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Beckstrom Observatory
Clear Sky Chart
History

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

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% 10% 4 9% 5% 5% 5% 4 5% 7% 33%	126108
	Night	13% 11% 4 9% 5% 5% 4 4 5% 7% 33%	124012
Transparency	Day	48% 7% 12% 20% 12% 1	129663
	Night	48% 6% 12% 20% 12% 1	128527
Seeing	Day	17% 15% 30% 23% 11% 4	16738
	Night	19% 12% 31% 27% 10% 1	26228

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	11% 16% 8% 16% 7% 5% 4 3 4 5% 21%	3617
	11th to 20th	Day	12% 17% 8% 15% 10% 10% 6% 3 4 4 11%	3620
	21st to end	Day	19% 16% 6% 13% 6% 4 4 3 5% 5% 18%	4308
February	1st to 10th	Day	12% 17% 5% 13% 7% 6% 5% 3 5% 6% 22%	1800
	11th to 20th	Day	12% 15% 5% 12% 8% 6% 4 3 4 5% 25%	1852
	21st to end	Day	13% 16% 4 11% 6% 4 4 3 5% 5% 27%	1650
March	1st to 10th	Day	13% 11% 4 10% 6% 5% 4 4 5% 6% 31%	4608
	11th to 20th	Day	12% 14% 5% 9% 5% 4 5% 4 5% 9% 29%	4980
	21st to end	Day	17% 14% 6% 10% 5% 4 4 3 5% 5% 26%	5465
April	1st to 10th	Day	16% 14% 4 9% 5% 4 4 3 4 6% 30%	2320
	11th to 20th	Day	16% 10% 3 6% 5% 4 4 4 5% 7% 36%	2563
	21st to end	Day	16% 9% 3 8% 5% 5% 4 3 5% 6% 36%	2600
May	1st to 10th	Day	17% 9% 3 7% 4 4 5% 3 5% 7% 36%	5862
	11th to 20th	Day	16% 9% 3 7% 4 5% 5% 4 6% 8% 34%	5861
	21st to end	Day	14% 8% 2 7% 5% 4 4 3 5% 7% 42%	6662
June	1st to 10th	Day	11% 7% 2 7% 4 5% 4 4 6% 9% 41%	3000
	11th to 20th	Day	10% 6% 3 7% 4 5% 5% 4 6% 9% 42%	3000
	21st to end	Day	11% 6% 2 6% 5% 5% 5% 5% 7% 9% 40%	3000
July	1st to 10th	Day	9% 6% 3 7% 5% 5% 6% 5% 7% 10% 38%	6438
	11th to 20th	Day	8% 5% 2 7% 4 5% 5% 4 6% 8% 46%	6360
	21st to end	Day	7% 5% 2 6% 4 5% 5% 4 7% 9% 47%	6616
August	1st to 10th	Day	11% 5% 2 6% 6% 6% 5% 5% 7% 8% 39%	4234
	11th to 20th	Day	10% 6% 2 7% 5% 5% 5% 4 7% 9% 39%	4174
	21st to end	Day	9% 6% 3 6% 4 5% 5% 5% 7% 9% 41%	4675
September	1st to 10th	Day	10% 5% 3 8% 5% 5% 6% 4 6% 8% 40%	2275
	11th to 20th	Day	12% 5% 2 5% 4 3 4 4 6% 9% 46%	2200
	21st to end	Day	15% 8% 3 7% 5% 5% 5% 4 6% 9% 35%	2200
October	1st to 10th	Day	15% 8% 2 6% 5% 4 4 3 5% 6% 41%	2124
	11th to 20th	Day	19% 11% 2 8% 5% 5% 4 4 6% 7% 30%	2000
	21st to end	Day	21% 12% 3 9% 4 4 5% 3 5% 8% 26%	2030
November	1st to 10th	Day	13% 13% 3 8% 6% 4 4 4 6% 6% 33%	1780
	11th to 20th	Day	15% 14% 5% 9% 6% 5% 4 4 6% 6% 26%	1776
	21st to end	Day	17% 16% 6% 12% 5% 5% 4 4 5% 5% 21%	1600
December	1st to 10th	Day	15% 16% 7% 14% 6% 5% 5% 4 6% 6% 17%	2600
	11th to 20th	Day	18% 19% 6% 10% 5% 4 4 3 4 5% 21%	2662
	21st to end	Day	25% 22% 8% 11% 6% 4 4 2 3 4 11%	3596
January	1st to 10th	Night	13% 12% 5% 13% 6% 6% 5% 4 6% 7% 25%	6348
	11th to 20th	Night	11% 15% 6% 15% 9% 10% 5% 4 5% 5% 15%	6360
	21st to end	Night	13% 15% 9% 11% 6% 4 5% 3 6% 6% 21%	6652
February	1st to 10th	Night	13% 17% 4 11% 6% 5% 5% 4 5% 6% 23%	2600
	11th to 20th	Night	13% 14% 5% 10% 6% 5% 5% 4 6% 5% 27%	2431
	21st to end	Night	16% 12% 4 11% 6% 5% 5% 4 5% 6% 26%	1980
March	1st to 10th	Night	13% 12% 4 8% 4 4 4 3 5% 6% 39%	5245
	11th to 20th	Night	9% 8% 3 9% 5% 4 5% 4 6% 9% 37%	4980
	21st to end	Night	15% 12% 4 8% 5% 5% 4 3 4 6% 35%	5467
April	1st to 10th	Night	18% 12% 4 7% 5% 3 4 3 5% 6% 33%	2095
	11th to 20th	Night	17% 8% 2 6% 5% 4 4 4 5% 7% 37%	1785
	21st to end	Night	16% 10% 3 8% 5% 5% 4 3 5% 6% 35%	1800
May	1st to 10th	Night	16% 10% 3 7% 5% 4 4 3 5% 7% 35%	4033
	11th to 20th	Night	14% 9% 3 9% 5% 4 4 2 5% 6% 38%	4032
	21st to end	Night	16% 6% 2 5% 4 4 4 3 5% 7% 44%	4162
June	1st to 10th	Night	12% 6% 3 6% 6% 5% 5% 4 6% 6% 44%	1400
	11th to 20th	Night	10% 6% 2 6% 5% 4 5% 4 7% 9% 43%	1400
	21st to end	Night	8% 6% 2 6% 4 5% 5% 4 6% 8% 47%	1400
July	1st to 10th	Night	8% 7% 3 7% 5% 4 4 4 5% 9% 45%	3521
	11th to 20th	Night	8% 4 2 4 3 4 4 4 6% 10% 51%	3617
	21st to end	Night	7% 5% 2 6% 4 5% 5% 4 7% 7% 49%	4243
August	1st to 10th	Night	10% 6% 2 6% 4 4 5% 4 5% 7% 47%	2915
	11th to 20th	Night	11% 6% 2 5% 4 4 5% 4 5% 8% 47%	2875
	21st to end	Night	7% 5% 2 6% 5% 5% 5% 4 6% 8% 47%	3223
September	1st to 10th	Night	9% 5% 2 7% 4 5% 5% 4 6% 7% 47%	2101
	11th to 20th	Night	10% 5% 2 4 3 3 4 3 4 7% 55%	2200
	21st to end	Night	14% 8% 3 7% 4 4 5% 3 5% 7% 42%	2200
October	1st to 10th	Night	14% 7% 3 6% 3 3 3 3 5% 7% 45%	2275
	11th to 20th	Night	19% 11% 3 8% 4 3 5% 3 5% 6% 34%	2400
	21st to end	Night	20% 10% 4 9% 4 5% 4 3 5% 7% 30%	2806
November	1st to 10th	Night	12% 10% 4 7% 4 4 4 4 6% 6% 39%	2563
	11th to 20th	Night	15% 13% 4 9% 6% 6% 6% 4 5% 7% 25%	2620
	21st to end	Night	19% 15% 4 8% 6% 4 5% 3 5% 6% 24%	2800
December	1st to 10th	Night	13% 16% 5% 11% 7% 5% 6% 4 6% 6% 21%	4580
	11th to 20th	Night	13% 19% 6% 11% 6% 5% 4 4 5% 5% 23%	5539
	21st to end	Night	23% 21% 7% 11% 5% 4 4 3 4 4 15%	7364

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	62% 7% 9% 13% 6% 1	3618
	11th to 20th	Day	75% 6% 7% 7% 5%	3620
	21st to end	Day	68% 5% 11% 8% 6% 1	4308
February	1st to 10th	Day	63% 6% 10% 15% 5%	1800
	11th to 20th	Day	62% 5% 10% 14% 9% 1	1861
	21st to end	Day	59% 5% 11% 16% 7% 1	1650
March	1st to 10th	Day	52% 5% 14% 18% 9% 2	4608
	11th to 20th	Day	52% 5% 15% 18% 9% 1	4980
	21st to end	Day	59% 5% 11% 15% 8% 1	5465
April	1st to 10th	Day	55% 5% 11% 18% 10% 1	2320
	11th to 20th	Day	48% 6% 12% 19% 14% 2	2563
	21st to end	Day	48% 7% 13% 19% 12% 1	2600
May	1st to 10th	Day	47% 6% 12% 17% 14% 4	5897
	11th to 20th	Day	46% 8% 11% 20% 14% 2	5865
	21st to end	Day	42% 6% 8% 19% 19% 6%	6677
June	1st to 10th	Day	39% 7% 12% 25% 15% 2	3000
	11th to 20th	Day	39% 7% 12% 23% 17% 2	3000
	21st to end	Day	39% 8% 13% 24% 15% 1	3000
July	1st to 10th	Day	39% 8% 12% 24% 15% 2	6369
	11th to 20th	Day	33% 8% 11% 31% 16% 1	6285
	21st to end	Day	32% 8% 13% 24% 21% 1	6428
August	1st to 10th	Day	38% 9% 13% 22% 16% 1	4221
	11th to 20th	Day	38% 8% 14% 28% 11% 1	4213
	21st to end	Day	37% 8% 13% 28% 13% 1	4674
September	1st to 10th	Day	40% 9% 12% 26% 11% 2	2275
	11th to 20th	Day	34% 8% 13% 30% 15% 1	2200
	21st to end	Day	47% 7% 13% 22% 12%	2200
October	1st to 10th	Day	33% 5% 10% 34% 16% 3	3462
	11th to 20th	Day	42% 6% 13% 25% 14%	3240
	21st to end	Day	50% 5% 13% 22% 9%	3254
November	1st to 10th	Day	49% 7% 12% 22% 9%	1780
	11th to 20th	Day	56% 6% 14% 15% 9%	1776
	21st to end	Day	63% 6% 14% 13% 4 1	1600
December	1st to 10th	Day	66% 8% 16% 9% 2	2600
	11th to 20th	Day	66% 5% 11% 13% 6%	2658
	21st to end	Day	78% 5% 6% 7% 4	3596
January	1st to 10th	Night	55% 6% 12% 17% 7% 3	6348
	11th to 20th	Night	68% 7% 9% 9% 6% 1	6360
	21st to end	Night	62% 6% 11% 15% 7%	6652
February	1st to 10th	Night	60% 6% 10% 16% 7% 1	2600
	11th to 20th	Night	56% 6% 11% 17% 9% 1	2433
	21st to end	Night	59% 5% 10% 17% 8%	1980
March	1st to 10th	Night	46% 4 16% 22% 11% 1	5245
	11th to 20th	Night	41% 5% 19% 22% 11% 1	4980
	21st to end	Night	50% 6% 13% 18% 12% 1	5467
April	1st to 10th	Night	51% 5% 10% 20% 12% 2	2095
	11th to 20th	Night	45% 6% 10% 19% 18% 2	1785
	21st to end	Night	48% 6% 10% 18% 15% 3	1800
May	1st to 10th	Night	47% 6% 10% 17% 14% 5%	4060
	11th to 20th	Night	46% 6% 10% 24% 12% 2	4033
	21st to end	Night	38% 6% 10% 25% 16% 5%	4169
June	1st to 10th	Night	38% 7% 10% 25% 19% 2	1400
	11th to 20th	Night	35% 8% 14% 22% 19% 3	1400
	21st to end	Night	33% 7% 13% 27% 18% 2	1400
July	1st to 10th	Night	35% 6% 13% 22% 18% 5%	3483
	11th to 20th	Night	29% 7% 12% 33% 18% 1	3572
	21st to end	Night	31% 8% 14% 24% 21% 1	4118
August	1st to 10th	Night	34% 7% 13% 29% 16% 1	2904
	11th to 20th	Night	34% 7% 14% 30% 13% 2	2902
	21st to end	Night	32% 9% 13% 32% 13% 1	3223
September	1st to 10th	Night	34% 9% 13% 29% 15% 1	2101
	11th to 20th	Night	28% 6% 12% 35% 17% 1	2200
	21st to end	Night	42% 7% 11% 28% 12% 1	2200
October	1st to 10th	Night	27% 5% 12% 32% 23% 1	3717
	11th to 20th	Night	43% 6% 10% 24% 16% 1	3899
	21st to end	Night	45% 5% 12% 27% 11% 1	4532
November	1st to 10th	Night	44% 6% 13% 25% 11% 1	2563
	11th to 20th	Night	55% 7% 13% 16% 8% 1	2620
	21st to end	Night	60% 6% 15% 15% 4 1	2800
December	1st to 10th	Night	60% 9% 17% 11% 3	4580
	11th to 20th	Night	62% 5% 11% 12% 9% 1	5542
	21st to end	Night	72% 5% 9% 8% 5%	7364

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% 24% 36% 13% 4	300
	11th to 20th	Day	21% 22% 40% 14% 2	300
	21st to end	Day	26% 20% 34% 16% 3 1	330
February	1st to 10th	Day	22% 29% 34% 11% 3 1	300
	11th to 20th	Day	21% 29% 37% 12% 1	300
	21st to end	Day	26% 30% 32% 10% 2	249
March	1st to 10th	Day	28% 23% 32% 15% 2	350
	11th to 20th	Day	23% 17% 33% 21% 6% 1	500
	21st to end	Day	28% 17% 31% 19% 5% 1	550
April	1st to 10th	Day	23% 22% 31% 17% 6% 1	500
	11th to 20th	Day	26% 21% 27% 16% 7% 4	492
	21st to end	Day	21% 18% 32% 19% 8% 2	500
May	1st to 10th	Day	22% 18% 29% 23% 7% 2	503
	11th to 20th	Day	17% 13% 32% 22% 11% 4	527
	21st to end	Day	13% 12% 31% 25% 15% 4	613
June	1st to 10th	Day	12% 13% 35% 28% 9% 3	730
	11th to 20th	Day	13% 12% 34% 27% 11% 3	730
	21st to end	Day	13% 10% 33% 29% 12% 3	730
July	1st to 10th	Day	9% 7% 28% 32% 17% 7%	721
	11th to 20th	Day	10% 9% 31% 30% 14% 7%	718
	21st to end	Day	7% 6% 29% 34% 17% 6%	664
August	1st to 10th	Day	9% 6% 22% 29% 22% 11%	526
	11th to 20th	Day	12% 5% 21% 29% 21% 11%	519
	21st to end	Day	12% 9% 21% 30% 21% 8%	583
September	1st to 10th	Day	13% 9% 26% 25% 17% 10%	530
	11th to 20th	Day	10% 6% 19% 26% 25% 14%	530
	21st to end	Day	17% 14% 22% 24% 16% 7%	530
October	1st to 10th	Day	18% 15% 25% 21% 16% 5%	530
	11th to 20th	Day	20% 19% 32% 23% 5% 1	530
	21st to end	Day	26% 18% 36% 15% 4 1	413
November	1st to 10th	Day	18% 24% 33% 18% 7%	306
	11th to 20th	Day	24% 23% 34% 14% 5% 1	287
	21st to end	Day	22% 22% 38% 14% 4	200
December	1st to 10th	Day	25% 25% 31% 17% 3 1	200
	11th to 20th	Day	29% 25% 35% 9% 2 1	200
	21st to end	Day	30% 23% 35% 9% 2	247
January	1st to 10th	Night	20% 20% 37% 19% 4 1	1000
	11th to 20th	Night	23% 18% 35% 20% 4 1	1000
	21st to end	Night	19% 14% 38% 24% 6%	951
February	1st to 10th	Night	22% 17% 38% 20% 3	800
	11th to 20th	Night	23% 18% 33% 21% 4	783
	21st to end	Night	22% 21% 35% 20% 2	660
March	1st to 10th	Night	20% 16% 35% 23% 6%	800
	11th to 20th	Night	16% 17% 38% 24% 5% 1	800
	21st to end	Night	23% 14% 31% 25% 7%	880
April	1st to 10th	Night	23% 16% 34% 21% 4	695
	11th to 20th	Night	22% 15% 33% 21% 7% 2	593
	21st to end	Night	21% 11% 32% 29% 7%	600
May	1st to 10th	Night	17% 7% 35% 32% 9% 1	600
	11th to 20th	Night	19% 9% 29% 32% 10% 1	591
	21st to end	Night	13% 5% 30% 34% 16% 2	586
June	1st to 10th	Night	12% 7% 32% 39% 10%	400
	11th to 20th	Night	13% 5% 33% 38% 10%	400
	21st to end	Night	11% 6% 29% 40% 15%	400
July	1st to 10th	Night	9% 3 19% 43% 24% 2	540
	11th to 20th	Night	10% 3 21% 42% 21% 3	582
	21st to end	Night	7% 2 26% 43% 21% 1	657
August	1st to 10th	Night	9% 2 16% 45% 26% 2	597
	11th to 20th	Night	9% 4 20% 36% 27% 4	594
	21st to end	Night	11% 4 23% 35% 23% 3	660
September	1st to 10th	Night	11% 4 28% 31% 21% 5%	720
	11th to 20th	Night	12% 4 18% 35% 27% 4	800
	21st to end	Night	16% 8% 23% 32% 17% 4	800
October	1st to 10th	Night	17% 10% 29% 28% 16% 1	800
	11th to 20th	Night	22% 15% 32% 24% 7% 1	800
	21st to end	Night	25% 13% 34% 23% 5%	880
November	1st to 10th	Night	19% 14% 31% 25% 9% 1	789
	11th to 20th	Night	22% 19% 33% 19% 6% 1	800
	21st to end	Night	26% 16% 36% 18% 3	800
December	1st to 10th	Night	25% 18% 34% 18% 5% 1	800
	11th to 20th	Night	22% 19% 37% 19% 2 1	970
	21st to end	Night	30% 15% 35% 18% 2	1100

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