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 2025-03-27.

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% 10% 4 9% 6% 5% 5% 5% 7% 8% 33%	111645
	Night	7% 12% 5% 11% 7% 6% 6% 5% 6% 7% 28%	116279
Transparency	Day	47% 7% 16% 23% 8%	111354
	Night	53% 6% 13% 19% 8% 1	116216
Seeing	Day	13% 6% 21% 30% 22% 7%	17485
	Night	14% 5% 20% 36% 22% 4	25320

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	6% 14% 6% 17% 10% 8% 8% 7% 6% 6% 12%	2259
	11th to 20th	Day	5% 12% 3 10% 10% 10% 7% 9% 7% 8% 20%	2638
	21st to end	Day	7% 15% 5% 13% 6% 4 5% 3 7% 8% 25%	3452
February	1st to 10th	Day	7% 11% 7% 12% 9% 8% 7% 5% 6% 7% 23%	1400
	11th to 20th	Day	5% 14% 5% 14% 8% 6% 5% 4 6% 8% 26%	1651
	21st to end	Day	5% 13% 6% 14% 7% 7% 6% 3 6% 6% 27%	1485
March	1st to 10th	Day	3 9% 4 13% 8% 5% 6% 5% 7% 8% 33%	3105
	11th to 20th	Day	5% 20% 7% 13% 6% 5% 5% 4 6% 7% 23%	3558
	21st to end	Day	4 11% 5% 11% 6% 5% 5% 3 6% 7% 37%	4076
April	1st to 10th	Day	6% 13% 4 9% 7% 5% 5% 5% 7% 7% 32%	2347
	11th to 20th	Day	7% 11% 4 8% 6% 4 5% 5% 8% 9% 34%	2527
	21st to end	Day	11% 13% 4 9% 6% 5% 5% 4 6% 7% 28%	2660
May	1st to 10th	Day	7% 7% 3 8% 6% 6% 6% 5% 7% 8% 39%	4720
	11th to 20th	Day	9% 8% 3 8% 5% 6% 6% 6% 8% 9% 33%	5013
	21st to end	Day	11% 7% 3 9% 6% 6% 6% 4 7% 9% 33%	5509
June	1st to 10th	Day	14% 8% 3 8% 6% 6% 5% 5% 7% 7% 32%	3211
	11th to 20th	Day	17% 9% 3 7% 5% 5% 6% 5% 7% 8% 28%	3230
	21st to end	Day	12% 7% 3 7% 6% 5% 5% 6% 7% 9% 34%	3230
July	1st to 10th	Day	10% 5% 2 6% 5% 4 4 4 6% 8% 45%	5380
	11th to 20th	Day	12% 7% 2 7% 5% 5% 5% 4 7% 9% 38%	5234
	21st to end	Day	9% 5% 2 6% 6% 5% 6% 5% 8% 10% 37%	5411
August	1st to 10th	Day	8% 4 2 5% 4 4 4 4 6% 8% 53%	4719
	11th to 20th	Day	10% 4 2 6% 4 4 4 4 6% 8% 48%	4403
	21st to end	Day	12% 7% 3 9% 5% 5% 5% 4 6% 8% 37%	4680
September	1st to 10th	Day	13% 7% 2 6% 4 6% 5% 4 7% 8% 38%	2347
	11th to 20th	Day	11% 8% 3 6% 6% 5% 5% 4 6% 8% 36%	2280
	21st to end	Day	13% 8% 3 8% 6% 5% 5% 5% 7% 7% 33%	2006
October	1st to 10th	Day	10% 13% 3 7% 6% 5% 5% 4 6% 8% 33%	3180
	11th to 20th	Day	11% 15% 4 9% 6% 4 4 4 6% 7% 29%	2921
	21st to end	Day	12% 14% 5% 11% 6% 5% 4 4 5% 5% 30%	2754
November	1st to 10th	Day	15% 20% 5% 10% 7% 5% 4 4 4 5% 21%	1493
	11th to 20th	Day	10% 18% 6% 16% 8% 5% 4 5% 4 5% 19%	1223
	21st to end	Day	6% 13% 6% 14% 9% 7% 6% 4 6% 6% 24%	1200
December	1st to 10th	Day	11% 15% 5% 11% 8% 6% 7% 5% 6% 9% 18%	1960
	11th to 20th	Day	4 13% 7% 13% 9% 7% 6% 5% 8% 8% 20%	1897
	21st to end	Day	4 10% 6% 13% 7% 7% 7% 5% 9% 8% 24%	2486
January	1st to 10th	Night	5% 13% 6% 16% 9% 8% 9% 8% 7% 5% 15%	7686
	11th to 20th	Night	5% 12% 4 11% 9% 7% 7% 7% 9% 9% 22%	7245
	21st to end	Night	7% 16% 6% 12% 7% 5% 4 3 4 6% 30%	7534
February	1st to 10th	Night	6% 14% 6% 14% 8% 6% 5% 4 6% 6% 25%	3000
	11th to 20th	Night	6% 14% 6% 13% 7% 5% 5% 4 5% 7% 29%	2633
	21st to end	Night	5% 14% 6% 11% 7% 7% 6% 5% 6% 6% 29%	2145
March	1st to 10th	Night	3 11% 4 14% 9% 7% 7% 5% 6% 6% 28%	3985
	11th to 20th	Night	4 18% 6% 12% 7% 6% 6% 4 6% 8% 25%	3639
	21st to end	Night	4 10% 5% 10% 5% 4 4 4 6% 7% 41%	3714
April	1st to 10th	Night	6% 15% 4 11% 6% 5% 6% 4 6% 7% 30%	1830
	11th to 20th	Night	5% 14% 4 8% 5% 5% 6% 5% 7% 6% 35%	1601
	21st to end	Night	13% 11% 3 7% 6% 6% 7% 5% 7% 6% 30%	1520
May	1st to 10th	Night	7% 9% 4 5% 5% 5% 4 4 7% 10% 40%	2237
	11th to 20th	Night	10% 8% 3 9% 6% 6% 6% 5% 7% 10% 31%	1882
	21st to end	Night	12% 8% 4 9% 6% 6% 6% 4 7% 9% 30%	2066
June	1st to 10th	Night	14% 6% 2 7% 8% 5% 6% 4 8% 8% 30%	967
	11th to 20th	Night	15% 9% 3 8% 7% 6% 7% 5% 8% 8% 25%	950
	21st to end	Night	11% 7% 2 8% 7% 6% 5% 5% 7% 10% 33%	950
July	1st to 10th	Night	10% 4 1 7% 5% 4 4 3 6% 10% 46%	1580
	11th to 20th	Night	11% 7% 2 7% 5% 6% 5% 4 6% 8% 38%	1726
	21st to end	Night	10% 6% 3 7% 6% 5% 6% 6% 9% 9% 33%	2128
August	1st to 10th	Night	8% 5% 2 5% 4 4 3 4 4 6% 53%	2211
	11th to 20th	Night	13% 5% 2 6% 4 5% 4 4 6% 8% 41%	2424
	21st to end	Night	13% 8% 3 8% 6% 6% 7% 5% 7% 9% 28%	2981
September	1st to 10th	Night	12% 6% 3 8% 5% 4 6% 5% 8% 8% 34%	1833
	11th to 20th	Night	9% 8% 2 8% 6% 4 5% 4 7% 11% 35%	1900
	21st to end	Night	16% 9% 3 8% 7% 7% 6% 5% 6% 6% 28%	2171
October	1st to 10th	Night	9% 12% 4 10% 5% 6% 5% 4 5% 8% 31%	3800
	11th to 20th	Night	10% 14% 4 9% 5% 5% 5% 4 5% 7% 32%	4006
	21st to end	Night	8% 14% 5% 11% 5% 5% 5% 3 6% 7% 31%	4794
November	1st to 10th	Night	12% 19% 5% 11% 6% 5% 4 3 3 5% 27%	2785
	11th to 20th	Night	8% 21% 7% 14% 8% 4 4 3 5% 4 20%	3175
	21st to end	Night	8% 13% 4 12% 9% 6% 5% 3 7% 7% 26%	3200
December	1st to 10th	Night	9% 14% 6% 11% 7% 6% 6% 4 6% 8% 22%	5220
	11th to 20th	Night	5% 14% 8% 16% 6% 6% 6% 5% 7% 8% 19%	6291
	21st to end	Night	4 11% 5% 14% 9% 8% 7% 5% 8% 9% 21%	8470

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	70% 5% 9% 8% 8%	2259
	11th to 20th	Day	59% 5% 8% 19% 7% 1	2656
	21st to end	Day	55% 5% 17% 15% 8%	3452
February	1st to 10th	Day	59% 7% 10% 16% 8% 1	1400
	11th to 20th	Day	55% 6% 15% 18% 6%	1659
	21st to end	Day	55% 6% 12% 19% 7%	1485
March	1st to 10th	Day	45% 7% 12% 25% 11%	3105
	11th to 20th	Day	59% 6% 14% 15% 6%	3558
	21st to end	Day	44% 7% 19% 25% 5%	4076
April	1st to 10th	Day	47% 8% 14% 21% 9% 1	2347
	11th to 20th	Day	43% 8% 14% 25% 11%	2527
	21st to end	Day	52% 7% 14% 19% 8%	2660
May	1st to 10th	Day	37% 7% 15% 24% 15% 2	4720
	11th to 20th	Day	40% 8% 13% 26% 12% 1	5017
	21st to end	Day	43% 9% 11% 22% 14% 1	5510
June	1st to 10th	Day	48% 9% 14% 21% 7% 1	3211
	11th to 20th	Day	50% 9% 13% 21% 6%	3230
	21st to end	Day	43% 10% 17% 25% 5%	3230
July	1st to 10th	Day	36% 6% 18% 34% 6%	5296
	11th to 20th	Day	41% 7% 23% 26% 3	5141
	21st to end	Day	37% 8% 21% 29% 4	5240
August	1st to 10th	Day	29% 7% 17% 27% 20%	4701
	11th to 20th	Day	34% 7% 17% 31% 12%	4446
	21st to end	Day	45% 6% 22% 24% 2	4679
September	1st to 10th	Day	41% 10% 21% 22% 6%	2347
	11th to 20th	Day	43% 9% 20% 24% 4	2280
	21st to end	Day	47% 9% 16% 20% 8%	2006
October	1st to 10th	Day	48% 5% 16% 26% 4	3180
	11th to 20th	Day	54% 5% 15% 22% 3	2921
	21st to end	Day	57% 4 13% 21% 5%	2754
November	1st to 10th	Day	65% 4 8% 18% 4	1493
	11th to 20th	Day	67% 7% 9% 12% 4	1223
	21st to end	Day	59% 5% 10% 17% 7% 2	1200
December	1st to 10th	Day	65% 6% 16% 9% 4	1960
	11th to 20th	Day	59% 5% 13% 17% 5%	1899
	21st to end	Day	55% 5% 12% 17% 9% 1	2486
January	1st to 10th	Night	68% 5% 10% 10% 6% 1	7686
	11th to 20th	Night	55% 6% 12% 17% 9% 1	7289
	21st to end	Night	56% 5% 13% 17% 8%	7534
February	1st to 10th	Night	57% 6% 9% 16% 10% 1	3000
	11th to 20th	Night	54% 6% 14% 18% 8%	2635
	21st to end	Night	53% 7% 11% 19% 8% 1	2145
March	1st to 10th	Night	52% 6% 9% 23% 10%	3985
	11th to 20th	Night	55% 7% 12% 21% 4	3640
	21st to end	Night	41% 7% 19% 27% 7%	3714
April	1st to 10th	Night	52% 8% 11% 19% 10%	1830
	11th to 20th	Night	45% 7% 11% 22% 15% 1	1601
	21st to end	Night	51% 7% 12% 19% 10% 1	1520
May	1st to 10th	Night	38% 8% 10% 24% 16% 5%	2237
	11th to 20th	Night	42% 8% 10% 25% 13% 2	1882
	21st to end	Night	47% 9% 11% 20% 13% 1	2066
June	1st to 10th	Night	47% 9% 15% 23% 7%	967
	11th to 20th	Night	51% 10% 13% 17% 8%	950
	21st to end	Night	45% 10% 15% 21% 9%	950
July	1st to 10th	Night	34% 7% 22% 32% 6%	1555
	11th to 20th	Night	43% 6% 20% 24% 7%	1696
	21st to end	Night	40% 9% 19% 26% 6%	2060
August	1st to 10th	Night	32% 6% 13% 31% 18%	2204
	11th to 20th	Night	38% 7% 13% 28% 14%	2447
	21st to end	Night	51% 7% 20% 19% 3	2981
September	1st to 10th	Night	42% 10% 16% 24% 7%	1833
	11th to 20th	Night	42% 9% 18% 25% 5%	1900
	21st to end	Night	52% 10% 12% 17% 8% 1	2171
October	1st to 10th	Night	52% 5% 16% 20% 7% 1	3800
	11th to 20th	Night	51% 5% 15% 24% 4	4006
	21st to end	Night	52% 5% 16% 23% 4	4794
November	1st to 10th	Night	62% 5% 7% 21% 6%	2785
	11th to 20th	Night	66% 6% 9% 14% 5%	3175
	21st to end	Night	56% 6% 11% 17% 8% 1	3200
December	1st to 10th	Night	61% 5% 14% 13% 6%	5220
	11th to 20th	Night	61% 4 14% 14% 6% 1	6288
	21st to end	Night	57% 6% 14% 14% 8% 1	8470

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	11% 3 15% 38% 23% 11%	200
	11th to 20th	Day	12% 4 25% 30% 22% 8%	200
	21st to end	Day	15% 9% 18% 37% 18% 3	220
February	1st to 10th	Day	8% 6% 20% 42% 20% 5%	200
	11th to 20th	Day	10% 6% 22% 35% 21% 5%	298
	21st to end	Day	8% 5% 27% 36% 21% 4	331
March	1st to 10th	Day	7% 5% 22% 35% 21% 10%	458
	11th to 20th	Day	11% 6% 21% 34% 22% 6%	500
	21st to end	Day	15% 6% 23% 26% 23% 6%	531
April	1st to 10th	Day	12% 9% 28% 30% 16% 5%	460
	11th to 20th	Day	11% 10% 24% 27% 22% 7%	452
	21st to end	Day	14% 10% 27% 29% 16% 3	460
May	1st to 10th	Day	13% 10% 29% 30% 13% 4	633
	11th to 20th	Day	12% 8% 23% 31% 21% 5%	862
	21st to end	Day	14% 6% 18% 30% 26% 7%	940
June	1st to 10th	Day	14% 7% 22% 29% 23% 5%	870
	11th to 20th	Day	20% 5% 18% 28% 23% 6%	870
	21st to end	Day	11% 4 19% 27% 28% 11%	870
July	1st to 10th	Day	18% 3 16% 29% 23% 10%	856
	11th to 20th	Day	12% 3 19% 27% 26% 13%	852
	21st to end	Day	11% 4 16% 32% 27% 9%	908
August	1st to 10th	Day	10% 4 19% 26% 28% 13%	676
	11th to 20th	Day	12% 9% 29% 28% 16% 6%	526
	21st to end	Day	19% 5% 21% 24% 20% 11%	539
September	1st to 10th	Day	14% 7% 26% 25% 20% 8%	490
	11th to 20th	Day	10% 5% 20% 32% 25% 8%	490
	21st to end	Day	15% 7% 29% 27% 18% 4	490
October	1st to 10th	Day	17% 7% 26% 33% 13% 4	490
	11th to 20th	Day	17% 8% 26% 30% 16% 4	373
	21st to end	Day	17% 8% 23% 26% 22% 5%	220
November	1st to 10th	Day	23% 4 15% 34% 21% 5%	200
	11th to 20th	Day	16% 3 12% 41% 22% 6%	200
	21st to end	Day	11% 6% 16% 40% 21% 8%	200
December	1st to 10th	Day	15% 6% 11% 41% 24% 5%	200
	11th to 20th	Day	11% 3 20% 42% 17% 7%	200
	21st to end	Day	7% 2 9% 41% 29% 12%	220
January	1st to 10th	Night	11% 5% 18% 38% 23% 5%	1100
	11th to 20th	Night	15% 4 20% 42% 16% 4	1100
	21st to end	Night	15% 6% 23% 38% 15% 2	1027
February	1st to 10th	Night	11% 5% 22% 37% 22% 2	900
	11th to 20th	Night	9% 7% 24% 37% 21% 1	787
	21st to end	Night	9% 4 21% 40% 23% 4	660
March	1st to 10th	Night	10% 5% 21% 36% 23% 7%	800
	11th to 20th	Night	12% 7% 24% 34% 22% 2	800
	21st to end	Night	13% 4 18% 33% 26% 5%	862
April	1st to 10th	Night	13% 8% 24% 28% 22% 4	690
	11th to 20th	Night	9% 7% 23% 34% 22% 4	473
	21st to end	Night	19% 5% 25% 32% 18% 1	380
May	1st to 10th	Night	14% 8% 26% 31% 18% 3	380
	11th to 20th	Night	15% 8% 23% 32% 22% 1	374
	21st to end	Night	15% 3 18% 31% 30% 3	411
June	1st to 10th	Night	16% 4 21% 32% 25% 2	380
	11th to 20th	Night	18% 3 16% 32% 28% 3	380
	21st to end	Night	13% 3 17% 25% 35% 7%	380
July	1st to 10th	Night	17% 4 13% 29% 33% 4	370
	11th to 20th	Night	15% 3 16% 30% 29% 7%	368
	21st to end	Night	12% 2 14% 35% 32% 6%	416
August	1st to 10th	Night	11% 2 14% 29% 38% 5%	378
	11th to 20th	Night	14% 2 23% 33% 24% 4	376
	21st to end	Night	17% 2 16% 30% 30% 5%	527
September	1st to 10th	Night	16% 4 19% 33% 23% 5%	693
	11th to 20th	Night	12% 4 19% 35% 25% 5%	760
	21st to end	Night	19% 5% 22% 34% 17% 3	760
October	1st to 10th	Night	18% 5% 24% 36% 16% 2	760
	11th to 20th	Night	16% 4 25% 38% 14% 2	760
	21st to end	Night	17% 7% 21% 34% 18% 4	836
November	1st to 10th	Night	20% 5% 19% 32% 19% 4	835
	11th to 20th	Night	16% 6% 20% 36% 20% 2	1087
	21st to end	Night	13% 5% 18% 45% 18% 3	1100
December	1st to 10th	Night	15% 5% 16% 43% 18% 2	1100
	11th to 20th	Night	12% 5% 20% 38% 19% 6%	1100
	21st to end	Night	11% 3 15% 41% 24% 6%	1210

Page generated at 2025-03-24 06:04:27. Version ac07

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