Banks
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Banks clear sky chart from 2005-11-06 to 2026-01-17.

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	16% 8% 3 7% 5% 4 4 3 5% 6% 39%	137129
	Night	19% 9% 3 6% 4 4 4 3 4 5% 39%	131877
Transparency	Day	46% 8% 12% 18% 12% 4	131897
	Night	48% 8% 12% 19% 11% 3	128106
Seeing	Day	20% 11% 24% 26% 15% 4	16412
	Night	22% 6% 22% 33% 15% 1	25076

By Month

Percentage Cloud Cover SW (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	36% 14% 4 10% 6% 5% 5% 3 4 4 10%	3719
	11th to 20th	Day	25% 11% 4 7% 5% 4 4 4 5% 6% 27%	3688
	21st to end	Day	25% 10% 3 8% 5% 4 4 3 4 4 30%	4417
February	1st to 10th	Day	27% 13% 4 10% 6% 5% 5% 4 5% 5% 16%	1800
	11th to 20th	Day	28% 14% 4 9% 7% 5% 5% 3 4 3 18%	1755
	21st to end	Day	26% 13% 5% 10% 7% 5% 4 3 5% 4 18%	1545
March	1st to 10th	Day	17% 14% 4 10% 7% 6% 6% 4 7% 6% 19%	4776
	11th to 20th	Day	25% 14% 4 9% 6% 4 4 3 3 3 26%	5000
	21st to end	Day	23% 14% 4 11% 7% 6% 6% 4 6% 6% 12%	5568
April	1st to 10th	Day	25% 12% 3 10% 7% 5% 5% 4 5% 5% 19%	2400
	11th to 20th	Day	21% 11% 3 8% 6% 5% 4 4 5% 5% 28%	2388
	21st to end	Day	19% 9% 4 8% 6% 6% 5% 5% 6% 7% 25%	2600
May	1st to 10th	Day	13% 7% 2 7% 4 4 4 4 5% 6% 43%	6188
	11th to 20th	Day	11% 9% 3 8% 5% 5% 6% 4 6% 7% 36%	6277
	21st to end	Day	14% 7% 3 8% 6% 5% 6% 5% 5% 8% 34%	6902
June	1st to 10th	Day	19% 9% 3 7% 6% 5% 4 3 5% 6% 35%	2800
	11th to 20th	Day	15% 9% 3 9% 6% 5% 6% 5% 6% 8% 29%	2855
	21st to end	Day	8% 5% 2 6% 4 5% 4 3 5% 7% 50%	3000
July	1st to 10th	Day	3 2 1 4 3 2 3 2 3 5% 72%	6554
	11th to 20th	Day	3 2 1 2 2 2 2 1 2 4 80%	6320
	21st to end	Day	5% 2 1 3 2 2 2 2 3 6% 71%	6880
August	1st to 10th	Day	4 3 1 4 3 3 3 2 4 7% 66%	5787
	11th to 20th	Day	6% 4 2 5% 3 4 5% 4 6% 9% 52%	5796
	21st to end	Day	8% 6% 2 5% 3 4 4 3 4 6% 53%	6204
September	1st to 10th	Day	6% 3 2 4 4 3 4 3 4 7% 60%	2400
	11th to 20th	Day	10% 6% 2 7% 4 5% 4 4 5% 8% 45%	2381
	21st to end	Day	16% 5% 2 5% 4 3 4 3 4 7% 46%	2114
October	1st to 10th	Day	13% 6% 2 4 3 4 3 3 5% 7% 49%	3280
	11th to 20th	Day	16% 8% 3 10% 6% 5% 6% 4 7% 6% 29%	3261
	21st to end	Day	25% 11% 3 7% 5% 4 4 4 5% 6% 25%	3501
November	1st to 10th	Day	28% 11% 4 9% 7% 5% 4 4 4 6% 17%	1713
	11th to 20th	Day	31% 12% 5% 7% 5% 4 5% 3 4 5% 19%	1680
	21st to end	Day	27% 12% 4 10% 5% 5% 4 3 5% 4 22%	1680
December	1st to 10th	Day	21% 10% 3 7% 5% 4 3 4 4 5% 33%	2700
	11th to 20th	Day	33% 15% 4 9% 6% 5% 4 3 4 4 12%	3108
	21st to end	Day	34% 16% 5% 12% 7% 5% 4 3 3 3 8%	4092
January	1st to 10th	Night	34% 13% 4 9% 6% 5% 5% 3 4 4 13%	6446
	11th to 20th	Night	24% 11% 3 6% 4 4 3 3 4 5% 33%	6404
	21st to end	Night	27% 11% 2 6% 4 3 3 2 3 4 34%	6556
February	1st to 10th	Night	24% 13% 3 9% 7% 5% 5% 4 5% 5% 21%	2600
	11th to 20th	Night	25% 12% 4 8% 5% 4 4 4 4 4 26%	2532
	21st to end	Night	24% 13% 4 8% 6% 4 5% 4 4 6% 23%	2085
March	1st to 10th	Night	16% 15% 4 8% 5% 7% 6% 4 5% 5% 24%	5116
	11th to 20th	Night	20% 15% 3 7% 5% 4 4 3 4 4 31%	5000
	21st to end	Night	26% 14% 4 10% 7% 6% 5% 3 4 6% 15%	5406
April	1st to 10th	Night	21% 12% 4 9% 5% 5% 5% 4 5% 5% 25%	2000
	11th to 20th	Night	15% 11% 3 8% 5% 5% 4 3 6% 6% 34%	1954
	21st to end	Night	18% 9% 3 6% 5% 4 4 4 5% 8% 34%	1800
May	1st to 10th	Night	15% 6% 2 4 3 3 4 3 4 6% 50%	3763
	11th to 20th	Night	11% 7% 2 6% 4 4 4 4 5% 7% 47%	3616
	21st to end	Night	10% 6% 2 6% 4 5% 5% 4 6% 8% 43%	3972
June	1st to 10th	Night	13% 7% 2 6% 7% 6% 4 4 4 6% 43%	1600
	11th to 20th	Night	11% 7% 3 6% 4 4 4 3 5% 6% 46%	1536
	21st to end	Night	6% 5% 2 4 3 3 3 2 4 6% 61%	1400
July	1st to 10th	Night	2 1 1 1 1 1 2 1 3 4 82%	3397
	11th to 20th	Night	2 2 1 2 2 1 2 1 2 3 82%	3640
	21st to end	Night	2 2 1 3 2 2 2 2 3 5% 76%	3970
August	1st to 10th	Night	3 3 1 3 2 2 3 3 5% 5% 70%	4022
	11th to 20th	Night	9% 4 2 4 2 3 3 3 5% 6% 58%	4052
	21st to end	Night	8% 4 2 5% 3 3 4 3 4 5% 58%	4784
September	1st to 10th	Night	6% 2 1 4 3 3 3 3 4 5% 66%	2000
	11th to 20th	Night	9% 6% 2 5% 5% 4 4 3 4 6% 52%	2015
	21st to end	Night	10% 5% 2 5% 4 4 3 3 5% 6% 55%	2285
October	1st to 10th	Night	10% 4 1 4 3 3 3 3 4 6% 58%	3920
	11th to 20th	Night	16% 7% 3 7% 4 4 4 4 6% 8% 37%	3897
	21st to end	Night	22% 10% 3 7% 4 5% 4 3 4 5% 32%	4306
November	1st to 10th	Night	27% 10% 3 8% 5% 5% 5% 3 5% 5% 24%	2801
	11th to 20th	Night	33% 10% 3 8% 4 5% 4 3 4 4 22%	2940
	21st to end	Night	24% 10% 3 8% 5% 5% 4 4 4 4 28%	2940
December	1st to 10th	Night	19% 11% 3 5% 4 4 3 3 4 6% 38%	4700
	11th to 20th	Night	37% 15% 4 9% 5% 4 4 2 3 4 13%	5316
	21st to end	Night	33% 16% 5% 10% 5% 5% 4 3 4 3 14%	7106

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	78% 7% 5% 6% 3	3719
	11th to 20th	Day	59% 7% 9% 18% 6% 2	3688
	21st to end	Day	58% 6% 7% 12% 15% 3	4417
February	1st to 10th	Day	68% 9% 10% 10% 3	1800
	11th to 20th	Day	70% 8% 6% 10% 5% 1	1764
	21st to end	Day	69% 7% 8% 8% 6% 1	1545
March	1st to 10th	Day	61% 9% 15% 11% 5%	4776
	11th to 20th	Day	63% 6% 7% 19% 6%	5000
	21st to end	Day	68% 11% 11% 7% 2	5568
April	1st to 10th	Day	66% 9% 12% 11% 2	2400
	11th to 20th	Day	57% 8% 11% 13% 9% 2	2391
	21st to end	Day	56% 10% 12% 13% 8% 1	2600
May	1st to 10th	Day	40% 8% 11% 25% 16% 1	6188
	11th to 20th	Day	42% 11% 19% 21% 7% 1	6280
	21st to end	Day	45% 12% 21% 18% 4	6904
June	1st to 10th	Day	50% 8% 15% 18% 8% 1	2800
	11th to 20th	Day	50% 10% 16% 17% 6%	2855
	21st to end	Day	33% 8% 17% 27% 11% 4	3000
July	1st to 10th	Day	16% 4 12% 26% 20% 22%	6485
	11th to 20th	Day	12% 3 8% 22% 41% 14%	6245
	21st to end	Day	16% 5% 17% 34% 25% 4	6681
August	1st to 10th	Day	17% 5% 11% 27% 27% 12%	4215
	11th to 20th	Day	26% 9% 21% 26% 13% 5%	4195
	21st to end	Day	31% 6% 12% 24% 20% 6%	4471
September	1st to 10th	Day	23% 7% 15% 26% 18% 11%	2400
	11th to 20th	Day	38% 9% 14% 18% 15% 6%	2381
	21st to end	Day	38% 8% 14% 20% 16% 4	2114
October	1st to 10th	Day	35% 6% 9% 29% 19% 2	3280
	11th to 20th	Day	52% 9% 13% 14% 12% 1	3261
	21st to end	Day	57% 7% 9% 17% 9% 1	3501
November	1st to 10th	Day	68% 9% 9% 10% 3	1713
	11th to 20th	Day	67% 8% 9% 9% 6% 1	1680
	21st to end	Day	65% 8% 8% 9% 10%	1680
December	1st to 10th	Day	55% 5% 10% 17% 14%	2700
	11th to 20th	Day	74% 7% 10% 8% 1	3108
	21st to end	Day	78% 10% 7% 4 2	4092
January	1st to 10th	Night	73% 9% 8% 6% 4 1	6446
	11th to 20th	Night	54% 7% 8% 20% 8% 1	6404
	21st to end	Night	55% 5% 6% 14% 15% 4	6556
February	1st to 10th	Night	64% 10% 10% 12% 4	2600
	11th to 20th	Night	61% 8% 11% 12% 5% 2	2534
	21st to end	Night	62% 8% 12% 11% 6% 1	2085
March	1st to 10th	Night	58% 8% 13% 15% 6%	5116
	11th to 20th	Night	54% 10% 11% 19% 6%	5000
	21st to end	Night	66% 11% 12% 9% 1	5406
April	1st to 10th	Night	60% 8% 14% 12% 4	2000
	11th to 20th	Night	50% 8% 14% 16% 9% 3	1954
	21st to end	Night	49% 9% 15% 18% 6% 2	1800
May	1st to 10th	Night	34% 8% 11% 32% 14% 1	3760
	11th to 20th	Night	36% 7% 19% 27% 10% 1	3616
	21st to end	Night	36% 12% 30% 18% 4	3972
June	1st to 10th	Night	43% 6% 18% 24% 7% 1	1600
	11th to 20th	Night	39% 8% 20% 27% 7%	1536
	21st to end	Night	25% 6% 20% 33% 12% 4	1400
July	1st to 10th	Night	9% 3 14% 31% 21% 22%	3358
	11th to 20th	Night	11% 3 8% 21% 38% 19%	3592
	21st to end	Night	13% 3 17% 34% 27% 5%	3857
August	1st to 10th	Night	11% 3 11% 31% 30% 13%	2917
	11th to 20th	Night	23% 7% 23% 30% 12% 5%	2929
	21st to end	Night	26% 6% 15% 26% 21% 6%	3442
September	1st to 10th	Night	20% 7% 14% 28% 20% 11%	2000
	11th to 20th	Night	33% 7% 15% 22% 14% 8%	2015
	21st to end	Night	30% 7% 16% 22% 18% 7%	2285
October	1st to 10th	Night	29% 6% 11% 33% 19% 2	3920
	11th to 20th	Night	43% 8% 18% 19% 11% 1	3897
	21st to end	Night	53% 8% 9% 19% 10% 1	4306
November	1st to 10th	Night	61% 9% 10% 14% 6%	2801
	11th to 20th	Night	66% 8% 9% 10% 6% 1	2940
	21st to end	Night	59% 9% 10% 10% 11% 2	2940
December	1st to 10th	Night	48% 6% 12% 21% 14%	4700
	11th to 20th	Night	76% 7% 7% 7% 3	5316
	21st to end	Night	72% 11% 9% 7% 1	7106

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	45% 13% 18% 13% 9% 2	220
	11th to 20th	Day	36% 5% 22% 17% 13% 7%	212
	21st to end	Day	33% 11% 18% 20% 15% 4	399
February	1st to 10th	Day	36% 10% 24% 18% 10% 3	400
	11th to 20th	Day	35% 13% 27% 15% 9% 1	395
	21st to end	Day	35% 16% 22% 19% 7% 2	361
March	1st to 10th	Day	30% 16% 25% 21% 8% 1	500
	11th to 20th	Day	29% 17% 22% 19% 9% 2	500
	21st to end	Day	34% 14% 23% 18% 10% 1	550
April	1st to 10th	Day	30% 14% 20% 23% 8% 4	500
	11th to 20th	Day	24% 19% 27% 18% 8% 4	492
	21st to end	Day	21% 15% 27% 21% 14% 2	500
May	1st to 10th	Day	15% 18% 28% 23% 13% 4	503
	11th to 20th	Day	22% 12% 27% 21% 13% 5%	528
	21st to end	Day	17% 11% 24% 26% 16% 5%	573
June	1st to 10th	Day	20% 11% 26% 25% 16% 3	530
	11th to 20th	Day	13% 17% 33% 27% 9% 2	585
	21st to end	Day	8% 9% 32% 31% 17% 3	730
July	1st to 10th	Day	7% 7% 30% 33% 17% 6%	630
	11th to 20th	Day	5% 9% 26% 35% 19% 6%	524
	21st to end	Day	3 5% 23% 39% 23% 7%	575
August	1st to 10th	Day	6% 4 20% 37% 24% 8%	527
	11th to 20th	Day	5% 3 20% 38% 25% 10%	517
	21st to end	Day	8% 7% 26% 33% 19% 6%	583
September	1st to 10th	Day	7% 6% 25% 35% 23% 4	530
	11th to 20th	Day	13% 6% 20% 33% 21% 6%	530
	21st to end	Day	17% 9% 24% 30% 16% 4	530
October	1st to 10th	Day	19% 10% 23% 26% 19% 4	530
	11th to 20th	Day	20% 11% 23% 24% 17% 5%	530
	21st to end	Day	30% 12% 25% 19% 11% 4	550
November	1st to 10th	Day	32% 11% 23% 22% 11% 2	256
	11th to 20th	Day	37% 11% 19% 17% 10% 5%	220
	21st to end	Day	30% 10% 23% 20% 13% 5%	220
December	1st to 10th	Day	36% 7% 17% 22% 13% 5%	220
	11th to 20th	Day	42% 10% 22% 16% 8% 1	220
	21st to end	Day	37% 13% 16% 23% 10% 2	242
January	1st to 10th	Night	38% 7% 18% 22% 13% 3	840
	11th to 20th	Night	34% 7% 14% 26% 18% 1	824
	21st to end	Night	30% 5% 13% 31% 17% 3	880
February	1st to 10th	Night	29% 8% 20% 28% 14% 2	800
	11th to 20th	Night	30% 9% 18% 26% 16% 1	784
	21st to end	Night	30% 8% 20% 30% 12%	660
March	1st to 10th	Night	28% 8% 20% 31% 13% 1	800
	11th to 20th	Night	27% 6% 19% 35% 13% 1	800
	21st to end	Night	29% 10% 20% 29% 12% 1	880
April	1st to 10th	Night	25% 10% 21% 29% 14% 1	800
	11th to 20th	Night	20% 8% 29% 31% 10% 1	766
	21st to end	Night	21% 8% 25% 33% 12% 1	600
May	1st to 10th	Night	15% 6% 34% 30% 13% 1	458
	11th to 20th	Night	14% 5% 29% 38% 14% 1	394
	21st to end	Night	14% 8% 27% 36% 15%	433
June	1st to 10th	Night	16% 6% 32% 37% 10%	400
	11th to 20th	Night	9% 9% 35% 37% 10% 1	400
	21st to end	Night	8% 4 38% 41% 9%	400
July	1st to 10th	Night	3 4 34% 48% 11% 1	390
	11th to 20th	Night	4 7% 40% 39% 10%	388
	21st to end	Night	3 3 37% 47% 11%	438
August	1st to 10th	Night	4 3 25% 43% 23% 1	584
	11th to 20th	Night	7% 2 18% 49% 23% 2	594
	21st to end	Night	8% 3 30% 40% 19% 1	779
September	1st to 10th	Night	6% 4 26% 46% 16% 2	800
	11th to 20th	Night	11% 3 21% 35% 27% 4	800
	21st to end	Night	11% 6% 22% 41% 19% 2	800
October	1st to 10th	Night	16% 5% 23% 37% 18% 2	800
	11th to 20th	Night	20% 5% 17% 33% 22% 3	800
	21st to end	Night	26% 7% 19% 30% 17% 2	880
November	1st to 10th	Night	31% 7% 15% 34% 13%	820
	11th to 20th	Night	37% 7% 16% 26% 13% 1	840
	21st to end	Night	29% 5% 16% 31% 17% 2	840
December	1st to 10th	Night	28% 6% 18% 30% 16% 2	840
	11th to 20th	Night	39% 6% 17% 25% 12% 1	840
	21st to end	Night	38% 9% 18% 24% 10% 1	924

Page generated at 2026-01-13 22:14:28. Version ac07

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