Boundary Bay
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Boundary Bay clear sky chart from 2005-11-06 to 2025-01-07.

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	19% 8% 3 7% 5% 4 4 3 5% 6% 35%	108858
	Night	24% 9% 3 7% 5% 4 4 3 5% 6% 29%	108782
Transparency	Day	50% 4 16% 19% 8% 1	108607
	Night	57% 4 13% 17% 7% 1	108679
Seeing	Day	22% 5% 14% 26% 24% 9%	15509
	Night	29% 6% 12% 25% 22% 6%	23816

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	38% 13% 3 8% 6% 5% 4 3 4 5% 10%	2261
	11th to 20th	Day	28% 13% 4 8% 5% 5% 4 2 3 6% 22%	2199
	21st to end	Day	50% 13% 3 7% 4 3 3 2 3 3 8%	2813
February	1st to 10th	Day	30% 12% 4 8% 6% 4 4 4 6% 6% 15%	1710
	11th to 20th	Day	27% 11% 3 10% 6% 5% 5% 3 4 6% 21%	1665
	21st to end	Day	23% 12% 4 9% 5% 4 5% 4 6% 6% 23%	1429
March	1st to 10th	Day	21% 14% 4 9% 7% 6% 5% 4 6% 7% 18%	3252
	11th to 20th	Day	20% 8% 3 8% 5% 4 4 3 5% 7% 32%	3490
	21st to end	Day	23% 13% 4 9% 4 4 4 3 5% 6% 25%	3917
April	1st to 10th	Day	23% 10% 3 8% 6% 6% 5% 5% 6% 7% 22%	2280
	11th to 20th	Day	19% 10% 3 7% 6% 5% 5% 4 6% 6% 30%	2344
	21st to end	Day	20% 10% 3 8% 5% 4 5% 3 5% 7% 30%	2519
May	1st to 10th	Day	13% 7% 2 7% 4 4 4 3 5% 6% 45%	4420
	11th to 20th	Day	10% 7% 2 7% 4 4 5% 4 7% 11% 40%	4377
	21st to end	Day	22% 9% 3 7% 5% 4 5% 4 6% 6% 27%	4896
June	1st to 10th	Day	18% 7% 2 7% 6% 5% 4 4 5% 7% 34%	2850
	11th to 20th	Day	17% 8% 3 8% 4 4 5% 4 6% 8% 33%	2850
	21st to end	Day	11% 7% 2 6% 5% 5% 5% 4 5% 8% 41%	2850
July	1st to 10th	Day	6% 3 2 4 3 3 2 3 4 5% 66%	4740
	11th to 20th	Day	5% 3 1 3 2 2 2 2 3 5% 72%	4740
	21st to end	Day	9% 4 2 4 3 3 3 3 6% 7% 57%	4863
August	1st to 10th	Day	5% 3 1 3 2 2 2 2 4 6% 69%	4350
	11th to 20th	Day	10% 5% 2 6% 4 4 5% 3 5% 8% 49%	4026
	21st to end	Day	12% 7% 3 6% 4 5% 4 3 5% 7% 44%	4411
September	1st to 10th	Day	11% 6% 3 6% 4 4 4 4 5% 8% 46%	2280
	11th to 20th	Day	16% 7% 3 7% 4 4 4 3 6% 7% 38%	2275
	21st to end	Day	21% 7% 2 6% 4 5% 4 4 5% 7% 35%	2000
October	1st to 10th	Day	19% 7% 3 7% 6% 5% 5% 5% 7% 7% 30%	3180
	11th to 20th	Day	32% 8% 2 7% 5% 3 4 3 5% 6% 24%	3161
	21st to end	Day	27% 12% 4 9% 5% 5% 4 3 5% 6% 19%	3202
November	1st to 10th	Day	35% 10% 3 8% 5% 5% 5% 3 4 4 18%	1568
	11th to 20th	Day	31% 11% 4 8% 6% 4 3 3 4 5% 21%	1600
	21st to end	Day	35% 11% 4 7% 5% 5% 4 4 5% 5% 16%	1600
December	1st to 10th	Day	23% 12% 4 11% 7% 6% 5% 4 4 6% 19%	2597
	11th to 20th	Day	37% 13% 4 9% 5% 4 4 4 4 4 12%	2645
	21st to end	Day	34% 15% 4 11% 6% 5% 5% 3 4 4 9%	3498
January	1st to 10th	Night	35% 13% 4 9% 6% 4 4 3 4 5% 13%	4816
	11th to 20th	Night	31% 9% 3 7% 4 4 4 3 5% 7% 24%	4699
	21st to end	Night	47% 13% 3 8% 3 3 3 2 3 4 11%	4837
February	1st to 10th	Night	31% 12% 4 7% 5% 4 4 4 5% 6% 19%	2470
	11th to 20th	Night	26% 13% 3 9% 4 3 3 3 4 5% 25%	2402
	21st to end	Night	27% 12% 3 7% 4 3 4 4 5% 5% 26%	2024
March	1st to 10th	Night	24% 15% 5% 9% 6% 4 4 3 4 6% 19%	3621
	11th to 20th	Night	20% 7% 3 5% 4 4 5% 3 7% 9% 32%	3489
	21st to end	Night	28% 15% 5% 8% 5% 4 3 2 5% 5% 19%	3736
April	1st to 10th	Night	24% 10% 3 8% 5% 5% 4 4 6% 7% 23%	1900
	11th to 20th	Night	20% 10% 3 7% 4 5% 5% 3 5% 6% 31%	1783
	21st to end	Night	20% 9% 3 11% 5% 5% 5% 4 6% 6% 26%	1660
May	1st to 10th	Night	14% 8% 3 6% 4 4 4 3 4 6% 45%	2540
	11th to 20th	Night	9% 6% 3 5% 4 3 4 4 6% 9% 46%	2516
	21st to end	Night	18% 9% 3 10% 6% 6% 5% 3 5% 6% 29%	2676
June	1st to 10th	Night	19% 9% 2 7% 5% 4 4 4 6% 7% 33%	1330
	11th to 20th	Night	13% 10% 3 10% 6% 5% 5% 4 5% 9% 31%	1330
	21st to end	Night	10% 6% 2 7% 5% 4 4 5% 6% 9% 42%	1330
July	1st to 10th	Night	5% 2 1 4 3 3 3 2 4 6% 67%	2220
	11th to 20th	Night	3 2 1 3 2 2 3 2 2 4 76%	2220
	21st to end	Night	7% 4 2 5% 4 3 3 3 5% 6% 58%	2689
August	1st to 10th	Night	4 2 1 2 2 1 2 2 4 6% 73%	2580
	11th to 20th	Night	11% 5% 2 6% 4 4 4 3 4 7% 51%	2812
	21st to end	Night	11% 4 2 6% 5% 4 5% 4 6% 9% 44%	3260
September	1st to 10th	Night	8% 3 1 4 3 4 4 4 5% 7% 57%	1900
	11th to 20th	Night	17% 5% 2 5% 5% 4 4 3 6% 6% 42%	1901
	21st to end	Night	18% 6% 3 7% 4 4 5% 3 5% 6% 39%	2180
October	1st to 10th	Night	17% 7% 2 7% 5% 5% 5% 4 6% 9% 33%	3800
	11th to 20th	Night	28% 7% 3 6% 5% 4 5% 3 5% 6% 28%	3777
	21st to end	Night	27% 10% 3 7% 5% 4 4 3 5% 7% 25%	4358
November	1st to 10th	Night	35% 10% 4 8% 5% 4 5% 4 4 5% 18%	2730
	11th to 20th	Night	33% 11% 2 6% 4 5% 4 3 5% 6% 21%	2800
	21st to end	Night	32% 12% 4 9% 5% 5% 4 3 4 4 19%	2800
December	1st to 10th	Night	24% 9% 3 9% 7% 6% 6% 5% 7% 7% 17%	4578
	11th to 20th	Night	40% 14% 4 8% 5% 4 4 3 4 4 11%	5560
	21st to end	Night	33% 14% 4 9% 5% 4 4 3 4 4 15%	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	77% 4 9% 7% 2 1	2261
	11th to 20th	Day	64% 4 8% 14% 6% 4	2220
	21st to end	Day	83% 3 7% 6% 2	2813
February	1st to 10th	Day	68% 5% 12% 9% 4 2	1710
	11th to 20th	Day	65% 5% 10% 13% 6% 1	1674
	21st to end	Day	61% 6% 13% 14% 4 2	1429
March	1st to 10th	Day	62% 5% 14% 12% 6%	3252
	11th to 20th	Day	52% 5% 14% 25% 4 1	3490
	21st to end	Day	62% 4 18% 13% 4	3917
April	1st to 10th	Day	60% 6% 18% 12% 3	2280
	11th to 20th	Day	55% 6% 11% 17% 9% 2	2344
	21st to end	Day	54% 5% 15% 16% 8% 1	2519
May	1st to 10th	Day	40% 5% 17% 25% 12% 1	4420
	11th to 20th	Day	37% 8% 25% 24% 6%	4380
	21st to end	Day	55% 5% 21% 16% 2	4897
June	1st to 10th	Day	49% 5% 20% 19% 6% 2	2850
	11th to 20th	Day	49% 6% 22% 19% 4	2850
	21st to end	Day	41% 5% 22% 21% 8% 2	2850
July	1st to 10th	Day	22% 3 12% 27% 24% 11%	4666
	11th to 20th	Day	17% 3 16% 37% 25% 2	4659
	21st to end	Day	28% 4 24% 29% 13% 3	4709
August	1st to 10th	Day	19% 3 15% 44% 17% 1	4333
	11th to 20th	Day	36% 3 31% 20% 8% 1	4065
	21st to end	Day	41% 4 21% 23% 10% 1	4410
September	1st to 10th	Day	36% 6% 19% 23% 11% 4	2280
	11th to 20th	Day	46% 4 16% 19% 13% 2	2275
	21st to end	Day	49% 6% 15% 19% 10% 1	2000
October	1st to 10th	Day	52% 4 18% 21% 5%	3180
	11th to 20th	Day	61% 4 13% 15% 7%	3161
	21st to end	Day	67% 3 11% 13% 5%	3202
November	1st to 10th	Day	69% 4 12% 10% 4 1	1568
	11th to 20th	Day	67% 5% 11% 12% 4 2	1600
	21st to end	Day	70% 4 10% 11% 5% 1	1600
December	1st to 10th	Day	68% 3 8% 16% 5%	2597
	11th to 20th	Day	77% 3 9% 8% 2 1	2648
	21st to end	Day	79% 3 9% 8% 1	3498
January	1st to 10th	Night	76% 4 8% 10% 2	4816
	11th to 20th	Night	60% 4 10% 16% 7% 3	4740
	21st to end	Night	79% 3 6% 8% 2 1	4837
February	1st to 10th	Night	66% 6% 11% 10% 4 3	2470
	11th to 20th	Night	63% 5% 10% 14% 7% 1	2404
	21st to end	Night	60% 5% 10% 15% 7% 2	2024
March	1st to 10th	Night	66% 4 10% 14% 6%	3621
	11th to 20th	Night	49% 5% 17% 23% 5%	3490
	21st to end	Night	69% 4 14% 11% 2	3736
April	1st to 10th	Night	61% 6% 13% 17% 4	1900
	11th to 20th	Night	54% 6% 11% 18% 8% 3	1783
	21st to end	Night	59% 5% 12% 14% 9%	1660
May	1st to 10th	Night	42% 5% 12% 28% 12% 1	2540
	11th to 20th	Night	33% 8% 26% 24% 8%	2516
	21st to end	Night	57% 5% 18% 15% 4	2676
June	1st to 10th	Night	51% 5% 18% 19% 5% 2	1330
	11th to 20th	Night	51% 6% 18% 20% 4	1330
	21st to end	Night	39% 5% 23% 21% 9% 3	1330
July	1st to 10th	Night	21% 3 10% 33% 24% 10%	2185
	11th to 20th	Night	16% 2 12% 41% 28% 2	2178
	21st to end	Night	29% 2 14% 31% 21% 3	2604
August	1st to 10th	Night	15% 3 14% 38% 27% 2	2572
	11th to 20th	Night	35% 3 29% 22% 9% 3	2839
	21st to end	Night	38% 4 21% 26% 11% 1	3260
September	1st to 10th	Night	26% 6% 19% 29% 15% 5%	1900
	11th to 20th	Night	44% 4 14% 18% 16% 4	1901
	21st to end	Night	46% 4 15% 24% 8% 2	2180
October	1st to 10th	Night	51% 3 19% 20% 6%	3800
	11th to 20th	Night	58% 3 12% 20% 6%	3777
	21st to end	Night	61% 3 15% 15% 6%	4358
November	1st to 10th	Night	69% 5% 10% 12% 3 1	2730
	11th to 20th	Night	66% 5% 11% 10% 5% 2	2800
	21st to end	Night	70% 4 9% 9% 7% 1	2800
December	1st to 10th	Night	66% 4 11% 13% 5% 1	4578
	11th to 20th	Night	79% 3 8% 8% 3	5556
	21st to end	Night	74% 3 8% 12% 2 1	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	41% 6% 19% 17% 11% 6%	192
	11th to 20th	Day	38% 14% 13% 15% 14% 5%	180
	21st to end	Day	49% 5% 13% 16% 12% 5%	226
February	1st to 10th	Day	39% 7% 15% 21% 15% 2	370
	11th to 20th	Day	34% 8% 15% 24% 16% 3	365
	21st to end	Day	28% 9% 18% 28% 11% 5%	316
March	1st to 10th	Day	30% 6% 15% 27% 17% 5%	460
	11th to 20th	Day	29% 7% 15% 21% 20% 9%	460
	21st to end	Day	31% 6% 18% 22% 18% 4	506
April	1st to 10th	Day	28% 9% 16% 24% 19% 5%	460
	11th to 20th	Day	23% 4 21% 28% 19% 6%	452
	21st to end	Day	23% 6% 20% 28% 18% 6%	460
May	1st to 10th	Day	15% 4 13% 35% 27% 6%	463
	11th to 20th	Day	16% 4 10% 28% 31% 11%	488
	21st to end	Day	16% 4 15% 30% 25% 10%	573
June	1st to 10th	Day	19% 3 15% 32% 25% 6%	680
	11th to 20th	Day	18% 4 18% 34% 21% 6%	680
	21st to end	Day	12% 3 14% 29% 32% 10%	680
July	1st to 10th	Day	11% 2 11% 29% 35% 12%	671
	11th to 20th	Day	5% 3 10% 32% 37% 12%	668
	21st to end	Day	6% 1 9% 25% 42% 16%	578
August	1st to 10th	Day	9% 3 6% 23% 40% 20%	487
	11th to 20th	Day	6% 2 6% 31% 38% 17%	477
	21st to end	Day	12% 2 13% 31% 30% 12%	539
September	1st to 10th	Day	14% 2 11% 31% 30% 12%	490
	11th to 20th	Day	18% 4 10% 29% 29% 10%	490
	21st to end	Day	25% 3 15% 23% 25% 9%	490
October	1st to 10th	Day	25% 5% 12% 23% 23% 12%	490
	11th to 20th	Day	33% 4 11% 21% 20% 11%	490
	21st to end	Day	32% 7% 17% 23% 15% 6%	408
November	1st to 10th	Day	36% 5% 19% 22% 15% 4	200
	11th to 20th	Day	33% 12% 15% 12% 17% 11%	200
	21st to end	Day	38% 9% 15% 20% 9% 10%	200
December	1st to 10th	Day	38% 8% 17% 13% 13% 13%	200
	11th to 20th	Day	44% 9% 11% 22% 13% 2	200
	21st to end	Day	39% 7% 21% 22% 8% 2	220
January	1st to 10th	Night	44% 8% 16% 18% 11% 3	880
	11th to 20th	Night	42% 6% 13% 20% 16% 3	850
	21st to end	Night	39% 8% 13% 22% 16% 3	839
February	1st to 10th	Night	37% 9% 18% 23% 11% 3	760
	11th to 20th	Night	33% 8% 17% 26% 15% 1	744
	21st to end	Night	32% 10% 18% 25% 12% 4	628
March	1st to 10th	Night	33% 7% 18% 21% 18% 3	760
	11th to 20th	Night	33% 7% 16% 23% 17% 4	760
	21st to end	Night	34% 8% 12% 25% 17% 5%	836
April	1st to 10th	Night	31% 6% 11% 28% 18% 5%	760
	11th to 20th	Night	24% 7% 13% 31% 18% 7%	655
	21st to end	Night	26% 6% 12% 31% 20% 5%	520
May	1st to 10th	Night	19% 3 12% 27% 33% 5%	380
	11th to 20th	Night	17% 3 6% 30% 37% 8%	374
	21st to end	Night	19% 3 6% 30% 34% 7%	411
June	1st to 10th	Night	22% 5% 10% 28% 31% 4	380
	11th to 20th	Night	18% 4 11% 35% 26% 6%	380
	21st to end	Night	13% 3 7% 34% 35% 9%	380
July	1st to 10th	Night	12% 1 5% 28% 45% 8%	370
	11th to 20th	Night	7% 3 6% 31% 42% 10%	368
	21st to end	Night	4 1 7% 27% 47% 14%	416
August	1st to 10th	Night	8% 2 5% 19% 45% 21%	403
	11th to 20th	Night	8% 2 4 23% 46% 18%	564
	21st to end	Night	11% 2 7% 24% 43% 12%	689
September	1st to 10th	Night	10% 1 5% 33% 37% 13%	760
	11th to 20th	Night	19% 4 10% 18% 33% 16%	760
	21st to end	Night	22% 2 13% 25% 30% 7%	760
October	1st to 10th	Night	22% 4 11% 26% 28% 10%	760
	11th to 20th	Night	32% 4 12% 23% 21% 9%	760
	21st to end	Night	34% 6% 14% 27% 14% 4	836
November	1st to 10th	Night	41% 7% 14% 26% 12% 1	780
	11th to 20th	Night	40% 7% 17% 21% 12% 4	800
	21st to end	Night	38% 8% 12% 24% 14% 4	800
December	1st to 10th	Night	33% 7% 14% 26% 16% 4	803
	11th to 20th	Night	47% 8% 15% 21% 8% 1	900
	21st to end	Night	46% 7% 19% 19% 7% 1	990

Page generated at 2025-01-04 00:24:00. Version ac07

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