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 2024-03-18.

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	20% 18% 5% 7% 6% 3 4 4 4 5% 25%	78387
	Night	18% 18% 5% 8% 6% 3 4 4 4 5% 24%	81345
Transparency	Day	61% 7% 10% 14% 7% 1	78102
	Night	63% 7% 10% 12% 7% 1	81210
Seeing	Day	33% 13% 29% 19% 6% 1	14125
	Night	32% 10% 28% 21% 7%	22703

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	19% 30% 9% 10% 4 3 3 3 3 3 13%	2219
	11th to 20th	Day	14% 19% 6% 12% 8% 3 3 3 3 5% 25%	2416
	21st to end	Day	19% 25% 7% 9% 7% 4 5% 3 4 4 13%	2794
February	1st to 10th	Day	22% 22% 6% 11% 6% 2 3 4 3 5% 15%	1667
	11th to 20th	Day	15% 26% 8% 8% 7% 3 3 4 3 4 19%	1665
	21st to end	Day	11% 27% 10% 12% 7% 3 4 4 3 3 16%	1489
March	1st to 10th	Day	13% 24% 11% 12% 6% 3 4 4 4 3 16%	1900
	11th to 20th	Day	14% 31% 8% 8% 7% 3 3 3 2 4 16%	2049
	21st to end	Day	18% 29% 8% 9% 6% 3 4 4 4 3 10%	2285
April	1st to 10th	Day	17% 32% 9% 8% 6% 3 4 3 4 3 11%	2160
	11th to 20th	Day	17% 29% 6% 9% 6% 4 5% 4 4 3 14%	2146
	21st to end	Day	23% 28% 4 7% 5% 3 4 4 3 4 15%	2450
May	1st to 10th	Day	18% 23% 5% 7% 7% 3 5% 3 4 5% 20%	2520
	11th to 20th	Day	20% 16% 4 7% 6% 4 5% 5% 6% 7% 20%	2477
	21st to end	Day	32% 15% 4 7% 5% 3 3 4 5% 5% 18%	2723
June	1st to 10th	Day	31% 16% 4 6% 6% 3 4 3 4 4 19%	2628
	11th to 20th	Day	39% 15% 5% 7% 5% 3 4 3 3 4 12%	2700
	21st to end	Day	25% 9% 3 7% 6% 3 4 4 5% 6% 29%	2700
July	1st to 10th	Day	17% 8% 3 6% 5% 3 5% 4 4 5% 41%	2700
	11th to 20th	Day	15% 6% 3 5% 5% 3 4 4 5% 6% 45%	2639
	21st to end	Day	10% 5% 2 4 4 2 3 3 4 5% 57%	2741
August	1st to 10th	Day	13% 6% 2 3 4 3 3 4 5% 8% 49%	2507
	11th to 20th	Day	11% 5% 2 4 4 3 3 4 5% 7% 52%	2350
	21st to end	Day	17% 7% 2 6% 6% 3 4 5% 5% 6% 38%	2462
September	1st to 10th	Day	16% 7% 3 5% 4 3 4 4 4 6% 45%	2160
	11th to 20th	Day	21% 9% 3 7% 5% 3 3 3 3 5% 38%	2160
	21st to end	Day	23% 10% 3 6% 5% 2 4 4 4 6% 33%	1917
October	1st to 10th	Day	21% 13% 4 6% 6% 3 5% 4 4 6% 30%	1800
	11th to 20th	Day	26% 13% 3 7% 6% 3 4 4 4 6% 26%	1800
	21st to end	Day	27% 14% 3 8% 6% 3 5% 4 5% 5% 22%	1810
November	1st to 10th	Day	29% 20% 5% 9% 4 2 5% 4 4 4 15%	1524
	11th to 20th	Day	25% 26% 6% 7% 4 2 3 2 2 4 20%	1520
	21st to end	Day	23% 22% 5% 7% 5% 2 5% 3 4 5% 19%	1520
December	1st to 10th	Day	17% 21% 8% 8% 5% 3 4 3 2 4 25%	1520
	11th to 20th	Day	14% 24% 7% 10% 6% 3 3 4 4 5% 19%	1827
	21st to end	Day	13% 25% 9% 12% 8% 4 4 4 5% 4 13%	2442
January	1st to 10th	Night	18% 28% 7% 11% 6% 3 4 3 3 4 13%	4726
	11th to 20th	Night	17% 18% 5% 9% 6% 3 4 4 4 6% 24%	4478
	21st to end	Night	17% 23% 6% 11% 8% 4 5% 5% 4 5% 12%	4862
February	1st to 10th	Night	22% 21% 7% 9% 6% 3 4 4 5% 5% 15%	2510
	11th to 20th	Night	14% 26% 7% 11% 6% 4 4 3 3 3 20%	2402
	21st to end	Night	13% 26% 6% 10% 7% 4 5% 4 4 4 16%	1965
March	1st to 10th	Night	14% 20% 7% 10% 7% 4 5% 4 4 5% 19%	2280
	11th to 20th	Night	15% 27% 7% 7% 5% 3 4 4 4 5% 19%	2075
	21st to end	Night	18% 23% 7% 9% 8% 4 5% 4 4 4 15%	2070
April	1st to 10th	Night	13% 26% 8% 10% 7% 3 4 4 4 5% 17%	1800
	11th to 20th	Night	16% 24% 6% 9% 7% 3 4 4 4 4 21%	1761
	21st to end	Night	20% 25% 5% 7% 6% 3 4 4 5% 4 17%	1509
May	1st to 10th	Night	17% 18% 4 8% 7% 3 5% 5% 3 5% 25%	1440
	11th to 20th	Night	16% 12% 4 7% 7% 3 6% 5% 7% 7% 25%	1416
	21st to end	Night	26% 12% 4 7% 4 3 5% 6% 5% 6% 23%	1552
June	1st to 10th	Night	27% 10% 4 6% 7% 4 5% 4 5% 5% 23%	1328
	11th to 20th	Night	23% 17% 5% 9% 6% 3 5% 5% 5% 5% 18%	1260
	21st to end	Night	19% 9% 3 5% 6% 3 4 4 5% 6% 36%	1260
July	1st to 10th	Night	15% 10% 2 4 5% 3 4 4 3 5% 44%	1260
	11th to 20th	Night	10% 6% 3 4 6% 3 4 5% 5% 5% 48%	1318
	21st to end	Night	9% 3 2 3 4 2 4 3 4 6% 60%	1576
August	1st to 10th	Night	10% 5% 1 4 4 2 4 3 5% 6% 56%	1432
	11th to 20th	Night	9% 5% 2 3 3 2 3 4 4 7% 58%	1547
	21st to end	Night	11% 7% 3 6% 6% 3 5% 5% 7% 8% 39%	1894
September	1st to 10th	Night	13% 8% 2 6% 6% 3 3 5% 4 5% 45%	1800
	11th to 20th	Night	21% 10% 3 5% 5% 2 4 3 4 4 38%	1800
	21st to end	Night	19% 10% 3 7% 6% 3 4 4 3 6% 33%	2039
October	1st to 10th	Night	20% 11% 4 7% 5% 3 4 5% 4 6% 31%	2160
	11th to 20th	Night	26% 14% 3 6% 5% 3 4 3 4 5% 26%	2160
	21st to end	Night	25% 16% 4 7% 6% 3 5% 4 4 4 23%	2545
November	1st to 10th	Night	29% 18% 4 9% 6% 3 4 4 3 5% 14%	2552
	11th to 20th	Night	25% 22% 5% 7% 5% 3 3 3 4 5% 18%	2660
	21st to end	Night	22% 22% 6% 8% 5% 3 5% 4 3 3 17%	2660
December	1st to 10th	Night	17% 19% 7% 7% 6% 3 4 5% 4 5% 23%	2660
	11th to 20th	Night	19% 24% 6% 10% 7% 4 5% 4 5% 4 13%	3374
	21st to end	Night	14% 23% 7% 10% 9% 5% 5% 6% 5% 5% 12%	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% 5% 6% 9% 3 1	2219
	11th to 20th	Day	64% 5% 9% 10% 8% 4	2440
	21st to end	Day	76% 5% 7% 7% 5%	2794
February	1st to 10th	Day	72% 7% 7% 8% 5% 1	1667
	11th to 20th	Day	69% 6% 7% 9% 6% 2	1674
	21st to end	Day	73% 6% 7% 10% 2 1	1489
March	1st to 10th	Day	72% 5% 8% 9% 4 1	1900
	11th to 20th	Day	72% 6% 8% 9% 3	2049
	21st to end	Day	78% 6% 9% 6% 1	2285
April	1st to 10th	Day	77% 7% 8% 7% 1	2160
	11th to 20th	Day	73% 6% 7% 7% 6% 1	2146
	21st to end	Day	74% 6% 8% 6% 5%	2450
May	1st to 10th	Day	64% 8% 11% 13% 3	2520
	11th to 20th	Day	61% 9% 15% 13% 3	2480
	21st to end	Day	67% 7% 12% 13% 1	2724
June	1st to 10th	Day	68% 7% 11% 10% 3 2	2628
	11th to 20th	Day	75% 7% 8% 8% 2	2700
	21st to end	Day	55% 7% 13% 18% 6% 1	2700
July	1st to 10th	Day	45% 8% 11% 24% 11% 2	2626
	11th to 20th	Day	38% 8% 13% 26% 13% 2	2558
	21st to end	Day	27% 6% 12% 33% 17% 5%	2589
August	1st to 10th	Day	32% 10% 16% 23% 16% 3	2490
	11th to 20th	Day	30% 8% 17% 27% 15% 3	2350
	21st to end	Day	45% 8% 13% 25% 7% 2	2462
September	1st to 10th	Day	41% 8% 11% 24% 12% 4	2160
	11th to 20th	Day	51% 4 8% 17% 18% 2	2160
	21st to end	Day	53% 7% 12% 16% 10% 2	1917
October	1st to 10th	Day	56% 7% 11% 17% 9%	1800
	11th to 20th	Day	61% 7% 8% 15% 8% 1	1800
	21st to end	Day	65% 5% 9% 12% 8%	1810
November	1st to 10th	Day	74% 6% 7% 8% 4 1	1524
	11th to 20th	Day	72% 5% 6% 10% 6% 2	1520
	21st to end	Day	68% 7% 5% 10% 7% 3	1520
December	1st to 10th	Day	66% 7% 8% 9% 9% 1	1520
	11th to 20th	Day	68% 9% 7% 11% 4 1	1829
	21st to end	Day	74% 8% 7% 8% 3	2442
January	1st to 10th	Night	76% 5% 6% 8% 3 1	4726
	11th to 20th	Night	61% 6% 9% 11% 10% 4	4516
	21st to end	Night	76% 7% 6% 6% 5% 1	4862
February	1st to 10th	Night	72% 5% 8% 9% 5% 1	2510
	11th to 20th	Night	71% 5% 6% 9% 6% 2	2404
	21st to end	Night	71% 6% 7% 13% 3	1965
March	1st to 10th	Night	66% 6% 11% 11% 5% 1	2280
	11th to 20th	Night	68% 5% 9% 13% 4	2076
	21st to end	Night	73% 7% 11% 7% 1	2070
April	1st to 10th	Night	71% 6% 11% 9% 2	1800
	11th to 20th	Night	66% 6% 11% 10% 6% 1	1761
	21st to end	Night	70% 7% 8% 10% 4 1	1509
May	1st to 10th	Night	62% 8% 11% 14% 5% 1	1440
	11th to 20th	Night	56% 10% 17% 12% 5%	1416
	21st to end	Night	59% 10% 15% 14% 2	1552
June	1st to 10th	Night	62% 8% 14% 11% 3 2	1328
	11th to 20th	Night	65% 11% 12% 10% 3	1260
	21st to end	Night	47% 10% 17% 19% 7% 1	1260
July	1st to 10th	Night	43% 8% 14% 21% 13% 2	1225
	11th to 20th	Night	34% 10% 16% 28% 10% 2	1276
	21st to end	Night	22% 7% 17% 28% 22% 4	1488
August	1st to 10th	Night	28% 8% 17% 26% 18% 2	1424
	11th to 20th	Night	26% 8% 19% 26% 18% 3	1547
	21st to end	Night	39% 9% 17% 24% 9% 2	1894
September	1st to 10th	Night	40% 8% 14% 20% 15% 3	1800
	11th to 20th	Night	49% 5% 9% 17% 17% 3	1800
	21st to end	Night	53% 7% 13% 16% 9% 2	2039
October	1st to 10th	Night	55% 7% 11% 19% 8%	2160
	11th to 20th	Night	61% 7% 8% 14% 9% 1	2160
	21st to end	Night	65% 6% 7% 12% 10%	2545
November	1st to 10th	Night	73% 5% 9% 8% 4 1	2552
	11th to 20th	Night	70% 6% 7% 8% 6% 2	2660
	21st to end	Night	72% 6% 5% 8% 8% 3	2660
December	1st to 10th	Night	64% 6% 10% 11% 8% 1	2660
	11th to 20th	Night	74% 7% 8% 7% 3 1	3371
	21st to end	Night	71% 10% 7% 7% 4 1	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	35% 22% 27% 11% 4 1	180
	11th to 20th	Day	32% 14% 32% 17% 6%	180
	21st to end	Day	35% 15% 26% 18% 6%	198
February	1st to 10th	Day	43% 17% 23% 14% 2	327
	11th to 20th	Day	38% 18% 29% 12% 3	365
	21st to end	Day	34% 17% 32% 13% 3	343
March	1st to 10th	Day	34% 17% 29% 17% 4	460
	11th to 20th	Day	37% 16% 24% 19% 5%	449
	21st to end	Day	40% 15% 25% 16% 3	462
April	1st to 10th	Day	42% 15% 22% 16% 4 1	420
	11th to 20th	Day	36% 17% 25% 14% 6% 1	412
	21st to end	Day	44% 14% 21% 16% 5%	420
May	1st to 10th	Day	31% 13% 30% 22% 4	423
	11th to 20th	Day	33% 9% 25% 22% 10% 1	448
	21st to end	Day	42% 9% 22% 18% 7% 3	485
June	1st to 10th	Day	37% 13% 27% 18% 4	558
	11th to 20th	Day	47% 11% 23% 14% 3 1	630
	21st to end	Day	30% 12% 27% 22% 7% 2	630
July	1st to 10th	Day	26% 10% 33% 24% 5% 1	621
	11th to 20th	Day	23% 11% 38% 19% 6% 2	557
	21st to end	Day	12% 11% 37% 27% 9% 3	487
August	1st to 10th	Day	17% 9% 33% 27% 10% 3	447
	11th to 20th	Day	12% 11% 35% 29% 10% 3	437
	21st to end	Day	21% 14% 35% 20% 9% 1	495
September	1st to 10th	Day	24% 14% 38% 20% 4 1	450
	11th to 20th	Day	34% 10% 28% 21% 7% 1	450
	21st to end	Day	32% 15% 26% 21% 7% 1	450
October	1st to 10th	Day	30% 11% 30% 21% 7% 1	450
	11th to 20th	Day	36% 14% 22% 18% 7% 1	450
	21st to end	Day	36% 14% 30% 18% 4	325
November	1st to 10th	Day	41% 9% 31% 16% 3 1	198
	11th to 20th	Day	41% 11% 26% 17% 6% 1	180
	21st to end	Day	39% 16% 22% 14% 8% 1	180
December	1st to 10th	Day	34% 14% 26% 18% 8%	180
	11th to 20th	Day	33% 16% 34% 11% 4 1	180
	21st to end	Day	39% 11% 31% 17% 1	198
January	1st to 10th	Night	43% 15% 29% 12% 1	850
	11th to 20th	Night	38% 12% 36% 13% 1	787
	21st to end	Night	33% 14% 37% 15% 1	836
February	1st to 10th	Night	38% 13% 34% 12% 3	760
	11th to 20th	Night	31% 14% 38% 15% 2	744
	21st to end	Night	34% 14% 33% 17% 2	628
March	1st to 10th	Night	29% 13% 33% 19% 6%	760
	11th to 20th	Night	32% 15% 29% 19% 4	752
	21st to end	Night	35% 11% 33% 15% 5% 1	792
April	1st to 10th	Night	34% 10% 29% 21% 5% 1	720
	11th to 20th	Night	33% 11% 28% 22% 5%	693
	21st to end	Night	41% 6% 27% 21% 5%	429
May	1st to 10th	Night	29% 6% 27% 26% 12% 1	360
	11th to 20th	Night	27% 4 19% 35% 14% 1	354
	21st to end	Night	36% 3 19% 31% 11%	389
June	1st to 10th	Night	35% 4 22% 27% 12% 1	360
	11th to 20th	Night	34% 5% 19% 29% 12% 1	360
	21st to end	Night	24% 4 20% 37% 15% 1	360
July	1st to 10th	Night	22% 3 15% 37% 23% 1	350
	11th to 20th	Night	15% 4 20% 38% 22% 1	348
	21st to end	Night	11% 4 14% 42% 28% 1	394
August	1st to 10th	Night	13% 4 11% 37% 34% 2	358
	11th to 20th	Night	13% 3 14% 44% 25% 1	479
	21st to end	Night	18% 4 22% 42% 13%	706
September	1st to 10th	Night	19% 5% 29% 32% 14%	720
	11th to 20th	Night	28% 4 19% 29% 18% 2	720
	21st to end	Night	29% 7% 24% 32% 8%	720
October	1st to 10th	Night	31% 8% 24% 29% 8%	720
	11th to 20th	Night	38% 8% 28% 22% 4	720
	21st to end	Night	40% 11% 32% 15% 2	792
November	1st to 10th	Night	43% 12% 35% 9% 1	740
	11th to 20th	Night	43% 13% 29% 13% 1	760
	21st to end	Night	41% 13% 30% 13% 2 1	760
December	1st to 10th	Night	31% 14% 38% 15% 1	760
	11th to 20th	Night	37% 17% 35% 9% 1	787
	21st to end	Night	39% 18% 33% 10%	935

Page generated at 2024-03-15 11:01:40. Version ac07

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