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 2024-03-18.

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% 4 7% 6% 3 5% 5% 5% 7% 34%	78271
	Night	12% 10% 4 8% 6% 4 5% 5% 5% 7% 33%	81534
Transparency	Day	48% 7% 14% 21% 10% 1	77977
	Night	49% 7% 13% 21% 9% 1	81362
Seeing	Day	17% 15% 34% 22% 9% 2	15082
	Night	17% 11% 30% 29% 11% 1	25320

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% 12% 6% 10% 8% 4 5% 4 5% 8% 30%	2527
	11th to 20th	Day	7% 9% 7% 15% 9% 4 4 4 6% 9% 27%	2530
	21st to end	Day	10% 15% 10% 13% 9% 5% 4 5% 4 6% 19%	2783
February	1st to 10th	Day	8% 11% 5% 10% 7% 3 6% 5% 5% 7% 34%	1621
	11th to 20th	Day	8% 11% 4 8% 6% 4 6% 5% 5% 6% 36%	1673
	21st to end	Day	8% 11% 6% 9% 7% 3 5% 4 6% 7% 34%	1570
March	1st to 10th	Day	9% 10% 4 7% 7% 4 5% 5% 5% 6% 38%	1898
	11th to 20th	Day	12% 10% 4 8% 7% 3 5% 5% 4 6% 36%	1971
	21st to end	Day	14% 10% 4 8% 8% 4 5% 5% 5% 6% 31%	2347
April	1st to 10th	Day	14% 10% 4 8% 8% 3 5% 5% 5% 7% 29%	2160
	11th to 20th	Day	17% 11% 5% 8% 5% 4 4 5% 5% 7% 29%	2134
	21st to end	Day	17% 12% 4 6% 6% 3 4 5% 4 5% 34%	2402
May	1st to 10th	Day	16% 11% 4 7% 6% 3 5% 5% 4 7% 32%	2520
	11th to 20th	Day	16% 9% 3 6% 6% 3 5% 4 5% 7% 35%	2479
	21st to end	Day	16% 8% 3 6% 5% 3 5% 5% 5% 7% 37%	2725
June	1st to 10th	Day	14% 7% 4 6% 5% 3 5% 5% 6% 8% 38%	2520
	11th to 20th	Day	13% 6% 3 6% 5% 3 6% 6% 6% 8% 38%	2520
	21st to end	Day	16% 6% 3 5% 5% 3 5% 5% 6% 9% 38%	2520
July	1st to 10th	Day	9% 4 2 5% 5% 4 6% 5% 6% 9% 44%	2520
	11th to 20th	Day	10% 6% 3 6% 5% 4 6% 6% 7% 9% 38%	2520
	21st to end	Day	8% 5% 3 5% 5% 4 5% 7% 6% 10% 42%	2743
August	1st to 10th	Day	7% 5% 3 5% 6% 4 6% 6% 8% 10% 38%	2507
	11th to 20th	Day	12% 5% 2 5% 4 3 5% 6% 7% 8% 43%	2429
	21st to end	Day	13% 5% 2 5% 5% 3 5% 5% 7% 8% 43%	2376
September	1st to 10th	Day	11% 6% 3 6% 6% 4 5% 4 5% 8% 42%	2160
	11th to 20th	Day	14% 7% 2 6% 6% 3 5% 5% 6% 9% 36%	2120
	21st to end	Day	16% 10% 3 7% 6% 3 4 5% 5% 7% 33%	1976
October	1st to 10th	Day	18% 11% 4 6% 6% 3 4 5% 5% 6% 32%	1800
	11th to 20th	Day	17% 12% 4 8% 6% 4 4 6% 5% 6% 26%	1751
	21st to end	Day	20% 13% 4 8% 6% 3 4 5% 5% 6% 27%	1782
November	1st to 10th	Day	13% 11% 5% 9% 8% 4 6% 6% 5% 6% 26%	1639
	11th to 20th	Day	15% 16% 6% 10% 7% 3 6% 5% 4 5% 25%	1520
	21st to end	Day	15% 13% 5% 7% 6% 3 5% 4 5% 8% 28%	1520
December	1st to 10th	Day	11% 12% 5% 10% 7% 4 6% 6% 6% 5% 29%	1427
	11th to 20th	Day	11% 15% 6% 10% 8% 4 4 3 4 6% 29%	1798
	21st to end	Day	24% 22% 5% 7% 5% 3 3 3 3 4 22%	2783
January	1st to 10th	Night	9% 14% 6% 8% 5% 4 5% 5% 5% 7% 33%	4428
	11th to 20th	Night	6% 10% 6% 16% 12% 6% 6% 5% 5% 7% 21%	4440
	21st to end	Night	8% 15% 8% 12% 8% 5% 5% 5% 4 6% 23%	4884
February	1st to 10th	Night	8% 10% 4 10% 7% 4 6% 6% 5% 6% 34%	2556
	11th to 20th	Night	9% 9% 4 6% 7% 5% 6% 6% 6% 7% 35%	2394
	21st to end	Night	9% 11% 5% 9% 8% 3 6% 7% 6% 8% 29%	1884
March	1st to 10th	Night	9% 9% 4 8% 8% 4 6% 6% 5% 6% 35%	2269
	11th to 20th	Night	11% 11% 3 7% 5% 3 5% 5% 6% 7% 37%	2168
	21st to end	Night	13% 10% 3 7% 7% 3 5% 5% 6% 6% 35%	2008
April	1st to 10th	Night	14% 11% 4 7% 6% 4 6% 4 6% 6% 33%	1800
	11th to 20th	Night	15% 11% 4 6% 5% 3 5% 5% 5% 6% 34%	1775
	21st to end	Night	17% 10% 3 7% 7% 4 5% 5% 5% 5% 32%	1553
May	1st to 10th	Night	19% 7% 3 6% 5% 3 4 5% 5% 7% 34%	1440
	11th to 20th	Night	13% 8% 4 6% 5% 3 5% 6% 5% 8% 36%	1416
	21st to end	Night	17% 7% 3 6% 4 3 6% 4 6% 8% 36%	1551
June	1st to 10th	Night	16% 7% 3 5% 5% 3 6% 5% 6% 6% 38%	1440
	11th to 20th	Night	12% 7% 3 6% 5% 3 5% 6% 5% 9% 40%	1440
	21st to end	Night	14% 6% 2 6% 5% 2 4 6% 5% 8% 41%	1440
July	1st to 10th	Night	8% 5% 2 4 5% 3 5% 5% 6% 8% 50%	1440
	11th to 20th	Night	7% 5% 2 4 6% 3 5% 5% 7% 6% 49%	1440
	21st to end	Night	7% 3 2 5% 5% 3 5% 5% 7% 9% 49%	1575
August	1st to 10th	Night	8% 4 2 4 4 3 6% 5% 6% 9% 48%	1432
	11th to 20th	Night	9% 5% 2 4 4 3 4 4 6% 8% 50%	1467
	21st to end	Night	15% 5% 2 4 4 3 3 4 6% 7% 47%	1980
September	1st to 10th	Night	12% 5% 2 4 4 3 4 5% 6% 6% 51%	1800
	11th to 20th	Night	12% 5% 3 5% 6% 3 5% 4 6% 7% 44%	1822
	21st to end	Night	17% 7% 3 6% 7% 3 4 5% 6% 8% 35%	1982
October	1st to 10th	Night	17% 7% 3 5% 7% 4 5% 4 6% 7% 36%	2160
	11th to 20th	Night	14% 11% 3 6% 6% 3 5% 5% 5% 8% 34%	2208
	21st to end	Night	17% 11% 5% 8% 6% 4 5% 5% 4 7% 29%	2574
November	1st to 10th	Night	14% 11% 4 7% 6% 4 5% 6% 6% 8% 30%	2434
	11th to 20th	Night	16% 12% 5% 11% 7% 4 4 5% 5% 6% 26%	2660
	21st to end	Night	14% 13% 5% 8% 7% 3 5% 5% 6% 7% 27%	2660
December	1st to 10th	Night	10% 11% 7% 10% 6% 3 5% 5% 5% 7% 31%	2741
	11th to 20th	Night	11% 13% 6% 9% 7% 4 4 4 5% 6% 32%	3389
	21st to end	Night	23% 20% 7% 10% 5% 2 3 3 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	46% 7% 12% 19% 14% 3	1900
	11th to 20th	Day	49% 7% 11% 21% 12% 2	1971
	21st to end	Day	52% 6% 13% 15% 13% 2	2347
April	1st to 10th	Day	52% 5% 11% 18% 11% 2	2160
	11th to 20th	Day	54% 5% 11% 19% 9% 1	2134
	21st to end	Day	52% 4 10% 17% 14% 2	2402
May	1st to 10th	Day	51% 6% 13% 19% 10% 1	2520
	11th to 20th	Day	46% 7% 13% 21% 11% 1	2483
	21st to end	Day	44% 9% 12% 20% 14% 1	2725
June	1st to 10th	Day	41% 8% 15% 21% 14% 1	2520
	11th to 20th	Day	40% 8% 14% 23% 14% 1	2520
	21st to end	Day	41% 9% 17% 22% 10% 1	2520
July	1st to 10th	Day	33% 9% 15% 27% 16% 1	2451
	11th to 20th	Day	37% 10% 14% 28% 11% 1	2445
	21st to end	Day	34% 9% 16% 29% 11% 1	2591
August	1st to 10th	Day	35% 10% 18% 27% 8% 1	2492
	11th to 20th	Day	35% 9% 18% 27% 12%	2429
	21st to end	Day	35% 8% 17% 28% 12%	2376
September	1st to 10th	Day	39% 7% 16% 28% 9%	2160
	11th to 20th	Day	42% 8% 22% 23% 4 1	2120
	21st to end	Day	48% 8% 17% 22% 5%	1976
October	1st to 10th	Day	51% 7% 15% 23% 4	1800
	11th to 20th	Day	55% 6% 14% 20% 5% 1	1751
	21st to end	Day	59% 7% 10% 19% 6%	1782
November	1st to 10th	Day	56% 6% 15% 15% 7% 1	1639
	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	48% 6% 12% 19% 12% 2	2280
	11th to 20th	Night	46% 6% 12% 21% 13% 2	2169
	21st to end	Night	47% 7% 11% 20% 12% 2	2008
April	1st to 10th	Night	51% 6% 8% 20% 14%	1800
	11th to 20th	Night	50% 5% 10% 21% 11% 2	1775
	21st to end	Night	52% 5% 8% 21% 13% 2	1553
May	1st to 10th	Night	48% 6% 11% 21% 12% 1	1440
	11th to 20th	Night	44% 8% 10% 21% 14% 2	1417
	21st to end	Night	46% 9% 9% 23% 13% 1	1552
June	1st to 10th	Night	46% 7% 11% 21% 14% 1	1440
	11th to 20th	Night	42% 7% 11% 23% 16% 1	1440
	21st to end	Night	39% 11% 15% 22% 12% 2	1440
July	1st to 10th	Night	32% 9% 14% 28% 16% 1	1400
	11th to 20th	Night	32% 10% 13% 30% 14% 1	1392
	21st to end	Night	29% 10% 18% 28% 13% 1	1487
August	1st to 10th	Night	31% 9% 20% 29% 10% 2	1423
	11th to 20th	Night	31% 8% 16% 33% 11% 1	1467
	21st to end	Night	37% 8% 15% 28% 13% 1	1980
September	1st to 10th	Night	32% 6% 17% 31% 12%	1800
	11th to 20th	Night	37% 9% 19% 31% 4	1822
	21st to end	Night	46% 8% 15% 26% 5%	1982
October	1st to 10th	Night	47% 8% 15% 23% 7%	2160
	11th to 20th	Night	48% 8% 15% 23% 6% 1	2208
	21st to end	Night	54% 7% 14% 18% 6%	2574
November	1st to 10th	Night	51% 8% 13% 20% 7%	2434
	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% 23% 36% 16% 7% 2	261
	21st to end	Day	18% 17% 34% 20% 9% 3	433
April	1st to 10th	Day	20% 15% 35% 23% 6% 1	420
	11th to 20th	Day	25% 15% 25% 22% 11% 1	412
	21st to end	Day	24% 16% 30% 19% 9% 2	496
May	1st to 10th	Day	22% 18% 32% 20% 7% 2	603
	11th to 20th	Day	17% 14% 37% 21% 8% 3	625
	21st to end	Day	20% 14% 32% 25% 7% 2	679
June	1st to 10th	Day	16% 13% 39% 22% 8% 2	630
	11th to 20th	Day	16% 13% 36% 23% 9% 3	630
	21st to end	Day	16% 11% 33% 26% 11% 2	630
July	1st to 10th	Day	10% 10% 37% 29% 10% 4	621
	11th to 20th	Day	11% 11% 38% 24% 11% 5%	618
	21st to end	Day	8% 9% 38% 26% 13% 5%	684
August	1st to 10th	Day	11% 11% 38% 27% 10% 3	626
	11th to 20th	Day	15% 9% 28% 28% 15% 4	614
	21st to end	Day	18% 11% 31% 23% 14% 4	495
September	1st to 10th	Day	12% 12% 31% 26% 16% 4	450
	11th to 20th	Day	17% 13% 31% 24% 12% 3	450
	21st to end	Day	22% 15% 29% 21% 11% 2	446
October	1st to 10th	Day	22% 21% 27% 20% 9% 1	270
	11th to 20th	Day	22% 19% 36% 18% 5% 1	270
	21st to end	Day	30% 20% 36% 11% 2	297
November	1st to 10th	Day	20% 24% 29% 22% 4 1	270
	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	16% 15% 31% 28% 9% 1	665
	21st to end	Night	17% 12% 25% 29% 14% 3	594
April	1st to 10th	Night	18% 11% 29% 31% 10% 1	540
	11th to 20th	Night	21% 11% 24% 30% 13% 1	533
	21st to end	Night	21% 5% 23% 36% 13% 2	540
May	1st to 10th	Night	22% 9% 23% 33% 12% 2	540
	11th to 20th	Night	17% 6% 27% 35% 13% 2	531
	21st to end	Night	19% 8% 23% 34% 15% 1	584
June	1st to 10th	Night	19% 6% 24% 36% 14% 1	540
	11th to 20th	Night	15% 7% 22% 40% 15% 1	540
	21st to end	Night	16% 5% 21% 39% 17% 1	540
July	1st to 10th	Night	11% 6% 23% 42% 18% 1	525
	11th to 20th	Night	9% 4 29% 36% 19% 2	522
	21st to end	Night	7% 4 25% 38% 24% 2	591
August	1st to 10th	Night	9% 4 29% 39% 18% 1	537
	11th to 20th	Night	12% 5% 20% 34% 25% 4	534
	21st to end	Night	17% 5% 25% 33% 18% 2	594
September	1st to 10th	Night	13% 4 26% 31% 20% 5%	540
	11th to 20th	Night	17% 6% 29% 32% 15% 2	562
	21st to end	Night	22% 8% 25% 29% 14% 2	720
October	1st to 10th	Night	22% 13% 27% 22% 15% 2	720
	11th to 20th	Night	19% 15% 31% 26% 8%	720
	21st to end	Night	21% 14% 32% 26% 8%	792
November	1st to 10th	Night	18% 13% 33% 26% 8% 1	768
	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

Page generated at 2024-03-15 14:01:45. Version ac07

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