Thompson observing site
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Thompson observing site clear sky chart from 2005-11-06 to 2024-02-25.

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	6% 5% 2 4 4 3 4 5% 6% 8% 52%	78348
	Night	6% 5% 2 4 4 3 4 4 6% 8% 53%	81006
Transparency	Day	29% 8% 15% 27% 17% 4	78052
	Night	29% 8% 14% 27% 17% 4	80824
Seeing	Day	11% 15% 34% 27% 11% 2	14872
	Night	9% 16% 34% 31% 9% 1	25049

By Month

Percentage Cloud Cover SW (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	1 4 2 5% 5% 3 5% 5% 5% 9% 55%	2538
	11th to 20th	Day	1 4 3 6% 5% 4 5% 5% 5% 8% 53%	2540
	21st to end	Day	2 4 2 5% 5% 3 3 5% 5% 7% 59%	2794
February	1st to 10th	Day	4 9% 4 7% 5% 4 5% 6% 6% 8% 43%	1691
	11th to 20th	Day	4 7% 4 7% 5% 3 6% 5% 6% 8% 46%	1812
	21st to end	Day	3 7% 4 5% 7% 3 6% 4 6% 8% 47%	1520
March	1st to 10th	Day	3 6% 3 5% 6% 4 5% 6% 6% 8% 48%	1800
	11th to 20th	Day	6% 5% 2 4 4 2 4 4 5% 9% 53%	1840
	21st to end	Day	9% 10% 4 5% 6% 4 4 5% 6% 9% 39%	2376
April	1st to 10th	Day	11% 7% 3 4 4 2 4 5% 6% 9% 44%	2160
	11th to 20th	Day	11% 7% 3 6% 5% 4 6% 5% 6% 7% 40%	2131
	21st to end	Day	13% 7% 2 5% 5% 3 5% 4 6% 8% 42%	2298
May	1st to 10th	Day	12% 5% 2 6% 5% 3 5% 5% 7% 8% 42%	2394
	11th to 20th	Day	14% 7% 2 5% 4 3 4 5% 6% 9% 40%	2478
	21st to end	Day	16% 6% 2 5% 4 2 5% 6% 7% 10% 37%	2724
June	1st to 10th	Day	9% 4 2 3 4 3 5% 6% 7% 10% 47%	2520
	11th to 20th	Day	8% 3 2 3 4 3 5% 6% 7% 10% 49%	2520
	21st to end	Day	4 3 2 4 3 2 4 5% 6% 7% 61%	2520
July	1st to 10th	Day	4 3 1 3 4 2 6% 6% 8% 9% 53%	2520
	11th to 20th	Day	4 2 1 2 3 2 4 4 7% 8% 62%	2520
	21st to end	Day	4 3 1 3 3 3 4 5% 7% 10% 59%	2742
August	1st to 10th	Day	5% 2 2 4 5% 2 5% 6% 8% 9% 52%	2507
	11th to 20th	Day	4 2 1 2 3 3 4 4 7% 9% 61%	2227
	21st to end	Day	5% 3 1 3 3 2 3 4 5% 7% 64%	2376
September	1st to 10th	Day	7% 3 2 4 3 2 3 3 5% 7% 63%	2160
	11th to 20th	Day	8% 3 1 3 3 2 3 4 5% 6% 63%	2080
	21st to end	Day	8% 4 2 3 3 2 2 3 4 7% 61%	1980
October	1st to 10th	Day	11% 5% 2 3 4 2 4 4 4 6% 55%	1894
	11th to 20th	Day	9% 6% 2 2 2 2 3 4 4 7% 58%	1760
	21st to end	Day	6% 5% 2 4 3 3 4 4 5% 7% 57%	1782
November	1st to 10th	Day	4 4 2 3 3 3 3 5% 6% 9% 57%	1673
	11th to 20th	Day	3 5% 2 5% 4 3 3 5% 5% 8% 57%	1710
	21st to end	Day	3 5% 2 5% 5% 2 4 5% 5% 9% 54%	1521
December	1st to 10th	Day	1 5% 3 6% 5% 4 4 6% 5% 9% 52%	1520
	11th to 20th	Day	2 4 2 4 3 3 5% 6% 6% 9% 57%	1926
	21st to end	Day	6% 9% 3 5% 5% 3 4 4 5% 6% 51%	2794
January	1st to 10th	Night	2 6% 3 5% 5% 3 6% 6% 6% 7% 51%	4417
	11th to 20th	Night	1 4 2 7% 7% 5% 6% 5% 6% 8% 49%	4430
	21st to end	Night	1 3 2 5% 6% 4 5% 5% 6% 8% 56%	4873
February	1st to 10th	Night	4 8% 4 7% 5% 3 4 5% 6% 7% 46%	2506
	11th to 20th	Night	3 6% 3 6% 6% 4 5% 5% 6% 8% 47%	2253
	21st to end	Night	4 7% 3 6% 5% 3 5% 5% 6% 8% 48%	1830
March	1st to 10th	Night	5% 6% 2 6% 5% 3 4 4 6% 8% 51%	2160
	11th to 20th	Night	6% 6% 3 5% 5% 3 4 4 6% 9% 49%	2118
	21st to end	Night	11% 8% 4 4 5% 3 4 3 6% 9% 43%	1980
April	1st to 10th	Night	11% 6% 2 5% 3 3 6% 5% 5% 8% 46%	1800
	11th to 20th	Night	12% 7% 3 4 4 3 4 4 4 8% 46%	1777
	21st to end	Night	12% 5% 2 5% 4 2 5% 5% 6% 8% 47%	1661
May	1st to 10th	Night	10% 6% 2 4 5% 3 4 4 5% 8% 50%	1564
	11th to 20th	Night	14% 6% 2 5% 4 2 3 5% 4 8% 48%	1417
	21st to end	Night	17% 5% 1 4 3 2 4 4 6% 10% 44%	1552
June	1st to 10th	Night	9% 4 2 4 3 3 3 5% 6% 9% 52%	1440
	11th to 20th	Night	10% 4 2 4 4 2 4 5% 8% 9% 48%	1440
	21st to end	Night	6% 3 1 3 4 3 4 6% 6% 8% 58%	1440
July	1st to 10th	Night	5% 3 1 2 4 3 5% 5% 6% 12% 55%	1440
	11th to 20th	Night	4 2 1 3 3 2 4 3 6% 9% 63%	1440
	21st to end	Night	3 2 1 2 4 3 4 5% 6% 8% 62%	1576
August	1st to 10th	Night	4 3 2 4 4 4 5% 6% 7% 8% 54%	1432
	11th to 20th	Night	4 3 1 3 3 2 3 4 5% 7% 63%	1669
	21st to end	Night	4 2 1 3 4 2 4 4 6% 6% 65%	1980
September	1st to 10th	Night	6% 2 1 3 3 2 4 3 4 7% 64%	1800
	11th to 20th	Night	7% 4 1 3 3 2 3 4 5% 7% 62%	1879
	21st to end	Night	11% 4 1 2 3 2 3 3 5% 5% 61%	1980
October	1st to 10th	Night	12% 6% 2 4 3 2 3 3 4 7% 53%	2064
	11th to 20th	Night	8% 4 2 4 3 2 3 4 6% 7% 56%	2180
	21st to end	Night	7% 6% 2 4 4 2 3 4 5% 8% 55%	2574
November	1st to 10th	Night	6% 5% 2 4 4 2 5% 4 7% 8% 52%	2405
	11th to 20th	Night	4 5% 2 4 5% 3 4 5% 5% 6% 57%	2470
	21st to end	Night	4 5% 2 4 4 3 4 4 6% 8% 56%	2655
December	1st to 10th	Night	3 5% 3 5% 4 3 5% 4 5% 9% 54%	2660
	11th to 20th	Night	2 4 1 4 4 3 4 5% 6% 9% 56%	3271
	21st to end	Night	6% 9% 3 6% 4 3 4 4 5% 6% 50%	4873

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	24% 11% 21% 29% 14% 1	2539
	11th to 20th	Day	28% 9% 18% 28% 12% 5%	2540
	21st to end	Day	23% 10% 19% 32% 14% 1	2794
February	1st to 10th	Day	39% 10% 14% 21% 14% 2	1691
	11th to 20th	Day	36% 8% 16% 22% 14% 4	1822
	21st to end	Day	35% 7% 17% 24% 14% 4	1520
March	1st to 10th	Day	33% 9% 16% 27% 14% 2	1800
	11th to 20th	Day	28% 7% 16% 31% 16% 2	1840
	21st to end	Day	42% 10% 14% 24% 9% 1	2376
April	1st to 10th	Day	36% 7% 19% 24% 11% 3	2160
	11th to 20th	Day	40% 9% 16% 22% 11% 2	2134
	21st to end	Day	40% 8% 17% 26% 8% 1	2298
May	1st to 10th	Day	37% 9% 16% 27% 10% 1	2394
	11th to 20th	Day	40% 9% 18% 24% 9%	2482
	21st to end	Day	41% 11% 18% 23% 6% 2	2724
June	1st to 10th	Day	30% 11% 19% 26% 12% 2	2520
	11th to 20th	Day	28% 9% 19% 26% 14% 4	2520
	21st to end	Day	20% 8% 15% 30% 22% 5%	2520
July	1st to 10th	Day	24% 9% 15% 29% 19% 4	2451
	11th to 20th	Day	18% 7% 12% 27% 25% 11%	2445
	21st to end	Day	19% 7% 12% 27% 25% 9%	2590
August	1st to 10th	Day	24% 10% 15% 26% 18% 7%	2491
	11th to 20th	Day	19% 7% 13% 32% 22% 6%	2227
	21st to end	Day	19% 6% 12% 24% 30% 9%	2376
September	1st to 10th	Day	22% 5% 10% 26% 26% 10%	2160
	11th to 20th	Day	23% 6% 11% 28% 25% 8%	2080
	21st to end	Day	25% 5% 10% 33% 25% 2	1980
October	1st to 10th	Day	31% 7% 13% 24% 21% 4	1894
	11th to 20th	Day	27% 5% 12% 30% 20% 6%	1760
	21st to end	Day	27% 7% 17% 28% 16% 5%	1782
November	1st to 10th	Day	23% 7% 16% 25% 22% 6%	1673
	11th to 20th	Day	26% 6% 13% 28% 21% 5%	1710
	21st to end	Day	27% 6% 15% 31% 17% 4	1521
December	1st to 10th	Day	30% 7% 17% 26% 18% 2	1520
	11th to 20th	Day	23% 9% 19% 35% 13% 2	1924
	21st to end	Day	35% 6% 10% 30% 18% 2	2794
January	1st to 10th	Night	30% 10% 14% 30% 14% 2	4417
	11th to 20th	Night	33% 8% 16% 27% 14% 2	4430
	21st to end	Night	23% 12% 17% 30% 15% 2	4873
February	1st to 10th	Night	36% 8% 17% 22% 14% 2	2506
	11th to 20th	Night	34% 8% 15% 25% 15% 4	2253
	21st to end	Night	33% 7% 14% 24% 15% 6%	1830
March	1st to 10th	Night	32% 7% 15% 28% 16% 2	2160
	11th to 20th	Night	31% 8% 14% 25% 20% 2	2118
	21st to end	Night	38% 8% 18% 23% 12% 1	1980
April	1st to 10th	Night	37% 9% 14% 22% 15% 3	1800
	11th to 20th	Night	38% 7% 14% 25% 14% 2	1777
	21st to end	Night	35% 8% 16% 27% 13% 1	1661
May	1st to 10th	Night	34% 9% 14% 30% 13% 1	1564
	11th to 20th	Night	35% 10% 16% 30% 10%	1418
	21st to end	Night	37% 8% 17% 25% 11% 1	1552
June	1st to 10th	Night	28% 9% 19% 29% 13% 2	1440
	11th to 20th	Night	31% 9% 21% 22% 14% 3	1440
	21st to end	Night	23% 9% 16% 25% 22% 5%	1440
July	1st to 10th	Night	22% 8% 16% 32% 17% 5%	1400
	11th to 20th	Night	18% 7% 14% 24% 26% 11%	1392
	21st to end	Night	18% 7% 13% 28% 25% 8%	1488
August	1st to 10th	Night	27% 8% 12% 27% 19% 7%	1424
	11th to 20th	Night	20% 6% 11% 30% 26% 8%	1669
	21st to end	Night	19% 6% 12% 24% 30% 9%	1980
September	1st to 10th	Night	21% 5% 9% 29% 27% 10%	1800
	11th to 20th	Night	22% 6% 12% 27% 26% 8%	1879
	21st to end	Night	26% 4 9% 30% 25% 6%	1980
October	1st to 10th	Night	32% 6% 12% 25% 20% 5%	2064
	11th to 20th	Night	28% 6% 10% 27% 22% 7%	2180
	21st to end	Night	28% 6% 16% 25% 19% 6%	2574
November	1st to 10th	Night	30% 6% 16% 22% 20% 6%	2405
	11th to 20th	Night	27% 6% 13% 27% 22% 5%	2470
	21st to end	Night	26% 8% 14% 28% 19% 5%	2655
December	1st to 10th	Night	29% 7% 15% 29% 17% 3	2660
	11th to 20th	Night	23% 7% 16% 37% 15% 1	3272
	21st to end	Night	35% 7% 11% 31% 14% 2	4873

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	5% 16% 36% 26% 14% 3	270
	11th to 20th	Day	11% 19% 36% 22% 10% 2	270
	21st to end	Day	9% 30% 36% 19% 6% 1	297
February	1st to 10th	Day	12% 24% 43% 15% 6%	270
	11th to 20th	Day	11% 21% 45% 17% 5%	270
	21st to end	Day	7% 25% 46% 18% 3 1	210
March	1st to 10th	Day	6% 25% 44% 16% 8% 2	240
	11th to 20th	Day	11% 19% 39% 20% 10% 2	266
	21st to end	Day	21% 21% 32% 18% 8% 2	462
April	1st to 10th	Day	17% 22% 38% 18% 4	420
	11th to 20th	Day	20% 29% 37% 11% 2	412
	21st to end	Day	17% 18% 33% 23% 8% 1	558
May	1st to 10th	Day	17% 19% 29% 23% 10% 2	603
	11th to 20th	Day	20% 13% 29% 25% 10% 2	625
	21st to end	Day	18% 13% 34% 21% 10% 3	679
June	1st to 10th	Day	10% 10% 31% 32% 14% 3	630
	11th to 20th	Day	13% 14% 37% 27% 8% 1	630
	21st to end	Day	8% 11% 34% 31% 14% 2	630
July	1st to 10th	Day	5% 5% 32% 41% 16% 1	621
	11th to 20th	Day	4 8% 36% 37% 15% 1	618
	21st to end	Day	5% 8% 28% 40% 17% 2	684
August	1st to 10th	Day	5% 6% 33% 39% 15% 2	626
	11th to 20th	Day	5% 6% 31% 41% 13% 4	512
	21st to end	Day	3 10% 30% 36% 16% 4	495
September	1st to 10th	Day	8% 9% 36% 33% 10% 4	450
	11th to 20th	Day	9% 9% 27% 36% 15% 4	370
	21st to end	Day	13% 18% 33% 21% 12% 3	270
October	1st to 10th	Day	19% 11% 33% 21% 13% 3	270
	11th to 20th	Day	20% 23% 28% 16% 9% 4	270
	21st to end	Day	11% 23% 37% 21% 7% 1	297
November	1st to 10th	Day	11% 24% 33% 23% 7% 3	270
	11th to 20th	Day	11% 20% 41% 17% 10% 1	270
	21st to end	Day	10% 21% 33% 25% 10% 1	270
December	1st to 10th	Day	9% 23% 39% 20% 7% 1	270
	11th to 20th	Day	10% 21% 41% 22% 5%	270
	21st to end	Day	10% 23% 37% 25% 5% 1	297
January	1st to 10th	Night	7% 20% 38% 24% 10% 1	950
	11th to 20th	Night	7% 27% 36% 22% 7% 1	950
	21st to end	Night	6% 25% 38% 25% 5%	1045
February	1st to 10th	Night	8% 23% 40% 22% 7%	796
	11th to 20th	Night	8% 21% 38% 26% 8% 1	744
	21st to end	Night	7% 19% 38% 28% 7% 1	610
March	1st to 10th	Night	8% 19% 38% 29% 5% 1	720
	11th to 20th	Night	9% 12% 38% 32% 6% 2	705
	21st to end	Night	16% 14% 29% 31% 8% 1	594
April	1st to 10th	Night	15% 13% 36% 31% 5%	540
	11th to 20th	Night	18% 15% 39% 23% 5%	533
	21st to end	Night	14% 16% 29% 31% 9% 1	540
May	1st to 10th	Night	13% 10% 31% 32% 13% 1	540
	11th to 20th	Night	17% 10% 29% 31% 12% 1	531
	21st to end	Night	19% 10% 36% 25% 10%	584
June	1st to 10th	Night	10% 11% 31% 33% 14% 1	540
	11th to 20th	Night	12% 10% 38% 34% 7%	540
	21st to end	Night	6% 7% 33% 42% 11%	540
July	1st to 10th	Night	6% 5% 28% 43% 17% 1	525
	11th to 20th	Night	4 4 29% 50% 12% 1	522
	21st to end	Night	3 3 29% 50% 15%	591
August	1st to 10th	Night	6% 3 26% 49% 14% 1	537
	11th to 20th	Night	4 4 27% 47% 18% 1	534
	21st to end	Night	4 6% 24% 46% 19% 1	594
September	1st to 10th	Night	7% 6% 27% 43% 16% 2	540
	11th to 20th	Night	7% 9% 28% 41% 14% 1	540
	21st to end	Night	15% 10% 30% 33% 11% 1	540
October	1st to 10th	Night	14% 13% 28% 34% 11% 1	624
	11th to 20th	Night	11% 16% 34% 29% 10% 1	740
	21st to end	Night	12% 20% 33% 27% 8% 1	990
November	1st to 10th	Night	8% 18% 37% 29% 6% 2	925
	11th to 20th	Night	10% 20% 35% 27% 6% 1	950
	21st to end	Night	7% 23% 37% 27% 5%	950
December	1st to 10th	Night	8% 25% 37% 25% 5%	950
	11th to 20th	Night	7% 20% 38% 27% 7% 1	950
	21st to end	Night	11% 23% 35% 24% 6%	1045

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