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 2025-07-06.

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% 51%	123926
	Night	3 6% 2 5% 3 3 3 3 5% 7% 59%	123800
Transparency	Day	30% 5% 8% 24% 23% 10%	123673
	Night	25% 3 9% 25% 27% 11%	123642
Seeing	Day	8% 12% 33% 30% 14% 4	16485
	Night	6% 10% 32% 37% 14% 2	26523

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	6% 3 7% 3 3 2 2 5% 8% 59%	3638
	11th to 20th	Day	1 4 2 7% 7% 5% 5% 3 5% 7% 53%	3802
	21st to end	Day	1 6% 3 9% 3 3 3 3 5% 7% 59%	4510
February	1st to 10th	Day	1 7% 4 8% 4 3 4 3 4 7% 55%	1800
	11th to 20th	Day	4 3 6% 4 4 3 3 6% 9% 57%	1912
	21st to end	Day	1 7% 3 6% 4 3 4 2 5% 7% 59%	1650
March	1st to 10th	Day	2 10% 4 6% 3 3 3 3 4 5% 57%	4500
	11th to 20th	Day	2 13% 4 9% 5% 4 4 3 5% 8% 43%	4990
	21st to end	Day	2 10% 3 6% 4 3 4 5% 7% 9% 47%	5789
April	1st to 10th	Day	4 9% 2 5% 3 4 4 3 7% 8% 51%	2400
	11th to 20th	Day	6% 11% 4 8% 4 3 4 4 5% 7% 43%	2371
	21st to end	Day	7% 11% 3 6% 5% 4 5% 4 6% 8% 41%	2400
May	1st to 10th	Day	10% 10% 3 8% 4 4 4 5% 6% 8% 39%	5718
	11th to 20th	Day	8% 7% 2 5% 4 5% 6% 5% 7% 8% 42%	6298
	21st to end	Day	6% 6% 2 5% 4 5% 5% 5% 8% 9% 45%	6925
June	1st to 10th	Day	5% 5% 2 5% 4 5% 6% 5% 8% 9% 46%	2800
	11th to 20th	Day	5% 4 2 5% 4 6% 5% 5% 8% 9% 49%	2800
	21st to end	Day	5% 4 2 5% 6% 5% 6% 6% 8% 9% 45%	2800
July	1st to 10th	Day	5% 4 2 4 5% 4 5% 5% 7% 8% 52%	4501
	11th to 20th	Day	3 4 2 6% 5% 6% 7% 6% 9% 11% 42%	4430
	21st to end	Day	6% 4 2 6% 5% 6% 5% 5% 6% 8% 48%	4677
August	1st to 10th	Day	9% 6% 2 7% 6% 7% 7% 6% 8% 9% 33%	4092
	11th to 20th	Day	4 4 2 6% 6% 6% 7% 6% 8% 9% 43%	3738
	21st to end	Day	5% 5% 2 6% 5% 6% 6% 6% 7% 8% 44%	4180
September	1st to 10th	Day	6% 4 1 6% 3 4 4 4 5% 7% 54%	2280
	11th to 20th	Day	7% 4 2 4 3 4 5% 3 5% 7% 56%	2271
	21st to end	Day	6% 4 2 4 3 3 3 2 4 6% 63%	2087
October	1st to 10th	Day	4 4 2 4 3 3 3 3 4 5% 67%	3170
	11th to 20th	Day	4 4 2 6% 4 4 4 4 6% 8% 52%	3150
	21st to end	Day	4 7% 2 3 3 2 2 2 4 6% 64%	3349
November	1st to 10th	Day	1 5% 2 5% 2 3 3 2 5% 7% 65%	1763
	11th to 20th	Day	1 4 2 4 3 3 4 3 5% 6% 64%	1800
	21st to end	Day	1 6% 2 7% 3 3 3 3 5% 7% 59%	1685
December	1st to 10th	Day	1 3 2 5% 3 3 2 2 4 6% 69%	2620
	11th to 20th	Day	1 3 2 3 2 2 3 3 4 7% 69%	3026
	21st to end	Day	1 5% 2 7% 3 4 5% 4 6% 9% 54%	4004
January	1st to 10th	Night	6% 3 8% 4 4 3 3 4 7% 57%	6327
	11th to 20th	Night	3 1 6% 4 4 4 4 5% 7% 62%	6220
	21st to end	Night	1 6% 2 5% 4 3 3 3 4 6% 62%	6468
February	1st to 10th	Night	1 7% 3 5% 4 3 4 3 4 9% 59%	2600
	11th to 20th	Night	1 5% 3 6% 4 4 4 4 6% 6% 57%	2373
	21st to end	Night	1 5% 3 6% 3 3 3 3 5% 5% 62%	1980
March	1st to 10th	Night	1 9% 3 6% 3 3 3 3 5% 6% 57%	5374
	11th to 20th	Night	2 13% 3 5% 3 3 3 3 4 7% 55%	5005
	21st to end	Night	4 7% 4 6% 4 3 3 2 5% 7% 53%	5171
April	1st to 10th	Night	3 7% 2 5% 4 3 3 3 6% 7% 58%	2000
	11th to 20th	Night	6% 10% 3 6% 2 2 3 3 5% 7% 52%	1977
	21st to end	Night	9% 9% 2 6% 3 3 3 3 4 6% 51%	2000
May	1st to 10th	Night	9% 11% 2 5% 3 3 4 3 5% 7% 46%	4258
	11th to 20th	Night	8% 7% 2 6% 4 3 4 3 5% 7% 51%	3617
	21st to end	Night	6% 5% 2 5% 3 3 3 3 4 7% 60%	3972
June	1st to 10th	Night	5% 3 1 3 3 3 4 3 5% 9% 60%	1600
	11th to 20th	Night	5% 2 1 3 3 3 3 4 5% 8% 64%	1600
	21st to end	Night	4 3 1 4 2 3 3 4 7% 9% 60%	1600
July	1st to 10th	Night	4 3 2 4 2 3 4 3 5% 7% 63%	2580
	11th to 20th	Night	1 2 1 4 3 3 6% 5% 8% 10% 58%	2540
	21st to end	Night	4 2 1 3 3 4 4 5% 8% 9% 59%	2881
August	1st to 10th	Night	5% 5% 2 5% 4 5% 6% 5% 9% 9% 46%	2847
	11th to 20th	Night	3 2 1 3 3 3 4 5% 8% 12% 56%	3100
	21st to end	Night	3 4 2 6% 4 4 5% 5% 6% 9% 52%	3487
September	1st to 10th	Night	7% 3 2 3 4 4 3 4 5% 7% 59%	1900
	11th to 20th	Night	6% 2 1 3 3 3 2 3 5% 7% 65%	1909
	21st to end	Night	6% 4 1 4 3 2 3 3 4 6% 64%	2090
October	1st to 10th	Night	4 4 1 2 2 2 2 2 3 5% 73%	3799
	11th to 20th	Night	3 5% 1 4 3 3 3 3 4 8% 64%	3778
	21st to end	Night	3 6% 3 4 2 2 2 2 4 5% 66%	4177
November	1st to 10th	Night	2 4 2 4 2 3 3 3 5% 7% 66%	2535
	11th to 20th	Night	1 5% 2 4 3 3 3 2 4 7% 67%	2600
	21st to end	Night	2 6% 2 6% 3 3 3 3 5% 6% 62%	2710
December	1st to 10th	Night	1 3 1 4 3 2 2 2 3 5% 75%	4570
	11th to 20th	Night	1 3 2 4 3 3 3 2 4 8% 67%	5181
	21st to end	Night	2 6% 3 6% 4 4 4 4 6% 8% 53%	6974

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	25% 2 6% 32% 24% 10%	3639
	11th to 20th	Day	31% 2 9% 22% 27% 10%	3802
	21st to end	Day	25% 2 7% 27% 24% 14%	4510
February	1st to 10th	Day	29% 3 10% 26% 22% 10%	1800
	11th to 20th	Day	24% 3 6% 30% 28% 9%	1922
	21st to end	Day	25% 3 6% 22% 29% 15%	1650
March	1st to 10th	Day	29% 2 4 20% 30% 15%	4500
	11th to 20th	Day	40% 3 7% 24% 21% 5%	4990
	21st to end	Day	30% 5% 9% 37% 16% 3	5789
April	1st to 10th	Day	30% 4 9% 26% 21% 10%	2400
	11th to 20th	Day	40% 5% 6% 20% 20% 9%	2374
	21st to end	Day	40% 4 8% 26% 19% 3	2400
May	1st to 10th	Day	42% 5% 7% 23% 16% 7%	5718
	11th to 20th	Day	36% 5% 11% 21% 18% 9%	6302
	21st to end	Day	33% 6% 11% 23% 22% 5%	6926
June	1st to 10th	Day	30% 6% 15% 23% 17% 8%	2800
	11th to 20th	Day	29% 5% 11% 19% 21% 15%	2800
	21st to end	Day	32% 7% 9% 21% 20% 12%	2800
July	1st to 10th	Day	26% 7% 8% 16% 30% 13%	4432
	11th to 20th	Day	29% 9% 11% 23% 20% 9%	4355
	21st to end	Day	31% 6% 10% 19% 19% 15%	4530
August	1st to 10th	Day	41% 10% 13% 18% 14% 4	4078
	11th to 20th	Day	32% 9% 11% 16% 20% 12%	3774
	21st to end	Day	31% 9% 10% 18% 23% 9%	4179
September	1st to 10th	Day	28% 5% 6% 21% 24% 16%	2280
	11th to 20th	Day	27% 4 6% 22% 29% 13%	2271
	21st to end	Day	24% 3 5% 23% 34% 11%	2087
October	1st to 10th	Day	19% 4 5% 19% 32% 20%	3170
	11th to 20th	Day	26% 5% 9% 26% 27% 7%	3150
	21st to end	Day	22% 3 5% 26% 33% 11%	3349
November	1st to 10th	Day	20% 3 7% 24% 30% 15%	1763
	11th to 20th	Day	21% 3 5% 30% 30% 11%	1800
	21st to end	Day	24% 3 6% 23% 31% 12%	1685
December	1st to 10th	Day	18% 3 6% 33% 30% 10%	2620
	11th to 20th	Day	16% 2 7% 33% 30% 11%	3024
	21st to end	Day	27% 3 6% 26% 28% 10%	4004
January	1st to 10th	Night	28% 2 6% 27% 29% 8%	6327
	11th to 20th	Night	22% 2 7% 25% 27% 17%	6220
	21st to end	Night	25% 2 8% 25% 25% 15%	6468
February	1st to 10th	Night	25% 2 9% 29% 25% 10%	2600
	11th to 20th	Night	26% 3 7% 30% 26% 8%	2373
	21st to end	Night	24% 2 4 25% 30% 15%	1980
March	1st to 10th	Night	27% 3 5% 17% 34% 14%	5374
	11th to 20th	Night	32% 2 7% 20% 26% 14%	5005
	21st to end	Night	31% 3 6% 36% 21% 3	5171
April	1st to 10th	Night	27% 3 10% 26% 25% 10%	2000
	11th to 20th	Night	33% 3 8% 26% 22% 8%	1977
	21st to end	Night	35% 3 8% 28% 21% 5%	2000
May	1st to 10th	Night	38% 5% 9% 26% 14% 8%	4255
	11th to 20th	Night	33% 3 12% 25% 20% 7%	3618
	21st to end	Night	26% 4 13% 27% 23% 6%	3972
June	1st to 10th	Night	22% 4 19% 30% 17% 9%	1600
	11th to 20th	Night	20% 4 11% 21% 24% 20%	1600
	21st to end	Night	20% 5% 10% 26% 28% 12%	1600
July	1st to 10th	Night	22% 5% 10% 19% 31% 14%	2540
	11th to 20th	Night	17% 8% 14% 30% 22% 9%	2492
	21st to end	Night	18% 6% 16% 28% 16% 17%	2791
August	1st to 10th	Night	30% 8% 17% 24% 17% 4	2838
	11th to 20th	Night	21% 8% 19% 22% 21% 10%	3130
	21st to end	Night	27% 6% 14% 22% 25% 5%	3487
September	1st to 10th	Night	25% 4 8% 24% 25% 15%	1900
	11th to 20th	Night	21% 3 9% 24% 27% 16%	1909
	21st to end	Night	22% 3 7% 24% 32% 11%	2090
October	1st to 10th	Night	16% 3 6% 21% 33% 20%	3799
	11th to 20th	Night	20% 4 9% 27% 30% 10%	3778
	21st to end	Night	21% 2 5% 25% 32% 14%	4177
November	1st to 10th	Night	19% 3 7% 24% 32% 15%	2535
	11th to 20th	Night	20% 2 6% 27% 34% 12%	2600
	21st to end	Night	24% 1 8% 23% 31% 13%	2710
December	1st to 10th	Night	15% 2 7% 31% 37% 9%	4570
	11th to 20th	Night	18% 2 6% 28% 35% 11%	5182
	21st to end	Night	29% 3 7% 24% 26% 11%	6974

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	6% 11% 38% 34% 9% 2	300
	11th to 20th	Day	5% 18% 42% 30% 5% 1	300
	21st to end	Day	4 18% 42% 28% 7% 2	330
February	1st to 10th	Day	4 30% 44% 18% 3 1	300
	11th to 20th	Day	3 21% 49% 24% 3 1	300
	21st to end	Day	6% 20% 45% 27% 3	249
March	1st to 10th	Day	3 18% 44% 24% 8% 2	300
	11th to 20th	Day	10% 21% 36% 25% 7% 1	490
	21st to end	Day	13% 24% 35% 22% 6%	550
April	1st to 10th	Day	10% 23% 36% 24% 6% 1	500
	11th to 20th	Day	15% 25% 37% 17% 4 1	492
	21st to end	Day	15% 18% 35% 22% 9% 1	500
May	1st to 10th	Day	15% 14% 35% 26% 8% 2	608
	11th to 20th	Day	16% 9% 31% 30% 12% 2	725
	21st to end	Day	8% 10% 31% 30% 17% 3	789
June	1st to 10th	Day	6% 7% 26% 35% 19% 6%	730
	11th to 20th	Day	6% 8% 33% 34% 15% 4	730
	21st to end	Day	6% 5% 34% 31% 18% 6%	730
July	1st to 10th	Day	7% 4 20% 34% 27% 7%	697
	11th to 20th	Day	4 3 20% 35% 27% 10%	668
	21st to end	Day	7% 4 17% 35% 29% 8%	663
August	1st to 10th	Day	9% 5% 23% 36% 21% 7%	487
	11th to 20th	Day	5% 4 27% 35% 22% 6%	477
	21st to end	Day	10% 2 28% 36% 19% 6%	539
September	1st to 10th	Day	8% 4 23% 40% 18% 6%	490
	11th to 20th	Day	7% 7% 27% 38% 18% 3	481
	21st to end	Day	6% 9% 32% 31% 17% 5%	300
October	1st to 10th	Day	9% 7% 31% 27% 19% 6%	300
	11th to 20th	Day	5% 13% 37% 34% 9% 3	300
	21st to end	Day	8% 13% 39% 30% 9% 1	330
November	1st to 10th	Day	6% 12% 40% 28% 11% 2	300
	11th to 20th	Day	7% 15% 36% 33% 6% 2	300
	21st to end	Day	5% 13% 46% 28% 6% 2	300
December	1st to 10th	Day	7% 17% 37% 30% 8% 1	300
	11th to 20th	Day	4 14% 49% 28% 4	300
	21st to end	Day	6% 18% 47% 22% 6% 1	330
January	1st to 10th	Night	3 17% 37% 32% 9% 2	1000
	11th to 20th	Night	3 22% 41% 26% 7% 2	945
	21st to end	Night	3 16% 40% 34% 6% 1	880
February	1st to 10th	Night	5% 17% 45% 28% 5%	800
	11th to 20th	Night	3 15% 45% 32% 5%	784
	21st to end	Night	4 14% 46% 33% 3	660
March	1st to 10th	Night	3 13% 43% 34% 7%	800
	11th to 20th	Night	8% 11% 35% 35% 9% 1	800
	21st to end	Night	7% 10% 45% 33% 5% 1	743
April	1st to 10th	Night	8% 13% 41% 34% 4	600
	11th to 20th	Night	12% 13% 35% 31% 8% 1	593
	21st to end	Night	13% 8% 37% 32% 10% 1	600
May	1st to 10th	Night	14% 6% 30% 36% 12% 2	600
	11th to 20th	Night	16% 4 22% 43% 13% 2	591
	21st to end	Night	9% 4 26% 40% 19% 2	650
June	1st to 10th	Night	6% 2 19% 47% 22% 4	600
	11th to 20th	Night	7% 2 27% 46% 16% 2	600
	21st to end	Night	4 2 18% 45% 29% 2	600
July	1st to 10th	Night	6% 3 12% 39% 36% 4	570
	11th to 20th	Night	2 1 10% 43% 39% 5%	552
	21st to end	Night	6% 2 7% 40% 38% 6%	624
August	1st to 10th	Night	6% 1 12% 41% 35% 4	567
	11th to 20th	Night	5% 1 13% 48% 28% 4	564
	21st to end	Night	6% 1 16% 40% 30% 6%	627
September	1st to 10th	Night	6% 2 14% 46% 26% 6%	570
	11th to 20th	Night	7% 2 22% 44% 21% 6%	570
	21st to end	Night	7% 4 24% 42% 18% 5%	570
October	1st to 10th	Night	9% 7% 28% 38% 14% 4	759
	11th to 20th	Night	4 5% 34% 42% 12% 3	760
	21st to end	Night	7% 10% 35% 34% 11% 2	869
November	1st to 10th	Night	4 9% 37% 39% 10% 2	975
	11th to 20th	Night	4 15% 38% 36% 6% 1	1000
	21st to end	Night	6% 10% 38% 38% 9%	1000
December	1st to 10th	Night	4 14% 38% 37% 6% 1	1000
	11th to 20th	Night	4 14% 43% 32% 7% 1	1000
	21st to end	Night	5% 15% 40% 32% 7% 1	1100

Page generated at 2025-07-02 23:30:12. Version ac07

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