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 2024-11-03.

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	13% 10% 3 8% 5% 5% 4 4 6% 7% 35%	101484
	Night	12% 10% 4 9% 5% 5% 4 4 5% 7% 35%	97639
Transparency	Day	46% 7% 14% 22% 10% 1	101243
	Night	48% 7% 13% 22% 9% 1	97498
Seeing	Day	17% 15% 34% 22% 9% 2	16080
	Night	17% 11% 30% 29% 12% 1	26053

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	9% 13% 6% 12% 6% 5% 5% 3 5% 7% 30%	2527
	11th to 20th	Day	7% 10% 7% 17% 7% 5% 4 4 6% 8% 27%	2530
	21st to end	Day	10% 16% 9% 15% 7% 6% 4 4 5% 6% 19%	2783
February	1st to 10th	Day	8% 12% 4 12% 5% 4 5% 4 6% 6% 34%	1621
	11th to 20th	Day	8% 12% 4 9% 5% 6% 5% 4 5% 6% 36%	1673
	21st to end	Day	7% 13% 5% 10% 7% 5% 4 3 6% 7% 34%	1570
March	1st to 10th	Day	11% 10% 3 7% 6% 5% 5% 3 5% 6% 41%	3118
	11th to 20th	Day	7% 10% 6% 12% 6% 5% 4 4 4 6% 36%	3342
	21st to end	Day	18% 14% 5% 10% 5% 4 3 3 4 4 29%	4122
April	1st to 10th	Day	14% 12% 4 10% 6% 5% 5% 4 6% 6% 30%	2280
	11th to 20th	Day	17% 12% 4 9% 5% 5% 4 4 5% 7% 29%	2254
	21st to end	Day	17% 13% 4 7% 5% 4 4 4 4 5% 33%	2537
May	1st to 10th	Day	16% 10% 3 7% 4 3 4 3 4 6% 38%	4428
	11th to 20th	Day	14% 9% 3 7% 6% 5% 4 4 5% 7% 38%	4399
	21st to end	Day	15% 9% 3 8% 5% 4 5% 3 6% 6% 37%	4837
June	1st to 10th	Day	14% 8% 3 6% 5% 4 5% 4 6% 8% 38%	2660
	11th to 20th	Day	14% 6% 3 7% 5% 5% 5% 5% 6% 8% 37%	2660
	21st to end	Day	16% 7% 2 6% 5% 4 5% 4 6% 8% 37%	2660
July	1st to 10th	Day	8% 5% 2 6% 5% 6% 6% 6% 8% 9% 38%	4440
	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% 5% 10% 7% 6% 5% 4 5% 6% 26%	1657
	11th to 20th	Day	14% 17% 5% 11% 6% 5% 5% 4 4 5% 25%	1520
	21st to end	Day	15% 14% 4 8% 5% 4 5% 3 6% 8% 28%	1520
December	1st to 10th	Day	10% 13% 4 11% 6% 6% 5% 5% 5% 5% 29%	1427
	11th to 20th	Day	11% 17% 5% 11% 7% 4 4 2 5% 5% 29%	1798
	21st to end	Day	24% 23% 5% 8% 4 3 3 2 4 3 21%	2783
January	1st to 10th	Night	9% 15% 5% 9% 4 5% 4 4 5% 7% 32%	4428
	11th to 20th	Night	6% 10% 6% 19% 10% 8% 5% 4 6% 7% 21%	4440
	21st to end	Night	8% 16% 8% 14% 7% 6% 5% 4 4 6% 23%	4884
February	1st to 10th	Night	8% 11% 3 12% 6% 6% 6% 5% 6% 6% 33%	2556
	11th to 20th	Night	9% 10% 4 8% 6% 6% 6% 5% 6% 7% 34%	2394
	21st to end	Night	8% 12% 4 10% 7% 4 6% 6% 7% 7% 29%	1884
March	1st to 10th	Night	10% 11% 3 9% 6% 4 5% 4 5% 6% 37%	3722
	11th to 20th	Night	9% 13% 4 9% 5% 6% 6% 5% 6% 7% 32%	3645
	21st to end	Night	17% 13% 4 9% 5% 4 4 3 5% 5% 30%	3508
April	1st to 10th	Night	13% 12% 4 8% 5% 6% 4 3 6% 6% 33%	1900
	11th to 20th	Night	15% 12% 4 7% 5% 4 5% 4 5% 6% 33%	1875
	21st to end	Night	17% 11% 3 7% 6% 5% 4 4 5% 5% 33%	1638
May	1st to 10th	Night	19% 7% 2 4 3 4 4 3 6% 7% 41%	2523
	11th to 20th	Night	12% 9% 4 8% 5% 5% 5% 4 5% 7% 36%	2506
	21st to end	Night	20% 6% 2 7% 4 4 4 3 6% 6% 38%	2750
June	1st to 10th	Night	15% 9% 2 6% 5% 5% 5% 4 5% 6% 37%	1520
	11th to 20th	Night	12% 8% 3 7% 4 4 5% 5% 5% 8% 40%	1520
	21st to end	Night	14% 7% 2 7% 5% 3 4 4 5% 8% 40%	1520
July	1st to 10th	Night	7% 7% 2 6% 5% 5% 5% 4 6% 9% 45%	2530
	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	13% 11% 3 8% 6% 5% 5% 5% 6% 7% 30%	2461
	11th to 20th	Night	15% 13% 4 12% 6% 5% 4 4 5% 6% 25%	2660
	21st to end	Night	14% 14% 4 9% 6% 5% 5% 4 6% 7% 26%	2660
December	1st to 10th	Night	10% 12% 6% 11% 6% 4 5% 4 5% 7% 31%	2741
	11th to 20th	Night	11% 14% 5% 11% 6% 5% 4 3 5% 6% 32%	3389
	21st to end	Night	23% 21% 7% 11% 4 3 2 2 3 4 20%	4884

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% 7% 12% 21% 8%	2528
	11th to 20th	Day	54% 5% 11% 26% 3	2530
	21st to end	Day	65% 9% 9% 11% 6%	2783
February	1st to 10th	Day	48% 6% 13% 20% 11% 2	1621
	11th to 20th	Day	48% 5% 13% 23% 10% 1	1682
	21st to end	Day	49% 6% 11% 18% 15% 1	1570
March	1st to 10th	Day	43% 6% 13% 20% 16% 2	3120
	11th to 20th	Day	46% 7% 13% 21% 11% 1	3342
	21st to end	Day	57% 4 8% 14% 14% 2	4122
April	1st to 10th	Day	52% 5% 11% 18% 12% 2	2280
	11th to 20th	Day	54% 6% 11% 20% 9% 1	2254
	21st to end	Day	52% 4 10% 18% 14% 1	2537
May	1st to 10th	Day	45% 5% 13% 20% 16% 1	4440
	11th to 20th	Day	43% 7% 13% 23% 13% 1	4403
	21st to end	Day	44% 10% 13% 15% 17% 1	4837
June	1st to 10th	Day	41% 8% 14% 21% 14% 1	2660
	11th to 20th	Day	41% 8% 14% 22% 13% 1	2660
	21st to end	Day	42% 8% 16% 21% 10% 1	2660
July	1st to 10th	Day	35% 10% 19% 24% 12% 1	4371
	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	50% 7% 15% 23% 5%	1900
	11th to 20th	Day	54% 6% 14% 21% 5% 1	1848
	21st to end	Day	58% 6% 11% 19% 5%	1881
November	1st to 10th	Day	56% 6% 15% 15% 7% 1	1657
	11th to 20th	Day	60% 7% 12% 16% 6%	1520
	21st to end	Day	54% 8% 16% 15% 7% 1	1520
December	1st to 10th	Day	53% 9% 15% 18% 6%	1427
	11th to 20th	Day	57% 4 13% 13% 9% 4	1799
	21st to end	Day	69% 3 7% 16% 4	2783
January	1st to 10th	Night	51% 6% 16% 20% 6%	4428
	11th to 20th	Night	60% 7% 11% 18% 4	4440
	21st to end	Night	62% 8% 12% 12% 6%	4884
February	1st to 10th	Night	49% 7% 13% 19% 10% 2	2556
	11th to 20th	Night	46% 6% 12% 22% 12% 1	2395
	21st to end	Night	50% 8% 11% 19% 11% 1	1884
March	1st to 10th	Night	46% 7% 12% 17% 15% 2	3733
	11th to 20th	Night	48% 6% 14% 20% 10% 1	3646
	21st to end	Night	54% 6% 9% 16% 14% 1	3508
April	1st to 10th	Night	51% 6% 8% 20% 14%	1900
	11th to 20th	Night	50% 5% 10% 21% 11% 2	1875
	21st to end	Night	51% 5% 8% 22% 13% 2	1638
May	1st to 10th	Night	41% 6% 12% 25% 15%	2530
	11th to 20th	Night	45% 8% 10% 22% 11% 4	2507
	21st to end	Night	44% 9% 10% 20% 15% 1	2751
June	1st to 10th	Night	46% 6% 11% 21% 14% 1	1520
	11th to 20th	Night	42% 7% 11% 23% 16% 1	1520
	21st to end	Night	39% 10% 15% 22% 12% 2	1520
July	1st to 10th	Night	34% 9% 21% 21% 14% 1	2490
	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	45% 8% 15% 23% 9%	2280
	11th to 20th	Night	47% 8% 15% 24% 6% 1	2331
	21st to end	Night	55% 7% 14% 18% 6%	2717
November	1st to 10th	Night	51% 8% 13% 21% 7%	2461
	11th to 20th	Night	58% 6% 13% 17% 6%	2660
	21st to end	Night	55% 8% 14% 16% 7%	2660
December	1st to 10th	Night	51% 8% 15% 21% 5%	2741
	11th to 20th	Night	52% 6% 13% 21% 7%	3387
	21st to end	Night	70% 4 8% 15% 4	4884

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	15% 14% 36% 27% 7% 1	270
	11th to 20th	Day	18% 14% 35% 24% 9% 1	270
	21st to end	Day	15% 16% 38% 26% 5%	297
February	1st to 10th	Day	14% 16% 43% 19% 6% 2	270
	11th to 20th	Day	14% 24% 40% 17% 4 1	270
	21st to end	Day	12% 27% 42% 17% 2	225
March	1st to 10th	Day	13% 21% 43% 19% 5%	270
	11th to 20th	Day	16% 24% 36% 16% 6% 2	270
	21st to end	Day	19% 16% 35% 20% 8% 2	476
April	1st to 10th	Day	19% 15% 35% 23% 7% 1	460
	11th to 20th	Day	25% 16% 27% 21% 10% 1	452
	21st to end	Day	24% 16% 30% 19% 9% 2	541
May	1st to 10th	Day	22% 19% 32% 20% 6% 2	653
	11th to 20th	Day	18% 15% 37% 21% 7% 3	675
	21st to end	Day	19% 14% 33% 24% 7% 2	734
June	1st to 10th	Day	17% 14% 38% 21% 8% 2	680
	11th to 20th	Day	17% 14% 36% 22% 9% 3	680
	21st to end	Day	17% 12% 33% 26% 10% 2	680
July	1st to 10th	Day	10% 10% 36% 29% 11% 4	671
	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% 24% 29% 22% 4 2	276
	11th to 20th	Day	24% 23% 37% 11% 4 2	270
	21st to end	Day	26% 22% 29% 16% 6%	270
December	1st to 10th	Day	20% 16% 35% 21% 5% 3	225
	11th to 20th	Day	22% 21% 39% 14% 5%	218
	21st to end	Day	23% 21% 34% 16% 5% 1	297
January	1st to 10th	Night	15% 15% 38% 26% 5%	950
	11th to 20th	Night	16% 13% 37% 25% 7% 1	950
	21st to end	Night	17% 10% 37% 27% 8%	1045
February	1st to 10th	Night	15% 12% 34% 30% 8% 1	950
	11th to 20th	Night	15% 14% 34% 27% 9% 1	920
	21st to end	Night	14% 13% 36% 29% 8%	628
March	1st to 10th	Night	13% 13% 32% 32% 9% 1	760
	11th to 20th	Night	17% 15% 31% 28% 9% 1	671
	21st to end	Night	18% 11% 26% 29% 14% 3	627
April	1st to 10th	Night	18% 11% 29% 32% 10% 1	570
	11th to 20th	Night	20% 12% 25% 30% 12% 1	563
	21st to end	Night	22% 6% 23% 36% 13% 2	570
May	1st to 10th	Night	22% 9% 23% 33% 11% 2	570
	11th to 20th	Night	16% 6% 29% 35% 12% 2	561
	21st to end	Night	19% 7% 23% 34% 15% 1	617
June	1st to 10th	Night	19% 7% 24% 35% 14% 1	570
	11th to 20th	Night	15% 8% 22% 41% 15% 1	570
	21st to end	Night	16% 5% 22% 39% 16% 1	570
July	1st to 10th	Night	10% 6% 23% 41% 19% 1	555
	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	18% 13% 32% 27% 9% 1	777
	11th to 20th	Night	22% 17% 31% 20% 8% 2	950
	21st to end	Night	23% 19% 30% 22% 7%	950
December	1st to 10th	Night	17% 17% 36% 22% 8% 1	950
	11th to 20th	Night	20% 16% 33% 22% 8% 1	950
	21st to end	Night	27% 13% 31% 22% 5% 1	1045

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