Grouse Gulch
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Grouse Gulch clear sky chart from 2005-11-06 to 2026-02-01.

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	4 8% 4 11% 6% 5% 5% 4 5% 6% 42%	136109
	Night	1 4 3 8% 5% 4 4 4 6% 8% 52%	133422
Transparency	Day	37% 9% 9% 17% 19% 9%	131018
	Night	24% 10% 11% 21% 23% 10%	129516
Seeing	Day	18% 22% 29% 19% 9% 2	16899
	Night	8% 13% 27% 32% 18% 3	27150

By Month

Percentage Cloud Cover SW (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	3 3 15% 9% 6% 4 5% 6% 6% 43%	4066
	11th to 20th	Day	4 2 11% 10% 6% 6% 3 5% 7% 45%	4190
	21st to end	Day	5% 5% 14% 6% 6% 4 3 5% 5% 47%	4609
February	1st to 10th	Day	5% 5% 16% 6% 5% 5% 4 4 5% 44%	1800
	11th to 20th	Day	1 7% 6% 13% 7% 5% 5% 4 5% 6% 41%	1812
	21st to end	Day	1 6% 4 15% 8% 5% 5% 4 5% 5% 42%	1650
March	1st to 10th	Day	6% 6% 21% 7% 3 3 3 4 5% 43%	4926
	11th to 20th	Day	1 7% 7% 21% 7% 5% 6% 4 5% 6% 32%	5000
	21st to end	Day	12% 10% 15% 5% 5% 4 4 4 6% 35%	5523
April	1st to 10th	Day	1 12% 5% 10% 7% 4 4 3 5% 6% 43%	2400
	11th to 20th	Day	1 13% 7% 12% 6% 5% 4 4 5% 6% 38%	2371
	21st to end	Day	1 12% 6% 11% 6% 5% 5% 4 5% 5% 40%	2400
May	1st to 10th	Day	2 17% 7% 12% 6% 5% 5% 3 4 4 34%	5717
	11th to 20th	Day	2 12% 5% 9% 5% 5% 5% 4 6% 7% 41%	5922
	21st to end	Day	2 7% 2 7% 5% 6% 6% 5% 7% 7% 47%	6925
June	1st to 10th	Day	4 6% 2 6% 5% 5% 6% 4 5% 7% 49%	2800
	11th to 20th	Day	6% 6% 2 6% 5% 4 3 3 5% 4 57%	2800
	21st to end	Day	5% 8% 2 8% 6% 4 4 3 5% 7% 47%	2800
July	1st to 10th	Day	7% 6% 3 7% 5% 5% 5% 4 5% 5% 49%	6301
	11th to 20th	Day	8% 7% 4 11% 8% 8% 8% 5% 7% 7% 27%	5880
	21st to end	Day	10% 9% 4 11% 8% 6% 5% 4 5% 5% 32%	6401
August	1st to 10th	Day	10% 9% 4 11% 7% 7% 6% 5% 5% 7% 30%	5779
	11th to 20th	Day	11% 10% 4 12% 7% 6% 6% 3 4 5% 32%	5587
	21st to end	Day	12% 13% 6% 14% 7% 6% 5% 4 5% 5% 23%	5984
September	1st to 10th	Day	13% 10% 3 9% 7% 5% 4 3 4 4 37%	2400
	11th to 20th	Day	11% 9% 3 8% 6% 5% 4 3 4 4 43%	2400
	21st to end	Day	9% 8% 3 8% 5% 3 3 3 3 5% 50%	2105
October	1st to 10th	Day	6% 10% 2 5% 4 3 3 3 3 4 56%	3270
	11th to 20th	Day	3 17% 4 9% 5% 4 3 2 3 5% 44%	3250
	21st to end	Day	2 6% 2 7% 3 3 2 2 3 5% 64%	3548
November	1st to 10th	Day	1 5% 3 6% 4 4 3 2 4 5% 63%	2026
	11th to 20th	Day	6% 2 6% 3 3 3 3 3 7% 63%	1889
	21st to end	Day	1 7% 4 8% 5% 4 3 3 4 5% 57%	1680
December	1st to 10th	Day	4 2 7% 5% 4 3 3 4 7% 61%	2700
	11th to 20th	Day	5% 3 11% 4 3 3 2 5% 7% 57%	3106
	21st to end	Day	4 5% 11% 6% 4 4 3 6% 8% 50%	4092
January	1st to 10th	Night	2 2 11% 10% 7% 7% 5% 6% 7% 41%	6161
	11th to 20th	Night	2 2 11% 8% 5% 6% 4 5% 7% 50%	6010
	21st to end	Night	3 3 12% 5% 5% 4 4 6% 7% 51%	6611
February	1st to 10th	Night	4 4 12% 7% 5% 5% 3 5% 8% 48%	2600
	11th to 20th	Night	4 3 10% 8% 6% 5% 5% 5% 7% 48%	2472
	21st to end	Night	4 3 11% 7% 7% 5% 5% 6% 7% 46%	1980
March	1st to 10th	Night	4 5% 13% 7% 4 4 3 5% 7% 48%	4950
	11th to 20th	Night	4 5% 13% 11% 7% 5% 4 6% 7% 38%	5000
	21st to end	Night	9% 7% 15% 7% 5% 4 3 5% 6% 39%	5449
April	1st to 10th	Night	1 7% 3 9% 6% 5% 4 4 5% 8% 48%	2000
	11th to 20th	Night	1 7% 4 10% 6% 5% 6% 5% 6% 7% 44%	1977
	21st to end	Night	1 7% 4 8% 4 6% 5% 3 6% 6% 51%	2000
May	1st to 10th	Night	1 8% 3 8% 6% 6% 5% 4 6% 7% 46%	4259
	11th to 20th	Night	1 5% 3 6% 4 3 4 4 6% 9% 56%	3992
	21st to end	Night	1 3 1 3 2 3 3 2 5% 7% 69%	3972
June	1st to 10th	Night	2 3 1 3 2 3 3 4 6% 9% 63%	1600
	11th to 20th	Night	2 3 1 3 3 3 3 3 4 5% 69%	1600
	21st to end	Night	2 3 1 4 4 5% 5% 5% 6% 7% 59%	1600
July	1st to 10th	Night	3 3 1 4 3 3 4 4 5% 6% 62%	3673
	11th to 20th	Night	2 3 1 4 4 4 5% 5% 9% 14% 50%	4100
	21st to end	Night	3 3 2 6% 4 5% 5% 5% 7% 10% 50%	4472
August	1st to 10th	Night	3 4 2 6% 4 4 6% 5% 7% 12% 47%	4033
	11th to 20th	Night	3 3 2 6% 4 4 5% 5% 7% 10% 52%	4260
	21st to end	Night	6% 7% 3 8% 5% 6% 5% 5% 7% 10% 36%	4993
September	1st to 10th	Night	5% 4 2 6% 5% 3 4 4 7% 8% 52%	2000
	11th to 20th	Night	3 5% 2 6% 4 3 6% 3 5% 6% 57%	2000
	21st to end	Night	4 5% 2 5% 3 4 3 2 4 6% 62%	2290
October	1st to 10th	Night	2 5% 1 3 2 2 2 2 3 5% 72%	3920
	11th to 20th	Night	1 12% 2 5% 3 3 3 3 4 7% 55%	3898
	21st to end	Night	1 6% 2 6% 3 2 3 2 4 6% 66%	4252
November	1st to 10th	Night	5% 2 5% 5% 4 4 3 6% 7% 60%	2470
	11th to 20th	Night	4 2 5% 3 3 3 4 5% 7% 63%	2730
	21st to end	Night	1 5% 4 8% 5% 4 4 3 5% 6% 55%	2939
December	1st to 10th	Night	2 1 4 4 4 4 3 4 7% 66%	4710
	11th to 20th	Night	3 2 6% 5% 4 4 4 5% 9% 58%	5321
	21st to end	Night	3 5% 13% 6% 5% 4 4 5% 6% 50%	7128

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	30% 9% 13% 23% 16% 9%	4067
	11th to 20th	Day	30% 9% 12% 17% 17% 16%	4190
	21st to end	Day	33% 7% 8% 21% 20% 12%	4609
February	1st to 10th	Day	38% 6% 15% 15% 15% 11%	1800
	11th to 20th	Day	39% 7% 9% 15% 20% 9%	1821
	21st to end	Day	40% 7% 6% 16% 20% 12%	1650
March	1st to 10th	Day	38% 8% 7% 15% 19% 13%	4926
	11th to 20th	Day	43% 11% 10% 20% 13% 3	5000
	21st to end	Day	45% 9% 10% 22% 12% 3	5523
April	1st to 10th	Day	39% 8% 9% 19% 19% 7%	2400
	11th to 20th	Day	43% 8% 9% 18% 18% 5%	2374
	21st to end	Day	40% 8% 9% 25% 15% 2	2400
May	1st to 10th	Day	42% 13% 9% 16% 15% 5%	5717
	11th to 20th	Day	34% 12% 10% 20% 19% 4	5925
	21st to end	Day	25% 14% 10% 20% 27% 4	6926
June	1st to 10th	Day	30% 9% 12% 25% 18% 7%	2800
	11th to 20th	Day	28% 6% 7% 17% 27% 16%	2800
	21st to end	Day	34% 7% 9% 15% 21% 14%	2800
July	1st to 10th	Day	32% 8% 8% 9% 15% 30%	6233
	11th to 20th	Day	45% 14% 13% 14% 10% 5%	5811
	21st to end	Day	46% 11% 10% 12% 14% 7%	6217
August	1st to 10th	Day	58% 12% 11% 7% 9% 3	4208
	11th to 20th	Day	52% 10% 8% 9% 16% 6%	4046
	21st to end	Day	54% 10% 9% 11% 13% 3	4311
September	1st to 10th	Day	48% 8% 8% 12% 18% 7%	2400
	11th to 20th	Day	41% 7% 7% 15% 22% 8%	2400
	21st to end	Day	36% 5% 6% 20% 25% 8%	2105
October	1st to 10th	Day	28% 6% 7% 17% 32% 10%	3270
	11th to 20th	Day	39% 7% 7% 18% 22% 7%	3250
	21st to end	Day	22% 4 6% 22% 39% 7%	3548
November	1st to 10th	Day	21% 5% 12% 25% 27% 9%	2026
	11th to 20th	Day	20% 6% 9% 28% 27% 10%	1889
	21st to end	Day	27% 5% 9% 23% 22% 13%	1680
December	1st to 10th	Day	20% 7% 10% 25% 29% 9%	2700
	11th to 20th	Day	25% 6% 12% 24% 26% 7%	3104
	21st to end	Day	25% 9% 11% 20% 21% 14%	4092
January	1st to 10th	Night	27% 12% 11% 21% 19% 10%	6161
	11th to 20th	Night	21% 14% 11% 17% 18% 18%	6010
	21st to end	Night	26% 7% 7% 22% 24% 15%	6611
February	1st to 10th	Night	30% 7% 14% 17% 21% 11%	2600
	11th to 20th	Night	29% 9% 10% 20% 22% 10%	2473
	21st to end	Night	30% 9% 9% 19% 22% 12%	1980
March	1st to 10th	Night	28% 10% 11% 15% 24% 12%	4950
	11th to 20th	Night	32% 15% 11% 18% 18% 7%	5000
	21st to end	Night	36% 13% 11% 27% 11% 2	5449
April	1st to 10th	Night	30% 9% 12% 23% 19% 7%	2000
	11th to 20th	Night	30% 11% 12% 22% 20% 4	1977
	21st to end	Night	26% 11% 14% 29% 19% 2	2000
May	1st to 10th	Night	25% 14% 12% 25% 22% 3	4256
	11th to 20th	Night	18% 10% 14% 33% 23% 2	3993
	21st to end	Night	12% 6% 13% 25% 37% 7%	3972
June	1st to 10th	Night	15% 7% 13% 30% 28% 7%	1600
	11th to 20th	Night	16% 6% 9% 18% 34% 17%	1600
	21st to end	Night	19% 8% 11% 19% 26% 17%	1600
July	1st to 10th	Night	19% 8% 9% 11% 18% 35%	3633
	11th to 20th	Night	19% 10% 19% 28% 16% 8%	4046
	21st to end	Night	25% 13% 18% 18% 18% 8%	4344
August	1st to 10th	Night	33% 16% 22% 11% 12% 5%	2928
	11th to 20th	Night	25% 15% 20% 12% 20% 8%	3077
	21st to end	Night	37% 10% 14% 17% 18% 4	3597
September	1st to 10th	Night	27% 10% 14% 20% 23% 6%	2000
	11th to 20th	Night	25% 9% 11% 21% 26% 9%	2000
	21st to end	Night	21% 7% 9% 27% 29% 7%	2290
October	1st to 10th	Night	16% 4 7% 20% 39% 14%	3920
	11th to 20th	Night	25% 7% 9% 29% 23% 7%	3898
	21st to end	Night	18% 5% 7% 20% 37% 14%	4252
November	1st to 10th	Night	20% 6% 12% 24% 26% 10%	2470
	11th to 20th	Night	17% 7% 11% 26% 28% 11%	2730
	21st to end	Night	27% 6% 9% 22% 24% 13%	2939
December	1st to 10th	Night	15% 8% 10% 25% 31% 11%	4710
	11th to 20th	Night	19% 8% 10% 24% 28% 11%	5322
	21st to end	Night	27% 10% 9% 17% 23% 13%	7128

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	19% 24% 32% 18% 6%	330
	11th to 20th	Day	22% 28% 27% 19% 4	330
	21st to end	Day	16% 26% 31% 19% 7% 1	363
February	1st to 10th	Day	26% 33% 26% 12% 2 1	300
	11th to 20th	Day	24% 34% 26% 13% 2	300
	21st to end	Day	31% 30% 22% 14% 3	249
March	1st to 10th	Day	25% 30% 27% 15% 3	485
	11th to 20th	Day	24% 27% 28% 16% 4	500
	21st to end	Day	26% 34% 27% 10% 2	550
April	1st to 10th	Day	19% 32% 34% 11% 4	500
	11th to 20th	Day	25% 37% 28% 7% 3 1	492
	21st to end	Day	21% 33% 31% 12% 3 1	500
May	1st to 10th	Day	21% 35% 30% 11% 4	503
	11th to 20th	Day	20% 27% 33% 15% 5% 1	557
	21st to end	Day	18% 23% 28% 19% 10% 3	789
June	1st to 10th	Day	9% 14% 31% 27% 16% 3	730
	11th to 20th	Day	10% 20% 34% 20% 13% 3	730
	21st to end	Day	10% 18% 29% 23% 15% 5%	730
July	1st to 10th	Day	10% 11% 24% 28% 22% 4	709
	11th to 20th	Day	17% 8% 26% 29% 15% 4	524
	21st to end	Day	19% 8% 30% 24% 15% 4	575
August	1st to 10th	Day	22% 9% 32% 22% 11% 4	527
	11th to 20th	Day	14% 13% 30% 29% 9% 5%	517
	21st to end	Day	15% 12% 32% 24% 13% 3	583
September	1st to 10th	Day	17% 14% 28% 22% 13% 6%	530
	11th to 20th	Day	16% 20% 34% 19% 8% 3	530
	21st to end	Day	18% 21% 26% 19% 11% 5%	430
October	1st to 10th	Day	22% 19% 23% 18% 14% 4	330
	11th to 20th	Day	20% 23% 25% 20% 11% 2	330
	21st to end	Day	19% 24% 29% 19% 9% 1	363
November	1st to 10th	Day	20% 27% 25% 18% 9% 2	330
	11th to 20th	Day	15% 25% 32% 17% 9% 2	330
	21st to end	Day	16% 23% 31% 20% 9%	330
December	1st to 10th	Day	16% 29% 27% 17% 10% 1	330
	11th to 20th	Day	22% 22% 31% 18% 7% 1	330
	21st to end	Day	25% 28% 26% 16% 5%	363
January	1st to 10th	Night	8% 15% 32% 30% 12% 3	913
	11th to 20th	Night	9% 15% 32% 30% 13% 2	840
	21st to end	Night	5% 16% 32% 34% 10% 3	924
February	1st to 10th	Night	10% 17% 35% 30% 7% 1	800
	11th to 20th	Night	9% 17% 38% 27% 8%	784
	21st to end	Night	11% 15% 43% 26% 6%	660
March	1st to 10th	Night	11% 19% 31% 29% 9% 1	800
	11th to 20th	Night	9% 15% 32% 31% 13% 1	800
	21st to end	Night	10% 20% 35% 27% 9%	865
April	1st to 10th	Night	9% 21% 35% 28% 7%	600
	11th to 20th	Night	11% 20% 31% 28% 9%	593
	21st to end	Night	7% 20% 32% 29% 11% 1	600
May	1st to 10th	Night	8% 15% 34% 31% 12% 1	600
	11th to 20th	Night	8% 10% 29% 34% 16% 4	591
	21st to end	Night	9% 11% 29% 31% 18% 2	650
June	1st to 10th	Night	3 8% 23% 38% 26% 2	600
	11th to 20th	Night	6% 10% 25% 38% 19% 3	600
	21st to end	Night	4 6% 22% 34% 30% 4	600
July	1st to 10th	Night	6% 3 12% 38% 39% 3	585
	11th to 20th	Night	5% 2 8% 35% 41% 8%	582
	21st to end	Night	7% 2 8% 31% 45% 7%	657
August	1st to 10th	Night	7% 2 11% 32% 42% 5%	597
	11th to 20th	Night	7% 2 11% 36% 37% 7%	594
	21st to end	Night	7% 5% 9% 38% 34% 7%	660
September	1st to 10th	Night	6% 4 13% 39% 35% 4	600
	11th to 20th	Night	5% 7% 20% 36% 29% 4	600
	21st to end	Night	8% 9% 17% 32% 27% 7%	790
October	1st to 10th	Night	10% 11% 25% 32% 20% 2	800
	11th to 20th	Night	10% 11% 25% 34% 18% 4	800
	21st to end	Night	9% 13% 26% 31% 18% 3	880
November	1st to 10th	Night	9% 14% 26% 32% 17% 2	830
	11th to 20th	Night	8% 12% 35% 31% 11% 3	1050
	21st to end	Night	11% 15% 27% 31% 13% 3	1050
December	1st to 10th	Night	8% 16% 32% 33% 10% 1	1050
	11th to 20th	Night	10% 18% 34% 26% 11% 2	1050
	21st to end	Night	12% 16% 35% 30% 6% 1	1155

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