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 2024-05-15.

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	10% 6% 2 6% 4 4 5% 4 6% 8% 43%	84025
	Night	11% 6% 2 6% 4 4 4 4 6% 8% 43%	85526
Transparency	Day	36% 9% 14% 19% 16% 6%	83748
	Night	38% 8% 13% 18% 17% 7%	85322
Seeing	Day	11% 5% 20% 29% 25% 9%	14790
	Night	12% 6% 21% 31% 24% 5%	24952

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% 3 5% 4 5% 6% 4 7% 8% 44%	2836
	11th to 20th	Day	12% 6% 2 6% 4 3 4 3 5% 9% 46%	2840
	21st to end	Day	13% 9% 3 5% 4 3 4 3 4 6% 45%	3124
February	1st to 10th	Day	14% 7% 3 5% 5% 5% 4 4 6% 7% 40%	1741
	11th to 20th	Day	17% 6% 2 4 3 3 3 4 5% 6% 46%	1848
	21st to end	Day	13% 6% 3 6% 5% 3 3 3 5% 7% 47%	1679
March	1st to 10th	Day	19% 10% 3 7% 4 4 4 3 5% 5% 37%	3435
	11th to 20th	Day	7% 3 1 4 4 4 4 4 6% 8% 55%	3480
	21st to end	Day	15% 6% 2 7% 4 4 4 3 4 6% 45%	3815
April	1st to 10th	Day	8% 4 1 4 4 4 4 4 6% 8% 53%	2090
	11th to 20th	Day	10% 5% 2 5% 4 3 4 3 5% 8% 51%	2230
	21st to end	Day	4 4 1 4 4 3 4 4 6% 8% 57%	2294
May	1st to 10th	Day	6% 4 1 4 4 4 5% 5% 7% 9% 51%	2470
	11th to 20th	Day	9% 6% 2 6% 4 4 4 4 7% 8% 45%	2353
	21st to end	Day	8% 5% 2 6% 4 5% 6% 6% 6% 11% 40%	2531
June	1st to 10th	Day	7% 6% 3 7% 5% 7% 6% 7% 9% 10% 34%	2340
	11th to 20th	Day	6% 5% 2 6% 6% 5% 7% 6% 8% 12% 39%	2340
	21st to end	Day	6% 4 2 6% 6% 6% 8% 6% 9% 14% 33%	2340
July	1st to 10th	Day	6% 5% 3 6% 6% 6% 5% 6% 9% 13% 34%	2340
	11th to 20th	Day	8% 5% 2 7% 6% 6% 7% 5% 10% 13% 30%	2340
	21st to end	Day	6% 6% 2 6% 6% 7% 7% 7% 9% 12% 32%	2547
August	1st to 10th	Day	8% 5% 2 7% 6% 6% 7% 7% 10% 12% 30%	2327
	11th to 20th	Day	7% 5% 3 8% 6% 7% 7% 6% 9% 12% 30%	2272
	21st to end	Day	13% 8% 3 9% 6% 5% 5% 6% 9% 10% 27%	2372
September	1st to 10th	Day	10% 6% 3 7% 6% 5% 7% 5% 7% 9% 35%	1980
	11th to 20th	Day	10% 8% 3 7% 6% 5% 6% 4 6% 8% 37%	1980
	21st to end	Day	13% 6% 3 6% 5% 4 5% 4 7% 8% 39%	1980
October	1st to 10th	Day	11% 6% 2 6% 4 4 3 3 6% 6% 48%	1871
	11th to 20th	Day	8% 5% 1 5% 3 2 3 3 4 7% 60%	1800
	21st to end	Day	10% 6% 1 6% 3 3 3 2 6% 6% 54%	1980
November	1st to 10th	Day	10% 5% 1 5% 4 2 4 3 5% 6% 54%	1857
	11th to 20th	Day	12% 6% 2 5% 4 3 3 3 4 6% 51%	1896
	21st to end	Day	13% 6% 2 5% 4 3 3 3 4 6% 50%	1710
December	1st to 10th	Day	11% 6% 3 6% 4 5% 4 5% 5% 7% 45%	1710
	11th to 20th	Day	15% 5% 2 4 4 2 3 2 5% 6% 52%	2153
	21st to end	Day	16% 9% 3 6% 3 3 4 3 4 6% 43%	3124
January	1st to 10th	Night	13% 6% 3 6% 4 5% 5% 4 6% 7% 43%	4109
	11th to 20th	Night	10% 7% 2 6% 5% 4 5% 4 6% 8% 43%	4120
	21st to end	Night	12% 10% 4 8% 4 4 3 3 4 5% 43%	4532
February	1st to 10th	Night	14% 7% 2 7% 5% 3 4 4 5% 6% 44%	2436
	11th to 20th	Night	15% 8% 2 5% 4 3 3 4 4 6% 45%	2218
	21st to end	Night	11% 6% 3 7% 4 5% 5% 4 5% 6% 44%	1774
March	1st to 10th	Night	19% 12% 4 7% 4 3 3 3 4 4 35%	3418
	11th to 20th	Night	10% 5% 3 5% 4 3 4 4 6% 8% 49%	3480
	21st to end	Night	15% 5% 2 5% 4 4 3 3 5% 8% 46%	3817
April	1st to 10th	Night	9% 6% 2 5% 4 4 4 4 5% 8% 50%	2090
	11th to 20th	Night	10% 4 2 5% 4 3 4 4 5% 7% 52%	1899
	21st to end	Night	6% 4 1 5% 4 4 3 4 6% 9% 55%	1900
May	1st to 10th	Night	5% 5% 2 5% 4 3 4 3 6% 9% 55%	1710
	11th to 20th	Night	7% 5% 2 5% 4 5% 4 4 7% 9% 48%	1628
	21st to end	Night	8% 5% 2 6% 5% 5% 6% 4 8% 10% 42%	1744
June	1st to 10th	Night	8% 6% 3 6% 6% 5% 5% 5% 8% 13% 34%	1620
	11th to 20th	Night	5% 5% 2 7% 5% 5% 6% 5% 9% 13% 37%	1620
	21st to end	Night	5% 4 2 6% 6% 6% 7% 8% 9% 14% 32%	1620
July	1st to 10th	Night	7% 5% 3 6% 5% 5% 7% 5% 8% 14% 34%	1620
	11th to 20th	Night	7% 6% 3 6% 6% 6% 6% 6% 9% 11% 34%	1620
	21st to end	Night	5% 4 2 7% 6% 6% 6% 6% 9% 12% 36%	1771
August	1st to 10th	Night	5% 5% 3 7% 6% 7% 7% 6% 10% 14% 32%	1611
	11th to 20th	Night	7% 5% 3 7% 7% 7% 9% 6% 8% 12% 28%	1610
	21st to end	Night	10% 8% 3 7% 5% 6% 7% 6% 8% 11% 29%	1980
September	1st to 10th	Night	9% 6% 3 8% 4 5% 6% 5% 8% 10% 36%	1980
	11th to 20th	Night	9% 5% 3 9% 5% 5% 6% 5% 6% 9% 37%	1980
	21st to end	Night	13% 8% 3 8% 5% 5% 4 4 6% 8% 36%	1980
October	1st to 10th	Night	11% 7% 2 6% 4 4 3 4 5% 6% 48%	2088
	11th to 20th	Night	9% 4 1 5% 3 3 4 4 4 7% 55%	2160
	21st to end	Night	8% 4 2 5% 4 4 4 3 5% 6% 54%	2376
November	1st to 10th	Night	9% 5% 2 4 4 3 4 3 5% 6% 55%	2221
	11th to 20th	Night	13% 7% 2 5% 4 3 3 3 5% 5% 50%	2280
	21st to end	Night	12% 7% 2 6% 4 4 4 3 6% 6% 47%	2470
December	1st to 10th	Night	13% 6% 2 6% 4 5% 4 4 6% 8% 41%	2470
	11th to 20th	Night	16% 7% 2 5% 3 3 3 2 4 6% 48%	3042
	21st to end	Night	17% 8% 2 6% 5% 4 4 3 5% 7% 39%	4532

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	35% 6% 10% 19% 21% 10%	2837
	11th to 20th	Day	35% 6% 8% 17% 25% 9%	2840
	21st to end	Day	40% 6% 7% 15% 22% 9%	3124
February	1st to 10th	Day	42% 6% 10% 18% 18% 7%	1741
	11th to 20th	Day	39% 6% 9% 15% 23% 8%	1858
	21st to end	Day	38% 6% 10% 17% 18% 11%	1679
March	1st to 10th	Day	48% 7% 9% 15% 14% 7%	3435
	11th to 20th	Day	25% 8% 14% 17% 23% 13%	3480
	21st to end	Day	40% 6% 9% 19% 19% 7%	3815
April	1st to 10th	Day	28% 7% 14% 22% 19% 10%	2090
	11th to 20th	Day	32% 6% 11% 22% 19% 10%	2230
	21st to end	Day	22% 7% 14% 31% 20% 6%	2294
May	1st to 10th	Day	26% 9% 14% 26% 19% 5%	2470
	11th to 20th	Day	34% 9% 18% 23% 13% 2	2357
	21st to end	Day	34% 12% 19% 20% 15% 1	2531
June	1st to 10th	Day	38% 16% 19% 17% 10% 1	2340
	11th to 20th	Day	33% 13% 21% 26% 7%	2340
	21st to end	Day	35% 17% 23% 17% 7% 1	2340
July	1st to 10th	Day	33% 14% 27% 18% 6% 1	2275
	11th to 20th	Day	39% 16% 25% 15% 5%	2271
	21st to end	Day	35% 18% 24% 20% 3 1	2405
August	1st to 10th	Day	38% 16% 24% 17% 4	2314
	11th to 20th	Day	40% 17% 24% 15% 4	2272
	21st to end	Day	46% 15% 19% 15% 5%	2372
September	1st to 10th	Day	40% 14% 17% 15% 11% 3	1980
	11th to 20th	Day	42% 11% 15% 16% 11% 3	1980
	21st to end	Day	40% 10% 14% 18% 14% 3	1980
October	1st to 10th	Day	35% 6% 12% 18% 21% 8%	1871
	11th to 20th	Day	25% 5% 12% 23% 28% 8%	1800
	21st to end	Day	30% 5% 9% 16% 26% 12%	1980
November	1st to 10th	Day	30% 6% 11% 17% 23% 13%	1857
	11th to 20th	Day	35% 6% 8% 18% 22% 11%	1896
	21st to end	Day	34% 5% 10% 23% 20% 8%	1710
December	1st to 10th	Day	37% 8% 13% 19% 15% 8%	1710
	11th to 20th	Day	34% 4 8% 21% 27% 6%	2150
	21st to end	Day	42% 6% 7% 18% 22% 4	3124
January	1st to 10th	Night	38% 6% 8% 14% 22% 11%	4109
	11th to 20th	Night	38% 6% 8% 17% 26% 6%	4120
	21st to end	Night	43% 5% 7% 12% 23% 11%	4532
February	1st to 10th	Night	41% 4 9% 19% 19% 8%	2436
	11th to 20th	Night	40% 5% 9% 14% 21% 10%	2219
	21st to end	Night	40% 6% 9% 18% 15% 12%	1774
March	1st to 10th	Night	52% 6% 10% 12% 13% 7%	3418
	11th to 20th	Night	32% 6% 12% 16% 22% 13%	3480
	21st to end	Night	38% 5% 12% 18% 19% 8%	3817
April	1st to 10th	Night	32% 6% 12% 21% 20% 9%	2090
	11th to 20th	Night	30% 7% 10% 22% 20% 11%	1899
	21st to end	Night	26% 7% 14% 26% 23% 6%	1900
May	1st to 10th	Night	26% 7% 15% 24% 22% 6%	1710
	11th to 20th	Night	30% 11% 19% 22% 15% 2	1629
	21st to end	Night	34% 11% 15% 26% 13% 2	1745
June	1st to 10th	Night	38% 12% 21% 19% 9%	1620
	11th to 20th	Night	35% 13% 21% 24% 7%	1620
	21st to end	Night	35% 17% 24% 19% 5%	1620
July	1st to 10th	Night	35% 16% 25% 18% 5% 2	1575
	11th to 20th	Night	38% 15% 27% 15% 5%	1566
	21st to end	Night	33% 14% 28% 21% 3	1672
August	1st to 10th	Night	36% 18% 24% 18% 3	1600
	11th to 20th	Night	42% 18% 20% 18% 3	1610
	21st to end	Night	45% 16% 19% 16% 4	1980
September	1st to 10th	Night	39% 14% 19% 16% 10% 2	1980
	11th to 20th	Night	41% 10% 16% 18% 12% 2	1980
	21st to end	Night	44% 8% 14% 16% 15% 2	1980
October	1st to 10th	Night	36% 7% 12% 18% 20% 7%	2088
	11th to 20th	Night	28% 7% 9% 22% 25% 9%	2160
	21st to end	Night	30% 6% 9% 17% 25% 14%	2376
November	1st to 10th	Night	30% 5% 10% 18% 24% 13%	2221
	11th to 20th	Night	36% 5% 7% 18% 22% 12%	2280
	21st to end	Night	38% 4 10% 18% 21% 9%	2470
December	1st to 10th	Night	40% 8% 14% 16% 17% 6%	2470
	11th to 20th	Night	38% 4 9% 16% 27% 6%	3044
	21st to end	Night	44% 5% 8% 18% 20% 4	4532

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% 10% 40% 34% 3 1	270
	11th to 20th	Day	9% 10% 35% 33% 12% 2	270
	21st to end	Day	21% 13% 39% 24% 3	297
February	1st to 10th	Day	16% 13% 31% 31% 8%	270
	11th to 20th	Day	24% 9% 34% 28% 5%	270
	21st to end	Day	14% 11% 39% 29% 6%	334
March	1st to 10th	Day	14% 10% 32% 30% 14% 1	460
	11th to 20th	Day	11% 8% 39% 32% 8% 2	460
	21st to end	Day	13% 8% 36% 31% 9% 2	506
April	1st to 10th	Day	12% 11% 31% 30% 14% 2	460
	11th to 20th	Day	10% 8% 28% 30% 18% 5%	452
	21st to end	Day	5% 5% 26% 37% 21% 6%	460
May	1st to 10th	Day	3 8% 30% 33% 21% 6%	463
	11th to 20th	Day	9% 4 17% 32% 29% 8%	463
	21st to end	Day	12% 2 13% 26% 36% 12%	485
June	1st to 10th	Day	10% 1 14% 32% 34% 10%	450
	11th to 20th	Day	8% 2 10% 28% 32% 20%	450
	21st to end	Day	5% 2 8% 31% 37% 16%	450
July	1st to 10th	Day	7% 2 4 23% 41% 22%	445
	11th to 20th	Day	8% 1 4 20% 44% 23%	444
	21st to end	Day	5% 1 8% 27% 45% 14%	488
August	1st to 10th	Day	9% 2 7% 24% 39% 19%	446
	11th to 20th	Day	6% 1 3 20% 49% 21%	439
	21st to end	Day	15% 2 7% 22% 33% 20%	495
September	1st to 10th	Day	11% 1 8% 24% 40% 16%	450
	11th to 20th	Day	14% 2 11% 27% 36% 10%	450
	21st to end	Day	14% 1 8% 28% 36% 14%	450
October	1st to 10th	Day	14% 4 18% 33% 22% 10%	450
	11th to 20th	Day	9% 4 17% 32% 28% 10%	450
	21st to end	Day	12% 6% 18% 32% 26% 6%	495
November	1st to 10th	Day	15% 5% 21% 32% 22% 6%	455
	11th to 20th	Day	16% 6% 32% 31% 13% 2	456
	21st to end	Day	12% 8% 26% 34% 17% 3	270
December	1st to 10th	Day	14% 10% 27% 32% 15% 2	270
	11th to 20th	Day	15% 10% 31% 30% 10% 3	270
	21st to end	Day	20% 10% 28% 28% 13% 2	297
January	1st to 10th	Night	14% 13% 34% 32% 7%	760
	11th to 20th	Night	14% 12% 30% 31% 12% 1	760
	21st to end	Night	17% 11% 39% 28% 5%	836
February	1st to 10th	Night	16% 11% 31% 33% 9%	760
	11th to 20th	Night	19% 12% 31% 33% 5%	743
	21st to end	Night	14% 8% 32% 37% 8%	628
March	1st to 10th	Night	13% 9% 30% 39% 9% 1	760
	11th to 20th	Night	13% 8% 30% 35% 13% 1	760
	21st to end	Night	12% 9% 29% 34% 15% 1	836
April	1st to 10th	Night	10% 8% 32% 35% 14% 1	760
	11th to 20th	Night	11% 7% 25% 35% 19% 2	751
	21st to end	Night	8% 5% 24% 40% 21% 3	760
May	1st to 10th	Night	6% 4 22% 37% 26% 4	570
	11th to 20th	Night	10% 3 18% 34% 28% 7%	543
	21st to end	Night	10% 2 10% 28% 39% 12%	584
June	1st to 10th	Night	11% 1 9% 36% 35% 7%	540
	11th to 20th	Night	7% 2 8% 29% 39% 15%	540
	21st to end	Night	5% 2 8% 33% 39% 13%	540
July	1st to 10th	Night	8% 2 7% 25% 46% 12%	525
	11th to 20th	Night	10% 1 5% 18% 51% 16%	522
	21st to end	Night	5% 2 6% 29% 49% 9%	591
August	1st to 10th	Night	7% 2 7% 25% 47% 12%	537
	11th to 20th	Night	7% 2 5% 25% 45% 15%	546
	21st to end	Night	12% 2 7% 22% 41% 15%	792
September	1st to 10th	Night	9% 3 6% 25% 43% 12%	720
	11th to 20th	Night	12% 3 9% 29% 40% 7%	720
	21st to end	Night	17% 2 12% 25% 37% 8%	720
October	1st to 10th	Night	15% 3 17% 29% 29% 8%	720
	11th to 20th	Night	11% 4 21% 29% 28% 6%	720
	21st to end	Night	11% 5% 22% 30% 28% 5%	792
November	1st to 10th	Night	13% 5% 20% 35% 24% 3	740
	11th to 20th	Night	16% 9% 26% 31% 17% 2	760
	21st to end	Night	14% 8% 24% 32% 19% 2	760
December	1st to 10th	Night	16% 8% 26% 33% 15% 1	760
	11th to 20th	Night	18% 8% 26% 31% 16% 1	760
	21st to end	Night	20% 10% 28% 28% 13%	836

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