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 2024-09-29.

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	14% 12% 5% 11% 7% 6% 6% 4 6% 7% 22%	102128
	Night	12% 13% 5% 13% 8% 7% 7% 5% 7% 6% 18%	96256
Transparency	Day	59% 7% 12% 16% 6%	101842
	Night	63% 7% 10% 15% 5%	96159
Seeing	Day	20% 8% 27% 28% 14% 4	15657
	Night	18% 6% 25% 32% 15% 3	24641

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	8% 9% 3 16% 11% 11% 12% 7% 7% 9% 6%	1899
	11th to 20th	Day	4 13% 8% 19% 8% 8% 9% 6% 10% 7% 7%	1900
	21st to end	Day	8% 19% 7% 15% 8% 7% 7% 6% 11% 8% 4	2500
February	1st to 10th	Day	4 11% 4 11% 10% 11% 11% 8% 10% 9% 10%	1520
	11th to 20th	Day	2 7% 6% 11% 9% 10% 9% 8% 11% 10% 16%	1493
	21st to end	Day	2 8% 6% 9% 7% 7% 9% 11% 13% 9% 19%	1513
March	1st to 10th	Day	5% 9% 4 13% 9% 8% 7% 8% 10% 11% 17%	3118
	11th to 20th	Day	6% 12% 5% 17% 13% 12% 9% 6% 7% 5% 9%	3245
	21st to end	Day	8% 13% 5% 9% 6% 11% 13% 9% 8% 6% 12%	4165
April	1st to 10th	Day	6% 10% 4 15% 10% 10% 9% 5% 5% 6% 20%	2280
	11th to 20th	Day	8% 11% 4 11% 10% 10% 6% 3 5% 6% 25%	2458
	21st to end	Day	10% 11% 5% 14% 10% 7% 5% 4 5% 7% 22%	2660
May	1st to 10th	Day	16% 16% 9% 23% 7% 4 3 2 3 4 13%	4577
	11th to 20th	Day	18% 13% 7% 13% 6% 4 4 3 4 6% 22%	4730
	21st to end	Day	16% 12% 6% 14% 5% 4 5% 4 6% 7% 22%	5521
June	1st to 10th	Day	14% 12% 5% 10% 7% 5% 4 4 6% 8% 27%	3040
	11th to 20th	Day	16% 10% 3 7% 4 4 4 4 7% 8% 33%	3040
	21st to end	Day	17% 10% 3 7% 5% 4 5% 4 5% 7% 32%	3040
July	1st to 10th	Day	16% 10% 3 8% 5% 5% 5% 4 6% 8% 30%	5070
	11th to 20th	Day	15% 9% 3 8% 5% 5% 5% 4 6% 8% 32%	5070
	21st to end	Day	14% 9% 4 9% 6% 5% 5% 4 6% 8% 31%	5418
August	1st to 10th	Day	18% 7% 2 8% 5% 4 4 3 5% 7% 36%	4433
	11th to 20th	Day	17% 9% 3 7% 5% 4 4 3 4 6% 37%	4356
	21st to end	Day	19% 9% 3 7% 5% 4 5% 3 5% 8% 31%	4673
September	1st to 10th	Day	25% 14% 4 9% 5% 5% 5% 3 5% 6% 19%	2294
	11th to 20th	Day	24% 15% 4 9% 5% 4 4 3 5% 7% 21%	2199
	21st to end	Day	24% 16% 3 8% 5% 4 4 3 4 5% 24%	2069
October	1st to 10th	Day	25% 22% 4 10% 5% 4 4 4 4 4 13%	1803
	11th to 20th	Day	22% 24% 5% 11% 5% 5% 4 3 4 4 13%	1620
	21st to end	Day	17% 24% 6% 11% 6% 4 4 3 4 4 16%	1771
November	1st to 10th	Day	16% 21% 5% 13% 7% 5% 6% 4 4 5% 15%	1351
	11th to 20th	Day	11% 18% 9% 14% 6% 7% 6% 4 6% 6% 14%	1330
	21st to end	Day	11% 18% 6% 15% 8% 8% 6% 5% 5% 7% 11%	1306
December	1st to 10th	Day	9% 12% 5% 18% 11% 7% 6% 6% 8% 6% 12%	1140
	11th to 20th	Day	5% 14% 5% 14% 9% 9% 7% 5% 8% 9% 15%	1436
	21st to end	Day	7% 13% 7% 14% 8% 11% 8% 4 9% 6% 13%	2090
January	1st to 10th	Night	6% 10% 5% 18% 13% 11% 11% 6% 8% 6% 5%	5057
	11th to 20th	Night	3 10% 6% 20% 12% 10% 9% 8% 8% 5% 8%	5070
	21st to end	Night	7% 15% 6% 17% 8% 7% 7% 8% 9% 8% 7%	5163
February	1st to 10th	Night	4 10% 5% 11% 11% 11% 12% 9% 11% 7% 9%	2660
	11th to 20th	Night	4 8% 5% 11% 9% 10% 11% 8% 12% 8% 12%	2570
	21st to end	Night	3 9% 4 10% 7% 8% 9% 10% 12% 9% 19%	1941
March	1st to 10th	Night	5% 10% 4 12% 7% 7% 9% 8% 10% 10% 18%	3722
	11th to 20th	Night	6% 12% 5% 15% 14% 10% 7% 5% 7% 6% 12%	3738
	21st to end	Night	10% 12% 3 11% 7% 10% 10% 7% 8% 6% 16%	3471
April	1st to 10th	Night	8% 12% 3 12% 8% 8% 7% 5% 5% 6% 25%	1900
	11th to 20th	Night	9% 13% 4 10% 8% 9% 6% 3 5% 5% 27%	1667
	21st to end	Night	10% 9% 4 13% 8% 7% 5% 5% 6% 6% 27%	1520
May	1st to 10th	Night	16% 16% 8% 22% 7% 3 2 2 3 3 16%	2373
	11th to 20th	Night	22% 14% 6% 13% 5% 3 4 3 3 5% 22%	2172
	21st to end	Night	17% 9% 6% 18% 5% 4 4 4 7% 7% 21%	2066
June	1st to 10th	Night	18% 10% 6% 8% 5% 5% 5% 5% 6% 6% 27%	1140
	11th to 20th	Night	20% 9% 4 7% 4 3 4 4 7% 7% 32%	1140
	21st to end	Night	17% 9% 4 6% 7% 4 5% 4 6% 7% 32%	1140
July	1st to 10th	Night	17% 12% 4 9% 6% 4 5% 4 6% 7% 25%	1900
	11th to 20th	Night	13% 9% 4 10% 6% 5% 7% 6% 9% 8% 22%	1900
	21st to end	Night	11% 10% 3 9% 6% 5% 5% 5% 8% 10% 28%	2144
August	1st to 10th	Night	16% 11% 4 11% 6% 5% 5% 4 5% 6% 28%	2504
	11th to 20th	Night	18% 9% 3 7% 5% 4 4 4 4 7% 36%	2483
	21st to end	Night	25% 9% 3 7% 4 4 4 3 5% 6% 30%	2987
September	1st to 10th	Night	21% 11% 4 9% 5% 5% 4 4 5% 6% 25%	1883
	11th to 20th	Night	22% 15% 4 8% 4 4 4 3 5% 7% 23%	1962
	21st to end	Night	22% 14% 5% 8% 4 4 4 2 5% 6% 26%	2079
October	1st to 10th	Night	26% 18% 5% 9% 4 4 4 2 5% 6% 18%	2154
	11th to 20th	Night	24% 21% 5% 9% 6% 4 4 3 3 5% 17%	2340
	21st to end	Night	20% 20% 6% 12% 5% 5% 4 3 4 5% 16%	2582
November	1st to 10th	Night	19% 19% 6% 12% 6% 6% 5% 4 5% 5% 15%	2727
	11th to 20th	Night	10% 17% 7% 14% 7% 6% 5% 4 6% 5% 18%	2850
	21st to end	Night	12% 17% 7% 15% 7% 6% 5% 4 6% 7% 13%	2873
December	1st to 10th	Night	7% 17% 6% 17% 9% 7% 7% 6% 7% 7% 11%	3040
	11th to 20th	Night	5% 13% 6% 16% 10% 9% 7% 5% 7% 7% 14%	3761
	21st to end	Night	7% 14% 6% 15% 9% 10% 8% 6% 6% 5% 14%	5577

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	69% 9% 10% 9% 3	1900
	11th to 20th	Day	70% 6% 15% 7% 2	1900
	21st to end	Day	70% 6% 7% 10% 7%	2500
February	1st to 10th	Day	63% 8% 10% 14% 6%	1520
	11th to 20th	Day	55% 7% 14% 17% 6%	1501
	21st to end	Day	49% 7% 10% 21% 13%	1513
March	1st to 10th	Day	56% 6% 9% 18% 11% 1	3120
	11th to 20th	Day	73% 6% 8% 9% 4	3245
	21st to end	Day	63% 7% 8% 14% 7%	4165
April	1st to 10th	Day	63% 5% 8% 16% 8%	2280
	11th to 20th	Day	59% 5% 9% 17% 9%	2458
	21st to end	Day	59% 6% 10% 16% 8% 1	2660
May	1st to 10th	Day	76% 5% 6% 10% 3	4589
	11th to 20th	Day	62% 7% 9% 15% 8%	4734
	21st to end	Day	58% 9% 12% 17% 4	5521
June	1st to 10th	Day	54% 8% 10% 18% 9%	3040
	11th to 20th	Day	48% 8% 13% 23% 7% 1	3040
	21st to end	Day	49% 9% 12% 22% 8%	3040
July	1st to 10th	Day	51% 8% 14% 21% 5%	4991
	11th to 20th	Day	49% 8% 18% 21% 4	4983
	21st to end	Day	49% 9% 15% 19% 7%	5246
August	1st to 10th	Day	48% 7% 21% 22% 2	4417
	11th to 20th	Day	47% 7% 13% 21% 11%	4398
	21st to end	Day	51% 7% 14% 23% 5%	4672
September	1st to 10th	Day	65% 9% 12% 12% 2	2294
	11th to 20th	Day	63% 6% 14% 14% 2	2199
	21st to end	Day	64% 6% 10% 16% 4	2069
October	1st to 10th	Day	73% 7% 9% 9% 2	1803
	11th to 20th	Day	76% 7% 10% 6% 1	1620
	21st to end	Day	71% 7% 9% 10% 2	1771
November	1st to 10th	Day	72% 7% 13% 7% 1	1351
	11th to 20th	Day	71% 8% 12% 8% 1	1330
	21st to end	Day	70% 8% 10% 11% 1	1306
December	1st to 10th	Day	68% 8% 9% 12% 3	1140
	11th to 20th	Day	61% 8% 11% 12% 7% 1	1436
	21st to end	Day	67% 6% 9% 15% 3	2090
January	1st to 10th	Night	74% 7% 10% 7% 2	5057
	11th to 20th	Night	70% 6% 13% 8% 3	5070
	21st to end	Night	69% 5% 8% 14% 5%	5163
February	1st to 10th	Night	64% 7% 9% 14% 6%	2660
	11th to 20th	Night	58% 8% 11% 19% 4	2572
	21st to end	Night	51% 6% 15% 18% 11%	1941
March	1st to 10th	Night	53% 7% 9% 22% 9%	3733
	11th to 20th	Night	70% 6% 8% 11% 4	3739
	21st to end	Night	61% 6% 7% 14% 12%	3471
April	1st to 10th	Night	58% 5% 7% 20% 10% 1	1900
	11th to 20th	Night	57% 5% 7% 18% 11% 1	1667
	21st to end	Night	55% 6% 8% 19% 11%	1520
May	1st to 10th	Night	73% 3 6% 12% 5%	2380
	11th to 20th	Night	65% 6% 8% 10% 12%	2173
	21st to end	Night	58% 7% 10% 16% 9%	2066
June	1st to 10th	Night	55% 8% 9% 17% 11% 1	1140
	11th to 20th	Night	49% 6% 13% 22% 11% 1	1140
	21st to end	Night	51% 8% 10% 23% 7%	1140
July	1st to 10th	Night	58% 7% 8% 22% 4	1870
	11th to 20th	Night	54% 10% 14% 20% 1	1864
	21st to end	Night	50% 9% 17% 20% 3	2076
August	1st to 10th	Night	58% 7% 14% 21%	2496
	11th to 20th	Night	48% 6% 11% 25% 10%	2507
	21st to end	Night	55% 6% 12% 18% 8% 1	2987
September	1st to 10th	Night	58% 9% 17% 13% 3	1883
	11th to 20th	Night	61% 7% 13% 17% 2	1962
	21st to end	Night	60% 6% 12% 18% 4	2079
October	1st to 10th	Night	68% 7% 10% 12% 2	2154
	11th to 20th	Night	71% 7% 13% 8% 1	2340
	21st to end	Night	70% 9% 11% 8% 2	2582
November	1st to 10th	Night	71% 8% 10% 10% 1	2727
	11th to 20th	Night	65% 10% 13% 11% 1	2850
	21st to end	Night	69% 8% 10% 12% 1	2873
December	1st to 10th	Night	68% 8% 10% 12% 2	3040
	11th to 20th	Night	66% 7% 9% 14% 4 1	3760
	21st to end	Night	67% 7% 8% 14% 3	5577

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% 4 29% 36% 18% 3	180
	11th to 20th	Day	9% 9% 31% 31% 14% 6%	180
	21st to end	Day	8% 6% 24% 36% 18% 8%	271
February	1st to 10th	Day	8% 5% 27% 38% 18% 4	270
	11th to 20th	Day	6% 6% 30% 36% 17% 5%	270
	21st to end	Day	5% 7% 28% 35% 19% 6%	225
March	1st to 10th	Day	8% 6% 26% 28% 18% 14%	270
	11th to 20th	Day	8% 6% 32% 33% 17% 5%	304
	21st to end	Day	11% 9% 27% 29% 16% 9%	506
April	1st to 10th	Day	11% 4 24% 33% 21% 7%	460
	11th to 20th	Day	14% 8% 26% 31% 15% 6%	471
	21st to end	Day	13% 8% 27% 31% 17% 4	650
May	1st to 10th	Day	20% 9% 28% 26% 13% 4	653
	11th to 20th	Day	18% 10% 31% 28% 10% 3	675
	21st to end	Day	22% 10% 34% 26% 8% 1	734
June	1st to 10th	Day	19% 9% 31% 27% 11% 4	680
	11th to 20th	Day	18% 10% 30% 27% 12% 3	680
	21st to end	Day	20% 7% 31% 27% 13% 3	680
July	1st to 10th	Day	20% 7% 28% 29% 12% 3	671
	11th to 20th	Day	20% 7% 30% 27% 12% 4	668
	21st to end	Day	25% 6% 21% 27% 17% 4	739
August	1st to 10th	Day	27% 7% 24% 24% 15% 4	676
	11th to 20th	Day	26% 7% 20% 28% 13% 6%	665
	21st to end	Day	30% 6% 21% 27% 12% 4	628
September	1st to 10th	Day	33% 9% 22% 23% 10% 1	490
	11th to 20th	Day	26% 10% 27% 27% 9% 2	490
	21st to end	Day	30% 8% 29% 20% 11% 2	483
October	1st to 10th	Day	29% 13% 28% 22% 8% 1	298
	11th to 20th	Day	29% 11% 25% 17% 13% 4	270
	21st to end	Day	25% 12% 26% 26% 10% 2	297
November	1st to 10th	Day	25% 16% 22% 21% 12% 4	205
	11th to 20th	Day	17% 11% 29% 28% 13% 2	180
	21st to end	Day	22% 7% 31% 22% 12% 6%	180
December	1st to 10th	Day	16% 4 26% 34% 17% 3	180
	11th to 20th	Day	14% 7% 24% 30% 20% 4	180
	21st to end	Day	8% 6% 19% 41% 19% 7%	198
January	1st to 10th	Night	13% 6% 26% 35% 15% 5%	950
	11th to 20th	Night	11% 5% 28% 35% 17% 3	950
	21st to end	Night	10% 5% 26% 33% 20% 6%	1045
February	1st to 10th	Night	8% 6% 25% 38% 20% 2	950
	11th to 20th	Night	6% 4 26% 39% 20% 5%	929
	21st to end	Night	6% 5% 25% 35% 23% 7%	685
March	1st to 10th	Night	8% 5% 22% 39% 23% 4	760
	11th to 20th	Night	11% 8% 29% 35% 15% 2	760
	21st to end	Night	11% 9% 21% 33% 20% 5%	632
April	1st to 10th	Night	12% 4 23% 38% 16% 6%	570
	11th to 20th	Night	15% 4 23% 36% 18% 3	563
	21st to end	Night	13% 6% 26% 36% 16% 3	570
May	1st to 10th	Night	20% 7% 23% 33% 15% 2	485
	11th to 20th	Night	23% 7% 28% 31% 11%	374
	21st to end	Night	24% 6% 30% 31% 9%	411
June	1st to 10th	Night	23% 6% 29% 30% 12%	380
	11th to 20th	Night	22% 5% 28% 33% 12% 1	380
	21st to end	Night	18% 6% 24% 38% 13% 1	380
July	1st to 10th	Night	22% 6% 21% 38% 13%	370
	11th to 20th	Night	19% 3 24% 37% 15% 2	368
	21st to end	Night	23% 5% 20% 36% 15% 1	416
August	1st to 10th	Night	25% 4 19% 32% 18% 2	554
	11th to 20th	Night	29% 4 17% 31% 17% 2	564
	21st to end	Night	31% 3 20% 33% 12% 1	627
September	1st to 10th	Night	26% 7% 25% 28% 13% 1	570
	11th to 20th	Night	30% 7% 27% 26% 10%	632
	21st to end	Night	29% 8% 26% 25% 11% 1	756
October	1st to 10th	Night	37% 8% 26% 23% 4 1	720
	11th to 20th	Night	35% 6% 20% 25% 12% 2	720
	21st to end	Night	27% 9% 27% 22% 12% 3	800
November	1st to 10th	Night	26% 9% 24% 25% 13% 3	925
	11th to 20th	Night	17% 9% 30% 30% 11% 3	950
	21st to end	Night	20% 9% 31% 27% 10% 3	950
December	1st to 10th	Night	14% 8% 34% 32% 9% 2	950
	11th to 20th	Night	12% 7% 24% 34% 19% 4	950
	21st to end	Night	12% 6% 24% 34% 18% 6%	1045

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