 6% 7% 40%	2654
December	1st to 10th	Night	14% 11% 4 7% 5% 4 4 3 6% 7% 35%	4580
	11th to 20th	Night	21% 9% 3 6% 4 4 4 3 4 6% 36%	5208
	21st to end	Night	18% 11% 4 7% 4 4 4 3 5% 6% 34%	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	47% 3 10% 28% 11% 1	3598
	11th to 20th	Day	44% 4 15% 20% 14% 3	3600
	21st to end	Day	45% 5% 8% 21% 17% 4	3960
February	1st to 10th	Day	50% 5% 14% 20% 10% 2	1808
	11th to 20th	Day	46% 5% 12% 22% 13% 3	1957
	21st to end	Day	51% 3 10% 20% 13% 2	1650
March	1st to 10th	Day	54% 3 9% 13% 11% 9%	4479
	11th to 20th	Day	47% 3 16% 22% 11% 2	4699
	21st to end	Day	50% 3 6% 17% 20% 4	5878
April	1st to 10th	Day	50% 4 8% 20% 15% 4	2400
	11th to 20th	Day	45% 4 11% 21% 17% 2	2374
	21st to end	Day	52% 4 8% 19% 15% 3	2595
May	1st to 10th	Day	62% 5% 10% 14% 8% 1	5897
	11th to 20th	Day	54% 4 14% 17% 9% 1	6177
	21st to end	Day	46% 5% 14% 22% 12% 1	6949
June	1st to 10th	Day	48% 6% 12% 22% 12% 1	2800
	11th to 20th	Day	44% 5% 13% 20% 16% 2	2800
	21st to end	Day	48% 6% 12% 19% 13% 1	2800
July	1st to 10th	Day	45% 5% 15% 21% 14% 1	6290
	11th to 20th	Day	39% 5% 17% 24% 14% 1	6285
	21st to end	Day	39% 3 13% 28% 16% 1	6728
August	1st to 10th	Day	44% 5% 16% 23% 12% 1	4390
	11th to 20th	Day	44% 4 16% 27% 8% 1	4120
	21st to end	Day	34% 5% 14% 30% 16% 1	4312
September	1st to 10th	Day	35% 5% 13% 28% 18% 2	2400
	11th to 20th	Day	34% 4 9% 28% 23% 2	2230
	21st to end	Day	39% 4 10% 26% 18% 3	2200
October	1st to 10th	Day	44% 4 8% 24% 18% 2	3590
	11th to 20th	Day	37% 3 11% 27% 16% 5%	3075
	21st to end	Day	36% 4 9% 32% 16% 3	3081
November	1st to 10th	Day	39% 4 14% 25% 16% 2	1761
	11th to 20th	Day	40% 4 11% 23% 20% 2	1800
	21st to end	Day	46% 4 16% 23% 10% 1	1745
December	1st to 10th	Day	47% 4 13% 29% 6% 1	2600
	11th to 20th	Day	46% 3 11% 19% 18% 2	2986
	21st to end	Day	52% 3 9% 18% 13% 5%	3960
January	1st to 10th	Night	43% 3 15% 26% 12% 1	6349
	11th to 20th	Night	48% 4 14% 21% 11% 2	6360
	21st to end	Night	48% 3 11% 17% 17% 4	6996
February	1st to 10th	Night	47% 5% 14% 21% 11% 2	2588
	11th to 20th	Night	44% 4 13% 23% 14% 2	2340
	21st to end	Night	44% 4 13% 24% 11% 4	1980
March	1st to 10th	Night	49% 4 10% 17% 16% 5%	5374
	11th to 20th	Night	48% 3 18% 19% 11% 2	5261
	21st to end	Night	50% 3 5% 16% 23% 3	5038
April	1st to 10th	Night	49% 4 11% 20% 15% 2	2000
	11th to 20th	Night	45% 4 11% 22% 17% 1	1975
	21st to end	Night	47% 3 11% 20% 16% 3	1804
May	1st to 10th	Night	61% 5% 12% 13% 8% 1	4060
	11th to 20th	Night	52% 6% 15% 17% 9% 1	3764
	21st to end	Night	50% 5% 16% 18% 10% 1	3927
June	1st to 10th	Night	47% 5% 13% 23% 12% 1	1600
	11th to 20th	Night	44% 6% 16% 20% 13% 2	1600
	21st to end	Night	44% 6% 14% 23% 12% 1	1600
July	1st to 10th	Night	38% 5% 17% 28% 12%	3560
	11th to 20th	Night	41% 6% 16% 21% 16% 1	3552
	21st to end	Night	33% 4 15% 34% 14% 1	3809
August	1st to 10th	Night	49% 5% 14% 21% 10% 1	2703
	11th to 20th	Night	43% 5% 18% 23% 11%	2990
	21st to end	Night	26% 3 17% 37% 17% 1	3586
September	1st to 10th	Night	32% 5% 14% 29% 19% 1	2000
	11th to 20th	Night	33% 4 11% 28% 23% 2	2170
	21st to end	Night	36% 5% 10% 30% 16% 2	2200
October	1st to 10th	Night	39% 4 16% 25% 15% 2	3590
	11th to 20th	Night	32% 3 12% 28% 21% 4	4016
	21st to end	Night	39% 3 11% 32% 13% 2	4467
November	1st to 10th	Night	41% 4 12% 24% 17% 3	2537
	11th to 20th	Night	41% 4 11% 24% 18% 3	2600
	21st to end	Night	41% 4 18% 25% 11% 1	2654
December	1st to 10th	Night	48% 4 12% 28% 8%	4580
	11th to 20th	Night	49% 3 12% 15% 18% 1	5208
	21st to end	Night	50% 3 10% 19% 13% 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	34% 22% 29% 12% 3	200
	11th to 20th	Day	34% 23% 33% 10% 1 1	200
	21st to end	Day	27% 29% 32% 9% 3	220
February	1st to 10th	Day	35% 31% 24% 9% 2	340
	11th to 20th	Day	35% 24% 28% 12% 1	496
	21st to end	Day	29% 26% 29% 13% 3	414
March	1st to 10th	Day	32% 22% 32% 11% 3	500
	11th to 20th	Day	30% 26% 27% 12% 3 1	500
	21st to end	Day	32% 22% 30% 9% 5% 2	550
April	1st to 10th	Day	36% 21% 26% 10% 6% 1	500
	11th to 20th	Day	34% 24% 26% 10% 5% 1	492
	21st to end	Day	29% 20% 31% 12% 7% 1	500
May	1st to 10th	Day	34% 20% 27% 13% 4 1	503
	11th to 20th	Day	21% 19% 38% 16% 5% 2	527
	21st to end	Day	21% 14% 33% 24% 6% 1	573
June	1st to 10th	Day	22% 15% 36% 17% 8% 2	530
	11th to 20th	Day	20% 16% 34% 20% 9% 2	530
	21st to end	Day	22% 15% 35% 19% 7% 3	530
July	1st to 10th	Day	17% 11% 34% 25% 10% 2	525
	11th to 20th	Day	15% 10% 34% 30% 9% 2	524
	21st to end	Day	16% 11% 35% 29% 7% 2	576
August	1st to 10th	Day	12% 12% 33% 27% 12% 5%	526
	11th to 20th	Day	16% 9% 34% 23% 13% 5%	519
	21st to end	Day	14% 11% 33% 26% 11% 5%	583
September	1st to 10th	Day	16% 13% 28% 22% 15% 5%	530
	11th to 20th	Day	18% 13% 26% 20% 15% 9%	530
	21st to end	Day	19% 14% 30% 21% 10% 5%	530
October	1st to 10th	Day	27% 14% 24% 20% 11% 4	530
	11th to 20th	Day	25% 19% 27% 20% 7% 2	474
	21st to end	Day	34% 14% 28% 18% 6% 1	429
November	1st to 10th	Day	24% 23% 26% 17% 10% 1	395
	11th to 20th	Day	32% 24% 22% 17% 5% 1	400
	21st to end	Day	34% 21% 26% 14% 4 1	345
December	1st to 10th	Day	25% 30% 27% 15% 3 1	200
	11th to 20th	Day	38% 25% 27% 8% 2	200
	21st to end	Day	31% 20% 30% 15% 4	220
January	1st to 10th	Night	25% 25% 31% 17% 2	800
	11th to 20th	Night	27% 23% 30% 18% 3	800
	21st to end	Night	20% 25% 34% 18% 4	880
February	1st to 10th	Night	24% 23% 34% 17% 2	800
	11th to 20th	Night	25% 25% 33% 15% 2	783
	21st to end	Night	27% 24% 30% 16% 2	660
March	1st to 10th	Night	30% 18% 32% 17% 3	800
	11th to 20th	Night	26% 22% 28% 20% 4	800
	21st to end	Night	27% 19% 30% 19% 6%	880
April	1st to 10th	Night	27% 19% 30% 20% 3	800
	11th to 20th	Night	24% 21% 31% 19% 5%	791
	21st to end	Night	27% 18% 28% 23% 4	604
May	1st to 10th	Night	28% 15% 31% 21% 4	600
	11th to 20th	Night	19% 16% 32% 27% 5%	478
	21st to end	Night	21% 9% 32% 29% 8% 1	433
June	1st to 10th	Night	22% 10% 32% 28% 8% 1	400
	11th to 20th	Night	21% 11% 31% 29% 9%	400
	21st to end	Night	19% 8% 31% 33% 9% 1	400
July	1st to 10th	Night	16% 8% 29% 38% 9%	390
	11th to 20th	Night	12% 4 28% 40% 13% 2	388
	21st to end	Night	13% 5% 34% 37% 11%	438
August	1st to 10th	Night	14% 6% 25% 40% 15% 1	473
	11th to 20th	Night	16% 7% 29% 31% 16% 2	644
	21st to end	Night	15% 7% 25% 36% 15% 2	880
September	1st to 10th	Night	14% 9% 30% 30% 17% 1	800
	11th to 20th	Night	16% 9% 25% 27% 19% 3	800
	21st to end	Night	17% 9% 27% 31% 14% 2	800
October	1st to 10th	Night	25% 12% 23% 27% 12% 2	800
	11th to 20th	Night	20% 16% 30% 24% 10% 1	793
	21st to end	Night	28% 19% 27% 20% 6%	836
November	1st to 10th	Night	19% 21% 30% 23% 7% 1	780
	11th to 20th	Night	24% 25% 26% 18% 6% 1	800
	21st to end	Night	25% 22% 28% 19% 6%	800
December	1st to 10th	Night	25% 22% 33% 16% 4	800
	11th to 20th	Night	27% 23% 31% 16% 3	800
	21st to end	Night	28% 24% 27% 19% 2	880

Page generated at 2025-10-16 01:50:28. Version ac07

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