Beaufort
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Beaufort clear sky chart from 2005-11-06 to 2025-12-03.

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	11% 6% 2 6% 4 4 5% 4 7% 9% 43%	126999
	Night	11% 6% 2 6% 4 4 4 4 6% 8% 44%	123701
Transparency	Day	23% 40% 10% 12% 10% 4	130645
	Night	23% 40% 9% 11% 12% 5%	128111
Seeing	Day	11% 5% 20% 29% 25% 9%	16951
	Night	12% 6% 21% 31% 25% 5%	27013

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% 5% 2 4 3 4 4 3 5% 6% 54%	4056
	11th to 20th	Day	14% 6% 2 5% 3 3 3 3 4 7% 48%	4060
	21st to end	Day	13% 9% 3 5% 3 3 3 3 5% 7% 45%	4466
February	1st to 10th	Day	13% 7% 3 5% 5% 5% 4 4 6% 7% 42%	1833
	11th to 20th	Day	19% 6% 2 4 3 3 3 4 5% 6% 45%	1948
	21st to end	Day	13% 6% 2 6% 5% 3 3 3 5% 7% 48%	1765
March	1st to 10th	Day	20% 9% 2 6% 3 3 4 3 5% 5% 39%	4935
	11th to 20th	Day	8% 3 1 4 3 3 4 4 6% 8% 56%	4980
	21st to end	Day	12% 6% 2 7% 4 4 4 4 4 5% 49%	5465
April	1st to 10th	Day	8% 4 1 4 4 4 4 4 6% 8% 52%	2200
	11th to 20th	Day	9% 5% 2 5% 4 3 4 3 5% 8% 52%	2349
	21st to end	Day	4 4 1 4 4 3 4 4 7% 8% 57%	2415
May	1st to 10th	Day	9% 5% 2 5% 4 5% 5% 4 8% 11% 42%	5862
	11th to 20th	Day	14% 6% 2 6% 4 4 4 4 7% 9% 39%	5861
	21st to end	Day	7% 4 2 6% 4 4 6% 5% 7% 10% 46%	6433
June	1st to 10th	Day	7% 5% 3 7% 5% 7% 6% 7% 9% 10% 34%	2600
	11th to 20th	Day	5% 4 2 5% 5% 5% 7% 6% 8% 12% 40%	2600
	21st to end	Day	6% 4 2 6% 5% 6% 8% 6% 9% 14% 35%	2600
July	1st to 10th	Day	10% 6% 3 8% 6% 7% 7% 7% 9% 11% 26%	5900
	11th to 20th	Day	4 3 2 6% 5% 6% 7% 6% 10% 17% 34%	5900
	21st to end	Day	4 4 2 6% 5% 6% 7% 6% 9% 15% 35%	6424
August	1st to 10th	Day	20% 6% 3 7% 5% 5% 6% 6% 8% 11% 23%	4234
	11th to 20th	Day	6% 4 2 6% 5% 5% 7% 6% 10% 13% 37%	4113
	21st to end	Day	10% 7% 3 7% 5% 4 5% 5% 8% 10% 36%	4293
September	1st to 10th	Day	11% 6% 3 6% 5% 5% 6% 5% 7% 9% 36%	2200
	11th to 20th	Day	11% 8% 3 7% 6% 5% 6% 4 6% 8% 37%	2200
	21st to end	Day	13% 6% 3 6% 5% 4 5% 4 7% 7% 40%	2200
October	1st to 10th	Day	12% 6% 2 7% 4 4 3 3 6% 6% 47%	2078
	11th to 20th	Day	8% 4 1 4 3 2 2 3 4 6% 62%	2000
	21st to end	Day	11% 5% 2 5% 3 3 3 2 5% 6% 56%	2200
November	1st to 10th	Day	10% 5% 1 5% 4 3 4 3 5% 6% 54%	2057
	11th to 20th	Day	11% 6% 2 5% 4 3 3 3 4 6% 54%	2095
	21st to end	Day	13% 6% 2 5% 4 3 3 3 5% 6% 51%	1890
December	1st to 10th	Day	7% 5% 2 5% 3 5% 4 4 5% 7% 53%	2948
	11th to 20th	Day	13% 6% 3 5% 4 3 4 2 5% 6% 49%	3373
	21st to end	Day	18% 9% 3 6% 3 3 3 3 5% 6% 40%	4466
January	1st to 10th	Night	9% 4 2 5% 3 3 4 3 4 6% 56%	5889
	11th to 20th	Night	12% 7% 3 6% 4 4 4 4 6% 7% 42%	5900
	21st to end	Night	12% 8% 3 7% 4 4 4 3 4 7% 44%	6490
February	1st to 10th	Night	14% 7% 2 7% 5% 3 4 4 5% 5% 45%	2564
	11th to 20th	Night	16% 8% 2 5% 4 3 3 4 4 6% 44%	2338
	21st to end	Night	11% 6% 3 7% 4 5% 5% 4 5% 6% 45%	1864
March	1st to 10th	Night	17% 9% 4 6% 4 3 4 3 4 5% 41%	4918
	11th to 20th	Night	9% 5% 2 5% 3 3 4 4 6% 9% 51%	4980
	21st to end	Night	12% 5% 2 5% 3 4 3 3 5% 7% 51%	5467
April	1st to 10th	Night	9% 5% 2 5% 4 4 4 4 5% 8% 50%	2200
	11th to 20th	Night	10% 4 2 5% 3 3 4 4 5% 7% 53%	2000
	21st to end	Night	6% 4 1 5% 4 4 3 4 6% 9% 54%	2000
May	1st to 10th	Night	6% 6% 2 7% 5% 4 5% 4 6% 11% 45%	4033
	11th to 20th	Night	13% 6% 2 5% 4 4 4 4 7% 11% 38%	4032
	21st to end	Night	9% 5% 2 6% 4 4 5% 4 7% 10% 45%	4421
June	1st to 10th	Night	8% 6% 3 6% 6% 5% 5% 5% 8% 13% 35%	1800
	11th to 20th	Night	5% 5% 2 7% 6% 5% 6% 5% 9% 13% 36%	1800
	21st to end	Night	4 4 1 6% 6% 6% 7% 7% 10% 14% 34%	1800
July	1st to 10th	Night	9% 6% 3 8% 6% 6% 8% 6% 9% 12% 28%	4058
	11th to 20th	Night	4 4 2 6% 5% 6% 7% 6% 9% 14% 35%	4057
	21st to end	Night	7% 6% 2 8% 6% 6% 7% 6% 9% 12% 31%	4427
August	1st to 10th	Night	17% 7% 3 7% 5% 6% 6% 5% 9% 13% 23%	2915
	11th to 20th	Night	6% 5% 2 6% 6% 6% 8% 6% 8% 12% 36%	2902
	21st to end	Night	8% 6% 3 6% 4 5% 5% 4 6% 9% 43%	3586
September	1st to 10th	Night	10% 6% 2 8% 4 5% 5% 5% 8% 10% 37%	2200
	11th to 20th	Night	9% 6% 3 9% 5% 5% 6% 5% 6% 9% 37%	2200
	21st to end	Night	13% 8% 3 8% 5% 5% 4 4 6% 8% 37%	2200
October	1st to 10th	Night	12% 7% 2 6% 5% 4 3 4 5% 6% 46%	2320
	11th to 20th	Night	9% 4 1 5% 3 3 4 3 4 6% 57%	2400
	21st to end	Night	9% 4 2 5% 4 3 4 3 5% 6% 55%	2640
November	1st to 10th	Night	10% 5% 2 5% 4 3 4 3 5% 6% 54%	2461
	11th to 20th	Night	13% 7% 2 6% 4 3 3 3 5% 6% 51%	2520
	21st to end	Night	11% 7% 2 6% 4 4 3 3 6% 7% 47%	2730
December	1st to 10th	Night	10% 5% 2 5% 3 4 3 3 5% 7% 54%	4277
	11th to 20th	Night	14% 8% 3 8% 5% 4 4 2 4 5% 44%	4822
	21st to end	Night	17% 7% 2 6% 4 4 4 3 5% 6% 43%	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	23% 27% 4 11% 26% 9%	4057
	11th to 20th	Day	28% 27% 4 11% 22% 8%	4060
	21st to end	Day	28% 30% 5% 14% 15% 8%	4466
February	1st to 10th	Day	15% 63% 4 9% 7% 2	1833
	11th to 20th	Day	17% 61% 4 7% 9% 2	1958
	21st to end	Day	17% 62% 3 5% 8% 5%	1765
March	1st to 10th	Day	38% 31% 5% 8% 12% 6%	4935
	11th to 20th	Day	16% 30% 8% 11% 16% 17%	4980
	21st to end	Day	27% 30% 6% 14% 13% 10%	5465
April	1st to 10th	Day	13% 63% 6% 8% 6% 5%	2200
	11th to 20th	Day	15% 62% 4 9% 6% 4	2349
	21st to end	Day	8% 63% 6% 13% 8% 2	2415
May	1st to 10th	Day	26% 34% 14% 19% 6% 1	5897
	11th to 20th	Day	30% 31% 14% 18% 6% 1	5865
	21st to end	Day	20% 33% 13% 16% 17%	6447
June	1st to 10th	Day	14% 65% 7% 8% 5%	2600
	11th to 20th	Day	10% 65% 10% 11% 4	2600
	21st to end	Day	12% 65% 12% 8% 3	2600
July	1st to 10th	Day	35% 35% 20% 10%	5835
	11th to 20th	Day	22% 37% 32% 8% 1	5831
	21st to end	Day	24% 35% 21% 18% 2	6240
August	1st to 10th	Day	38% 41% 17% 4	4221
	11th to 20th	Day	16% 42% 17% 18% 7%	4152
	21st to end	Day	20% 40% 11% 27% 2	4293
September	1st to 10th	Day	18% 61% 7% 7% 6% 2	2200
	11th to 20th	Day	19% 59% 5% 11% 5% 1	2200
	21st to end	Day	18% 60% 8% 9% 4 1	2200
October	1st to 10th	Day	29% 36% 6% 7% 18% 4	3377
	11th to 20th	Day	7% 35% 3 8% 22% 25%	3240
	21st to end	Day	11% 35% 6% 14% 27% 6%	3538
November	1st to 10th	Day	18% 54% 6% 7% 10% 5%	2057
	11th to 20th	Day	17% 55% 4 8% 11% 5%	2095
	21st to end	Day	14% 55% 6% 10% 10% 4	1890
December	1st to 10th	Day	20% 37% 5% 11% 17% 10%	2948
	11th to 20th	Day	24% 34% 5% 17% 18% 4	3370
	21st to end	Day	35% 28% 4 16% 16% 1	4466
January	1st to 10th	Night	21% 26% 4 9% 30% 9%	5889
	11th to 20th	Night	31% 27% 4 13% 20% 5%	5900
	21st to end	Night	29% 29% 5% 8% 19% 8%	6490
February	1st to 10th	Night	15% 61% 4 9% 8% 2	2564
	11th to 20th	Night	18% 61% 4 7% 8% 3	2339
	21st to end	Night	17% 63% 3 4 7% 6%	1864
March	1st to 10th	Night	37% 30% 5% 9% 12% 8%	4918
	11th to 20th	Night	21% 29% 6% 9% 21% 15%	4980
	21st to end	Night	24% 30% 7% 12% 17% 10%	5467
April	1st to 10th	Night	13% 62% 5% 9% 6% 4	2200
	11th to 20th	Night	12% 63% 5% 11% 5% 5%	2000
	21st to end	Night	12% 63% 5% 11% 8% 2	2000
May	1st to 10th	Night	24% 33% 14% 22% 6% 1	4060
	11th to 20th	Night	30% 32% 16% 16% 6%	4033
	21st to end	Night	24% 32% 11% 21% 13% 1	4429
June	1st to 10th	Night	14% 65% 9% 9% 3	1800
	11th to 20th	Night	12% 65% 9% 10% 3	1800
	21st to end	Night	12% 65% 11% 9% 3	1800
July	1st to 10th	Night	33% 35% 21% 10%	4015
	11th to 20th	Night	26% 35% 31% 6% 1	4006
	21st to end	Night	32% 34% 16% 16% 2	4297
August	1st to 10th	Night	38% 43% 15% 4	2904
	11th to 20th	Night	19% 42% 17% 16% 6%	2929
	21st to end	Night	18% 39% 13% 28% 2	3586
September	1st to 10th	Night	17% 61% 8% 8% 5% 1	2200
	11th to 20th	Night	19% 59% 7% 9% 5% 1	2200
	21st to end	Night	19% 58% 8% 10% 4	2200
October	1st to 10th	Night	25% 36% 8% 9% 18% 4	3788
	11th to 20th	Night	7% 35% 2 8% 19% 29%	3899
	21st to end	Night	12% 36% 4 13% 25% 10%	4252
November	1st to 10th	Night	18% 54% 5% 8% 10% 5%	2461
	11th to 20th	Night	17% 55% 4 8% 12% 5%	2520
	21st to end	Night	15% 55% 5% 9% 12% 4	2730
December	1st to 10th	Night	19% 36% 4 10% 23% 9%	4277
	11th to 20th	Night	31% 32% 7% 9% 15% 6%	4824
	21st to end	Night	33% 28% 5% 14% 19% 1	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	11% 9% 40% 35% 4 1	300
	11th to 20th	Day	10% 9% 34% 33% 12% 2	300
	21st to end	Day	20% 13% 39% 25% 4	330
February	1st to 10th	Day	15% 12% 30% 34% 9% 1	300
	11th to 20th	Day	26% 9% 33% 27% 4	300
	21st to end	Day	13% 10% 38% 31% 8%	364
March	1st to 10th	Day	14% 10% 32% 30% 13% 1	500
	11th to 20th	Day	11% 8% 38% 32% 9% 2	500
	21st to end	Day	13% 9% 36% 32% 9% 1	550
April	1st to 10th	Day	12% 11% 30% 31% 14% 2	500
	11th to 20th	Day	9% 8% 27% 32% 20% 5%	492
	21st to end	Day	5% 4 24% 37% 24% 7%	500
May	1st to 10th	Day	4 7% 29% 33% 20% 6%	503
	11th to 20th	Day	9% 4 19% 33% 27% 7%	527
	21st to end	Day	11% 2 14% 29% 34% 10%	573
June	1st to 10th	Day	9% 2 14% 33% 33% 10%	530
	11th to 20th	Day	7% 1 10% 28% 34% 19%	530
	21st to end	Day	5% 2 7% 28% 38% 18%	530
July	1st to 10th	Day	7% 2 5% 21% 41% 24%	525
	11th to 20th	Day	6% 1 4 18% 44% 27%	524
	21st to end	Day	4 1 7% 24% 45% 19%	576
August	1st to 10th	Day	12% 2 8% 22% 38% 18%	526
	11th to 20th	Day	6% 1 3 20% 47% 23%	519
	21st to end	Day	16% 2 7% 22% 33% 20%	583
September	1st to 10th	Day	14% 2 8% 24% 38% 15%	530
	11th to 20th	Day	15% 2 11% 28% 35% 10%	530
	21st to end	Day	15% 2 7% 27% 36% 13%	530
October	1st to 10th	Day	15% 4 17% 33% 22% 9%	530
	11th to 20th	Day	9% 3 17% 33% 27% 9%	530
	21st to end	Day	13% 5% 17% 32% 27% 6%	583
November	1st to 10th	Day	15% 5% 20% 32% 22% 6%	535
	11th to 20th	Day	15% 5% 31% 31% 15% 3	535
	21st to end	Day	11% 8% 28% 33% 18% 3	330
December	1st to 10th	Day	14% 9% 29% 33% 14% 2	306
	11th to 20th	Day	15% 10% 31% 30% 11% 3	300
	21st to end	Day	22% 9% 27% 28% 13% 2	330
January	1st to 10th	Night	13% 13% 34% 32% 7%	800
	11th to 20th	Night	15% 12% 30% 31% 12% 1	800
	21st to end	Night	17% 11% 38% 29% 5%	880
February	1st to 10th	Night	15% 10% 30% 35% 10%	800
	11th to 20th	Night	19% 11% 32% 32% 6%	783
	21st to end	Night	14% 8% 32% 38% 7%	660
March	1st to 10th	Night	13% 9% 31% 38% 9% 1	800
	11th to 20th	Night	13% 8% 30% 36% 13% 1	800
	21st to end	Night	12% 9% 29% 35% 14% 1	880
April	1st to 10th	Night	10% 8% 31% 36% 15% 1	800
	11th to 20th	Night	10% 7% 25% 36% 20% 2	791
	21st to end	Night	8% 5% 23% 40% 22% 3	800
May	1st to 10th	Night	6% 4 22% 38% 26% 4	600
	11th to 20th	Night	9% 3 19% 36% 27% 7%	591
	21st to end	Night	9% 2 11% 30% 37% 10%	650
June	1st to 10th	Night	11% 1 9% 37% 35% 7%	600
	11th to 20th	Night	7% 2 8% 29% 40% 14%	600
	21st to end	Night	5% 2 7% 32% 41% 13%	600
July	1st to 10th	Night	8% 2 7% 25% 45% 14%	585
	11th to 20th	Night	9% 1 5% 17% 51% 17%	582
	21st to end	Night	5% 2 5% 27% 49% 12%	657
August	1st to 10th	Night	10% 2 8% 23% 46% 12%	597
	11th to 20th	Night	7% 1 5% 24% 46% 16%	608
	21st to end	Night	13% 3 8% 23% 39% 15%	880
September	1st to 10th	Night	10% 3 7% 27% 41% 12%	800
	11th to 20th	Night	12% 3 9% 30% 39% 7%	800
	21st to end	Night	17% 2 12% 25% 37% 8%	800
October	1st to 10th	Night	15% 3 16% 30% 28% 7%	800
	11th to 20th	Night	11% 5% 21% 30% 28% 6%	800
	21st to end	Night	11% 5% 22% 29% 28% 5%	880
November	1st to 10th	Night	12% 5% 20% 35% 24% 4	820
	11th to 20th	Night	15% 9% 25% 31% 18% 2	840
	21st to end	Night	13% 9% 25% 32% 19% 2	840
December	1st to 10th	Night	16% 9% 27% 33% 14% 1	809
	11th to 20th	Night	18% 8% 27% 30% 16% 2	800
	21st to end	Night	20% 10% 29% 28% 13%	880

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