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 2025-02-06.

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	21% 10% 3 7% 5% 4 4 3 5% 6% 33%	112079
	Night	23% 11% 3 7% 5% 4 4 3 4 5% 31%	114806
Transparency	Day	54% 4 15% 18% 8% 2	111833
	Night	57% 4 15% 16% 7% 1	114698
Seeing	Day	26% 11% 26% 24% 10% 3	15375
	Night	31% 6% 17% 27% 17% 2	23845

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	31% 18% 4 9% 5% 4 4 3 4 4 14%	3179
	11th to 20th	Day	19% 11% 4 6% 4 3 3 3 5% 5% 37%	3339
	21st to end	Day	28% 13% 3 8% 3 3 3 2 3 4 29%	4004
February	1st to 10th	Day	31% 16% 4 7% 5% 4 3 3 3 6% 17%	1731
	11th to 20th	Day	26% 15% 4 8% 5% 5% 4 3 3 6% 22%	1665
	21st to end	Day	24% 19% 4 9% 3 6% 4 3 4 3 20%	1470
March	1st to 10th	Day	18% 17% 4 10% 6% 5% 5% 4 5% 6% 21%	3163
	11th to 20th	Day	18% 13% 3 8% 5% 4 4 2 3 5% 33%	3490
	21st to end	Day	23% 14% 4 10% 5% 5% 5% 4 5% 4 20%	3979
April	1st to 10th	Day	24% 16% 4 10% 6% 4 5% 4 4 5% 17%	2280
	11th to 20th	Day	22% 13% 4 8% 6% 6% 5% 4 6% 5% 22%	2293
	21st to end	Day	25% 14% 4 8% 6% 5% 5% 4 5% 5% 20%	2568
May	1st to 10th	Day	18% 11% 3 7% 5% 5% 4 3 4 5% 36%	4420
	11th to 20th	Day	17% 8% 2 6% 5% 5% 5% 5% 6% 7% 34%	4377
	21st to end	Day	32% 10% 4 8% 6% 5% 4 3 4 5% 18%	4813
June	1st to 10th	Day	27% 9% 4 8% 5% 4 5% 3 5% 5% 25%	2821
	11th to 20th	Day	28% 10% 3 9% 6% 5% 5% 4 6% 6% 19%	2850
	21st to end	Day	17% 9% 3 7% 5% 4 4 4 5% 7% 34%	2850
July	1st to 10th	Day	9% 4 1 4 2 2 3 3 3 4 64%	4740
	11th to 20th	Day	7% 3 1 3 3 2 2 2 4 5% 69%	4692
	21st to end	Day	13% 3 1 4 3 4 3 2 4 6% 56%	4790
August	1st to 10th	Day	7% 3 1 3 3 2 3 3 4 7% 64%	4401
	11th to 20th	Day	12% 5% 3 6% 4 4 5% 4 6% 8% 42%	4068
	21st to end	Day	16% 6% 3 6% 5% 5% 5% 3 4 6% 41%	4349
September	1st to 10th	Day	12% 6% 3 6% 4 4 3 3 4 6% 49%	2280
	11th to 20th	Day	17% 8% 3 7% 5% 4 4 3 5% 5% 39%	2280
	21st to end	Day	24% 7% 2 5% 4 4 3 3 4 4 38%	2014
October	1st to 10th	Day	21% 7% 1 6% 5% 5% 5% 4 6% 8% 33%	3180
	11th to 20th	Day	32% 8% 3 7% 5% 4 4 4 5% 6% 24%	3161
	21st to end	Day	26% 9% 3 11% 7% 7% 5% 4 5% 6% 19%	3173
November	1st to 10th	Day	34% 12% 3 8% 4 4 4 4 4 5% 18%	1582
	11th to 20th	Day	34% 12% 3 9% 4 4 4 3 4 3 22%	1600
	21st to end	Day	32% 15% 4 8% 4 3 4 3 5% 4 19%	1600
December	1st to 10th	Day	20% 10% 2 8% 5% 5% 6% 3 4 6% 30%	2620
	11th to 20th	Day	26% 16% 6% 12% 6% 4 4 3 4 4 15%	2759
	21st to end	Day	26% 14% 5% 12% 8% 6% 6% 4 5% 5% 11%	3498
January	1st to 10th	Night	30% 16% 4 10% 5% 4 4 2 3 3 17%	6766
	11th to 20th	Night	20% 12% 3 5% 3 3 3 3 4 7% 37%	6556
	21st to end	Night	25% 12% 4 9% 5% 4 3 2 3 3 30%	6952
February	1st to 10th	Night	31% 14% 3 8% 5% 5% 4 3 4 4 19%	2555
	11th to 20th	Night	26% 19% 4 7% 5% 4 4 2 4 4 23%	2402
	21st to end	Night	25% 17% 4 8% 5% 3 4 3 4 4 24%	1984
March	1st to 10th	Night	21% 16% 4 9% 5% 5% 4 3 5% 6% 23%	3709
	11th to 20th	Night	21% 12% 3 6% 4 3 3 3 4 6% 36%	3489
	21st to end	Night	26% 14% 4 9% 5% 4 4 3 4 4 22%	3674
April	1st to 10th	Night	23% 15% 3 10% 5% 5% 5% 3 5% 6% 20%	1900
	11th to 20th	Night	22% 11% 4 8% 5% 5% 5% 4 5% 5% 26%	1835
	21st to end	Night	28% 11% 4 9% 5% 4 5% 3 6% 5% 20%	1610
May	1st to 10th	Night	19% 9% 3 6% 4 4 3 2 3 4 43%	2540
	11th to 20th	Night	16% 8% 3 7% 4 4 4 3 5% 7% 39%	2516
	21st to end	Night	26% 13% 4 8% 4 5% 4 3 5% 4 24%	2762
June	1st to 10th	Night	19% 8% 3 10% 6% 6% 5% 4 5% 5% 29%	1355
	11th to 20th	Night	22% 9% 4 10% 5% 4 4 5% 6% 6% 23%	1330
	21st to end	Night	16% 5% 3 6% 4 4 4 3 5% 8% 42%	1330
July	1st to 10th	Night	8% 4 2 3 2 2 2 2 4 5% 67%	2220
	11th to 20th	Night	6% 3 1 2 2 2 2 2 2 4 75%	2252
	21st to end	Night	12% 3 1 4 3 3 3 3 4 4 62%	2762
August	1st to 10th	Night	5% 2 1 2 2 2 2 4 5% 7% 67%	2529
	11th to 20th	Night	9% 5% 2 6% 4 3 4 4 5% 9% 49%	2766
	21st to end	Night	15% 5% 2 6% 4 5% 4 3 5% 8% 41%	3324
September	1st to 10th	Night	12% 4 2 5% 4 4 4 3 5% 7% 50%	1900
	11th to 20th	Night	18% 6% 3 7% 5% 4 3 3 5% 5% 41%	1900
	21st to end	Night	20% 7% 2 6% 4 4 5% 3 4 5% 39%	2163
October	1st to 10th	Night	19% 8% 3 8% 6% 6% 5% 4 6% 7% 29%	3800
	11th to 20th	Night	31% 9% 3 6% 3 3 3 3 4 6% 29%	3777
	21st to end	Night	27% 10% 4 8% 5% 5% 4 3 4 6% 25%	4387
November	1st to 10th	Night	34% 12% 2 8% 5% 4 4 3 4 5% 18%	2715
	11th to 20th	Night	37% 11% 3 6% 5% 4 3 2 3 5% 21%	2800
	21st to end	Night	33% 14% 3 7% 4 4 4 3 3 4 20%	2800
December	1st to 10th	Night	20% 10% 3 8% 5% 5% 4 3 4 6% 31%	4560
	11th to 20th	Night	31% 15% 4 10% 6% 4 4 3 4 4 15%	5428
	21st to end	Night	28% 16% 5% 12% 6% 6% 5% 3 4 4 12%	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	75% 3 9% 11% 2	3179
	11th to 20th	Day	50% 4 17% 20% 6% 3	3361
	21st to end	Day	62% 2 10% 19% 7%	4004
February	1st to 10th	Day	71% 4 9% 9% 6% 1	1731
	11th to 20th	Day	66% 4 10% 12% 7% 2	1674
	21st to end	Day	70% 4 9% 12% 4 2	1470
March	1st to 10th	Day	65% 3 13% 14% 5%	3163
	11th to 20th	Day	57% 3 13% 24% 3	3490
	21st to end	Day	67% 3 15% 10% 5%	3979
April	1st to 10th	Day	71% 5% 13% 10% 1	2280
	11th to 20th	Day	65% 4 11% 10% 9% 2	2293
	21st to end	Day	67% 4 12% 10% 7%	2568
May	1st to 10th	Day	53% 3 13% 23% 8%	4420
	11th to 20th	Day	49% 4 27% 16% 4	4380
	21st to end	Day	69% 3 17% 10% 1	4814
June	1st to 10th	Day	62% 4 15% 13% 4 2	2821
	11th to 20th	Day	67% 4 16% 11% 3	2850
	21st to end	Day	50% 4 18% 18% 8% 1	2850
July	1st to 10th	Day	25% 4 11% 25% 25% 10%	4666
	11th to 20th	Day	21% 3 12% 36% 24% 3	4611
	21st to end	Day	30% 4 21% 30% 12% 3	4639
August	1st to 10th	Day	21% 4 16% 39% 19% 1	4384
	11th to 20th	Day	38% 7% 26% 19% 9% 2	4108
	21st to end	Day	46% 4 16% 23% 10% 1	4348
September	1st to 10th	Day	38% 5% 16% 23% 13% 5%	2280
	11th to 20th	Day	48% 4 12% 16% 17% 3	2280
	21st to end	Day	51% 4 17% 17% 10% 2	2014
October	1st to 10th	Day	50% 4 18% 20% 7%	3180
	11th to 20th	Day	61% 5% 13% 14% 7% 1	3161
	21st to end	Day	65% 6% 12% 12% 5%	3173
November	1st to 10th	Day	70% 4 11% 12% 3 1	1582
	11th to 20th	Day	71% 3 9% 10% 5% 2	1600
	21st to end	Day	68% 5% 8% 11% 8% 1	1600
December	1st to 10th	Day	56% 7% 14% 14% 9%	2620
	11th to 20th	Day	73% 4 11% 9% 2 1	2762
	21st to end	Day	75% 4 13% 7% 2	3498
January	1st to 10th	Night	73% 2 10% 12% 2	6766
	11th to 20th	Night	49% 6% 17% 18% 6% 4	6596
	21st to end	Night	61% 3 14% 18% 4	6952
February	1st to 10th	Night	70% 4 10% 10% 5% 1	2555
	11th to 20th	Night	68% 4 9% 11% 7% 1	2404
	21st to end	Night	65% 3 10% 15% 6% 1	1984
March	1st to 10th	Night	62% 3 12% 14% 8% 1	3709
	11th to 20th	Night	53% 4 16% 24% 3	3490
	21st to end	Night	67% 3 18% 9% 2	3674
April	1st to 10th	Night	67% 5% 15% 10% 2	1900
	11th to 20th	Night	60% 5% 12% 13% 8% 2	1835
	21st to end	Night	66% 4 13% 12% 5% 1	1610
May	1st to 10th	Night	48% 3 15% 25% 9% 1	2540
	11th to 20th	Night	46% 5% 27% 18% 4	2516
	21st to end	Night	65% 3 19% 11% 2	2762
June	1st to 10th	Night	57% 4 18% 14% 5% 2	1355
	11th to 20th	Night	61% 6% 18% 12% 2	1330
	21st to end	Night	43% 5% 25% 18% 5% 3	1330
July	1st to 10th	Night	23% 3 14% 30% 23% 8%	2185
	11th to 20th	Night	18% 2 12% 38% 28% 2	2210
	21st to end	Night	28% 3 18% 27% 21% 3	2674
August	1st to 10th	Night	17% 4 23% 36% 18% 2	2521
	11th to 20th	Night	32% 9% 29% 18% 9% 3	2792
	21st to end	Night	42% 8% 21% 20% 9%	3324
September	1st to 10th	Night	35% 5% 18% 23% 15% 4	1900
	11th to 20th	Night	46% 3 15% 17% 14% 4	1900
	21st to end	Night	50% 4 18% 17% 9% 2	2163
October	1st to 10th	Night	54% 6% 19% 15% 5%	3800
	11th to 20th	Night	58% 4 13% 18% 7%	3777
	21st to end	Night	61% 6% 14% 14% 5%	4387
November	1st to 10th	Night	70% 4 10% 11% 4	2715
	11th to 20th	Night	70% 4 10% 9% 6% 2	2800
	21st to end	Night	69% 4 8% 9% 9% 1	2800
December	1st to 10th	Night	54% 6% 19% 15% 6%	4560
	11th to 20th	Night	73% 5% 11% 8% 2 1	5424
	21st to end	Night	76% 4 10% 7% 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	37% 12% 25% 17% 8% 3	200
	11th to 20th	Day	34% 10% 20% 22% 14% 2	200
	21st to end	Day	33% 9% 22% 25% 8% 3	220
February	1st to 10th	Day	43% 15% 19% 17% 4 1	361
	11th to 20th	Day	37% 13% 28% 16% 5% 1	365
	21st to end	Day	35% 16% 27% 16% 6% 1	334
March	1st to 10th	Day	31% 15% 26% 20% 7%	460
	11th to 20th	Day	30% 11% 25% 21% 12% 1	460
	21st to end	Day	34% 16% 24% 20% 7% 1	506
April	1st to 10th	Day	36% 12% 27% 17% 7% 1	460
	11th to 20th	Day	28% 17% 28% 18% 8% 1	452
	21st to end	Day	34% 16% 24% 16% 8% 2	460
May	1st to 10th	Day	23% 13% 30% 26% 6% 1	463
	11th to 20th	Day	22% 11% 24% 28% 12% 3	488
	21st to end	Day	29% 13% 24% 22% 9% 3	529
June	1st to 10th	Day	25% 15% 30% 22% 7% 1	651
	11th to 20th	Day	28% 14% 30% 22% 6% 1	680
	21st to end	Day	20% 12% 26% 28% 12% 2	680
July	1st to 10th	Day	18% 8% 30% 32% 10% 2	671
	11th to 20th	Day	11% 10% 28% 35% 13% 3	646
	21st to end	Day	9% 8% 30% 33% 15% 5%	531
August	1st to 10th	Day	11% 8% 23% 32% 16% 9%	487
	11th to 20th	Day	11% 6% 26% 33% 17% 7%	477
	21st to end	Day	16% 11% 26% 27% 14% 5%	539
September	1st to 10th	Day	16% 11% 27% 29% 12% 5%	490
	11th to 20th	Day	22% 8% 24% 28% 14% 4	490
	21st to end	Day	27% 11% 24% 23% 13% 2	490
October	1st to 10th	Day	26% 11% 19% 26% 16% 3	490
	11th to 20th	Day	35% 8% 19% 23% 12% 2	490
	21st to end	Day	35% 10% 25% 21% 8% 2	378
November	1st to 10th	Day	34% 9% 29% 23% 5%	207
	11th to 20th	Day	38% 12% 23% 17% 9% 3	200
	21st to end	Day	39% 9% 24% 15% 12% 2	200
December	1st to 10th	Day	37% 8% 18% 18% 15% 5%	200
	11th to 20th	Day	39% 9% 26% 20% 4 3	200
	21st to end	Day	40% 10% 25% 20% 4 1	220
January	1st to 10th	Night	43% 9% 20% 18% 9% 1	900
	11th to 20th	Night	39% 7% 18% 24% 11%	860
	21st to end	Night	36% 6% 22% 26% 8% 1	880
February	1st to 10th	Night	38% 10% 24% 23% 5% 1	780
	11th to 20th	Night	37% 9% 21% 25% 7%	744
	21st to end	Night	35% 13% 18% 23% 10% 1	628
March	1st to 10th	Night	32% 7% 19% 25% 17% 1	760
	11th to 20th	Night	34% 8% 20% 24% 15% 1	760
	21st to end	Night	35% 9% 20% 25% 9% 2	836
April	1st to 10th	Night	32% 5% 20% 30% 12% 2	760
	11th to 20th	Night	28% 6% 21% 32% 11% 1	707
	21st to end	Night	34% 6% 16% 31% 11% 1	470
May	1st to 10th	Night	24% 6% 14% 34% 21% 1	380
	11th to 20th	Night	19% 4 11% 33% 29% 4	374
	21st to end	Night	26% 5% 6% 35% 26% 2	411
June	1st to 10th	Night	23% 3 16% 35% 21% 2	380
	11th to 20th	Night	26% 4 14% 29% 22% 4	380
	21st to end	Night	18% 4 11% 35% 31% 2	380
July	1st to 10th	Night	15% 3 8% 32% 38% 4	370
	11th to 20th	Night	12% 4 7% 31% 40% 6%	368
	21st to end	Night	10% 1 3 33% 46% 7%	416
August	1st to 10th	Night	12% 1 3 25% 48% 10%	378
	11th to 20th	Night	9% 2 5% 32% 44% 8%	530
	21st to end	Night	14% 3 11% 35% 35% 4	727
September	1st to 10th	Night	14% 3 14% 36% 30% 3	760
	11th to 20th	Night	21% 4 12% 25% 32% 6%	760
	21st to end	Night	24% 5% 14% 31% 24% 2	760
October	1st to 10th	Night	24% 5% 18% 30% 22% 1	760
	11th to 20th	Night	35% 5% 15% 27% 16% 2	760
	21st to end	Night	37% 9% 20% 24% 8% 2	836
November	1st to 10th	Night	42% 6% 23% 24% 4	780
	11th to 20th	Night	44% 7% 22% 17% 10% 1	800
	21st to end	Night	42% 11% 17% 20% 9% 1	800
December	1st to 10th	Night	31% 9% 24% 26% 10% 1	800
	11th to 20th	Night	43% 7% 25% 20% 5%	860
	21st to end	Night	43% 9% 23% 21% 3	990

Page generated at 2025-02-03 00:05:50. Version ac07

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