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 2024-04-03.

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	25% 8% 3 6% 5% 3 4 4 5% 6% 32%	78378
	Night	30% 9% 3 6% 5% 3 4 4 4 5% 29%	81704
Transparency	Day	54% 9% 13% 16% 7% 1	78087
	Night	58% 7% 12% 15% 7% 1	81575
Seeing	Day	22% 4 15% 26% 26% 8%	14510
	Night	29% 5% 13% 24% 23% 6%	22911

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	44% 11% 3 6% 6% 3 3 3 5% 5% 10%	2219
	11th to 20th	Day	31% 12% 4 6% 4 2 3 3 3 6% 26%	2199
	21st to end	Day	57% 10% 3 4 4 2 2 2 3 3 9%	2779
February	1st to 10th	Day	34% 10% 4 7% 7% 3 4 3 5% 6% 17%	1710
	11th to 20th	Day	31% 9% 3 6% 6% 3 5% 4 5% 4 23%	1665
	21st to end	Day	25% 11% 5% 7% 6% 3 4 5% 4 6% 23%	1422
March	1st to 10th	Day	28% 12% 4 8% 6% 3 4 4 4 6% 20%	1968
	11th to 20th	Day	33% 12% 3 5% 5% 3 4 3 3 4 24%	2090
	21st to end	Day	29% 11% 4 7% 6% 3 4 4 4 5% 23%	2352
April	1st to 10th	Day	26% 10% 3 7% 6% 3 6% 5% 5% 6% 21%	2185
	11th to 20th	Day	22% 9% 4 7% 7% 3 4 5% 5% 6% 28%	2227
	21st to end	Day	22% 9% 4 6% 6% 3 5% 4 5% 6% 29%	2387
May	1st to 10th	Day	16% 7% 3 6% 6% 3 5% 5% 6% 6% 37%	2520
	11th to 20th	Day	16% 7% 3 6% 6% 4 5% 5% 6% 7% 36%	2477
	21st to end	Day	18% 8% 4 7% 5% 4 5% 4 5% 6% 35%	2785
June	1st to 10th	Day	21% 6% 2 7% 6% 3 5% 4 5% 7% 33%	2700
	11th to 20th	Day	20% 8% 3 7% 6% 3 5% 5% 6% 7% 30%	2700
	21st to end	Day	14% 6% 2 6% 5% 3 5% 5% 6% 7% 41%	2700
July	1st to 10th	Day	11% 5% 2 5% 4 2 4 4 4 7% 51%	2700
	11th to 20th	Day	11% 4 2 4 4 2 3 4 5% 6% 55%	2700
	21st to end	Day	8% 2 1 3 3 2 3 4 4 6% 65%	2804
August	1st to 10th	Day	10% 4 2 4 4 2 4 3 4 5% 58%	2482
	11th to 20th	Day	8% 5% 2 4 4 2 3 3 4 6% 58%	2298
	21st to end	Day	12% 4 2 5% 5% 3 4 4 5% 8% 48%	2524
September	1st to 10th	Day	14% 6% 3 4 6% 3 3 4 4 6% 48%	2160
	11th to 20th	Day	18% 8% 3 6% 5% 3 4 4 5% 6% 39%	2152
	21st to end	Day	24% 6% 2 5% 4 2 4 5% 5% 7% 37%	1900
October	1st to 10th	Day	24% 7% 3 5% 4 3 4 4 5% 7% 35%	1800
	11th to 20th	Day	33% 7% 4 6% 5% 3 4 4 4 7% 23%	1800
	21st to end	Day	35% 8% 3 6% 5% 3 4 4 5% 6% 23%	1831
November	1st to 10th	Day	41% 8% 2 5% 5% 2 4 4 4 6% 18%	1488
	11th to 20th	Day	37% 10% 4 5% 5% 3 3 4 3 5% 21%	1520
	21st to end	Day	41% 9% 3 6% 4 3 4 3 5% 5% 19%	1520
December	1st to 10th	Day	33% 12% 3 5% 5% 2 4 4 3 4 24%	1487
	11th to 20th	Day	37% 14% 4 9% 6% 3 4 4 4 5% 12%	1685
	21st to end	Day	44% 11% 4 7% 6% 3 3 3 3 3 11%	2442
January	1st to 10th	Night	40% 12% 3 7% 6% 3 3 4 3 4 14%	4726
	11th to 20th	Night	34% 8% 3 5% 4 2 4 4 5% 8% 24%	4699
	21st to end	Night	53% 10% 3 5% 4 2 2 2 3 4 12%	4875
February	1st to 10th	Night	34% 10% 4 7% 5% 3 4 4 4 5% 20%	2470
	11th to 20th	Night	30% 11% 3 6% 5% 3 3 4 3 6% 26%	2402
	21st to end	Night	31% 9% 4 6% 5% 2 3 4 4 5% 27%	2032
March	1st to 10th	Night	31% 11% 3 6% 6% 3 4 4 4 5% 24%	2209
	11th to 20th	Night	34% 9% 3 6% 5% 3 4 3 5% 5% 24%	2089
	21st to end	Night	31% 11% 4 7% 6% 4 4 4 4 4 21%	2246
April	1st to 10th	Night	27% 10% 4 6% 6% 3 5% 4 4 6% 24%	1820
	11th to 20th	Night	23% 10% 3 7% 6% 3 4 4 5% 6% 28%	1681
	21st to end	Night	23% 9% 3 7% 6% 4 4 4 6% 6% 28%	1572
May	1st to 10th	Night	17% 5% 3 6% 5% 3 5% 4 7% 6% 40%	1440
	11th to 20th	Night	16% 6% 3 5% 6% 3 5% 6% 6% 9% 36%	1416
	21st to end	Night	17% 7% 3 7% 6% 3 5% 4 5% 7% 36%	1487
June	1st to 10th	Night	21% 7% 2 6% 5% 4 5% 4 5% 5% 35%	1260
	11th to 20th	Night	18% 8% 3 6% 5% 4 6% 6% 5% 7% 32%	1260
	21st to end	Night	12% 5% 3 5% 5% 4 6% 4 5% 9% 43%	1260
July	1st to 10th	Night	11% 4 2 5% 5% 2 3 3 4 6% 55%	1260
	11th to 20th	Night	7% 4 2 3 4 2 4 3 4 7% 59%	1260
	21st to end	Night	4 3 1 3 3 2 3 4 4 5% 68%	1513
August	1st to 10th	Night	8% 4 1 3 3 2 3 3 4 5% 64%	1453
	11th to 20th	Night	6% 3 1 3 3 1 3 4 4 8% 64%	1602
	21st to end	Night	10% 4 3 5% 5% 3 4 5% 6% 9% 46%	1831
September	1st to 10th	Night	9% 3 2 4 4 3 4 4 5% 7% 56%	1800
	11th to 20th	Night	19% 6% 2 5% 4 3 3 4 5% 5% 43%	1805
	21st to end	Night	22% 6% 3 5% 5% 3 3 3 5% 5% 41%	2060
October	1st to 10th	Night	20% 6% 3 6% 4 3 4 4 5% 5% 40%	2160
	11th to 20th	Night	31% 8% 3 5% 3 2 3 4 4 6% 31%	2160
	21st to end	Night	35% 7% 3 5% 4 2 3 3 4 6% 29%	2524
November	1st to 10th	Night	39% 8% 3 6% 5% 3 4 4 4 5% 19%	2590
	11th to 20th	Night	37% 9% 3 4 4 2 4 4 3 5% 24%	2660
	21st to end	Night	37% 9% 4 7% 5% 3 4 3 3 4 21%	2660
December	1st to 10th	Night	33% 10% 3 6% 5% 2 4 4 3 5% 24%	2688
	11th to 20th	Night	45% 12% 4 7% 5% 2 3 3 3 4 13%	3520
	21st to end	Night	43% 13% 3 6% 4 2 3 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	65% 8% 13% 10% 4	1968
	11th to 20th	Day	66% 6% 10% 13% 4 1	2090
	21st to end	Day	65% 9% 12% 11% 3	2352
April	1st to 10th	Day	63% 10% 13% 10% 3	2185
	11th to 20th	Day	58% 10% 9% 14% 8% 1	2227
	21st to end	Day	57% 9% 12% 13% 7% 1	2387
May	1st to 10th	Day	48% 11% 15% 18% 7% 1	2520
	11th to 20th	Day	48% 10% 17% 19% 5%	2480
	21st to end	Day	51% 10% 16% 20% 3	2786
June	1st to 10th	Day	52% 9% 15% 16% 5% 2	2700
	11th to 20th	Day	55% 13% 15% 13% 4	2700
	21st to end	Day	43% 10% 18% 18% 9% 2	2700
July	1st to 10th	Day	36% 9% 16% 25% 11% 3	2626
	11th to 20th	Day	31% 9% 19% 29% 11% 1	2619
	21st to end	Day	21% 9% 18% 33% 16% 3	2648
August	1st to 10th	Day	30% 9% 17% 29% 12% 2	2465
	11th to 20th	Day	28% 8% 21% 25% 15% 3	2298
	21st to end	Day	35% 11% 19% 27% 7% 1	2524
September	1st to 10th	Day	38% 8% 15% 24% 11% 4	2160
	11th to 20th	Day	46% 8% 12% 18% 14% 2	2152
	21st to end	Day	48% 10% 14% 17% 9% 1	1900
October	1st to 10th	Day	50% 9% 16% 17% 9%	1800
	11th to 20th	Day	62% 9% 13% 13% 3 1	1800
	21st to end	Day	63% 8% 11% 11% 6% 1	1831
November	1st to 10th	Day	69% 9% 9% 9% 4 1	1488
	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	62% 7% 13% 12% 5%	2209
	11th to 20th	Night	62% 8% 9% 14% 6% 1	2090
	21st to end	Night	67% 8% 12% 11% 2	2246
April	1st to 10th	Night	62% 6% 13% 16% 3	1820
	11th to 20th	Night	56% 9% 11% 16% 7% 2	1681
	21st to end	Night	57% 9% 13% 13% 8%	1572
May	1st to 10th	Night	43% 9% 19% 19% 9% 1	1440
	11th to 20th	Night	43% 9% 20% 20% 7% 1	1416
	21st to end	Night	46% 8% 20% 20% 6%	1487
June	1st to 10th	Night	50% 9% 17% 17% 4 3	1260
	11th to 20th	Night	50% 10% 20% 17% 4	1260
	21st to end	Night	38% 9% 22% 19% 9% 3	1260
July	1st to 10th	Night	33% 7% 15% 27% 14% 4	1225
	11th to 20th	Night	26% 7% 22% 30% 13% 2	1218
	21st to end	Night	18% 6% 16% 36% 20% 5%	1429
August	1st to 10th	Night	24% 7% 17% 30% 18% 3	1445
	11th to 20th	Night	20% 6% 26% 28% 13% 6%	1602
	21st to end	Night	33% 9% 19% 29% 10% 1	1831
September	1st to 10th	Night	28% 9% 18% 27% 15% 4	1800
	11th to 20th	Night	43% 6% 12% 18% 16% 5%	1805
	21st to end	Night	46% 8% 14% 22% 9% 2	2060
October	1st to 10th	Night	46% 8% 15% 20% 10% 1	2160
	11th to 20th	Night	56% 8% 13% 16% 6%	2160
	21st to end	Night	59% 7% 10% 15% 9%	2524
November	1st to 10th	Night	69% 7% 9% 11% 3 1	2590
	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% 19% 22% 22% 4	429
	11th to 20th	Day	25% 3 19% 27% 18% 7%	412
	21st to end	Day	21% 5% 20% 29% 20% 5%	420
May	1st to 10th	Day	15% 4 15% 32% 30% 5%	423
	11th to 20th	Day	16% 3 9% 26% 33% 12%	448
	21st to end	Day	14% 3 14% 33% 27% 10%	547
June	1st to 10th	Day	19% 2 15% 31% 28% 6%	630
	11th to 20th	Day	18% 3 17% 33% 25% 5%	630
	21st to end	Day	12% 3 14% 29% 34% 8%	630
July	1st to 10th	Day	11% 1 10% 29% 37% 11%	621
	11th to 20th	Day	6% 2 11% 31% 40% 10%	618
	21st to end	Day	5% 1 7% 24% 49% 14%	550
August	1st to 10th	Day	10% 2 9% 20% 41% 18%	447
	11th to 20th	Day	6% 2 9% 27% 41% 15%	437
	21st to end	Day	11% 3 15% 30% 31% 11%	495
September	1st to 10th	Day	14% 2 15% 33% 28% 8%	450
	11th to 20th	Day	19% 3 13% 28% 27% 10%	450
	21st to end	Day	22% 4 16% 23% 27% 8%	450
October	1st to 10th	Day	27% 4 13% 25% 22% 10%	450
	11th to 20th	Day	34% 5% 15% 18% 19% 9%	450
	21st to end	Day	33% 8% 17% 23% 14% 5%	366
November	1st to 10th	Day	38% 4 19% 20% 14% 3	180
	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	31% 5% 10% 28% 21% 5%	728
	11th to 20th	Night	24% 5% 15% 31% 18% 6%	613
	21st to end	Night	24% 4 16% 31% 20% 5%	492
May	1st to 10th	Night	17% 3 13% 28% 32% 7%	360
	11th to 20th	Night	18% 2 8% 24% 38% 11%	354
	21st to end	Night	19% 3 7% 29% 35% 7%	389
June	1st to 10th	Night	21% 4 10% 28% 32% 5%	360
	11th to 20th	Night	16% 5% 14% 33% 26% 6%	360
	21st to end	Night	13% 2 8% 32% 39% 7%	360
July	1st to 10th	Night	12% 1 8% 24% 45% 11%	350
	11th to 20th	Night	7% 3 7% 26% 45% 12%	348
	21st to end	Night	5% 1 6% 23% 49% 16%	394
August	1st to 10th	Night	8% 1 6% 19% 44% 21%	379
	11th to 20th	Night	7% 4 22% 50% 18%	534
	21st to end	Night	12% 2 8% 22% 42% 13%	643
September	1st to 10th	Night	10% 1 7% 32% 36% 14%	720
	11th to 20th	Night	20% 3 9% 20% 31% 17%	720
	21st to end	Night	22% 4 13% 24% 29% 8%	720
October	1st to 10th	Night	22% 3 14% 26% 25% 12%	720
	11th to 20th	Night	34% 3 12% 23% 21% 7%	720
	21st to end	Night	37% 5% 13% 25% 14% 6%	792
November	1st to 10th	Night	40% 6% 15% 23% 15% 1	740
	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 2024-03-31 03:08:55. Version ac07

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