Glacier Bay Visitor Center
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Glacier Bay Visitor Center clear sky chart from 2005-11-06 to 2025-12-03.

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	32% 15% 4 8% 4 4 3 3 4 4 18%	132548
	Night	34% 17% 4 8% 4 3 3 2 3 4 17%	126750
Transparency	Day	46% 39% 6% 6% 3	132212
	Night	47% 38% 5% 6% 4 1	126664
Seeing	Day	38% 9% 16% 19% 14% 4	18199
	Night	42% 9% 15% 18% 13% 3	24472

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	31% 22% 5% 8% 3 3 3 3 3 3 17%	2719
	11th to 20th	Day	30% 20% 5% 8% 4 3 2 1 2 2 22%	2702
	21st to end	Day	37% 24% 4 9% 5% 2 3 2 2 3 9%	3313
February	1st to 10th	Day	26% 17% 4 6% 4 4 3 3 4 4 24%	1465
	11th to 20th	Day	29% 20% 4 9% 5% 3 2 2 3 3 21%	1610
	21st to end	Day	21% 20% 3 7% 4 4 2 3 4 4 28%	1485
March	1st to 10th	Day	20% 23% 4 10% 6% 4 4 2 3 3 21%	4442
	11th to 20th	Day	19% 21% 5% 9% 5% 4 4 2 3 3 23%	4794
	21st to end	Day	15% 16% 4 8% 5% 4 4 3 6% 6% 29%	5864
April	1st to 10th	Day	25% 19% 6% 8% 5% 5% 4 3 4 3 18%	2403
	11th to 20th	Day	22% 17% 4 9% 5% 4 4 3 4 5% 24%	2710
	21st to end	Day	29% 17% 3 8% 4 5% 4 3 4 5% 19%	2800
May	1st to 10th	Day	35% 14% 4 9% 5% 4 4 2 4 4 14%	6688
	11th to 20th	Day	27% 15% 5% 9% 5% 5% 4 3 4 5% 18%	7193
	21st to end	Day	28% 16% 5% 10% 6% 5% 4 4 4 5% 13%	7907
June	1st to 10th	Day	31% 12% 4 8% 5% 5% 6% 3 5% 5% 16%	3225
	11th to 20th	Day	30% 13% 4 9% 4 4 4 3 5% 5% 20%	3400
	21st to end	Day	31% 12% 3 9% 5% 4 4 4 4 5% 18%	3400
July	1st to 10th	Day	29% 14% 5% 10% 5% 5% 4 3 5% 5% 15%	7611
	11th to 20th	Day	42% 10% 3 7% 4 3 3 2 3 5% 18%	7240
	21st to end	Day	36% 11% 3 7% 4 4 4 3 4 5% 20%	7876
August	1st to 10th	Day	28% 8% 3 6% 3 3 3 2 4 5% 36%	4855
	11th to 20th	Day	35% 10% 3 7% 3 3 3 3 3 5% 25%	4480
	21st to end	Day	49% 12% 3 7% 3 3 3 2 3 3 13%	4931
September	1st to 10th	Day	46% 11% 4 7% 4 3 3 2 4 4 13%	2414
	11th to 20th	Day	39% 12% 3 9% 4 2 3 2 3 4 19%	2400
	21st to end	Day	48% 10% 4 8% 5% 4 3 2 3 3 11%	2124
October	1st to 10th	Day	41% 16% 5% 8% 4 4 3 2 3 3 11%	3270
	11th to 20th	Day	44% 16% 3 7% 3 3 3 2 3 2 13%	3012
	21st to end	Day	33% 12% 4 9% 5% 4 3 2 4 4 19%	2874
November	1st to 10th	Day	35% 15% 4 8% 6% 4 3 3 3 3 17%	1538
	11th to 20th	Day	33% 17% 5% 9% 4 2 4 2 3 4 18%	1365
	21st to end	Day	35% 17% 5% 8% 4 4 2 2 4 3 16%	1260
December	1st to 10th	Day	48% 19% 3 6% 3 3 2 2 2 3 9%	1972
	11th to 20th	Day	24% 21% 4 10% 6% 4 3 2 3 3 19%	2265
	21st to end	Day	37% 20% 4 9% 4 3 2 2 3 3 13%	2941
January	1st to 10th	Night	34% 17% 4 7% 3 3 2 2 3 4 20%	7226
	11th to 20th	Night	33% 17% 4 7% 4 3 3 2 3 3 22%	7196
	21st to end	Night	40% 25% 4 9% 4 3 2 2 2 2 8%	7656
February	1st to 10th	Night	27% 18% 4 6% 4 3 3 2 4 4 25%	2935
	11th to 20th	Night	32% 22% 3 7% 4 3 2 2 2 2 21%	2674
	21st to end	Night	21% 21% 4 7% 3 3 3 3 4 4 28%	2145
March	1st to 10th	Night	21% 23% 4 9% 5% 4 4 2 3 4 21%	5432
	11th to 20th	Night	22% 20% 4 11% 6% 4 4 3 4 4 19%	5169
	21st to end	Night	21% 17% 3 8% 4 4 4 3 4 7% 26%	5084
April	1st to 10th	Night	26% 18% 4 6% 5% 3 3 3 4 4 23%	1995
	11th to 20th	Night	22% 16% 5% 10% 5% 4 3 3 4 4 24%	1639
	21st to end	Night	32% 16% 3 9% 5% 4 3 3 3 3 20%	1600
May	1st to 10th	Night	37% 12% 3 9% 5% 4 3 3 5% 4 15%	3279
	11th to 20th	Night	27% 18% 5% 9% 5% 4 4 3 4 4 17%	2702
	21st to end	Night	31% 20% 6% 11% 5% 4 3 3 4 3 10%	2968
June	1st to 10th	Night	32% 13% 4 9% 7% 5% 4 4 4 4 14%	1171
	11th to 20th	Night	29% 13% 3 10% 6% 4 6% 3 5% 6% 17%	1000
	21st to end	Night	33% 13% 3 7% 6% 4 5% 3 5% 4 18%	1000
July	1st to 10th	Night	34% 17% 4 10% 5% 4 4 3 4 4 11%	2347
	11th to 20th	Night	41% 13% 4 8% 4 3 3 2 3 5% 14%	2720
	21st to end	Night	40% 15% 4 8% 4 3 3 2 2 4 14%	2967
August	1st to 10th	Night	30% 9% 2 7% 4 3 4 3 4 4 30%	2302
	11th to 20th	Night	36% 13% 3 7% 4 3 2 2 4 5% 22%	2578
	21st to end	Night	51% 12% 2 5% 3 3 2 2 2 3 15%	2976
September	1st to 10th	Night	46% 9% 3 9% 3 3 3 2 3 4 15%	1986
	11th to 20th	Night	39% 11% 4 6% 4 3 4 3 4 4 19%	2000
	21st to end	Night	47% 13% 4 8% 4 3 3 2 2 3 11%	2273
October	1st to 10th	Night	41% 15% 4 9% 4 3 4 2 3 3 12%	3920
	11th to 20th	Night	45% 13% 3 8% 5% 4 4 2 3 3 10%	4136
	21st to end	Night	34% 15% 3 8% 4 4 4 2 3 4 20%	4926
November	1st to 10th	Night	36% 16% 4 8% 4 3 3 2 3 3 18%	2978
	11th to 20th	Night	32% 18% 4 7% 5% 3 3 2 3 4 18%	3254
	21st to end	Night	36% 19% 4 7% 4 4 3 3 3 4 14%	3360
December	1st to 10th	Night	49% 15% 2 5% 3 2 2 2 3 4 12%	5252
	11th to 20th	Night	29% 19% 4 8% 4 4 3 3 4 5% 17%	5940
	21st to end	Night	37% 20% 4 9% 4 3 3 2 3 3 13%	7964

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% 28% 2 6% 6% 4	2719
	11th to 20th	Day	53% 26% 2 5% 7% 7%	2720
	21st to end	Day	62% 28% 4 2 2 1	3313
February	1st to 10th	Day	16% 62% 3 8% 7% 3	1465
	11th to 20th	Day	28% 60% 2 4 6%	1618
	21st to end	Day	23% 62% 3 6% 4 1	1485
March	1st to 10th	Day	51% 30% 4 5% 9% 1	4442
	11th to 20th	Day	48% 28% 5% 7% 12%	4794
	21st to end	Day	36% 31% 14% 15% 4	5864
April	1st to 10th	Day	30% 62% 3 5% 1	2403
	11th to 20th	Day	27% 62% 2 4 4	2710
	21st to end	Day	29% 63% 3 4 2	2800
May	1st to 10th	Day	56% 30% 5% 7% 2	6688
	11th to 20th	Day	56% 31% 5% 6% 2	7196
	21st to end	Day	58% 30% 7% 4 1	7908
June	1st to 10th	Day	35% 61% 3 1	3225
	11th to 20th	Day	28% 63% 6% 4	3400
	21st to end	Day	30% 61% 4 4	3400
July	1st to 10th	Day	60% 29% 6% 5%	7528
	11th to 20th	Day	56% 29% 12% 3	7153
	21st to end	Day	53% 30% 15% 3	7655
August	1st to 10th	Day	28% 38% 11% 15% 9%	4836
	11th to 20th	Day	40% 38% 11% 9% 1	4522
	21st to end	Day	51% 36% 6% 6% 1	4930
September	1st to 10th	Day	30% 58% 6% 5% 1	2414
	11th to 20th	Day	34% 57% 3 4 2	2400
	21st to end	Day	35% 57% 3 3 1	2124
October	1st to 10th	Day	53% 36% 4 4 2	3270
	11th to 20th	Day	52% 36% 4 6% 3	3012
	21st to end	Day	46% 38% 10% 5% 1	2874
November	1st to 10th	Day	38% 54% 3 4 2	1538
	11th to 20th	Day	33% 54% 4 6% 2 1	1365
	21st to end	Day	36% 54% 3 3 3 1	1260
December	1st to 10th	Day	58% 36% 3 2 1	1972
	11th to 20th	Day	53% 33% 3 7% 4	2268
	21st to end	Day	61% 26% 3 4 6%	2941
January	1st to 10th	Night	50% 28% 4 9% 8% 3	7226
	11th to 20th	Night	49% 27% 5% 4 10% 5%	7240
	21st to end	Night	65% 28% 3 2 2	7656
February	1st to 10th	Night	16% 62% 4 7% 7% 3	2935
	11th to 20th	Night	28% 60% 1 5% 6% 1	2677
	21st to end	Night	24% 61% 3 6% 4 2	2145
March	1st to 10th	Night	46% 29% 5% 9% 10% 1	5432
	11th to 20th	Night	49% 28% 5% 7% 11% 1	5171
	21st to end	Night	40% 31% 11% 13% 5% 1	5084
April	1st to 10th	Night	29% 61% 3 4 2	1995
	11th to 20th	Night	26% 62% 3 4 4 1	1639
	21st to end	Night	30% 62% 2 4 3	1600
May	1st to 10th	Night	52% 31% 6% 9% 2	3276
	11th to 20th	Night	57% 33% 5% 4 2	2702
	21st to end	Night	60% 32% 4 3 1	2968
June	1st to 10th	Night	34% 62% 3 1	1171
	11th to 20th	Night	28% 63% 6% 3	1000
	21st to end	Night	28% 62% 6% 3 1	1000
July	1st to 10th	Night	64% 29% 5% 2	2321
	11th to 20th	Night	60% 30% 9% 1 1	2684
	21st to end	Night	56% 30% 11% 3	2883
August	1st to 10th	Night	29% 38% 12% 17% 5%	2296
	11th to 20th	Night	42% 38% 13% 3 4	2602
	21st to end	Night	50% 37% 6% 7% 1	2976
September	1st to 10th	Night	30% 57% 6% 5% 1	1986
	11th to 20th	Night	33% 58% 3 4 2	2000
	21st to end	Night	35% 57% 3 4 2	2273
October	1st to 10th	Night	51% 37% 3 7% 3	3920
	11th to 20th	Night	55% 37% 3 5% 1	4136
	21st to end	Night	48% 37% 7% 5% 3	4926
November	1st to 10th	Night	38% 54% 3 4 1	2978
	11th to 20th	Night	33% 54% 2 7% 3	3254
	21st to end	Night	36% 55% 3 3 3 1	3360
December	1st to 10th	Night	54% 36% 4 3 3	5252
	11th to 20th	Night	51% 33% 5% 8% 3	5936
	21st to end	Night	60% 27% 3 6% 4	7964

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	31% 11% 18% 22% 16% 3	200
	11th to 20th	Day	47% 11% 16% 16% 8% 2	200
	21st to end	Day	46% 16% 14% 14% 8% 2	359
February	1st to 10th	Day	32% 12% 25% 17% 12% 2	400
	11th to 20th	Day	36% 11% 22% 17% 11% 4	395
	21st to end	Day	34% 15% 23% 18% 6% 5%	331
March	1st to 10th	Day	25% 18% 21% 20% 13% 4	400
	11th to 20th	Day	37% 13% 21% 16% 10% 4	400
	21st to end	Day	29% 13% 18% 17% 14% 9%	525
April	1st to 10th	Day	33% 14% 17% 21% 13% 2	503
	11th to 20th	Day	28% 10% 14% 21% 21% 6%	690
	21st to end	Day	39% 7% 14% 18% 18% 5%	700
May	1st to 10th	Day	39% 7% 17% 21% 13% 4	703
	11th to 20th	Day	29% 6% 16% 22% 20% 7%	725
	21st to end	Day	28% 7% 18% 25% 17% 5%	790
June	1st to 10th	Day	33% 9% 18% 20% 14% 5%	730
	11th to 20th	Day	32% 7% 15% 25% 17% 4	730
	21st to end	Day	32% 5% 13% 20% 23% 7%	730
July	1st to 10th	Day	30% 6% 13% 24% 20% 8%	721
	11th to 20th	Day	42% 6% 15% 17% 16% 4	718
	21st to end	Day	41% 6% 12% 18% 17% 6%	794
August	1st to 10th	Day	38% 7% 12% 21% 17% 5%	726
	11th to 20th	Day	47% 9% 13% 17% 10% 3	715
	21st to end	Day	50% 8% 10% 16% 13% 3	803
September	1st to 10th	Day	44% 9% 12% 16% 16% 4	544
	11th to 20th	Day	43% 6% 13% 20% 14% 5%	530
	21st to end	Day	53% 8% 16% 13% 8% 3	469
October	1st to 10th	Day	44% 11% 15% 15% 12% 3	420
	11th to 20th	Day	51% 11% 16% 14% 6% 2	420
	21st to end	Day	40% 11% 20% 20% 7% 2	462
November	1st to 10th	Day	44% 10% 21% 15% 9% 1	316
	11th to 20th	Day	45% 9% 16% 16% 10% 4	220
	21st to end	Day	45% 17% 17% 14% 6%	220
December	1st to 10th	Day	53% 14% 9% 14% 7% 3	204
	11th to 20th	Day	41% 11% 22% 13% 10% 5%	200
	21st to end	Day	43% 12% 20% 14% 8% 3	206
January	1st to 10th	Night	42% 10% 18% 19% 11% 1	900
	11th to 20th	Night	48% 13% 19% 13% 6% 1	900
	21st to end	Night	50% 12% 14% 14% 8% 1	990
February	1st to 10th	Night	37% 10% 19% 21% 11% 2	900
	11th to 20th	Night	42% 11% 16% 16% 13% 2	884
	21st to end	Night	32% 12% 22% 21% 10% 3	743
March	1st to 10th	Night	29% 14% 20% 20% 13% 5%	814
	11th to 20th	Night	34% 9% 22% 21% 11% 3	687
	21st to end	Night	30% 8% 15% 23% 19% 6%	660
April	1st to 10th	Night	35% 9% 16% 19% 16% 6%	600
	11th to 20th	Night	32% 8% 12% 24% 19% 6%	593
	21st to end	Night	36% 6% 15% 20% 19% 6%	600
May	1st to 10th	Night	44% 7% 9% 19% 16% 5%	521
	11th to 20th	Night	32% 5% 12% 23% 23% 5%	394
	21st to end	Night	25% 5% 16% 25% 25% 4	433
June	1st to 10th	Night	34% 8% 14% 19% 20% 6%	371
	11th to 20th	Night	29% 5% 12% 23% 28% 4	200
	21st to end	Night	36% 4 7% 21% 23% 10%	200
July	1st to 10th	Night	34% 5% 10% 20% 25% 7%	229
	11th to 20th	Night	35% 7% 14% 19% 22% 3	388
	21st to end	Night	42% 5% 8% 18% 23% 4	438
August	1st to 10th	Night	37% 8% 10% 20% 20% 5%	587
	11th to 20th	Night	46% 8% 12% 15% 13% 5%	594
	21st to end	Night	52% 4 9% 16% 14% 4	660
September	1st to 10th	Night	50% 6% 10% 19% 13% 3	600
	11th to 20th	Night	43% 7% 11% 19% 15% 5%	600
	21st to end	Night	51% 10% 14% 13% 9% 3	766
October	1st to 10th	Night	45% 9% 14% 18% 12% 3	800
	11th to 20th	Night	53% 7% 16% 13% 9% 2	800
	21st to end	Night	43% 9% 20% 16% 11% 1	990
November	1st to 10th	Night	46% 11% 16% 18% 8%	930
	11th to 20th	Night	41% 12% 18% 17% 8% 3	950
	21st to end	Night	47% 12% 16% 15% 8% 1	950
December	1st to 10th	Night	49% 10% 14% 14% 11% 2	910
	11th to 20th	Night	43% 11% 16% 18% 10% 2	900
	21st to end	Night	47% 11% 15% 16% 8% 3	990

Page generated at 2025-11-29 21:00:52. Version ac07

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