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 2026-07-31.

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% 12% 5% 12% 8% 7% 6% 5% 6% 7% 23%	143122
	Night	8% 13% 5% 14% 9% 8% 7% 5% 6% 6% 19%	138640
Transparency	Day	56% 10% 15% 14% 4	142760
	Night	61% 9% 12% 12% 6% 1	138585
Seeing	Day	14% 8% 17% 24% 22% 16%	19906
	Night	13% 7% 18% 31% 24% 6%	24415

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	5% 11% 6% 18% 10% 7% 8% 7% 9% 9% 9%	2361
	11th to 20th	Day	7% 15% 5% 16% 10% 9% 6% 5% 7% 6% 15%	2863
	21st to end	Day	6% 13% 6% 15% 10% 10% 8% 5% 6% 6% 15%	3871
February	1st to 10th	Day	6% 17% 5% 14% 11% 7% 6% 6% 6% 8% 15%	1380
	11th to 20th	Day	6% 18% 7% 14% 10% 8% 8% 6% 5% 5% 15%	1639
	21st to end	Day	3 11% 6% 13% 9% 7% 7% 5% 8% 9% 23%	1447
March	1st to 10th	Day	2 10% 5% 17% 10% 10% 9% 6% 6% 7% 18%	4528
	11th to 20th	Day	3 8% 5% 17% 10% 8% 7% 5% 7% 6% 22%	4841
	21st to end	Day	3 10% 5% 13% 9% 7% 7% 5% 7% 8% 27%	5959
April	1st to 10th	Day	3 12% 5% 13% 7% 7% 7% 6% 8% 8% 24%	2667
	11th to 20th	Day	2 10% 4 10% 6% 6% 5% 5% 7% 7% 37%	2814
	21st to end	Day	5% 10% 4 9% 7% 6% 6% 5% 6% 7% 34%	3113
May	1st to 10th	Day	7% 18% 6% 13% 8% 7% 7% 4 6% 6% 18%	9056
	11th to 20th	Day	9% 17% 5% 10% 7% 5% 6% 4 5% 6% 25%	9829
	21st to end	Day	12% 10% 4 11% 8% 8% 7% 5% 6% 7% 21%	11036
June	1st to 10th	Day	11% 11% 4 10% 7% 6% 6% 5% 7% 8% 24%	3780
	11th to 20th	Day	14% 8% 3 8% 6% 6% 6% 5% 7% 8% 28%	3863
	21st to end	Day	15% 8% 3 9% 6% 6% 7% 5% 8% 9% 25%	3795
July	1st to 10th	Day	13% 8% 3 9% 7% 7% 7% 5% 6% 8% 26%	8340
	11th to 20th	Day	12% 7% 3 10% 8% 7% 7% 6% 8% 9% 23%	8163
	21st to end	Day	18% 9% 4 8% 6% 6% 6% 5% 6% 8% 25%	8311
August	1st to 10th	Day	14% 9% 3 9% 6% 6% 6% 4 6% 6% 30%	5151
	11th to 20th	Day	16% 8% 3 8% 5% 6% 5% 4 7% 7% 30%	4715
	21st to end	Day	17% 12% 5% 10% 6% 5% 6% 5% 7% 7% 20%	4924
September	1st to 10th	Day	17% 10% 4 11% 7% 6% 6% 5% 5% 7% 22%	2585
	11th to 20th	Day	15% 15% 5% 9% 6% 5% 5% 4 6% 5% 23%	2306
	21st to end	Day	16% 14% 4 11% 6% 6% 5% 5% 5% 6% 23%	2200
October	1st to 10th	Day	12% 14% 5% 14% 7% 6% 5% 5% 7% 7% 19%	3120
	11th to 20th	Day	6% 11% 4 13% 7% 5% 6% 5% 6% 6% 31%	2924
	21st to end	Day	10% 15% 5% 13% 8% 8% 7% 4 6% 6% 17%	2690
November	1st to 10th	Day	8% 20% 8% 16% 10% 8% 6% 4 5% 4 11%	1441
	11th to 20th	Day	8% 15% 6% 15% 10% 8% 7% 4 5% 5% 17%	1171
	21st to end	Day	6% 16% 7% 17% 10% 6% 6% 6% 6% 8% 12%	1050
December	1st to 10th	Day	10% 12% 6% 17% 12% 9% 6% 4 5% 5% 13%	1586
	11th to 20th	Day	6% 17% 8% 18% 13% 8% 7% 5% 6% 4 8%	1557
	21st to end	Day	3 15% 6% 21% 11% 11% 10% 5% 4 4 10%	2046
January	1st to 10th	Night	4 12% 6% 17% 10% 8% 10% 6% 9% 8% 9%	10564
	11th to 20th	Night	5% 14% 6% 18% 10% 8% 7% 5% 7% 6% 15%	10040
	21st to end	Night	6% 14% 5% 14% 10% 10% 7% 5% 6% 7% 15%	10342
February	1st to 10th	Night	6% 13% 5% 13% 9% 7% 6% 5% 6% 7% 23%	3240
	11th to 20th	Night	5% 16% 6% 15% 8% 7% 6% 6% 5% 7% 19%	2868
	21st to end	Night	4 11% 5% 14% 7% 6% 6% 5% 7% 7% 28%	2359
March	1st to 10th	Night	2 12% 6% 16% 9% 7% 5% 3 5% 7% 27%	5545
	11th to 20th	Night	4 9% 5% 13% 8% 7% 6% 5% 7% 8% 27%	5336
	21st to end	Night	2 10% 4 14% 7% 7% 7% 5% 7% 9% 29%	5213
April	1st to 10th	Night	2 10% 5% 13% 7% 7% 7% 7% 8% 8% 26%	1953
	11th to 20th	Night	3 11% 4 9% 8% 6% 6% 5% 7% 8% 33%	1752
	21st to end	Night	5% 12% 3 9% 6% 6% 5% 4 7% 8% 34%	1507
May	1st to 10th	Night	7% 17% 5% 12% 7% 8% 9% 5% 6% 6% 18%	3892
	11th to 20th	Night	12% 14% 5% 10% 7% 6% 5% 5% 5% 5% 26%	3037
	21st to end	Night	13% 9% 4 10% 7% 6% 7% 6% 6% 8% 24%	3136
June	1st to 10th	Night	15% 9% 5% 9% 6% 6% 4 6% 7% 9% 24%	840
	11th to 20th	Night	13% 10% 5% 10% 6% 7% 6% 3 6% 7% 26%	747
	21st to end	Night	18% 8% 3 10% 7% 5% 7% 5% 5% 6% 25%	813
July	1st to 10th	Night	20% 8% 4 10% 8% 6% 6% 5% 5% 7% 20%	1860
	11th to 20th	Night	18% 9% 4 9% 6% 7% 7% 5% 7% 7% 21%	2029
	21st to end	Night	20% 11% 4 10% 6% 6% 6% 4 6% 8% 20%	2764
August	1st to 10th	Night	15% 9% 4 11% 5% 6% 6% 4 5% 6% 29%	2009
	11th to 20th	Night	17% 10% 4 11% 5% 6% 5% 5% 6% 8% 23%	2340
	21st to end	Night	18% 11% 4 12% 8% 6% 6% 4 6% 6% 20%	2985
September	1st to 10th	Night	20% 11% 3 10% 7% 6% 6% 4 6% 6% 22%	1813
	11th to 20th	Night	17% 14% 4 11% 7% 6% 6% 5% 6% 5% 19%	2094
	21st to end	Night	16% 15% 4 11% 7% 6% 5% 4 5% 6% 21%	2200
October	1st to 10th	Night	13% 16% 5% 11% 7% 7% 7% 5% 7% 6% 17%	4064
	11th to 20th	Night	7% 10% 5% 9% 5% 5% 5% 5% 7% 7% 34%	4224
	21st to end	Night	12% 16% 4 13% 8% 7% 7% 4 5% 5% 18%	5110
November	1st to 10th	Night	9% 19% 8% 15% 9% 5% 5% 4 5% 5% 17%	3077
	11th to 20th	Night	7% 15% 7% 16% 9% 6% 5% 5% 6% 7% 17%	3448
	21st to end	Night	6% 16% 6% 17% 9% 8% 7% 6% 6% 6% 12%	3570
December	1st to 10th	Night	10% 15% 6% 17% 10% 7% 6% 4 6% 5% 13%	5815
	11th to 20th	Night	4 18% 7% 18% 10% 10% 9% 5% 5% 5% 9%	6880
	21st to end	Night	5% 16% 8% 20% 11% 11% 10% 4 4 3 9%	9174

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	61% 8% 9% 8% 12% 2	2361
	11th to 20th	Day	64% 9% 10% 10% 6% 1	2878
	21st to end	Day	66% 8% 7% 8% 9% 1	3872
February	1st to 10th	Day	61% 10% 8% 11% 7% 2	1380
	11th to 20th	Day	67% 8% 8% 10% 6%	1646
	21st to end	Day	52% 9% 11% 16% 12%	1447
March	1st to 10th	Day	61% 8% 9% 12% 9% 2	4528
	11th to 20th	Day	55% 9% 9% 15% 12%	4841
	21st to end	Day	50% 11% 13% 15% 11% 1	5959
April	1st to 10th	Day	53% 10% 17% 16% 4	2667
	11th to 20th	Day	42% 10% 18% 23% 6%	2814
	21st to end	Day	46% 9% 15% 22% 8%	3113
May	1st to 10th	Day	61% 9% 9% 16% 5%	9056
	11th to 20th	Day	55% 8% 13% 18% 6%	9832
	21st to end	Day	57% 9% 11% 19% 4	11037
June	1st to 10th	Day	53% 12% 15% 17% 3	3780
	11th to 20th	Day	50% 12% 16% 18% 3	3863
	21st to end	Day	52% 12% 19% 16% 1	3795
July	1st to 10th	Day	53% 10% 25% 11% 1	8251
	11th to 20th	Day	52% 11% 26% 12%	8064
	21st to end	Day	54% 10% 23% 12% 1	8082
August	1st to 10th	Day	53% 10% 21% 14% 3	5132
	11th to 20th	Day	48% 11% 22% 17% 2	4760
	21st to end	Day	60% 10% 20% 10% 1	4923
September	1st to 10th	Day	59% 11% 17% 12% 1	2585
	11th to 20th	Day	60% 10% 19% 10% 1	2306
	21st to end	Day	59% 10% 17% 12% 2	2200
October	1st to 10th	Day	62% 9% 15% 13% 2	3120
	11th to 20th	Day	50% 9% 15% 15% 12%	2924
	21st to end	Day	63% 11% 14% 10% 1	2690
November	1st to 10th	Day	73% 10% 8% 8% 2	1441
	11th to 20th	Day	67% 8% 11% 11% 4	1171
	21st to end	Day	65% 10% 11% 9% 4	1050
December	1st to 10th	Day	71% 9% 7% 10% 2	1586
	11th to 20th	Day	74% 8% 8% 7% 2	1560
	21st to end	Day	72% 9% 7% 6% 5% 2	2046
January	1st to 10th	Night	62% 9% 8% 8% 12% 1	10564
	11th to 20th	Night	65% 10% 8% 10% 6% 1	10087
	21st to end	Night	64% 9% 8% 8% 9% 2	10342
February	1st to 10th	Night	58% 7% 10% 14% 8% 3	3240
	11th to 20th	Night	60% 10% 11% 11% 8%	2872
	21st to end	Night	50% 8% 13% 14% 14%	2359
March	1st to 10th	Night	56% 6% 9% 15% 13% 1	5545
	11th to 20th	Night	50% 8% 11% 15% 16%	5338
	21st to end	Night	46% 10% 14% 18% 12% 1	5213
April	1st to 10th	Night	50% 10% 16% 19% 5%	1953
	11th to 20th	Night	45% 10% 16% 21% 8%	1752
	21st to end	Night	45% 7% 17% 22% 8%	1507
May	1st to 10th	Night	60% 9% 10% 16% 4	3889
	11th to 20th	Night	56% 9% 12% 19% 4	3037
	21st to end	Night	51% 10% 13% 21% 5%	3140
June	1st to 10th	Night	53% 11% 17% 18% 2	840
	11th to 20th	Night	54% 12% 14% 18% 3	747
	21st to end	Night	53% 15% 15% 17% 1	813
July	1st to 10th	Night	61% 10% 21% 7% 1	1840
	11th to 20th	Night	57% 10% 22% 11%	2005
	21st to end	Night	60% 10% 20% 10%	2688
August	1st to 10th	Night	54% 11% 22% 12% 1	2003
	11th to 20th	Night	57% 10% 15% 18%	2361
	21st to end	Night	63% 10% 17% 9% 1	2985
September	1st to 10th	Night	61% 11% 17% 11%	1813
	11th to 20th	Night	63% 11% 13% 12% 1	2094
	21st to end	Night	62% 9% 15% 13% 2	2200
October	1st to 10th	Night	63% 9% 14% 12% 1	4064
	11th to 20th	Night	46% 10% 17% 18% 9%	4224
	21st to end	Night	66% 8% 12% 12% 2	5110
November	1st to 10th	Night	68% 8% 10% 10% 3	3077
	11th to 20th	Night	64% 9% 13% 12% 3	3448
	21st to end	Night	66% 10% 11% 8% 4	3570
December	1st to 10th	Night	70% 9% 8% 10% 3	5815
	11th to 20th	Night	75% 6% 8% 7% 4	6876
	21st to end	Night	75% 8% 7% 5% 4	9174

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% 35% 34% 12%	110
	11th to 20th	Day	12% 7% 12% 38% 21% 12%	120
	21st to end	Day	14% 6% 12% 26% 27% 15%	352
February	1st to 10th	Day	12% 5% 11% 27% 27% 18%	370
	11th to 20th	Day	9% 9% 16% 30% 24% 12%	425
	21st to end	Day	7% 5% 15% 38% 21% 14%	355
March	1st to 10th	Day	5% 6% 16% 28% 25% 20%	430
	11th to 20th	Day	6% 3 18% 24% 26% 23%	430
	21st to end	Day	5% 5% 13% 28% 24% 25%	640
April	1st to 10th	Day	4 8% 20% 30% 25% 14%	717
	11th to 20th	Day	6% 8% 15% 24% 29% 16%	740
	21st to end	Day	8% 9% 15% 27% 26% 14%	750
May	1st to 10th	Day	12% 12% 22% 23% 19% 12%	753
	11th to 20th	Day	12% 10% 20% 22% 21% 16%	775
	21st to end	Day	15% 8% 21% 22% 21% 13%	868
June	1st to 10th	Day	16% 8% 18% 21% 21% 16%	990
	11th to 20th	Day	14% 5% 16% 25% 23% 17%	1073
	21st to end	Day	14% 6% 15% 25% 21% 18%	1005
July	1st to 10th	Day	12% 6% 17% 25% 23% 17%	976
	11th to 20th	Day	17% 9% 16% 22% 20% 17%	899
	21st to end	Day	19% 5% 16% 19% 20% 22%	845
August	1st to 10th	Day	17% 10% 18% 20% 17% 18%	726
	11th to 20th	Day	22% 11% 17% 20% 17% 12%	715
	21st to end	Day	24% 10% 16% 19% 17% 14%	803
September	1st to 10th	Day	18% 14% 19% 20% 17% 11%	722
	11th to 20th	Day	18% 13% 19% 21% 17% 13%	526
	21st to end	Day	23% 10% 12% 15% 21% 19%	420
October	1st to 10th	Day	23% 12% 16% 17% 16% 16%	420
	11th to 20th	Day	18% 7% 14% 20% 21% 20%	420
	21st to end	Day	14% 6% 14% 24% 27% 15%	462
November	1st to 10th	Day	14% 7% 14% 27% 26% 12%	425
	11th to 20th	Day	14% 3 16% 27% 21% 20%	193
	21st to end	Day	7% 10% 15% 30% 22% 15%	110
December	1st to 10th	Day	13% 6% 17% 28% 27% 8%	110
	11th to 20th	Day	9% 6% 14% 39% 23% 9%	110
	21st to end	Day	9% 9% 18% 29% 24% 11%	121
January	1st to 10th	Night	8% 3 14% 34% 31% 10%	1060
	11th to 20th	Night	15% 7% 16% 36% 20% 7%	969
	21st to end	Night	15% 6% 17% 33% 22% 8%	1045
February	1st to 10th	Night	9% 5% 13% 39% 27% 7%	950
	11th to 20th	Night	10% 7% 21% 36% 21% 4	934
	21st to end	Night	7% 7% 18% 40% 21% 7%	783
March	1st to 10th	Night	7% 6% 21% 35% 25% 6%	761
	11th to 20th	Night	7% 4 20% 31% 30% 8%	690
	21st to end	Night	4 6% 16% 30% 33% 11%	693
April	1st to 10th	Night	4 5% 24% 34% 26% 6%	630
	11th to 20th	Night	7% 6% 20% 33% 29% 4	623
	21st to end	Night	8% 8% 15% 34% 30% 3	630
May	1st to 10th	Night	13% 9% 21% 32% 22% 2	550
	11th to 20th	Night	14% 8% 22% 33% 21% 1	253
	21st to end	Night	15% 6% 20% 32% 26%	228
June	1st to 10th	Night	19% 7% 17% 35% 20% 2	210
	11th to 20th	Night	18% 5% 22% 30% 24% 1	210
	21st to end	Night	18% 5% 18% 30% 27% 3	210
July	1st to 10th	Night	16% 7% 18% 34% 21% 4	205
	11th to 20th	Night	19% 7% 21% 26% 26%	204
	21st to end	Night	20% 5% 16% 29% 27% 3	337
August	1st to 10th	Night	21% 7% 19% 25% 23% 4	427
	11th to 20th	Night	25% 9% 22% 23% 17% 4	594
	21st to end	Night	22% 8% 20% 24% 23% 3	660
September	1st to 10th	Night	23% 10% 23% 26% 15% 3	600
	11th to 20th	Night	20% 12% 24% 22% 19% 4	600
	21st to end	Night	18% 10% 19% 25% 22% 6%	600
October	1st to 10th	Night	23% 11% 22% 22% 19% 4	812
	11th to 20th	Night	13% 7% 18% 29% 26% 6%	910
	21st to end	Night	17% 6% 15% 29% 27% 5%	1001
November	1st to 10th	Night	16% 6% 18% 32% 23% 5%	930
	11th to 20th	Night	13% 4 14% 35% 24% 10%	950
	21st to end	Night	11% 7% 18% 31% 23% 10%	950
December	1st to 10th	Night	18% 8% 14% 29% 23% 8%	980
	11th to 20th	Night	11% 5% 18% 32% 25% 9%	1060
	21st to end	Night	12% 6% 17% 32% 25% 9%	1166

Page generated at 2026-07-27 23:25:23. Version ac07

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