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 2024-11-06.

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	8% 10% 4 9% 6% 6% 6% 5% 7% 8% 33%	105464
	Night	8% 11% 4 10% 7% 6% 6% 5% 7% 7% 29%	102144
Transparency	Day	45% 9% 16% 20% 9% 1	105209
	Night	50% 10% 15% 17% 8% 1	102045
Seeing	Day	13% 10% 24% 27% 19% 7%	16054
	Night	14% 9% 23% 33% 18% 3	24036

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% 16% 5% 14% 9% 7% 7% 4 7% 7% 18%	2219
	11th to 20th	Day	10% 14% 5% 11% 6% 5% 5% 5% 8% 9% 23%	2199
	21st to end	Day	8% 17% 7% 14% 8% 6% 5% 4 6% 7% 17%	2570
February	1st to 10th	Day	11% 16% 5% 14% 8% 5% 5% 4 5% 5% 24%	1598
	11th to 20th	Day	7% 14% 6% 10% 7% 5% 6% 6% 7% 8% 26%	1665
	21st to end	Day	4 14% 6% 11% 9% 7% 5% 5% 6% 8% 26%	1422
March	1st to 10th	Day	3 10% 5% 10% 6% 6% 6% 6% 8% 10% 28%	3138
	11th to 20th	Day	3 10% 5% 9% 6% 6% 7% 5% 8% 9% 31%	3490
	21st to end	Day	4 10% 4 10% 7% 7% 7% 5% 6% 9% 31%	3996
April	1st to 10th	Day	5% 13% 5% 13% 7% 7% 8% 5% 7% 7% 24%	2280
	11th to 20th	Day	6% 12% 5% 12% 8% 7% 6% 5% 6% 8% 27%	2368
	21st to end	Day	8% 11% 4 10% 7% 7% 6% 5% 9% 7% 27%	2630
May	1st to 10th	Day	8% 9% 4 8% 6% 5% 5% 4 6% 8% 36%	4420
	11th to 20th	Day	9% 10% 3 10% 6% 6% 7% 6% 7% 9% 27%	4377
	21st to end	Day	13% 12% 5% 12% 7% 7% 6% 5% 7% 7% 21%	5198
June	1st to 10th	Day	12% 10% 3 9% 7% 6% 6% 6% 6% 7% 28%	3040
	11th to 20th	Day	11% 10% 4 10% 7% 7% 7% 5% 7% 9% 24%	3040
	21st to end	Day	9% 7% 4 9% 6% 7% 7% 5% 6% 8% 32%	3040
July	1st to 10th	Day	6% 4 2 4 4 4 4 4 6% 8% 55%	4850
	11th to 20th	Day	4 4 1 4 3 3 4 3 6% 8% 61%	4740
	21st to end	Day	8% 5% 2 5% 5% 5% 5% 4 7% 8% 45%	5059
August	1st to 10th	Day	5% 5% 1 4 3 3 4 4 6% 8% 57%	4401
	11th to 20th	Day	4 4 2 6% 5% 4 6% 5% 8% 10% 46%	4215
	21st to end	Day	9% 8% 3 8% 6% 4 5% 4 7% 9% 36%	4441
September	1st to 10th	Day	9% 6% 3 6% 4 4 5% 3 6% 7% 46%	2280
	11th to 20th	Day	11% 9% 2 7% 4 4 5% 4 6% 7% 41%	2280
	21st to end	Day	13% 9% 3 7% 5% 5% 5% 4 6% 8% 35%	2011
October	1st to 10th	Day	11% 13% 4 10% 5% 6% 5% 4 6% 8% 29%	3180
	11th to 20th	Day	14% 12% 3 9% 5% 5% 5% 4 6% 7% 30%	3161
	21st to end	Day	8% 13% 4 10% 6% 6% 6% 4 7% 5% 30%	2963
November	1st to 10th	Day	11% 15% 4 9% 6% 5% 6% 5% 7% 8% 23%	1528
	11th to 20th	Day	8% 12% 5% 12% 8% 7% 7% 4 7% 7% 23%	1520
	21st to end	Day	9% 19% 5% 11% 8% 6% 5% 5% 6% 8% 19%	1417
December	1st to 10th	Day	9% 13% 5% 14% 8% 8% 6% 4 7% 6% 19%	1150
	11th to 20th	Day	8% 16% 7% 14% 7% 6% 9% 6% 7% 7% 13%	1442
	21st to end	Day	6% 14% 5% 12% 6% 7% 6% 5% 7% 7% 25%	2136
January	1st to 10th	Night	8% 16% 6% 15% 8% 7% 6% 5% 6% 7% 17%	4726
	11th to 20th	Night	8% 15% 7% 14% 7% 5% 5% 4 7% 6% 23%	4699
	21st to end	Night	8% 14% 5% 12% 8% 8% 9% 6% 7% 7% 16%	5084
February	1st to 10th	Night	10% 19% 5% 11% 7% 6% 5% 4 4 6% 22%	2580
	11th to 20th	Night	8% 15% 5% 11% 5% 5% 6% 5% 6% 6% 28%	2402
	21st to end	Night	6% 15% 4 11% 7% 6% 5% 4 6% 6% 29%	2032
March	1st to 10th	Night	4 12% 4 11% 7% 5% 7% 6% 9% 10% 25%	3736
	11th to 20th	Night	2 9% 3 8% 6% 7% 6% 6% 8% 10% 35%	3489
	21st to end	Night	3 11% 4 10% 7% 6% 7% 5% 7% 8% 31%	3657
April	1st to 10th	Night	5% 12% 4 10% 7% 6% 7% 4 8% 10% 27%	1900
	11th to 20th	Night	6% 10% 4 10% 7% 6% 6% 6% 8% 7% 31%	1762
	21st to end	Night	7% 11% 4 11% 8% 6% 5% 5% 7% 7% 29%	1549
May	1st to 10th	Night	7% 11% 4 10% 7% 6% 5% 4 5% 7% 36%	2540
	11th to 20th	Night	12% 11% 3 8% 6% 5% 4 4 7% 9% 29%	2516
	21st to end	Night	18% 13% 5% 10% 6% 5% 6% 4 7% 6% 20%	2374
June	1st to 10th	Night	12% 10% 3 7% 7% 5% 7% 7% 7% 7% 29%	1140
	11th to 20th	Night	17% 10% 3 9% 8% 5% 5% 6% 6% 9% 22%	1140
	21st to end	Night	9% 8% 3 11% 8% 6% 6% 5% 6% 8% 30%	1140
July	1st to 10th	Night	8% 5% 2 7% 4 3 4 4 5% 6% 51%	2092
	11th to 20th	Night	5% 3 2 5% 3 4 5% 4 6% 8% 56%	2220
	21st to end	Night	8% 5% 2 7% 5% 5% 5% 5% 7% 7% 42%	2492
August	1st to 10th	Night	5% 4 2 5% 4 4 4 4 5% 7% 56%	2529
	11th to 20th	Night	6% 5% 3 8% 6% 6% 6% 4 7% 8% 40%	2614
	21st to end	Night	13% 8% 3 9% 5% 5% 5% 5% 7% 7% 33%	3229
September	1st to 10th	Night	7% 6% 3 7% 6% 5% 4 4 6% 7% 44%	1900
	11th to 20th	Night	9% 6% 2 7% 5% 4 5% 4 6% 8% 43%	1900
	21st to end	Night	13% 8% 2 7% 6% 4 5% 4 6% 7% 37%	2166
October	1st to 10th	Night	12% 13% 4 9% 6% 6% 5% 4 5% 7% 30%	3800
	11th to 20th	Night	10% 9% 3 9% 6% 6% 6% 6% 7% 10% 29%	3777
	21st to end	Night	9% 11% 4 10% 6% 7% 7% 5% 9% 7% 27%	4577
November	1st to 10th	Night	10% 14% 4 11% 7% 7% 6% 6% 7% 7% 22%	2672
	11th to 20th	Night	9% 11% 5% 10% 8% 7% 6% 4 8% 7% 24%	2660
	21st to end	Night	8% 16% 6% 11% 8% 7% 6% 5% 6% 8% 20%	2741
December	1st to 10th	Night	8% 14% 6% 11% 9% 7% 7% 6% 6% 7% 21%	3025
	11th to 20th	Night	10% 18% 6% 14% 7% 6% 6% 5% 7% 6% 14%	3750
	21st to end	Night	6% 13% 5% 12% 8% 7% 7% 5% 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	59% 13% 14% 13% 1	2219
	11th to 20th	Day	54% 9% 18% 7% 4 7%	2220
	21st to end	Day	64% 12% 11% 10% 2 1	2570
February	1st to 10th	Day	61% 8% 11% 12% 8%	1598
	11th to 20th	Day	52% 11% 17% 14% 7%	1674
	21st to end	Day	51% 10% 16% 16% 6% 1	1422
March	1st to 10th	Day	43% 13% 14% 20% 11%	3138
	11th to 20th	Day	42% 9% 18% 25% 5%	3490
	21st to end	Day	45% 10% 16% 18% 10%	3996
April	1st to 10th	Day	53% 11% 14% 16% 6%	2280
	11th to 20th	Day	52% 9% 11% 17% 9% 1	2368
	21st to end	Day	50% 11% 11% 19% 8% 1	2630
May	1st to 10th	Day	40% 8% 16% 26% 10%	4420
	11th to 20th	Day	45% 11% 15% 21% 7% 1	4380
	21st to end	Day	55% 12% 13% 15% 5% 1	5199
June	1st to 10th	Day	50% 10% 12% 15% 10% 3	3040
	11th to 20th	Day	52% 11% 15% 17% 5%	3040
	21st to end	Day	47% 10% 17% 17% 8% 1	3040
July	1st to 10th	Day	25% 7% 11% 27% 22% 8%	4776
	11th to 20th	Day	21% 7% 15% 31% 22% 4	4659
	21st to end	Day	33% 7% 19% 24% 14% 2	4899
August	1st to 10th	Day	24% 6% 12% 37% 20%	4384
	11th to 20th	Day	29% 7% 25% 29% 9% 1	4257
	21st to end	Day	42% 7% 28% 18% 4 1	4440
September	1st to 10th	Day	34% 9% 20% 25% 10% 2	2280
	11th to 20th	Day	40% 9% 16% 24% 10% 1	2280
	21st to end	Day	46% 8% 18% 18% 9% 1	2011
October	1st to 10th	Day	53% 7% 20% 16% 4	3180
	11th to 20th	Day	51% 7% 17% 22% 3	3161
	21st to end	Day	52% 8% 19% 18% 3	2963
November	1st to 10th	Day	54% 11% 13% 17% 5%	1528
	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	61% 11% 14% 12% 2	4726
	11th to 20th	Night	59% 8% 13% 9% 6% 5%	4740
	21st to end	Night	58% 15% 14% 9% 3	5084
February	1st to 10th	Night	63% 7% 11% 12% 6%	2580
	11th to 20th	Night	53% 9% 15% 15% 7% 1	2404
	21st to end	Night	52% 8% 16% 17% 7% 1	2032
March	1st to 10th	Night	44% 13% 15% 17% 11% 1	3736
	11th to 20th	Night	37% 11% 18% 26% 8% 1	3490
	21st to end	Night	44% 12% 15% 16% 12%	3657
April	1st to 10th	Night	48% 10% 14% 19% 8%	1900
	11th to 20th	Night	46% 10% 14% 20% 10%	1762
	21st to end	Night	50% 10% 11% 20% 9% 1	1549
May	1st to 10th	Night	44% 8% 13% 23% 10% 1	2540
	11th to 20th	Night	47% 10% 14% 23% 6%	2516
	21st to end	Night	58% 11% 13% 13% 4	2374
June	1st to 10th	Night	48% 11% 12% 18% 9% 2	1140
	11th to 20th	Night	55% 12% 15% 13% 5%	1140
	21st to end	Night	49% 9% 14% 17% 8% 2	1140
July	1st to 10th	Night	32% 7% 9% 21% 21% 9%	2057
	11th to 20th	Night	25% 7% 11% 33% 22% 1	2178
	21st to end	Night	35% 8% 15% 23% 12% 7%	2413
August	1st to 10th	Night	26% 7% 11% 35% 20% 1	2521
	11th to 20th	Night	38% 8% 18% 26% 9% 1	2638
	21st to end	Night	48% 7% 24% 16% 4	3229
September	1st to 10th	Night	37% 8% 15% 27% 12% 2	1900
	11th to 20th	Night	38% 7% 14% 27% 11% 3	1900
	21st to end	Night	45% 8% 16% 21% 10% 1	2166
October	1st to 10th	Night	53% 6% 19% 16% 6%	3800
	11th to 20th	Night	48% 7% 18% 22% 5%	3777
	21st to end	Night	50% 10% 17% 18% 4	4577
November	1st to 10th	Night	57% 10% 13% 13% 6%	2672
	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% 29% 24% 13% 4	180
	21st to end	Day	14% 15% 24% 27% 17% 3	198
February	1st to 10th	Day	19% 16% 28% 23% 11% 2	258
	11th to 20th	Day	14% 9% 28% 30% 15% 3	365
	21st to end	Day	11% 15% 30% 27% 13% 4	312
March	1st to 10th	Day	13% 18% 26% 24% 15% 4	460
	11th to 20th	Day	12% 16% 28% 24% 17% 3	460
	21st to end	Day	13% 14% 33% 21% 12% 8%	506
April	1st to 10th	Day	15% 17% 26% 26% 13% 3	460
	11th to 20th	Day	12% 18% 26% 23% 14% 8%	452
	21st to end	Day	12% 15% 27% 24% 16% 6%	460
May	1st to 10th	Day	14% 12% 28% 26% 16% 5%	463
	11th to 20th	Day	12% 10% 22% 25% 22% 8%	488
	21st to end	Day	15% 9% 23% 28% 18% 6%	746
June	1st to 10th	Day	15% 12% 27% 28% 16% 3	870
	11th to 20th	Day	14% 8% 25% 28% 19% 6%	870
	21st to end	Day	11% 9% 19% 30% 23% 8%	870
July	1st to 10th	Day	11% 5% 23% 31% 21% 10%	742
	11th to 20th	Day	7% 5% 23% 29% 27% 8%	668
	21st to end	Day	6% 5% 19% 31% 29% 9%	696
August	1st to 10th	Day	7% 4 14% 29% 31% 15%	487
	11th to 20th	Day	5% 6% 17% 29% 30% 13%	477
	21st to end	Day	11% 6% 22% 27% 23% 11%	539
September	1st to 10th	Day	13% 6% 20% 27% 24% 11%	490
	11th to 20th	Day	16% 6% 19% 24% 23% 11%	490
	21st to end	Day	20% 10% 25% 28% 12% 6%	490
October	1st to 10th	Day	20% 11% 19% 23% 18% 10%	490
	11th to 20th	Day	19% 13% 25% 22% 15% 5%	490
	21st to end	Day	20% 11% 30% 27% 9% 3	289
November	1st to 10th	Day	22% 14% 27% 22% 8% 6%	190
	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	17% 11% 26% 31% 14% 1	850
	21st to end	Night	15% 8% 31% 29% 15% 1	897
February	1st to 10th	Night	19% 12% 23% 31% 13% 3	760
	11th to 20th	Night	15% 7% 26% 38% 13% 2	744
	21st to end	Night	12% 9% 23% 34% 19% 3	628
March	1st to 10th	Night	13% 12% 26% 32% 16% 2	760
	11th to 20th	Night	8% 13% 30% 32% 16% 2	760
	21st to end	Night	11% 9% 22% 30% 24% 3	836
April	1st to 10th	Night	11% 9% 25% 32% 19% 4	760
	11th to 20th	Night	11% 9% 23% 39% 16% 3	634
	21st to end	Night	10% 13% 28% 30% 15% 3	409
May	1st to 10th	Night	13% 7% 24% 37% 17% 1	380
	11th to 20th	Night	13% 8% 21% 39% 18% 2	374
	21st to end	Night	16% 6% 21% 28% 26% 3	411
June	1st to 10th	Night	16% 8% 26% 31% 18% 1	380
	11th to 20th	Night	19% 5% 19% 35% 19% 3	380
	21st to end	Night	9% 5% 20% 37% 25% 4	380
July	1st to 10th	Night	11% 3 19% 37% 28% 2	370
	11th to 20th	Night	9% 3 18% 37% 31% 2	368
	21st to end	Night	8% 3 16% 38% 29% 5%	416
August	1st to 10th	Night	5% 4 13% 33% 38% 6%	378
	11th to 20th	Night	7% 3 12% 37% 36% 6%	441
	21st to end	Night	16% 5% 14% 33% 29% 3	671
September	1st to 10th	Night	11% 5% 14% 38% 29% 2	760
	11th to 20th	Night	12% 5% 14% 32% 31% 6%	760
	21st to end	Night	19% 9% 22% 30% 18% 2	760
October	1st to 10th	Night	17% 9% 19% 29% 22% 3	760
	11th to 20th	Night	18% 7% 23% 33% 17% 3	760
	21st to end	Night	20% 10% 25% 30% 13% 2	836
November	1st to 10th	Night	20% 13% 24% 30% 11% 2	764
	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 2024-11-03 10:19:06. Version ac07

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