Evergreen
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Evergreen clear sky chart from 2005-11-06 to 2026-10-29.

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	4 6% 2 6% 4 4 5% 4 6% 8% 50%	158288
	Night	3 5% 2 5% 3 3 4 3 5% 8% 59%	151393
Transparency	Day	29% 6% 9% 23% 22% 10%	153242
	Night	24% 4 10% 25% 26% 11%	147461
Seeing	Day	8% 12% 34% 30% 13% 3	18297
	Night	6% 9% 34% 36% 13% 2	28246

By Month

Percentage Cloud Cover SW (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	1 7% 3 8% 3 3 2 2 5% 7% 59%	4720
	11th to 20th	Day	3 2 6% 6% 5% 4 3 5% 7% 60%	4958
	21st to end	Day	5% 2 8% 3 4 3 3 5% 7% 60%	5844
February	1st to 10th	Day	1 7% 4 8% 4 3 4 3 4 7% 56%	1890
	11th to 20th	Day	4 3 6% 4 4 4 3 6% 9% 57%	2010
	21st to end	Day	1 7% 3 6% 3 3 3 2 5% 7% 60%	1730
March	1st to 10th	Day	2 11% 4 6% 3 3 3 2 4 5% 57%	4600
	11th to 20th	Day	2 12% 4 9% 5% 4 4 3 5% 8% 44%	5099
	21st to end	Day	2 10% 3 6% 4 3 4 5% 7% 9% 46%	5917
April	1st to 10th	Day	4 8% 2 5% 3 4 4 3 7% 8% 51%	2520
	11th to 20th	Day	6% 11% 4 8% 4 3 4 4 5% 7% 43%	2491
	21st to end	Day	7% 11% 3 6% 5% 4 5% 4 6% 8% 41%	2520
May	1st to 10th	Day	9% 11% 3 8% 4 4 4 4 5% 7% 40%	7403
	11th to 20th	Day	9% 8% 3 6% 4 5% 6% 5% 7% 8% 41%	8191
	21st to end	Day	5% 6% 2 6% 4 6% 7% 5% 9% 9% 41%	9026
June	1st to 10th	Day	5% 5% 2 5% 4 5% 6% 5% 8% 9% 46%	2940
	11th to 20th	Day	5% 4 2 5% 4 5% 5% 5% 7% 9% 50%	2940
	21st to end	Day	4 4 2 5% 5% 5% 6% 5% 8% 9% 46%	2940
July	1st to 10th	Day	3 3 1 5% 5% 5% 6% 5% 7% 9% 49%	8249
	11th to 20th	Day	2 3 1 5% 4 5% 6% 5% 8% 10% 51%	8250
	21st to end	Day	4 4 2 7% 6% 7% 7% 6% 8% 9% 40%	8720
August	1st to 10th	Day	6% 4 2 6% 5% 6% 7% 5% 8% 10% 42%	7549
	11th to 20th	Day	3 4 2 6% 5% 6% 7% 6% 8% 9% 43%	7028
	21st to end	Day	6% 5% 2 7% 6% 6% 8% 6% 8% 9% 37%	7751
September	1st to 10th	Day	5% 4 1 6% 3 4 4 4 6% 7% 54%	2520
	11th to 20th	Day	6% 4 2 4 3 4 5% 4 5% 7% 54%	2509
	21st to end	Day	7% 5% 2 5% 3 3 3 2 5% 6% 60%	2307
October	1st to 10th	Day	4 4 2 4 3 3 3 3 4 5% 67%	3370
	11th to 20th	Day	5% 4 2 6% 4 4 4 4 6% 8% 53%	3350
	21st to end	Day	4 7% 2 4 3 2 2 2 4 6% 64%	3536
November	1st to 10th	Day	1 4 2 4 2 3 3 2 5% 7% 66%	1853
	11th to 20th	Day	1 4 2 4 3 3 4 3 5% 6% 64%	1890
	21st to end	Day	1 6% 2 7% 3 3 3 3 5% 8% 59%	1769
December	1st to 10th	Day	1 3 2 6% 3 3 2 2 4 6% 69%	2700
	11th to 20th	Day	1 3 2 3 2 2 3 3 5% 8% 69%	3106
	21st to end	Day	1 5% 2 7% 3 4 5% 4 6% 9% 54%	4092
January	1st to 10th	Night	6% 3 8% 4 3 3 3 4 7% 58%	8201
	11th to 20th	Night	3 2 7% 4 3 4 3 4 7% 63%	8088
	21st to end	Night	1 5% 2 6% 4 4 4 3 4 5% 63%	8370
February	1st to 10th	Night	1 6% 2 5% 4 3 4 3 4 9% 59%	2730
	11th to 20th	Night	1 5% 2 5% 4 4 4 4 6% 6% 57%	2495
	21st to end	Night	1 5% 3 6% 3 3 3 3 5% 6% 63%	2076
March	1st to 10th	Night	1 9% 3 6% 3 3 3 3 5% 6% 57%	5494
	11th to 20th	Night	2 13% 3 5% 3 3 3 3 4 7% 55%	5115
	21st to end	Night	4 7% 4 6% 4 3 3 2 5% 7% 53%	5285
April	1st to 10th	Night	3 7% 2 5% 4 3 3 3 6% 7% 58%	2100
	11th to 20th	Night	6% 10% 3 6% 3 2 3 3 5% 7% 52%	2077
	21st to end	Night	9% 10% 2 6% 3 3 3 3 4 6% 50%	2100
May	1st to 10th	Night	10% 12% 2 5% 3 3 4 3 5% 7% 46%	5540
	11th to 20th	Night	8% 8% 2 6% 3 3 4 3 5% 7% 51%	4701
	21st to end	Night	6% 6% 2 6% 4 4 3 3 4 6% 56%	5181
June	1st to 10th	Night	5% 4 1 3 3 3 4 3 5% 9% 60%	1680
	11th to 20th	Night	5% 3 1 3 3 3 3 4 5% 8% 64%	1680
	21st to end	Night	4 3 1 4 2 3 3 4 6% 9% 61%	1680
July	1st to 10th	Night	2 2 1 3 2 3 3 3 6% 8% 67%	4740
	11th to 20th	Night	2 2 1 3 3 3 5% 4 7% 10% 61%	4740
	21st to end	Night	3 2 1 4 3 5% 6% 6% 10% 13% 48%	5384
August	1st to 10th	Night	4 3 1 3 3 4 5% 3 7% 9% 58%	5273
	11th to 20th	Night	3 3 1 3 3 3 4 5% 7% 10% 56%	5814
	21st to end	Night	4 4 2 6% 4 4 6% 5% 8% 11% 46%	6465
September	1st to 10th	Night	6% 3 2 4 4 4 3 4 5% 6% 60%	2100
	11th to 20th	Night	6% 2 1 4 2 3 3 3 5% 7% 63%	2110
	21st to end	Night	6% 5% 1 5% 3 3 4 3 4 6% 61%	2310
October	1st to 10th	Night	4 4 1 2 2 2 2 2 3 5% 72%	4039
	11th to 20th	Night	4 5% 1 4 3 3 3 3 4 7% 64%	4018
	21st to end	Night	3 6% 3 4 2 2 2 2 4 5% 66%	4407
November	1st to 10th	Night	2 4 2 4 2 3 2 3 4 7% 68%	2665
	11th to 20th	Night	1 5% 2 4 3 3 3 2 4 7% 66%	2730
	21st to end	Night	2 6% 2 5% 3 3 3 3 5% 7% 61%	2846
December	1st to 10th	Night	1 3 1 4 3 2 2 2 3 5% 74%	4710
	11th to 20th	Night	1 3 2 4 3 3 3 2 4 8% 67%	5321
	21st to end	Night	2 6% 2 6% 4 4 4 4 6% 8% 54%	7128

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	27% 2 5% 30% 26% 9%	4723
	11th to 20th	Day	24% 2 10% 26% 28% 11%	4958
	21st to end	Day	24% 3 7% 26% 25% 14%	5844
February	1st to 10th	Day	28% 3 10% 27% 22% 10%	1890
	11th to 20th	Day	24% 3 6% 30% 28% 9%	2020
	21st to end	Day	24% 3 6% 22% 30% 15%	1730
March	1st to 10th	Day	29% 2 4 19% 30% 15%	4600
	11th to 20th	Day	39% 3 7% 24% 21% 6%	5099
	21st to end	Day	30% 5% 9% 37% 16% 3	5917
April	1st to 10th	Day	29% 5% 9% 25% 22% 10%	2520
	11th to 20th	Day	39% 5% 6% 20% 20% 9%	2494
	21st to end	Day	40% 5% 8% 25% 19% 4	2520
May	1st to 10th	Day	40% 6% 8% 24% 17% 5%	7403
	11th to 20th	Day	38% 6% 10% 20% 19% 7%	8195
	21st to end	Day	34% 8% 12% 22% 18% 6%	9027
June	1st to 10th	Day	30% 7% 15% 23% 17% 8%	2940
	11th to 20th	Day	28% 5% 10% 20% 21% 15%	2940
	21st to end	Day	31% 7% 9% 21% 20% 13%	2940
July	1st to 10th	Day	23% 8% 9% 15% 26% 18%	8181
	11th to 20th	Day	23% 8% 10% 20% 19% 19%	8175
	21st to end	Day	34% 10% 14% 18% 14% 9%	8526
August	1st to 10th	Day	34% 9% 11% 17% 17% 12%	5978
	11th to 20th	Day	31% 12% 15% 18% 17% 8%	5544
	21st to end	Day	31% 12% 14% 20% 17% 6%	6078
September	1st to 10th	Day	27% 5% 7% 22% 24% 15%	2520
	11th to 20th	Day	28% 5% 7% 22% 27% 11%	2509
	21st to end	Day	26% 4 5% 23% 32% 11%	2307
October	1st to 10th	Day	19% 4 5% 20% 32% 20%	3370
	11th to 20th	Day	25% 5% 9% 25% 27% 9%	3350
	21st to end	Day	22% 3 6% 26% 33% 11%	3536
November	1st to 10th	Day	20% 3 7% 24% 31% 15%	1853
	11th to 20th	Day	21% 3 5% 30% 30% 11%	1890
	21st to end	Day	24% 3 7% 24% 30% 12%	1769
December	1st to 10th	Day	19% 3 6% 33% 30% 10%	2700
	11th to 20th	Day	15% 2 7% 33% 30% 12%	3104
	21st to end	Day	26% 3 6% 26% 28% 10%	4092
January	1st to 10th	Night	27% 2 5% 28% 31% 7%	8202
	11th to 20th	Night	21% 3 8% 25% 27% 17%	8088
	21st to end	Night	23% 3 8% 23% 28% 16%	8370
February	1st to 10th	Night	24% 2 9% 28% 27% 10%	2730
	11th to 20th	Night	25% 3 7% 30% 26% 9%	2495
	21st to end	Night	23% 2 4 25% 30% 16%	2076
March	1st to 10th	Night	27% 3 4 17% 34% 14%	5494
	11th to 20th	Night	31% 2 7% 20% 26% 14%	5115
	21st to end	Night	31% 3 6% 36% 21% 3	5285
April	1st to 10th	Night	26% 3 10% 26% 25% 10%	2100
	11th to 20th	Night	32% 3 8% 26% 23% 8%	2077
	21st to end	Night	35% 3 8% 27% 20% 5%	2100
May	1st to 10th	Night	38% 5% 9% 26% 15% 6%	5538
	11th to 20th	Night	33% 4 11% 26% 20% 6%	4702
	21st to end	Night	30% 5% 13% 25% 20% 6%	5182
June	1st to 10th	Night	22% 4 18% 30% 17% 10%	1680
	11th to 20th	Night	20% 4 11% 21% 24% 21%	1680
	21st to end	Night	19% 5% 10% 25% 27% 13%	1680
July	1st to 10th	Night	16% 4 8% 18% 38% 16%	4700
	11th to 20th	Night	16% 7% 15% 25% 23% 15%	4692
	21st to end	Night	23% 10% 22% 22% 14% 10%	5267
August	1st to 10th	Night	23% 6% 13% 25% 24% 7%	4168
	11th to 20th	Night	21% 10% 23% 23% 16% 7%	4586
	21st to end	Night	25% 8% 21% 24% 17% 4	5069
September	1st to 10th	Night	24% 4 8% 24% 26% 14%	2100
	11th to 20th	Night	22% 4 9% 24% 27% 15%	2110
	21st to end	Night	24% 3 7% 24% 30% 11%	2310
October	1st to 10th	Night	17% 3 6% 21% 33% 20%	4039
	11th to 20th	Night	20% 4 8% 26% 30% 11%	4018
	21st to end	Night	21% 2 5% 25% 33% 14%	4407
November	1st to 10th	Night	18% 3 7% 24% 32% 16%	2665
	11th to 20th	Night	20% 2 5% 28% 34% 11%	2730
	21st to end	Night	24% 2 8% 24% 30% 12%	2846
December	1st to 10th	Night	15% 2 7% 31% 36% 9%	4710
	11th to 20th	Night	18% 2 6% 28% 35% 11%	5322
	21st to end	Night	29% 2 7% 24% 26% 11%	7128

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	8% 12% 39% 32% 8% 2	330
	11th to 20th	Day	6% 19% 42% 28% 5% 1	330
	21st to end	Day	4 20% 42% 26% 6% 1	363
February	1st to 10th	Day	5% 28% 45% 18% 3 1	330
	11th to 20th	Day	4 20% 50% 23% 3 1	330
	21st to end	Day	7% 20% 45% 25% 3	273
March	1st to 10th	Day	4 17% 47% 23% 8% 2	330
	11th to 20th	Day	10% 23% 36% 23% 7% 1	529
	21st to end	Day	13% 23% 36% 23% 5%	594
April	1st to 10th	Day	11% 22% 36% 24% 6% 1	540
	11th to 20th	Day	14% 26% 37% 17% 4 1	532
	21st to end	Day	16% 18% 36% 21% 8% 1	540
May	1st to 10th	Day	15% 15% 36% 25% 8% 2	653
	11th to 20th	Day	16% 9% 31% 30% 12% 2	775
	21st to end	Day	8% 10% 31% 31% 16% 3	844
June	1st to 10th	Day	6% 7% 27% 35% 19% 6%	780
	11th to 20th	Day	6% 9% 35% 33% 14% 4	780
	21st to end	Day	6% 6% 34% 31% 17% 6%	780
July	1st to 10th	Day	7% 5% 21% 35% 25% 7%	771
	11th to 20th	Day	4 3 20% 37% 27% 9%	768
	21st to end	Day	6% 5% 19% 35% 28% 7%	765
August	1st to 10th	Day	8% 5% 25% 36% 19% 6%	567
	11th to 20th	Day	5% 4 27% 37% 21% 6%	557
	21st to end	Day	9% 2 28% 36% 18% 6%	627
September	1st to 10th	Day	7% 5% 27% 39% 17% 5%	570
	11th to 20th	Day	7% 7% 30% 37% 17% 3	559
	21st to end	Day	8% 8% 32% 31% 16% 5%	360
October	1st to 10th	Day	9% 7% 32% 28% 19% 6%	360
	11th to 20th	Day	6% 14% 36% 32% 9% 2	360
	21st to end	Day	7% 14% 43% 27% 8% 1	387
November	1st to 10th	Day	6% 13% 42% 26% 11% 2	330
	11th to 20th	Day	7% 15% 38% 33% 5% 2	330
	21st to end	Day	5% 13% 46% 28% 6% 2	330
December	1st to 10th	Day	7% 19% 39% 27% 7% 1	330
	11th to 20th	Day	6% 14% 48% 27% 4 1	330
	21st to end	Day	6% 18% 48% 22% 5% 1	363
January	1st to 10th	Night	3 17% 39% 31% 8% 2	1050
	11th to 20th	Night	4 22% 42% 24% 7% 2	992
	21st to end	Night	3 16% 42% 33% 5% 1	924
February	1st to 10th	Night	4 18% 46% 27% 5%	840
	11th to 20th	Night	4 14% 47% 31% 4	824
	21st to end	Night	4 14% 48% 31% 3	692
March	1st to 10th	Night	3 14% 43% 33% 6%	840
	11th to 20th	Night	8% 11% 37% 33% 9% 1	840
	21st to end	Night	7% 10% 46% 32% 5% 1	780
April	1st to 10th	Night	8% 12% 42% 33% 4	630
	11th to 20th	Night	11% 13% 36% 30% 8% 1	623
	21st to end	Night	12% 8% 38% 31% 10% 1	630
May	1st to 10th	Night	14% 7% 31% 35% 12% 2	630
	11th to 20th	Night	16% 4 23% 43% 13% 2	621
	21st to end	Night	9% 4 27% 39% 18% 2	683
June	1st to 10th	Night	6% 2 21% 46% 21% 3	630
	11th to 20th	Night	6% 3 29% 45% 16% 2	630
	21st to end	Night	4 2 20% 44% 28% 2	630
July	1st to 10th	Night	6% 3 15% 40% 33% 4	615
	11th to 20th	Night	2 1 11% 44% 37% 4	612
	21st to end	Night	6% 2 9% 42% 36% 5%	690
August	1st to 10th	Night	6% 1 15% 42% 32% 4	627
	11th to 20th	Night	5% 2 14% 50% 26% 4	624
	21st to end	Night	6% 2 16% 43% 28% 6%	693
September	1st to 10th	Night	6% 2 17% 46% 23% 6%	630
	11th to 20th	Night	7% 2 24% 43% 19% 5%	630
	21st to end	Night	8% 4 25% 42% 17% 5%	630
October	1st to 10th	Night	9% 7% 29% 38% 13% 3	839
	11th to 20th	Night	5% 5% 37% 40% 11% 3	840
	21st to end	Night	7% 10% 37% 33% 10% 2	947
November	1st to 10th	Night	4 8% 39% 38% 9% 2	1025
	11th to 20th	Night	4 14% 40% 35% 5% 1	1050
	21st to end	Night	6% 10% 40% 36% 8%	1050
December	1st to 10th	Night	4 14% 40% 36% 6%	1050
	11th to 20th	Night	5% 14% 44% 30% 6% 1	1050
	21st to end	Night	5% 15% 42% 30% 6% 1	1155

Page generated at 2026-10-26 02:37:49. Version ac07

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