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 2024-11-10.

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 7% 9% 42%	101192
	Night	11% 6% 2 6% 4 4 5% 4 6% 8% 42%	97898
Transparency	Day	20% 49% 9% 11% 9% 3	104882
	Night	21% 48% 7% 10% 10% 4	102334
Seeing	Day	11% 5% 20% 29% 25% 9%	15506
	Night	12% 6% 21% 31% 25% 5%	25574

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 2 5% 4 5% 5% 4 8% 11% 47%	4082
	11th to 20th	Day	13% 7% 2 6% 4 4 4 4 7% 8% 41%	4081
	21st to end	Day	5% 4 1 4 3 4 5% 5% 7% 11% 50%	4476
June	1st to 10th	Day	7% 5% 3 7% 5% 7% 6% 7% 9% 10% 34%	2470
	11th to 20th	Day	6% 4 2 5% 5% 5% 6% 6% 8% 11% 40%	2470
	21st to end	Day	6% 5% 2 6% 6% 6% 8% 6% 9% 14% 33%	2470
July	1st to 10th	Day	12% 6% 3 7% 6% 6% 6% 7% 9% 12% 28%	4120
	11th to 20th	Day	5% 4 2 7% 6% 6% 7% 5% 10% 16% 31%	4120
	21st to end	Day	5% 5% 2 7% 7% 7% 8% 7% 10% 16% 26%	4466
August	1st to 10th	Day	19% 6% 3 7% 5% 5% 6% 6% 8% 11% 24%	4104
	11th to 20th	Day	6% 4 2 6% 5% 6% 7% 6% 9% 13% 37%	3985
	21st to end	Day	10% 7% 3 7% 5% 4 5% 5% 8% 11% 36%	4162
September	1st to 10th	Day	11% 6% 3 7% 5% 5% 7% 5% 7% 9% 35%	2090
	11th to 20th	Day	11% 8% 3 7% 6% 5% 6% 4 6% 8% 36%	2090
	21st to end	Day	12% 6% 3 6% 5% 4 5% 4 7% 7% 40%	2090
October	1st to 10th	Day	11% 7% 2 6% 4 4 3 3 6% 6% 48%	1974
	11th to 20th	Day	8% 4 1 4 3 2 2 3 4 7% 61%	1900
	21st to end	Day	10% 5% 2 5% 3 3 3 2 5% 6% 55%	2090
November	1st to 10th	Day	10% 5% 1 5% 4 3 4 4 5% 6% 53%	1947
	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% 4 2 5% 4 4 5% 4 6% 11% 49%	2813
	11th to 20th	Night	11% 6% 2 6% 5% 5% 4 4 6% 9% 43%	2812
	21st to end	Night	6% 4 2 5% 4 4 5% 4 7% 10% 52%	3079
June	1st to 10th	Night	8% 6% 3 6% 6% 5% 5% 5% 8% 13% 34%	1710
	11th to 20th	Night	5% 5% 2 6% 5% 5% 6% 6% 9% 13% 37%	1710
	21st to end	Night	5% 4 2 6% 6% 6% 7% 8% 10% 14% 33%	1710
July	1st to 10th	Night	10% 6% 3 7% 6% 6% 7% 6% 8% 13% 29%	2840
	11th to 20th	Night	6% 5% 3 7% 6% 7% 7% 7% 10% 14% 30%	2840
	21st to end	Night	8% 6% 3 8% 7% 7% 7% 7% 10% 13% 25%	3087
August	1st to 10th	Night	16% 7% 3 7% 4 6% 6% 5% 9% 14% 24%	2825
	11th to 20th	Night	6% 5% 2 6% 6% 6% 8% 5% 8% 12% 36%	2811
	21st to end	Night	7% 6% 3 7% 4 5% 5% 4 6% 9% 43%	3476
September	1st to 10th	Night	10% 6% 3 8% 4 5% 6% 5% 8% 10% 35%	2090
	11th to 20th	Night	10% 6% 3 9% 5% 5% 6% 5% 6% 9% 36%	2090
	21st to end	Night	13% 8% 3 8% 5% 5% 4 4 6% 8% 37%	2090
October	1st to 10th	Night	12% 7% 2 6% 4 4 3 4 5% 6% 47%	2204
	11th to 20th	Night	8% 4 1 5% 3 3 4 3 4 6% 57%	2280
	21st to end	Night	8% 4 2 5% 4 4 4 3 5% 6% 55%	2508
November	1st to 10th	Night	10% 5% 2 5% 4 3 4 3 5% 6% 53%	2336
	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	24% 39% 5% 11% 13% 8%	2837
	11th to 20th	Day	23% 38% 5% 10% 17% 7%	2840
	21st to end	Day	24% 42% 4 9% 15% 6%	3124
February	1st to 10th	Day	15% 66% 4 7% 6% 2	1741
	11th to 20th	Day	15% 64% 4 6% 9% 2	1858
	21st to end	Day	17% 65% 4 4 7% 5%	1679
March	1st to 10th	Day	36% 44% 5% 8% 5% 2	3435
	11th to 20th	Day	13% 43% 8% 9% 17% 10%	3480
	21st to end	Day	26% 42% 4 10% 13% 5%	3815
April	1st to 10th	Day	12% 66% 5% 7% 6% 5%	2090
	11th to 20th	Day	15% 66% 4 8% 4 3	2230
	21st to end	Day	7% 66% 5% 12% 8% 2	2294
May	1st to 10th	Day	17% 45% 11% 19% 7% 1	4120
	11th to 20th	Day	25% 42% 7% 17% 8% 1	4085
	21st to end	Day	11% 43% 14% 18% 15%	4489
June	1st to 10th	Day	13% 68% 7% 7% 5%	2470
	11th to 20th	Day	9% 67% 8% 11% 4	2470
	21st to end	Day	12% 68% 11% 7% 3	2470
July	1st to 10th	Day	29% 43% 16% 12% 1	4055
	11th to 20th	Day	21% 45% 27% 6% 1	4051
	21st to end	Day	27% 46% 19% 7% 1	4324
August	1st to 10th	Day	37% 42% 17% 4	4091
	11th to 20th	Day	16% 43% 17% 18% 7%	4024
	21st to end	Day	19% 41% 11% 26% 2	4162
September	1st to 10th	Day	18% 64% 7% 6% 4 2	2090
	11th to 20th	Day	20% 62% 5% 9% 5% 1	2090
	21st to end	Day	16% 62% 8% 9% 5% 1	2090
October	1st to 10th	Day	28% 37% 5% 7% 18% 5%	3273
	11th to 20th	Day	6% 36% 3 8% 21% 25%	3140
	21st to end	Day	10% 37% 6% 14% 27% 6%	3428
November	1st to 10th	Day	17% 58% 6% 7% 8% 4	1947
	11th to 20th	Day	17% 61% 4 6% 9% 4	1896
	21st to end	Day	13% 60% 5% 11% 8% 3	1710
December	1st to 10th	Day	19% 61% 5% 7% 6% 3	1710
	11th to 20th	Day	15% 48% 3 11% 19% 4	2150
	21st to end	Day	29% 39% 4 11% 16% 1	3124
January	1st to 10th	Night	27% 38% 5% 7% 14% 9%	4109
	11th to 20th	Night	25% 38% 5% 11% 18% 4	4120
	21st to end	Night	27% 41% 3 6% 15% 7%	4532
February	1st to 10th	Night	15% 64% 4 8% 7% 2	2436
	11th to 20th	Night	16% 64% 3 6% 8% 3	2219
	21st to end	Night	17% 66% 3 3 6% 5%	1774
March	1st to 10th	Night	40% 42% 6% 6% 4 2	3418
	11th to 20th	Night	20% 41% 5% 9% 16% 9%	3480
	21st to end	Night	24% 41% 6% 11% 12% 6%	3817
April	1st to 10th	Night	12% 65% 5% 7% 6% 5%	2090
	11th to 20th	Night	12% 66% 5% 9% 4 4	1899
	21st to end	Night	10% 66% 4 10% 8% 2	1900
May	1st to 10th	Night	15% 45% 11% 20% 7% 1	2840
	11th to 20th	Night	24% 43% 8% 16% 8% 1	2813
	21st to end	Night	13% 42% 9% 21% 13% 1	3087
June	1st to 10th	Night	13% 68% 8% 8% 2	1710
	11th to 20th	Night	10% 68% 8% 11% 3	1710
	21st to end	Night	12% 68% 10% 8% 3	1710
July	1st to 10th	Night	28% 44% 14% 13% 1	2795
	11th to 20th	Night	26% 45% 23% 5% 1	2786
	21st to end	Night	33% 45% 15% 6% 1	2988
August	1st to 10th	Night	36% 44% 16% 4	2814
	11th to 20th	Night	19% 43% 17% 16% 6%	2838
	21st to end	Night	17% 40% 13% 28% 2	3476
September	1st to 10th	Night	17% 64% 8% 7% 4 1	2090
	11th to 20th	Night	19% 62% 6% 8% 4 1	2090
	21st to end	Night	19% 61% 7% 9% 5%	2090
October	1st to 10th	Night	24% 37% 8% 8% 18% 4	3672
	11th to 20th	Night	6% 37% 2 7% 18% 29%	3779
	21st to end	Night	12% 37% 4 13% 25% 10%	4120
November	1st to 10th	Night	18% 57% 5% 8% 9% 3	2336
	11th to 20th	Night	16% 60% 4 7% 9% 4	2280
	21st to end	Night	14% 60% 4 9% 10% 3	2470
December	1st to 10th	Night	19% 61% 6% 5% 7% 1	2470
	11th to 20th	Night	18% 48% 3 8% 19% 4	3044
	21st to end	Night	31% 38% 5% 11% 14% 1	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 19% 32% 28% 8%	487
	21st to end	Day	11% 2 13% 28% 35% 11%	529
June	1st to 10th	Day	9% 1 14% 33% 33% 10%	490
	11th to 20th	Day	8% 1 10% 29% 32% 19%	490
	21st to end	Day	6% 2 8% 31% 37% 16%	490
July	1st to 10th	Day	8% 2 4 22% 41% 24%	485
	11th to 20th	Day	7% 1 4 19% 44% 25%	484
	21st to end	Day	5% 1 7% 26% 45% 16%	532
August	1st to 10th	Day	11% 2 7% 23% 38% 19%	486
	11th to 20th	Day	6% 1 4 21% 48% 21%	479
	21st to end	Day	15% 2 7% 22% 33% 21%	539
September	1st to 10th	Day	14% 2 8% 23% 38% 15%	490
	11th to 20th	Day	16% 2 11% 26% 35% 10%	490
	21st to end	Day	14% 2 7% 28% 36% 13%	490
October	1st to 10th	Day	14% 4 18% 32% 23% 9%	490
	11th to 20th	Day	9% 3 18% 34% 26% 9%	490
	21st to end	Day	11% 6% 17% 31% 28% 7%	539
November	1st to 10th	Day	15% 5% 20% 30% 23% 6%	491
	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	9% 3 19% 35% 27% 7%	561
	21st to end	Night	9% 2 10% 30% 38% 11%	617
June	1st to 10th	Night	11% 1 9% 37% 35% 7%	570
	11th to 20th	Night	7% 2 8% 30% 39% 14%	570
	21st to end	Night	5% 2 8% 33% 39% 13%	570
July	1st to 10th	Night	8% 2 7% 24% 45% 14%	555
	11th to 20th	Night	9% 1 5% 18% 51% 16%	552
	21st to end	Night	5% 2 5% 29% 50% 10%	624
August	1st to 10th	Night	8% 2 7% 24% 47% 12%	567
	11th to 20th	Night	7% 2 6% 26% 45% 15%	577
	21st to end	Night	12% 3 8% 22% 40% 16%	836
September	1st to 10th	Night	11% 3 6% 26% 42% 12%	760
	11th to 20th	Night	13% 3 10% 28% 39% 7%	760
	21st to end	Night	16% 2 12% 25% 37% 8%	760
October	1st to 10th	Night	15% 3 16% 30% 29% 7%	760
	11th to 20th	Night	11% 4 21% 30% 27% 6%	760
	21st to end	Night	10% 5% 21% 30% 29% 5%	836
November	1st to 10th	Night	13% 5% 20% 34% 25% 4	779
	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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