Thompson
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Thompson clear sky chart from 2005-11-06 to 2026-05-02.

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	11% 10% 4 9% 6% 5% 5% 4 6% 7% 31%	134966
	Night	10% 12% 5% 12% 7% 6% 6% 5% 6% 7% 25%	136243
Transparency	Day	51% 6% 13% 22% 8%	134649
	Night	56% 6% 12% 19% 7%	136140
Seeing	Day	19% 12% 32% 25% 10% 2	17858
	Night	18% 9% 31% 29% 11% 1	27099

By Month

Percentage Cloud Cover NW (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	5% 10% 6% 17% 10% 9% 9% 6% 10% 6% 13%	3527
	11th to 20th	Day	4 11% 6% 18% 8% 7% 8% 8% 12% 7% 10%	4055
	21st to end	Day	5% 14% 7% 15% 7% 7% 6% 5% 8% 9% 16%	4532
February	1st to 10th	Day	6% 9% 5% 11% 8% 6% 9% 7% 7% 7% 25%	1675
	11th to 20th	Day	6% 12% 3 8% 7% 7% 10% 8% 8% 7% 23%	1750
	21st to end	Day	4 8% 4 11% 10% 8% 8% 6% 9% 8% 26%	1567
March	1st to 10th	Day	5% 11% 5% 12% 6% 7% 7% 5% 6% 8% 28%	4600
	11th to 20th	Day	9% 12% 4 10% 7% 7% 8% 5% 4 5% 28%	5060
	21st to end	Day	3 10% 4 10% 6% 7% 8% 5% 6% 8% 33%	5649
April	1st to 10th	Day	8% 10% 4 11% 7% 5% 6% 5% 5% 7% 33%	2671
	11th to 20th	Day	11% 13% 5% 11% 6% 5% 4 4 5% 5% 30%	2702
	21st to end	Day	7% 10% 4 9% 5% 5% 5% 4 6% 6% 37%	2814
May	1st to 10th	Day	14% 14% 4 9% 5% 3 4 3 4 6% 33%	6816
	11th to 20th	Day	21% 12% 5% 7% 4 4 4 3 5% 6% 30%	6755
	21st to end	Day	17% 10% 2 5% 3 3 4 3 5% 7% 41%	7428
June	1st to 10th	Day	18% 11% 3 7% 6% 4 4 3 5% 6% 33%	3327
	11th to 20th	Day	17% 7% 3 8% 6% 5% 6% 4 7% 9% 29%	3400
	21st to end	Day	13% 8% 3 7% 6% 6% 5% 5% 6% 7% 34%	3400
July	1st to 10th	Day	8% 5% 2 5% 4 4 4 5% 8% 11% 45%	7379
	11th to 20th	Day	8% 5% 2 6% 5% 4 5% 4 7% 9% 44%	7030
	21st to end	Day	11% 5% 2 6% 4 4 4 4 7% 9% 45%	7400
August	1st to 10th	Day	10% 6% 2 6% 5% 4 5% 4 7% 9% 41%	4881
	11th to 20th	Day	11% 7% 2 6% 4 5% 5% 4 7% 8% 41%	4367
	21st to end	Day	21% 8% 3 7% 5% 4 5% 4 5% 9% 30%	4675
September	1st to 10th	Day	15% 10% 3 8% 5% 6% 5% 4 5% 8% 31%	2590
	11th to 20th	Day	21% 11% 4 8% 4 5% 5% 4 5% 6% 28%	2210
	21st to end	Day	16% 13% 3 8% 5% 4 4 3 6% 9% 29%	2200
October	1st to 10th	Day	17% 14% 5% 11% 6% 5% 5% 4 5% 6% 22%	3299
	11th to 20th	Day	15% 16% 6% 9% 6% 5% 5% 3 5% 6% 25%	2955
	21st to end	Day	19% 19% 7% 13% 6% 4 4 3 4 5% 16%	3006
November	1st to 10th	Day	16% 18% 7% 13% 8% 6% 5% 3 3 4 15%	1618
	11th to 20th	Day	12% 20% 6% 13% 8% 6% 4 5% 6% 5% 16%	1470
	21st to end	Day	9% 16% 8% 18% 9% 6% 5% 4 4 5% 15%	1438
December	1st to 10th	Day	5% 16% 7% 15% 11% 10% 6% 6% 8% 7% 10%	2020
	11th to 20th	Day	12% 13% 5% 11% 9% 7% 7% 8% 6% 5% 16%	2038
	21st to end	Day	9% 17% 7% 19% 6% 6% 5% 4 6% 4 16%	2662
January	1st to 10th	Night	4 12% 6% 16% 10% 7% 8% 7% 7% 5% 16%	9366
	11th to 20th	Night	5% 11% 6% 18% 10% 9% 9% 8% 8% 5% 10%	8867
	21st to end	Night	8% 16% 6% 15% 8% 6% 7% 5% 7% 7% 16%	9681
February	1st to 10th	Night	6% 12% 4 9% 8% 8% 8% 7% 8% 8% 23%	2942
	11th to 20th	Night	5% 10% 4 10% 8% 7% 9% 7% 8% 8% 24%	2756
	21st to end	Night	3 12% 6% 11% 8% 7% 5% 5% 7% 9% 27%	2249
March	1st to 10th	Night	7% 10% 4 10% 6% 7% 6% 4 5% 6% 34%	5503
	11th to 20th	Night	9% 11% 4 13% 7% 7% 6% 6% 5% 5% 28%	5119
	21st to end	Night	4 7% 4 9% 7% 7% 10% 8% 7% 6% 32%	5513
April	1st to 10th	Night	8% 12% 5% 9% 6% 5% 5% 4 6% 6% 36%	1949
	11th to 20th	Night	10% 12% 4 10% 7% 5% 5% 3 4 6% 34%	1869
	21st to end	Night	7% 10% 4 7% 5% 4 5% 5% 6% 7% 40%	1804
May	1st to 10th	Night	13% 11% 3 7% 4 5% 4 3 5% 7% 37%	3187
	11th to 20th	Night	20% 11% 5% 8% 5% 4 4 3 5% 7% 27%	3161
	21st to end	Night	17% 10% 3 5% 3 3 3 2 3 5% 44%	3470
June	1st to 10th	Night	17% 7% 3 8% 4 4 4 3 6% 7% 36%	1068
	11th to 20th	Night	17% 7% 3 7% 6% 5% 5% 5% 6% 8% 31%	1000
	21st to end	Night	13% 6% 3 8% 5% 4 5% 5% 6% 6% 39%	1000
July	1st to 10th	Night	8% 4 2 7% 4 6% 6% 6% 8% 10% 38%	2577
	11th to 20th	Night	7% 6% 2 6% 4 5% 5% 5% 7% 11% 42%	2937
	21st to end	Night	6% 5% 2 5% 4 5% 5% 5% 8% 9% 47%	3466
August	1st to 10th	Night	10% 5% 2 5% 4 5% 5% 4 6% 8% 47%	2279
	11th to 20th	Night	11% 5% 2 6% 5% 4 5% 4 6% 8% 45%	2689
	21st to end	Night	17% 8% 3 7% 5% 5% 5% 4 6% 8% 32%	3234
September	1st to 10th	Night	17% 9% 2 7% 4 4 5% 3 5% 5% 40%	1810
	11th to 20th	Night	19% 12% 3 7% 5% 4 4 4 5% 6% 31%	2189
	21st to end	Night	15% 10% 3 7% 4 5% 4 4 5% 8% 35%	2200
October	1st to 10th	Night	15% 12% 4 10% 6% 6% 5% 4 6% 7% 24%	3848
	11th to 20th	Night	17% 12% 4 7% 5% 5% 5% 4 6% 7% 27%	4189
	21st to end	Night	17% 19% 7% 12% 6% 4 5% 3 5% 5% 19%	4794
November	1st to 10th	Night	15% 16% 6% 15% 8% 6% 5% 4 4 4 17%	2897
	11th to 20th	Night	14% 17% 7% 14% 7% 6% 5% 3 5% 6% 15%	3150
	21st to end	Night	9% 18% 7% 16% 9% 7% 5% 3 5% 5% 15%	3181
December	1st to 10th	Night	6% 16% 7% 17% 9% 8% 8% 6% 7% 7% 9%	5390
	11th to 20th	Night	9% 14% 5% 12% 9% 7% 8% 8% 6% 6% 17%	6375
	21st to end	Night	7% 15% 7% 19% 7% 5% 5% 4 5% 5% 21%	8534

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	65% 5% 9% 15% 5% 1	3527
	11th to 20th	Day	63% 5% 9% 18% 5%	4076
	21st to end	Day	62% 4 9% 16% 8%	4532
February	1st to 10th	Day	55% 5% 12% 20% 8%	1675
	11th to 20th	Day	54% 7% 13% 17% 8% 1	1758
	21st to end	Day	51% 5% 11% 21% 12%	1567
March	1st to 10th	Day	53% 5% 10% 19% 14%	4600
	11th to 20th	Day	58% 3 7% 15% 16%	5062
	21st to end	Day	46% 5% 8% 24% 17% 1	5649
April	1st to 10th	Day	49% 5% 12% 22% 12%	2671
	11th to 20th	Day	55% 4 10% 19% 11%	2702
	21st to end	Day	45% 6% 10% 26% 13%	2814
May	1st to 10th	Day	52% 5% 14% 21% 7%	6813
	11th to 20th	Day	55% 5% 10% 22% 8% 1	6759
	21st to end	Day	44% 4 9% 23% 18% 2	7428
June	1st to 10th	Day	52% 5% 11% 21% 10%	3327
	11th to 20th	Day	50% 7% 11% 24% 8% 1	3400
	21st to end	Day	46% 8% 12% 24% 9%	3400
July	1st to 10th	Day	30% 8% 24% 31% 6%	7300
	11th to 20th	Day	33% 8% 19% 35% 4	6943
	21st to end	Day	34% 9% 21% 31% 5%	7193
August	1st to 10th	Day	37% 9% 22% 29% 4	4864
	11th to 20th	Day	38% 9% 23% 25% 5%	4409
	21st to end	Day	51% 7% 17% 22% 2	4674
September	1st to 10th	Day	50% 9% 18% 20% 3	2590
	11th to 20th	Day	56% 7% 17% 19% 1	2210
	21st to end	Day	53% 6% 19% 17% 5%	2200
October	1st to 10th	Day	62% 7% 11% 16% 4	3299
	11th to 20th	Day	59% 6% 11% 18% 6%	2955
	21st to end	Day	69% 7% 11% 11% 2	3006
November	1st to 10th	Day	74% 5% 7% 12% 2	1618
	11th to 20th	Day	69% 6% 13% 11% 1	1470
	21st to end	Day	71% 5% 11% 11% 3	1438
December	1st to 10th	Day	71% 6% 10% 10% 3	2020
	11th to 20th	Day	67% 4 8% 17% 3	2038
	21st to end	Day	68% 5% 9% 14% 4	2662
January	1st to 10th	Night	65% 5% 11% 12% 7% 1	9367
	11th to 20th	Night	68% 5% 8% 14% 5%	8908
	21st to end	Night	66% 4 8% 15% 7%	9682
February	1st to 10th	Night	55% 7% 13% 20% 6%	2942
	11th to 20th	Night	54% 5% 12% 21% 8%	2758
	21st to end	Night	53% 4 9% 21% 12%	2249
March	1st to 10th	Night	50% 5% 10% 21% 10% 3	5503
	11th to 20th	Night	58% 3 8% 17% 14% 1	5119
	21st to end	Night	46% 6% 10% 22% 15% 1	5513
April	1st to 10th	Night	48% 5% 10% 23% 14% 1	1949
	11th to 20th	Night	51% 5% 8% 22% 13% 1	1869
	21st to end	Night	41% 6% 10% 30% 12% 2	1804
May	1st to 10th	Night	46% 6% 16% 24% 7%	3187
	11th to 20th	Night	56% 6% 9% 20% 8% 1	3161
	21st to end	Night	44% 4 8% 22% 21% 2	3470
June	1st to 10th	Night	46% 7% 13% 19% 15% 1	1068
	11th to 20th	Night	47% 9% 8% 26% 10% 1	1000
	21st to end	Night	43% 7% 11% 27% 12%	1000
July	1st to 10th	Night	38% 9% 22% 25% 6%	2547
	11th to 20th	Night	34% 9% 21% 32% 4	2901
	21st to end	Night	29% 10% 19% 35% 7%	3368
August	1st to 10th	Night	33% 8% 19% 36% 4	2272
	11th to 20th	Night	36% 8% 23% 29% 5%	2713
	21st to end	Night	48% 9% 19% 20% 4	3234
September	1st to 10th	Night	45% 8% 18% 23% 5% 1	1810
	11th to 20th	Night	53% 7% 14% 22% 3	2189
	21st to end	Night	48% 7% 18% 22% 5%	2200
October	1st to 10th	Night	58% 7% 14% 17% 4	3848
	11th to 20th	Night	54% 7% 14% 22% 3	4189
	21st to end	Night	66% 7% 11% 13% 2	4794
November	1st to 10th	Night	71% 5% 9% 12% 3	2897
	11th to 20th	Night	71% 5% 12% 10% 2	3150
	21st to end	Night	72% 5% 9% 13% 2	3181
December	1st to 10th	Night	71% 7% 12% 8% 2	5390
	11th to 20th	Night	65% 5% 9% 18% 4	6374
	21st to end	Night	65% 4 9% 17% 4	8534

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	10% 10% 37% 31% 11% 1	220
	11th to 20th	Day	12% 10% 41% 28% 7% 1	317
	21st to end	Day	18% 11% 37% 26% 7% 1	363
February	1st to 10th	Day	10% 15% 38% 29% 6% 2	330
	11th to 20th	Day	11% 11% 41% 25% 9% 2	330
	21st to end	Day	7% 12% 37% 30% 10% 3	273
March	1st to 10th	Day	11% 14% 32% 25% 12% 6%	330
	11th to 20th	Day	12% 16% 39% 22% 9% 2	330
	21st to end	Day	11% 14% 41% 23% 8% 3	389
April	1st to 10th	Day	11% 12% 37% 28% 10% 2	691
	11th to 20th	Day	16% 16% 35% 21% 10% 1	740
	21st to end	Day	14% 13% 34% 26% 11% 1	750
May	1st to 10th	Day	22% 13% 35% 22% 7% 1	709
	11th to 20th	Day	23% 16% 30% 23% 7% 1	725
	21st to end	Day	24% 11% 32% 23% 7% 2	789
June	1st to 10th	Day	28% 12% 27% 22% 8% 3	730
	11th to 20th	Day	21% 12% 32% 22% 10% 3	730
	21st to end	Day	21% 10% 29% 26% 12% 3	730
July	1st to 10th	Day	15% 12% 32% 27% 11% 3	721
	11th to 20th	Day	16% 9% 26% 29% 14% 6%	718
	21st to end	Day	19% 10% 27% 26% 15% 4	794
August	1st to 10th	Day	15% 12% 28% 31% 12% 3	726
	11th to 20th	Day	20% 11% 28% 25% 13% 4	715
	21st to end	Day	24% 9% 25% 26% 13% 3	803
September	1st to 10th	Day	21% 14% 33% 23% 8% 1	720
	11th to 20th	Day	23% 11% 32% 21% 11% 1	340
	21st to end	Day	23% 14% 31% 23% 8%	330
October	1st to 10th	Day	28% 18% 29% 16% 7% 2	330
	11th to 20th	Day	38% 13% 28% 12% 6% 2	330
	21st to end	Day	26% 13% 33% 19% 7% 1	363
November	1st to 10th	Day	22% 14% 26% 25% 9% 2	330
	11th to 20th	Day	21% 15% 40% 16% 6% 2	330
	21st to end	Day	20% 12% 37% 23% 7% 1	313
December	1st to 10th	Day	10% 10% 35% 35% 8% 2	220
	11th to 20th	Day	18% 13% 31% 24% 14% 1	148
	21st to end	Day	11% 9% 35% 30% 13% 1	151
January	1st to 10th	Night	14% 11% 34% 32% 9% 1	1050
	11th to 20th	Night	13% 11% 37% 26% 10% 2	1050
	21st to end	Night	16% 10% 35% 30% 8% 1	1155
February	1st to 10th	Night	11% 9% 40% 30% 9% 2	1050
	11th to 20th	Night	10% 8% 34% 31% 14% 3	1029
	21st to end	Night	8% 11% 35% 32% 12% 1	865
March	1st to 10th	Night	10% 8% 30% 36% 14% 2	849
	11th to 20th	Night	18% 12% 33% 26% 10% 1	646
	21st to end	Night	10% 8% 37% 28% 13% 2	693
April	1st to 10th	Night	13% 9% 33% 31% 12% 2	630
	11th to 20th	Night	14% 10% 32% 31% 12% 2	623
	21st to end	Night	12% 12% 36% 29% 10% 1	630
May	1st to 10th	Night	22% 10% 34% 25% 8%	603
	11th to 20th	Night	22% 11% 33% 24% 8% 1	591
	21st to end	Night	24% 9% 33% 26% 9%	650
June	1st to 10th	Night	22% 9% 22% 32% 12% 1	268
	11th to 20th	Night	18% 9% 26% 33% 14% 2	200
	21st to end	Night	15% 8% 29% 35% 14%	200
July	1st to 10th	Night	13% 5% 30% 35% 16% 1	325
	11th to 20th	Night	15% 6% 25% 30% 22% 3	475
	21st to end	Night	14% 4 20% 35% 24% 3	657
August	1st to 10th	Night	16% 5% 23% 34% 20% 1	597
	11th to 20th	Night	16% 7% 24% 34% 18% 1	594
	21st to end	Night	22% 6% 25% 33% 12% 1	660
September	1st to 10th	Night	23% 10% 28% 27% 11% 1	600
	11th to 20th	Night	27% 10% 26% 26% 12% 1	600
	21st to end	Night	22% 9% 33% 25% 11% 2	600
October	1st to 10th	Night	26% 9% 31% 25% 8% 1	754
	11th to 20th	Night	34% 8% 24% 23% 10% 2	975
	21st to end	Night	33% 11% 29% 18% 8% 1	1100
November	1st to 10th	Night	24% 12% 30% 22% 11% 1	1025
	11th to 20th	Night	24% 10% 34% 23% 9% 1	1050
	21st to end	Night	18% 13% 32% 26% 8% 2	1050
December	1st to 10th	Night	13% 8% 37% 32% 9% 1	1050
	11th to 20th	Night	17% 10% 28% 32% 11% 1	1050
	21st to end	Night	15% 7% 29% 33% 12% 3	1155

Page generated at 2026-04-29 01:21:46. Version ac07

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