Nanaimo
Clear Sky Chart
History

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

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	15% 10% 3 8% 5% 5% 5% 4 5% 7% 34%	123667
	Night	17% 10% 3 8% 5% 5% 4 4 5% 6% 32%	123836
Transparency	Day	49% 8% 17% 18% 8% 1	123413
	Night	51% 7% 15% 18% 7% 1	123733
Seeing	Day	19% 9% 24% 28% 16% 4	16356
	Night	23% 8% 18% 27% 20% 4	24471

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	25% 17% 4 10% 6% 6% 4 4 6% 6% 12%	3179
	11th to 20th	Day	15% 11% 3 8% 6% 5% 5% 5% 8% 8% 26%	3159
	21st to end	Day	24% 15% 4 11% 6% 5% 4 3 5% 4 19%	3978
February	1st to 10th	Day	20% 17% 5% 10% 7% 4 5% 4 6% 6% 17%	1800
	11th to 20th	Day	19% 14% 4 8% 6% 5% 5% 3 5% 6% 23%	1755
	21st to end	Day	18% 15% 3 9% 7% 6% 5% 4 7% 7% 19%	1485
March	1st to 10th	Day	17% 15% 5% 11% 7% 7% 6% 5% 5% 6% 17%	4733
	11th to 20th	Day	15% 14% 4 9% 6% 6% 5% 4 5% 6% 26%	5000
	21st to end	Day	21% 14% 4 10% 6% 6% 6% 4 5% 6% 16%	5546
April	1st to 10th	Day	18% 13% 4 9% 7% 6% 5% 5% 7% 6% 20%	2400
	11th to 20th	Day	16% 10% 4 9% 6% 6% 6% 5% 6% 6% 28%	2519
	21st to end	Day	16% 12% 3 9% 5% 5% 5% 4 6% 7% 28%	2614
May	1st to 10th	Day	10% 7% 3 6% 5% 6% 6% 4 6% 7% 42%	6320
	11th to 20th	Day	11% 9% 3 8% 5% 5% 4 4 6% 10% 33%	6277
	21st to end	Day	15% 8% 3 9% 5% 6% 6% 5% 7% 7% 29%	7178
June	1st to 10th	Day	16% 8% 3 8% 5% 5% 6% 5% 6% 8% 31%	3000
	11th to 20th	Day	12% 8% 4 10% 6% 6% 6% 4 7% 8% 28%	3000
	21st to end	Day	11% 8% 3 7% 5% 5% 5% 4 6% 8% 38%	3000
July	1st to 10th	Day	5% 3 1 4 3 2 3 3 4 5% 67%	4816
	11th to 20th	Day	4 3 1 3 2 2 3 2 4 5% 71%	4740
	21st to end	Day	6% 4 1 4 3 3 4 4 6% 8% 56%	4963
August	1st to 10th	Day	4 3 1 3 3 2 3 2 4 6% 69%	4279
	11th to 20th	Day	11% 7% 3 6% 3 4 4 3 6% 8% 44%	4026
	21st to end	Day	10% 6% 2 7% 4 4 4 4 5% 7% 47%	4473
September	1st to 10th	Day	8% 5% 2 5% 3 4 4 4 5% 8% 53%	2280
	11th to 20th	Day	15% 7% 3 8% 4 5% 4 3 5% 6% 42%	2228
	21st to end	Day	16% 7% 2 7% 5% 4 4 4 6% 6% 39%	2033
October	1st to 10th	Day	17% 8% 3 8% 6% 5% 6% 5% 7% 7% 30%	3180
	11th to 20th	Day	23% 9% 3 8% 5% 5% 5% 4 5% 6% 28%	3161
	21st to end	Day	19% 11% 3 10% 7% 5% 5% 4 5% 7% 22%	3165
November	1st to 10th	Day	25% 12% 4 9% 8% 5% 6% 3 4 5% 20%	1568
	11th to 20th	Day	21% 12% 4 9% 6% 6% 5% 4 5% 5% 23%	1600
	21st to end	Day	22% 14% 4 10% 6% 5% 5% 4 5% 6% 20%	1600
December	1st to 10th	Day	15% 13% 4 10% 5% 5% 4 4 5% 7% 28%	2469
	11th to 20th	Day	29% 16% 3 9% 5% 5% 5% 4 4 5% 14%	2645
	21st to end	Day	27% 17% 5% 12% 5% 6% 5% 4 4 4 10%	3498
January	1st to 10th	Night	24% 17% 5% 11% 7% 6% 5% 4 5% 4 14%	6766
	11th to 20th	Night	15% 10% 3 7% 5% 5% 5% 4 7% 8% 32%	6739
	21st to end	Night	23% 12% 4 9% 5% 4 5% 4 5% 4 24%	6979
February	1st to 10th	Night	20% 13% 4 11% 6% 5% 5% 4 5% 5% 22%	2600
	11th to 20th	Night	19% 13% 3 7% 5% 5% 4 3 5% 7% 28%	2532
	21st to end	Night	19% 15% 4 9% 5% 3 4 4 4 6% 26%	2145
March	1st to 10th	Night	16% 14% 4 8% 6% 5% 4 4 5% 7% 27%	5158
	11th to 20th	Night	13% 10% 3 7% 5% 5% 5% 4 7% 8% 32%	4999
	21st to end	Night	24% 14% 3 9% 5% 5% 4 4 6% 6% 21%	5429
April	1st to 10th	Night	16% 11% 3 9% 6% 5% 6% 6% 5% 7% 26%	2000
	11th to 20th	Night	14% 10% 3 8% 4 5% 5% 4 6% 7% 34%	1827
	21st to end	Night	14% 9% 3 8% 5% 5% 5% 4 7% 7% 33%	1785
May	1st to 10th	Night	11% 6% 2 6% 4 4 5% 3 5% 7% 49%	3640
	11th to 20th	Night	11% 6% 3 6% 5% 4 4 4 6% 10% 40%	3616
	21st to end	Night	12% 9% 3 9% 7% 6% 6% 4 6% 7% 31%	3680
June	1st to 10th	Night	12% 8% 2 8% 7% 4 5% 4 6% 6% 38%	1400
	11th to 20th	Night	11% 7% 2 8% 5% 5% 6% 6% 6% 8% 37%	1400
	21st to end	Night	7% 5% 2 6% 4 5% 5% 4 7% 9% 46%	1400
July	1st to 10th	Night	4 3 1 3 3 3 4 3 3 4 69%	2255
	11th to 20th	Night	2 2 1 2 2 2 2 2 3 5% 78%	2220
	21st to end	Night	7% 5% 2 3 3 3 4 4 4 6% 59%	2586
August	1st to 10th	Night	3 2 1 3 2 2 2 2 3 5% 76%	2651
	11th to 20th	Night	14% 8% 2 5% 3 3 4 3 5% 6% 48%	2812
	21st to end	Night	11% 4 1 4 4 4 5% 4 5% 8% 49%	3196
September	1st to 10th	Night	6% 2 1 4 3 3 4 3 5% 7% 62%	1900
	11th to 20th	Night	13% 7% 2 5% 4 4 5% 4 6% 6% 45%	1948
	21st to end	Night	14% 7% 3 7% 4 4 4 3 4 7% 42%	2147
October	1st to 10th	Night	13% 8% 3 9% 5% 4 5% 4 6% 6% 38%	3800
	11th to 20th	Night	21% 9% 3 7% 5% 5% 6% 4 6% 7% 29%	3777
	21st to end	Night	21% 9% 3 8% 5% 4 4 3 5% 6% 32%	4391
November	1st to 10th	Night	26% 13% 3 7% 6% 5% 5% 4 5% 5% 21%	2730
	11th to 20th	Night	20% 13% 4 8% 5% 5% 5% 4 5% 6% 24%	2800
	21st to end	Night	22% 11% 3 9% 7% 6% 5% 4 5% 5% 23%	2800
December	1st to 10th	Night	16% 13% 5% 9% 6% 5% 4 4 5% 7% 27%	4710
	11th to 20th	Night	29% 18% 4 9% 6% 5% 4 4 4 5% 13%	5560
	21st to end	Night	25% 15% 4 11% 6% 5% 5% 4 5% 5% 16%	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	71% 7% 8% 12% 2	3179
	11th to 20th	Day	50% 15% 16% 15% 3 1	3180
	21st to end	Day	69% 5% 10% 12% 3	3978
February	1st to 10th	Day	68% 6% 11% 11% 4 1	1800
	11th to 20th	Day	62% 7% 13% 14% 5%	1764
	21st to end	Day	62% 7% 15% 11% 4 1	1485
March	1st to 10th	Day	64% 8% 11% 12% 4	4733
	11th to 20th	Day	57% 7% 13% 19% 4	5000
	21st to end	Day	65% 8% 16% 8% 2	5546
April	1st to 10th	Day	61% 7% 17% 12% 3	2400
	11th to 20th	Day	55% 6% 14% 13% 9% 2	2519
	21st to end	Day	53% 8% 15% 16% 7% 1	2614
May	1st to 10th	Day	39% 7% 21% 21% 12% 1	6320
	11th to 20th	Day	44% 8% 26% 18% 4	6280
	21st to end	Day	49% 11% 22% 14% 4 1	7179
June	1st to 10th	Day	49% 7% 21% 15% 5% 2	3000
	11th to 20th	Day	49% 9% 23% 15% 4	3000
	21st to end	Day	43% 8% 21% 19% 8% 2	3000
July	1st to 10th	Day	20% 5% 14% 26% 25% 9%	4742
	11th to 20th	Day	15% 5% 16% 38% 24% 2	4659
	21st to end	Day	24% 8% 28% 24% 14% 2	4806
August	1st to 10th	Day	18% 4 16% 45% 16% 1	4262
	11th to 20th	Day	36% 9% 29% 16% 9% 1	4065
	21st to end	Day	36% 8% 23% 23% 9% 1	4472
September	1st to 10th	Day	31% 8% 21% 25% 13% 2	2280
	11th to 20th	Day	43% 7% 17% 19% 13% 2	2228
	21st to end	Day	45% 7% 17% 19% 10% 1	2033
October	1st to 10th	Day	50% 9% 18% 19% 4	3180
	11th to 20th	Day	57% 8% 13% 17% 6%	3161
	21st to end	Day	59% 9% 13% 13% 5%	3165
November	1st to 10th	Day	70% 5% 11% 9% 4 1	1568
	11th to 20th	Day	64% 7% 10% 14% 5% 1	1600
	21st to end	Day	65% 8% 8% 12% 5%	1600
December	1st to 10th	Day	55% 7% 14% 19% 4	2469
	11th to 20th	Day	72% 7% 12% 7% 2	2648
	21st to end	Day	75% 7% 8% 6% 3	3498
January	1st to 10th	Night	72% 7% 8% 10% 2	6766
	11th to 20th	Night	47% 11% 19% 19% 3 1	6780
	21st to end	Night	63% 7% 11% 16% 4	6979
February	1st to 10th	Night	63% 7% 11% 12% 6% 1	2600
	11th to 20th	Night	57% 7% 14% 15% 7%	2534
	21st to end	Night	59% 6% 11% 16% 7% 1	2145
March	1st to 10th	Night	56% 6% 13% 17% 8%	5158
	11th to 20th	Night	48% 7% 15% 24% 7%	5000
	21st to end	Night	63% 9% 13% 12% 3	5429
April	1st to 10th	Night	55% 8% 16% 17% 4	2000
	11th to 20th	Night	49% 7% 13% 19% 10% 2	1827
	21st to end	Night	49% 7% 16% 18% 10% 1	1785
May	1st to 10th	Night	35% 5% 15% 31% 14% 1	3637
	11th to 20th	Night	39% 7% 26% 23% 6%	3616
	21st to end	Night	50% 7% 19% 19% 4	3680
June	1st to 10th	Night	46% 7% 19% 18% 7% 2	1400
	11th to 20th	Night	42% 8% 23% 22% 5% 1	1400
	21st to end	Night	33% 8% 23% 24% 9% 3	1400
July	1st to 10th	Night	20% 4 13% 29% 23% 11%	2220
	11th to 20th	Night	12% 3 16% 39% 27% 3	2178
	21st to end	Night	24% 7% 18% 30% 19% 2	2504
August	1st to 10th	Night	14% 3 14% 38% 29% 2	2643
	11th to 20th	Night	37% 7% 28% 17% 9% 3	2839
	21st to end	Night	32% 8% 22% 28% 10%	3196
September	1st to 10th	Night	23% 7% 19% 32% 15% 4	1900
	11th to 20th	Night	39% 8% 15% 18% 17% 4	1948
	21st to end	Night	43% 6% 17% 22% 8% 3	2147
October	1st to 10th	Night	45% 8% 20% 18% 8% 1	3800
	11th to 20th	Night	54% 6% 15% 20% 5%	3777
	21st to end	Night	53% 6% 16% 18% 6%	4391
November	1st to 10th	Night	66% 7% 11% 12% 4 1	2730
	11th to 20th	Night	61% 7% 13% 11% 6% 1	2800
	21st to end	Night	63% 7% 11% 13% 6% 1	2800
December	1st to 10th	Night	57% 7% 14% 19% 3	4710
	11th to 20th	Night	74% 7% 11% 6% 2	5556
	21st to end	Night	69% 7% 10% 10% 3	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	36% 13% 23% 15% 7% 7%	200
	11th to 20th	Day	33% 16% 21% 18% 9% 4	200
	21st to end	Day	31% 13% 21% 21% 10% 3	291
February	1st to 10th	Day	30% 15% 27% 20% 8% 1	400
	11th to 20th	Day	26% 15% 27% 23% 9% 1	395
	21st to end	Day	25% 17% 29% 20% 8% 1	331
March	1st to 10th	Day	24% 13% 24% 27% 10% 2	490
	11th to 20th	Day	26% 11% 24% 25% 12% 2	500
	21st to end	Day	28% 13% 28% 20% 8% 3	550
April	1st to 10th	Day	26% 11% 30% 23% 9% 2	500
	11th to 20th	Day	20% 13% 32% 21% 14% 1	492
	21st to end	Day	20% 10% 30% 25% 13% 2	500
May	1st to 10th	Day	14% 9% 26% 35% 14% 3	503
	11th to 20th	Day	17% 5% 23% 31% 20% 4	528
	21st to end	Day	14% 6% 26% 33% 16% 5%	692
June	1st to 10th	Day	18% 6% 26% 33% 14% 2	730
	11th to 20th	Day	13% 10% 28% 32% 15% 2	730
	21st to end	Day	11% 7% 23% 37% 18% 3	730
July	1st to 10th	Day	11% 5% 23% 31% 25% 6%	697
	11th to 20th	Day	5% 4 19% 41% 24% 6%	668
	21st to end	Day	4 4 15% 39% 33% 5%	639
August	1st to 10th	Day	7% 2 16% 33% 31% 10%	487
	11th to 20th	Day	7% 4 17% 41% 24% 7%	477
	21st to end	Day	11% 6% 23% 37% 20% 4	539
September	1st to 10th	Day	11% 7% 23% 35% 21% 3	490
	11th to 20th	Day	18% 9% 20% 29% 21% 4	490
	21st to end	Day	20% 11% 26% 26% 15% 3	490
October	1st to 10th	Day	22% 10% 22% 26% 18% 2	490
	11th to 20th	Day	28% 10% 23% 21% 14% 4	490
	21st to end	Day	25% 14% 31% 18% 9% 3	417
November	1st to 10th	Day	32% 16% 29% 16% 7%	200
	11th to 20th	Day	28% 13% 22% 18% 16% 5%	200
	21st to end	Day	34% 14% 21% 15% 11% 7%	200
December	1st to 10th	Day	31% 13% 16% 16% 18% 8%	200
	11th to 20th	Day	33% 12% 24% 22% 9% 1	200
	21st to end	Day	40% 14% 20% 20% 5% 1	220
January	1st to 10th	Night	35% 12% 19% 19% 11% 2	900
	11th to 20th	Night	30% 12% 16% 23% 17% 2	900
	21st to end	Night	32% 10% 14% 26% 16% 2	920
February	1st to 10th	Night	29% 11% 20% 26% 12% 2	800
	11th to 20th	Night	28% 9% 21% 24% 17% 1	784
	21st to end	Night	28% 10% 20% 26% 12% 4	660
March	1st to 10th	Night	24% 10% 22% 25% 16% 4	800
	11th to 20th	Night	24% 11% 22% 25% 15% 3	800
	21st to end	Night	25% 9% 20% 24% 18% 4	880
April	1st to 10th	Night	21% 10% 19% 29% 18% 4	800
	11th to 20th	Night	19% 8% 23% 30% 17% 3	639
	21st to end	Night	18% 7% 21% 31% 19% 3	585
May	1st to 10th	Night	14% 3 23% 32% 25% 4	400
	11th to 20th	Night	14% 7% 14% 32% 24% 9%	394
	21st to end	Night	14% 6% 14% 34% 26% 6%	433
June	1st to 10th	Night	16% 6% 19% 38% 17% 4	400
	11th to 20th	Night	14% 5% 21% 33% 24% 4	400
	21st to end	Night	10% 4 17% 38% 26% 6%	400
July	1st to 10th	Night	8% 6% 12% 33% 33% 8%	380
	11th to 20th	Night	4 4 18% 33% 35% 7%	368
	21st to end	Night	6% 3 11% 31% 42% 7%	416
August	1st to 10th	Night	8% 2 11% 28% 38% 12%	448
	11th to 20th	Night	5% 2 12% 31% 39% 10%	564
	21st to end	Night	10% 4 15% 31% 32% 7%	651
September	1st to 10th	Night	8% 3 15% 34% 33% 7%	760
	11th to 20th	Night	16% 6% 14% 21% 31% 11%	760
	21st to end	Night	18% 6% 19% 28% 24% 5%	760
October	1st to 10th	Night	18% 6% 18% 27% 25% 6%	760
	11th to 20th	Night	27% 6% 16% 26% 19% 5%	760
	21st to end	Night	29% 9% 20% 26% 13% 3	836
November	1st to 10th	Night	34% 9% 18% 27% 11% 1	780
	11th to 20th	Night	28% 12% 21% 23% 13% 2	800
	21st to end	Night	29% 12% 17% 25% 14% 3	800
December	1st to 10th	Night	26% 11% 15% 27% 17% 2	843
	11th to 20th	Night	35% 13% 20% 21% 10% 1	900
	21st to end	Night	37% 12% 23% 20% 8% 1	990

Page generated at 2025-07-02 22:30:18. Version ac07

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