Persephone Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Persephone Observatory clear sky chart from 2005-11-06 to 2026-04-02.

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	15% 8% 3 8% 5% 5% 5% 4 6% 7% 35%	131534
	Night	14% 8% 3 8% 5% 4 5% 4 5% 7% 37%	130295
Transparency	Day	46% 5% 11% 22% 13% 2	135129
	Night	45% 5% 11% 23% 14% 2	134770
Seeing	Day	25% 16% 26% 20% 10% 4	16558
	Night	21% 14% 25% 28% 11% 1	26185

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	14% 12% 5% 14% 6% 5% 5% 3 4 5% 28%	4779
	11th to 20th	Day	10% 14% 6% 15% 7% 6% 5% 4 5% 5% 23%	5280
	21st to end	Day	13% 14% 5% 10% 6% 5% 4 3 4 5% 32%	5780
February	1st to 10th	Day	14% 14% 4 8% 6% 5% 4 3 6% 5% 31%	1890
	11th to 20th	Day	15% 10% 4 11% 6% 5% 4 3 4 6% 32%	1843
	21st to end	Day	16% 11% 4 11% 5% 3 5% 4 5% 6% 32%	1715
March	1st to 10th	Day	18% 12% 2 6% 4 4 4 3 4 6% 36%	5042
	11th to 20th	Day	11% 8% 3 7% 5% 5% 5% 4 5% 8% 40%	5090
	21st to end	Day	13% 9% 3 7% 5% 5% 5% 4 5% 6% 38%	5617
April	1st to 10th	Day	17% 11% 3 7% 5% 5% 4 3 5% 7% 33%	2412
	11th to 20th	Day	14% 11% 3 7% 5% 3 4 4 5% 5% 39%	2374
	21st to end	Day	19% 6% 2 7% 5% 4 4 3 6% 5% 37%	2463
May	1st to 10th	Day	23% 7% 2 7% 5% 5% 5% 4 5% 8% 29%	5862
	11th to 20th	Day	25% 7% 3 6% 4 5% 4 4 5% 8% 30%	6151
	21st to end	Day	17% 6% 3 6% 5% 5% 5% 5% 7% 7% 34%	6934
June	1st to 10th	Day	13% 6% 2 5% 5% 6% 5% 5% 6% 9% 37%	2800
	11th to 20th	Day	14% 6% 2 7% 5% 5% 5% 5% 6% 9% 36%	2800
	21st to end	Day	11% 4 2 7% 5% 5% 6% 5% 7% 11% 39%	2800
July	1st to 10th	Day	6% 4 2 7% 6% 6% 6% 5% 7% 9% 43%	6360
	11th to 20th	Day	7% 6% 2 8% 7% 7% 7% 5% 8% 10% 33%	6183
	21st to end	Day	10% 6% 3 7% 5% 5% 5% 4 6% 8% 42%	6424
August	1st to 10th	Day	17% 6% 2 6% 5% 6% 6% 5% 7% 9% 32%	4234
	11th to 20th	Day	11% 5% 3 7% 6% 6% 6% 5% 7% 8% 38%	4117
	21st to end	Day	14% 5% 2 5% 4 4 5% 4 6% 7% 45%	4312
September	1st to 10th	Day	16% 6% 2 5% 4 4 4 3 4 7% 46%	2400
	11th to 20th	Day	14% 5% 2 5% 3 4 3 4 5% 7% 48%	2377
	21st to end	Day	21% 9% 2 6% 4 3 4 3 5% 7% 35%	2099
October	1st to 10th	Day	17% 6% 2 6% 4 4 4 3 5% 6% 43%	2000
	11th to 20th	Day	16% 6% 2 6% 4 5% 5% 4 5% 7% 40%	2000
	21st to end	Day	16% 7% 3 6% 4 4 4 3 5% 7% 41%	2200
November	1st to 10th	Day	10% 7% 3 5% 4 4 4 4 5% 7% 47%	1962
	11th to 20th	Day	14% 10% 3 7% 4 4 5% 3 6% 6% 38%	1756
	21st to end	Day	15% 10% 3 7% 4 5% 5% 4 5% 6% 35%	1680
December	1st to 10th	Day	13% 8% 7% 10% 5% 4 5% 5% 7% 6% 31%	2680
	11th to 20th	Day	21% 12% 4 8% 4 4 3 3 4 6% 31%	3070
	21st to end	Day	18% 14% 5% 10% 5% 6% 5% 4 6% 6% 21%	4048
January	1st to 10th	Night	10% 11% 5% 15% 5% 5% 5% 4 5% 7% 28%	8099
	11th to 20th	Night	11% 10% 5% 12% 5% 5% 5% 4 5% 6% 32%	7680
	21st to end	Night	14% 10% 3 8% 7% 5% 4 3 5% 6% 34%	8401
February	1st to 10th	Night	14% 10% 4 8% 6% 3 5% 5% 5% 7% 33%	2730
	11th to 20th	Night	16% 9% 4 9% 5% 5% 5% 4 5% 7% 31%	2663
	21st to end	Night	14% 11% 4 9% 5% 5% 4 3 5% 6% 34%	2089
March	1st to 10th	Night	18% 10% 3 7% 4 4 4 2 4 6% 38%	5028
	11th to 20th	Night	10% 7% 3 6% 4 4 4 3 5% 6% 47%	5090
	21st to end	Night	11% 8% 2 7% 5% 5% 5% 4 6% 6% 42%	5555
April	1st to 10th	Night	17% 12% 3 9% 6% 4 5% 3 5% 5% 33%	2011
	11th to 20th	Night	15% 7% 2 7% 4 4 4 3 6% 8% 41%	1975
	21st to end	Night	17% 6% 2 5% 3 4 5% 4 5% 8% 41%	1954
May	1st to 10th	Night	25% 9% 3 7% 4 4 5% 4 5% 7% 27%	4033
	11th to 20th	Night	26% 8% 2 5% 4 3 3 4 5% 8% 32%	3741
	21st to end	Night	20% 7% 2 6% 4 4 5% 4 5% 8% 35%	3920
June	1st to 10th	Night	13% 6% 2 5% 4 3 4 5% 5% 7% 45%	1600
	11th to 20th	Night	12% 6% 2 5% 4 5% 5% 4 7% 9% 40%	1600
	21st to end	Night	10% 4 2 5% 4 4 5% 3 6% 11% 44%	1600
July	1st to 10th	Night	8% 5% 2 7% 4 4 5% 4 7% 10% 44%	3598
	11th to 20th	Night	9% 5% 2 8% 5% 6% 6% 5% 7% 10% 35%	3771
	21st to end	Night	9% 6% 2 7% 4 5% 5% 4 6% 7% 44%	4427
August	1st to 10th	Night	17% 5% 2 6% 5% 4 5% 4 6% 9% 37%	2915
	11th to 20th	Night	12% 5% 2 7% 4 4 5% 4 5% 7% 44%	2898
	21st to end	Night	15% 5% 1 5% 3 3 4 3 5% 9% 46%	3586
September	1st to 10th	Night	15% 5% 2 6% 4 4 3 3 5% 6% 47%	2000
	11th to 20th	Night	11% 6% 2 5% 3 3 5% 3 4 8% 51%	2019
	21st to end	Night	19% 6% 3 5% 4 4 4 3 6% 6% 40%	2300
October	1st to 10th	Night	16% 7% 2 5% 3 4 4 3 5% 6% 46%	2400
	11th to 20th	Night	15% 7% 2 6% 3 4 4 3 5% 7% 45%	2400
	21st to end	Night	17% 6% 2 5% 4 4 4 4 5% 6% 43%	2640
November	1st to 10th	Night	9% 7% 2 5% 5% 3 4 4 5% 6% 51%	2556
	11th to 20th	Night	15% 10% 3 7% 4 4 4 4 5% 6% 38%	2861
	21st to end	Night	15% 9% 3 6% 6% 5% 5% 4 6% 6% 36%	2940
December	1st to 10th	Night	10% 8% 5% 8% 5% 5% 5% 4 6% 8% 36%	4720
	11th to 20th	Night	20% 11% 4 7% 5% 5% 4 3 5% 6% 31%	5345
	21st to end	Night	17% 13% 5% 9% 5% 6% 5% 4 5% 6% 25%	7150

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	57% 6% 10% 18% 8% 1	4780
	11th to 20th	Day	60% 6% 8% 13% 9% 4	5280
	21st to end	Day	56% 3 10% 21% 9% 1	5780
February	1st to 10th	Day	55% 3 10% 21% 9% 1	1890
	11th to 20th	Day	54% 5% 10% 21% 9% 1	1852
	21st to end	Day	53% 4 12% 21% 8% 3	1715
March	1st to 10th	Day	50% 4 9% 22% 11% 5%	5042
	11th to 20th	Day	42% 5% 12% 27% 10% 4	5090
	21st to end	Day	47% 4 11% 20% 16% 3	5617
April	1st to 10th	Day	51% 4 11% 18% 14% 2	2412
	11th to 20th	Day	46% 4 9% 20% 17% 3	2374
	21st to end	Day	48% 5% 7% 23% 14% 2	2463
May	1st to 10th	Day	53% 5% 12% 17% 11% 2	5897
	11th to 20th	Day	53% 4 9% 24% 9% 1	6155
	21st to end	Day	46% 6% 12% 21% 14% 2	6949
June	1st to 10th	Day	42% 6% 13% 24% 15% 1	2800
	11th to 20th	Day	43% 7% 15% 20% 14% 1	2800
	21st to end	Day	38% 6% 13% 27% 15% 1	2800
July	1st to 10th	Day	37% 8% 12% 26% 15% 2	6290
	11th to 20th	Day	42% 8% 16% 22% 11% 1	6108
	21st to end	Day	39% 6% 9% 24% 21% 1	6240
August	1st to 10th	Day	47% 7% 14% 22% 10%	4221
	11th to 20th	Day	42% 7% 11% 27% 13% 1	4156
	21st to end	Day	37% 5% 11% 29% 16% 2	4312
September	1st to 10th	Day	39% 5% 10% 23% 19% 3	2400
	11th to 20th	Day	35% 4 10% 28% 19% 3	2377
	21st to end	Day	49% 4 11% 16% 18% 2	2099
October	1st to 10th	Day	36% 4 8% 26% 21% 4	3260
	11th to 20th	Day	35% 5% 11% 26% 17% 5%	3240
	21st to end	Day	28% 3 13% 30% 24% 1	3538
November	1st to 10th	Day	36% 4 12% 24% 19% 4	1962
	11th to 20th	Day	46% 5% 9% 23% 15% 3	1756
	21st to end	Day	48% 6% 11% 25% 9% 1	1680
December	1st to 10th	Day	52% 5% 15% 20% 5% 3	2680
	11th to 20th	Day	56% 3 13% 20% 7%	3066
	21st to end	Day	62% 5% 12% 13% 8% 1	4048
January	1st to 10th	Night	53% 6% 11% 19% 10% 1	8100
	11th to 20th	Night	49% 7% 9% 17% 14% 4	7680
	21st to end	Night	50% 4 10% 25% 10% 1	8401
February	1st to 10th	Night	49% 4 13% 23% 10% 1	2730
	11th to 20th	Night	52% 5% 11% 18% 13% 1	2665
	21st to end	Night	51% 4 11% 21% 11% 3	2089
March	1st to 10th	Night	48% 4 9% 25% 11% 4	5028
	11th to 20th	Night	37% 4 14% 30% 11% 4	5090
	21st to end	Night	41% 5% 12% 20% 20% 3	5555
April	1st to 10th	Night	52% 5% 9% 18% 14% 2	2011
	11th to 20th	Night	41% 5% 11% 23% 16% 3	1975
	21st to end	Night	42% 5% 10% 25% 15% 3	1954
May	1st to 10th	Night	57% 4 10% 16% 10% 3	4060
	11th to 20th	Night	51% 5% 10% 25% 9% 1	3742
	21st to end	Night	47% 5% 13% 24% 10% 2	3927
June	1st to 10th	Night	37% 6% 11% 28% 15% 2	1600
	11th to 20th	Night	39% 6% 14% 26% 13% 2	1600
	21st to end	Night	33% 6% 17% 27% 16% 1	1600
July	1st to 10th	Night	35% 8% 15% 23% 15% 3	3560
	11th to 20th	Night	42% 9% 17% 20% 12% 1	3726
	21st to end	Night	37% 6% 11% 22% 23% 1	4297
August	1st to 10th	Night	44% 6% 16% 24% 9%	2904
	11th to 20th	Night	39% 5% 12% 29% 15% 1	2925
	21st to end	Night	36% 4 13% 31% 15% 2	3586
September	1st to 10th	Night	38% 4 12% 23% 19% 3	2000
	11th to 20th	Night	34% 4 9% 32% 21% 2	2019
	21st to end	Night	44% 4 10% 23% 16% 2	2300
October	1st to 10th	Night	36% 4 10% 27% 22% 1	3920
	11th to 20th	Night	30% 4 10% 28% 21% 6%	3899
	21st to end	Night	29% 3 12% 31% 23% 2	4252
November	1st to 10th	Night	35% 3 11% 27% 20% 4	2556
	11th to 20th	Night	45% 5% 7% 24% 16% 3	2861
	21st to end	Night	46% 5% 11% 25% 11% 1	2940
December	1st to 10th	Night	45% 5% 16% 26% 8%	4720
	11th to 20th	Night	54% 4 12% 20% 9% 1	5348
	21st to end	Night	59% 5% 11% 16% 8%	7150

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	38% 25% 26% 10% 1	242
	11th to 20th	Day	38% 28% 27% 7% 1	330
	21st to end	Day	33% 30% 28% 6% 2	363
February	1st to 10th	Day	34% 29% 27% 8% 2	330
	11th to 20th	Day	46% 25% 25% 4 1	330
	21st to end	Day	34% 26% 26% 13% 2	431
March	1st to 10th	Day	34% 21% 24% 17% 3	540
	11th to 20th	Day	34% 23% 24% 14% 4 1	540
	21st to end	Day	32% 21% 31% 11% 4 2	594
April	1st to 10th	Day	32% 24% 26% 13% 4 1	504
	11th to 20th	Day	27% 21% 26% 14% 9% 3	492
	21st to end	Day	29% 19% 29% 14% 8% 1	500
May	1st to 10th	Day	32% 17% 25% 15% 7% 3	503
	11th to 20th	Day	28% 17% 26% 17% 9% 3	527
	21st to end	Day	22% 12% 28% 21% 12% 6%	573
June	1st to 10th	Day	18% 18% 25% 22% 14% 3	530
	11th to 20th	Day	20% 12% 29% 24% 12% 4	530
	21st to end	Day	13% 12% 30% 26% 15% 5%	530
July	1st to 10th	Day	13% 5% 22% 31% 21% 8%	525
	11th to 20th	Day	8% 6% 29% 30% 18% 8%	524
	21st to end	Day	12% 7% 29% 31% 16% 6%	576
August	1st to 10th	Day	14% 6% 23% 29% 19% 9%	526
	11th to 20th	Day	13% 5% 24% 31% 17% 11%	519
	21st to end	Day	15% 7% 22% 28% 18% 10%	583
September	1st to 10th	Day	19% 10% 17% 28% 19% 8%	530
	11th to 20th	Day	15% 5% 21% 30% 20% 9%	530
	21st to end	Day	32% 12% 19% 21% 12% 4	530
October	1st to 10th	Day	24% 9% 22% 25% 16% 5%	530
	11th to 20th	Day	22% 18% 28% 19% 12% 2	530
	21st to end	Day	22% 17% 33% 19% 8% 2	583
November	1st to 10th	Day	20% 16% 31% 21% 10% 2	485
	11th to 20th	Day	29% 19% 30% 14% 6% 1	296
	21st to end	Day	32% 20% 32% 13% 2	220
December	1st to 10th	Day	34% 24% 30% 7% 4 1	220
	11th to 20th	Day	32% 22% 29% 14% 2	220
	21st to end	Day	33% 20% 29% 15% 3	242
January	1st to 10th	Night	23% 23% 33% 17% 3	840
	11th to 20th	Night	26% 20% 31% 20% 3	840
	21st to end	Night	23% 23% 34% 19% 2	924
February	1st to 10th	Night	24% 20% 29% 24% 3	840
	11th to 20th	Night	29% 25% 29% 16% 1	823
	21st to end	Night	29% 20% 30% 19% 2	692
March	1st to 10th	Night	25% 20% 32% 20% 3	840
	11th to 20th	Night	29% 18% 27% 21% 4	840
	21st to end	Night	22% 19% 35% 19% 5% 1	924
April	1st to 10th	Night	27% 20% 29% 21% 3	805
	11th to 20th	Night	20% 16% 29% 26% 8%	791
	21st to end	Night	22% 15% 25% 31% 7%	754
May	1st to 10th	Night	25% 8% 24% 30% 14% 1	600
	11th to 20th	Night	21% 9% 27% 33% 9% 1	469
	21st to end	Night	17% 7% 23% 34% 18% 2	433
June	1st to 10th	Night	15% 5% 27% 33% 20% 1	400
	11th to 20th	Night	15% 6% 21% 39% 19% 1	400
	21st to end	Night	10% 4 25% 44% 17% 1	400
July	1st to 10th	Night	13% 3 17% 39% 25% 4	390
	11th to 20th	Night	9% 3 18% 38% 28% 3	458
	21st to end	Night	10% 2 17% 43% 25% 3	657
August	1st to 10th	Night	11% 3 13% 42% 28% 2	597
	11th to 20th	Night	13% 3 13% 41% 26% 4	604
	21st to end	Night	14% 4 17% 36% 26% 3	880
September	1st to 10th	Night	17% 4 17% 35% 24% 4	800
	11th to 20th	Night	14% 6% 15% 32% 29% 5%	800
	21st to end	Night	22% 7% 21% 27% 20% 3	800
October	1st to 10th	Night	20% 7% 19% 32% 21% 3	800
	11th to 20th	Night	20% 15% 27% 27% 11% 1	800
	21st to end	Night	23% 12% 27% 30% 8%	880
November	1st to 10th	Night	14% 14% 30% 32% 10% 1	820
	11th to 20th	Night	24% 20% 25% 23% 7% 1	840
	21st to end	Night	23% 21% 28% 22% 5%	840
December	1st to 10th	Night	24% 23% 26% 22% 5%	840
	11th to 20th	Night	26% 18% 28% 25% 4	840
	21st to end	Night	23% 18% 30% 27% 2	924

Page generated at 2026-03-30 02:18:42. Version ac07

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