Shawnigan Lake School Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Shawnigan Lake School Observatory clear sky chart from 2005-11-06 to 2025-05-11.

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	14% 11% 4 9% 6% 5% 5% 4 6% 6% 31%	117417
	Night	15% 10% 3 8% 5% 5% 5% 4 5% 7% 34%	120220
Transparency	Day	50% 6% 15% 20% 8% 1	117165
	Night	50% 6% 15% 20% 9% 1	120118
Seeing	Day	19% 12% 24% 25% 15% 4	16022
	Night	21% 9% 17% 28% 20% 4	24353

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	24% 18% 6% 10% 6% 4 4 3 5% 5% 14%	3179
	11th to 20th	Day	14% 11% 4 8% 5% 5% 5% 4 7% 7% 31%	3159
	21st to end	Day	21% 16% 5% 9% 6% 5% 5% 3 4 5% 22%	4118
February	1st to 10th	Day	19% 17% 5% 11% 6% 6% 5% 4 5% 7% 15%	1800
	11th to 20th	Day	18% 18% 5% 9% 6% 5% 4 4 4 5% 21%	1755
	21st to end	Day	15% 17% 4 11% 8% 6% 5% 4 7% 6% 16%	1485
March	1st to 10th	Day	12% 17% 5% 13% 7% 6% 6% 5% 6% 6% 17%	4751
	11th to 20th	Day	14% 15% 4 9% 6% 5% 6% 4 6% 6% 25%	5000
	21st to end	Day	18% 14% 5% 12% 8% 6% 7% 4 5% 6% 15%	5566
April	1st to 10th	Day	18% 16% 4 11% 7% 6% 6% 5% 6% 6% 17%	2400
	11th to 20th	Day	16% 12% 4 9% 7% 6% 6% 4 6% 5% 23%	2494
	21st to end	Day	18% 14% 4 10% 5% 5% 5% 3 5% 6% 25%	2611
May	1st to 10th	Day	14% 9% 3 7% 6% 5% 5% 4 6% 6% 36%	4560
	11th to 20th	Day	9% 8% 3 7% 6% 6% 6% 5% 7% 11% 34%	4380
	21st to end	Day	17% 11% 4 11% 6% 6% 6% 5% 6% 6% 22%	4924
June	1st to 10th	Day	17% 10% 3 8% 6% 5% 6% 5% 6% 8% 26%	2850
	11th to 20th	Day	12% 10% 4 10% 8% 7% 6% 5% 7% 7% 24%	2850
	21st to end	Day	10% 7% 3 8% 5% 6% 5% 5% 7% 7% 37%	2850
July	1st to 10th	Day	5% 4 1 4 3 3 3 3 4 6% 62%	4740
	11th to 20th	Day	4 3 1 3 3 2 3 2 4 5% 70%	4740
	21st to end	Day	6% 4 2 5% 4 4 5% 4 7% 9% 51%	4891
August	1st to 10th	Day	4 3 1 4 3 3 3 3 4 6% 67%	4262
	11th to 20th	Day	9% 9% 3 10% 6% 5% 5% 4 7% 8% 35%	4026
	21st to end	Day	10% 6% 2 7% 5% 5% 5% 4 6% 7% 42%	4442
September	1st to 10th	Day	8% 6% 2 6% 4 4 4 4 6% 9% 48%	2280
	11th to 20th	Day	12% 8% 3 8% 5% 5% 4 4 5% 6% 41%	2242
	21st to end	Day	16% 7% 2 7% 5% 5% 4 4 6% 7% 37%	2024
October	1st to 10th	Day	13% 8% 3 9% 6% 6% 6% 5% 8% 8% 29%	3180
	11th to 20th	Day	20% 10% 4 10% 6% 6% 5% 3 5% 6% 25%	3161
	21st to end	Day	16% 11% 4 9% 7% 7% 6% 4 6% 7% 23%	3217
November	1st to 10th	Day	24% 13% 5% 13% 6% 6% 5% 4 6% 4 15%	1568
	11th to 20th	Day	21% 13% 4 10% 6% 5% 5% 4 5% 5% 22%	1600
	21st to end	Day	22% 14% 4 10% 5% 5% 6% 4 5% 5% 20%	1600
December	1st to 10th	Day	14% 14% 5% 11% 6% 5% 4 4 5% 6% 28%	2569
	11th to 20th	Day	27% 16% 5% 10% 5% 5% 5% 3 4 6% 15%	2645
	21st to end	Day	23% 21% 5% 10% 6% 6% 5% 4 5% 5% 11%	3498
January	1st to 10th	Night	22% 19% 6% 11% 7% 6% 5% 3 5% 4 13%	6766
	11th to 20th	Night	15% 9% 3 6% 4 4 4 4 6% 8% 37%	6739
	21st to end	Night	21% 12% 3 9% 5% 5% 5% 4 4 5% 27%	6843
February	1st to 10th	Night	20% 14% 5% 9% 7% 6% 4 4 4 6% 21%	2600
	11th to 20th	Night	17% 14% 4 8% 5% 5% 4 3 5% 6% 29%	2532
	21st to end	Night	17% 15% 4 8% 7% 5% 4 4 6% 6% 25%	2145
March	1st to 10th	Night	10% 11% 3 9% 6% 6% 7% 6% 7% 9% 26%	5126
	11th to 20th	Night	10% 10% 3 8% 5% 6% 6% 5% 7% 9% 32%	4999
	21st to end	Night	17% 14% 4 11% 6% 5% 5% 4 6% 6% 21%	5408
April	1st to 10th	Night	16% 9% 3 9% 7% 6% 5% 4 6% 7% 28%	2000
	11th to 20th	Night	14% 9% 3 7% 6% 4 5% 4 7% 7% 35%	1853
	21st to end	Night	14% 10% 2 8% 5% 5% 5% 3 7% 7% 34%	1788
May	1st to 10th	Night	10% 7% 2 5% 4 3 4 3 5% 5% 50%	2620
	11th to 20th	Night	7% 5% 2 6% 4 3 5% 4 6% 8% 52%	2519
	21st to end	Night	12% 8% 3 8% 6% 5% 5% 4 6% 8% 34%	2648
June	1st to 10th	Night	12% 8% 3 7% 6% 4 4 4 4 7% 41%	1330
	11th to 20th	Night	10% 4 2 6% 5% 4 5% 5% 6% 10% 43%	1330
	21st to end	Night	6% 3 2 6% 4 3 5% 4 7% 11% 50%	1330
July	1st to 10th	Night	3 3 1 3 3 3 3 2 3 6% 71%	2220
	11th to 20th	Night	2 2 1 2 2 1 1 2 3 5% 79%	2220
	21st to end	Night	6% 3 1 3 3 3 3 3 4 6% 66%	2658
August	1st to 10th	Night	2 2 1 3 1 2 2 2 3 6% 75%	2656
	11th to 20th	Night	8% 5% 2 5% 4 4 5% 4 5% 8% 49%	2812
	21st to end	Night	10% 4 1 4 3 3 4 3 5% 9% 54%	3228
September	1st to 10th	Night	5% 2 1 4 3 3 3 3 6% 8% 62%	1900
	11th to 20th	Night	12% 5% 2 5% 5% 4 4 3 5% 7% 47%	1936
	21st to end	Night	11% 6% 3 6% 4 5% 4 4 7% 8% 44%	2156
October	1st to 10th	Night	10% 6% 2 7% 6% 6% 5% 4 6% 7% 42%	3800
	11th to 20th	Night	17% 8% 4 8% 6% 6% 6% 4 6% 7% 30%	3777
	21st to end	Night	18% 10% 3 9% 6% 5% 5% 3 6% 6% 30%	4340
November	1st to 10th	Night	24% 12% 4 8% 6% 6% 5% 3 5% 6% 22%	2730
	11th to 20th	Night	22% 11% 3 8% 6% 6% 5% 3 5% 5% 25%	2800
	21st to end	Night	20% 11% 4 9% 6% 6% 5% 4 5% 6% 24%	2800
December	1st to 10th	Night	12% 11% 5% 10% 7% 6% 5% 5% 5% 6% 27%	4593
	11th to 20th	Night	26% 17% 4 10% 6% 5% 5% 4 4 4 13%	5560
	21st to end	Night	20% 14% 4 10% 7% 7% 6% 4 6% 6% 15%	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	72% 4 8% 12% 4	3179
	11th to 20th	Day	51% 6% 18% 19% 4 1	3180
	21st to end	Day	67% 4 8% 15% 5%	4118
February	1st to 10th	Day	69% 6% 10% 10% 5% 1	1800
	11th to 20th	Day	65% 5% 9% 15% 5% 1	1764
	21st to end	Day	65% 7% 12% 10% 4 2	1485
March	1st to 10th	Day	64% 6% 11% 13% 5%	4751
	11th to 20th	Day	57% 5% 11% 22% 5% 1	5000
	21st to end	Day	67% 6% 13% 12% 3	5566
April	1st to 10th	Day	62% 10% 13% 13% 3	2400
	11th to 20th	Day	57% 9% 11% 14% 8% 2	2494
	21st to end	Day	57% 8% 12% 16% 6% 1	2611
May	1st to 10th	Day	46% 7% 14% 24% 9% 1	4560
	11th to 20th	Day	40% 9% 25% 20% 6%	4383
	21st to end	Day	58% 6% 17% 15% 3	4925
June	1st to 10th	Day	51% 9% 17% 16% 6% 2	2850
	11th to 20th	Day	52% 10% 20% 15% 4	2850
	21st to end	Day	41% 9% 20% 20% 8% 1	2850
July	1st to 10th	Day	21% 5% 10% 31% 22% 12%	4666
	11th to 20th	Day	16% 5% 12% 41% 25% 1	4659
	21st to end	Day	24% 6% 25% 28% 15% 2	4736
August	1st to 10th	Day	18% 4 14% 44% 19% 1	4245
	11th to 20th	Day	38% 9% 26% 17% 8% 1	4065
	21st to end	Day	38% 7% 19% 27% 8% 1	4441
September	1st to 10th	Day	31% 8% 19% 26% 13% 3	2280
	11th to 20th	Day	42% 6% 15% 20% 15% 2	2242
	21st to end	Day	45% 8% 16% 20% 10% 1	2024
October	1st to 10th	Day	50% 7% 16% 20% 7%	3180
	11th to 20th	Day	58% 6% 13% 16% 6%	3161
	21st to end	Day	57% 7% 14% 15% 6% 1	3217
November	1st to 10th	Day	71% 5% 11% 9% 4 1	1568
	11th to 20th	Day	64% 5% 10% 13% 7% 1	1600
	21st to end	Day	66% 5% 10% 13% 5%	1600
December	1st to 10th	Day	58% 6% 11% 19% 6%	2569
	11th to 20th	Day	70% 5% 11% 11% 2	2648
	21st to end	Day	73% 7% 9% 8% 3	3498
January	1st to 10th	Night	73% 5% 8% 11% 3	6766
	11th to 20th	Night	45% 7% 18% 23% 6% 1	6780
	21st to end	Night	60% 4 11% 17% 7%	6843
February	1st to 10th	Night	64% 6% 12% 11% 6% 1	2600
	11th to 20th	Night	56% 7% 13% 16% 8% 1	2534
	21st to end	Night	58% 7% 9% 17% 7% 1	2145
March	1st to 10th	Night	52% 7% 15% 18% 8%	5126
	11th to 20th	Night	48% 6% 13% 26% 7%	5000
	21st to end	Night	63% 6% 13% 15% 3	5408
April	1st to 10th	Night	54% 7% 17% 17% 6%	2000
	11th to 20th	Night	46% 7% 15% 20% 9% 2	1853
	21st to end	Night	48% 7% 17% 18% 9% 1	1788
May	1st to 10th	Night	36% 5% 14% 27% 17% 1	2620
	11th to 20th	Night	29% 6% 26% 31% 8%	2519
	21st to end	Night	47% 6% 21% 21% 4	2648
June	1st to 10th	Night	44% 4 19% 23% 7% 3	1330
	11th to 20th	Night	35% 7% 27% 24% 7%	1330
	21st to end	Night	28% 7% 27% 24% 11% 3	1330
July	1st to 10th	Night	18% 3 12% 33% 19% 14%	2185
	11th to 20th	Night	11% 3 13% 38% 33% 1	2178
	21st to end	Night	21% 4 18% 32% 24% 2	2574
August	1st to 10th	Night	13% 3 15% 41% 26% 2	2648
	11th to 20th	Night	34% 8% 26% 19% 8% 4	2839
	21st to end	Night	27% 6% 21% 33% 12% 1	3228
September	1st to 10th	Night	21% 6% 21% 29% 18% 5%	1900
	11th to 20th	Night	35% 7% 16% 20% 17% 5%	1936
	21st to end	Night	38% 7% 17% 24% 12% 3	2156
October	1st to 10th	Night	41% 5% 20% 23% 9% 1	3800
	11th to 20th	Night	52% 5% 16% 21% 6%	3777
	21st to end	Night	53% 7% 14% 17% 10%	4340
November	1st to 10th	Night	64% 6% 10% 14% 5% 1	2730
	11th to 20th	Night	60% 7% 11% 13% 7% 2	2800
	21st to end	Night	62% 6% 12% 13% 8%	2800
December	1st to 10th	Night	56% 7% 13% 20% 5%	4593
	11th to 20th	Night	74% 5% 11% 8% 2	5556
	21st to end	Night	65% 7% 12% 12% 4	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% 18% 18% 16% 7% 5%	200
	11th to 20th	Day	38% 11% 15% 20% 15% 2	200
	21st to end	Day	31% 15% 22% 19% 10% 4	299
February	1st to 10th	Day	31% 19% 25% 18% 6% 1	400
	11th to 20th	Day	31% 19% 28% 17% 5%	395
	21st to end	Day	31% 21% 25% 17% 5% 1	331
March	1st to 10th	Day	26% 18% 26% 19% 9% 1	499
	11th to 20th	Day	25% 19% 22% 23% 10% 2	500
	21st to end	Day	27% 18% 24% 22% 8% 1	550
April	1st to 10th	Day	26% 17% 27% 19% 9% 2	500
	11th to 20th	Day	21% 15% 31% 19% 11% 3	492
	21st to end	Day	22% 17% 25% 22% 11% 3	500
May	1st to 10th	Day	13% 12% 31% 28% 13% 3	503
	11th to 20th	Day	17% 9% 25% 25% 17% 5%	489
	21st to end	Day	15% 11% 26% 30% 14% 4	588
June	1st to 10th	Day	18% 10% 30% 26% 14% 2	680
	11th to 20th	Day	12% 15% 32% 26% 13% 3	680
	21st to end	Day	10% 8% 27% 32% 20% 3	680
July	1st to 10th	Day	11% 5% 20% 34% 25% 4	671
	11th to 20th	Day	5% 6% 23% 38% 21% 7%	668
	21st to end	Day	5% 5% 21% 33% 28% 8%	593
August	1st to 10th	Day	8% 6% 16% 30% 31% 10%	487
	11th to 20th	Day	5% 5% 19% 35% 27% 10%	477
	21st to end	Day	11% 7% 24% 33% 16% 9%	539
September	1st to 10th	Day	10% 6% 27% 34% 17% 6%	490
	11th to 20th	Day	18% 10% 21% 29% 18% 4	490
	21st to end	Day	18% 13% 26% 26% 16% 2	490
October	1st to 10th	Day	20% 9% 26% 25% 18% 2	490
	11th to 20th	Day	26% 13% 21% 24% 14% 3	490
	21st to end	Day	25% 20% 25% 17% 10% 3	431
November	1st to 10th	Day	33% 16% 26% 19% 6% 1	200
	11th to 20th	Day	29% 16% 21% 18% 11% 5%	200
	21st to end	Day	31% 17% 21% 13% 12% 8%	200
December	1st to 10th	Day	30% 15% 18% 13% 19% 7%	200
	11th to 20th	Day	33% 15% 19% 23% 9% 2	200
	21st to end	Day	36% 15% 24% 18% 6% 2	220
January	1st to 10th	Night	33% 16% 20% 18% 10% 3	900
	11th to 20th	Night	29% 10% 17% 26% 17% 2	900
	21st to end	Night	29% 10% 17% 27% 15% 3	893
February	1st to 10th	Night	30% 14% 18% 23% 13% 3	800
	11th to 20th	Night	26% 13% 18% 27% 15% 1	784
	21st to end	Night	25% 14% 21% 25% 13% 3	660
March	1st to 10th	Night	20% 16% 19% 28% 15% 2	800
	11th to 20th	Night	21% 15% 21% 26% 15% 2	800
	21st to end	Night	24% 12% 19% 29% 14% 3	880
April	1st to 10th	Night	22% 10% 20% 28% 17% 3	800
	11th to 20th	Night	17% 11% 21% 30% 17% 3	665
	21st to end	Night	18% 8% 20% 35% 17% 3	588
May	1st to 10th	Night	11% 9% 18% 38% 21% 3	400
	11th to 20th	Night	13% 6% 15% 34% 27% 5%	375
	21st to end	Night	13% 4 20% 36% 24% 4	411
June	1st to 10th	Night	15% 9% 18% 32% 24% 2	380
	11th to 20th	Night	10% 8% 23% 35% 22% 3	380
	21st to end	Night	7% 5% 20% 33% 31% 5%	380
July	1st to 10th	Night	6% 3 12% 33% 40% 6%	370
	11th to 20th	Night	4 3 13% 35% 38% 7%	368
	21st to end	Night	4 2 10% 30% 44% 10%	416
August	1st to 10th	Night	6% 2 6% 26% 45% 15%	453
	11th to 20th	Night	5% 1 6% 26% 49% 13%	564
	21st to end	Night	9% 4 13% 32% 36% 6%	670
September	1st to 10th	Night	6% 3 10% 37% 34% 9%	760
	11th to 20th	Night	15% 4 13% 23% 32% 13%	760
	21st to end	Night	16% 5% 17% 29% 28% 5%	760
October	1st to 10th	Night	18% 4 14% 32% 26% 6%	760
	11th to 20th	Night	21% 8% 17% 26% 21% 7%	760
	21st to end	Night	28% 9% 19% 27% 15% 4	836
November	1st to 10th	Night	32% 12% 16% 29% 10% 1	780
	11th to 20th	Night	29% 14% 19% 22% 14% 2	800
	21st to end	Night	27% 11% 20% 22% 15% 4	800
December	1st to 10th	Night	25% 12% 16% 26% 18% 3	810
	11th to 20th	Night	32% 12% 20% 27% 9% 1	900
	21st to end	Night	34% 14% 21% 24% 6% 1	990

Page generated at 2025-05-08 06:33:59. Version ac07

Copyright © 2025 Attilla Danko.