Backus Mountain Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Backus Mountain Observatory clear sky chart from 2005-11-06 to 2025-09-19.

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 9% 5% 5% 5% 4 6% 7% 31%	126335
	Night	14% 11% 4 9% 5% 4 4 3 5% 6% 33%	122735
Transparency	Day	47% 8% 11% 19% 12% 2	129967
	Night	47% 7% 11% 20% 12% 2	127172
Seeing	Day	24% 15% 24% 21% 12% 4	16328
	Night	22% 13% 22% 28% 14% 1	26378

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	12% 18% 6% 14% 10% 4 4 3 4 5% 21%	4056
	11th to 20th	Day	11% 16% 8% 20% 5% 4 3 2 2 2 27%	4060
	21st to end	Day	15% 13% 7% 10% 5% 3 3 2 4 5% 33%	4466
February	1st to 10th	Day	16% 12% 5% 11% 6% 4 4 3 4 7% 27%	1800
	11th to 20th	Day	20% 11% 6% 11% 5% 4 5% 3 5% 5% 26%	1847
	21st to end	Day	18% 13% 5% 11% 5% 4 3 3 4 5% 30%	1670
March	1st to 10th	Day	16% 12% 8% 8% 5% 4 4 3 4 6% 30%	4935
	11th to 20th	Day	11% 9% 4 7% 4 4 3 3 4 7% 42%	4980
	21st to end	Day	15% 11% 4 8% 5% 5% 5% 4 6% 6% 32%	5465
April	1st to 10th	Day	15% 12% 4 7% 4 4 4 4 4 7% 35%	2232
	11th to 20th	Day	14% 10% 4 8% 4 5% 4 3 5% 6% 38%	2404
	21st to end	Day	17% 9% 3 7% 5% 4 4 3 5% 6% 35%	2600
May	1st to 10th	Day	16% 9% 4 9% 6% 6% 7% 6% 7% 7% 23%	5862
	11th to 20th	Day	20% 10% 3 9% 6% 5% 6% 4 7% 8% 23%	5861
	21st to end	Day	14% 8% 3 8% 6% 6% 5% 5% 7% 9% 28%	6433
June	1st to 10th	Day	12% 6% 3 8% 6% 5% 5% 4 7% 9% 35%	2600
	11th to 20th	Day	10% 8% 2 8% 6% 6% 6% 6% 8% 8% 33%	2600
	21st to end	Day	10% 7% 2 7% 6% 6% 6% 5% 7% 9% 36%	2600
July	1st to 10th	Day	8% 6% 2 8% 6% 6% 6% 5% 7% 9% 37%	5900
	11th to 20th	Day	10% 7% 3 9% 6% 6% 7% 6% 8% 11% 29%	5900
	21st to end	Day	13% 9% 4 9% 6% 6% 6% 5% 8% 9% 24%	6424
August	1st to 10th	Day	13% 8% 3 7% 5% 6% 7% 6% 9% 10% 26%	4234
	11th to 20th	Day	11% 6% 3 8% 6% 7% 6% 6% 9% 10% 29%	4174
	21st to end	Day	6% 4 2 6% 5% 5% 5% 5% 7% 9% 46%	4451
September	1st to 10th	Day	12% 6% 2 7% 6% 4 5% 5% 5% 6% 43%	2290
	11th to 20th	Day	12% 8% 3 8% 6% 5% 5% 4 4 7% 38%	2178
	21st to end	Day	20% 9% 2 7% 5% 5% 5% 4 5% 7% 32%	2090
October	1st to 10th	Day	16% 6% 2 7% 4 3 4 3 5% 7% 42%	1952
	11th to 20th	Day	15% 10% 2 6% 3 4 4 3 5% 5% 42%	1900
	21st to end	Day	17% 12% 2 6% 4 3 4 3 4 6% 39%	2090
November	1st to 10th	Day	11% 10% 3 4 4 4 3 3 6% 6% 45%	1912
	11th to 20th	Day	14% 12% 4 7% 5% 4 4 3 4 7% 35%	1800
	21st to end	Day	15% 13% 4 7% 4 4 4 3 5% 6% 35%	1800
December	1st to 10th	Day	16% 14% 5% 9% 8% 3 4 3 3 5% 29%	2930
	11th to 20th	Day	22% 14% 5% 9% 4 3 4 3 4 6% 26%	3373
	21st to end	Day	17% 13% 5% 13% 6% 6% 5% 3 5% 6% 21%	4466
January	1st to 10th	Night	11% 16% 8% 18% 9% 4 4 3 3 3 20%	5889
	11th to 20th	Night	14% 12% 6% 16% 7% 5% 4 3 3 4 26%	5900
	21st to end	Night	16% 12% 4 9% 5% 6% 4 3 4 4 33%	6490
February	1st to 10th	Night	15% 13% 5% 11% 6% 5% 4 4 5% 5% 28%	2600
	11th to 20th	Night	19% 12% 6% 12% 7% 4 4 3 4 4 24%	2437
	21st to end	Night	17% 14% 5% 11% 5% 4 4 3 5% 5% 27%	1960
March	1st to 10th	Night	18% 12% 4 8% 5% 4 3 3 4 5% 35%	4918
	11th to 20th	Night	11% 9% 4 8% 4 3 3 3 5% 6% 43%	4980
	21st to end	Night	13% 12% 3 8% 6% 5% 5% 4 5% 7% 33%	5467
April	1st to 10th	Night	14% 13% 3 7% 5% 5% 4 3 5% 7% 33%	2166
	11th to 20th	Night	12% 9% 4 6% 5% 4 4 4 6% 8% 38%	1963
	21st to end	Night	15% 7% 3 8% 6% 5% 4 3 5% 7% 38%	1800
May	1st to 10th	Night	20% 10% 3 8% 5% 5% 4 4 5% 7% 28%	4033
	11th to 20th	Night	19% 11% 4 9% 5% 4 4 4 6% 8% 26%	4032
	21st to end	Night	19% 12% 4 9% 5% 4 4 4 5% 7% 28%	4421
June	1st to 10th	Night	9% 7% 3 6% 5% 4 5% 4 7% 9% 41%	1800
	11th to 20th	Night	10% 6% 2 7% 5% 4 6% 5% 7% 10% 38%	1800
	21st to end	Night	9% 7% 3 6% 4 4 5% 4 6% 10% 42%	1800
July	1st to 10th	Night	8% 7% 3 8% 5% 6% 6% 5% 7% 9% 37%	4058
	11th to 20th	Night	10% 9% 3 8% 5% 6% 4 4 6% 10% 36%	4057
	21st to end	Night	11% 11% 4 9% 6% 5% 6% 5% 7% 9% 29%	4427
August	1st to 10th	Night	12% 9% 4 10% 5% 4 5% 4 7% 8% 32%	2915
	11th to 20th	Night	13% 8% 3 7% 4 4 5% 4 7% 7% 38%	2875
	21st to end	Night	6% 5% 2 6% 4 4 4 3 6% 8% 52%	3412
September	1st to 10th	Night	11% 7% 2 6% 4 5% 4 3 5% 7% 46%	2105
	11th to 20th	Night	13% 6% 2 6% 4 4 4 4 5% 7% 45%	2179
	21st to end	Night	15% 9% 3 7% 5% 4 4 4 5% 7% 38%	2090
October	1st to 10th	Night	14% 7% 2 5% 4 5% 4 3 5% 6% 45%	2228
	11th to 20th	Night	12% 11% 3 6% 3 3 4 3 4 6% 43%	2280
	21st to end	Night	15% 11% 3 7% 4 4 2 2 5% 5% 43%	2508
November	1st to 10th	Night	12% 7% 3 7% 5% 3 5% 4 4 6% 43%	2383
	11th to 20th	Night	14% 12% 4 8% 5% 4 5% 3 4 5% 36%	2600
	21st to end	Night	13% 12% 5% 9% 5% 4 4 3 5% 5% 35%	2600
December	1st to 10th	Night	13% 12% 6% 12% 8% 4 4 3 4 6% 28%	4250
	11th to 20th	Night	19% 13% 8% 12% 5% 3 4 3 3 6% 25%	4822
	21st to end	Night	16% 14% 9% 12% 5% 5% 5% 3 5% 7% 21%	6490

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	63% 5% 6% 16% 9%	4057
	11th to 20th	Day	64% 4 5% 11% 13% 4	4060
	21st to end	Day	52% 5% 9% 22% 11% 1	4466
February	1st to 10th	Day	56% 4 10% 19% 10% 1	1800
	11th to 20th	Day	59% 4 10% 16% 10% 2	1856
	21st to end	Day	57% 5% 8% 16% 13% 3	1670
March	1st to 10th	Day	52% 7% 9% 17% 11% 5%	4935
	11th to 20th	Day	39% 6% 9% 20% 21% 4	4980
	21st to end	Day	48% 8% 9% 19% 14% 2	5465
April	1st to 10th	Day	48% 5% 11% 19% 13% 3	2232
	11th to 20th	Day	45% 6% 10% 21% 15% 3	2404
	21st to end	Day	49% 7% 12% 21% 11% 2	2600
May	1st to 10th	Day	53% 11% 11% 16% 8% 1	5897
	11th to 20th	Day	53% 9% 12% 19% 7% 1	5865
	21st to end	Day	45% 11% 15% 17% 9% 2	6447
June	1st to 10th	Day	42% 11% 12% 21% 14%	2600
	11th to 20th	Day	43% 11% 13% 18% 14% 1	2600
	21st to end	Day	41% 11% 14% 22% 12% 1	2600
July	1st to 10th	Day	39% 10% 15% 21% 11% 2	5835
	11th to 20th	Day	44% 12% 15% 23% 6% 1	5831
	21st to end	Day	51% 9% 14% 17% 9%	6240
August	1st to 10th	Day	47% 11% 18% 18% 6%	4221
	11th to 20th	Day	44% 11% 13% 23% 9%	4213
	21st to end	Day	31% 9% 16% 29% 14% 1	4451
September	1st to 10th	Day	39% 8% 12% 19% 18% 3	2290
	11th to 20th	Day	44% 7% 11% 20% 16% 1	2178
	21st to end	Day	49% 8% 11% 17% 14% 1	2090
October	1st to 10th	Day	30% 5% 11% 20% 28% 5%	3238
	11th to 20th	Day	33% 6% 10% 23% 15% 13%	3140
	21st to end	Day	32% 4 9% 30% 15% 11%	3428
November	1st to 10th	Day	38% 6% 10% 22% 21% 3	1912
	11th to 20th	Day	48% 4 8% 18% 19% 3	1800
	21st to end	Day	49% 6% 12% 18% 14% 2	1800
December	1st to 10th	Day	56% 6% 8% 19% 11% 1	2930
	11th to 20th	Day	58% 5% 9% 15% 12% 1	3370
	21st to end	Day	60% 8% 10% 16% 6%	4466
January	1st to 10th	Night	65% 7% 6% 14% 7% 1	5889
	11th to 20th	Night	59% 6% 7% 14% 12% 3	5900
	21st to end	Night	52% 6% 9% 20% 12% 1	6490
February	1st to 10th	Night	56% 5% 11% 16% 11% 1	2600
	11th to 20th	Night	62% 5% 8% 14% 10% 2	2439
	21st to end	Night	57% 6% 10% 15% 9% 4	1960
March	1st to 10th	Night	48% 7% 7% 21% 14% 3	4918
	11th to 20th	Night	38% 6% 8% 23% 21% 3	4980
	21st to end	Night	47% 7% 10% 20% 15% 1	5467
April	1st to 10th	Night	49% 7% 11% 18% 13% 2	2166
	11th to 20th	Night	42% 6% 12% 22% 14% 4	1963
	21st to end	Night	45% 7% 14% 22% 11% 1	1800
May	1st to 10th	Night	52% 9% 14% 16% 8% 1	4060
	11th to 20th	Night	53% 10% 14% 18% 5%	4033
	21st to end	Night	50% 11% 15% 17% 4 3	4429
June	1st to 10th	Night	36% 12% 16% 22% 14% 1	1800
	11th to 20th	Night	36% 12% 18% 20% 13% 1	1800
	21st to end	Night	35% 11% 16% 24% 14% 1	1800
July	1st to 10th	Night	40% 10% 16% 22% 10% 2	4015
	11th to 20th	Night	42% 9% 18% 24% 6% 1	4006
	21st to end	Night	47% 10% 14% 20% 9%	4297
August	1st to 10th	Night	47% 10% 16% 21% 6%	2904
	11th to 20th	Night	42% 9% 12% 26% 10%	2902
	21st to end	Night	27% 8% 13% 34% 17% 1	3412
September	1st to 10th	Night	36% 8% 15% 20% 17% 4	2105
	11th to 20th	Night	37% 8% 12% 22% 18% 2	2179
	21st to end	Night	42% 9% 13% 22% 13% 1	2090
October	1st to 10th	Night	32% 6% 11% 21% 27% 4	3722
	11th to 20th	Night	34% 6% 8% 21% 19% 12%	3779
	21st to end	Night	28% 4 10% 28% 18% 12%	4120
November	1st to 10th	Night	40% 6% 10% 19% 21% 4	2383
	11th to 20th	Night	48% 5% 9% 17% 17% 4	2600
	21st to end	Night	48% 6% 10% 19% 15% 3	2600
December	1st to 10th	Night	56% 6% 8% 22% 8% 1	4250
	11th to 20th	Night	61% 4 11% 16% 8% 1	4824
	21st to end	Night	59% 9% 11% 13% 8%	6490

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	40% 26% 22% 8% 3	300
	11th to 20th	Day	43% 25% 24% 8%	300
	21st to end	Day	39% 23% 26% 11% 1	330
February	1st to 10th	Day	38% 27% 23% 10% 2	300
	11th to 20th	Day	54% 20% 19% 6% 1	300
	21st to end	Day	42% 22% 28% 7% 1	269
March	1st to 10th	Day	33% 22% 26% 15% 4	500
	11th to 20th	Day	30% 26% 28% 12% 4 1	500
	21st to end	Day	32% 24% 26% 15% 4	550
April	1st to 10th	Day	33% 20% 25% 16% 6%	500
	11th to 20th	Day	27% 18% 29% 16% 8% 2	492
	21st to end	Day	26% 20% 28% 17% 8% 2	500
May	1st to 10th	Day	28% 20% 26% 16% 7% 3	503
	11th to 20th	Day	24% 16% 27% 17% 13% 3	527
	21st to end	Day	20% 11% 25% 23% 15% 7%	573
June	1st to 10th	Day	12% 12% 28% 24% 17% 6%	530
	11th to 20th	Day	13% 10% 27% 27% 17% 7%	530
	21st to end	Day	13% 9% 25% 27% 18% 7%	530
July	1st to 10th	Day	14% 6% 22% 27% 21% 11%	525
	11th to 20th	Day	7% 5% 23% 32% 22% 10%	524
	21st to end	Day	13% 6% 20% 30% 22% 9%	576
August	1st to 10th	Day	14% 8% 23% 26% 20% 10%	526
	11th to 20th	Day	13% 8% 20% 26% 24% 9%	519
	21st to end	Day	11% 7% 21% 28% 25% 9%	583
September	1st to 10th	Day	13% 9% 19% 27% 22% 9%	530
	11th to 20th	Day	15% 6% 18% 28% 26% 7%	522
	21st to end	Day	25% 13% 20% 26% 14% 3	490
October	1st to 10th	Day	18% 9% 23% 27% 19% 4	490
	11th to 20th	Day	22% 15% 23% 25% 13% 3	490
	21st to end	Day	25% 15% 25% 22% 12% 1	539
November	1st to 10th	Day	21% 15% 29% 23% 9% 2	450
	11th to 20th	Day	33% 17% 25% 18% 7%	300
	21st to end	Day	30% 20% 27% 19% 4	300
December	1st to 10th	Day	38% 22% 26% 11% 3 1	300
	11th to 20th	Day	33% 21% 25% 17% 4	300
	21st to end	Day	32% 24% 25% 14% 5%	330
January	1st to 10th	Night	31% 23% 26% 17% 2	800
	11th to 20th	Night	37% 18% 25% 18% 2	800
	21st to end	Night	28% 23% 31% 17% 1	880
February	1st to 10th	Night	28% 19% 28% 23% 2	800
	11th to 20th	Night	35% 22% 25% 16% 1	783
	21st to end	Night	30% 22% 31% 16% 1	660
March	1st to 10th	Night	29% 18% 28% 23% 3	800
	11th to 20th	Night	30% 20% 24% 19% 6%	800
	21st to end	Night	27% 18% 28% 22% 5%	880
April	1st to 10th	Night	25% 19% 26% 26% 5%	800
	11th to 20th	Night	19% 16% 27% 26% 10% 1	779
	21st to end	Night	19% 16% 24% 33% 8% 1	600
May	1st to 10th	Night	22% 11% 25% 29% 12% 1	600
	11th to 20th	Night	19% 12% 22% 31% 16%	591
	21st to end	Night	16% 5% 18% 34% 24% 3	650
June	1st to 10th	Night	10% 6% 20% 38% 24% 3	600
	11th to 20th	Night	12% 6% 15% 34% 31% 3	600
	21st to end	Night	11% 3 15% 38% 30% 3	600
July	1st to 10th	Night	11% 3 12% 36% 34% 4	585
	11th to 20th	Night	8% 3 12% 32% 39% 6%	582
	21st to end	Night	11% 5% 11% 35% 34% 5%	657
August	1st to 10th	Night	14% 4 12% 33% 35% 2	597
	11th to 20th	Night	14% 4 10% 36% 34% 3	594
	21st to end	Night	12% 3 11% 38% 31% 5%	771
September	1st to 10th	Night	13% 5% 13% 34% 30% 4	800
	11th to 20th	Night	14% 6% 15% 32% 31% 3	793
	21st to end	Night	19% 9% 17% 31% 22% 2	760
October	1st to 10th	Night	16% 10% 20% 30% 22% 3	760
	11th to 20th	Night	21% 13% 20% 32% 12% 1	760
	21st to end	Night	22% 13% 21% 31% 12% 1	836
November	1st to 10th	Night	17% 12% 25% 33% 12% 1	780
	11th to 20th	Night	27% 15% 22% 28% 7%	800
	21st to end	Night	23% 17% 22% 30% 7% 1	800
December	1st to 10th	Night	31% 19% 27% 18% 5%	800
	11th to 20th	Night	26% 20% 25% 25% 4	800
	21st to end	Night	28% 17% 28% 24% 3	880

Page generated at 2025-09-16 01:49:12. Version ac07

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