Cranbrook
Clear Sky Chart
History

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

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	4 6% 3 8% 6% 6% 6% 5% 7% 9% 40%	135903
	Night	3 7% 3 9% 6% 6% 6% 5% 7% 9% 40%	135933
Transparency	Day	38% 11% 18% 21% 10% 2	135619
	Night	39% 12% 19% 20% 9% 2	135795
Seeing	Day	11% 18% 30% 26% 12% 2	17124
	Night	8% 12% 28% 35% 16% 2	26306

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	1 8% 6% 17% 9% 10% 8% 7% 9% 9% 17%	4167
	11th to 20th	Day	1 7% 3 9% 6% 6% 7% 5% 8% 10% 36%	4716
	21st to end	Day	1 6% 3 9% 7% 8% 9% 6% 9% 11% 31%	5186
February	1st to 10th	Day	3 7% 3 13% 8% 7% 8% 6% 9% 10% 26%	1680
	11th to 20th	Day	2 9% 4 12% 9% 7% 6% 5% 9% 10% 28%	1872
	21st to end	Day	1 7% 4 10% 8% 7% 8% 7% 10% 9% 29%	1730
March	1st to 10th	Day	2 9% 3 7% 5% 6% 6% 6% 9% 12% 35%	4618
	11th to 20th	Day	1 6% 3 8% 6% 6% 7% 7% 8% 11% 38%	4921
	21st to end	Day	3 9% 3 9% 8% 7% 8% 7% 10% 11% 25%	5871
April	1st to 10th	Day	5% 8% 4 10% 7% 7% 7% 6% 8% 9% 30%	2520
	11th to 20th	Day	4 11% 3 10% 7% 5% 6% 5% 7% 8% 34%	2663
	21st to end	Day	6% 10% 4 8% 7% 7% 5% 5% 7% 8% 34%	2730
May	1st to 10th	Day	6% 9% 4 8% 5% 5% 5% 4 6% 7% 40%	6297
	11th to 20th	Day	8% 9% 3 9% 7% 7% 8% 6% 8% 9% 26%	6575
	21st to end	Day	7% 6% 2 6% 5% 5% 6% 5% 7% 9% 40%	7570
June	1st to 10th	Day	11% 9% 3 7% 7% 6% 6% 4 6% 8% 33%	3001
	11th to 20th	Day	9% 7% 3 8% 7% 8% 7% 6% 8% 9% 28%	3000
	21st to end	Day	6% 3 1 6% 6% 6% 6% 5% 8% 9% 43%	3000
July	1st to 10th	Day	3 3 1 4 4 4 4 4 5% 7% 59%	6780
	11th to 20th	Day	4 2 1 3 3 3 4 4 6% 8% 64%	6625
	21st to end	Day	4 2 1 3 3 3 4 3 6% 9% 62%	6874
August	1st to 10th	Day	3 3 1 4 3 5% 5% 5% 7% 11% 55%	4541
	11th to 20th	Day	4 3 1 3 3 4 4 4 7% 8% 58%	4251
	21st to end	Day	3 3 1 3 3 4 5% 5% 7% 10% 54%	4610
September	1st to 10th	Day	4 3 2 5% 3 4 5% 5% 6% 8% 56%	2400
	11th to 20th	Day	7% 5% 2 5% 4 5% 5% 4 6% 8% 49%	2237
	21st to end	Day	6% 4 2 5% 5% 4 6% 4 8% 10% 47%	2200
October	1st to 10th	Day	3 4 2 5% 4 4 5% 5% 7% 10% 53%	3554
	11th to 20th	Day	2 4 2 6% 5% 5% 7% 6% 7% 11% 46%	2932
	21st to end	Day	4 8% 4 7% 5% 6% 6% 5% 8% 9% 40%	3200
November	1st to 10th	Day	5% 11% 4 12% 8% 6% 7% 4 7% 9% 27%	1854
	11th to 20th	Day	2 8% 4 10% 6% 6% 8% 7% 8% 9% 32%	1611
	21st to end	Day	2 10% 3 11% 8% 7% 7% 6% 8% 10% 29%	1470
December	1st to 10th	Day	2 7% 3 12% 9% 9% 9% 7% 9% 9% 25%	2360
	11th to 20th	Day	2 10% 7% 17% 9% 8% 8% 7% 7% 7% 18%	2712
	21st to end	Day	2 10% 6% 17% 10% 9% 8% 5% 6% 7% 20%	3575
January	1st to 10th	Night	1 12% 7% 14% 9% 9% 8% 6% 8% 9% 18%	8768
	11th to 20th	Night	1 7% 3 10% 7% 8% 8% 6% 8% 9% 34%	8185
	21st to end	Night	1 8% 4 10% 6% 8% 8% 6% 9% 10% 30%	8994
February	1st to 10th	Night	3 11% 5% 11% 8% 6% 6% 5% 8% 9% 28%	2940
	11th to 20th	Night	2 8% 4 10% 7% 6% 7% 6% 8% 8% 34%	2633
	21st to end	Night	2 8% 4 10% 6% 6% 6% 6% 8% 10% 33%	2076
March	1st to 10th	Night	1 6% 3 8% 6% 5% 6% 5% 8% 11% 42%	5496
	11th to 20th	Night	1 5% 3 8% 7% 6% 7% 7% 8% 8% 40%	5297
	21st to end	Night	4 8% 3 8% 7% 6% 7% 6% 8% 11% 30%	5345
April	1st to 10th	Night	4 7% 2 7% 5% 5% 5% 4 7% 11% 43%	2100
	11th to 20th	Night	3 8% 4 7% 6% 5% 6% 5% 7% 10% 40%	1907
	21st to end	Night	5% 8% 3 7% 5% 6% 5% 4 7% 9% 41%	1890
May	1st to 10th	Night	6% 9% 3 7% 5% 4 5% 4 6% 7% 43%	3874
	11th to 20th	Night	6% 7% 3 8% 6% 6% 5% 5% 8% 10% 37%	3522
	21st to end	Night	6% 7% 2 6% 4 4 5% 4 5% 9% 47%	3547
June	1st to 10th	Night	8% 4 2 7% 6% 5% 4 4 7% 10% 43%	1400
	11th to 20th	Night	8% 6% 2 8% 4 5% 7% 6% 7% 9% 39%	1400
	21st to end	Night	4 4 2 5% 3 4 5% 4 7% 8% 53%	1400
July	1st to 10th	Night	3 3 1 3 2 2 3 3 5% 7% 67%	3180
	11th to 20th	Night	4 2 1 4 3 3 3 2 4 6% 67%	3318
	21st to end	Night	3 2 1 2 2 3 3 3 4 6% 71%	3969
August	1st to 10th	Night	4 3 1 3 2 2 4 3 5% 9% 63%	2609
	11th to 20th	Night	2 2 2 4 3 3 4 3 6% 10% 60%	2803
	21st to end	Night	3 3 1 3 3 3 4 4 7% 9% 60%	3297
September	1st to 10th	Night	4 3 1 3 2 3 4 4 5% 7% 64%	2000
	11th to 20th	Night	5% 5% 2 5% 3 4 4 3 5% 7% 57%	2162
	21st to end	Night	4 4 2 5% 3 3 4 5% 6% 8% 56%	2200
October	1st to 10th	Night	3 3 1 4 2 3 4 3 6% 8% 61%	3643
	11th to 20th	Night	2 4 1 5% 4 5% 5% 4 6% 9% 55%	4216
	21st to end	Night	2 6% 2 6% 5% 5% 5% 5% 8% 9% 47%	4600
November	1st to 10th	Night	4 9% 3 9% 6% 5% 7% 5% 8% 11% 33%	2665
	11th to 20th	Night	2 9% 5% 9% 7% 6% 7% 5% 8% 9% 32%	2984
	21st to end	Night	2 14% 4 11% 7% 6% 7% 6% 8% 8% 26%	3150
December	1st to 10th	Night	2 7% 3 11% 9% 8% 10% 6% 8% 10% 28%	5040
	11th to 20th	Night	1 10% 6% 20% 11% 8% 8% 5% 6% 7% 18%	5700
	21st to end	Night	1 11% 6% 16% 10% 9% 8% 6% 7% 7% 19%	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	58% 14% 13% 10% 5%	4167
	11th to 20th	Day	39% 13% 22% 18% 4 2	4740
	21st to end	Day	47% 14% 22% 11% 6%	5187
February	1st to 10th	Day	50% 15% 17% 10% 7% 1	1680
	11th to 20th	Day	50% 15% 18% 12% 5%	1881
	21st to end	Day	46% 16% 19% 13% 5%	1730
March	1st to 10th	Day	39% 10% 21% 21% 9%	4618
	11th to 20th	Day	36% 12% 28% 17% 7%	4922
	21st to end	Day	49% 14% 19% 12% 5%	5871
April	1st to 10th	Day	49% 12% 20% 16% 3	2520
	11th to 20th	Day	46% 11% 18% 20% 5%	2663
	21st to end	Day	49% 10% 16% 20% 5% 1	2730
May	1st to 10th	Day	42% 9% 12% 24% 12% 1	6297
	11th to 20th	Day	46% 14% 15% 20% 5%	6578
	21st to end	Day	39% 10% 21% 23% 6%	7571
June	1st to 10th	Day	50% 8% 15% 17% 9% 1	3001
	11th to 20th	Day	50% 11% 15% 16% 7% 1	3000
	21st to end	Day	34% 11% 18% 24% 12% 1	3000
July	1st to 10th	Day	22% 8% 12% 25% 27% 6%	6706
	11th to 20th	Day	18% 7% 14% 24% 28% 10%	6544
	21st to end	Day	19% 7% 15% 30% 18% 11%	6681
August	1st to 10th	Day	21% 8% 19% 27% 20% 5%	4524
	11th to 20th	Day	22% 8% 23% 32% 14% 2	4292
	21st to end	Day	23% 9% 21% 33% 13% 1	4609
September	1st to 10th	Day	27% 6% 16% 32% 17% 2	2400
	11th to 20th	Day	33% 8% 16% 30% 11% 1	2237
	21st to end	Day	33% 9% 24% 24% 10% 1	2200
October	1st to 10th	Day	27% 10% 22% 36% 5%	3554
	11th to 20th	Day	30% 13% 23% 26% 7%	2932
	21st to end	Day	38% 13% 22% 19% 8%	3200
November	1st to 10th	Day	54% 15% 16% 12% 3	1854
	11th to 20th	Day	48% 17% 20% 10% 5%	1611
	21st to end	Day	50% 20% 20% 8% 3	1470
December	1st to 10th	Day	49% 20% 17% 12% 1	2360
	11th to 20th	Day	63% 11% 15% 8% 3	2714
	21st to end	Day	58% 15% 15% 9% 3 1	3575
January	1st to 10th	Night	58% 12% 16% 11% 2	8769
	11th to 20th	Night	41% 15% 19% 14% 8% 2	8223
	21st to end	Night	47% 15% 19% 15% 4	8994
February	1st to 10th	Night	50% 14% 16% 12% 7% 1	2940
	11th to 20th	Night	45% 14% 19% 16% 6%	2634
	21st to end	Night	42% 14% 21% 16% 6% 1	2076
March	1st to 10th	Night	35% 11% 26% 18% 10%	5496
	11th to 20th	Night	37% 12% 26% 19% 6%	5297
	21st to end	Night	43% 16% 23% 15% 3	5345
April	1st to 10th	Night	34% 13% 26% 19% 7%	2100
	11th to 20th	Night	39% 9% 21% 23% 7%	1907
	21st to end	Night	39% 10% 18% 25% 7% 1	1890
May	1st to 10th	Night	38% 9% 15% 23% 14% 1	3871
	11th to 20th	Night	38% 11% 18% 26% 7%	3522
	21st to end	Night	33% 8% 21% 29% 9%	3547
June	1st to 10th	Night	38% 9% 21% 21% 9% 2	1400
	11th to 20th	Night	40% 10% 19% 21% 9% 1	1400
	21st to end	Night	26% 9% 20% 28% 15% 2	1400
July	1st to 10th	Night	16% 6% 17% 25% 27% 9%	3145
	11th to 20th	Night	19% 5% 13% 22% 27% 14%	3276
	21st to end	Night	15% 5% 17% 31% 20% 13%	3857
August	1st to 10th	Night	19% 5% 20% 26% 24% 6%	2601
	11th to 20th	Night	19% 7% 24% 31% 17% 2	2828
	21st to end	Night	18% 8% 21% 32% 19% 2	3297
September	1st to 10th	Night	20% 7% 14% 36% 18% 5%	2000
	11th to 20th	Night	26% 8% 19% 31% 14% 2	2162
	21st to end	Night	26% 9% 23% 27% 13% 1	2200
October	1st to 10th	Night	21% 7% 22% 39% 11%	3643
	11th to 20th	Night	27% 11% 18% 35% 9% 1	4216
	21st to end	Night	30% 13% 25% 27% 4	4600
November	1st to 10th	Night	45% 17% 20% 14% 4 1	2665
	11th to 20th	Night	47% 15% 20% 13% 5%	2984
	21st to end	Night	53% 16% 16% 12% 3	3150
December	1st to 10th	Night	45% 20% 19% 14% 3	5040
	11th to 20th	Night	64% 14% 12% 8% 2	5697
	21st to end	Night	59% 14% 14% 9% 3	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	7% 25% 30% 26% 9% 3	220
	11th to 20th	Day	6% 12% 38% 28% 14% 1	220
	21st to end	Day	3 11% 44% 28% 10% 3	242
February	1st to 10th	Day	8% 17% 46% 19% 9% 1	220
	11th to 20th	Day	8% 16% 34% 29% 13% 1	382
	21st to end	Day	14% 20% 34% 25% 6% 1	446
March	1st to 10th	Day	12% 29% 29% 19% 10% 1	540
	11th to 20th	Day	14% 25% 26% 25% 9% 1	540
	21st to end	Day	16% 24% 34% 18% 6% 2	594
April	1st to 10th	Day	14% 27% 30% 21% 6% 1	540
	11th to 20th	Day	15% 28% 26% 22% 8% 2	532
	21st to end	Day	14% 23% 29% 22% 9% 2	540
May	1st to 10th	Day	12% 23% 32% 22% 9% 2	543
	11th to 20th	Day	16% 16% 29% 25% 12% 3	635
	21st to end	Day	17% 14% 23% 28% 16% 2	844
June	1st to 10th	Day	17% 19% 26% 26% 10% 1	731
	11th to 20th	Day	14% 21% 27% 24% 12% 1	730
	21st to end	Day	9% 11% 29% 32% 16% 3	730
July	1st to 10th	Day	6% 10% 28% 33% 21% 2	721
	11th to 20th	Day	8% 15% 29% 30% 16% 2	654
	21st to end	Day	5% 15% 34% 29% 14% 2	575
August	1st to 10th	Day	6% 11% 32% 31% 18% 3	527
	11th to 20th	Day	5% 13% 30% 29% 19% 4	517
	21st to end	Day	8% 15% 28% 30% 13% 5%	583
September	1st to 10th	Day	7% 15% 29% 27% 15% 6%	530
	11th to 20th	Day	9% 17% 25% 28% 16% 5%	530
	21st to end	Day	15% 23% 26% 25% 11% 1	530
October	1st to 10th	Day	11% 17% 30% 29% 12% 1	525
	11th to 20th	Day	12% 20% 29% 22% 13% 3	330
	21st to end	Day	18% 17% 37% 19% 8% 1	363
November	1st to 10th	Day	17% 17% 28% 30% 7%	330
	11th to 20th	Day	15% 21% 34% 22% 6% 1	278
	21st to end	Day	9% 18% 32% 28% 11% 2	220
December	1st to 10th	Day	9% 14% 32% 28% 12% 5%	220
	11th to 20th	Day	11% 21% 32% 23% 11% 1	220
	21st to end	Day	8% 10% 38% 32% 10% 1	242
January	1st to 10th	Night	9% 19% 31% 31% 8% 2	1050
	11th to 20th	Night	6% 12% 37% 33% 11% 2	843
	21st to end	Night	6% 10% 35% 39% 10% 2	924
February	1st to 10th	Night	7% 15% 33% 34% 11% 1	840
	11th to 20th	Night	6% 13% 29% 39% 13% 1	824
	21st to end	Night	9% 13% 28% 33% 14% 2	692
March	1st to 10th	Night	8% 13% 31% 33% 14% 1	840
	11th to 20th	Night	8% 18% 28% 30% 13% 2	840
	21st to end	Night	9% 14% 30% 32% 12% 2	924
April	1st to 10th	Night	8% 13% 28% 33% 18% 1	840
	11th to 20th	Night	7% 15% 33% 31% 12% 1	659
	21st to end	Night	9% 11% 34% 30% 16%	630
May	1st to 10th	Night	8% 12% 26% 36% 17% 1	496
	11th to 20th	Night	9% 10% 24% 32% 24% 2	414
	21st to end	Night	13% 10% 21% 31% 23% 2	455
June	1st to 10th	Night	13% 8% 24% 36% 18% 2	400
	11th to 20th	Night	12% 7% 29% 35% 18% 1	400
	21st to end	Night	4 8% 22% 40% 24% 2	400
July	1st to 10th	Night	3 4 17% 46% 28% 2	390
	11th to 20th	Night	3 4 22% 48% 20% 3	388
	21st to end	Night	3 4 21% 42% 28% 2	438
August	1st to 10th	Night	6% 4 14% 40% 33% 4	398
	11th to 20th	Night	3 4 21% 38% 30% 4	532
	21st to end	Night	5% 6% 20% 42% 25% 2	686
September	1st to 10th	Night	6% 4 22% 38% 26% 4	800
	11th to 20th	Night	6% 6% 18% 35% 30% 4	800
	21st to end	Night	7% 9% 25% 38% 18% 3	800
October	1st to 10th	Night	9% 9% 27% 35% 19% 1	800
	11th to 20th	Night	6% 13% 27% 34% 18% 2	800
	21st to end	Night	10% 13% 31% 34% 10% 2	880
November	1st to 10th	Night	10% 14% 33% 36% 7%	820
	11th to 20th	Night	9% 21% 31% 30% 10% 1	998
	21st to end	Night	8% 17% 28% 35% 11% 1	1050
December	1st to 10th	Night	9% 11% 33% 33% 13% 1	1050
	11th to 20th	Night	9% 13% 34% 35% 9%	1050
	21st to end	Night	8% 14% 34% 34% 10%	1155

Page generated at 2026-05-28 23:53:31. Version ac07

Copyright © 2026 Attilla Danko.