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 2024-07-31.

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	14% 12% 4 10% 6% 5% 5% 4 6% 7% 27%	89712
	Night	11% 12% 5% 11% 7% 6% 6% 5% 6% 6% 25%	90148
Transparency	Day	55% 6% 13% 19% 7%	89401
	Night	58% 6% 12% 18% 6%	90040
Seeing	Day	19% 12% 32% 24% 10% 3	15591
	Night	18% 9% 31% 29% 12% 2	24657

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	7% 11% 5% 19% 12% 8% 7% 6% 8% 5% 12%	1899
	11th to 20th	Day	5% 11% 6% 22% 11% 7% 8% 4 8% 7% 11%	2163
	21st to end	Day	8% 20% 9% 16% 8% 5% 5% 5% 6% 6% 13%	2442
February	1st to 10th	Day	5% 9% 4 11% 8% 6% 9% 7% 7% 8% 26%	1515
	11th to 20th	Day	6% 12% 3 8% 7% 7% 9% 7% 8% 7% 24%	1578
	21st to end	Day	4 8% 4 10% 10% 8% 8% 6% 10% 8% 25%	1422
March	1st to 10th	Day	6% 11% 4 11% 6% 7% 6% 4 6% 7% 32%	3130
	11th to 20th	Day	10% 14% 5% 11% 7% 6% 8% 6% 5% 5% 23%	3464
	21st to end	Day	5% 11% 4 10% 5% 6% 9% 4 6% 7% 33%	3863
April	1st to 10th	Day	8% 10% 4 10% 7% 5% 6% 5% 5% 7% 34%	2417
	11th to 20th	Day	10% 12% 5% 11% 7% 5% 4 4 5% 5% 32%	2442
	21st to end	Day	6% 10% 4 9% 5% 5% 5% 5% 6% 6% 39%	2546
May	1st to 10th	Day	16% 15% 5% 9% 5% 4 4 4 5% 7% 27%	4750
	11th to 20th	Day	22% 9% 3 7% 5% 4 4 3 5% 6% 32%	4705
	21st to end	Day	24% 15% 3 7% 3 3 4 3 4 6% 29%	5173
June	1st to 10th	Day	18% 11% 3 7% 5% 4 4 3 5% 6% 34%	3160
	11th to 20th	Day	18% 7% 3 8% 6% 5% 6% 4 7% 9% 28%	3230
	21st to end	Day	13% 7% 3 7% 6% 6% 5% 5% 6% 7% 34%	3230
July	1st to 10th	Day	15% 7% 2 6% 6% 5% 5% 5% 7% 8% 34%	3098
	11th to 20th	Day	13% 7% 2 7% 5% 4 5% 4 7% 9% 36%	2955
	21st to end	Day	16% 8% 3 7% 5% 5% 5% 4 6% 8% 33%	3093
August	1st to 10th	Day	13% 7% 2 7% 6% 4 6% 5% 6% 8% 36%	2686
	11th to 20th	Day	16% 8% 3 8% 5% 6% 5% 4 7% 8% 31%	2417
	21st to end	Day	24% 8% 3 7% 5% 4 5% 4 5% 7% 29%	2574
September	1st to 10th	Day	16% 10% 3 8% 5% 6% 4 4 5% 8% 30%	2332
	11th to 20th	Day	21% 12% 4 8% 4 4 5% 4 6% 6% 26%	1990
	21st to end	Day	17% 13% 3 8% 5% 4 4 3 6% 8% 28%	1980
October	1st to 10th	Day	22% 16% 4 8% 5% 4 4 3 4 5% 23%	1824
	11th to 20th	Day	23% 21% 6% 9% 5% 4 4 3 4 5% 17%	1639
	21st to end	Day	19% 23% 7% 13% 6% 3 3 3 4 5% 13%	1684
November	1st to 10th	Day	16% 18% 7% 13% 9% 6% 5% 3 3 4 15%	1462
	11th to 20th	Day	11% 20% 5% 12% 8% 6% 5% 5% 6% 5% 16%	1330
	21st to end	Day	9% 16% 7% 18% 9% 6% 5% 4 4 5% 15%	1302
December	1st to 10th	Day	5% 14% 7% 13% 10% 7% 6% 7% 7% 8% 15%	1140
	11th to 20th	Day	13% 16% 6% 11% 10% 5% 5% 5% 6% 7% 15%	1263
	21st to end	Day	7% 11% 5% 17% 6% 7% 7% 6% 8% 6% 21%	1814
January	1st to 10th	Night	5% 11% 5% 16% 11% 8% 8% 6% 8% 7% 16%	5046
	11th to 20th	Night	5% 9% 5% 20% 14% 11% 9% 5% 7% 6% 10%	4741
	21st to end	Night	12% 20% 7% 15% 8% 6% 6% 4 5% 6% 11%	5225
February	1st to 10th	Night	5% 12% 4 8% 8% 8% 8% 7% 8% 8% 23%	2662
	11th to 20th	Night	5% 10% 4 10% 8% 7% 8% 6% 9% 8% 24%	2488
	21st to end	Night	3 12% 6% 11% 8% 7% 4 5% 8% 9% 27%	2041
March	1st to 10th	Night	7% 9% 4 8% 6% 5% 6% 4 5% 6% 39%	3743
	11th to 20th	Night	9% 13% 5% 14% 8% 7% 6% 7% 5% 5% 22%	3494
	21st to end	Night	5% 9% 4 8% 6% 7% 8% 8% 6% 6% 33%	3769
April	1st to 10th	Night	8% 12% 5% 9% 5% 5% 6% 4 6% 6% 36%	1763
	11th to 20th	Night	10% 11% 4 9% 7% 5% 5% 3 4 6% 35%	1689
	21st to end	Night	6% 9% 4 6% 5% 4 5% 5% 6% 8% 42%	1633
May	1st to 10th	Night	16% 14% 4 8% 5% 5% 5% 4 5% 6% 30%	2220
	11th to 20th	Night	21% 10% 3 8% 6% 4 4 3 6% 7% 28%	2201
	21st to end	Night	25% 14% 4 6% 3 2 3 2 3 5% 33%	2414
June	1st to 10th	Night	17% 7% 3 8% 4 4 4 3 6% 7% 37%	1015
	11th to 20th	Night	18% 7% 3 7% 6% 5% 5% 5% 7% 8% 30%	950
	21st to end	Night	13% 5% 3 8% 5% 4 5% 5% 6% 7% 39%	950
July	1st to 10th	Night	12% 5% 3 7% 5% 5% 4 5% 8% 8% 38%	1078
	11th to 20th	Night	12% 7% 2 6% 4 6% 5% 5% 7% 10% 36%	1222
	21st to end	Night	11% 6% 2 5% 4 5% 5% 5% 9% 10% 38%	1456
August	1st to 10th	Night	14% 6% 2 5% 4 4 4 4 6% 6% 45%	1253
	11th to 20th	Night	12% 7% 2 7% 6% 5% 6% 5% 6% 6% 37%	1481
	21st to end	Night	19% 8% 3 7% 5% 5% 5% 4 5% 8% 31%	1782
September	1st to 10th	Night	19% 8% 1 6% 4 3 5% 3 5% 5% 40%	1628
	11th to 20th	Night	20% 13% 3 8% 5% 4 3 4 5% 6% 30%	1969
	21st to end	Night	16% 11% 3 7% 4 5% 4 4 5% 7% 34%	1980
October	1st to 10th	Night	21% 13% 4 7% 4 5% 4 4 5% 7% 27%	2118
	11th to 20th	Night	23% 17% 5% 8% 4 4 4 4 5% 5% 21%	2317
	21st to end	Night	21% 22% 7% 12% 5% 3 4 3 4 5% 15%	2672
November	1st to 10th	Night	17% 16% 6% 14% 8% 6% 5% 3 4 4 17%	2614
	11th to 20th	Night	13% 17% 6% 14% 7% 6% 5% 4 6% 7% 15%	2850
	21st to end	Night	9% 18% 7% 15% 9% 7% 5% 3 5% 6% 16%	2877
December	1st to 10th	Night	6% 17% 7% 16% 9% 7% 6% 4 6% 7% 13%	3040
	11th to 20th	Night	8% 14% 5% 12% 10% 7% 7% 6% 6% 7% 17%	3927
	21st to end	Night	7% 11% 5% 16% 7% 6% 6% 4 6% 6% 26%	5840

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	70% 6% 10% 13% 1	1899
	11th to 20th	Day	71% 5% 10% 12% 3	2184
	21st to end	Day	70% 5% 9% 11% 5%	2442
February	1st to 10th	Day	52% 6% 13% 20% 9%	1515
	11th to 20th	Day	52% 7% 13% 18% 9% 1	1586
	21st to end	Day	50% 6% 11% 21% 12%	1422
March	1st to 10th	Day	51% 4 13% 23% 9%	3130
	11th to 20th	Day	63% 4 10% 15% 8% 1	3466
	21st to end	Day	50% 4 7% 22% 16% 1	3863
April	1st to 10th	Day	49% 5% 12% 22% 12%	2417
	11th to 20th	Day	53% 4 10% 20% 12%	2442
	21st to end	Day	43% 6% 10% 27% 13%	2546
May	1st to 10th	Day	55% 7% 15% 19% 5%	4750
	11th to 20th	Day	53% 5% 10% 21% 10% 1	4709
	21st to end	Day	58% 4 10% 19% 8%	5173
June	1st to 10th	Day	51% 6% 11% 22% 10%	3160
	11th to 20th	Day	50% 7% 12% 23% 8% 1	3230
	21st to end	Day	46% 8% 12% 24% 10%	3230
July	1st to 10th	Day	44% 9% 15% 25% 7%	3019
	11th to 20th	Day	42% 9% 19% 25% 6%	2868
	21st to end	Day	48% 9% 18% 22% 3	2930
August	1st to 10th	Day	44% 10% 19% 23% 4	2669
	11th to 20th	Day	49% 11% 18% 19% 3	2417
	21st to end	Day	55% 8% 19% 15% 4	2574
September	1st to 10th	Day	50% 9% 18% 19% 4	2332
	11th to 20th	Day	57% 7% 17% 17% 1	1990
	21st to end	Day	54% 6% 19% 17% 4	1980
October	1st to 10th	Day	64% 6% 10% 14% 5%	1824
	11th to 20th	Day	70% 6% 11% 12% 1	1639
	21st to end	Day	74% 6% 8% 9% 3	1684
November	1st to 10th	Day	74% 5% 8% 11% 1	1462
	11th to 20th	Day	68% 6% 13% 12% 1	1330
	21st to end	Day	71% 5% 11% 11% 2	1302
December	1st to 10th	Day	63% 8% 13% 15% 2	1140
	11th to 20th	Day	68% 4 10% 14% 4	1263
	21st to end	Day	59% 6% 9% 20% 5%	1814
January	1st to 10th	Night	64% 6% 16% 11% 4	5046
	11th to 20th	Night	72% 5% 10% 11% 2	4782
	21st to end	Night	74% 5% 8% 9% 5%	5225
February	1st to 10th	Night	53% 7% 13% 20% 7%	2662
	11th to 20th	Night	53% 5% 12% 22% 8%	2490
	21st to end	Night	52% 5% 9% 21% 13%	2041
March	1st to 10th	Night	46% 5% 13% 27% 9%	3743
	11th to 20th	Night	63% 4 9% 16% 7% 1	3494
	21st to end	Night	48% 5% 10% 22% 15% 1	3769
April	1st to 10th	Night	48% 6% 10% 22% 15% 1	1763
	11th to 20th	Night	49% 5% 8% 23% 14% 1	1689
	21st to end	Night	38% 6% 11% 31% 13% 2	1633
May	1st to 10th	Night	54% 5% 15% 20% 5%	2220
	11th to 20th	Night	55% 6% 9% 18% 11% 1	2201
	21st to end	Night	56% 4 8% 23% 9%	2414
June	1st to 10th	Night	45% 7% 13% 19% 15% 1	1015
	11th to 20th	Night	48% 9% 8% 25% 9% 1	950
	21st to end	Night	43% 7% 11% 27% 12%	950
July	1st to 10th	Night	42% 8% 16% 26% 8%	1048
	11th to 20th	Night	42% 9% 18% 26% 5%	1186
	21st to end	Night	36% 13% 21% 27% 4	1379
August	1st to 10th	Night	39% 9% 20% 28% 4	1246
	11th to 20th	Night	45% 11% 18% 22% 4	1481
	21st to end	Night	50% 9% 19% 17% 4	1782
September	1st to 10th	Night	46% 8% 18% 21% 6% 1	1628
	11th to 20th	Night	55% 7% 14% 22% 3	1969
	21st to end	Night	50% 7% 15% 23% 5%	1980
October	1st to 10th	Night	56% 8% 12% 19% 5%	2118
	11th to 20th	Night	66% 6% 12% 14% 2	2317
	21st to end	Night	73% 7% 9% 10% 2	2672
November	1st to 10th	Night	71% 5% 9% 12% 3	2614
	11th to 20th	Night	69% 6% 13% 10% 2	2850
	21st to end	Night	70% 5% 9% 14% 2	2877
December	1st to 10th	Night	68% 5% 11% 12% 3	3040
	11th to 20th	Night	64% 6% 11% 15% 4	3926
	21st to end	Night	57% 5% 10% 21% 6% 1	5840

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	12% 9% 37% 30% 11% 1	180
	11th to 20th	Day	13% 9% 37% 31% 8% 1	259
	21st to end	Day	21% 9% 34% 27% 8% 1	297
February	1st to 10th	Day	9% 14% 39% 29% 6% 2	270
	11th to 20th	Day	11% 11% 43% 23% 9% 2	270
	21st to end	Day	7% 14% 36% 29% 11% 3	225
March	1st to 10th	Day	12% 14% 29% 23% 14% 7%	270
	11th to 20th	Day	11% 17% 38% 22% 9% 3	270
	21st to end	Day	13% 14% 41% 21% 7% 3	321
April	1st to 10th	Day	12% 12% 36% 28% 10% 2	597
	11th to 20th	Day	14% 14% 36% 23% 11% 2	640
	21st to end	Day	12% 14% 34% 27% 12% 2	650
May	1st to 10th	Day	23% 13% 33% 23% 7% 1	653
	11th to 20th	Day	23% 16% 30% 23% 7% 1	675
	21st to end	Day	26% 12% 32% 21% 7% 1	734
June	1st to 10th	Day	28% 11% 27% 23% 8% 3	680
	11th to 20th	Day	22% 12% 32% 21% 9% 3	680
	21st to end	Day	20% 9% 30% 26% 11% 3	680
July	1st to 10th	Day	16% 11% 32% 27% 11% 3	671
	11th to 20th	Day	17% 9% 26% 28% 14% 6%	668
	21st to end	Day	19% 9% 25% 26% 16% 4	736
August	1st to 10th	Day	15% 12% 27% 30% 13% 3	626
	11th to 20th	Day	19% 12% 27% 25% 13% 4	615
	21st to end	Day	25% 8% 26% 25% 13% 3	693
September	1st to 10th	Day	23% 14% 33% 21% 8%	622
	11th to 20th	Day	24% 13% 32% 21% 10% 1	280
	21st to end	Day	25% 11% 30% 24% 9%	270
October	1st to 10th	Day	28% 17% 27% 17% 8% 2	270
	11th to 20th	Day	38% 13% 28% 12% 7% 3	270
	21st to end	Day	27% 11% 32% 20% 9% 1	297
November	1st to 10th	Day	21% 11% 26% 27% 11% 3	270
	11th to 20th	Day	19% 14% 40% 17% 7% 3	270
	21st to end	Day	23% 12% 39% 19% 7% 1	257
December	1st to 10th	Day	10% 11% 34% 34% 8% 3	180
	11th to 20th	Day	20% 12% 27% 25% 15% 1	122
	21st to end	Day	12% 7% 33% 31% 15% 2	123
January	1st to 10th	Night	14% 9% 35% 32% 9% 1	950
	11th to 20th	Night	13% 10% 37% 27% 10% 2	950
	21st to end	Night	17% 9% 34% 31% 8% 1	1045
February	1st to 10th	Night	10% 9% 40% 30% 10% 2	950
	11th to 20th	Night	10% 8% 34% 30% 15% 3	929
	21st to end	Night	8% 11% 34% 34% 12% 2	785
March	1st to 10th	Night	10% 7% 28% 37% 16% 2	769
	11th to 20th	Night	19% 13% 33% 24% 10% 1	584
	21st to end	Night	11% 9% 36% 28% 13% 2	627
April	1st to 10th	Night	14% 8% 32% 31% 12% 2	570
	11th to 20th	Night	13% 9% 31% 31% 13% 2	563
	21st to end	Night	11% 11% 37% 30% 10% 1	570
May	1st to 10th	Night	23% 10% 33% 25% 9% 1	570
	11th to 20th	Night	22% 11% 33% 24% 8% 1	561
	21st to end	Night	25% 10% 33% 24% 8%	617
June	1st to 10th	Night	22% 9% 23% 32% 13% 1	255
	11th to 20th	Night	19% 9% 27% 31% 13% 2	190
	21st to end	Night	15% 7% 29% 35% 14%	190
July	1st to 10th	Night	13% 5% 29% 36% 17% 1	308
	11th to 20th	Night	15% 6% 26% 29% 21% 3	450
	21st to end	Night	15% 4 19% 34% 25% 3	624
August	1st to 10th	Night	17% 5% 23% 33% 21% 1	537
	11th to 20th	Night	16% 6% 25% 34% 17% 1	534
	21st to end	Night	23% 5% 27% 32% 12% 1	594
September	1st to 10th	Night	25% 10% 28% 26% 10% 1	540
	11th to 20th	Night	28% 10% 26% 25% 11% 1	540
	21st to end	Night	22% 9% 32% 25% 10% 2	540
October	1st to 10th	Night	27% 8% 30% 25% 9% 1	678
	11th to 20th	Night	35% 7% 23% 23% 10% 2	877
	21st to end	Night	33% 11% 27% 19% 8% 1	990
November	1st to 10th	Night	25% 10% 29% 22% 11% 2	925
	11th to 20th	Night	22% 9% 34% 24% 9% 1	950
	21st to end	Night	19% 13% 31% 27% 8% 2	950
December	1st to 10th	Night	13% 8% 35% 32% 10% 1	950
	11th to 20th	Night	18% 10% 26% 33% 12% 1	950
	21st to end	Night	15% 7% 28% 34% 13% 4	1045

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