Chilliwack Lake
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Chilliwack Lake clear sky chart from 2005-11-06 to 2026-10-14.

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% 17% 4 8% 4 4 4 3 4 5% 31%	153943
	Night	15% 18% 5% 9% 5% 4 4 3 4 5% 28%	146514
Transparency	Day	36% 34% 9% 12% 8% 1	153659
	Night	37% 35% 8% 11% 7% 1	146380
Seeing	Day	32% 14% 29% 18% 5% 1	17577
	Night	32% 11% 28% 22% 7%	25796

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	14% 35% 8% 11% 5% 4 3 2 3 3 12%	4125
	11th to 20th	Day	11% 13% 5% 9% 4 3 3 3 4 6% 39%	4532
	21st to end	Day	13% 19% 5% 9% 6% 6% 5% 3 3 3 28%	5186
February	1st to 10th	Day	20% 21% 6% 12% 6% 4 3 3 4 6% 16%	1843
	11th to 20th	Day	14% 26% 7% 9% 6% 4 4 3 4 5% 19%	1845
	21st to end	Day	11% 29% 8% 13% 6% 4 3 4 3 3 15%	1641
March	1st to 10th	Day	8% 22% 9% 14% 6% 5% 5% 3 4 4 20%	4618
	11th to 20th	Day	9% 33% 7% 9% 5% 4 3 2 2 3 21%	5099
	21st to end	Day	15% 30% 6% 11% 6% 4 4 3 3 4 14%	5875
April	1st to 10th	Day	16% 32% 7% 9% 5% 5% 4 3 4 4 12%	2520
	11th to 20th	Day	14% 30% 5% 9% 5% 5% 5% 3 4 4 17%	2509
	21st to end	Day	21% 28% 4 8% 4 4 3 3 3 4 18%	2859
May	1st to 10th	Day	11% 17% 3 6% 4 5% 5% 4 5% 5% 35%	8195
	11th to 20th	Day	18% 24% 4 8% 5% 5% 4 3 5% 6% 19%	8160
	21st to end	Day	23% 18% 3 7% 4 4 4 3 5% 7% 22%	8993
June	1st to 10th	Day	29% 17% 3 7% 5% 4 4 3 4 5% 21%	3067
	11th to 20th	Day	34% 16% 4 8% 5% 4 3 3 4 5% 16%	3150
	21st to end	Day	25% 11% 3 7% 5% 4 4 3 5% 6% 28%	3150
July	1st to 10th	Day	10% 5% 2 4 3 4 4 3 5% 7% 53%	8820
	11th to 20th	Day	7% 4 1 4 3 3 3 3 4 6% 64%	8605
	21st to end	Day	10% 4 2 5% 4 4 4 3 5% 7% 52%	8916
August	1st to 10th	Day	6% 4 1 4 3 3 3 3 5% 6% 62%	6441
	11th to 20th	Day	8% 6% 2 7% 5% 5% 5% 5% 6% 8% 44%	6053
	21st to end	Day	17% 9% 3 9% 5% 4 4 3 4 5% 35%	6281
September	1st to 10th	Day	14% 6% 2 6% 4 4 4 4 4 6% 45%	2520
	11th to 20th	Day	19% 10% 3 8% 4 4 4 3 3 5% 39%	2520
	21st to end	Day	23% 10% 3 6% 4 4 4 3 4 6% 33%	2235
October	1st to 10th	Day	20% 12% 3 6% 4 5% 6% 4 6% 6% 29%	3380
	11th to 20th	Day	28% 14% 3 7% 4 4 4 3 4 5% 24%	3292
	21st to end	Day	22% 19% 4 10% 6% 5% 6% 3 4 4 16%	3242
November	1st to 10th	Day	27% 23% 5% 9% 3 3 4 3 4 4 15%	1688
	11th to 20th	Day	25% 27% 5% 8% 3 3 3 2 2 3 19%	1680
	21st to end	Day	22% 23% 4 9% 4 4 4 3 4 5% 18%	1680
December	1st to 10th	Day	14% 18% 5% 7% 4 6% 5% 4 4 5% 29%	2700
	11th to 20th	Day	16% 28% 7% 11% 5% 4 3 3 4 4 14%	2948
	21st to end	Day	12% 30% 7% 13% 6% 5% 4 3 5% 5% 11%	3575
January	1st to 10th	Night	13% 31% 6% 12% 5% 5% 4 3 4 4 12%	8770
	11th to 20th	Night	14% 13% 4 9% 4 3 4 3 4 6% 36%	8362
	21st to end	Night	12% 17% 4 9% 6% 6% 5% 3 4 5% 29%	8994
February	1st to 10th	Night	19% 21% 6% 10% 5% 4 4 4 5% 5% 16%	2774
	11th to 20th	Night	13% 26% 7% 12% 5% 4 4 2 3 3 20%	2662
	21st to end	Night	13% 28% 6% 11% 6% 5% 4 3 4 4 15%	2165
March	1st to 10th	Night	7% 18% 8% 12% 6% 5% 4 4 5% 6% 23%	5496
	11th to 20th	Night	9% 27% 8% 10% 5% 4 4 3 3 4 24%	5118
	21st to end	Night	19% 24% 6% 11% 6% 4 4 3 4 4 15%	5341
April	1st to 10th	Night	12% 27% 7% 10% 6% 5% 4 4 4 4 17%	2100
	11th to 20th	Night	14% 24% 4 9% 6% 4 4 3 4 5% 23%	2058
	21st to end	Night	18% 26% 4 8% 5% 4 3 3 5% 4 20%	1760
May	1st to 10th	Night	9% 15% 3 7% 4 5% 5% 4 4 4 41%	4724
	11th to 20th	Night	15% 22% 5% 8% 5% 4 4 4 5% 6% 22%	4700
	21st to end	Night	14% 16% 4 7% 4 6% 6% 5% 6% 7% 26%	5178
June	1st to 10th	Night	25% 11% 3 7% 5% 5% 4 4 5% 6% 25%	1549
	11th to 20th	Night	21% 16% 4 9% 5% 4 5% 4 5% 5% 22%	1470
	21st to end	Night	20% 10% 3 6% 5% 4 4 3 5% 6% 34%	1470
July	1st to 10th	Night	10% 9% 2 4 3 3 3 3 3 5% 55%	4140
	11th to 20th	Night	6% 5% 2 4 4 3 3 3 3 6% 62%	4338
	21st to end	Night	9% 6% 2 5% 3 4 4 2 5% 6% 53%	5152
August	1st to 10th	Night	5% 3 1 3 2 3 3 2 5% 8% 66%	3709
	11th to 20th	Night	8% 7% 3 6% 5% 4 4 4 5% 9% 45%	4003
	21st to end	Night	20% 9% 3 7% 4 4 4 3 5% 6% 37%	4879
September	1st to 10th	Night	13% 7% 3 7% 5% 4 3 5% 4 5% 45%	2100
	11th to 20th	Night	20% 12% 3 6% 5% 3 3 2 4 5% 38%	2100
	21st to end	Night	20% 10% 3 7% 5% 5% 4 3 4 7% 32%	2381
October	1st to 10th	Night	18% 10% 3 8% 5% 5% 5% 4 6% 7% 28%	4040
	11th to 20th	Night	27% 14% 2 6% 3 4 3 3 4 6% 28%	3933
	21st to end	Night	22% 18% 4 10% 5% 5% 5% 3 4 5% 20%	4558
November	1st to 10th	Night	27% 20% 4 10% 5% 4 4 3 4 5% 14%	2828
	11th to 20th	Night	24% 24% 4 7% 5% 4 3 3 4 4 17%	2940
	21st to end	Night	21% 23% 6% 10% 4 5% 5% 3 3 4 16%	2940
December	1st to 10th	Night	12% 18% 5% 8% 5% 4 4 3 4 6% 29%	4700
	11th to 20th	Night	17% 25% 6% 11% 6% 5% 5% 4 4 5% 12%	5459
	21st to end	Night	14% 28% 7% 12% 7% 5% 4 4 4 5% 10%	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	64% 22% 4 8% 2	4125
	11th to 20th	Day	34% 21% 8% 18% 15% 3	4556
	21st to end	Day	49% 23% 4 10% 13%	5187
February	1st to 10th	Day	30% 59% 2 3 4 2	1843
	11th to 20th	Day	30% 58% 3 5% 3 1	1854
	21st to end	Day	31% 59% 4 4 2	1641
March	1st to 10th	Day	47% 30% 8% 8% 7%	4618
	11th to 20th	Day	49% 29% 6% 12% 3	5099
	21st to end	Day	53% 30% 7% 5% 4	5875
April	1st to 10th	Day	31% 60% 3 5%	2520
	11th to 20th	Day	27% 61% 4 5% 2 1	2509
	21st to end	Day	28% 59% 4 5% 4	2859
May	1st to 10th	Day	35% 25% 11% 23% 6%	8195
	11th to 20th	Day	53% 27% 13% 7% 1	8163
	21st to end	Day	46% 26% 14% 12% 1	8994
June	1st to 10th	Day	24% 61% 6% 6% 3	3067
	11th to 20th	Day	25% 60% 4 7% 4	3150
	21st to end	Day	24% 58% 6% 9% 2	3150
July	1st to 10th	Day	22% 23% 13% 16% 21% 7%	8746
	11th to 20th	Day	16% 21% 9% 22% 26% 7%	8524
	21st to end	Day	25% 23% 16% 21% 14% 1	8723
August	1st to 10th	Day	13% 26% 9% 17% 28% 6%	6424
	11th to 20th	Day	28% 32% 18% 18% 4 1	6095
	21st to end	Day	39% 28% 7% 19% 6% 1	6280
September	1st to 10th	Day	21% 56% 5% 10% 7% 1	2520
	11th to 20th	Day	25% 55% 4 9% 8%	2520
	21st to end	Day	25% 56% 7% 9% 3	2235
October	1st to 10th	Day	37% 38% 8% 12% 5%	3380
	11th to 20th	Day	44% 36% 6% 9% 5%	3292
	21st to end	Day	49% 38% 5% 4 4	3242
November	1st to 10th	Day	34% 55% 4 5% 2	1688
	11th to 20th	Day	33% 54% 3 5% 3 1	1680
	21st to end	Day	30% 55% 3 7% 3 2	1680
December	1st to 10th	Day	37% 36% 7% 11% 7% 2	2700
	11th to 20th	Day	53% 35% 4 6% 2	2950
	21st to end	Day	57% 30% 7% 4 1	3575
January	1st to 10th	Night	61% 23% 5% 7% 3 1	8770
	11th to 20th	Night	35% 21% 12% 14% 15% 2	8400
	21st to end	Night	45% 24% 6% 12% 11% 1	8994
February	1st to 10th	Night	29% 59% 3 2 5% 2	2774
	11th to 20th	Night	31% 58% 2 5% 3 1	2664
	21st to end	Night	30% 60% 3 6% 1	2165
March	1st to 10th	Night	40% 31% 10% 9% 10% 1	5496
	11th to 20th	Night	45% 30% 9% 12% 4	5119
	21st to end	Night	51% 32% 10% 5% 2	5341
April	1st to 10th	Night	28% 59% 5% 6% 1	2100
	11th to 20th	Night	26% 61% 6% 5% 2 1	2058
	21st to end	Night	26% 60% 4 5% 3 1	1760
May	1st to 10th	Night	31% 25% 12% 24% 7%	4721
	11th to 20th	Night	50% 28% 13% 8% 1	4700
	21st to end	Night	40% 28% 17% 14% 1	5182
June	1st to 10th	Night	21% 61% 7% 8% 3	1549
	11th to 20th	Night	21% 61% 5% 9% 4	1470
	21st to end	Night	22% 59% 6% 11% 1	1470
July	1st to 10th	Night	23% 22% 14% 16% 18% 7%	4105
	11th to 20th	Night	17% 21% 11% 19% 25% 7%	4296
	21st to end	Night	26% 23% 17% 21% 12% 2	5040
August	1st to 10th	Night	11% 27% 8% 25% 23% 6%	3701
	11th to 20th	Night	28% 32% 18% 18% 4	4027
	21st to end	Night	37% 30% 8% 17% 7% 1	4879
September	1st to 10th	Night	22% 57% 5% 8% 7% 1	2100
	11th to 20th	Night	26% 54% 4 9% 6% 1	2100
	21st to end	Night	26% 57% 6% 7% 4	2381
October	1st to 10th	Night	36% 38% 10% 13% 3	4040
	11th to 20th	Night	38% 37% 6% 11% 7%	3933
	21st to end	Night	41% 38% 10% 8% 4	4558
November	1st to 10th	Night	36% 54% 5% 3 2	2828
	11th to 20th	Night	32% 56% 2 5% 3 1	2940
	21st to end	Night	32% 55% 2 4 5% 2	2940
December	1st to 10th	Night	36% 37% 8% 11% 7% 2	4700
	11th to 20th	Night	53% 35% 4 6% 1	5456
	21st to end	Night	56% 32% 5% 4 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	37% 22% 27% 9% 4 1	220
	11th to 20th	Day	30% 16% 33% 17% 5%	220
	21st to end	Day	33% 17% 26% 19% 5%	242
February	1st to 10th	Day	40% 18% 24% 15% 2	383
	11th to 20th	Day	36% 18% 30% 13% 4	425
	21st to end	Day	37% 17% 31% 12% 3	399
March	1st to 10th	Day	34% 18% 29% 16% 4	540
	11th to 20th	Day	41% 15% 22% 17% 4	540
	21st to end	Day	40% 18% 24% 15% 3	594
April	1st to 10th	Day	39% 17% 23% 15% 4 1	540
	11th to 20th	Day	34% 18% 27% 14% 5% 1	532
	21st to end	Day	43% 14% 21% 15% 6% 1	540
May	1st to 10th	Day	29% 13% 32% 21% 4 1	543
	11th to 20th	Day	36% 11% 25% 18% 8% 1	568
	21st to end	Day	41% 12% 22% 17% 6% 2	617
June	1st to 10th	Day	37% 14% 28% 18% 4	697
	11th to 20th	Day	45% 14% 23% 14% 3 1	780
	21st to end	Day	31% 13% 27% 20% 6% 1	780
July	1st to 10th	Day	24% 12% 37% 22% 5% 1	771
	11th to 20th	Day	21% 13% 40% 18% 6% 2	696
	21st to end	Day	13% 12% 39% 25% 7% 4	619
August	1st to 10th	Day	16% 11% 35% 25% 11% 3	567
	11th to 20th	Day	14% 13% 34% 27% 9% 3	557
	21st to end	Day	22% 15% 32% 22% 9% 1	627
September	1st to 10th	Day	21% 14% 34% 22% 8% 2	570
	11th to 20th	Day	30% 12% 29% 21% 7% 1	570
	21st to end	Day	33% 16% 26% 19% 5% 1	570
October	1st to 10th	Day	30% 13% 31% 20% 6% 1	570
	11th to 20th	Day	39% 15% 22% 17% 6% 1	543
	21st to end	Day	37% 16% 28% 16% 3	393
November	1st to 10th	Day	42% 11% 31% 14% 2	242
	11th to 20th	Day	46% 12% 23% 14% 5%	220
	21st to end	Day	39% 19% 23% 12% 7% 1	220
December	1st to 10th	Day	35% 13% 26% 19% 7%	220
	11th to 20th	Day	41% 15% 30% 10% 3 1	220
	21st to end	Day	40% 14% 29% 16% 1	242
January	1st to 10th	Night	41% 18% 29% 11% 1	950
	11th to 20th	Night	35% 14% 36% 13% 1	873
	21st to end	Night	31% 15% 37% 16% 1	924
February	1st to 10th	Night	36% 15% 33% 12% 3	840
	11th to 20th	Night	30% 15% 38% 15% 2	824
	21st to end	Night	35% 15% 31% 16% 2	692
March	1st to 10th	Night	30% 13% 32% 19% 6%	840
	11th to 20th	Night	33% 16% 27% 19% 4	840
	21st to end	Night	35% 11% 33% 15% 5%	924
April	1st to 10th	Night	33% 10% 28% 22% 5% 1	840
	11th to 20th	Night	31% 12% 29% 23% 5%	810
	21st to end	Night	38% 7% 26% 23% 6% 1	500
May	1st to 10th	Night	27% 5% 26% 28% 13%	420
	11th to 20th	Night	28% 6% 20% 33% 13%	414
	21st to end	Night	34% 4 20% 32% 11%	455
June	1st to 10th	Night	34% 4 22% 28% 12% 1	420
	11th to 20th	Night	32% 5% 20% 30% 12%	420
	21st to end	Night	26% 5% 19% 36% 14%	420
July	1st to 10th	Night	20% 3 14% 38% 23% 1	410
	11th to 20th	Night	13% 6% 20% 38% 23%	408
	21st to end	Night	12% 3 13% 41% 30% 1	460
August	1st to 10th	Night	13% 3 10% 38% 33% 2	418
	11th to 20th	Night	14% 3 14% 42% 26% 1	560
	21st to end	Night	19% 5% 21% 41% 13%	824
September	1st to 10th	Night	18% 5% 28% 33% 16% 1	840
	11th to 20th	Night	27% 5% 19% 29% 18% 2	840
	21st to end	Night	29% 8% 24% 31% 8%	840
October	1st to 10th	Night	30% 10% 25% 28% 7%	840
	11th to 20th	Night	38% 9% 27% 21% 4	812
	21st to end	Night	40% 12% 32% 14% 2	880
November	1st to 10th	Night	43% 13% 34% 9% 1	820
	11th to 20th	Night	44% 14% 28% 12% 1	840
	21st to end	Night	40% 15% 29% 13% 2 1	840
December	1st to 10th	Night	32% 14% 37% 15% 1	840
	11th to 20th	Night	39% 18% 33% 9% 1	873
	21st to end	Night	38% 19% 33% 9%	1045

Page generated at 2026-10-11 02:06:26. Version ac07

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