Fort Severn
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Fort Severn clear sky chart from 2005-11-06 to 2026-07-01.

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	12% 12% 5% 12% 7% 7% 6% 5% 6% 7% 22%	138763
	Night	10% 12% 5% 13% 8% 8% 7% 6% 7% 6% 17%	133936
Transparency	Day	60% 6% 11% 17% 6%	141995
	Night	64% 6% 10% 15% 5%	138791
Seeing	Day	19% 8% 27% 28% 13% 4	17918
	Night	18% 7% 26% 32% 15% 3	27004

By Month

Percentage Cloud Cover NE (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% 11% 9% 10% 6% 8% 7% 5%	3527
	11th to 20th	Day	3 10% 6% 16% 11% 12% 13% 7% 10% 7% 5%	3516
	21st to end	Day	5% 12% 5% 13% 10% 12% 10% 9% 12% 6% 6%	4649
February	1st to 10th	Day	4 12% 5% 11% 10% 11% 10% 8% 11% 9% 9%	1680
	11th to 20th	Day	3 7% 6% 12% 9% 11% 10% 8% 11% 9% 15%	1655
	21st to end	Day	2 8% 6% 9% 7% 7% 9% 11% 13% 9% 19%	1667
March	1st to 10th	Day	4 7% 4 16% 10% 9% 8% 8% 10% 8% 16%	4578
	11th to 20th	Day	6% 9% 4 13% 11% 12% 11% 7% 8% 6% 12%	4750
	21st to end	Day	5% 10% 4 9% 7% 12% 15% 11% 9% 7% 12%	6101
April	1st to 10th	Day	5% 10% 4 15% 10% 10% 9% 5% 5% 6% 21%	2520
	11th to 20th	Day	8% 12% 4 11% 10% 10% 6% 3 5% 6% 25%	2720
	21st to end	Day	9% 11% 5% 14% 10% 7% 5% 4 5% 7% 22%	2940
May	1st to 10th	Day	13% 17% 9% 19% 8% 5% 4 3 4 5% 16%	8514
	11th to 20th	Day	17% 11% 8% 20% 5% 4 4 3 4 5% 18%	8831
	21st to end	Day	11% 10% 5% 13% 5% 4 4 3 5% 7% 33%	10338
June	1st to 10th	Day	14% 11% 5% 10% 7% 5% 4 4 5% 8% 29%	3360
	11th to 20th	Day	16% 10% 3 7% 4 4 4 4 7% 7% 33%	3360
	21st to end	Day	17% 10% 4 7% 5% 4 4 4 5% 8% 33%	3360
July	1st to 10th	Day	12% 9% 3 8% 5% 5% 5% 4 6% 8% 34%	7260
	11th to 20th	Day	15% 8% 3 8% 5% 5% 5% 4 7% 8% 32%	7260
	21st to end	Day	11% 8% 3 9% 7% 6% 6% 4 6% 8% 32%	7788
August	1st to 10th	Day	18% 8% 2 8% 5% 4 4 3 5% 7% 36%	4573
	11th to 20th	Day	17% 9% 3 7% 5% 4 4 3 4 6% 37%	4496
	21st to end	Day	20% 9% 3 7% 5% 4 5% 3 5% 8% 31%	4820
September	1st to 10th	Day	25% 15% 4 9% 5% 5% 5% 3 5% 6% 18%	2415
	11th to 20th	Day	23% 15% 3 9% 5% 4 4 3 5% 7% 21%	2315
	21st to end	Day	23% 16% 3 9% 5% 5% 4 3 4 5% 24%	2200
October	1st to 10th	Day	24% 21% 4 10% 5% 4 4 4 4 4 14%	2001
	11th to 20th	Day	21% 23% 5% 11% 5% 5% 4 3 4 5% 13%	1800
	21st to end	Day	16% 23% 6% 12% 6% 4 4 3 4 4 17%	1967
November	1st to 10th	Day	16% 21% 5% 13% 6% 6% 6% 4 4 4 16%	1496
	11th to 20th	Day	11% 20% 9% 15% 6% 7% 5% 3 6% 6% 13%	1470
	21st to end	Day	11% 18% 7% 16% 8% 7% 6% 5% 5% 7% 11%	1442
December	1st to 10th	Day	6% 14% 5% 17% 10% 8% 7% 6% 7% 6% 15%	2020
	11th to 20th	Day	5% 15% 5% 15% 9% 10% 9% 6% 9% 7% 10%	2316
	21st to end	Day	10% 18% 9% 13% 6% 9% 7% 4 8% 5% 11%	3058
January	1st to 10th	Night	6% 13% 6% 18% 11% 9% 9% 7% 9% 6% 6%	9398
	11th to 20th	Night	2 8% 6% 19% 13% 12% 10% 9% 8% 6% 7%	9399
	21st to end	Night	4 9% 5% 15% 11% 11% 11% 9% 12% 6% 8%	9561
February	1st to 10th	Night	5% 11% 5% 11% 11% 11% 12% 8% 11% 7% 9%	2940
	11th to 20th	Night	4 8% 5% 12% 9% 10% 13% 8% 12% 8% 12%	2848
	21st to end	Night	3 9% 4 10% 7% 8% 9% 10% 12% 9% 19%	2139
March	1st to 10th	Night	3 7% 3 11% 9% 10% 10% 8% 12% 9% 18%	5482
	11th to 20th	Night	8% 10% 4 13% 13% 9% 9% 7% 8% 6% 14%	5468
	21st to end	Night	7% 10% 3 11% 7% 11% 13% 9% 8% 7% 14%	5077
April	1st to 10th	Night	7% 12% 4 12% 8% 8% 9% 4 5% 7% 25%	2100
	11th to 20th	Night	10% 13% 5% 10% 7% 9% 6% 3 5% 5% 26%	1845
	21st to end	Night	10% 10% 4 13% 8% 7% 5% 5% 6% 6% 26%	1680
May	1st to 10th	Night	14% 15% 7% 18% 9% 5% 3 3 4 4 19%	4414
	11th to 20th	Night	22% 13% 7% 18% 5% 3 3 3 3 5% 18%	4058
	21st to end	Night	11% 8% 5% 17% 5% 5% 4 3 6% 6% 29%	3870
June	1st to 10th	Night	17% 9% 5% 9% 5% 5% 5% 5% 6% 6% 28%	1260
	11th to 20th	Night	20% 9% 4 7% 3 3 4 4 7% 7% 32%	1260
	21st to end	Night	17% 9% 4 6% 7% 4 5% 4 6% 7% 32%	1260
July	1st to 10th	Night	13% 10% 4 8% 6% 4 5% 4 6% 7% 33%	2720
	11th to 20th	Night	15% 9% 4 10% 6% 5% 7% 6% 8% 8% 23%	2720
	21st to end	Night	13% 9% 3 8% 6% 5% 5% 5% 8% 9% 30%	3084
August	1st to 10th	Night	16% 11% 4 11% 6% 5% 5% 4 5% 6% 28%	2584
	11th to 20th	Night	18% 9% 3 7% 5% 4 4 4 4 7% 35%	2563
	21st to end	Night	24% 9% 3 7% 4 4 4 3 5% 6% 31%	3082
September	1st to 10th	Night	21% 11% 4 9% 5% 5% 4 4 5% 6% 25%	1982
	11th to 20th	Night	21% 15% 4 8% 4 4 4 3 5% 7% 24%	2065
	21st to end	Night	22% 15% 5% 8% 5% 4 4 2 5% 6% 26%	2200
October	1st to 10th	Night	26% 18% 4 9% 4 4 4 2 5% 6% 18%	2396
	11th to 20th	Night	23% 20% 5% 9% 6% 4 4 3 3 5% 17%	2600
	21st to end	Night	19% 20% 6% 12% 5% 4 4 3 5% 5% 16%	2870
November	1st to 10th	Night	18% 19% 6% 12% 6% 6% 4 4 5% 5% 15%	3022
	11th to 20th	Night	11% 18% 7% 13% 7% 5% 5% 4 6% 5% 17%	3150
	21st to end	Night	12% 19% 8% 16% 7% 6% 5% 4 5% 7% 12%	3176
December	1st to 10th	Night	5% 15% 6% 17% 10% 9% 7% 5% 8% 7% 12%	5390
	11th to 20th	Night	5% 15% 6% 16% 10% 9% 8% 6% 7% 6% 12%	6111
	21st to end	Night	10% 17% 8% 15% 8% 8% 7% 5% 6% 4 12%	8162

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	74% 6% 6% 8% 6%	3528
	11th to 20th	Day	73% 4 9% 11% 3	3540
	21st to end	Day	69% 4 9% 13% 5%	4649
February	1st to 10th	Day	63% 7% 10% 14% 6%	1680
	11th to 20th	Day	57% 7% 15% 16% 5%	1663
	21st to end	Day	49% 7% 10% 21% 13%	1667
March	1st to 10th	Day	60% 4 7% 14% 12% 3	4580
	11th to 20th	Day	69% 5% 7% 10% 9%	4750
	21st to end	Day	65% 6% 6% 14% 9%	6101
April	1st to 10th	Day	62% 5% 8% 18% 8%	2520
	11th to 20th	Day	60% 5% 9% 16% 9%	2720
	21st to end	Day	60% 6% 10% 16% 7%	2940
May	1st to 10th	Day	72% 4 6% 11% 7%	8523
	11th to 20th	Day	68% 5% 9% 12% 5%	8835
	21st to end	Day	50% 6% 8% 21% 14% 1	10339
June	1st to 10th	Day	53% 8% 10% 19% 10% 1	3360
	11th to 20th	Day	48% 8% 12% 24% 7% 1	3360
	21st to end	Day	49% 8% 11% 24% 8%	3360
July	1st to 10th	Day	46% 7% 13% 29% 6%	7182
	11th to 20th	Day	49% 7% 19% 21% 3	7173
	21st to end	Day	47% 8% 17% 22% 6%	7563
August	1st to 10th	Day	48% 7% 20% 23% 2	4557
	11th to 20th	Day	48% 7% 13% 20% 11%	4538
	21st to end	Day	51% 7% 14% 23% 5%	4819
September	1st to 10th	Day	66% 9% 12% 12% 2	2415
	11th to 20th	Day	63% 7% 14% 14% 2	2315
	21st to end	Day	63% 6% 12% 15% 4	2200
October	1st to 10th	Day	76% 5% 7% 9% 2	3248
	11th to 20th	Day	64% 7% 13% 14% 1	2912
	21st to end	Day	67% 6% 10% 14% 3	3156
November	1st to 10th	Day	72% 7% 13% 7% 2	1496
	11th to 20th	Day	72% 7% 11% 8% 1	1470
	21st to end	Day	72% 8% 9% 10% 1	1442
December	1st to 10th	Day	67% 6% 8% 16% 2	2020
	11th to 20th	Day	68% 6% 9% 12% 5%	2316
	21st to end	Day	71% 5% 9% 12% 2	3058
January	1st to 10th	Night	74% 5% 7% 9% 5%	9398
	11th to 20th	Night	73% 5% 10% 11% 2	9450
	21st to end	Night	67% 4 9% 16% 4	9561
February	1st to 10th	Night	66% 6% 8% 14% 6%	2940
	11th to 20th	Night	60% 7% 11% 18% 4	2850
	21st to end	Night	51% 6% 14% 18% 12%	2139
March	1st to 10th	Night	54% 5% 7% 18% 13% 3	5493
	11th to 20th	Night	66% 5% 6% 13% 9%	5469
	21st to end	Night	64% 5% 6% 14% 11%	5077
April	1st to 10th	Night	58% 5% 7% 20% 10% 1	2100
	11th to 20th	Night	59% 4 7% 19% 10% 1	1845
	21st to end	Night	56% 6% 9% 19% 10%	1680
May	1st to 10th	Night	70% 3 5% 14% 7%	4421
	11th to 20th	Night	70% 5% 8% 11% 6%	4059
	21st to end	Night	54% 5% 8% 19% 12% 2	3870
June	1st to 10th	Night	54% 8% 8% 18% 11% 1	1260
	11th to 20th	Night	49% 5% 12% 23% 10% 1	1260
	21st to end	Night	51% 8% 10% 24% 7%	1260
July	1st to 10th	Night	50% 6% 8% 27% 8% 1	2690
	11th to 20th	Night	55% 9% 14% 20% 1	2684
	21st to end	Night	48% 8% 20% 21% 3	2995
August	1st to 10th	Night	57% 7% 14% 21% 1	2576
	11th to 20th	Night	49% 6% 11% 24% 9%	2587
	21st to end	Night	54% 6% 13% 19% 7% 1	3082
September	1st to 10th	Night	59% 9% 17% 13% 3	1982
	11th to 20th	Night	60% 7% 14% 17% 2	2065
	21st to end	Night	60% 6% 12% 18% 4	2200
October	1st to 10th	Night	69% 5% 9% 13% 3	3929
	11th to 20th	Night	63% 7% 13% 15% 2	4227
	21st to end	Night	69% 7% 9% 12% 3	4632
November	1st to 10th	Night	71% 8% 9% 10% 2	3022
	11th to 20th	Night	66% 9% 12% 11% 1	3150
	21st to end	Night	71% 7% 9% 11% 1	3176
December	1st to 10th	Night	68% 7% 11% 13% 2	5390
	11th to 20th	Night	70% 6% 9% 12% 3	6110
	21st to end	Night	72% 5% 8% 13% 2	8162

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% 5% 31% 35% 16% 3	220
	11th to 20th	Day	9% 9% 30% 32% 15% 6%	220
	21st to end	Day	8% 6% 23% 38% 17% 8%	332
February	1st to 10th	Day	9% 6% 27% 38% 16% 3	330
	11th to 20th	Day	7% 6% 27% 37% 16% 8%	330
	21st to end	Day	5% 7% 30% 35% 18% 6%	273
March	1st to 10th	Day	8% 7% 25% 28% 20% 12%	330
	11th to 20th	Day	7% 7% 31% 35% 17% 4	368
	21st to end	Day	9% 8% 28% 30% 16% 8%	594
April	1st to 10th	Day	9% 4 24% 35% 21% 6%	540
	11th to 20th	Day	14% 9% 27% 31% 14% 5%	553
	21st to end	Day	13% 9% 28% 31% 15% 4	750
May	1st to 10th	Day	20% 10% 28% 25% 12% 5%	753
	11th to 20th	Day	19% 11% 30% 26% 11% 3	775
	21st to end	Day	20% 9% 33% 26% 10% 1	844
June	1st to 10th	Day	19% 9% 31% 26% 11% 4	780
	11th to 20th	Day	19% 10% 29% 28% 12% 2	780
	21st to end	Day	19% 7% 29% 28% 13% 4	780
July	1st to 10th	Day	20% 8% 28% 29% 12% 4	722
	11th to 20th	Day	19% 7% 31% 27% 12% 3	718
	21st to end	Day	24% 6% 23% 27% 16% 4	794
August	1st to 10th	Day	27% 6% 23% 24% 15% 4	726
	11th to 20th	Day	26% 7% 20% 28% 13% 6%	715
	21st to end	Day	29% 6% 21% 28% 12% 4	676
September	1st to 10th	Day	34% 10% 22% 24% 10% 1	530
	11th to 20th	Day	25% 9% 27% 27% 10% 2	530
	21st to end	Day	29% 9% 29% 20% 12% 2	530
October	1st to 10th	Day	28% 14% 30% 21% 7% 1	360
	11th to 20th	Day	26% 12% 28% 17% 13% 4	330
	21st to end	Day	22% 14% 28% 26% 9% 2	363
November	1st to 10th	Day	25% 16% 26% 19% 10% 4	250
	11th to 20th	Day	18% 11% 30% 27% 12% 1	220
	21st to end	Day	23% 7% 30% 24% 11% 5%	220
December	1st to 10th	Day	14% 5% 29% 32% 18% 2	220
	11th to 20th	Day	15% 7% 26% 30% 18% 5%	220
	21st to end	Day	11% 5% 23% 38% 17% 6%	242
January	1st to 10th	Night	13% 6% 27% 35% 14% 4	1050
	11th to 20th	Night	10% 5% 29% 36% 17% 3	1050
	21st to end	Night	9% 5% 26% 34% 19% 6%	1155
February	1st to 10th	Night	9% 7% 25% 38% 19% 2	1050
	11th to 20th	Night	6% 4 24% 39% 21% 5%	1029
	21st to end	Night	6% 5% 26% 35% 22% 6%	755
March	1st to 10th	Night	7% 5% 23% 39% 22% 3	840
	11th to 20th	Night	11% 7% 28% 36% 15% 2	840
	21st to end	Night	10% 9% 22% 34% 19% 5%	698
April	1st to 10th	Night	11% 4 24% 39% 16% 5%	630
	11th to 20th	Night	16% 5% 24% 35% 17% 3	623
	21st to end	Night	13% 6% 27% 36% 15% 3	630
May	1st to 10th	Night	20% 7% 24% 31% 15% 3	536
	11th to 20th	Night	23% 7% 29% 30% 11%	414
	21st to end	Night	22% 6% 30% 30% 10% 1	455
June	1st to 10th	Night	21% 6% 30% 30% 12%	420
	11th to 20th	Night	22% 5% 27% 33% 12%	420
	21st to end	Night	19% 6% 24% 37% 14% 1	420
July	1st to 10th	Night	21% 6% 22% 38% 13%	390
	11th to 20th	Night	19% 4 24% 37% 14% 2	388
	21st to end	Night	23% 4 21% 35% 16% 1	438
August	1st to 10th	Night	26% 4 19% 32% 18% 2	584
	11th to 20th	Night	30% 4 16% 31% 16% 2	594
	21st to end	Night	31% 3 19% 33% 13% 1	660
September	1st to 10th	Night	27% 7% 25% 28% 12% 1	600
	11th to 20th	Night	29% 7% 26% 28% 10%	665
	21st to end	Night	29% 7% 26% 26% 11% 1	800
October	1st to 10th	Night	37% 10% 28% 21% 4 1	800
	11th to 20th	Night	34% 7% 22% 24% 12% 2	800
	21st to end	Night	27% 10% 28% 22% 12% 2	890
November	1st to 10th	Night	26% 10% 24% 24% 12% 3	1025
	11th to 20th	Night	18% 9% 30% 29% 11% 3	1050
	21st to end	Night	20% 9% 31% 26% 10% 3	1050
December	1st to 10th	Night	13% 8% 35% 32% 10% 2	1050
	11th to 20th	Night	12% 7% 25% 34% 18% 4	1050
	21st to end	Night	12% 7% 25% 33% 17% 6%	1155

Page generated at 2026-06-28 02:10:46. Version ac07

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