Evergreen Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Evergreen Observatory clear sky chart from 2005-11-06 to 2026-05-17.

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	16% 12% 5% 10% 5% 4 4 3 5% 6% 29%	131999
	Night	16% 15% 5% 11% 5% 4 4 3 4 6% 26%	130812
Transparency	Day	54% 6% 11% 18% 9% 1	135599
	Night	58% 5% 10% 16% 9% 1	135283
Seeing	Day	23% 14% 29% 22% 9% 3	16449
	Night	26% 14% 25% 23% 11% 1	26152

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	17% 24% 9% 28% 4 3 2 2 3 2 6%	5021
	11th to 20th	Day	11% 23% 11% 23% 7% 6% 4 2 3 3 8%	5280
	21st to end	Day	16% 18% 8% 18% 9% 7% 5% 2 3 3 11%	5780
February	1st to 10th	Day	17% 19% 6% 15% 11% 5% 3 3 3 4 15%	1890
	11th to 20th	Day	15% 18% 9% 15% 8% 5% 4 2 3 3 16%	1843
	21st to end	Day	19% 17% 5% 13% 4 4 4 3 4 6% 20%	1692
March	1st to 10th	Day	15% 15% 6% 12% 5% 4 3 3 4 6% 27%	5041
	11th to 20th	Day	12% 16% 6% 10% 4 4 4 3 4 6% 31%	5090
	21st to end	Day	21% 15% 6% 10% 4 4 4 2 3 5% 26%	5586
April	1st to 10th	Day	18% 14% 4 7% 4 4 3 3 4 7% 31%	2420
	11th to 20th	Day	19% 12% 3 6% 4 4 4 4 5% 6% 33%	2595
	21st to end	Day	20% 10% 3 7% 5% 3 4 3 5% 6% 35%	2730
May	1st to 10th	Day	20% 10% 3 8% 5% 4 5% 4 5% 6% 31%	5992
	11th to 20th	Day	17% 9% 3 7% 4 4 5% 4 6% 7% 33%	5980
	21st to end	Day	19% 8% 3 7% 4 4 5% 3 5% 6% 36%	6934
June	1st to 10th	Day	16% 6% 2 5% 4 4 5% 4 6% 8% 40%	2800
	11th to 20th	Day	13% 6% 3 7% 4 5% 4 4 6% 9% 38%	2800
	21st to end	Day	14% 4 2 5% 4 4 4 4 7% 9% 42%	2800
July	1st to 10th	Day	9% 5% 3 6% 4 4 5% 4 7% 10% 43%	6360
	11th to 20th	Day	12% 6% 3 6% 4 4 4 3 7% 9% 42%	5930
	21st to end	Day	12% 5% 2 6% 4 4 4 4 6% 7% 47%	6424
August	1st to 10th	Day	15% 6% 2 6% 5% 4 5% 4 7% 9% 38%	4234
	11th to 20th	Day	14% 6% 3 7% 5% 5% 5% 4 8% 9% 34%	4174
	21st to end	Day	11% 5% 2 6% 4 4 5% 4 6% 9% 42%	4519
September	1st to 10th	Day	16% 7% 2 5% 4 4 4 4 5% 8% 40%	2398
	11th to 20th	Day	12% 6% 2 5% 4 5% 5% 3 6% 8% 46%	2200
	21st to end	Day	18% 8% 3 8% 5% 4 5% 3 4 6% 34%	2200
October	1st to 10th	Day	18% 6% 2 7% 4 4 4 3 6% 7% 38%	2006
	11th to 20th	Day	23% 9% 3 8% 4 5% 4 3 6% 6% 29%	2000
	21st to end	Day	28% 12% 3 8% 5% 4 4 4 4 6% 22%	2200
November	1st to 10th	Day	17% 15% 4 8% 4 4 4 3 4 6% 30%	1902
	11th to 20th	Day	19% 21% 4 9% 5% 3 3 3 4 5% 22%	1700
	21st to end	Day	20% 27% 5% 9% 5% 5% 3 3 3 6% 13%	1680
December	1st to 10th	Day	18% 33% 12% 13% 4 3 3 2 2 2 8%	2680
	11th to 20th	Day	21% 27% 8% 13% 5% 3 3 3 3 4 10%	3070
	21st to end	Day	23% 24% 7% 16% 5% 4 3 2 3 3 9%	4048
January	1st to 10th	Night	16% 24% 12% 28% 4 3 2 2 2 2 6%	7855
	11th to 20th	Night	10% 23% 11% 22% 8% 5% 4 2 3 3 8%	7680
	21st to end	Night	17% 19% 7% 15% 8% 5% 4 3 4 4 14%	8401
February	1st to 10th	Night	18% 20% 6% 14% 8% 4 3 2 4 4 17%	2730
	11th to 20th	Night	17% 18% 8% 14% 6% 5% 4 3 4 4 17%	2663
	21st to end	Night	19% 16% 6% 15% 6% 4 4 3 3 5% 21%	2113
March	1st to 10th	Night	19% 17% 5% 9% 5% 3 3 3 4 5% 27%	5031
	11th to 20th	Night	13% 16% 5% 7% 4 3 4 3 5% 6% 34%	5090
	21st to end	Night	17% 14% 5% 11% 5% 4 4 3 4 4 29%	5588
April	1st to 10th	Night	19% 15% 4 8% 5% 4 4 4 5% 5% 28%	2200
	11th to 20th	Night	20% 11% 3 7% 4 5% 4 3 6% 7% 31%	1992
	21st to end	Night	15% 11% 3 6% 5% 5% 5% 4 5% 7% 34%	1890
May	1st to 10th	Night	24% 10% 3 7% 4 5% 4 4 4 6% 29%	4123
	11th to 20th	Night	15% 8% 3 8% 5% 5% 4 4 5% 7% 36%	4045
	21st to end	Night	20% 7% 2 5% 3 3 4 3 6% 9% 38%	3920
June	1st to 10th	Night	11% 7% 2 7% 6% 5% 4 4 6% 8% 40%	1600
	11th to 20th	Night	12% 6% 3 8% 5% 4 4 4 6% 8% 41%	1600
	21st to end	Night	12% 6% 3 6% 3 5% 5% 4 7% 8% 41%	1600
July	1st to 10th	Night	11% 7% 2 6% 4 5% 4 4 6% 9% 41%	3598
	11th to 20th	Night	12% 5% 2 6% 5% 5% 5% 4 7% 9% 41%	4009
	21st to end	Night	8% 5% 2 6% 4 5% 5% 4 6% 9% 47%	4427
August	1st to 10th	Night	10% 7% 3 8% 5% 5% 5% 4 6% 8% 39%	2915
	11th to 20th	Night	15% 8% 3 8% 5% 4 4 3 5% 6% 38%	2875
	21st to end	Night	10% 6% 3 6% 4 4 5% 4 6% 9% 41%	3345
September	1st to 10th	Night	14% 7% 2 6% 3 4 4 3 6% 7% 46%	2000
	11th to 20th	Night	13% 4 3 5% 3 4 4 4 5% 7% 49%	2199
	21st to end	Night	18% 6% 2 8% 6% 4 5% 4 5% 7% 35%	2200
October	1st to 10th	Night	19% 7% 2 7% 3 3 4 3 5% 7% 40%	2393
	11th to 20th	Night	24% 9% 3 7% 4 4 4 3 5% 6% 29%	2400
	21st to end	Night	30% 11% 4 7% 4 3 4 3 4 5% 27%	2640
November	1st to 10th	Night	19% 15% 4 7% 4 4 4 3 4 5% 31%	2616
	11th to 20th	Night	19% 21% 4 9% 5% 4 4 3 5% 5% 20%	2919
	21st to end	Night	22% 25% 6% 8% 4 3 3 3 3 4 18%	2940
December	1st to 10th	Night	16% 31% 10% 11% 5% 4 4 2 3 3 11%	4720
	11th to 20th	Night	19% 28% 8% 12% 4 3 3 2 3 4 12%	5345
	21st to end	Night	21% 24% 6% 14% 5% 5% 4 3 4 5% 11%	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	86% 3 6% 4 1	5022
	11th to 20th	Day	83% 3 7% 4 3	5280
	21st to end	Day	79% 3 7% 7% 3	5780
February	1st to 10th	Day	75% 5% 7% 9% 5%	1890
	11th to 20th	Day	74% 3 7% 11% 5%	1852
	21st to end	Day	64% 5% 11% 13% 4 2	1692
March	1st to 10th	Day	59% 5% 13% 19% 4	5041
	11th to 20th	Day	54% 5% 15% 18% 5% 1	5090
	21st to end	Day	62% 4 8% 16% 9% 1	5586
April	1st to 10th	Day	54% 5% 14% 15% 11%	2420
	11th to 20th	Day	51% 6% 10% 19% 12% 2	2595
	21st to end	Day	51% 5% 10% 21% 11% 3	2730
May	1st to 10th	Day	53% 7% 9% 19% 9% 4	6027
	11th to 20th	Day	48% 9% 10% 21% 10% 1	5984
	21st to end	Day	47% 7% 12% 19% 11% 3	6949
June	1st to 10th	Day	42% 7% 13% 24% 13% 1	2800
	11th to 20th	Day	42% 7% 14% 21% 14% 1	2800
	21st to end	Day	38% 9% 14% 23% 15% 1	2800
July	1st to 10th	Day	35% 8% 13% 25% 17% 2	6290
	11th to 20th	Day	38% 7% 13% 25% 16% 1	5860
	21st to end	Day	36% 7% 12% 26% 19% 1	6240
August	1st to 10th	Day	42% 7% 16% 23% 12%	4221
	11th to 20th	Day	43% 9% 15% 25% 6% 1	4213
	21st to end	Day	37% 8% 14% 28% 13% 1	4519
September	1st to 10th	Day	42% 7% 13% 23% 13% 2	2398
	11th to 20th	Day	36% 8% 13% 27% 16%	2200
	21st to end	Day	50% 7% 11% 21% 11% 1	2200
October	1st to 10th	Day	37% 6% 14% 24% 18% 1	3266
	11th to 20th	Day	48% 5% 13% 19% 13% 2	3240
	21st to end	Day	48% 5% 14% 24% 8% 1	3538
November	1st to 10th	Day	54% 5% 10% 19% 10% 2	1902
	11th to 20th	Day	64% 4 11% 13% 6% 2	1700
	21st to end	Day	74% 4 8% 9% 4	1680
December	1st to 10th	Day	84% 5% 6% 5% 1	2680
	11th to 20th	Day	78% 5% 7% 8% 1	3066
	21st to end	Day	82% 4 7% 6% 1	4048
January	1st to 10th	Night	87% 3 5% 4 1	7856
	11th to 20th	Night	83% 4 6% 5% 2	7680
	21st to end	Night	74% 5% 8% 10% 4	8401
February	1st to 10th	Night	72% 5% 9% 11% 4	2730
	11th to 20th	Night	71% 4 9% 11% 4	2665
	21st to end	Night	68% 5% 9% 11% 7%	2113
March	1st to 10th	Night	60% 4 12% 15% 8%	5031
	11th to 20th	Night	51% 5% 13% 21% 10% 1	5090
	21st to end	Night	58% 4 9% 15% 12% 1	5588
April	1st to 10th	Night	58% 5% 10% 15% 10% 1	2200
	11th to 20th	Night	52% 6% 10% 17% 13% 2	1992
	21st to end	Night	49% 6% 10% 21% 11% 3	1890
May	1st to 10th	Night	56% 5% 8% 14% 11% 6%	4150
	11th to 20th	Night	47% 7% 9% 24% 11% 2	4046
	21st to end	Night	42% 8% 11% 23% 14% 2	3927
June	1st to 10th	Night	41% 7% 11% 22% 15% 3	1600
	11th to 20th	Night	40% 7% 13% 21% 17% 2	1600
	21st to end	Night	39% 8% 12% 24% 16% 1	1600
July	1st to 10th	Night	38% 7% 12% 23% 17% 3	3560
	11th to 20th	Night	36% 8% 12% 25% 16% 2	3960
	21st to end	Night	33% 8% 13% 21% 25% 1	4297
August	1st to 10th	Night	41% 8% 11% 27% 13% 1	2904
	11th to 20th	Night	46% 7% 12% 26% 10% 1	2902
	21st to end	Night	37% 8% 15% 27% 12% 1	3345
September	1st to 10th	Night	37% 8% 13% 27% 14% 2	2000
	11th to 20th	Night	34% 6% 13% 26% 20% 1	2199
	21st to end	Night	47% 6% 12% 20% 14% 1	2200
October	1st to 10th	Night	39% 6% 9% 23% 21% 1	3913
	11th to 20th	Night	50% 5% 11% 16% 14% 4	3899
	21st to end	Night	46% 4 14% 21% 13% 2	4252
November	1st to 10th	Night	54% 7% 10% 18% 9% 2	2616
	11th to 20th	Night	64% 6% 9% 12% 6% 2	2919
	21st to end	Night	70% 5% 8% 12% 4	2940
December	1st to 10th	Night	79% 4 9% 6% 2	4720
	11th to 20th	Night	76% 4 10% 9% 1	5348
	21st to end	Night	76% 6% 8% 9% 1	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	32% 26% 31% 8% 4	286
	11th to 20th	Day	34% 28% 28% 7% 2	330
	21st to end	Day	28% 30% 31% 9% 2	363
February	1st to 10th	Day	29% 24% 34% 12% 1	330
	11th to 20th	Day	32% 27% 31% 9% 1	330
	21st to end	Day	31% 29% 30% 9%	273
March	1st to 10th	Day	27% 22% 35% 11% 4	536
	11th to 20th	Day	30% 22% 32% 12% 3 1	540
	21st to end	Day	33% 20% 30% 14% 3	594
April	1st to 10th	Day	24% 24% 33% 13% 5% 1	540
	11th to 20th	Day	30% 17% 29% 18% 5% 2	532
	21st to end	Day	26% 14% 34% 20% 5% 1	540
May	1st to 10th	Day	28% 13% 34% 17% 4 2	543
	11th to 20th	Day	22% 12% 31% 24% 9% 2	551
	21st to end	Day	17% 9% 29% 29% 13% 4	573
June	1st to 10th	Day	18% 8% 36% 27% 8% 2	530
	11th to 20th	Day	16% 10% 28% 29% 14% 2	530
	21st to end	Day	16% 8% 30% 30% 14% 2	530
July	1st to 10th	Day	10% 5% 26% 33% 20% 6%	525
	11th to 20th	Day	15% 4 27% 32% 17% 5%	524
	21st to end	Day	10% 6% 28% 35% 16% 5%	576
August	1st to 10th	Day	12% 6% 23% 34% 18% 7%	526
	11th to 20th	Day	15% 6% 20% 34% 18% 6%	519
	21st to end	Day	15% 7% 23% 33% 17% 6%	583
September	1st to 10th	Day	18% 9% 22% 28% 17% 7%	530
	11th to 20th	Day	11% 5% 22% 30% 23% 9%	530
	21st to end	Day	19% 9% 28% 26% 13% 5%	530
October	1st to 10th	Day	19% 10% 28% 24% 15% 5%	530
	11th to 20th	Day	28% 16% 29% 20% 6% 2	530
	21st to end	Day	37% 16% 28% 17% 2	583
November	1st to 10th	Day	31% 22% 24% 16% 6% 1	380
	11th to 20th	Day	39% 20% 24% 11% 5%	230
	21st to end	Day	40% 19% 31% 10%	220
December	1st to 10th	Day	36% 23% 29% 10% 2	220
	11th to 20th	Day	33% 30% 26% 10%	220
	21st to end	Day	38% 20% 29% 10% 2	242
January	1st to 10th	Night	33% 25% 28% 11% 3	840
	11th to 20th	Night	35% 23% 28% 13% 2	840
	21st to end	Night	28% 23% 32% 14% 2	924
February	1st to 10th	Night	33% 17% 28% 18% 3	840
	11th to 20th	Night	28% 22% 30% 17% 2	823
	21st to end	Night	30% 22% 28% 18% 1	692
March	1st to 10th	Night	28% 18% 29% 22% 3	840
	11th to 20th	Night	27% 17% 30% 20% 5%	840
	21st to end	Night	27% 15% 29% 23% 6% 1	924
April	1st to 10th	Night	27% 15% 29% 23% 5%	840
	11th to 20th	Night	26% 11% 32% 22% 8% 1	748
	21st to end	Night	20% 12% 27% 32% 8% 1	630
May	1st to 10th	Night	25% 8% 22% 33% 11% 1	630
	11th to 20th	Night	19% 8% 25% 35% 11% 1	594
	21st to end	Night	15% 6% 25% 30% 22% 3	433
June	1st to 10th	Night	14% 5% 31% 33% 16% 2	400
	11th to 20th	Night	14% 3 26% 37% 19% 1	400
	21st to end	Night	12% 8% 25% 31% 23% 1	400
July	1st to 10th	Night	13% 3 18% 40% 20% 5%	390
	11th to 20th	Night	13% 4 21% 33% 27% 4	562
	21st to end	Night	12% 3 18% 39% 26% 2	657
August	1st to 10th	Night	10% 2 19% 40% 25% 4	597
	11th to 20th	Night	14% 3 13% 35% 30% 4	594
	21st to end	Night	15% 5% 21% 30% 26% 4	730
September	1st to 10th	Night	18% 7% 17% 30% 24% 5%	800
	11th to 20th	Night	13% 5% 15% 32% 30% 5%	800
	21st to end	Night	20% 9% 21% 28% 17% 5%	800
October	1st to 10th	Night	23% 9% 23% 24% 19% 3	800
	11th to 20th	Night	31% 12% 29% 20% 8% 1	800
	21st to end	Night	38% 13% 28% 18% 3	880
November	1st to 10th	Night	30% 13% 29% 21% 7%	820
	11th to 20th	Night	36% 20% 24% 15% 5% 1	840
	21st to end	Night	39% 19% 28% 11% 3	840
December	1st to 10th	Night	40% 23% 23% 10% 4	840
	11th to 20th	Night	36% 26% 23% 12% 3	840
	21st to end	Night	33% 22% 29% 14% 1	924

Page generated at 2026-05-14 01:58:55. Version ac07

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