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 2024-11-02.

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% 8% 39%	100853
	Night	13% 7% 2 6% 4 4 4 4 5% 7% 42%	98035
Transparency	Day	42% 7% 12% 22% 14% 3	100665
	Night	39% 7% 11% 23% 16% 4	97893
Seeing	Day	21% 16% 26% 21% 12% 5%	15029
	Night	17% 14% 24% 28% 14% 2	24911

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	15% 7% 3 6% 4 4 4 3 4 7% 44%	2836
	11th to 20th	Day	13% 10% 3 8% 5% 5% 5% 4 4 4 41%	2840
	21st to end	Day	19% 9% 3 8% 4 3 4 3 4 5% 38%	3124
February	1st to 10th	Day	11% 8% 3 8% 5% 5% 5% 4 5% 6% 40%	1710
	11th to 20th	Day	15% 8% 3 8% 5% 4 5% 4 6% 6% 38%	1663
	21st to end	Day	18% 9% 2 6% 6% 4 4 3 5% 5% 39%	1650
March	1st to 10th	Day	18% 11% 3 7% 4 4 4 3 5% 7% 34%	3435
	11th to 20th	Day	9% 6% 2 6% 4 4 4 4 5% 7% 48%	3480
	21st to end	Day	16% 9% 3 8% 5% 4 5% 4 5% 5% 36%	3815
April	1st to 10th	Day	9% 7% 3 7% 5% 5% 4 4 6% 7% 44%	2211
	11th to 20th	Day	12% 7% 2 6% 4 4 5% 3 5% 7% 43%	2254
	21st to end	Day	15% 9% 2 6% 6% 5% 4 3 5% 5% 39%	2353
May	1st to 10th	Day	16% 7% 3 9% 6% 6% 6% 4 6% 7% 29%	4082
	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	12% 6% 2 4 3 3 3 3 4 5% 55%	1867
	11th to 20th	Day	14% 5% 2 5% 4 3 4 3 5% 6% 48%	1729
	21st to end	Day	14% 7% 2 6% 3 4 3 3 4 6% 49%	1520
December	1st to 10th	Day	15% 9% 3 8% 4 5% 5% 3 6% 6% 36%	1520
	11th to 20th	Day	15% 5% 2 5% 4 3 3 2 4 6% 51%	1910
	21st to end	Day	17% 8% 2 6% 4 4 5% 4 6% 7% 34%	2946
January	1st to 10th	Night	12% 8% 2 5% 4 4 4 4 4 6% 47%	4109
	11th to 20th	Night	14% 9% 2 6% 4 4 4 3 4 6% 45%	4120
	21st to end	Night	19% 8% 2 6% 4 4 4 3 4 5% 41%	4532
February	1st to 10th	Night	11% 7% 2 8% 5% 4 5% 4 6% 7% 40%	2470
	11th to 20th	Night	18% 8% 2 6% 4 4 3 3 4 6% 40%	2403
	21st to end	Night	13% 8% 3 5% 5% 5% 4 4 6% 8% 39%	1799
March	1st to 10th	Night	19% 8% 3 8% 5% 5% 5% 4 5% 5% 34%	3418
	11th to 20th	Night	11% 5% 2 5% 4 4 4 4 6% 8% 49%	3480
	21st to end	Night	14% 7% 3 6% 4 4 4 2 5% 7% 46%	3817
April	1st to 10th	Night	11% 8% 2 5% 5% 4 4 4 7% 8% 42%	1967
	11th to 20th	Night	12% 7% 2 5% 3 4 4 4 5% 8% 47%	1875
	21st to end	Night	15% 6% 3 6% 4 4 4 3 6% 7% 42%	1842
May	1st to 10th	Night	17% 8% 4 7% 4 4 5% 4 6% 8% 33%	2813
	11th to 20th	Night	23% 9% 3 7% 4 4 3 3 5% 6% 35%	2812
	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	12% 8% 3 6% 4 3 3 3 3 4 52%	2234
	11th to 20th	Night	13% 6% 2 5% 5% 3 4 4 4 6% 49%	2446
	21st to end	Night	13% 6% 2 5% 4 4 4 3 5% 6% 48%	2660
December	1st to 10th	Night	16% 9% 2 8% 5% 4 4 3 4 5% 41%	2660
	11th to 20th	Night	13% 7% 3 7% 5% 4 4 3 4 6% 45%	3285
	21st to end	Night	15% 5% 2 7% 5% 5% 5% 4 6% 7% 38%	4692

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	39% 6% 5% 26% 18% 6%	2837
	11th to 20th	Day	47% 4 6% 18% 20% 5%	2840
	21st to end	Day	48% 4 7% 20% 17% 3	3124
February	1st to 10th	Day	44% 5% 10% 23% 16% 3	1710
	11th to 20th	Day	45% 5% 9% 19% 17% 4	1672
	21st to end	Day	47% 4 9% 18% 16% 5%	1650
March	1st to 10th	Day	49% 5% 9% 19% 13% 5%	3435
	11th to 20th	Day	34% 6% 8% 26% 19% 7%	3480
	21st to end	Day	49% 6% 8% 18% 15% 4	3815
April	1st to 10th	Day	38% 5% 12% 21% 18% 7%	2211
	11th to 20th	Day	39% 6% 11% 22% 18% 4	2254
	21st to end	Day	46% 5% 10% 22% 13% 4	2353
May	1st to 10th	Day	51% 7% 12% 19% 9% 1	4120
	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	39% 5% 8% 26% 18% 5%	1900
	11th to 20th	Day	35% 4 9% 26% 21% 5%	1900
	21st to end	Day	37% 3 7% 27% 20% 6%	2090
November	1st to 10th	Day	33% 4 8% 23% 27% 5%	1867
	11th to 20th	Day	36% 4 10% 20% 22% 8%	1729
	21st to end	Day	37% 4 9% 24% 23% 3	1520
December	1st to 10th	Day	47% 6% 9% 20% 15% 4	1520
	11th to 20th	Day	35% 4 7% 25% 21% 7%	1906
	21st to end	Day	46% 5% 10% 23% 14% 2	2946
January	1st to 10th	Night	36% 6% 6% 24% 23% 4	4109
	11th to 20th	Night	39% 6% 7% 21% 21% 6%	4120
	21st to end	Night	46% 4 6% 23% 16% 5%	4532
February	1st to 10th	Night	40% 6% 11% 23% 17% 2	2470
	11th to 20th	Night	45% 4 7% 19% 18% 6%	2405
	21st to end	Night	43% 5% 10% 23% 14% 5%	1799
March	1st to 10th	Night	49% 7% 8% 20% 14% 3	3418
	11th to 20th	Night	33% 5% 6% 23% 28% 5%	3480
	21st to end	Night	39% 6% 10% 22% 19% 5%	3817
April	1st to 10th	Night	39% 5% 12% 19% 19% 5%	1967
	11th to 20th	Night	34% 6% 14% 22% 20% 5%	1875
	21st to end	Night	40% 5% 11% 25% 15% 3	1842
May	1st to 10th	Night	48% 7% 13% 19% 11% 2	2840
	11th to 20th	Night	49% 7% 10% 24% 9% 1	2813
	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	39% 5% 11% 25% 16% 4	2280
	11th to 20th	Night	36% 5% 8% 26% 21% 5%	2280
	21st to end	Night	32% 4 9% 24% 25% 6%	2508
November	1st to 10th	Night	36% 4 7% 22% 24% 7%	2234
	11th to 20th	Night	36% 4 8% 22% 24% 6%	2446
	21st to end	Night	37% 5% 8% 24% 21% 5%	2660
December	1st to 10th	Night	45% 5% 8% 23% 15% 3	2660
	11th to 20th	Night	40% 5% 7% 19% 22% 7%	3288
	21st to end	Night	42% 6% 9% 23% 19% 2	4692

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	32% 32% 21% 12% 3	270
	11th to 20th	Day	37% 28% 27% 8% 1	270
	21st to end	Day	31% 32% 29% 7% 1	297
February	1st to 10th	Day	27% 33% 31% 9% 1	270
	11th to 20th	Day	45% 24% 26% 4	270
	21st to end	Day	36% 25% 23% 14% 1	342
March	1st to 10th	Day	30% 23% 27% 15% 4	460
	11th to 20th	Day	27% 27% 27% 14% 4 1	460
	21st to end	Day	24% 25% 30% 15% 6%	506
April	1st to 10th	Day	25% 28% 24% 14% 8%	460
	11th to 20th	Day	24% 17% 29% 17% 10% 3	452
	21st to end	Day	25% 21% 28% 16% 8% 2	460
May	1st to 10th	Day	24% 20% 29% 16% 7% 3	463
	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	19% 15% 31% 24% 10% 2	459
	11th to 20th	Day	25% 21% 26% 20% 7% 1	321
	21st to end	Day	28% 17% 35% 14% 4 1	180
December	1st to 10th	Day	27% 26% 32% 11% 3 1	180
	11th to 20th	Day	28% 23% 29% 17% 2 1	180
	21st to end	Day	28% 22% 33% 14% 4	249
January	1st to 10th	Night	21% 26% 30% 19% 3	760
	11th to 20th	Night	24% 21% 29% 23% 4	760
	21st to end	Night	20% 23% 32% 23% 2	836
February	1st to 10th	Night	18% 21% 31% 27% 3	760
	11th to 20th	Night	27% 27% 28% 16% 1	743
	21st to end	Night	23% 21% 34% 19% 2	628
March	1st to 10th	Night	18% 22% 31% 24% 4 1	760
	11th to 20th	Night	27% 21% 24% 21% 7% 1	760
	21st to end	Night	20% 18% 32% 25% 5%	836
April	1st to 10th	Night	18% 23% 31% 23% 5%	760
	11th to 20th	Night	17% 18% 29% 25% 10% 1	751
	21st to end	Night	20% 12% 31% 28% 8% 1	702
May	1st to 10th	Night	19% 12% 25% 28% 15% 1	570
	11th to 20th	Night	18% 8% 26% 31% 14% 2	561
	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	16% 13% 27% 30% 13% 3	745
	11th to 20th	Night	19% 16% 24% 29% 10% 1	760
	21st to end	Night	18% 18% 25% 28% 10% 1	760
December	1st to 10th	Night	21% 21% 29% 22% 7%	760
	11th to 20th	Night	15% 19% 29% 31% 6%	760
	21st to end	Night	19% 20% 27% 31% 3	836

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