Glennallen
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Glennallen clear sky chart from 2005-11-06 to 2025-03-27.

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	10% 11% 5% 12% 8% 7% 6% 5% 6% 7% 23%	111183
	Night	8% 14% 5% 14% 8% 7% 7% 5% 6% 6% 19%	116835
Transparency	Day	56% 10% 16% 14% 4	110870
	Night	61% 9% 12% 12% 5% 1	116795
Seeing	Day	14% 8% 16% 24% 22% 16%	17902
	Night	14% 6% 18% 31% 25% 6%	22922

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	6% 12% 6% 17% 11% 8% 9% 9% 10% 4 9%	1819
	11th to 20th	Day	8% 14% 5% 13% 8% 9% 7% 5% 8% 7% 16%	2195
	21st to end	Day	6% 14% 6% 14% 9% 7% 7% 5% 7% 8% 18%	2992
February	1st to 10th	Day	5% 16% 5% 14% 11% 7% 6% 6% 6% 8% 16%	1314
	11th to 20th	Day	6% 18% 7% 14% 9% 8% 8% 6% 5% 5% 14%	1559
	21st to end	Day	3 11% 6% 13% 9% 7% 7% 5% 8% 9% 21%	1380
March	1st to 10th	Day	2 12% 6% 17% 10% 9% 8% 4 5% 6% 19%	3181
	11th to 20th	Day	4 10% 5% 15% 10% 8% 7% 5% 7% 7% 24%	3412
	21st to end	Day	4 13% 6% 15% 9% 7% 7% 4 6% 7% 23%	4141
April	1st to 10th	Day	3 12% 5% 13% 7% 7% 7% 5% 8% 8% 25%	2413
	11th to 20th	Day	2 9% 4 9% 6% 6% 5% 5% 7% 8% 39%	2542
	21st to end	Day	5% 9% 4 9% 7% 6% 5% 5% 6% 7% 36%	2817
May	1st to 10th	Day	8% 18% 6% 13% 8% 7% 6% 4 6% 5% 19%	4873
	11th to 20th	Day	7% 13% 4 10% 8% 6% 6% 5% 6% 7% 30%	5263
	21st to end	Day	15% 11% 4 9% 6% 6% 5% 5% 6% 8% 26%	5904
June	1st to 10th	Day	11% 11% 4 9% 7% 6% 7% 6% 7% 8% 25%	3420
	11th to 20th	Day	15% 9% 3 8% 6% 6% 7% 6% 7% 8% 26%	3495
	21st to end	Day	14% 8% 3 9% 6% 6% 7% 5% 8% 9% 26%	3434
July	1st to 10th	Day	15% 8% 4 10% 8% 7% 7% 5% 6% 8% 22%	5700
	11th to 20th	Day	13% 7% 3 10% 8% 7% 7% 6% 8% 9% 22%	5582
	21st to end	Day	19% 8% 3 8% 6% 6% 6% 5% 6% 8% 26%	5666
August	1st to 10th	Day	14% 9% 3 9% 7% 6% 6% 4 6% 6% 30%	4993
	11th to 20th	Day	16% 8% 3 8% 5% 5% 5% 4 7% 7% 31%	4568
	21st to end	Day	18% 12% 5% 11% 6% 5% 5% 5% 6% 7% 20%	4773
September	1st to 10th	Day	17% 11% 4 11% 7% 6% 6% 5% 5% 6% 21%	2456
	11th to 20th	Day	15% 15% 5% 9% 6% 6% 5% 5% 6% 6% 24%	2191
	21st to end	Day	16% 14% 4 10% 6% 6% 5% 5% 6% 6% 24%	2090
October	1st to 10th	Day	11% 14% 5% 14% 7% 6% 5% 5% 7% 7% 18%	3025
	11th to 20th	Day	6% 11% 4 12% 6% 5% 6% 5% 6% 6% 32%	2834
	21st to end	Day	10% 14% 5% 13% 8% 8% 7% 4 6% 6% 17%	2607
November	1st to 10th	Day	8% 20% 7% 16% 9% 8% 6% 4 5% 4 12%	1371
	11th to 20th	Day	8% 16% 6% 15% 10% 7% 7% 4 5% 5% 18%	1115
	21st to end	Day	5% 16% 7% 17% 9% 6% 6% 7% 6% 8% 12%	1000
December	1st to 10th	Day	10% 11% 6% 18% 13% 9% 6% 4 5% 5% 12%	1539
	11th to 20th	Day	6% 17% 9% 18% 13% 8% 7% 5% 7% 4 6%	1517
	21st to end	Day	3 15% 6% 21% 11% 11% 10% 5% 4 4 9%	2002
January	1st to 10th	Night	4 14% 5% 17% 10% 9% 11% 7% 10% 4 8%	8146
	11th to 20th	Night	6% 14% 5% 17% 9% 9% 8% 5% 8% 6% 15%	7705
	21st to end	Night	6% 15% 5% 13% 8% 7% 6% 5% 7% 9% 19%	7986
February	1st to 10th	Night	6% 13% 5% 12% 9% 7% 6% 5% 6% 7% 24%	3086
	11th to 20th	Night	5% 16% 6% 15% 8% 7% 6% 6% 6% 7% 18%	2728
	21st to end	Night	4 12% 5% 14% 7% 6% 6% 5% 7% 7% 28%	2250
March	1st to 10th	Night	3 14% 7% 18% 9% 5% 4 3 4 6% 26%	3891
	11th to 20th	Night	6% 11% 5% 12% 8% 7% 6% 5% 7% 7% 28%	3765
	21st to end	Night	2 12% 5% 15% 8% 6% 7% 5% 6% 8% 27%	3627
April	1st to 10th	Night	2 10% 4 13% 8% 7% 7% 7% 7% 8% 27%	1767
	11th to 20th	Night	3 10% 4 8% 8% 6% 6% 5% 8% 8% 34%	1584
	21st to end	Night	5% 12% 3 9% 6% 6% 4 4 7% 8% 36%	1363
May	1st to 10th	Night	8% 16% 5% 13% 8% 8% 7% 5% 6% 6% 20%	2096
	11th to 20th	Night	10% 12% 5% 9% 6% 6% 4 4 5% 6% 32%	1633
	21st to end	Night	19% 11% 4 9% 6% 5% 5% 4 4 7% 27%	1670
June	1st to 10th	Night	15% 8% 5% 9% 6% 6% 4 6% 7% 10% 25%	760
	11th to 20th	Night	14% 10% 5% 10% 6% 6% 6% 3 6% 7% 25%	675
	21st to end	Night	17% 7% 3 10% 7% 6% 7% 5% 6% 6% 25%	735
July	1st to 10th	Night	19% 9% 4 10% 8% 6% 6% 6% 5% 8% 20%	1270
	11th to 20th	Night	16% 9% 4 9% 6% 8% 9% 6% 7% 8% 18%	1384
	21st to end	Night	18% 11% 4 10% 7% 6% 7% 5% 7% 8% 17%	1892
August	1st to 10th	Night	15% 9% 4 10% 5% 6% 6% 4 5% 6% 29%	1947
	11th to 20th	Night	17% 10% 4 11% 5% 6% 5% 4 6% 8% 23%	2267
	21st to end	Night	18% 11% 4 12% 8% 6% 6% 4 6% 5% 19%	2894
September	1st to 10th	Night	20% 12% 3 10% 7% 5% 5% 4 6% 6% 21%	1722
	11th to 20th	Night	17% 14% 4 11% 7% 6% 6% 5% 6% 5% 20%	1989
	21st to end	Night	16% 15% 4 11% 6% 6% 5% 4 5% 6% 22%	2090
October	1st to 10th	Night	13% 17% 5% 11% 7% 7% 7% 5% 7% 6% 17%	3939
	11th to 20th	Night	6% 10% 5% 9% 6% 5% 5% 5% 7% 7% 35%	4094
	21st to end	Night	12% 16% 4 13% 8% 7% 7% 4 5% 5% 18%	4951
November	1st to 10th	Night	9% 18% 7% 15% 9% 6% 5% 4 5% 5% 17%	2927
	11th to 20th	Night	7% 16% 7% 16% 8% 6% 5% 5% 6% 7% 18%	3284
	21st to end	Night	6% 16% 6% 17% 9% 7% 7% 6% 6% 6% 13%	3400
December	1st to 10th	Night	10% 16% 6% 17% 10% 7% 6% 4 6% 5% 12%	5642
	11th to 20th	Night	4 18% 7% 19% 10% 10% 9% 5% 5% 5% 7%	6700
	21st to end	Night	5% 16% 8% 20% 11% 11% 10% 4 4 3 8%	8976

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	64% 9% 11% 9% 6% 1	1819
	11th to 20th	Day	60% 9% 11% 12% 7% 1	2210
	21st to end	Day	58% 10% 9% 10% 12% 1	2992
February	1st to 10th	Day	60% 10% 9% 11% 8% 3	1314
	11th to 20th	Day	67% 9% 8% 10% 6%	1566
	21st to end	Day	53% 9% 11% 16% 11%	1380
March	1st to 10th	Day	62% 8% 10% 9% 9% 3	3181
	11th to 20th	Day	53% 10% 11% 16% 11%	3412
	21st to end	Day	55% 13% 14% 12% 5% 1	4141
April	1st to 10th	Day	52% 10% 18% 15% 5%	2413
	11th to 20th	Day	41% 11% 19% 23% 6%	2542
	21st to end	Day	45% 9% 16% 23% 9%	2817
May	1st to 10th	Day	63% 9% 11% 14% 3	4873
	11th to 20th	Day	48% 10% 12% 22% 7%	5266
	21st to end	Day	54% 10% 13% 19% 4	5905
June	1st to 10th	Day	53% 12% 15% 17% 3	3420
	11th to 20th	Day	52% 12% 15% 17% 3	3495
	21st to end	Day	51% 12% 20% 16% 1	3434
July	1st to 10th	Day	57% 11% 20% 11% 2	5611
	11th to 20th	Day	52% 11% 23% 13%	5483
	21st to end	Day	53% 11% 24% 11% 1	5487
August	1st to 10th	Day	54% 10% 20% 13% 3	4974
	11th to 20th	Day	47% 11% 23% 18% 2	4613
	21st to end	Day	60% 10% 19% 10% 1	4772
September	1st to 10th	Day	60% 12% 16% 11% 1	2456
	11th to 20th	Day	59% 11% 19% 10% 1	2191
	21st to end	Day	58% 10% 18% 12% 2	2090
October	1st to 10th	Day	62% 9% 15% 12% 2	3025
	11th to 20th	Day	49% 9% 15% 15% 12%	2834
	21st to end	Day	63% 12% 14% 10% 1	2607
November	1st to 10th	Day	72% 11% 8% 8% 1	1371
	11th to 20th	Day	65% 9% 11% 11% 4	1115
	21st to end	Day	64% 11% 12% 10% 4	1000
December	1st to 10th	Day	71% 10% 8% 10% 1	1539
	11th to 20th	Day	76% 8% 8% 7% 1	1520
	21st to end	Day	73% 9% 7% 5% 4 2	2002
January	1st to 10th	Night	64% 11% 10% 9% 6% 1	8146
	11th to 20th	Night	63% 10% 8% 11% 6% 1	7752
	21st to end	Night	56% 10% 9% 10% 12% 2	7986
February	1st to 10th	Night	56% 7% 10% 15% 8% 3	3086
	11th to 20th	Night	60% 10% 11% 11% 7%	2732
	21st to end	Night	51% 8% 13% 14% 12%	2250
March	1st to 10th	Night	58% 6% 9% 14% 12% 1	3891
	11th to 20th	Night	50% 8% 13% 15% 13%	3767
	21st to end	Night	48% 11% 15% 15% 10% 1	3627
April	1st to 10th	Night	49% 11% 16% 19% 6%	1767
	11th to 20th	Night	43% 10% 17% 22% 8%	1584
	21st to end	Night	44% 7% 17% 23% 9%	1363
May	1st to 10th	Night	59% 11% 11% 17% 2	2096
	11th to 20th	Night	49% 10% 12% 23% 5%	1633
	21st to end	Night	54% 11% 12% 18% 5%	1670
June	1st to 10th	Night	53% 11% 17% 18% 2	760
	11th to 20th	Night	55% 12% 13% 17% 3	675
	21st to end	Night	53% 16% 14% 17% 1	735
July	1st to 10th	Night	60% 11% 19% 10% 1	1250
	11th to 20th	Night	58% 13% 19% 11%	1360
	21st to end	Night	60% 11% 21% 7%	1832
August	1st to 10th	Night	53% 11% 23% 12% 1	1941
	11th to 20th	Night	57% 9% 15% 18%	2288
	21st to end	Night	63% 10% 17% 9% 1	2894
September	1st to 10th	Night	61% 11% 17% 11%	1722
	11th to 20th	Night	63% 11% 13% 12% 1	1989
	21st to end	Night	61% 9% 15% 13% 2	2090
October	1st to 10th	Night	64% 9% 14% 12% 1	3939
	11th to 20th	Night	45% 10% 17% 18% 10%	4094
	21st to end	Night	66% 8% 13% 12% 2	4951
November	1st to 10th	Night	68% 9% 10% 10% 3	2927
	11th to 20th	Night	62% 9% 13% 12% 3	3284
	21st to end	Night	65% 11% 11% 9% 4	3400
December	1st to 10th	Night	70% 9% 8% 10% 2	5642
	11th to 20th	Night	77% 7% 8% 6% 3	6696
	21st to end	Night	76% 8% 7% 5% 4	8976

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	5% 6% 9% 36% 33% 11%	100
	11th to 20th	Day	11% 6% 11% 39% 21% 13%	109
	21st to end	Day	14% 6% 12% 27% 28% 13%	330
February	1st to 10th	Day	10% 5% 10% 28% 28% 19%	345
	11th to 20th	Day	9% 9% 17% 28% 24% 13%	395
	21st to end	Day	7% 5% 14% 38% 21% 15%	331
March	1st to 10th	Day	5% 6% 17% 29% 23% 21%	400
	11th to 20th	Day	6% 4 18% 24% 26% 22%	400
	21st to end	Day	6% 6% 14% 27% 25% 23%	581
April	1st to 10th	Day	4 6% 19% 32% 26% 13%	623
	11th to 20th	Day	5% 7% 15% 24% 31% 18%	640
	21st to end	Day	9% 7% 14% 28% 27% 16%	650
May	1st to 10th	Day	11% 10% 22% 24% 20% 12%	653
	11th to 20th	Day	11% 8% 20% 23% 22% 16%	675
	21st to end	Day	17% 9% 20% 21% 21% 13%	756
June	1st to 10th	Day	16% 7% 16% 23% 22% 17%	870
	11th to 20th	Day	16% 5% 16% 24% 22% 17%	945
	21st to end	Day	14% 5% 16% 25% 20% 20%	884
July	1st to 10th	Day	12% 6% 17% 24% 22% 19%	856
	11th to 20th	Day	18% 7% 14% 22% 20% 18%	786
	21st to end	Day	18% 5% 16% 18% 20% 23%	739
August	1st to 10th	Day	18% 9% 18% 20% 17% 18%	676
	11th to 20th	Day	22% 11% 18% 20% 17% 11%	665
	21st to end	Day	24% 10% 15% 19% 17% 15%	748
September	1st to 10th	Day	19% 13% 18% 20% 19% 12%	673
	11th to 20th	Day	18% 14% 19% 19% 17% 13%	491
	21st to end	Day	24% 10% 12% 16% 20% 18%	390
October	1st to 10th	Day	22% 12% 15% 17% 17% 17%	390
	11th to 20th	Day	17% 7% 14% 20% 21% 21%	390
	21st to end	Day	15% 6% 14% 25% 25% 16%	429
November	1st to 10th	Day	14% 7% 15% 27% 24% 13%	395
	11th to 20th	Day	15% 2 18% 26% 21% 18%	177
	21st to end	Day	5% 9% 17% 32% 23% 14%	100
December	1st to 10th	Day	13% 7% 17% 29% 25% 9%	100
	11th to 20th	Day	10% 4 15% 39% 23% 9%	100
	21st to end	Day	9% 8% 19% 28% 24% 12%	110
January	1st to 10th	Night	8% 4 14% 33% 32% 10%	1000
	11th to 20th	Night	15% 7% 15% 35% 21% 7%	917
	21st to end	Night	15% 6% 18% 34% 21% 7%	990
February	1st to 10th	Night	9% 5% 13% 39% 28% 7%	900
	11th to 20th	Night	10% 7% 21% 36% 21% 5%	884
	21st to end	Night	7% 6% 18% 41% 21% 8%	743
March	1st to 10th	Night	7% 6% 21% 34% 26% 6%	720
	11th to 20th	Night	7% 5% 20% 30% 30% 8%	654
	21st to end	Night	4 6% 17% 30% 32% 11%	646
April	1st to 10th	Night	5% 5% 23% 35% 27% 5%	570
	11th to 20th	Night	7% 5% 19% 33% 32% 4	563
	21st to end	Night	8% 6% 15% 34% 33% 4	570
May	1st to 10th	Night	13% 8% 20% 33% 24% 2	498
	11th to 20th	Night	14% 7% 22% 34% 22% 1	229
	21st to end	Night	16% 6% 19% 31% 28%	206
June	1st to 10th	Night	18% 6% 17% 35% 21% 3	190
	11th to 20th	Night	19% 4 21% 31% 25% 1	190
	21st to end	Night	17% 5% 18% 27% 29% 3	190
July	1st to 10th	Night	16% 8% 17% 34% 22% 4	185
	11th to 20th	Night	20% 5% 20% 27% 28% 1	184
	21st to end	Night	20% 5% 15% 29% 28% 3	307
August	1st to 10th	Night	21% 7% 20% 25% 23% 4	405
	11th to 20th	Night	25% 9% 23% 23% 16% 4	564
	21st to end	Night	23% 8% 19% 23% 23% 4	627
September	1st to 10th	Night	24% 9% 21% 27% 16% 3	570
	11th to 20th	Night	21% 12% 24% 21% 19% 4	570
	21st to end	Night	19% 10% 19% 24% 22% 6%	570
October	1st to 10th	Night	23% 9% 21% 22% 20% 4	767
	11th to 20th	Night	13% 7% 19% 29% 27% 7%	860
	21st to end	Night	17% 6% 15% 30% 27% 5%	946
November	1st to 10th	Night	16% 6% 19% 31% 23% 5%	880
	11th to 20th	Night	13% 5% 14% 35% 23% 9%	900
	21st to end	Night	11% 7% 18% 31% 23% 9%	900
December	1st to 10th	Night	18% 9% 13% 29% 24% 8%	927
	11th to 20th	Night	12% 6% 18% 31% 25% 10%	1000
	21st to end	Night	12% 5% 17% 31% 25% 9%	1100

Page generated at 2025-03-24 05:04:30. Version ac07

Copyright © 2025 Attilla Danko.