Thunder Bay
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Thunder Bay clear sky chart from 2005-11-06 to 2025-09-04.

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% 9% 3 8% 5% 5% 5% 4 6% 8% 37%	126132
	Night	12% 11% 4 9% 5% 5% 5% 4 6% 7% 33%	122816
Transparency	Day	45% 6% 14% 23% 10% 1	129523
	Night	50% 5% 13% 22% 10% 1	127511
Seeing	Day	16% 12% 32% 26% 11% 3	17186
	Night	18% 10% 29% 30% 13% 2	26991

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	7% 11% 4 11% 6% 7% 6% 5% 6% 7% 31%	3617
	11th to 20th	Day	6% 11% 6% 13% 7% 4 4 4 8% 11% 25%	3589
	21st to end	Day	8% 13% 6% 14% 7% 6% 6% 4 5% 7% 23%	3982
February	1st to 10th	Day	7% 10% 4 12% 5% 4 5% 4 7% 6% 35%	1690
	11th to 20th	Day	8% 11% 3 8% 7% 6% 5% 4 5% 6% 38%	1774
	21st to end	Day	6% 12% 5% 9% 7% 5% 5% 4 6% 7% 34%	1650
March	1st to 10th	Day	10% 10% 3 7% 4 5% 5% 4 5% 6% 40%	4478
	11th to 20th	Day	8% 10% 4 10% 5% 6% 5% 4 6% 5% 36%	4745
	21st to end	Day	18% 11% 4 9% 5% 4 4 3 5% 5% 32%	5923
April	1st to 10th	Day	13% 10% 3 7% 6% 5% 6% 4 6% 7% 33%	2400
	11th to 20th	Day	17% 9% 3 9% 4 4 6% 4 6% 6% 31%	2374
	21st to end	Day	17% 10% 3 8% 5% 4 4 3 5% 6% 35%	2673
May	1st to 10th	Day	13% 7% 2 6% 4 3 3 3 5% 6% 49%	6348
	11th to 20th	Day	15% 8% 2 6% 4 4 4 4 5% 7% 40%	6319
	21st to end	Day	10% 5% 2 7% 5% 5% 5% 5% 7% 8% 41%	6949
June	1st to 10th	Day	15% 6% 3 6% 4 5% 5% 5% 6% 8% 38%	2800
	11th to 20th	Day	13% 6% 2 6% 4 5% 5% 4 6% 9% 39%	2989
	21st to end	Day	16% 7% 3 6% 5% 4 5% 5% 6% 8% 35%	2804
July	1st to 10th	Day	11% 4 2 5% 3 4 5% 4 7% 10% 45%	6359
	11th to 20th	Day	10% 6% 2 5% 4 4 5% 4 7% 10% 42%	6360
	21st to end	Day	8% 5% 2 5% 4 4 5% 4 7% 11% 45%	6926
August	1st to 10th	Day	9% 6% 3 8% 6% 5% 6% 5% 7% 9% 36%	4563
	11th to 20th	Day	12% 3 1 4 3 3 5% 4 6% 9% 49%	4410
	21st to end	Day	9% 6% 2 6% 4 4 4 4 7% 10% 43%	4312
September	1st to 10th	Day	12% 5% 3 6% 5% 5% 5% 4 5% 7% 44%	2316
	11th to 20th	Day	13% 6% 3 7% 4 4 5% 4 6% 9% 40%	2228
	21st to end	Day	14% 8% 3 7% 5% 5% 4 4 6% 8% 35%	2090
October	1st to 10th	Day	16% 9% 3 8% 4 4 5% 4 6% 7% 34%	1910
	11th to 20th	Day	17% 11% 3 7% 6% 5% 5% 5% 7% 7% 28%	1835
	21st to end	Day	20% 11% 3 7% 6% 5% 5% 3 5% 6% 27%	1881
November	1st to 10th	Day	13% 10% 4 10% 7% 5% 5% 4 5% 9% 27%	1738
	11th to 20th	Day	16% 14% 4 10% 6% 5% 5% 4 5% 6% 24%	1600
	21st to end	Day	16% 13% 3 8% 6% 5% 5% 4 7% 8% 24%	1600
December	1st to 10th	Day	11% 12% 5% 10% 8% 6% 6% 4 7% 7% 23%	2298
	11th to 20th	Day	9% 15% 6% 13% 7% 5% 4 4 6% 8% 24%	2621
	21st to end	Day	22% 20% 5% 9% 4 4 4 3 5% 5% 19%	3981
January	1st to 10th	Night	7% 13% 4 11% 8% 6% 5% 4 5% 5% 32%	6348
	11th to 20th	Night	5% 12% 6% 16% 9% 5% 5% 5% 7% 7% 21%	6316
	21st to end	Night	6% 15% 7% 13% 7% 6% 5% 4 5% 7% 26%	6996
February	1st to 10th	Night	8% 10% 4 11% 6% 6% 5% 5% 5% 7% 33%	2707
	11th to 20th	Night	9% 9% 3 9% 6% 6% 7% 4 6% 6% 35%	2512
	21st to end	Night	7% 12% 5% 10% 8% 4 5% 4 7% 7% 30%	1980
March	1st to 10th	Night	10% 8% 4 8% 5% 5% 6% 4 6% 7% 38%	5362
	11th to 20th	Night	9% 12% 4 10% 6% 5% 6% 4 6% 8% 28%	5258
	21st to end	Night	18% 12% 4 9% 5% 4 4 3 4 6% 30%	5008
April	1st to 10th	Night	14% 10% 4 8% 6% 5% 5% 4 5% 7% 32%	2000
	11th to 20th	Night	17% 11% 3 6% 4 4 5% 5% 5% 7% 33%	1974
	21st to end	Night	17% 11% 3 7% 5% 4 4 3 6% 6% 35%	1725
May	1st to 10th	Night	16% 6% 2 4 3 3 3 3 5% 7% 47%	3613
	11th to 20th	Night	14% 9% 3 7% 5% 4 4 3 5% 6% 39%	3596
	21st to end	Night	14% 7% 3 7% 4 4 4 4 5% 6% 43%	3949
June	1st to 10th	Night	15% 9% 3 6% 4 5% 5% 4 6% 7% 37%	1600
	11th to 20th	Night	13% 6% 3 7% 5% 6% 5% 4 6% 7% 39%	1400
	21st to end	Night	14% 8% 3 7% 4 4 4 5% 6% 8% 37%	1583
July	1st to 10th	Night	7% 7% 2 7% 5% 4 5% 4 6% 9% 44%	3620
	11th to 20th	Night	10% 6% 3 8% 4 4 5% 4 7% 8% 43%	3620
	21st to end	Night	11% 6% 2 6% 5% 4 4 5% 6% 9% 42%	3948
August	1st to 10th	Night	12% 6% 2 6% 4 4 5% 4 6% 10% 42%	2597
	11th to 20th	Night	11% 4 2 5% 4 3 5% 4 6% 8% 47%	2643
	21st to end	Night	15% 8% 3 6% 4 4 4 3 5% 7% 42%	3586
September	1st to 10th	Night	12% 5% 2 5% 3 4 4 4 5% 7% 49%	1930
	11th to 20th	Night	13% 7% 3 6% 3 3 4 3 5% 9% 43%	1933
	21st to end	Night	16% 8% 3 7% 4 4 4 4 5% 8% 37%	2090
October	1st to 10th	Night	17% 8% 2 6% 4 5% 5% 4 5% 7% 37%	2268
	11th to 20th	Night	14% 11% 4 8% 5% 4 5% 4 5% 7% 34%	2344
	21st to end	Night	18% 12% 3 9% 5% 5% 5% 4 5% 6% 28%	2717
November	1st to 10th	Night	15% 10% 4 8% 6% 5% 6% 4 7% 6% 29%	2556
	11th to 20th	Night	17% 13% 4 11% 6% 5% 5% 4 5% 6% 24%	2800
	21st to end	Night	14% 15% 4 10% 6% 5% 5% 4 6% 7% 23%	2800
December	1st to 10th	Night	11% 15% 5% 12% 6% 6% 6% 5% 7% 7% 21%	4873
	11th to 20th	Night	9% 12% 6% 14% 6% 5% 5% 4 6% 7% 26%	5568
	21st to end	Night	22% 21% 5% 9% 4 4 3 3 5% 6% 18%	6996

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	52% 5% 11% 22% 10% 1	3618
	11th to 20th	Day	52% 5% 10% 28% 5%	3620
	21st to end	Day	61% 6% 9% 16% 8% 1	3982
February	1st to 10th	Day	47% 7% 14% 19% 11% 1	1690
	11th to 20th	Day	48% 6% 13% 23% 9% 2	1783
	21st to end	Day	48% 7% 14% 16% 13% 1	1650
March	1st to 10th	Day	44% 5% 13% 17% 16% 4	4480
	11th to 20th	Day	48% 7% 11% 21% 13% 1	4745
	21st to end	Day	54% 5% 7% 18% 13% 2	5923
April	1st to 10th	Day	47% 9% 11% 20% 12% 2	2400
	11th to 20th	Day	50% 8% 11% 21% 8% 1	2374
	21st to end	Day	49% 6% 11% 18% 14% 1	2673
May	1st to 10th	Day	37% 5% 14% 27% 17% 1	6357
	11th to 20th	Day	43% 6% 12% 20% 18% 1	6323
	21st to end	Day	37% 7% 13% 23% 18% 2	6949
June	1st to 10th	Day	40% 9% 15% 21% 15% 1	2800
	11th to 20th	Day	40% 7% 15% 24% 12% 1	2989
	21st to end	Day	45% 7% 16% 22% 10% 1	2804
July	1st to 10th	Day	35% 6% 18% 30% 11% 1	6291
	11th to 20th	Day	38% 4 21% 32% 6%	6285
	21st to end	Day	33% 4 19% 34% 9% 1	6728
August	1st to 10th	Day	42% 6% 17% 26% 9% 1	4548
	11th to 20th	Day	32% 6% 23% 30% 9%	4452
	21st to end	Day	37% 6% 16% 32% 9%	4311
September	1st to 10th	Day	38% 7% 17% 28% 9%	2316
	11th to 20th	Day	40% 10% 24% 22% 4 1	2228
	21st to end	Day	46% 9% 17% 20% 6% 1	2090
October	1st to 10th	Day	39% 6% 17% 26% 11% 2	3170
	11th to 20th	Day	48% 6% 14% 27% 5%	3025
	21st to end	Day	56% 5% 15% 19% 5%	3081
November	1st to 10th	Day	53% 8% 17% 15% 6% 1	1738
	11th to 20th	Day	60% 7% 13% 16% 4	1600
	21st to end	Day	56% 7% 16% 15% 5%	1600
December	1st to 10th	Day	59% 6% 18% 14% 3	2298
	11th to 20th	Day	60% 5% 11% 10% 12% 2	2621
	21st to end	Day	69% 3 7% 16% 4	3981
January	1st to 10th	Night	52% 4 13% 21% 9% 1	6348
	11th to 20th	Night	61% 5% 8% 18% 8%	6360
	21st to end	Night	57% 6% 11% 17% 8%	6996
February	1st to 10th	Night	50% 5% 13% 21% 10% 1	2707
	11th to 20th	Night	48% 6% 12% 21% 12% 1	2513
	21st to end	Night	51% 7% 13% 18% 10% 1	1980
March	1st to 10th	Night	45% 5% 12% 14% 17% 6%	5373
	11th to 20th	Night	52% 6% 14% 19% 9% 1	5259
	21st to end	Night	55% 5% 7% 19% 13% 1	5008
April	1st to 10th	Night	51% 7% 9% 20% 14% 1	2000
	11th to 20th	Night	50% 6% 10% 22% 11% 2	1974
	21st to end	Night	48% 6% 10% 22% 12% 2	1725
May	1st to 10th	Night	37% 5% 13% 28% 15% 3	3620
	11th to 20th	Night	44% 6% 9% 22% 16% 3	3597
	21st to end	Night	40% 6% 9% 26% 19% 1	3950
June	1st to 10th	Night	44% 9% 12% 21% 14% 2	1600
	11th to 20th	Night	42% 7% 12% 22% 15% 1	1400
	21st to end	Night	44% 7% 15% 21% 10% 2	1583
July	1st to 10th	Night	37% 4 19% 26% 13% 1	3580
	11th to 20th	Night	39% 4 19% 31% 7%	3572
	21st to end	Night	39% 5% 14% 32% 10% 1	3832
August	1st to 10th	Night	38% 5% 17% 30% 9% 1	2588
	11th to 20th	Night	34% 5% 18% 33% 9%	2667
	21st to end	Night	44% 5% 14% 28% 10%	3586
September	1st to 10th	Night	34% 7% 16% 30% 13%	1930
	11th to 20th	Night	37% 8% 21% 30% 3	1933
	21st to end	Night	45% 8% 16% 25% 5%	2090
October	1st to 10th	Night	35% 5% 15% 28% 17%	3788
	11th to 20th	Night	45% 5% 13% 28% 9%	3893
	21st to end	Night	58% 5% 15% 17% 6%	4467
November	1st to 10th	Night	52% 8% 13% 20% 6%	2556
	11th to 20th	Night	59% 7% 13% 16% 5%	2800
	21st to end	Night	58% 8% 14% 16% 5%	2800
December	1st to 10th	Night	61% 5% 18% 14% 2	4873
	11th to 20th	Night	57% 4 13% 17% 8%	5567
	21st to end	Night	68% 4 8% 17% 4	6996

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	13% 11% 36% 30% 9% 1	300
	11th to 20th	Day	15% 9% 39% 26% 9% 2	300
	21st to end	Day	14% 12% 39% 28% 6% 1	330
February	1st to 10th	Day	14% 14% 41% 22% 8% 2	300
	11th to 20th	Day	11% 18% 39% 22% 8% 1	300
	21st to end	Day	14% 20% 45% 17% 4	249
March	1st to 10th	Day	11% 18% 39% 25% 6% 1	300
	11th to 20th	Day	14% 20% 35% 23% 7% 1	300
	21st to end	Day	20% 11% 37% 22% 8% 2	530
April	1st to 10th	Day	16% 14% 33% 29% 7% 1	500
	11th to 20th	Day	24% 14% 27% 23% 11% 1	492
	21st to end	Day	24% 13% 28% 24% 10% 2	573
May	1st to 10th	Day	18% 14% 36% 21% 9% 3	703
	11th to 20th	Day	16% 10% 36% 25% 9% 3	725
	21st to end	Day	17% 11% 34% 24% 11% 3	789
June	1st to 10th	Day	16% 11% 36% 24% 11% 2	730
	11th to 20th	Day	15% 9% 33% 29% 12% 2	730
	21st to end	Day	18% 7% 28% 31% 14% 2	730
July	1st to 10th	Day	9% 6% 32% 31% 17% 4	721
	11th to 20th	Day	12% 8% 32% 28% 14% 7%	718
	21st to end	Day	8% 6% 31% 32% 17% 6%	794
August	1st to 10th	Day	12% 6% 33% 31% 14% 5%	726
	11th to 20th	Day	13% 6% 26% 31% 18% 6%	703
	21st to end	Day	16% 9% 25% 28% 17% 4	583
September	1st to 10th	Day	14% 10% 28% 27% 16% 6%	502
	11th to 20th	Day	18% 9% 29% 24% 16% 4	490
	21st to end	Day	18% 13% 29% 24% 14% 3	490
October	1st to 10th	Day	22% 15% 28% 27% 8% 1	310
	11th to 20th	Day	19% 16% 37% 20% 7% 1	300
	21st to end	Day	31% 19% 32% 14% 3 1	330
November	1st to 10th	Day	20% 22% 27% 26% 4 2	300
	11th to 20th	Day	28% 20% 33% 14% 5% 1	300
	21st to end	Day	27% 17% 28% 22% 6%	300
December	1st to 10th	Day	23% 11% 35% 22% 8% 2	208
	11th to 20th	Day	21% 22% 35% 19% 5% 1	200
	21st to end	Day	22% 17% 30% 22% 8% 1	330
January	1st to 10th	Night	15% 12% 37% 28% 7%	1000
	11th to 20th	Night	16% 13% 37% 27% 7% 1	1000
	21st to end	Night	17% 8% 39% 27% 8% 1	1100
February	1st to 10th	Night	14% 12% 36% 29% 8% 1	1000
	11th to 20th	Night	13% 12% 32% 31% 11% 1	958
	21st to end	Night	13% 11% 35% 32% 9%	660
March	1st to 10th	Night	12% 11% 33% 34% 9% 1	800
	11th to 20th	Night	17% 12% 32% 30% 9% 1	723
	21st to end	Night	17% 10% 29% 27% 15% 2	660
April	1st to 10th	Night	19% 9% 30% 33% 9% 1	600
	11th to 20th	Night	21% 11% 24% 30% 14% 1	593
	21st to end	Night	22% 6% 23% 33% 14% 2	600
May	1st to 10th	Night	23% 7% 23% 32% 14% 2	600
	11th to 20th	Night	17% 6% 26% 35% 14% 3	591
	21st to end	Night	20% 7% 21% 35% 17% 1	650
June	1st to 10th	Night	19% 6% 20% 39% 15% 2	600
	11th to 20th	Night	15% 5% 20% 44% 16% 1	400
	21st to end	Night	18% 3 17% 40% 20% 2	583
July	1st to 10th	Night	11% 2 21% 40% 23% 3	585
	11th to 20th	Night	10% 3 23% 36% 25% 3	582
	21st to end	Night	9% 3 21% 36% 27% 4	657
August	1st to 10th	Night	11% 2 23% 40% 22% 2	597
	11th to 20th	Night	13% 4 17% 31% 30% 5%	594
	21st to end	Night	17% 5% 20% 34% 21% 4	660
September	1st to 10th	Night	14% 4 23% 32% 21% 6%	579
	11th to 20th	Night	16% 5% 26% 32% 18% 3	603
	21st to end	Night	20% 9% 23% 29% 18% 2	760
October	1st to 10th	Night	20% 10% 31% 23% 15% 1	760
	11th to 20th	Night	19% 14% 28% 28% 9% 1	760
	21st to end	Night	25% 12% 28% 27% 7%	836
November	1st to 10th	Night	20% 11% 33% 26% 9% 1	800
	11th to 20th	Night	24% 14% 31% 20% 9% 1	1000
	21st to end	Night	24% 17% 31% 23% 6%	1000
December	1st to 10th	Night	17% 16% 35% 24% 7% 1	1000
	11th to 20th	Night	19% 15% 34% 22% 8% 1	1000
	21st to end	Night	26% 13% 31% 24% 5% 1	1100

Page generated at 2025-09-01 01:51:03. Version ac07

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