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 2026-09-14.

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	12% 10% 3 8% 5% 5% 5% 4 6% 7% 35%	153631
	Night	13% 9% 3 8% 5% 5% 5% 4 5% 7% 36%	146179
Transparency	Day	46% 6% 17% 21% 9% 1	153338
	Night	47% 6% 16% 21% 9% 1	146054
Seeing	Day	19% 12% 24% 26% 15% 4	17883
	Night	21% 9% 17% 28% 20% 4	25924

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	21% 18% 6% 11% 7% 5% 5% 4 5% 5% 13%	4125
	11th to 20th	Day	13% 10% 4 7% 5% 5% 5% 4 7% 6% 34%	4119
	21st to end	Day	21% 15% 5% 9% 6% 5% 5% 3 4 5% 22%	5341
February	1st to 10th	Day	19% 16% 5% 11% 6% 6% 5% 4 5% 6% 15%	1890
	11th to 20th	Day	18% 18% 5% 10% 6% 5% 4 4 4 6% 21%	1845
	21st to end	Day	15% 17% 5% 11% 8% 6% 5% 4 7% 6% 16%	1557
March	1st to 10th	Day	12% 17% 5% 13% 7% 6% 6% 5% 6% 6% 17%	4856
	11th to 20th	Day	14% 15% 4 10% 6% 5% 6% 4 6% 6% 25%	5110
	21st to end	Day	18% 14% 5% 12% 8% 6% 7% 4 6% 6% 15%	5689
April	1st to 10th	Day	18% 15% 4 10% 7% 6% 6% 5% 6% 6% 18%	2520
	11th to 20th	Day	16% 12% 4 10% 7% 6% 6% 4 6% 5% 23%	2620
	21st to end	Day	17% 13% 4 10% 5% 5% 5% 3 5% 6% 26%	2742
May	1st to 10th	Day	9% 6% 2 6% 6% 6% 6% 4 6% 7% 41%	8195
	11th to 20th	Day	9% 10% 3 8% 6% 6% 7% 5% 8% 10% 29%	8160
	21st to end	Day	11% 7% 3 8% 5% 5% 6% 5% 7% 8% 33%	9188
June	1st to 10th	Day	16% 10% 3 8% 6% 5% 6% 5% 6% 8% 28%	3150
	11th to 20th	Day	11% 9% 4 10% 7% 6% 7% 5% 7% 7% 27%	3150
	21st to end	Day	10% 8% 3 8% 5% 6% 5% 5% 7% 7% 36%	3150
July	1st to 10th	Day	5% 4 2 6% 4 4 4 4 5% 7% 54%	8820
	11th to 20th	Day	4 3 1 4 3 3 3 3 5% 6% 64%	8820
	21st to end	Day	5% 3 2 5% 4 4 5% 4 7% 9% 53%	9099
August	1st to 10th	Day	3 3 1 4 2 3 3 3 5% 5% 69%	6246
	11th to 20th	Day	6% 6% 2 8% 5% 4 5% 4 7% 8% 45%	5923
	21st to end	Day	8% 6% 2 8% 5% 5% 6% 4 7% 8% 40%	6462
September	1st to 10th	Day	8% 6% 2 6% 4 4 4 4 6% 9% 48%	2520
	11th to 20th	Day	11% 8% 3 8% 5% 4 4 4 5% 7% 41%	2397
	21st to end	Day	16% 7% 2 7% 5% 5% 4 4 6% 7% 37%	2130
October	1st to 10th	Day	13% 8% 3 8% 6% 6% 6% 5% 8% 8% 28%	3280
	11th to 20th	Day	20% 10% 4 10% 6% 5% 5% 3 5% 6% 26%	3261
	21st to end	Day	16% 11% 4 9% 7% 7% 6% 4 6% 7% 22%	3320
November	1st to 10th	Day	23% 13% 5% 12% 6% 6% 5% 4 6% 4 15%	1648
	11th to 20th	Day	21% 13% 4 10% 6% 5% 5% 4 5% 5% 21%	1680
	21st to end	Day	22% 14% 4 10% 6% 5% 5% 4 5% 5% 19%	1680
December	1st to 10th	Day	14% 14% 5% 11% 6% 5% 4 4 5% 6% 27%	2648
	11th to 20th	Day	26% 16% 5% 10% 5% 5% 5% 3 4 6% 14%	2715
	21st to end	Day	23% 21% 5% 10% 6% 6% 5% 4 5% 5% 11%	3575
January	1st to 10th	Night	22% 19% 5% 11% 6% 6% 5% 3 5% 4 13%	8770
	11th to 20th	Night	14% 9% 4 6% 3 4 4 4 5% 7% 39%	8779
	21st to end	Night	19% 12% 4 9% 6% 5% 5% 4 5% 6% 27%	8848
February	1st to 10th	Night	19% 14% 5% 9% 7% 6% 4 4 4 6% 21%	2730
	11th to 20th	Night	17% 14% 4 8% 5% 5% 4 3 5% 6% 29%	2662
	21st to end	Night	16% 15% 4 9% 6% 5% 4 4 5% 6% 25%	2249
March	1st to 10th	Night	10% 11% 3 9% 6% 6% 7% 6% 7% 9% 26%	5241
	11th to 20th	Night	10% 10% 3 9% 6% 6% 6% 5% 7% 8% 31%	5109
	21st to end	Night	17% 14% 4 12% 6% 5% 5% 4 6% 6% 21%	5527
April	1st to 10th	Night	15% 9% 3 9% 7% 5% 5% 5% 6% 7% 28%	2100
	11th to 20th	Night	14% 9% 3 7% 6% 4 5% 4 7% 7% 35%	1947
	21st to end	Night	14% 9% 2 8% 5% 5% 5% 3 7% 7% 35%	1877
May	1st to 10th	Night	7% 5% 2 6% 5% 4 4 3 4 6% 54%	4724
	11th to 20th	Night	7% 5% 2 8% 5% 4 5% 4 7% 9% 44%	4700
	21st to end	Night	9% 7% 3 7% 5% 5% 5% 4 7% 10% 39%	4980
June	1st to 10th	Night	12% 7% 3 6% 6% 4 4 3 5% 7% 42%	1470
	11th to 20th	Night	9% 4 2 6% 5% 4 5% 5% 6% 9% 45%	1470
	21st to end	Night	6% 3 2 6% 4 3 5% 4 6% 11% 50%	1470
July	1st to 10th	Night	3 2 1 3 3 3 4 3 4 8% 66%	4140
	11th to 20th	Night	2 2 1 2 2 1 2 2 3 5% 77%	4140
	21st to end	Night	4 3 1 4 3 3 3 3 5% 7% 64%	4966
August	1st to 10th	Night	2 2 1 2 1 2 2 1 3 6% 78%	3896
	11th to 20th	Night	6% 4 2 4 4 4 5% 3 5% 8% 56%	4142
	21st to end	Night	8% 4 1 4 3 3 4 3 5% 9% 56%	4691
September	1st to 10th	Night	5% 3 1 4 3 3 3 4 6% 8% 61%	2100
	11th to 20th	Night	12% 5% 2 5% 5% 4 4 3 5% 7% 47%	2068
	21st to end	Night	11% 6% 3 6% 4 4 4 4 6% 7% 44%	2270
October	1st to 10th	Night	10% 6% 2 7% 6% 6% 5% 4 6% 7% 42%	3920
	11th to 20th	Night	17% 8% 4 8% 6% 6% 5% 4 6% 7% 31%	3897
	21st to end	Night	17% 10% 3 9% 6% 5% 5% 3 6% 6% 30%	4479
November	1st to 10th	Night	23% 12% 4 8% 6% 6% 5% 4 5% 6% 21%	2870
	11th to 20th	Night	21% 12% 4 8% 6% 6% 5% 3 5% 5% 24%	2940
	21st to end	Night	21% 11% 4 9% 6% 6% 5% 4 5% 6% 24%	2940
December	1st to 10th	Night	12% 11% 5% 10% 7% 6% 5% 5% 5% 6% 26%	4734
	11th to 20th	Night	26% 17% 4 10% 6% 6% 5% 4 5% 4 13%	5710
	21st to end	Night	20% 14% 4 10% 7% 7% 6% 4 6% 6% 15%	7623

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	71% 5% 9% 12% 3	4125
	11th to 20th	Day	48% 6% 16% 23% 7%	4140
	21st to end	Day	66% 4 8% 16% 6%	5342
February	1st to 10th	Day	69% 6% 10% 10% 5% 1	1890
	11th to 20th	Day	64% 5% 10% 15% 5% 1	1854
	21st to end	Day	65% 7% 13% 10% 4 1	1557
March	1st to 10th	Day	63% 6% 12% 14% 6%	4856
	11th to 20th	Day	58% 5% 10% 22% 5% 1	5110
	21st to end	Day	66% 6% 13% 12% 3	5689
April	1st to 10th	Day	61% 9% 13% 14% 3	2520
	11th to 20th	Day	57% 9% 11% 13% 8% 2	2620
	21st to end	Day	55% 8% 13% 17% 6% 1	2742
May	1st to 10th	Day	39% 6% 17% 27% 10% 1	8195
	11th to 20th	Day	44% 9% 24% 18% 5%	8163
	21st to end	Day	43% 6% 24% 20% 7% 1	9189
June	1st to 10th	Day	48% 8% 18% 18% 6% 2	3150
	11th to 20th	Day	49% 9% 19% 17% 5%	3150
	21st to end	Day	41% 9% 20% 20% 8% 1	3150
July	1st to 10th	Day	25% 5% 22% 27% 15% 6%	8746
	11th to 20th	Day	18% 4 19% 37% 21% 1	8739
	21st to end	Day	24% 5% 30% 28% 11% 1	8902
August	1st to 10th	Day	16% 4 14% 40% 23% 3	6229
	11th to 20th	Day	29% 7% 32% 25% 6% 1	5962
	21st to end	Day	37% 8% 24% 25% 7% 1	6461
September	1st to 10th	Day	31% 8% 21% 26% 12% 3	2520
	11th to 20th	Day	42% 6% 16% 21% 15% 2	2397
	21st to end	Day	45% 8% 16% 20% 10% 1	2130
October	1st to 10th	Day	50% 7% 16% 20% 6%	3280
	11th to 20th	Day	58% 6% 13% 17% 6%	3261
	21st to end	Day	57% 7% 14% 15% 6% 1	3320
November	1st to 10th	Day	70% 6% 11% 9% 4 1	1648
	11th to 20th	Day	64% 5% 10% 12% 7% 1	1680
	21st to end	Day	67% 6% 10% 13% 5%	1680
December	1st to 10th	Day	58% 6% 11% 19% 6%	2648
	11th to 20th	Day	70% 5% 11% 12% 2	2718
	21st to end	Day	73% 7% 9% 8% 3	3575
January	1st to 10th	Night	73% 4 8% 11% 4	8770
	11th to 20th	Night	45% 5% 15% 22% 12% 1	8820
	21st to end	Night	59% 4 11% 19% 7%	8848
February	1st to 10th	Night	63% 6% 12% 11% 6% 1	2730
	11th to 20th	Night	56% 6% 13% 16% 9% 1	2664
	21st to end	Night	59% 7% 10% 17% 6% 1	2249
March	1st to 10th	Night	51% 7% 15% 18% 8%	5241
	11th to 20th	Night	49% 6% 13% 25% 7%	5110
	21st to end	Night	63% 6% 13% 15% 3	5527
April	1st to 10th	Night	53% 7% 16% 18% 6%	2100
	11th to 20th	Night	46% 7% 15% 20% 9% 2	1947
	21st to end	Night	46% 7% 17% 19% 9% 1	1877
May	1st to 10th	Night	33% 4 15% 30% 18% 1	4721
	11th to 20th	Night	35% 7% 21% 28% 8%	4700
	21st to end	Night	40% 4 24% 26% 5%	4984
June	1st to 10th	Night	42% 4 19% 24% 8% 3	1470
	11th to 20th	Night	34% 6% 26% 26% 8%	1470
	21st to end	Night	28% 7% 27% 24% 12% 3	1470
July	1st to 10th	Night	18% 5% 21% 33% 16% 7%	4105
	11th to 20th	Night	12% 3 17% 40% 26% 2	4098
	21st to end	Night	21% 3 28% 31% 16% 1	4858
August	1st to 10th	Night	10% 2 17% 41% 25% 3	3888
	11th to 20th	Night	28% 7% 30% 26% 6% 3	4169
	21st to end	Night	25% 8% 23% 32% 10% 1	4691
September	1st to 10th	Night	21% 6% 22% 29% 17% 5%	2100
	11th to 20th	Night	35% 7% 16% 20% 17% 5%	2068
	21st to end	Night	38% 7% 17% 23% 12% 2	2270
October	1st to 10th	Night	41% 5% 21% 23% 9%	3920
	11th to 20th	Night	51% 5% 16% 21% 7%	3897
	21st to end	Night	53% 7% 14% 17% 9%	4479
November	1st to 10th	Night	64% 6% 11% 14% 5% 1	2870
	11th to 20th	Night	61% 6% 11% 13% 7% 2	2940
	21st to end	Night	62% 5% 12% 13% 7%	2940
December	1st to 10th	Night	57% 7% 12% 19% 5%	4734
	11th to 20th	Night	73% 5% 11% 8% 2	5706
	21st to end	Night	65% 7% 12% 12% 3	7623

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	36% 18% 19% 16% 7% 4	220
	11th to 20th	Day	35% 11% 15% 21% 15% 3	220
	21st to end	Day	30% 15% 21% 21% 10% 4	325
February	1st to 10th	Day	30% 18% 25% 18% 7% 1	430
	11th to 20th	Day	30% 18% 27% 19% 6%	425
	21st to end	Day	31% 22% 26% 15% 5% 1	355
March	1st to 10th	Day	26% 19% 26% 19% 8% 1	539
	11th to 20th	Day	26% 20% 22% 21% 9% 1	540
	21st to end	Day	27% 18% 24% 22% 8% 1	594
April	1st to 10th	Day	25% 16% 27% 19% 11% 3	540
	11th to 20th	Day	21% 15% 29% 19% 12% 3	532
	21st to end	Day	21% 17% 26% 23% 11% 3	540
May	1st to 10th	Day	13% 11% 30% 29% 13% 3	543
	11th to 20th	Day	17% 11% 27% 25% 16% 5%	568
	21st to end	Day	14% 11% 27% 30% 14% 4	682
June	1st to 10th	Day	17% 11% 29% 26% 14% 3	780
	11th to 20th	Day	11% 14% 33% 26% 14% 3	780
	21st to end	Day	11% 7% 26% 33% 20% 3	780
July	1st to 10th	Day	11% 5% 21% 34% 25% 5%	771
	11th to 20th	Day	5% 6% 24% 38% 21% 6%	768
	21st to end	Day	5% 5% 20% 34% 27% 9%	687
August	1st to 10th	Day	7% 5% 17% 30% 30% 10%	567
	11th to 20th	Day	5% 6% 18% 33% 28% 11%	557
	21st to end	Day	10% 7% 23% 31% 19% 9%	627
September	1st to 10th	Day	10% 6% 25% 35% 18% 6%	570
	11th to 20th	Day	17% 9% 21% 29% 19% 5%	543
	21st to end	Day	17% 13% 28% 25% 16% 2	530
October	1st to 10th	Day	20% 8% 27% 24% 18% 3	530
	11th to 20th	Day	26% 12% 21% 24% 14% 3	530
	21st to end	Day	26% 19% 25% 17% 9% 3	468
November	1st to 10th	Day	32% 17% 26% 18% 6% 1	220
	11th to 20th	Day	29% 15% 22% 19% 10% 5%	220
	21st to end	Day	30% 17% 20% 15% 11% 7%	220
December	1st to 10th	Day	30% 14% 20% 12% 17% 6%	220
	11th to 20th	Day	34% 16% 19% 21% 8% 2	220
	21st to end	Day	33% 15% 24% 20% 7% 2	242
January	1st to 10th	Night	33% 15% 20% 18% 10% 3	950
	11th to 20th	Night	29% 10% 16% 26% 17% 2	950
	21st to end	Night	28% 9% 16% 28% 16% 3	938
February	1st to 10th	Night	30% 13% 18% 23% 13% 3	840
	11th to 20th	Night	25% 13% 19% 27% 16% 1	824
	21st to end	Night	25% 14% 21% 25% 12% 3	692
March	1st to 10th	Night	19% 16% 19% 29% 15% 2	840
	11th to 20th	Night	22% 15% 22% 25% 14% 1	840
	21st to end	Night	23% 11% 20% 29% 14% 3	924
April	1st to 10th	Night	21% 9% 20% 28% 19% 3	840
	11th to 20th	Night	17% 11% 21% 30% 18% 3	699
	21st to end	Night	18% 8% 20% 36% 17% 2	617
May	1st to 10th	Night	11% 9% 16% 39% 22% 4	420
	11th to 20th	Night	13% 6% 16% 34% 26% 5%	414
	21st to end	Night	12% 3 20% 39% 23% 3	455
June	1st to 10th	Night	14% 9% 18% 32% 25% 2	420
	11th to 20th	Night	10% 7% 22% 37% 22% 3	420
	21st to end	Night	8% 5% 19% 34% 30% 5%	420
July	1st to 10th	Night	6% 3 12% 35% 40% 5%	410
	11th to 20th	Night	4 3 13% 37% 37% 6%	408
	21st to end	Night	4 2 10% 30% 44% 10%	460
August	1st to 10th	Night	6% 2 7% 28% 44% 14%	501
	11th to 20th	Night	5% 1 7% 26% 49% 13%	624
	21st to end	Night	8% 4 12% 32% 37% 6%	740
September	1st to 10th	Night	6% 3 10% 38% 35% 9%	840
	11th to 20th	Night	15% 4 13% 24% 32% 13%	812
	21st to end	Night	16% 6% 16% 30% 27% 5%	800
October	1st to 10th	Night	17% 5% 14% 33% 26% 5%	800
	11th to 20th	Night	21% 8% 18% 27% 20% 7%	800
	21st to end	Night	27% 9% 19% 28% 14% 4	880
November	1st to 10th	Night	32% 12% 17% 29% 10% 1	820
	11th to 20th	Night	28% 14% 19% 23% 13% 2	840
	21st to end	Night	27% 11% 20% 23% 14% 4	840
December	1st to 10th	Night	26% 12% 16% 26% 17% 2	851
	11th to 20th	Night	32% 13% 20% 26% 8% 1	950
	21st to end	Night	33% 14% 21% 24% 7% 1	1045

Page generated at 2026-09-11 01:11:56. Version ac07

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