Williams Lake
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Williams Lake clear sky chart from 2005-11-06 to 2025-11-03.

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	8% 10% 4 9% 6% 6% 6% 5% 7% 8% 33%	130464
	Night	8% 12% 4 10% 7% 6% 6% 5% 7% 8% 28%	128224
Transparency	Day	46% 9% 16% 20% 9% 1	130164
	Night	51% 9% 15% 17% 7% 1	128101
Seeing	Day	13% 10% 24% 27% 19% 7%	17381
	Night	14% 9% 23% 33% 19% 3	25318

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	6% 17% 7% 16% 8% 7% 6% 4 6% 6% 16%	3179
	11th to 20th	Day	8% 12% 5% 10% 7% 6% 6% 5% 8% 8% 26%	3159
	21st to end	Day	6% 13% 6% 12% 7% 6% 6% 5% 6% 8% 24%	3696
February	1st to 10th	Day	10% 15% 5% 14% 8% 6% 5% 4 5% 5% 24%	1683
	11th to 20th	Day	7% 14% 6% 10% 7% 5% 6% 6% 7% 7% 26%	1755
	21st to end	Day	4 14% 6% 11% 8% 6% 6% 5% 6% 8% 26%	1495
March	1st to 10th	Day	4 13% 5% 11% 6% 6% 7% 6% 7% 9% 26%	4518
	11th to 20th	Day	2 9% 4 11% 7% 7% 8% 6% 8% 10% 28%	5000
	21st to end	Day	7% 12% 4 11% 7% 6% 7% 4 6% 8% 27%	5722
April	1st to 10th	Day	5% 12% 5% 12% 7% 7% 8% 5% 7% 7% 25%	2400
	11th to 20th	Day	6% 12% 4 12% 8% 7% 6% 5% 6% 7% 28%	2494
	21st to end	Day	8% 10% 4 9% 7% 7% 6% 5% 9% 7% 28%	2769
May	1st to 10th	Day	7% 8% 3 8% 6% 6% 5% 4 6% 8% 37%	6320
	11th to 20th	Day	8% 11% 4 11% 7% 7% 7% 6% 7% 9% 23%	6277
	21st to end	Day	11% 11% 4 11% 7% 7% 7% 5% 7% 8% 21%	7442
June	1st to 10th	Day	11% 10% 3 9% 6% 6% 5% 5% 7% 7% 30%	3200
	11th to 20th	Day	10% 10% 4 10% 7% 7% 7% 6% 7% 9% 24%	3200
	21st to end	Day	9% 7% 4 9% 6% 7% 7% 5% 6% 8% 32%	3200
July	1st to 10th	Day	7% 5% 2 5% 5% 5% 4 4 6% 9% 48%	6946
	11th to 20th	Day	5% 5% 2 5% 4 4 5% 4 6% 8% 53%	6780
	21st to end	Day	7% 5% 2 5% 5% 4 5% 5% 7% 9% 46%	7268
August	1st to 10th	Day	5% 4 1 5% 3 3 4 4 6% 8% 56%	4541
	11th to 20th	Day	4 4 2 6% 5% 4 6% 5% 8% 10% 46%	4351
	21st to end	Day	9% 8% 3 8% 5% 4 5% 4 7% 9% 38%	4581
September	1st to 10th	Day	9% 6% 3 6% 4 4 5% 4 6% 8% 47%	2400
	11th to 20th	Day	11% 8% 2 7% 5% 4 5% 4 6% 7% 42%	2400
	21st to end	Day	14% 9% 3 7% 5% 5% 5% 4 6% 8% 34%	2116
October	1st to 10th	Day	11% 14% 4 10% 5% 5% 5% 4 6% 7% 29%	3280
	11th to 20th	Day	13% 12% 3 9% 5% 5% 5% 3 6% 7% 31%	3261
	21st to end	Day	8% 13% 4 10% 6% 6% 6% 4 7% 5% 30%	3058
November	1st to 10th	Day	11% 15% 4 9% 6% 5% 6% 5% 7% 9% 23%	1584
	11th to 20th	Day	8% 12% 5% 12% 8% 6% 7% 5% 7% 7% 23%	1600
	21st to end	Day	9% 19% 4 12% 8% 6% 5% 4 5% 8% 18%	1491
December	1st to 10th	Day	8% 15% 5% 14% 7% 7% 7% 5% 6% 7% 20%	1970
	11th to 20th	Day	7% 13% 6% 16% 8% 8% 9% 7% 7% 6% 13%	2262
	21st to end	Day	6% 16% 5% 12% 7% 7% 6% 4 6% 7% 23%	3066
January	1st to 10th	Night	8% 18% 7% 15% 7% 6% 5% 5% 6% 6% 15%	6766
	11th to 20th	Night	6% 13% 7% 14% 7% 5% 6% 5% 7% 7% 23%	6739
	21st to end	Night	6% 15% 5% 11% 6% 6% 7% 6% 8% 9% 19%	7258
February	1st to 10th	Night	10% 18% 5% 11% 7% 7% 5% 4 5% 6% 22%	2715
	11th to 20th	Night	8% 15% 5% 11% 5% 5% 6% 5% 6% 6% 28%	2532
	21st to end	Night	5% 15% 4 11% 7% 6% 6% 4 6% 7% 29%	2135
March	1st to 10th	Night	3 12% 5% 12% 6% 6% 7% 6% 9% 10% 24%	5376
	11th to 20th	Night	2 9% 4 11% 7% 7% 7% 6% 8% 9% 30%	4999
	21st to end	Night	6% 13% 4 10% 7% 6% 7% 5% 8% 8% 25%	5240
April	1st to 10th	Night	5% 12% 4 10% 7% 7% 7% 4 8% 10% 26%	2000
	11th to 20th	Night	6% 9% 4 10% 7% 6% 6% 5% 7% 8% 32%	1855
	21st to end	Night	6% 11% 4 11% 8% 6% 5% 5% 7% 7% 29%	1630
May	1st to 10th	Night	9% 12% 4 9% 6% 5% 5% 3 6% 7% 33%	3640
	11th to 20th	Night	12% 12% 4 9% 6% 6% 4 5% 7% 8% 27%	3616
	21st to end	Night	15% 12% 4 10% 7% 6% 7% 5% 7% 5% 22%	3416
June	1st to 10th	Night	11% 10% 3 7% 7% 5% 6% 6% 7% 7% 31%	1200
	11th to 20th	Night	16% 10% 3 9% 8% 5% 5% 6% 6% 9% 22%	1200
	21st to end	Night	9% 9% 3 11% 7% 6% 6% 5% 6% 8% 29%	1200
July	1st to 10th	Night	8% 5% 2 7% 5% 4 5% 5% 7% 9% 43%	2996
	11th to 20th	Night	7% 4 2 6% 4 4 5% 4 6% 8% 50%	3180
	21st to end	Night	8% 6% 2 6% 5% 4 5% 5% 7% 8% 46%	3574
August	1st to 10th	Night	5% 4 2 5% 4 4 4 4 5% 7% 55%	2609
	11th to 20th	Night	6% 5% 3 8% 6% 6% 6% 4 7% 8% 40%	2698
	21st to end	Night	13% 8% 3 9% 5% 5% 5% 5% 7% 7% 35%	3331
September	1st to 10th	Night	7% 6% 2 7% 6% 5% 4 4 6% 7% 45%	2000
	11th to 20th	Night	9% 6% 2 7% 5% 4 5% 4 6% 8% 44%	2000
	21st to end	Night	13% 9% 3 8% 6% 4 5% 4 6% 7% 36%	2281
October	1st to 10th	Night	12% 13% 4 9% 6% 6% 4 4 5% 7% 30%	3920
	11th to 20th	Night	9% 9% 3 9% 6% 6% 6% 6% 7% 10% 30%	3897
	21st to end	Night	8% 11% 4 10% 6% 7% 6% 5% 9% 7% 27%	4724
November	1st to 10th	Night	10% 14% 4 11% 7% 6% 6% 5% 7% 7% 23%	2758
	11th to 20th	Night	9% 12% 5% 10% 8% 8% 6% 4 8% 7% 23%	2800
	21st to end	Night	8% 16% 6% 11% 8% 7% 6% 5% 6% 8% 20%	2886
December	1st to 10th	Night	7% 13% 5% 11% 8% 6% 8% 6% 9% 8% 19%	5205
	11th to 20th	Night	9% 19% 7% 15% 7% 6% 6% 4 6% 6% 14%	5930
	21st to end	Night	5% 13% 5% 12% 8% 7% 7% 5% 6% 7% 24%	7918

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	65% 10% 13% 11% 1	3179
	11th to 20th	Day	51% 9% 17% 12% 6% 5%	3180
	21st to end	Day	55% 10% 17% 16% 2	3696
February	1st to 10th	Day	60% 7% 11% 12% 10%	1683
	11th to 20th	Day	52% 10% 17% 14% 6%	1764
	21st to end	Day	51% 10% 15% 16% 7% 1	1495
March	1st to 10th	Day	49% 10% 13% 18% 9%	4518
	11th to 20th	Day	44% 9% 13% 23% 10%	5000
	21st to end	Day	52% 9% 13% 19% 8%	5722
April	1st to 10th	Day	52% 11% 14% 17% 6%	2400
	11th to 20th	Day	52% 9% 11% 18% 9% 1	2494
	21st to end	Day	48% 10% 12% 20% 8% 1	2769
May	1st to 10th	Day	40% 7% 13% 25% 14% 1	6320
	11th to 20th	Day	50% 10% 14% 20% 6%	6280
	21st to end	Day	54% 10% 15% 17% 4	7443
June	1st to 10th	Day	48% 10% 13% 16% 11% 3	3200
	11th to 20th	Day	51% 11% 14% 17% 5%	3200
	21st to end	Day	47% 9% 17% 17% 8% 1	3200
July	1st to 10th	Day	32% 6% 14% 25% 18% 5%	6872
	11th to 20th	Day	28% 7% 18% 27% 17% 3	6699
	21st to end	Day	31% 6% 22% 26% 14% 2	7063
August	1st to 10th	Day	25% 6% 13% 36% 19%	4524
	11th to 20th	Day	30% 7% 25% 29% 8% 1	4393
	21st to end	Day	41% 7% 27% 19% 5% 1	4580
September	1st to 10th	Day	33% 9% 20% 25% 11% 2	2400
	11th to 20th	Day	40% 9% 16% 24% 10% 1	2400
	21st to end	Day	47% 8% 18% 17% 9% 1	2116
October	1st to 10th	Day	53% 7% 20% 15% 4	3280
	11th to 20th	Day	50% 7% 17% 22% 3	3261
	21st to end	Day	52% 7% 19% 18% 3	3058
November	1st to 10th	Day	54% 11% 13% 17% 5%	1584
	11th to 20th	Day	55% 12% 12% 15% 6%	1600
	21st to end	Day	62% 12% 13% 10% 3	1491
December	1st to 10th	Day	62% 9% 18% 9% 2	1970
	11th to 20th	Day	64% 11% 13% 8% 4	2264
	21st to end	Day	56% 10% 15% 15% 4	3066
January	1st to 10th	Night	65% 9% 13% 11% 3	6766
	11th to 20th	Night	56% 9% 14% 12% 6% 4	6780
	21st to end	Night	54% 13% 19% 13% 2	7258
February	1st to 10th	Night	62% 7% 11% 13% 7%	2715
	11th to 20th	Night	53% 9% 16% 15% 6% 1	2534
	21st to end	Night	52% 8% 16% 17% 8% 1	2135
March	1st to 10th	Night	47% 11% 14% 18% 10%	5376
	11th to 20th	Night	44% 9% 15% 22% 10%	5000
	21st to end	Night	51% 11% 14% 15% 10%	5240
April	1st to 10th	Night	49% 10% 14% 19% 8%	2000
	11th to 20th	Night	46% 9% 14% 20% 10%	1855
	21st to end	Night	49% 10% 11% 21% 8%	1630
May	1st to 10th	Night	47% 6% 11% 24% 11% 1	3637
	11th to 20th	Night	50% 8% 13% 23% 5%	3616
	21st to end	Night	58% 9% 13% 16% 4	3416
June	1st to 10th	Night	47% 10% 12% 19% 11% 2	1200
	11th to 20th	Night	55% 11% 15% 14% 5%	1200
	21st to end	Night	50% 9% 15% 17% 7% 2	1200
July	1st to 10th	Night	37% 6% 15% 19% 16% 6%	2961
	11th to 20th	Night	31% 7% 13% 32% 16% 1	3138
	21st to end	Night	33% 6% 18% 27% 11% 5%	3474
August	1st to 10th	Night	27% 7% 11% 35% 20% 1	2601
	11th to 20th	Night	39% 8% 17% 26% 8% 1	2722
	21st to end	Night	46% 7% 23% 17% 6% 1	3331
September	1st to 10th	Night	35% 8% 15% 27% 13% 2	2000
	11th to 20th	Night	37% 7% 15% 28% 11% 3	2000
	21st to end	Night	46% 8% 16% 20% 10% 1	2281
October	1st to 10th	Night	53% 6% 19% 16% 6%	3920
	11th to 20th	Night	47% 7% 18% 22% 5%	3897
	21st to end	Night	50% 10% 17% 19% 4	4724
November	1st to 10th	Night	58% 10% 13% 14% 6%	2758
	11th to 20th	Night	54% 12% 15% 13% 7%	2800
	21st to end	Night	59% 11% 14% 12% 3 1	2886
December	1st to 10th	Night	57% 13% 18% 10% 3	5205
	11th to 20th	Night	68% 9% 12% 9% 2	5927
	21st to end	Night	55% 10% 15% 15% 4	7918

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	13% 13% 37% 23% 11% 4	200
	11th to 20th	Day	13% 14% 30% 27% 13% 4	200
	21st to end	Day	13% 14% 24% 29% 16% 4	220
February	1st to 10th	Day	17% 15% 27% 23% 12% 5%	283
	11th to 20th	Day	14% 9% 27% 31% 16% 3	395
	21st to end	Day	13% 15% 29% 27% 12% 4	337
March	1st to 10th	Day	13% 18% 26% 23% 16% 4	500
	11th to 20th	Day	11% 17% 27% 25% 17% 3	500
	21st to end	Day	15% 14% 31% 21% 12% 7%	550
April	1st to 10th	Day	15% 16% 27% 26% 13% 3	500
	11th to 20th	Day	11% 18% 26% 24% 14% 7%	492
	21st to end	Day	12% 14% 26% 24% 17% 7%	500
May	1st to 10th	Day	14% 13% 28% 26% 15% 4	503
	11th to 20th	Day	13% 9% 22% 25% 23% 8%	528
	21st to end	Day	15% 9% 24% 27% 18% 6%	801
June	1st to 10th	Day	14% 12% 27% 28% 16% 3	930
	11th to 20th	Day	14% 8% 24% 29% 20% 5%	930
	21st to end	Day	11% 9% 18% 29% 24% 8%	930
July	1st to 10th	Day	10% 5% 23% 31% 21% 10%	796
	11th to 20th	Day	8% 6% 23% 28% 27% 8%	718
	21st to end	Day	6% 5% 18% 30% 30% 11%	749
August	1st to 10th	Day	7% 4 14% 29% 32% 14%	527
	11th to 20th	Day	6% 7% 18% 28% 29% 12%	517
	21st to end	Day	10% 6% 20% 26% 23% 15%	583
September	1st to 10th	Day	12% 6% 18% 26% 25% 14%	530
	11th to 20th	Day	15% 6% 19% 24% 24% 12%	530
	21st to end	Day	21% 10% 25% 27% 12% 5%	530
October	1st to 10th	Day	19% 12% 19% 23% 18% 9%	530
	11th to 20th	Day	20% 12% 25% 22% 15% 5%	530
	21st to end	Day	21% 11% 30% 25% 9% 3	318
November	1st to 10th	Day	22% 15% 29% 21% 8% 5%	204
	11th to 20th	Day	18% 10% 30% 24% 15% 4	200
	21st to end	Day	15% 17% 22% 27% 12% 8%	200
December	1st to 10th	Day	13% 9% 21% 27% 24% 8%	200
	11th to 20th	Day	13% 10% 22% 37% 14% 6%	200
	21st to end	Day	20% 10% 22% 30% 15% 3	220
January	1st to 10th	Night	18% 14% 28% 27% 12% 1	900
	11th to 20th	Night	16% 11% 26% 33% 14% 1	900
	21st to end	Night	15% 8% 30% 30% 16% 1	947
February	1st to 10th	Night	19% 11% 24% 30% 13% 3	800
	11th to 20th	Night	14% 7% 26% 38% 13% 2	784
	21st to end	Night	12% 9% 24% 33% 18% 3	660
March	1st to 10th	Night	13% 12% 26% 32% 16% 2	800
	11th to 20th	Night	8% 13% 30% 33% 15% 2	800
	21st to end	Night	12% 9% 22% 30% 24% 3	880
April	1st to 10th	Night	11% 10% 24% 32% 20% 3	800
	11th to 20th	Night	11% 9% 23% 39% 16% 3	667
	21st to end	Night	10% 13% 28% 31% 16% 3	430
May	1st to 10th	Night	14% 7% 24% 37% 17% 1	400
	11th to 20th	Night	13% 8% 20% 39% 19% 2	394
	21st to end	Night	15% 6% 22% 29% 25% 3	433
June	1st to 10th	Night	16% 8% 26% 32% 18% 1	400
	11th to 20th	Night	18% 5% 19% 35% 20% 3	400
	21st to end	Night	10% 5% 20% 37% 25% 4	400
July	1st to 10th	Night	11% 3 19% 38% 28% 2	390
	11th to 20th	Night	9% 3 19% 37% 30% 2	388
	21st to end	Night	9% 3 16% 37% 29% 5%	438
August	1st to 10th	Night	5% 4 13% 33% 39% 6%	398
	11th to 20th	Night	7% 3 13% 37% 35% 5%	465
	21st to end	Night	15% 5% 13% 32% 31% 3	707
September	1st to 10th	Night	11% 5% 13% 37% 30% 3	800
	11th to 20th	Night	12% 5% 14% 31% 32% 6%	800
	21st to end	Night	19% 9% 22% 30% 17% 2	800
October	1st to 10th	Night	17% 9% 19% 30% 22% 3	800
	11th to 20th	Night	18% 7% 23% 33% 17% 3	800
	21st to end	Night	20% 10% 25% 29% 13% 2	880
November	1st to 10th	Night	20% 13% 24% 30% 11% 2	788
	11th to 20th	Night	17% 13% 26% 31% 12% 2	800
	21st to end	Night	15% 13% 26% 31% 11% 3	886
December	1st to 10th	Night	13% 9% 21% 35% 18% 2	1093
	11th to 20th	Night	16% 11% 25% 34% 12% 2	1100
	21st to end	Night	16% 11% 27% 34% 12% 2	1190

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