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 2025-07-06.

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% 4 8% 5% 5% 4 4 6% 7% 35%	119969
	Night	12% 11% 4 9% 5% 5% 5% 4 5% 7% 34%	119047
Transparency	Day	46% 7% 14% 22% 11% 1	123419
	Night	48% 6% 12% 21% 11% 1	123756
Seeing	Day	17% 15% 34% 22% 9% 2	16983
	Night	17% 11% 30% 29% 11% 1	27023

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% 6% 13% 6% 6% 5% 3 5% 7% 33%	3617
	11th to 20th	Day	6% 11% 7% 17% 6% 4 3 4 6% 10% 27%	3589
	21st to end	Day	7% 14% 9% 14% 7% 7% 5% 4 5% 7% 21%	3982
February	1st to 10th	Day	7% 11% 4 12% 5% 5% 6% 4 5% 6% 33%	1707
	11th to 20th	Day	8% 12% 4 10% 6% 6% 5% 4 5% 6% 36%	1764
	21st to end	Day	7% 14% 5% 11% 7% 5% 4 3 6% 7% 32%	1650
March	1st to 10th	Day	10% 10% 3 8% 6% 5% 4 3 5% 6% 39%	4478
	11th to 20th	Day	8% 11% 6% 12% 6% 6% 5% 4 4 6% 33%	4786
	21st to end	Day	19% 15% 4 10% 4 4 3 3 4 4 31%	5912
April	1st to 10th	Day	13% 12% 4 10% 6% 5% 5% 4 6% 6% 30%	2400
	11th to 20th	Day	17% 12% 4 10% 5% 5% 4 4 5% 7% 28%	2374
	21st to end	Day	18% 13% 4 7% 5% 4 4 4 4 5% 33%	2671
May	1st to 10th	Day	14% 8% 2 6% 3 3 4 3 4 6% 47%	6348
	11th to 20th	Day	15% 9% 2 6% 5% 4 4 3 5% 7% 40%	6319
	21st to end	Day	11% 8% 3 7% 5% 5% 5% 4 6% 8% 39%	6949
June	1st to 10th	Day	14% 8% 3 6% 5% 4 4 4 6% 8% 38%	2800
	11th to 20th	Day	13% 6% 3 7% 5% 5% 5% 5% 6% 8% 38%	2800
	21st to end	Day	16% 7% 2 6% 5% 4 5% 4 7% 8% 36%	2800
July	1st to 10th	Day	8% 5% 2 6% 5% 6% 6% 6% 8% 9% 38%	4511
	11th to 20th	Day	9% 5% 2 5% 4 5% 5% 4 8% 10% 43%	4440
	21st to end	Day	10% 5% 2 5% 4 4 4 4 7% 10% 46%	4814
August	1st to 10th	Day	10% 7% 3 8% 6% 5% 5% 5% 7% 10% 34%	4423
	11th to 20th	Day	13% 4 1 4 3 4 5% 4 6% 8% 47%	4250
	21st to end	Day	11% 6% 2 6% 4 5% 5% 4 7% 9% 41%	4180
September	1st to 10th	Day	11% 6% 3 7% 5% 5% 5% 3 6% 7% 43%	2280
	11th to 20th	Day	14% 7% 2 8% 5% 4 5% 4 7% 9% 37%	2237
	21st to end	Day	15% 10% 3 8% 5% 4 4 4 5% 7% 35%	2085
October	1st to 10th	Day	17% 11% 3 7% 5% 4 4 4 5% 6% 33%	1900
	11th to 20th	Day	17% 13% 4 10% 5% 5% 4 5% 5% 6% 27%	1848
	21st to end	Day	20% 14% 4 9% 5% 4 4 4 4 6% 27%	1881
November	1st to 10th	Day	13% 13% 4 10% 7% 6% 5% 4 5% 6% 26%	1727
	11th to 20th	Day	14% 17% 5% 11% 6% 5% 5% 4 4 5% 24%	1600
	21st to end	Day	15% 14% 5% 9% 5% 4 4 3 6% 8% 27%	1600
December	1st to 10th	Day	10% 13% 4 14% 8% 7% 6% 4 5% 5% 23%	2447
	11th to 20th	Day	7% 17% 7% 13% 6% 4 3 2 4 5% 31%	2818
	21st to end	Day	19% 21% 6% 9% 4 4 3 3 4 5% 22%	3982
January	1st to 10th	Night	6% 12% 5% 11% 6% 6% 5% 4 5% 6% 33%	6348
	11th to 20th	Night	6% 11% 6% 17% 9% 7% 5% 5% 6% 7% 21%	6316
	21st to end	Night	6% 14% 7% 14% 7% 6% 5% 4 5% 7% 26%	6996
February	1st to 10th	Night	8% 10% 3 11% 6% 6% 6% 5% 6% 6% 33%	2690
	11th to 20th	Night	9% 10% 4 8% 6% 6% 6% 5% 7% 7% 34%	2523
	21st to end	Night	9% 12% 4 10% 7% 5% 6% 6% 7% 7% 29%	1980
March	1st to 10th	Night	10% 9% 3 9% 6% 4 5% 5% 6% 6% 38%	5362
	11th to 20th	Night	8% 14% 5% 11% 6% 5% 6% 5% 6% 7% 27%	5215
	21st to end	Night	18% 14% 4 9% 5% 4 4 4 5% 5% 31%	5018
April	1st to 10th	Night	13% 12% 4 8% 5% 6% 4 3 6% 6% 34%	2000
	11th to 20th	Night	15% 12% 4 7% 5% 4 5% 4 5% 6% 34%	1975
	21st to end	Night	17% 12% 3 7% 6% 5% 4 4 5% 5% 33%	1723
May	1st to 10th	Night	16% 6% 1 4 4 3 3 3 5% 6% 48%	3613
	11th to 20th	Night	13% 10% 3 8% 4 5% 4 3 5% 7% 39%	3596
	21st to end	Night	14% 7% 3 7% 4 4 4 3 6% 6% 43%	3949
June	1st to 10th	Night	15% 9% 2 6% 5% 5% 5% 4 6% 6% 38%	1600
	11th to 20th	Night	12% 8% 3 7% 5% 4 5% 5% 5% 8% 40%	1600
	21st to end	Night	14% 7% 2 7% 4 3 4 4 5% 8% 41%	1600
July	1st to 10th	Night	6% 6% 2 6% 5% 5% 5% 4 6% 9% 46%	2570
	11th to 20th	Night	5% 5% 2 5% 5% 4 4 4 7% 8% 51%	2530
	21st to end	Night	9% 4 2 5% 4 3 4 4 6% 10% 51%	2750
August	1st to 10th	Night	13% 5% 2 4 4 4 5% 3 7% 10% 44%	2517
	11th to 20th	Night	11% 4 1 5% 3 4 4 4 7% 8% 49%	2583
	21st to end	Night	16% 6% 3 6% 4 3 3 3 4 6% 46%	3476
September	1st to 10th	Night	12% 5% 2 5% 3 4 3 4 6% 6% 51%	1900
	11th to 20th	Night	12% 4 3 6% 5% 4 5% 4 6% 8% 44%	1924
	21st to end	Night	16% 8% 2 7% 5% 4 4 4 6% 8% 37%	2093
October	1st to 10th	Night	16% 7% 2 7% 5% 5% 4 3 6% 6% 38%	2280
	11th to 20th	Night	13% 12% 3 7% 5% 4 5% 4 5% 8% 34%	2331
	21st to end	Night	17% 11% 4 9% 5% 5% 4 4 5% 7% 29%	2717
November	1st to 10th	Night	14% 12% 3 8% 6% 5% 5% 5% 6% 7% 30%	2566
	11th to 20th	Night	15% 14% 4 12% 6% 4 4 4 5% 6% 26%	2800
	21st to end	Night	13% 14% 4 9% 6% 5% 5% 4 6% 7% 25%	2800
December	1st to 10th	Night	11% 14% 6% 13% 7% 6% 6% 4 5% 6% 22%	4731
	11th to 20th	Night	8% 13% 6% 14% 6% 5% 4 3 5% 5% 30%	5379
	21st to end	Night	19% 20% 6% 11% 5% 4 3 3 4 4 22%	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	50% 6% 10% 20% 13% 2	3618
	11th to 20th	Day	52% 5% 9% 27% 6%	3620
	21st to end	Day	61% 8% 9% 13% 8% 1	3982
February	1st to 10th	Day	49% 6% 13% 19% 11% 2	1707
	11th to 20th	Day	48% 5% 12% 23% 10% 1	1773
	21st to end	Day	51% 6% 11% 17% 14% 1	1650
March	1st to 10th	Day	45% 6% 13% 17% 16% 4	4480
	11th to 20th	Day	50% 7% 11% 18% 14% 1	4786
	21st to end	Day	57% 4 7% 16% 14% 2	5912
April	1st to 10th	Day	52% 5% 11% 19% 12% 2	2400
	11th to 20th	Day	54% 5% 11% 20% 9% 1	2374
	21st to end	Day	53% 4 10% 18% 14% 1	2671
May	1st to 10th	Day	38% 4 15% 24% 17% 1	6357
	11th to 20th	Day	43% 6% 12% 20% 18% 2	6323
	21st to end	Day	39% 8% 13% 20% 19% 2	6949
June	1st to 10th	Day	41% 7% 15% 21% 14% 1	2800
	11th to 20th	Day	40% 8% 15% 23% 12% 1	2800
	21st to end	Day	43% 8% 17% 21% 10% 1	2800
July	1st to 10th	Day	35% 10% 19% 24% 12% 1	4442
	11th to 20th	Day	31% 8% 20% 33% 7%	4365
	21st to end	Day	32% 7% 16% 35% 10% 1	4662
August	1st to 10th	Day	42% 8% 16% 25% 9% 1	4408
	11th to 20th	Day	32% 8% 23% 28% 10%	4292
	21st to end	Day	35% 8% 18% 30% 9%	4179
September	1st to 10th	Day	39% 7% 17% 28% 9%	2280
	11th to 20th	Day	41% 8% 23% 23% 4 1	2237
	21st to end	Day	47% 8% 16% 22% 6% 1	2085
October	1st to 10th	Day	40% 5% 14% 29% 10% 1	3160
	11th to 20th	Day	49% 4 14% 27% 5%	3051
	21st to end	Day	56% 6% 13% 20% 5%	3081
November	1st to 10th	Day	56% 6% 16% 16% 7% 1	1727
	11th to 20th	Day	60% 7% 12% 16% 6%	1600
	21st to end	Day	55% 8% 15% 15% 7% 1	1600
December	1st to 10th	Day	60% 8% 16% 13% 3	2447
	11th to 20th	Day	57% 4 10% 12% 15% 3	2819
	21st to end	Day	66% 4 7% 18% 5%	3982
January	1st to 10th	Night	52% 5% 13% 18% 11% 1	6348
	11th to 20th	Night	59% 7% 9% 18% 8%	6360
	21st to end	Night	57% 7% 10% 15% 9%	6996
February	1st to 10th	Night	49% 7% 12% 20% 11% 2	2690
	11th to 20th	Night	46% 7% 12% 22% 12% 1	2524
	21st to end	Night	50% 9% 10% 19% 11% 1	1980
March	1st to 10th	Night	44% 6% 12% 14% 17% 6%	5373
	11th to 20th	Night	52% 7% 14% 17% 9% 1	5216
	21st to end	Night	55% 5% 7% 17% 15% 1	5018
April	1st to 10th	Night	51% 6% 8% 20% 15% 1	2000
	11th to 20th	Night	49% 5% 10% 22% 11% 2	1975
	21st to end	Night	51% 5% 8% 22% 12% 2	1723
May	1st to 10th	Night	37% 6% 13% 26% 16% 3	3620
	11th to 20th	Night	44% 7% 8% 21% 16% 3	3597
	21st to end	Night	40% 7% 9% 24% 18% 1	3950
June	1st to 10th	Night	46% 7% 11% 21% 14% 1	1600
	11th to 20th	Night	41% 7% 11% 24% 15% 1	1600
	21st to end	Night	39% 10% 15% 22% 12% 2	1600
July	1st to 10th	Night	34% 9% 21% 21% 14% 1	2530
	11th to 20th	Night	29% 9% 17% 34% 11%	2482
	21st to end	Night	29% 7% 14% 36% 13% 1	2662
August	1st to 10th	Night	35% 7% 18% 30% 9% 1	2508
	11th to 20th	Night	32% 7% 19% 32% 9%	2607
	21st to end	Night	41% 6% 15% 28% 10%	3476
September	1st to 10th	Night	32% 6% 17% 32% 12%	1900
	11th to 20th	Night	36% 9% 20% 31% 4	1924
	21st to end	Night	44% 8% 16% 26% 6%	2093
October	1st to 10th	Night	34% 6% 13% 30% 16%	3800
	11th to 20th	Night	44% 6% 14% 27% 9%	3867
	21st to end	Night	55% 6% 15% 18% 6%	4467
November	1st to 10th	Night	51% 8% 13% 21% 7%	2566
	11th to 20th	Night	58% 6% 13% 17% 6%	2800
	21st to end	Night	57% 8% 13% 16% 6%	2800
December	1st to 10th	Night	61% 6% 16% 14% 3	4731
	11th to 20th	Night	55% 6% 12% 18% 10%	5377
	21st to end	Night	67% 5% 7% 16% 5%	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	14% 13% 38% 27% 7% 1	300
	11th to 20th	Day	17% 15% 36% 24% 8% 1	300
	21st to end	Day	15% 17% 39% 24% 5%	330
February	1st to 10th	Day	14% 18% 41% 20% 5% 2	300
	11th to 20th	Day	13% 23% 42% 18% 4 1	300
	21st to end	Day	14% 27% 41% 16% 2	249
March	1st to 10th	Day	14% 21% 42% 18% 5%	300
	11th to 20th	Day	16% 25% 37% 15% 6% 2	300
	21st to end	Day	20% 17% 33% 19% 8% 2	519
April	1st to 10th	Day	20% 15% 35% 23% 7% 1	500
	11th to 20th	Day	26% 16% 26% 20% 11% 1	492
	21st to end	Day	25% 16% 30% 18% 10% 2	586
May	1st to 10th	Day	21% 18% 31% 20% 7% 2	703
	11th to 20th	Day	18% 14% 36% 21% 7% 3	725
	21st to end	Day	18% 14% 33% 25% 8% 2	789
June	1st to 10th	Day	16% 14% 38% 21% 8% 2	730
	11th to 20th	Day	16% 14% 37% 22% 8% 2	730
	21st to end	Day	18% 12% 33% 25% 10% 2	730
July	1st to 10th	Day	10% 10% 37% 28% 11% 4	697
	11th to 20th	Day	11% 11% 37% 25% 11% 5%	668
	21st to end	Day	9% 9% 37% 26% 13% 6%	739
August	1st to 10th	Day	11% 12% 37% 27% 11% 3	676
	11th to 20th	Day	15% 9% 27% 28% 16% 5%	663
	21st to end	Day	18% 11% 32% 23% 13% 4	539
September	1st to 10th	Day	12% 12% 30% 26% 16% 4	490
	11th to 20th	Day	16% 12% 31% 24% 13% 3	490
	21st to end	Day	20% 15% 28% 22% 12% 2	485
October	1st to 10th	Day	23% 22% 27% 19% 8% 1	300
	11th to 20th	Day	23% 18% 37% 17% 4 1	300
	21st to end	Day	30% 20% 37% 11% 2	330
November	1st to 10th	Day	20% 23% 30% 22% 4 1	300
	11th to 20th	Day	24% 23% 37% 10% 4 2	300
	21st to end	Day	25% 22% 31% 16% 5%	300
December	1st to 10th	Day	19% 14% 38% 21% 5% 3	250
	11th to 20th	Day	20% 21% 41% 14% 5%	243
	21st to end	Day	23% 20% 33% 17% 6% 2	330
January	1st to 10th	Night	15% 15% 38% 27% 6%	1000
	11th to 20th	Night	17% 13% 38% 25% 7% 1	1000
	21st to end	Night	17% 11% 37% 28% 8%	1100
February	1st to 10th	Night	15% 13% 34% 29% 8% 1	1000
	11th to 20th	Night	14% 14% 35% 27% 9% 1	969
	21st to end	Night	14% 13% 35% 30% 8%	660
March	1st to 10th	Night	13% 13% 33% 31% 9% 1	800
	11th to 20th	Night	16% 15% 31% 28% 9% 1	706
	21st to end	Night	18% 12% 26% 28% 13% 3	660
April	1st to 10th	Night	18% 10% 30% 31% 10% 1	600
	11th to 20th	Night	20% 11% 25% 31% 12% 1	593
	21st to end	Night	22% 5% 24% 36% 12% 2	600
May	1st to 10th	Night	22% 8% 23% 33% 12% 2	600
	11th to 20th	Night	17% 6% 29% 34% 13% 2	591
	21st to end	Night	19% 7% 23% 34% 16% 1	650
June	1st to 10th	Night	19% 7% 24% 36% 14% 1	600
	11th to 20th	Night	14% 7% 21% 42% 14% 1	600
	21st to end	Night	17% 5% 22% 39% 15% 1	600
July	1st to 10th	Night	10% 6% 23% 41% 19% 1	570
	11th to 20th	Night	9% 5% 28% 37% 20% 2	552
	21st to end	Night	7% 4 25% 38% 25% 2	624
August	1st to 10th	Night	10% 4 28% 40% 17% 1	567
	11th to 20th	Night	12% 5% 19% 34% 26% 4	564
	21st to end	Night	17% 5% 24% 33% 18% 2	627
September	1st to 10th	Night	12% 5% 26% 32% 20% 5%	570
	11th to 20th	Night	16% 6% 28% 33% 16% 2	594
	21st to end	Night	21% 8% 24% 30% 15% 1	760
October	1st to 10th	Night	21% 13% 28% 22% 14% 1	760
	11th to 20th	Night	19% 14% 32% 27% 8%	760
	21st to end	Night	22% 13% 32% 26% 7%	836
November	1st to 10th	Night	19% 13% 32% 27% 8% 1	810
	11th to 20th	Night	23% 16% 30% 21% 8% 2	1000
	21st to end	Night	23% 19% 30% 22% 6%	1000
December	1st to 10th	Night	17% 16% 35% 23% 8% 1	1000
	11th to 20th	Night	19% 16% 33% 22% 8% 1	1000
	21st to end	Night	26% 13% 31% 24% 6% 1	1100

Page generated at 2025-07-03 01:30:12. Version ac07

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