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 2024-01-12.

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% 16% 4 7% 5% 3 3 3 3 4 16%	74723
	Night	35% 18% 4 7% 5% 2 3 3 3 3 17%	74965
Transparency	Day	74% 4 8% 9% 4 1	74378
	Night	75% 4 6% 9% 5% 1	74829
Seeing	Day	49% 7% 12% 17% 12% 2	15749
	Night	51% 8% 16% 15% 9% 2	22585

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	32% 21% 4 6% 3 2 2 3 3 4 20%	1139
	11th to 20th	Day	43% 20% 4 5% 4 2 2 3 2 3 13%	1086
	21st to end	Day	33% 23% 4 6% 4 2 3 3 3 4 15%	1274
February	1st to 10th	Day	25% 23% 4 7% 4 2 3 3 2 3 24%	1368
	11th to 20th	Day	28% 26% 5% 7% 5% 2 1 2 3 3 18%	1405
	21st to end	Day	18% 22% 5% 6% 4 2 3 2 2 4 31%	1345
March	1st to 10th	Day	17% 22% 5% 7% 5% 3 4 3 4 4 28%	1800
	11th to 20th	Day	21% 25% 7% 8% 6% 3 4 3 3 4 16%	1854
	21st to end	Day	19% 23% 7% 7% 7% 3 4 4 4 5% 18%	2364
April	1st to 10th	Day	26% 23% 6% 10% 5% 3 4 3 3 3 15%	2160
	11th to 20th	Day	24% 17% 5% 8% 6% 3 4 4 4 5% 21%	2422
	21st to end	Day	30% 20% 5% 7% 6% 2 4 3 3 4 17%	2520
May	1st to 10th	Day	36% 14% 4 8% 6% 4 5% 4 4 5% 12%	2636
	11th to 20th	Day	29% 12% 4 7% 6% 3 4 5% 3 5% 22%	2833
	21st to end	Day	26% 10% 4 7% 6% 4 4 5% 5% 6% 24%	3111
June	1st to 10th	Day	36% 11% 3 8% 6% 3 5% 4 4 6% 15%	2880
	11th to 20th	Day	39% 11% 4 8% 6% 3 4 4 4 4 14%	3082
	21st to end	Day	41% 12% 4 6% 6% 3 4 4 4 4 13%	3120
July	1st to 10th	Day	37% 12% 3 6% 5% 3 4 3 4 4 19%	2880
	11th to 20th	Day	46% 13% 4 7% 5% 3 4 3 2 4 9%	2880
	21st to end	Day	46% 11% 4 7% 4 2 3 3 3 3 14%	3135
August	1st to 10th	Day	40% 11% 4 6% 5% 3 3 3 4 5% 16%	2672
	11th to 20th	Day	48% 12% 3 7% 6% 3 3 2 3 2 11%	2476
	21st to end	Day	54% 12% 3 5% 4 2 2 2 2 3 11%	2738
September	1st to 10th	Day	53% 9% 3 6% 4 3 3 3 3 4 10%	2160
	11th to 20th	Day	41% 10% 4 7% 5% 2 3 3 3 4 18%	2154
	21st to end	Day	52% 13% 4 7% 4 2 3 3 2 2 8%	1931
October	1st to 10th	Day	41% 13% 5% 9% 5% 3 4 2 2 3 14%	1800
	11th to 20th	Day	53% 15% 4 7% 4 2 2 2 1 2 9%	1634
	21st to end	Day	40% 17% 4 6% 5% 2 2 2 2 4 15%	1651
November	1st to 10th	Day	35% 20% 5% 6% 5% 2 3 2 2 3 17%	1352
	11th to 20th	Day	35% 20% 4 6% 4 2 2 3 2 3 17%	1292
	21st to end	Day	35% 26% 5% 6% 4 3 2 1 2 4 11%	1140
December	1st to 10th	Day	34% 21% 5% 6% 4 2 2 3 2 4 16%	1129
	11th to 20th	Day	28% 23% 4 6% 4 3 4 4 3 3 19%	1210
	21st to end	Day	39% 25% 6% 7% 6% 2 2 2 1 2 8%	2090
January	1st to 10th	Night	31% 21% 5% 6% 4 2 2 3 2 4 21%	3026
	11th to 20th	Night	41% 19% 4 6% 4 2 3 3 2 2 13%	2902
	21st to end	Night	36% 21% 5% 7% 5% 2 3 3 3 4 13%	3078
February	1st to 10th	Night	29% 19% 4 7% 5% 2 3 3 3 3 22%	2592
	11th to 20th	Night	31% 25% 4 6% 5% 2 2 2 2 2 18%	2438
	21st to end	Night	20% 24% 6% 5% 4 2 3 2 2 2 30%	1911
March	1st to 10th	Night	18% 25% 5% 6% 4 2 3 3 2 3 29%	2160
	11th to 20th	Night	21% 26% 7% 10% 6% 2 3 3 4 3 15%	2103
	21st to end	Night	19% 21% 5% 8% 5% 3 5% 4 5% 4 20%	1991
April	1st to 10th	Night	27% 23% 5% 8% 6% 2 3 3 3 4 16%	1800
	11th to 20th	Night	21% 17% 5% 10% 8% 4 4 4 3 3 22%	1484
	21st to end	Night	31% 19% 5% 6% 5% 2 3 3 3 3 19%	1440
May	1st to 10th	Night	40% 11% 3 7% 6% 3 3 4 4 5% 16%	1322
	11th to 20th	Night	27% 11% 4 7% 5% 2 5% 4 4 5% 25%	1062
	21st to end	Night	26% 11% 4 7% 5% 1 4 4 7% 6% 26%	1164
June	1st to 10th	Night	34% 13% 4 8% 6% 2 3 3 4 4 20%	1076
	11th to 20th	Night	41% 11% 4 8% 5% 2 4 2 3 5% 14%	869
	21st to end	Night	43% 8% 3 5% 6% 4 4 4 5% 4 13%	827
July	1st to 10th	Night	38% 10% 3 5% 4 2 4 3 4 6% 20%	1080
	11th to 20th	Night	47% 9% 4 5% 6% 2 4 3 3 5% 13%	1080
	21st to end	Night	48% 8% 3 6% 4 2 2 2 3 3 18%	1182
August	1st to 10th	Night	39% 11% 4 6% 6% 2 2 3 3 5% 18%	1264
	11th to 20th	Night	50% 10% 4 5% 4 3 3 2 3 4 13%	1424
	21st to end	Night	52% 11% 3 5% 4 2 2 2 2 3 15%	1616
September	1st to 10th	Night	52% 10% 4 5% 5% 2 2 2 2 3 13%	1800
	11th to 20th	Night	41% 11% 3 7% 5% 3 4 3 3 3 18%	1805
	21st to end	Night	53% 14% 4 6% 3 1 2 2 2 2 10%	2026
October	1st to 10th	Night	44% 11% 4 6% 5% 2 2 4 3 3 15%	2160
	11th to 20th	Night	52% 15% 4 7% 4 2 2 2 2 3 9%	2326
	21st to end	Night	39% 18% 4 6% 4 2 3 2 2 3 17%	2704
November	1st to 10th	Night	37% 18% 4 7% 4 1 3 3 2 3 17%	2726
	11th to 20th	Night	34% 19% 5% 8% 5% 2 3 3 3 3 16%	2888
	21st to end	Night	35% 24% 4 7% 5% 3 3 3 3 4 11%	3040
December	1st to 10th	Night	35% 20% 4 6% 4 2 3 3 2 4 17%	3040
	11th to 20th	Night	28% 22% 5% 7% 6% 3 4 3 3 3 16%	3975
	21st to end	Night	36% 26% 5% 7% 5% 2 3 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	71% 3 4 15% 4 4	1139
	11th to 20th	Day	82% 1 3 7% 5% 1	1086
	21st to end	Day	75% 4 4 11% 6% 1	1274
February	1st to 10th	Day	68% 3 4 10% 12% 3	1368
	11th to 20th	Day	73% 3 5% 8% 9% 2	1413
	21st to end	Day	61% 2 8% 15% 10% 3	1345
March	1st to 10th	Day	62% 4 5% 10% 14% 5%	1800
	11th to 20th	Day	74% 4 4 9% 8% 1	1854
	21st to end	Day	70% 4 7% 12% 6%	2364
April	1st to 10th	Day	77% 4 5% 9% 4	2160
	11th to 20th	Day	65% 4 9% 13% 8%	2422
	21st to end	Day	74% 3 8% 11% 4	2520
May	1st to 10th	Day	74% 5% 7% 10% 3	2636
	11th to 20th	Day	64% 4 7% 18% 6% 1	2836
	21st to end	Day	60% 6% 15% 14% 5%	3112
June	1st to 10th	Day	73% 5% 10% 9% 3 1	2880
	11th to 20th	Day	76% 5% 9% 10% 1	3082
	21st to end	Day	75% 5% 12% 7% 1	3120
July	1st to 10th	Day	69% 6% 14% 9% 2	2801
	11th to 20th	Day	83% 5% 9% 3	2793
	21st to end	Day	75% 5% 11% 8%	2961
August	1st to 10th	Day	72% 6% 12% 9% 1	2653
	11th to 20th	Day	80% 4 8% 7% 1	2476
	21st to end	Day	82% 4 9% 5% 1	2738
September	1st to 10th	Day	81% 4 7% 8%	2160
	11th to 20th	Day	71% 5% 6% 13% 4	2154
	21st to end	Day	86% 3 4 6% 1	1931
October	1st to 10th	Day	78% 4 7% 8% 4	1800
	11th to 20th	Day	86% 2 4 4 5%	1634
	21st to end	Day	77% 2 5% 11% 5%	1651
November	1st to 10th	Day	76% 2 5% 12% 5%	1352
	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	71% 3 4 13% 6% 3	3026
	11th to 20th	Night	80% 2 4 7% 5% 1	2902
	21st to end	Night	78% 3 5% 8% 5% 1	3078
February	1st to 10th	Night	69% 3 5% 10% 9% 4	2592
	11th to 20th	Night	76% 2 4 6% 10% 2	2441
	21st to end	Night	63% 2 5% 17% 9% 3	1911
March	1st to 10th	Night	63% 3 5% 10% 14% 4	2160
	11th to 20th	Night	76% 4 3 9% 7% 1	2105
	21st to end	Night	68% 5% 9% 12% 5% 1	1991
April	1st to 10th	Night	75% 5% 6% 11% 4	1800
	11th to 20th	Night	67% 4 9% 11% 9%	1484
	21st to end	Night	72% 4 9% 9% 6%	1440
May	1st to 10th	Night	71% 6% 9% 10% 4	1322
	11th to 20th	Night	62% 5% 8% 18% 7%	1062
	21st to end	Night	56% 7% 16% 18% 3	1164
June	1st to 10th	Night	71% 5% 11% 8% 4 1	1076
	11th to 20th	Night	74% 6% 10% 9% 1	869
	21st to end	Night	75% 5% 12% 6% 1	827
July	1st to 10th	Night	65% 8% 14% 11% 2	1050
	11th to 20th	Night	76% 7% 14% 3	1044
	21st to end	Night	73% 3 15% 7% 1	1116
August	1st to 10th	Night	71% 8% 14% 6% 1	1258
	11th to 20th	Night	79% 5% 8% 7% 1	1424
	21st to end	Night	79% 3 9% 7% 1	1616
September	1st to 10th	Night	80% 4 8% 7% 1	1800
	11th to 20th	Night	73% 6% 5% 11% 6%	1805
	21st to end	Night	84% 2 5% 6% 2	2026
October	1st to 10th	Night	77% 4 6% 10% 4	2160
	11th to 20th	Night	86% 2 4 5% 3	2326
	21st to end	Night	76% 3 5% 11% 5%	2704
November	1st to 10th	Night	75% 3 5% 12% 5%	2726
	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	60% 9% 15% 7% 7% 1	162
	21st to end	Day	54% 8% 14% 16% 7%	262
February	1st to 10th	Day	44% 14% 22% 14% 5%	340
	11th to 20th	Day	46% 12% 19% 17% 4 1	335
	21st to end	Day	38% 11% 28% 15% 8%	280
March	1st to 10th	Day	31% 15% 22% 24% 8% 1	340
	11th to 20th	Day	43% 10% 21% 17% 7% 1	340
	21st to end	Day	37% 9% 18% 20% 12% 3	458
April	1st to 10th	Day	43% 12% 18% 15% 10% 2	420
	11th to 20th	Day	35% 8% 19% 20% 16% 3	542
	21st to end	Day	48% 8% 15% 16% 12% 3	600
May	1st to 10th	Day	50% 6% 9% 21% 11% 4	603
	11th to 20th	Day	34% 5% 10% 25% 22% 4	625
	21st to end	Day	36% 3 11% 25% 21% 4	680
June	1st to 10th	Day	49% 4 8% 17% 19% 3	630
	11th to 20th	Day	50% 5% 9% 19% 16% 3	810
	21st to end	Day	47% 4 6% 17% 21% 5%	721
July	1st to 10th	Day	46% 4 7% 21% 18% 4	621
	11th to 20th	Day	62% 4 7% 12% 13% 1	618
	21st to end	Day	58% 4 6% 16% 11% 5%	684
August	1st to 10th	Day	50% 5% 10% 16% 15% 4	626
	11th to 20th	Day	62% 7% 6% 13% 10% 2	615
	21st to end	Day	63% 5% 7% 16% 8% 1	667
September	1st to 10th	Day	55% 6% 12% 18% 10%	450
	11th to 20th	Day	52% 5% 11% 18% 12% 2	450
	21st to end	Day	68% 5% 12% 11% 4 1	426
October	1st to 10th	Day	53% 7% 16% 16% 8% 1	360
	11th to 20th	Day	71% 8% 10% 6% 3 2	360
	21st to end	Day	47% 9% 19% 18% 6% 1	396
November	1st to 10th	Day	51% 10% 20% 16% 3	230
	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% 25% 12% 3	850
	11th to 20th	Night	61% 10% 16% 9% 3 1	807
	21st to end	Night	52% 10% 18% 13% 6%	880
February	1st to 10th	Night	44% 11% 25% 14% 5%	800
	11th to 20th	Night	48% 11% 20% 16% 5% 1	784
	21st to end	Night	38% 11% 24% 20% 6%	652
March	1st to 10th	Night	37% 13% 25% 16% 9% 1	720
	11th to 20th	Night	42% 10% 23% 18% 5% 1	665
	21st to end	Night	37% 6% 20% 20% 14% 4	594
April	1st to 10th	Night	47% 6% 14% 15% 13% 4	540
	11th to 20th	Night	36% 5% 20% 21% 16% 2	533
	21st to end	Night	47% 6% 12% 16% 14% 3	540
May	1st to 10th	Night	48% 7% 8% 17% 16% 3	432
	11th to 20th	Night	35% 6% 12% 19% 23% 4	354
	21st to end	Night	33% 5% 6% 26% 24% 6%	389
June	1st to 10th	Night	45% 7% 4 17% 22% 5%	360
	11th to 20th	Night	49% 6% 6% 16% 18% 5%	329
	21st to end	Night	49% 6% 6% 11% 24% 4	204
July	1st to 10th	Night	44% 4 5% 23% 19% 6%	350
	11th to 20th	Night	55% 3 4 16% 16% 5%	348
	21st to end	Night	55% 3 5% 16% 15% 6%	394
August	1st to 10th	Night	50% 4 7% 14% 18% 7%	485
	11th to 20th	Night	58% 7% 6% 12% 13% 4	534
	21st to end	Night	63% 5% 4 14% 12% 2	594
September	1st to 10th	Night	61% 4 8% 13% 12% 2	540
	11th to 20th	Night	54% 6% 9% 13% 15% 3	545
	21st to end	Night	68% 6% 10% 8% 7% 2	720
October	1st to 10th	Night	56% 5% 12% 17% 9% 2	720
	11th to 20th	Night	71% 5% 10% 8% 5% 1	720
	21st to end	Night	56% 6% 19% 13% 5% 1	857
November	1st to 10th	Night	56% 8% 20% 14% 2	830
	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 2024-01-09 03:14:39. Version ac07

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