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 2024-11-10.

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	17% 11% 4 9% 6% 5% 4 4 5% 6% 30%	105572
	Night	20% 12% 4 9% 5% 5% 4 3 5% 5% 27%	102163
Transparency	Day	55% 8% 12% 16% 8% 1	105315
	Night	58% 8% 12% 14% 7% 1	102066
Seeing	Day	23% 15% 24% 23% 12% 3	15702
	Night	25% 10% 20% 25% 17% 3	23683

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% 6% 10% 5% 4 4 4 4 5% 9%	2219
	11th to 20th	Day	25% 14% 5% 11% 9% 4 4 4 6% 6% 12%	2199
	21st to end	Day	39% 17% 5% 12% 6% 4 3 2 3 2 7%	2721
February	1st to 10th	Day	26% 17% 5% 9% 8% 5% 4 4 5% 5% 12%	1710
	11th to 20th	Day	23% 16% 5% 11% 6% 5% 5% 3 4 5% 18%	1665
	21st to end	Day	20% 19% 4 11% 6% 6% 5% 4 6% 4 17%	1413
March	1st to 10th	Day	13% 19% 5% 11% 8% 6% 6% 4 4 6% 17%	3304
	11th to 20th	Day	17% 14% 5% 9% 6% 5% 4 4 4 6% 26%	3490
	21st to end	Day	24% 17% 4 11% 6% 5% 4 3 4 5% 17%	3825
April	1st to 10th	Day	20% 19% 5% 11% 8% 5% 5% 3 4 4 16%	2273
	11th to 20th	Day	18% 17% 5% 11% 7% 5% 4 3 4 5% 21%	2436
	21st to end	Day	22% 15% 3 10% 6% 5% 5% 4 5% 5% 20%	2470
May	1st to 10th	Day	15% 11% 3 8% 5% 5% 5% 4 5% 5% 32%	4384
	11th to 20th	Day	11% 9% 3 7% 5% 5% 6% 4 6% 9% 33%	4377
	21st to end	Day	19% 11% 4 11% 6% 6% 6% 4 5% 5% 21%	5115
June	1st to 10th	Day	21% 12% 4 9% 7% 5% 5% 4 5% 6% 23%	2850
	11th to 20th	Day	15% 13% 4 10% 6% 6% 6% 5% 7% 7% 21%	2850
	21st to end	Day	14% 9% 4 10% 5% 5% 5% 4 4 7% 32%	2850
July	1st to 10th	Day	7% 4 2 4 3 3 3 2 3 4 65%	4740
	11th to 20th	Day	5% 3 1 3 2 2 2 2 4 6% 69%	4740
	21st to end	Day	6% 5% 2 5% 4 4 4 3 5% 7% 53%	5056
August	1st to 10th	Day	6% 3 1 3 2 2 2 3 4 8% 66%	4199
	11th to 20th	Day	7% 6% 3 8% 6% 6% 5% 5% 8% 8% 38%	4026
	21st to end	Day	12% 7% 3 7% 5% 4 4 3 6% 8% 41%	4521
September	1st to 10th	Day	11% 7% 3 6% 4 3 5% 4 5% 8% 46%	2289
	11th to 20th	Day	15% 9% 3 8% 4 4 4 3 5% 6% 38%	2183
	21st to end	Day	19% 9% 3 8% 4 4 4 4 6% 6% 33%	2062
October	1st to 10th	Day	19% 11% 4 10% 7% 5% 5% 3 6% 7% 22%	3180
	11th to 20th	Day	27% 10% 4 9% 6% 5% 5% 4 5% 6% 19%	3161
	21st to end	Day	21% 12% 4 12% 9% 7% 5% 4 5% 6% 16%	3159
November	1st to 10th	Day	30% 16% 5% 11% 6% 6% 4 3 3 4 11%	1560
	11th to 20th	Day	29% 15% 5% 9% 5% 5% 4 3 5% 4 15%	1520
	21st to end	Day	29% 15% 4 11% 8% 4 3 3 4 4 15%	1520
December	1st to 10th	Day	23% 15% 6% 12% 6% 6% 4 3 4 6% 14%	1378
	11th to 20th	Day	31% 18% 6% 17% 6% 4 4 2 2 3 6%	1685
	21st to end	Day	32% 17% 5% 11% 6% 5% 4 3 4 3 9%	2442
January	1st to 10th	Night	30% 21% 6% 11% 5% 4 4 3 3 4 9%	4726
	11th to 20th	Night	26% 12% 5% 11% 6% 4 5% 4 5% 5% 16%	4699
	21st to end	Night	36% 18% 5% 12% 6% 4 3 2 3 3 8%	4920
February	1st to 10th	Night	24% 16% 5% 10% 6% 5% 4 3 5% 5% 17%	2470
	11th to 20th	Night	20% 16% 4 9% 5% 4 4 3 4 5% 25%	2402
	21st to end	Night	20% 16% 3 10% 7% 5% 3 3 4 5% 23%	2041
March	1st to 10th	Night	14% 15% 5% 11% 6% 6% 6% 4 5% 5% 23%	3563
	11th to 20th	Night	11% 13% 4 9% 6% 6% 6% 5% 5% 5% 30%	3489
	21st to end	Night	20% 17% 5% 11% 5% 5% 5% 3 4 5% 20%	3828
April	1st to 10th	Night	17% 14% 4 10% 6% 6% 6% 4 5% 7% 22%	1906
	11th to 20th	Night	15% 12% 5% 10% 6% 6% 6% 3 6% 7% 25%	1691
	21st to end	Night	21% 12% 4 9% 5% 6% 5% 4 6% 6% 23%	1710
May	1st to 10th	Night	12% 10% 4 7% 5% 4 3 3 4 5% 44%	2577
	11th to 20th	Night	10% 7% 3 6% 4 5% 5% 5% 7% 9% 40%	2516
	21st to end	Night	18% 13% 3 8% 6% 5% 5% 4 5% 7% 26%	2458
June	1st to 10th	Night	17% 9% 4 8% 5% 5% 5% 4 5% 7% 30%	1330
	11th to 20th	Night	12% 7% 3 9% 7% 7% 7% 5% 6% 8% 29%	1330
	21st to end	Night	9% 7% 3 7% 7% 6% 5% 5% 7% 8% 37%	1330
July	1st to 10th	Night	6% 5% 1 3 3 3 2 2 4 5% 65%	2220
	11th to 20th	Night	4 3 1 2 3 1 2 3 4 7% 71%	2220
	21st to end	Night	7% 3 2 4 3 3 4 4 6% 7% 58%	2494
August	1st to 10th	Night	3 3 1 3 3 2 2 3 3 6% 71%	2736
	11th to 20th	Night	7% 4 2 6% 5% 5% 5% 4 7% 9% 45%	2812
	21st to end	Night	11% 6% 2 7% 5% 4 4 4 6% 7% 45%	3146
September	1st to 10th	Night	7% 5% 2 5% 3 4 4 4 5% 8% 53%	1890
	11th to 20th	Night	14% 9% 2 9% 5% 5% 5% 4 5% 6% 36%	1995
	21st to end	Night	19% 8% 3 8% 5% 4 4 3 4 4 37%	2117
October	1st to 10th	Night	18% 10% 4 11% 6% 5% 4 3 4 6% 29%	3800
	11th to 20th	Night	23% 10% 4 10% 6% 6% 6% 4 5% 6% 20%	3777
	21st to end	Night	22% 12% 5% 11% 6% 6% 5% 3 4 4 22%	4397
November	1st to 10th	Night	30% 15% 4 10% 6% 4 4 4 5% 5% 14%	2722
	11th to 20th	Night	28% 16% 4 10% 6% 5% 4 3 3 5% 16%	2660
	21st to end	Night	28% 12% 3 10% 7% 5% 4 4 5% 6% 15%	2660
December	1st to 10th	Night	23% 16% 5% 12% 7% 6% 5% 3 4 5% 15%	2797
	11th to 20th	Night	31% 18% 6% 13% 5% 4 5% 3 4 4 7%	3520
	21st to end	Night	30% 18% 5% 11% 7% 5% 4 2 3 3 10%	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	76% 9% 5% 7% 3	2219
	11th to 20th	Day	65% 7% 15% 7% 5% 1	2220
	21st to end	Day	85% 5% 4 4 2	2721
February	1st to 10th	Day	73% 9% 7% 7% 4 1	1710
	11th to 20th	Day	69% 7% 10% 10% 5% 1	1674
	21st to end	Day	69% 7% 10% 8% 4 1	1413
March	1st to 10th	Day	68% 8% 10% 7% 6%	3304
	11th to 20th	Day	60% 6% 10% 17% 7%	3490
	21st to end	Day	70% 9% 11% 8% 2	3825
April	1st to 10th	Day	73% 6% 9% 8% 3	2273
	11th to 20th	Day	67% 6% 7% 10% 8% 2	2436
	21st to end	Day	67% 8% 9% 11% 5%	2470
May	1st to 10th	Day	53% 8% 14% 18% 7%	4384
	11th to 20th	Day	46% 13% 19% 18% 4	4380
	21st to end	Day	65% 9% 10% 11% 5%	5116
June	1st to 10th	Day	63% 9% 11% 10% 5% 2	2850
	11th to 20th	Day	60% 11% 14% 10% 4	2850
	21st to end	Day	52% 9% 15% 16% 8%	2850
July	1st to 10th	Day	25% 6% 8% 27% 22% 12%	4666
	11th to 20th	Day	19% 6% 12% 40% 22% 1	4659
	21st to end	Day	29% 9% 20% 25% 15% 2	4896
August	1st to 10th	Day	20% 6% 13% 41% 20% 1	4182
	11th to 20th	Day	37% 16% 23% 15% 9% 1	4065
	21st to end	Day	41% 8% 20% 22% 8% 1	4520
September	1st to 10th	Day	36% 10% 14% 26% 12% 2	2289
	11th to 20th	Day	48% 8% 12% 16% 15% 1	2183
	21st to end	Day	52% 9% 14% 15% 10% 1	2062
October	1st to 10th	Day	59% 9% 14% 13% 6%	3180
	11th to 20th	Day	63% 8% 10% 16% 3	3161
	21st to end	Day	63% 11% 11% 10% 5%	3159
November	1st to 10th	Day	78% 7% 5% 4 4 1	1560
	11th to 20th	Day	71% 8% 8% 6% 6% 1	1520
	21st to end	Day	70% 7% 11% 6% 6%	1520
December	1st to 10th	Day	71% 7% 7% 8% 6% 1	1378
	11th to 20th	Day	79% 7% 8% 4 2 1	1688
	21st to end	Day	80% 6% 7% 5% 2	2442
January	1st to 10th	Night	79% 8% 7% 5% 2	4726
	11th to 20th	Night	62% 7% 15% 9% 5% 1	4740
	21st to end	Night	84% 4 4 5% 3	4920
February	1st to 10th	Night	70% 7% 9% 7% 6% 1	2470
	11th to 20th	Night	64% 7% 10% 11% 8%	2404
	21st to end	Night	64% 6% 11% 13% 5% 1	2041
March	1st to 10th	Night	62% 7% 11% 13% 5% 1	3563
	11th to 20th	Night	55% 9% 12% 16% 8% 1	3490
	21st to end	Night	69% 9% 10% 10% 2	3828
April	1st to 10th	Night	63% 9% 12% 10% 5%	1906
	11th to 20th	Night	59% 7% 10% 13% 8% 1	1691
	21st to end	Night	63% 8% 9% 13% 6% 1	1710
May	1st to 10th	Night	44% 10% 15% 23% 8%	2577
	11th to 20th	Night	41% 12% 25% 17% 5%	2516
	21st to end	Night	57% 11% 15% 15% 3	2458
June	1st to 10th	Night	54% 9% 18% 11% 5% 2	1330
	11th to 20th	Night	54% 10% 21% 11% 4	1330
	21st to end	Night	43% 10% 21% 16% 7% 2	1330
July	1st to 10th	Night	24% 7% 10% 29% 16% 14%	2185
	11th to 20th	Night	16% 6% 15% 42% 20% 2	2178
	21st to end	Night	24% 10% 21% 29% 15% 2	2415
August	1st to 10th	Night	18% 5% 12% 35% 28% 1	2728
	11th to 20th	Night	32% 14% 29% 15% 9% 2	2839
	21st to end	Night	38% 11% 19% 24% 7% 1	3146
September	1st to 10th	Night	30% 10% 16% 27% 14% 3	1890
	11th to 20th	Night	49% 9% 9% 14% 16% 3	1995
	21st to end	Night	50% 7% 12% 18% 10% 3	2117
October	1st to 10th	Night	54% 9% 13% 15% 7% 1	3800
	11th to 20th	Night	63% 7% 13% 13% 4	3777
	21st to end	Night	62% 9% 12% 12% 5% 1	4397
November	1st to 10th	Night	73% 8% 8% 7% 4 1	2722
	11th to 20th	Night	72% 6% 9% 7% 4 2	2660
	21st to end	Night	68% 8% 9% 9% 5% 1	2660
December	1st to 10th	Night	70% 8% 10% 8% 4 1	2797
	11th to 20th	Night	77% 9% 6% 5% 2 1	3516
	21st to end	Night	80% 4 7% 6% 3	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% 16% 16% 18% 6% 5%	180
	11th to 20th	Day	32% 22% 19% 14% 11% 1	180
	21st to end	Day	35% 17% 21% 14% 11% 2	302
February	1st to 10th	Day	36% 19% 23% 15% 7%	370
	11th to 20th	Day	33% 16% 26% 19% 5% 1	365
	21st to end	Day	28% 25% 22% 17% 5% 2	307
March	1st to 10th	Day	30% 17% 20% 21% 10% 2	436
	11th to 20th	Day	28% 21% 17% 23% 8% 3	460
	21st to end	Day	33% 19% 24% 17% 6% 2	506
April	1st to 10th	Day	33% 19% 23% 17% 7% 1	460
	11th to 20th	Day	26% 21% 24% 17% 9% 3	452
	21st to end	Day	31% 19% 24% 16% 8% 2	460
May	1st to 10th	Day	19% 15% 26% 29% 10% 2	463
	11th to 20th	Day	20% 11% 23% 28% 13% 4	488
	21st to end	Day	16% 15% 28% 27% 12% 3	701
June	1st to 10th	Day	25% 16% 28% 20% 9% 2	680
	11th to 20th	Day	19% 19% 31% 21% 8% 2	680
	21st to end	Day	16% 14% 25% 29% 12% 3	680
July	1st to 10th	Day	12% 10% 26% 35% 15% 2	671
	11th to 20th	Day	7% 12% 27% 32% 18% 4	668
	21st to end	Day	8% 10% 23% 33% 22% 5%	693
August	1st to 10th	Day	12% 7% 19% 29% 24% 9%	487
	11th to 20th	Day	8% 6% 22% 31% 25% 8%	477
	21st to end	Day	15% 9% 27% 29% 14% 6%	539
September	1st to 10th	Day	14% 9% 27% 31% 14% 5%	490
	11th to 20th	Day	21% 11% 22% 27% 14% 4	490
	21st to end	Day	24% 13% 26% 21% 13% 3	490
October	1st to 10th	Day	27% 14% 20% 21% 13% 5%	490
	11th to 20th	Day	33% 12% 20% 19% 11% 4	490
	21st to end	Day	30% 19% 22% 17% 9% 4	431
November	1st to 10th	Day	35% 21% 14% 20% 9% 1	198
	11th to 20th	Day	32% 14% 17% 16% 14% 6%	180
	21st to end	Day	37% 19% 17% 12% 6% 9%	180
December	1st to 10th	Day	28% 17% 19% 17% 11% 7%	180
	11th to 20th	Day	34% 13% 18% 18% 15% 1	180
	21st to end	Day	40% 15% 17% 18% 9% 1	198
January	1st to 10th	Night	42% 13% 17% 17% 8% 2	850
	11th to 20th	Night	36% 13% 17% 20% 13% 1	850
	21st to end	Night	35% 13% 20% 19% 11% 2	900
February	1st to 10th	Night	32% 14% 23% 22% 9% 1	760
	11th to 20th	Night	27% 13% 22% 26% 12%	744
	21st to end	Night	27% 12% 24% 23% 11% 3	628
March	1st to 10th	Night	24% 13% 24% 21% 16% 3	760
	11th to 20th	Night	26% 17% 22% 20% 14% 1	760
	21st to end	Night	26% 16% 21% 23% 11% 2	836
April	1st to 10th	Night	26% 12% 22% 26% 13% 2	760
	11th to 20th	Night	18% 10% 27% 33% 10% 2	563
	21st to end	Night	25% 8% 22% 31% 12% 2	570
May	1st to 10th	Night	14% 9% 19% 36% 22%	404
	11th to 20th	Night	16% 6% 16% 33% 24% 4	374
	21st to end	Night	16% 7% 18% 36% 22% 1	411
June	1st to 10th	Night	18% 11% 21% 28% 22% 1	380
	11th to 20th	Night	15% 7% 24% 37% 17% 1	380
	21st to end	Night	12% 6% 24% 34% 19% 5%	380
July	1st to 10th	Night	11% 4 16% 35% 31% 4	370
	11th to 20th	Night	6% 7% 16% 35% 34% 3	368
	21st to end	Night	7% 5% 13% 31% 39% 4	416
August	1st to 10th	Night	8% 4 11% 28% 38% 10%	494
	11th to 20th	Night	6% 3 10% 32% 39% 10%	564
	21st to end	Night	13% 4 17% 30% 32% 5%	627
September	1st to 10th	Night	10% 4 16% 35% 29% 6%	750
	11th to 20th	Night	18% 7% 17% 23% 26% 9%	760
	21st to end	Night	22% 10% 18% 26% 19% 4	760
October	1st to 10th	Night	23% 6% 18% 26% 25% 3	760
	11th to 20th	Night	31% 8% 18% 22% 16% 5%	760
	21st to end	Night	30% 10% 22% 24% 11% 2	836
November	1st to 10th	Night	38% 13% 19% 24% 7%	778
	11th to 20th	Night	36% 13% 20% 20% 9% 2	760
	21st to end	Night	34% 10% 21% 19% 13% 3	760
December	1st to 10th	Night	28% 13% 20% 23% 14% 2	825
	11th to 20th	Night	37% 11% 22% 22% 7% 1	850
	21st to end	Night	37% 12% 24% 21% 6%	935

Page generated at 2024-11-07 03:55:47. Version ac07

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