Vancouver
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Vancouver clear sky chart from 2005-11-06 to 2024-11-04.

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	22% 8% 3 7% 4 4 4 3 5% 6% 35%	105507
	Night	27% 9% 3 6% 4 4 3 3 4 5% 31%	102113
Transparency	Day	50% 9% 14% 18% 8% 1	105255
	Night	55% 7% 13% 16% 7% 1	102011
Seeing	Day	22% 4 15% 26% 25% 8%	15411
	Night	29% 5% 13% 24% 23% 6%	23534

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	43% 12% 3 8% 5% 4 3 3 5% 5% 10%	2219
	11th to 20th	Day	30% 13% 3 7% 4 3 3 2 3 6% 26%	2199
	21st to end	Day	56% 10% 3 5% 3 2 3 2 3 3 9%	2779
February	1st to 10th	Day	34% 11% 4 8% 5% 4 3 3 6% 6% 16%	1710
	11th to 20th	Day	30% 10% 3 7% 6% 4 5% 3 5% 4 23%	1665
	21st to end	Day	25% 13% 4 8% 6% 4 5% 3 4 6% 23%	1422
March	1st to 10th	Day	24% 14% 4 10% 6% 4 5% 4 5% 6% 18%	3254
	11th to 20th	Day	23% 10% 2 7% 4 4 4 3 4 6% 34%	3490
	21st to end	Day	27% 13% 3 8% 5% 4 3 3 4 6% 24%	3913
April	1st to 10th	Day	26% 11% 3 9% 5% 5% 5% 4 5% 6% 20%	2280
	11th to 20th	Day	21% 10% 4 7% 6% 4 4 4 5% 6% 29%	2353
	21st to end	Day	23% 10% 4 7% 5% 4 4 3 5% 7% 29%	2520
May	1st to 10th	Day	15% 8% 3 6% 4 4 4 3 4 5% 44%	4420
	11th to 20th	Day	11% 6% 3 7% 4 4 4 4 7% 9% 40%	4377
	21st to end	Day	26% 9% 3 7% 4 5% 4 3 5% 6% 26%	4931
June	1st to 10th	Day	21% 6% 2 8% 5% 4 4 4 6% 7% 32%	2850
	11th to 20th	Day	20% 8% 3 8% 5% 4 4 4 6% 7% 30%	2850
	21st to end	Day	14% 7% 2 7% 5% 5% 4 4 6% 7% 40%	2850
July	1st to 10th	Day	7% 4 2 4 3 2 3 2 3 6% 64%	4740
	11th to 20th	Day	6% 3 1 2 2 2 2 2 3 5% 71%	4740
	21st to end	Day	11% 4 2 4 3 3 3 3 5% 7% 55%	4895
August	1st to 10th	Day	6% 3 1 3 2 2 3 2 4 5% 69%	4350
	11th to 20th	Day	11% 6% 3 6% 4 4 5% 3 4 7% 47%	4026
	21st to end	Day	14% 6% 3 6% 4 4 4 3 5% 7% 44%	4432
September	1st to 10th	Day	13% 5% 3 6% 4 3 4 3 4 7% 48%	2280
	11th to 20th	Day	18% 8% 3 7% 4 4 4 3 5% 6% 38%	2271
	21st to end	Day	24% 6% 1 6% 4 4 4 3 5% 7% 36%	2005
October	1st to 10th	Day	22% 7% 3 8% 5% 5% 4 4 6% 6% 29%	3180
	11th to 20th	Day	36% 7% 3 6% 3 4 3 3 4 6% 24%	3161
	21st to end	Day	30% 11% 4 9% 6% 4 4 3 5% 6% 17%	3179
November	1st to 10th	Day	41% 8% 2 7% 4 3 4 4 4 5% 18%	1512
	11th to 20th	Day	37% 12% 3 6% 4 3 3 3 3 5% 21%	1520
	21st to end	Day	40% 10% 2 6% 3 4 3 3 5% 5% 19%	1520
December	1st to 10th	Day	33% 13% 2 6% 5% 4 3 3 3 4 24%	1487
	11th to 20th	Day	36% 15% 4 10% 4 4 4 3 4 5% 12%	1685
	21st to end	Day	43% 12% 4 9% 5% 4 3 2 3 3 11%	2442
January	1st to 10th	Night	40% 13% 3 8% 5% 4 3 3 3 4 14%	4726
	11th to 20th	Night	34% 9% 3 5% 3 3 4 4 5% 8% 24%	4699
	21st to end	Night	53% 11% 2 6% 3 2 2 2 3 4 12%	4875
February	1st to 10th	Night	33% 11% 4 8% 4 4 4 3 4 5% 20%	2470
	11th to 20th	Night	30% 12% 3 7% 4 4 3 3 3 6% 26%	2402
	21st to end	Night	30% 11% 3 7% 4 3 3 3 4 5% 27%	2032
March	1st to 10th	Night	29% 14% 3 8% 5% 4 4 3 4 5% 21%	3617
	11th to 20th	Night	24% 7% 2 6% 4 4 4 2 5% 6% 36%	3489
	21st to end	Night	31% 14% 4 8% 5% 4 3 3 3 4 22%	3740
April	1st to 10th	Night	26% 12% 4 7% 6% 5% 5% 3 4 5% 23%	1900
	11th to 20th	Night	22% 11% 3 8% 5% 4 4 3 5% 6% 29%	1775
	21st to end	Night	23% 10% 3 8% 5% 5% 4 4 6% 6% 27%	1659
May	1st to 10th	Night	17% 7% 2 6% 3 3 3 2 5% 4 47%	2540
	11th to 20th	Night	11% 5% 2 5% 4 4 4 4 6% 10% 45%	2516
	21st to end	Night	22% 10% 4 9% 6% 4 4 3 4 5% 29%	2641
June	1st to 10th	Night	21% 8% 2 7% 5% 5% 5% 3 5% 5% 34%	1330
	11th to 20th	Night	17% 9% 2 7% 5% 6% 5% 5% 5% 8% 31%	1330
	21st to end	Night	12% 7% 2 7% 4 6% 4 3 5% 9% 42%	1330
July	1st to 10th	Night	6% 3 1 3 3 3 3 2 3 6% 66%	2220
	11th to 20th	Night	4 3 1 2 2 2 2 2 3 5% 75%	2220
	21st to end	Night	7% 6% 2 5% 3 3 4 3 5% 5% 56%	2657
August	1st to 10th	Night	4 2 2 2 2 2 2 4 6% 73%	2578
	11th to 20th	Night	11% 5% 2 6% 3 3 3 3 5% 7% 51%	2812
	21st to end	Night	13% 5% 3 6% 5% 4 4 4 6% 8% 44%	3237
September	1st to 10th	Night	9% 3 2 4 3 4 4 3 5% 7% 56%	1900
	11th to 20th	Night	19% 6% 2 5% 4 3 3 4 5% 5% 42%	1906
	21st to end	Night	22% 6% 3 6% 5% 4 3 2 5% 5% 40%	2175
October	1st to 10th	Night	19% 8% 3 8% 5% 5% 5% 4 5% 6% 31%	3800
	11th to 20th	Night	30% 9% 3 6% 4 4 4 3 4 6% 28%	3777
	21st to end	Night	32% 9% 4 7% 3 4 4 3 4 5% 25%	4380
November	1st to 10th	Night	39% 9% 3 8% 4 4 3 3 3 5% 19%	2638
	11th to 20th	Night	37% 10% 2 5% 4 3 4 3 3 5% 23%	2660
	21st to end	Night	37% 10% 3 7% 5% 4 4 3 3 4 21%	2660
December	1st to 10th	Night	33% 11% 3 6% 4 3 3 3 3 5% 24%	2688
	11th to 20th	Night	45% 13% 3 7% 4 3 3 2 3 3 13%	3520
	21st to end	Night	43% 14% 3 7% 4 3 2 2 3 3 17%	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	75% 9% 7% 6% 2	2219
	11th to 20th	Day	63% 6% 14% 10% 4 4	2220
	21st to end	Day	82% 4 6% 6% 2	2779
February	1st to 10th	Day	69% 7% 10% 8% 3 2	1710
	11th to 20th	Day	65% 7% 10% 12% 6% 1	1674
	21st to end	Day	62% 9% 11% 12% 4 2	1422
March	1st to 10th	Day	62% 11% 14% 9% 4	3254
	11th to 20th	Day	52% 9% 13% 23% 3 1	3490
	21st to end	Day	62% 10% 15% 9% 3	3913
April	1st to 10th	Day	63% 10% 13% 10% 3	2280
	11th to 20th	Day	57% 10% 10% 14% 9% 2	2353
	21st to end	Day	57% 9% 12% 14% 7%	2520
May	1st to 10th	Day	41% 10% 16% 23% 10%	4420
	11th to 20th	Day	38% 12% 23% 22% 5%	4380
	21st to end	Day	55% 13% 16% 14% 2	4932
June	1st to 10th	Day	52% 10% 15% 16% 5% 2	2850
	11th to 20th	Day	54% 13% 15% 14% 4	2850
	21st to end	Day	44% 10% 18% 18% 8% 2	2850
July	1st to 10th	Day	24% 6% 11% 27% 23% 9%	4666
	11th to 20th	Day	18% 5% 15% 39% 21% 1	4659
	21st to end	Day	28% 8% 21% 27% 13% 2	4740
August	1st to 10th	Day	19% 6% 14% 44% 16% 1	4333
	11th to 20th	Day	37% 9% 27% 18% 8% 2	4065
	21st to end	Day	41% 10% 18% 21% 10% 1	4431
September	1st to 10th	Day	38% 8% 15% 23% 11% 4	2280
	11th to 20th	Day	47% 8% 13% 17% 13% 2	2271
	21st to end	Day	49% 10% 14% 17% 9% 1	2005
October	1st to 10th	Day	54% 8% 17% 16% 5%	3180
	11th to 20th	Day	61% 6% 13% 14% 5%	3161
	21st to end	Day	66% 8% 10% 11% 4	3179
November	1st to 10th	Day	69% 9% 9% 9% 4 1	1512
	11th to 20th	Day	68% 7% 8% 9% 5% 2	1520
	21st to end	Day	68% 6% 9% 10% 6% 1	1520
December	1st to 10th	Day	66% 7% 6% 12% 8% 1	1487
	11th to 20th	Day	72% 10% 8% 6% 2 1	1688
	21st to end	Day	77% 7% 7% 6% 2 1	2442
January	1st to 10th	Night	74% 7% 7% 9% 2 1	4726
	11th to 20th	Night	58% 6% 15% 11% 7% 2	4740
	21st to end	Night	79% 4 6% 8% 2 1	4875
February	1st to 10th	Night	67% 8% 10% 9% 4 3	2470
	11th to 20th	Night	62% 8% 10% 12% 7% 1	2404
	21st to end	Night	60% 6% 11% 14% 7% 2	2032
March	1st to 10th	Night	63% 8% 12% 10% 6%	3617
	11th to 20th	Night	47% 11% 14% 22% 5%	3490
	21st to end	Night	67% 8% 12% 10% 2	3740
April	1st to 10th	Night	62% 7% 12% 15% 3	1900
	11th to 20th	Night	54% 9% 11% 16% 7% 3	1775
	21st to end	Night	57% 9% 13% 13% 8%	1659
May	1st to 10th	Night	40% 7% 16% 26% 11% 1	2540
	11th to 20th	Night	32% 11% 27% 23% 7%	2516
	21st to end	Night	56% 9% 18% 14% 3	2641
June	1st to 10th	Night	50% 9% 17% 16% 4 3	1330
	11th to 20th	Night	49% 10% 20% 17% 4	1330
	21st to end	Night	40% 9% 21% 18% 9% 3	1330
July	1st to 10th	Night	21% 5% 10% 33% 24% 7%	2185
	11th to 20th	Night	15% 4 13% 44% 23% 1	2178
	21st to end	Night	28% 6% 15% 31% 18% 3	2573
August	1st to 10th	Night	15% 5% 15% 41% 23% 2	2570
	11th to 20th	Night	33% 7% 30% 19% 8% 4	2839
	21st to end	Night	38% 8% 21% 22% 10% 1	3237
September	1st to 10th	Night	27% 10% 18% 26% 15% 4	1900
	11th to 20th	Night	43% 7% 13% 17% 15% 4	1906
	21st to end	Night	47% 8% 14% 21% 8% 2	2175
October	1st to 10th	Night	52% 8% 18% 16% 6%	3800
	11th to 20th	Night	58% 6% 11% 18% 6%	3777
	21st to end	Night	62% 7% 11% 14% 6%	4380
November	1st to 10th	Night	70% 7% 9% 10% 3 1	2638
	11th to 20th	Night	66% 6% 11% 9% 6% 2	2660
	21st to end	Night	68% 6% 9% 9% 8% 1	2660
December	1st to 10th	Night	64% 7% 11% 12% 6% 1	2688
	11th to 20th	Night	76% 6% 7% 6% 4 1	3516
	21st to end	Night	73% 7% 6% 10% 4 1	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	38% 7% 17% 19% 12% 7%	180
	11th to 20th	Day	40% 12% 13% 16% 14% 6%	180
	21st to end	Day	47% 5% 11% 18% 15% 4	226
February	1st to 10th	Day	39% 6% 15% 23% 16% 2	370
	11th to 20th	Day	34% 2 21% 23% 18% 2	365
	21st to end	Day	27% 7% 18% 29% 13% 5%	312
March	1st to 10th	Day	29% 7% 15% 28% 16% 5%	460
	11th to 20th	Day	29% 5% 17% 21% 23% 5%	460
	21st to end	Day	31% 8% 18% 23% 16% 4	506
April	1st to 10th	Day	27% 6% 20% 23% 21% 4	460
	11th to 20th	Day	23% 4 20% 28% 17% 7%	452
	21st to end	Day	22% 5% 20% 28% 19% 5%	460
May	1st to 10th	Day	15% 4 14% 31% 30% 5%	463
	11th to 20th	Day	16% 3 10% 26% 33% 11%	488
	21st to end	Day	16% 3 15% 31% 26% 9%	595
June	1st to 10th	Day	19% 3 15% 31% 27% 5%	680
	11th to 20th	Day	17% 4 17% 32% 25% 5%	680
	21st to end	Day	12% 3 16% 28% 33% 8%	680
July	1st to 10th	Day	11% 2 11% 29% 36% 11%	671
	11th to 20th	Day	6% 2 10% 32% 40% 10%	668
	21st to end	Day	5% 2 9% 24% 46% 13%	597
August	1st to 10th	Day	9% 2 8% 22% 40% 18%	487
	11th to 20th	Day	6% 2 9% 27% 41% 14%	477
	21st to end	Day	11% 3 15% 29% 30% 11%	539
September	1st to 10th	Day	12% 2 14% 33% 28% 9%	490
	11th to 20th	Day	19% 4 14% 29% 26% 10%	490
	21st to end	Day	23% 4 16% 23% 26% 8%	490
October	1st to 10th	Day	27% 4 14% 25% 21% 9%	490
	11th to 20th	Day	34% 6% 14% 19% 19% 8%	490
	21st to end	Day	32% 8% 18% 24% 13% 4	401
November	1st to 10th	Day	39% 5% 19% 20% 14% 3	186
	11th to 20th	Day	37% 6% 15% 14% 19% 9%	180
	21st to end	Day	39% 8% 14% 17% 9% 12%	180
December	1st to 10th	Day	42% 4 20% 10% 14% 10%	180
	11th to 20th	Day	37% 12% 12% 21% 16% 2	180
	21st to end	Day	41% 5% 19% 22% 10% 3	198
January	1st to 10th	Night	45% 9% 15% 17% 11% 4	850
	11th to 20th	Night	43% 6% 10% 20% 18% 2	850
	21st to end	Night	40% 5% 14% 24% 15% 3	849
February	1st to 10th	Night	35% 7% 17% 25% 14% 3	760
	11th to 20th	Night	33% 5% 17% 28% 15% 2	744
	21st to end	Night	33% 5% 18% 25% 16% 3	628
March	1st to 10th	Night	32% 6% 17% 22% 19% 3	760
	11th to 20th	Night	34% 6% 16% 23% 18% 4	760
	21st to end	Night	34% 6% 15% 23% 17% 6%	836
April	1st to 10th	Night	30% 5% 11% 29% 21% 4	760
	11th to 20th	Night	23% 6% 15% 32% 18% 6%	647
	21st to end	Night	24% 4 17% 31% 19% 5%	519
May	1st to 10th	Night	17% 3 13% 29% 32% 7%	380
	11th to 20th	Night	18% 2 7% 25% 37% 10%	374
	21st to end	Night	18% 4 8% 29% 35% 6%	411
June	1st to 10th	Night	21% 4 10% 29% 32% 5%	380
	11th to 20th	Night	16% 5% 14% 34% 26% 7%	380
	21st to end	Night	12% 3 8% 32% 38% 7%	380
July	1st to 10th	Night	12% 1 8% 25% 44% 10%	370
	11th to 20th	Night	7% 3 7% 26% 45% 12%	368
	21st to end	Night	5% 1 6% 24% 48% 15%	416
August	1st to 10th	Night	8% 1 5% 19% 45% 20%	401
	11th to 20th	Night	7% 4 21% 49% 18%	564
	21st to end	Night	13% 3 8% 23% 41% 12%	679
September	1st to 10th	Night	10% 1 7% 31% 36% 15%	760
	11th to 20th	Night	20% 3 9% 21% 31% 16%	760
	21st to end	Night	22% 4 13% 24% 29% 8%	760
October	1st to 10th	Night	21% 3 13% 26% 25% 11%	760
	11th to 20th	Night	33% 4 12% 23% 22% 7%	760
	21st to end	Night	36% 6% 13% 25% 13% 6%	836
November	1st to 10th	Night	40% 6% 15% 23% 15% 1	754
	11th to 20th	Night	38% 7% 16% 21% 14% 4	760
	21st to end	Night	40% 4 16% 21% 14% 5%	760
December	1st to 10th	Night	35% 5% 14% 26% 16% 4	773
	11th to 20th	Night	49% 6% 14% 21% 9% 1	850
	21st to end	Night	45% 7% 19% 19% 9% 1	935

Page generated at 2024-11-01 19:47:38. Version ac07

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