Big Spy Mountain Overlook
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Big Spy Mountain Overlook clear sky chart from 2005-11-06 to 2025-05-11.

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	13% 7% 2 7% 5% 5% 5% 4 6% 7% 40%	113733
	Night	13% 7% 2 6% 4 4 4 4 5% 7% 44%	115195
Transparency	Day	40% 7% 11% 22% 16% 5%	117383
	Night	37% 7% 11% 23% 18% 5%	119684
Seeing	Day	21% 16% 26% 21% 11% 4	15650
	Night	18% 14% 24% 28% 14% 2	25659

By Month

Percentage Cloud Cover SE (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	12% 7% 3 6% 4 4 4 3 4 6% 47%	4056
	11th to 20th	Day	11% 8% 3 6% 4 4 4 3 3 3 50%	4060
	21st to end	Day	16% 8% 3 6% 4 3 3 3 5% 6% 45%	4466
February	1st to 10th	Day	11% 9% 3 7% 5% 5% 5% 4 5% 6% 39%	1800
	11th to 20th	Day	15% 8% 3 8% 5% 4 4 4 6% 6% 37%	1753
	21st to end	Day	17% 8% 2 6% 6% 4 4 3 5% 5% 40%	1735
March	1st to 10th	Day	15% 8% 2 7% 4 5% 5% 4 6% 7% 38%	4935
	11th to 20th	Day	10% 6% 2 6% 4 4 4 3 5% 7% 49%	4980
	21st to end	Day	13% 8% 3 9% 5% 4 5% 4 5% 5% 39%	5465
April	1st to 10th	Day	10% 8% 3 7% 5% 4 4 4 5% 6% 44%	2328
	11th to 20th	Day	12% 7% 2 6% 4 5% 5% 3 5% 7% 43%	2374
	21st to end	Day	15% 9% 2 6% 5% 5% 5% 4 5% 5% 39%	2477
May	1st to 10th	Day	16% 7% 3 9% 6% 6% 6% 4 6% 8% 29%	4212
	11th to 20th	Day	21% 9% 2 6% 5% 4 5% 4 6% 8% 31%	4081
	21st to end	Day	9% 5% 2 6% 5% 4 5% 4 7% 10% 41%	4525
June	1st to 10th	Day	12% 5% 2 5% 5% 5% 5% 5% 7% 9% 40%	2660
	11th to 20th	Day	11% 6% 2 6% 6% 5% 6% 5% 8% 10% 35%	2660
	21st to end	Day	8% 5% 2 5% 5% 6% 7% 5% 8% 9% 38%	2605
July	1st to 10th	Day	7% 4 2 7% 6% 6% 6% 6% 7% 10% 39%	4120
	11th to 20th	Day	9% 6% 2 6% 6% 6% 7% 5% 9% 12% 33%	4120
	21st to end	Day	12% 9% 3 9% 7% 7% 7% 5% 8% 9% 25%	4466
August	1st to 10th	Day	15% 6% 2 7% 5% 6% 6% 5% 7% 10% 30%	4104
	11th to 20th	Day	9% 5% 2 7% 6% 7% 7% 6% 8% 10% 33%	4017
	21st to end	Day	8% 5% 2 6% 4 4 6% 5% 7% 10% 43%	4180
September	1st to 10th	Day	12% 6% 2 7% 5% 4 4 4 5% 7% 45%	2280
	11th to 20th	Day	15% 8% 2 6% 5% 4 4 4 5% 7% 40%	2144
	21st to end	Day	21% 8% 3 6% 4 5% 4 4 6% 5% 34%	2056
October	1st to 10th	Day	15% 7% 2 6% 4 4 3 3 5% 6% 45%	1900
	11th to 20th	Day	14% 6% 2 5% 4 3 3 3 5% 6% 50%	1900
	21st to end	Day	13% 7% 2 5% 3 4 3 3 4 6% 49%	2090
November	1st to 10th	Day	11% 6% 2 5% 4 3 3 3 5% 5% 54%	1957
	11th to 20th	Day	14% 5% 2 5% 4 3 4 3 6% 6% 48%	1819
	21st to end	Day	13% 7% 2 6% 3 4 3 3 4 6% 48%	1600
December	1st to 10th	Day	10% 7% 2 7% 4 4 4 3 5% 6% 47%	2600
	11th to 20th	Day	17% 6% 3 6% 4 3 4 3 5% 7% 43%	2990
	21st to end	Day	18% 9% 3 7% 4 4 4 4 6% 6% 35%	4218
January	1st to 10th	Night	11% 6% 2 5% 4 4 4 3 4 5% 52%	5889
	11th to 20th	Night	10% 8% 2 5% 3 4 3 2 4 5% 54%	5900
	21st to end	Night	14% 7% 2 6% 4 4 3 2 4 5% 50%	6490
February	1st to 10th	Night	11% 8% 2 8% 5% 4 5% 4 6% 7% 40%	2600
	11th to 20th	Night	18% 9% 2 7% 4 4 3 3 4 6% 40%	2533
	21st to end	Night	13% 8% 2 5% 5% 5% 4 4 6% 8% 40%	1890
March	1st to 10th	Night	15% 6% 2 6% 4 4 4 3 5% 5% 45%	4918
	11th to 20th	Night	10% 4 1 5% 4 4 4 3 5% 7% 52%	4980
	21st to end	Night	12% 6% 3 7% 5% 5% 4 3 5% 7% 42%	5467
April	1st to 10th	Night	12% 9% 2 5% 4 4 5% 3 7% 8% 41%	2070
	11th to 20th	Night	11% 7% 2 5% 3 4 4 4 5% 8% 46%	1975
	21st to end	Night	15% 6% 3 6% 4 4 4 3 6% 7% 42%	1939
May	1st to 10th	Night	17% 8% 4 7% 4 4 5% 4 6% 8% 33%	2903
	11th to 20th	Night	22% 9% 3 7% 4 4 3 3 5% 6% 35%	2818
	21st to end	Night	10% 5% 2 7% 3 4 5% 4 7% 10% 43%	3026
June	1st to 10th	Night	9% 7% 2 6% 4 4 6% 5% 7% 9% 42%	1520
	11th to 20th	Night	9% 7% 2 6% 4 4 5% 5% 7% 8% 42%	1520
	21st to end	Night	8% 5% 1 5% 4 4 4 4 7% 10% 47%	1567
July	1st to 10th	Night	6% 5% 2 5% 5% 4 6% 5% 9% 11% 43%	2840
	11th to 20th	Night	8% 6% 2 7% 4 5% 6% 5% 8% 12% 37%	2840
	21st to end	Night	11% 9% 3 8% 6% 7% 6% 4 7% 8% 31%	3087
August	1st to 10th	Night	15% 6% 3 7% 5% 4 5% 5% 6% 9% 34%	2825
	11th to 20th	Night	10% 7% 2 7% 5% 6% 6% 4 7% 8% 38%	2785
	21st to end	Night	9% 7% 2 5% 5% 4 4 4 6% 8% 45%	3469
September	1st to 10th	Night	11% 6% 2 6% 4 4 4 3 6% 6% 47%	1900
	11th to 20th	Night	13% 7% 2 6% 4 4 3 4 6% 8% 43%	2031
	21st to end	Night	21% 9% 2 7% 4 3 3 3 4 6% 38%	2123
October	1st to 10th	Night	15% 7% 3 7% 3 3 3 3 5% 5% 45%	2280
	11th to 20th	Night	13% 6% 3 5% 4 4 4 3 5% 6% 49%	2280
	21st to end	Night	13% 5% 1 4 3 3 4 3 5% 6% 53%	2508
November	1st to 10th	Night	11% 8% 3 6% 4 3 3 3 4 4 51%	2341
	11th to 20th	Night	13% 6% 2 5% 5% 3 4 4 4 6% 49%	2576
	21st to end	Night	12% 5% 2 5% 4 4 4 4 5% 6% 48%	2800
December	1st to 10th	Night	10% 6% 2 6% 4 4 4 4 5% 6% 51%	4580
	11th to 20th	Night	15% 8% 3 7% 5% 4 4 3 4 6% 41%	5205
	21st to end	Night	15% 6% 2 6% 4 5% 5% 4 6% 7% 39%	6720

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	36% 5% 5% 21% 23% 9%	4057
	11th to 20th	Day	38% 3 5% 18% 24% 11%	4060
	21st to end	Day	40% 4 6% 25% 20% 5%	4466
February	1st to 10th	Day	44% 5% 10% 23% 15% 3	1800
	11th to 20th	Day	46% 5% 9% 19% 17% 4	1762
	21st to end	Day	46% 4 9% 19% 16% 6%	1735
March	1st to 10th	Day	41% 6% 9% 18% 17% 9%	4935
	11th to 20th	Day	32% 7% 7% 25% 21% 9%	4980
	21st to end	Day	46% 7% 8% 17% 17% 6%	5465
April	1st to 10th	Day	39% 5% 11% 21% 17% 7%	2328
	11th to 20th	Day	39% 6% 11% 23% 18% 4	2374
	21st to end	Day	45% 6% 10% 23% 13% 4	2477
May	1st to 10th	Day	50% 8% 12% 20% 9% 1	4250
	11th to 20th	Day	49% 8% 12% 22% 9% 1	4085
	21st to end	Day	35% 8% 16% 22% 15% 3	4539
June	1st to 10th	Day	38% 9% 15% 25% 13% 1	2660
	11th to 20th	Day	40% 9% 16% 21% 12% 1	2660
	21st to end	Day	38% 10% 16% 23% 12% 1	2605
July	1st to 10th	Day	36% 12% 19% 19% 9% 5%	4055
	11th to 20th	Day	39% 11% 18% 24% 8% 1	4051
	21st to end	Day	51% 10% 13% 18% 8%	4324
August	1st to 10th	Day	44% 10% 19% 21% 6%	4091
	11th to 20th	Day	39% 12% 15% 25% 9%	4056
	21st to end	Day	34% 10% 19% 27% 10% 1	4180
September	1st to 10th	Day	39% 6% 12% 18% 20% 4	2280
	11th to 20th	Day	43% 6% 6% 25% 17% 3	2144
	21st to end	Day	49% 7% 8% 18% 17% 2	2056
October	1st to 10th	Day	32% 6% 8% 19% 29% 7%	3160
	11th to 20th	Day	25% 4 9% 25% 23% 15%	3140
	21st to end	Day	23% 3 9% 31% 20% 14%	3428
November	1st to 10th	Day	32% 5% 8% 24% 26% 5%	1957
	11th to 20th	Day	36% 4 10% 19% 23% 8%	1819
	21st to end	Day	36% 4 9% 24% 23% 3	1600
December	1st to 10th	Day	34% 8% 10% 21% 20% 7%	2600
	11th to 20th	Day	40% 6% 10% 20% 19% 5%	2986
	21st to end	Day	45% 7% 10% 21% 15% 1	4218
January	1st to 10th	Night	32% 5% 6% 23% 24% 10%	5889
	11th to 20th	Night	32% 5% 6% 20% 26% 10%	5900
	21st to end	Night	38% 3 7% 25% 21% 6%	6490
February	1st to 10th	Night	41% 6% 11% 23% 17% 2	2600
	11th to 20th	Night	46% 4 8% 19% 18% 6%	2535
	21st to end	Night	41% 5% 10% 23% 14% 6%	1890
March	1st to 10th	Night	37% 7% 8% 21% 19% 8%	4918
	11th to 20th	Night	29% 6% 7% 22% 30% 6%	4980
	21st to end	Night	39% 7% 11% 19% 18% 5%	5467
April	1st to 10th	Night	40% 6% 13% 19% 19% 5%	2070
	11th to 20th	Night	35% 6% 14% 22% 19% 4	1975
	21st to end	Night	40% 5% 11% 26% 15% 3	1939
May	1st to 10th	Night	47% 8% 13% 19% 11% 2	2930
	11th to 20th	Night	49% 7% 10% 24% 9% 1	2819
	21st to end	Night	34% 8% 18% 26% 11% 3	3033
June	1st to 10th	Night	36% 9% 13% 29% 13% 1	1520
	11th to 20th	Night	36% 9% 17% 24% 12% 2	1520
	21st to end	Night	31% 10% 17% 25% 16% 2	1567
July	1st to 10th	Night	32% 12% 21% 18% 14% 2	2795
	11th to 20th	Night	35% 13% 21% 23% 7% 1	2786
	21st to end	Night	46% 11% 15% 20% 8%	2988
August	1st to 10th	Night	43% 9% 20% 22% 5% 1	2814
	11th to 20th	Night	39% 10% 16% 25% 10% 1	2812
	21st to end	Night	33% 8% 18% 31% 7% 2	3469
September	1st to 10th	Night	36% 7% 14% 20% 17% 5%	1900
	11th to 20th	Night	37% 8% 10% 25% 17% 3	2031
	21st to end	Night	48% 6% 8% 20% 16% 3	2123
October	1st to 10th	Night	37% 7% 10% 19% 21% 6%	3800
	11th to 20th	Night	27% 5% 8% 21% 24% 15%	3779
	21st to end	Night	20% 3 9% 30% 24% 13%	4120
November	1st to 10th	Night	36% 4 8% 22% 24% 7%	2341
	11th to 20th	Night	37% 4 8% 21% 24% 6%	2576
	21st to end	Night	36% 6% 9% 24% 21% 5%	2800
December	1st to 10th	Night	31% 5% 11% 27% 22% 3	4580
	11th to 20th	Night	43% 6% 10% 17% 17% 6%	5208
	21st to end	Night	39% 7% 11% 24% 17% 2	6720

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	36% 31% 19% 11% 2	300
	11th to 20th	Day	41% 26% 25% 8% 1	300
	21st to end	Day	33% 32% 27% 6% 1	330
February	1st to 10th	Day	29% 34% 29% 8% 1	300
	11th to 20th	Day	49% 23% 24% 4	300
	21st to end	Day	35% 26% 24% 13% 1	372
March	1st to 10th	Day	32% 22% 27% 14% 4	500
	11th to 20th	Day	27% 26% 28% 14% 4 1	500
	21st to end	Day	27% 25% 29% 13% 6%	550
April	1st to 10th	Day	27% 28% 24% 13% 8%	500
	11th to 20th	Day	25% 18% 29% 16% 10% 3	492
	21st to end	Day	24% 21% 30% 16% 8% 2	500
May	1st to 10th	Day	23% 22% 30% 15% 7% 3	503
	11th to 20th	Day	23% 15% 29% 20% 11% 3	487
	21st to end	Day	18% 9% 28% 22% 16% 6%	529
June	1st to 10th	Day	13% 12% 30% 26% 14% 6%	490
	11th to 20th	Day	13% 9% 28% 28% 16% 7%	490
	21st to end	Day	9% 12% 32% 29% 13% 5%	490
July	1st to 10th	Day	11% 6% 22% 32% 20% 8%	485
	11th to 20th	Day	7% 5% 23% 35% 20% 10%	484
	21st to end	Day	14% 7% 22% 30% 19% 9%	532
August	1st to 10th	Day	14% 7% 25% 24% 21% 9%	486
	11th to 20th	Day	11% 7% 24% 27% 21% 10%	479
	21st to end	Day	11% 4 19% 29% 21% 14%	539
September	1st to 10th	Day	13% 6% 21% 25% 21% 13%	490
	11th to 20th	Day	17% 6% 20% 29% 20% 9%	490
	21st to end	Day	27% 11% 23% 21% 14% 4	490
October	1st to 10th	Day	20% 10% 22% 29% 14% 4	490
	11th to 20th	Day	18% 18% 23% 25% 14% 2	490
	21st to end	Day	18% 17% 34% 20% 8% 2	539
November	1st to 10th	Day	18% 16% 32% 23% 9% 2	495
	11th to 20th	Day	25% 21% 28% 19% 7% 1	351
	21st to end	Day	33% 18% 33% 13% 4 1	200
December	1st to 10th	Day	27% 29% 31% 11% 3 1	200
	11th to 20th	Day	29% 23% 29% 17% 2 1	200
	21st to end	Day	29% 22% 32% 14% 3	277
January	1st to 10th	Night	24% 26% 29% 18% 3	800
	11th to 20th	Night	25% 21% 29% 22% 4	800
	21st to end	Night	20% 24% 32% 21% 2	880
February	1st to 10th	Night	19% 22% 31% 25% 3	800
	11th to 20th	Night	28% 27% 28% 16% 1	783
	21st to end	Night	24% 22% 34% 19% 2	660
March	1st to 10th	Night	20% 22% 31% 23% 4 1	800
	11th to 20th	Night	27% 21% 25% 20% 7% 1	800
	21st to end	Night	21% 19% 31% 24% 5%	880
April	1st to 10th	Night	19% 24% 31% 22% 5%	800
	11th to 20th	Night	17% 19% 29% 24% 10% 1	791
	21st to end	Night	20% 12% 30% 29% 8% 1	739
May	1st to 10th	Night	18% 13% 26% 28% 15% 1	600
	11th to 20th	Night	18% 8% 26% 31% 14% 2	563
	21st to end	Night	16% 4 18% 32% 26% 4	589
June	1st to 10th	Night	8% 3 21% 44% 22% 2	380
	11th to 20th	Night	11% 6% 19% 34% 28% 2	380
	21st to end	Night	9% 5% 19% 42% 21% 3	427
July	1st to 10th	Night	12% 2 17% 32% 30% 7%	555
	11th to 20th	Night	6% 1 13% 35% 39% 6%	552
	21st to end	Night	7% 4 17% 35% 32% 5%	624
August	1st to 10th	Night	12% 2 13% 36% 32% 5%	567
	11th to 20th	Night	14% 3 14% 32% 32% 5%	564
	21st to end	Night	13% 3 13% 32% 34% 5%	829
September	1st to 10th	Night	14% 4 14% 31% 31% 7%	760
	11th to 20th	Night	15% 5% 14% 31% 29% 6%	760
	21st to end	Night	23% 9% 19% 27% 20% 3	760
October	1st to 10th	Night	17% 10% 18% 30% 21% 5%	760
	11th to 20th	Night	16% 14% 23% 29% 15% 2	760
	21st to end	Night	16% 13% 26% 31% 13% 2	836
November	1st to 10th	Night	15% 13% 28% 29% 12% 3	780
	11th to 20th	Night	20% 17% 25% 28% 9% 1	800
	21st to end	Night	19% 19% 25% 27% 9% 1	800
December	1st to 10th	Night	22% 21% 29% 21% 6%	800
	11th to 20th	Night	16% 20% 28% 30% 6%	800
	21st to end	Night	19% 20% 27% 31% 3	880

Page generated at 2025-05-08 09:34:00. Version ac07

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