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 2025-09-04.

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	21% 8% 3 7% 4 4 4 3 5% 6% 35%	130276
	Night	26% 9% 3 7% 4 4 4 3 4 6% 31%	127172
Transparency	Day	50% 8% 15% 18% 7% 1	129982
	Night	55% 7% 14% 16% 7% 1	127043
Seeing	Day	21% 4 15% 26% 25% 8%	16482
	Night	29% 5% 13% 25% 23% 6%	24553

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	39% 12% 3 8% 5% 5% 4 3 5% 5% 10%	3179
	11th to 20th	Day	21% 10% 3 6% 5% 5% 4 3 6% 8% 29%	3159
	21st to end	Day	45% 8% 2 5% 3 2 3 2 4 4 21%	4003
February	1st to 10th	Day	32% 11% 4 8% 5% 5% 3 3 6% 6% 16%	1800
	11th to 20th	Day	30% 10% 3 7% 6% 4 5% 3 5% 4 24%	1755
	21st to end	Day	26% 12% 4 9% 6% 4 5% 3 4 5% 22%	1495
March	1st to 10th	Day	25% 14% 4 11% 6% 5% 5% 4 4 5% 17%	4686
	11th to 20th	Day	25% 13% 4 7% 4 4 4 3 4 5% 26%	5000
	21st to end	Day	30% 12% 3 9% 5% 5% 4 4 4 5% 18%	5613
April	1st to 10th	Day	26% 11% 3 9% 5% 5% 5% 4 6% 7% 20%	2400
	11th to 20th	Day	20% 9% 3 7% 6% 4 4 4 5% 6% 31%	2479
	21st to end	Day	23% 9% 4 7% 4 4 4 3 5% 7% 30%	2653
May	1st to 10th	Day	13% 8% 3 6% 4 4 4 3 4 6% 45%	6320
	11th to 20th	Day	16% 8% 3 8% 4 4 4 4 6% 8% 33%	6277
	21st to end	Day	21% 8% 3 7% 4 5% 5% 4 7% 8% 29%	7077
June	1st to 10th	Day	20% 6% 2 8% 5% 4 4 4 6% 7% 34%	3000
	11th to 20th	Day	19% 8% 3 8% 5% 4 5% 4 6% 7% 30%	3000
	21st to end	Day	14% 7% 2 7% 5% 5% 4 4 6% 7% 39%	3000
July	1st to 10th	Day	8% 4 2 4 3 3 3 3 4 6% 60%	6780
	11th to 20th	Day	5% 3 1 3 2 2 2 2 4 6% 71%	6780
	21st to end	Day	8% 4 2 4 2 3 3 3 4 7% 61%	7021
August	1st to 10th	Day	6% 3 1 3 2 3 3 2 4 5% 68%	4488
	11th to 20th	Day	11% 6% 3 6% 5% 4 5% 3 4 7% 47%	4156
	21st to end	Day	14% 6% 3 6% 4 4 4 3 5% 7% 44%	4572
September	1st to 10th	Day	13% 5% 3 6% 4 3 4 3 4 7% 48%	2317
	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	40% 9% 2 7% 5% 3 4 4 4 5% 17%	1568
	11th to 20th	Day	37% 12% 3 6% 4 4 3 3 4 5% 21%	1600
	21st to end	Day	40% 10% 2 7% 3 4 4 3 5% 5% 18%	1600
December	1st to 10th	Day	25% 10% 3 8% 6% 6% 5% 5% 6% 6% 20%	2559
	11th to 20th	Day	42% 13% 3 10% 4 4 3 3 3 4 10%	2645
	21st to end	Day	42% 13% 4 9% 6% 5% 4 3 3 3 9%	3498
January	1st to 10th	Night	39% 13% 4 9% 5% 4 3 3 3 4 13%	6766
	11th to 20th	Night	24% 8% 3 6% 4 4 5% 5% 6% 8% 29%	6739
	21st to end	Night	40% 9% 2 5% 3 2 3 3 5% 6% 23%	6953
February	1st to 10th	Night	32% 12% 4 8% 5% 4 4 4 4 5% 21%	2600
	11th to 20th	Night	30% 12% 3 7% 4 4 3 3 4 5% 26%	2532
	21st to end	Night	30% 11% 3 7% 4 3 3 3 4 5% 26%	2135
March	1st to 10th	Night	27% 13% 3 8% 5% 4 4 3 5% 6% 22%	5205
	11th to 20th	Night	27% 10% 3 7% 4 4 4 3 5% 6% 28%	4999
	21st to end	Night	37% 13% 4 8% 4 4 3 3 3 4 17%	5362
April	1st to 10th	Night	27% 12% 4 7% 5% 5% 5% 3 4 5% 23%	2000
	11th to 20th	Night	21% 10% 3 8% 5% 4 4 3 5% 6% 30%	1869
	21st to end	Night	22% 9% 3 8% 5% 5% 3 4 6% 6% 29%	1746
May	1st to 10th	Night	16% 7% 3 6% 3 3 3 2 5% 4 49%	3640
	11th to 20th	Night	15% 7% 3 7% 4 4 4 4 7% 9% 36%	3616
	21st to end	Night	18% 9% 4 10% 6% 5% 5% 3 5% 6% 29%	3795
June	1st to 10th	Night	20% 8% 2 7% 5% 5% 5% 3 5% 5% 36%	1400
	11th to 20th	Night	16% 9% 3 7% 5% 6% 5% 4 5% 8% 32%	1400
	21st to end	Night	12% 7% 2 7% 4 6% 4 3 5% 9% 41%	1400
July	1st to 10th	Night	9% 4 2 4 3 3 3 2 4 6% 62%	3180
	11th to 20th	Night	3 2 1 2 2 2 2 2 3 5% 77%	3180
	21st to end	Night	6% 4 1 4 3 3 4 3 5% 6% 62%	3822
August	1st to 10th	Night	5% 2 2 2 2 2 2 4 6% 73%	2660
	11th to 20th	Night	11% 5% 2 6% 3 3 3 3 5% 7% 51%	2902
	21st to end	Night	13% 5% 2 6% 5% 4 4 4 6% 8% 44%	3339
September	1st to 10th	Night	8% 3 2 4 3 4 4 3 5% 7% 56%	1930
	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% 18%	2730
	11th to 20th	Night	37% 10% 3 5% 4 4 3 3 3 5% 22%	2800
	21st to end	Night	37% 11% 3 7% 5% 4 4 3 3 4 20%	2800
December	1st to 10th	Night	27% 11% 3 7% 5% 5% 7% 5% 6% 6% 19%	4616
	11th to 20th	Night	45% 12% 3 8% 4 3 3 3 3 4 11%	5560
	21st to end	Night	41% 14% 4 8% 4 4 3 2 3 3 14%	7458

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	74% 8% 8% 8% 2	3179
	11th to 20th	Day	53% 7% 15% 16% 6% 3	3180
	21st to end	Day	68% 4 9% 15% 5%	4003
February	1st to 10th	Day	68% 7% 10% 9% 4 3	1800
	11th to 20th	Day	65% 7% 9% 13% 6% 1	1764
	21st to end	Day	64% 9% 11% 11% 4 2	1495
March	1st to 10th	Day	65% 9% 12% 11% 3	4686
	11th to 20th	Day	60% 7% 11% 19% 3	5000
	21st to end	Day	68% 9% 13% 8% 2	5613
April	1st to 10th	Day	63% 10% 14% 10% 3	2400
	11th to 20th	Day	55% 10% 10% 16% 8% 2	2479
	21st to end	Day	55% 9% 13% 15% 7%	2653
May	1st to 10th	Day	40% 8% 17% 24% 11%	6320
	11th to 20th	Day	47% 11% 19% 19% 4	6280
	21st to end	Day	49% 11% 20% 17% 3	7078
June	1st to 10th	Day	50% 9% 16% 17% 6% 2	3000
	11th to 20th	Day	54% 12% 16% 14% 4	3000
	21st to end	Day	45% 10% 18% 18% 8% 2	3000
July	1st to 10th	Day	27% 6% 19% 24% 18% 6%	6706
	11th to 20th	Day	17% 6% 21% 35% 20% 1	6699
	21st to end	Day	23% 7% 30% 27% 11% 2	6824
August	1st to 10th	Day	20% 6% 15% 43% 16% 1	4471
	11th to 20th	Day	37% 9% 27% 18% 8% 2	4195
	21st to end	Day	40% 10% 18% 21% 11% 1	4571
September	1st to 10th	Day	37% 8% 16% 23% 11% 4	2317
	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% 8% 9% 4 1	1568
	11th to 20th	Day	69% 7% 8% 9% 5% 2	1600
	21st to end	Day	68% 6% 10% 10% 6% 1	1600
December	1st to 10th	Day	62% 8% 10% 15% 5% 1	2559
	11th to 20th	Day	76% 8% 7% 6% 2 1	2648
	21st to end	Day	79% 7% 7% 6% 1	3498
January	1st to 10th	Night	74% 6% 8% 9% 2	6766
	11th to 20th	Night	50% 9% 16% 17% 6% 2	6780
	21st to end	Night	64% 5% 12% 16% 3	6953
February	1st to 10th	Night	66% 8% 10% 9% 5% 3	2600
	11th to 20th	Night	62% 8% 10% 12% 7% 2	2534
	21st to end	Night	61% 6% 11% 14% 6% 2	2135
March	1st to 10th	Night	61% 8% 13% 12% 6%	5205
	11th to 20th	Night	56% 9% 11% 19% 4	5000
	21st to end	Night	71% 7% 12% 8% 1	5362
April	1st to 10th	Night	63% 7% 13% 15% 3	2000
	11th to 20th	Night	53% 9% 12% 16% 7% 2	1869
	21st to end	Night	55% 9% 14% 14% 8%	1746
May	1st to 10th	Night	39% 5% 16% 27% 12%	3637
	11th to 20th	Night	41% 11% 23% 19% 5%	3616
	21st to end	Night	54% 8% 18% 18% 2	3795
June	1st to 10th	Night	48% 9% 18% 17% 5% 3	1400
	11th to 20th	Night	49% 10% 19% 19% 4	1400
	21st to end	Night	41% 9% 21% 18% 9% 3	1400
July	1st to 10th	Night	26% 5% 19% 28% 17% 5%	3145
	11th to 20th	Night	13% 4 23% 37% 22% 2	3138
	21st to end	Night	23% 5% 25% 32% 14% 2	3714
August	1st to 10th	Night	16% 5% 15% 40% 22% 2	2652
	11th to 20th	Night	33% 7% 30% 19% 8% 4	2929
	21st to end	Night	37% 8% 21% 22% 11% 1	3339
September	1st to 10th	Night	26% 10% 18% 27% 15% 4	1930
	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	2730
	11th to 20th	Night	67% 6% 10% 9% 6% 2	2800
	21st to end	Night	69% 6% 10% 8% 7% 1	2800
December	1st to 10th	Night	64% 9% 12% 10% 4 1	4616
	11th to 20th	Night	77% 6% 7% 7% 3	5556
	21st to end	Night	75% 7% 6% 9% 3	7458

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	40% 7% 16% 21% 12% 7%	200
	11th to 20th	Day	37% 11% 13% 19% 16% 6%	200
	21st to end	Day	44% 5% 11% 19% 14% 6%	250
February	1st to 10th	Day	37% 6% 17% 22% 16% 2	400
	11th to 20th	Day	34% 2 21% 24% 17% 2	395
	21st to end	Day	29% 7% 18% 28% 13% 5%	337
March	1st to 10th	Day	29% 7% 16% 27% 16% 5%	500
	11th to 20th	Day	30% 7% 17% 21% 21% 5%	500
	21st to end	Day	32% 8% 19% 23% 15% 4	550
April	1st to 10th	Day	27% 6% 20% 23% 21% 4	500
	11th to 20th	Day	22% 5% 21% 28% 17% 6%	492
	21st to end	Day	21% 6% 20% 28% 19% 6%	500
May	1st to 10th	Day	15% 5% 16% 31% 29% 5%	503
	11th to 20th	Day	17% 4 12% 26% 31% 10%	528
	21st to end	Day	16% 3 17% 31% 25% 8%	643
June	1st to 10th	Day	18% 3 16% 32% 27% 5%	730
	11th to 20th	Day	17% 4 17% 32% 25% 5%	730
	21st to end	Day	13% 3 16% 28% 32% 8%	730
July	1st to 10th	Day	11% 2 13% 30% 35% 10%	721
	11th to 20th	Day	6% 3 11% 33% 38% 10%	718
	21st to end	Day	5% 2 9% 25% 46% 13%	644
August	1st to 10th	Day	9% 3 9% 23% 39% 17%	527
	11th to 20th	Day	7% 3 11% 27% 39% 13%	517
	21st to end	Day	10% 3 15% 29% 31% 12%	583
September	1st to 10th	Day	12% 2 14% 34% 28% 9%	503
	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% 6% 19% 20% 15% 3	200
	11th to 20th	Day	37% 7% 16% 15% 19% 8%	200
	21st to end	Day	40% 9% 14% 17% 9% 12%	200
December	1st to 10th	Day	40% 4 19% 11% 17% 11%	200
	11th to 20th	Day	40% 11% 14% 19% 16% 2	200
	21st to end	Day	42% 6% 19% 20% 10% 3	220
January	1st to 10th	Night	45% 8% 15% 18% 11% 3	900
	11th to 20th	Night	40% 7% 11% 22% 19% 2	900
	21st to end	Night	38% 4 14% 24% 16% 4	894
February	1st to 10th	Night	34% 7% 18% 25% 14% 3	800
	11th to 20th	Night	33% 5% 17% 27% 16% 2	784
	21st to end	Night	33% 6% 18% 24% 15% 3	660
March	1st to 10th	Night	32% 6% 17% 23% 19% 3	800
	11th to 20th	Night	34% 6% 16% 23% 17% 4	800
	21st to end	Night	35% 6% 15% 22% 17% 5%	880
April	1st to 10th	Night	30% 5% 11% 29% 20% 4	800
	11th to 20th	Night	23% 6% 16% 32% 18% 6%	681
	21st to end	Night	23% 4 17% 31% 20% 5%	546
May	1st to 10th	Night	17% 3 12% 30% 32% 7%	400
	11th to 20th	Night	19% 3 7% 25% 36% 10%	394
	21st to end	Night	18% 3 9% 29% 35% 6%	433
June	1st to 10th	Night	20% 4 11% 30% 31% 5%	400
	11th to 20th	Night	15% 5% 14% 34% 26% 7%	400
	21st to end	Night	13% 3 8% 32% 38% 7%	400
July	1st to 10th	Night	12% 1 9% 25% 43% 10%	390
	11th to 20th	Night	6% 3 8% 28% 44% 11%	388
	21st to end	Night	5% 1 7% 24% 48% 16%	438
August	1st to 10th	Night	8% 1 6% 20% 44% 20%	423
	11th to 20th	Night	8% 5% 23% 47% 17%	594
	21st to end	Night	12% 3 8% 22% 43% 12%	715
September	1st to 10th	Night	10% 1 7% 31% 37% 15%	772
	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	41% 6% 14% 23% 15% 1	780
	11th to 20th	Night	39% 7% 16% 21% 13% 4	800
	21st to end	Night	40% 5% 16% 21% 14% 5%	800
December	1st to 10th	Night	34% 5% 14% 26% 17% 4	815
	11th to 20th	Night	49% 6% 15% 21% 9% 1	900
	21st to end	Night	45% 7% 19% 19% 8% 1	990

Page generated at 2025-08-31 22:51:04. Version ac07

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