Port Alberni
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Port Alberni clear sky chart from 2005-11-06 to 2025-12-03.

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	16% 11% 4 9% 6% 5% 5% 4 5% 6% 30%	131139
	Night	18% 12% 4 10% 6% 5% 5% 4 5% 5% 26%	128272
Transparency	Day	53% 8% 14% 16% 7% 1	130837
	Night	57% 9% 14% 13% 6% 1	128151
Seeing	Day	22% 15% 24% 23% 12% 3	17057
	Night	25% 10% 20% 26% 17% 3	25051

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	30% 20% 7% 13% 6% 4 3 3 3 3 7%	3179
	11th to 20th	Day	17% 11% 4 10% 10% 8% 6% 5% 7% 7% 16%	3159
	21st to end	Day	30% 14% 4 10% 5% 4 4 4 4 3 18%	3917
February	1st to 10th	Day	25% 17% 5% 9% 8% 6% 5% 4 5% 5% 12%	1800
	11th to 20th	Day	23% 16% 5% 11% 6% 5% 4 3 4 5% 18%	1755
	21st to end	Day	20% 19% 4 11% 6% 6% 5% 4 5% 4 16%	1485
March	1st to 10th	Day	17% 20% 5% 11% 7% 6% 6% 4 4 5% 15%	4746
	11th to 20th	Day	15% 17% 6% 11% 7% 6% 5% 4 4 5% 19%	5000
	21st to end	Day	22% 17% 5% 14% 7% 6% 5% 4 4 5% 12%	5486
April	1st to 10th	Day	20% 19% 5% 11% 8% 6% 5% 3 4 4 16%	2393
	11th to 20th	Day	17% 16% 5% 11% 7% 5% 5% 3 5% 5% 22%	2566
	21st to end	Day	21% 14% 3 9% 6% 5% 5% 4 5% 5% 22%	2600
May	1st to 10th	Day	12% 10% 3 9% 6% 5% 6% 4 5% 5% 33%	6271
	11th to 20th	Day	14% 11% 3 8% 5% 5% 6% 4 6% 9% 29%	6277
	21st to end	Day	16% 11% 4 11% 7% 7% 6% 4 6% 6% 23%	7331
June	1st to 10th	Day	20% 11% 4 8% 6% 5% 5% 4 5% 6% 25%	3000
	11th to 20th	Day	14% 13% 4 10% 6% 6% 6% 5% 8% 7% 21%	3000
	21st to end	Day	14% 9% 4 10% 5% 5% 5% 4 4 7% 32%	3000
July	1st to 10th	Day	7% 3 2 4 3 3 3 3 4 6% 63%	6780
	11th to 20th	Day	3 3 1 3 2 2 2 2 3 6% 72%	6780
	21st to end	Day	4 4 2 4 4 3 3 3 4 7% 63%	7251
August	1st to 10th	Day	6% 3 1 3 2 2 3 3 4 8% 65%	4332
	11th to 20th	Day	7% 6% 3 8% 6% 6% 5% 5% 8% 8% 38%	4156
	21st to end	Day	12% 7% 3 6% 5% 4 4 3 6% 9% 42%	4664
September	1st to 10th	Day	10% 6% 2 6% 4 3 5% 4 5% 8% 46%	2409
	11th to 20th	Day	15% 9% 3 8% 4 4 4 4 5% 6% 38%	2298
	21st to end	Day	19% 9% 4 8% 4 4 4 4 6% 6% 32%	2170
October	1st to 10th	Day	19% 11% 5% 11% 7% 5% 5% 4 6% 7% 22%	3280
	11th to 20th	Day	26% 10% 4 9% 6% 5% 5% 4 5% 6% 20%	3261
	21st to end	Day	21% 12% 4 11% 9% 7% 5% 4 5% 6% 16%	3260
November	1st to 10th	Day	30% 15% 5% 11% 7% 6% 4 3 3 4 11%	1648
	11th to 20th	Day	29% 15% 5% 10% 5% 5% 5% 3 5% 4 14%	1680
	21st to end	Day	28% 14% 4 11% 8% 5% 3 3 4 4 14%	1680
December	1st to 10th	Day	18% 14% 6% 16% 10% 9% 5% 3 4 5% 11%	2382
	11th to 20th	Day	33% 19% 6% 15% 6% 5% 4 2 2 3 5%	2645
	21st to end	Day	29% 19% 6% 13% 7% 6% 5% 3 3 3 7%	3498
January	1st to 10th	Night	27% 23% 8% 13% 6% 5% 4 3 3 3 6%	6766
	11th to 20th	Night	19% 9% 4 9% 7% 9% 7% 5% 5% 5% 20%	6739
	21st to end	Night	27% 14% 4 10% 5% 5% 7% 6% 4 3 15%	7030
February	1st to 10th	Night	23% 16% 5% 10% 6% 5% 5% 3 5% 5% 18%	2600
	11th to 20th	Night	20% 16% 5% 10% 5% 4 4 3 4 5% 24%	2532
	21st to end	Night	20% 16% 3 10% 7% 5% 4 3 4 5% 22%	2145
March	1st to 10th	Night	17% 15% 5% 11% 6% 5% 6% 4 5% 5% 21%	5138
	11th to 20th	Night	10% 14% 4 11% 7% 7% 6% 5% 6% 6% 24%	4999
	21st to end	Night	20% 20% 5% 12% 6% 5% 4 3 4 4 15%	5489
April	1st to 10th	Night	17% 14% 4 10% 6% 6% 6% 4 5% 7% 22%	2006
	11th to 20th	Night	14% 11% 5% 9% 6% 6% 6% 3 6% 7% 27%	1781
	21st to end	Night	20% 12% 4 9% 5% 5% 5% 4 6% 6% 25%	1800
May	1st to 10th	Night	11% 10% 3 6% 5% 4 4 3 5% 6% 43%	3691
	11th to 20th	Night	12% 8% 4 8% 5% 6% 6% 5% 7% 8% 32%	3616
	21st to end	Night	16% 12% 3 10% 6% 5% 5% 3 6% 7% 26%	3542
June	1st to 10th	Night	16% 9% 4 8% 5% 5% 5% 4 5% 7% 32%	1400
	11th to 20th	Night	12% 7% 3 10% 7% 7% 7% 5% 6% 8% 29%	1400
	21st to end	Night	9% 7% 3 8% 6% 6% 5% 5% 7% 8% 37%	1400
July	1st to 10th	Night	6% 4 1 4 3 3 3 3 5% 7% 61%	3180
	11th to 20th	Night	3 2 1 2 2 1 2 2 4 6% 75%	3180
	21st to end	Night	5% 2 1 3 3 3 3 3 5% 7% 65%	3590
August	1st to 10th	Night	3 3 1 3 3 2 3 3 3 6% 70%	2822
	11th to 20th	Night	8% 4 2 6% 5% 5% 6% 4 7% 9% 45%	2902
	21st to end	Night	10% 6% 2 7% 5% 4 4 4 6% 7% 46%	3245
September	1st to 10th	Night	7% 5% 2 5% 3 4 4 4 5% 8% 53%	1990
	11th to 20th	Night	14% 8% 2 9% 5% 5% 5% 4 5% 6% 37%	2100
	21st to end	Night	19% 8% 3 8% 5% 4 4 3 4 5% 38%	2229
October	1st to 10th	Night	18% 10% 4 11% 6% 5% 4 3 5% 6% 29%	3920
	11th to 20th	Night	23% 10% 4 10% 6% 6% 6% 4 5% 6% 21%	3897
	21st to end	Night	22% 12% 4 12% 6% 6% 5% 3 4 4 22%	4538
November	1st to 10th	Night	29% 15% 4 10% 6% 5% 4 4 5% 5% 14%	2870
	11th to 20th	Night	28% 16% 5% 10% 6% 5% 4 3 3 4 15%	2940
	21st to end	Night	27% 13% 4 10% 7% 5% 5% 4 5% 6% 15%	2940
December	1st to 10th	Night	19% 16% 7% 14% 11% 7% 5% 3 3 4 10%	4837
	11th to 20th	Night	31% 20% 6% 14% 6% 4 4 2 4 3 6%	5560
	21st to end	Night	27% 18% 5% 12% 8% 6% 5% 3 4 3 8%	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% 10% 5% 6% 2	3179
	11th to 20th	Day	51% 14% 19% 11% 4 1	3180
	21st to end	Day	69% 6% 10% 9% 6%	3917
February	1st to 10th	Day	72% 9% 7% 7% 5% 1	1800
	11th to 20th	Day	68% 6% 10% 10% 4 1	1764
	21st to end	Day	70% 7% 10% 8% 4 1	1485
March	1st to 10th	Day	70% 8% 9% 8% 5%	4746
	11th to 20th	Day	65% 6% 10% 14% 5%	5000
	21st to end	Day	74% 9% 9% 6% 1	5486
April	1st to 10th	Day	73% 7% 9% 8% 3	2393
	11th to 20th	Day	66% 7% 7% 11% 7% 2	2566
	21st to end	Day	65% 8% 11% 12% 5%	2600
May	1st to 10th	Day	50% 8% 14% 20% 8%	6271
	11th to 20th	Day	50% 11% 20% 15% 3	6280
	21st to end	Day	61% 9% 14% 10% 5% 1	7332
June	1st to 10th	Day	61% 9% 12% 11% 6% 2	3000
	11th to 20th	Day	60% 11% 15% 10% 3	3000
	21st to end	Day	52% 10% 15% 16% 8%	3000
July	1st to 10th	Day	25% 7% 18% 26% 16% 8%	6706
	11th to 20th	Day	16% 5% 19% 37% 20% 1	6699
	21st to end	Day	22% 8% 29% 27% 13% 2	7046
August	1st to 10th	Day	20% 6% 13% 40% 19% 1	4315
	11th to 20th	Day	38% 15% 22% 15% 9% 1	4195
	21st to end	Day	40% 8% 20% 22% 9% 1	4663
September	1st to 10th	Day	35% 10% 15% 26% 11% 2	2409
	11th to 20th	Day	47% 8% 13% 16% 15% 1	2298
	21st to end	Day	52% 9% 14% 14% 11% 1	2170
October	1st to 10th	Day	58% 9% 14% 13% 6%	3280
	11th to 20th	Day	63% 8% 10% 16% 4	3261
	21st to end	Day	64% 11% 11% 10% 5%	3260
November	1st to 10th	Day	78% 8% 6% 4 4 1	1648
	11th to 20th	Day	71% 8% 8% 6% 5% 1	1680
	21st to end	Day	70% 8% 11% 6% 5%	1680
December	1st to 10th	Day	66% 11% 11% 8% 4	2382
	11th to 20th	Day	81% 7% 7% 3 1	2648
	21st to end	Day	81% 7% 7% 3 1	3498
January	1st to 10th	Night	81% 7% 7% 3 1	6766
	11th to 20th	Night	49% 13% 19% 12% 6% 1	6780
	21st to end	Night	67% 7% 12% 8% 6%	7030
February	1st to 10th	Night	69% 7% 9% 7% 7% 1	2600
	11th to 20th	Night	64% 7% 10% 11% 8% 1	2534
	21st to end	Night	64% 6% 11% 12% 5% 1	2145
March	1st to 10th	Night	62% 7% 12% 13% 5%	5138
	11th to 20th	Night	58% 9% 12% 15% 6%	5000
	21st to end	Night	72% 8% 10% 8% 2	5489
April	1st to 10th	Night	63% 9% 13% 10% 5%	2006
	11th to 20th	Night	57% 8% 11% 14% 8% 1	1781
	21st to end	Night	61% 8% 11% 13% 6% 1	1800
May	1st to 10th	Night	42% 8% 15% 25% 9%	3688
	11th to 20th	Night	49% 11% 23% 13% 3	3616
	21st to end	Night	57% 9% 15% 17% 2	3542
June	1st to 10th	Night	52% 9% 20% 11% 7% 2	1400
	11th to 20th	Night	53% 10% 21% 11% 4	1400
	21st to end	Night	43% 11% 21% 16% 7% 2	1400
July	1st to 10th	Night	24% 8% 20% 26% 12% 10%	3145
	11th to 20th	Night	12% 7% 24% 36% 20% 1	3138
	21st to end	Night	18% 8% 34% 25% 13% 1	3490
August	1st to 10th	Night	18% 6% 13% 35% 27% 1	2814
	11th to 20th	Night	32% 14% 28% 15% 9% 2	2929
	21st to end	Night	37% 10% 20% 24% 8% 1	3245
September	1st to 10th	Night	30% 10% 17% 27% 14% 3	1990
	11th to 20th	Night	48% 9% 10% 14% 16% 3	2100
	21st to end	Night	50% 7% 13% 17% 10% 2	2229
October	1st to 10th	Night	54% 9% 14% 15% 7% 1	3920
	11th to 20th	Night	62% 7% 13% 13% 5%	3897
	21st to end	Night	62% 9% 12% 11% 5% 1	4538
November	1st to 10th	Night	73% 8% 8% 7% 4 1	2870
	11th to 20th	Night	72% 6% 8% 7% 3 2	2940
	21st to end	Night	68% 8% 10% 8% 5% 1	2940
December	1st to 10th	Night	66% 12% 12% 7% 3	4837
	11th to 20th	Night	79% 8% 7% 4 1 1	5556
	21st to end	Night	78% 7% 8% 5% 2	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	38% 16% 18% 19% 6% 5%	200
	11th to 20th	Day	29% 20% 21% 17% 13% 1	200
	21st to end	Day	33% 16% 21% 15% 12% 2	330
February	1st to 10th	Day	34% 19% 24% 16% 7% 1	400
	11th to 20th	Day	33% 16% 26% 19% 5% 1	395
	21st to end	Day	29% 25% 24% 16% 5% 2	331
March	1st to 10th	Day	30% 17% 21% 21% 10% 1	473
	11th to 20th	Day	29% 21% 17% 22% 9% 3	500
	21st to end	Day	33% 20% 24% 16% 6% 2	550
April	1st to 10th	Day	32% 20% 24% 16% 7% 1	500
	11th to 20th	Day	25% 20% 25% 18% 9% 3	492
	21st to end	Day	30% 18% 24% 17% 9% 3	500
May	1st to 10th	Day	20% 15% 26% 28% 9% 2	503
	11th to 20th	Day	21% 13% 23% 27% 13% 4	528
	21st to end	Day	17% 15% 28% 26% 11% 3	754
June	1st to 10th	Day	23% 15% 29% 22% 9% 2	730
	11th to 20th	Day	18% 19% 31% 21% 9% 2	730
	21st to end	Day	16% 14% 25% 30% 12% 3	730
July	1st to 10th	Day	12% 10% 27% 34% 15% 2	721
	11th to 20th	Day	7% 12% 28% 33% 17% 4	718
	21st to end	Day	7% 9% 23% 33% 22% 6%	745
August	1st to 10th	Day	12% 7% 20% 29% 24% 8%	527
	11th to 20th	Day	9% 8% 22% 31% 23% 7%	517
	21st to end	Day	14% 8% 25% 31% 16% 7%	583
September	1st to 10th	Day	13% 9% 26% 32% 15% 5%	530
	11th to 20th	Day	20% 11% 22% 27% 15% 4	530
	21st to end	Day	23% 14% 27% 21% 12% 3	530
October	1st to 10th	Day	26% 15% 21% 21% 13% 5%	530
	11th to 20th	Day	32% 13% 21% 19% 11% 4	530
	21st to end	Day	31% 19% 22% 16% 9% 4	466
November	1st to 10th	Day	34% 25% 14% 19% 8% 1	220
	11th to 20th	Day	30% 18% 19% 15% 12% 5%	220
	21st to end	Day	35% 18% 19% 14% 7% 8%	220
December	1st to 10th	Day	27% 17% 19% 17% 14% 7%	204
	11th to 20th	Day	37% 14% 18% 17% 14% 1	200
	21st to end	Day	40% 16% 18% 16% 8% 1	220
January	1st to 10th	Night	41% 13% 18% 18% 8% 2	900
	11th to 20th	Night	34% 12% 18% 22% 13% 1	900
	21st to end	Night	33% 13% 21% 20% 11% 3	951
February	1st to 10th	Night	30% 14% 23% 23% 9% 1	800
	11th to 20th	Night	27% 13% 21% 26% 12%	784
	21st to end	Night	28% 13% 23% 23% 10% 2	660
March	1st to 10th	Night	24% 13% 24% 21% 15% 2	800
	11th to 20th	Night	25% 18% 22% 21% 13% 1	800
	21st to end	Night	27% 16% 22% 23% 11% 2	880
April	1st to 10th	Night	25% 12% 23% 26% 13% 2	800
	11th to 20th	Night	18% 9% 27% 33% 10% 2	593
	21st to end	Night	24% 8% 22% 32% 13% 2	600
May	1st to 10th	Night	14% 9% 19% 36% 22%	425
	11th to 20th	Night	16% 7% 18% 32% 23% 4	394
	21st to end	Night	16% 7% 18% 36% 21% 1	433
June	1st to 10th	Night	17% 10% 22% 28% 23% 1	400
	11th to 20th	Night	14% 7% 25% 36% 18% 1	400
	21st to end	Night	12% 6% 23% 34% 20% 5%	400
July	1st to 10th	Night	11% 4 17% 35% 31% 3	390
	11th to 20th	Night	5% 6% 15% 37% 34% 3	388
	21st to end	Night	6% 5% 13% 32% 40% 4	438
August	1st to 10th	Night	8% 4 11% 30% 37% 10%	520
	11th to 20th	Night	7% 4 10% 32% 38% 9%	594
	21st to end	Night	13% 4 16% 29% 33% 5%	660
September	1st to 10th	Night	10% 4 16% 36% 29% 7%	790
	11th to 20th	Night	17% 7% 17% 24% 26% 9%	800
	21st to end	Night	22% 11% 19% 27% 18% 4	800
October	1st to 10th	Night	22% 6% 19% 26% 25% 2	800
	11th to 20th	Night	31% 9% 19% 22% 16% 4	800
	21st to end	Night	30% 11% 22% 24% 11% 2	880
November	1st to 10th	Night	37% 13% 19% 24% 7%	820
	11th to 20th	Night	35% 14% 19% 21% 9% 2	840
	21st to end	Night	33% 11% 21% 20% 12% 3	840
December	1st to 10th	Night	28% 12% 20% 23% 15% 2	881
	11th to 20th	Night	36% 13% 22% 21% 7% 1	900
	21st to end	Night	37% 13% 23% 20% 6%	990

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