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 2024-11-30.

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	10% 9% 3 9% 6% 5% 5% 4 7% 8% 34%	106088
	Night	8% 10% 4 10% 6% 6% 6% 5% 7% 8% 32%	102051
Transparency	Day	46% 9% 18% 21% 7%	105818
	Night	47% 8% 17% 20% 7% 1	101967
Seeing	Day	14% 15% 31% 25% 12% 3	16444
	Night	13% 13% 31% 29% 13% 2	24739

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	3 10% 5% 16% 13% 8% 9% 6% 6% 8% 15%	1899
	11th to 20th	Day	3 9% 4 10% 8% 6% 7% 7% 11% 8% 27%	2013
	21st to end	Day	4 11% 4 10% 6% 6% 6% 5% 6% 10% 32%	2576
February	1st to 10th	Day	4 11% 5% 11% 8% 6% 8% 6% 8% 9% 23%	1520
	11th to 20th	Day	3 11% 6% 13% 7% 6% 6% 5% 8% 8% 28%	1528
	21st to end	Day	3 13% 4 13% 8% 8% 6% 6% 7% 6% 25%	1479
March	1st to 10th	Day	3 8% 5% 11% 8% 8% 7% 6% 8% 8% 29%	3138
	11th to 20th	Day	3 8% 4 11% 6% 5% 5% 5% 6% 10% 36%	3384
	21st to end	Day	6% 13% 5% 17% 8% 5% 5% 4 6% 7% 22%	3979
April	1st to 10th	Day	7% 12% 5% 9% 7% 5% 6% 4 7% 9% 29%	2356
	11th to 20th	Day	8% 12% 3 9% 7% 5% 6% 4 6% 8% 32%	2440
	21st to end	Day	12% 12% 4 9% 6% 5% 5% 5% 6% 7% 29%	2552
May	1st to 10th	Day	15% 9% 2 6% 5% 4 5% 4 5% 7% 38%	4698
	11th to 20th	Day	13% 11% 3 7% 5% 5% 5% 4 7% 8% 32%	4694
	21st to end	Day	12% 9% 3 8% 6% 6% 7% 6% 8% 8% 26%	5209
June	1st to 10th	Day	15% 7% 3 7% 6% 6% 6% 4 7% 9% 30%	3040
	11th to 20th	Day	20% 9% 3 6% 6% 5% 6% 5% 7% 9% 26%	3040
	21st to end	Day	11% 7% 3 8% 5% 6% 5% 4 7% 8% 36%	3040
July	1st to 10th	Day	9% 5% 2 5% 4 5% 5% 5% 7% 8% 45%	5060
	11th to 20th	Day	8% 5% 2 6% 4 4 5% 4 7% 9% 47%	5033
	21st to end	Day	15% 5% 2 5% 4 4 5% 3 6% 8% 44%	5138
August	1st to 10th	Day	13% 6% 2 6% 4 5% 4 4 5% 8% 43%	4561
	11th to 20th	Day	11% 7% 2 6% 4 4 5% 4 6% 9% 43%	4299
	21st to end	Day	13% 5% 2 7% 5% 6% 5% 4 7% 9% 38%	4521
September	1st to 10th	Day	14% 6% 3 7% 4 4 4 4 6% 8% 40%	2395
	11th to 20th	Day	12% 9% 3 7% 4 5% 5% 4 6% 9% 37%	2132
	21st to end	Day	9% 8% 3 7% 6% 4 5% 4 6% 8% 40%	2090
October	1st to 10th	Day	10% 11% 4 9% 5% 5% 5% 5% 7% 8% 30%	3256
	11th to 20th	Day	9% 11% 4 9% 5% 5% 5% 4 7% 9% 31%	2842
	21st to end	Day	7% 16% 6% 11% 5% 5% 5% 4 7% 8% 26%	3049
November	1st to 10th	Day	11% 13% 5% 11% 8% 5% 5% 4 5% 7% 26%	1483
	11th to 20th	Day	6% 12% 6% 16% 8% 6% 6% 4 7% 7% 24%	1400
	21st to end	Day	3 11% 5% 13% 8% 5% 6% 4 6% 10% 30%	1396
December	1st to 10th	Day	5% 11% 5% 11% 9% 8% 7% 5% 5% 7% 28%	1316
	11th to 20th	Day	7% 11% 5% 10% 6% 7% 7% 6% 8% 9% 25%	1442
	21st to end	Day	2 6% 3 9% 6% 4 5% 5% 6% 10% 43%	2090
January	1st to 10th	Night	3 10% 5% 16% 11% 9% 7% 5% 7% 7% 21%	5046
	11th to 20th	Night	3 10% 3 9% 7% 8% 8% 6% 10% 8% 27%	4884
	21st to end	Night	6% 10% 5% 10% 7% 6% 7% 5% 7% 11% 27%	5109
February	1st to 10th	Night	4 13% 4 12% 9% 7% 7% 5% 6% 8% 25%	2660
	11th to 20th	Night	5% 11% 4 10% 8% 7% 6% 6% 6% 7% 29%	2537
	21st to end	Night	4 10% 5% 12% 6% 6% 6% 6% 7% 8% 29%	1971
March	1st to 10th	Night	2 8% 5% 13% 7% 8% 8% 6% 7% 8% 28%	3736
	11th to 20th	Night	4 10% 4 8% 6% 5% 6% 4 7% 10% 37%	3594
	21st to end	Night	6% 13% 5% 15% 7% 6% 5% 3 5% 7% 27%	3661
April	1st to 10th	Night	6% 9% 3 8% 6% 6% 6% 6% 7% 10% 34%	1824
	11th to 20th	Night	8% 11% 5% 8% 6% 6% 7% 5% 8% 6% 30%	1691
	21st to end	Night	13% 13% 3 8% 6% 5% 5% 4 6% 7% 30%	1625
May	1st to 10th	Night	15% 10% 3 8% 5% 5% 5% 3 6% 6% 34%	2262
	11th to 20th	Night	13% 10% 4 9% 5% 5% 5% 5% 6% 7% 31%	2199
	21st to end	Night	14% 9% 3 8% 6% 5% 5% 5% 7% 10% 29%	2364
June	1st to 10th	Night	12% 6% 3 8% 8% 6% 6% 6% 7% 8% 30%	1140
	11th to 20th	Night	19% 9% 2 8% 4 6% 6% 4 5% 7% 27%	1140
	21st to end	Night	9% 4 2 7% 6% 6% 7% 5% 8% 9% 38%	1140
July	1st to 10th	Night	8% 4 2 6% 5% 5% 5% 4 7% 9% 45%	1900
	11th to 20th	Night	7% 6% 2 6% 5% 5% 5% 3 6% 10% 45%	1927
	21st to end	Night	14% 5% 2 4 3 3 4 4 5% 9% 46%	2414
August	1st to 10th	Night	11% 6% 2 7% 4 4 4 4 5% 6% 46%	2367
	11th to 20th	Night	14% 6% 2 6% 4 4 4 4 6% 7% 42%	2529
	21st to end	Night	16% 6% 3 6% 5% 5% 4 4 6% 7% 38%	3146
September	1st to 10th	Night	12% 6% 3 6% 4 4 5% 3 7% 8% 43%	1781
	11th to 20th	Night	11% 8% 2 8% 4 4 4 4 7% 8% 41%	2048
	21st to end	Night	9% 8% 2 6% 5% 5% 5% 4 6% 8% 41%	2090
October	1st to 10th	Night	9% 13% 3 10% 5% 5% 5% 4 7% 8% 33%	3715
	11th to 20th	Night	6% 8% 3 9% 5% 5% 5% 5% 8% 10% 35%	4086
	21st to end	Night	9% 11% 6% 11% 7% 6% 6% 5% 7% 7% 26%	4478
November	1st to 10th	Night	9% 15% 6% 12% 7% 6% 4 3 5% 6% 26%	2811
	11th to 20th	Night	7% 14% 4 15% 9% 6% 6% 4 6% 6% 24%	3000
	21st to end	Night	5% 12% 4 13% 8% 7% 7% 5% 8% 8% 24%	3000
December	1st to 10th	Night	5% 12% 5% 10% 7% 8% 6% 4 6% 8% 29%	2860
	11th to 20th	Night	6% 13% 5% 12% 7% 7% 7% 6% 7% 7% 24%	3750
	21st to end	Night	2 7% 3 8% 6% 5% 6% 4 7% 10% 41%	5566

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	63% 6% 16% 11% 4	1899
	11th to 20th	Day	46% 6% 16% 14% 16% 2	2032
	21st to end	Day	45% 9% 26% 14% 7% 1	2576
February	1st to 10th	Day	52% 10% 16% 14% 8% 1	1520
	11th to 20th	Day	50% 10% 16% 18% 7%	1536
	21st to end	Day	55% 8% 15% 14% 8%	1479
March	1st to 10th	Day	48% 9% 17% 20% 6%	3138
	11th to 20th	Day	42% 9% 21% 22% 7%	3385
	21st to end	Day	56% 13% 13% 13% 5%	3979
April	1st to 10th	Day	49% 10% 15% 17% 8%	2356
	11th to 20th	Day	48% 10% 14% 19% 9%	2440
	21st to end	Day	53% 9% 14% 17% 7% 1	2552
May	1st to 10th	Day	44% 8% 12% 23% 12%	4698
	11th to 20th	Day	47% 8% 11% 26% 7%	4698
	21st to end	Day	48% 11% 18% 16% 6%	5210
June	1st to 10th	Day	48% 10% 15% 21% 7%	3040
	11th to 20th	Day	54% 11% 14% 16% 5%	3040
	21st to end	Day	43% 11% 19% 20% 7%	3040
July	1st to 10th	Day	35% 7% 24% 26% 8% 1	4981
	11th to 20th	Day	32% 9% 16% 31% 11%	4946
	21st to end	Day	37% 7% 24% 26% 6%	4976
August	1st to 10th	Day	41% 8% 20% 28% 2	4543
	11th to 20th	Day	38% 7% 21% 26% 8%	4341
	21st to end	Day	43% 7% 28% 19% 3	4520
September	1st to 10th	Day	41% 10% 19% 25% 5% 1	2395
	11th to 20th	Day	43% 8% 24% 19% 5%	2132
	21st to end	Day	42% 9% 18% 22% 8%	2090
October	1st to 10th	Day	50% 6% 21% 18% 4	3256
	11th to 20th	Day	49% 5% 21% 21% 4	2842
	21st to end	Day	55% 6% 19% 16% 3	3049
November	1st to 10th	Day	58% 5% 17% 15% 4 1	1483
	11th to 20th	Day	58% 9% 16% 12% 4	1400
	21st to end	Day	51% 8% 20% 16% 3 1	1396
December	1st to 10th	Day	54% 8% 14% 17% 7% 1	1316
	11th to 20th	Day	52% 10% 20% 15% 4	1444
	21st to end	Day	35% 8% 12% 30% 14% 1	2090
January	1st to 10th	Night	57% 8% 15% 14% 5% 1	5046
	11th to 20th	Night	47% 7% 13% 13% 18% 2	4927
	21st to end	Night	49% 9% 23% 13% 5%	5109
February	1st to 10th	Night	55% 8% 15% 15% 7% 1	2660
	11th to 20th	Night	49% 10% 16% 16% 7% 1	2539
	21st to end	Night	48% 9% 16% 18% 8%	1971
March	1st to 10th	Night	48% 10% 15% 20% 7%	3736
	11th to 20th	Night	40% 8% 22% 22% 8%	3594
	21st to end	Night	52% 11% 11% 20% 6%	3661
April	1st to 10th	Night	43% 8% 15% 21% 13%	1824
	11th to 20th	Night	49% 9% 10% 19% 10% 2	1691
	21st to end	Night	51% 7% 10% 20% 10% 1	1625
May	1st to 10th	Night	48% 6% 13% 21% 11%	2262
	11th to 20th	Night	47% 8% 13% 20% 11% 1	2199
	21st to end	Night	47% 10% 20% 16% 7%	2364
June	1st to 10th	Night	48% 11% 14% 18% 9%	1140
	11th to 20th	Night	55% 8% 15% 17% 6%	1140
	21st to end	Night	39% 11% 18% 23% 8%	1140
July	1st to 10th	Night	35% 7% 21% 26% 11% 1	1870
	11th to 20th	Night	35% 6% 18% 26% 14%	1891
	21st to end	Night	36% 6% 24% 30% 5%	2337
August	1st to 10th	Night	38% 7% 21% 32% 2	2360
	11th to 20th	Night	40% 8% 19% 25% 9%	2553
	21st to end	Night	45% 5% 26% 21% 3	3146
September	1st to 10th	Night	37% 10% 19% 26% 6% 2	1781
	11th to 20th	Night	40% 8% 22% 24% 7%	2048
	21st to end	Night	39% 9% 17% 25% 9% 1	2090
October	1st to 10th	Night	48% 5% 18% 21% 7%	3715
	11th to 20th	Night	43% 6% 22% 25% 5%	4086
	21st to end	Night	55% 6% 21% 14% 3	4478
November	1st to 10th	Night	59% 5% 14% 19% 4	2811
	11th to 20th	Night	60% 7% 14% 14% 4	3000
	21st to end	Night	56% 9% 18% 13% 3 1	3000
December	1st to 10th	Night	52% 8% 13% 17% 9% 1	2860
	11th to 20th	Night	56% 9% 16% 14% 5%	3747
	21st to end	Night	36% 7% 17% 26% 12% 1	5566

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	7% 17% 34% 26% 12% 4	180
	11th to 20th	Day	11% 22% 32% 24% 10% 1	180
	21st to end	Day	15% 24% 31% 21% 8% 2	198
February	1st to 10th	Day	14% 23% 37% 18% 8%	180
	11th to 20th	Day	8% 19% 42% 23% 6% 1	203
	21st to end	Day	9% 24% 32% 27% 5% 4	291
March	1st to 10th	Day	7% 23% 33% 29% 7%	460
	11th to 20th	Day	9% 19% 40% 21% 10% 2	460
	21st to end	Day	16% 18% 34% 25% 7% 1	506
April	1st to 10th	Day	13% 19% 38% 22% 7% 2	460
	11th to 20th	Day	15% 18% 30% 24% 9% 4	452
	21st to end	Day	17% 19% 36% 20% 7% 1	460
May	1st to 10th	Day	14% 16% 29% 26% 11% 3	624
	11th to 20th	Day	12% 14% 31% 27% 13% 3	675
	21st to end	Day	16% 11% 26% 30% 14% 4	756
June	1st to 10th	Day	16% 13% 34% 24% 11% 2	870
	11th to 20th	Day	23% 10% 25% 24% 15% 4	870
	21st to end	Day	12% 9% 26% 30% 20% 4	870
July	1st to 10th	Day	13% 7% 26% 33% 17% 4	856
	11th to 20th	Day	13% 11% 26% 28% 16% 5%	838
	21st to end	Day	13% 6% 29% 29% 15% 7%	739
August	1st to 10th	Day	17% 7% 27% 25% 18% 7%	582
	11th to 20th	Day	12% 16% 34% 25% 10% 3	477
	21st to end	Day	13% 13% 32% 27% 13% 2	539
September	1st to 10th	Day	18% 17% 29% 21% 11% 3	490
	11th to 20th	Day	18% 15% 31% 25% 8% 2	490
	21st to end	Day	16% 19% 33% 19% 11% 1	490
October	1st to 10th	Day	18% 19% 29% 24% 10%	404
	11th to 20th	Day	14% 18% 37% 22% 6% 2	300
	21st to end	Day	22% 21% 29% 21% 7% 1	330
November	1st to 10th	Day	16% 19% 35% 22% 6% 2	257
	11th to 20th	Day	15% 20% 37% 17% 10% 2	200
	21st to end	Day	10% 24% 38% 20% 6% 2	199
December	1st to 10th	Day	11% 17% 42% 19% 7% 4	180
	11th to 20th	Day	12% 23% 34% 24% 4 2	180
	21st to end	Day	5% 16% 30% 33% 11% 5%	198
January	1st to 10th	Night	10% 18% 34% 28% 8% 1	1040
	11th to 20th	Night	11% 17% 36% 27% 8% 1	1001
	21st to end	Night	15% 20% 33% 24% 7% 2	979
February	1st to 10th	Night	12% 14% 32% 27% 14% 1	760
	11th to 20th	Night	13% 13% 36% 26% 11% 1	744
	21st to end	Night	7% 11% 35% 31% 15% 2	628
March	1st to 10th	Night	8% 14% 35% 28% 13% 2	760
	11th to 20th	Night	10% 13% 33% 32% 10% 2	760
	21st to end	Night	12% 12% 30% 31% 13% 1	836
April	1st to 10th	Night	10% 11% 31% 33% 14% 2	684
	11th to 20th	Night	13% 11% 30% 31% 12% 3	563
	21st to end	Night	19% 11% 28% 30% 12% 1	485
May	1st to 10th	Night	13% 9% 26% 33% 18% 2	380
	11th to 20th	Night	13% 11% 29% 29% 16% 2	374
	21st to end	Night	16% 8% 23% 32% 18% 2	411
June	1st to 10th	Night	14% 8% 30% 28% 18% 2	380
	11th to 20th	Night	22% 7% 21% 23% 24% 3	380
	21st to end	Night	9% 6% 21% 36% 24% 4	380
July	1st to 10th	Night	13% 5% 23% 36% 22% 1	370
	11th to 20th	Night	12% 7% 23% 34% 20% 5%	368
	21st to end	Night	13% 5% 21% 30% 26% 5%	416
August	1st to 10th	Night	11% 5% 17% 34% 24% 8%	378
	11th to 20th	Night	10% 8% 24% 33% 24% 2	395
	21st to end	Night	18% 7% 23% 32% 19% 2	627
September	1st to 10th	Night	15% 8% 25% 29% 21% 2	641
	11th to 20th	Night	12% 11% 23% 36% 16% 2	760
	21st to end	Night	13% 15% 29% 27% 15% 1	760
October	1st to 10th	Night	18% 12% 29% 30% 10% 1	760
	11th to 20th	Night	14% 14% 31% 30% 11%	760
	21st to end	Night	17% 20% 30% 25% 8%	845
November	1st to 10th	Night	17% 15% 35% 25% 8% 1	975
	11th to 20th	Night	15% 18% 33% 24% 8% 2	1000
	21st to end	Night	12% 20% 38% 22% 7% 1	1000
December	1st to 10th	Night	12% 17% 35% 24% 10% 2	955
	11th to 20th	Night	12% 19% 34% 28% 6% 1	1040
	21st to end	Night	8% 15% 33% 29% 12% 3	1144

Page generated at 2024-11-27 15:30:55. Version ac07

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