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 2024-01-15.

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	16% 18% 5% 8% 6% 3 4 4 4 5% 25%	74741
	Night	14% 19% 6% 9% 7% 3 4 4 4 6% 23%	75060
Transparency	Day	61% 7% 10% 13% 7% 1	74427
	Night	62% 8% 11% 12% 6% 1	74892
Seeing	Day	23% 17% 27% 22% 9% 2	14266
	Night	23% 12% 28% 25% 11% 1	22557

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	20% 29% 8% 7% 6% 2 4 4 4 4 13%	1329
	11th to 20th	Day	16% 23% 7% 10% 6% 3 4 3 4 5% 20%	1290
	21st to end	Day	12% 26% 7% 11% 6% 4 4 3 3 5% 18%	1516
February	1st to 10th	Day	13% 27% 8% 9% 7% 3 4 4 4 7% 16%	1610
	11th to 20th	Day	9% 23% 9% 11% 7% 3 3 3 4 5% 21%	1575
	21st to end	Day	8% 23% 10% 13% 7% 4 4 5% 5% 5% 16%	1336
March	1st to 10th	Day	11% 23% 8% 11% 7% 4 4 4 5% 5% 16%	1849
	11th to 20th	Day	11% 31% 9% 11% 6% 4 4 3 3 5% 13%	1980
	21st to end	Day	12% 31% 7% 11% 7% 4 4 4 4 3 13%	2238
April	1st to 10th	Day	13% 29% 9% 10% 7% 4 4 4 4 4 11%	2160
	11th to 20th	Day	12% 29% 7% 10% 8% 4 4 3 3 4 16%	2237
	21st to end	Day	17% 25% 7% 10% 8% 3 4 3 4 5% 14%	2416
May	1st to 10th	Day	13% 22% 6% 9% 7% 4 4 4 4 5% 22%	2520
	11th to 20th	Day	15% 18% 5% 9% 7% 3 4 6% 6% 8% 20%	2477
	21st to end	Day	23% 17% 5% 8% 5% 3 5% 4 4 5% 20%	2830
June	1st to 10th	Day	27% 17% 4 7% 7% 3 4 4 5% 5% 17%	2700
	11th to 20th	Day	25% 17% 5% 8% 8% 5% 5% 4 4 5% 14%	2700
	21st to end	Day	22% 11% 5% 7% 7% 3 4 4 4 6% 28%	2700
July	1st to 10th	Day	14% 9% 3 7% 6% 3 5% 4 4 6% 39%	2700
	11th to 20th	Day	13% 6% 3 5% 5% 3 5% 5% 5% 7% 43%	2700
	21st to end	Day	9% 6% 3 5% 5% 3 4 5% 5% 7% 48%	2840
August	1st to 10th	Day	11% 6% 3 4 5% 3 4 4 5% 7% 48%	2507
	11th to 20th	Day	9% 5% 2 4 4 3 4 4 4 7% 54%	2311
	21st to end	Day	15% 8% 2 6% 6% 4 5% 6% 6% 8% 35%	2530
September	1st to 10th	Day	13% 8% 3 6% 5% 3 3 4 6% 6% 43%	2160
	11th to 20th	Day	21% 10% 3 6% 4 2 4 3 3 5% 37%	2160
	21st to end	Day	21% 14% 3 6% 4 2 5% 3 5% 5% 31%	1892
October	1st to 10th	Day	18% 14% 4 7% 6% 3 4 4 4 6% 31%	1800
	11th to 20th	Day	24% 17% 4 7% 5% 3 4 4 3 6% 23%	1800
	21st to end	Day	21% 20% 5% 8% 5% 3 3 4 4 4 22%	1792
November	1st to 10th	Day	22% 23% 7% 7% 6% 3 4 4 5% 4 16%	1488
	11th to 20th	Day	16% 30% 8% 8% 6% 2 4 3 3 4 18%	1520
	21st to end	Day	18% 23% 6% 9% 7% 4 4 3 4 5% 16%	1520
December	1st to 10th	Day	14% 19% 8% 9% 5% 3 5% 4 4 5% 24%	1431
	11th to 20th	Day	16% 26% 9% 12% 7% 4 4 4 4 3 11%	1685
	21st to end	Day	16% 29% 7% 10% 7% 4 3 3 4 5% 11%	2442
January	1st to 10th	Night	18% 27% 7% 10% 7% 3 4 3 3 4 14%	2836
	11th to 20th	Night	16% 22% 8% 9% 7% 3 5% 4 4 5% 17%	2775
	21st to end	Night	13% 21% 8% 11% 7% 4 4 4 4 7% 16%	2839
February	1st to 10th	Night	14% 23% 8% 10% 7% 4 6% 4 5% 5% 14%	2349
	11th to 20th	Night	10% 20% 8% 13% 8% 4 4 5% 4 5% 19%	2272
	21st to end	Night	10% 20% 10% 11% 6% 4 5% 4 4 6% 20%	1920
March	1st to 10th	Night	11% 21% 10% 11% 8% 4 5% 3 4 6% 17%	2109
	11th to 20th	Night	9% 27% 9% 11% 7% 4 4 4 4 4 17%	1979
	21st to end	Night	11% 26% 8% 11% 8% 3 4 4 4 4 16%	2117
April	1st to 10th	Night	8% 23% 8% 10% 9% 5% 6% 4 5% 6% 16%	1800
	11th to 20th	Night	11% 19% 7% 10% 8% 4 5% 6% 5% 6% 20%	1671
	21st to end	Night	12% 20% 7% 12% 7% 4 5% 4 4 6% 19%	1544
May	1st to 10th	Night	11% 17% 5% 9% 6% 3 4 4 5% 7% 30%	1440
	11th to 20th	Night	13% 13% 5% 8% 5% 3 7% 6% 6% 9% 26%	1416
	21st to end	Night	18% 15% 4 7% 7% 3 3 5% 5% 6% 27%	1441
June	1st to 10th	Night	19% 15% 5% 8% 6% 3 7% 4 5% 6% 23%	1260
	11th to 20th	Night	18% 14% 5% 9% 8% 4 5% 4 5% 8% 20%	1260
	21st to end	Night	15% 10% 4 9% 6% 4 4 4 5% 5% 34%	1260
July	1st to 10th	Night	10% 7% 4 7% 5% 3 4 4 5% 6% 45%	1260
	11th to 20th	Night	9% 6% 3 5% 4 3 5% 4 5% 8% 49%	1260
	21st to end	Night	8% 5% 3 4 3 3 3 4 4 6% 57%	1476
August	1st to 10th	Night	9% 6% 2 3 3 2 3 3 5% 7% 56%	1432
	11th to 20th	Night	6% 6% 2 4 3 3 4 4 4 8% 56%	1586
	21st to end	Night	11% 8% 4 7% 6% 3 4 5% 6% 7% 39%	1826
September	1st to 10th	Night	10% 7% 2 7% 7% 2 5% 5% 6% 6% 44%	1800
	11th to 20th	Night	17% 11% 4 6% 5% 3 4 4 5% 6% 37%	1800
	21st to end	Night	16% 13% 4 7% 5% 3 4 4 5% 7% 31%	2063
October	1st to 10th	Night	17% 15% 4 8% 7% 3 4 3 4 5% 31%	2160
	11th to 20th	Night	21% 17% 5% 7% 6% 4 4 4 4 5% 24%	2160
	21st to end	Night	20% 19% 6% 9% 6% 3 4 4 4 4 21%	2560
November	1st to 10th	Night	24% 24% 7% 8% 6% 3 4 3 4 4 14%	2590
	11th to 20th	Night	20% 24% 6% 10% 7% 4 5% 4 4 3 15%	2660
	21st to end	Night	17% 23% 7% 11% 7% 3 5% 3 4 6% 15%	2660
December	1st to 10th	Night	13% 19% 7% 11% 8% 4 4 5% 4 7% 20%	2745
	11th to 20th	Night	16% 26% 9% 12% 8% 4 4 5% 3 3 9%	3520
	21st to end	Night	16% 30% 7% 10% 6% 3 4 4 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	78% 6% 6% 4 3 2	1329
	11th to 20th	Day	70% 7% 8% 9% 4 2	1290
	21st to end	Day	71% 7% 9% 8% 4 1	1516
February	1st to 10th	Day	71% 7% 9% 6% 6% 1	1610
	11th to 20th	Day	68% 8% 9% 10% 5% 1	1584
	21st to end	Day	72% 8% 9% 7% 4 1	1336
March	1st to 10th	Day	70% 9% 8% 8% 5%	1849
	11th to 20th	Day	77% 6% 6% 8% 3 1	1980
	21st to end	Day	77% 7% 8% 7% 2	2238
April	1st to 10th	Day	77% 7% 9% 6% 1	2160
	11th to 20th	Day	74% 5% 7% 7% 6%	2237
	21st to end	Day	73% 7% 8% 8% 4	2416
May	1st to 10th	Day	64% 8% 12% 12% 4	2520
	11th to 20th	Day	60% 10% 13% 14% 4	2480
	21st to end	Day	64% 8% 10% 14% 4	2831
June	1st to 10th	Day	67% 8% 10% 8% 4 3	2700
	11th to 20th	Day	71% 9% 10% 8% 3	2700
	21st to end	Day	58% 7% 12% 15% 7% 1	2700
July	1st to 10th	Day	46% 7% 12% 21% 12% 2	2626
	11th to 20th	Day	39% 8% 16% 24% 13% 1	2619
	21st to end	Day	33% 7% 13% 26% 17% 4	2682
August	1st to 10th	Day	35% 7% 15% 26% 15% 2	2490
	11th to 20th	Day	30% 7% 15% 27% 17% 3	2311
	21st to end	Day	44% 9% 14% 23% 8% 2	2530
September	1st to 10th	Day	42% 8% 12% 23% 11% 4	2160
	11th to 20th	Day	51% 5% 9% 16% 16% 3	2160
	21st to end	Day	55% 6% 14% 14% 9% 2	1892
October	1st to 10th	Day	56% 6% 13% 16% 10%	1800
	11th to 20th	Day	65% 6% 9% 15% 6%	1800
	21st to end	Day	66% 7% 9% 11% 6% 1	1792
November	1st to 10th	Day	71% 9% 7% 8% 4 1	1488
	11th to 20th	Day	72% 6% 7% 6% 6% 1	1520
	21st to end	Day	71% 6% 9% 7% 5% 2	1520
December	1st to 10th	Day	66% 7% 10% 9% 8%	1431
	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	76% 7% 7% 6% 3 1	2836
	11th to 20th	Night	69% 9% 8% 8% 5% 1	2775
	21st to end	Night	70% 8% 10% 7% 4 1	2839
February	1st to 10th	Night	70% 10% 8% 5% 6% 1	2349
	11th to 20th	Night	66% 10% 9% 10% 4 1	2274
	21st to end	Night	66% 8% 9% 10% 5% 2	1920
March	1st to 10th	Night	68% 10% 9% 8% 4	2109
	11th to 20th	Night	72% 8% 9% 8% 3	1980
	21st to end	Night	73% 8% 10% 7% 2	2117
April	1st to 10th	Night	69% 9% 11% 10% 1	1800
	11th to 20th	Night	66% 7% 10% 12% 5%	1671
	21st to end	Night	65% 9% 11% 10% 5%	1544
May	1st to 10th	Night	53% 12% 14% 16% 5% 1	1440
	11th to 20th	Night	56% 11% 17% 14% 3	1416
	21st to end	Night	57% 10% 14% 15% 4	1441
June	1st to 10th	Night	62% 7% 17% 8% 5% 2	1260
	11th to 20th	Night	63% 10% 15% 10% 3	1260
	21st to end	Night	52% 10% 16% 16% 5% 1	1260
July	1st to 10th	Night	37% 10% 16% 22% 11% 3	1225
	11th to 20th	Night	31% 9% 23% 26% 10%	1218
	21st to end	Night	26% 7% 15% 34% 14% 4	1394
August	1st to 10th	Night	28% 8% 15% 30% 17% 2	1424
	11th to 20th	Night	27% 9% 19% 27% 15% 3	1586
	21st to end	Night	40% 10% 19% 24% 6% 2	1826
September	1st to 10th	Night	37% 10% 16% 24% 10% 3	1800
	11th to 20th	Night	46% 9% 9% 17% 16% 3	1800
	21st to end	Night	51% 8% 15% 16% 8% 2	2063
October	1st to 10th	Night	55% 7% 12% 18% 7% 1	2160
	11th to 20th	Night	63% 7% 9% 14% 7%	2160
	21st to end	Night	67% 6% 7% 12% 8%	2560
November	1st to 10th	Night	75% 7% 6% 7% 3 1	2590
	11th to 20th	Night	73% 7% 7% 6% 5% 1	2660
	21st to end	Night	71% 6% 9% 7% 5% 2	2660
December	1st to 10th	Night	65% 9% 10% 9% 5% 1	2745
	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	34% 13% 20% 24% 9%	169
	21st to end	Day	23% 13% 26% 27% 10% 2	176
February	1st to 10th	Day	28% 23% 29% 15% 5%	330
	11th to 20th	Day	21% 21% 32% 17% 7% 1	335
	21st to end	Day	15% 24% 32% 20% 7% 1	282
March	1st to 10th	Day	22% 21% 28% 20% 8% 1	420
	11th to 20th	Day	24% 21% 26% 17% 11% 2	420
	21st to end	Day	30% 19% 26% 16% 7% 2	462
April	1st to 10th	Day	30% 23% 22% 16% 8% 1	420
	11th to 20th	Day	25% 21% 23% 18% 6% 6%	412
	21st to end	Day	28% 21% 21% 17% 10% 3	420
May	1st to 10th	Day	21% 15% 25% 25% 13% 1	423
	11th to 20th	Day	22% 10% 24% 26% 15% 4	448
	21st to end	Day	23% 10% 29% 24% 10% 3	592
June	1st to 10th	Day	27% 18% 28% 18% 7% 1	630
	11th to 20th	Day	27% 19% 29% 17% 7% 1	630
	21st to end	Day	20% 15% 31% 24% 10% 1	630
July	1st to 10th	Day	18% 13% 30% 28% 9% 2	621
	11th to 20th	Day	14% 15% 32% 29% 8% 3	618
	21st to end	Day	9% 10% 34% 31% 12% 4	586
August	1st to 10th	Day	12% 11% 25% 30% 16% 5%	447
	11th to 20th	Day	11% 12% 29% 31% 14% 3	437
	21st to end	Day	17% 14% 34% 22% 9% 3	495
September	1st to 10th	Day	19% 22% 30% 20% 7% 2	450
	11th to 20th	Day	26% 15% 25% 24% 8% 2	450
	21st to end	Day	29% 16% 25% 21% 9% 1	450
October	1st to 10th	Day	27% 16% 25% 18% 10% 2	450
	11th to 20th	Day	34% 16% 23% 15% 10% 2	450
	21st to end	Day	34% 18% 21% 19% 7% 1	335
November	1st to 10th	Day	31% 17% 26% 18% 8%	180
	11th to 20th	Day	36% 14% 19% 16% 11% 4	180
	21st to end	Day	27% 23% 21% 17% 7% 6%	180
December	1st to 10th	Day	24% 17% 18% 22% 15% 3	180
	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% 23% 7%	825
	21st to end	Night	23% 16% 31% 21% 8%	821
February	1st to 10th	Night	27% 14% 35% 20% 4	720
	11th to 20th	Night	18% 16% 36% 23% 7%	704
	21st to end	Night	21% 15% 33% 21% 10%	592
March	1st to 10th	Night	25% 13% 31% 20% 10% 1	720
	11th to 20th	Night	24% 19% 29% 19% 10%	720
	21st to end	Night	24% 15% 29% 22% 9% 1	792
April	1st to 10th	Night	23% 12% 30% 23% 10% 1	720
	11th to 20th	Night	20% 15% 30% 24% 9% 1	603
	21st to end	Night	21% 10% 32% 24% 12% 2	464
May	1st to 10th	Night	16% 9% 24% 34% 15% 1	360
	11th to 20th	Night	19% 6% 14% 40% 19% 2	354
	21st to end	Night	23% 6% 26% 31% 13% 1	389
June	1st to 10th	Night	23% 8% 30% 30% 9% 1	360
	11th to 20th	Night	18% 10% 33% 30% 9% 1	360
	21st to end	Night	17% 9% 26% 33% 15% 1	360
July	1st to 10th	Night	13% 4 24% 37% 21% 1	350
	11th to 20th	Night	9% 4 25% 43% 19%	348
	21st to end	Night	6% 5% 19% 43% 27% 1	394
August	1st to 10th	Night	9% 4 16% 42% 28% 1	358
	11th to 20th	Night	7% 4 21% 40% 26% 2	518
	21st to end	Night	14% 8% 26% 33% 19%	638
September	1st to 10th	Night	13% 7% 30% 33% 16% 1	720
	11th to 20th	Night	18% 8% 24% 24% 23% 3	720
	21st to end	Night	24% 11% 27% 25% 12% 1	720
October	1st to 10th	Night	24% 9% 27% 23% 15% 1	720
	11th to 20th	Night	29% 10% 27% 21% 12% 1	720
	21st to end	Night	32% 17% 26% 18% 7% 1	792
November	1st to 10th	Night	37% 14% 26% 20% 3	740
	11th to 20th	Night	35% 16% 25% 16% 8%	760
	21st to end	Night	31% 16% 26% 21% 6% 1	760
December	1st to 10th	Night	22% 15% 31% 23% 9% 1	800
	11th to 20th	Night	31% 15% 31% 19% 3	850
	21st to end	Night	28% 15% 34% 19% 3	935

Page generated at 2024-01-12 13:17:46. Version ac07

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