Atlin
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Atlin clear sky chart from 2005-11-06 to 2026-05-17.

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% 10% 4 11% 8% 7% 7% 6% 8% 9% 26%	135744
	Night	4 11% 5% 13% 9% 8% 8% 6% 8% 8% 21%	135725
Transparency	Day	52% 11% 15% 16% 5%	135404
	Night	55% 11% 14% 13% 6% 1	135646
Seeing	Day	15% 15% 27% 23% 15% 5%	19193
	Night	12% 12% 25% 29% 19% 4	25727

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	2 6% 4 14% 13% 12% 11% 7% 10% 11% 10%	3356
	11th to 20th	Day	3 12% 5% 10% 6% 8% 9% 6% 11% 8% 22%	3522
	21st to end	Day	4 14% 5% 10% 7% 8% 7% 5% 7% 9% 24%	3914
February	1st to 10th	Day	2 12% 5% 14% 8% 7% 8% 6% 9% 10% 19%	1570
	11th to 20th	Day	2 11% 5% 13% 9% 7% 6% 5% 8% 9% 24%	1640
	21st to end	Day	1 11% 5% 12% 10% 8% 8% 6% 7% 8% 23%	1528
March	1st to 10th	Day	1 6% 4 13% 10% 10% 10% 6% 9% 9% 23%	4596
	11th to 20th	Day	2 7% 5% 15% 10% 10% 8% 6% 7% 8% 19%	4759
	21st to end	Day	2 7% 5% 13% 9% 7% 7% 6% 8% 10% 29%	6101
April	1st to 10th	Day	1 9% 5% 12% 9% 8% 8% 6% 8% 9% 24%	2546
	11th to 20th	Day	3 12% 4 11% 8% 7% 7% 6% 7% 8% 26%	2853
	21st to end	Day	4 13% 4 10% 7% 7% 6% 5% 7% 7% 28%	2940
May	1st to 10th	Day	5% 9% 3 9% 7% 7% 9% 7% 10% 9% 25%	7053
	11th to 20th	Day	3 13% 5% 12% 8% 8% 7% 6% 8% 7% 22%	7290
	21st to end	Day	4 8% 3 9% 8% 7% 8% 6% 8% 10% 28%	8061
June	1st to 10th	Day	11% 9% 3 9% 6% 6% 6% 5% 7% 8% 30%	3450
	11th to 20th	Day	10% 10% 3 8% 6% 6% 7% 6% 8% 8% 27%	3600
	21st to end	Day	7% 8% 4 9% 6% 6% 7% 5% 8% 10% 30%	3600
July	1st to 10th	Day	7% 6% 2 7% 6% 6% 6% 6% 8% 11% 35%	8050
	11th to 20th	Day	9% 8% 3 9% 6% 7% 7% 5% 8% 10% 28%	7240
	21st to end	Day	10% 7% 2 8% 5% 6% 7% 5% 7% 9% 33%	7876
August	1st to 10th	Day	6% 4 2 6% 4 4 5% 3 6% 7% 53%	5023
	11th to 20th	Day	12% 8% 3 7% 5% 5% 5% 4 6% 8% 36%	4480
	21st to end	Day	15% 11% 5% 10% 7% 5% 5% 4 6% 8% 23%	4971
September	1st to 10th	Day	14% 12% 3 10% 8% 7% 5% 4 6% 7% 23%	2443
	11th to 20th	Day	11% 11% 4 10% 6% 6% 5% 5% 6% 7% 28%	2375
	21st to end	Day	15% 15% 5% 11% 8% 7% 6% 5% 5% 5% 18%	2165
October	1st to 10th	Day	6% 11% 5% 12% 9% 8% 8% 6% 8% 9% 19%	3238
	11th to 20th	Day	5% 15% 7% 13% 6% 6% 7% 4 7% 9% 20%	2933
	21st to end	Day	7% 16% 5% 14% 8% 8% 7% 4 6% 6% 17%	2918
November	1st to 10th	Day	5% 16% 5% 14% 10% 11% 8% 7% 7% 7% 12%	1442
	11th to 20th	Day	5% 20% 6% 17% 11% 10% 7% 4 6% 6% 9%	1381
	21st to end	Day	4 15% 7% 17% 12% 8% 7% 6% 8% 5% 10%	1176
December	1st to 10th	Day	4 17% 5% 14% 10% 9% 8% 5% 6% 6% 16%	1680
	11th to 20th	Day	1 7% 3 16% 12% 11% 12% 7% 10% 10% 10%	1932
	21st to end	Day	2 10% 7% 18% 11% 11% 10% 6% 8% 7% 10%	2042
January	1st to 10th	Night	2 9% 5% 15% 13% 14% 11% 7% 8% 8% 9%	9590
	11th to 20th	Night	3 11% 5% 11% 7% 8% 8% 7% 11% 11% 18%	9376
	21st to end	Night	4 11% 5% 15% 8% 7% 7% 5% 8% 9% 20%	10268
February	1st to 10th	Night	3 14% 5% 15% 10% 7% 7% 6% 8% 8% 20%	3050
	11th to 20th	Night	2 11% 6% 14% 8% 9% 6% 6% 8% 8% 21%	2867
	21st to end	Night	2 12% 5% 13% 11% 7% 7% 5% 7% 9% 24%	2278
March	1st to 10th	Night	1 7% 4 15% 8% 10% 9% 6% 10% 9% 21%	5498
	11th to 20th	Night	2 10% 5% 16% 11% 10% 7% 5% 8% 9% 17%	5435
	21st to end	Night	2 9% 6% 13% 7% 6% 6% 6% 9% 10% 25%	5094
April	1st to 10th	Night	2 9% 5% 11% 9% 7% 7% 6% 8% 8% 28%	2074
	11th to 20th	Night	3 10% 4 11% 7% 6% 6% 5% 8% 9% 30%	1713
	21st to end	Night	4 12% 3 10% 7% 5% 6% 6% 7% 8% 30%	1680
May	1st to 10th	Night	2 8% 2 9% 6% 7% 6% 5% 8% 10% 36%	3125
	11th to 20th	Night	2 10% 4 11% 7% 7% 7% 6% 9% 9% 28%	2738
	21st to end	Night	3 7% 3 8% 6% 7% 7% 6% 9% 8% 35%	2812
June	1st to 10th	Night	11% 9% 3 9% 6% 6% 4 4 7% 10% 32%	945
	11th to 20th	Night	9% 10% 3 7% 6% 5% 6% 5% 8% 10% 32%	800
	21st to end	Night	5% 7% 3 8% 5% 6% 7% 6% 10% 11% 33%	800
July	1st to 10th	Night	5% 4 2 6% 6% 6% 7% 6% 8% 8% 42%	1906
	11th to 20th	Night	6% 7% 3 9% 6% 5% 7% 5% 8% 10% 34%	2720
	21st to end	Night	12% 7% 3 8% 6% 6% 7% 5% 7% 8% 32%	2967
August	1st to 10th	Night	7% 4 1 5% 2 3 5% 4 7% 10% 52%	2128
	11th to 20th	Night	9% 6% 3 7% 4 5% 4 4 7% 9% 43%	2578
	21st to end	Night	16% 12% 5% 9% 6% 5% 5% 3 5% 6% 29%	2938
September	1st to 10th	Night	11% 12% 4 11% 6% 7% 5% 5% 7% 7% 25%	1955
	11th to 20th	Night	9% 11% 4 8% 7% 5% 5% 4 6% 6% 33%	2025
	21st to end	Night	14% 12% 5% 11% 7% 7% 6% 5% 7% 6% 20%	2233
October	1st to 10th	Night	8% 9% 4 11% 7% 8% 8% 6% 8% 8% 24%	3952
	11th to 20th	Night	6% 15% 7% 16% 7% 5% 6% 5% 7% 8% 19%	4215
	21st to end	Night	6% 15% 4 11% 7% 7% 8% 5% 8% 9% 20%	4878
November	1st to 10th	Night	6% 14% 5% 16% 9% 10% 7% 5% 8% 7% 12%	3076
	11th to 20th	Night	5% 18% 8% 19% 9% 9% 7% 5% 6% 6% 8%	3237
	21st to end	Night	3 13% 7% 17% 12% 11% 8% 6% 8% 6% 10%	3423
December	1st to 10th	Night	4 15% 7% 15% 10% 9% 9% 6% 8% 6% 12%	5720
	11th to 20th	Night	2 10% 6% 17% 12% 11% 9% 7% 8% 9% 10%	6480
	21st to end	Night	2 12% 6% 17% 11% 9% 9% 7% 8% 8% 12%	9151

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	53% 10% 18% 9% 7% 3	3356
	11th to 20th	Day	50% 11% 14% 9% 13% 3	3540
	21st to end	Day	55% 11% 15% 11% 8%	3915
February	1st to 10th	Day	55% 10% 11% 11% 11% 1	1570
	11th to 20th	Day	54% 11% 14% 10% 9% 3	1648
	21st to end	Day	54% 10% 14% 14% 7% 1	1528
March	1st to 10th	Day	51% 9% 17% 13% 11% 1	4596
	11th to 20th	Day	57% 8% 18% 9% 7%	4759
	21st to end	Day	46% 11% 16% 20% 7%	6101
April	1st to 10th	Day	59% 15% 11% 12% 3	2546
	11th to 20th	Day	55% 12% 11% 14% 7% 1	2853
	21st to end	Day	56% 11% 15% 14% 4	2940
May	1st to 10th	Day	48% 14% 12% 22% 4	7053
	11th to 20th	Day	56% 11% 11% 16% 5%	7293
	21st to end	Day	46% 10% 13% 24% 7%	8062
June	1st to 10th	Day	55% 12% 15% 14% 4 1	3450
	11th to 20th	Day	55% 12% 14% 15% 3	3600
	21st to end	Day	50% 14% 16% 17% 2	3600
July	1st to 10th	Day	42% 11% 23% 20% 4	7963
	11th to 20th	Day	51% 12% 16% 19% 2	7153
	21st to end	Day	45% 14% 22% 17% 2	7655
August	1st to 10th	Day	33% 9% 15% 26% 16% 1	5004
	11th to 20th	Day	46% 11% 15% 19% 9%	4522
	21st to end	Day	58% 13% 13% 14% 2	4970
September	1st to 10th	Day	63% 11% 12% 12% 2	2443
	11th to 20th	Day	56% 11% 10% 19% 3	2375
	21st to end	Day	70% 11% 9% 9% 2	2165
October	1st to 10th	Day	57% 15% 11% 15% 2	3238
	11th to 20th	Day	62% 11% 13% 12% 2	2933
	21st to end	Day	62% 12% 19% 6% 2	2918
November	1st to 10th	Day	62% 14% 12% 8% 3	1442
	11th to 20th	Day	70% 11% 11% 6% 3	1381
	21st to end	Day	64% 13% 14% 7% 3	1176
December	1st to 10th	Day	64% 11% 13% 8% 3	1680
	11th to 20th	Day	56% 10% 18% 11% 5%	1934
	21st to end	Day	66% 10% 10% 7% 6% 1	2042
January	1st to 10th	Night	63% 9% 14% 6% 7% 1	9591
	11th to 20th	Night	51% 11% 14% 10% 10% 4	9420
	21st to end	Night	57% 12% 12% 13% 6%	10268
February	1st to 10th	Night	58% 9% 12% 10% 9% 1	3050
	11th to 20th	Night	57% 11% 10% 11% 9% 3	2870
	21st to end	Night	54% 8% 13% 17% 7% 1	2278
March	1st to 10th	Night	51% 7% 17% 13% 11% 1	5498
	11th to 20th	Night	57% 8% 15% 9% 10%	5437
	21st to end	Night	43% 14% 16% 18% 9%	5094
April	1st to 10th	Night	52% 10% 16% 18% 3	2074
	11th to 20th	Night	48% 11% 15% 16% 8% 1	1713
	21st to end	Night	51% 11% 16% 16% 6%	1680
May	1st to 10th	Night	41% 9% 12% 28% 9%	3122
	11th to 20th	Night	50% 14% 12% 17% 7% 1	2738
	21st to end	Night	44% 10% 12% 29% 5%	2812
June	1st to 10th	Night	48% 11% 19% 17% 5% 1	945
	11th to 20th	Night	50% 14% 17% 17% 3	800
	21st to end	Night	43% 15% 22% 19% 1	800
July	1st to 10th	Night	37% 10% 28% 23% 2	1885
	11th to 20th	Night	43% 12% 23% 20% 1	2684
	21st to end	Night	46% 14% 26% 13% 1	2883
August	1st to 10th	Night	29% 11% 17% 28% 13% 2	2122
	11th to 20th	Night	39% 9% 20% 24% 8%	2602
	21st to end	Night	58% 10% 12% 17% 2	2938
September	1st to 10th	Night	58% 13% 15% 12% 2	1955
	11th to 20th	Night	53% 10% 12% 20% 4	2025
	21st to end	Night	65% 11% 9% 12% 3	2233
October	1st to 10th	Night	56% 12% 13% 15% 5%	3952
	11th to 20th	Night	64% 11% 9% 13% 2	4215
	21st to end	Night	55% 12% 19% 12% 2	4878
November	1st to 10th	Night	62% 14% 13% 8% 3	3076
	11th to 20th	Night	71% 10% 10% 6% 3	3237
	21st to end	Night	66% 11% 13% 7% 2	3423
December	1st to 10th	Night	63% 12% 11% 10% 3	5720
	11th to 20th	Night	60% 9% 15% 11% 4	6477
	21st to end	Night	65% 10% 10% 7% 8% 1	9151

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	6% 9% 27% 30% 21% 8%	213
	11th to 20th	Day	13% 16% 24% 23% 20% 4	220
	21st to end	Day	17% 17% 22% 24% 14% 6%	258
February	1st to 10th	Day	10% 10% 29% 35% 13% 3	430
	11th to 20th	Day	10% 15% 28% 24% 17% 6%	425
	21st to end	Day	6% 16% 36% 25% 13% 4	355
March	1st to 10th	Day	8% 16% 30% 25% 16% 6%	430
	11th to 20th	Day	8% 16% 36% 22% 14% 3	436
	21st to end	Day	8% 15% 27% 26% 19% 6%	594
April	1st to 10th	Day	7% 19% 35% 25% 12% 3	540
	11th to 20th	Day	9% 18% 27% 25% 18% 4	687
	21st to end	Day	10% 17% 33% 21% 15% 3	750
May	1st to 10th	Day	13% 18% 30% 22% 15% 3	753
	11th to 20th	Day	10% 12% 22% 27% 19% 11%	756
	21st to end	Day	10% 12% 27% 24% 21% 6%	853
June	1st to 10th	Day	16% 13% 23% 24% 18% 5%	930
	11th to 20th	Day	15% 11% 25% 26% 17% 6%	930
	21st to end	Day	10% 11% 25% 25% 22% 6%	930
July	1st to 10th	Day	13% 10% 26% 25% 20% 6%	892
	11th to 20th	Day	19% 14% 27% 24% 12% 4	718
	21st to end	Day	18% 16% 25% 21% 15% 6%	794
August	1st to 10th	Day	16% 13% 30% 23% 13% 5%	726
	11th to 20th	Day	28% 21% 25% 17% 8% 2	715
	21st to end	Day	29% 15% 27% 19% 9% 2	773
September	1st to 10th	Day	24% 20% 22% 22% 11% 1	530
	11th to 20th	Day	24% 18% 24% 20% 12% 3	530
	21st to end	Day	33% 23% 24% 13% 6% 2	511
October	1st to 10th	Day	26% 17% 30% 18% 7% 1	420
	11th to 20th	Day	24% 22% 31% 14% 6% 3	420
	21st to end	Day	18% 15% 27% 25% 12% 2	450
November	1st to 10th	Day	14% 16% 29% 25% 12% 3	220
	11th to 20th	Day	14% 12% 30% 23% 16% 5%	220
	21st to end	Day	8% 20% 30% 25% 13% 5%	220
December	1st to 10th	Day	10% 19% 28% 25% 13% 5%	220
	11th to 20th	Day	8% 11% 21% 27% 25% 8%	220
	21st to end	Day	5% 17% 20% 28% 23% 7%	124
January	1st to 10th	Night	8% 12% 26% 29% 20% 5%	1023
	11th to 20th	Night	13% 17% 29% 24% 14% 3	950
	21st to end	Night	11% 19% 27% 24% 16% 3	1045
February	1st to 10th	Night	10% 11% 28% 34% 14% 3	950
	11th to 20th	Night	7% 14% 30% 29% 15% 4	934
	21st to end	Night	5% 12% 30% 33% 17% 4	772
March	1st to 10th	Night	6% 13% 27% 30% 19% 4	840
	11th to 20th	Night	7% 10% 27% 31% 20% 3	803
	21st to end	Night	6% 8% 22% 31% 24% 9%	693
April	1st to 10th	Night	4 9% 24% 33% 25% 5%	630
	11th to 20th	Night	7% 9% 26% 29% 25% 5%	623
	21st to end	Night	9% 8% 26% 30% 22% 6%	630
May	1st to 10th	Night	8% 8% 25% 27% 27% 5%	425
	11th to 20th	Night	8% 8% 19% 31% 26% 8%	406
	21st to end	Night	6% 6% 18% 32% 30% 8%	433
June	1st to 10th	Night	16% 6% 19% 30% 28% 1	345
	11th to 20th	Night	14% 3 23% 28% 31% 3	200
	21st to end	Night	8% 3 19% 31% 37% 3	200
July	1st to 10th	Night	13% 3 21% 34% 29% 1	214
	11th to 20th	Night	16% 9% 18% 30% 24% 2	388
	21st to end	Night	12% 8% 20% 29% 28% 4	438
August	1st to 10th	Night	13% 6% 17% 34% 27% 3	452
	11th to 20th	Night	20% 11% 21% 26% 19% 4	594
	21st to end	Night	26% 8% 18% 28% 17% 3	660
September	1st to 10th	Night	20% 11% 22% 29% 15% 2	600
	11th to 20th	Night	18% 14% 19% 27% 17% 5%	625
	21st to end	Night	26% 18% 21% 21% 11% 4	800
October	1st to 10th	Night	19% 15% 24% 24% 15% 3	800
	11th to 20th	Night	23% 18% 24% 23% 11% 3	800
	21st to end	Night	16% 11% 30% 27% 13% 3	965
November	1st to 10th	Night	13% 14% 29% 29% 13% 2	930
	11th to 20th	Night	13% 12% 25% 28% 17% 4	950
	21st to end	Night	11% 14% 27% 28% 16% 3	1013
December	1st to 10th	Night	12% 15% 26% 28% 18% 2	1160
	11th to 20th	Night	11% 9% 25% 33% 19% 4	1160
	21st to end	Night	11% 11% 25% 29% 19% 5%	1276

Page generated at 2026-05-13 22:59:01. Version ac07

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