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 2025-06-10.

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% 18% 4 8% 5% 4 4 3 4 5% 28%	123456
	Night	15% 18% 5% 9% 5% 4 4 3 4 5% 26%	123469
Transparency	Day	35% 41% 6% 10% 7% 1	123213
	Night	36% 41% 7% 9% 6% 1	123355
Seeing	Day	32% 14% 29% 19% 6% 1	15752
	Night	32% 11% 28% 22% 7%	24187

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	16% 31% 8% 11% 4 4 3 2 3 3 14%	3179
	11th to 20th	Day	10% 16% 6% 11% 5% 4 4 3 4 6% 32%	3474
	21st to end	Day	14% 20% 5% 9% 5% 5% 5% 3 3 3 28%	4004
February	1st to 10th	Day	20% 21% 6% 12% 5% 4 3 3 4 5% 16%	1755
	11th to 20th	Day	14% 26% 7% 9% 6% 4 3 3 3 5% 19%	1755
	21st to end	Day	10% 29% 8% 13% 6% 4 3 4 3 3 15%	1565
March	1st to 10th	Day	8% 22% 9% 14% 6% 5% 5% 3 4 4 20%	4518
	11th to 20th	Day	8% 33% 7% 9% 5% 4 3 2 2 3 22%	4990
	21st to end	Day	15% 30% 6% 11% 5% 4 4 3 3 4 14%	5748
April	1st to 10th	Day	16% 33% 7% 9% 5% 4 4 3 4 3 11%	2400
	11th to 20th	Day	14% 30% 5% 9% 5% 5% 4 3 4 4 17%	2388
	21st to end	Day	21% 29% 4 8% 4 4 3 3 3 4 16%	2723
May	1st to 10th	Day	13% 20% 4 7% 4 4 5% 3 4 4 32%	6320
	11th to 20th	Day	19% 24% 4 8% 5% 4 4 3 5% 6% 18%	6277
	21st to end	Day	25% 20% 3 8% 5% 4 3 3 5% 6% 19%	6903
June	1st to 10th	Day	29% 16% 3 7% 5% 4 4 3 4 5% 21%	2910
	11th to 20th	Day	37% 16% 4 8% 5% 4 3 3 4 4 12%	2850
	21st to end	Day	25% 11% 3 7% 5% 4 4 3 5% 6% 28%	2850
July	1st to 10th	Day	10% 5% 2 4 3 3 3 2 4 4 60%	4740
	11th to 20th	Day	9% 4 1 3 3 2 3 3 4 4 64%	4623
	21st to end	Day	13% 5% 2 4 3 4 3 3 5% 6% 53%	4790
August	1st to 10th	Day	7% 4 1 3 3 3 3 3 5% 7% 60%	4401
	11th to 20th	Day	11% 8% 2 7% 5% 5% 4 4 5% 8% 40%	4114
	21st to end	Day	17% 9% 3 9% 5% 5% 4 3 4 4 36%	4318
September	1st to 10th	Day	15% 7% 2 6% 4 4 4 3 4 6% 46%	2280
	11th to 20th	Day	20% 10% 3 8% 4 4 4 3 3 5% 37%	2280
	21st to end	Day	23% 10% 3 6% 4 4 4 3 4 5% 33%	2023
October	1st to 10th	Day	21% 12% 3 6% 4 5% 6% 4 5% 6% 28%	3180
	11th to 20th	Day	29% 14% 3 7% 4 4 4 3 4 5% 23%	3161
	21st to end	Day	23% 19% 4 10% 5% 5% 5% 3 4 4 16%	3142
November	1st to 10th	Day	27% 22% 5% 9% 3 3 5% 3 4 4 15%	1606
	11th to 20th	Day	23% 28% 5% 9% 3 3 3 2 2 3 19%	1600
	21st to end	Day	21% 24% 4 9% 5% 4 4 3 5% 5% 18%	1600
December	1st to 10th	Day	13% 18% 5% 7% 4 6% 5% 4 4 5% 30%	2620
	11th to 20th	Day	15% 28% 7% 12% 5% 4 3 3 4 4 14%	2871
	21st to end	Day	12% 30% 7% 13% 6% 5% 4 3 5% 4 11%	3498
January	1st to 10th	Night	15% 27% 6% 12% 6% 5% 4 3 4 4 15%	6766
	11th to 20th	Night	12% 15% 5% 10% 4 3 4 3 5% 7% 33%	6420
	21st to end	Night	12% 18% 4 10% 5% 5% 5% 4 4 5% 28%	6952
February	1st to 10th	Night	20% 22% 6% 10% 5% 4 4 3 5% 5% 16%	2642
	11th to 20th	Night	14% 27% 7% 12% 5% 4 4 2 3 3 20%	2532
	21st to end	Night	13% 28% 6% 11% 6% 5% 4 3 4 4 16%	2065
March	1st to 10th	Night	7% 18% 8% 12% 6% 5% 4 4 5% 6% 23%	5376
	11th to 20th	Night	8% 27% 8% 10% 5% 4 4 3 4 4 24%	5008
	21st to end	Night	19% 24% 6% 11% 6% 4 4 3 4 4 15%	5226
April	1st to 10th	Night	13% 28% 7% 10% 6% 5% 4 4 4 4 16%	2000
	11th to 20th	Night	14% 24% 5% 9% 6% 4 3 3 4 4 24%	1959
	21st to end	Night	19% 26% 4 8% 5% 4 4 4 5% 4 18%	1676
May	1st to 10th	Night	11% 16% 3 7% 5% 5% 6% 3 3 3 37%	3640
	11th to 20th	Night	17% 20% 4 8% 5% 4 4 3 5% 5% 23%	3616
	21st to end	Night	17% 19% 5% 8% 4 5% 5% 4 6% 6% 21%	3972
June	1st to 10th	Night	25% 11% 3 8% 5% 5% 4 4 5% 6% 25%	1470
	11th to 20th	Night	23% 17% 4 9% 5% 4 5% 4 5% 5% 19%	1330
	21st to end	Night	19% 10% 3 6% 5% 4 4 3 5% 6% 35%	1330
July	1st to 10th	Night	9% 8% 2 3 3 2 3 2 3 4 61%	2220
	11th to 20th	Night	6% 4 2 3 3 3 2 2 4 4 67%	2334
	21st to end	Night	13% 5% 2 4 4 4 3 2 4 5% 54%	2762
August	1st to 10th	Night	6% 3 1 3 3 3 3 3 5% 9% 63%	2529
	11th to 20th	Night	10% 10% 4 8% 5% 4 3 4 5% 8% 39%	2718
	21st to end	Night	18% 7% 3 6% 4 4 4 3 6% 7% 38%	3356
September	1st to 10th	Night	13% 7% 2 7% 5% 3 3 5% 4 5% 46%	1900
	11th to 20th	Night	20% 12% 3 6% 4 3 3 2 4 4 37%	1900
	21st to end	Night	20% 11% 3 8% 5% 4 4 3 4 7% 33%	2153
October	1st to 10th	Night	18% 10% 3 9% 5% 5% 5% 4 6% 7% 28%	3800
	11th to 20th	Night	28% 13% 2 6% 4 4 3 3 5% 6% 27%	3777
	21st to end	Night	22% 18% 4 10% 5% 5% 4 3 4 5% 20%	4416
November	1st to 10th	Night	28% 20% 4 10% 5% 4 4 3 4 4 14%	2690
	11th to 20th	Night	24% 24% 4 8% 5% 4 3 3 4 5% 17%	2800
	21st to end	Night	21% 24% 6% 10% 4 5% 5% 4 3 4 16%	2800
December	1st to 10th	Night	12% 18% 5% 8% 6% 4 4 3 4 6% 30%	4560
	11th to 20th	Night	17% 25% 6% 11% 6% 5% 5% 4 4 5% 12%	5316
	21st to end	Night	14% 28% 7% 12% 6% 5% 4 4 4 5% 10%	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	57% 27% 4 9% 3	3179
	11th to 20th	Day	36% 27% 10% 15% 10% 2	3498
	21st to end	Day	44% 30% 5% 11% 11%	4004
February	1st to 10th	Day	29% 61% 2 2 4 2	1755
	11th to 20th	Day	29% 61% 3 4 2 1	1764
	21st to end	Day	29% 62% 4 4 1 1	1565
March	1st to 10th	Day	47% 31% 8% 8% 7%	4518
	11th to 20th	Day	48% 30% 6% 12% 3	4990
	21st to end	Day	53% 31% 7% 5% 4	5748
April	1st to 10th	Day	30% 63% 3 4	2400
	11th to 20th	Day	25% 64% 4 5% 2 1	2388
	21st to end	Day	28% 62% 3 4 2	2723
May	1st to 10th	Day	36% 31% 9% 18% 6%	6320
	11th to 20th	Day	51% 32% 11% 5%	6280
	21st to end	Day	47% 32% 10% 10% 1	6904
June	1st to 10th	Day	21% 64% 6% 6% 3	2910
	11th to 20th	Day	24% 66% 4 4 2	2850
	21st to end	Day	20% 64% 6% 8% 2	2850
July	1st to 10th	Day	13% 36% 4 10% 25% 12%	4666
	11th to 20th	Day	11% 36% 4 15% 27% 8%	4542
	21st to end	Day	22% 37% 10% 19% 11% 1	4639
August	1st to 10th	Day	9% 38% 5% 19% 27% 1	4384
	11th to 20th	Day	28% 42% 13% 13% 3 1	4156
	21st to end	Day	34% 38% 5% 14% 8% 1	4317
September	1st to 10th	Day	20% 61% 4 9% 5% 1	2280
	11th to 20th	Day	24% 60% 3 5% 7%	2280
	21st to end	Day	23% 61% 6% 7% 3	2023
October	1st to 10th	Day	36% 40% 7% 12% 5%	3180
	11th to 20th	Day	44% 38% 6% 9% 4	3161
	21st to end	Day	48% 39% 5% 4 5%	3142
November	1st to 10th	Day	33% 58% 3 5% 2	1606
	11th to 20th	Day	30% 57% 3 5% 3 1	1600
	21st to end	Day	28% 58% 2 6% 3 2	1600
December	1st to 10th	Day	35% 37% 7% 11% 7% 2	2620
	11th to 20th	Day	52% 36% 4 6% 2	2873
	21st to end	Day	57% 31% 7% 4 1	3498
January	1st to 10th	Night	54% 29% 5% 7% 4 1	6766
	11th to 20th	Night	34% 28% 12% 13% 11% 2	6458
	21st to end	Night	41% 31% 7% 13% 7%	6952
February	1st to 10th	Night	28% 61% 3 2 4 1	2642
	11th to 20th	Night	29% 61% 2 5% 2 1	2534
	21st to end	Night	28% 62% 2 6% 1	2065
March	1st to 10th	Night	39% 32% 9% 9% 10% 1	5376
	11th to 20th	Night	44% 31% 9% 13% 4	5009
	21st to end	Night	51% 32% 9% 5% 2	5226
April	1st to 10th	Night	27% 62% 4 6% 1	2000
	11th to 20th	Night	24% 64% 5% 5% 3 1	1959
	21st to end	Night	26% 63% 4 5% 2	1676
May	1st to 10th	Night	32% 31% 12% 17% 8%	3637
	11th to 20th	Night	47% 34% 11% 7% 1	3616
	21st to end	Night	43% 35% 12% 10% 1	3972
June	1st to 10th	Night	19% 64% 7% 8% 3	1470
	11th to 20th	Night	20% 67% 5% 6% 3	1330
	21st to end	Night	18% 65% 6% 10% 1	1330
July	1st to 10th	Night	12% 36% 4 15% 21% 12%	2185
	11th to 20th	Night	10% 36% 5% 11% 27% 12%	2292
	21st to end	Night	22% 39% 10% 12% 14% 3	2674
August	1st to 10th	Night	8% 38% 7% 23% 23% 2	2521
	11th to 20th	Night	31% 42% 11% 11% 4	2742
	21st to end	Night	30% 39% 9% 15% 7% 1	3356
September	1st to 10th	Night	20% 62% 5% 7% 6% 1	1900
	11th to 20th	Night	25% 60% 3 6% 4 1	1900
	21st to end	Night	24% 61% 5% 6% 4	2153
October	1st to 10th	Night	36% 40% 10% 12% 3	3800
	11th to 20th	Night	38% 39% 6% 11% 6%	3777
	21st to end	Night	40% 39% 10% 8% 3	4416
November	1st to 10th	Night	34% 56% 4 3 2	2690
	11th to 20th	Night	29% 58% 2 5% 3 2	2800
	21st to end	Night	30% 57% 2 4 5% 2	2800
December	1st to 10th	Night	34% 38% 8% 11% 8% 2	4560
	11th to 20th	Night	52% 36% 4 6% 1	5313
	21st to end	Night	56% 32% 5% 4 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	36% 22% 28% 10% 4 1	200
	11th to 20th	Day	30% 15% 33% 17% 5%	200
	21st to end	Day	33% 17% 25% 20% 5%	220
February	1st to 10th	Day	41% 18% 24% 15% 2	355
	11th to 20th	Day	37% 18% 30% 12% 3	395
	21st to end	Day	34% 18% 32% 12% 3	371
March	1st to 10th	Day	33% 18% 29% 17% 4	500
	11th to 20th	Day	37% 15% 24% 19% 5%	500
	21st to end	Day	41% 16% 24% 15% 3	550
April	1st to 10th	Day	40% 17% 23% 15% 4 1	500
	11th to 20th	Day	34% 19% 27% 14% 5% 1	492
	21st to end	Day	43% 14% 20% 16% 6% 1	500
May	1st to 10th	Day	31% 13% 31% 22% 4	503
	11th to 20th	Day	35% 10% 25% 20% 9% 1	528
	21st to end	Day	43% 11% 21% 17% 6% 2	573
June	1st to 10th	Day	35% 13% 29% 18% 4	648
	11th to 20th	Day	47% 12% 23% 14% 4 1	680
	21st to end	Day	31% 12% 27% 21% 7% 1	680
July	1st to 10th	Day	25% 11% 34% 24% 5% 1	671
	11th to 20th	Day	21% 11% 39% 20% 7% 2	603
	21st to end	Day	14% 12% 37% 26% 8% 4	531
August	1st to 10th	Day	16% 11% 33% 27% 11% 3	487
	11th to 20th	Day	13% 13% 34% 28% 9% 3	477
	21st to end	Day	23% 14% 33% 21% 9% 1	539
September	1st to 10th	Day	22% 13% 37% 21% 6% 1	490
	11th to 20th	Day	32% 11% 29% 20% 6% 1	490
	21st to end	Day	32% 15% 26% 20% 6% 1	490
October	1st to 10th	Day	31% 11% 30% 21% 6% 1	490
	11th to 20th	Day	37% 15% 22% 18% 7% 1	490
	21st to end	Day	35% 15% 30% 17% 3	359
November	1st to 10th	Day	40% 11% 31% 15% 2	220
	11th to 20th	Day	43% 13% 24% 15% 5% 1	200
	21st to end	Day	38% 18% 23% 14% 8% 1	200
December	1st to 10th	Day	32% 13% 27% 21% 8%	200
	11th to 20th	Day	36% 17% 33% 11% 4 1	200
	21st to end	Day	40% 12% 30% 16% 1	220
January	1st to 10th	Night	41% 17% 30% 11% 1	900
	11th to 20th	Night	36% 13% 36% 13% 1	830
	21st to end	Night	32% 14% 37% 16% 1	880
February	1st to 10th	Night	37% 14% 33% 13% 3	800
	11th to 20th	Night	31% 14% 39% 15% 2	784
	21st to end	Night	35% 14% 32% 16% 2	660
March	1st to 10th	Night	29% 13% 32% 20% 6%	800
	11th to 20th	Night	31% 16% 28% 20% 4	800
	21st to end	Night	36% 11% 33% 15% 5%	880
April	1st to 10th	Night	34% 10% 29% 21% 5% 1	800
	11th to 20th	Night	32% 12% 30% 22% 5%	771
	21st to end	Night	38% 7% 26% 22% 7% 1	476
May	1st to 10th	Night	28% 6% 27% 27% 12% 1	400
	11th to 20th	Night	27% 5% 21% 34% 13% 1	394
	21st to end	Night	35% 4 20% 31% 11%	433
June	1st to 10th	Night	33% 4 22% 27% 14% 1	399
	11th to 20th	Night	35% 5% 19% 28% 12% 1	380
	21st to end	Night	25% 5% 20% 36% 14% 1	380
July	1st to 10th	Night	21% 3 15% 38% 23% 1	370
	11th to 20th	Night	14% 4 19% 38% 25% 1	368
	21st to end	Night	12% 4 14% 42% 28% 1	416
August	1st to 10th	Night	12% 3 10% 38% 34% 2	378
	11th to 20th	Night	13% 3 14% 43% 25% 1	506
	21st to end	Night	19% 5% 21% 41% 13%	746
September	1st to 10th	Night	19% 5% 29% 33% 15%	760
	11th to 20th	Night	28% 5% 19% 28% 17% 2	760
	21st to end	Night	29% 7% 24% 31% 9%	760
October	1st to 10th	Night	31% 9% 24% 28% 8%	760
	11th to 20th	Night	38% 8% 28% 22% 4	760
	21st to end	Night	40% 11% 32% 14% 2	836
November	1st to 10th	Night	42% 13% 35% 9% 1	780
	11th to 20th	Night	44% 14% 29% 13% 1	800
	21st to end	Night	40% 14% 29% 14% 2 1	800
December	1st to 10th	Night	30% 14% 38% 16% 1	800
	11th to 20th	Night	37% 18% 35% 9% 1	830
	21st to end	Night	39% 18% 33% 9%	990

Page generated at 2025-06-07 07:28:17. Version ac07

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