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 2024-11-05.

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	36% 17% 4 8% 4 3 3 2 3 4 16%	106323
	Night	35% 19% 4 7% 4 3 3 2 3 3 17%	101214
Transparency	Day	73% 5% 7% 11% 4 1	106038
	Night	73% 4 6% 10% 5% 1	101146
Seeing	Day	49% 7% 13% 17% 12% 2	16971
	Night	51% 8% 16% 15% 9% 2	23551

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	24% 19% 4 8% 4 3 4 3 3 4 24%	1899
	11th to 20th	Day	31% 23% 3 4 2 2 2 1 1 2 28%	1882
	21st to end	Day	42% 27% 4 6% 3 2 2 2 2 2 8%	2246
February	1st to 10th	Day	25% 25% 4 8% 3 2 2 2 2 4 23%	1444
	11th to 20th	Day	26% 27% 5% 7% 4 3 2 2 3 4 19%	1485
	21st to end	Day	18% 23% 5% 7% 4 2 3 2 3 4 30%	1427
March	1st to 10th	Day	13% 28% 4 7% 4 3 4 3 4 4 27%	3126
	11th to 20th	Day	26% 24% 5% 9% 5% 3 3 2 2 3 16%	3263
	21st to end	Day	16% 19% 5% 8% 4 3 3 3 4 5% 31%	4153
April	1st to 10th	Day	25% 25% 6% 11% 4 4 3 2 3 3 14%	2280
	11th to 20th	Day	22% 19% 4 9% 5% 4 4 3 4 4 23%	2560
	21st to end	Day	28% 22% 5% 8% 5% 3 4 2 3 4 18%	2660
May	1st to 10th	Day	33% 16% 3 7% 4 4 3 2 3 4 19%	4638
	11th to 20th	Day	29% 20% 4 10% 6% 4 4 3 3 3 14%	5013
	21st to end	Day	25% 14% 4 10% 7% 6% 4 4 5% 5% 16%	5509
June	1st to 10th	Day	35% 11% 4 9% 5% 5% 4 3 4 5% 14%	3040
	11th to 20th	Day	37% 11% 4 9% 6% 4 4 3 4 4 15%	3254
	21st to end	Day	40% 13% 4 7% 5% 4 3 3 4 4 14%	3293
July	1st to 10th	Day	44% 13% 3 7% 4 3 3 3 4 4 13%	5060
	11th to 20th	Day	56% 12% 3 7% 4 3 3 2 2 3 6%	5060
	21st to end	Day	53% 11% 3 7% 3 3 3 2 3 3 10%	5486
August	1st to 10th	Day	28% 9% 2 6% 4 3 3 2 4 5% 33%	4684
	11th to 20th	Day	37% 10% 3 8% 5% 3 4 2 4 4 19%	4340
	21st to end	Day	55% 11% 3 7% 3 3 2 1 2 3 10%	4798
September	1st to 10th	Day	52% 10% 3 7% 4 4 3 3 3 3 10%	2280
	11th to 20th	Day	41% 10% 4 8% 4 3 3 2 3 4 17%	2273
	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%	1382
	11th to 20th	Day	34% 22% 4 7% 3 2 3 2 2 3 17%	1292
	21st to end	Day	35% 26% 5% 6% 3 4 1 1 3 3 11%	1140
December	1st to 10th	Day	34% 22% 5% 7% 4 2 3 2 3 4 16%	1129
	11th to 20th	Day	27% 24% 3 7% 3 4 4 3 3 3 19%	1210
	21st to end	Day	38% 26% 5% 8% 4 3 2 2 1 2 8%	2090
January	1st to 10th	Night	24% 22% 4 7% 4 2 2 2 3 4 25%	5046
	11th to 20th	Night	28% 21% 3 6% 3 3 3 2 3 2 27%	5016
	21st to end	Night	46% 23% 4 6% 3 2 2 1 2 3 8%	5411
February	1st to 10th	Night	28% 20% 4 8% 4 3 3 2 3 3 21%	2736
	11th to 20th	Night	29% 27% 4 7% 5% 3 2 1 2 2 19%	2578
	21st to end	Night	19% 26% 5% 7% 4 3 3 2 2 3 28%	2027
March	1st to 10th	Night	14% 29% 3 5% 4 3 2 2 3 4 31%	3738
	11th to 20th	Night	26% 27% 6% 10% 5% 3 2 2 3 2 13%	3687
	21st to end	Night	21% 15% 3 7% 3 3 3 2 5% 6% 31%	3488
April	1st to 10th	Night	25% 26% 5% 10% 5% 3 3 2 3 3 15%	1900
	11th to 20th	Night	19% 19% 5% 11% 6% 4 4 3 3 3 24%	1566
	21st to end	Night	29% 21% 4 8% 4 3 3 3 3 3 20%	1520
May	1st to 10th	Night	36% 14% 2 6% 3 3 3 2 3 5% 22%	2310
	11th to 20th	Night	23% 23% 6% 10% 5% 4 5% 3 3 4 15%	1882
	21st to end	Night	29% 14% 5% 10% 6% 3 4 2 5% 5% 18%	2066
June	1st to 10th	Night	34% 14% 4 9% 5% 3 3 3 4 4 19%	1135
	11th to 20th	Night	39% 11% 3 9% 5% 4 3 2 4 4 16%	917
	21st to end	Night	42% 9% 3 6% 5% 4 5% 3 5% 4 13%	874
July	1st to 10th	Night	49% 11% 2 5% 3 3 3 2 3 4 15%	1900
	11th to 20th	Night	61% 9% 3 5% 4 2 2 2 2 3 7%	1900
	21st to end	Night	61% 8% 2 5% 3 2 2 1 3 2 11%	2066
August	1st to 10th	Night	30% 8% 3 6% 4 3 2 2 3 5% 34%	2251
	11th to 20th	Night	39% 10% 3 6% 3 3 3 3 3 5% 23%	2498
	21st to end	Night	57% 11% 2 5% 3 2 2 2 2 3 11%	2867
September	1st to 10th	Night	52% 10% 4 7% 4 3 3 2 2 3 12%	1900
	11th to 20th	Night	41% 12% 3 8% 4 4 3 3 3 3 17%	1906
	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 3 3 17%	2796
	11th to 20th	Night	33% 20% 4 8% 5% 3 3 2 3 3 16%	2888
	21st to end	Night	35% 24% 4 8% 4 3 2 2 4 3 10%	3040
December	1st to 10th	Night	35% 21% 4 7% 3 3 2 2 2 4 17%	3040
	11th to 20th	Night	28% 23% 5% 9% 5% 4 3 2 3 3 16%	3975
	21st to end	Night	35% 27% 5% 8% 4 3 2 2 2 2 11%	5584

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	66% 4 5% 16% 4 6%	1899
	11th to 20th	Day	67% 1 3 11% 7% 11%	1900
	21st to end	Day	85% 3 2 7% 3	2246
February	1st to 10th	Day	68% 3 4 10% 12% 3	1444
	11th to 20th	Day	72% 3 5% 8% 9% 3	1493
	21st to end	Day	62% 2 7% 15% 11% 3	1427
March	1st to 10th	Day	63% 4 4 10% 16% 3	3126
	11th to 20th	Day	75% 3 4 10% 7% 1	3263
	21st to end	Day	55% 5% 8% 27% 5%	4153
April	1st to 10th	Day	77% 4 5% 9% 4	2280
	11th to 20th	Day	64% 4 8% 13% 9% 1	2560
	21st to end	Day	73% 4 8% 12% 4	2660
May	1st to 10th	Day	68% 6% 5% 17% 5%	4638
	11th to 20th	Day	71% 6% 5% 12% 5%	5016
	21st to end	Day	65% 7% 11% 13% 4	5510
June	1st to 10th	Day	73% 5% 10% 9% 3 1	3040
	11th to 20th	Day	73% 5% 10% 10% 1	3254
	21st to end	Day	74% 5% 13% 7% 1	3293
July	1st to 10th	Day	70% 10% 12% 7% 1	4981
	11th to 20th	Day	87% 4 6% 3	4973
	21st to end	Day	80% 5% 9% 6%	5313
August	1st to 10th	Day	53% 7% 11% 20% 9%	4665
	11th to 20th	Day	66% 6% 12% 13% 3	4382
	21st to end	Day	84% 3 6% 6% 1	4797
September	1st to 10th	Day	82% 4 6% 8%	2280
	11th to 20th	Day	71% 5% 6% 13% 4	2273
	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	76% 2 5% 12% 4	1382
	11th to 20th	Day	75% 3 6% 10% 6%	1292
	21st to end	Day	82% 2 3 6% 6% 1	1140
December	1st to 10th	Day	75% 2 9% 9% 5%	1129
	11th to 20th	Day	72% 4 4 9% 10% 1	1212
	21st to end	Day	87% 2 2 4 3 2	2090
January	1st to 10th	Night	65% 3 5% 19% 7% 2	5046
	11th to 20th	Night	64% 2 4 10% 12% 8%	5060
	21st to end	Night	86% 2 3 5% 3	5411
February	1st to 10th	Night	70% 3 4 10% 9% 4	2736
	11th to 20th	Night	74% 2 4 6% 10% 3	2581
	21st to end	Night	63% 2 5% 17% 9% 4	2027
March	1st to 10th	Night	58% 3 4 17% 16% 3	3738
	11th to 20th	Night	79% 4 2 10% 4	3689
	21st to end	Night	55% 5% 12% 24% 4	3488
April	1st to 10th	Night	75% 5% 5% 11% 4	1900
	11th to 20th	Night	65% 4 8% 11% 10% 1	1566
	21st to end	Night	71% 4 9% 10% 6%	1520
May	1st to 10th	Night	65% 4 8% 17% 6%	2310
	11th to 20th	Night	70% 8% 6% 12% 4	1882
	21st to end	Night	66% 7% 11% 15% 2	2066
June	1st to 10th	Night	72% 5% 10% 8% 4 1	1135
	11th to 20th	Night	72% 6% 12% 9% 1	917
	21st to end	Night	76% 5% 12% 6% 1	874
July	1st to 10th	Night	72% 8% 11% 7% 1	1870
	11th to 20th	Night	86% 4 8% 2	1864
	21st to end	Night	81% 3 11% 5%	2000
August	1st to 10th	Night	51% 9% 14% 15% 10%	2245
	11th to 20th	Night	62% 8% 16% 12% 3	2522
	21st to end	Night	81% 4 7% 7% 1	2867
September	1st to 10th	Night	81% 4 8% 6% 1	1900
	11th to 20th	Night	73% 6% 5% 10% 6%	1906
	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% 3 5% 12% 5%	2796
	11th to 20th	Night	76% 4 4 10% 6% 1	2888
	21st to end	Night	81% 3 4 7% 5% 1	3040
December	1st to 10th	Night	75% 3 8% 9% 5%	3040
	11th to 20th	Night	76% 4 4 8% 9% 1	3972
	21st to end	Night	82% 4 2 6% 4 2	5584

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	39% 12% 26% 17% 5% 1	180
	11th to 20th	Day	57% 10% 19% 7% 7% 1	180
	21st to end	Day	57% 8% 13% 15% 7%	289
February	1st to 10th	Day	44% 15% 22% 14% 5% 1	370
	11th to 20th	Day	44% 12% 20% 20% 4 1	365
	21st to end	Day	37% 12% 27% 15% 8%	307
March	1st to 10th	Day	31% 15% 24% 23% 7% 1	370
	11th to 20th	Day	44% 10% 21% 17% 6% 1	370
	21st to end	Day	36% 9% 18% 22% 12% 4	502
April	1st to 10th	Day	42% 13% 18% 17% 10% 2	460
	11th to 20th	Day	33% 7% 19% 21% 17% 4	590
	21st to end	Day	45% 8% 14% 16% 14% 4	650
May	1st to 10th	Day	49% 6% 9% 21% 11% 4	653
	11th to 20th	Day	35% 5% 10% 24% 21% 4	675
	21st to end	Day	36% 4 12% 24% 21% 4	735
June	1st to 10th	Day	49% 5% 9% 17% 17% 3	680
	11th to 20th	Day	47% 5% 9% 20% 16% 3	870
	21st to end	Day	46% 4 6% 18% 20% 5%	776
July	1st to 10th	Day	46% 5% 8% 21% 16% 4	671
	11th to 20th	Day	64% 4 7% 12% 12% 1	668
	21st to end	Day	58% 5% 7% 15% 11% 4	739
August	1st to 10th	Day	47% 5% 11% 17% 16% 4	676
	11th to 20th	Day	60% 7% 7% 14% 10% 2	665
	21st to end	Day	63% 5% 7% 16% 8% 1	720
September	1st to 10th	Day	57% 6% 11% 16% 9% 1	490
	11th to 20th	Day	53% 6% 11% 17% 11% 2	490
	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	52% 10% 19% 16% 3	240
	11th to 20th	Day	58% 4 19% 12% 7%	180
	21st to end	Day	63% 13% 12% 10% 3	180
December	1st to 10th	Day	51% 8% 19% 16% 5% 1	180
	11th to 20th	Day	52% 13% 19% 11% 4 1	180
	21st to end	Day	48% 12% 16% 16% 8% 1	198
January	1st to 10th	Night	46% 13% 24% 12% 3	850
	11th to 20th	Night	59% 11% 18% 9% 3 1	850
	21st to end	Night	54% 10% 18% 12% 6%	935
February	1st to 10th	Night	44% 12% 25% 14% 5%	850
	11th to 20th	Night	46% 11% 21% 16% 5%	834
	21st to end	Night	38% 11% 25% 20% 6%	696
March	1st to 10th	Night	36% 12% 26% 16% 9% 1	760
	11th to 20th	Night	42% 10% 23% 17% 5% 1	701
	21st to end	Night	36% 6% 19% 21% 15% 4	627
April	1st to 10th	Night	48% 6% 15% 15% 13% 4	570
	11th to 20th	Night	34% 5% 20% 23% 15% 2	563
	21st to end	Night	46% 7% 12% 17% 15% 3	570
May	1st to 10th	Night	48% 7% 8% 18% 16% 3	455
	11th to 20th	Night	35% 6% 13% 20% 22% 3	374
	21st to end	Night	33% 6% 6% 26% 23% 6%	411
June	1st to 10th	Night	45% 8% 5% 17% 21% 4	380
	11th to 20th	Night	48% 6% 6% 18% 18% 4	347
	21st to end	Night	48% 7% 6% 13% 22% 4	216
July	1st to 10th	Night	44% 5% 5% 22% 18% 6%	370
	11th to 20th	Night	57% 3 4 15% 15% 5%	368
	21st to end	Night	56% 3 5% 16% 14% 6%	416
August	1st to 10th	Night	48% 4 7% 16% 19% 6%	513
	11th to 20th	Night	57% 7% 6% 13% 13% 4	564
	21st to end	Night	63% 5% 4 14% 12% 2	627
September	1st to 10th	Night	61% 4 8% 13% 11% 2	570
	11th to 20th	Night	54% 6% 9% 13% 15% 3	576
	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% 8% 19% 14% 2	854
	11th to 20th	Night	51% 10% 19% 14% 5% 1	850
	21st to end	Night	60% 8% 16% 11% 5%	850
December	1st to 10th	Night	52% 6% 22% 15% 4	850
	11th to 20th	Night	50% 10% 21% 14% 4	1025
	21st to end	Night	53% 10% 20% 11% 6%	940

Page generated at 2024-11-02 22:11:12. Version ac07

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