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 2025-04-11.

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 5% 4 4 4 4 6% 8% 53%	117255
	Night	5% 5% 2 5% 4 4 4 3 6% 8% 53%	119874
Transparency	Day	28% 8% 15% 27% 18% 4	116999
	Night	29% 7% 14% 27% 19% 4	119720
Seeing	Day	11% 16% 34% 26% 10% 2	16424
	Night	9% 16% 34% 31% 9% 1	26577

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 6% 4 10% 5% 5% 4 4 6% 10% 46%	3638
	11th to 20th	Day	1 5% 3 8% 6% 5% 5% 4 5% 6% 51%	3640
	21st to end	Day	1 3 2 6% 4 4 3 4 5% 6% 61%	4004
February	1st to 10th	Day	3 9% 4 8% 5% 5% 5% 5% 6% 7% 43%	1780
	11th to 20th	Day	4 7% 4 8% 4 5% 5% 5% 5% 7% 46%	1911
	21st to end	Day	3 7% 3 6% 6% 5% 5% 3 6% 7% 49%	1650
March	1st to 10th	Day	4 8% 2 5% 4 4 4 4 5% 6% 55%	4500
	11th to 20th	Day	6% 6% 2 4 3 2 3 3 5% 8% 57%	4662
	21st to end	Day	12% 11% 3 7% 4 4 4 3 6% 8% 39%	5969
April	1st to 10th	Day	11% 8% 2 4 4 3 4 4 6% 8% 45%	2400
	11th to 20th	Day	10% 8% 3 6% 5% 5% 6% 4 6% 7% 40%	2252
	21st to end	Day	12% 7% 2 6% 4 4 5% 3 6% 8% 42%	2426
May	1st to 10th	Day	14% 8% 3 8% 4 4 5% 4 6% 7% 37%	4213
	11th to 20th	Day	9% 5% 2 5% 3 4 5% 4 7% 9% 47%	4388
	21st to end	Day	14% 6% 2 5% 4 4 4 4 7% 9% 41%	4825
June	1st to 10th	Day	9% 4 1 4 4 4 5% 5% 7% 10% 47%	2660
	11th to 20th	Day	9% 3 2 4 4 4 5% 5% 7% 10% 48%	2660
	21st to end	Day	5% 3 2 4 3 3 4 4 6% 7% 61%	2660
July	1st to 10th	Day	4 3 1 4 4 4 6% 4 7% 10% 53%	4430
	11th to 20th	Day	3 2 1 4 3 4 4 4 7% 9% 60%	4430
	21st to end	Day	3 2 1 3 2 4 4 4 7% 9% 62%	4802
August	1st to 10th	Day	6% 3 1 5% 4 4 5% 5% 8% 9% 51%	4411
	11th to 20th	Day	6% 3 1 4 3 5% 5% 4 8% 10% 52%	3895
	21st to end	Day	5% 2 1 3 2 3 3 3 5% 7% 66%	4180
September	1st to 10th	Day	6% 3 1 4 2 3 3 2 5% 6% 63%	2280
	11th to 20th	Day	7% 3 1 3 3 3 3 3 5% 6% 63%	2195
	21st to end	Day	8% 4 1 3 3 3 2 3 4 6% 62%	2090
October	1st to 10th	Day	6% 3 1 2 2 2 3 2 3 5% 71%	3329
	11th to 20th	Day	9% 4 1 2 2 3 3 3 4 7% 63%	3070
	21st to end	Day	7% 6% 1 4 2 4 4 3 6% 8% 55%	3091
November	1st to 10th	Day	4 5% 2 4 3 3 4 3 7% 9% 57%	1763
	11th to 20th	Day	3 5% 2 5% 4 3 3 5% 5% 7% 57%	1800
	21st to end	Day	3 6% 2 6% 5% 3 4 4 6% 9% 54%	1601
December	1st to 10th	Day	1 4 2 6% 4 4 5% 4 6% 9% 54%	2620
	11th to 20th	Day	1 3 1 6% 4 5% 6% 5% 6% 8% 54%	3026
	21st to end	Day	5% 8% 2 6% 4 4 4 3 5% 7% 51%	4004
January	1st to 10th	Night	4 9% 4 8% 4 4 5% 4 5% 6% 46%	6327
	11th to 20th	Night	2 4 3 9% 6% 7% 6% 4 5% 7% 49%	6340
	21st to end	Night	1 3 2 6% 4 4 4 3 5% 7% 62%	6974
February	1st to 10th	Night	4 9% 4 8% 4 5% 4 4 7% 7% 44%	2637
	11th to 20th	Night	2 7% 3 7% 6% 5% 5% 4 7% 8% 47%	2374
	21st to end	Night	3 7% 3 6% 4 3 4 4 7% 8% 49%	1980
March	1st to 10th	Night	5% 7% 2 6% 4 3 3 3 5% 7% 54%	5374
	11th to 20th	Night	4 6% 3 5% 4 3 3 4 6% 9% 52%	5318
	21st to end	Night	11% 11% 4 7% 4 4 4 3 7% 8% 37%	4984
April	1st to 10th	Night	11% 7% 2 5% 4 4 5% 4 5% 8% 45%	2000
	11th to 20th	Night	11% 7% 3 5% 4 4 4 3 5% 7% 48%	1882
	21st to end	Night	11% 6% 2 5% 3 4 5% 3 6% 8% 47%	1753
May	1st to 10th	Night	12% 7% 3 7% 4 4 4 4 5% 6% 45%	2742
	11th to 20th	Night	8% 4 1 4 3 3 4 4 6% 9% 53%	2517
	21st to end	Night	14% 6% 1 5% 3 4 5% 3 6% 8% 43%	2762
June	1st to 10th	Night	9% 4 1 4 3 4 4 3 6% 9% 52%	1520
	11th to 20th	Night	9% 4 1 4 4 3 5% 4 8% 9% 49%	1520
	21st to end	Night	6% 3 1 3 3 3 4 4 7% 8% 57%	1520
July	1st to 10th	Night	5% 3 1 3 4 3 3 4 6% 10% 59%	2540
	11th to 20th	Night	3 2 1 3 3 3 4 4 7% 11% 60%	2540
	21st to end	Night	3 2 1 3 3 4 4 4 6% 8% 61%	2761
August	1st to 10th	Night	7% 4 1 5% 4 4 6% 4 7% 8% 49%	2528
	11th to 20th	Night	5% 4 2 4 3 3 4 4 6% 10% 55%	2939
	21st to end	Night	3 2 1 5% 3 3 4 3 5% 6% 64%	3487
September	1st to 10th	Night	6% 3 1 3 3 2 4 2 4 7% 65%	1900
	11th to 20th	Night	6% 4 1 3 2 2 3 3 6% 7% 62%	1984
	21st to end	Night	11% 4 1 2 2 2 3 2 5% 5% 63%	2090
October	1st to 10th	Night	7% 3 1 3 2 1 2 2 3 5% 72%	3634
	11th to 20th	Night	7% 4 1 4 2 3 3 3 6% 8% 59%	3825
	21st to end	Night	8% 6% 2 5% 3 3 3 3 5% 8% 55%	4467
November	1st to 10th	Night	6% 5% 2 6% 3 4 5% 4 7% 8% 52%	2535
	11th to 20th	Night	4 6% 1 4 4 4 4 3 5% 6% 57%	2600
	21st to end	Night	3 5% 2 5% 3 4 4 3 6% 8% 56%	2795
December	1st to 10th	Night	2 4 2 5% 4 4 5% 3 6% 9% 56%	4570
	11th to 20th	Night	2 4 2 6% 5% 6% 5% 4 6% 8% 51%	5181
	21st to end	Night	5% 8% 3 6% 4 4 5% 4 6% 7% 48%	6974

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	32% 10% 19% 27% 12% 1	3639
	11th to 20th	Day	31% 8% 16% 24% 16% 3	3640
	21st to end	Day	22% 9% 15% 33% 17% 4	4004
February	1st to 10th	Day	40% 10% 14% 21% 14% 2	1780
	11th to 20th	Day	36% 8% 15% 21% 15% 4	1921
	21st to end	Day	33% 6% 17% 25% 15% 4	1650
March	1st to 10th	Day	28% 5% 11% 24% 28% 4	4500
	11th to 20th	Day	24% 6% 16% 31% 21% 1	4662
	21st to end	Day	43% 9% 15% 22% 10%	5969
April	1st to 10th	Day	36% 7% 18% 24% 12% 3	2400
	11th to 20th	Day	40% 8% 15% 22% 12% 2	2255
	21st to end	Day	39% 9% 17% 26% 9% 1	2426
May	1st to 10th	Day	42% 8% 16% 23% 9% 2	4213
	11th to 20th	Day	31% 8% 17% 29% 13% 3	4392
	21st to end	Day	36% 10% 16% 26% 11% 1	4825
June	1st to 10th	Day	29% 11% 20% 25% 12% 2	2660
	11th to 20th	Day	28% 9% 19% 26% 15% 3	2660
	21st to end	Day	20% 8% 15% 30% 22% 5%	2660
July	1st to 10th	Day	23% 8% 13% 31% 22% 3	4361
	11th to 20th	Day	18% 7% 12% 30% 25% 8%	4355
	21st to end	Day	16% 8% 12% 27% 28% 10%	4651
August	1st to 10th	Day	25% 10% 17% 20% 18% 9%	4396
	11th to 20th	Day	25% 9% 16% 28% 18% 4	3934
	21st to end	Day	19% 5% 11% 25% 28% 12%	4179
September	1st to 10th	Day	22% 5% 10% 26% 27% 10%	2280
	11th to 20th	Day	22% 6% 11% 29% 25% 8%	2195
	21st to end	Day	24% 5% 9% 33% 26% 4	2090
October	1st to 10th	Day	18% 4 10% 19% 32% 17%	3329
	11th to 20th	Day	23% 4 11% 34% 24% 4	3070
	21st to end	Day	27% 7% 15% 31% 16% 4	3091
November	1st to 10th	Day	24% 7% 16% 25% 22% 6%	1763
	11th to 20th	Day	26% 6% 13% 27% 23% 5%	1800
	21st to end	Day	27% 6% 15% 31% 17% 4	1601
December	1st to 10th	Day	27% 7% 20% 27% 18% 1	2620
	11th to 20th	Day	26% 8% 19% 32% 13% 1	3024
	21st to end	Day	33% 5% 10% 34% 17% 1	4004
January	1st to 10th	Night	37% 9% 12% 27% 13% 2	6327
	11th to 20th	Night	34% 7% 15% 23% 17% 3	6340
	21st to end	Night	21% 9% 13% 30% 23% 4	6974
February	1st to 10th	Night	37% 8% 16% 21% 14% 2	2637
	11th to 20th	Night	34% 8% 15% 24% 16% 3	2374
	21st to end	Night	32% 7% 14% 26% 15% 6%	1980
March	1st to 10th	Night	29% 6% 10% 27% 24% 4	5374
	11th to 20th	Night	27% 7% 17% 26% 21% 1	5318
	21st to end	Night	42% 9% 19% 20% 10% 1	4984
April	1st to 10th	Night	37% 8% 14% 24% 14% 3	2000
	11th to 20th	Night	36% 7% 13% 26% 16% 2	1882
	21st to end	Night	34% 8% 16% 27% 13% 1	1753
May	1st to 10th	Night	38% 8% 13% 23% 16% 2	2742
	11th to 20th	Night	25% 10% 16% 37% 11%	2518
	21st to end	Night	36% 9% 14% 28% 12% 1	2762
June	1st to 10th	Night	28% 10% 19% 29% 13% 2	1520
	11th to 20th	Night	30% 9% 21% 22% 14% 3	1520
	21st to end	Night	23% 9% 16% 26% 21% 5%	1520
July	1st to 10th	Night	20% 7% 16% 30% 24% 3	2500
	11th to 20th	Night	17% 7% 16% 32% 22% 6%	2492
	21st to end	Night	18% 7% 14% 30% 24% 7%	2674
August	1st to 10th	Night	29% 9% 15% 19% 19% 9%	2520
	11th to 20th	Night	24% 8% 19% 25% 19% 4	2966
	21st to end	Night	21% 6% 12% 24% 29% 8%	3487
September	1st to 10th	Night	20% 4 8% 29% 28% 10%	1900
	11th to 20th	Night	22% 6% 11% 27% 26% 8%	1984
	21st to end	Night	25% 4 9% 29% 25% 8%	2090
October	1st to 10th	Night	18% 4 8% 22% 31% 17%	3634
	11th to 20th	Night	25% 5% 11% 30% 26% 5%	3825
	21st to end	Night	28% 5% 13% 28% 18% 8%	4467
November	1st to 10th	Night	30% 6% 15% 23% 20% 6%	2535
	11th to 20th	Night	27% 6% 13% 26% 22% 6%	2600
	21st to end	Night	26% 8% 15% 28% 18% 4	2795
December	1st to 10th	Night	25% 7% 18% 30% 18% 3	4570
	11th to 20th	Night	29% 8% 19% 30% 13% 2	5182
	21st to end	Night	35% 6% 10% 32% 15% 1	6974

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% 35% 27% 14% 3	300
	11th to 20th	Day	11% 20% 36% 22% 9% 2	300
	21st to end	Day	9% 31% 35% 19% 6% 1	330
February	1st to 10th	Day	12% 25% 44% 15% 5%	300
	11th to 20th	Day	10% 20% 47% 17% 5%	300
	21st to end	Day	9% 25% 45% 18% 3 1	249
March	1st to 10th	Day	7% 24% 42% 17% 8% 2	300
	11th to 20th	Day	10% 19% 41% 19% 8% 1	330
	21st to end	Day	22% 21% 33% 16% 7% 1	550
April	1st to 10th	Day	18% 23% 37% 18% 4	500
	11th to 20th	Day	21% 30% 35% 12% 2	453
	21st to end	Day	17% 19% 32% 24% 8% 1	606
May	1st to 10th	Day	18% 20% 30% 21% 9% 2	653
	11th to 20th	Day	20% 13% 29% 24% 11% 3	675
	21st to end	Day	17% 14% 35% 21% 10% 2	734
June	1st to 10th	Day	9% 11% 32% 32% 14% 3	680
	11th to 20th	Day	13% 15% 37% 26% 8% 1	680
	21st to end	Day	8% 11% 35% 31% 14% 2	680
July	1st to 10th	Day	4 6% 31% 41% 16% 2	671
	11th to 20th	Day	3 8% 35% 37% 16% 1	668
	21st to end	Day	5% 8% 28% 40% 17% 3	739
August	1st to 10th	Day	5% 7% 34% 38% 15% 2	676
	11th to 20th	Day	6% 6% 33% 39% 12% 3	556
	21st to end	Day	4 11% 32% 35% 15% 4	539
September	1st to 10th	Day	7% 9% 34% 33% 12% 5%	490
	11th to 20th	Day	8% 10% 29% 36% 14% 3	405
	21st to end	Day	13% 18% 32% 21% 12% 3	300
October	1st to 10th	Day	17% 12% 32% 22% 13% 4	300
	11th to 20th	Day	19% 21% 28% 18% 10% 4	300
	21st to end	Day	12% 22% 37% 20% 8% 1	330
November	1st to 10th	Day	11% 23% 33% 23% 6% 3	300
	11th to 20th	Day	13% 19% 40% 17% 10% 1	300
	21st to end	Day	10% 21% 33% 25% 10% 1	300
December	1st to 10th	Day	10% 24% 38% 20% 7% 1	300
	11th to 20th	Day	10% 20% 42% 23% 5%	300
	21st to end	Day	10% 22% 36% 26% 5% 1	330
January	1st to 10th	Night	8% 20% 37% 25% 10% 1	1000
	11th to 20th	Night	7% 27% 37% 21% 7% 1	1000
	21st to end	Night	6% 25% 38% 25% 5%	1100
February	1st to 10th	Night	8% 22% 41% 22% 6%	837
	11th to 20th	Night	7% 20% 38% 27% 7% 1	784
	21st to end	Night	7% 18% 40% 27% 7% 1	660
March	1st to 10th	Night	8% 18% 38% 30% 5% 1	800
	11th to 20th	Night	9% 13% 38% 32% 6% 1	783
	21st to end	Night	15% 15% 30% 32% 7% 1	660
April	1st to 10th	Night	14% 14% 36% 30% 6%	600
	11th to 20th	Night	18% 17% 37% 24% 5%	565
	21st to end	Night	14% 16% 29% 31% 9% 1	570
May	1st to 10th	Night	13% 12% 32% 30% 13% 1	570
	11th to 20th	Night	16% 10% 29% 32% 11% 1	561
	21st to end	Night	18% 10% 37% 25% 9%	617
June	1st to 10th	Night	10% 11% 33% 33% 13% 1	570
	11th to 20th	Night	11% 10% 38% 34% 6%	570
	21st to end	Night	7% 8% 33% 41% 11%	570
July	1st to 10th	Night	6% 5% 29% 43% 17% 1	555
	11th to 20th	Night	4 4 29% 51% 12% 1	552
	21st to end	Night	3 3 28% 51% 16%	624
August	1st to 10th	Night	6% 3 26% 49% 14% 1	567
	11th to 20th	Night	4 4 29% 46% 17% 1	564
	21st to end	Night	4 6% 25% 46% 18% 1	627
September	1st to 10th	Night	7% 6% 26% 43% 16% 2	570
	11th to 20th	Night	7% 9% 28% 41% 14% 1	570
	21st to end	Night	14% 10% 30% 33% 11% 1	570
October	1st to 10th	Night	14% 12% 28% 34% 11% 1	659
	11th to 20th	Night	11% 15% 33% 30% 10% 1	782
	21st to end	Night	12% 19% 32% 28% 8% 1	1045
November	1st to 10th	Night	8% 18% 38% 29% 6% 2	975
	11th to 20th	Night	10% 20% 35% 27% 6% 1	1000
	21st to end	Night	7% 23% 37% 29% 5%	1000
December	1st to 10th	Night	8% 25% 37% 25% 5%	1000
	11th to 20th	Night	7% 20% 38% 27% 7% 1	1000
	21st to end	Night	11% 22% 36% 24% 6%	1100

Page generated at 2025-04-08 07:17:29. Version ac07

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