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 2024-01-12.

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	15% 9% 4 7% 6% 3 5% 5% 5% 7% 32%	74923
	Night	14% 10% 5% 8% 6% 3 5% 5% 5% 7% 34%	74817
Transparency	Day	49% 8% 11% 18% 11% 2	74645
	Night	48% 8% 12% 19% 12% 2	74614
Seeing	Day	23% 14% 24% 21% 13% 4	14026
	Night	21% 13% 22% 28% 15% 1	24173

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	15% 12% 6% 12% 10% 4 4 4 4 5% 24%	1706
	11th to 20th	Day	17% 17% 8% 12% 8% 3 4 3 3 3 22%	1635
	21st to end	Day	16% 15% 8% 9% 6% 2 3 3 4 5% 29%	1782
February	1st to 10th	Day	17% 11% 6% 10% 7% 3 4 4 5% 7% 25%	1620
	11th to 20th	Day	19% 11% 7% 10% 6% 2 5% 4 5% 5% 26%	1658
	21st to end	Day	20% 13% 5% 10% 6% 3 4 3 4 4 28%	1498
March	1st to 10th	Day	16% 9% 6% 7% 4 3 4 4 4 6% 37%	1980
	11th to 20th	Day	18% 12% 5% 9% 6% 2 4 4 4 6% 30%	1980
	21st to end	Day	17% 12% 5% 8% 5% 3 5% 4 4 6% 31%	2178
April	1st to 10th	Day	14% 10% 5% 6% 5% 3 4 4 5% 7% 37%	2008
	11th to 20th	Day	14% 9% 4 7% 5% 3 4 4 5% 6% 39%	2160
	21st to end	Day	19% 8% 4 6% 6% 3 5% 4 5% 6% 34%	2340
May	1st to 10th	Day	18% 8% 4 6% 8% 4 5% 6% 6% 8% 28%	2340
	11th to 20th	Day	20% 9% 3 6% 7% 4 6% 5% 6% 7% 27%	2301
	21st to end	Day	12% 8% 3 7% 7% 5% 6% 6% 7% 9% 31%	2531
June	1st to 10th	Day	13% 6% 3 7% 7% 4 5% 5% 6% 9% 35%	2340
	11th to 20th	Day	10% 7% 3 7% 6% 4 6% 7% 7% 8% 33%	2340
	21st to end	Day	11% 6% 3 7% 7% 5% 6% 6% 6% 9% 35%	2340
July	1st to 10th	Day	12% 6% 3 7% 8% 4 6% 6% 7% 9% 32%	2340
	11th to 20th	Day	10% 5% 3 7% 7% 4 7% 7% 7% 9% 33%	2340
	21st to end	Day	12% 8% 3 7% 7% 4 7% 6% 7% 9% 28%	2547
August	1st to 10th	Day	14% 7% 3 6% 7% 5% 6% 8% 9% 9% 25%	2327
	11th to 20th	Day	15% 6% 3 7% 8% 5% 7% 7% 6% 9% 26%	2303
	21st to end	Day	11% 6% 3 6% 7% 5% 6% 7% 7% 10% 32%	2455
September	1st to 10th	Day	13% 6% 2 6% 6% 3 5% 6% 6% 6% 41%	2062
	11th to 20th	Day	12% 7% 3 7% 7% 4 5% 6% 4 8% 38%	1980
	21st to end	Day	19% 8% 3 7% 5% 3 6% 5% 5% 7% 33%	1980
October	1st to 10th	Day	17% 6% 2 6% 6% 2 3 5% 5% 7% 41%	1850
	11th to 20th	Day	16% 10% 2 5% 4 2 5% 5% 5% 6% 40%	1800
	21st to end	Day	18% 12% 3 6% 5% 2 4 4 3 5% 38%	1980
November	1st to 10th	Day	11% 10% 3 4 4 3 4 4 4 7% 46%	1815
	11th to 20th	Day	14% 11% 5% 7% 5% 3 5% 4 4 7% 36%	1710
	21st to end	Day	16% 11% 4 7% 5% 3 4 4 5% 6% 36%	1710
December	1st to 10th	Day	22% 15% 5% 8% 5% 2 4 3 4 6% 24%	1710
	11th to 20th	Day	18% 14% 5% 9% 4 2 4 5% 4 6% 30%	2153
	21st to end	Day	17% 15% 6% 10% 8% 4 6% 5% 5% 6% 19%	3124
January	1st to 10th	Night	15% 12% 6% 13% 8% 4 5% 4 4 5% 24%	2459
	11th to 20th	Night	19% 13% 7% 13% 6% 3 5% 4 4 5% 21%	2365
	21st to end	Night	15% 13% 9% 11% 7% 4 4 4 4 4 26%	2574
February	1st to 10th	Night	15% 12% 6% 10% 7% 4 5% 5% 5% 6% 27%	2340
	11th to 20th	Night	19% 11% 6% 10% 9% 3 5% 4 4 5% 24%	2187
	21st to end	Night	19% 14% 6% 11% 5% 3 4 4 4 5% 26%	1758
March	1st to 10th	Night	16% 10% 4 7% 7% 3 4 4 4 5% 36%	1980
	11th to 20th	Night	18% 11% 4 9% 7% 3 5% 4 5% 5% 29%	1980
	21st to end	Night	15% 12% 4 8% 6% 3 5% 5% 4 6% 31%	2178
April	1st to 10th	Night	14% 10% 4 6% 5% 3 5% 4 5% 8% 35%	1950
	11th to 20th	Night	13% 8% 4 5% 6% 4 4 5% 6% 8% 38%	1765
	21st to end	Night	15% 7% 2 8% 7% 4 4 4 4 7% 38%	1620
May	1st to 10th	Night	17% 8% 3 7% 5% 4 4 5% 5% 6% 35%	1620
	11th to 20th	Night	15% 9% 4 7% 6% 4 5% 5% 5% 9% 31%	1592
	21st to end	Night	11% 8% 3 7% 6% 3 5% 5% 6% 8% 38%	1744
June	1st to 10th	Night	10% 6% 3 5% 6% 3 4 5% 7% 10% 41%	1620
	11th to 20th	Night	11% 6% 2 6% 5% 2 6% 7% 6% 9% 40%	1620
	21st to end	Night	10% 6% 3 5% 5% 3 4 6% 6% 10% 42%	1620
July	1st to 10th	Night	9% 6% 4 6% 6% 4 5% 5% 7% 8% 39%	1620
	11th to 20th	Night	7% 5% 2 5% 5% 3 5% 5% 7% 10% 45%	1620
	21st to end	Night	8% 6% 3 5% 5% 4 5% 7% 8% 11% 38%	1771
August	1st to 10th	Night	10% 7% 3 7% 6% 3 5% 7% 7% 8% 36%	1611
	11th to 20th	Night	14% 6% 3 5% 4 3 5% 5% 7% 7% 40%	1598
	21st to end	Night	9% 7% 2 5% 5% 4 4 5% 6% 9% 44%	1881
September	1st to 10th	Night	11% 6% 3 5% 5% 3 5% 4 5% 7% 45%	1894
	11th to 20th	Night	13% 6% 2 5% 5% 3 5% 5% 5% 7% 46%	1980
	21st to end	Night	13% 9% 3 6% 5% 3 4 5% 5% 7% 39%	1980
October	1st to 10th	Night	15% 7% 2 4 5% 3 5% 4 5% 6% 45%	2110
	11th to 20th	Night	13% 11% 3 6% 4 3 5% 4 4 6% 42%	2160
	21st to end	Night	15% 12% 3 6% 5% 3 3 3 4 5% 40%	2376
November	1st to 10th	Night	12% 7% 4 6% 6% 2 5% 4 4 6% 44%	2260
	11th to 20th	Night	14% 10% 5% 7% 6% 3 5% 4 4 5% 37%	2470
	21st to end	Night	14% 10% 5% 8% 6% 3 4 4 5% 6% 36%	2470
December	1st to 10th	Night	19% 14% 7% 10% 7% 3 5% 4 4 5% 23%	2470
	11th to 20th	Night	15% 12% 8% 12% 7% 3 4 4 3 6% 26%	3042
	21st to end	Night	14% 16% 9% 10% 7% 4 5% 4 4 6% 21%	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	62% 5% 8% 15% 9% 1	1707
	11th to 20th	Day	66% 4 9% 13% 7% 1	1635
	21st to end	Day	59% 4 9% 17% 10% 2	1782
February	1st to 10th	Day	58% 4 11% 19% 7% 1	1620
	11th to 20th	Day	59% 4 10% 17% 9% 1	1667
	21st to end	Day	59% 5% 8% 14% 11% 3	1498
March	1st to 10th	Day	47% 5% 10% 24% 12% 2	1980
	11th to 20th	Day	55% 6% 9% 16% 11% 3	1980
	21st to end	Day	54% 7% 8% 20% 10% 2	2178
April	1st to 10th	Day	46% 5% 12% 20% 13% 4	2008
	11th to 20th	Day	45% 6% 10% 20% 16% 3	2160
	21st to end	Day	49% 7% 12% 20% 10% 2	2340
May	1st to 10th	Day	50% 9% 12% 17% 10% 1	2340
	11th to 20th	Day	53% 9% 12% 18% 6% 1	2305
	21st to end	Day	47% 11% 15% 18% 9%	2531
June	1st to 10th	Day	43% 11% 12% 21% 13%	2340
	11th to 20th	Day	43% 12% 13% 17% 14% 1	2340
	21st to end	Day	42% 11% 13% 21% 12% 1	2340
July	1st to 10th	Day	45% 12% 13% 17% 11% 2	2275
	11th to 20th	Day	41% 12% 15% 18% 13% 1	2271
	21st to end	Day	47% 11% 14% 18% 9% 1	2405
August	1st to 10th	Day	48% 13% 17% 15% 7%	2314
	11th to 20th	Day	50% 12% 12% 17% 8%	2303
	21st to end	Day	43% 11% 15% 18% 10% 2	2455
September	1st to 10th	Day	40% 9% 13% 20% 15% 3	2062
	11th to 20th	Day	43% 7% 12% 20% 17% 1	1980
	21st to end	Day	47% 8% 11% 17% 15% 1	1980
October	1st to 10th	Day	41% 6% 13% 22% 15% 3	1850
	11th to 20th	Day	44% 7% 10% 21% 17% 2	1800
	21st to end	Day	47% 6% 8% 23% 14% 3	1980
November	1st to 10th	Day	37% 6% 10% 22% 21% 3	1815
	11th to 20th	Day	48% 4 8% 19% 18% 3	1710
	21st to end	Day	48% 6% 11% 19% 14% 2	1710
December	1st to 10th	Day	61% 5% 9% 16% 7% 1	1710
	11th to 20th	Day	53% 5% 9% 17% 14% 1	2150
	21st to end	Day	60% 9% 10% 16% 5%	3124
January	1st to 10th	Night	60% 6% 8% 14% 10% 2	2459
	11th to 20th	Night	64% 5% 8% 12% 9% 1	2365
	21st to end	Night	61% 4 9% 17% 8% 1	2574
February	1st to 10th	Night	57% 6% 11% 16% 9% 1	2340
	11th to 20th	Night	61% 5% 8% 14% 10% 2	2189
	21st to end	Night	59% 5% 10% 13% 8% 4	1758
March	1st to 10th	Night	50% 4 9% 20% 14% 3	1980
	11th to 20th	Night	54% 7% 9% 20% 9% 1	1980
	21st to end	Night	53% 7% 9% 16% 13% 2	2178
April	1st to 10th	Night	47% 7% 11% 19% 13% 3	1950
	11th to 20th	Night	41% 6% 13% 20% 15% 5%	1765
	21st to end	Night	46% 6% 14% 22% 11% 1	1620
May	1st to 10th	Night	48% 8% 13% 18% 12% 2	1620
	11th to 20th	Night	48% 10% 15% 19% 6% 1	1593
	21st to end	Night	41% 10% 17% 24% 7% 1	1745
June	1st to 10th	Night	36% 11% 17% 23% 12% 1	1620
	11th to 20th	Night	36% 12% 18% 19% 13% 1	1620
	21st to end	Night	35% 11% 15% 24% 13% 1	1620
July	1st to 10th	Night	40% 12% 16% 15% 13% 4	1575
	11th to 20th	Night	32% 11% 18% 22% 13% 3	1566
	21st to end	Night	33% 13% 16% 25% 12% 1	1672
August	1st to 10th	Night	41% 12% 16% 21% 10%	1600
	11th to 20th	Night	39% 11% 15% 23% 11% 1	1598
	21st to end	Night	35% 10% 17% 25% 13% 2	1881
September	1st to 10th	Night	37% 8% 16% 21% 15% 4	1894
	11th to 20th	Night	36% 9% 11% 23% 18% 2	1980
	21st to end	Night	41% 10% 13% 22% 13% 1	1980
October	1st to 10th	Night	39% 7% 12% 22% 17% 3	2110
	11th to 20th	Night	41% 7% 11% 22% 16% 3	2160
	21st to end	Night	45% 6% 8% 23% 15% 4	2376
November	1st to 10th	Night	39% 6% 11% 19% 21% 4	2260
	11th to 20th	Night	48% 5% 9% 17% 17% 4	2470
	21st to end	Night	47% 6% 10% 20% 15% 3	2470
December	1st to 10th	Night	62% 5% 7% 16% 7% 2	2470
	11th to 20th	Night	58% 5% 9% 17% 10% 1	3044
	21st to end	Night	60% 10% 9% 13% 8%	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	35% 27% 25% 9% 4	270
	11th to 20th	Day	37% 27% 28% 8%	245
	21st to end	Day	41% 22% 24% 12% 1	264
February	1st to 10th	Day	39% 28% 24% 8% 2	240
	11th to 20th	Day	51% 22% 20% 6%	240
	21st to end	Day	44% 22% 25% 8% 1	216
March	1st to 10th	Day	31% 24% 28% 14% 4	420
	11th to 20th	Day	31% 25% 26% 12% 4 1	420
	21st to end	Day	31% 22% 25% 17% 5%	462
April	1st to 10th	Day	29% 20% 26% 18% 7%	420
	11th to 20th	Day	26% 16% 29% 19% 9% 2	412
	21st to end	Day	28% 20% 25% 17% 8% 2	420
May	1st to 10th	Day	29% 19% 26% 16% 7% 4	423
	11th to 20th	Day	23% 15% 27% 18% 13% 3	447
	21st to end	Day	17% 9% 25% 24% 17% 8%	485
June	1st to 10th	Day	13% 10% 26% 26% 18% 7%	450
	11th to 20th	Day	14% 10% 25% 26% 17% 8%	450
	21st to end	Day	14% 10% 26% 28% 17% 6%	450
July	1st to 10th	Day	14% 6% 19% 26% 22% 12%	445
	11th to 20th	Day	7% 4 23% 33% 23% 10%	444
	21st to end	Day	12% 5% 20% 30% 22% 10%	488
August	1st to 10th	Day	14% 8% 22% 24% 22% 11%	446
	11th to 20th	Day	14% 6% 18% 27% 25% 11%	439
	21st to end	Day	11% 6% 21% 27% 25% 10%	495
September	1st to 10th	Day	13% 7% 19% 26% 25% 9%	450
	11th to 20th	Day	15% 6% 18% 28% 28% 7%	450
	21st to end	Day	23% 12% 20% 27% 15% 3	450
October	1st to 10th	Day	19% 9% 22% 26% 20% 4	450
	11th to 20th	Day	21% 16% 24% 25% 12% 2	450
	21st to end	Day	26% 16% 24% 21% 12% 1	495
November	1st to 10th	Day	22% 15% 27% 24% 10% 3	413
	11th to 20th	Day	33% 16% 24% 20% 7%	270
	21st to end	Day	27% 20% 28% 21% 4	270
December	1st to 10th	Day	39% 20% 25% 12% 4 1	270
	11th to 20th	Day	31% 22% 25% 17% 4	270
	21st to end	Day	32% 23% 26% 14% 5%	297
January	1st to 10th	Night	29% 23% 27% 18% 2	760
	11th to 20th	Night	35% 18% 25% 20% 3	728
	21st to end	Night	28% 23% 31% 17% 1	792
February	1st to 10th	Night	28% 19% 28% 22% 2	720
	11th to 20th	Night	33% 22% 27% 17%	703
	21st to end	Night	30% 23% 30% 15% 1	592
March	1st to 10th	Night	27% 18% 29% 23% 3	720
	11th to 20th	Night	30% 20% 24% 18% 7%	720
	21st to end	Night	27% 16% 28% 23% 6%	792
April	1st to 10th	Night	24% 19% 24% 28% 5%	720
	11th to 20th	Night	19% 16% 25% 28% 11% 1	701
	21st to end	Night	19% 15% 23% 34% 8% 1	540
May	1st to 10th	Night	23% 12% 24% 28% 12% 1	540
	11th to 20th	Night	19% 11% 22% 31% 16%	531
	21st to end	Night	14% 4 17% 36% 25% 4	584
June	1st to 10th	Night	10% 5% 18% 39% 25% 3	540
	11th to 20th	Night	12% 5% 15% 34% 31% 3	540
	21st to end	Night	12% 3 15% 40% 27% 2	540
July	1st to 10th	Night	12% 2 12% 35% 34% 5%	525
	11th to 20th	Night	7% 2 12% 33% 39% 6%	522
	21st to end	Night	9% 5% 11% 35% 37% 4	591
August	1st to 10th	Night	13% 3 11% 34% 36% 2	537
	11th to 20th	Night	14% 3 9% 35% 35% 3	534
	21st to end	Night	12% 3 10% 37% 33% 6%	693
September	1st to 10th	Night	13% 5% 12% 33% 32% 5%	720
	11th to 20th	Night	14% 5% 15% 32% 31% 3	720
	21st to end	Night	18% 9% 17% 32% 23% 2	720
October	1st to 10th	Night	16% 9% 20% 29% 22% 3	720
	11th to 20th	Night	21% 13% 21% 32% 12% 1	720
	21st to end	Night	24% 14% 21% 29% 12% 1	792
November	1st to 10th	Night	16% 12% 25% 33% 13% 1	740
	11th to 20th	Night	25% 15% 23% 28% 8%	760
	21st to end	Night	22% 16% 22% 31% 7% 1	760
December	1st to 10th	Night	32% 18% 27% 18% 5%	760
	11th to 20th	Night	25% 20% 26% 25% 4	760
	21st to end	Night	28% 17% 28% 24% 3	836

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