Thunder Bay Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Thunder Bay Observatory clear sky chart from 2005-11-06 to 2026-10-29.

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	12% 10% 3 8% 5% 5% 5% 4 6% 7% 36%	150104
	Night	11% 10% 4 9% 5% 5% 5% 4 6% 7% 34%	142350
Transparency	Day	44% 6% 14% 23% 11% 1	153509
	Night	48% 6% 13% 22% 10% 1	147031
Seeing	Day	17% 15% 34% 22% 9% 2	18852
	Night	17% 11% 30% 30% 11% 1	28783

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% 12% 6% 12% 6% 6% 5% 4 5% 6% 31%	4691
	11th to 20th	Day	6% 13% 8% 16% 6% 5% 4 4 6% 9% 24%	4679
	21st to end	Day	6% 11% 7% 12% 7% 6% 6% 5% 6% 7% 26%	5156
February	1st to 10th	Day	7% 12% 5% 12% 5% 5% 6% 4 5% 6% 33%	1793
	11th to 20th	Day	8% 12% 4 9% 5% 6% 5% 4 5% 7% 36%	1855
	21st to end	Day	7% 14% 5% 11% 6% 5% 4 3 6% 7% 32%	1730
March	1st to 10th	Day	10% 11% 4 8% 6% 5% 4 3 5% 6% 39%	4578
	11th to 20th	Day	8% 11% 6% 12% 6% 6% 5% 4 4 6% 33%	4891
	21st to end	Day	18% 16% 4 10% 4 4 3 3 4 4 31%	6042
April	1st to 10th	Day	14% 12% 4 10% 6% 5% 5% 4 5% 6% 30%	2520
	11th to 20th	Day	17% 12% 4 10% 5% 5% 4 4 5% 6% 28%	2494
	21st to end	Day	18% 13% 4 7% 5% 4 4 4 4 5% 32%	2804
May	1st to 10th	Day	11% 10% 3 7% 4 4 4 3 4 6% 42%	8241
	11th to 20th	Day	16% 10% 2 6% 4 3 4 3 5% 6% 40%	8222
	21st to end	Day	10% 7% 2 6% 4 4 4 4 6% 8% 45%	9061
June	1st to 10th	Day	14% 8% 3 6% 5% 4 4 4 6% 8% 38%	2940
	11th to 20th	Day	13% 6% 2 7% 5% 5% 5% 4 6% 8% 37%	2940
	21st to end	Day	16% 6% 2 6% 5% 4 5% 4 7% 8% 37%	2940
July	1st to 10th	Day	9% 4 2 5% 4 4 5% 4 7% 10% 47%	8279
	11th to 20th	Day	12% 6% 2 5% 4 5% 5% 4 8% 10% 40%	8280
	21st to end	Day	9% 5% 2 5% 4 4 5% 4 7% 10% 43%	8992
August	1st to 10th	Day	8% 6% 2 7% 5% 5% 5% 5% 7% 10% 40%	6483
	11th to 20th	Day	12% 4 2 5% 4 4 5% 4 7% 9% 44%	6262
	21st to end	Day	13% 6% 2 6% 4 5% 5% 4 7% 9% 39%	6080
September	1st to 10th	Day	12% 6% 3 8% 5% 5% 5% 3 6% 7% 41%	2520
	11th to 20th	Day	13% 7% 3 8% 5% 4 5% 4 7% 9% 36%	2473
	21st to end	Day	14% 10% 3 7% 6% 4 4 4 5% 7% 35%	2305
October	1st to 10th	Day	16% 10% 3 7% 4 4 4 4 5% 7% 36%	2100
	11th to 20th	Day	17% 12% 3 10% 5% 5% 4 4 5% 6% 29%	2042
	21st to end	Day	19% 13% 4 9% 5% 4 4 4 5% 7% 28%	2052
November	1st to 10th	Day	13% 13% 5% 10% 7% 6% 5% 4 5% 6% 27%	1815
	11th to 20th	Day	14% 17% 5% 11% 6% 5% 5% 4 4 5% 25%	1680
	21st to end	Day	15% 14% 5% 9% 5% 4 4 3 5% 8% 28%	1680
December	1st to 10th	Day	10% 13% 4 14% 8% 7% 6% 4 5% 5% 24%	2522
	11th to 20th	Day	7% 17% 7% 13% 5% 4 3 2 4 5% 31%	2892
	21st to end	Day	19% 21% 6% 10% 4 4 3 2 4 5% 22%	4070
January	1st to 10th	Night	7% 12% 5% 12% 7% 6% 5% 4 5% 6% 33%	8233
	11th to 20th	Night	6% 13% 6% 18% 9% 7% 5% 5% 6% 6% 19%	8236
	21st to end	Night	5% 11% 6% 13% 7% 7% 7% 4 5% 7% 29%	9058
February	1st to 10th	Night	9% 11% 3 11% 5% 6% 5% 5% 6% 6% 33%	2824
	11th to 20th	Night	9% 10% 4 8% 5% 6% 6% 5% 7% 7% 33%	2652
	21st to end	Night	9% 12% 4 10% 7% 5% 6% 6% 7% 7% 28%	2076
March	1st to 10th	Night	10% 9% 3 9% 5% 4 5% 5% 6% 6% 37%	5482
	11th to 20th	Night	9% 14% 5% 11% 6% 6% 6% 5% 6% 7% 27%	5330
	21st to end	Night	18% 14% 4 9% 5% 4 4 4 4 5% 31%	5130
April	1st to 10th	Night	13% 12% 4 8% 5% 6% 5% 3 6% 6% 33%	2100
	11th to 20th	Night	15% 12% 4 7% 5% 4 5% 4 5% 6% 34%	2075
	21st to end	Night	17% 11% 3 7% 6% 5% 4 4 5% 5% 33%	1810
May	1st to 10th	Night	14% 7% 2 5% 5% 4 4 4 5% 7% 44%	4688
	11th to 20th	Night	16% 10% 3 7% 4 4 4 3 5% 6% 38%	4670
	21st to end	Night	13% 6% 2 6% 4 4 4 3 5% 6% 48%	5148
June	1st to 10th	Night	14% 9% 2 6% 5% 5% 5% 4 5% 6% 38%	1680
	11th to 20th	Night	12% 8% 3 7% 5% 4 5% 5% 5% 8% 39%	1680
	21st to end	Night	14% 7% 2 7% 4 3 4 4 5% 8% 41%	1680
July	1st to 10th	Night	5% 5% 2 5% 5% 5% 5% 4 6% 10% 48%	4710
	11th to 20th	Night	9% 5% 2 6% 5% 4 4 4 6% 8% 47%	4710
	21st to end	Night	11% 4 2 7% 4 4 5% 4 6% 9% 44%	5118
August	1st to 10th	Night	10% 4 2 4 4 4 5% 4 7% 10% 46%	3687
	11th to 20th	Night	10% 5% 2 5% 4 5% 5% 4 7% 8% 46%	3798
	21st to end	Night	16% 7% 3 7% 4 4 4 3 5% 6% 42%	5046
September	1st to 10th	Night	12% 5% 2 4 3 4 3 4 6% 7% 49%	2100
	11th to 20th	Night	12% 5% 3 6% 5% 4 4 3 6% 8% 43%	2126
	21st to end	Night	15% 8% 3 7% 5% 4 4 4 6% 8% 38%	2313
October	1st to 10th	Night	15% 7% 2 7% 5% 5% 4 3 6% 7% 39%	2520
	11th to 20th	Night	14% 12% 3 7% 5% 4 4 3 5% 7% 35%	2577
	21st to end	Night	17% 11% 4 9% 5% 5% 4 4 5% 7% 30%	2965
November	1st to 10th	Night	13% 12% 3 8% 5% 5% 5% 5% 6% 7% 31%	2698
	11th to 20th	Night	15% 13% 5% 12% 6% 4 5% 4 5% 6% 26%	2940
	21st to end	Night	13% 14% 4 10% 6% 5% 5% 4 6% 7% 25%	2940
December	1st to 10th	Night	11% 14% 6% 13% 7% 6% 6% 4 5% 6% 22%	4876
	11th to 20th	Night	8% 13% 6% 14% 6% 5% 4 3 5% 6% 29%	5524
	21st to end	Night	19% 20% 6% 11% 5% 4 3 3 4 4 22%	7150

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% 6% 10% 19% 11% 1	4692
	11th to 20th	Day	56% 5% 9% 22% 7%	4710
	21st to end	Day	54% 7% 11% 17% 10% 1	5156
February	1st to 10th	Day	50% 6% 13% 19% 11% 2	1793
	11th to 20th	Day	48% 5% 12% 24% 10% 1	1864
	21st to end	Day	51% 6% 11% 17% 15% 1	1730
March	1st to 10th	Day	45% 5% 13% 17% 16% 4	4580
	11th to 20th	Day	50% 7% 11% 18% 14% 1	4891
	21st to end	Day	57% 4 7% 15% 14% 2	6042
April	1st to 10th	Day	52% 5% 10% 19% 12% 2	2520
	11th to 20th	Day	55% 5% 11% 19% 8% 1	2494
	21st to end	Day	53% 4 9% 19% 13% 1	2804
May	1st to 10th	Day	40% 5% 13% 26% 15% 1	8250
	11th to 20th	Day	43% 6% 11% 21% 18% 2	8226
	21st to end	Day	34% 7% 13% 25% 19% 2	9061
June	1st to 10th	Day	41% 7% 15% 21% 14% 1	2940
	11th to 20th	Day	41% 8% 15% 23% 12% 1	2940
	21st to end	Day	42% 8% 17% 21% 10% 1	2940
July	1st to 10th	Day	30% 7% 18% 33% 11% 1	8211
	11th to 20th	Day	37% 8% 19% 30% 6%	8205
	21st to end	Day	34% 7% 19% 32% 8%	8794
August	1st to 10th	Day	36% 8% 18% 29% 9%	6468
	11th to 20th	Day	33% 8% 25% 28% 7%	6304
	21st to end	Day	38% 8% 18% 30% 6%	6079
September	1st to 10th	Day	40% 7% 17% 28% 9%	2520
	11th to 20th	Day	42% 8% 23% 23% 4 1	2473
	21st to end	Day	46% 7% 16% 22% 7% 1	2305
October	1st to 10th	Day	39% 5% 15% 29% 11% 1	3360
	11th to 20th	Day	49% 4 13% 27% 5% 1	3245
	21st to end	Day	55% 6% 13% 20% 5%	3252
November	1st to 10th	Day	55% 6% 15% 16% 7% 1	1815
	11th to 20th	Day	59% 6% 11% 17% 6%	1680
	21st to end	Day	55% 7% 15% 15% 7% 1	1680
December	1st to 10th	Day	58% 8% 16% 14% 4	2522
	11th to 20th	Day	56% 4 10% 12% 15% 3	2893
	21st to end	Day	66% 4 6% 18% 5%	4070
January	1st to 10th	Night	53% 5% 12% 17% 11% 2	8233
	11th to 20th	Night	61% 6% 9% 16% 8%	8280
	21st to end	Night	53% 6% 12% 18% 10%	9058
February	1st to 10th	Night	50% 7% 12% 20% 10% 2	2824
	11th to 20th	Night	47% 6% 12% 22% 12% 1	2653
	21st to end	Night	51% 9% 10% 19% 10% 1	2076
March	1st to 10th	Night	45% 6% 12% 14% 17% 6%	5493
	11th to 20th	Night	53% 7% 14% 17% 9% 1	5331
	21st to end	Night	55% 5% 8% 17% 15% 1	5130
April	1st to 10th	Night	51% 6% 7% 20% 14% 1	2100
	11th to 20th	Night	50% 5% 11% 21% 11% 2	2075
	21st to end	Night	51% 5% 8% 22% 12% 2	1810
May	1st to 10th	Night	37% 6% 12% 27% 15% 2	4695
	11th to 20th	Night	46% 6% 8% 22% 14% 3	4671
	21st to end	Night	36% 6% 10% 30% 17% 1	5149
June	1st to 10th	Night	46% 6% 11% 21% 14% 2	1680
	11th to 20th	Night	42% 7% 11% 24% 15% 1	1680
	21st to end	Night	39% 10% 15% 23% 11% 2	1680
July	1st to 10th	Night	31% 8% 18% 26% 15% 2	4670
	11th to 20th	Night	35% 6% 19% 31% 10%	4662
	21st to end	Night	37% 7% 15% 32% 8%	5002
August	1st to 10th	Night	31% 7% 18% 32% 11% 1	3678
	11th to 20th	Night	34% 7% 21% 31% 7%	3822
	21st to end	Night	43% 6% 15% 29% 7%	5046
September	1st to 10th	Night	33% 6% 18% 31% 11%	2100
	11th to 20th	Night	38% 9% 20% 29% 4	2126
	21st to end	Night	44% 7% 16% 26% 7% 1	2313
October	1st to 10th	Night	34% 6% 14% 29% 16%	4040
	11th to 20th	Night	44% 5% 13% 27% 10%	4113
	21st to end	Night	55% 6% 15% 19% 5%	4715
November	1st to 10th	Night	50% 8% 13% 21% 7%	2698
	11th to 20th	Night	58% 5% 13% 17% 6%	2940
	21st to end	Night	56% 8% 14% 16% 7%	2940
December	1st to 10th	Night	60% 6% 16% 15% 3	4876
	11th to 20th	Night	54% 6% 12% 18% 10%	5522
	21st to end	Night	66% 4 8% 16% 5%	7150

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	14% 14% 36% 28% 7% 1	330
	11th to 20th	Day	16% 15% 36% 23% 8% 1	330
	21st to end	Day	13% 17% 40% 25% 5% 1	363
February	1st to 10th	Day	15% 19% 40% 20% 5% 2	330
	11th to 20th	Day	14% 22% 41% 19% 4 1	330
	21st to end	Day	15% 27% 40% 16% 2	273
March	1st to 10th	Day	15% 23% 40% 17% 5%	330
	11th to 20th	Day	16% 27% 36% 14% 5% 2	330
	21st to end	Day	21% 18% 33% 19% 7% 2	561
April	1st to 10th	Day	20% 16% 34% 21% 7% 1	540
	11th to 20th	Day	27% 15% 27% 20% 11% 1	532
	21st to end	Day	26% 16% 29% 18% 10% 2	630
May	1st to 10th	Day	21% 19% 31% 20% 7% 2	753
	11th to 20th	Day	19% 15% 36% 20% 7% 3	775
	21st to end	Day	17% 15% 33% 25% 8% 2	844
June	1st to 10th	Day	16% 14% 38% 22% 8% 2	780
	11th to 20th	Day	17% 14% 37% 22% 8% 2	780
	21st to end	Day	17% 11% 34% 25% 10% 2	780
July	1st to 10th	Day	9% 10% 37% 28% 11% 4	771
	11th to 20th	Day	12% 11% 39% 24% 10% 5%	768
	21st to end	Day	9% 9% 38% 26% 13% 5%	849
August	1st to 10th	Day	10% 11% 37% 27% 11% 3	776
	11th to 20th	Day	14% 9% 29% 27% 15% 4	763
	21st to end	Day	17% 11% 31% 24% 13% 3	627
September	1st to 10th	Day	13% 12% 30% 25% 15% 5%	570
	11th to 20th	Day	17% 12% 30% 24% 13% 4	570
	21st to end	Day	19% 15% 27% 23% 13% 3	565
October	1st to 10th	Day	21% 23% 30% 18% 7% 1	360
	11th to 20th	Day	24% 18% 35% 18% 4 1	360
	21st to end	Day	28% 18% 36% 13% 3 2	387
November	1st to 10th	Day	21% 22% 31% 20% 4 1	330
	11th to 20th	Day	25% 24% 35% 10% 4 2	330
	21st to end	Day	25% 21% 32% 16% 5%	330
December	1st to 10th	Day	18% 16% 38% 21% 5% 3	275
	11th to 20th	Day	19% 22% 40% 14% 4	267
	21st to end	Day	25% 19% 33% 17% 6% 1	363
January	1st to 10th	Night	15% 16% 37% 27% 6%	1050
	11th to 20th	Night	16% 14% 38% 25% 7% 1	1050
	21st to end	Night	16% 10% 38% 28% 8%	1155
February	1st to 10th	Night	16% 13% 34% 29% 7% 1	1050
	11th to 20th	Night	14% 14% 34% 27% 9% 1	1018
	21st to end	Night	14% 13% 35% 29% 8%	692
March	1st to 10th	Night	13% 13% 33% 31% 8% 1	840
	11th to 20th	Night	17% 15% 31% 28% 8% 1	741
	21st to end	Night	18% 13% 27% 28% 13% 2	693
April	1st to 10th	Night	19% 10% 30% 31% 9% 1	630
	11th to 20th	Night	21% 11% 25% 31% 12% 1	623
	21st to end	Night	22% 5% 24% 36% 12% 1	630
May	1st to 10th	Night	21% 8% 23% 34% 12% 2	630
	11th to 20th	Night	18% 6% 29% 33% 12% 2	621
	21st to end	Night	18% 7% 23% 35% 16% 1	683
June	1st to 10th	Night	18% 6% 24% 37% 14% 1	630
	11th to 20th	Night	14% 7% 22% 41% 14% 1	630
	21st to end	Night	17% 5% 22% 40% 16% 1	630
July	1st to 10th	Night	10% 5% 23% 42% 19% 1	615
	11th to 20th	Night	9% 4 29% 37% 19% 2	612
	21st to end	Night	8% 5% 24% 39% 23% 2	690
August	1st to 10th	Night	9% 4 27% 41% 18% 1	627
	11th to 20th	Night	12% 5% 20% 35% 24% 4	624
	21st to end	Night	17% 5% 24% 34% 19% 2	693
September	1st to 10th	Night	13% 5% 25% 32% 20% 4	630
	11th to 20th	Night	16% 5% 27% 34% 15% 2	656
	21st to end	Night	20% 8% 24% 31% 16% 1	840
October	1st to 10th	Night	20% 13% 29% 23% 13% 1	840
	11th to 20th	Night	20% 14% 32% 26% 8%	840
	21st to end	Night	21% 12% 31% 27% 9%	913
November	1st to 10th	Night	19% 13% 32% 27% 8% 1	852
	11th to 20th	Night	22% 17% 30% 21% 8% 2	1050
	21st to end	Night	22% 19% 31% 22% 6%	1050
December	1st to 10th	Night	17% 16% 35% 23% 7% 1	1050
	11th to 20th	Night	20% 16% 33% 22% 8% 1	1050
	21st to end	Night	26% 14% 30% 23% 5% 1	1155

Page generated at 2026-10-26 04:37:47. Version ac07

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