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 2026-08-15.

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	7% 5% 2 5% 3 4 4 4 6% 8% 52%	152771
	Night	6% 5% 2 5% 4 4 4 4 6% 8% 52%	146613
Transparency	Day	28% 7% 14% 27% 19% 4	147535
	Night	29% 7% 14% 27% 19% 4	142873
Seeing	Day	11% 16% 34% 26% 10% 2	18431
	Night	9% 15% 34% 32% 9% 1	28319

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	2 6% 4 8% 4 4 4 4 7% 10% 48%	4720
	11th to 20th	Day	1 4 3 7% 6% 5% 5% 5% 7% 8% 48%	4740
	21st to end	Day	1 4 2 8% 4 4 4 4 6% 7% 56%	5188
February	1st to 10th	Day	3 9% 4 7% 5% 5% 5% 5% 7% 7% 43%	1869
	11th to 20th	Day	4 7% 4 8% 4 4 5% 4 5% 8% 47%	2009
	21st to end	Day	3 6% 3 6% 6% 5% 5% 4 6% 8% 49%	1730
March	1st to 10th	Day	3 8% 2 5% 4 4 4 4 5% 6% 55%	4600
	11th to 20th	Day	6% 6% 2 4 3 2 3 3 5% 9% 57%	4764
	21st to end	Day	12% 11% 3 7% 3 4 4 3 6% 8% 39%	6101
April	1st to 10th	Day	12% 8% 2 4 4 3 4 4 6% 8% 45%	2520
	11th to 20th	Day	10% 8% 3 6% 5% 4 6% 4 6% 7% 42%	2491
	21st to end	Day	13% 8% 2 6% 4 4 4 3 6% 7% 41%	2682
May	1st to 10th	Day	8% 6% 3 6% 4 4 4 4 6% 8% 48%	7825
	11th to 20th	Day	13% 7% 2 4 3 3 4 3 6% 8% 48%	8191
	21st to end	Day	14% 6% 2 5% 3 4 4 4 6% 8% 44%	9026
June	1st to 10th	Day	9% 4 1 4 4 4 5% 5% 7% 10% 48%	2940
	11th to 20th	Day	9% 4 2 4 4 4 4 4 7% 10% 49%	2940
	21st to end	Day	5% 3 2 4 3 3 4 4 6% 7% 60%	2940
July	1st to 10th	Day	4 3 1 4 4 4 5% 5% 8% 10% 49%	8249
	11th to 20th	Day	7% 2 1 3 2 3 4 3 6% 7% 60%	8250
	21st to end	Day	6% 3 1 4 3 4 5% 5% 7% 10% 52%	8957
August	1st to 10th	Day	6% 3 1 4 3 4 5% 5% 9% 10% 49%	6372
	11th to 20th	Day	5% 3 1 4 3 4 5% 4 8% 10% 54%	5659
	21st to end	Day	8% 4 1 4 3 3 4 3 5% 8% 56%	5984
September	1st to 10th	Day	6% 3 1 4 2 3 3 2 5% 7% 64%	2400
	11th to 20th	Day	8% 3 1 3 3 3 4 3 5% 6% 61%	2310
	21st to end	Day	8% 4 1 4 3 3 2 3 4 6% 62%	2200
October	1st to 10th	Day	6% 3 1 2 2 2 3 2 3 5% 71%	3434
	11th to 20th	Day	9% 4 1 2 2 3 3 3 4 7% 63%	3168
	21st to end	Day	7% 6% 2 4 2 4 4 3 6% 8% 56%	3190
November	1st to 10th	Day	4 4 2 4 3 3 4 4 7% 9% 56%	1853
	11th to 20th	Day	3 5% 2 5% 4 3 3 5% 6% 7% 57%	1890
	21st to end	Day	3 6% 2 6% 5% 3 4 4 6% 9% 53%	1681
December	1st to 10th	Day	1 4 3 6% 4 4 5% 4 5% 9% 54%	2700
	11th to 20th	Day	1 3 1 6% 4 5% 6% 4 6% 8% 54%	3106
	21st to end	Day	5% 8% 2 6% 4 4 4 3 5% 7% 51%	4092
January	1st to 10th	Night	4 9% 4 8% 5% 4 5% 4 5% 6% 46%	8201
	11th to 20th	Night	1 3 2 9% 5% 6% 6% 4 6% 9% 49%	8250
	21st to end	Night	1 3 2 7% 5% 5% 5% 3 5% 7% 57%	9026
February	1st to 10th	Night	3 9% 4 8% 4 5% 4 4 7% 8% 45%	2769
	11th to 20th	Night	2 7% 3 7% 6% 5% 5% 3 6% 8% 48%	2496
	21st to end	Night	3 7% 3 6% 4 4 4 4 7% 8% 50%	2076
March	1st to 10th	Night	5% 7% 2 6% 4 3 3 3 5% 7% 54%	5494
	11th to 20th	Night	4 7% 3 5% 4 4 3 4 6% 9% 52%	5435
	21st to end	Night	11% 11% 4 7% 4 4 4 3 7% 9% 37%	5094
April	1st to 10th	Night	11% 7% 2 5% 4 4 5% 4 5% 8% 45%	2100
	11th to 20th	Night	11% 6% 3 4 4 4 4 3 5% 7% 49%	2077
	21st to end	Night	11% 6% 2 5% 3 4 4 4 6% 8% 47%	1937
May	1st to 10th	Night	7% 5% 2 6% 4 4 4 3 6% 8% 51%	5107
	11th to 20th	Night	11% 6% 2 5% 3 3 4 3 5% 7% 50%	4701
	21st to end	Night	16% 7% 1 4 3 3 4 3 5% 8% 45%	5181
June	1st to 10th	Night	8% 5% 1 4 3 4 4 4 6% 9% 52%	1680
	11th to 20th	Night	9% 4 1 5% 4 3 5% 4 8% 8% 48%	1680
	21st to end	Night	7% 3 1 3 3 3 4 4 6% 8% 56%	1680
July	1st to 10th	Night	6% 4 1 4 4 3 3 4 6% 10% 55%	4740
	11th to 20th	Night	5% 2 1 3 3 3 4 3 6% 10% 61%	4740
	21st to end	Night	9% 4 1 4 3 4 5% 4 7% 9% 49%	5152
August	1st to 10th	Night	7% 3 1 4 4 4 5% 3 6% 8% 54%	3660
	11th to 20th	Night	5% 4 1 4 3 3 4 4 6% 10% 58%	4261
	21st to end	Night	6% 3 2 6% 4 4 4 4 5% 6% 55%	4993
September	1st to 10th	Night	6% 3 1 3 3 2 3 2 4 7% 66%	2000
	11th to 20th	Night	7% 4 1 3 3 2 3 3 6% 8% 60%	2089
	21st to end	Night	10% 3 1 2 2 2 3 2 5% 5% 63%	2200
October	1st to 10th	Night	7% 3 1 3 2 1 2 2 3 5% 72%	3749
	11th to 20th	Night	7% 4 2 4 2 3 3 3 6% 7% 59%	3946
	21st to end	Night	8% 6% 2 5% 3 3 3 3 5% 8% 55%	4610
November	1st to 10th	Night	6% 6% 2 5% 3 4 5% 3 6% 7% 52%	2665
	11th to 20th	Night	4 6% 1 4 4 4 4 3 5% 6% 57%	2730
	21st to end	Night	3 5% 2 5% 4 4 4 3 6% 8% 55%	2935
December	1st to 10th	Night	2 4 2 5% 4 4 5% 3 6% 9% 56%	4710
	11th to 20th	Night	2 4 2 6% 5% 6% 5% 4 7% 9% 50%	5321
	21st to end	Night	5% 8% 3 6% 4 4 5% 4 6% 7% 48%	7128

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	31% 9% 18% 28% 13% 1	4723
	11th to 20th	Day	30% 9% 16% 26% 15% 3	4740
	21st to end	Day	26% 9% 14% 31% 17% 3	5188
February	1st to 10th	Day	38% 10% 14% 21% 14% 2	1869
	11th to 20th	Day	36% 8% 15% 22% 17% 4	2019
	21st to end	Day	32% 6% 17% 26% 15% 4	1730
March	1st to 10th	Day	28% 5% 11% 24% 27% 4	4600
	11th to 20th	Day	24% 6% 16% 31% 21% 2	4764
	21st to end	Day	43% 9% 15% 23% 10%	6101
April	1st to 10th	Day	37% 7% 18% 24% 11% 3	2520
	11th to 20th	Day	38% 8% 14% 22% 14% 3	2494
	21st to end	Day	40% 8% 16% 25% 9% 1	2682
May	1st to 10th	Day	32% 7% 13% 27% 18% 1	7825
	11th to 20th	Day	33% 7% 13% 24% 19% 5%	8195
	21st to end	Day	35% 8% 13% 26% 17% 1	9027
June	1st to 10th	Day	29% 11% 20% 25% 13% 2	2940
	11th to 20th	Day	28% 9% 19% 25% 16% 3	2940
	21st to end	Day	21% 8% 15% 29% 22% 5%	2940
July	1st to 10th	Day	24% 9% 15% 30% 21% 2	8181
	11th to 20th	Day	20% 7% 12% 27% 20% 14%	8175
	21st to end	Day	24% 9% 16% 26% 20% 6%	8759
August	1st to 10th	Day	24% 10% 17% 21% 18% 9%	4676
	11th to 20th	Day	25% 9% 16% 28% 18% 4	4114
	21st to end	Day	19% 5% 11% 25% 28% 12%	4311
September	1st to 10th	Day	21% 4 11% 28% 27% 9%	2400
	11th to 20th	Day	23% 6% 11% 28% 24% 7%	2310
	21st to end	Day	24% 5% 9% 33% 26% 4	2200
October	1st to 10th	Day	18% 4 10% 19% 32% 17%	3434
	11th to 20th	Day	23% 4 11% 34% 23% 4	3168
	21st to end	Day	27% 7% 15% 31% 16% 4	3190
November	1st to 10th	Day	24% 8% 16% 26% 22% 6%	1853
	11th to 20th	Day	25% 7% 14% 27% 22% 5%	1890
	21st to end	Day	27% 6% 15% 31% 16% 4	1681
December	1st to 10th	Day	27% 7% 20% 28% 18% 1	2700
	11th to 20th	Day	26% 8% 19% 32% 13% 1	3104
	21st to end	Day	33% 5% 10% 34% 16% 1	4092
January	1st to 10th	Night	37% 8% 14% 29% 11% 1	8202
	11th to 20th	Night	32% 7% 18% 24% 16% 3	8250
	21st to end	Night	25% 9% 14% 28% 22% 3	9026
February	1st to 10th	Night	36% 8% 16% 23% 14% 2	2769
	11th to 20th	Night	33% 7% 14% 24% 17% 3	2496
	21st to end	Night	31% 7% 14% 27% 16% 5%	2076
March	1st to 10th	Night	29% 6% 10% 27% 24% 4	5494
	11th to 20th	Night	28% 7% 17% 26% 21% 1	5435
	21st to end	Night	41% 9% 19% 20% 10% 1	5094
April	1st to 10th	Night	38% 8% 14% 24% 13% 3	2100
	11th to 20th	Night	35% 7% 13% 25% 18% 2	2077
	21st to end	Night	35% 8% 16% 27% 13% 1	1937
May	1st to 10th	Night	31% 8% 12% 26% 23% 1	5108
	11th to 20th	Night	33% 8% 14% 27% 17% 2	4702
	21st to end	Night	36% 7% 12% 27% 17% 1	5182
June	1st to 10th	Night	28% 10% 19% 28% 14% 2	1680
	11th to 20th	Night	31% 9% 20% 22% 15% 3	1680
	21st to end	Night	24% 9% 16% 25% 21% 5%	1680
July	1st to 10th	Night	23% 6% 18% 29% 21% 2	4700
	11th to 20th	Night	19% 6% 14% 27% 19% 15%	4692
	21st to end	Night	30% 7% 15% 27% 17% 4	5038
August	1st to 10th	Night	28% 9% 15% 20% 20% 9%	2680
	11th to 20th	Night	24% 8% 19% 25% 19% 4	3094
	21st to end	Night	21% 6% 12% 24% 29% 8%	3597
September	1st to 10th	Night	20% 5% 9% 30% 27% 9%	2000
	11th to 20th	Night	23% 6% 12% 26% 25% 7%	2089
	21st to end	Night	24% 4 9% 30% 25% 8%	2200
October	1st to 10th	Night	19% 4 8% 22% 31% 17%	3749
	11th to 20th	Night	25% 5% 10% 30% 26% 4	3946
	21st to end	Night	28% 5% 13% 28% 19% 8%	4610
November	1st to 10th	Night	30% 6% 15% 23% 20% 6%	2665
	11th to 20th	Night	27% 6% 14% 26% 22% 5%	2730
	21st to end	Night	27% 8% 14% 29% 18% 4	2935
December	1st to 10th	Night	26% 7% 17% 30% 17% 3	4710
	11th to 20th	Night	30% 8% 19% 30% 13% 2	5322
	21st to end	Night	35% 6% 10% 32% 15% 1	7128

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% 37% 25% 12% 3	330
	11th to 20th	Day	14% 19% 37% 20% 8% 2	330
	21st to end	Day	10% 29% 35% 19% 6% 1	363
February	1st to 10th	Day	11% 24% 43% 18% 5%	330
	11th to 20th	Day	10% 19% 46% 18% 7% 1	330
	21st to end	Day	9% 25% 45% 18% 3 1	273
March	1st to 10th	Day	8% 23% 40% 19% 8% 2	330
	11th to 20th	Day	11% 20% 41% 18% 8% 1	361
	21st to end	Day	21% 21% 35% 17% 7% 1	594
April	1st to 10th	Day	18% 24% 37% 17% 4	540
	11th to 20th	Day	21% 28% 34% 14% 3	532
	21st to end	Day	18% 19% 33% 22% 7% 1	702
May	1st to 10th	Day	16% 19% 31% 23% 9% 2	753
	11th to 20th	Day	21% 15% 28% 23% 11% 2	775
	21st to end	Day	17% 14% 35% 22% 9% 3	844
June	1st to 10th	Day	10% 12% 32% 30% 14% 2	780
	11th to 20th	Day	13% 15% 37% 27% 7% 1	780
	21st to end	Day	9% 12% 35% 30% 13% 1	780
July	1st to 10th	Day	4 6% 29% 42% 17% 2	771
	11th to 20th	Day	5% 8% 34% 37% 16% 2	768
	21st to end	Day	5% 8% 28% 39% 18% 2	849
August	1st to 10th	Day	5% 7% 34% 38% 15% 2	776
	11th to 20th	Day	7% 6% 32% 39% 13% 4	621
	21st to end	Day	4 11% 32% 35% 15% 3	583
September	1st to 10th	Day	7% 9% 34% 33% 13% 5%	530
	11th to 20th	Day	9% 9% 29% 35% 14% 3	440
	21st to end	Day	12% 16% 32% 23% 13% 4	330
October	1st to 10th	Day	17% 12% 32% 23% 13% 3	330
	11th to 20th	Day	21% 21% 28% 17% 9% 3	330
	21st to end	Day	12% 24% 35% 19% 9% 2	363
November	1st to 10th	Day	12% 23% 33% 23% 7% 3	330
	11th to 20th	Day	12% 18% 40% 19% 9% 1	330
	21st to end	Day	10% 20% 34% 24% 11% 2	330
December	1st to 10th	Day	10% 25% 39% 19% 6% 1	330
	11th to 20th	Day	11% 21% 42% 21% 5%	330
	21st to end	Day	10% 21% 38% 25% 5% 1	363
January	1st to 10th	Night	8% 20% 37% 26% 10% 1	1050
	11th to 20th	Night	9% 26% 37% 21% 6% 1	1050
	21st to end	Night	6% 25% 38% 25% 5%	1155
February	1st to 10th	Night	8% 22% 41% 22% 6%	879
	11th to 20th	Night	7% 19% 38% 27% 8% 1	824
	21st to end	Night	7% 18% 41% 27% 7% 1	692
March	1st to 10th	Night	8% 18% 38% 30% 5% 1	840
	11th to 20th	Night	9% 13% 39% 32% 6% 1	822
	21st to end	Night	15% 15% 31% 31% 7% 1	693
April	1st to 10th	Night	15% 13% 37% 30% 6%	630
	11th to 20th	Night	17% 16% 38% 24% 5%	623
	21st to end	Night	14% 16% 29% 32% 8% 1	630
May	1st to 10th	Night	12% 11% 31% 32% 13% 1	630
	11th to 20th	Night	17% 11% 29% 32% 11% 1	621
	21st to end	Night	19% 10% 37% 25% 8%	683
June	1st to 10th	Night	9% 12% 33% 33% 12% 1	630
	11th to 20th	Night	11% 10% 39% 33% 6%	630
	21st to end	Night	8% 8% 34% 40% 10%	630
July	1st to 10th	Night	6% 5% 28% 45% 16% 1	615
	11th to 20th	Night	4 4 29% 51% 11% 1	612
	21st to end	Night	3 3 28% 50% 15%	690
August	1st to 10th	Night	5% 4 28% 49% 13% 1	627
	11th to 20th	Night	4 4 28% 47% 17% 1	606
	21st to end	Night	4 7% 25% 46% 18% 1	660
September	1st to 10th	Night	6% 6% 25% 44% 16% 2	600
	11th to 20th	Night	8% 9% 28% 41% 14% 1	600
	21st to end	Night	14% 9% 29% 35% 12% 1	600
October	1st to 10th	Night	13% 12% 27% 36% 11% 1	694
	11th to 20th	Night	11% 16% 34% 29% 9%	823
	21st to end	Night	12% 19% 33% 27% 9% 1	1100
November	1st to 10th	Night	8% 18% 38% 29% 5% 2	1025
	11th to 20th	Night	10% 19% 35% 28% 7% 1	1050
	21st to end	Night	8% 22% 37% 29% 5%	1050
December	1st to 10th	Night	8% 24% 37% 25% 5%	1050
	11th to 20th	Night	8% 20% 39% 26% 7%	1050
	21st to end	Night	11% 22% 36% 25% 6%	1155

Page generated at 2026-08-12 01:37:20. Version ac07

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