Chilliwack
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Chilliwack clear sky chart from 2005-11-06 to 2026-06-01.

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% 9% 3 7% 5% 4 4 3 5% 6% 34%	135859
	Night	22% 11% 3 8% 5% 4 4 3 4 5% 31%	135984
Transparency	Day	52% 4 16% 18% 8% 1	135572
	Night	56% 5% 16% 16% 7% 1	135849
Seeing	Day	25% 12% 26% 24% 10% 3	17105
	Night	30% 7% 17% 27% 17% 2	25479

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% 18% 5% 10% 6% 5% 4 3 4 4 13%	4125
	11th to 20th	Day	18% 9% 3 5% 4 3 3 2 4 5% 42%	4355
	21st to end	Day	25% 12% 4 9% 4 4 3 2 3 4 30%	5186
February	1st to 10th	Day	30% 16% 4 7% 5% 4 3 3 4 6% 19%	1864
	11th to 20th	Day	24% 15% 3 7% 5% 5% 4 3 4 6% 23%	1845
	21st to end	Day	24% 19% 4 9% 4 6% 5% 3 4 3 19%	1620
March	1st to 10th	Day	19% 16% 4 9% 5% 5% 5% 4 5% 6% 22%	4657
	11th to 20th	Day	17% 14% 4 10% 7% 6% 5% 4 5% 5% 23%	5110
	21st to end	Day	22% 12% 3 9% 6% 6% 6% 5% 6% 7% 18%	5831
April	1st to 10th	Day	22% 15% 4 10% 6% 5% 6% 4 5% 6% 18%	2520
	11th to 20th	Day	21% 12% 4 8% 5% 5% 5% 4 6% 5% 23%	2537
	21st to end	Day	23% 13% 4 8% 5% 5% 5% 4 5% 5% 23%	2838
May	1st to 10th	Day	15% 9% 3 6% 5% 5% 4 3 4 6% 38%	6460
	11th to 20th	Day	19% 10% 3 8% 6% 6% 5% 5% 6% 6% 25%	6417
	21st to end	Day	24% 8% 3 7% 5% 5% 4 4 6% 8% 26%	7057
June	1st to 10th	Day	25% 9% 3 7% 5% 4 5% 3 5% 6% 27%	2971
	11th to 20th	Day	27% 10% 3 9% 6% 5% 5% 4 6% 6% 19%	3000
	21st to end	Day	17% 9% 4 7% 5% 4 4 4 5% 6% 34%	3000
July	1st to 10th	Day	9% 4 1 3 2 3 3 3 4 6% 62%	6780
	11th to 20th	Day	6% 3 1 3 3 2 2 2 3 5% 69%	6718
	21st to end	Day	10% 3 1 4 3 3 3 3 5% 7% 59%	6874
August	1st to 10th	Day	7% 3 1 4 3 2 3 3 4 7% 63%	4541
	11th to 20th	Day	12% 5% 3 6% 4 4 5% 4 7% 8% 42%	4200
	21st to end	Day	15% 6% 3 6% 5% 5% 5% 3 4 6% 42%	4487
September	1st to 10th	Day	12% 6% 3 6% 4 4 3 3 4 6% 49%	2400
	11th to 20th	Day	17% 8% 3 7% 5% 4 4 3 5% 6% 40%	2400
	21st to end	Day	23% 7% 3 5% 4 4 3 4 4 4 38%	2120
October	1st to 10th	Day	21% 7% 1 6% 5% 5% 5% 4 6% 8% 33%	3280
	11th to 20th	Day	32% 8% 3 7% 4 4 4 4 5% 6% 24%	3261
	21st to end	Day	26% 9% 3 11% 7% 7% 5% 4 5% 6% 19%	3274
November	1st to 10th	Day	33% 12% 3 8% 4 5% 4 4 4 5% 18%	1663
	11th to 20th	Day	35% 12% 3 8% 4 4 4 3 4 3 21%	1680
	21st to end	Day	32% 14% 4 8% 4 4 4 3 5% 4 19%	1680
December	1st to 10th	Day	21% 10% 2 8% 5% 5% 6% 3 4 6% 30%	2700
	11th to 20th	Day	26% 15% 6% 12% 6% 4 4 3 4 4 15%	2833
	21st to end	Day	26% 14% 5% 12% 8% 6% 6% 4 5% 5% 11%	3575
January	1st to 10th	Night	27% 16% 5% 12% 6% 5% 5% 3 4 3 14%	8770
	11th to 20th	Night	21% 10% 3 4 2 3 3 2 4 6% 41%	8540
	21st to end	Night	22% 11% 4 9% 5% 4 3 3 3 4 32%	8994
February	1st to 10th	Night	29% 14% 3 9% 5% 5% 4 3 4 4 21%	2751
	11th to 20th	Night	25% 18% 3 7% 5% 4 4 2 3 4 24%	2662
	21st to end	Night	25% 16% 4 9% 5% 3 4 3 4 4 24%	2186
March	1st to 10th	Night	18% 15% 4 9% 5% 5% 4 4 5% 7% 24%	5455
	11th to 20th	Night	20% 14% 4 9% 5% 5% 4 3 4 6% 26%	5109
	21st to end	Night	29% 13% 4 9% 5% 4 4 3 4 5% 19%	5386
April	1st to 10th	Night	22% 15% 3 10% 5% 4 5% 4 5% 6% 21%	2100
	11th to 20th	Night	21% 10% 3 8% 5% 6% 5% 4 5% 5% 27%	2030
	21st to end	Night	27% 11% 3 8% 5% 3 4 3 5% 5% 24%	1780
May	1st to 10th	Night	16% 8% 3 6% 5% 5% 4 2 3 4 44%	3720
	11th to 20th	Night	18% 11% 3 9% 5% 5% 5% 3 5% 6% 30%	3696
	21st to end	Night	19% 9% 3 7% 5% 5% 5% 4 6% 7% 29%	4060
June	1st to 10th	Night	18% 8% 3 9% 6% 5% 5% 4 5% 5% 31%	1426
	11th to 20th	Night	22% 9% 4 10% 5% 5% 4 5% 6% 6% 24%	1400
	21st to end	Night	17% 5% 3 6% 4 4 4 3 5% 8% 41%	1400
July	1st to 10th	Night	9% 4 2 3 2 2 2 2 4 6% 65%	3180
	11th to 20th	Night	5% 3 1 2 2 2 2 2 3 5% 76%	3226
	21st to end	Night	9% 3 1 3 3 3 3 2 4 5% 65%	3969
August	1st to 10th	Night	5% 3 1 2 2 2 2 4 5% 7% 67%	2609
	11th to 20th	Night	10% 5% 2 6% 4 3 4 4 5% 9% 48%	2854
	21st to end	Night	15% 5% 2 6% 4 5% 4 3 5% 8% 42%	3428
September	1st to 10th	Night	12% 3 2 5% 4 4 4 3 5% 7% 50%	2000
	11th to 20th	Night	17% 6% 3 6% 5% 4 3 3 5% 6% 42%	2000
	21st to end	Night	20% 7% 2 6% 5% 4 5% 3 4 5% 39%	2277
October	1st to 10th	Night	19% 8% 3 8% 6% 6% 5% 4 5% 7% 30%	3920
	11th to 20th	Night	31% 9% 3 6% 3 4 3 3 4 6% 29%	3897
	21st to end	Night	26% 10% 4 8% 5% 5% 4 4 4 6% 25%	4528
November	1st to 10th	Night	34% 12% 3 8% 5% 5% 4 3 4 5% 18%	2854
	11th to 20th	Night	37% 11% 3 6% 5% 4 4 2 3 5% 20%	2940
	21st to end	Night	33% 14% 3 7% 4 4 4 3 3 4 20%	2940
December	1st to 10th	Night	21% 10% 3 7% 5% 5% 4 3 4 6% 30%	4700
	11th to 20th	Night	31% 15% 4 10% 6% 4 4 3 4 4 15%	5574
	21st to end	Night	27% 16% 5% 12% 6% 6% 5% 3 4 4 12%	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	77% 4 9% 9% 2	4125
	11th to 20th	Day	45% 4 14% 27% 8% 2	4377
	21st to end	Day	60% 2 9% 20% 7%	5187
February	1st to 10th	Day	69% 4 9% 9% 8% 2	1864
	11th to 20th	Day	64% 4 10% 14% 7% 2	1854
	21st to end	Day	70% 4 9% 11% 4 2	1620
March	1st to 10th	Day	63% 4 11% 16% 6%	4657
	11th to 20th	Day	62% 5% 12% 18% 2	5110
	21st to end	Day	65% 5% 17% 9% 4	5831
April	1st to 10th	Day	68% 6% 14% 11% 1	2520
	11th to 20th	Day	62% 4 12% 11% 8% 2	2537
	21st to end	Day	63% 4 14% 11% 7% 1	2838
May	1st to 10th	Day	47% 4 14% 26% 8%	6460
	11th to 20th	Day	57% 6% 22% 12% 2	6420
	21st to end	Day	56% 5% 21% 16% 1	7058
June	1st to 10th	Day	59% 4 16% 14% 5% 2	2971
	11th to 20th	Day	66% 4 16% 12% 3	3000
	21st to end	Day	51% 4 18% 18% 7% 1	3000
July	1st to 10th	Day	25% 4 21% 23% 20% 7%	6706
	11th to 20th	Day	19% 3 19% 32% 23% 4	6637
	21st to end	Day	26% 4 27% 31% 10% 2	6681
August	1st to 10th	Day	22% 4 16% 38% 18% 1	4524
	11th to 20th	Day	38% 8% 25% 19% 9% 2	4240
	21st to end	Day	45% 4 16% 23% 10% 1	4486
September	1st to 10th	Day	37% 5% 17% 23% 13% 5%	2400
	11th to 20th	Day	47% 4 13% 17% 17% 3	2400
	21st to end	Day	50% 4 17% 17% 10% 2	2120
October	1st to 10th	Day	50% 4 18% 20% 7%	3280
	11th to 20th	Day	61% 5% 12% 14% 7% 1	3261
	21st to end	Day	65% 6% 12% 12% 5%	3274
November	1st to 10th	Day	69% 4 11% 12% 3 1	1663
	11th to 20th	Day	71% 4 9% 10% 5% 2	1680
	21st to end	Day	68% 5% 8% 11% 7% 1	1680
December	1st to 10th	Day	57% 6% 14% 14% 8%	2700
	11th to 20th	Day	73% 5% 11% 9% 2 1	2836
	21st to end	Day	75% 4 13% 7% 2	3575
January	1st to 10th	Night	75% 3 10% 10% 2	8770
	11th to 20th	Night	45% 5% 15% 22% 10% 3	8580
	21st to end	Night	58% 3 13% 19% 6%	8994
February	1st to 10th	Night	68% 4 9% 10% 6% 2	2751
	11th to 20th	Night	66% 4 9% 12% 7% 1	2664
	21st to end	Night	65% 4 10% 15% 5% 1	2186
March	1st to 10th	Night	60% 5% 13% 14% 7%	5455
	11th to 20th	Night	60% 5% 14% 18% 2	5110
	21st to end	Night	69% 4 18% 7% 2	5386
April	1st to 10th	Night	65% 5% 16% 12% 2	2100
	11th to 20th	Night	59% 5% 14% 13% 7% 2	2030
	21st to end	Night	62% 4 15% 13% 6% 1	1780
May	1st to 10th	Night	46% 3 16% 25% 9%	3717
	11th to 20th	Night	54% 6% 23% 13% 3	3696
	21st to end	Night	53% 6% 24% 14% 2	4060
June	1st to 10th	Night	54% 4 19% 14% 6% 2	1426
	11th to 20th	Night	60% 6% 18% 14% 2	1400
	21st to end	Night	44% 5% 25% 18% 5% 3	1400
July	1st to 10th	Night	24% 3 22% 28% 18% 5%	3145
	11th to 20th	Night	16% 2 19% 36% 25% 2	3184
	21st to end	Night	23% 3 26% 30% 16% 2	3857
August	1st to 10th	Night	17% 4 23% 36% 17% 1	2601
	11th to 20th	Night	33% 9% 29% 18% 9% 3	2880
	21st to end	Night	41% 8% 21% 20% 9% 1	3428
September	1st to 10th	Night	35% 5% 19% 23% 14% 4	2000
	11th to 20th	Night	45% 4 16% 17% 14% 4	2000
	21st to end	Night	50% 4 17% 18% 9% 2	2277
October	1st to 10th	Night	54% 6% 20% 15% 5%	3920
	11th to 20th	Night	58% 4 13% 17% 7%	3897
	21st to end	Night	61% 6% 14% 14% 5%	4528
November	1st to 10th	Night	70% 4 11% 11% 4	2854
	11th to 20th	Night	69% 4 10% 9% 5% 2	2940
	21st to end	Night	69% 4 8% 9% 8% 1	2940
December	1st to 10th	Night	55% 6% 19% 14% 6%	4700
	11th to 20th	Night	74% 4 11% 8% 2 1	5570
	21st to end	Night	76% 5% 10% 7% 2	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	36% 11% 25% 17% 8% 2	220
	11th to 20th	Day	32% 9% 21% 21% 15% 1	220
	21st to end	Day	31% 9% 22% 25% 9% 3	242
February	1st to 10th	Day	41% 14% 19% 19% 5% 1	404
	11th to 20th	Day	34% 12% 28% 19% 6%	425
	21st to end	Day	35% 16% 26% 16% 6% 1	388
March	1st to 10th	Day	32% 14% 24% 21% 8% 1	540
	11th to 20th	Day	33% 12% 24% 19% 11% 1	540
	21st to end	Day	32% 16% 25% 20% 6% 1	594
April	1st to 10th	Day	31% 12% 28% 17% 9% 2	540
	11th to 20th	Day	26% 17% 27% 19% 9% 2	532
	21st to end	Day	30% 16% 25% 16% 9% 3	540
May	1st to 10th	Day	22% 13% 29% 26% 9% 1	543
	11th to 20th	Day	23% 12% 25% 26% 12% 3	568
	21st to end	Day	27% 13% 25% 22% 10% 3	617
June	1st to 10th	Day	24% 15% 30% 23% 8% 1	701
	11th to 20th	Day	27% 13% 30% 23% 6% 1	730
	21st to end	Day	20% 12% 26% 27% 13% 2	730
July	1st to 10th	Day	17% 8% 30% 33% 10% 2	721
	11th to 20th	Day	11% 10% 30% 34% 13% 3	695
	21st to end	Day	9% 7% 29% 34% 15% 6%	575
August	1st to 10th	Day	11% 9% 23% 31% 17% 9%	527
	11th to 20th	Day	11% 6% 27% 33% 16% 7%	517
	21st to end	Day	15% 10% 26% 27% 15% 7%	583
September	1st to 10th	Day	15% 10% 26% 29% 13% 6%	530
	11th to 20th	Day	21% 8% 24% 29% 14% 5%	530
	21st to end	Day	26% 12% 24% 23% 13% 2	530
October	1st to 10th	Day	24% 12% 20% 26% 15% 3	530
	11th to 20th	Day	35% 8% 19% 23% 12% 2	530
	21st to end	Day	34% 11% 26% 20% 8% 1	413
November	1st to 10th	Day	32% 11% 31% 21% 5%	228
	11th to 20th	Day	39% 11% 22% 17% 9% 2	220
	21st to end	Day	39% 10% 23% 17% 10% 2	220
December	1st to 10th	Day	40% 8% 17% 18% 13% 5%	220
	11th to 20th	Day	40% 10% 26% 19% 4 2	220
	21st to end	Day	38% 10% 26% 20% 5% 1	242
January	1st to 10th	Night	42% 9% 21% 19% 9% 1	950
	11th to 20th	Night	39% 7% 18% 25% 11%	906
	21st to end	Night	35% 6% 22% 27% 9% 1	924
February	1st to 10th	Night	36% 10% 23% 25% 5% 1	840
	11th to 20th	Night	35% 9% 20% 27% 8%	824
	21st to end	Night	35% 13% 18% 24% 9% 1	692
March	1st to 10th	Night	32% 8% 18% 26% 16%	840
	11th to 20th	Night	34% 9% 19% 24% 14% 1	840
	21st to end	Night	34% 10% 20% 25% 9% 2	924
April	1st to 10th	Night	30% 6% 19% 31% 13% 2	840
	11th to 20th	Night	27% 7% 21% 32% 12% 1	782
	21st to end	Night	33% 6% 15% 32% 12% 2	520
May	1st to 10th	Night	22% 5% 14% 35% 23% 1	420
	11th to 20th	Night	19% 5% 12% 33% 28% 3	414
	21st to end	Night	24% 5% 7% 36% 26% 2	455
June	1st to 10th	Night	22% 3 16% 36% 22% 2	400
	11th to 20th	Night	25% 4 14% 30% 23% 4	400
	21st to end	Night	18% 5% 10% 34% 31% 2	400
July	1st to 10th	Night	14% 4 8% 32% 38% 4	390
	11th to 20th	Night	11% 4 7% 33% 39% 6%	388
	21st to end	Night	9% 1 3 32% 47% 7%	438
August	1st to 10th	Night	12% 2 4 26% 48% 10%	398
	11th to 20th	Night	10% 2 6% 32% 43% 7%	558
	21st to end	Night	13% 3 10% 34% 36% 4	765
September	1st to 10th	Night	14% 3 13% 36% 30% 5%	800
	11th to 20th	Night	20% 4 12% 25% 33% 6%	800
	21st to end	Night	23% 5% 15% 32% 24% 2	800
October	1st to 10th	Night	23% 6% 18% 30% 22% 1	800
	11th to 20th	Night	35% 5% 16% 27% 16% 2	800
	21st to end	Night	36% 9% 21% 24% 8% 2	880
November	1st to 10th	Night	42% 6% 23% 25% 4	820
	11th to 20th	Night	43% 7% 22% 18% 9% 1	840
	21st to end	Night	41% 11% 17% 21% 8% 1	840
December	1st to 10th	Night	33% 9% 23% 25% 10% 1	840
	11th to 20th	Night	44% 8% 24% 19% 5%	906
	21st to end	Night	42% 9% 24% 21% 4	1045

Page generated at 2026-05-28 22:53:25. Version ac07

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