Whistler
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Whistler clear sky chart from 2005-11-06 to 2026-03-18.

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	12% 17% 5% 10% 5% 5% 4 3 4 6% 29%	134770
	Night	11% 18% 6% 11% 6% 5% 5% 3 4 6% 27%	135423
Transparency	Day	54% 9% 12% 14% 9% 2	134474
	Night	56% 10% 12% 14% 8% 1	135297
Seeing	Day	23% 18% 27% 21% 9% 2	17052
	Night	23% 13% 28% 25% 11% 1	25345

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	11% 28% 8% 14% 7% 4 4 3 4 4 13%	4125
	11th to 20th	Day	7% 15% 4 10% 5% 4 3 2 4 5% 41%	4119
	21st to end	Day	14% 24% 6% 11% 5% 4 2 2 2 3 27%	4964
February	1st to 10th	Day	12% 26% 6% 10% 7% 4 4 3 5% 6% 17%	1879
	11th to 20th	Day	8% 23% 8% 12% 6% 4 3 3 4 5% 23%	1845
	21st to end	Day	7% 27% 8% 14% 6% 5% 4 4 4 5% 15%	1563
March	1st to 10th	Day	7% 24% 8% 14% 6% 5% 5% 4 5% 6% 18%	4748
	11th to 20th	Day	5% 24% 9% 14% 6% 5% 4 3 4 5% 20%	5078
	21st to end	Day	8% 29% 7% 16% 7% 5% 4 3 4 4 13%	5649
April	1st to 10th	Day	11% 31% 7% 12% 7% 5% 4 4 5% 4 11%	2400
	11th to 20th	Day	10% 28% 6% 11% 6% 5% 5% 2 3 4 19%	2489
	21st to end	Day	15% 27% 6% 11% 6% 5% 3 3 4 5% 16%	2685
May	1st to 10th	Day	9% 17% 5% 11% 5% 4 3 3 4 5% 34%	6320
	11th to 20th	Day	10% 20% 5% 12% 7% 5% 5% 4 6% 8% 19%	6277
	21st to end	Day	16% 22% 4 11% 6% 5% 5% 3 5% 6% 17%	7192
June	1st to 10th	Day	24% 18% 4 8% 5% 4 3 3 5% 5% 20%	3000
	11th to 20th	Day	22% 18% 5% 10% 8% 6% 4 4 5% 5% 14%	3000
	21st to end	Day	20% 12% 4 10% 6% 4 4 3 4 6% 27%	3000
July	1st to 10th	Day	9% 6% 2 5% 4 3 4 3 4 6% 56%	6780
	11th to 20th	Day	6% 4 1 4 3 3 3 3 4 6% 65%	6780
	21st to end	Day	7% 4 2 4 4 4 5% 5% 6% 8% 52%	7113
August	1st to 10th	Day	7% 4 2 4 3 3 4 3 6% 8% 55%	4541
	11th to 20th	Day	8% 7% 2 7% 6% 5% 6% 4 6% 8% 40%	4170
	21st to end	Day	14% 8% 3 7% 5% 5% 5% 4 6% 7% 38%	4578
September	1st to 10th	Day	12% 7% 3 7% 4 4 3 3 6% 7% 45%	2400
	11th to 20th	Day	19% 11% 3 7% 4 3 4 3 4 5% 38%	2400
	21st to end	Day	20% 15% 3 7% 4 4 4 3 5% 6% 30%	2102
October	1st to 10th	Day	15% 13% 5% 11% 7% 7% 5% 4 4 5% 25%	3280
	11th to 20th	Day	25% 16% 4 9% 5% 4 4 3 5% 6% 22%	3261
	21st to end	Day	13% 20% 7% 14% 8% 7% 5% 4 4 4 14%	3198
November	1st to 10th	Day	20% 25% 6% 8% 5% 4 4 3 5% 4 16%	1648
	11th to 20th	Day	15% 31% 7% 9% 5% 4 3 2 3 4 16%	1680
	21st to end	Day	16% 24% 5% 11% 7% 5% 4 3 4 4 16%	1680
December	1st to 10th	Day	10% 16% 6% 11% 6% 5% 5% 4 4 5% 28%	2536
	11th to 20th	Day	11% 25% 9% 17% 7% 6% 6% 3 4 4 8%	2715
	21st to end	Day	12% 28% 7% 13% 6% 5% 4 3 5% 6% 12%	3575
January	1st to 10th	Night	9% 25% 8% 17% 8% 5% 5% 3 4 4 13%	8770
	11th to 20th	Night	10% 15% 4 7% 3 3 4 4 5% 7% 39%	8779
	21st to end	Night	12% 21% 6% 11% 5% 4 4 2 3 5% 28%	9201
February	1st to 10th	Night	12% 23% 6% 11% 7% 7% 5% 3 5% 6% 15%	2740
	11th to 20th	Night	8% 20% 7% 14% 8% 5% 5% 4 4 5% 21%	2662
	21st to end	Night	9% 25% 8% 14% 6% 5% 4 3 4 5% 18%	2243
March	1st to 10th	Night	6% 20% 8% 14% 8% 6% 5% 4 5% 6% 18%	5364
	11th to 20th	Night	4 22% 7% 13% 6% 6% 5% 3 4 5% 23%	5076
	21st to end	Night	9% 24% 7% 13% 6% 5% 5% 4 5% 5% 17%	5325
April	1st to 10th	Night	7% 24% 8% 12% 8% 7% 5% 3 5% 6% 15%	2000
	11th to 20th	Night	9% 20% 5% 11% 7% 5% 5% 4 5% 6% 22%	1858
	21st to end	Night	10% 22% 6% 13% 5% 5% 5% 3 4 6% 20%	1715
May	1st to 10th	Night	7% 16% 5% 9% 4 4 4 2 4 5% 40%	3640
	11th to 20th	Night	10% 16% 6% 10% 5% 5% 5% 4 6% 9% 25%	3616
	21st to end	Night	12% 19% 5% 11% 6% 5% 5% 4 5% 6% 22%	3679
June	1st to 10th	Night	17% 16% 4 9% 5% 5% 5% 3 5% 6% 25%	1400
	11th to 20th	Night	16% 16% 5% 11% 7% 6% 4 3 5% 7% 21%	1400
	21st to end	Night	14% 12% 4 11% 5% 5% 4 4 5% 5% 32%	1400
July	1st to 10th	Night	8% 6% 2 5% 3 3 4 3 4 6% 56%	3180
	11th to 20th	Night	4 4 1 3 2 3 3 2 3 5% 70%	3180
	21st to end	Night	6% 6% 3 6% 3 4 4 4 5% 7% 53%	3729
August	1st to 10th	Night	5% 4 1 3 2 3 3 2 4 9% 64%	2609
	11th to 20th	Night	8% 10% 3 7% 4 5% 5% 3 5% 8% 41%	2885
	21st to end	Night	14% 8% 3 7% 4 4 3 3 5% 6% 42%	3334
September	1st to 10th	Night	9% 6% 2 8% 5% 3 5% 4 6% 6% 45%	2000
	11th to 20th	Night	15% 11% 4 7% 4 4 3 4 5% 6% 37%	2000
	21st to end	Night	17% 13% 4 8% 4 3 5% 3 5% 6% 31%	2293
October	1st to 10th	Night	14% 14% 5% 11% 7% 6% 7% 3 5% 5% 23%	3920
	11th to 20th	Night	18% 14% 5% 9% 5% 5% 5% 4 5% 6% 23%	3897
	21st to end	Night	14% 19% 6% 11% 6% 5% 6% 4 4 5% 21%	4585
November	1st to 10th	Night	22% 24% 6% 10% 6% 4 4 3 4 4 14%	2870
	11th to 20th	Night	18% 25% 6% 11% 6% 6% 4 3 4 3 14%	2940
	21st to end	Night	15% 24% 6% 13% 6% 4 5% 3 4 5% 15%	2940
December	1st to 10th	Night	11% 17% 5% 11% 6% 4 4 4 5% 7% 24%	4860
	11th to 20th	Night	13% 26% 9% 14% 6% 5% 4 3 4 4 11%	5710
	21st to end	Night	12% 27% 7% 14% 7% 6% 4 3 4 4 11%	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	71% 11% 8% 6% 3 1	4125
	11th to 20th	Day	45% 7% 12% 18% 15% 4	4140
	21st to end	Day	66% 4 5% 12% 12% 1	4965
February	1st to 10th	Day	68% 7% 9% 8% 6% 2	1879
	11th to 20th	Day	64% 8% 9% 11% 7% 1	1854
	21st to end	Day	72% 9% 8% 6% 3 2	1563
March	1st to 10th	Day	65% 9% 10% 11% 6% 1	4748
	11th to 20th	Day	61% 10% 9% 15% 4	5078
	21st to end	Day	73% 9% 9% 7% 3	5649
April	1st to 10th	Day	75% 8% 9% 7% 1	2400
	11th to 20th	Day	69% 6% 8% 9% 7% 1	2489
	21st to end	Day	70% 8% 9% 9% 4	2685
May	1st to 10th	Day	49% 8% 16% 19% 8%	6320
	11th to 20th	Day	54% 13% 19% 12% 2	6280
	21st to end	Day	60% 13% 14% 10% 3	7193
June	1st to 10th	Day	64% 8% 11% 10% 4 3	3000
	11th to 20th	Day	69% 10% 10% 8% 3	3000
	21st to end	Day	58% 8% 13% 14% 7% 1	3000
July	1st to 10th	Day	29% 7% 15% 18% 21% 10%	6706
	11th to 20th	Day	21% 6% 15% 31% 23% 3	6699
	21st to end	Day	26% 10% 21% 23% 16% 3	6913
August	1st to 10th	Day	24% 9% 13% 26% 26% 1	4524
	11th to 20th	Day	36% 14% 18% 20% 10% 1	4209
	21st to end	Day	43% 9% 16% 22% 8% 1	4577
September	1st to 10th	Day	39% 8% 12% 24% 12% 4	2400
	11th to 20th	Day	49% 5% 10% 18% 15% 3	2400
	21st to end	Day	56% 7% 14% 13% 8% 2	2102
October	1st to 10th	Day	57% 10% 13% 14% 7%	3280
	11th to 20th	Day	62% 8% 10% 16% 4	3261
	21st to end	Day	65% 13% 10% 8% 4	3198
November	1st to 10th	Day	71% 9% 7% 8% 4 1	1648
	11th to 20th	Day	73% 7% 7% 6% 6% 1	1680
	21st to end	Day	71% 7% 9% 7% 5% 2	1680
December	1st to 10th	Day	60% 7% 12% 13% 8%	2536
	11th to 20th	Day	78% 9% 8% 3 2 1	2718
	21st to end	Day	73% 10% 10% 5% 2	3575
January	1st to 10th	Night	71% 10% 8% 6% 5%	8770
	11th to 20th	Night	42% 9% 11% 16% 20% 3	8820
	21st to end	Night	59% 7% 7% 12% 12% 2	9201
February	1st to 10th	Night	67% 10% 8% 6% 8% 1	2740
	11th to 20th	Night	63% 9% 9% 11% 5% 2	2664
	21st to end	Night	67% 9% 9% 9% 5% 2	2243
March	1st to 10th	Night	60% 10% 10% 12% 7%	5364
	11th to 20th	Night	56% 12% 13% 15% 4	5077
	21st to end	Night	64% 14% 11% 8% 3 1	5325
April	1st to 10th	Night	68% 10% 12% 10% 1	2000
	11th to 20th	Night	61% 9% 11% 13% 6%	1858
	21st to end	Night	62% 10% 12% 11% 5%	1715
May	1st to 10th	Night	45% 9% 14% 23% 9%	3637
	11th to 20th	Night	49% 15% 22% 13% 1	3616
	21st to end	Night	56% 13% 15% 13% 3	3679
June	1st to 10th	Night	59% 8% 17% 9% 6% 2	1400
	11th to 20th	Night	61% 10% 14% 11% 4	1400
	21st to end	Night	53% 10% 15% 16% 5% 1	1400
July	1st to 10th	Night	28% 9% 18% 21% 16% 8%	3145
	11th to 20th	Night	16% 7% 21% 35% 20% 1	3138
	21st to end	Night	26% 10% 24% 23% 13% 4	3624
August	1st to 10th	Night	19% 7% 12% 37% 23% 2	2601
	11th to 20th	Night	37% 13% 19% 20% 9% 2	2912
	21st to end	Night	39% 9% 22% 21% 8% 1	3334
September	1st to 10th	Night	35% 10% 17% 24% 12% 3	2000
	11th to 20th	Night	45% 9% 11% 18% 15% 3	2000
	21st to end	Night	52% 8% 16% 14% 8% 2	2293
October	1st to 10th	Night	56% 11% 13% 14% 5% 1	3920
	11th to 20th	Night	58% 9% 10% 15% 7%	3897
	21st to end	Night	61% 9% 10% 14% 6%	4585
November	1st to 10th	Night	73% 8% 8% 7% 3 1	2870
	11th to 20th	Night	73% 8% 7% 6% 5% 1	2940
	21st to end	Night	71% 7% 9% 7% 5% 2	2940
December	1st to 10th	Night	59% 10% 12% 14% 5% 1	4860
	11th to 20th	Night	76% 8% 7% 7% 2 1	5706
	21st to end	Night	74% 10% 8% 6% 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	26% 24% 24% 17% 8% 2	220
	11th to 20th	Day	29% 14% 23% 25% 9%	220
	21st to end	Day	23% 16% 26% 24% 10% 1	242
February	1st to 10th	Day	25% 23% 29% 17% 5% 1	419
	11th to 20th	Day	19% 21% 33% 18% 7% 1	425
	21st to end	Day	21% 25% 29% 17% 7% 1	359
March	1st to 10th	Day	23% 22% 28% 20% 8% 1	540
	11th to 20th	Day	24% 22% 24% 16% 12% 2	529
	21st to end	Day	29% 19% 27% 15% 7% 2	550
April	1st to 10th	Day	28% 24% 23% 17% 7% 1	500
	11th to 20th	Day	23% 21% 26% 18% 7% 5%	492
	21st to end	Day	26% 21% 22% 18% 10% 3	500
May	1st to 10th	Day	20% 17% 24% 25% 12% 1	503
	11th to 20th	Day	22% 13% 24% 24% 14% 4	528
	21st to end	Day	24% 13% 30% 22% 9% 3	693
June	1st to 10th	Day	27% 19% 28% 18% 6% 1	730
	11th to 20th	Day	27% 21% 29% 16% 6% 1	730
	21st to end	Day	22% 18% 29% 22% 8% 1	730
July	1st to 10th	Day	19% 15% 31% 25% 9% 2	721
	11th to 20th	Day	13% 16% 34% 28% 7% 3	718
	21st to end	Day	10% 12% 34% 29% 11% 4	684
August	1st to 10th	Day	11% 12% 28% 28% 16% 5%	527
	11th to 20th	Day	13% 15% 28% 29% 12% 3	517
	21st to end	Day	16% 14% 32% 25% 10% 3	583
September	1st to 10th	Day	16% 19% 30% 22% 10% 2	530
	11th to 20th	Day	25% 15% 26% 24% 8% 2	530
	21st to end	Day	32% 19% 23% 19% 8% 1	530
October	1st to 10th	Day	28% 19% 25% 17% 9% 2	530
	11th to 20th	Day	35% 17% 22% 15% 9% 2	530
	21st to end	Day	35% 20% 21% 17% 6% 1	400
November	1st to 10th	Day	33% 20% 26% 15% 7%	220
	11th to 20th	Day	35% 19% 20% 14% 9% 3	220
	21st to end	Day	26% 24% 23% 16% 6% 5%	220
December	1st to 10th	Day	25% 18% 17% 23% 15% 3	220
	11th to 20th	Day	32% 16% 23% 18% 10% 1	220
	21st to end	Day	23% 16% 30% 25% 5%	242
January	1st to 10th	Night	31% 19% 28% 17% 5%	950
	11th to 20th	Night	29% 13% 29% 22% 6%	950
	21st to end	Night	23% 17% 31% 21% 8%	963
February	1st to 10th	Night	25% 16% 35% 20% 4	840
	11th to 20th	Night	16% 15% 36% 25% 7%	824
	21st to end	Night	22% 17% 32% 20% 9%	692
March	1st to 10th	Night	24% 14% 30% 22% 10% 1	840
	11th to 20th	Night	23% 18% 29% 19% 10%	828
	21st to end	Night	23% 16% 29% 23% 9% 1	880
April	1st to 10th	Night	21% 14% 31% 23% 10% 1	800
	11th to 20th	Night	19% 14% 31% 25% 9% 1	670
	21st to end	Night	20% 10% 30% 25% 13% 2	515
May	1st to 10th	Night	15% 10% 24% 35% 15% 1	400
	11th to 20th	Night	19% 8% 15% 38% 18% 2	394
	21st to end	Night	23% 7% 27% 31% 12%	433
June	1st to 10th	Night	22% 8% 30% 31% 9% 1	400
	11th to 20th	Night	17% 11% 33% 30% 9% 1	400
	21st to end	Night	17% 9% 27% 33% 14% 1	400
July	1st to 10th	Night	12% 5% 23% 38% 21% 1	390
	11th to 20th	Night	8% 4 23% 43% 22%	388
	21st to end	Night	5% 6% 18% 42% 27% 1	438
August	1st to 10th	Night	9% 5% 15% 42% 29% 1	398
	11th to 20th	Night	8% 4 21% 39% 25% 2	577
	21st to end	Night	14% 7% 25% 34% 20%	710
September	1st to 10th	Night	12% 7% 30% 34% 18% 1	800
	11th to 20th	Night	17% 8% 25% 26% 22% 3	800
	21st to end	Night	25% 12% 27% 24% 11% 1	800
October	1st to 10th	Night	23% 11% 28% 23% 14% 1	800
	11th to 20th	Night	29% 11% 28% 21% 11% 1	800
	21st to end	Night	31% 17% 27% 18% 7%	880
November	1st to 10th	Night	35% 16% 26% 19% 3	820
	11th to 20th	Night	34% 16% 26% 16% 7%	840
	21st to end	Night	29% 17% 27% 20% 6% 1	840
December	1st to 10th	Night	22% 16% 30% 23% 9%	890
	11th to 20th	Night	32% 17% 30% 18% 3	950
	21st to end	Night	27% 17% 34% 18% 3	1045

Page generated at 2026-03-14 23:23:58. Version ac07

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