Beman Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Beman Observatory clear sky chart from 2005-11-06 to 2026-03-03.

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	13% 11% 4 9% 6% 5% 5% 4 5% 7% 32%	130648
	Night	14% 12% 4 10% 5% 5% 4 3 5% 6% 31%	130726
Transparency	Day	49% 6% 12% 20% 12% 1	134190
	Night	51% 6% 11% 19% 12% 1	135232
Seeing	Day	18% 12% 29% 23% 13% 5%	17488
	Night	19% 10% 30% 29% 11% 1	27466

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	13% 19% 7% 17% 7% 5% 4 4 4 5% 15%	4691
	11th to 20th	Day	11% 16% 9% 20% 13% 11% 5% 2 3 3 9%	4710
	21st to end	Day	16% 17% 6% 13% 10% 6% 7% 4 3 4 15%	5305
February	1st to 10th	Day	13% 15% 5% 13% 9% 7% 5% 4 4 4 20%	1928
	11th to 20th	Day	13% 13% 6% 15% 8% 5% 4 4 4 4 24%	2048
	21st to end	Day	14% 16% 6% 12% 6% 5% 5% 3 5% 5% 23%	1730
March	1st to 10th	Day	14% 13% 4 11% 6% 5% 4 3 4 5% 30%	4500
	11th to 20th	Day	11% 10% 4 9% 6% 5% 5% 3 6% 8% 35%	4926
	21st to end	Day	16% 16% 5% 9% 6% 5% 4 3 5% 5% 25%	5544
April	1st to 10th	Day	17% 14% 4 8% 5% 4 4 4 5% 5% 30%	2400
	11th to 20th	Day	16% 11% 3 6% 5% 4 4 4 4 7% 35%	2461
	21st to end	Day	16% 9% 3 7% 5% 5% 5% 3 6% 7% 35%	2600
May	1st to 10th	Day	20% 9% 3 7% 4 4 4 4 6% 6% 33%	5862
	11th to 20th	Day	14% 7% 3 7% 5% 5% 5% 4 6% 8% 35%	5861
	21st to end	Day	15% 7% 2 6% 4 4 4 3 6% 8% 40%	6725
June	1st to 10th	Day	9% 6% 2 6% 5% 5% 5% 5% 7% 9% 42%	2800
	11th to 20th	Day	10% 5% 2 6% 4 5% 5% 4 7% 9% 42%	2800
	21st to end	Day	9% 6% 2 7% 6% 5% 6% 6% 8% 9% 38%	2800
July	1st to 10th	Day	10% 5% 2 6% 5% 5% 6% 5% 7% 10% 39%	6360
	11th to 20th	Day	7% 6% 2 6% 4 4 5% 4 7% 9% 45%	6360
	21st to end	Day	7% 6% 2 6% 4 4 5% 4 6% 10% 44%	6799
August	1st to 10th	Day	11% 6% 2 5% 4 4 5% 5% 6% 9% 42%	4234
	11th to 20th	Day	10% 6% 2 6% 5% 5% 5% 4 5% 8% 44%	4174
	21st to end	Day	7% 5% 2 7% 4 5% 5% 5% 7% 10% 43%	4427
September	1st to 10th	Day	11% 6% 2 7% 5% 5% 5% 4 5% 8% 43%	2330
	11th to 20th	Day	9% 5% 2 5% 4 4 5% 4 5% 9% 48%	2200
	21st to end	Day	12% 8% 3 6% 5% 5% 5% 3 6% 8% 39%	2200
October	1st to 10th	Day	12% 7% 3 8% 5% 4 4 4 5% 7% 43%	2200
	11th to 20th	Day	17% 9% 3 8% 4 5% 5% 4 6% 6% 32%	1977
	21st to end	Day	19% 12% 3 9% 5% 4 5% 4 5% 6% 28%	1980
November	1st to 10th	Day	11% 12% 3 8% 5% 5% 4 3 5% 7% 39%	1852
	11th to 20th	Day	14% 17% 5% 11% 6% 4 5% 4 4 6% 25%	1890
	21st to end	Day	18% 16% 6% 9% 5% 4 4 3 4 6% 25%	1890
December	1st to 10th	Day	10% 18% 12% 15% 6% 6% 5% 3 4 5% 16%	2926
	11th to 20th	Day	17% 22% 10% 12% 8% 4 3 3 4 4 14%	3088
	21st to end	Day	23% 22% 7% 13% 6% 4 3 3 4 4 12%	4070
January	1st to 10th	Night	13% 16% 8% 19% 6% 5% 4 3 4 5% 17%	8232
	11th to 20th	Night	10% 16% 9% 20% 10% 10% 6% 3 4 4 9%	8280
	21st to end	Night	14% 15% 7% 14% 8% 7% 5% 3 4 5% 17%	8899
February	1st to 10th	Night	13% 18% 5% 12% 9% 6% 4 3 5% 4 22%	2688
	11th to 20th	Night	13% 14% 6% 11% 7% 6% 4 4 5% 5% 26%	2458
	21st to end	Night	16% 13% 5% 10% 5% 4 4 4 5% 7% 26%	2076
March	1st to 10th	Night	14% 11% 4 8% 5% 4 4 3 5% 7% 36%	5398
	11th to 20th	Night	10% 8% 3 6% 4 4 4 3 5% 8% 44%	5032
	21st to end	Night	16% 12% 4 10% 5% 4 3 3 4 5% 34%	5427
April	1st to 10th	Night	15% 11% 3 7% 5% 4 4 4 5% 7% 36%	2000
	11th to 20th	Night	18% 9% 3 7% 4 5% 4 3 5% 6% 37%	1887
	21st to end	Night	16% 9% 2 7% 5% 4 4 4 5% 7% 38%	1800
May	1st to 10th	Night	19% 11% 3 8% 5% 5% 4 3 5% 7% 30%	4033
	11th to 20th	Night	16% 8% 2 6% 4 4 3 3 5% 6% 41%	4032
	21st to end	Night	14% 6% 3 6% 4 4 4 3 5% 7% 43%	4125
June	1st to 10th	Night	11% 6% 2 5% 4 4 4 2 7% 10% 46%	1600
	11th to 20th	Night	11% 6% 2 6% 5% 4 4 3 7% 10% 43%	1600
	21st to end	Night	8% 5% 2 5% 5% 3 5% 5% 7% 9% 45%	1600
July	1st to 10th	Night	8% 5% 2 6% 4 4 5% 4 8% 11% 45%	3618
	11th to 20th	Night	8% 5% 2 5% 4 4 4 4 6% 8% 51%	3617
	21st to end	Night	10% 6% 2 6% 4 4 4 3 6% 8% 47%	4064
August	1st to 10th	Night	10% 5% 2 5% 4 4 4 4 6% 9% 48%	2915
	11th to 20th	Night	11% 6% 2 6% 5% 4 3 5% 6% 8% 44%	2875
	21st to end	Night	9% 6% 2 7% 4 5% 4 4 5% 8% 46%	3454
September	1st to 10th	Night	9% 6% 2 6% 4 4 4 3 5% 8% 49%	2050
	11th to 20th	Night	9% 5% 2 4 4 3 3 3 4 8% 56%	2200
	21st to end	Night	14% 7% 3 6% 4 4 4 3 5% 7% 43%	2200
October	1st to 10th	Night	14% 7% 2 6% 3 4 4 3 4 6% 47%	2200
	11th to 20th	Night	17% 10% 3 8% 5% 4 4 4 4 6% 35%	2419
	21st to end	Night	20% 9% 3 7% 5% 4 4 3 5% 6% 32%	2860
November	1st to 10th	Night	13% 11% 3 8% 5% 4 4 3 4 6% 39%	2666
	11th to 20th	Night	13% 15% 4 9% 5% 4 5% 5% 6% 7% 27%	2730
	21st to end	Night	18% 16% 5% 10% 4 5% 3 3 5% 6% 25%	2730
December	1st to 10th	Night	12% 16% 8% 16% 6% 5% 5% 3 4 5% 19%	4472
	11th to 20th	Night	16% 19% 7% 16% 8% 5% 4 3 4 4 14%	5339
	21st to end	Night	25% 21% 7% 11% 4 4 3 3 4 5% 13%	7150

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	70% 7% 11% 11% 2	4692
	11th to 20th	Day	81% 5% 5% 5% 4	4710
	21st to end	Day	72% 6% 10% 9% 3	5305
February	1st to 10th	Day	66% 6% 8% 14% 6%	1928
	11th to 20th	Day	63% 4 9% 14% 9% 1	2058
	21st to end	Day	61% 6% 11% 14% 7% 1	1730
March	1st to 10th	Day	55% 5% 12% 18% 9% 1	4500
	11th to 20th	Day	46% 6% 16% 22% 9% 2	4926
	21st to end	Day	60% 5% 9% 14% 10% 1	5546
April	1st to 10th	Day	56% 5% 9% 20% 10%	2400
	11th to 20th	Day	48% 6% 10% 20% 13% 2	2461
	21st to end	Day	48% 6% 12% 18% 14% 2	2600
May	1st to 10th	Day	49% 6% 10% 18% 15% 1	5897
	11th to 20th	Day	44% 8% 14% 22% 12% 1	5865
	21st to end	Day	41% 6% 10% 22% 19% 2	6739
June	1st to 10th	Day	36% 7% 12% 28% 15% 2	2800
	11th to 20th	Day	37% 6% 15% 23% 18% 2	2800
	21st to end	Day	38% 9% 13% 21% 16% 2	2800
July	1st to 10th	Day	37% 8% 14% 24% 15% 2	6291
	11th to 20th	Day	32% 7% 14% 27% 18% 1	6285
	21st to end	Day	32% 8% 15% 24% 20% 1	6604
August	1st to 10th	Day	35% 8% 14% 26% 16% 1	4221
	11th to 20th	Day	37% 7% 11% 30% 14% 1	4213
	21st to end	Day	34% 7% 14% 30% 13% 2	4426
September	1st to 10th	Day	38% 6% 14% 24% 15% 2	2330
	11th to 20th	Day	31% 7% 13% 30% 19% 1	2200
	21st to end	Day	42% 6% 12% 23% 17%	2200
October	1st to 10th	Day	30% 5% 9% 36% 19% 1	3590
	11th to 20th	Day	40% 5% 10% 27% 17%	3180
	21st to end	Day	48% 5% 12% 23% 11% 2	3180
November	1st to 10th	Day	45% 6% 12% 23% 14% 1	1852
	11th to 20th	Day	60% 5% 10% 17% 7% 1	1890
	21st to end	Day	62% 4 13% 15% 5%	1890
December	1st to 10th	Day	69% 8% 12% 8% 3	2926
	11th to 20th	Day	74% 4 9% 11% 2	3085
	21st to end	Day	76% 4 7% 9% 3	4070
January	1st to 10th	Night	68% 6% 8% 13% 5% 1	8233
	11th to 20th	Night	76% 6% 8% 6% 3	8280
	21st to end	Night	68% 5% 10% 11% 5%	8899
February	1st to 10th	Night	64% 5% 10% 14% 6% 1	2688
	11th to 20th	Night	59% 5% 9% 18% 8% 1	2459
	21st to end	Night	56% 5% 13% 16% 7% 2	2076
March	1st to 10th	Night	48% 5% 13% 24% 9% 1	5398
	11th to 20th	Night	37% 5% 14% 27% 15% 1	5032
	21st to end	Night	52% 5% 10% 19% 13% 1	5427
April	1st to 10th	Night	47% 6% 11% 22% 13% 1	2000
	11th to 20th	Night	47% 5% 10% 19% 17% 2	1887
	21st to end	Night	44% 6% 12% 21% 14% 3	1800
May	1st to 10th	Night	52% 7% 10% 15% 14% 2	4060
	11th to 20th	Night	43% 6% 11% 23% 15% 2	4033
	21st to end	Night	39% 6% 11% 22% 19% 2	4133
June	1st to 10th	Night	33% 6% 11% 29% 18% 3	1600
	11th to 20th	Night	36% 6% 15% 25% 17% 2	1600
	21st to end	Night	32% 8% 15% 26% 17% 2	1600
July	1st to 10th	Night	31% 8% 14% 26% 16% 5%	3580
	11th to 20th	Night	29% 7% 15% 27% 20% 1	3572
	21st to end	Night	34% 8% 13% 20% 23% 2	3947
August	1st to 10th	Night	32% 6% 15% 30% 15% 1	2904
	11th to 20th	Night	34% 7% 13% 30% 14% 1	2902
	21st to end	Night	35% 7% 11% 29% 17% 2	3454
September	1st to 10th	Night	33% 7% 11% 28% 19% 2	2050
	11th to 20th	Night	28% 5% 10% 33% 23% 2	2200
	21st to end	Night	40% 6% 11% 25% 16% 1	2200
October	1st to 10th	Night	26% 4 10% 34% 24% 1	3590
	11th to 20th	Night	43% 6% 10% 20% 20%	3929
	21st to end	Night	40% 5% 10% 30% 15% 1	4610
November	1st to 10th	Night	46% 5% 10% 23% 14% 1	2666
	11th to 20th	Night	53% 8% 13% 17% 8% 2	2730
	21st to end	Night	59% 6% 12% 18% 5%	2730
December	1st to 10th	Night	65% 7% 12% 12% 4	4472
	11th to 20th	Night	72% 6% 9% 10% 3	5341
	21st to end	Night	74% 5% 8% 8% 4	7150

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	22% 15% 40% 19% 4 1	330
	11th to 20th	Day	21% 24% 41% 9% 4 1	330
	21st to end	Day	28% 18% 34% 16% 4	363
February	1st to 10th	Day	23% 25% 32% 15% 3 1	330
	11th to 20th	Day	26% 20% 37% 16% 2	330
	21st to end	Day	25% 28% 36% 10% 2	273
March	1st to 10th	Day	23% 21% 41% 11% 4	306
	11th to 20th	Day	22% 14% 35% 21% 7%	472
	21st to end	Day	26% 16% 32% 19% 5% 1	550
April	1st to 10th	Day	25% 15% 32% 21% 7%	500
	11th to 20th	Day	26% 18% 29% 16% 7% 3	492
	21st to end	Day	22% 11% 34% 22% 10% 1	500
May	1st to 10th	Day	24% 14% 27% 23% 9% 3	503
	11th to 20th	Day	20% 11% 26% 23% 13% 7%	527
	21st to end	Day	15% 8% 26% 27% 18% 6%	696
June	1st to 10th	Day	12% 8% 30% 31% 13% 5%	730
	11th to 20th	Day	12% 6% 29% 31% 17% 5%	730
	21st to end	Day	12% 8% 30% 32% 13% 5%	730
July	1st to 10th	Day	9% 6% 22% 31% 22% 12%	721
	11th to 20th	Day	10% 7% 26% 27% 20% 9%	718
	21st to end	Day	8% 4 25% 33% 23% 8%	744
August	1st to 10th	Day	9% 4 18% 27% 25% 17%	526
	11th to 20th	Day	12% 4 17% 28% 28% 10%	519
	21st to end	Day	9% 5% 22% 29% 24% 12%	583
September	1st to 10th	Day	14% 8% 20% 25% 22% 12%	530
	11th to 20th	Day	8% 4 16% 27% 31% 15%	530
	21st to end	Day	17% 12% 20% 25% 18% 9%	530
October	1st to 10th	Day	15% 8% 28% 28% 15% 7%	530
	11th to 20th	Day	18% 17% 28% 29% 7% 1	489
	21st to end	Day	26% 15% 39% 15% 5% 1	363
November	1st to 10th	Day	17% 19% 36% 21% 5% 2	330
	11th to 20th	Day	25% 21% 33% 15% 5%	330
	21st to end	Day	28% 18% 40% 12% 2	330
December	1st to 10th	Day	25% 22% 34% 12% 7%	330
	11th to 20th	Day	25% 23% 36% 14% 2	330
	21st to end	Day	27% 23% 34% 13% 2	363
January	1st to 10th	Night	22% 15% 37% 22% 5% 1	1050
	11th to 20th	Night	23% 16% 36% 21% 4 1	1050
	21st to end	Night	20% 14% 34% 27% 4	1100
February	1st to 10th	Night	23% 12% 38% 23% 3	840
	11th to 20th	Night	20% 16% 36% 26% 3	823
	21st to end	Night	24% 13% 41% 19% 2	692
March	1st to 10th	Night	21% 11% 36% 24% 6% 1	809
	11th to 20th	Night	16% 14% 38% 26% 6% 1	800
	21st to end	Night	23% 11% 33% 27% 7%	865
April	1st to 10th	Night	18% 12% 37% 28% 5% 1	600
	11th to 20th	Night	22% 10% 36% 24% 7% 1	593
	21st to end	Night	20% 7% 35% 31% 7% 1	600
May	1st to 10th	Night	22% 7% 31% 32% 9% 1	600
	11th to 20th	Night	21% 7% 29% 30% 13% 1	591
	21st to end	Night	13% 3 26% 38% 18% 2	650
June	1st to 10th	Night	11% 5% 25% 42% 17%	600
	11th to 20th	Night	14% 4 25% 40% 18% 1	600
	21st to end	Night	10% 2 24% 42% 21% 2	600
July	1st to 10th	Night	8% 3 17% 40% 27% 5%	585
	11th to 20th	Night	9% 3 18% 44% 23% 3	582
	21st to end	Night	8% 2 16% 46% 28% 1	657
August	1st to 10th	Night	9% 2 12% 45% 29% 3	597
	11th to 20th	Night	10% 2 15% 40% 28% 5%	594
	21st to end	Night	11% 2 17% 38% 28% 4	660
September	1st to 10th	Night	11% 4 19% 37% 24% 6%	650
	11th to 20th	Night	10% 4 16% 36% 31% 4	800
	21st to end	Night	17% 6% 22% 29% 21% 5%	800
October	1st to 10th	Night	16% 6% 24% 33% 19% 2	800
	11th to 20th	Night	22% 11% 28% 28% 9% 1	800
	21st to end	Night	24% 13% 31% 26% 6%	880
November	1st to 10th	Night	20% 11% 30% 29% 9% 1	820
	11th to 20th	Night	21% 14% 36% 23% 5% 1	840
	21st to end	Night	25% 13% 33% 25% 4	840
December	1st to 10th	Night	22% 15% 36% 21% 6%	893
	11th to 20th	Night	24% 15% 37% 21% 3	1050
	21st to end	Night	31% 13% 34% 21% 2	1155

Page generated at 2026-02-28 01:40:20. Version ac07

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