Slave Lake
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Slave Lake clear sky chart from 2005-11-06 to 2026-02-16.

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% 3 9% 6% 5% 6% 5% 7% 8% 34%	134569
	Night	7% 10% 4 10% 7% 6% 6% 5% 7% 8% 31%	134944
Transparency	Day	46% 7% 19% 21% 7%	134255
	Night	49% 7% 17% 19% 8% 1	134839
Seeing	Day	14% 15% 31% 25% 12% 3	17960
	Night	13% 13% 31% 28% 13% 2	26459

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	4 13% 5% 13% 9% 6% 8% 6% 7% 9% 20%	3527
	11th to 20th	Day	2 8% 3 11% 8% 5% 6% 5% 8% 8% 36%	3779
	21st to end	Day	3 11% 5% 12% 8% 6% 7% 5% 7% 8% 28%	4803
February	1st to 10th	Day	4 11% 5% 11% 8% 6% 8% 6% 8% 9% 25%	1680
	11th to 20th	Day	3 11% 5% 12% 6% 6% 6% 5% 8% 9% 29%	1651
	21st to end	Day	3 13% 4 13% 8% 8% 6% 7% 7% 7% 25%	1555
March	1st to 10th	Day	3 8% 4 11% 9% 9% 7% 6% 7% 8% 28%	4518
	11th to 20th	Day	3 10% 4 13% 8% 6% 6% 5% 6% 9% 31%	4855
	21st to end	Day	7% 13% 5% 15% 8% 5% 5% 4 6% 8% 24%	5705
April	1st to 10th	Day	7% 12% 5% 9% 7% 5% 6% 4 7% 8% 31%	2480
	11th to 20th	Day	8% 12% 3 9% 6% 5% 6% 4 6% 8% 33%	2570
	21st to end	Day	12% 12% 3 9% 6% 5% 5% 5% 7% 7% 29%	2687
May	1st to 10th	Day	11% 7% 2 6% 5% 4 5% 4 6% 7% 43%	6724
	11th to 20th	Day	11% 11% 3 8% 6% 6% 6% 5% 8% 8% 28%	6734
	21st to end	Day	10% 7% 3 7% 5% 5% 6% 5% 7% 8% 37%	7467
June	1st to 10th	Day	14% 7% 3 7% 6% 6% 6% 4 8% 9% 30%	3200
	11th to 20th	Day	21% 8% 3 6% 6% 5% 6% 5% 7% 8% 26%	3200
	21st to end	Day	10% 7% 3 8% 5% 6% 5% 4 7% 8% 36%	3200
July	1st to 10th	Day	8% 4 2 6% 5% 5% 6% 5% 8% 10% 41%	7240
	11th to 20th	Day	6% 4 2 6% 5% 5% 6% 5% 8% 9% 45%	7199
	21st to end	Day	12% 4 2 5% 4 4 5% 3 6% 9% 46%	7376
August	1st to 10th	Day	13% 6% 2 6% 4 5% 4 4 5% 8% 42%	4706
	11th to 20th	Day	11% 6% 2 6% 4 4 5% 4 6% 9% 42%	4438
	21st to end	Day	13% 5% 2 7% 5% 5% 5% 4 7% 9% 39%	4664
September	1st to 10th	Day	13% 6% 3 7% 4 4 4 4 6% 8% 41%	2521
	11th to 20th	Day	11% 9% 3 6% 4 5% 5% 3 6% 8% 39%	2244
	21st to end	Day	9% 8% 3 7% 6% 4 5% 4 6% 7% 40%	2200
October	1st to 10th	Day	10% 11% 4 9% 5% 5% 6% 5% 7% 8% 30%	3358
	11th to 20th	Day	9% 11% 4 10% 5% 5% 5% 4 7% 9% 31%	2932
	21st to end	Day	7% 16% 6% 11% 5% 5% 5% 4 7% 8% 27%	3146
November	1st to 10th	Day	10% 14% 5% 11% 8% 5% 4 4 5% 7% 27%	1559
	11th to 20th	Day	6% 11% 5% 15% 8% 6% 6% 4 7% 8% 24%	1470
	21st to end	Day	3 11% 5% 13% 8% 5% 5% 4 6% 10% 31%	1470
December	1st to 10th	Day	6% 14% 6% 13% 8% 8% 7% 6% 7% 6% 19%	2331
	11th to 20th	Day	7% 15% 7% 10% 5% 5% 7% 5% 8% 9% 22%	2322
	21st to end	Day	2 7% 4 12% 8% 5% 6% 5% 6% 9% 36%	3058
January	1st to 10th	Night	3 12% 5% 15% 10% 9% 7% 5% 6% 6% 22%	9366
	11th to 20th	Night	2 8% 3 9% 7% 7% 6% 5% 8% 9% 35%	9104
	21st to end	Night	4 10% 5% 12% 8% 7% 7% 5% 7% 9% 27%	9434
February	1st to 10th	Night	4 12% 4 11% 9% 7% 8% 5% 6% 8% 26%	2940
	11th to 20th	Night	5% 10% 4 10% 8% 7% 7% 6% 7% 7% 30%	2744
	21st to end	Night	4 10% 5% 12% 7% 7% 6% 6% 7% 8% 29%	2071
March	1st to 10th	Night	2 7% 4 12% 7% 7% 7% 6% 7% 8% 32%	5376
	11th to 20th	Night	4 10% 4 11% 6% 6% 7% 5% 7% 10% 31%	5143
	21st to end	Night	7% 13% 6% 14% 7% 5% 5% 3 5% 8% 27%	5257
April	1st to 10th	Night	6% 9% 3 8% 6% 6% 6% 6% 7% 9% 35%	1920
	11th to 20th	Night	8% 11% 5% 8% 6% 6% 7% 5% 8% 6% 30%	1781
	21st to end	Night	12% 13% 3 7% 5% 5% 5% 4 6% 7% 30%	1710
May	1st to 10th	Night	13% 9% 3 7% 4 5% 5% 4 6% 7% 38%	3236
	11th to 20th	Night	12% 10% 4 10% 6% 5% 6% 6% 7% 7% 26%	3159
	21st to end	Night	13% 8% 2 7% 5% 4 4 4 6% 10% 36%	3406
June	1st to 10th	Night	12% 7% 3 8% 8% 6% 6% 6% 7% 8% 31%	1200
	11th to 20th	Night	20% 9% 2 8% 4 6% 6% 4 6% 8% 27%	1200
	21st to end	Night	9% 4 2 7% 6% 5% 7% 5% 9% 9% 37%	1200
July	1st to 10th	Night	9% 4 2 7% 6% 6% 6% 5% 8% 9% 39%	2720
	11th to 20th	Night	6% 6% 3 8% 5% 6% 5% 4 6% 9% 43%	2761
	21st to end	Night	11% 4 2 5% 3 4 5% 4 6% 8% 48%	3467
August	1st to 10th	Night	11% 6% 2 7% 4 4 5% 4 6% 6% 45%	2442
	11th to 20th	Night	14% 6% 2 6% 4 4 4 4 6% 7% 41%	2610
	21st to end	Night	15% 6% 2 6% 5% 5% 5% 4 6% 8% 38%	3245
September	1st to 10th	Night	11% 6% 3 6% 4 4 5% 3 7% 8% 43%	1875
	11th to 20th	Night	10% 7% 2 7% 4 4 4 4 7% 8% 43%	2156
	21st to end	Night	10% 8% 2 7% 5% 5% 5% 4 6% 8% 41%	2200
October	1st to 10th	Night	9% 12% 3 10% 5% 5% 5% 4 7% 8% 33%	3833
	11th to 20th	Night	6% 8% 3 9% 5% 5% 5% 5% 8% 10% 35%	4216
	21st to end	Night	9% 11% 6% 11% 7% 6% 6% 5% 7% 7% 26%	4622
November	1st to 10th	Night	9% 15% 6% 12% 7% 6% 4 3 5% 6% 26%	2955
	11th to 20th	Night	7% 13% 4 14% 9% 6% 6% 4 6% 7% 25%	3150
	21st to end	Night	5% 12% 4 13% 8% 6% 7% 4 7% 8% 25%	3150
December	1st to 10th	Night	6% 15% 6% 12% 8% 8% 6% 5% 6% 7% 20%	5065
	11th to 20th	Night	7% 14% 5% 13% 7% 7% 8% 6% 7% 7% 19%	6090
	21st to end	Night	2 6% 4 13% 7% 6% 7% 5% 8% 9% 34%	8140

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% 5% 14% 13% 10%	3527
	11th to 20th	Day	43% 5% 21% 14% 15% 2	3798
	21st to end	Day	51% 8% 23% 12% 5%	4804
February	1st to 10th	Day	51% 9% 16% 15% 9%	1680
	11th to 20th	Day	48% 9% 16% 18% 8%	1659
	21st to end	Day	55% 8% 16% 14% 8%	1555
March	1st to 10th	Day	50% 7% 16% 20% 7%	4518
	11th to 20th	Day	49% 7% 18% 21% 6%	4856
	21st to end	Day	54% 10% 11% 17% 8%	5705
April	1st to 10th	Day	48% 10% 14% 18% 9%	2480
	11th to 20th	Day	47% 10% 14% 19% 10% 1	2570
	21st to end	Day	51% 9% 14% 19% 7% 1	2687
May	1st to 10th	Day	40% 6% 11% 27% 14% 1	6724
	11th to 20th	Day	50% 6% 14% 24% 5%	6738
	21st to end	Day	40% 8% 23% 21% 7% 1	7468
June	1st to 10th	Day	47% 10% 15% 21% 7%	3200
	11th to 20th	Day	54% 11% 14% 16% 5%	3200
	21st to end	Day	43% 11% 19% 20% 7%	3200
July	1st to 10th	Day	37% 6% 27% 23% 8% 1	7161
	11th to 20th	Day	34% 7% 16% 35% 9%	7112
	21st to end	Day	36% 5% 26% 28% 5%	7169
August	1st to 10th	Day	41% 8% 20% 27% 2	4688
	11th to 20th	Day	38% 7% 22% 25% 8%	4480
	21st to end	Day	42% 6% 28% 20% 4	4663
September	1st to 10th	Day	40% 9% 20% 25% 4 1	2521
	11th to 20th	Day	41% 8% 24% 21% 6%	2244
	21st to end	Day	42% 9% 18% 22% 8%	2200
October	1st to 10th	Day	50% 6% 21% 19% 4	3358
	11th to 20th	Day	49% 5% 20% 22% 4	2932
	21st to end	Day	55% 6% 20% 16% 3	3146
November	1st to 10th	Day	57% 6% 17% 15% 5% 1	1559
	11th to 20th	Day	58% 9% 16% 12% 5%	1470
	21st to end	Day	51% 8% 19% 17% 4 1	1470
December	1st to 10th	Day	64% 7% 15% 10% 4	2331
	11th to 20th	Day	56% 7% 21% 14% 3	2324
	21st to end	Day	45% 7% 13% 25% 10% 1	3058
January	1st to 10th	Night	61% 5% 12% 14% 7% 1	9367
	11th to 20th	Night	42% 6% 17% 17% 17% 1	9147
	21st to end	Night	51% 9% 22% 13% 4	9434
February	1st to 10th	Night	53% 8% 14% 14% 9% 1	2940
	11th to 20th	Night	49% 9% 15% 17% 8% 1	2746
	21st to end	Night	49% 9% 15% 18% 8%	2071
March	1st to 10th	Night	45% 8% 17% 20% 10%	5376
	11th to 20th	Night	46% 6% 20% 21% 6%	5143
	21st to end	Night	54% 8% 10% 18% 9%	5257
April	1st to 10th	Night	42% 8% 15% 21% 13% 1	1920
	11th to 20th	Night	49% 9% 10% 19% 10% 2	1781
	21st to end	Night	51% 7% 11% 20% 10% 1	1710
May	1st to 10th	Night	43% 5% 13% 23% 13% 2	3235
	11th to 20th	Night	52% 6% 13% 20% 8% 1	3159
	21st to end	Night	42% 7% 22% 22% 7%	3406
June	1st to 10th	Night	47% 10% 14% 18% 10%	1200
	11th to 20th	Night	54% 7% 17% 16% 6%	1200
	21st to end	Night	39% 11% 19% 23% 8%	1200
July	1st to 10th	Night	40% 7% 21% 22% 10%	2690
	11th to 20th	Night	38% 5% 15% 31% 11%	2725
	21st to end	Night	35% 5% 26% 31% 4	3369
August	1st to 10th	Night	38% 7% 21% 32% 2	2435
	11th to 20th	Night	41% 7% 19% 24% 8%	2634
	21st to end	Night	44% 5% 26% 22% 4	3245
September	1st to 10th	Night	36% 10% 20% 26% 6% 1	1875
	11th to 20th	Night	38% 7% 21% 26% 7%	2156
	21st to end	Night	40% 9% 16% 25% 9% 1	2200
October	1st to 10th	Night	48% 5% 18% 21% 7%	3833
	11th to 20th	Night	43% 6% 21% 25% 5%	4216
	21st to end	Night	55% 6% 21% 15% 3	4622
November	1st to 10th	Night	59% 5% 14% 18% 5%	2955
	11th to 20th	Night	59% 7% 14% 15% 5%	3150
	21st to end	Night	55% 9% 17% 13% 5% 1	3150
December	1st to 10th	Night	62% 6% 14% 12% 5% 1	5065
	11th to 20th	Night	63% 6% 16% 11% 4	6087
	21st to end	Night	46% 6% 15% 23% 9% 1	8140

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	9% 16% 34% 27% 10% 4	220
	11th to 20th	Day	16% 20% 33% 22% 8% 1	220
	21st to end	Day	16% 25% 33% 19% 7% 1	242
February	1st to 10th	Day	12% 23% 35% 20% 8% 1	220
	11th to 20th	Day	8% 19% 44% 22% 6% 1	236
	21st to end	Day	9% 23% 31% 28% 4 3	319
March	1st to 10th	Day	7% 23% 34% 29% 7%	500
	11th to 20th	Day	8% 18% 40% 22% 10% 2	500
	21st to end	Day	16% 18% 33% 25% 7% 1	550
April	1st to 10th	Day	13% 18% 37% 23% 7% 2	500
	11th to 20th	Day	15% 20% 29% 24% 9% 3	492
	21st to end	Day	16% 18% 36% 21% 8% 1	500
May	1st to 10th	Day	14% 16% 29% 26% 11% 3	673
	11th to 20th	Day	12% 13% 31% 27% 13% 3	725
	21st to end	Day	15% 11% 27% 29% 14% 4	812
June	1st to 10th	Day	17% 14% 33% 23% 11% 2	930
	11th to 20th	Day	23% 10% 24% 25% 14% 4	930
	21st to end	Day	12% 8% 26% 30% 20% 4	930
July	1st to 10th	Day	13% 7% 27% 32% 17% 4	916
	11th to 20th	Day	12% 12% 26% 28% 16% 5%	897
	21st to end	Day	13% 6% 28% 29% 17% 7%	794
August	1st to 10th	Day	17% 7% 26% 25% 19% 6%	627
	11th to 20th	Day	13% 16% 34% 24% 9% 3	517
	21st to end	Day	13% 12% 29% 27% 15% 3	583
September	1st to 10th	Day	17% 16% 29% 23% 12% 3	530
	11th to 20th	Day	17% 14% 32% 26% 9% 2	530
	21st to end	Day	16% 21% 34% 18% 10% 1	530
October	1st to 10th	Day	18% 19% 30% 23% 10% 1	439
	11th to 20th	Day	15% 17% 38% 22% 6% 2	330
	21st to end	Day	23% 21% 29% 20% 6% 1	363
November	1st to 10th	Day	17% 20% 36% 21% 5% 1	283
	11th to 20th	Day	15% 20% 38% 17% 9% 1	220
	21st to end	Day	9% 23% 37% 22% 7% 2	220
December	1st to 10th	Day	13% 19% 41% 19% 6% 3	220
	11th to 20th	Day	13% 23% 37% 22% 4 1	220
	21st to end	Day	7% 16% 31% 32% 10% 4	242
January	1st to 10th	Night	10% 18% 35% 28% 8% 1	1160
	11th to 20th	Night	12% 17% 36% 26% 7% 1	1111
	21st to end	Night	16% 19% 34% 23% 6% 1	1081
February	1st to 10th	Night	12% 13% 32% 27% 14% 1	840
	11th to 20th	Night	12% 13% 37% 26% 11% 1	804
	21st to end	Night	7% 11% 35% 32% 14% 2	660
March	1st to 10th	Night	8% 14% 34% 29% 13% 2	800
	11th to 20th	Night	10% 13% 33% 33% 10% 2	800
	21st to end	Night	12% 12% 30% 32% 13% 1	880
April	1st to 10th	Night	10% 10% 31% 33% 14% 2	720
	11th to 20th	Night	13% 12% 31% 30% 12% 3	593
	21st to end	Night	18% 11% 27% 32% 11% 1	510
May	1st to 10th	Night	13% 9% 26% 33% 17% 2	400
	11th to 20th	Night	13% 11% 30% 29% 16% 2	394
	21st to end	Night	17% 8% 24% 32% 18% 2	433
June	1st to 10th	Night	14% 8% 31% 28% 17% 2	400
	11th to 20th	Night	23% 7% 21% 22% 25% 3	400
	21st to end	Night	10% 5% 21% 36% 25% 4	400
July	1st to 10th	Night	14% 5% 23% 36% 22% 1	390
	11th to 20th	Night	11% 7% 22% 34% 21% 4	388
	21st to end	Night	13% 5% 21% 30% 27% 4	438
August	1st to 10th	Night	11% 5% 17% 34% 25% 8%	398
	11th to 20th	Night	10% 7% 24% 33% 23% 2	416
	21st to end	Night	17% 7% 22% 32% 20% 2	660
September	1st to 10th	Night	15% 8% 24% 30% 21% 2	675
	11th to 20th	Night	12% 10% 22% 37% 17% 2	800
	21st to end	Night	13% 16% 30% 26% 14% 1	800
October	1st to 10th	Night	18% 12% 29% 30% 10% 1	800
	11th to 20th	Night	14% 14% 31% 30% 10% 1	800
	21st to end	Night	17% 19% 31% 25% 8%	890
November	1st to 10th	Night	18% 15% 35% 24% 8% 1	1025
	11th to 20th	Night	15% 18% 34% 23% 8% 2	1050
	21st to end	Night	11% 20% 38% 22% 7% 2	1050
December	1st to 10th	Night	13% 18% 35% 23% 9% 2	1057
	11th to 20th	Night	13% 19% 34% 27% 6% 1	1160
	21st to end	Night	8% 15% 34% 29% 11% 3	1276

Page generated at 2026-02-12 23:42:16. Version ac07

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