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 2026-09-14.

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	12% 7% 2 7% 5% 5% 5% 4 6% 8% 39%	148188
	Night	12% 7% 2 6% 4 4 4 4 5% 7% 43%	142854
Transparency	Day	40% 8% 12% 22% 15% 4	151794
	Night	38% 7% 12% 22% 17% 4	147320
Seeing	Day	22% 16% 27% 20% 11% 4	17475
	Night	18% 15% 25% 27% 14% 2	27348

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% 8% 3 6% 4 4 4 3 4 6% 45%	5258
	11th to 20th	Day	9% 8% 3 8% 4 3 3 3 3 3 52%	5280
	21st to end	Day	14% 7% 3 8% 5% 3 3 2 4 5% 46%	5780
February	1st to 10th	Day	11% 9% 3 8% 5% 5% 5% 3 5% 6% 40%	1890
	11th to 20th	Day	16% 8% 3 8% 5% 3 4 4 6% 6% 38%	1843
	21st to end	Day	17% 8% 2 6% 6% 4 4 3 5% 5% 40%	1819
March	1st to 10th	Day	15% 8% 2 7% 4 5% 5% 4 6% 7% 38%	5045
	11th to 20th	Day	10% 6% 2 6% 4 4 4 3 5% 7% 49%	5090
	21st to end	Day	13% 8% 3 9% 5% 4 5% 4 5% 5% 39%	5586
April	1st to 10th	Day	10% 8% 3 7% 4 4 4 4 6% 7% 45%	2444
	11th to 20th	Day	12% 7% 2 6% 4 4 5% 3 5% 8% 44%	2494
	21st to end	Day	14% 9% 2 6% 5% 5% 4 3 5% 5% 40%	2601
May	1st to 10th	Day	13% 6% 2 8% 6% 6% 6% 5% 7% 8% 34%	7616
	11th to 20th	Day	17% 7% 2 7% 5% 5% 5% 4 6% 8% 36%	7626
	21st to end	Day	19% 7% 2 6% 5% 4 5% 4 6% 8% 34%	8468
June	1st to 10th	Day	11% 5% 2 6% 5% 5% 5% 5% 7% 9% 40%	2940
	11th to 20th	Day	11% 5% 2 6% 6% 5% 6% 5% 8% 10% 36%	2940
	21st to end	Day	8% 5% 2 6% 5% 6% 7% 5% 8% 9% 38%	2879
July	1st to 10th	Day	7% 5% 3 8% 6% 6% 6% 5% 7% 10% 37%	7680
	11th to 20th	Day	10% 6% 2 8% 6% 7% 7% 6% 9% 11% 28%	7680
	21st to end	Day	10% 7% 3 8% 6% 6% 7% 6% 9% 11% 28%	8340
August	1st to 10th	Day	14% 8% 3 8% 6% 7% 7% 5% 8% 10% 25%	6014
	11th to 20th	Day	8% 5% 3 8% 6% 7% 8% 6% 8% 11% 30%	5910
	21st to end	Day	8% 4 2 6% 5% 4 6% 5% 7% 10% 43%	6080
September	1st to 10th	Day	11% 6% 2 7% 5% 4 4 4 5% 7% 46%	2520
	11th to 20th	Day	15% 8% 2 7% 5% 4 4 4 5% 7% 40%	2293
	21st to end	Day	22% 8% 3 7% 4 5% 4 4 6% 6% 33%	2164
October	1st to 10th	Day	14% 7% 2 6% 4 4 3 3 5% 6% 46%	2000
	11th to 20th	Day	14% 6% 2 5% 4 3 3 3 5% 6% 50%	2000
	21st to end	Day	14% 7% 2 5% 3 4 3 3 4 6% 49%	2200
November	1st to 10th	Day	11% 6% 2 5% 4 3 3 3 5% 6% 54%	2057
	11th to 20th	Day	13% 5% 2 5% 4 3 4 3 6% 6% 49%	1909
	21st to end	Day	13% 7% 3 6% 3 4 3 3 4 6% 48%	1680
December	1st to 10th	Day	10% 7% 2 7% 4 4 5% 3 5% 6% 46%	2680
	11th to 20th	Day	16% 7% 3 6% 4 3 4 3 5% 6% 43%	3070
	21st to end	Day	17% 9% 3 8% 4 4 4 4 6% 7% 35%	4312
January	1st to 10th	Night	11% 6% 2 5% 4 5% 5% 4 5% 5% 48%	7635
	11th to 20th	Night	9% 7% 2 6% 3 4 3 2 4 5% 54%	7680
	21st to end	Night	13% 6% 2 7% 4 3 3 2 4 5% 51%	8401
February	1st to 10th	Night	11% 8% 2 8% 5% 4 5% 3 6% 7% 41%	2730
	11th to 20th	Night	17% 9% 2 7% 4 4 3 3 4 6% 40%	2663
	21st to end	Night	12% 8% 3 5% 5% 5% 4 4 6% 8% 40%	1982
March	1st to 10th	Night	15% 6% 2 6% 4 4 4 3 5% 5% 45%	5028
	11th to 20th	Night	10% 4 1 5% 4 3 4 3 5% 7% 52%	5090
	21st to end	Night	12% 6% 3 6% 5% 5% 4 3 5% 7% 42%	5588
April	1st to 10th	Night	11% 8% 2 5% 4 4 5% 3 7% 8% 42%	2174
	11th to 20th	Night	11% 7% 2 5% 3 4 4 4 5% 8% 46%	2075
	21st to end	Night	15% 5% 3 6% 5% 4 4 3 6% 7% 43%	2036
May	1st to 10th	Night	13% 7% 3 7% 4 5% 6% 4 6% 8% 38%	5237
	11th to 20th	Night	16% 7% 2 7% 4 4 4 4 6% 7% 37%	5234
	21st to end	Night	19% 7% 3 8% 4 5% 5% 4 6% 8% 33%	5666
June	1st to 10th	Night	8% 7% 2 6% 4 5% 6% 4 6% 9% 42%	1680
	11th to 20th	Night	9% 7% 2 6% 4 4 5% 5% 7% 9% 42%	1680
	21st to end	Night	8% 5% 1 6% 4 4 4 4 7% 9% 46%	1733
July	1st to 10th	Night	9% 6% 3 6% 5% 4 5% 4 7% 10% 40%	5278
	11th to 20th	Night	12% 8% 3 9% 5% 6% 6% 5% 7% 10% 30%	5277
	21st to end	Night	10% 8% 4 9% 6% 7% 6% 5% 6% 9% 32%	5736
August	1st to 10th	Night	16% 8% 3 8% 5% 4 6% 5% 7% 10% 28%	4135
	11th to 20th	Night	10% 7% 3 9% 6% 6% 6% 5% 8% 9% 32%	4095
	21st to end	Night	9% 7% 3 7% 5% 4 5% 4 6% 8% 43%	5038
September	1st to 10th	Night	10% 6% 2 6% 3 4 4 3 5% 6% 50%	2100
	11th to 20th	Night	12% 7% 2 6% 4 4 3 4 6% 8% 43%	2169
	21st to end	Night	22% 9% 3 7% 4 3 3 4 4 6% 36%	2235
October	1st to 10th	Night	14% 7% 3 7% 3 3 3 3 5% 5% 47%	2400
	11th to 20th	Night	13% 6% 3 5% 4 4 4 3 5% 5% 49%	2400
	21st to end	Night	13% 5% 2 5% 3 3 4 3 5% 6% 52%	2640
November	1st to 10th	Night	11% 8% 3 6% 3 3 3 3 4 4 52%	2461
	11th to 20th	Night	13% 6% 2 5% 5% 3 4 4 4 6% 49%	2705
	21st to end	Night	12% 6% 2 5% 5% 4 4 4 5% 6% 48%	2940
December	1st to 10th	Night	10% 6% 2 6% 4 4 4 3 5% 6% 50%	4720
	11th to 20th	Night	15% 8% 3 7% 5% 4 4 3 4 6% 41%	5345
	21st to end	Night	15% 6% 2 6% 4 5% 5% 4 6% 7% 40%	6868

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	38% 5% 8% 22% 20% 7%	5259
	11th to 20th	Day	36% 3 5% 17% 25% 13%	5280
	21st to end	Day	40% 4 6% 27% 17% 5%	5780
February	1st to 10th	Day	43% 5% 10% 22% 16% 3	1890
	11th to 20th	Day	46% 5% 9% 19% 17% 4	1852
	21st to end	Day	46% 4 9% 19% 15% 6%	1819
March	1st to 10th	Day	41% 6% 9% 18% 17% 9%	5045
	11th to 20th	Day	32% 7% 7% 25% 20% 8%	5090
	21st to end	Day	45% 7% 8% 17% 17% 6%	5586
April	1st to 10th	Day	38% 5% 12% 21% 17% 6%	2444
	11th to 20th	Day	38% 6% 11% 23% 18% 4	2494
	21st to end	Day	45% 6% 10% 24% 13% 3	2601
May	1st to 10th	Day	43% 9% 12% 24% 11% 1	7651
	11th to 20th	Day	43% 8% 13% 24% 10% 1	7630
	21st to end	Day	45% 7% 12% 20% 13% 3	8483
June	1st to 10th	Day	37% 9% 15% 24% 14% 1	2940
	11th to 20th	Day	39% 9% 16% 22% 13% 1	2940
	21st to end	Day	37% 11% 16% 24% 11% 1	2879
July	1st to 10th	Day	37% 13% 18% 21% 9% 3	7615
	11th to 20th	Day	42% 14% 17% 20% 6%	7611
	21st to end	Day	42% 12% 16% 20% 10%	8156
August	1st to 10th	Day	47% 14% 18% 17% 4	6001
	11th to 20th	Day	40% 13% 17% 22% 7%	5949
	21st to end	Day	32% 11% 20% 25% 11% 1	6080
September	1st to 10th	Day	37% 6% 14% 19% 20% 4	2520
	11th to 20th	Day	43% 6% 6% 25% 17% 3	2293
	21st to end	Day	50% 7% 9% 17% 16% 2	2164
October	1st to 10th	Day	31% 6% 8% 19% 29% 7%	3260
	11th to 20th	Day	25% 4 9% 25% 23% 15%	3240
	21st to end	Day	24% 3 9% 30% 21% 13%	3538
November	1st to 10th	Day	31% 5% 9% 24% 26% 5%	2057
	11th to 20th	Day	35% 4 10% 20% 23% 7%	1909
	21st to end	Day	37% 5% 9% 24% 22% 3	1680
December	1st to 10th	Day	34% 8% 10% 21% 20% 7%	2680
	11th to 20th	Day	40% 6% 10% 20% 19% 5%	3066
	21st to end	Day	45% 7% 10% 21% 15% 1	4312
January	1st to 10th	Night	34% 6% 8% 23% 21% 8%	7636
	11th to 20th	Night	31% 6% 6% 20% 27% 10%	7680
	21st to end	Night	37% 3 7% 25% 22% 6%	8401
February	1st to 10th	Night	40% 6% 11% 23% 18% 3	2730
	11th to 20th	Night	46% 4 8% 19% 18% 6%	2665
	21st to end	Night	42% 5% 10% 22% 15% 6%	1982
March	1st to 10th	Night	37% 7% 8% 21% 19% 8%	5028
	11th to 20th	Night	29% 6% 7% 23% 29% 6%	5090
	21st to end	Night	39% 7% 11% 20% 18% 5%	5588
April	1st to 10th	Night	38% 6% 13% 19% 19% 4	2174
	11th to 20th	Night	34% 6% 14% 22% 19% 4	2075
	21st to end	Night	40% 6% 11% 26% 15% 3	2036
May	1st to 10th	Night	42% 9% 15% 21% 12% 2	5264
	11th to 20th	Night	43% 9% 13% 25% 11%	5235
	21st to end	Night	46% 8% 14% 19% 10% 3	5675
June	1st to 10th	Night	36% 9% 13% 28% 14% 1	1680
	11th to 20th	Night	36% 9% 17% 24% 12% 2	1680
	21st to end	Night	32% 10% 16% 24% 15% 2	1733
July	1st to 10th	Night	37% 12% 18% 22% 11% 1	5235
	11th to 20th	Night	45% 14% 16% 19% 6%	5226
	21st to end	Night	43% 13% 15% 18% 11%	5606
August	1st to 10th	Night	47% 12% 18% 19% 4	4124
	11th to 20th	Night	42% 12% 18% 20% 8%	4122
	21st to end	Night	34% 10% 18% 28% 8% 1	5038
September	1st to 10th	Night	34% 6% 15% 22% 17% 5%	2100
	11th to 20th	Night	37% 8% 10% 25% 17% 2	2169
	21st to end	Night	49% 7% 9% 19% 15% 3	2235
October	1st to 10th	Night	37% 7% 10% 20% 22% 6%	3920
	11th to 20th	Night	27% 5% 8% 21% 24% 15%	3899
	21st to end	Night	21% 3 9% 30% 24% 13%	4252
November	1st to 10th	Night	36% 4 8% 22% 24% 6%	2461
	11th to 20th	Night	36% 4 8% 22% 24% 6%	2705
	21st to end	Night	36% 6% 9% 23% 21% 5%	2940
December	1st to 10th	Night	32% 5% 11% 27% 22% 3	4720
	11th to 20th	Night	43% 6% 10% 17% 17% 6%	5348
	21st to end	Night	39% 7% 11% 24% 17% 2	6868

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% 32% 18% 11% 2	330
	11th to 20th	Day	44% 26% 23% 7% 1	330
	21st to end	Day	37% 30% 26% 6% 1	363
February	1st to 10th	Day	29% 34% 28% 8% 1	330
	11th to 20th	Day	50% 23% 23% 4	330
	21st to end	Day	36% 26% 25% 12% 1	402
March	1st to 10th	Day	31% 22% 29% 14% 4	540
	11th to 20th	Day	29% 26% 28% 13% 4 1	540
	21st to end	Day	27% 24% 31% 13% 5%	594
April	1st to 10th	Day	26% 27% 25% 14% 8%	540
	11th to 20th	Day	26% 18% 30% 15% 9% 3	532
	21st to end	Day	24% 21% 30% 17% 8% 1	540
May	1st to 10th	Day	24% 24% 29% 14% 6% 3	543
	11th to 20th	Day	24% 16% 28% 20% 10% 2	567
	21st to end	Day	22% 12% 27% 19% 14% 5%	617
June	1st to 10th	Day	12% 13% 32% 26% 12% 5%	570
	11th to 20th	Day	13% 11% 29% 27% 14% 6%	570
	21st to end	Day	9% 12% 32% 29% 14% 5%	570
July	1st to 10th	Day	10% 5% 23% 32% 21% 9%	565
	11th to 20th	Day	7% 5% 24% 34% 21% 9%	564
	21st to end	Day	13% 7% 23% 29% 19% 9%	620
August	1st to 10th	Day	15% 7% 26% 25% 19% 8%	566
	11th to 20th	Day	11% 8% 26% 27% 20% 9%	559
	21st to end	Day	11% 6% 21% 30% 19% 13%	627
September	1st to 10th	Day	12% 7% 24% 25% 20% 12%	570
	11th to 20th	Day	17% 6% 20% 29% 19% 8%	542
	21st to end	Day	27% 11% 24% 21% 13% 4	530
October	1st to 10th	Day	19% 10% 22% 30% 15% 5%	530
	11th to 20th	Day	20% 18% 23% 24% 13% 2	530
	21st to end	Day	22% 17% 32% 19% 8% 2	583
November	1st to 10th	Day	18% 19% 31% 22% 9% 2	535
	11th to 20th	Day	27% 22% 27% 18% 6% 1	381
	21st to end	Day	35% 18% 30% 13% 4 1	220
December	1st to 10th	Day	29% 29% 30% 10% 2	220
	11th to 20th	Day	30% 23% 30% 15% 2 1	220
	21st to end	Day	30% 23% 31% 13% 3	305
January	1st to 10th	Night	24% 26% 29% 17% 3	840
	11th to 20th	Night	27% 22% 28% 21% 3	840
	21st to end	Night	22% 24% 32% 21% 2	924
February	1st to 10th	Night	19% 22% 32% 24% 3	840
	11th to 20th	Night	28% 27% 29% 15% 1	823
	21st to end	Night	25% 22% 33% 18% 2	692
March	1st to 10th	Night	19% 23% 32% 22% 3 1	840
	11th to 20th	Night	27% 21% 26% 20% 6%	840
	21st to end	Night	20% 20% 31% 24% 5%	924
April	1st to 10th	Night	19% 23% 32% 22% 5%	840
	11th to 20th	Night	17% 19% 29% 24% 9% 1	831
	21st to end	Night	20% 13% 30% 28% 9% 1	776
May	1st to 10th	Night	18% 15% 26% 26% 14% 1	630
	11th to 20th	Night	18% 9% 27% 31% 13% 2	621
	21st to end	Night	18% 6% 20% 29% 24% 4	652
June	1st to 10th	Night	8% 5% 22% 44% 20% 1	420
	11th to 20th	Night	12% 7% 21% 34% 25% 2	420
	21st to end	Night	9% 5% 20% 42% 21% 3	473
July	1st to 10th	Night	11% 2 17% 32% 31% 7%	615
	11th to 20th	Night	6% 2 13% 34% 37% 6%	612
	21st to end	Night	8% 4 18% 35% 31% 5%	690
August	1st to 10th	Night	13% 4 13% 36% 30% 5%	627
	11th to 20th	Night	13% 4 15% 33% 31% 5%	624
	21st to end	Night	12% 4 15% 33% 31% 5%	917
September	1st to 10th	Night	13% 3 15% 33% 29% 6%	840
	11th to 20th	Night	15% 5% 15% 32% 28% 5%	813
	21st to end	Night	23% 10% 19% 27% 19% 2	800
October	1st to 10th	Night	17% 10% 18% 29% 22% 5%	800
	11th to 20th	Night	17% 14% 24% 28% 15% 2	800
	21st to end	Night	18% 13% 26% 30% 12% 2	880
November	1st to 10th	Night	16% 14% 28% 29% 11% 2	820
	11th to 20th	Night	21% 18% 25% 27% 9% 1	840
	21st to end	Night	20% 19% 25% 26% 9% 1	840
December	1st to 10th	Night	21% 22% 30% 20% 6%	840
	11th to 20th	Night	18% 20% 28% 29% 6%	840
	21st to end	Night	19% 21% 27% 30% 3	924

Page generated at 2026-09-11 04:11:52. Version ac07

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