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 2025-01-22.

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% 11% 4 11% 8% 7% 7% 5% 7% 8% 25%	108780
	Night	5% 12% 5% 13% 9% 8% 7% 5% 7% 8% 20%	109118
Transparency	Day	54% 11% 14% 14% 5%	108487
	Night	58% 11% 14% 12% 5% 1	109059
Seeing	Day	16% 15% 27% 23% 15% 5%	17400
	Night	13% 11% 24% 29% 19% 4	23997

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	3 9% 5% 11% 13% 15% 12% 8% 8% 8% 8%	1840
	11th to 20th	Day	3 18% 6% 13% 6% 8% 6% 5% 10% 8% 16%	1942
	21st to end	Day	7% 22% 6% 13% 9% 7% 6% 4 7% 6% 12%	2110
February	1st to 10th	Day	2 13% 5% 15% 8% 7% 8% 6% 9% 10% 18%	1420
	11th to 20th	Day	3 11% 5% 12% 9% 7% 7% 5% 8% 9% 23%	1480
	21st to end	Day	1 11% 5% 12% 10% 8% 8% 6% 7% 9% 23%	1386
March	1st to 10th	Day	1 6% 4 13% 12% 12% 11% 7% 9% 9% 17%	3126
	11th to 20th	Day	3 10% 6% 16% 10% 9% 7% 6% 7% 7% 20%	3247
	21st to end	Day	2 10% 6% 15% 10% 7% 8% 6% 7% 7% 22%	4165
April	1st to 10th	Day	1 9% 5% 13% 9% 8% 7% 6% 8% 9% 24%	2304
	11th to 20th	Day	3 13% 4 11% 9% 7% 7% 6% 7% 8% 26%	2578
	21st to end	Day	4 14% 4 11% 7% 7% 6% 5% 7% 7% 28%	2660
May	1st to 10th	Day	6% 12% 4 9% 7% 6% 7% 6% 8% 8% 28%	4819
	11th to 20th	Day	4 13% 6% 13% 8% 7% 6% 6% 7% 7% 22%	5013
	21st to end	Day	6% 9% 4 9% 7% 5% 6% 6% 7% 9% 32%	5621
June	1st to 10th	Day	11% 9% 3 9% 6% 6% 6% 5% 7% 8% 30%	3277
	11th to 20th	Day	11% 10% 3 8% 5% 6% 7% 6% 8% 9% 27%	3420
	21st to end	Day	8% 8% 4 9% 6% 6% 7% 5% 8% 10% 29%	3420
July	1st to 10th	Day	6% 5% 2 6% 5% 5% 6% 5% 8% 11% 41%	5618
	11th to 20th	Day	12% 9% 4 10% 7% 7% 7% 5% 7% 8% 23%	5060
	21st to end	Day	11% 8% 2 8% 5% 6% 7% 4 7% 8% 33%	5486
August	1st to 10th	Day	6% 4 2 6% 4 4 5% 3 6% 7% 54%	4868
	11th to 20th	Day	12% 8% 3 7% 5% 4 5% 4 6% 8% 37%	4340
	21st to end	Day	15% 11% 5% 10% 7% 5% 5% 4 6% 8% 22%	4819
September	1st to 10th	Day	15% 12% 3 10% 8% 7% 6% 5% 6% 7% 21%	2321
	11th to 20th	Day	11% 11% 4 9% 6% 6% 5% 5% 6% 7% 28%	2256
	21st to end	Day	16% 16% 5% 11% 7% 6% 6% 5% 5% 5% 18%	2057
October	1st to 10th	Day	6% 11% 5% 12% 9% 8% 8% 6% 8% 9% 18%	3139
	11th to 20th	Day	5% 16% 7% 13% 6% 6% 7% 4 7% 9% 20%	2843
	21st to end	Day	7% 17% 6% 15% 8% 8% 7% 4 6% 6% 16%	2828
November	1st to 10th	Day	5% 16% 5% 14% 10% 10% 7% 7% 7% 6% 12%	1372
	11th to 20th	Day	5% 20% 6% 17% 11% 10% 7% 4 5% 6% 8%	1315
	21st to end	Day	4 15% 7% 17% 12% 8% 7% 6% 8% 6% 10%	1120
December	1st to 10th	Day	4 16% 5% 14% 10% 9% 8% 5% 7% 6% 16%	1630
	11th to 20th	Day	1 7% 3 16% 12% 11% 12% 7% 9% 10% 10%	1882
	21st to end	Day	2 10% 7% 18% 11% 11% 10% 6% 8% 7% 9%	1998
January	1st to 10th	Night	3 12% 5% 15% 15% 16% 10% 5% 5% 6% 8%	5351
	11th to 20th	Night	3 13% 7% 13% 9% 8% 5% 5% 12% 9% 15%	5176
	21st to end	Night	7% 18% 7% 17% 9% 7% 7% 5% 6% 6% 12%	5568
February	1st to 10th	Night	3 15% 5% 14% 10% 6% 7% 5% 8% 8% 20%	2760
	11th to 20th	Night	2 12% 6% 14% 9% 9% 6% 6% 8% 8% 20%	2587
	21st to end	Night	2 12% 5% 13% 10% 7% 6% 5% 7% 9% 25%	2068
March	1st to 10th	Night	1 8% 5% 13% 8% 10% 9% 6% 9% 8% 22%	3738
	11th to 20th	Night	3 13% 7% 15% 9% 8% 7% 5% 7% 8% 19%	3718
	21st to end	Night	3 12% 7% 16% 8% 6% 6% 5% 7% 8% 22%	3477
April	1st to 10th	Night	2 10% 5% 11% 9% 7% 7% 6% 8% 8% 28%	1876
	11th to 20th	Night	3 11% 4 11% 7% 6% 6% 4 8% 10% 31%	1549
	21st to end	Night	5% 13% 3 10% 8% 6% 6% 7% 7% 7% 29%	1520
May	1st to 10th	Night	3 10% 3 10% 7% 6% 6% 5% 8% 7% 36%	2139
	11th to 20th	Night	3 11% 4 12% 8% 7% 7% 6% 7% 8% 26%	1882
	21st to end	Night	5% 9% 3 8% 6% 6% 5% 5% 8% 8% 38%	1952
June	1st to 10th	Night	12% 9% 3 10% 6% 6% 4 4 7% 10% 31%	898
	11th to 20th	Night	9% 9% 3 7% 6% 5% 6% 5% 8% 10% 33%	760
	21st to end	Night	5% 7% 3 8% 5% 6% 7% 6% 10% 11% 32%	760
July	1st to 10th	Night	6% 4 2 6% 6% 5% 7% 6% 6% 7% 45%	1338
	11th to 20th	Night	9% 8% 3 11% 7% 6% 7% 5% 8% 10% 26%	1900
	21st to end	Night	10% 8% 3 10% 7% 7% 7% 4 6% 6% 32%	2066
August	1st to 10th	Night	6% 4 2 5% 2 3 5% 4 7% 10% 53%	2063
	11th to 20th	Night	9% 6% 3 7% 4 5% 4 4 7% 10% 43%	2498
	21st to end	Night	17% 12% 5% 10% 6% 5% 5% 3 5% 6% 27%	2848
September	1st to 10th	Night	11% 13% 4 11% 6% 7% 5% 5% 7% 6% 24%	1857
	11th to 20th	Night	9% 11% 4 9% 7% 5% 5% 4 6% 6% 33%	1924
	21st to end	Night	14% 12% 5% 11% 7% 6% 6% 5% 6% 7% 21%	2121
October	1st to 10th	Night	8% 9% 4 11% 7% 8% 8% 6% 8% 8% 23%	3831
	11th to 20th	Night	7% 15% 7% 16% 7% 5% 5% 5% 7% 8% 18%	4085
	21st to end	Night	6% 15% 4 11% 7% 8% 8% 5% 8% 8% 20%	4726
November	1st to 10th	Night	6% 15% 5% 16% 10% 10% 6% 5% 8% 7% 12%	2926
	11th to 20th	Night	5% 19% 9% 19% 9% 10% 7% 5% 6% 5% 7%	3083
	21st to end	Night	3 13% 7% 17% 11% 10% 8% 6% 8% 6% 9%	3260
December	1st to 10th	Night	4 15% 7% 15% 10% 9% 9% 6% 8% 6% 12%	5550
	11th to 20th	Night	2 10% 6% 16% 12% 11% 10% 7% 8% 9% 10%	6310
	21st to end	Night	2 12% 6% 17% 11% 9% 9% 6% 8% 8% 12%	8953

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	52% 10% 20% 12% 5% 2	1840
	11th to 20th	Day	57% 8% 10% 6% 14% 5%	1960
	21st to end	Day	72% 8% 9% 7% 4	2110
February	1st to 10th	Day	56% 10% 11% 10% 11% 1	1420
	11th to 20th	Day	55% 11% 14% 10% 8% 3	1488
	21st to end	Day	54% 10% 14% 14% 7% 1	1386
March	1st to 10th	Day	56% 8% 17% 11% 8% 1	3126
	11th to 20th	Day	62% 9% 14% 10% 4	3247
	21st to end	Day	54% 12% 16% 12% 5%	4165
April	1st to 10th	Day	60% 15% 11% 11% 3	2304
	11th to 20th	Day	57% 12% 11% 14% 6% 1	2578
	21st to end	Day	58% 11% 13% 13% 4	2660
May	1st to 10th	Day	49% 13% 13% 21% 4	4819
	11th to 20th	Day	58% 11% 11% 15% 5%	5016
	21st to end	Day	45% 9% 16% 24% 7%	5622
June	1st to 10th	Day	55% 12% 15% 14% 4 1	3277
	11th to 20th	Day	55% 12% 14% 15% 3	3420
	21st to end	Day	51% 15% 16% 16% 2	3420
July	1st to 10th	Day	38% 12% 27% 19% 4	5531
	11th to 20th	Day	60% 12% 13% 14% 1	4973
	21st to end	Day	46% 14% 17% 20% 3	5313
August	1st to 10th	Day	32% 10% 15% 25% 16% 2	4849
	11th to 20th	Day	45% 10% 15% 19% 10%	4382
	21st to end	Day	60% 13% 12% 13% 2	4818
September	1st to 10th	Day	66% 11% 11% 11% 2	2321
	11th to 20th	Day	57% 11% 11% 18% 3	2256
	21st to end	Day	70% 10% 9% 8% 2	2057
October	1st to 10th	Day	58% 15% 11% 15% 2	3139
	11th to 20th	Day	62% 11% 13% 12% 2	2843
	21st to end	Day	63% 12% 19% 5% 2	2828
November	1st to 10th	Day	62% 14% 12% 7% 4	1372
	11th to 20th	Day	71% 11% 11% 5% 2	1315
	21st to end	Day	64% 13% 14% 7% 3	1120
December	1st to 10th	Day	64% 12% 13% 8% 3	1630
	11th to 20th	Day	55% 11% 18% 11% 4	1884
	21st to end	Day	66% 10% 10% 6% 6% 1	1998
January	1st to 10th	Night	65% 7% 17% 6% 4 1	5351
	11th to 20th	Night	56% 7% 9% 8% 12% 7%	5220
	21st to end	Night	71% 11% 7% 7% 4	5568
February	1st to 10th	Night	59% 9% 12% 9% 9% 2	2760
	11th to 20th	Night	58% 11% 10% 10% 7% 3	2590
	21st to end	Night	53% 8% 13% 17% 7% 1	2068
March	1st to 10th	Night	52% 6% 16% 14% 11% 2	3738
	11th to 20th	Night	61% 10% 14% 10% 6%	3720
	21st to end	Night	47% 15% 20% 14% 4	3477
April	1st to 10th	Night	53% 10% 16% 18% 4	1876
	11th to 20th	Night	48% 11% 15% 17% 8% 2	1549
	21st to end	Night	53% 11% 15% 15% 6%	1520
May	1st to 10th	Night	44% 11% 13% 27% 5%	2139
	11th to 20th	Night	51% 15% 12% 15% 6% 1	1882
	21st to end	Night	42% 11% 14% 28% 6%	1952
June	1st to 10th	Night	49% 11% 18% 17% 5% 1	898
	11th to 20th	Night	49% 14% 17% 16% 3	760
	21st to end	Night	44% 16% 21% 17% 1	760
July	1st to 10th	Night	37% 10% 31% 19% 3	1317
	11th to 20th	Night	53% 14% 18% 14% 1	1864
	21st to end	Night	49% 14% 20% 16% 1	2000
August	1st to 10th	Night	28% 11% 17% 28% 13% 2	2057
	11th to 20th	Night	38% 9% 20% 24% 8%	2522
	21st to end	Night	60% 10% 12% 16% 2	2848
September	1st to 10th	Night	60% 13% 14% 11% 2	1857
	11th to 20th	Night	53% 10% 13% 20% 4	1924
	21st to end	Night	65% 11% 9% 12% 3	2121
October	1st to 10th	Night	56% 12% 13% 15% 4	3831
	11th to 20th	Night	65% 11% 9% 13% 2	4085
	21st to end	Night	55% 11% 19% 12% 2	4726
November	1st to 10th	Night	63% 13% 13% 7% 3	2926
	11th to 20th	Night	72% 10% 10% 5% 3	3083
	21st to end	Night	66% 12% 13% 7% 2	3260
December	1st to 10th	Night	63% 12% 11% 11% 3	5550
	11th to 20th	Night	60% 9% 15% 11% 4	6307
	21st to end	Night	65% 10% 10% 6% 7% 1	8953

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% 8% 28% 30% 20% 7%	193
	11th to 20th	Day	12% 15% 24% 25% 21% 5%	200
	21st to end	Day	20% 17% 19% 24% 14% 6%	214
February	1st to 10th	Day	11% 10% 28% 35% 13% 4	370
	11th to 20th	Day	10% 16% 28% 24% 15% 6%	365
	21st to end	Day	6% 16% 36% 24% 14% 5%	307
March	1st to 10th	Day	8% 16% 30% 25% 16% 6%	370
	11th to 20th	Day	10% 17% 37% 23% 11% 2	375
	21st to end	Day	10% 18% 27% 25% 15% 5%	506
April	1st to 10th	Day	8% 20% 33% 25% 11% 3	460
	11th to 20th	Day	10% 16% 24% 27% 19% 4	592
	21st to end	Day	11% 17% 31% 21% 17% 3	650
May	1st to 10th	Day	13% 17% 28% 22% 16% 3	653
	11th to 20th	Day	10% 11% 21% 27% 19% 11%	675
	21st to end	Day	10% 10% 26% 25% 22% 7%	795
June	1st to 10th	Day	16% 11% 23% 25% 20% 5%	870
	11th to 20th	Day	16% 11% 25% 25% 17% 6%	870
	21st to end	Day	11% 11% 26% 24% 22% 7%	870
July	1st to 10th	Day	13% 10% 25% 26% 20% 6%	833
	11th to 20th	Day	20% 14% 28% 23% 12% 4	668
	21st to end	Day	18% 16% 24% 21% 15% 6%	739
August	1st to 10th	Day	15% 13% 30% 23% 13% 6%	676
	11th to 20th	Day	27% 22% 26% 17% 7% 1	665
	21st to end	Day	31% 16% 26% 18% 8% 2	720
September	1st to 10th	Day	25% 21% 21% 21% 10% 1	490
	11th to 20th	Day	24% 16% 24% 20% 13% 3	490
	21st to end	Day	34% 21% 23% 13% 6% 2	473
October	1st to 10th	Day	26% 16% 31% 18% 7% 1	390
	11th to 20th	Day	24% 21% 31% 15% 6% 3	390
	21st to end	Day	18% 15% 27% 25% 12% 2	418
November	1st to 10th	Day	15% 17% 29% 23% 14% 3	200
	11th to 20th	Day	15% 12% 29% 22% 18% 6%	200
	21st to end	Day	9% 20% 32% 25% 12% 5%	200
December	1st to 10th	Day	10% 19% 25% 28% 14% 6%	200
	11th to 20th	Day	9% 12% 22% 28% 24% 7%	200
	21st to end	Day	5% 18% 19% 27% 22% 8%	113
January	1st to 10th	Night	8% 12% 26% 30% 20% 4	970
	11th to 20th	Night	13% 16% 29% 25% 14% 3	900
	21st to end	Night	12% 19% 25% 25% 15% 4	940
February	1st to 10th	Night	11% 11% 27% 35% 14% 3	850
	11th to 20th	Night	8% 15% 30% 29% 15% 4	834
	21st to end	Night	6% 11% 28% 33% 18% 4	694
March	1st to 10th	Night	6% 12% 27% 30% 20% 4	760
	11th to 20th	Night	8% 11% 28% 31% 19% 3	727
	21st to end	Night	7% 8% 22% 29% 24% 10%	627
April	1st to 10th	Night	4 9% 22% 33% 27% 5%	570
	11th to 20th	Night	7% 8% 24% 29% 27% 5%	563
	21st to end	Night	9% 7% 22% 30% 24% 6%	570
May	1st to 10th	Night	9% 8% 21% 27% 29% 5%	385
	11th to 20th	Night	8% 8% 18% 30% 26% 9%	374
	21st to end	Night	6% 6% 17% 32% 31% 8%	411
June	1st to 10th	Night	16% 5% 19% 30% 29% 1	328
	11th to 20th	Night	13% 3 22% 28% 32% 3	190
	21st to end	Night	8% 3 19% 31% 36% 3	190
July	1st to 10th	Night	13% 3 19% 33% 30% 1	203
	11th to 20th	Night	17% 8% 17% 30% 25% 2	368
	21st to end	Night	11% 8% 20% 28% 29% 4	416
August	1st to 10th	Night	14% 5% 16% 34% 29% 3	429
	11th to 20th	Night	20% 11% 21% 26% 18% 4	564
	21st to end	Night	27% 8% 18% 26% 18% 3	627
September	1st to 10th	Night	21% 12% 22% 28% 16% 2	570
	11th to 20th	Night	19% 13% 18% 27% 18% 5%	594
	21st to end	Night	26% 17% 21% 21% 11% 4	760
October	1st to 10th	Night	19% 14% 23% 25% 15% 3	760
	11th to 20th	Night	23% 16% 23% 24% 11% 3	760
	21st to end	Night	15% 12% 30% 27% 13% 3	913
November	1st to 10th	Night	13% 12% 29% 29% 14% 2	880
	11th to 20th	Night	14% 12% 24% 28% 18% 4	900
	21st to end	Night	12% 15% 27% 28% 15% 3	960
December	1st to 10th	Night	12% 14% 25% 28% 19% 2	1100
	11th to 20th	Night	12% 9% 25% 33% 17% 4	1100
	21st to end	Night	11% 11% 26% 29% 19% 5%	1210

Page generated at 2025-01-19 00:06:50. Version ac07

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