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 2025-01-22.

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% 13% 4 9% 5% 4 4 3 5% 7% 30%	104994
	Night	17% 15% 5% 10% 5% 4 4 3 4 6% 27%	104315
Transparency	Day	53% 6% 12% 18% 10% 1	108638
	Night	57% 6% 10% 16% 9% 1	108811
Seeing	Day	23% 14% 29% 22% 10% 3	14739
	Night	26% 13% 25% 23% 11% 1	24449

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	18% 28% 8% 16% 5% 4 3 2 3 3 10%	2795
	11th to 20th	Day	12% 24% 10% 20% 6% 4 3 2 3 4 12%	2930
	21st to end	Day	23% 21% 10% 18% 7% 4 3 2 3 3 7%	3133
February	1st to 10th	Day	18% 19% 6% 15% 11% 5% 3 3 3 4 14%	1710
	11th to 20th	Day	16% 19% 8% 15% 8% 5% 4 2 3 3 16%	1663
	21st to end	Day	20% 16% 5% 13% 4 4 4 4 4 6% 21%	1535
March	1st to 10th	Day	19% 16% 5% 8% 4 3 4 3 5% 6% 27%	3431
	11th to 20th	Day	12% 19% 6% 9% 4 4 4 3 4 6% 29%	3480
	21st to end	Day	20% 14% 5% 10% 4 4 4 3 4 5% 28%	3815
April	1st to 10th	Day	17% 14% 4 7% 4 4 3 3 4 7% 32%	2190
	11th to 20th	Day	20% 12% 3 6% 4 3 4 3 5% 6% 34%	2345
	21st to end	Day	21% 10% 3 7% 5% 4 4 3 4 6% 34%	2470
May	1st to 10th	Day	19% 9% 3 7% 5% 4 4 4 5% 7% 32%	4082
	11th to 20th	Day	18% 9% 3 7% 4 4 4 3 6% 6% 34%	4108
	21st to end	Day	11% 6% 2 6% 5% 4 5% 4 6% 7% 43%	4823
June	1st to 10th	Day	16% 6% 2 5% 4 4 5% 4 6% 9% 39%	2660
	11th to 20th	Day	13% 5% 2 7% 4 5% 4 4 7% 9% 40%	2660
	21st to end	Day	14% 5% 2 5% 4 4 4 4 7% 9% 42%	2660
July	1st to 10th	Day	9% 5% 2 6% 4 5% 5% 4 7% 11% 43%	4440
	11th to 20th	Day	12% 6% 2 5% 4 4 4 3 7% 10% 45%	4136
	21st to end	Day	9% 5% 2 6% 5% 5% 5% 4 7% 8% 44%	4466
August	1st to 10th	Day	15% 6% 2 6% 4 4 5% 4 6% 9% 37%	4104
	11th to 20th	Day	14% 6% 3 7% 6% 5% 5% 5% 8% 9% 34%	4044
	21st to end	Day	12% 5% 2 6% 4 4 5% 4 6% 9% 42%	4381
September	1st to 10th	Day	17% 7% 2 5% 4 4 4 4 5% 8% 39%	2278
	11th to 20th	Day	12% 6% 2 5% 4 5% 5% 3 6% 8% 44%	2090
	21st to end	Day	19% 8% 3 8% 5% 4 5% 3 4 6% 34%	2090
October	1st to 10th	Day	19% 6% 2 8% 4 5% 5% 3 6% 7% 36%	1906
	11th to 20th	Day	24% 10% 3 8% 4 5% 4 3 6% 6% 28%	1900
	21st to end	Day	28% 12% 3 9% 5% 4 4 4 4 6% 21%	2090
November	1st to 10th	Day	17% 16% 4 8% 4 4 4 4 4 6% 30%	1810
	11th to 20th	Day	18% 22% 3 9% 5% 3 3 3 4 5% 23%	1619
	21st to end	Day	21% 27% 5% 9% 5% 5% 3 3 3 6% 13%	1600
December	1st to 10th	Day	18% 33% 12% 13% 4 3 3 2 2 2 8%	2600
	11th to 20th	Day	21% 27% 8% 12% 5% 3 4 3 3 4 10%	2990
	21st to end	Day	23% 24% 7% 16% 5% 4 3 2 3 3 9%	3960
January	1st to 10th	Night	17% 27% 12% 17% 5% 3 2 2 2 3 9%	4366
	11th to 20th	Night	13% 24% 9% 17% 7% 5% 4 3 4 4 10%	4250
	21st to end	Night	25% 23% 7% 16% 5% 4 3 2 2 3 10%	4546
February	1st to 10th	Night	19% 20% 5% 14% 8% 4 3 2 4 4 16%	2470
	11th to 20th	Night	17% 18% 7% 14% 6% 5% 4 3 4 4 18%	2403
	21st to end	Night	19% 16% 6% 14% 6% 3 4 3 3 5% 21%	1918
March	1st to 10th	Night	21% 16% 4 8% 4 3 3 3 3 5% 29%	3421
	11th to 20th	Night	17% 19% 6% 8% 3 3 3 3 4 5% 29%	3480
	21st to end	Night	17% 13% 5% 10% 5% 3 4 3 4 4 34%	3817
April	1st to 10th	Night	19% 15% 4 7% 4 4 4 4 5% 5% 29%	1990
	11th to 20th	Night	20% 10% 3 7% 4 4 4 3 6% 7% 32%	1800
	21st to end	Night	16% 11% 3 6% 5% 5% 5% 4 5% 7% 33%	1710
May	1st to 10th	Night	24% 9% 3 6% 4 4 4 3 5% 6% 32%	2813
	11th to 20th	Night	16% 8% 3 7% 5% 4 4 3 5% 8% 35%	2785
	21st to end	Night	12% 5% 2 5% 3 3 4 4 7% 10% 45%	2732
June	1st to 10th	Night	11% 7% 2 7% 6% 5% 4 4 6% 9% 39%	1520
	11th to 20th	Night	11% 6% 3 8% 5% 4 4 4 6% 7% 42%	1520
	21st to end	Night	13% 6% 3 6% 3 5% 5% 4 7% 8% 40%	1520
July	1st to 10th	Night	12% 7% 2 6% 4 4 4 3 6% 9% 42%	2520
	11th to 20th	Night	11% 5% 2 6% 5% 4 5% 3 7% 10% 43%	2806
	21st to end	Night	8% 5% 2 6% 5% 5% 5% 5% 6% 9% 44%	3087
August	1st to 10th	Night	10% 8% 3 8% 5% 5% 5% 4 6% 8% 38%	2825
	11th to 20th	Night	15% 8% 3 9% 5% 4 4 3 5% 6% 37%	2785
	21st to end	Night	10% 6% 3 6% 4 4 5% 5% 6% 9% 42%	3242
September	1st to 10th	Night	15% 7% 2 6% 3 3 4 3 6% 6% 45%	1900
	11th to 20th	Night	13% 5% 3 5% 3 4 4 4 5% 7% 47%	2089
	21st to end	Night	18% 6% 2 8% 6% 4 5% 4 5% 7% 35%	2090
October	1st to 10th	Night	19% 7% 2 7% 3 3 4 3 5% 7% 38%	2273
	11th to 20th	Night	25% 9% 3 7% 5% 4 4 3 5% 6% 28%	2280
	21st to end	Night	29% 11% 4 7% 4 3 4 3 4 5% 26%	2508
November	1st to 10th	Night	19% 15% 4 7% 4 4 4 3 4 5% 31%	2488
	11th to 20th	Night	19% 21% 4 8% 5% 4 4 3 5% 6% 21%	2780
	21st to end	Night	22% 25% 6% 8% 4 3 3 3 3 4 18%	2800
December	1st to 10th	Night	16% 31% 10% 11% 5% 4 4 2 3 3 11%	4580
	11th to 20th	Night	19% 29% 8% 11% 4 3 3 2 3 4 12%	5205
	21st to end	Night	21% 24% 6% 13% 5% 5% 4 3 4 5% 11%	6996

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	80% 4 8% 6% 2	2796
	11th to 20th	Day	79% 3 9% 7% 2	2930
	21st to end	Day	85% 3 6% 4 2	3133
February	1st to 10th	Day	76% 5% 8% 8% 4	1710
	11th to 20th	Day	74% 3 7% 10% 6%	1672
	21st to end	Day	64% 6% 11% 13% 5% 2	1535
March	1st to 10th	Day	58% 5% 11% 22% 4	3431
	11th to 20th	Day	56% 6% 17% 15% 5% 1	3480
	21st to end	Day	59% 5% 10% 16% 10% 1	3815
April	1st to 10th	Day	53% 5% 13% 16% 12%	2190
	11th to 20th	Day	51% 5% 11% 19% 12% 2	2345
	21st to end	Day	52% 5% 10% 19% 11% 3	2470
May	1st to 10th	Day	51% 7% 10% 22% 10% 2	4120
	11th to 20th	Day	49% 8% 12% 22% 8% 1	4112
	21st to end	Day	38% 8% 13% 21% 15% 5%	4837
June	1st to 10th	Day	42% 8% 13% 24% 13% 1	2660
	11th to 20th	Day	41% 7% 14% 22% 14% 1	2660
	21st to end	Day	38% 9% 13% 24% 15% 1	2660
July	1st to 10th	Day	35% 8% 14% 24% 17% 2	4370
	11th to 20th	Day	35% 7% 14% 27% 17% 1	4066
	21st to end	Day	37% 8% 14% 23% 17% 1	4324
August	1st to 10th	Day	43% 8% 15% 22% 12%	4091
	11th to 20th	Day	44% 10% 15% 25% 6% 1	4083
	21st to end	Day	37% 8% 13% 28% 13% 1	4381
September	1st to 10th	Day	43% 7% 14% 22% 13% 2	2278
	11th to 20th	Day	38% 8% 13% 24% 16%	2090
	21st to end	Day	50% 7% 11% 21% 10% 1	2090
October	1st to 10th	Day	38% 6% 15% 23% 17% 1	3166
	11th to 20th	Day	48% 5% 13% 18% 13% 3	3140
	21st to end	Day	48% 5% 14% 24% 8%	3428
November	1st to 10th	Day	54% 5% 11% 18% 10% 2	1810
	11th to 20th	Day	63% 4 11% 13% 7% 2	1619
	21st to end	Day	74% 4 9% 9% 4	1600
December	1st to 10th	Day	84% 4 6% 5% 1	2600
	11th to 20th	Day	78% 5% 7% 9% 1	2986
	21st to end	Day	82% 4 7% 6% 1	3960
January	1st to 10th	Night	83% 3 7% 5% 2	4366
	11th to 20th	Night	79% 4 9% 6% 1	4250
	21st to end	Night	82% 4 7% 6% 1	4546
February	1st to 10th	Night	72% 5% 9% 10% 3	2470
	11th to 20th	Night	71% 4 9% 12% 4	2405
	21st to end	Night	67% 5% 9% 11% 7%	1918
March	1st to 10th	Night	59% 4 12% 18% 8%	3421
	11th to 20th	Night	58% 5% 13% 17% 7% 1	3480
	21st to end	Night	54% 4 11% 16% 13% 2	3817
April	1st to 10th	Night	56% 5% 10% 16% 11% 1	1990
	11th to 20th	Night	51% 6% 11% 17% 13% 2	1800
	21st to end	Night	50% 6% 10% 20% 11% 3	1710
May	1st to 10th	Night	52% 5% 9% 19% 10% 4	2840
	11th to 20th	Night	47% 8% 11% 23% 10% 2	2786
	21st to end	Night	32% 9% 13% 26% 17% 3	2739
June	1st to 10th	Night	41% 8% 11% 23% 15% 2	1520
	11th to 20th	Night	39% 7% 12% 22% 18% 2	1520
	21st to end	Night	40% 9% 12% 24% 15% 1	1520
July	1st to 10th	Night	39% 7% 12% 21% 18% 4	2480
	11th to 20th	Night	35% 8% 11% 27% 17% 3	2754
	21st to end	Night	35% 9% 14% 22% 20% 1	2988
August	1st to 10th	Night	42% 8% 10% 27% 12% 1	2814
	11th to 20th	Night	46% 7% 12% 25% 9% 1	2812
	21st to end	Night	37% 8% 15% 27% 12% 2	3242
September	1st to 10th	Night	38% 7% 13% 25% 14% 2	1900
	11th to 20th	Night	35% 6% 13% 25% 20% 1	2089
	21st to end	Night	48% 6% 11% 20% 13% 1	2090
October	1st to 10th	Night	40% 6% 10% 23% 21% 1	3793
	11th to 20th	Night	51% 5% 12% 16% 14% 3	3779
	21st to end	Night	45% 4 14% 22% 13% 1	4120
November	1st to 10th	Night	54% 7% 10% 18% 9% 2	2488
	11th to 20th	Night	64% 6% 9% 12% 6% 2	2780
	21st to end	Night	70% 6% 9% 12% 4	2800
December	1st to 10th	Night	79% 4 9% 6% 1	4580
	11th to 20th	Night	76% 4 10% 9% 1	5208
	21st to end	Night	76% 6% 8% 9% 1	6996

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	30% 26% 31% 9% 4	260
	11th to 20th	Day	32% 27% 30% 8% 3	300
	21st to end	Day	28% 28% 32% 9% 2	300
February	1st to 10th	Day	31% 23% 35% 11%	270
	11th to 20th	Day	32% 27% 31% 9% 1	270
	21st to end	Day	32% 31% 28% 9%	225
March	1st to 10th	Day	29% 21% 35% 10% 4	456
	11th to 20th	Day	28% 21% 35% 12% 3 1	460
	21st to end	Day	33% 19% 29% 15% 3	506
April	1st to 10th	Day	22% 25% 34% 13% 5% 1	460
	11th to 20th	Day	30% 17% 28% 18% 5% 2	452
	21st to end	Day	27% 15% 34% 19% 5% 1	460
May	1st to 10th	Day	28% 14% 32% 19% 5% 3	463
	11th to 20th	Day	22% 11% 31% 24% 10% 2	487
	21st to end	Day	15% 7% 29% 30% 14% 5%	529
June	1st to 10th	Day	18% 7% 35% 28% 9% 3	490
	11th to 20th	Day	15% 10% 27% 31% 15% 2	490
	21st to end	Day	17% 8% 31% 29% 12% 2	490
July	1st to 10th	Day	10% 5% 26% 33% 20% 6%	485
	11th to 20th	Day	15% 4 27% 32% 17% 5%	484
	21st to end	Day	10% 6% 30% 34% 16% 5%	532
August	1st to 10th	Day	13% 6% 24% 33% 18% 7%	486
	11th to 20th	Day	15% 6% 22% 33% 18% 6%	479
	21st to end	Day	14% 7% 22% 33% 17% 7%	539
September	1st to 10th	Day	18% 8% 23% 28% 17% 6%	490
	11th to 20th	Day	11% 5% 23% 30% 22% 9%	490
	21st to end	Day	20% 10% 29% 24% 12% 5%	490
October	1st to 10th	Day	19% 10% 29% 23% 15% 4	490
	11th to 20th	Day	29% 15% 28% 20% 6% 2	490
	21st to end	Day	37% 16% 28% 17% 2	539
November	1st to 10th	Day	30% 21% 25% 17% 7% 1	348
	11th to 20th	Day	37% 21% 26% 11% 4	209
	21st to end	Day	38% 20% 33% 10%	200
December	1st to 10th	Day	36% 24% 28% 10% 2 1	200
	11th to 20th	Day	33% 30% 27% 11% 1	200
	21st to end	Day	38% 19% 30% 10% 3	220
January	1st to 10th	Night	33% 25% 29% 11% 4	800
	11th to 20th	Night	35% 21% 28% 13% 2	800
	21st to end	Night	29% 22% 32% 15% 2	841
February	1st to 10th	Night	34% 18% 29% 17% 3	760
	11th to 20th	Night	28% 22% 31% 17% 2	743
	21st to end	Night	31% 22% 28% 18% 1	628
March	1st to 10th	Night	28% 18% 28% 23% 4	760
	11th to 20th	Night	26% 18% 30% 20% 5% 1	760
	21st to end	Night	27% 14% 28% 24% 6% 1	836
April	1st to 10th	Night	26% 14% 30% 24% 6%	760
	11th to 20th	Night	26% 11% 32% 23% 8% 1	676
	21st to end	Night	21% 12% 28% 31% 8% 1	570
May	1st to 10th	Night	25% 7% 22% 33% 12% 1	570
	11th to 20th	Night	19% 7% 25% 36% 12% 1	547
	21st to end	Night	13% 5% 25% 31% 23% 3	411
June	1st to 10th	Night	15% 4 31% 33% 16% 2	380
	11th to 20th	Night	13% 3 26% 38% 19% 1	380
	21st to end	Night	13% 8% 25% 32% 21% 1	380
July	1st to 10th	Night	13% 3 19% 38% 21% 6%	370
	11th to 20th	Night	13% 4 21% 32% 26% 4	533
	21st to end	Night	12% 3 18% 39% 26% 2	624
August	1st to 10th	Night	11% 2 19% 39% 25% 4	567
	11th to 20th	Night	14% 3 13% 35% 30% 4	564
	21st to end	Night	15% 4 21% 30% 26% 4	693
September	1st to 10th	Night	18% 7% 16% 30% 23% 5%	760
	11th to 20th	Night	14% 5% 16% 32% 28% 5%	760
	21st to end	Night	20% 9% 21% 28% 17% 5%	760
October	1st to 10th	Night	23% 9% 24% 23% 18% 3	760
	11th to 20th	Night	31% 12% 27% 20% 8% 1	760
	21st to end	Night	38% 13% 27% 18% 3	836
November	1st to 10th	Night	29% 12% 28% 22% 7% 1	780
	11th to 20th	Night	36% 20% 24% 14% 5% 1	800
	21st to end	Night	39% 18% 28% 11% 4	800
December	1st to 10th	Night	41% 23% 23% 10% 4	800
	11th to 20th	Night	37% 25% 23% 13% 3	800
	21st to end	Night	33% 22% 29% 15% 1	880

Page generated at 2025-01-19 03:06:44. Version ac07

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