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 2024-03-23.

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	5% 6% 3 5% 5% 3 5% 5% 6% 8% 50%	78790
	Night	3 5% 2 4 4 2 4 4 5% 8% 58%	81158
Transparency	Day	31% 4 9% 24% 22% 9%	78499
	Night	26% 3 9% 26% 26% 10%	80972
Seeing	Day	8% 11% 32% 30% 14% 4	14632
	Night	6% 9% 31% 37% 14% 2	24856

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	4 3 6% 5% 2 4 4 6% 10% 56%	2538
	11th to 20th	Day	1 4 3 7% 6% 3 5% 5% 6% 8% 52%	2646
	21st to end	Day	1 7% 4 6% 4 3 3 4 5% 8% 55%	3146
February	1st to 10th	Day	1 7% 4 7% 4 2 4 4 4 7% 55%	1710
	11th to 20th	Day	4 3 5% 4 2 4 4 6% 10% 58%	1813
	21st to end	Day	1 7% 3 6% 4 2 4 3 4 7% 59%	1570
March	1st to 10th	Day	2 7% 3 5% 5% 2 4 4 5% 7% 55%	1900
	11th to 20th	Day	4 10% 4 6% 5% 2 4 4 5% 9% 48%	2080
	21st to end	Day	4 11% 3 5% 5% 2 4 5% 6% 10% 46%	2323
April	1st to 10th	Day	5% 8% 3 4 3 3 5% 4 6% 8% 52%	2160
	11th to 20th	Day	7% 11% 3 7% 5% 2 5% 4 5% 7% 43%	2131
	21st to end	Day	8% 11% 3 5% 5% 3 5% 5% 7% 8% 41%	2160
May	1st to 10th	Day	11% 8% 3 7% 7% 3 6% 6% 5% 8% 37%	2258
	11th to 20th	Day	13% 9% 3 5% 6% 3 6% 6% 6% 8% 35%	2478
	21st to end	Day	8% 6% 2 5% 6% 4 6% 6% 8% 8% 41%	2724
June	1st to 10th	Day	5% 4 2 4 5% 3 6% 7% 8% 10% 47%	2520
	11th to 20th	Day	6% 4 2 4 5% 4 5% 6% 7% 9% 49%	2520
	21st to end	Day	5% 4 2 4 6% 4 7% 7% 8% 9% 45%	2520
July	1st to 10th	Day	7% 5% 2 4 6% 3 6% 8% 8% 9% 40%	2520
	11th to 20th	Day	4 5% 2 5% 5% 3 6% 6% 7% 9% 46%	2520
	21st to end	Day	8% 4 2 6% 6% 4 7% 7% 7% 9% 39%	2670
August	1st to 10th	Day	8% 5% 3 6% 7% 5% 7% 7% 9% 9% 35%	2328
	11th to 20th	Day	5% 4 2 5% 6% 3 6% 6% 8% 9% 46%	2135
	21st to end	Day	7% 4 2 5% 5% 4 6% 8% 7% 7% 45%	2376
September	1st to 10th	Day	6% 4 2 5% 4 3 5% 5% 6% 7% 54%	2160
	11th to 20th	Day	7% 4 2 3 4 3 5% 4 5% 7% 56%	2152
	21st to end	Day	6% 4 2 3 4 2 3 3 4 6% 63%	1978
October	1st to 10th	Day	6% 5% 2 5% 4 3 3 5% 5% 6% 56%	1800
	11th to 20th	Day	4 3 1 3 3 2 3 3 4 6% 66%	1800
	21st to end	Day	5% 7% 2 4 4 2 2 3 4 7% 60%	1930
November	1st to 10th	Day	1 4 2 3 3 2 3 3 5% 8% 66%	1673
	11th to 20th	Day	2 4 3 4 4 2 4 3 5% 7% 63%	1710
	21st to end	Day	1 6% 2 7% 3 1 3 4 5% 7% 59%	1601
December	1st to 10th	Day	1 5% 3 5% 4 3 3 3 5% 9% 59%	1520
	11th to 20th	Day	2 4 2 4 3 2 3 4 5% 9% 61%	1926
	21st to end	Day	1 6% 2 7% 5% 3 5% 4 6% 8% 53%	2794
January	1st to 10th	Night	5% 3 8% 5% 3 4 3 4 8% 55%	4417
	11th to 20th	Night	1 3 2 4 4 3 4 5% 5% 8% 61%	4352
	21st to end	Night	2 7% 3 5% 5% 3 3 4 4 6% 58%	4521
February	1st to 10th	Night	1 7% 3 4 4 3 4 4 4 8% 59%	2470
	11th to 20th	Night	1 5% 3 5% 4 3 5% 5% 6% 6% 58%	2252
	21st to end	Night	1 5% 4 5% 4 2 3 4 4 6% 62%	1884
March	1st to 10th	Night	1 5% 3 5% 4 2 4 4 6% 8% 58%	2280
	11th to 20th	Night	4 9% 3 4 3 2 3 4 4 7% 58%	2095
	21st to end	Night	4 7% 4 5% 3 3 4 3 5% 6% 56%	2077
April	1st to 10th	Night	3 7% 3 5% 3 2 3 4 6% 7% 59%	1800
	11th to 20th	Night	6% 9% 3 5% 3 2 3 4 5% 7% 53%	1777
	21st to end	Night	9% 8% 3 5% 3 2 4 4 4 6% 51%	1800
May	1st to 10th	Night	8% 9% 3 5% 3 3 4 4 5% 8% 46%	1698
	11th to 20th	Night	12% 8% 2 5% 3 2 4 5% 5% 8% 45%	1417
	21st to end	Night	7% 4 2 4 4 2 4 4 6% 9% 54%	1552
June	1st to 10th	Night	5% 3 1 2 3 2 4 4 5% 10% 61%	1440
	11th to 20th	Night	6% 2 1 2 3 2 3 4 4 8% 63%	1440
	21st to end	Night	4 2 1 3 3 2 4 5% 6% 10% 61%	1440
July	1st to 10th	Night	6% 3 3 4 3 3 5% 5% 7% 9% 54%	1440
	11th to 20th	Night	1 2 1 3 4 2 5% 6% 7% 9% 61%	1440
	21st to end	Night	6% 2 2 4 4 3 6% 6% 9% 11% 48%	1643
August	1st to 10th	Night	4 4 2 4 5% 3 6% 6% 7% 9% 51%	1611
	11th to 20th	Night	5% 3 1 3 4 2 4 4 7% 9% 58%	1765
	21st to end	Night	4 4 2 4 4 2 4 5% 5% 8% 56%	1980
September	1st to 10th	Night	7% 3 2 3 4 2 4 5% 5% 7% 59%	1800
	11th to 20th	Night	7% 2 1 2 3 2 2 4 5% 7% 66%	1808
	21st to end	Night	6% 4 1 4 3 2 3 4 4 6% 63%	1980
October	1st to 10th	Night	6% 5% 2 3 3 2 3 4 4 7% 60%	2159
	11th to 20th	Night	3 4 1 3 3 2 2 3 3 6% 71%	2160
	21st to end	Night	4 7% 3 4 4 2 2 3 4 6% 60%	2407
November	1st to 10th	Night	1 4 2 3 3 2 3 3 4 7% 68%	2405
	11th to 20th	Night	2 5% 2 4 3 3 3 2 4 7% 66%	2470
	21st to end	Night	2 6% 2 5% 4 2 3 3 4 8% 62%	2574
December	1st to 10th	Night	1 4 2 5% 5% 2 3 3 4 8% 62%	2660
	11th to 20th	Night	2 5% 2 4 4 3 4 4 5% 10% 58%	3271
	21st to end	Night	2 6% 3 5% 4 3 4 5% 6% 9% 54%	4873

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	26% 2 8% 31% 24% 10%	2539
	11th to 20th	Day	32% 2 10% 27% 21% 8%	2646
	21st to end	Day	29% 2 8% 32% 23% 6%	3146
February	1st to 10th	Day	30% 3 10% 27% 21% 9%	1710
	11th to 20th	Day	23% 3 6% 30% 29% 9%	1823
	21st to end	Day	26% 3 6% 23% 28% 13%	1570
March	1st to 10th	Day	29% 3 7% 27% 27% 7%	1900
	11th to 20th	Day	35% 3 8% 26% 23% 6%	2080
	21st to end	Day	33% 4 11% 25% 20% 7%	2323
April	1st to 10th	Day	29% 5% 10% 25% 21% 10%	2160
	11th to 20th	Day	41% 4 7% 20% 20% 8%	2134
	21st to end	Day	41% 4 8% 25% 19% 3	2160
May	1st to 10th	Day	44% 5% 10% 22% 15% 4	2258
	11th to 20th	Day	45% 6% 14% 22% 11% 3	2482
	21st to end	Day	38% 5% 12% 22% 15% 7%	2724
June	1st to 10th	Day	30% 6% 15% 23% 17% 9%	2520
	11th to 20th	Day	30% 5% 11% 19% 21% 15%	2520
	21st to end	Day	32% 6% 9% 22% 20% 11%	2520
July	1st to 10th	Day	35% 8% 10% 18% 19% 10%	2451
	11th to 20th	Day	32% 7% 10% 24% 20% 7%	2445
	21st to end	Day	38% 7% 14% 24% 12% 5%	2522
August	1st to 10th	Day	41% 7% 13% 19% 13% 6%	2313
	11th to 20th	Day	32% 6% 9% 21% 22% 10%	2135
	21st to end	Day	34% 7% 8% 21% 23% 7%	2376
September	1st to 10th	Day	29% 5% 6% 21% 24% 16%	2160
	11th to 20th	Day	28% 4 6% 22% 28% 12%	2152
	21st to end	Day	24% 3 5% 24% 34% 11%	1978
October	1st to 10th	Day	29% 4 7% 24% 26% 11%	1800
	11th to 20th	Day	20% 3 6% 25% 33% 13%	1800
	21st to end	Day	26% 3 7% 27% 25% 13%	1930
November	1st to 10th	Day	19% 3 7% 24% 31% 16%	1673
	11th to 20th	Day	21% 3 5% 31% 30% 10%	1710
	21st to end	Day	24% 3 6% 23% 31% 13%	1601
December	1st to 10th	Day	24% 3 10% 30% 24% 9%	1520
	11th to 20th	Day	21% 2 9% 31% 29% 8%	1924
	21st to end	Day	31% 2 6% 25% 27% 9%	2794
January	1st to 10th	Night	31% 1 6% 27% 30% 6%	4417
	11th to 20th	Night	21% 2 9% 28% 28% 11%	4352
	21st to end	Night	29% 1 9% 31% 22% 7%	4521
February	1st to 10th	Night	25% 2 9% 29% 25% 9%	2470
	11th to 20th	Night	25% 2 7% 30% 27% 9%	2252
	21st to end	Night	25% 2 4 26% 30% 14%	1884
March	1st to 10th	Night	24% 3 7% 25% 32% 9%	2280
	11th to 20th	Night	29% 2 7% 28% 28% 6%	2095
	21st to end	Night	30% 3 9% 27% 25% 6%	2077
April	1st to 10th	Night	26% 3 10% 25% 25% 11%	1800
	11th to 20th	Night	32% 3 9% 27% 22% 7%	1777
	21st to end	Night	35% 3 8% 28% 21% 5%	1800
May	1st to 10th	Night	37% 5% 10% 26% 16% 6%	1698
	11th to 20th	Night	37% 4 18% 23% 15% 3	1418
	21st to end	Night	28% 4 17% 24% 19% 8%	1552
June	1st to 10th	Night	21% 4 20% 30% 16% 10%	1440
	11th to 20th	Night	21% 3 12% 20% 24% 20%	1440
	21st to end	Night	19% 4 10% 26% 28% 12%	1440
July	1st to 10th	Night	27% 5% 14% 24% 21% 9%	1400
	11th to 20th	Night	18% 6% 13% 29% 24% 10%	1392
	21st to end	Night	25% 6% 22% 29% 13% 3	1552
August	1st to 10th	Night	28% 6% 20% 24% 15% 6%	1602
	11th to 20th	Night	24% 4 10% 27% 25% 9%	1765
	21st to end	Night	27% 4 11% 26% 25% 8%	1980
September	1st to 10th	Night	25% 4 9% 23% 24% 16%	1800
	11th to 20th	Night	21% 3 9% 24% 27% 16%	1808
	21st to end	Night	23% 3 6% 25% 32% 11%	1980
October	1st to 10th	Night	25% 4 9% 22% 27% 12%	2159
	11th to 20th	Night	17% 3 8% 22% 34% 17%	2160
	21st to end	Night	26% 3 6% 25% 29% 11%	2407
November	1st to 10th	Night	17% 3 8% 25% 31% 16%	2405
	11th to 20th	Night	20% 2 6% 28% 33% 10%	2470
	21st to end	Night	24% 1 8% 23% 31% 13%	2574
December	1st to 10th	Night	22% 3 11% 26% 28% 11%	2660
	11th to 20th	Night	25% 3 8% 26% 30% 8%	3272
	21st to end	Night	31% 2 7% 25% 25% 10%	4873

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	4 11% 37% 36% 10% 2	270
	11th to 20th	Day	5% 18% 40% 30% 5% 1	270
	21st to end	Day	2 18% 41% 29% 8% 2	297
February	1st to 10th	Day	4 30% 43% 18% 4 1	270
	11th to 20th	Day	3 20% 48% 24% 3 1	270
	21st to end	Day	6% 19% 45% 27% 3	225
March	1st to 10th	Day	3 18% 42% 26% 9% 2	270
	11th to 20th	Day	9% 21% 35% 26% 8%	450
	21st to end	Day	14% 24% 35% 21% 5%	471
April	1st to 10th	Day	11% 23% 36% 23% 7% 1	420
	11th to 20th	Day	16% 26% 34% 18% 5% 1	412
	21st to end	Day	15% 18% 36% 21% 8% 1	420
May	1st to 10th	Day	15% 12% 35% 28% 8% 2	517
	11th to 20th	Day	17% 9% 29% 30% 13% 3	625
	21st to end	Day	9% 10% 30% 30% 18% 3	679
June	1st to 10th	Day	7% 7% 25% 35% 20% 7%	630
	11th to 20th	Day	6% 7% 33% 34% 16% 4	630
	21st to end	Day	6% 4 32% 32% 19% 7%	630
July	1st to 10th	Day	8% 4 18% 33% 29% 8%	621
	11th to 20th	Day	5% 3 21% 33% 28% 10%	618
	21st to end	Day	7% 4 17% 35% 30% 8%	612
August	1st to 10th	Day	9% 5% 22% 35% 22% 6%	447
	11th to 20th	Day	6% 4 27% 35% 23% 6%	437
	21st to end	Day	10% 2 27% 36% 19% 6%	495
September	1st to 10th	Day	8% 5% 25% 40% 17% 5%	450
	11th to 20th	Day	7% 6% 25% 40% 18% 3	442
	21st to end	Day	6% 8% 33% 30% 18% 6%	270
October	1st to 10th	Day	10% 7% 30% 27% 19% 6%	270
	11th to 20th	Day	4 12% 37% 34% 9% 3	270
	21st to end	Day	7% 13% 38% 31% 10% 1	297
November	1st to 10th	Day	5% 11% 40% 29% 12% 3	270
	11th to 20th	Day	7% 15% 34% 35% 6% 3	270
	21st to end	Day	5% 13% 44% 29% 6% 2	270
December	1st to 10th	Day	7% 18% 37% 29% 8% 1	270
	11th to 20th	Day	4 14% 47% 30% 5%	270
	21st to end	Day	6% 19% 46% 23% 6% 1	297
January	1st to 10th	Night	2 17% 36% 33% 9% 2	950
	11th to 20th	Night	3 21% 41% 26% 7% 2	898
	21st to end	Night	3 15% 39% 36% 6% 1	836
February	1st to 10th	Night	4 18% 43% 29% 5%	760
	11th to 20th	Night	3 16% 43% 33% 5%	744
	21st to end	Night	4 14% 45% 34% 3	628
March	1st to 10th	Night	3 14% 41% 35% 7%	760
	11th to 20th	Night	8% 11% 34% 36% 10% 1	760
	21st to end	Night	6% 10% 44% 34% 5% 1	677
April	1st to 10th	Night	9% 11% 40% 35% 5%	540
	11th to 20th	Night	11% 13% 33% 33% 9% 1	533
	21st to end	Night	13% 8% 35% 32% 11% 1	540
May	1st to 10th	Night	14% 6% 29% 36% 13% 2	540
	11th to 20th	Night	17% 3 21% 43% 15% 2	531
	21st to end	Night	9% 4 24% 40% 20% 3	584
June	1st to 10th	Night	5% 1 16% 49% 24% 4	540
	11th to 20th	Night	7% 2 23% 48% 18% 2	540
	21st to end	Night	4 2 14% 45% 32% 3	540
July	1st to 10th	Night	6% 3 11% 38% 38% 4	525
	11th to 20th	Night	2 1 10% 41% 41% 5%	522
	21st to end	Night	7% 1 7% 39% 40% 6%	591
August	1st to 10th	Night	6% 1 11% 41% 37% 4	537
	11th to 20th	Night	6% 1 11% 48% 29% 4	534
	21st to end	Night	6% 1 15% 40% 32% 7%	594
September	1st to 10th	Night	6% 2 14% 44% 27% 7%	540
	11th to 20th	Night	7% 1 19% 45% 22% 6%	540
	21st to end	Night	7% 3 23% 42% 19% 5%	540
October	1st to 10th	Night	10% 7% 26% 39% 15% 4	719
	11th to 20th	Night	4 6% 33% 42% 12% 3	720
	21st to end	Night	8% 10% 33% 35% 12% 2	823
November	1st to 10th	Night	3 8% 35% 41% 10% 2	925
	11th to 20th	Night	4 15% 36% 38% 6% 1	950
	21st to end	Night	5% 10% 36% 39% 9%	950
December	1st to 10th	Night	4 15% 37% 37% 6% 1	950
	11th to 20th	Night	4 14% 41% 33% 7% 1	950
	21st to end	Night	5% 16% 39% 32% 7% 1	1045

Page generated at 2024-03-19 21:31:08. Version ac07

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