Fort Nelson
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Fort Nelson clear sky chart from 2005-11-06 to 2026-07-31.

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% 4 9% 6% 5% 5% 5% 7% 8% 33%	142844
	Night	7% 12% 5% 11% 7% 6% 6% 5% 6% 7% 28%	138808
Transparency	Day	47% 6% 16% 23% 8% 1	142506
	Night	53% 6% 13% 19% 8% 1	138729
Seeing	Day	13% 6% 22% 30% 21% 7%	19542
	Night	14% 5% 20% 36% 22% 4	26946

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	5% 13% 7% 20% 11% 7% 7% 7% 7% 6% 11%	2929
	11th to 20th	Day	4 10% 2 9% 9% 9% 8% 8% 8% 8% 26%	3444
	21st to end	Day	7% 14% 5% 14% 8% 5% 6% 3 6% 8% 24%	4469
February	1st to 10th	Day	7% 11% 7% 13% 9% 8% 6% 5% 5% 7% 22%	1470
	11th to 20th	Day	4 13% 6% 15% 8% 6% 5% 4 6% 8% 25%	1736
	21st to end	Day	5% 12% 6% 15% 7% 7% 6% 3 6% 6% 27%	1557
March	1st to 10th	Day	2 10% 5% 13% 8% 5% 6% 4 7% 8% 32%	4429
	11th to 20th	Day	5% 19% 8% 16% 9% 5% 4 3 5% 7% 20%	5054
	21st to end	Day	4 12% 5% 11% 5% 5% 4 3 6% 7% 37%	5873
April	1st to 10th	Day	6% 14% 4 9% 6% 5% 5% 5% 7% 8% 32%	2595
	11th to 20th	Day	7% 11% 4 8% 6% 4 6% 5% 8% 8% 34%	2797
	21st to end	Day	10% 13% 4 9% 6% 5% 6% 4 7% 7% 30%	2940
May	1st to 10th	Day	4 5% 2 6% 5% 6% 6% 5% 7% 9% 45%	8757
	11th to 20th	Day	10% 10% 4 9% 6% 6% 6% 5% 7% 8% 30%	9352
	21st to end	Day	11% 7% 3 8% 6% 6% 6% 5% 8% 10% 30%	10305
June	1st to 10th	Day	13% 8% 3 8% 6% 6% 6% 5% 7% 7% 33%	3549
	11th to 20th	Day	17% 9% 3 8% 5% 5% 6% 5% 7% 8% 27%	3570
	21st to end	Day	12% 7% 3 7% 6% 5% 6% 6% 7% 9% 33%	3570
July	1st to 10th	Day	12% 6% 2 7% 5% 5% 5% 5% 7% 9% 36%	7870
	11th to 20th	Day	10% 8% 2 7% 5% 5% 5% 4 7% 9% 37%	7664
	21st to end	Day	9% 6% 2 6% 5% 5% 5% 5% 7% 10% 40%	7934
August	1st to 10th	Day	8% 4 2 5% 4 4 4 4 6% 8% 53%	4869
	11th to 20th	Day	10% 4 2 6% 4 4 4 4 6% 8% 47%	4545
	21st to end	Day	12% 7% 3 8% 5% 5% 5% 4 6% 8% 38%	4828
September	1st to 10th	Day	12% 7% 2 6% 4 5% 5% 4 7% 8% 39%	2470
	11th to 20th	Day	11% 8% 3 7% 6% 5% 5% 4 6% 8% 36%	2400
	21st to end	Day	13% 8% 3 8% 6% 5% 5% 5% 6% 7% 34%	2111
October	1st to 10th	Day	10% 13% 4 8% 6% 5% 5% 4 6% 8% 33%	3280
	11th to 20th	Day	11% 15% 4 9% 6% 4 4 4 6% 7% 29%	3013
	21st to end	Day	12% 14% 5% 11% 6% 5% 4 4 5% 5% 30%	2842
November	1st to 10th	Day	14% 20% 5% 11% 7% 5% 4 4 4 5% 21%	1569
	11th to 20th	Day	9% 19% 6% 15% 8% 5% 4 5% 4 5% 20%	1284
	21st to end	Day	5% 12% 6% 14% 8% 7% 7% 4 7% 7% 23%	1260
December	1st to 10th	Day	11% 16% 5% 12% 8% 6% 6% 5% 6% 8% 18%	2020
	11th to 20th	Day	5% 13% 7% 13% 8% 7% 6% 5% 8% 8% 20%	1948
	21st to end	Day	4 10% 6% 13% 8% 7% 7% 4 9% 8% 24%	2541
January	1st to 10th	Night	4 14% 7% 18% 9% 8% 8% 7% 7% 5% 13%	9964
	11th to 20th	Night	4 10% 3 10% 8% 6% 7% 7% 9% 10% 25%	9439
	21st to end	Night	6% 16% 7% 13% 8% 6% 5% 4 5% 6% 26%	9755
February	1st to 10th	Night	6% 14% 6% 15% 7% 6% 5% 4 6% 6% 24%	3150
	11th to 20th	Night	5% 14% 7% 13% 7% 5% 5% 4 5% 7% 28%	2768
	21st to end	Night	5% 14% 6% 12% 7% 7% 6% 4 6% 6% 28%	2249
March	1st to 10th	Night	2 10% 4 14% 8% 6% 6% 4 6% 7% 32%	5677
	11th to 20th	Night	3 17% 8% 17% 8% 6% 5% 4 5% 7% 20%	5162
	21st to end	Night	4 11% 5% 10% 5% 4 5% 4 6% 7% 38%	5343
April	1st to 10th	Night	6% 14% 5% 11% 7% 5% 6% 4 6% 7% 29%	2022
	11th to 20th	Night	5% 14% 4 8% 5% 5% 6% 4 7% 7% 34%	1771
	21st to end	Night	12% 11% 3 7% 6% 6% 7% 5% 7% 6% 31%	1680
May	1st to 10th	Night	4 6% 3 5% 4 4 4 4 7% 11% 46%	4145
	11th to 20th	Night	12% 11% 3 10% 5% 5% 5% 5% 6% 8% 29%	3510
	21st to end	Night	11% 8% 3 8% 5% 6% 6% 5% 8% 8% 33%	3866
June	1st to 10th	Night	13% 6% 2 7% 7% 5% 6% 4 8% 8% 32%	1069
	11th to 20th	Night	15% 9% 3 9% 7% 6% 7% 5% 8% 8% 24%	1050
	21st to end	Night	10% 7% 3 8% 6% 7% 4 5% 7% 9% 34%	1050
July	1st to 10th	Night	13% 5% 2 7% 5% 5% 5% 4 6% 10% 37%	2310
	11th to 20th	Night	10% 7% 3 7% 6% 6% 6% 5% 7% 9% 34%	2516
	21st to end	Night	12% 7% 3 6% 5% 4 5% 5% 8% 9% 36%	3117
August	1st to 10th	Night	8% 6% 2 6% 4 4 4 5% 5% 6% 52%	2281
	11th to 20th	Night	13% 6% 2 6% 4 5% 5% 4 6% 8% 41%	2502
	21st to end	Night	13% 8% 3 8% 6% 6% 7% 5% 7% 9% 29%	3075
September	1st to 10th	Night	11% 6% 3 8% 5% 4 5% 5% 8% 8% 36%	1930
	11th to 20th	Night	10% 8% 2 8% 6% 5% 5% 4 7% 11% 34%	2000
	21st to end	Night	15% 8% 3 8% 6% 7% 6% 5% 6% 6% 29%	2286
October	1st to 10th	Night	9% 12% 4 10% 5% 6% 5% 4 5% 8% 31%	3920
	11th to 20th	Night	9% 14% 4 9% 5% 5% 5% 4 6% 7% 33%	4134
	21st to end	Night	8% 14% 5% 11% 5% 5% 5% 3 6% 7% 31%	4948
November	1st to 10th	Night	12% 19% 5% 11% 6% 5% 5% 3 3 5% 27%	2928
	11th to 20th	Night	8% 21% 8% 14% 7% 5% 4 3 5% 4 21%	3334
	21st to end	Night	7% 13% 4 12% 9% 6% 6% 4 7% 7% 26%	3360
December	1st to 10th	Night	9% 15% 6% 12% 7% 6% 6% 4 6% 8% 21%	5380
	11th to 20th	Night	5% 14% 7% 16% 7% 6% 6% 5% 7% 8% 20%	6460
	21st to end	Night	4 11% 5% 14% 9% 8% 7% 5% 8% 9% 21%	8657

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% 9% 7%	2929
	11th to 20th	Day	54% 4 14% 19% 6% 3	3462
	21st to end	Day	58% 4 18% 13% 7%	4470
February	1st to 10th	Day	60% 6% 10% 16% 7% 1	1470
	11th to 20th	Day	56% 6% 14% 18% 6%	1744
	21st to end	Day	57% 6% 11% 19% 7%	1557
March	1st to 10th	Day	47% 6% 12% 24% 11%	4429
	11th to 20th	Day	63% 5% 13% 14% 5%	5054
	21st to end	Day	45% 6% 15% 26% 8%	5873
April	1st to 10th	Day	47% 8% 14% 21% 9% 1	2595
	11th to 20th	Day	43% 7% 14% 24% 11%	2797
	21st to end	Day	51% 6% 14% 20% 8%	2940
May	1st to 10th	Day	33% 5% 16% 28% 15% 4	8757
	11th to 20th	Day	49% 5% 11% 24% 10%	9356
	21st to end	Day	45% 7% 11% 23% 14% 1	10306
June	1st to 10th	Day	47% 8% 14% 22% 8% 1	3549
	11th to 20th	Day	51% 9% 13% 20% 6%	3570
	21st to end	Day	43% 9% 18% 25% 5%	3570
July	1st to 10th	Day	43% 5% 19% 28% 4	7786
	11th to 20th	Day	42% 6% 23% 26% 3	7571
	21st to end	Day	37% 6% 25% 28% 3	7715
August	1st to 10th	Day	30% 7% 17% 26% 20%	4851
	11th to 20th	Day	34% 7% 17% 30% 12%	4588
	21st to end	Day	44% 6% 22% 24% 3	4827
September	1st to 10th	Day	40% 9% 22% 23% 6%	2470
	11th to 20th	Day	44% 9% 21% 23% 4	2400
	21st to end	Day	46% 9% 15% 21% 9%	2111
October	1st to 10th	Day	48% 5% 16% 26% 4	3280
	11th to 20th	Day	54% 5% 15% 22% 4	3013
	21st to end	Day	57% 4 13% 21% 5%	2842
November	1st to 10th	Day	66% 4 8% 18% 5%	1569
	11th to 20th	Day	66% 7% 9% 13% 5%	1284
	21st to end	Day	59% 5% 10% 17% 8% 2	1260
December	1st to 10th	Day	65% 6% 16% 9% 4	2020
	11th to 20th	Day	59% 5% 13% 17% 5%	1950
	21st to end	Day	56% 5% 12% 17% 9% 1	2541
January	1st to 10th	Night	70% 5% 9% 10% 6%	9965
	11th to 20th	Night	52% 5% 15% 17% 9% 2	9483
	21st to end	Night	60% 4 15% 15% 6%	9755
February	1st to 10th	Night	59% 6% 9% 16% 9% 1	3150
	11th to 20th	Night	55% 6% 13% 18% 8%	2770
	21st to end	Night	54% 7% 11% 19% 8% 1	2249
March	1st to 10th	Night	49% 5% 10% 23% 12%	5677
	11th to 20th	Night	63% 5% 11% 18% 3	5163
	21st to end	Night	44% 6% 16% 25% 10%	5343
April	1st to 10th	Night	52% 8% 12% 18% 10%	2022
	11th to 20th	Night	46% 6% 11% 22% 14% 1	1771
	21st to end	Night	50% 7% 13% 20% 10% 1	1680
May	1st to 10th	Night	30% 5% 15% 30% 15% 4	4142
	11th to 20th	Night	50% 6% 11% 22% 10% 1	3510
	21st to end	Night	46% 6% 11% 22% 12% 2	3870
June	1st to 10th	Night	45% 9% 15% 22% 9%	1069
	11th to 20th	Night	53% 10% 13% 17% 7%	1050
	21st to end	Night	45% 9% 15% 22% 8%	1050
July	1st to 10th	Night	42% 6% 21% 26% 5%	2285
	11th to 20th	Night	45% 6% 21% 23% 6%	2486
	21st to end	Night	40% 7% 22% 26% 4	3031
August	1st to 10th	Night	33% 6% 13% 31% 18%	2274
	11th to 20th	Night	39% 7% 13% 27% 14%	2525
	21st to end	Night	49% 7% 20% 19% 4	3075
September	1st to 10th	Night	41% 10% 17% 24% 7%	1930
	11th to 20th	Night	42% 9% 18% 25% 5%	2000
	21st to end	Night	51% 9% 12% 18% 9% 1	2286
October	1st to 10th	Night	51% 5% 16% 20% 7%	3920
	11th to 20th	Night	51% 5% 15% 24% 4	4134
	21st to end	Night	52% 5% 16% 23% 4	4948
November	1st to 10th	Night	62% 5% 7% 20% 6%	2928
	11th to 20th	Night	66% 6% 9% 14% 6%	3334
	21st to end	Night	56% 6% 11% 18% 8% 1	3360
December	1st to 10th	Night	63% 5% 14% 13% 6%	5380
	11th to 20th	Night	62% 4 13% 14% 6% 1	6457
	21st to end	Night	58% 6% 14% 14% 7% 1	8657

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	10% 3 15% 40% 22% 10%	220
	11th to 20th	Day	11% 5% 26% 31% 20% 7%	220
	21st to end	Day	15% 9% 17% 37% 19% 2	242
February	1st to 10th	Day	8% 7% 20% 43% 19% 4	220
	11th to 20th	Day	10% 7% 22% 36% 21% 5%	323
	21st to end	Day	8% 5% 27% 35% 21% 3	355
March	1st to 10th	Day	6% 6% 24% 34% 20% 9%	494
	11th to 20th	Day	11% 5% 22% 33% 23% 6%	540
	21st to end	Day	13% 6% 24% 26% 22% 8%	594
April	1st to 10th	Day	13% 9% 26% 31% 16% 5%	540
	11th to 20th	Day	12% 10% 24% 27% 21% 6%	532
	21st to end	Day	13% 11% 27% 30% 16% 4	540
May	1st to 10th	Day	13% 11% 29% 30% 12% 4	732
	11th to 20th	Day	13% 9% 23% 30% 21% 4	982
	21st to end	Day	15% 6% 21% 29% 23% 6%	1072
June	1st to 10th	Day	14% 7% 23% 29% 21% 5%	990
	11th to 20th	Day	20% 5% 17% 29% 23% 7%	990
	21st to end	Day	11% 4 18% 27% 27% 13%	990
July	1st to 10th	Day	19% 4 18% 28% 23% 9%	976
	11th to 20th	Day	12% 4 18% 28% 26% 12%	972
	21st to end	Day	12% 4 16% 31% 28% 9%	1032
August	1st to 10th	Day	10% 4 19% 27% 27% 12%	726
	11th to 20th	Day	12% 9% 29% 28% 15% 6%	568
	21st to end	Day	17% 4 20% 25% 21% 13%	583
September	1st to 10th	Day	14% 7% 26% 25% 20% 9%	530
	11th to 20th	Day	10% 5% 21% 32% 24% 7%	530
	21st to end	Day	15% 8% 29% 28% 17% 4	530
October	1st to 10th	Day	17% 8% 25% 32% 14% 5%	530
	11th to 20th	Day	17% 7% 29% 29% 15% 3	405
	21st to end	Day	18% 8% 22% 26% 21% 5%	242
November	1st to 10th	Day	23% 4 15% 34% 21% 4	220
	11th to 20th	Day	16% 4 12% 41% 22% 5%	220
	21st to end	Day	10% 5% 15% 42% 20% 8%	220
December	1st to 10th	Day	15% 7% 12% 39% 22% 4	220
	11th to 20th	Day	12% 3 19% 42% 17% 7%	220
	21st to end	Day	7% 2 10% 41% 28% 12%	242
January	1st to 10th	Night	11% 5% 18% 38% 23% 5%	1160
	11th to 20th	Night	15% 4 21% 42% 15% 4	1160
	21st to end	Night	15% 6% 22% 39% 15% 2	1084
February	1st to 10th	Night	11% 5% 22% 38% 22% 2	950
	11th to 20th	Night	9% 7% 24% 38% 20% 1	827
	21st to end	Night	9% 4 21% 40% 22% 3	692
March	1st to 10th	Night	10% 6% 21% 35% 23% 6%	840
	11th to 20th	Night	12% 6% 23% 34% 22% 3	840
	21st to end	Night	12% 4 18% 34% 26% 6%	924
April	1st to 10th	Night	13% 8% 23% 30% 22% 4	762
	11th to 20th	Night	10% 7% 23% 35% 21% 4	523
	21st to end	Night	17% 5% 24% 34% 18% 1	420
May	1st to 10th	Night	13% 8% 27% 31% 17% 3	420
	11th to 20th	Night	14% 8% 22% 33% 22%	414
	21st to end	Night	15% 4 20% 31% 28% 2	455
June	1st to 10th	Night	15% 5% 21% 33% 25% 1	420
	11th to 20th	Night	18% 3 15% 33% 29% 3	420
	21st to end	Night	12% 3 17% 24% 36% 7%	420
July	1st to 10th	Night	18% 4 13% 31% 31% 4	410
	11th to 20th	Night	14% 3 15% 33% 28% 6%	408
	21st to end	Night	13% 2 13% 34% 33% 5%	458
August	1st to 10th	Night	11% 3 15% 29% 38% 5%	398
	11th to 20th	Night	14% 3 23% 33% 24% 4	396
	21st to end	Night	17% 2 15% 32% 31% 4	555
September	1st to 10th	Night	15% 4 20% 32% 25% 5%	730
	11th to 20th	Night	13% 4 19% 36% 25% 5%	800
	21st to end	Night	18% 5% 23% 35% 16% 3	800
October	1st to 10th	Night	18% 5% 23% 35% 17% 2	800
	11th to 20th	Night	16% 4 25% 38% 14% 2	800
	21st to end	Night	17% 7% 21% 35% 18% 4	880
November	1st to 10th	Night	20% 5% 19% 33% 19% 4	878
	11th to 20th	Night	15% 6% 20% 37% 20% 2	1146
	21st to end	Night	12% 4 17% 44% 18% 3	1160
December	1st to 10th	Night	15% 5% 17% 43% 17% 2	1160
	11th to 20th	Night	11% 5% 20% 38% 19% 6%	1160
	21st to end	Night	11% 3 15% 41% 25% 6%	1276

Page generated at 2026-07-28 00:25:26. Version ac07

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