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 2025-07-21.

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	9% 9% 3 6% 4 4 4 3 4 6% 49%	123821
	Night	11% 10% 3 6% 4 4 4 3 5% 6% 47%	124104
Transparency	Day	35% 6% 10% 19% 19% 11%	123566
	Night	37% 6% 11% 20% 16% 9%	123946
Seeing	Day	14% 12% 28% 29% 15% 3	15294
	Night	15% 4 18% 39% 21% 2	24584

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	14% 22% 7% 8% 5% 4 4 3 4 5% 24%	3639
	11th to 20th	Day	11% 14% 5% 7% 4 4 3 3 5% 6% 40%	3640
	21st to end	Day	15% 15% 3 6% 3 2 2 2 3 6% 42%	4004
February	1st to 10th	Day	16% 18% 5% 9% 6% 4 4 2 4 5% 28%	1736
	11th to 20th	Day	13% 20% 4 7% 6% 4 4 3 5% 5% 28%	1950
	21st to end	Day	13% 19% 6% 10% 5% 5% 4 4 5% 5% 25%	1650
March	1st to 10th	Day	8% 15% 7% 9% 6% 5% 5% 4 6% 6% 29%	4518
	11th to 20th	Day	9% 15% 4 8% 5% 4 4 3 4 4 41%	4818
	21st to end	Day	12% 17% 4 10% 6% 7% 6% 4 5% 6% 23%	5737
April	1st to 10th	Day	13% 14% 4 8% 6% 5% 5% 4 5% 6% 32%	2400
	11th to 20th	Day	10% 11% 3 7% 4 5% 5% 4 4 7% 39%	2427
	21st to end	Day	11% 10% 3 7% 5% 5% 4 4 6% 6% 40%	2600
May	1st to 10th	Day	7% 10% 2 6% 4 4 3 3 4 5% 52%	5880
	11th to 20th	Day	9% 7% 2 5% 4 4 4 3 5% 6% 49%	6057
	21st to end	Day	6% 6% 2 4 3 4 4 3 6% 7% 56%	6902
June	1st to 10th	Day	11% 7% 2 6% 5% 5% 6% 5% 6% 7% 40%	2800
	11th to 20th	Day	11% 5% 2 4 4 4 4 4 6% 8% 49%	2800
	21st to end	Day	4 3 1 4 3 3 3 4 4 5% 67%	2800
July	1st to 10th	Day	1 1 1 2 1 2 2 2 3 4 81%	4560
	11th to 20th	Day	2 1 1 2 2 2 3 2 3 4 78%	4560
	21st to end	Day	3 2 1 3 2 2 3 2 3 5% 75%	4791
August	1st to 10th	Day	5% 4 2 6% 3 3 4 3 4 6% 61%	4248
	11th to 20th	Day	2 1 1 2 2 2 2 2 3 5% 77%	4028
	21st to end	Day	3 3 1 3 2 2 3 2 3 4 73%	4225
September	1st to 10th	Day	4 2 1 2 2 3 2 3 4 5% 72%	2280
	11th to 20th	Day	7% 4 2 4 3 3 3 3 5% 6% 60%	2104
	21st to end	Day	7% 4 2 3 3 3 3 3 4 5% 62%	2090
October	1st to 10th	Day	7% 5% 2 4 3 2 2 2 4 5% 65%	3490
	11th to 20th	Day	10% 6% 2 6% 3 3 4 3 5% 6% 52%	3020
	21st to end	Day	12% 11% 3 6% 4 4 3 3 5% 6% 43%	3101
November	1st to 10th	Day	18% 12% 3 6% 4 4 4 3 6% 6% 35%	1764
	11th to 20th	Day	16% 15% 3 7% 4 3 4 3 5% 7% 33%	1800
	21st to end	Day	13% 12% 3 8% 5% 4 5% 3 5% 7% 35%	1750
December	1st to 10th	Day	12% 9% 3 5% 3 2 3 2 4 6% 51%	2620
	11th to 20th	Day	19% 18% 5% 8% 5% 5% 4 4 5% 5% 20%	3028
	21st to end	Day	17% 17% 4 8% 4 4 4 3 5% 6% 29%	4004
January	1st to 10th	Night	17% 18% 4 8% 4 5% 4 3 5% 5% 26%	6306
	11th to 20th	Night	15% 15% 5% 6% 3 3 3 2 4 5% 39%	6320
	21st to end	Night	14% 13% 3 7% 4 3 4 3 4 5% 40%	6952
February	1st to 10th	Night	17% 16% 4 8% 5% 3 4 3 4 5% 31%	2657
	11th to 20th	Night	14% 16% 4 7% 5% 4 4 3 4 5% 34%	2338
	21st to end	Night	14% 18% 6% 7% 4 5% 5% 3 4 6% 30%	1980
March	1st to 10th	Night	9% 16% 4 6% 4 3 4 3 5% 7% 39%	5376
	11th to 20th	Night	13% 16% 3 6% 4 3 3 3 4 5% 41%	5181
	21st to end	Night	11% 13% 3 8% 5% 5% 6% 5% 7% 8% 30%	5236
April	1st to 10th	Night	13% 11% 2 5% 3 4 4 4 5% 7% 42%	2000
	11th to 20th	Night	12% 8% 3 6% 5% 4 4 4 5% 4 47%	1921
	21st to end	Night	9% 7% 2 6% 4 4 5% 3 5% 6% 49%	1800
May	1st to 10th	Night	9% 7% 1 3 2 3 4 3 5% 7% 57%	4100
	11th to 20th	Night	8% 4 2 7% 4 4 5% 3 5% 7% 52%	3830
	21st to end	Night	6% 5% 1 5% 4 3 3 2 3 5% 62%	3972
June	1st to 10th	Night	11% 5% 2 5% 4 4 4 4 6% 8% 46%	1600
	11th to 20th	Night	9% 5% 1 4 4 4 3 3 4 5% 58%	1600
	21st to end	Night	3 4 1 3 3 3 4 4 6% 5% 66%	1600
July	1st to 10th	Night	1 1 2 2 2 2 3 4 4 79%	2620
	11th to 20th	Night	1 1 1 2 2 3 2 4 5% 78%	2620
	21st to end	Night	2 3 1 3 3 3 3 3 5% 7% 67%	2761
August	1st to 10th	Night	4 3 1 5% 4 4 5% 4 5% 6% 59%	2684
	11th to 20th	Night	1 1 1 2 2 2 3 3 4 5% 75%	2811
	21st to end	Night	4 3 1 3 1 2 2 2 2 4 75%	3450
September	1st to 10th	Night	3 2 1 3 2 2 2 2 4 4 74%	1900
	11th to 20th	Night	6% 4 1 5% 3 3 3 3 4 6% 61%	2076
	21st to end	Night	8% 4 1 4 2 2 3 3 4 5% 66%	2090
October	1st to 10th	Night	5% 3 1 4 3 3 3 2 4 5% 67%	3490
	11th to 20th	Night	9% 5% 1 4 3 4 3 3 4 6% 58%	3907
	21st to end	Night	11% 8% 2 6% 4 4 4 4 6% 7% 45%	4457
November	1st to 10th	Night	17% 9% 3 6% 4 4 3 3 5% 7% 39%	2535
	11th to 20th	Night	20% 9% 3 6% 5% 4 4 4 5% 6% 35%	2600
	21st to end	Night	14% 12% 3 6% 4 4 5% 3 4 6% 40%	2646
December	1st to 10th	Night	10% 10% 2 5% 3 3 3 3 4 7% 49%	4560
	11th to 20th	Night	18% 19% 5% 9% 5% 5% 4 5% 5% 6% 20%	5176
	21st to end	Night	18% 15% 4 8% 5% 4 5% 3 5% 6% 27%	6952

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	61% 9% 12% 14% 3	3639
	11th to 20th	Day	45% 6% 18% 24% 6%	3640
	21st to end	Day	44% 7% 13% 20% 12% 3	4004
February	1st to 10th	Day	58% 8% 14% 15% 4 1	1736
	11th to 20th	Day	56% 8% 15% 15% 6%	1960
	21st to end	Day	58% 8% 11% 16% 6% 1	1650
March	1st to 10th	Day	50% 7% 11% 19% 10% 3	4518
	11th to 20th	Day	44% 6% 9% 30% 10%	4818
	21st to end	Day	54% 10% 15% 16% 5%	5737
April	1st to 10th	Day	49% 7% 15% 19% 9% 1	2400
	11th to 20th	Day	42% 6% 13% 22% 15% 2	2430
	21st to end	Day	41% 7% 14% 23% 14%	2600
May	1st to 10th	Day	32% 6% 8% 26% 25% 4	5880
	11th to 20th	Day	31% 5% 9% 26% 22% 6%	6060
	21st to end	Day	24% 6% 11% 29% 25% 4	6904
June	1st to 10th	Day	39% 8% 13% 23% 17% 1	2800
	11th to 20th	Day	31% 6% 10% 22% 23% 9%	2800
	21st to end	Day	19% 4 7% 18% 35% 17%	2800
July	1st to 10th	Day	9% 2 4 10% 20% 54%	4491
	11th to 20th	Day	11% 1 4 6% 22% 55%	4485
	21st to end	Day	13% 2 4 9% 30% 42%	4640
August	1st to 10th	Day	24% 3 7% 9% 26% 31%	4232
	11th to 20th	Day	11% 2 4 13% 44% 26%	4067
	21st to end	Day	16% 2 4 19% 33% 25%	4224
September	1st to 10th	Day	15% 4 6% 17% 32% 26%	2280
	11th to 20th	Day	23% 5% 10% 20% 28% 13%	2104
	21st to end	Day	23% 4 9% 26% 28% 10%	2090
October	1st to 10th	Day	24% 4 7% 17% 39% 9%	3490
	11th to 20th	Day	33% 4 12% 24% 25% 3	3020
	21st to end	Day	41% 6% 9% 25% 19% 1	3101
November	1st to 10th	Day	48% 6% 13% 21% 11% 1	1764
	11th to 20th	Day	50% 8% 17% 20% 6%	1800
	21st to end	Day	48% 6% 16% 18% 10% 1	1750
December	1st to 10th	Day	35% 6% 17% 38% 4	2620
	11th to 20th	Day	64% 8% 15% 11% 2	3028
	21st to end	Day	56% 6% 16% 17% 5%	4004
January	1st to 10th	Night	58% 9% 15% 14% 4	6306
	11th to 20th	Night	48% 5% 15% 21% 9% 2	6320
	21st to end	Night	44% 8% 11% 22% 10% 4	6952
February	1st to 10th	Night	56% 7% 15% 17% 4 1	2657
	11th to 20th	Night	50% 7% 14% 18% 9% 1	2339
	21st to end	Night	54% 8% 11% 18% 9% 1	1980
March	1st to 10th	Night	42% 6% 10% 26% 13% 3	5376
	11th to 20th	Night	45% 5% 7% 26% 16% 1	5181
	21st to end	Night	45% 12% 18% 19% 7%	5236
April	1st to 10th	Night	40% 7% 13% 25% 14% 2	2000
	11th to 20th	Night	38% 7% 12% 25% 17% 2	1921
	21st to end	Night	34% 6% 15% 26% 18% 1	1800
May	1st to 10th	Night	26% 5% 9% 27% 28% 4	4097
	11th to 20th	Night	30% 6% 10% 27% 25% 3	3830
	21st to end	Night	24% 5% 8% 32% 25% 5%	3972
June	1st to 10th	Night	33% 8% 13% 28% 16% 1	1600
	11th to 20th	Night	28% 5% 9% 24% 27% 8%	1600
	21st to end	Night	18% 5% 8% 19% 34% 17%	1600
July	1st to 10th	Night	9% 3 4 9% 22% 53%	2580
	11th to 20th	Night	10% 2 4 8% 20% 57%	2572
	21st to end	Night	16% 2 5% 8% 29% 39%	2674
August	1st to 10th	Night	24% 4 7% 8% 28% 30%	2676
	11th to 20th	Night	11% 2 5% 11% 36% 35%	2838
	21st to end	Night	15% 2 4 17% 31% 30%	3450
September	1st to 10th	Night	15% 3 5% 15% 32% 30%	1900
	11th to 20th	Night	24% 4 8% 17% 28% 19%	2076
	21st to end	Night	21% 4 6% 24% 29% 15%	2090
October	1st to 10th	Night	21% 3 7% 19% 33% 18%	3490
	11th to 20th	Night	28% 3 12% 23% 28% 6%	3907
	21st to end	Night	37% 5% 12% 24% 19% 2	4457
November	1st to 10th	Night	43% 6% 13% 22% 15% 1	2535
	11th to 20th	Night	49% 7% 13% 23% 9%	2600
	21st to end	Night	45% 6% 14% 24% 10% 2	2646
December	1st to 10th	Night	35% 5% 23% 30% 7%	4560
	11th to 20th	Night	63% 6% 13% 15% 2	5176
	21st to end	Night	56% 7% 17% 15% 5%	6952

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	20% 18% 26% 23% 13% 2	200
	11th to 20th	Day	20% 10% 25% 24% 14% 8%	200
	21st to end	Day	19% 10% 24% 29% 13% 5%	220
February	1st to 10th	Day	27% 10% 33% 17% 12% 2	288
	11th to 20th	Day	22% 13% 31% 25% 8% 1	495
	21st to end	Day	21% 17% 28% 25% 8% 1	414
March	1st to 10th	Day	18% 19% 32% 24% 7%	500
	11th to 20th	Day	20% 14% 28% 24% 12% 2	500
	21st to end	Day	21% 18% 30% 22% 8% 1	550
April	1st to 10th	Day	21% 17% 27% 22% 13% 1	500
	11th to 20th	Day	17% 20% 30% 22% 9% 1	492
	21st to end	Day	14% 19% 31% 25% 10% 2	500
May	1st to 10th	Day	12% 16% 33% 25% 12% 2	503
	11th to 20th	Day	22% 13% 28% 23% 13% 2	528
	21st to end	Day	11% 12% 24% 31% 17% 5%	573
June	1st to 10th	Day	11% 13% 29% 27% 17% 3	530
	11th to 20th	Day	13% 17% 32% 25% 11% 2	530
	21st to end	Day	5% 9% 33% 31% 18% 3	530
July	1st to 10th	Day	4 6% 34% 38% 16% 3	526
	11th to 20th	Day	1 10% 32% 40% 15% 2	524
	21st to end	Day	2 4 30% 41% 19% 3	532
August	1st to 10th	Day	5% 7% 31% 36% 18% 2	487
	11th to 20th	Day	2 6% 27% 38% 23% 3	477
	21st to end	Day	6% 9% 24% 36% 22% 2	539
September	1st to 10th	Day	6% 6% 23% 40% 22% 3	490
	11th to 20th	Day	9% 9% 25% 34% 21% 1	490
	21st to end	Day	8% 8% 28% 34% 18% 4	490
October	1st to 10th	Day	11% 10% 22% 31% 23% 4	490
	11th to 20th	Day	14% 7% 25% 30% 17% 6%	371
	21st to end	Day	18% 12% 22% 32% 13% 4	330
November	1st to 10th	Day	22% 10% 23% 29% 15% 2	300
	11th to 20th	Day	18% 10% 25% 30% 14% 3	300
	21st to end	Day	17% 8% 24% 29% 19% 4	275
December	1st to 10th	Day	31% 7% 17% 18% 20% 9%	200
	11th to 20th	Day	27% 10% 23% 27% 12% 2	200
	21st to end	Day	20% 13% 24% 28% 13% 2	220
January	1st to 10th	Night	26% 5% 20% 31% 15% 3	800
	11th to 20th	Night	24% 6% 20% 28% 19% 4	800
	21st to end	Night	21% 3 19% 34% 21% 3	880
February	1st to 10th	Night	24% 7% 27% 30% 12% 1	800
	11th to 20th	Night	21% 5% 22% 35% 15% 2	784
	21st to end	Night	21% 7% 22% 36% 13% 2	660
March	1st to 10th	Night	21% 6% 24% 35% 13% 2	800
	11th to 20th	Night	18% 6% 20% 38% 18% 1	800
	21st to end	Night	19% 7% 24% 38% 12% 1	880
April	1st to 10th	Night	17% 7% 21% 36% 17% 2	800
	11th to 20th	Night	14% 8% 27% 37% 13% 1	733
	21st to end	Night	11% 6% 23% 44% 17% 1	600
May	1st to 10th	Night	10% 6% 21% 43% 20%	600
	11th to 20th	Night	10% 6% 18% 34% 29% 2	491
	21st to end	Night	11% 3 18% 37% 28% 3	433
June	1st to 10th	Night	12% 5% 13% 44% 25% 3	400
	11th to 20th	Night	10% 3 17% 45% 24% 1	400
	21st to end	Night	4 2 12% 51% 32% 1	400
July	1st to 10th	Night	1 2 14% 48% 34% 2	390
	11th to 20th	Night	2 1 10% 62% 25% 1	388
	21st to end	Night	4 1 7% 52% 34% 1	416
August	1st to 10th	Night	5% 2 9% 50% 32% 2	468
	11th to 20th	Night	1 1 9% 48% 38% 2	564
	21st to end	Night	4 2 12% 46% 33% 3	801
September	1st to 10th	Night	4 1 9% 52% 32% 2	760
	11th to 20th	Night	7% 3 11% 42% 34% 4	760
	21st to end	Night	9% 2 15% 39% 29% 6%	760
October	1st to 10th	Night	10% 2 16% 43% 26% 2	760
	11th to 20th	Night	12% 4 16% 41% 25% 3	760
	21st to end	Night	15% 3 21% 39% 19% 3	836
November	1st to 10th	Night	21% 4 19% 37% 17% 2	780
	11th to 20th	Night	25% 4 20% 34% 16% 1	800
	21st to end	Night	19% 5% 21% 37% 15% 4	800
December	1st to 10th	Night	21% 4 21% 34% 17% 3	800
	11th to 20th	Night	27% 5% 19% 34% 13% 2	800
	21st to end	Night	30% 5% 24% 30% 10% 1	880

Page generated at 2025-07-17 23:09:59. Version ac07

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