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 2024-12-06.

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	13% 19% 5% 9% 5% 4 4 3 4 5% 28%	105761
	Night	13% 20% 6% 10% 5% 4 4 3 4 5% 25%	102542
Transparency	Day	56% 8% 11% 14% 9% 2	105507
	Night	59% 9% 11% 13% 7% 1	102442
Seeing	Day	23% 17% 27% 21% 9% 2	15486
	Night	23% 12% 28% 25% 11% 1	23624

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	14% 34% 7% 8% 5% 3 3 2 4 5% 14%	2219
	11th to 20th	Day	13% 23% 6% 12% 5% 3 3 2 3 4 27%	2199
	21st to end	Day	22% 35% 6% 9% 4 4 2 2 2 3 11%	2670
February	1st to 10th	Day	12% 28% 7% 10% 6% 4 3 3 4 6% 16%	1699
	11th to 20th	Day	9% 24% 8% 12% 6% 4 3 2 4 5% 22%	1665
	21st to end	Day	8% 27% 8% 14% 6% 5% 4 4 5% 5% 15%	1418
March	1st to 10th	Day	6% 26% 10% 15% 6% 4 4 3 5% 6% 15%	3226
	11th to 20th	Day	7% 27% 7% 9% 4 4 3 3 4 6% 26%	3490
	21st to end	Day	9% 31% 7% 14% 6% 5% 4 3 4 3 16%	3936
April	1st to 10th	Day	12% 32% 7% 12% 7% 5% 4 4 4 4 11%	2280
	11th to 20th	Day	10% 29% 7% 11% 7% 4 4 2 3 4 17%	2363
	21st to end	Day	15% 28% 6% 11% 7% 5% 3 3 4 5% 14%	2551
May	1st to 10th	Day	9% 21% 5% 10% 5% 4 4 3 5% 5% 29%	4420
	11th to 20th	Day	12% 17% 4 9% 6% 5% 5% 4 6% 9% 22%	4377
	21st to end	Day	19% 29% 5% 10% 5% 4 4 2 4 4 14%	5018
June	1st to 10th	Day	25% 19% 4 9% 5% 4 3 3 5% 5% 17%	2850
	11th to 20th	Day	23% 18% 5% 9% 7% 6% 4 4 5% 5% 14%	2850
	21st to end	Day	20% 12% 4 9% 6% 4 4 3 4 6% 26%	2850
July	1st to 10th	Day	8% 6% 2 5% 4 3 3 2 3 4 61%	4740
	11th to 20th	Day	7% 4 2 3 3 3 3 2 4 5% 65%	4740
	21st to end	Day	8% 5% 2 5% 4 4 5% 5% 7% 8% 46%	4959
August	1st to 10th	Day	7% 4 2 4 3 3 4 3 5% 8% 56%	4401
	11th to 20th	Day	8% 7% 2 7% 6% 5% 6% 4 6% 8% 41%	4039
	21st to end	Day	14% 8% 3 7% 5% 5% 5% 4 6% 7% 37%	4438
September	1st to 10th	Day	13% 7% 3 7% 4 4 3 3 6% 6% 44%	2280
	11th to 20th	Day	20% 11% 3 7% 4 3 4 3 3 5% 37%	2280
	21st to end	Day	20% 15% 3 7% 4 4 4 3 5% 6% 30%	1997
October	1st to 10th	Day	15% 13% 5% 11% 7% 7% 5% 4 4 5% 25%	3180
	11th to 20th	Day	25% 16% 4 9% 4 4 4 3 5% 6% 22%	3161
	21st to end	Day	13% 20% 7% 14% 8% 7% 5% 4 4 4 14%	3099
November	1st to 10th	Day	20% 25% 6% 8% 5% 4 4 3 5% 4 16%	1568
	11th to 20th	Day	15% 31% 7% 9% 5% 4 3 2 3 4 17%	1600
	21st to end	Day	16% 25% 5% 11% 7% 5% 4 3 4 4 16%	1600
December	1st to 10th	Day	13% 20% 7% 9% 5% 4 4 3 4 5% 25%	1471
	11th to 20th	Day	16% 27% 8% 13% 6% 5% 4 3 4 3 11%	1685
	21st to end	Day	15% 30% 6% 12% 6% 5% 3 3 4 5% 11%	2442
January	1st to 10th	Night	13% 30% 7% 14% 6% 4 3 2 4 4 12%	4726
	11th to 20th	Night	14% 21% 5% 10% 5% 3 4 3 4 6% 25%	4699
	21st to end	Night	20% 31% 8% 11% 5% 4 3 2 3 4 9%	4985
February	1st to 10th	Night	12% 25% 7% 11% 6% 6% 5% 3 5% 6% 13%	2480
	11th to 20th	Night	9% 21% 7% 14% 7% 5% 4 3 4 6% 20%	2402
	21st to end	Night	9% 24% 8% 13% 5% 5% 4 3 4 5% 19%	2036
March	1st to 10th	Night	7% 22% 10% 14% 8% 6% 5% 3 5% 5% 14%	3646
	11th to 20th	Night	6% 24% 6% 10% 5% 4 4 3 4 5% 29%	3489
	21st to end	Night	9% 25% 7% 12% 6% 4 4 3 4 5% 21%	3716
April	1st to 10th	Night	8% 25% 7% 12% 8% 7% 5% 3 5% 6% 15%	1900
	11th to 20th	Night	9% 21% 5% 11% 7% 5% 5% 5% 5% 6% 21%	1764
	21st to end	Night	11% 23% 6% 13% 6% 5% 5% 3 5% 6% 18%	1629
May	1st to 10th	Night	7% 17% 6% 10% 4 3 3 2 4 5% 39%	2540
	11th to 20th	Night	10% 15% 5% 9% 5% 4 5% 4 6% 10% 28%	2516
	21st to end	Night	15% 26% 6% 11% 5% 3 3 3 4 5% 19%	2553
June	1st to 10th	Night	18% 17% 4 9% 5% 5% 5% 3 5% 6% 23%	1330
	11th to 20th	Night	16% 16% 5% 10% 7% 6% 5% 3 5% 7% 20%	1330
	21st to end	Night	15% 11% 4 10% 5% 5% 4 4 5% 6% 33%	1330
July	1st to 10th	Night	7% 6% 2 5% 3 2 3 2 3 5% 60%	2220
	11th to 20th	Night	5% 4 1 3 2 3 2 2 4 5% 69%	2220
	21st to end	Night	7% 6% 3 7% 3 4 4 4 5% 6% 49%	2592
August	1st to 10th	Night	5% 4 1 3 2 3 3 2 4 9% 65%	2529
	11th to 20th	Night	8% 11% 3 7% 4 5% 5% 3 5% 8% 41%	2796
	21st to end	Night	14% 8% 3 7% 4 4 3 3 5% 7% 41%	3232
September	1st to 10th	Night	9% 7% 2 8% 6% 3 5% 4 6% 6% 44%	1900
	11th to 20th	Night	16% 11% 4 7% 4 4 3 4 5% 6% 36%	1900
	21st to end	Night	17% 13% 4 8% 4 4 5% 3 5% 7% 30%	2178
October	1st to 10th	Night	14% 14% 5% 11% 7% 6% 7% 3 5% 5% 23%	3800
	11th to 20th	Night	18% 14% 5% 9% 5% 5% 5% 4 6% 6% 23%	3777
	21st to end	Night	15% 19% 6% 11% 5% 5% 5% 4 4 4 21%	4442
November	1st to 10th	Night	22% 25% 7% 9% 5% 4 4 3 4 3 14%	2730
	11th to 20th	Night	18% 25% 6% 11% 6% 5% 4 3 4 3 14%	2800
	21st to end	Night	15% 24% 6% 13% 6% 4 5% 3 4 6% 15%	2800
December	1st to 10th	Night	12% 20% 6% 11% 7% 4 4 4 4 7% 20%	2821
	11th to 20th	Night	16% 28% 8% 14% 6% 5% 4 4 3 3 9%	3520
	21st to end	Night	16% 31% 6% 12% 5% 4 4 3 4 4 11%	5214

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	73% 8% 9% 5% 3 1	2219
	11th to 20th	Day	64% 6% 10% 9% 6% 5%	2220
	21st to end	Day	82% 5% 6% 5% 2 1	2670
February	1st to 10th	Day	71% 7% 9% 7% 6% 1	1699
	11th to 20th	Day	66% 8% 9% 11% 6% 1	1674
	21st to end	Day	72% 9% 8% 7% 4 1	1418
March	1st to 10th	Day	68% 9% 10% 8% 4	3226
	11th to 20th	Day	58% 8% 10% 18% 5%	3490
	21st to end	Day	73% 7% 10% 6% 4	3936
April	1st to 10th	Day	76% 7% 9% 6% 1	2280
	11th to 20th	Day	71% 6% 7% 8% 7% 1	2363
	21st to end	Day	72% 7% 8% 8% 4	2551
May	1st to 10th	Day	53% 8% 16% 18% 5%	4420
	11th to 20th	Day	48% 13% 21% 15% 3	4380
	21st to end	Day	66% 11% 10% 10% 3	5019
June	1st to 10th	Day	67% 8% 10% 9% 4 3	2850
	11th to 20th	Day	69% 9% 10% 8% 3	2850
	21st to end	Day	58% 8% 12% 14% 7% 1	2850
July	1st to 10th	Day	27% 4 8% 19% 28% 14%	4666
	11th to 20th	Day	22% 5% 10% 31% 26% 5%	4659
	21st to end	Day	30% 10% 18% 23% 15% 5%	4802
August	1st to 10th	Day	24% 9% 13% 26% 27% 2	4384
	11th to 20th	Day	36% 14% 18% 20% 10% 2	4078
	21st to end	Day	44% 9% 16% 22% 7% 1	4437
September	1st to 10th	Day	41% 8% 12% 24% 12% 4	2280
	11th to 20th	Day	51% 5% 9% 17% 16% 3	2280
	21st to end	Day	56% 7% 14% 14% 8% 2	1997
October	1st to 10th	Day	57% 9% 12% 14% 7%	3180
	11th to 20th	Day	62% 8% 10% 16% 4	3161
	21st to end	Day	64% 13% 10% 8% 4	3099
November	1st to 10th	Day	72% 9% 7% 8% 4 1	1568
	11th to 20th	Day	72% 7% 7% 6% 6% 1	1600
	21st to end	Day	71% 7% 9% 7% 5% 2	1600
December	1st to 10th	Day	65% 6% 11% 8% 9%	1471
	11th to 20th	Day	78% 9% 7% 3 2 2	1688
	21st to end	Day	76% 8% 8% 4 2 1	2442
January	1st to 10th	Night	75% 8% 8% 6% 2 1	4726
	11th to 20th	Night	60% 8% 10% 11% 8% 4	4740
	21st to end	Night	81% 6% 6% 4 2 1	4985
February	1st to 10th	Night	69% 10% 8% 6% 6% 1	2480
	11th to 20th	Night	65% 10% 9% 10% 5% 1	2404
	21st to end	Night	66% 9% 9% 9% 5% 2	2036
March	1st to 10th	Night	64% 11% 10% 9% 6%	3646
	11th to 20th	Night	53% 11% 15% 18% 3	3490
	21st to end	Night	63% 13% 12% 8% 4 1	3716
April	1st to 10th	Night	68% 9% 11% 10% 1	1900
	11th to 20th	Night	63% 7% 10% 12% 6% 1	1764
	21st to end	Night	65% 10% 11% 10% 5%	1629
May	1st to 10th	Night	46% 10% 14% 24% 7%	2540
	11th to 20th	Night	44% 14% 23% 17% 1	2516
	21st to end	Night	61% 14% 11% 11% 2	2553
June	1st to 10th	Night	62% 7% 16% 8% 5% 2	1330
	11th to 20th	Night	62% 10% 14% 10% 3	1330
	21st to end	Night	52% 10% 15% 17% 5% 1	1330
July	1st to 10th	Night	25% 7% 11% 22% 22% 12%	2185
	11th to 20th	Night	18% 5% 14% 38% 25% 1	2178
	21st to end	Night	30% 10% 15% 27% 12% 6%	2510
August	1st to 10th	Night	18% 7% 12% 38% 24% 2	2521
	11th to 20th	Night	37% 13% 19% 20% 9% 2	2823
	21st to end	Night	41% 9% 22% 21% 7% 1	3232
September	1st to 10th	Night	35% 10% 16% 24% 11% 3	1900
	11th to 20th	Night	46% 9% 10% 17% 15% 3	1900
	21st to end	Night	52% 9% 15% 15% 8% 2	2178
October	1st to 10th	Night	57% 11% 12% 14% 5% 1	3800
	11th to 20th	Night	58% 10% 10% 15% 7%	3777
	21st to end	Night	61% 9% 10% 14% 6%	4442
November	1st to 10th	Night	74% 7% 7% 7% 3 1	2730
	11th to 20th	Night	73% 8% 7% 6% 5% 1	2800
	21st to end	Night	71% 6% 9% 7% 5% 2	2800
December	1st to 10th	Night	65% 9% 11% 9% 6% 1	2821
	11th to 20th	Night	80% 7% 7% 3 2 1	3516
	21st to end	Night	77% 8% 6% 5% 4	5214

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	27% 21% 23% 18% 9% 2	180
	11th to 20th	Day	33% 14% 20% 24% 9%	180
	21st to end	Day	26% 15% 23% 25% 9% 2	198
February	1st to 10th	Day	26% 22% 28% 17% 6% 1	359
	11th to 20th	Day	20% 21% 32% 18% 7% 2	365
	21st to end	Day	18% 25% 30% 19% 8% 1	310
March	1st to 10th	Day	21% 21% 28% 21% 9% 1	460
	11th to 20th	Day	22% 21% 25% 18% 13% 2	460
	21st to end	Day	29% 18% 27% 16% 8% 2	506
April	1st to 10th	Day	29% 23% 23% 17% 7% 1	460
	11th to 20th	Day	24% 20% 25% 19% 7% 6%	452
	21st to end	Day	28% 21% 22% 17% 10% 3	460
May	1st to 10th	Day	20% 15% 24% 26% 13% 1	463
	11th to 20th	Day	21% 11% 24% 25% 15% 4	488
	21st to end	Day	24% 11% 29% 23% 10% 3	643
June	1st to 10th	Day	27% 18% 28% 18% 6% 1	680
	11th to 20th	Day	28% 20% 29% 16% 7% 1	680
	21st to end	Day	21% 16% 30% 22% 9% 1	680
July	1st to 10th	Day	19% 14% 30% 26% 9% 2	671
	11th to 20th	Day	13% 14% 34% 29% 8% 3	668
	21st to end	Day	11% 11% 34% 29% 11% 4	635
August	1st to 10th	Day	11% 12% 26% 29% 16% 5%	487
	11th to 20th	Day	11% 13% 28% 31% 13% 3	477
	21st to end	Day	18% 15% 34% 22% 9% 3	539
September	1st to 10th	Day	18% 21% 30% 21% 9% 2	490
	11th to 20th	Day	26% 16% 26% 23% 8% 2	490
	21st to end	Day	31% 18% 24% 19% 8% 1	490
October	1st to 10th	Day	29% 17% 25% 17% 10% 2	490
	11th to 20th	Day	35% 16% 23% 16% 10% 2	490
	21st to end	Day	33% 19% 22% 19% 6% 1	367
November	1st to 10th	Day	31% 20% 26% 16% 8%	200
	11th to 20th	Day	36% 16% 19% 16% 10% 4	200
	21st to end	Day	25% 23% 23% 17% 7% 6%	200
December	1st to 10th	Day	24% 16% 17% 24% 15% 4	190
	11th to 20th	Day	28% 14% 24% 22% 12% 1	180
	21st to end	Day	22% 16% 30% 27% 5% 1	198
January	1st to 10th	Night	33% 18% 27% 16% 5%	850
	11th to 20th	Night	32% 12% 27% 22% 7%	850
	21st to end	Night	24% 17% 30% 20% 8%	869
February	1st to 10th	Night	26% 14% 34% 21% 5% 1	760
	11th to 20th	Night	17% 15% 36% 24% 7%	744
	21st to end	Night	21% 16% 32% 21% 10%	628
March	1st to 10th	Night	24% 13% 30% 21% 11% 1	760
	11th to 20th	Night	23% 19% 28% 20% 11%	760
	21st to end	Night	23% 15% 29% 23% 9% 1	836
April	1st to 10th	Night	22% 14% 30% 23% 10% 1	760
	11th to 20th	Night	19% 15% 31% 24% 10% 1	636
	21st to end	Night	21% 10% 31% 24% 12% 2	489
May	1st to 10th	Night	15% 10% 24% 35% 15% 1	380
	11th to 20th	Night	18% 7% 15% 39% 18% 2	374
	21st to end	Night	23% 7% 27% 30% 12%	411
June	1st to 10th	Night	23% 8% 31% 29% 8% 1	380
	11th to 20th	Night	17% 11% 33% 30% 9% 1	380
	21st to end	Night	17% 9% 27% 32% 14% 1	380
July	1st to 10th	Night	12% 4 24% 39% 21% 1	370
	11th to 20th	Night	8% 4 24% 42% 22%	368
	21st to end	Night	6% 6% 19% 42% 26% 1	416
August	1st to 10th	Night	8% 4 15% 43% 29% 1	378
	11th to 20th	Night	8% 4 22% 39% 26% 2	548
	21st to end	Night	14% 8% 26% 33% 19%	674
September	1st to 10th	Night	12% 7% 30% 34% 17% 1	760
	11th to 20th	Night	18% 8% 25% 25% 23% 3	760
	21st to end	Night	24% 12% 27% 24% 12% 1	760
October	1st to 10th	Night	24% 10% 28% 23% 14% 1	760
	11th to 20th	Night	28% 11% 28% 20% 12% 1	760
	21st to end	Night	31% 16% 27% 18% 7%	836
November	1st to 10th	Night	36% 15% 26% 20% 3	780
	11th to 20th	Night	35% 16% 25% 16% 8%	800
	21st to end	Night	29% 16% 26% 21% 6% 1	800
December	1st to 10th	Night	21% 15% 31% 24% 9%	822
	11th to 20th	Night	31% 15% 31% 19% 3	850
	21st to end	Night	28% 15% 34% 19% 3	935

Page generated at 2024-12-03 03:45:15. Version ac07

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