Banff
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Banff clear sky chart from 2005-11-06 to 2026-08-15.

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	5% 9% 4 10% 7% 6% 6% 5% 7% 8% 33%	148546
	Night	3 7% 4 10% 7% 7% 6% 5% 8% 9% 33%	142037
Transparency	Day	42% 10% 19% 19% 9% 1	148259
	Night	38% 12% 22% 20% 8% 1	141908
Seeing	Day	17% 21% 33% 20% 7% 1	17576
	Night	10% 19% 38% 24% 8% 1	26489

By Month

Percentage Cloud Cover NW (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	3 3 16% 12% 10% 9% 6% 10% 10% 21%	4125
	11th to 20th	Day	4 3 10% 8% 7% 7% 4 6% 7% 44%	4435
	21st to end	Day	4 3 8% 7% 6% 7% 5% 8% 12% 38%	5186
February	1st to 10th	Day	1 8% 5% 11% 10% 9% 8% 6% 10% 8% 24%	1680
	11th to 20th	Day	1 8% 4 15% 8% 8% 7% 5% 7% 10% 26%	1789
	21st to end	Day	1 9% 6% 17% 10% 8% 6% 5% 8% 8% 21%	1730
March	1st to 10th	Day	7% 6% 18% 9% 7% 6% 4 5% 8% 31%	4618
	11th to 20th	Day	2 9% 6% 12% 8% 8% 9% 6% 7% 7% 27%	4924
	21st to end	Day	2 18% 9% 17% 8% 6% 6% 5% 6% 6% 16%	5867
April	1st to 10th	Day	4 17% 8% 16% 9% 7% 6% 4 5% 6% 18%	2520
	11th to 20th	Day	4 18% 9% 16% 7% 6% 6% 4 5% 6% 20%	2698
	21st to end	Day	7% 20% 7% 14% 6% 5% 5% 4 5% 6% 21%	2730
May	1st to 10th	Day	7% 14% 4 7% 4 4 4 4 7% 9% 36%	8136
	11th to 20th	Day	8% 16% 5% 12% 7% 8% 7% 5% 7% 7% 19%	8655
	21st to end	Day	11% 13% 3 9% 6% 5% 6% 4 5% 9% 29%	9649
June	1st to 10th	Day	17% 15% 4 10% 6% 5% 5% 4 6% 6% 23%	3150
	11th to 20th	Day	19% 14% 4 10% 7% 7% 6% 5% 5% 6% 18%	3150
	21st to end	Day	12% 10% 3 11% 7% 6% 6% 5% 6% 8% 26%	3150
July	1st to 10th	Day	5% 4 2 6% 5% 5% 6% 5% 7% 8% 48%	8820
	11th to 20th	Day	7% 4 2 6% 6% 5% 5% 5% 8% 9% 44%	8820
	21st to end	Day	5% 4 2 5% 4 5% 5% 5% 8% 10% 47%	9093
August	1st to 10th	Day	6% 5% 3 8% 6% 6% 7% 5% 7% 8% 38%	4681
	11th to 20th	Day	7% 5% 2 7% 5% 6% 6% 6% 8% 10% 38%	4368
	21st to end	Day	8% 6% 3 7% 5% 5% 6% 5% 7% 9% 41%	4664
September	1st to 10th	Day	9% 9% 3 8% 6% 4 4 3 6% 7% 42%	2415
	11th to 20th	Day	9% 9% 3 9% 5% 6% 4 3 5% 8% 39%	2240
	21st to end	Day	7% 11% 3 8% 6% 6% 4 5% 6% 8% 37%	2200
October	1st to 10th	Day	7% 9% 3 9% 6% 5% 5% 4 6% 7% 39%	3491
	11th to 20th	Day	3 7% 3 9% 7% 6% 7% 5% 8% 11% 34%	2932
	21st to end	Day	3 9% 3 9% 7% 6% 7% 5% 7% 7% 37%	3200
November	1st to 10th	Day	3 10% 4 14% 8% 6% 6% 5% 8% 10% 26%	1796
	11th to 20th	Day	1 7% 4 10% 7% 7% 7% 6% 11% 9% 31%	1517
	21st to end	Day	1 9% 3 11% 7% 6% 7% 6% 10% 11% 30%	1470
December	1st to 10th	Day	1 4 4 10% 7% 6% 6% 6% 9% 11% 36%	2360
	11th to 20th	Day	5% 4 13% 9% 8% 8% 6% 10% 12% 26%	2712
	21st to end	Day	6% 3 9% 8% 9% 9% 6% 9% 11% 30%	3575
January	1st to 10th	Night	5% 4 19% 11% 10% 10% 6% 7% 8% 20%	8768
	11th to 20th	Night	3 3 10% 9% 7% 7% 6% 8% 9% 37%	8508
	21st to end	Night	5% 4 11% 8% 7% 7% 6% 8% 10% 33%	8994
February	1st to 10th	Night	2 8% 4 14% 10% 9% 9% 6% 9% 10% 19%	2940
	11th to 20th	Night	1 8% 6% 15% 7% 8% 7% 6% 8% 9% 26%	2714
	21st to end	Night	8% 5% 14% 11% 8% 7% 7% 9% 7% 25%	2076
March	1st to 10th	Night	7% 6% 14% 9% 8% 7% 6% 8% 9% 28%	5496
	11th to 20th	Night	1 7% 4 9% 6% 7% 8% 7% 8% 10% 30%	5295
	21st to end	Night	2 16% 8% 16% 10% 7% 7% 5% 6% 7% 17%	5346
April	1st to 10th	Night	3 13% 5% 12% 8% 7% 5% 4 7% 9% 26%	2100
	11th to 20th	Night	2 12% 5% 12% 6% 5% 5% 5% 7% 9% 30%	1872
	21st to end	Night	4 16% 6% 11% 6% 5% 4 5% 6% 6% 30%	1890
May	1st to 10th	Night	6% 14% 3 8% 4 4 3 3 5% 7% 43%	4784
	11th to 20th	Night	5% 13% 4 9% 6% 5% 6% 5% 8% 10% 27%	4190
	21st to end	Night	8% 9% 3 7% 5% 4 5% 4 7% 9% 39%	4522
June	1st to 10th	Night	13% 11% 3 9% 4 4 5% 4 7% 7% 33%	1470
	11th to 20th	Night	13% 13% 3 7% 6% 5% 4 5% 6% 9% 30%	1470
	21st to end	Night	8% 5% 2 6% 5% 4 5% 4 8% 11% 43%	1470
July	1st to 10th	Night	4 4 2 5% 4 4 5% 4 7% 8% 53%	4140
	11th to 20th	Night	5% 3 2 4 3 4 4 4 6% 8% 57%	4140
	21st to end	Night	5% 5% 2 6% 4 3 3 3 5% 8% 56%	4971
August	1st to 10th	Night	7% 7% 3 8% 5% 5% 5% 4 6% 9% 41%	2689
	11th to 20th	Night	6% 4 2 5% 4 5% 5% 5% 7% 11% 48%	2773
	21st to end	Night	7% 6% 2 5% 4 4 4 4 6% 8% 51%	3245
September	1st to 10th	Night	7% 7% 2 5% 3 4 4 4 6% 9% 50%	1982
	11th to 20th	Night	8% 8% 3 5% 5% 4 5% 4 6% 7% 43%	2159
	21st to end	Night	6% 8% 3 5% 4 5% 4 4 7% 9% 45%	2200
October	1st to 10th	Night	4 8% 3 9% 5% 4 5% 4 6% 8% 42%	3703
	11th to 20th	Night	2 7% 3 9% 6% 6% 6% 4 7% 9% 41%	4216
	21st to end	Night	2 9% 4 10% 6% 6% 5% 5% 7% 9% 36%	4600
November	1st to 10th	Night	2 12% 5% 12% 9% 7% 6% 5% 10% 10% 21%	2701
	11th to 20th	Night	1 7% 5% 12% 9% 9% 9% 6% 8% 10% 24%	3100
	21st to end	Night	9% 5% 11% 9% 9% 7% 7% 9% 10% 23%	3150
December	1st to 10th	Night	1 6% 3 10% 7% 7% 7% 6% 11% 13% 30%	5040
	11th to 20th	Night	4 4 14% 12% 11% 9% 8% 10% 11% 17%	5700
	21st to end	Night	1 7% 3 12% 10% 10% 10% 7% 9% 10% 21%	7623

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	40% 14% 25% 16% 5% 1	4125
	11th to 20th	Day	36% 10% 21% 18% 12% 3	4458
	21st to end	Day	29% 18% 27% 18% 7%	5187
February	1st to 10th	Day	47% 14% 18% 11% 7% 3	1680
	11th to 20th	Day	49% 12% 16% 15% 7% 1	1797
	21st to end	Day	55% 11% 16% 11% 6% 1	1730
March	1st to 10th	Day	46% 10% 16% 15% 13%	4618
	11th to 20th	Day	44% 10% 20% 21% 5%	4925
	21st to end	Day	59% 11% 14% 10% 5%	5867
April	1st to 10th	Day	63% 10% 16% 9% 2	2520
	11th to 20th	Day	61% 9% 15% 11% 4	2698
	21st to end	Day	61% 9% 13% 14% 2	2730
May	1st to 10th	Day	42% 6% 23% 22% 7%	8136
	11th to 20th	Day	54% 9% 18% 15% 4	8659
	21st to end	Day	47% 9% 23% 19% 2	9650
June	1st to 10th	Day	59% 9% 11% 15% 5% 1	3150
	11th to 20th	Day	63% 10% 11% 12% 4	3150
	21st to end	Day	51% 10% 16% 15% 7% 1	3150
July	1st to 10th	Day	30% 6% 12% 26% 22% 4	8746
	11th to 20th	Day	33% 6% 13% 25% 15% 8%	8739
	21st to end	Day	28% 5% 16% 29% 19% 3	8897
August	1st to 10th	Day	37% 9% 14% 26% 12% 1	4664
	11th to 20th	Day	36% 8% 22% 26% 7% 1	4410
	21st to end	Day	34% 8% 24% 25% 9% 1	4663
September	1st to 10th	Day	40% 8% 17% 22% 11% 2	2415
	11th to 20th	Day	42% 9% 14% 23% 10% 1	2240
	21st to end	Day	42% 11% 19% 20% 8% 1	2200
October	1st to 10th	Day	41% 8% 19% 26% 6%	3491
	11th to 20th	Day	34% 10% 28% 20% 7% 1	2932
	21st to end	Day	37% 9% 26% 23% 5%	3200
November	1st to 10th	Day	48% 16% 20% 10% 5%	1796
	11th to 20th	Day	37% 20% 24% 13% 6%	1517
	21st to end	Day	41% 16% 22% 14% 6% 1	1470
December	1st to 10th	Day	35% 20% 29% 13% 4	2360
	11th to 20th	Day	38% 17% 28% 13% 4	2714
	21st to end	Day	33% 13% 23% 20% 10% 1	3575
January	1st to 10th	Night	45% 12% 23% 14% 5% 1	8769
	11th to 20th	Night	36% 11% 21% 15% 12% 5%	8547
	21st to end	Night	33% 18% 23% 19% 6% 1	8994
February	1st to 10th	Night	50% 16% 13% 12% 7% 2	2940
	11th to 20th	Night	47% 13% 19% 12% 8% 1	2716
	21st to end	Night	49% 10% 17% 15% 8% 1	2076
March	1st to 10th	Night	41% 15% 21% 14% 9%	5496
	11th to 20th	Night	35% 11% 21% 23% 9%	5295
	21st to end	Night	54% 12% 15% 13% 6%	5346
April	1st to 10th	Night	49% 13% 20% 14% 4	2100
	11th to 20th	Night	46% 13% 22% 13% 6%	1872
	21st to end	Night	51% 10% 16% 17% 5% 1	1890
May	1st to 10th	Night	40% 6% 18% 31% 4 1	4781
	11th to 20th	Night	43% 10% 20% 23% 3	4190
	21st to end	Night	36% 10% 27% 23% 4	4526
June	1st to 10th	Night	48% 11% 14% 20% 6%	1470
	11th to 20th	Night	50% 10% 15% 16% 8%	1470
	21st to end	Night	34% 11% 22% 25% 8% 1	1470
July	1st to 10th	Night	26% 5% 17% 31% 17% 4	4105
	11th to 20th	Night	22% 5% 20% 28% 17% 7%	4098
	21st to end	Night	25% 5% 18% 33% 16% 3	4863
August	1st to 10th	Night	37% 8% 16% 29% 9% 1	2681
	11th to 20th	Night	26% 8% 32% 27% 6%	2797
	21st to end	Night	28% 8% 32% 26% 7%	3245
September	1st to 10th	Night	29% 10% 22% 27% 11% 1	1982
	11th to 20th	Night	35% 12% 19% 23% 10% 1	2159
	21st to end	Night	32% 12% 23% 24% 9%	2200
October	1st to 10th	Night	34% 8% 24% 25% 9%	3703
	11th to 20th	Night	31% 11% 26% 25% 7%	4216
	21st to end	Night	35% 10% 26% 25% 4	4600
November	1st to 10th	Night	48% 18% 17% 11% 5% 1	2701
	11th to 20th	Night	44% 18% 20% 11% 6% 1	3100
	21st to end	Night	43% 17% 20% 13% 5% 2	3150
December	1st to 10th	Night	34% 17% 33% 11% 4	5040
	11th to 20th	Night	43% 20% 23% 10% 4	5697
	21st to end	Night	38% 15% 23% 16% 6% 1	7623

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	13% 21% 37% 21% 5% 2	220
	11th to 20th	Day	10% 23% 47% 14% 6%	220
	21st to end	Day	10% 29% 41% 14% 5% 2	242
February	1st to 10th	Day	22% 24% 34% 14% 5% 1	220
	11th to 20th	Day	11% 19% 44% 21% 5% 1	312
	21st to end	Day	20% 25% 32% 17% 4 1	446
March	1st to 10th	Day	18% 29% 31% 16% 4 2	540
	11th to 20th	Day	17% 27% 34% 16% 4 2	540
	21st to end	Day	20% 25% 33% 16% 5% 2	594
April	1st to 10th	Day	18% 26% 36% 14% 5%	540
	11th to 20th	Day	15% 26% 32% 19% 6% 1	532
	21st to end	Day	17% 24% 31% 18% 7% 2	540
May	1st to 10th	Day	19% 19% 29% 22% 10% 1	543
	11th to 20th	Day	18% 12% 31% 25% 11% 2	738
	21st to end	Day	23% 14% 25% 27% 10% 2	844
June	1st to 10th	Day	23% 21% 30% 19% 7% 1	780
	11th to 20th	Day	25% 21% 28% 19% 7% 1	780
	21st to end	Day	16% 16% 32% 24% 10% 2	780
July	1st to 10th	Day	15% 15% 29% 25% 14% 2	771
	11th to 20th	Day	16% 18% 31% 24% 9% 1	768
	21st to end	Day	9% 19% 35% 26% 10% 2	681
August	1st to 10th	Day	13% 16% 35% 25% 9% 2	567
	11th to 20th	Day	12% 19% 37% 22% 9% 2	534
	21st to end	Day	15% 20% 33% 20% 10% 2	583
September	1st to 10th	Day	17% 22% 31% 18% 9% 3	530
	11th to 20th	Day	18% 19% 36% 19% 7% 1	530
	21st to end	Day	27% 28% 26% 16% 2 1	530
October	1st to 10th	Day	23% 20% 31% 17% 9% 1	501
	11th to 20th	Day	19% 26% 28% 22% 5% 1	330
	21st to end	Day	20% 27% 36% 14% 2 1	363
November	1st to 10th	Day	16% 32% 35% 13% 3 1	330
	11th to 20th	Day	16% 26% 47% 8% 3 1	245
	21st to end	Day	15% 25% 32% 15% 10% 3	220
December	1st to 10th	Day	13% 23% 40% 17% 5% 3	220
	11th to 20th	Day	11% 28% 41% 14% 5%	220
	21st to end	Day	8% 25% 42% 21% 3 1	242
January	1st to 10th	Night	10% 26% 47% 15% 2 1	1050
	11th to 20th	Night	7% 26% 46% 17% 3 1	959
	21st to end	Night	7% 28% 46% 16% 4	924
February	1st to 10th	Night	11% 22% 39% 21% 7%	840
	11th to 20th	Night	7% 20% 45% 23% 5%	824
	21st to end	Night	9% 19% 41% 22% 8% 1	692
March	1st to 10th	Night	7% 20% 44% 22% 6% 1	840
	11th to 20th	Night	8% 20% 46% 20% 5% 1	840
	21st to end	Night	8% 19% 38% 24% 9% 1	924
April	1st to 10th	Night	10% 19% 35% 28% 7%	840
	11th to 20th	Night	7% 17% 38% 30% 7%	624
	21st to end	Night	11% 18% 35% 26% 9% 1	630
May	1st to 10th	Night	13% 10% 30% 37% 10%	441
	11th to 20th	Night	11% 9% 27% 34% 19% 1	414
	21st to end	Night	20% 7% 25% 30% 17% 2	455
June	1st to 10th	Night	18% 13% 32% 25% 11% 1	420
	11th to 20th	Night	21% 11% 30% 27% 10%	420
	21st to end	Night	10% 12% 25% 34% 19%	420
July	1st to 10th	Night	9% 7% 24% 41% 18%	410
	11th to 20th	Night	10% 12% 28% 34% 16% 1	408
	21st to end	Night	7% 5% 20% 48% 18% 1	460
August	1st to 10th	Night	9% 7% 19% 43% 22% 1	418
	11th to 20th	Night	10% 10% 21% 38% 19% 2	503
	21st to end	Night	11% 10% 28% 33% 17% 1	660
September	1st to 10th	Night	13% 13% 27% 30% 15% 2	782
	11th to 20th	Night	13% 13% 28% 29% 15% 3	800
	21st to end	Night	14% 19% 35% 22% 9% 1	800
October	1st to 10th	Night	14% 13% 36% 28% 8% 1	800
	11th to 20th	Night	12% 19% 39% 26% 5% 1	800
	21st to end	Night	11% 22% 42% 21% 4	880
November	1st to 10th	Night	11% 26% 45% 14% 3	856
	11th to 20th	Night	10% 28% 45% 14% 3	1050
	21st to end	Night	8% 25% 44% 19% 4 1	1050
December	1st to 10th	Night	8% 25% 45% 17% 5%	1050
	11th to 20th	Night	7% 26% 48% 14% 4	1050
	21st to end	Night	6% 23% 49% 17% 5% 1	1155

Page generated at 2026-08-12 01:37:17. Version ac07

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