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 2026-05-02.

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% 7% 35%	135441
	Night	23% 9% 3 7% 5% 4 4 3 5% 6% 29%	135779
Transparency	Day	50% 4 17% 20% 8% 1	135149
	Night	57% 3 13% 18% 7% 1	135649
Seeing	Day	22% 5% 14% 26% 24% 9%	17193
	Night	29% 5% 12% 25% 22% 6%	25533

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	36% 13% 4 9% 5% 5% 5% 3 5% 5% 9%	4125
	11th to 20th	Day	20% 9% 3 8% 4 5% 5% 5% 6% 8% 27%	4119
	21st to end	Day	37% 10% 3 7% 4 4 4 3 4 4 17%	5261
February	1st to 10th	Day	29% 12% 4 9% 6% 4 4 4 7% 6% 15%	1890
	11th to 20th	Day	26% 11% 3 10% 6% 4 4 3 4 6% 22%	1845
	21st to end	Day	24% 12% 4 9% 6% 4 5% 4 5% 6% 22%	1575
March	1st to 10th	Day	23% 13% 4 10% 6% 6% 5% 4 5% 7% 18%	4787
	11th to 20th	Day	22% 11% 4 9% 5% 4 5% 3 5% 7% 25%	5110
	21st to end	Day	25% 13% 4 9% 5% 5% 5% 4 6% 5% 19%	5741
April	1st to 10th	Day	23% 10% 3 7% 5% 6% 6% 5% 6% 7% 22%	2520
	11th to 20th	Day	18% 9% 3 7% 6% 5% 5% 4 7% 7% 30%	2594
	21st to end	Day	18% 10% 3 8% 5% 4 4 4 5% 7% 32%	2785
May	1st to 10th	Day	12% 8% 2 7% 4 4 4 3 5% 7% 45%	6335
	11th to 20th	Day	14% 9% 3 8% 5% 4 5% 3 7% 9% 33%	6277
	21st to end	Day	18% 8% 3 6% 5% 5% 6% 5% 6% 8% 31%	7028
June	1st to 10th	Day	17% 7% 2 7% 6% 5% 4 4 5% 7% 36%	3000
	11th to 20th	Day	17% 8% 3 8% 4 4 5% 4 6% 8% 33%	3000
	21st to end	Day	11% 7% 2 7% 5% 5% 5% 4 5% 9% 40%	3000
July	1st to 10th	Day	7% 3 2 4 3 3 3 3 5% 6% 62%	6780
	11th to 20th	Day	4 3 1 3 2 2 2 2 3 6% 72%	6780
	21st to end	Day	7% 4 1 3 3 2 3 3 6% 7% 61%	6975
August	1st to 10th	Day	5% 3 1 3 2 3 3 2 4 6% 68%	4488
	11th to 20th	Day	10% 5% 2 6% 4 4 5% 3 5% 7% 48%	4156
	21st to end	Day	12% 7% 3 6% 4 5% 4 4 5% 7% 45%	4551
September	1st to 10th	Day	10% 6% 3 6% 4 4 4 4 5% 8% 47%	2400
	11th to 20th	Day	15% 7% 3 6% 4 4 4 3 6% 7% 39%	2395
	21st to end	Day	21% 7% 2 6% 4 5% 4 4 5% 7% 35%	2105
October	1st to 10th	Day	19% 7% 3 7% 6% 5% 5% 4 7% 7% 29%	3280
	11th to 20th	Day	32% 8% 2 7% 5% 3 4 3 5% 6% 24%	3261
	21st to end	Day	27% 12% 4 9% 5% 5% 4 3 5% 6% 19%	3304
November	1st to 10th	Day	34% 10% 3 8% 5% 5% 5% 3 5% 4 18%	1648
	11th to 20th	Day	33% 11% 4 8% 6% 4 3 3 4 5% 20%	1680
	21st to end	Day	35% 11% 4 7% 5% 5% 4 4 5% 5% 16%	1680
December	1st to 10th	Day	24% 12% 4 11% 7% 5% 5% 4 4 6% 19%	2676
	11th to 20th	Day	37% 14% 4 9% 5% 4 4 4 4 4 11%	2715
	21st to end	Day	33% 15% 5% 11% 6% 5% 5% 3 4 4 9%	3575
January	1st to 10th	Night	35% 14% 4 10% 5% 5% 4 3 4 5% 11%	8770
	11th to 20th	Night	22% 8% 3 7% 4 4 5% 5% 7% 8% 26%	8779
	21st to end	Night	34% 10% 3 7% 4 4 4 4 6% 8% 16%	8917
February	1st to 10th	Night	29% 13% 4 8% 5% 4 4 4 5% 6% 20%	2730
	11th to 20th	Night	26% 13% 3 9% 4 3 3 3 4 5% 25%	2662
	21st to end	Night	27% 12% 3 7% 4 3 4 4 6% 5% 24%	2230
March	1st to 10th	Night	23% 13% 4 9% 5% 4 4 4 5% 7% 23%	5325
	11th to 20th	Night	24% 10% 3 6% 4 4 5% 4 6% 8% 25%	5109
	21st to end	Night	32% 13% 5% 8% 5% 4 4 3 5% 5% 16%	5476
April	1st to 10th	Night	24% 11% 3 8% 5% 5% 4 4 6% 7% 23%	2100
	11th to 20th	Night	18% 10% 3 8% 4 4 5% 4 6% 6% 31%	1973
	21st to end	Night	19% 8% 3 10% 4 5% 5% 4 6% 6% 29%	1834
May	1st to 10th	Night	15% 8% 3 6% 4 3 3 3 4 6% 47%	3648
	11th to 20th	Night	15% 9% 3 7% 5% 3 4 4 6% 8% 36%	3616
	21st to end	Night	15% 9% 3 10% 6% 6% 6% 4 5% 7% 29%	3844
June	1st to 10th	Night	19% 8% 2 7% 5% 4 4 4 6% 7% 35%	1400
	11th to 20th	Night	12% 10% 3 9% 6% 5% 5% 4 5% 9% 32%	1400
	21st to end	Night	10% 6% 2 7% 6% 4 5% 4 6% 9% 41%	1400
July	1st to 10th	Night	8% 3 2 4 2 3 3 2 4 7% 62%	3180
	11th to 20th	Night	3 2 1 2 2 1 3 2 3 4 77%	3180
	21st to end	Night	5% 3 2 4 3 3 3 3 5% 7% 64%	3868
August	1st to 10th	Night	4 2 1 2 2 1 2 2 4 6% 73%	2662
	11th to 20th	Night	11% 5% 2 6% 4 4 4 3 4 7% 50%	2902
	21st to end	Night	11% 4 2 6% 5% 4 5% 4 6% 9% 45%	3362
September	1st to 10th	Night	8% 3 2 4 3 4 4 4 5% 7% 57%	2000
	11th to 20th	Night	16% 6% 2 5% 5% 4 4 4 6% 6% 43%	2001
	21st to end	Night	19% 6% 3 7% 4 4 4 3 5% 6% 39%	2295
October	1st to 10th	Night	17% 6% 2 7% 5% 5% 5% 4 6% 9% 33%	3920
	11th to 20th	Night	28% 7% 3 6% 5% 4 5% 3 5% 6% 28%	3897
	21st to end	Night	27% 10% 3 7% 5% 4 4 3 5% 7% 25%	4498
November	1st to 10th	Night	34% 10% 4 8% 5% 4 5% 4 4 5% 17%	2870
	11th to 20th	Night	34% 11% 2 7% 4 5% 4 3 4 6% 20%	2940
	21st to end	Night	32% 12% 4 9% 5% 5% 4 3 4 4 19%	2940
December	1st to 10th	Night	25% 9% 3 9% 7% 6% 6% 5% 6% 7% 17%	4718
	11th to 20th	Night	40% 14% 4 9% 5% 4 4 3 4 4 11%	5710
	21st to end	Night	33% 14% 4 9% 5% 4 4 3 4 4 15%	7623

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	79% 2 8% 8% 1	4125
	11th to 20th	Day	55% 3 9% 24% 8% 2	4140
	21st to end	Day	70% 2 8% 14% 4 1	5262
February	1st to 10th	Day	68% 4 11% 9% 4 3	1890
	11th to 20th	Day	65% 5% 9% 14% 6% 1	1854
	21st to end	Day	62% 6% 13% 13% 4 2	1575
March	1st to 10th	Day	64% 4 12% 15% 5%	4787
	11th to 20th	Day	60% 3 11% 21% 4	5110
	21st to end	Day	67% 4 16% 10% 3	5741
April	1st to 10th	Day	59% 6% 19% 13% 3	2520
	11th to 20th	Day	54% 5% 12% 18% 9% 2	2594
	21st to end	Day	52% 5% 17% 18% 8% 1	2785
May	1st to 10th	Day	40% 4 17% 26% 13% 1	6335
	11th to 20th	Day	48% 5% 21% 21% 4	6280
	21st to end	Day	50% 4 23% 19% 4	7029
June	1st to 10th	Day	47% 5% 20% 19% 6% 2	3000
	11th to 20th	Day	49% 6% 22% 19% 4	3000
	21st to end	Day	42% 5% 23% 21% 8% 2	3000
July	1st to 10th	Day	25% 3 21% 25% 19% 7%	6706
	11th to 20th	Day	17% 2 22% 34% 22% 3	6699
	21st to end	Day	24% 3 32% 29% 11% 2	6779
August	1st to 10th	Day	20% 3 16% 43% 17% 1	4471
	11th to 20th	Day	36% 3 31% 20% 8% 1	4195
	21st to end	Day	41% 4 21% 23% 11% 1	4550
September	1st to 10th	Day	36% 6% 20% 23% 11% 4	2400
	11th to 20th	Day	45% 4 16% 20% 12% 2	2395
	21st to end	Day	49% 6% 15% 19% 10% 1	2105
October	1st to 10th	Day	53% 4 18% 21% 5%	3280
	11th to 20th	Day	61% 4 12% 15% 7%	3261
	21st to end	Day	67% 3 11% 13% 5%	3304
November	1st to 10th	Day	69% 4 13% 10% 4	1648
	11th to 20th	Day	68% 4 10% 11% 4 2	1680
	21st to end	Day	70% 4 10% 11% 5% 1	1680
December	1st to 10th	Day	68% 3 8% 16% 5%	2676
	11th to 20th	Day	78% 3 9% 8% 2 1	2718
	21st to end	Day	79% 3 9% 7% 1	3575
January	1st to 10th	Night	78% 2 7% 10% 2	8770
	11th to 20th	Night	56% 4 10% 19% 9% 1	8820
	21st to end	Night	67% 2 8% 17% 5%	8917
February	1st to 10th	Night	66% 5% 10% 10% 5% 3	2730
	11th to 20th	Night	63% 5% 9% 14% 8% 2	2664
	21st to end	Night	61% 5% 10% 15% 7% 2	2230
March	1st to 10th	Night	63% 3 12% 15% 7%	5325
	11th to 20th	Night	58% 4 12% 21% 4	5110
	21st to end	Night	73% 3 12% 10% 2	5476
April	1st to 10th	Night	61% 5% 13% 17% 3	2100
	11th to 20th	Night	53% 6% 12% 18% 8% 3	1973
	21st to end	Night	56% 4 14% 16% 10%	1834
May	1st to 10th	Night	41% 4 11% 31% 13% 1	3645
	11th to 20th	Night	45% 6% 23% 21% 6%	3616
	21st to end	Night	56% 4 18% 18% 3	3844
June	1st to 10th	Night	49% 5% 19% 19% 6% 2	1400
	11th to 20th	Night	51% 6% 18% 21% 4	1400
	21st to end	Night	40% 5% 23% 21% 9% 3	1400
July	1st to 10th	Night	26% 2 18% 30% 18% 7%	3145
	11th to 20th	Night	14% 2 21% 36% 25% 3	3138
	21st to end	Night	23% 2 23% 34% 16% 2	3759
August	1st to 10th	Night	16% 3 15% 38% 27% 2	2654
	11th to 20th	Night	35% 2 29% 21% 9% 3	2929
	21st to end	Night	38% 4 21% 26% 12% 1	3362
September	1st to 10th	Night	26% 6% 20% 29% 14% 5%	2000
	11th to 20th	Night	43% 4 14% 19% 15% 4	2001
	21st to end	Night	47% 4 15% 24% 8% 2	2295
October	1st to 10th	Night	50% 3 20% 20% 6%	3920
	11th to 20th	Night	58% 3 12% 20% 6%	3897
	21st to end	Night	61% 3 15% 16% 6%	4498
November	1st to 10th	Night	69% 4 10% 12% 3 1	2870
	11th to 20th	Night	66% 5% 11% 10% 5% 2	2940
	21st to end	Night	70% 4 9% 10% 6% 1	2940
December	1st to 10th	Night	67% 4 11% 13% 5%	4718
	11th to 20th	Night	79% 3 8% 7% 3	5706
	21st to end	Night	74% 3 9% 12% 2 1	7623

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	42% 6% 18% 18% 10% 5%	220
	11th to 20th	Day	35% 12% 11% 20% 16% 6%	220
	21st to end	Day	46% 4 12% 16% 14% 7%	274
February	1st to 10th	Day	38% 7% 16% 20% 16% 3	430
	11th to 20th	Day	33% 7% 14% 24% 18% 4	425
	21st to end	Day	30% 8% 19% 28% 11% 4	366
March	1st to 10th	Day	31% 6% 14% 26% 17% 5%	540
	11th to 20th	Day	32% 7% 16% 19% 18% 8%	540
	21st to end	Day	31% 5% 18% 25% 17% 4	594
April	1st to 10th	Day	27% 7% 15% 24% 20% 6%	540
	11th to 20th	Day	21% 4 20% 27% 22% 6%	532
	21st to end	Day	21% 5% 18% 30% 18% 7%	540
May	1st to 10th	Day	14% 4 14% 34% 28% 6%	508
	11th to 20th	Day	18% 4 10% 28% 30% 11%	528
	21st to end	Day	15% 4 15% 30% 26% 10%	620
June	1st to 10th	Day	18% 3 15% 32% 26% 7%	730
	11th to 20th	Day	17% 3 17% 32% 24% 7%	730
	21st to end	Day	13% 3 13% 28% 33% 10%	730
July	1st to 10th	Day	11% 2 11% 29% 35% 12%	721
	11th to 20th	Day	5% 3 11% 32% 37% 12%	718
	21st to end	Day	6% 1 9% 25% 42% 17%	624
August	1st to 10th	Day	9% 3 6% 23% 41% 19%	527
	11th to 20th	Day	7% 2 6% 32% 37% 16%	517
	21st to end	Day	11% 2 12% 30% 31% 14%	583
September	1st to 10th	Day	13% 2 10% 29% 32% 14%	530
	11th to 20th	Day	18% 4 10% 29% 29% 11%	530
	21st to end	Day	25% 3 15% 24% 24% 8%	530
October	1st to 10th	Day	25% 5% 13% 24% 22% 12%	530
	11th to 20th	Day	33% 5% 12% 21% 19% 10%	530
	21st to end	Day	33% 7% 18% 22% 14% 5%	444
November	1st to 10th	Day	35% 6% 19% 23% 14% 3	220
	11th to 20th	Day	35% 11% 15% 12% 17% 10%	220
	21st to end	Day	40% 9% 15% 19% 9% 9%	220
December	1st to 10th	Day	40% 7% 17% 13% 11% 12%	220
	11th to 20th	Day	46% 9% 12% 20% 11% 2	220
	21st to end	Day	39% 8% 20% 22% 9% 2	242
January	1st to 10th	Night	44% 8% 16% 18% 10% 3	950
	11th to 20th	Night	39% 6% 12% 22% 17% 3	950
	21st to end	Night	37% 7% 12% 23% 17% 3	927
February	1st to 10th	Night	36% 9% 18% 24% 11% 3	840
	11th to 20th	Night	33% 7% 17% 25% 16% 1	824
	21st to end	Night	33% 10% 18% 25% 12% 3	692
March	1st to 10th	Night	33% 7% 17% 22% 18% 3	840
	11th to 20th	Night	36% 7% 16% 23% 15% 4	840
	21st to end	Night	33% 8% 14% 25% 16% 4	924
April	1st to 10th	Night	31% 6% 10% 28% 19% 5%	840
	11th to 20th	Night	22% 7% 14% 31% 19% 7%	725
	21st to end	Night	24% 6% 11% 33% 21% 6%	574
May	1st to 10th	Night	19% 3 12% 27% 33% 5%	402
	11th to 20th	Night	18% 3 6% 29% 37% 8%	394
	21st to end	Night	19% 3 7% 30% 34% 6%	433
June	1st to 10th	Night	21% 5% 11% 29% 30% 4	400
	11th to 20th	Night	18% 4 10% 36% 27% 5%	400
	21st to end	Night	14% 3 6% 33% 35% 10%	400
July	1st to 10th	Night	13% 1 5% 28% 45% 8%	390
	11th to 20th	Night	7% 3 6% 32% 41% 10%	388
	21st to end	Night	4 1 7% 27% 47% 15%	438
August	1st to 10th	Night	9% 2 5% 20% 44% 21%	425
	11th to 20th	Night	8% 2 4 25% 44% 17%	594
	21st to end	Night	11% 2 6% 23% 44% 13%	725
September	1st to 10th	Night	10% 1 5% 33% 37% 14%	800
	11th to 20th	Night	18% 4 11% 19% 33% 16%	800
	21st to end	Night	23% 2 13% 26% 29% 7%	800
October	1st to 10th	Night	22% 4 11% 26% 27% 10%	800
	11th to 20th	Night	31% 4 13% 24% 20% 8%	800
	21st to end	Night	34% 6% 15% 27% 15% 4	880
November	1st to 10th	Night	41% 7% 14% 26% 12%	820
	11th to 20th	Night	40% 7% 16% 22% 12% 3	840
	21st to end	Night	38% 7% 12% 25% 14% 4	840
December	1st to 10th	Night	34% 7% 14% 26% 16% 4	843
	11th to 20th	Night	47% 8% 15% 20% 8% 1	950
	21st to end	Night	45% 7% 20% 20% 8% 1	1045

Page generated at 2026-04-28 23:21:42. Version ac07

Copyright © 2026 Attilla Danko.