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 2024-11-10.

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	33% 15% 4 8% 4 4 3 3 4 4 18%	106790
	Night	35% 16% 4 8% 4 3 3 2 3 4 18%	100862
Transparency	Day	40% 47% 5% 6% 2	106502
	Night	39% 47% 4 5% 3 1	100797
Seeing	Day	38% 9% 16% 19% 14% 5%	16809
	Night	42% 9% 15% 18% 13% 3	23075

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% 22% 4 8% 3 4 3 3 3 4 18%	1899
	11th to 20th	Day	34% 18% 3 5% 3 3 2 2 3 2 25%	1882
	21st to end	Day	45% 21% 3 7% 4 2 2 2 2 3 9%	2313
February	1st to 10th	Day	28% 18% 5% 7% 4 3 3 3 3 4 21%	1391
	11th to 20th	Day	29% 20% 4 8% 5% 2 2 2 3 3 22%	1527
	21st to end	Day	21% 19% 3 7% 3 4 2 3 4 4 29%	1413
March	1st to 10th	Day	17% 21% 4 9% 5% 4 3 3 3 3 28%	3086
	11th to 20th	Day	26% 21% 6% 10% 5% 4 3 2 3 3 17%	3346
	21st to end	Day	16% 17% 4 8% 4 3 4 3 6% 7% 28%	4088
April	1st to 10th	Day	25% 20% 6% 8% 5% 5% 4 3 4 3 18%	2283
	11th to 20th	Day	22% 16% 4 8% 5% 4 3 3 4 5% 25%	2573
	21st to end	Day	28% 17% 3 8% 5% 5% 3 3 4 5% 20%	2660
May	1st to 10th	Day	33% 13% 4 7% 5% 4 4 2 4 5% 19%	4670
	11th to 20th	Day	27% 16% 5% 9% 5% 4 4 3 4 5% 18%	5013
	21st to end	Day	28% 12% 3 8% 5% 4 5% 4 5% 6% 19%	5509
June	1st to 10th	Day	31% 12% 4 8% 5% 5% 6% 3 5% 5% 17%	3064
	11th to 20th	Day	31% 13% 4 9% 4 3 4 3 5% 5% 19%	3230
	21st to end	Day	31% 12% 3 9% 5% 5% 4 4 5% 5% 18%	3230
July	1st to 10th	Day	29% 14% 4 10% 6% 5% 4 3 4 4 17%	5319
	11th to 20th	Day	53% 11% 3 7% 4 3 3 2 2 3 9%	5060
	21st to end	Day	42% 12% 3 7% 4 3 4 3 4 4 13%	5486
August	1st to 10th	Day	28% 8% 2 6% 3 3 3 2 4 5% 36%	4706
	11th to 20th	Day	35% 10% 3 7% 3 3 3 3 3 5% 25%	4340
	21st to end	Day	50% 12% 3 7% 3 3 2 2 2 3 13%	4780
September	1st to 10th	Day	47% 11% 4 7% 4 3 3 2 3 4 12%	2294
	11th to 20th	Day	39% 12% 3 9% 4 2 3 3 3 4 19%	2280
	21st to end	Day	47% 10% 4 8% 5% 4 2 2 3 3 12%	2019
October	1st to 10th	Day	41% 16% 5% 8% 4 4 3 2 3 3 11%	3170
	11th to 20th	Day	44% 16% 3 7% 3 3 3 2 3 2 13%	2919
	21st to end	Day	34% 12% 4 9% 5% 4 3 2 4 4 19%	2785
November	1st to 10th	Day	35% 15% 4 7% 6% 4 3 2 3 3 18%	1456
	11th to 20th	Day	34% 15% 5% 9% 4 2 3 2 3 4 19%	1235
	21st to end	Day	38% 16% 5% 8% 4 4 2 2 4 4 14%	1140
December	1st to 10th	Day	38% 19% 4 6% 3 3 2 2 3 4 15%	1140
	11th to 20th	Day	28% 19% 3 11% 5% 3 3 2 3 4 17%	1445
	21st to end	Day	38% 20% 4 10% 4 3 3 2 3 3 9%	2039
January	1st to 10th	Night	28% 17% 4 8% 4 3 3 2 4 6% 20%	5046
	11th to 20th	Night	33% 14% 3 5% 3 2 2 2 3 4 28%	5016
	21st to end	Night	49% 19% 3 6% 3 2 2 2 2 3 9%	5349
February	1st to 10th	Night	28% 19% 4 6% 4 3 3 2 4 4 23%	2789
	11th to 20th	Night	33% 21% 3 7% 3 3 2 2 2 2 21%	2537
	21st to end	Night	20% 21% 4 7% 3 4 3 3 4 4 29%	2041
March	1st to 10th	Night	17% 22% 3 8% 4 4 3 2 3 5% 29%	3778
	11th to 20th	Night	28% 20% 4 10% 6% 4 4 2 3 4 15%	3613
	21st to end	Night	23% 15% 3 6% 4 4 4 3 5% 7% 27%	3551
April	1st to 10th	Night	26% 18% 4 6% 5% 3 3 3 4 4 23%	1895
	11th to 20th	Night	22% 16% 5% 10% 5% 4 3 3 4 4 25%	1556
	21st to end	Night	31% 15% 3 8% 5% 4 3 3 3 3 21%	1520
May	1st to 10th	Night	34% 10% 3 7% 4 4 3 3 6% 5% 20%	2291
	11th to 20th	Night	26% 18% 5% 9% 5% 4 4 3 5% 5% 17%	1882
	21st to end	Night	28% 18% 5% 10% 5% 4 4 3 5% 4 14%	2066
June	1st to 10th	Night	31% 13% 3 9% 7% 5% 4 4 5% 5% 15%	1113
	11th to 20th	Night	29% 14% 3 10% 6% 4 5% 3 5% 6% 17%	950
	21st to end	Night	32% 13% 3 7% 6% 4 5% 3 5% 4 18%	950
July	1st to 10th	Night	37% 17% 3 8% 5% 4 3 3 3 4 13%	1639
	11th to 20th	Night	51% 13% 4 8% 4 3 3 2 2 3 7%	1900
	21st to end	Night	46% 14% 4 8% 4 4 3 2 2 4 9%	2066
August	1st to 10th	Night	29% 9% 2 7% 4 3 4 3 4 4 31%	2231
	11th to 20th	Night	36% 13% 3 7% 4 3 2 2 4 5% 22%	2498
	21st to end	Night	52% 12% 2 5% 3 3 2 1 2 3 14%	2885
September	1st to 10th	Night	47% 9% 3 9% 3 3 3 2 3 4 14%	1886
	11th to 20th	Night	39% 11% 4 6% 4 4 4 3 3 4 19%	1900
	21st to end	Night	48% 12% 3 8% 4 3 3 2 2 3 12%	2158
October	1st to 10th	Night	41% 14% 5% 9% 4 3 4 2 3 3 11%	3800
	11th to 20th	Night	45% 14% 3 9% 5% 4 4 2 3 3 10%	4009
	21st to end	Night	34% 14% 3 8% 4 4 4 3 3 4 20%	4773
November	1st to 10th	Night	37% 15% 4 8% 3 3 3 3 3 4 18%	2824
	11th to 20th	Night	32% 17% 4 7% 5% 3 3 2 3 4 19%	2944
	21st to end	Night	38% 18% 4 7% 4 3 3 3 3 4 12%	3040
December	1st to 10th	Night	39% 18% 3 7% 3 2 2 2 3 4 16%	3040
	11th to 20th	Night	31% 19% 3 8% 4 3 3 2 4 4 18%	3760
	21st to end	Night	37% 21% 4 8% 5% 3 3 2 3 3 12%	5566

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	41% 40% 3 9% 4 3	1899
	11th to 20th	Day	37% 37% 3 6% 7% 9%	1900
	21st to end	Day	53% 39% 3 3 2	2313
February	1st to 10th	Day	17% 65% 3 5% 7% 3	1391
	11th to 20th	Day	25% 64% 2 3 6%	1535
	21st to end	Day	20% 65% 3 6% 5% 2	1413
March	1st to 10th	Day	35% 42% 2 7% 13% 1	3086
	11th to 20th	Day	50% 39% 3 6% 3	3346
	21st to end	Day	29% 43% 14% 12% 2	4088
April	1st to 10th	Day	27% 65% 3 4 1	2283
	11th to 20th	Day	24% 65% 2 4 4	2573
	21st to end	Day	26% 66% 3 4 2	2660
May	1st to 10th	Day	41% 42% 5% 9% 3	4670
	11th to 20th	Day	45% 43% 4 6% 2	5016
	21st to end	Day	43% 42% 8% 6% 1	5510
June	1st to 10th	Day	32% 64% 3 1	3064
	11th to 20th	Day	26% 65% 5% 3	3230
	21st to end	Day	28% 65% 4 3	3230
July	1st to 10th	Day	51% 37% 6% 5%	5236
	11th to 20th	Day	59% 37% 3 1	4973
	21st to end	Day	53% 39% 6% 3	5313
August	1st to 10th	Day	27% 39% 11% 15% 9%	4687
	11th to 20th	Day	39% 40% 11% 9% 1	4382
	21st to end	Day	52% 37% 5% 6% 1	4779
September	1st to 10th	Day	30% 60% 5% 4	2294
	11th to 20th	Day	32% 60% 3 4 2	2280
	21st to end	Day	33% 60% 3 3 1	2019
October	1st to 10th	Day	53% 37% 4 4 2	3170
	11th to 20th	Day	51% 37% 4 5% 3	2919
	21st to end	Day	45% 39% 10% 5% 1	2785
November	1st to 10th	Day	35% 56% 3 4 2	1456
	11th to 20th	Day	28% 60% 4 6% 2	1235
	21st to end	Day	34% 59% 3 2 1 1	1140
December	1st to 10th	Day	32% 60% 3 4 2	1140
	11th to 20th	Day	44% 48% 3 4 1	1448
	21st to end	Day	57% 36% 2 2 3 1	2039
January	1st to 10th	Night	38% 40% 5% 12% 4 1	5046
	11th to 20th	Night	34% 36% 4 6% 14% 7%	5060
	21st to end	Night	54% 40% 2 3 1	5349
February	1st to 10th	Night	16% 65% 3 6% 7% 3	2789
	11th to 20th	Night	25% 63% 1 4 6% 1	2540
	21st to end	Night	20% 64% 4 6% 4 2	2041
March	1st to 10th	Night	32% 40% 3 10% 14% 1	3778
	11th to 20th	Night	49% 40% 3 7% 1	3615
	21st to end	Night	29% 43% 14% 12% 2 1	3551
April	1st to 10th	Night	27% 64% 2 5% 2	1895
	11th to 20th	Night	23% 65% 3 5% 4 1	1556
	21st to end	Night	26% 64% 2 4 3	1520
May	1st to 10th	Night	36% 43% 7% 12% 3	2291
	11th to 20th	Night	47% 45% 3 3 2	1882
	21st to end	Night	46% 45% 4 3 1	2066
June	1st to 10th	Night	30% 65% 3 1	1113
	11th to 20th	Night	27% 65% 6% 3	950
	21st to end	Night	26% 65% 6% 3 1	950
July	1st to 10th	Night	55% 38% 4 2	1613
	11th to 20th	Night	61% 37% 2	1864
	21st to end	Night	54% 39% 5% 2	2000
August	1st to 10th	Night	27% 39% 12% 17% 6%	2225
	11th to 20th	Night	41% 39% 12% 3 4	2522
	21st to end	Night	51% 38% 5% 7% 1	2885
September	1st to 10th	Night	29% 60% 5% 5% 1	1886
	11th to 20th	Night	30% 60% 3 4 2	1900
	21st to end	Night	31% 60% 3 4 2	2158
October	1st to 10th	Night	51% 38% 3 6% 2	3800
	11th to 20th	Night	54% 38% 3 5%	4009
	21st to end	Night	46% 38% 7% 5% 3	4773
November	1st to 10th	Night	35% 57% 3 4 1	2824
	11th to 20th	Night	29% 59% 2 7% 3	2944
	21st to end	Night	34% 60% 2 2 1	3040
December	1st to 10th	Night	33% 60% 3 3 2	3040
	11th to 20th	Night	43% 49% 3 3 3	3756
	21st to end	Night	55% 37% 2 3 3 1	5566

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	27% 11% 20% 23% 16% 3	180
	11th to 20th	Day	47% 8% 17% 17% 9% 2	180
	21st to end	Day	47% 14% 14% 14% 8% 2	330
February	1st to 10th	Day	35% 11% 25% 16% 11% 2	370
	11th to 20th	Day	37% 11% 21% 16% 11% 4	365
	21st to end	Day	33% 13% 24% 19% 6% 6%	307
March	1st to 10th	Day	23% 17% 22% 20% 13% 4	370
	11th to 20th	Day	38% 12% 21% 15% 8% 4	370
	21st to end	Day	30% 12% 18% 15% 15% 10%	483
April	1st to 10th	Day	32% 13% 17% 21% 14% 2	463
	11th to 20th	Day	28% 9% 13% 21% 23% 7%	640
	21st to end	Day	37% 6% 14% 18% 19% 5%	650
May	1st to 10th	Day	39% 6% 16% 21% 13% 4	653
	11th to 20th	Day	28% 5% 16% 22% 20% 8%	675
	21st to end	Day	27% 6% 18% 26% 18% 5%	735
June	1st to 10th	Day	32% 8% 19% 21% 14% 6%	680
	11th to 20th	Day	33% 7% 15% 24% 18% 4	680
	21st to end	Day	32% 5% 14% 21% 21% 7%	680
July	1st to 10th	Day	28% 5% 13% 24% 20% 8%	671
	11th to 20th	Day	43% 6% 14% 16% 17% 4	668
	21st to end	Day	43% 6% 11% 17% 17% 6%	739
August	1st to 10th	Day	38% 7% 12% 21% 17% 5%	676
	11th to 20th	Day	48% 10% 13% 16% 9% 3	665
	21st to end	Day	53% 7% 10% 14% 13% 3	748
September	1st to 10th	Day	46% 9% 12% 15% 14% 4	504
	11th to 20th	Day	42% 6% 13% 20% 15% 5%	490
	21st to end	Day	52% 8% 15% 13% 9% 3	435
October	1st to 10th	Day	45% 10% 15% 15% 12% 3	390
	11th to 20th	Day	51% 11% 15% 14% 6% 2	390
	21st to end	Day	39% 10% 20% 21% 8% 2	429
November	1st to 10th	Day	43% 11% 21% 15% 9% 1	289
	11th to 20th	Day	46% 7% 17% 17% 10% 4	180
	21st to end	Day	49% 14% 19% 12% 6% 1	180
December	1st to 10th	Day	50% 14% 9% 16% 8% 3	180
	11th to 20th	Day	42% 11% 23% 12% 8% 4	180
	21st to end	Day	42% 11% 21% 14% 8% 4	184
January	1st to 10th	Night	41% 10% 18% 19% 11% 1	850
	11th to 20th	Night	47% 13% 20% 13% 6% 1	850
	21st to end	Night	51% 10% 14% 15% 8% 1	935
February	1st to 10th	Night	39% 11% 19% 20% 10% 2	850
	11th to 20th	Night	42% 11% 16% 16% 13% 1	834
	21st to end	Night	31% 12% 22% 22% 11% 3	703
March	1st to 10th	Night	28% 13% 20% 20% 13% 5%	773
	11th to 20th	Night	36% 9% 23% 19% 10% 3	653
	21st to end	Night	30% 7% 14% 23% 19% 7%	627
April	1st to 10th	Night	34% 9% 16% 18% 16% 6%	570
	11th to 20th	Night	31% 7% 12% 23% 20% 6%	563
	21st to end	Night	35% 5% 14% 21% 19% 6%	570
May	1st to 10th	Night	43% 6% 9% 19% 17% 5%	495
	11th to 20th	Night	32% 5% 12% 23% 23% 5%	374
	21st to end	Night	24% 5% 15% 26% 26% 4	411
June	1st to 10th	Night	34% 7% 14% 19% 21% 6%	353
	11th to 20th	Night	29% 5% 12% 22% 29% 3	190
	21st to end	Night	35% 4 6% 22% 24% 9%	190
July	1st to 10th	Night	34% 5% 9% 20% 25% 7%	217
	11th to 20th	Night	36% 6% 13% 18% 23% 3	368
	21st to end	Night	44% 4 8% 16% 24% 4	416
August	1st to 10th	Night	36% 8% 9% 20% 21% 6%	557
	11th to 20th	Night	47% 8% 12% 15% 13% 5%	564
	21st to end	Night	55% 3 9% 15% 14% 4	627
September	1st to 10th	Night	51% 6% 10% 19% 12% 3	570
	11th to 20th	Night	42% 6% 11% 20% 15% 5%	570
	21st to end	Night	51% 9% 14% 14% 9% 3	727
October	1st to 10th	Night	46% 8% 14% 18% 12% 3	760
	11th to 20th	Night	53% 7% 16% 14% 9% 2	760
	21st to end	Night	43% 8% 20% 17% 11% 1	936
November	1st to 10th	Night	46% 10% 16% 19% 9%	877
	11th to 20th	Night	40% 11% 19% 17% 9% 4	850
	21st to end	Night	49% 11% 16% 15% 8% 1	850
December	1st to 10th	Night	47% 10% 14% 15% 12% 2	850
	11th to 20th	Night	44% 11% 16% 18% 10% 2	850
	21st to end	Night	46% 11% 15% 16% 8% 3	935

Page generated at 2024-11-07 02:55:44. Version ac07

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