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 2025-04-11.

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	6% 10% 4 11% 7% 6% 6% 5% 7% 8% 30%	116959
	Night	3 8% 4 11% 8% 7% 7% 5% 8% 9% 31%	120041
Transparency	Day	45% 11% 18% 17% 8% 1	116713
	Night	40% 13% 21% 18% 7% 1	119934
Seeing	Day	17% 22% 33% 20% 8% 1	15696
	Night	10% 19% 39% 23% 8% 1	24940

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	4 3 16% 11% 9% 8% 6% 10% 10% 22%	3179
	11th to 20th	Day	4 3 11% 9% 8% 8% 5% 8% 9% 35%	3405
	21st to end	Day	6% 4 8% 7% 6% 8% 6% 8% 10% 37%	4004
February	1st to 10th	Day	2 8% 5% 11% 10% 9% 8% 6% 10% 8% 23%	1600
	11th to 20th	Day	1 8% 4 16% 9% 8% 7% 6% 7% 9% 26%	1702
	21st to end	Day	1 9% 6% 17% 10% 8% 7% 6% 8% 8% 21%	1650
March	1st to 10th	Day	7% 6% 19% 9% 7% 6% 4 5% 8% 31%	4518
	11th to 20th	Day	2 9% 5% 12% 8% 8% 9% 6% 7% 7% 27%	4818
	21st to end	Day	2 19% 9% 17% 8% 6% 5% 5% 6% 6% 16%	5740
April	1st to 10th	Day	4 18% 8% 16% 9% 7% 5% 4 5% 6% 18%	2400
	11th to 20th	Day	4 19% 9% 16% 7% 5% 5% 4 5% 6% 19%	2440
	21st to end	Day	7% 21% 7% 14% 6% 5% 5% 4 6% 6% 20%	2470
May	1st to 10th	Day	12% 24% 6% 9% 6% 4 5% 4 5% 5% 21%	4387
	11th to 20th	Day	11% 16% 4 10% 6% 5% 5% 4 7% 7% 24%	4648
	21st to end	Day	16% 22% 5% 12% 7% 6% 5% 3 4 5% 14%	5161
June	1st to 10th	Day	18% 15% 4 10% 7% 5% 5% 4 6% 6% 21%	2850
	11th to 20th	Day	21% 15% 4 10% 7% 6% 6% 4 4 6% 17%	2850
	21st to end	Day	12% 10% 4 11% 6% 6% 6% 5% 6% 7% 27%	2850
July	1st to 10th	Day	7% 6% 3 7% 6% 5% 5% 5% 6% 6% 45%	4740
	11th to 20th	Day	6% 4 2 6% 6% 5% 5% 5% 7% 8% 46%	4740
	21st to end	Day	4 3 2 6% 5% 6% 6% 5% 9% 9% 45%	4885
August	1st to 10th	Day	6% 5% 3 8% 6% 6% 7% 5% 7% 8% 38%	4401
	11th to 20th	Day	7% 5% 2 7% 5% 5% 6% 6% 8% 10% 37%	4176
	21st to end	Day	8% 6% 3 7% 5% 5% 6% 5% 7% 8% 40%	4521
September	1st to 10th	Day	9% 9% 3 8% 6% 4 4 3 5% 7% 41%	2294
	11th to 20th	Day	10% 9% 4 9% 5% 6% 4 4 6% 8% 37%	2128
	21st to end	Day	7% 11% 3 8% 6% 6% 4 4 6% 8% 36%	2090
October	1st to 10th	Day	7% 9% 3 9% 6% 5% 5% 4 6% 7% 38%	3385
	11th to 20th	Day	3 7% 3 9% 7% 7% 7% 5% 8% 11% 34%	2842
	21st to end	Day	3 9% 3 9% 7% 6% 7% 5% 7% 7% 37%	3101
November	1st to 10th	Day	3 11% 4 13% 8% 6% 5% 5% 8% 10% 27%	1709
	11th to 20th	Day	1 8% 4 10% 7% 7% 7% 6% 11% 9% 31%	1445
	21st to end	Day	1 10% 3 11% 7% 6% 7% 6% 10% 11% 30%	1400
December	1st to 10th	Day	1 4 4 9% 6% 6% 6% 6% 9% 11% 37%	2290
	11th to 20th	Day	5% 4 13% 9% 8% 8% 6% 10% 12% 26%	2642
	21st to end	Day	6% 3 9% 8% 9% 9% 6% 9% 11% 30%	3498
January	1st to 10th	Night	6% 5% 19% 11% 10% 9% 6% 7% 8% 19%	6766
	11th to 20th	Night	4 3 12% 11% 7% 7% 7% 9% 9% 30%	6538
	21st to end	Night	1 7% 5% 12% 9% 7% 6% 5% 7% 9% 32%	6952
February	1st to 10th	Night	2 8% 4 15% 10% 9% 9% 6% 9% 9% 19%	2800
	11th to 20th	Night	1 8% 6% 15% 7% 8% 7% 6% 8% 9% 25%	2582
	21st to end	Night	8% 5% 14% 11% 7% 7% 7% 8% 6% 25%	1980
March	1st to 10th	Night	7% 6% 14% 9% 8% 7% 6% 8% 9% 27%	5376
	11th to 20th	Night	2 8% 4 9% 6% 7% 8% 7% 8% 10% 30%	5181
	21st to end	Night	2 16% 7% 16% 10% 7% 7% 5% 6% 7% 17%	5231
April	1st to 10th	Night	3 14% 5% 12% 8% 7% 5% 5% 7% 8% 26%	2000
	11th to 20th	Night	2 13% 5% 12% 6% 5% 5% 5% 7% 9% 29%	1696
	21st to end	Night	5% 16% 6% 11% 6% 6% 4 5% 6% 6% 30%	1710
May	1st to 10th	Night	11% 22% 5% 10% 4 4 3 3 5% 5% 28%	2572
	11th to 20th	Night	6% 12% 3 8% 6% 5% 6% 4 8% 10% 32%	2244
	21st to end	Night	12% 15% 5% 10% 6% 4 5% 4 6% 7% 27%	2414
June	1st to 10th	Night	13% 12% 3 9% 4 4 5% 4 7% 8% 32%	1330
	11th to 20th	Night	14% 13% 3 7% 6% 5% 4 5% 5% 9% 29%	1330
	21st to end	Night	7% 5% 2 5% 5% 4 5% 5% 8% 11% 44%	1330
July	1st to 10th	Night	4 4 2 5% 4 4 5% 4 6% 7% 54%	2220
	11th to 20th	Night	4 4 2 3 3 4 3 4 6% 8% 59%	2220
	21st to end	Night	2 3 1 5% 5% 4 4 4 5% 7% 60%	2663
August	1st to 10th	Night	7% 7% 3 8% 5% 5% 5% 4 6% 9% 42%	2529
	11th to 20th	Night	6% 4 2 4 4 4 5% 5% 7% 11% 48%	2656
	21st to end	Night	7% 6% 2 5% 4 4 4 4 6% 8% 50%	3146
September	1st to 10th	Night	7% 7% 2 6% 3 4 4 4 6% 9% 49%	1883
	11th to 20th	Night	8% 9% 4 6% 5% 4 5% 4 6% 7% 42%	2051
	21st to end	Night	6% 8% 3 5% 5% 5% 4 4 7% 9% 44%	2090
October	1st to 10th	Night	4 8% 3 9% 5% 4 5% 4 6% 8% 42%	3589
	11th to 20th	Night	2 7% 3 9% 6% 6% 6% 4 7% 9% 41%	4086
	21st to end	Night	2 9% 4 10% 6% 6% 5% 5% 8% 9% 35%	4457
November	1st to 10th	Night	2 12% 5% 12% 9% 7% 6% 5% 10% 10% 21%	2569
	11th to 20th	Night	1 7% 5% 12% 9% 9% 9% 6% 8% 10% 24%	2952
	21st to end	Night	9% 5% 12% 10% 9% 7% 6% 9% 10% 22%	3000
December	1st to 10th	Night	1 6% 3 9% 7% 7% 7% 6% 11% 14% 31%	4890
	11th to 20th	Night	4 4 14% 12% 11% 10% 8% 10% 11% 18%	5550
	21st to end	Night	1 7% 3 12% 10% 10% 10% 7% 9% 10% 21%	7458

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% 17% 22% 14% 6% 1	3179
	11th to 20th	Day	42% 12% 18% 14% 12% 3	3428
	21st to end	Day	30% 18% 28% 17% 7%	4004
February	1st to 10th	Day	48% 14% 18% 10% 7% 3	1600
	11th to 20th	Day	50% 13% 17% 14% 6% 1	1710
	21st to end	Day	55% 12% 16% 10% 6% 1	1650
March	1st to 10th	Day	46% 11% 16% 14% 13%	4518
	11th to 20th	Day	43% 10% 20% 21% 5%	4819
	21st to end	Day	60% 11% 14% 9% 5%	5740
April	1st to 10th	Day	64% 10% 15% 8% 2	2400
	11th to 20th	Day	62% 9% 14% 11% 4	2440
	21st to end	Day	62% 10% 13% 13% 2	2470
May	1st to 10th	Day	60% 9% 14% 14% 3	4387
	11th to 20th	Day	51% 12% 17% 16% 4	4652
	21st to end	Day	64% 13% 14% 7% 2	5162
June	1st to 10th	Day	60% 10% 11% 14% 4 1	2850
	11th to 20th	Day	65% 10% 11% 10% 4	2850
	21st to end	Day	51% 11% 15% 15% 7% 1	2850
July	1st to 10th	Day	37% 7% 10% 18% 22% 6%	4666
	11th to 20th	Day	31% 8% 12% 26% 17% 5%	4659
	21st to end	Day	30% 7% 12% 26% 22% 4	4731
August	1st to 10th	Day	38% 9% 14% 27% 12% 1	4384
	11th to 20th	Day	36% 8% 22% 26% 7% 1	4218
	21st to end	Day	35% 8% 24% 25% 8% 1	4520
September	1st to 10th	Day	41% 8% 16% 21% 11% 2	2294
	11th to 20th	Day	43% 10% 14% 22% 10% 1	2128
	21st to end	Day	43% 11% 18% 19% 8% 1	2090
October	1st to 10th	Day	42% 8% 19% 25% 7%	3385
	11th to 20th	Day	35% 11% 28% 20% 6%	2842
	21st to end	Day	37% 9% 26% 23% 5%	3101
November	1st to 10th	Day	48% 16% 20% 11% 5%	1709
	11th to 20th	Day	37% 21% 24% 12% 6%	1445
	21st to end	Day	41% 16% 22% 13% 6% 1	1400
December	1st to 10th	Day	34% 20% 29% 12% 4	2290
	11th to 20th	Day	38% 17% 28% 13% 4	2644
	21st to end	Day	34% 13% 22% 20% 10% 1	3498
January	1st to 10th	Night	47% 14% 21% 13% 3 1	6766
	11th to 20th	Night	41% 13% 16% 13% 11% 5%	6577
	21st to end	Night	36% 18% 22% 17% 7% 1	6952
February	1st to 10th	Night	51% 17% 13% 11% 7% 2	2800
	11th to 20th	Night	48% 14% 20% 11% 7% 1	2584
	21st to end	Night	49% 11% 16% 15% 8% 1	1980
March	1st to 10th	Night	41% 15% 21% 13% 9%	5376
	11th to 20th	Night	34% 11% 21% 24% 9%	5181
	21st to end	Night	54% 12% 16% 12% 6%	5231
April	1st to 10th	Night	49% 13% 19% 13% 5%	2000
	11th to 20th	Night	46% 14% 21% 13% 6%	1696
	21st to end	Night	51% 10% 17% 16% 5% 1	1710
May	1st to 10th	Night	56% 8% 13% 19% 4	2572
	11th to 20th	Night	40% 15% 19% 23% 4	2244
	21st to end	Night	48% 14% 19% 14% 4	2414
June	1st to 10th	Night	48% 12% 14% 20% 5%	1330
	11th to 20th	Night	52% 11% 15% 15% 7%	1330
	21st to end	Night	33% 12% 21% 24% 9% 1	1330
July	1st to 10th	Night	27% 8% 16% 23% 21% 5%	2185
	11th to 20th	Night	20% 7% 19% 31% 17% 5%	2178
	21st to end	Night	22% 6% 14% 31% 22% 5%	2579
August	1st to 10th	Night	37% 9% 15% 29% 9% 1	2521
	11th to 20th	Night	25% 8% 33% 28% 6%	2680
	21st to end	Night	28% 8% 32% 25% 6%	3146
September	1st to 10th	Night	30% 10% 21% 26% 11% 1	1883
	11th to 20th	Night	36% 13% 19% 22% 10% 1	2051
	21st to end	Night	33% 13% 22% 23% 9%	2090
October	1st to 10th	Night	34% 8% 23% 25% 10%	3589
	11th to 20th	Night	31% 11% 26% 25% 6%	4086
	21st to end	Night	35% 10% 26% 25% 4	4457
November	1st to 10th	Night	48% 18% 17% 11% 5% 1	2569
	11th to 20th	Night	44% 19% 19% 11% 6% 1	2952
	21st to end	Night	44% 17% 19% 12% 5% 2	3000
December	1st to 10th	Night	34% 18% 33% 11% 4	4890
	11th to 20th	Night	43% 20% 23% 10% 4	5547
	21st to end	Night	38% 15% 23% 16% 7% 1	7458

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	11% 21% 38% 22% 6% 3	200
	11th to 20th	Day	8% 25% 47% 15% 6% 1	200
	21st to end	Day	10% 30% 39% 15% 5% 2	220
February	1st to 10th	Day	23% 24% 31% 15% 6% 2	200
	11th to 20th	Day	11% 19% 42% 22% 5% 1	285
	21st to end	Day	19% 26% 32% 18% 4 1	414
March	1st to 10th	Day	16% 29% 31% 16% 5% 2	500
	11th to 20th	Day	13% 29% 35% 17% 4 2	500
	21st to end	Day	17% 25% 33% 17% 5% 2	550
April	1st to 10th	Day	18% 28% 34% 15% 5%	500
	11th to 20th	Day	15% 26% 30% 20% 7% 1	453
	21st to end	Day	17% 26% 32% 18% 6% 1	460
May	1st to 10th	Day	19% 19% 28% 22% 10% 2	463
	11th to 20th	Day	18% 11% 30% 27% 12% 2	642
	21st to end	Day	24% 13% 24% 26% 10% 1	734
June	1st to 10th	Day	23% 20% 31% 19% 7% 1	680
	11th to 20th	Day	26% 21% 28% 18% 7% 1	680
	21st to end	Day	15% 17% 32% 23% 11% 2	680
July	1st to 10th	Day	16% 14% 29% 26% 15% 2	671
	11th to 20th	Day	15% 18% 32% 23% 10% 2	668
	21st to end	Day	9% 17% 35% 27% 10% 2	587
August	1st to 10th	Day	13% 17% 35% 24% 9% 2	487
	11th to 20th	Day	11% 19% 38% 23% 8% 2	477
	21st to end	Day	16% 21% 34% 20% 9% 1	539
September	1st to 10th	Day	18% 23% 32% 16% 9% 1	490
	11th to 20th	Day	19% 20% 37% 17% 6% 1	490
	21st to end	Day	27% 28% 26% 16% 3 1	490
October	1st to 10th	Day	24% 20% 30% 18% 8% 1	463
	11th to 20th	Day	18% 27% 27% 22% 5% 1	300
	21st to end	Day	20% 26% 36% 15% 2 1	330
November	1st to 10th	Day	14% 34% 35% 13% 4 1	300
	11th to 20th	Day	15% 28% 46% 8% 3 1	223
	21st to end	Day	15% 27% 32% 15% 9% 4	200
December	1st to 10th	Day	11% 25% 40% 17% 5% 3	200
	11th to 20th	Day	7% 29% 44% 15% 6%	200
	21st to end	Day	8% 26% 39% 22% 3 1	220
January	1st to 10th	Night	9% 26% 47% 15% 2 1	1000
	11th to 20th	Night	7% 26% 47% 17% 3 1	914
	21st to end	Night	7% 29% 45% 15% 4	880
February	1st to 10th	Night	11% 22% 39% 21% 7% 1	800
	11th to 20th	Night	7% 20% 45% 22% 5%	784
	21st to end	Night	8% 19% 42% 22% 8% 1	660
March	1st to 10th	Night	6% 20% 45% 22% 6% 1	800
	11th to 20th	Night	7% 20% 46% 20% 6% 1	800
	21st to end	Night	8% 18% 39% 24% 10% 1	880
April	1st to 10th	Night	11% 19% 35% 28% 7% 1	800
	11th to 20th	Night	7% 17% 37% 31% 7% 1	566
	21st to end	Night	12% 18% 37% 25% 8% 1	570
May	1st to 10th	Night	14% 11% 32% 33% 10%	399
	11th to 20th	Night	11% 8% 27% 34% 19% 1	374
	21st to end	Night	21% 8% 26% 28% 16% 1	411
June	1st to 10th	Night	18% 13% 32% 23% 12% 1	380
	11th to 20th	Night	22% 11% 31% 26% 10%	380
	21st to end	Night	10% 13% 26% 32% 20%	380
July	1st to 10th	Night	9% 8% 25% 39% 18%	370
	11th to 20th	Night	10% 12% 28% 33% 16% 1	368
	21st to end	Night	6% 6% 21% 49% 18%	416
August	1st to 10th	Night	8% 7% 19% 44% 22% 1	378
	11th to 20th	Night	10% 10% 21% 38% 19% 2	470
	21st to end	Night	11% 11% 29% 33% 15% 1	627
September	1st to 10th	Night	13% 13% 28% 29% 14% 2	743
	11th to 20th	Night	13% 14% 29% 28% 13% 3	760
	21st to end	Night	14% 20% 35% 21% 9% 1	760
October	1st to 10th	Night	15% 13% 36% 27% 8%	760
	11th to 20th	Night	12% 19% 39% 25% 5% 1	760
	21st to end	Night	11% 22% 42% 21% 4	836
November	1st to 10th	Night	11% 27% 45% 14% 3	814
	11th to 20th	Night	10% 29% 46% 13% 2 1	1000
	21st to end	Night	9% 25% 44% 18% 4 1	1000
December	1st to 10th	Night	8% 26% 45% 17% 5%	1000
	11th to 20th	Night	6% 26% 48% 15% 5%	1000
	21st to end	Night	6% 24% 49% 16% 5% 1	1100

Page generated at 2025-04-08 07:17:30. Version ac07

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