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 2024-11-30.

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 9% 4 11% 6% 5% 5% 4 5% 6% 42%	105284
	Night	1 5% 3 8% 5% 5% 5% 4 6% 8% 51%	103066
Transparency	Day	38% 9% 10% 17% 19% 8%	105044
	Night	26% 10% 12% 21% 23% 9%	102895
Seeing	Day	18% 22% 29% 20% 10% 2	15357
	Night	8% 12% 27% 32% 18% 3	25527

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 4 10% 9% 7% 5% 6% 7% 7% 42%	2766
	11th to 20th	Day	6% 3 15% 8% 6% 5% 3 5% 7% 41%	2860
	21st to end	Day	7% 7% 14% 6% 6% 4 3 5% 6% 42%	3146
February	1st to 10th	Day	5% 5% 17% 7% 5% 5% 4 4 5% 43%	1710
	11th to 20th	Day	1 7% 5% 12% 7% 5% 6% 4 5% 6% 42%	1719
	21st to end	Day	1 6% 4 16% 8% 6% 5% 4 5% 5% 41%	1570
March	1st to 10th	Day	7% 7% 17% 7% 3 3 3 4 5% 43%	3430
	11th to 20th	Day	1 9% 8% 19% 6% 5% 5% 4 5% 6% 33%	3490
	21st to end	Day	1 13% 13% 17% 6% 5% 4 4 4 5% 30%	3849
April	1st to 10th	Day	1 13% 5% 9% 7% 5% 4 3 5% 6% 42%	2280
	11th to 20th	Day	1 13% 6% 12% 6% 5% 4 4 5% 6% 38%	2251
	21st to end	Day	1 12% 6% 11% 6% 5% 5% 4 4 5% 39%	2280
May	1st to 10th	Day	2 13% 7% 10% 6% 4 5% 3 5% 5% 39%	3999
	11th to 20th	Day	2 12% 3 8% 6% 5% 6% 5% 7% 8% 38%	4124
	21st to end	Day	3 9% 2 6% 5% 6% 5% 4 7% 7% 46%	4825
June	1st to 10th	Day	3 6% 2 6% 5% 5% 6% 4 5% 6% 51%	2660
	11th to 20th	Day	6% 6% 2 6% 5% 4 3 3 5% 4 56%	2660
	21st to end	Day	5% 8% 2 8% 6% 4 4 3 5% 7% 47%	2660
July	1st to 10th	Day	7% 6% 3 7% 5% 5% 4 3 4 4 53%	4406
	11th to 20th	Day	8% 6% 4 11% 8% 8% 8% 6% 8% 7% 26%	4110
	21st to end	Day	11% 9% 4 10% 8% 5% 5% 4 5% 5% 34%	4454
August	1st to 10th	Day	14% 11% 5% 13% 7% 7% 7% 5% 6% 6% 20%	4092
	11th to 20th	Day	10% 10% 4 13% 8% 6% 6% 4 4 5% 30%	3882
	21st to end	Day	12% 13% 5% 13% 6% 6% 5% 4 5% 5% 25%	4180
September	1st to 10th	Day	13% 10% 3 9% 7% 5% 4 3 4 4 37%	2280
	11th to 20th	Day	12% 9% 3 8% 6% 5% 4 3 4 4 43%	2280
	21st to end	Day	9% 8% 3 8% 5% 3 3 3 3 5% 51%	2000
October	1st to 10th	Day	6% 10% 2 5% 4 3 3 3 4 4 56%	3170
	11th to 20th	Day	3 18% 5% 9% 5% 4 3 2 3 5% 44%	3150
	21st to end	Day	2 6% 3 7% 3 3 2 2 3 5% 64%	3438
November	1st to 10th	Day	1 5% 3 6% 4 4 3 2 4 5% 62%	1928
	11th to 20th	Day	6% 2 5% 3 3 3 3 3 6% 64%	1799
	21st to end	Day	1 7% 4 8% 6% 4 3 3 3 6% 57%	1596
December	1st to 10th	Day	6% 3 9% 6% 5% 4 4 4 6% 52%	1520
	11th to 20th	Day	7% 4 13% 6% 5% 4 3 6% 7% 44%	1926
	21st to end	Day	5% 4 11% 6% 4 4 4 6% 7% 50%	2794
January	1st to 10th	Night	2 2 11% 11% 8% 7% 5% 6% 6% 41%	4216
	11th to 20th	Night	3 3 14% 9% 4 7% 4 6% 8% 42%	4110
	21st to end	Night	4 4 13% 6% 5% 5% 4 6% 7% 46%	4521
February	1st to 10th	Night	4 4 12% 7% 5% 5% 3 5% 8% 47%	2470
	11th to 20th	Night	4 3 10% 8% 6% 5% 5% 5% 7% 48%	2345
	21st to end	Night	4 4 11% 7% 7% 5% 5% 6% 7% 45%	1884
March	1st to 10th	Night	3 5% 11% 6% 4 4 4 7% 8% 46%	3440
	11th to 20th	Night	5% 5% 14% 9% 7% 6% 5% 6% 7% 36%	3490
	21st to end	Night	10% 7% 15% 7% 6% 5% 4 6% 7% 34%	3802
April	1st to 10th	Night	1 7% 3 9% 6% 5% 4 4 5% 8% 47%	1900
	11th to 20th	Night	1 7% 4 9% 6% 5% 6% 5% 6% 6% 44%	1877
	21st to end	Night	1 7% 4 8% 5% 6% 5% 3 6% 6% 50%	1900
May	1st to 10th	Night	1 7% 3 7% 5% 5% 4 3 5% 7% 51%	2967
	11th to 20th	Night	1 5% 2 5% 4 3 4 4 6% 9% 56%	2780
	21st to end	Night	2 4 1 4 3 3 3 3 5% 6% 63%	2762
June	1st to 10th	Night	1 3 1 3 2 3 3 4 6% 9% 64%	1520
	11th to 20th	Night	3 3 1 3 3 3 3 3 5% 5% 68%	1520
	21st to end	Night	2 3 1 4 4 5% 5% 5% 6% 7% 59%	1520
July	1st to 10th	Night	4 4 1 5% 3 3 4 3 4 5% 63%	2559
	11th to 20th	Night	3 3 1 4 4 4 6% 5% 7% 12% 52%	2860
	21st to end	Night	4 3 2 6% 5% 5% 5% 5% 7% 9% 50%	3109
August	1st to 10th	Night	4 5% 2 7% 6% 6% 7% 6% 9% 13% 35%	2847
	11th to 20th	Night	4 3 2 6% 4 5% 5% 6% 8% 10% 47%	2955
	21st to end	Night	7% 8% 3 8% 5% 5% 5% 5% 6% 10% 40%	3487
September	1st to 10th	Night	5% 4 2 6% 5% 3 4 4 7% 8% 51%	1900
	11th to 20th	Night	3 5% 2 6% 4 3 6% 3 5% 6% 57%	1900
	21st to end	Night	4 5% 2 4 3 4 3 2 4 6% 64%	2175
October	1st to 10th	Night	2 5% 1 3 2 2 2 2 3 5% 72%	3800
	11th to 20th	Night	1 12% 2 5% 3 3 3 3 5% 7% 55%	3778
	21st to end	Night	1 6% 2 6% 3 2 3 2 4 6% 65%	4120
November	1st to 10th	Night	1 5% 2 5% 5% 4 4 3 6% 7% 58%	2349
	11th to 20th	Night	4 2 5% 3 3 3 4 5% 7% 63%	2600
	21st to end	Night	1 5% 4 8% 5% 4 4 3 5% 6% 55%	2799
December	1st to 10th	Night	4 2 7% 5% 5% 5% 4 5% 8% 54%	2660
	11th to 20th	Night	5% 4 8% 7% 6% 5% 5% 6% 10% 45%	3271
	21st to end	Night	3 3 11% 7% 5% 4 4 5% 7% 50%	4873

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	28% 9% 17% 23% 17% 6%	2767
	11th to 20th	Day	32% 11% 16% 22% 16% 4	2860
	21st to end	Day	36% 8% 9% 23% 18% 6%	3146
February	1st to 10th	Day	39% 6% 15% 15% 15% 10%	1710
	11th to 20th	Day	39% 7% 9% 15% 20% 9%	1728
	21st to end	Day	42% 7% 6% 16% 19% 10%	1570
March	1st to 10th	Day	37% 9% 8% 20% 18% 8%	3430
	11th to 20th	Day	42% 13% 11% 21% 10% 2	3490
	21st to end	Day	52% 9% 8% 15% 13% 3	3849
April	1st to 10th	Day	38% 8% 9% 19% 18% 7%	2280
	11th to 20th	Day	43% 8% 9% 19% 17% 5%	2254
	21st to end	Day	41% 8% 9% 25% 15% 2	2280
May	1st to 10th	Day	35% 14% 10% 17% 17% 7%	3999
	11th to 20th	Day	34% 14% 12% 19% 18% 3	4127
	21st to end	Day	27% 14% 10% 19% 26% 4	4825
June	1st to 10th	Day	29% 8% 12% 25% 19% 8%	2660
	11th to 20th	Day	28% 6% 7% 17% 27% 14%	2660
	21st to end	Day	34% 7% 9% 16% 20% 14%	2660
July	1st to 10th	Day	32% 6% 7% 8% 14% 33%	4337
	11th to 20th	Day	45% 14% 13% 14% 11% 4	4041
	21st to end	Day	45% 11% 10% 12% 14% 9%	4314
August	1st to 10th	Day	60% 12% 11% 6% 9% 3	4078
	11th to 20th	Day	52% 10% 8% 8% 15% 6%	3921
	21st to end	Day	54% 10% 9% 11% 13% 3	4179
September	1st to 10th	Day	48% 7% 8% 11% 19% 7%	2280
	11th to 20th	Day	41% 7% 7% 15% 21% 8%	2280
	21st to end	Day	35% 5% 6% 21% 26% 7%	2000
October	1st to 10th	Day	28% 6% 7% 18% 32% 10%	3170
	11th to 20th	Day	40% 7% 7% 19% 21% 7%	3150
	21st to end	Day	22% 4 6% 22% 39% 7%	3438
November	1st to 10th	Day	21% 5% 12% 24% 27% 10%	1928
	11th to 20th	Day	19% 6% 9% 27% 28% 10%	1799
	21st to end	Day	27% 6% 9% 22% 22% 14%	1596
December	1st to 10th	Day	29% 9% 14% 22% 19% 7%	1520
	11th to 20th	Day	35% 9% 11% 18% 22% 6%	1924
	21st to end	Day	28% 9% 10% 17% 19% 17%	2794
January	1st to 10th	Night	26% 15% 13% 23% 15% 9%	4216
	11th to 20th	Night	25% 18% 13% 21% 18% 5%	4110
	21st to end	Night	31% 8% 8% 25% 23% 6%	4521
February	1st to 10th	Night	32% 7% 14% 17% 21% 9%	2470
	11th to 20th	Night	29% 9% 9% 21% 22% 10%	2346
	21st to end	Night	32% 9% 9% 19% 21% 10%	1884
March	1st to 10th	Night	26% 12% 14% 20% 24% 4	3440
	11th to 20th	Night	31% 19% 12% 22% 14% 3	3490
	21st to end	Night	40% 13% 11% 20% 13% 3	3802
April	1st to 10th	Night	30% 10% 13% 22% 18% 7%	1900
	11th to 20th	Night	30% 12% 12% 22% 20% 4	1877
	21st to end	Night	27% 11% 15% 28% 17% 2	1900
May	1st to 10th	Night	22% 13% 10% 23% 28% 4	2967
	11th to 20th	Night	20% 10% 16% 30% 23% 1	2781
	21st to end	Night	16% 8% 12% 23% 33% 9%	2762
June	1st to 10th	Night	15% 7% 13% 29% 29% 7%	1520
	11th to 20th	Night	17% 6% 9% 19% 34% 16%	1520
	21st to end	Night	20% 7% 11% 20% 27% 15%	1520
July	1st to 10th	Night	22% 8% 8% 8% 20% 34%	2519
	11th to 20th	Night	20% 10% 18% 28% 18% 6%	2806
	21st to end	Night	25% 12% 18% 19% 16% 11%	3011
August	1st to 10th	Night	34% 16% 23% 10% 13% 4	2838
	11th to 20th	Night	25% 15% 20% 12% 20% 8%	2982
	21st to end	Night	37% 10% 14% 17% 18% 5%	3487
September	1st to 10th	Night	28% 10% 13% 19% 24% 7%	1900
	11th to 20th	Night	25% 9% 11% 21% 25% 9%	1900
	21st to end	Night	21% 6% 8% 28% 30% 7%	2175
October	1st to 10th	Night	16% 4 7% 20% 39% 14%	3800
	11th to 20th	Night	25% 7% 9% 30% 23% 7%	3778
	21st to end	Night	18% 5% 7% 20% 36% 14%	4120
November	1st to 10th	Night	22% 7% 13% 23% 25% 11%	2349
	11th to 20th	Night	17% 7% 11% 25% 29% 11%	2600
	21st to end	Night	27% 6% 9% 20% 25% 14%	2799
December	1st to 10th	Night	23% 12% 14% 19% 21% 11%	2660
	11th to 20th	Night	28% 10% 12% 19% 23% 8%	3272
	21st to end	Night	29% 9% 10% 15% 21% 16%	4873

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	16% 21% 34% 21% 7%	270
	11th to 20th	Day	20% 28% 26% 21% 4	270
	21st to end	Day	11% 24% 34% 23% 8% 1	297
February	1st to 10th	Day	20% 34% 29% 13% 2 1	270
	11th to 20th	Day	21% 34% 27% 15% 3	270
	21st to end	Day	30% 32% 21% 14% 3	225
March	1st to 10th	Day	24% 30% 27% 15% 3	446
	11th to 20th	Day	22% 27% 28% 17% 4	460
	21st to end	Day	26% 34% 28% 10% 2	506
April	1st to 10th	Day	19% 33% 32% 11% 4	460
	11th to 20th	Day	24% 37% 28% 7% 3 1	452
	21st to end	Day	21% 32% 31% 12% 4 1	460
May	1st to 10th	Day	20% 36% 30% 10% 4	463
	11th to 20th	Day	19% 26% 33% 16% 5% 1	515
	21st to end	Day	18% 23% 28% 18% 10% 3	734
June	1st to 10th	Day	8% 13% 31% 28% 17% 3	680
	11th to 20th	Day	10% 20% 34% 20% 13% 4	680
	21st to end	Day	9% 18% 30% 24% 16% 5%	680
July	1st to 10th	Day	10% 11% 23% 29% 22% 4	660
	11th to 20th	Day	18% 7% 26% 28% 16% 4	484
	21st to end	Day	19% 7% 29% 24% 16% 4	531
August	1st to 10th	Day	23% 7% 30% 24% 12% 4	487
	11th to 20th	Day	14% 12% 31% 29% 9% 5%	477
	21st to end	Day	16% 11% 33% 24% 13% 4	539
September	1st to 10th	Day	17% 13% 28% 23% 14% 6%	490
	11th to 20th	Day	16% 19% 34% 19% 9% 3	490
	21st to end	Day	18% 19% 27% 20% 11% 5%	395
October	1st to 10th	Day	23% 17% 23% 18% 15% 4	300
	11th to 20th	Day	15% 24% 26% 21% 12% 2	300
	21st to end	Day	19% 23% 28% 19% 9% 1	330
November	1st to 10th	Day	21% 25% 25% 18% 9% 2	300
	11th to 20th	Day	14% 26% 31% 18% 9% 2	300
	21st to end	Day	15% 22% 31% 21% 10%	299
December	1st to 10th	Day	15% 29% 29% 18% 9% 1	270
	11th to 20th	Day	19% 23% 30% 19% 7%	270
	21st to end	Day	21% 29% 27% 18% 5%	297
January	1st to 10th	Night	7% 14% 31% 32% 13% 3	827
	11th to 20th	Night	8% 14% 32% 31% 13% 2	760
	21st to end	Night	4 14% 30% 37% 11% 3	836
February	1st to 10th	Night	6% 17% 37% 32% 7% 1	760
	11th to 20th	Night	7% 16% 39% 28% 8%	744
	21st to end	Night	12% 14% 43% 26% 6%	628
March	1st to 10th	Night	10% 19% 31% 29% 9% 1	760
	11th to 20th	Night	7% 14% 32% 32% 14% 1	760
	21st to end	Night	9% 19% 35% 27% 9%	822
April	1st to 10th	Night	9% 21% 35% 28% 7%	570
	11th to 20th	Night	11% 20% 31% 29% 9%	563
	21st to end	Night	7% 20% 33% 28% 11% 1	570
May	1st to 10th	Night	8% 14% 34% 30% 13% 1	570
	11th to 20th	Night	6% 8% 30% 35% 17% 4	561
	21st to end	Night	10% 12% 29% 31% 17% 2	617
June	1st to 10th	Night	2 8% 23% 38% 27% 2	570
	11th to 20th	Night	6% 10% 24% 38% 19% 3	570
	21st to end	Night	3 5% 21% 35% 31% 4	570
July	1st to 10th	Night	6% 3 12% 36% 40% 3	555
	11th to 20th	Night	5% 2 8% 34% 42% 9%	552
	21st to end	Night	7% 2 8% 30% 46% 8%	624
August	1st to 10th	Night	8% 2 10% 31% 44% 5%	567
	11th to 20th	Night	7% 2 11% 36% 37% 7%	564
	21st to end	Night	7% 5% 9% 38% 34% 7%	627
September	1st to 10th	Night	6% 4 13% 37% 36% 4	570
	11th to 20th	Night	6% 6% 19% 35% 30% 4	570
	21st to end	Night	8% 9% 17% 32% 27% 7%	750
October	1st to 10th	Night	10% 11% 23% 34% 20% 2	760
	11th to 20th	Night	7% 11% 25% 35% 19% 4	760
	21st to end	Night	9% 13% 25% 31% 18% 4	836
November	1st to 10th	Night	8% 13% 26% 32% 18% 2	789
	11th to 20th	Night	8% 12% 33% 32% 12% 3	1000
	21st to end	Night	11% 15% 27% 32% 13% 3	1000
December	1st to 10th	Night	8% 14% 33% 34% 9% 1	950
	11th to 20th	Night	8% 18% 35% 27% 11% 2	950
	21st to end	Night	9% 16% 36% 32% 6% 1	1045

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