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 2024-01-16.

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	9% 9% 4 8% 7% 4 6% 6% 6% 8% 31%	74525
	Night	9% 11% 5% 9% 8% 4 6% 6% 6% 8% 28%	75263
Transparency	Day	48% 10% 16% 19% 7% 1	74207
	Night	52% 10% 15% 16% 7% 1	75099
Seeing	Day	13% 10% 24% 27% 19% 7%	14862
	Night	14% 9% 23% 33% 18% 3	23094

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	7% 16% 7% 11% 10% 6% 7% 6% 7% 7% 16%	1329
	11th to 20th	Day	8% 13% 6% 10% 11% 5% 6% 7% 7% 8% 19%	1295
	21st to end	Day	8% 14% 7% 14% 9% 5% 6% 5% 5% 8% 19%	1458
February	1st to 10th	Day	11% 15% 6% 12% 9% 4 5% 5% 4 5% 24%	1514
	11th to 20th	Day	7% 13% 7% 10% 7% 5% 6% 7% 7% 8% 25%	1575
	21st to end	Day	4 11% 7% 10% 9% 5% 5% 7% 6% 8% 27%	1340
March	1st to 10th	Day	6% 13% 6% 10% 7% 4 6% 7% 6% 8% 29%	1800
	11th to 20th	Day	5% 11% 6% 8% 8% 5% 8% 6% 7% 8% 28%	1980
	21st to end	Day	6% 10% 5% 9% 9% 5% 8% 7% 6% 9% 26%	2272
April	1st to 10th	Day	5% 12% 6% 11% 8% 5% 8% 6% 7% 7% 24%	2160
	11th to 20th	Day	6% 10% 6% 10% 8% 5% 7% 6% 6% 8% 27%	2241
	21st to end	Day	9% 10% 5% 8% 8% 5% 7% 6% 8% 7% 27%	2491
May	1st to 10th	Day	11% 9% 5% 8% 8% 4 6% 6% 6% 7% 30%	2520
	11th to 20th	Day	10% 9% 3 8% 7% 4 6% 7% 7% 8% 32%	2477
	21st to end	Day	11% 9% 4 9% 7% 4 6% 6% 7% 8% 28%	2926
June	1st to 10th	Day	12% 9% 4 8% 7% 4 7% 6% 6% 7% 28%	2880
	11th to 20th	Day	11% 9% 4 9% 8% 5% 8% 7% 7% 9% 24%	2880
	21st to end	Day	10% 7% 4 8% 6% 5% 8% 7% 6% 8% 33%	2880
July	1st to 10th	Day	10% 6% 3 5% 6% 4 6% 7% 7% 8% 39%	2768
	11th to 20th	Day	7% 5% 3 5% 5% 3 6% 6% 8% 9% 43%	2700
	21st to end	Day	6% 4 2 4 5% 3 5% 6% 7% 7% 49%	2898
August	1st to 10th	Day	6% 5% 2 4 5% 3 5% 6% 6% 9% 48%	2507
	11th to 20th	Day	5% 4 2 4 6% 3 5% 5% 6% 9% 51%	2412
	21st to end	Day	10% 6% 3 6% 7% 4 5% 6% 8% 9% 35%	2533
September	1st to 10th	Day	10% 6% 3 6% 5% 3 5% 5% 6% 7% 45%	2160
	11th to 20th	Day	12% 7% 2 5% 6% 3 5% 5% 6% 7% 42%	2160
	21st to end	Day	13% 8% 3 6% 6% 4 5% 5% 6% 9% 36%	1906
October	1st to 10th	Day	12% 10% 3 7% 6% 4 5% 4 5% 8% 36%	1800
	11th to 20th	Day	13% 9% 3 7% 7% 4 5% 5% 7% 7% 33%	1800
	21st to end	Day	12% 13% 4 7% 6% 4 6% 5% 6% 6% 31%	1710
November	1st to 10th	Day	12% 14% 4 8% 7% 4 7% 6% 6% 9% 23%	1488
	11th to 20th	Day	9% 10% 6% 11% 9% 5% 7% 6% 6% 7% 24%	1520
	21st to end	Day	9% 17% 6% 10% 10% 4 5% 6% 6% 8% 19%	1417
December	1st to 10th	Day	9% 12% 6% 12% 10% 6% 7% 6% 7% 6% 20%	1150
	11th to 20th	Day	8% 15% 7% 12% 9% 5% 8% 8% 7% 7% 14%	1442
	21st to end	Day	6% 13% 5% 10% 8% 5% 7% 6% 7% 7% 26%	2136
January	1st to 10th	Night	10% 15% 6% 12% 10% 6% 6% 6% 6% 7% 17%	2836
	11th to 20th	Night	9% 14% 7% 12% 9% 5% 6% 6% 7% 7% 18%	2775
	21st to end	Night	8% 13% 6% 11% 9% 5% 7% 7% 6% 8% 20%	2896
February	1st to 10th	Night	11% 18% 5% 9% 8% 5% 5% 5% 4 6% 23%	2444
	11th to 20th	Night	8% 15% 6% 10% 6% 4 6% 6% 6% 6% 27%	2272
	21st to end	Night	6% 13% 5% 10% 8% 4 6% 5% 6% 7% 30%	1916
March	1st to 10th	Night	6% 12% 5% 10% 8% 4 7% 6% 7% 6% 29%	2160
	11th to 20th	Night	3 11% 5% 9% 9% 6% 7% 7% 7% 8% 30%	1979
	21st to end	Night	5% 11% 4 8% 8% 4 7% 6% 8% 8% 28%	2084
April	1st to 10th	Night	5% 11% 5% 10% 7% 4 7% 5% 7% 11% 27%	1800
	11th to 20th	Night	7% 9% 5% 9% 7% 5% 6% 7% 7% 8% 30%	1669
	21st to end	Night	7% 9% 4 9% 9% 5% 6% 6% 6% 7% 30%	1468
May	1st to 10th	Night	11% 12% 5% 8% 8% 4 6% 5% 5% 7% 30%	1440
	11th to 20th	Night	10% 9% 4 7% 8% 4 6% 7% 7% 9% 30%	1416
	21st to end	Night	13% 9% 4 7% 6% 4 7% 7% 8% 8% 27%	1346
June	1st to 10th	Night	12% 9% 3 6% 8% 4 6% 8% 7% 8% 29%	1080
	11th to 20th	Night	18% 9% 4 8% 9% 5% 5% 6% 6% 9% 22%	1080
	21st to end	Night	9% 8% 3 10% 8% 5% 6% 7% 6% 8% 30%	1080
July	1st to 10th	Night	10% 6% 3 8% 7% 4 7% 6% 7% 9% 34%	1188
	11th to 20th	Night	9% 5% 3 7% 5% 5% 7% 6% 7% 10% 36%	1260
	21st to end	Night	7% 4 2 6% 7% 4 4 6% 7% 8% 46%	1418
August	1st to 10th	Night	6% 4 3 6% 6% 3 5% 6% 6% 9% 46%	1432
	11th to 20th	Night	6% 4 3 6% 5% 3 6% 5% 6% 7% 48%	1485
	21st to end	Night	11% 7% 3 7% 8% 4 6% 6% 7% 8% 33%	1823
September	1st to 10th	Night	8% 7% 3 6% 7% 4 5% 5% 7% 7% 42%	1800
	11th to 20th	Night	9% 5% 2 6% 6% 3 5% 5% 6% 8% 44%	1800
	21st to end	Night	13% 8% 2 6% 7% 3 5% 5% 6% 8% 37%	2051
October	1st to 10th	Night	11% 9% 4 7% 7% 4 4 5% 5% 8% 36%	2160
	11th to 20th	Night	12% 10% 3 7% 7% 4 5% 6% 5% 8% 32%	2160
	21st to end	Night	12% 12% 4 8% 6% 3 5% 6% 6% 7% 30%	2645
November	1st to 10th	Night	11% 13% 4 10% 8% 5% 6% 7% 7% 8% 22%	2590
	11th to 20th	Night	10% 10% 6% 9% 9% 6% 6% 6% 7% 8% 24%	2660
	21st to end	Night	8% 14% 7% 10% 8% 5% 7% 7% 6% 8% 20%	2741
December	1st to 10th	Night	9% 12% 7% 9% 10% 5% 7% 7% 6% 7% 21%	3025
	11th to 20th	Night	11% 17% 6% 12% 9% 5% 6% 6% 6% 7% 15%	3750
	21st to end	Night	6% 13% 5% 10% 10% 5% 7% 7% 7% 7% 23%	5534

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% 12% 13% 8% 2 1	1329
	11th to 20th	Day	58% 12% 18% 7% 5% 1	1295
	21st to end	Day	62% 9% 13% 12% 3 1	1458
February	1st to 10th	Day	61% 7% 11% 12% 8%	1514
	11th to 20th	Day	54% 11% 17% 14% 5%	1584
	21st to end	Day	52% 10% 16% 16% 6% 1	1340
March	1st to 10th	Day	52% 9% 14% 14% 11%	1800
	11th to 20th	Day	50% 9% 15% 21% 6%	1980
	21st to end	Day	52% 10% 17% 16% 4	2272
April	1st to 10th	Day	54% 11% 14% 15% 6%	2160
	11th to 20th	Day	53% 9% 11% 17% 8% 1	2241
	21st to end	Day	50% 11% 11% 19% 8% 1	2491
May	1st to 10th	Day	50% 8% 16% 17% 10%	2520
	11th to 20th	Day	46% 10% 15% 21% 8% 1	2480
	21st to end	Day	50% 10% 13% 19% 6% 1	2927
June	1st to 10th	Day	50% 10% 13% 15% 10% 3	2880
	11th to 20th	Day	53% 11% 15% 16% 5%	2880
	21st to end	Day	47% 10% 17% 18% 8% 1	2880
July	1st to 10th	Day	39% 11% 15% 23% 12% 2	2694
	11th to 20th	Day	34% 12% 20% 23% 12% 1	2619
	21st to end	Day	29% 9% 18% 30% 13% 1	2737
August	1st to 10th	Day	31% 9% 19% 30% 10% 1	2490
	11th to 20th	Day	28% 9% 22% 29% 12% 1	2412
	21st to end	Day	41% 10% 21% 21% 6% 1	2533
September	1st to 10th	Day	36% 10% 20% 24% 9% 2	2160
	11th to 20th	Day	39% 9% 16% 23% 10% 1	2160
	21st to end	Day	45% 8% 18% 19% 9% 1	1906
October	1st to 10th	Day	46% 9% 20% 21% 4	1800
	11th to 20th	Day	47% 9% 18% 23% 4	1800
	21st to end	Day	52% 9% 15% 18% 6%	1710
November	1st to 10th	Day	55% 11% 13% 16% 5%	1488
	11th to 20th	Day	56% 12% 13% 14% 6%	1520
	21st to end	Day	61% 12% 14% 10% 4	1417
December	1st to 10th	Day	61% 12% 13% 11% 3	1150
	11th to 20th	Day	61% 15% 12% 8% 3	1444
	21st to end	Day	54% 10% 16% 15% 4	2136
January	1st to 10th	Night	65% 9% 13% 10% 2	2836
	11th to 20th	Night	61% 11% 13% 10% 5% 1	2775
	21st to end	Night	58% 11% 13% 12% 5%	2896
February	1st to 10th	Night	63% 6% 12% 12% 7%	2444
	11th to 20th	Night	54% 9% 16% 15% 6% 1	2274
	21st to end	Night	51% 8% 16% 17% 7% 1	1916
March	1st to 10th	Night	51% 9% 16% 15% 8% 1	2160
	11th to 20th	Night	48% 10% 12% 21% 8% 1	1980
	21st to end	Night	49% 11% 17% 18% 5%	2084
April	1st to 10th	Night	50% 10% 14% 19% 8%	1800
	11th to 20th	Night	48% 9% 14% 20% 9%	1669
	21st to end	Night	50% 10% 11% 20% 9% 1	1468
May	1st to 10th	Night	54% 6% 14% 17% 8% 1	1440
	11th to 20th	Night	47% 10% 14% 20% 8% 1	1416
	21st to end	Night	50% 11% 15% 16% 6% 1	1346
June	1st to 10th	Night	48% 11% 12% 18% 9% 2	1080
	11th to 20th	Night	56% 12% 15% 12% 5%	1080
	21st to end	Night	49% 10% 15% 17% 7% 2	1080
July	1st to 10th	Night	44% 12% 16% 16% 10% 1	1153
	11th to 20th	Night	41% 12% 16% 21% 9% 1	1218
	21st to end	Night	31% 10% 17% 28% 12% 2	1339
August	1st to 10th	Night	33% 10% 16% 27% 13% 1	1424
	11th to 20th	Night	31% 9% 19% 26% 12% 2	1485
	21st to end	Night	45% 10% 16% 22% 6% 1	1823
September	1st to 10th	Night	38% 8% 15% 26% 10% 2	1800
	11th to 20th	Night	37% 8% 14% 28% 11% 3	1800
	21st to end	Night	44% 8% 16% 21% 10%	2051
October	1st to 10th	Night	45% 8% 19% 20% 7%	2160
	11th to 20th	Night	48% 10% 15% 20% 6%	2160
	21st to end	Night	50% 9% 15% 21% 5%	2645
November	1st to 10th	Night	57% 10% 13% 13% 6%	2590
	11th to 20th	Night	54% 12% 15% 13% 7%	2660
	21st to end	Night	58% 11% 14% 12% 4 1	2741
December	1st to 10th	Night	58% 11% 16% 11% 5%	3025
	11th to 20th	Night	65% 12% 12% 8% 2	3747
	21st to end	Night	56% 12% 14% 12% 5%	5534

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	12% 12% 36% 24% 12% 4	180
	11th to 20th	Day	14% 14% 28% 26% 14% 5%	170
	21st to end	Day	14% 14% 24% 27% 16% 3	176
February	1st to 10th	Day	21% 17% 29% 23% 9% 1	234
	11th to 20th	Day	15% 10% 30% 30% 15% 1	335
	21st to end	Day	11% 12% 32% 28% 12% 5%	284
March	1st to 10th	Day	13% 18% 27% 24% 14% 4	420
	11th to 20th	Day	12% 18% 28% 24% 15% 3	420
	21st to end	Day	14% 15% 32% 21% 12% 7%	462
April	1st to 10th	Day	16% 16% 26% 25% 13% 4	420
	11th to 20th	Day	12% 18% 26% 22% 14% 8%	412
	21st to end	Day	13% 15% 28% 23% 16% 5%	420
May	1st to 10th	Day	15% 12% 27% 26% 15% 4	423
	11th to 20th	Day	13% 8% 22% 26% 23% 9%	448
	21st to end	Day	15% 9% 23% 28% 19% 7%	689
June	1st to 10th	Day	15% 11% 27% 29% 16% 3	810
	11th to 20th	Day	15% 8% 25% 28% 20% 5%	810
	21st to end	Day	11% 9% 19% 30% 24% 8%	810
July	1st to 10th	Day	11% 5% 24% 30% 21% 9%	689
	11th to 20th	Day	8% 5% 24% 29% 26% 8%	618
	21st to end	Day	6% 5% 18% 31% 30% 9%	644
August	1st to 10th	Day	7% 4 14% 30% 31% 13%	447
	11th to 20th	Day	6% 6% 17% 29% 29% 13%	437
	21st to end	Day	11% 6% 22% 27% 23% 11%	495
September	1st to 10th	Day	14% 7% 21% 27% 23% 9%	450
	11th to 20th	Day	16% 6% 19% 24% 23% 12%	450
	21st to end	Day	18% 9% 24% 29% 14% 6%	450
October	1st to 10th	Day	19% 10% 18% 23% 19% 10%	450
	11th to 20th	Day	18% 13% 24% 22% 16% 6%	450
	21st to end	Day	20% 11% 32% 25% 9% 4	261
November	1st to 10th	Day	22% 14% 27% 22% 9% 6%	180
	11th to 20th	Day	18% 11% 30% 23% 15% 4	180
	21st to end	Day	14% 18% 22% 26% 12% 8%	180
December	1st to 10th	Day	14% 8% 22% 26% 22% 9%	180
	11th to 20th	Day	12% 11% 21% 38% 13% 6%	180
	21st to end	Day	18% 10% 23% 31% 15% 3	198
January	1st to 10th	Night	18% 14% 29% 26% 11% 2	850
	11th to 20th	Night	16% 12% 26% 30% 14% 1	825
	21st to end	Night	15% 7% 31% 30% 16% 1	846
February	1st to 10th	Night	20% 11% 23% 30% 12% 3	720
	11th to 20th	Night	15% 7% 27% 38% 12% 2	704
	21st to end	Night	12% 8% 24% 35% 19% 3	592
March	1st to 10th	Night	13% 12% 26% 31% 16% 2	720
	11th to 20th	Night	8% 13% 31% 31% 16% 2	720
	21st to end	Night	12% 10% 22% 30% 24% 3	792
April	1st to 10th	Night	11% 9% 24% 33% 20% 4	720
	11th to 20th	Night	12% 9% 23% 37% 16% 3	601
	21st to end	Night	10% 14% 29% 30% 15% 3	388
May	1st to 10th	Night	14% 7% 25% 36% 17% 1	360
	11th to 20th	Night	12% 8% 20% 39% 19% 2	354
	21st to end	Night	15% 5% 21% 29% 27% 4	389
June	1st to 10th	Night	17% 8% 25% 31% 19% 1	360
	11th to 20th	Night	19% 5% 20% 35% 19% 3	360
	21st to end	Night	9% 5% 21% 37% 25% 3	360
July	1st to 10th	Night	11% 3 21% 36% 27% 2	350
	11th to 20th	Night	9% 3 19% 36% 30% 2	348
	21st to end	Night	9% 3 16% 38% 29% 5%	394
August	1st to 10th	Night	5% 4 14% 33% 38% 6%	358
	11th to 20th	Night	7% 3 12% 37% 35% 6%	417
	21st to end	Night	16% 5% 13% 33% 29% 3	635
September	1st to 10th	Night	12% 6% 14% 39% 27% 3	720
	11th to 20th	Night	12% 5% 14% 31% 31% 6%	720
	21st to end	Night	18% 9% 23% 30% 19% 2	720
October	1st to 10th	Night	16% 9% 19% 29% 23% 3	720
	11th to 20th	Night	18% 7% 23% 33% 17% 3	720
	21st to end	Night	20% 10% 26% 29% 14% 2	792
November	1st to 10th	Night	20% 13% 24% 30% 11% 3	740
	11th to 20th	Night	17% 13% 26% 31% 12% 1	760
	21st to end	Night	16% 14% 26% 31% 11% 3	841
December	1st to 10th	Night	14% 9% 21% 35% 18% 3	1033
	11th to 20th	Night	16% 11% 26% 33% 12% 2	1040
	21st to end	Night	15% 11% 27% 34% 11% 1	1125

Page generated at 2024-01-12 22:38:09. Version ac07

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