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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 2024-01-16.

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	14% 10% 4 7% 6% 3 5% 5% 5% 7% 33%	74632
	Night	15% 10% 4 7% 6% 3 5% 5% 5% 6% 33%	75149
Transparency	Day	50% 7% 12% 19% 11% 1	74339
	Night	50% 7% 12% 20% 11% 1	74963
Seeing	Day	18% 15% 30% 22% 11% 4	14237
	Night	19% 12% 31% 27% 10% 1	23924

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	16% 16% 7% 12% 8% 4 4 4 5% 5% 20%	1517
	11th to 20th	Day	15% 15% 9% 11% 9% 5% 5% 4 4 5% 16%	1480
	21st to end	Day	15% 16% 8% 11% 9% 4 5% 4 5% 5% 18%	1715
February	1st to 10th	Day	12% 16% 6% 11% 9% 4 5% 4 4 6% 21%	1620
	11th to 20th	Day	13% 13% 6% 10% 9% 5% 5% 5% 4 5% 25%	1662
	21st to end	Day	15% 15% 5% 9% 8% 3 3 4 5% 5% 27%	1480
March	1st to 10th	Day	14% 11% 5% 10% 7% 4 5% 4 5% 6% 28%	1844
	11th to 20th	Day	12% 13% 6% 9% 6% 3 5% 4 4 7% 31%	1980
	21st to end	Day	18% 13% 4 9% 6% 3 4 4 4 6% 29%	2178
April	1st to 10th	Day	16% 13% 4 8% 7% 3 5% 4 4 7% 29%	2088
	11th to 20th	Day	16% 10% 3 6% 5% 3 4 5% 5% 8% 35%	2303
	21st to end	Day	17% 9% 3 7% 6% 4 4 4 5% 6% 35%	2340
May	1st to 10th	Day	16% 8% 3 5% 6% 3 6% 4 5% 7% 37%	2340
	11th to 20th	Day	17% 8% 4 6% 5% 4 4 3 5% 6% 38%	2301
	21st to end	Day	11% 7% 3 5% 6% 3 5% 4 6% 8% 42%	2621
June	1st to 10th	Day	12% 6% 3 6% 4 4 5% 5% 6% 9% 41%	2700
	11th to 20th	Day	10% 6% 3 6% 5% 3 5% 5% 6% 8% 43%	2700
	21st to end	Day	11% 5% 3 5% 5% 3 5% 6% 7% 10% 41%	2700
July	1st to 10th	Day	6% 5% 3 5% 4 3 5% 5% 6% 9% 47%	2556
	11th to 20th	Day	10% 5% 2 5% 5% 3 5% 6% 7% 8% 43%	2520
	21st to end	Day	8% 5% 3 6% 6% 4 5% 5% 6% 10% 42%	2627
August	1st to 10th	Day	10% 6% 2 6% 6% 4 6% 7% 7% 8% 39%	2327
	11th to 20th	Day	9% 5% 2 5% 6% 4 7% 5% 6% 8% 42%	2303
	21st to end	Day	12% 7% 3 5% 4 4 5% 5% 5% 9% 42%	2574
September	1st to 10th	Day	12% 5% 3 7% 6% 4 6% 6% 6% 8% 38%	2048
	11th to 20th	Day	13% 5% 2 5% 5% 3 4 5% 6% 9% 44%	1980
	21st to end	Day	15% 7% 3 6% 6% 4 6% 5% 6% 9% 35%	1980
October	1st to 10th	Day	16% 8% 3 5% 7% 3 4 5% 5% 6% 39%	1912
	11th to 20th	Day	21% 11% 3 7% 6% 3 4 5% 6% 6% 28%	1800
	21st to end	Day	23% 12% 3 8% 6% 2 5% 4 4 8% 25%	1828
November	1st to 10th	Day	13% 11% 4 7% 7% 3 4 4 6% 6% 33%	1672
	11th to 20th	Day	15% 13% 5% 9% 7% 4 4 5% 6% 6% 27%	1688
	21st to end	Day	17% 15% 6% 11% 6% 3 5% 5% 5% 5% 22%	1520
December	1st to 10th	Day	13% 18% 8% 13% 8% 4 5% 4 4 4 19%	1520
	11th to 20th	Day	17% 14% 6% 10% 7% 3 4 4 5% 5% 24%	1699
	21st to end	Day	26% 21% 9% 9% 6% 3 4 4 4 5% 11%	2509
January	1st to 10th	Night	16% 12% 6% 11% 6% 4 4 5% 5% 7% 23%	2648
	11th to 20th	Night	17% 16% 7% 10% 8% 4 4 5% 4 5% 20%	2591
	21st to end	Night	12% 13% 9% 11% 8% 3 6% 4 4 5% 24%	2638
February	1st to 10th	Night	14% 16% 5% 9% 8% 4 6% 4 5% 6% 23%	2340
	11th to 20th	Night	14% 13% 5% 9% 7% 4 6% 5% 6% 5% 26%	2182
	21st to end	Night	17% 12% 5% 9% 7% 4 5% 5% 5% 5% 26%	1776
March	1st to 10th	Night	13% 12% 5% 8% 6% 3 4 4 4 6% 35%	2116
	11th to 20th	Night	11% 9% 5% 7% 7% 3 6% 5% 5% 7% 37%	1980
	21st to end	Night	18% 10% 4 7% 6% 3 5% 4 4 6% 33%	2178
April	1st to 10th	Night	18% 10% 5% 6% 6% 2 4 4 5% 7% 34%	1886
	11th to 20th	Night	18% 8% 2 5% 5% 3 4 5% 5% 7% 38%	1605
	21st to end	Night	17% 9% 3 6% 5% 3 4 4 5% 6% 36%	1620
May	1st to 10th	Night	12% 10% 4 6% 7% 4 5% 5% 5% 7% 35%	1620
	11th to 20th	Night	16% 7% 3 6% 5% 3 4 3 5% 6% 41%	1592
	21st to end	Night	10% 5% 2 4 4 4 5% 5% 6% 7% 47%	1647
June	1st to 10th	Night	12% 5% 3 5% 5% 3 5% 5% 5% 7% 44%	1260
	11th to 20th	Night	10% 6% 2 4 5% 2 4 5% 6% 9% 45%	1260
	21st to end	Night	8% 5% 2 6% 4 3 5% 5% 6% 9% 48%	1260
July	1st to 10th	Night	6% 5% 2 3 4 2 3 5% 5% 8% 56%	1399
	11th to 20th	Night	8% 5% 2 4 4 3 5% 5% 6% 10% 50%	1440
	21st to end	Night	7% 4 2 5% 4 3 5% 6% 6% 7% 49%	1691
August	1st to 10th	Night	8% 4 1 4 4 4 5% 6% 6% 9% 48%	1611
	11th to 20th	Night	8% 4 2 4 4 4 4 6% 5% 9% 49%	1598
	21st to end	Night	10% 5% 3 5% 5% 3 4 4 5% 7% 49%	1782
September	1st to 10th	Night	9% 5% 2 6% 6% 3 6% 5% 6% 6% 45%	1890
	11th to 20th	Night	11% 5% 2 4 4 2 4 5% 4 8% 52%	1980
	21st to end	Night	13% 7% 3 6% 5% 2 4 4 4 7% 43%	1980
October	1st to 10th	Night	16% 7% 3 5% 4 2 3 4 5% 7% 43%	2047
	11th to 20th	Night	19% 10% 3 7% 5% 3 5% 4 5% 6% 32%	2160
	21st to end	Night	22% 10% 4 8% 5% 4 4 4 5% 7% 28%	2524
November	1st to 10th	Night	13% 9% 4 6% 5% 3 4 5% 6% 6% 39%	2406
	11th to 20th	Night	15% 11% 4 8% 7% 5% 6% 5% 5% 7% 26%	2489
	21st to end	Night	20% 14% 5% 7% 6% 4 4 4 5% 6% 25%	2660
December	1st to 10th	Night	18% 15% 6% 9% 7% 4 5% 5% 6% 5% 21%	2660
	11th to 20th	Night	14% 14% 6% 11% 8% 4 5% 5% 5% 6% 22%	3493
	21st to end	Night	25% 21% 7% 9% 6% 3 4 3 4 4 13%	5140

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	66% 6% 11% 12% 5%	1518
	11th to 20th	Day	70% 7% 10% 9% 4	1480
	21st to end	Day	67% 6% 9% 12% 5% 1	1715
February	1st to 10th	Day	65% 6% 10% 14% 4	1620
	11th to 20th	Day	62% 5% 10% 14% 8% 1	1671
	21st to end	Day	60% 6% 10% 16% 8% 1	1480
March	1st to 10th	Day	56% 6% 13% 16% 8% 1	1844
	11th to 20th	Day	53% 6% 9% 17% 11% 3	1980
	21st to end	Day	58% 5% 10% 19% 7% 1	2178
April	1st to 10th	Day	55% 5% 11% 17% 10% 1	2088
	11th to 20th	Day	49% 6% 12% 18% 14% 2	2303
	21st to end	Day	49% 7% 13% 18% 12% 1	2340
May	1st to 10th	Day	47% 7% 10% 23% 11% 2	2340
	11th to 20th	Day	47% 5% 12% 19% 14% 2	2305
	21st to end	Day	40% 7% 12% 22% 16% 3	2621
June	1st to 10th	Day	38% 7% 12% 25% 15% 2	2700
	11th to 20th	Day	38% 7% 12% 23% 18% 2	2700
	21st to end	Day	38% 8% 12% 25% 15% 1	2700
July	1st to 10th	Day	33% 8% 12% 26% 17% 4	2487
	11th to 20th	Day	36% 8% 13% 24% 16% 2	2445
	21st to end	Day	37% 8% 13% 22% 17% 3	2481
August	1st to 10th	Day	39% 10% 14% 24% 12% 2	2314
	11th to 20th	Day	38% 9% 15% 24% 12% 2	2303
	21st to end	Day	39% 7% 13% 26% 14% 1	2574
September	1st to 10th	Day	42% 9% 13% 23% 11% 2	2048
	11th to 20th	Day	36% 8% 13% 31% 12%	1980
	21st to end	Day	46% 7% 14% 23% 10%	1980
October	1st to 10th	Day	46% 6% 10% 23% 13% 2	1912
	11th to 20th	Day	56% 6% 12% 18% 7% 1	1800
	21st to end	Day	57% 6% 15% 16% 6%	1828
November	1st to 10th	Day	49% 8% 12% 21% 10%	1672
	11th to 20th	Day	56% 6% 14% 15% 9%	1688
	21st to end	Day	62% 6% 15% 13% 4 1	1520
December	1st to 10th	Day	68% 7% 12% 10% 3	1520
	11th to 20th	Day	61% 6% 14% 16% 4	1695
	21st to end	Day	78% 6% 8% 6% 3	2509
January	1st to 10th	Night	60% 6% 12% 14% 7%	2648
	11th to 20th	Night	65% 7% 11% 10% 5%	2591
	21st to end	Night	61% 5% 12% 15% 6%	2638
February	1st to 10th	Night	61% 6% 10% 16% 6% 1	2340
	11th to 20th	Night	57% 6% 11% 16% 9% 1	2184
	21st to end	Night	59% 5% 10% 17% 9%	1776
March	1st to 10th	Night	52% 5% 11% 22% 9% 1	2116
	11th to 20th	Night	45% 6% 12% 21% 14% 2	1980
	21st to end	Night	53% 5% 9% 21% 11% 1	2178
April	1st to 10th	Night	50% 5% 10% 21% 13% 1	1886
	11th to 20th	Night	44% 6% 11% 19% 19% 2	1605
	21st to end	Night	48% 6% 10% 19% 15% 3	1620
May	1st to 10th	Night	47% 7% 10% 20% 13% 2	1620
	11th to 20th	Night	45% 4 10% 20% 17% 4	1593
	21st to end	Night	34% 8% 13% 24% 16% 5%	1648
June	1st to 10th	Night	38% 7% 10% 25% 18% 2	1260
	11th to 20th	Night	34% 8% 14% 22% 20% 3	1260
	21st to end	Night	33% 7% 13% 27% 18% 2	1260
July	1st to 10th	Night	27% 7% 11% 30% 21% 5%	1359
	11th to 20th	Night	29% 9% 14% 29% 18% 2	1392
	21st to end	Night	31% 10% 16% 24% 18% 2	1597
August	1st to 10th	Night	30% 8% 16% 30% 15% 1	1600
	11th to 20th	Night	30% 10% 14% 29% 15% 3	1598
	21st to end	Night	33% 8% 12% 29% 17% 1	1782
September	1st to 10th	Night	35% 9% 13% 27% 15% 1	1890
	11th to 20th	Night	30% 6% 12% 35% 16% 1	1980
	21st to end	Night	41% 7% 11% 29% 11% 1	1980
October	1st to 10th	Night	41% 7% 12% 24% 15% 1	2047
	11th to 20th	Night	51% 7% 12% 21% 8% 1	2160
	21st to end	Night	57% 6% 13% 16% 8%	2524
November	1st to 10th	Night	43% 6% 14% 25% 11% 1	2406
	11th to 20th	Night	55% 7% 13% 17% 7% 1	2489
	21st to end	Night	59% 6% 15% 15% 4 1	2660
December	1st to 10th	Night	64% 7% 12% 13% 4	2660
	11th to 20th	Night	61% 6% 14% 13% 5%	3496
	21st to end	Night	75% 6% 10% 6% 3	5140

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	24% 24% 34% 13% 4	270
	11th to 20th	Day	22% 22% 38% 15% 3	255
	21st to end	Day	24% 20% 36% 15% 4 1	264
February	1st to 10th	Day	24% 29% 36% 9% 2	240
	11th to 20th	Day	22% 29% 38% 10% 1	240
	21st to end	Day	26% 32% 30% 11% 2	198
March	1st to 10th	Day	29% 21% 33% 15% 2	284
	11th to 20th	Day	22% 18% 33% 20% 6% 1	420
	21st to end	Day	28% 16% 29% 20% 6% 1	462
April	1st to 10th	Day	21% 21% 31% 18% 7% 1	420
	11th to 20th	Day	26% 20% 25% 17% 8% 4	412
	21st to end	Day	21% 18% 32% 18% 8% 2	420
May	1st to 10th	Day	22% 18% 27% 23% 8% 2	423
	11th to 20th	Day	18% 13% 34% 19% 12% 4	447
	21st to end	Day	12% 11% 29% 27% 15% 5%	521
June	1st to 10th	Day	13% 12% 34% 29% 10% 3	630
	11th to 20th	Day	13% 12% 32% 27% 12% 4	630
	21st to end	Day	13% 10% 32% 29% 12% 4	630
July	1st to 10th	Day	8% 7% 28% 31% 18% 8%	621
	11th to 20th	Day	11% 9% 30% 30% 14% 7%	618
	21st to end	Day	8% 6% 30% 33% 17% 6%	568
August	1st to 10th	Day	10% 7% 23% 28% 22% 11%	446
	11th to 20th	Day	12% 5% 21% 28% 22% 12%	439
	21st to end	Day	13% 10% 21% 28% 21% 6%	495
September	1st to 10th	Day	14% 8% 26% 26% 18% 8%	450
	11th to 20th	Day	12% 7% 21% 26% 23% 11%	450
	21st to end	Day	16% 15% 22% 23% 16% 7%	450
October	1st to 10th	Day	19% 15% 25% 21% 15% 5%	450
	11th to 20th	Day	21% 21% 31% 22% 3 1	450
	21st to end	Day	29% 18% 36% 13% 3 1	343
November	1st to 10th	Day	17% 25% 34% 17% 7%	270
	11th to 20th	Day	22% 24% 35% 14% 5%	259
	21st to end	Day	22% 21% 39% 14% 4	180
December	1st to 10th	Day	24% 25% 31% 16% 3 1	180
	11th to 20th	Day	28% 24% 37% 8% 2 1	180
	21st to end	Day	28% 24% 36% 9% 2	222
January	1st to 10th	Night	21% 20% 35% 19% 5% 1	950
	11th to 20th	Night	24% 17% 34% 19% 4 1	926
	21st to end	Night	18% 13% 38% 25% 6%	856
February	1st to 10th	Night	23% 17% 38% 20% 2	720
	11th to 20th	Night	23% 18% 32% 22% 5%	703
	21st to end	Night	23% 22% 33% 20% 3	592
March	1st to 10th	Night	20% 15% 35% 24% 6%	720
	11th to 20th	Night	17% 15% 38% 24% 6% 1	720
	21st to end	Night	23% 13% 30% 26% 7% 1	792
April	1st to 10th	Night	23% 15% 34% 22% 5%	626
	11th to 20th	Night	22% 13% 34% 22% 7% 2	533
	21st to end	Night	22% 10% 32% 29% 7%	540
May	1st to 10th	Night	17% 7% 34% 33% 9% 1	540
	11th to 20th	Night	20% 8% 28% 33% 10% 1	531
	21st to end	Night	13% 5% 29% 35% 17% 2	527
June	1st to 10th	Night	12% 7% 31% 39% 11%	360
	11th to 20th	Night	14% 5% 33% 38% 11%	360
	21st to end	Night	10% 6% 29% 40% 16%	360
July	1st to 10th	Night	8% 3 18% 43% 26% 2	484
	11th to 20th	Night	10% 2 22% 42% 21% 3	522
	21st to end	Night	7% 2 26% 43% 20% 1	591
August	1st to 10th	Night	9% 2 16% 45% 26% 2	537
	11th to 20th	Night	8% 4 21% 36% 28% 4	534
	21st to end	Night	12% 4 24% 33% 24% 3	594
September	1st to 10th	Night	12% 4 27% 33% 19% 5%	648
	11th to 20th	Night	13% 5% 20% 36% 23% 3	720
	21st to end	Night	16% 8% 24% 31% 17% 4	720
October	1st to 10th	Night	19% 10% 28% 26% 16% 1	720
	11th to 20th	Night	22% 16% 32% 23% 7%	720
	21st to end	Night	28% 14% 33% 21% 4	792
November	1st to 10th	Night	19% 14% 31% 25% 10% 1	740
	11th to 20th	Night	21% 19% 34% 19% 7% 1	760
	21st to end	Night	26% 16% 36% 19% 4	760
December	1st to 10th	Night	26% 18% 34% 18% 4 1	760
	11th to 20th	Night	22% 19% 37% 19% 2 1	921
	21st to end	Night	30% 15% 35% 18% 2	1045

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