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 2024-02-25.

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	11% 11% 5% 10% 8% 5% 7% 6% 6% 7% 24%	76705
	Night	9% 13% 6% 12% 10% 5% 7% 6% 6% 7% 19%	82561
Transparency	Day	56% 11% 15% 14% 4	76347
	Night	60% 10% 12% 12% 5% 1	82500
Seeing	Day	14% 7% 16% 24% 22% 16%	16475
	Night	14% 6% 18% 31% 25% 6%	21529

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% 15% 16% 7% 8% 10% 6% 4 10%	1269
	11th to 20th	Day	11% 15% 5% 12% 10% 4 5% 6% 7% 6% 17%	1514
	21st to end	Day	9% 18% 8% 15% 10% 4 6% 5% 5% 6% 13%	2090
February	1st to 10th	Day	6% 15% 6% 13% 12% 5% 8% 7% 6% 8% 15%	1247
	11th to 20th	Day	6% 17% 7% 13% 11% 6% 9% 8% 5% 5% 14%	1479
	21st to end	Day	4 11% 6% 11% 10% 6% 7% 6% 8% 9% 23%	1272
March	1st to 10th	Day	4 10% 5% 12% 10% 6% 8% 6% 6% 8% 26%	1779
	11th to 20th	Day	3 9% 5% 8% 9% 5% 7% 7% 8% 9% 29%	1877
	21st to end	Day	3 11% 5% 9% 9% 5% 7% 6% 7% 8% 31%	2321
April	1st to 10th	Day	3 10% 6% 11% 8% 5% 8% 6% 8% 8% 26%	2286
	11th to 20th	Day	3 8% 5% 8% 7% 4 6% 6% 7% 8% 38%	2406
	21st to end	Day	6% 9% 4 8% 8% 5% 6% 6% 6% 7% 35%	2668
May	1st to 10th	Day	8% 12% 6% 10% 9% 5% 7% 6% 6% 7% 25%	2767
	11th to 20th	Day	9% 11% 4 8% 7% 4 5% 6% 6% 6% 34%	2969
	21st to end	Day	11% 10% 4 7% 6% 4 6% 6% 7% 8% 32%	3324
June	1st to 10th	Day	11% 10% 4 8% 8% 4 7% 7% 7% 8% 25%	3240
	11th to 20th	Day	15% 8% 4 7% 7% 5% 7% 7% 7% 8% 26%	3311
	21st to end	Day	15% 8% 3 8% 8% 4 7% 6% 8% 9% 25%	3254
July	1st to 10th	Day	13% 8% 4 7% 7% 4 7% 6% 6% 8% 29%	3240
	11th to 20th	Day	15% 8% 4 9% 8% 4 7% 7% 7% 8% 22%	3178
	21st to end	Day	16% 8% 4 7% 7% 4 7% 6% 6% 8% 26%	3239
August	1st to 10th	Day	17% 8% 4 8% 8% 5% 6% 6% 5% 7% 25%	2840
	11th to 20th	Day	20% 10% 4 8% 7% 5% 5% 5% 6% 6% 24%	2607
	21st to end	Day	18% 11% 5% 10% 6% 4 6% 6% 7% 7% 21%	2717
September	1st to 10th	Day	18% 10% 5% 10% 8% 4 6% 6% 5% 6% 22%	2327
	11th to 20th	Day	15% 14% 5% 8% 7% 4 5% 6% 6% 6% 25%	2076
	21st to end	Day	16% 13% 4 9% 7% 4 5% 6% 6% 6% 24%	1980
October	1st to 10th	Day	17% 14% 6% 10% 8% 5% 6% 5% 6% 6% 18%	1728
	11th to 20th	Day	10% 14% 5% 15% 10% 4 6% 5% 6% 5% 19%	1620
	21st to end	Day	10% 14% 6% 12% 10% 5% 8% 5% 7% 6% 18%	1507
November	1st to 10th	Day	8% 20% 7% 13% 12% 6% 7% 6% 5% 4 12%	1301
	11th to 20th	Day	8% 15% 6% 12% 12% 5% 7% 6% 5% 6% 18%	1060
	21st to end	Day	6% 16% 8% 16% 10% 5% 6% 8% 5% 9% 11%	950
December	1st to 10th	Day	13% 14% 6% 12% 11% 6% 8% 5% 7% 6% 12%	898
	11th to 20th	Day	5% 20% 9% 16% 11% 5% 6% 6% 8% 5% 8%	967
	21st to end	Day	4 12% 5% 17% 13% 7% 12% 6% 5% 6% 13%	1397
January	1st to 10th	Night	4 13% 6% 17% 15% 8% 10% 8% 6% 4 9%	5686
	11th to 20th	Night	8% 13% 6% 14% 11% 6% 8% 7% 8% 6% 13%	5384
	21st to end	Night	8% 19% 8% 13% 9% 4 6% 5% 6% 8% 15%	5577
February	1st to 10th	Night	6% 12% 6% 11% 10% 6% 7% 6% 6% 7% 23%	2933
	11th to 20th	Night	6% 15% 7% 14% 10% 5% 7% 7% 6% 7% 18%	2588
	21st to end	Night	4 11% 5% 12% 8% 4 6% 6% 7% 8% 30%	2078
March	1st to 10th	Night	4 11% 5% 8% 7% 4 6% 5% 6% 9% 34%	2178
	11th to 20th	Night	4 8% 5% 8% 6% 4 6% 7% 8% 9% 35%	2080
	21st to end	Night	3 10% 5% 9% 8% 4 5% 5% 6% 9% 36%	2034
April	1st to 10th	Night	2 9% 5% 11% 8% 4 8% 8% 8% 8% 27%	1674
	11th to 20th	Night	3 9% 5% 7% 9% 5% 6% 6% 8% 9% 33%	1500
	21st to end	Night	5% 11% 4 8% 8% 5% 4 5% 7% 9% 35%	1292
May	1st to 10th	Night	9% 13% 6% 9% 8% 5% 9% 5% 5% 6% 25%	1192
	11th to 20th	Night	10% 9% 2 8% 7% 5% 6% 7% 6% 8% 32%	925
	21st to end	Night	13% 9% 3 7% 9% 4 6% 5% 5% 8% 31%	948
June	1st to 10th	Night	15% 8% 5% 7% 7% 5% 4 6% 7% 10% 25%	720
	11th to 20th	Night	15% 9% 6% 9% 8% 5% 5% 4 6% 7% 25%	640
	21st to end	Night	18% 7% 4 8% 8% 4 7% 7% 6% 6% 24%	696
July	1st to 10th	Night	14% 8% 5% 7% 8% 4 5% 8% 7% 11% 23%	720
	11th to 20th	Night	18% 9% 4 8% 8% 6% 9% 5% 7% 8% 16%	782
	21st to end	Night	19% 11% 5% 9% 7% 5% 8% 5% 5% 7% 19%	1078
August	1st to 10th	Night	16% 8% 4 7% 8% 5% 8% 7% 6% 7% 23%	1099
	11th to 20th	Night	22% 11% 4 12% 7% 5% 5% 5% 5% 6% 18%	1290
	21st to end	Night	20% 11% 3 7% 7% 4 5% 5% 7% 7% 22%	1639
September	1st to 10th	Night	20% 12% 4 9% 8% 4 5% 5% 5% 7% 22%	1631
	11th to 20th	Night	17% 13% 4 10% 8% 5% 6% 6% 6% 6% 20%	1884
	21st to end	Night	17% 14% 4 10% 8% 4 5% 5% 5% 6% 21%	1980
October	1st to 10th	Night	17% 16% 6% 9% 7% 5% 6% 5% 5% 5% 19%	2229
	11th to 20th	Night	9% 12% 6% 10% 8% 4 6% 6% 7% 7% 25%	2340
	21st to end	Night	10% 16% 5% 10% 9% 5% 6% 6% 6% 6% 20%	2848
November	1st to 10th	Night	9% 18% 8% 12% 11% 4 6% 4 5% 5% 18%	2777
	11th to 20th	Night	8% 15% 6% 14% 10% 5% 5% 6% 6% 7% 18%	3119
	21st to end	Night	7% 16% 7% 15% 11% 5% 7% 8% 6% 7% 12%	3230
December	1st to 10th	Night	14% 16% 7% 12% 10% 5% 7% 6% 6% 6% 13%	3280
	11th to 20th	Night	6% 18% 8% 15% 11% 5% 7% 7% 7% 7% 10%	4240
	21st to end	Night	5% 15% 8% 16% 12% 7% 10% 7% 5% 4 10%	6270

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	67% 10% 11% 6% 4 1	1269
	11th to 20th	Day	62% 5% 9% 13% 9% 2	1529
	21st to end	Day	67% 8% 5% 9% 10% 1	2090
February	1st to 10th	Day	61% 10% 7% 11% 8% 3	1247
	11th to 20th	Day	68% 9% 8% 10% 5%	1486
	21st to end	Day	52% 9% 11% 17% 11%	1272
March	1st to 10th	Day	53% 8% 13% 13% 12%	1779
	11th to 20th	Day	45% 10% 12% 20% 12%	1877
	21st to end	Day	47% 11% 19% 15% 7% 1	2321
April	1st to 10th	Day	52% 10% 18% 15% 5%	2286
	11th to 20th	Day	42% 11% 20% 22% 5%	2406
	21st to end	Day	46% 9% 15% 21% 9%	2668
May	1st to 10th	Day	57% 9% 14% 17% 4	2767
	11th to 20th	Day	47% 9% 13% 21% 9% 1	2972
	21st to end	Day	48% 8% 16% 22% 5%	3325
June	1st to 10th	Day	53% 12% 16% 16% 3	3240
	11th to 20th	Day	53% 12% 15% 17% 4	3311
	21st to end	Day	53% 13% 19% 14% 1	3254
July	1st to 10th	Day	52% 11% 18% 17% 3	3151
	11th to 20th	Day	57% 14% 20% 9%	3079
	21st to end	Day	52% 14% 22% 10% 1	3059
August	1st to 10th	Day	57% 12% 19% 11% 2	2821
	11th to 20th	Day	59% 11% 18% 11% 1	2607
	21st to end	Day	60% 11% 17% 10% 1	2717
September	1st to 10th	Day	60% 12% 16% 12% 1	2327
	11th to 20th	Day	58% 11% 20% 10% 1	2076
	21st to end	Day	59% 10% 17% 12% 2	1980
October	1st to 10th	Day	66% 9% 13% 10% 2	1728
	11th to 20th	Day	65% 11% 13% 8% 2	1620
	21st to end	Day	61% 13% 12% 12% 2	1507
November	1st to 10th	Day	71% 11% 8% 8% 2	1301
	11th to 20th	Day	65% 9% 11% 11% 3	1060
	21st to end	Day	64% 11% 12% 9% 4	950
December	1st to 10th	Day	69% 10% 7% 11% 3	898
	11th to 20th	Day	71% 8% 9% 10% 2	970
	21st to end	Day	63% 11% 9% 8% 6% 3	1397
January	1st to 10th	Night	67% 12% 9% 7% 4 1	5686
	11th to 20th	Night	62% 11% 7% 11% 8% 2	5431
	21st to end	Night	63% 7% 8% 11% 9% 1	5577
February	1st to 10th	Night	58% 7% 9% 14% 8% 4	2933
	11th to 20th	Night	60% 11% 11% 11% 6%	2592
	21st to end	Night	49% 9% 13% 15% 13%	2078
March	1st to 10th	Night	44% 8% 14% 19% 14%	2178
	11th to 20th	Night	40% 7% 13% 21% 17% 1	2082
	21st to end	Night	43% 9% 14% 20% 12% 2	2034
April	1st to 10th	Night	50% 11% 15% 18% 6%	1674
	11th to 20th	Night	44% 11% 17% 21% 7%	1500
	21st to end	Night	45% 7% 17% 22% 9%	1292
May	1st to 10th	Night	58% 8% 12% 19% 3	1192
	11th to 20th	Night	47% 11% 13% 21% 9%	925
	21st to end	Night	51% 10% 13% 21% 6%	948
June	1st to 10th	Night	52% 12% 17% 17% 2	720
	11th to 20th	Night	56% 13% 12% 16% 3	640
	21st to end	Night	54% 16% 14% 15% 1	696
July	1st to 10th	Night	51% 13% 17% 16% 2	700
	11th to 20th	Night	62% 15% 15% 7%	758
	21st to end	Night	63% 12% 16% 9%	1018
August	1st to 10th	Night	56% 15% 17% 10% 2	1093
	11th to 20th	Night	67% 10% 10% 13%	1290
	21st to end	Night	58% 13% 17% 11% 1	1639
September	1st to 10th	Night	60% 11% 18% 11%	1631
	11th to 20th	Night	62% 11% 13% 12% 1	1884
	21st to end	Night	62% 9% 14% 13% 2	1980
October	1st to 10th	Night	65% 10% 11% 12% 2	2229
	11th to 20th	Night	55% 12% 19% 12% 2	2340
	21st to end	Night	62% 9% 13% 13% 3	2848
November	1st to 10th	Night	67% 9% 10% 11% 3	2777
	11th to 20th	Night	62% 10% 13% 13% 3	3119
	21st to end	Night	66% 11% 11% 8% 4	3230
December	1st to 10th	Night	69% 10% 8% 11% 3	3280
	11th to 20th	Night	69% 8% 11% 8% 3	4236
	21st to end	Night	69% 10% 8% 7% 5%	6270

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	3 4 9% 38% 36% 10%	90
	11th to 20th	Day	11% 3 11% 39% 21% 14%	98
	21st to end	Day	13% 5% 11% 28% 29% 14%	308
February	1st to 10th	Day	11% 6% 11% 28% 28% 17%	320
	11th to 20th	Day	10% 9% 18% 30% 23% 11%	365
	21st to end	Day	8% 5% 14% 39% 21% 14%	293
March	1st to 10th	Day	6% 6% 16% 30% 21% 20%	340
	11th to 20th	Day	7% 4 18% 25% 27% 20%	340
	21st to end	Day	5% 6% 15% 28% 25% 21%	517
April	1st to 10th	Day	4 6% 18% 32% 26% 13%	576
	11th to 20th	Day	5% 7% 15% 24% 30% 19%	590
	21st to end	Day	9% 7% 15% 28% 26% 15%	600
May	1st to 10th	Day	12% 10% 21% 24% 21% 13%	603
	11th to 20th	Day	11% 8% 20% 23% 21% 16%	625
	21st to end	Day	16% 9% 20% 21% 21% 13%	699
June	1st to 10th	Day	16% 7% 16% 23% 21% 17%	810
	11th to 20th	Day	16% 5% 17% 25% 22% 16%	881
	21st to end	Day	15% 6% 17% 24% 20% 19%	824
July	1st to 10th	Day	12% 6% 16% 23% 23% 20%	796
	11th to 20th	Day	18% 7% 14% 22% 22% 18%	730
	21st to end	Day	17% 4 15% 18% 20% 25%	684
August	1st to 10th	Day	18% 8% 17% 21% 17% 19%	626
	11th to 20th	Day	22% 11% 18% 20% 18% 12%	615
	21st to end	Day	24% 10% 14% 18% 17% 16%	693
September	1st to 10th	Day	19% 13% 18% 18% 18% 13%	624
	11th to 20th	Day	17% 13% 19% 19% 18% 14%	456
	21st to end	Day	25% 10% 13% 17% 20% 16%	360
October	1st to 10th	Day	23% 12% 16% 17% 17% 15%	360
	11th to 20th	Day	17% 8% 14% 19% 21% 21%	360
	21st to end	Day	14% 6% 15% 23% 25% 17%	396
November	1st to 10th	Day	14% 6% 15% 26% 25% 13%	365
	11th to 20th	Day	16% 2 17% 27% 21% 18%	162
	21st to end	Day	6% 9% 18% 34% 20% 13%	90
December	1st to 10th	Day	13% 6% 16% 31% 28% 7%	90
	11th to 20th	Day	10% 4 16% 39% 22% 9%	90
	21st to end	Day	7% 9% 20% 28% 24% 11%	99
January	1st to 10th	Night	7% 3 14% 34% 32% 10%	940
	11th to 20th	Night	15% 5% 15% 35% 21% 7%	866
	21st to end	Night	14% 5% 17% 34% 22% 8%	935
February	1st to 10th	Night	9% 5% 14% 38% 27% 7%	850
	11th to 20th	Night	11% 8% 22% 36% 19% 5%	834
	21st to end	Night	7% 5% 17% 41% 21% 8%	680
March	1st to 10th	Night	8% 7% 21% 33% 25% 6%	638
	11th to 20th	Night	7% 4 21% 30% 30% 8%	584
	21st to end	Night	5% 5% 17% 30% 31% 11%	594
April	1st to 10th	Night	5% 6% 22% 35% 28% 5%	540
	11th to 20th	Night	7% 5% 19% 32% 32% 5%	533
	21st to end	Night	9% 6% 16% 34% 31% 4	540
May	1st to 10th	Night	13% 7% 21% 32% 25% 2	472
	11th to 20th	Night	15% 6% 22% 34% 22% 1	217
	21st to end	Night	15% 5% 19% 31% 29%	195
June	1st to 10th	Night	18% 6% 17% 35% 21% 3	180
	11th to 20th	Night	19% 4 22% 31% 23% 1	180
	21st to end	Night	18% 4 18% 26% 29% 3	180
July	1st to 10th	Night	16% 7% 17% 33% 23% 5%	175
	11th to 20th	Night	19% 5% 20% 28% 27% 1	174
	21st to end	Night	20% 4 15% 28% 29% 3	290
August	1st to 10th	Night	21% 7% 18% 25% 24% 4	383
	11th to 20th	Night	25% 9% 22% 23% 16% 4	534
	21st to end	Night	23% 8% 19% 23% 24% 4	594
September	1st to 10th	Night	24% 9% 21% 27% 16% 4	540
	11th to 20th	Night	21% 12% 24% 21% 19% 4	540
	21st to end	Night	19% 10% 20% 24% 20% 6%	540
October	1st to 10th	Night	24% 10% 20% 22% 20% 4	722
	11th to 20th	Night	13% 7% 17% 29% 27% 7%	810
	21st to end	Night	17% 6% 15% 29% 29% 5%	891
November	1st to 10th	Night	17% 5% 18% 31% 24% 5%	830
	11th to 20th	Night	14% 4 15% 35% 23% 9%	850
	21st to end	Night	12% 8% 19% 30% 22% 10%	850
December	1st to 10th	Night	19% 8% 12% 29% 24% 8%	874
	11th to 20th	Night	12% 5% 18% 29% 25% 9%	940
	21st to end	Night	12% 5% 17% 31% 25% 9%	1034

Page generated at 2024-02-22 02:05:46. Version ac07

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