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Peach Mountain Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Peach Mountain Observatory clear sky chart from 2005-11-06 to 2025-11-03.

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% 10% 4 8% 5% 5% 5% 4 6% 7% 33%	126074
	Night	14% 11% 4 9% 5% 4 4 3 5% 6% 34%	124093
Transparency	Day	48% 6% 12% 21% 13% 1	129644
	Night	48% 6% 11% 21% 13% 1	128593
Seeing	Day	17% 12% 29% 23% 14% 5%	16934
	Night	19% 10% 29% 29% 12% 1	26861

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% 17% 7% 17% 5% 5% 4 3 4 5% 22%	3617
	11th to 20th	Day	15% 15% 6% 18% 8% 8% 7% 4 5% 4 10%	3620
	21st to end	Day	22% 18% 6% 12% 5% 4 4 3 4 5% 16%	4304
February	1st to 10th	Day	14% 15% 5% 13% 8% 4 5% 3 6% 6% 21%	1800
	11th to 20th	Day	14% 13% 5% 14% 9% 4 5% 2 4 6% 24%	1926
	21st to end	Day	15% 16% 5% 12% 6% 5% 4 3 5% 6% 25%	1650
March	1st to 10th	Day	15% 12% 5% 11% 5% 4 5% 3 5% 5% 30%	4526
	11th to 20th	Day	12% 12% 4 8% 5% 4 5% 3 5% 7% 36%	4980
	21st to end	Day	18% 14% 4 10% 5% 4 4 3 5% 6% 28%	5465
April	1st to 10th	Day	17% 12% 4 6% 6% 4 5% 4 5% 7% 30%	2361
	11th to 20th	Day	16% 9% 4 7% 5% 5% 4 4 5% 7% 34%	2507
	21st to end	Day	16% 10% 3 7% 5% 4 4 2 5% 7% 36%	2600
May	1st to 10th	Day	19% 10% 2 7% 4 4 4 3 5% 6% 34%	5862
	11th to 20th	Day	16% 8% 3 7% 5% 5% 5% 4 6% 8% 33%	5861
	21st to end	Day	14% 7% 3 7% 4 4 4 3 5% 8% 41%	6515
June	1st to 10th	Day	10% 6% 2 6% 5% 5% 5% 4 6% 9% 42%	2800
	11th to 20th	Day	10% 7% 3 6% 4 6% 5% 4 7% 9% 40%	2800
	21st to end	Day	9% 6% 2 7% 5% 5% 6% 5% 8% 9% 39%	2800
July	1st to 10th	Day	9% 6% 2 6% 5% 5% 6% 5% 8% 11% 38%	6360
	11th to 20th	Day	6% 5% 2 6% 4 5% 5% 4 6% 10% 45%	6360
	21st to end	Day	7% 6% 2 6% 5% 5% 4 4 6% 9% 45%	6616
August	1st to 10th	Day	11% 5% 2 6% 4 5% 5% 4 6% 9% 42%	4234
	11th to 20th	Day	10% 7% 3 7% 4 5% 5% 4 7% 9% 40%	4174
	21st to end	Day	7% 6% 2 6% 6% 5% 6% 4 7% 10% 41%	4553
September	1st to 10th	Day	10% 6% 2 7% 5% 5% 5% 4 5% 8% 42%	2285
	11th to 20th	Day	9% 5% 2 4 4 4 5% 4 6% 8% 49%	2200
	21st to end	Day	12% 7% 3 8% 4 4 6% 4 6% 8% 38%	2200
October	1st to 10th	Day	12% 7% 3 6% 5% 4 5% 4 5% 7% 43%	2170
	11th to 20th	Day	17% 8% 3 9% 6% 5% 4 4 5% 6% 34%	2000
	21st to end	Day	19% 10% 4 7% 5% 4 4 4 4 7% 31%	2005
November	1st to 10th	Day	11% 10% 3 8% 5% 4 4 4 5% 6% 39%	1780
	11th to 20th	Day	15% 15% 5% 9% 5% 5% 4 3 5% 6% 29%	1800
	21st to end	Day	20% 12% 5% 8% 6% 5% 4 4 5% 6% 27%	1800
December	1st to 10th	Day	12% 15% 7% 16% 7% 5% 5% 3 5% 5% 20%	2637
	11th to 20th	Day	19% 21% 7% 9% 6% 5% 4 4 4 5% 16%	2924
	21st to end	Day	25% 21% 7% 12% 5% 4 4 2 3 4 13%	3982
January	1st to 10th	Night	14% 15% 6% 14% 6% 5% 4 3 5% 5% 23%	6348
	11th to 20th	Night	13% 15% 7% 15% 8% 6% 6% 4 5% 6% 15%	6360
	21st to end	Night	18% 16% 8% 10% 5% 5% 5% 3 4 5% 21%	6656
February	1st to 10th	Night	16% 16% 5% 10% 8% 5% 5% 3 5% 5% 23%	2600
	11th to 20th	Night	14% 13% 6% 10% 7% 4 5% 3 5% 5% 27%	2358
	21st to end	Night	17% 11% 4 11% 5% 4 4 3 6% 6% 28%	1980
March	1st to 10th	Night	15% 11% 4 8% 4 4 3 3 5% 6% 35%	5327
	11th to 20th	Night	9% 9% 3 5% 4 4 5% 5% 6% 8% 42%	4980
	21st to end	Night	16% 11% 3 8% 5% 4 4 3 4 6% 36%	5467
April	1st to 10th	Night	17% 9% 3 7% 5% 4 4 4 4 6% 36%	2055
	11th to 20th	Night	18% 9% 2 5% 5% 5% 4 4 6% 6% 36%	1840
	21st to end	Night	16% 9% 3 6% 5% 4 5% 4 5% 6% 38%	1800
May	1st to 10th	Night	20% 10% 3 8% 5% 4 4 3 5% 6% 31%	4033
	11th to 20th	Night	17% 9% 3 7% 4 4 4 3 4 6% 39%	4032
	21st to end	Night	14% 6% 2 6% 4 4 4 3 6% 7% 44%	4338
June	1st to 10th	Night	11% 5% 2 5% 4 4 5% 3 6% 8% 47%	1600
	11th to 20th	Night	11% 6% 2 6% 4 5% 4 4 6% 9% 43%	1600
	21st to end	Night	9% 5% 3 7% 3 4 4 3 6% 11% 47%	1600
July	1st to 10th	Night	7% 4 2 5% 5% 5% 5% 4 7% 10% 47%	3618
	11th to 20th	Night	8% 4 2 6% 4 4 4 4 6% 8% 50%	3617
	21st to end	Night	8% 6% 3 6% 4 4 4 3 7% 8% 48%	4243
August	1st to 10th	Night	10% 6% 2 6% 3 4 4 3 5% 9% 48%	2915
	11th to 20th	Night	9% 6% 2 5% 4 4 4 4 6% 7% 49%	2875
	21st to end	Night	8% 6% 2 6% 5% 5% 4 4 5% 7% 47%	3341
September	1st to 10th	Night	9% 5% 2 6% 4 4 4 4 5% 7% 50%	2095
	11th to 20th	Night	8% 5% 2 4 4 3 4 2 5% 7% 56%	2200
	21st to end	Night	14% 6% 2 6% 4 4 5% 4 5% 6% 44%	2200
October	1st to 10th	Night	12% 7% 3 6% 4 4 4 3 4 6% 48%	2228
	11th to 20th	Night	18% 9% 3 8% 5% 4 4 3 5% 6% 36%	2400
	21st to end	Night	20% 9% 3 7% 4 5% 4 3 5% 6% 35%	2830
November	1st to 10th	Night	13% 9% 3 7% 5% 4 4 3 5% 6% 42%	2563
	11th to 20th	Night	15% 11% 4 8% 5% 5% 5% 4 6% 7% 31%	2600
	21st to end	Night	18% 15% 4 9% 5% 4 4 3 5% 5% 28%	2600
December	1st to 10th	Night	12% 14% 5% 15% 7% 5% 5% 3 5% 6% 23%	4532
	11th to 20th	Night	15% 19% 6% 13% 6% 5% 4 4 5% 5% 18%	5266
	21st to end	Night	25% 23% 6% 10% 5% 4 4 3 4 4 14%	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	64% 5% 10% 15% 5%	3618
	11th to 20th	Day	74% 7% 10% 5% 4	3620
	21st to end	Day	71% 4 11% 10% 5%	4304
February	1st to 10th	Day	62% 6% 11% 13% 7%	1800
	11th to 20th	Day	63% 4 9% 14% 8% 1	1936
	21st to end	Day	60% 5% 11% 15% 8% 1	1650
March	1st to 10th	Day	56% 5% 12% 16% 9% 2	4526
	11th to 20th	Day	47% 6% 16% 22% 8% 2	4980
	21st to end	Day	58% 4 10% 15% 12% 1	5465
April	1st to 10th	Day	54% 5% 10% 19% 11%	2361
	11th to 20th	Day	48% 6% 11% 21% 12% 2	2507
	21st to end	Day	48% 6% 13% 18% 14% 1	2600
May	1st to 10th	Day	49% 7% 10% 19% 14% 2	5897
	11th to 20th	Day	46% 8% 12% 20% 12% 1	5865
	21st to end	Day	41% 5% 10% 23% 19% 3	6529
June	1st to 10th	Day	37% 7% 12% 27% 14% 2	2800
	11th to 20th	Day	39% 8% 13% 23% 17% 2	2800
	21st to end	Day	37% 9% 14% 22% 16% 2	2800
July	1st to 10th	Day	37% 7% 14% 26% 15% 2	6291
	11th to 20th	Day	32% 6% 13% 28% 18% 1	6285
	21st to end	Day	34% 7% 14% 23% 21% 1	6428
August	1st to 10th	Day	36% 7% 13% 25% 17% 1	4221
	11th to 20th	Day	38% 8% 10% 31% 12% 1	4213
	21st to end	Day	36% 7% 13% 30% 12% 1	4552
September	1st to 10th	Day	38% 8% 14% 24% 14% 2	2285
	11th to 20th	Day	31% 7% 11% 31% 17% 1	2200
	21st to end	Day	43% 7% 11% 24% 15%	2200
October	1st to 10th	Day	31% 5% 10% 31% 22% 1	3534
	11th to 20th	Day	40% 5% 11% 25% 19%	3240
	21st to end	Day	46% 4 13% 24% 12% 2	3217
November	1st to 10th	Day	44% 6% 12% 22% 14% 1	1780
	11th to 20th	Day	56% 5% 12% 19% 8% 2	1800
	21st to end	Day	58% 6% 12% 17% 7%	1800
December	1st to 10th	Day	65% 7% 14% 11% 3	2637
	11th to 20th	Day	69% 6% 10% 12% 3	2921
	21st to end	Day	76% 4 7% 8% 4	3982
January	1st to 10th	Night	61% 6% 9% 17% 7% 1	6348
	11th to 20th	Night	67% 6% 11% 10% 6%	6360
	21st to end	Night	65% 5% 12% 11% 7%	6656
February	1st to 10th	Night	63% 7% 10% 13% 6% 1	2600
	11th to 20th	Night	58% 5% 11% 17% 9% 1	2359
	21st to end	Night	56% 4 14% 17% 8% 2	1980
March	1st to 10th	Night	48% 5% 14% 22% 10% 1	5327
	11th to 20th	Night	37% 6% 14% 26% 16% 1	4980
	21st to end	Night	49% 5% 12% 19% 14% 2	5467
April	1st to 10th	Night	48% 6% 10% 21% 14% 2	2055
	11th to 20th	Night	46% 6% 11% 18% 18% 1	1840
	21st to end	Night	46% 6% 12% 18% 15% 3	1800
May	1st to 10th	Night	53% 5% 10% 16% 13% 4	4060
	11th to 20th	Night	45% 6% 9% 25% 12% 3	4033
	21st to end	Night	37% 7% 12% 23% 18% 3	4346
June	1st to 10th	Night	34% 6% 11% 27% 20% 2	1600
	11th to 20th	Night	36% 8% 14% 25% 16% 2	1600
	21st to end	Night	32% 6% 13% 27% 20% 1	1600
July	1st to 10th	Night	32% 6% 14% 28% 15% 5%	3580
	11th to 20th	Night	30% 6% 14% 29% 19% 2	3572
	21st to end	Night	32% 7% 14% 21% 24% 2	4118
August	1st to 10th	Night	34% 6% 13% 28% 17% 1	2904
	11th to 20th	Night	32% 7% 10% 37% 12% 2	2902
	21st to end	Night	35% 7% 11% 31% 16% 1	3341
September	1st to 10th	Night	33% 7% 12% 28% 18% 2	2095
	11th to 20th	Night	28% 6% 10% 31% 22% 2	2200
	21st to end	Night	39% 7% 12% 27% 15% 1	2200
October	1st to 10th	Night	26% 4 9% 33% 27% 1	3644
	11th to 20th	Night	42% 4 11% 23% 19%	3899
	21st to end	Night	39% 4 10% 31% 14% 1	4568
November	1st to 10th	Night	42% 6% 11% 24% 15% 1	2563
	11th to 20th	Night	51% 5% 14% 19% 9% 1	2600
	21st to end	Night	57% 6% 12% 18% 7% 1	2600
December	1st to 10th	Night	62% 6% 13% 15% 4	4532
	11th to 20th	Night	66% 6% 11% 12% 5%	5268
	21st to end	Night	75% 4 8% 7% 6%	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	22% 18% 37% 17% 5% 1	300
	11th to 20th	Day	24% 16% 43% 14% 2 1	300
	21st to end	Day	29% 16% 35% 15% 4 1	330
February	1st to 10th	Day	23% 23% 36% 13% 3 1	300
	11th to 20th	Day	26% 21% 36% 14% 2	300
	21st to end	Day	25% 27% 34% 13% 1	249
March	1st to 10th	Day	24% 21% 37% 17% 2	320
	11th to 20th	Day	24% 12% 35% 22% 7%	500
	21st to end	Day	27% 15% 33% 19% 6% 1	550
April	1st to 10th	Day	22% 16% 34% 20% 8% 1	500
	11th to 20th	Day	24% 19% 32% 13% 7% 5%	492
	21st to end	Day	23% 13% 32% 19% 11% 2	500
May	1st to 10th	Day	23% 13% 31% 19% 10% 3	503
	11th to 20th	Day	19% 9% 31% 21% 13% 6%	527
	21st to end	Day	16% 9% 23% 26% 18% 8%	603
June	1st to 10th	Day	12% 8% 30% 31% 16% 4	730
	11th to 20th	Day	12% 8% 31% 28% 17% 4	730
	21st to end	Day	10% 9% 29% 30% 15% 5%	730
July	1st to 10th	Day	9% 5% 21% 30% 25% 10%	721
	11th to 20th	Day	10% 6% 26% 27% 21% 8%	718
	21st to end	Day	9% 4 22% 32% 24% 9%	664
August	1st to 10th	Day	8% 4 18% 30% 24% 16%	526
	11th to 20th	Day	13% 4 17% 29% 26% 11%	519
	21st to end	Day	10% 3 22% 29% 25% 12%	583
September	1st to 10th	Day	13% 5% 22% 23% 21% 15%	530
	11th to 20th	Day	9% 3 16% 24% 29% 18%	530
	21st to end	Day	16% 10% 22% 23% 19% 9%	530
October	1st to 10th	Day	15% 9% 26% 25% 17% 7%	530
	11th to 20th	Day	19% 14% 29% 28% 9% 1	530
	21st to end	Day	28% 17% 32% 17% 6%	388
November	1st to 10th	Day	14% 19% 35% 24% 8% 1	306
	11th to 20th	Day	22% 22% 33% 17% 5% 1	300
	21st to end	Day	26% 19% 39% 13% 4	300
December	1st to 10th	Day	22% 20% 34% 15% 8%	212
	11th to 20th	Day	26% 21% 38% 11% 4	253
	21st to end	Day	28% 23% 32% 15% 2	330
January	1st to 10th	Night	22% 16% 35% 22% 5% 1	1000
	11th to 20th	Night	25% 14% 34% 21% 5% 1	1000
	21st to end	Night	20% 13% 36% 26% 5%	955
February	1st to 10th	Night	26% 12% 37% 23% 3	800
	11th to 20th	Night	23% 15% 35% 23% 4	783
	21st to end	Night	24% 13% 40% 20% 3	660
March	1st to 10th	Night	21% 13% 35% 24% 6% 1	800
	11th to 20th	Night	17% 17% 36% 24% 5% 1	800
	21st to end	Night	24% 12% 31% 26% 7%	880
April	1st to 10th	Night	20% 15% 36% 25% 4	655
	11th to 20th	Night	22% 11% 34% 24% 7% 1	593
	21st to end	Night	21% 8% 33% 29% 8%	600
May	1st to 10th	Night	21% 8% 31% 30% 9% 1	600
	11th to 20th	Night	21% 5% 29% 31% 13% 1	591
	21st to end	Night	12% 4 25% 37% 20% 2	650
June	1st to 10th	Night	12% 4 25% 44% 15% 1	600
	11th to 20th	Night	13% 3 22% 44% 18%	600
	21st to end	Night	10% 3 22% 43% 21% 1	600
July	1st to 10th	Night	8% 3 15% 42% 28% 4	585
	11th to 20th	Night	10% 3 17% 41% 25% 3	582
	21st to end	Night	5% 2 18% 48% 26% 2	657
August	1st to 10th	Night	8% 2 16% 42% 29% 3	597
	11th to 20th	Night	9% 3 19% 37% 29% 4	594
	21st to end	Night	9% 1 16% 39% 31% 3	660
September	1st to 10th	Night	11% 5% 19% 33% 26% 5%	695
	11th to 20th	Night	11% 3 18% 31% 33% 5%	800
	21st to end	Night	15% 7% 21% 31% 21% 5%	800
October	1st to 10th	Night	14% 7% 26% 33% 17% 3	800
	11th to 20th	Night	21% 13% 29% 28% 9% 1	800
	21st to end	Night	24% 13% 33% 24% 6%	880
November	1st to 10th	Night	18% 12% 31% 28% 11% 1	789
	11th to 20th	Night	22% 15% 35% 21% 7% 1	800
	21st to end	Night	25% 14% 34% 23% 5%	800
December	1st to 10th	Night	23% 17% 35% 21% 6%	800
	11th to 20th	Night	24% 13% 38% 22% 3	955
	21st to end	Night	33% 11% 33% 19% 2	1100

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