Girdwood
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Girdwood clear sky chart from 2005-11-06 to 2024-12-06.

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	18% 17% 5% 10% 6% 5% 5% 4 5% 6% 20%	106661
	Night	14% 17% 5% 10% 6% 5% 5% 4 5% 6% 23%	101671
Transparency	Day	67% 7% 11% 10% 4 1	106355
	Night	64% 6% 9% 10% 8% 2	101626
Seeing	Day	25% 14% 22% 24% 12% 3	18033
	Night	23% 13% 22% 24% 14% 3	22557

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	4 25% 7% 11% 7% 7% 5% 3 5% 8% 18%	1279
	11th to 20th	Day	12% 16% 5% 8% 4 4 3 3 4 6% 35%	1765
	21st to end	Day	8% 15% 6% 12% 7% 7% 6% 3 5% 7% 24%	2090
February	1st to 10th	Day	7% 17% 5% 9% 6% 6% 4 3 6% 7% 30%	1329
	11th to 20th	Day	12% 23% 6% 14% 7% 6% 5% 4 5% 5% 14%	1479
	21st to end	Day	6% 16% 6% 12% 8% 5% 4 4 6% 6% 28%	1356
March	1st to 10th	Day	6% 18% 7% 15% 9% 5% 5% 3 5% 5% 22%	3100
	11th to 20th	Day	5% 18% 6% 13% 7% 7% 7% 5% 7% 7% 20%	3244
	21st to end	Day	17% 23% 6% 12% 5% 4 4 3 5% 4 15%	4163
April	1st to 10th	Day	7% 22% 7% 14% 8% 6% 4 3 4 6% 18%	2312
	11th to 20th	Day	6% 17% 4 11% 7% 5% 6% 4 6% 7% 27%	2621
	21st to end	Day	7% 18% 5% 10% 7% 7% 5% 4 5% 7% 26%	2688
May	1st to 10th	Day	15% 29% 7% 12% 6% 5% 4 3 4 4 11%	4964
	11th to 20th	Day	9% 15% 4 10% 6% 6% 5% 4 6% 7% 29%	5023
	21st to end	Day	16% 17% 5% 11% 6% 6% 6% 4 6% 5% 18%	5869
June	1st to 10th	Day	14% 15% 4 9% 7% 7% 6% 5% 7% 7% 20%	3420
	11th to 20th	Day	13% 14% 5% 9% 7% 6% 6% 6% 7% 6% 22%	3420
	21st to end	Day	16% 14% 4 10% 8% 6% 6% 5% 6% 6% 20%	3420
July	1st to 10th	Day	28% 15% 4 9% 6% 6% 5% 3 5% 5% 14%	5700
	11th to 20th	Day	32% 16% 3 8% 5% 4 5% 4 5% 6% 12%	5450
	21st to end	Day	24% 11% 4 8% 5% 5% 5% 4 7% 7% 21%	5499
August	1st to 10th	Day	31% 10% 4 9% 5% 4 4 4 5% 6% 17%	5024
	11th to 20th	Day	30% 11% 4 8% 4 5% 4 3 5% 6% 19%	4384
	21st to end	Day	29% 14% 3 8% 5% 4 5% 3 5% 6% 17%	4884
September	1st to 10th	Day	34% 16% 4 10% 5% 5% 4 3 4 4 12%	2336
	11th to 20th	Day	33% 16% 4 8% 5% 5% 3 4 4 3 15%	2271
	21st to end	Day	26% 18% 5% 9% 5% 5% 4 3 4 5% 16%	2034
October	1st to 10th	Day	22% 19% 5% 10% 6% 6% 5% 4 4 5% 13%	3112
	11th to 20th	Day	16% 16% 4 9% 5% 4 4 2 4 5% 32%	2834
	21st to end	Day	19% 17% 5% 9% 4 3 4 3 4 5% 27%	2680
November	1st to 10th	Day	12% 18% 7% 11% 5% 5% 5% 4 6% 6% 21%	1371
	11th to 20th	Day	10% 15% 4 9% 5% 7% 5% 5% 7% 9% 23%	1230
	21st to end	Day	11% 20% 5% 12% 6% 6% 5% 3 6% 7% 20%	1037
December	1st to 10th	Day	19% 19% 6% 11% 6% 4 5% 4 4 8% 14%	909
	11th to 20th	Day	8% 24% 6% 10% 8% 6% 5% 4 7% 8% 14%	967
	21st to end	Day	6% 17% 6% 11% 7% 6% 6% 3 7% 6% 26%	1397
January	1st to 10th	Night	7% 24% 5% 10% 6% 6% 5% 4 6% 7% 19%	5672
	11th to 20th	Night	14% 14% 5% 9% 5% 4 3 3 4 4 34%	5145
	21st to end	Night	9% 14% 5% 11% 7% 6% 5% 4 6% 6% 28%	5577
February	1st to 10th	Night	6% 16% 6% 9% 6% 5% 5% 4 5% 6% 30%	2851
	11th to 20th	Night	14% 22% 5% 11% 7% 6% 6% 4 5% 6% 13%	2588
	21st to end	Night	8% 15% 5% 10% 6% 5% 5% 5% 7% 6% 28%	2098
March	1st to 10th	Night	5% 21% 7% 12% 7% 5% 4 3 5% 4 28%	3754
	11th to 20th	Night	5% 17% 6% 13% 6% 5% 5% 5% 7% 9% 23%	3712
	21st to end	Night	10% 23% 7% 11% 5% 5% 4 3 5% 6% 21%	3467
April	1st to 10th	Night	7% 21% 6% 12% 7% 5% 5% 4 6% 4 23%	1868
	11th to 20th	Night	8% 15% 3 7% 6% 4 6% 6% 8% 8% 30%	1505
	21st to end	Night	6% 16% 6% 10% 6% 6% 5% 4 6% 7% 29%	1492
May	1st to 10th	Night	15% 23% 6% 11% 7% 5% 6% 4 4 5% 14%	2006
	11th to 20th	Night	11% 17% 5% 10% 5% 6% 5% 4 6% 6% 26%	1882
	21st to end	Night	13% 16% 5% 10% 7% 5% 6% 5% 6% 7% 19%	1706
June	1st to 10th	Night	14% 14% 5% 10% 5% 5% 6% 4 8% 8% 21%	760
	11th to 20th	Night	14% 16% 4 8% 4 4 5% 6% 9% 7% 24%	760
	21st to end	Night	17% 11% 4 10% 8% 7% 6% 4 5% 8% 20%	760
July	1st to 10th	Night	30% 12% 3 7% 5% 6% 4 4 5% 6% 19%	1270
	11th to 20th	Night	33% 13% 2 6% 4 4 3 4 5% 6% 20%	1520
	21st to end	Night	25% 11% 3 7% 4 5% 6% 3 6% 8% 24%	2066
August	1st to 10th	Night	32% 11% 3 7% 5% 5% 4 3 4 6% 22%	1915
	11th to 20th	Night	29% 9% 2 6% 4 3 5% 3 5% 8% 26%	2454
	21st to end	Night	29% 11% 3 9% 6% 5% 4 3 4 5% 20%	2783
September	1st to 10th	Night	32% 14% 5% 10% 5% 5% 5% 3 3 4 14%	1840
	11th to 20th	Night	32% 14% 4 8% 5% 4 4 3 5% 4 17%	1909
	21st to end	Night	26% 14% 4 10% 5% 5% 4 3 5% 4 19%	2143
October	1st to 10th	Night	22% 17% 4 11% 7% 6% 5% 3 4 5% 16%	3850
	11th to 20th	Night	14% 16% 4 9% 5% 4 4 3 4 5% 32%	4094
	21st to end	Night	14% 17% 4 9% 6% 5% 4 3 4 5% 27%	4877
November	1st to 10th	Night	14% 19% 5% 11% 6% 5% 5% 3 5% 6% 20%	2927
	11th to 20th	Night	9% 15% 3 8% 5% 5% 6% 5% 8% 10% 26%	3166
	21st to end	Night	11% 19% 5% 11% 7% 4 5% 4 6% 6% 22%	3362
December	1st to 10th	Night	15% 21% 5% 10% 6% 6% 6% 3 5% 7% 16%	3382
	11th to 20th	Night	8% 22% 8% 13% 7% 6% 5% 4 7% 6% 15%	4240
	21st to end	Night	7% 18% 5% 10% 6% 5% 5% 4 5% 6% 27%	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	66% 6% 4 8% 12% 4	1279
	11th to 20th	Day	55% 2 4 13% 22% 3	1783
	21st to end	Day	58% 8% 5% 8% 16% 6%	2090
February	1st to 10th	Day	57% 5% 8% 13% 10% 6%	1329
	11th to 20th	Day	75% 4 4 9% 6% 2	1486
	21st to end	Day	55% 8% 7% 13% 15% 2	1356
March	1st to 10th	Day	65% 5% 5% 9% 9% 6%	3100
	11th to 20th	Day	62% 9% 6% 12% 10% 1	3244
	21st to end	Day	72% 6% 8% 7% 5% 1	4163
April	1st to 10th	Day	69% 8% 8% 10% 5% 1	2312
	11th to 20th	Day	57% 8% 12% 15% 8%	2621
	21st to end	Day	62% 6% 10% 15% 7% 1	2688
May	1st to 10th	Day	80% 5% 6% 6% 2	4964
	11th to 20th	Day	58% 6% 10% 22% 3	5026
	21st to end	Day	69% 5% 11% 13% 2	5870
June	1st to 10th	Day	66% 7% 15% 11% 1	3420
	11th to 20th	Day	62% 9% 14% 13% 2	3420
	21st to end	Day	64% 8% 20% 8%	3420
July	1st to 10th	Day	69% 10% 15% 5%	5611
	11th to 20th	Day	71% 11% 16% 3	5352
	21st to end	Day	62% 10% 21% 7%	5326
August	1st to 10th	Day	71% 7% 18% 5%	5005
	11th to 20th	Day	67% 8% 12% 12% 1	4426
	21st to end	Day	68% 7% 13% 10% 1	4883
September	1st to 10th	Day	80% 5% 7% 6% 2	2336
	11th to 20th	Day	77% 6% 8% 9% 1	2271
	21st to end	Day	72% 6% 9% 11% 3	2034
October	1st to 10th	Day	76% 7% 9% 7% 1	3112
	11th to 20th	Day	61% 6% 14% 10% 9%	2834
	21st to end	Day	63% 6% 10% 14% 6% 1	2680
November	1st to 10th	Day	67% 6% 7% 11% 6% 3	1371
	11th to 20th	Day	61% 4 7% 17% 9% 1	1230
	21st to end	Day	70% 3 8% 9% 8% 3	1037
December	1st to 10th	Day	78% 3 6% 7% 5% 1	909
	11th to 20th	Day	71% 8% 5% 10% 5% 1	970
	21st to end	Day	54% 10% 6% 8% 13% 10%	1397
January	1st to 10th	Night	66% 6% 5% 9% 11% 3	5672
	11th to 20th	Night	58% 2 4 11% 18% 6%	5190
	21st to end	Night	55% 6% 4 10% 17% 7%	5577
February	1st to 10th	Night	58% 5% 7% 11% 13% 6%	2851
	11th to 20th	Night	74% 4 6% 9% 6% 1	2592
	21st to end	Night	54% 7% 9% 13% 16% 2	2098
March	1st to 10th	Night	61% 4 5% 9% 13% 8%	3754
	11th to 20th	Night	57% 8% 9% 12% 13% 1	3714
	21st to end	Night	67% 6% 10% 9% 6% 2	3467
April	1st to 10th	Night	63% 6% 10% 14% 6% 1	1868
	11th to 20th	Night	53% 7% 14% 17% 8%	1505
	21st to end	Night	59% 6% 10% 16% 9% 1	1492
May	1st to 10th	Night	75% 7% 7% 8% 3	2006
	11th to 20th	Night	58% 7% 14% 17% 4	1883
	21st to end	Night	67% 7% 12% 12% 2	1706
June	1st to 10th	Night	66% 7% 14% 11% 1	760
	11th to 20th	Night	58% 9% 19% 11% 2	760
	21st to end	Night	62% 9% 22% 6% 1	760
July	1st to 10th	Night	68% 10% 15% 6%	1250
	11th to 20th	Night	69% 9% 19% 3	1495
	21st to end	Night	64% 10% 23% 4	2000
August	1st to 10th	Night	69% 7% 18% 6%	1909
	11th to 20th	Night	63% 9% 16% 10% 3	2478
	21st to end	Night	69% 7% 11% 11% 2	2783
September	1st to 10th	Night	79% 5% 8% 7% 1	1840
	11th to 20th	Night	74% 5% 9% 8% 3	1909
	21st to end	Night	69% 5% 11% 12% 3	2143
October	1st to 10th	Night	73% 6% 9% 10% 2	3850
	11th to 20th	Night	59% 8% 10% 13% 9%	4094
	21st to end	Night	64% 7% 12% 11% 5% 1	4877
November	1st to 10th	Night	69% 4 7% 9% 7% 3	2927
	11th to 20th	Night	59% 6% 7% 16% 11% 1	3166
	21st to end	Night	67% 5% 6% 10% 9% 3	3362
December	1st to 10th	Night	75% 4 6% 8% 7% 1	3382
	11th to 20th	Night	70% 6% 6% 9% 9% 1	4236
	21st to end	Night	57% 7% 5% 9% 13% 10%	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	20% 14% 16% 22% 23% 5%	100
	11th to 20th	Day	21% 11% 29% 23% 12% 3	280
	21st to end	Day	21% 16% 26% 20% 12% 4	308
February	1st to 10th	Day	15% 14% 26% 24% 14% 7%	293
	11th to 20th	Day	30% 14% 25% 21% 7% 2	365
	21st to end	Day	20% 18% 27% 22% 6% 7%	307
March	1st to 10th	Day	18% 11% 33% 20% 14% 4	370
	11th to 20th	Day	15% 17% 25% 24% 14% 4	415
	21st to end	Day	17% 18% 26% 22% 13% 3	616
April	1st to 10th	Day	16% 23% 27% 24% 9% 1	576
	11th to 20th	Day	14% 17% 20% 30% 14% 3	640
	21st to end	Day	18% 14% 22% 30% 13% 4	650
May	1st to 10th	Day	27% 18% 25% 21% 9% 2	653
	11th to 20th	Day	20% 12% 26% 26% 14% 2	675
	21st to end	Day	22% 10% 30% 25% 11% 1	774
June	1st to 10th	Day	19% 13% 20% 28% 17% 3	870
	11th to 20th	Day	16% 10% 21% 36% 15% 2	870
	21st to end	Day	19% 12% 20% 31% 15% 3	870
July	1st to 10th	Day	22% 14% 21% 29% 13% 2	856
	11th to 20th	Day	28% 11% 19% 27% 11% 2	811
	21st to end	Day	29% 12% 19% 25% 14% 1	739
August	1st to 10th	Day	32% 14% 21% 23% 8% 2	676
	11th to 20th	Day	37% 16% 20% 18% 8% 2	665
	21st to end	Day	36% 13% 20% 21% 9% 1	748
September	1st to 10th	Day	39% 17% 17% 17% 8% 1	605
	11th to 20th	Day	42% 13% 17% 16% 10% 2	571
	21st to end	Day	35% 16% 17% 18% 11% 3	390
October	1st to 10th	Day	38% 16% 18% 16% 9% 2	390
	11th to 20th	Day	28% 14% 23% 22% 10% 4	390
	21st to end	Day	31% 13% 23% 18% 12% 3	429
November	1st to 10th	Day	24% 14% 24% 19% 15% 5%	395
	11th to 20th	Day	17% 13% 26% 24% 13% 6%	315
	21st to end	Day	21% 12% 19% 31% 12% 4	137
December	1st to 10th	Day	24% 15% 19% 21% 17% 4	95
	11th to 20th	Day	23% 17% 27% 26% 6% 2	90
	21st to end	Day	22% 17% 31% 15% 10% 4	99
January	1st to 10th	Night	15% 17% 20% 26% 17% 4	940
	11th to 20th	Night	22% 15% 26% 21% 12% 4	882
	21st to end	Night	21% 13% 28% 22% 13% 3	935
February	1st to 10th	Night	14% 13% 28% 29% 13% 4	850
	11th to 20th	Night	29% 16% 28% 19% 8% 1	834
	21st to end	Night	17% 15% 32% 21% 11% 4	603
March	1st to 10th	Night	17% 11% 28% 28% 15% 2	660
	11th to 20th	Night	12% 13% 24% 33% 14% 4	660
	21st to end	Night	17% 11% 26% 24% 17% 5%	627
April	1st to 10th	Night	18% 16% 22% 26% 16% 3	570
	11th to 20th	Night	16% 10% 19% 28% 23% 3	563
	21st to end	Night	14% 15% 21% 30% 17% 3	570
May	1st to 10th	Night	19% 15% 23% 24% 18% 2	447
	11th to 20th	Night	19% 10% 15% 31% 23% 3	374
	21st to end	Night	17% 10% 23% 26% 21% 4	247
June	1st to 10th	Night	19% 13% 15% 29% 19% 4	190
	11th to 20th	Night	21% 10% 12% 32% 24% 2	190
	21st to end	Night	22% 9% 14% 34% 17% 4	190
July	1st to 10th	Night	22% 8% 16% 28% 24% 3	185
	11th to 20th	Night	29% 8% 15% 25% 21% 2	279
	21st to end	Night	30% 7% 16% 28% 17% 3	416
August	1st to 10th	Night	35% 12% 18% 20% 12% 2	378
	11th to 20th	Night	36% 11% 23% 18% 11% 1	542
	21st to end	Night	34% 9% 16% 26% 12% 2	627
September	1st to 10th	Night	37% 14% 21% 16% 11% 1	570
	11th to 20th	Night	39% 13% 15% 19% 14% 1	570
	21st to end	Night	33% 11% 15% 22% 16% 3	598
October	1st to 10th	Night	34% 13% 18% 20% 13% 2	694
	11th to 20th	Night	28% 12% 20% 24% 13% 3	860
	21st to end	Night	26% 11% 23% 23% 13% 3	946
November	1st to 10th	Night	25% 14% 23% 23% 13% 2	880
	11th to 20th	Night	17% 11% 25% 28% 15% 3	900
	21st to end	Night	23% 13% 23% 24% 12% 5%	900
December	1st to 10th	Night	27% 13% 22% 20% 13% 5%	906
	11th to 20th	Night	20% 13% 26% 26% 13% 3	940
	21st to end	Night	24% 19% 22% 19% 12% 4	1034

Page generated at 2024-12-03 02:45:09. Version ac07

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