South Mountain
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the South Mountain clear sky chart from 2005-11-06 to 2025-12-18.

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	17% 10% 4 8% 5% 5% 4 4 5% 7% 31%	127484
	Night	17% 12% 4 9% 5% 4 4 3 5% 6% 30%	123541
Transparency	Day	51% 6% 12% 21% 10% 1	130949
	Night	53% 6% 12% 19% 10% 1	128163
Seeing	Day	23% 15% 31% 20% 8% 3	15888
	Night	24% 13% 29% 26% 9% 1	25003

By Month

Percentage Cloud Cover NE (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	16% 18% 6% 13% 6% 6% 4 3 4 4 19%	3597
	11th to 20th	Day	12% 17% 7% 15% 7% 5% 4 4 5% 6% 20%	3570
	21st to end	Day	13% 16% 7% 13% 7% 7% 5% 4 4 5% 20%	4121
February	1st to 10th	Day	16% 15% 5% 13% 7% 5% 4 3 5% 4 23%	1800
	11th to 20th	Day	15% 15% 5% 11% 6% 5% 4 4 5% 4 26%	1783
	21st to end	Day	19% 15% 5% 10% 6% 5% 4 3 4 4 24%	1650
March	1st to 10th	Day	19% 13% 4 11% 6% 4 5% 3 4 6% 25%	4476
	11th to 20th	Day	10% 13% 4 8% 6% 6% 6% 4 5% 7% 30%	4933
	21st to end	Day	18% 10% 3 8% 5% 4 3 3 4 5% 37%	5803
April	1st to 10th	Day	22% 13% 3 7% 5% 4 3 3 4 6% 31%	2400
	11th to 20th	Day	21% 10% 3 8% 5% 3 3 3 3 5% 36%	2374
	21st to end	Day	23% 9% 3 7% 5% 4 4 3 5% 6% 31%	2511
May	1st to 10th	Day	24% 11% 4 9% 5% 4 5% 3 5% 6% 24%	6346
	11th to 20th	Day	17% 9% 4 10% 6% 5% 5% 4 6% 7% 30%	6318
	21st to end	Day	21% 9% 3 7% 4 4 4 4 5% 7% 33%	6949
June	1st to 10th	Day	21% 8% 3 7% 5% 4 5% 4 6% 7% 30%	2800
	11th to 20th	Day	15% 5% 2 7% 5% 5% 4 4 7% 8% 38%	2800
	21st to end	Day	19% 8% 3 7% 6% 5% 4 3 7% 7% 32%	2800
July	1st to 10th	Day	13% 6% 3 8% 5% 5% 6% 5% 7% 9% 34%	6360
	11th to 20th	Day	10% 5% 2 6% 4 5% 5% 5% 7% 8% 41%	6360
	21st to end	Day	10% 5% 2 5% 4 5% 5% 4 6% 10% 45%	6926
August	1st to 10th	Day	14% 8% 3 7% 5% 4 5% 4 5% 8% 37%	4506
	11th to 20th	Day	16% 6% 3 6% 4 4 4 4 5% 9% 39%	4080
	21st to end	Day	16% 6% 2 6% 4 4 5% 4 6% 9% 39%	4312
September	1st to 10th	Day	13% 6% 2 6% 4 4 5% 4 7% 7% 42%	2400
	11th to 20th	Day	13% 5% 2 5% 4 4 4 3 5% 7% 48%	2385
	21st to end	Day	19% 5% 2 6% 4 4 4 4 6% 8% 39%	2158
October	1st to 10th	Day	19% 7% 3 6% 5% 5% 5% 3 4 7% 37%	2000
	11th to 20th	Day	19% 9% 4 10% 5% 5% 4 4 5% 6% 29%	2000
	21st to end	Day	26% 10% 3 7% 5% 5% 4 3 4 5% 28%	2030
November	1st to 10th	Day	16% 10% 4 6% 6% 5% 5% 4 6% 6% 32%	1851
	11th to 20th	Day	16% 13% 5% 8% 6% 5% 4 3 6% 5% 29%	1723
	21st to end	Day	21% 15% 4 10% 6% 5% 4 4 5% 5% 21%	1680
December	1st to 10th	Day	20% 20% 6% 11% 6% 5% 4 5% 5% 6% 14%	2680
	11th to 20th	Day	18% 16% 5% 12% 6% 5% 4 3 4 4 23%	3042
	21st to end	Day	22% 16% 4 12% 6% 5% 4 3 4 4 20%	3960
January	1st to 10th	Night	14% 21% 8% 13% 6% 4 4 2 4 4 20%	6349
	11th to 20th	Night	11% 17% 7% 16% 8% 6% 6% 4 5% 4 17%	6315
	21st to end	Night	16% 16% 6% 13% 6% 5% 4 3 4 5% 20%	6830
February	1st to 10th	Night	15% 16% 6% 11% 6% 6% 4 4 5% 6% 22%	2600
	11th to 20th	Night	13% 15% 5% 12% 7% 5% 4 3 4 5% 27%	2503
	21st to end	Night	16% 15% 5% 10% 5% 4 4 4 4 5% 28%	1980
March	1st to 10th	Night	21% 14% 4 8% 5% 5% 4 2 4 4 29%	5364
	11th to 20th	Night	14% 12% 4 8% 4 4 4 4 6% 8% 31%	5025
	21st to end	Night	19% 11% 3 8% 5% 4 4 3 5% 5% 34%	5128
April	1st to 10th	Night	21% 11% 3 7% 6% 4 4 3 4 5% 33%	2000
	11th to 20th	Night	20% 10% 3 7% 4 5% 4 3 5% 5% 34%	1975
	21st to end	Night	22% 9% 3 7% 5% 4 4 3 4 6% 34%	1903
May	1st to 10th	Night	24% 11% 3 8% 5% 5% 5% 5% 5% 6% 23%	3594
	11th to 20th	Night	16% 9% 4 8% 4 4 4 3 5% 8% 36%	3575
	21st to end	Night	16% 10% 3 7% 5% 5% 4 4 6% 7% 32%	3926
June	1st to 10th	Night	17% 7% 3 7% 5% 6% 5% 4 6% 8% 31%	1600
	11th to 20th	Night	15% 6% 2 6% 4 3 5% 4 6% 9% 40%	1600
	21st to end	Night	19% 9% 2 5% 5% 4 5% 3 6% 7% 35%	1600
July	1st to 10th	Night	13% 7% 3 8% 5% 5% 5% 4 7% 10% 34%	3600
	11th to 20th	Night	12% 6% 2 6% 4 4 4 4 6% 8% 42%	3600
	21st to end	Night	9% 5% 2 6% 4 5% 6% 4 6% 8% 45%	3926
August	1st to 10th	Night	13% 8% 3 6% 6% 4 4 4 5% 9% 40%	2640
	11th to 20th	Night	13% 7% 2 7% 5% 4 5% 4 5% 7% 41%	2935
	21st to end	Night	11% 6% 2 6% 5% 5% 6% 5% 6% 8% 41%	3586
September	1st to 10th	Night	13% 6% 2 6% 4 5% 4 3 5% 6% 46%	2000
	11th to 20th	Night	13% 4 3 5% 3 4 3 3 5% 7% 51%	2014
	21st to end	Night	15% 6% 3 6% 5% 4 4 3 5% 7% 42%	2240
October	1st to 10th	Night	17% 7% 3 7% 4 4 4 4 5% 7% 39%	2400
	11th to 20th	Night	19% 9% 3 7% 4 4 4 4 6% 6% 34%	2400
	21st to end	Night	24% 10% 3 7% 5% 4 4 3 5% 6% 28%	2810
November	1st to 10th	Night	19% 10% 3 8% 5% 4 4 3 4 5% 36%	2667
	11th to 20th	Night	17% 12% 4 8% 5% 4 4 3 4 6% 32%	2893
	21st to end	Night	22% 14% 5% 9% 5% 5% 4 3 4 6% 23%	2940
December	1st to 10th	Night	20% 17% 6% 12% 7% 5% 5% 3 4 5% 16%	4720
	11th to 20th	Night	23% 15% 4 10% 5% 4 4 3 3 4 25%	5307
	21st to end	Night	26% 16% 4 11% 5% 4 4 3 4 4 19%	6996

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	67% 6% 11% 12% 4 1	3598
	11th to 20th	Day	65% 6% 11% 10% 8%	3600
	21st to end	Day	66% 7% 9% 11% 7% 1	4121
February	1st to 10th	Day	64% 4 7% 18% 5% 1	1800
	11th to 20th	Day	61% 5% 11% 15% 7% 1	1792
	21st to end	Day	64% 5% 9% 12% 8% 1	1650
March	1st to 10th	Day	60% 5% 8% 15% 9% 3	4479
	11th to 20th	Day	52% 6% 17% 14% 10% 1	4933
	21st to end	Day	50% 4 6% 19% 18% 2	5803
April	1st to 10th	Day	55% 4 8% 19% 13% 3	2400
	11th to 20th	Day	52% 4 8% 19% 16% 2	2374
	21st to end	Day	54% 5% 9% 15% 15% 3	2511
May	1st to 10th	Day	60% 6% 9% 17% 8% 1	6357
	11th to 20th	Day	53% 7% 11% 18% 10% 1	6322
	21st to end	Day	49% 7% 11% 22% 10% 1	6949
June	1st to 10th	Day	51% 7% 10% 19% 12% 1	2800
	11th to 20th	Day	43% 6% 12% 25% 12% 2	2800
	21st to end	Day	49% 8% 11% 20% 11% 1	2800
July	1st to 10th	Day	43% 8% 14% 25% 10%	6290
	11th to 20th	Day	37% 8% 14% 28% 14%	6285
	21st to end	Day	33% 8% 14% 37% 9%	6728
August	1st to 10th	Day	45% 6% 15% 24% 9%	4492
	11th to 20th	Day	41% 6% 15% 29% 8%	4119
	21st to end	Day	40% 7% 16% 26% 10%	4312
September	1st to 10th	Day	39% 7% 13% 29% 12%	2400
	11th to 20th	Day	36% 6% 12% 33% 14%	2385
	21st to end	Day	43% 7% 13% 26% 11%	2158
October	1st to 10th	Day	44% 6% 12% 28% 10%	3260
	11th to 20th	Day	46% 7% 15% 24% 7% 1	3240
	21st to end	Day	47% 5% 17% 23% 7%	3255
November	1st to 10th	Day	50% 6% 13% 23% 7% 1	1851
	11th to 20th	Day	56% 6% 12% 16% 9% 1	1723
	21st to end	Day	64% 7% 12% 12% 5%	1680
December	1st to 10th	Day	71% 7% 11% 10% 2	2680
	11th to 20th	Day	64% 7% 9% 14% 6%	3042
	21st to end	Day	68% 5% 8% 15% 4	3960
January	1st to 10th	Night	68% 5% 12% 11% 4	6349
	11th to 20th	Night	69% 6% 10% 10% 5%	6360
	21st to end	Night	66% 5% 9% 9% 10% 1	6830
February	1st to 10th	Night	64% 5% 9% 16% 6% 1	2600
	11th to 20th	Night	60% 5% 10% 19% 7%	2505
	21st to end	Night	60% 5% 8% 17% 10% 1	1980
March	1st to 10th	Night	59% 4 7% 16% 11% 2	5374
	11th to 20th	Night	49% 6% 19% 15% 8% 3	5026
	21st to end	Night	52% 4 8% 21% 13% 2	5128
April	1st to 10th	Night	54% 4 9% 19% 13% 2	2000
	11th to 20th	Night	51% 5% 9% 19% 15% 1	1975
	21st to end	Night	52% 4 9% 17% 15% 3	1903
May	1st to 10th	Night	58% 8% 8% 16% 8% 2	3600
	11th to 20th	Night	46% 7% 14% 19% 13% 1	3576
	21st to end	Night	49% 7% 12% 18% 13% 1	3927
June	1st to 10th	Night	49% 8% 12% 18% 12% 1	1600
	11th to 20th	Night	41% 6% 12% 21% 17% 2	1600
	21st to end	Night	47% 7% 13% 19% 13% 1	1600
July	1st to 10th	Night	42% 8% 17% 20% 13% 1	3560
	11th to 20th	Night	36% 8% 14% 30% 11%	3552
	21st to end	Night	35% 8% 13% 28% 15%	3809
August	1st to 10th	Night	42% 7% 15% 25% 10%	2630
	11th to 20th	Night	42% 6% 15% 28% 8% 1	2962
	21st to end	Night	37% 9% 15% 25% 12% 1	3586
September	1st to 10th	Night	39% 5% 12% 30% 14% 1	2000
	11th to 20th	Night	33% 6% 13% 35% 13% 1	2014
	21st to end	Night	42% 5% 11% 28% 14% 1	2240
October	1st to 10th	Night	40% 7% 14% 28% 11% 1	3920
	11th to 20th	Night	42% 5% 16% 27% 10%	3899
	21st to end	Night	48% 6% 16% 21% 8%	4535
November	1st to 10th	Night	51% 5% 13% 21% 10% 1	2667
	11th to 20th	Night	54% 5% 12% 18% 10% 1	2893
	21st to end	Night	62% 6% 11% 16% 5%	2940
December	1st to 10th	Night	69% 6% 11% 12% 2	4720
	11th to 20th	Night	63% 6% 7% 15% 9% 1	5307
	21st to end	Night	68% 5% 7% 12% 8%	6996

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	25% 19% 35% 16% 5%	200
	11th to 20th	Day	31% 16% 33% 14% 5% 2	200
	21st to end	Day	23% 19% 36% 16% 5%	255
February	1st to 10th	Day	32% 21% 32% 11% 3	300
	11th to 20th	Day	27% 26% 36% 12%	330
	21st to end	Day	30% 18% 35% 15% 2	414
March	1st to 10th	Day	27% 17% 34% 17% 5%	500
	11th to 20th	Day	25% 18% 35% 18% 4 1	500
	21st to end	Day	30% 19% 28% 16% 5% 1	550
April	1st to 10th	Day	32% 17% 30% 14% 5% 1	500
	11th to 20th	Day	33% 18% 25% 15% 6% 2	492
	21st to end	Day	31% 18% 32% 11% 6% 1	500
May	1st to 10th	Day	25% 17% 31% 18% 7% 1	503
	11th to 20th	Day	20% 16% 36% 18% 8% 3	527
	21st to end	Day	20% 18% 27% 24% 10% 1	573
June	1st to 10th	Day	26% 12% 32% 21% 7% 1	530
	11th to 20th	Day	15% 16% 34% 23% 8% 3	530
	21st to end	Day	24% 13% 34% 18% 9% 2	530
July	1st to 10th	Day	14% 8% 33% 31% 12% 2	525
	11th to 20th	Day	14% 11% 31% 31% 11% 3	524
	21st to end	Day	13% 10% 33% 29% 12% 2	576
August	1st to 10th	Day	15% 9% 26% 34% 13% 3	526
	11th to 20th	Day	16% 7% 25% 30% 15% 7%	519
	21st to end	Day	16% 12% 31% 23% 13% 6%	583
September	1st to 10th	Day	15% 11% 28% 26% 12% 8%	530
	11th to 20th	Day	13% 7% 23% 26% 18% 13%	530
	21st to end	Day	18% 11% 32% 19% 15% 4	530
October	1st to 10th	Day	21% 14% 27% 20% 13% 5%	530
	11th to 20th	Day	26% 18% 28% 21% 5% 2	530
	21st to end	Day	34% 13% 27% 21% 5%	489
November	1st to 10th	Day	28% 20% 28% 17% 6% 1	425
	11th to 20th	Day	25% 21% 33% 13% 8% 1	263
	21st to end	Day	30% 23% 30% 13% 4	220
December	1st to 10th	Day	35% 16% 33% 14% 3	220
	11th to 20th	Day	30% 27% 32% 7% 4	214
	21st to end	Day	35% 17% 27% 16% 4 2	220
January	1st to 10th	Night	29% 16% 33% 19% 4	800
	11th to 20th	Night	30% 17% 27% 22% 4 1	800
	21st to end	Night	23% 12% 34% 25% 5%	880
February	1st to 10th	Night	25% 19% 32% 21% 3	800
	11th to 20th	Night	25% 18% 34% 21% 3	783
	21st to end	Night	25% 16% 33% 22% 3	660
March	1st to 10th	Night	27% 13% 33% 22% 4	800
	11th to 20th	Night	25% 18% 31% 21% 5%	800
	21st to end	Night	25% 13% 30% 25% 7%	880
April	1st to 10th	Night	28% 14% 29% 26% 4	800
	11th to 20th	Night	27% 16% 29% 20% 7%	791
	21st to end	Night	26% 14% 29% 24% 7%	703
May	1st to 10th	Night	26% 10% 28% 27% 8% 2	400
	11th to 20th	Night	18% 10% 35% 31% 7%	394
	21st to end	Night	15% 10% 39% 25% 11%	433
June	1st to 10th	Night	17% 9% 30% 36% 7% 1	400
	11th to 20th	Night	16% 10% 31% 31% 12% 1	400
	21st to end	Night	21% 8% 30% 29% 12% 1	400
July	1st to 10th	Night	9% 8% 30% 41% 12%	390
	11th to 20th	Night	16% 5% 23% 40% 16%	388
	21st to end	Night	11% 5% 27% 46% 11% 1	438
August	1st to 10th	Night	16% 5% 21% 40% 17%	426
	11th to 20th	Night	14% 4 19% 39% 23% 1	628
	21st to end	Night	15% 7% 27% 33% 17% 2	880
September	1st to 10th	Night	16% 7% 29% 32% 16% 1	800
	11th to 20th	Night	15% 7% 21% 31% 23% 4	800
	21st to end	Night	18% 8% 25% 27% 20% 2	800
October	1st to 10th	Night	21% 10% 28% 24% 15% 2	800
	11th to 20th	Night	25% 12% 30% 24% 8% 1	800
	21st to end	Night	30% 14% 25% 26% 5%	880
November	1st to 10th	Night	28% 15% 26% 23% 7%	820
	11th to 20th	Night	27% 15% 29% 21% 9%	840
	21st to end	Night	32% 16% 26% 20% 5%	840
December	1st to 10th	Night	30% 17% 28% 20% 4	840
	11th to 20th	Night	28% 18% 30% 20% 4 1	829
	21st to end	Night	34% 14% 28% 21% 3	880

Page generated at 2025-12-15 01:13:53. Version ac07

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