University of British Columbia
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the University of British Columbia clear sky chart from 2005-11-06 to 2026-04-02.

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	21% 8% 3 7% 4 4 4 3 5% 6% 36%	135024
	Night	26% 9% 3 7% 4 4 4 3 4 6% 30%	135481
Transparency	Day	31% 40% 11% 12% 5% 1	134731
	Night	36% 38% 9% 11% 4 1	135352
Seeing	Day	22% 3 10% 25% 29% 11%	17125
	Night	29% 4 11% 24% 23% 8%	25446

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	41% 12% 3 8% 5% 4 4 3 4 4 11%	4125
	11th to 20th	Day	22% 8% 3 5% 4 4 4 4 5% 8% 33%	4119
	21st to end	Day	41% 9% 3 7% 4 4 3 2 3 7% 17%	5167
February	1st to 10th	Day	32% 10% 4 8% 5% 5% 4 5% 6% 6% 17%	1890
	11th to 20th	Day	27% 11% 3 8% 6% 5% 4 3 4 6% 24%	1845
	21st to end	Day	25% 11% 3 9% 5% 5% 5% 3 4 6% 23%	1562
March	1st to 10th	Day	24% 13% 4 10% 6% 5% 5% 4 5% 5% 19%	4797
	11th to 20th	Day	24% 13% 3 8% 5% 4 4 4 4 6% 26%	5110
	21st to end	Day	28% 12% 4 9% 6% 5% 4 4 4 5% 20%	5732
April	1st to 10th	Day	23% 11% 3 8% 5% 5% 6% 4 6% 6% 23%	2413
	11th to 20th	Day	19% 9% 3 7% 5% 5% 5% 4 6% 6% 32%	2485
	21st to end	Day	20% 10% 4 7% 5% 4 4 4 5% 6% 33%	2648
May	1st to 10th	Day	12% 8% 3 7% 4 4 4 3 5% 6% 46%	6320
	11th to 20th	Day	16% 8% 3 8% 4 4 4 4 7% 9% 34%	6277
	21st to end	Day	19% 7% 2 6% 4 5% 5% 5% 6% 8% 32%	7087
June	1st to 10th	Day	18% 7% 2 7% 5% 4 4 4 6% 8% 36%	3000
	11th to 20th	Day	16% 8% 3 7% 6% 4 5% 4 6% 7% 33%	3000
	21st to end	Day	13% 6% 2 7% 4 5% 5% 4 6% 7% 41%	3000
July	1st to 10th	Day	8% 4 2 4 3 3 3 2 4 5% 62%	6780
	11th to 20th	Day	4 3 1 3 2 2 2 2 4 5% 72%	6780
	21st to end	Day	8% 4 2 4 2 2 3 2 5% 6% 62%	7042
August	1st to 10th	Day	5% 3 1 3 2 2 2 2 4 6% 68%	4483
	11th to 20th	Day	10% 6% 2 7% 4 4 4 3 5% 7% 48%	4156
	21st to end	Day	14% 5% 2 7% 4 4 4 3 5% 7% 45%	4577
September	1st to 10th	Day	10% 5% 3 6% 4 4 4 3 5% 8% 49%	2400
	11th to 20th	Day	16% 7% 2 7% 5% 4 4 4 5% 7% 40%	2384
	21st to end	Day	22% 6% 2 7% 5% 4 4 3 5% 7% 37%	2114
October	1st to 10th	Day	21% 7% 3 8% 5% 5% 5% 4 5% 7% 30%	3280
	11th to 20th	Day	34% 8% 3 5% 4 4 4 3 4 6% 25%	3261
	21st to end	Day	29% 13% 4 8% 5% 5% 3 3 5% 6% 20%	3278
November	1st to 10th	Day	37% 11% 2 6% 4 4 5% 3 5% 5% 19%	1648
	11th to 20th	Day	35% 11% 4 6% 5% 3 4 3 4 4 21%	1680
	21st to end	Day	39% 10% 3 5% 5% 3 3 4 5% 5% 18%	1680
December	1st to 10th	Day	26% 10% 3 8% 6% 5% 5% 3 5% 6% 22%	2614
	11th to 20th	Day	42% 12% 3 10% 5% 3 3 3 3 4 11%	2715
	21st to end	Day	40% 13% 4 9% 5% 5% 4 3 3 3 11%	3575
January	1st to 10th	Night	39% 13% 4 9% 5% 4 4 3 3 4 13%	8770
	11th to 20th	Night	24% 7% 2 6% 4 4 4 3 5% 8% 31%	8779
	21st to end	Night	36% 8% 3 8% 4 3 4 3 5% 7% 20%	8989
February	1st to 10th	Night	31% 11% 4 8% 5% 5% 4 3 4 6% 20%	2730
	11th to 20th	Night	27% 12% 3 8% 4 4 3 2 4 6% 26%	2662
	21st to end	Night	28% 11% 2 8% 5% 4 4 3 5% 5% 24%	2244
March	1st to 10th	Night	26% 11% 3 8% 5% 4 4 4 5% 6% 23%	5316
	11th to 20th	Night	26% 11% 3 7% 4 4 4 3 5% 6% 28%	5109
	21st to end	Night	35% 13% 4 9% 5% 4 3 3 4 4 17%	5484
April	1st to 10th	Night	25% 11% 4 7% 6% 5% 5% 3 4 6% 22%	2010
	11th to 20th	Night	20% 9% 3 8% 5% 5% 4 4 5% 7% 30%	1861
	21st to end	Night	20% 9% 4 8% 6% 5% 4 3 7% 7% 29%	1750
May	1st to 10th	Night	15% 7% 3 5% 3 3 3 2 4 5% 48%	3640
	11th to 20th	Night	15% 7% 3 8% 4 5% 4 4 6% 8% 36%	3616
	21st to end	Night	17% 9% 4 10% 7% 6% 5% 3 5% 6% 28%	3772
June	1st to 10th	Night	20% 6% 2 7% 5% 5% 4 3 6% 5% 35%	1400
	11th to 20th	Night	15% 9% 3 8% 5% 4 6% 4 6% 8% 31%	1400
	21st to end	Night	11% 6% 2 7% 5% 5% 6% 4 6% 9% 40%	1400
July	1st to 10th	Night	9% 4 2 4 3 3 3 3 4 6% 60%	3180
	11th to 20th	Night	3 2 1 2 2 1 2 2 3 5% 76%	3180
	21st to end	Night	7% 4 1 4 3 3 4 3 4 6% 60%	3786
August	1st to 10th	Night	4 2 1 2 2 2 2 2 3 7% 72%	2666
	11th to 20th	Night	13% 5% 2 5% 3 3 4 3 4 7% 50%	2902
	21st to end	Night	12% 6% 2 6% 4 4 4 4 6% 7% 45%	3334
September	1st to 10th	Night	8% 4 2 4 3 4 4 4 6% 7% 56%	2000
	11th to 20th	Night	17% 6% 2 6% 3 3 4 3 5% 6% 44%	2014
	21st to end	Night	21% 7% 2 7% 4 3 4 3 4 6% 39%	2285
October	1st to 10th	Night	20% 9% 4 8% 4 4 4 4 5% 6% 32%	3920
	11th to 20th	Night	29% 8% 3 7% 4 3 4 3 5% 6% 28%	3897
	21st to end	Night	30% 10% 3 7% 4 4 3 3 4 5% 27%	4520
November	1st to 10th	Night	37% 10% 3 7% 4 4 4 3 4 5% 18%	2870
	11th to 20th	Night	35% 11% 3 6% 4 5% 4 3 4 4 22%	2940
	21st to end	Night	36% 11% 3 7% 6% 4 3 2 4 5% 20%	2940
December	1st to 10th	Night	28% 11% 3 8% 6% 6% 4 3 4 6% 20%	4782
	11th to 20th	Night	44% 13% 4 7% 4 4 3 3 3 4 12%	5710
	21st to end	Night	38% 14% 4 8% 4 4 3 3 3 4 14%	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	60% 25% 7% 6% 1	4125
	11th to 20th	Day	37% 24% 14% 17% 5% 2	4140
	21st to end	Day	54% 24% 7% 12% 4	5168
February	1st to 10th	Day	28% 59% 4 3 2 2	1890
	11th to 20th	Day	29% 59% 4 6% 2 1	1854
	21st to end	Day	26% 61% 5% 5% 2 1	1562
March	1st to 10th	Day	45% 35% 9% 9% 2	4797
	11th to 20th	Day	42% 32% 8% 16% 2	5110
	21st to end	Day	50% 33% 10% 5% 2	5732
April	1st to 10th	Day	24% 65% 5% 5% 1	2413
	11th to 20th	Day	21% 64% 5% 7% 2 1	2485
	21st to end	Day	20% 64% 6% 7% 3	2648
May	1st to 10th	Day	25% 32% 15% 18% 10%	6320
	11th to 20th	Day	33% 36% 15% 13% 2	6280
	21st to end	Day	33% 37% 16% 12% 2	7088
June	1st to 10th	Day	16% 65% 7% 8% 3	3000
	11th to 20th	Day	18% 64% 6% 7% 4	3000
	21st to end	Day	17% 63% 8% 9% 3	3000
July	1st to 10th	Day	18% 28% 14% 18% 16% 6%	6706
	11th to 20th	Day	9% 28% 16% 28% 18% 2	6699
	21st to end	Day	18% 29% 26% 20% 7% 1	6845
August	1st to 10th	Day	9% 37% 8% 33% 12% 1	4466
	11th to 20th	Day	26% 39% 21% 10% 4	4195
	21st to end	Day	27% 40% 11% 12% 9% 1	4576
September	1st to 10th	Day	19% 60% 7% 10% 3 1	2400
	11th to 20th	Day	22% 59% 5% 7% 6% 1	2384
	21st to end	Day	22% 58% 7% 8% 5%	2114
October	1st to 10th	Day	37% 39% 12% 11% 1	3280
	11th to 20th	Day	40% 37% 7% 11% 4	3261
	21st to end	Day	44% 38% 6% 10% 2	3278
November	1st to 10th	Day	32% 55% 6% 5% 2	1648
	11th to 20th	Day	30% 56% 5% 5% 2 1	1680
	21st to end	Day	29% 56% 6% 5% 4 1	1680
December	1st to 10th	Day	41% 39% 7% 12% 2	2614
	11th to 20th	Day	56% 34% 5% 4 1	2718
	21st to end	Day	60% 30% 6% 3 1	3575
January	1st to 10th	Night	60% 25% 6% 7% 2	8770
	11th to 20th	Night	39% 26% 13% 14% 6% 1	8820
	21st to end	Night	51% 26% 9% 12% 2	8989
February	1st to 10th	Night	28% 60% 3 3 4 2	2730
	11th to 20th	Night	28% 59% 4 5% 3 1	2664
	21st to end	Night	24% 60% 6% 6% 3 1	2244
March	1st to 10th	Night	42% 34% 9% 9% 5%	5316
	11th to 20th	Night	37% 34% 9% 18% 2	5110
	21st to end	Night	52% 32% 9% 6% 1	5484
April	1st to 10th	Night	24% 63% 4 7% 2	2010
	11th to 20th	Night	20% 63% 5% 7% 3 1	1861
	21st to end	Night	19% 63% 6% 7% 5%	1750
May	1st to 10th	Night	25% 31% 10% 23% 10%	3637
	11th to 20th	Night	30% 35% 18% 14% 3	3616
	21st to end	Night	40% 32% 14% 13% 1	3772
June	1st to 10th	Night	16% 63% 8% 9% 3 1	1400
	11th to 20th	Night	16% 63% 7% 11% 3	1400
	21st to end	Night	17% 63% 8% 8% 4	1400
July	1st to 10th	Night	18% 27% 15% 21% 16% 4	3145
	11th to 20th	Night	7% 26% 17% 29% 19% 2	3138
	21st to end	Night	19% 26% 22% 24% 9%	3678
August	1st to 10th	Night	6% 36% 9% 29% 19% 1	2658
	11th to 20th	Night	27% 38% 20% 11% 3 1	2929
	21st to end	Night	26% 39% 13% 13% 8% 1	3334
September	1st to 10th	Night	15% 60% 10% 10% 5%	2000
	11th to 20th	Night	20% 60% 6% 7% 6% 1	2014
	21st to end	Night	21% 58% 7% 10% 4	2285
October	1st to 10th	Night	35% 41% 12% 10% 2	3920
	11th to 20th	Night	40% 38% 6% 13% 4	3897
	21st to end	Night	40% 39% 8% 10% 3	4520
November	1st to 10th	Night	32% 55% 6% 5% 2	2870
	11th to 20th	Night	31% 55% 4 6% 3 2	2940
	21st to end	Night	31% 56% 4 4 5%	2940
December	1st to 10th	Night	45% 40% 6% 7% 2	4782
	11th to 20th	Night	56% 33% 5% 5% 2	5706
	21st to end	Night	55% 30% 6% 7% 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	39% 7% 12% 25% 11% 7%	220
	11th to 20th	Day	39% 4 14% 20% 19% 5%	220
	21st to end	Day	44% 4 7% 22% 18% 5%	279
February	1st to 10th	Day	35% 5% 13% 26% 18% 3	430
	11th to 20th	Day	32% 4 12% 29% 20% 3	425
	21st to end	Day	30% 4 15% 29% 15% 7%	358
March	1st to 10th	Day	31% 3 13% 28% 20% 6%	540
	11th to 20th	Day	32% 4 14% 20% 23% 6%	540
	21st to end	Day	33% 5% 15% 25% 17% 6%	594
April	1st to 10th	Day	27% 4 15% 25% 22% 7%	505
	11th to 20th	Day	24% 3 13% 31% 20% 9%	492
	21st to end	Day	21% 4 11% 32% 24% 9%	500
May	1st to 10th	Day	15% 3 10% 29% 34% 9%	503
	11th to 20th	Day	16% 2 6% 27% 34% 15%	528
	21st to end	Day	16% 3 8% 29% 31% 13%	653
June	1st to 10th	Day	17% 2 8% 31% 33% 9%	730
	11th to 20th	Day	17% 3 9% 28% 32% 11%	730
	21st to end	Day	13% 3 8% 25% 38% 14%	730
July	1st to 10th	Day	11% 1 4 27% 39% 17%	721
	11th to 20th	Day	6% 2 8% 23% 47% 15%	718
	21st to end	Day	6% 2 5% 18% 48% 22%	652
August	1st to 10th	Day	10% 1 4 17% 43% 25%	527
	11th to 20th	Day	8% 2 4 25% 42% 18%	517
	21st to end	Day	10% 2 9% 25% 40% 13%	583
September	1st to 10th	Day	12% 3 6% 30% 37% 12%	530
	11th to 20th	Day	18% 5% 8% 25% 31% 13%	530
	21st to end	Day	22% 2 14% 27% 26% 9%	530
October	1st to 10th	Day	26% 6% 10% 26% 22% 11%	530
	11th to 20th	Day	35% 5% 12% 18% 22% 9%	530
	21st to end	Day	34% 6% 15% 25% 15% 4	438
November	1st to 10th	Day	40% 5% 14% 24% 12% 4	220
	11th to 20th	Day	37% 6% 14% 19% 18% 7%	220
	21st to end	Day	42% 7% 12% 17% 13% 8%	220
December	1st to 10th	Day	40% 3 14% 16% 17% 10%	220
	11th to 20th	Day	42% 6% 12% 23% 15% 2	220
	21st to end	Day	43% 5% 8% 31% 10% 3	242
January	1st to 10th	Night	47% 7% 10% 19% 13% 4	950
	11th to 20th	Night	39% 4 11% 21% 23% 2	950
	21st to end	Night	38% 3 10% 25% 18% 6%	945
February	1st to 10th	Night	34% 6% 13% 27% 17% 4	840
	11th to 20th	Night	33% 4 13% 27% 20% 2	824
	21st to end	Night	32% 5% 16% 26% 15% 5%	692
March	1st to 10th	Night	33% 4 13% 24% 21% 5%	840
	11th to 20th	Night	36% 5% 15% 22% 17% 6%	840
	21st to end	Night	34% 6% 12% 23% 19% 7%	924
April	1st to 10th	Night	29% 4 10% 28% 22% 6%	804
	11th to 20th	Night	23% 6% 15% 29% 21% 7%	673
	21st to end	Night	24% 4 10% 31% 24% 7%	550
May	1st to 10th	Night	17% 2 12% 29% 31% 10%	400
	11th to 20th	Night	19% 2 9% 21% 35% 13%	394
	21st to end	Night	19% 4 6% 25% 37% 8%	433
June	1st to 10th	Night	21% 3 9% 29% 32% 7%	400
	11th to 20th	Night	15% 6% 10% 33% 27% 9%	400
	21st to end	Night	13% 2 5% 30% 40% 11%	400
July	1st to 10th	Night	13% 2 6% 23% 40% 16%	390
	11th to 20th	Night	6% 1 7% 27% 43% 17%	388
	21st to end	Night	5% 3 4 20% 49% 18%	438
August	1st to 10th	Night	9% 1 4 18% 44% 24%	429
	11th to 20th	Night	8% 1 3 24% 45% 19%	594
	21st to end	Night	12% 2 8% 21% 39% 17%	710
September	1st to 10th	Night	10% 3 6% 27% 36% 19%	800
	11th to 20th	Night	20% 3 8% 21% 29% 19%	800
	21st to end	Night	24% 3 12% 23% 28% 10%	800
October	1st to 10th	Night	22% 2 11% 28% 24% 13%	800
	11th to 20th	Night	32% 3 13% 23% 22% 8%	800
	21st to end	Night	36% 5% 13% 25% 16% 5%	880
November	1st to 10th	Night	41% 6% 11% 25% 15% 1	820
	11th to 20th	Night	38% 5% 15% 23% 13% 5%	840
	21st to end	Night	41% 4 12% 23% 15% 5%	840
December	1st to 10th	Night	36% 5% 12% 24% 20% 4	863
	11th to 20th	Night	50% 6% 12% 21% 11% 1	950
	21st to end	Night	44% 7% 17% 21% 10% 2	1045

Page generated at 2026-03-29 23:18:41. Version ac07

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