Sproat Lake (east)
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Sproat Lake (east) 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	16% 10% 3 8% 5% 5% 5% 4 5% 6% 31%	153716
	Night	20% 13% 4 9% 5% 4 5% 4 5% 6% 27%	146117
Transparency	Day	51% 9% 15% 17% 7% 1	153414
	Night	55% 9% 14% 15% 6% 1	146001
Seeing	Day	24% 12% 23% 24% 13% 4	18150
	Night	27% 8% 19% 27% 17% 2	26020

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	29% 19% 7% 12% 5% 5% 5% 3 4 4 7%	4125
	11th to 20th	Day	20% 8% 3 9% 7% 5% 5% 4 6% 7% 24%	4119
	21st to end	Day	34% 15% 4 7% 4 4 3 3 3 4 20%	5081
February	1st to 10th	Day	27% 17% 5% 11% 6% 5% 5% 4 4 5% 11%	1890
	11th to 20th	Day	25% 16% 4 10% 5% 5% 4 3 4 5% 18%	1845
	21st to end	Day	22% 18% 4 10% 6% 5% 4 4 5% 4 16%	1557
March	1st to 10th	Day	19% 20% 5% 12% 7% 6% 5% 3 4 5% 14%	4851
	11th to 20th	Day	17% 16% 5% 11% 7% 6% 6% 4 5% 6% 18%	5110
	21st to end	Day	24% 18% 5% 12% 7% 6% 5% 4 5% 5% 11%	5607
April	1st to 10th	Day	25% 16% 4 10% 6% 6% 5% 4 4 5% 17%	2514
	11th to 20th	Day	20% 15% 4 11% 6% 5% 5% 4 4 5% 21%	2698
	21st to end	Day	22% 13% 4 9% 5% 5% 6% 4 4 5% 23%	2730
May	1st to 10th	Day	10% 7% 3 8% 6% 6% 6% 4 6% 7% 38%	8115
	11th to 20th	Day	11% 9% 3 9% 7% 6% 6% 5% 8% 10% 27%	8160
	21st to end	Day	12% 9% 3 9% 6% 6% 6% 5% 6% 7% 30%	9586
June	1st to 10th	Day	21% 10% 3 9% 6% 5% 6% 3 6% 6% 25%	3150
	11th to 20th	Day	16% 10% 4 10% 7% 5% 6% 6% 6% 7% 24%	3150
	21st to end	Day	15% 9% 3 9% 5% 5% 5% 4 6% 6% 31%	3150
July	1st to 10th	Day	6% 4 2 5% 5% 5% 4 3 5% 7% 53%	8820
	11th to 20th	Day	4 3 1 3 3 3 3 3 5% 7% 65%	8820
	21st to end	Day	5% 4 2 6% 4 4 5% 4 6% 7% 53%	9452
August	1st to 10th	Day	4 3 1 3 2 2 4 3 5% 8% 64%	6134
	11th to 20th	Day	6% 5% 2 7% 4 5% 6% 5% 7% 9% 43%	5923
	21st to end	Day	9% 6% 3 7% 5% 5% 5% 4 7% 9% 38%	6569
September	1st to 10th	Day	11% 7% 2 7% 4 4 4 4 6% 7% 43%	2536
	11th to 20th	Day	17% 11% 3 7% 4 5% 4 3 4 5% 36%	2327
	21st to end	Day	21% 8% 3 8% 6% 5% 4 3 4 6% 32%	2176
October	1st to 10th	Day	18% 12% 4 10% 6% 5% 6% 5% 6% 7% 22%	3280
	11th to 20th	Day	28% 8% 4 9% 6% 5% 5% 3 5% 6% 22%	3261
	21st to end	Day	25% 12% 4 11% 6% 6% 6% 4 6% 6% 16%	3269
November	1st to 10th	Day	34% 16% 6% 11% 6% 4 4 2 3 3 11%	1648
	11th to 20th	Day	33% 16% 4 8% 5% 4 4 3 4 5% 14%	1680
	21st to end	Day	33% 14% 5% 10% 6% 5% 4 3 4 4 13%	1680
December	1st to 10th	Day	21% 16% 7% 12% 6% 5% 6% 4 4 4 15%	2413
	11th to 20th	Day	38% 20% 6% 11% 5% 4 3 2 3 3 6%	2715
	21st to end	Day	35% 19% 5% 10% 7% 5% 4 3 3 3 7%	3575
January	1st to 10th	Night	29% 22% 6% 12% 6% 5% 4 3 3 3 6%	8770
	11th to 20th	Night	21% 11% 4 8% 5% 5% 4 4 5% 6% 27%	8779
	21st to end	Night	32% 16% 4 6% 3 3 4 3 5% 4 21%	9114
February	1st to 10th	Night	28% 18% 4 8% 5% 4 4 3 4 5% 17%	2730
	11th to 20th	Night	23% 16% 4 9% 5% 4 4 3 4 6% 23%	2662
	21st to end	Night	22% 17% 3 9% 5% 5% 4 3 5% 5% 22%	2249
March	1st to 10th	Night	18% 21% 5% 9% 5% 4 5% 4 5% 5% 18%	5255
	11th to 20th	Night	13% 15% 5% 9% 6% 6% 6% 5% 6% 6% 22%	5109
	21st to end	Night	24% 20% 5% 11% 6% 5% 5% 3 4 4 12%	5610
April	1st to 10th	Night	22% 12% 4 9% 5% 5% 4 4 6% 6% 23%	2102
	11th to 20th	Night	17% 12% 4 9% 6% 6% 4 3 6% 6% 27%	1871
	21st to end	Night	22% 11% 3 8% 5% 5% 5% 3 6% 6% 25%	1890
May	1st to 10th	Night	9% 8% 3 6% 4 4 5% 3 5% 7% 46%	4808
	11th to 20th	Night	10% 7% 3 9% 6% 5% 6% 6% 7% 9% 31%	4700
	21st to end	Night	13% 10% 3 8% 6% 5% 5% 4 6% 7% 32%	4582
June	1st to 10th	Night	17% 10% 3 8% 5% 5% 5% 3 4 8% 32%	1470
	11th to 20th	Night	12% 8% 3 9% 6% 8% 6% 4 5% 7% 31%	1470
	21st to end	Night	9% 7% 2 7% 6% 6% 5% 5% 8% 9% 37%	1470
July	1st to 10th	Night	5% 3 1 5% 4 4 5% 4 7% 9% 53%	4140
	11th to 20th	Night	3 3 1 2 3 2 2 3 5% 7% 70%	4140
	21st to end	Night	5% 4 1 5% 3 3 5% 4 6% 8% 55%	4614
August	1st to 10th	Night	3 2 1 3 2 2 2 2 4 8% 70%	4013
	11th to 20th	Night	7% 4 2 5% 4 5% 5% 4 6% 9% 48%	4142
	21st to end	Night	9% 5% 2 6% 4 5% 5% 5% 7% 8% 42%	4579
September	1st to 10th	Night	9% 5% 2 5% 4 3 4 4 5% 8% 50%	2084
	11th to 20th	Night	16% 9% 3 8% 5% 4 5% 4 5% 6% 36%	2138
	21st to end	Night	21% 10% 3 7% 4 4 3 3 4 5% 36%	2223
October	1st to 10th	Night	16% 11% 4 11% 6% 6% 6% 3 4 6% 28%	3920
	11th to 20th	Night	25% 11% 4 10% 5% 4 5% 4 5% 5% 22%	3897
	21st to end	Night	26% 12% 4 11% 6% 5% 5% 3 4 4 19%	4531
November	1st to 10th	Night	33% 13% 4 10% 5% 4 4 3 4 5% 13%	2870
	11th to 20th	Night	32% 15% 5% 9% 5% 5% 5% 3 4 4 14%	2940
	21st to end	Night	31% 12% 4 11% 6% 4 5% 4 4 5% 14%	2940
December	1st to 10th	Night	20% 16% 8% 15% 6% 5% 6% 3 4 4 13%	4972
	11th to 20th	Night	37% 21% 6% 11% 4 3 4 2 3 3 6%	5710
	21st to end	Night	36% 19% 5% 11% 5% 4 4 3 3 3 8%	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	77% 9% 6% 6% 2	4125
	11th to 20th	Day	49% 15% 11% 19% 5% 1	4140
	21st to end	Day	69% 5% 7% 15% 4	5082
February	1st to 10th	Day	72% 8% 7% 7% 5% 1	1890
	11th to 20th	Day	68% 6% 9% 12% 5%	1854
	21st to end	Day	67% 8% 9% 10% 4 1	1557
March	1st to 10th	Day	68% 8% 10% 10% 4	4851
	11th to 20th	Day	63% 8% 9% 15% 4	5110
	21st to end	Day	73% 8% 11% 6% 1	5607
April	1st to 10th	Day	70% 8% 10% 10% 3	2514
	11th to 20th	Day	64% 6% 10% 11% 7% 1	2698
	21st to end	Day	63% 5% 12% 14% 5%	2730
May	1st to 10th	Day	43% 8% 17% 23% 8%	8115
	11th to 20th	Day	47% 13% 23% 13% 3	8163
	21st to end	Day	51% 9% 17% 15% 8% 1	9587
June	1st to 10th	Day	60% 8% 13% 11% 7% 1	3150
	11th to 20th	Day	57% 10% 16% 13% 4	3150
	21st to end	Day	50% 9% 16% 18% 7%	3150
July	1st to 10th	Day	29% 10% 19% 24% 13% 5%	8746
	11th to 20th	Day	19% 6% 20% 36% 19% 1	8739
	21st to end	Day	28% 9% 26% 24% 12% 2	9246
August	1st to 10th	Day	18% 6% 15% 39% 20% 2	6117
	11th to 20th	Day	31% 13% 25% 24% 6% 1	5962
	21st to end	Day	39% 12% 23% 20% 6% 1	6568
September	1st to 10th	Day	38% 8% 17% 25% 10% 2	2536
	11th to 20th	Day	49% 7% 13% 17% 14% 1	2327
	21st to end	Day	53% 7% 13% 16% 10% 1	2176
October	1st to 10th	Day	57% 10% 15% 12% 5%	3280
	11th to 20th	Day	63% 7% 10% 16% 4	3261
	21st to end	Day	64% 10% 10% 11% 4	3269
November	1st to 10th	Day	79% 5% 7% 5% 4 1	1648
	11th to 20th	Day	72% 7% 8% 7% 5% 1	1680
	21st to end	Day	72% 9% 8% 6% 5% 1	1680
December	1st to 10th	Day	69% 9% 9% 10% 4	2413
	11th to 20th	Day	81% 9% 6% 3 1 1	2718
	21st to end	Day	80% 8% 5% 4 1	3575
January	1st to 10th	Night	78% 9% 7% 4 2	8770
	11th to 20th	Night	51% 12% 10% 19% 7% 1	8820
	21st to end	Night	65% 7% 9% 15% 4	9114
February	1st to 10th	Night	69% 7% 8% 10% 5% 1	2730
	11th to 20th	Night	63% 6% 11% 12% 7% 1	2664
	21st to end	Night	64% 7% 9% 14% 4 1	2249
March	1st to 10th	Night	62% 8% 13% 12% 4	5255
	11th to 20th	Night	57% 10% 13% 16% 3	5110
	21st to end	Night	72% 9% 9% 8% 2	5610
April	1st to 10th	Night	61% 8% 13% 13% 4	2102
	11th to 20th	Night	56% 8% 13% 14% 8% 1	1871
	21st to end	Night	58% 8% 12% 15% 6% 1	1890
May	1st to 10th	Night	37% 7% 19% 28% 8%	4805
	11th to 20th	Night	45% 15% 22% 15% 4	4700
	21st to end	Night	49% 9% 20% 20% 3	4586
June	1st to 10th	Night	51% 6% 23% 12% 7% 2	1470
	11th to 20th	Night	51% 8% 21% 16% 5%	1470
	21st to end	Night	42% 11% 22% 18% 6% 1	1470
July	1st to 10th	Night	25% 11% 23% 24% 13% 4	4105
	11th to 20th	Night	14% 6% 24% 39% 16%	4098
	21st to end	Night	25% 9% 32% 23% 10% 1	4515
August	1st to 10th	Night	15% 6% 19% 38% 21% 1	4005
	11th to 20th	Night	29% 12% 31% 21% 6% 1	4169
	21st to end	Night	36% 15% 23% 21% 5% 1	4579
September	1st to 10th	Night	31% 8% 19% 27% 12% 2	2084
	11th to 20th	Night	49% 7% 11% 17% 13% 2	2138
	21st to end	Night	50% 7% 15% 17% 9% 2	2223
October	1st to 10th	Night	55% 10% 15% 13% 7%	3920
	11th to 20th	Night	62% 7% 12% 15% 4	3897
	21st to end	Night	63% 11% 11% 10% 4 1	4531
November	1st to 10th	Night	74% 6% 9% 7% 3 1	2870
	11th to 20th	Night	73% 6% 10% 6% 3 2	2940
	21st to end	Night	70% 8% 8% 8% 5% 1	2940
December	1st to 10th	Night	69% 10% 9% 9% 2	4972
	11th to 20th	Night	82% 7% 5% 4 2	5706
	21st to end	Night	80% 7% 7% 5% 2	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	40% 11% 19% 20% 6% 3	220
	11th to 20th	Day	37% 11% 13% 21% 13% 5%	220
	21st to end	Day	36% 11% 18% 19% 12% 4	365
February	1st to 10th	Day	35% 13% 26% 18% 7% 1	430
	11th to 20th	Day	32% 11% 24% 24% 7% 1	425
	21st to end	Day	35% 21% 23% 15% 5% 1	355
March	1st to 10th	Day	31% 15% 20% 21% 11% 2	507
	11th to 20th	Day	33% 17% 19% 19% 9% 3	540
	21st to end	Day	33% 16% 24% 18% 6% 3	594
April	1st to 10th	Day	35% 14% 23% 18% 8% 2	540
	11th to 20th	Day	26% 16% 26% 17% 12% 4	532
	21st to end	Day	30% 15% 24% 18% 10% 4	540
May	1st to 10th	Day	18% 14% 24% 30% 11% 3	543
	11th to 20th	Day	20% 11% 26% 26% 14% 4	568
	21st to end	Day	19% 11% 29% 27% 11% 3	820
June	1st to 10th	Day	25% 14% 27% 22% 8% 3	780
	11th to 20th	Day	17% 18% 32% 21% 9% 3	780
	21st to end	Day	17% 12% 25% 28% 14% 4	780
July	1st to 10th	Day	13% 9% 25% 34% 15% 3	771
	11th to 20th	Day	9% 10% 26% 32% 18% 5%	768
	21st to end	Day	8% 10% 24% 32% 20% 6%	810
August	1st to 10th	Day	11% 6% 18% 30% 25% 10%	567
	11th to 20th	Day	8% 7% 20% 31% 23% 11%	557
	21st to end	Day	14% 8% 25% 30% 15% 10%	627
September	1st to 10th	Day	16% 7% 23% 30% 16% 8%	570
	11th to 20th	Day	22% 9% 23% 22% 18% 5%	543
	21st to end	Day	25% 12% 25% 20% 15% 3	530
October	1st to 10th	Day	25% 12% 21% 23% 13% 7%	530
	11th to 20th	Day	33% 9% 21% 21% 11% 5%	530
	21st to end	Day	34% 14% 24% 16% 9% 4	466
November	1st to 10th	Day	39% 16% 14% 20% 10% 2	220
	11th to 20th	Day	39% 11% 15% 17% 14% 4	220
	21st to end	Day	40% 13% 15% 16% 8% 8%	220
December	1st to 10th	Day	32% 13% 20% 17% 12% 6%	220
	11th to 20th	Day	43% 11% 16% 16% 13% 1	220
	21st to end	Day	44% 12% 17% 18% 9%	242
January	1st to 10th	Night	44% 11% 19% 17% 8% 1	950
	11th to 20th	Night	37% 7% 17% 24% 14% 1	950
	21st to end	Night	37% 7% 19% 22% 12% 2	1009
February	1st to 10th	Night	36% 8% 21% 27% 8% 1	840
	11th to 20th	Night	31% 9% 19% 29% 12% 1	824
	21st to end	Night	30% 10% 25% 24% 10% 2	692
March	1st to 10th	Night	27% 9% 25% 22% 14% 2	840
	11th to 20th	Night	27% 14% 23% 22% 12% 1	840
	21st to end	Night	32% 10% 20% 26% 10% 2	924
April	1st to 10th	Night	27% 8% 21% 27% 15% 2	830
	11th to 20th	Night	19% 8% 26% 34% 11% 2	623
	21st to end	Night	24% 6% 23% 32% 13% 2	630
May	1st to 10th	Night	15% 6% 17% 38% 24%	452
	11th to 20th	Night	16% 5% 19% 35% 22% 3	414
	21st to end	Night	17% 4 17% 39% 21% 1	455
June	1st to 10th	Night	18% 8% 21% 32% 20%	420
	11th to 20th	Night	15% 5% 24% 37% 19% 1	420
	21st to end	Night	12% 5% 22% 37% 20% 3	420
July	1st to 10th	Night	10% 3 15% 39% 32% 2	410
	11th to 20th	Night	7% 4 17% 37% 33% 2	408
	21st to end	Night	6% 5% 13% 35% 38% 4	460
August	1st to 10th	Night	9% 3 9% 31% 39% 9%	553
	11th to 20th	Night	6% 4 9% 34% 38% 10%	624
	21st to end	Night	13% 3 16% 29% 32% 6%	693
September	1st to 10th	Night	12% 4 15% 35% 28% 6%	824
	11th to 20th	Night	18% 6% 15% 25% 28% 8%	812
	21st to end	Night	24% 7% 20% 26% 19% 4	800
October	1st to 10th	Night	24% 6% 16% 28% 23% 3	800
	11th to 20th	Night	31% 7% 18% 24% 16% 4	800
	21st to end	Night	34% 7% 21% 25% 10% 2	880
November	1st to 10th	Night	38% 10% 20% 25% 7%	820
	11th to 20th	Night	36% 11% 23% 19% 10% 2	840
	21st to end	Night	36% 7% 21% 22% 11% 3	840
December	1st to 10th	Night	31% 10% 19% 23% 15% 2	928
	11th to 20th	Night	42% 8% 22% 21% 6% 1	950
	21st to end	Night	40% 10% 24% 21% 6%	1045

Page generated at 2026-09-11 01:11:57. Version ac07

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