Juneau
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Juneau clear sky chart from 2005-11-06 to 2025-09-19.

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	34% 18% 4 8% 4 4 3 2 3 4 16%	131208
	Night	33% 20% 4 8% 4 3 3 2 3 3 17%	126430
Transparency	Day	73% 5% 7% 10% 4 1	130875
	Night	74% 4 6% 10% 6% 1	126341
Seeing	Day	49% 7% 13% 17% 11% 2	18179
	Night	51% 8% 16% 15% 9% 2	24658

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	28% 21% 3 8% 4 3 3 2 2 3 22%	2719
	11th to 20th	Day	30% 28% 3 6% 2 2 2 1 1 2 22%	2702
	21st to end	Day	34% 29% 4 9% 4 3 2 2 2 2 7%	3218
February	1st to 10th	Day	23% 24% 3 8% 3 2 2 2 2 4 26%	1520
	11th to 20th	Day	26% 26% 5% 7% 4 3 2 2 3 3 18%	1566
	21st to end	Day	19% 23% 5% 7% 4 3 3 2 2 4 29%	1500
March	1st to 10th	Day	14% 29% 5% 9% 6% 5% 4 3 3 3 20%	4496
	11th to 20th	Day	20% 25% 5% 9% 5% 4 4 2 3 3 21%	4672
	21st to end	Day	13% 18% 4 9% 5% 4 4 3 3 5% 31%	5957
April	1st to 10th	Day	24% 25% 6% 11% 4 4 3 2 3 3 14%	2400
	11th to 20th	Day	22% 19% 4 9% 5% 4 4 3 3 4 22%	2697
	21st to end	Day	29% 22% 5% 7% 5% 3 4 2 3 4 17%	2800
May	1st to 10th	Day	34% 21% 4 9% 4 4 3 2 3 3 13%	6640
	11th to 20th	Day	30% 21% 4 10% 6% 4 4 3 3 3 12%	7193
	21st to end	Day	28% 17% 5% 12% 7% 5% 4 3 3 4 12%	7907
June	1st to 10th	Day	36% 11% 4 9% 5% 4 4 3 4 5% 14%	3200
	11th to 20th	Day	36% 12% 4 9% 6% 4 4 3 4 4 15%	3426
	21st to end	Day	40% 12% 4 7% 5% 4 3 3 4 4 14%	3466
July	1st to 10th	Day	43% 13% 3 8% 4 4 4 3 4 4 10%	7240
	11th to 20th	Day	48% 13% 3 7% 4 3 3 3 3 4 10%	7240
	21st to end	Day	45% 10% 3 7% 3 3 3 3 3 4 17%	7876
August	1st to 10th	Day	29% 9% 2 7% 4 3 3 2 4 5% 32%	4833
	11th to 20th	Day	37% 10% 4 8% 5% 3 4 2 4 5% 19%	4480
	21st to end	Day	54% 11% 3 7% 3 3 2 1 2 3 11%	4949
September	1st to 10th	Day	50% 10% 3 7% 3 3 3 3 3 4 11%	2400
	11th to 20th	Day	42% 10% 4 8% 4 3 3 2 3 4 17%	2370
	21st to end	Day	52% 14% 3 8% 4 4 2 2 2 2 8%	2038
October	1st to 10th	Day	41% 18% 4 10% 4 4 3 1 2 2 11%	3170
	11th to 20th	Day	47% 18% 4 7% 3 3 2 2 2 2 10%	2857
	21st to end	Day	34% 16% 3 7% 4 3 3 2 3 5% 18%	2862
November	1st to 10th	Day	34% 22% 4 7% 4 3 2 2 2 3 16%	1424
	11th to 20th	Day	33% 22% 4 8% 4 3 3 2 2 3 17%	1359
	21st to end	Day	33% 26% 5% 6% 3 4 1 1 3 3 14%	1200
December	1st to 10th	Day	36% 20% 5% 10% 5% 4 4 2 2 3 10%	1935
	11th to 20th	Day	21% 23% 4 9% 4 5% 4 3 3 4 20%	1904
	21st to end	Day	32% 26% 5% 9% 5% 3 2 2 1 2 12%	2992
January	1st to 10th	Night	29% 21% 4 7% 3 2 2 2 2 3 24%	7226
	11th to 20th	Night	30% 23% 4 6% 3 3 3 2 3 3 21%	7196
	21st to end	Night	38% 29% 5% 9% 4 2 2 1 1 2 6%	7744
February	1st to 10th	Night	27% 19% 4 8% 4 3 2 2 3 3 24%	2879
	11th to 20th	Night	28% 26% 4 7% 5% 3 2 1 2 2 19%	2717
	21st to end	Night	20% 26% 5% 7% 4 3 3 2 2 3 27%	2130
March	1st to 10th	Night	17% 28% 4 7% 4 4 3 2 3 4 24%	5378
	11th to 20th	Night	19% 23% 6% 10% 5% 4 3 3 4 4 19%	5285
	21st to end	Night	19% 17% 3 8% 4 4 3 2 4 5% 30%	4995
April	1st to 10th	Night	25% 25% 4 10% 5% 3 4 2 3 3 16%	2000
	11th to 20th	Night	19% 20% 5% 11% 6% 4 4 3 3 3 23%	1649
	21st to end	Night	29% 22% 4 8% 4 3 3 2 3 3 19%	1600
May	1st to 10th	Night	38% 19% 3 7% 4 3 3 2 2 4 15%	3312
	11th to 20th	Night	26% 24% 5% 9% 4 4 5% 3 3 3 14%	2702
	21st to end	Night	36% 17% 5% 10% 5% 3 3 2 4 4 13%	2968
June	1st to 10th	Night	35% 14% 4 9% 5% 3 3 2 4 4 18%	1195
	11th to 20th	Night	38% 12% 4 8% 4 4 4 2 4 5% 16%	965
	21st to end	Night	42% 10% 3 6% 5% 4 4 3 4 4 13%	921
July	1st to 10th	Night	49% 12% 3 6% 3 3 3 2 2 3 11%	2720
	11th to 20th	Night	51% 10% 3 6% 3 3 3 2 2 4 14%	2720
	21st to end	Night	53% 8% 2 6% 3 3 2 2 3 3 17%	2967
August	1st to 10th	Night	30% 8% 3 6% 4 3 2 2 3 5% 34%	2322
	11th to 20th	Night	39% 10% 3 6% 3 4 3 3 3 5% 23%	2578
	21st to end	Night	55% 11% 2 5% 3 2 2 2 2 3 12%	2958
September	1st to 10th	Night	51% 10% 3 7% 4 3 3 2 2 3 13%	2000
	11th to 20th	Night	41% 12% 3 8% 4 4 3 3 3 3 17%	1986
	21st to end	Night	53% 14% 4 6% 3 2 2 2 2 2 9%	2139
October	1st to 10th	Night	42% 16% 4 9% 4 3 3 3 2 2 10%	3800
	11th to 20th	Night	50% 19% 4 8% 4 3 2 1 2 2 7%	4071
	21st to end	Night	33% 21% 3 6% 4 3 3 2 2 3 20%	4696
November	1st to 10th	Night	36% 20% 4 8% 3 2 2 3 2 3 16%	2874
	11th to 20th	Night	32% 20% 4 9% 5% 3 3 2 3 3 16%	3040
	21st to end	Night	33% 24% 4 7% 4 3 2 2 4 3 13%	3200
December	1st to 10th	Night	41% 17% 4 8% 4 3 3 2 2 3 14%	5220
	11th to 20th	Night	23% 20% 4 9% 5% 5% 4 3 4 4 19%	6295
	21st to end	Night	29% 25% 5% 10% 5% 4 4 2 2 2 12%	7982

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% 3 4 11% 3 8%	2719
	11th to 20th	Day	72% 2 2 8% 7% 8%	2720
	21st to end	Day	87% 2 2 5% 4 1	3218
February	1st to 10th	Day	65% 3 4 12% 13% 3	1520
	11th to 20th	Day	72% 3 5% 8% 9% 3	1574
	21st to end	Day	64% 2 7% 14% 10% 3	1500
March	1st to 10th	Day	70% 3 5% 8% 12% 2	4496
	11th to 20th	Day	69% 3 4 8% 16%	4672
	21st to end	Day	56% 4 7% 25% 7% 1	5957
April	1st to 10th	Day	77% 4 6% 9% 4	2400
	11th to 20th	Day	65% 4 8% 13% 9% 1	2697
	21st to end	Day	74% 4 8% 11% 4	2800
May	1st to 10th	Day	77% 5% 4 12% 3	6640
	11th to 20th	Day	74% 6% 5% 10% 4	7196
	21st to end	Day	74% 6% 8% 9% 3	7908
June	1st to 10th	Day	74% 5% 9% 8% 2 1	3200
	11th to 20th	Day	73% 5% 10% 11% 1	3426
	21st to end	Day	74% 5% 13% 7% 1	3466
July	1st to 10th	Day	76% 8% 10% 6% 1	7161
	11th to 20th	Day	78% 7% 11% 4	7153
	21st to end	Day	72% 5% 18% 4	7655
August	1st to 10th	Day	53% 7% 11% 20% 9%	4814
	11th to 20th	Day	66% 6% 12% 13% 3	4522
	21st to end	Day	82% 3 7% 7% 1	4948
September	1st to 10th	Day	80% 4 7% 8% 1	2400
	11th to 20th	Day	72% 5% 6% 13% 4	2370
	21st to end	Day	86% 3 4 6% 1	2038
October	1st to 10th	Day	83% 3 5% 7% 2	3170
	11th to 20th	Day	84% 3 3 6% 3	2857
	21st to end	Day	72% 3 10% 13% 3	2862
November	1st to 10th	Day	77% 2 5% 11% 4	1424
	11th to 20th	Day	75% 3 6% 10% 6%	1359
	21st to end	Day	79% 2 3 8% 7% 1	1200
December	1st to 10th	Day	83% 2 6% 6% 3	1935
	11th to 20th	Day	69% 6% 5% 10% 7% 3	1906
	21st to end	Day	83% 2 2 6% 5% 1	2992
January	1st to 10th	Night	68% 2 4 14% 7% 5%	7226
	11th to 20th	Night	69% 4 4 9% 9% 6%	7240
	21st to end	Night	89% 1 2 4 2 1	7744
February	1st to 10th	Night	67% 3 4 11% 11% 4	2879
	11th to 20th	Night	74% 2 4 6% 11% 3	2720
	21st to end	Night	65% 2 5% 16% 9% 4	2130
March	1st to 10th	Night	66% 3 4 14% 11% 2	5378
	11th to 20th	Night	69% 4 4 8% 16%	5287
	21st to end	Night	58% 4 9% 22% 6% 1	4995
April	1st to 10th	Night	74% 5% 6% 11% 4	2000
	11th to 20th	Night	66% 5% 8% 11% 9% 1	1649
	21st to end	Night	73% 4 8% 10% 5%	1600
May	1st to 10th	Night	75% 4 5% 12% 4	3309
	11th to 20th	Night	72% 8% 6% 10% 4	2702
	21st to end	Night	74% 6% 8% 11% 1	2968
June	1st to 10th	Night	73% 5% 10% 8% 4 1	1195
	11th to 20th	Night	72% 6% 12% 9% 1	965
	21st to end	Night	76% 5% 12% 6% 1	921
July	1st to 10th	Night	76% 8% 10% 6% 1	2690
	11th to 20th	Night	72% 9% 16% 2 1	2684
	21st to end	Night	74% 5% 17% 3	2883
August	1st to 10th	Night	52% 9% 14% 15% 9%	2316
	11th to 20th	Night	62% 8% 16% 12% 3	2602
	21st to end	Night	79% 4 8% 7% 1	2958
September	1st to 10th	Night	79% 4 9% 7% 2	2000
	11th to 20th	Night	74% 6% 5% 10% 6%	1986
	21st to end	Night	85% 2 5% 6% 2	2139
October	1st to 10th	Night	83% 3 4 7% 3	3800
	11th to 20th	Night	89% 2 3 5% 2	4071
	21st to end	Night	73% 2 7% 11% 6%	4696
November	1st to 10th	Night	76% 2 5% 12% 5%	2874
	11th to 20th	Night	76% 4 4 10% 6% 1	3040
	21st to end	Night	79% 3 4 8% 6% 1	3200
December	1st to 10th	Night	79% 2 6% 8% 5%	5220
	11th to 20th	Night	70% 4 4 12% 10% 1	6292
	21st to end	Night	81% 4 2 8% 4 1	7982

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	40% 13% 25% 16% 5% 1	200
	11th to 20th	Day	58% 11% 20% 6% 6% 1	200
	21st to end	Day	56% 8% 16% 15% 6%	316
February	1st to 10th	Day	42% 15% 24% 14% 5% 1	400
	11th to 20th	Day	43% 11% 20% 21% 4 1	395
	21st to end	Day	40% 11% 26% 15% 8%	331
March	1st to 10th	Day	33% 15% 23% 22% 7% 1	400
	11th to 20th	Day	42% 11% 21% 19% 6% 1	400
	21st to end	Day	36% 9% 19% 21% 11% 3	546
April	1st to 10th	Day	42% 13% 18% 16% 9% 1	500
	11th to 20th	Day	34% 8% 19% 20% 16% 3	638
	21st to end	Day	46% 9% 13% 15% 13% 4	700
May	1st to 10th	Day	51% 7% 9% 20% 11% 4	703
	11th to 20th	Day	36% 5% 10% 24% 20% 4	725
	21st to end	Day	38% 4 11% 23% 19% 4	790
June	1st to 10th	Day	51% 5% 9% 16% 16% 3	730
	11th to 20th	Day	46% 5% 10% 20% 15% 2	930
	21st to end	Day	47% 4 7% 18% 20% 4	831
July	1st to 10th	Day	48% 5% 8% 20% 16% 4	721
	11th to 20th	Day	62% 5% 9% 12% 11% 1	718
	21st to end	Day	57% 5% 8% 16% 10% 4	794
August	1st to 10th	Day	47% 5% 12% 17% 15% 4	726
	11th to 20th	Day	59% 8% 8% 14% 10% 2	715
	21st to end	Day	60% 5% 8% 17% 9% 1	773
September	1st to 10th	Day	54% 7% 12% 17% 10% 1	530
	11th to 20th	Day	54% 5% 10% 17% 11% 2	523
	21st to end	Day	68% 5% 12% 11% 4 1	463
October	1st to 10th	Day	54% 8% 16% 15% 7% 1	390
	11th to 20th	Day	69% 9% 11% 6% 3 2	390
	21st to end	Day	46% 10% 21% 18% 5% 1	429
November	1st to 10th	Day	53% 10% 18% 15% 3	252
	11th to 20th	Day	56% 5% 21% 12% 7%	200
	21st to end	Day	62% 14% 14% 9% 3	200
December	1st to 10th	Day	53% 8% 20% 15% 5% 1	200
	11th to 20th	Day	49% 13% 21% 11% 6% 1	200
	21st to end	Day	48% 14% 15% 16% 7%	220
January	1st to 10th	Night	47% 13% 24% 13% 3	900
	11th to 20th	Night	59% 11% 18% 9% 3 1	900
	21st to end	Night	53% 10% 19% 12% 6%	990
February	1st to 10th	Night	42% 13% 27% 14% 5%	900
	11th to 20th	Night	45% 11% 22% 17% 4	884
	21st to end	Night	40% 12% 24% 19% 6%	735
March	1st to 10th	Night	37% 13% 25% 17% 8% 1	800
	11th to 20th	Night	41% 10% 22% 20% 6% 1	738
	21st to end	Night	36% 6% 19% 21% 14% 3	660
April	1st to 10th	Night	47% 7% 16% 15% 12% 4	600
	11th to 20th	Night	34% 6% 20% 23% 15% 2	593
	21st to end	Night	46% 8% 12% 17% 15% 3	600
May	1st to 10th	Night	49% 7% 8% 17% 15% 3	479
	11th to 20th	Night	36% 6% 13% 20% 22% 4	394
	21st to end	Night	35% 6% 6% 24% 22% 5%	433
June	1st to 10th	Night	47% 8% 5% 17% 20% 4	400
	11th to 20th	Night	47% 6% 7% 19% 17% 4	365
	21st to end	Night	50% 7% 6% 12% 22% 4	228
July	1st to 10th	Night	45% 5% 6% 21% 17% 6%	390
	11th to 20th	Night	55% 3 5% 16% 16% 5%	388
	21st to end	Night	56% 3 6% 16% 14% 5%	438
August	1st to 10th	Night	49% 4 6% 17% 18% 6%	540
	11th to 20th	Night	56% 7% 6% 14% 12% 4	594
	21st to end	Night	61% 5% 5% 16% 12% 2	660
September	1st to 10th	Night	60% 4 8% 13% 13% 2	600
	11th to 20th	Night	55% 6% 9% 13% 15% 3	600
	21st to end	Night	68% 6% 10% 8% 7% 2	760
October	1st to 10th	Night	57% 5% 11% 17% 8% 1	760
	11th to 20th	Night	70% 5% 10% 8% 4 1	760
	21st to end	Night	55% 8% 19% 14% 4 1	909
November	1st to 10th	Night	56% 9% 19% 14% 2	880
	11th to 20th	Night	50% 10% 20% 14% 6% 1	900
	21st to end	Night	59% 9% 16% 11% 4	900
December	1st to 10th	Night	53% 6% 22% 15% 3	900
	11th to 20th	Night	49% 10% 22% 14% 4	1085
	21st to end	Night	54% 10% 19% 11% 5%	995

Page generated at 2025-09-15 21:49:10. Version ac07

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