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 2024-11-02.

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% 4 4 4 3 4 5% 29%	101737
	Night	16% 18% 5% 9% 5% 4 4 3 4 5% 26%	97315
Transparency	Day	29% 49% 6% 8% 7% 1	101494
	Night	30% 50% 6% 7% 5% 1	97204
Seeing	Day	33% 13% 29% 19% 6% 1	15072
	Night	32% 10% 28% 22% 7%	23380

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% 32% 8% 10% 4 3 3 2 3 3 13%	2219
	11th to 20th	Day	14% 19% 6% 14% 5% 4 3 2 3 5% 25%	2416
	21st to end	Day	19% 26% 7% 11% 5% 5% 4 3 4 4 13%	2794
February	1st to 10th	Day	21% 22% 6% 12% 5% 3 3 3 3 5% 15%	1667
	11th to 20th	Day	15% 26% 7% 9% 6% 4 3 3 3 4 18%	1665
	21st to end	Day	10% 30% 8% 13% 7% 4 3 4 3 3 15%	1489
March	1st to 10th	Day	8% 24% 11% 15% 6% 4 4 3 4 3 19%	3138
	11th to 20th	Day	9% 28% 6% 7% 5% 3 2 2 2 4 30%	3480
	21st to end	Day	14% 31% 7% 11% 6% 4 4 3 3 2 15%	4009
April	1st to 10th	Day	16% 33% 7% 9% 5% 4 4 3 4 3 11%	2280
	11th to 20th	Day	15% 30% 5% 10% 5% 5% 5% 3 4 3 16%	2267
	21st to end	Day	22% 30% 4 8% 4 4 3 3 3 4 14%	2587
May	1st to 10th	Day	14% 25% 4 8% 5% 4 3 2 4 4 28%	4420
	11th to 20th	Day	16% 18% 4 8% 4 4 4 3 5% 7% 26%	4377
	21st to end	Day	29% 25% 4 9% 5% 4 3 3 4 4 11%	4813
June	1st to 10th	Day	30% 17% 3 7% 5% 4 4 3 4 5% 19%	2775
	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% 13% 4 6% 5% 4 5% 3 4 6% 30%	1900
	11th to 20th	Day	26% 13% 3 8% 4 4 4 3 4 6% 25%	1900
	21st to end	Day	27% 15% 3 9% 5% 4 5% 3 4 4 21%	1910
November	1st to 10th	Day	28% 22% 5% 10% 3 3 5% 3 4 4 14%	1533
	11th to 20th	Day	23% 28% 5% 8% 3 3 2 2 2 3 20%	1520
	21st to end	Day	22% 24% 4 8% 4 4 4 3 4 5% 19%	1520
December	1st to 10th	Day	16% 24% 6% 9% 4 4 3 3 3 3 25%	1520
	11th to 20th	Day	14% 25% 6% 12% 5% 4 3 3 4 5% 19%	1827
	21st to end	Day	12% 26% 8% 13% 7% 5% 4 3 5% 4 13%	2442
January	1st to 10th	Night	17% 30% 6% 12% 5% 4 4 2 3 4 13%	4726
	11th to 20th	Night	16% 19% 5% 11% 5% 4 4 3 4 6% 24%	4478
	21st to end	Night	17% 24% 6% 13% 6% 5% 5% 4 4 5% 12%	4862
February	1st to 10th	Night	21% 23% 6% 10% 5% 4 4 3 5% 5% 15%	2510
	11th to 20th	Night	14% 27% 7% 12% 5% 4 4 2 3 3 20%	2402
	21st to end	Night	13% 28% 6% 11% 6% 5% 4 3 4 4 16%	1965
March	1st to 10th	Night	9% 18% 8% 13% 7% 6% 5% 4 5% 5% 20%	3736
	11th to 20th	Night	9% 24% 6% 7% 4 3 4 3 4 5% 33%	3498
	21st to end	Night	15% 24% 5% 10% 7% 5% 5% 3 4 4 18%	3643
April	1st to 10th	Night	13% 27% 7% 10% 6% 4 3 4 4 4 16%	1900
	11th to 20th	Night	14% 25% 5% 9% 6% 4 4 3 4 4 23%	1860
	21st to end	Night	19% 27% 4 8% 5% 4 4 4 5% 4 16%	1592
May	1st to 10th	Night	12% 19% 4 9% 5% 4 4 3 3 4 33%	2540
	11th to 20th	Night	11% 16% 4 8% 5% 4 4 4 6% 7% 32%	2516
	21st to end	Night	21% 22% 5% 8% 4 6% 5% 4 4 4 15%	2762
June	1st to 10th	Night	26% 12% 3 8% 5% 5% 4 4 4 5% 23%	1401
	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	20% 11% 4 8% 4 4 4 4 5% 6% 31%	2280
	11th to 20th	Night	26% 15% 3 7% 4 4 4 3 4 5% 26%	2280
	21st to end	Night	25% 16% 4 8% 5% 4 5% 3 4 4 22%	2687
November	1st to 10th	Night	28% 19% 4 11% 4 4 4 3 3 4 14%	2577
	11th to 20th	Night	24% 23% 4 7% 5% 4 3 3 4 5% 18%	2660
	21st to end	Night	22% 24% 6% 9% 4 5% 5% 3 3 3 17%	2660
December	1st to 10th	Night	16% 21% 6% 8% 6% 4 4 4 4 5% 23%	2660
	11th to 20th	Night	19% 25% 5% 11% 6% 5% 5% 4 5% 5% 12%	3374
	21st to end	Night	13% 24% 7% 12% 7% 6% 5% 5% 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	48% 38% 4 6% 3 1	2219
	11th to 20th	Day	40% 37% 7% 7% 6% 3	2440
	21st to end	Day	52% 40% 4 3 1	2794
February	1st to 10th	Day	28% 64% 1 2 3 1	1667
	11th to 20th	Day	27% 64% 3 3 2 1	1674
	21st to end	Day	26% 65% 4 4 1 1	1489
March	1st to 10th	Day	40% 42% 7% 8% 4	3138
	11th to 20th	Day	30% 42% 8% 17% 4	3480
	21st to end	Day	44% 42% 6% 3 5%	4009
April	1st to 10th	Day	28% 66% 3 3	2280
	11th to 20th	Day	24% 67% 3 3 2	2267
	21st to end	Day	27% 65% 3 2 3	2587
May	1st to 10th	Day	31% 43% 7% 13% 5%	4420
	11th to 20th	Day	33% 45% 14% 7% 1	4380
	21st to end	Day	49% 42% 6% 3	4814
June	1st to 10th	Day	21% 67% 6% 5% 2	2775
	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	25% 62% 4 5% 4	1900
	11th to 20th	Day	28% 61% 3 5% 3	1900
	21st to end	Day	28% 59% 3 5% 5%	1910
November	1st to 10th	Day	31% 60% 3 4 2	1533
	11th to 20th	Day	28% 60% 3 6% 3 1	1520
	21st to end	Day	25% 60% 2 6% 3 2	1520
December	1st to 10th	Day	28% 60% 3 3 5% 1	1520
	11th to 20th	Day	32% 53% 4 7% 2 1	1829
	21st to end	Day	47% 41% 6% 5% 1	2442
January	1st to 10th	Night	48% 39% 5% 5% 2 1	4726
	11th to 20th	Night	37% 38% 6% 8% 8% 3	4516
	21st to end	Night	51% 43% 3 3 1	4862
February	1st to 10th	Night	27% 65% 3 2 3 1	2510
	11th to 20th	Night	27% 64% 2 4 2 1	2404
	21st to end	Night	25% 65% 2 6% 1	1965
March	1st to 10th	Night	35% 44% 8% 5% 7% 1	3736
	11th to 20th	Night	27% 43% 9% 17% 4	3499
	21st to end	Night	40% 43% 10% 5% 3	3643
April	1st to 10th	Night	25% 65% 3 6% 1	1900
	11th to 20th	Night	23% 67% 4 4 2 1	1860
	21st to end	Night	25% 67% 3 3 2	1592
May	1st to 10th	Night	27% 44% 7% 16% 6%	2540
	11th to 20th	Night	28% 47% 15% 10% 1	2516
	21st to end	Night	43% 46% 7% 4	2762
June	1st to 10th	Night	19% 67% 5% 7% 2	1401
	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	23% 62% 5% 6% 4	2280
	11th to 20th	Night	26% 61% 3 6% 4	2280
	21st to end	Night	26% 60% 3 6% 4	2687
November	1st to 10th	Night	33% 58% 4 3 2	2577
	11th to 20th	Night	27% 61% 2 5% 3 2	2660
	21st to end	Night	28% 60% 1 4 5% 2	2660
December	1st to 10th	Night	27% 61% 4 3 4 1	2660
	11th to 20th	Night	39% 52% 3 3 2 1	3371
	21st to end	Night	44% 44% 5% 3 3	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	36% 15% 24% 20% 5%	460
	21st to end	Day	41% 14% 25% 16% 4	506
April	1st to 10th	Day	42% 16% 23% 15% 4 1	460
	11th to 20th	Day	36% 17% 27% 14% 5% 1	452
	21st to end	Day	45% 13% 20% 16% 5% 1	460
May	1st to 10th	Day	31% 12% 31% 23% 4	463
	11th to 20th	Day	33% 9% 26% 21% 10% 1	488
	21st to end	Day	44% 9% 21% 18% 6% 2	529
June	1st to 10th	Day	37% 13% 28% 18% 4	605
	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% 29% 17% 3	359
November	1st to 10th	Day	42% 9% 31% 15% 2	201
	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% 28% 21% 4	760
	21st to end	Night	35% 10% 33% 15% 6%	836
April	1st to 10th	Night	34% 10% 29% 21% 5% 1	760
	11th to 20th	Night	32% 11% 29% 22% 5%	732
	21st to end	Night	40% 6% 27% 21% 5% 1	452
May	1st to 10th	Night	28% 5% 27% 26% 12% 1	380
	11th to 20th	Night	27% 4 20% 35% 14% 1	374
	21st to end	Night	36% 3 20% 30% 11%	411
June	1st to 10th	Night	35% 4 22% 27% 12% 1	380
	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	43% 13% 34% 9% 1	748
	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 2024-10-30 10:19:23. Version ac07

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