Echo Hill Outdoor School
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Echo Hill Outdoor School clear sky chart from 2005-11-06 to 2026-02-01.

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	14% 6% 2 5% 4 4 4 4 6% 8% 43%	127500
	Night	15% 6% 2 6% 4 4 4 3 5% 7% 43%	124508
Transparency	Day	25% 38% 8% 16% 11% 2	131087
	Night	25% 38% 7% 15% 12% 3	128989
Seeing	Day	16% 8% 23% 27% 19% 7%	16531
	Night	17% 8% 27% 32% 14% 2	26022

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	14% 4 1 5% 4 3 3 3 5% 7% 51%	3677
	11th to 20th	Day	14% 9% 3 7% 4 3 3 3 3 4 45%	4023
	21st to end	Day	16% 7% 2 5% 4 4 4 3 4 6% 45%	4565
February	1st to 10th	Day	15% 7% 2 6% 4 4 5% 4 5% 7% 40%	1800
	11th to 20th	Day	18% 8% 2 5% 4 4 4 3 5% 6% 41%	1865
	21st to end	Day	18% 8% 2 6% 3 4 3 3 5% 5% 41%	1650
March	1st to 10th	Day	19% 4 1 4 3 3 3 3 5% 6% 48%	4654
	11th to 20th	Day	11% 7% 2 5% 4 4 4 3 5% 6% 49%	4980
	21st to end	Day	17% 5% 2 6% 4 4 4 4 5% 7% 42%	5689
April	1st to 10th	Day	15% 6% 2 5% 4 4 5% 4 6% 7% 42%	2400
	11th to 20th	Day	13% 5% 2 5% 4 4 4 3 6% 7% 46%	2374
	21st to end	Day	17% 5% 3 5% 3 3 4 3 6% 7% 44%	2400
May	1st to 10th	Day	22% 7% 2 5% 4 4 4 4 6% 8% 34%	5672
	11th to 20th	Day	21% 6% 2 6% 4 4 4 4 6% 9% 33%	6318
	21st to end	Day	16% 5% 2 6% 4 4 4 4 6% 8% 41%	6934
June	1st to 10th	Day	10% 5% 2 5% 5% 5% 5% 5% 7% 10% 42%	2800
	11th to 20th	Day	11% 5% 2 5% 4 4 5% 5% 6% 9% 43%	2800
	21st to end	Day	9% 3 2 5% 3 4 4 4 7% 10% 48%	2800
July	1st to 10th	Day	7% 4 2 5% 4 5% 5% 5% 8% 11% 44%	6360
	11th to 20th	Day	10% 6% 2 7% 5% 5% 6% 5% 9% 12% 34%	6360
	21st to end	Day	8% 5% 2 5% 4 5% 6% 5% 8% 10% 42%	6526
August	1st to 10th	Day	15% 6% 2 5% 3 4 5% 5% 7% 10% 36%	4234
	11th to 20th	Day	11% 4 2 6% 4 4 5% 4 6% 10% 44%	3897
	21st to end	Day	9% 4 2 6% 3 4 4 4 6% 8% 50%	4312
September	1st to 10th	Day	13% 4 2 5% 4 3 4 4 5% 8% 48%	2400
	11th to 20th	Day	12% 5% 1 4 3 4 4 4 6% 8% 50%	2400
	21st to end	Day	18% 8% 3 6% 4 4 4 3 5% 7% 37%	2144
October	1st to 10th	Day	15% 6% 2 5% 3 3 3 3 6% 7% 48%	2000
	11th to 20th	Day	15% 6% 2 4 3 3 3 3 5% 6% 51%	2000
	21st to end	Day	14% 5% 2 6% 3 3 4 3 5% 6% 49%	2200
November	1st to 10th	Day	13% 5% 2 4 3 3 4 3 5% 6% 53%	1856
	11th to 20th	Day	13% 6% 2 4 3 5% 4 3 5% 7% 49%	1890
	21st to end	Day	14% 6% 2 5% 3 3 4 3 5% 5% 47%	1722
December	1st to 10th	Day	13% 5% 2 6% 4 3 4 3 5% 7% 47%	2680
	11th to 20th	Day	19% 9% 2 5% 3 3 3 3 5% 6% 42%	3070
	21st to end	Day	14% 6% 2 6% 4 5% 4 4 5% 8% 42%	4048
January	1st to 10th	Night	13% 7% 2 5% 4 4 4 3 4 6% 48%	6489
	11th to 20th	Night	14% 7% 2 5% 3 3 3 3 4 5% 50%	6153
	21st to end	Night	16% 8% 2 6% 4 3 3 3 4 5% 46%	6633
February	1st to 10th	Night	17% 8% 2 5% 5% 4 4 3 5% 6% 41%	2601
	11th to 20th	Night	19% 7% 2 6% 4 3 4 3 4 5% 44%	2419
	21st to end	Night	17% 8% 2 6% 4 3 4 4 4 5% 42%	1980
March	1st to 10th	Night	17% 7% 2 5% 3 3 3 3 4 5% 48%	5195
	11th to 20th	Night	10% 6% 2 5% 4 3 3 4 5% 7% 51%	4980
	21st to end	Night	15% 7% 2 6% 4 4 4 4 5% 6% 44%	5243
April	1st to 10th	Night	16% 7% 3 6% 3 4 4 4 5% 6% 41%	2000
	11th to 20th	Night	14% 7% 3 6% 4 4 4 4 5% 6% 44%	1975
	21st to end	Night	18% 6% 2 4 3 4 3 3 5% 8% 45%	2000
May	1st to 10th	Night	21% 8% 3 7% 5% 5% 5% 4 6% 7% 30%	4249
	11th to 20th	Night	21% 9% 3 6% 4 4 4 4 6% 8% 31%	3575
	21st to end	Night	19% 6% 2 7% 4 4 4 3 5% 8% 37%	3920
June	1st to 10th	Night	11% 5% 2 6% 4 4 4 3 5% 9% 45%	1600
	11th to 20th	Night	10% 7% 2 7% 5% 5% 5% 4 6% 9% 40%	1600
	21st to end	Night	10% 5% 2 6% 4 5% 5% 4 7% 11% 42%	1600
July	1st to 10th	Night	9% 5% 3 7% 5% 5% 6% 5% 8% 9% 38%	3598
	11th to 20th	Night	11% 7% 3 8% 6% 5% 6% 5% 7% 11% 31%	3597
	21st to end	Night	11% 5% 3 6% 5% 4 5% 5% 6% 9% 42%	4307
August	1st to 10th	Night	16% 6% 3 7% 4 4 6% 4 7% 9% 33%	2915
	11th to 20th	Night	12% 7% 3 7% 5% 5% 6% 4 6% 8% 38%	3104
	21st to end	Night	10% 5% 2 5% 4 4 5% 3 6% 7% 49%	3586
September	1st to 10th	Night	12% 6% 3 7% 3 4 4 3 6% 7% 46%	2000
	11th to 20th	Night	11% 6% 2 5% 4 3 3 3 5% 6% 52%	2000
	21st to end	Night	18% 7% 2 7% 5% 5% 4 3 5% 6% 38%	2252
October	1st to 10th	Night	17% 7% 2 6% 4 3 3 3 4 5% 45%	2400
	11th to 20th	Night	16% 4 2 4 4 3 3 3 5% 6% 50%	2400
	21st to end	Night	16% 5% 1 4 3 4 3 3 4 6% 50%	2640
November	1st to 10th	Night	12% 5% 2 5% 3 3 4 3 5% 6% 53%	2659
	11th to 20th	Night	15% 7% 2 5% 3 3 4 3 5% 6% 48%	2730
	21st to end	Night	13% 5% 1 6% 4 4 4 3 5% 6% 47%	2893
December	1st to 10th	Night	12% 5% 2 6% 4 4 4 3 5% 7% 49%	4720
	11th to 20th	Night	22% 5% 2 5% 3 4 4 3 5% 7% 40%	5345
	21st to end	Night	16% 7% 3 6% 4 4 5% 4 6% 7% 39%	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	24% 26% 4 21% 21% 4	3678
	11th to 20th	Day	33% 26% 4 17% 15% 6%	4023
	21st to end	Day	30% 28% 10% 21% 10% 2	4565
February	1st to 10th	Day	18% 63% 7% 8% 3 1	1800
	11th to 20th	Day	18% 61% 3 9% 7% 2	1874
	21st to end	Day	17% 62% 5% 8% 6% 2	1650
March	1st to 10th	Day	28% 29% 6% 17% 14% 7%	4654
	11th to 20th	Day	24% 30% 5% 23% 14% 4	4980
	21st to end	Day	29% 30% 7% 12% 16% 6%	5689
April	1st to 10th	Day	18% 63% 4 8% 6% 2	2400
	11th to 20th	Day	15% 63% 5% 9% 6% 1	2374
	21st to end	Day	15% 62% 5% 9% 8% 1	2400
May	1st to 10th	Day	37% 30% 12% 14% 6% 1	5704
	11th to 20th	Day	37% 30% 9% 18% 6%	6322
	21st to end	Day	30% 30% 9% 17% 12% 1	6949
June	1st to 10th	Day	16% 63% 4 10% 7% 1	2800
	11th to 20th	Day	14% 63% 4 11% 7% 1	2800
	21st to end	Day	11% 62% 7% 11% 9%	2800
July	1st to 10th	Day	25% 31% 17% 12% 12% 3	6290
	11th to 20th	Day	34% 30% 15% 16% 4	6285
	21st to end	Day	26% 28% 9% 18% 19% 1	6339
August	1st to 10th	Day	29% 38% 10% 18% 4	4221
	11th to 20th	Day	22% 38% 8% 23% 8%	3934
	21st to end	Day	18% 37% 8% 21% 15% 1	4312
September	1st to 10th	Day	15% 57% 7% 12% 7% 2	2400
	11th to 20th	Day	15% 57% 4 13% 9% 2	2400
	21st to end	Day	24% 57% 4 10% 4	2144
October	1st to 10th	Day	22% 36% 5% 11% 22% 4	3260
	11th to 20th	Day	12% 35% 5% 19% 20% 8%	3240
	21st to end	Day	9% 35% 6% 23% 22% 5%	3538
November	1st to 10th	Day	17% 54% 7% 11% 9% 3	1856
	11th to 20th	Day	14% 55% 4 12% 13% 2	1890
	21st to end	Day	17% 54% 5% 16% 7% 1	1722
December	1st to 10th	Day	25% 35% 11% 17% 8% 3	2680
	11th to 20th	Day	31% 31% 8% 20% 8% 1	3066
	21st to end	Day	31% 28% 11% 16% 13% 1	4048
January	1st to 10th	Night	29% 26% 5% 18% 18% 4	6489
	11th to 20th	Night	26% 26% 5% 20% 17% 6%	6153
	21st to end	Night	30% 27% 5% 21% 14% 2	6633
February	1st to 10th	Night	17% 62% 7% 9% 4 1	2601
	11th to 20th	Night	18% 60% 2 8% 10% 2	2421
	21st to end	Night	17% 61% 4 8% 6% 3	1980
March	1st to 10th	Night	31% 29% 5% 18% 8% 9%	5195
	11th to 20th	Night	21% 29% 10% 22% 13% 5%	4980
	21st to end	Night	30% 29% 5% 13% 20% 3	5243
April	1st to 10th	Night	18% 62% 2 9% 8% 1	2000
	11th to 20th	Night	16% 63% 5% 9% 6% 2	1975
	21st to end	Night	16% 62% 4 8% 9% 1	2000
May	1st to 10th	Night	41% 31% 9% 12% 6% 2	4279
	11th to 20th	Night	38% 31% 10% 15% 6%	3576
	21st to end	Night	36% 29% 9% 15% 9% 2	3927
June	1st to 10th	Night	16% 63% 4 9% 8%	1600
	11th to 20th	Night	15% 64% 4 10% 6%	1600
	21st to end	Night	13% 62% 6% 12% 7% 1	1600
July	1st to 10th	Night	31% 30% 14% 12% 11% 2	3560
	11th to 20th	Night	37% 30% 16% 13% 3 1	3552
	21st to end	Night	29% 28% 10% 18% 16%	4180
August	1st to 10th	Night	33% 39% 12% 12% 4	2904
	11th to 20th	Night	27% 38% 7% 20% 7%	3132
	21st to end	Night	19% 37% 6% 22% 16% 1	3586
September	1st to 10th	Night	15% 58% 6% 12% 6% 2	2000
	11th to 20th	Night	15% 58% 4 11% 11% 2	2000
	21st to end	Night	24% 58% 5% 7% 6%	2252
October	1st to 10th	Night	28% 37% 5% 12% 15% 3	3920
	11th to 20th	Night	12% 36% 7% 15% 25% 5%	3899
	21st to end	Night	11% 35% 7% 24% 17% 5%	4252
November	1st to 10th	Night	16% 54% 6% 12% 9% 3	2659
	11th to 20th	Night	17% 54% 3 10% 13% 3	2730
	21st to end	Night	16% 54% 5% 14% 9% 1	2893
December	1st to 10th	Night	22% 35% 10% 21% 10% 2	4720
	11th to 20th	Night	31% 31% 9% 12% 12% 4	5348
	21st to end	Night	33% 27% 9% 15% 14% 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	20% 19% 35% 18% 6% 2	220
	11th to 20th	Day	18% 24% 33% 17% 7%	298
	21st to end	Day	17% 15% 44% 17% 6% 1	363
February	1st to 10th	Day	21% 10% 39% 20% 8% 1	300
	11th to 20th	Day	26% 17% 35% 19% 3	413
	21st to end	Day	25% 13% 33% 23% 5%	414
March	1st to 10th	Day	16% 16% 33% 29% 6% 1	500
	11th to 20th	Day	19% 12% 34% 24% 9% 2	500
	21st to end	Day	20% 11% 31% 28% 8% 2	550
April	1st to 10th	Day	22% 9% 35% 24% 7% 3	500
	11th to 20th	Day	15% 14% 26% 25% 15% 4	492
	21st to end	Day	20% 9% 23% 32% 14% 2	500
May	1st to 10th	Day	23% 10% 24% 26% 14% 3	503
	11th to 20th	Day	18% 7% 23% 29% 18% 5%	527
	21st to end	Day	18% 3 17% 24% 27% 12%	573
June	1st to 10th	Day	10% 5% 18% 28% 26% 12%	530
	11th to 20th	Day	14% 4 17% 30% 26% 10%	530
	21st to end	Day	10% 3 15% 32% 27% 13%	530
July	1st to 10th	Day	11% 1 10% 30% 31% 17%	525
	11th to 20th	Day	5% 1 7% 33% 35% 18%	524
	21st to end	Day	10% 2 11% 31% 36% 11%	576
August	1st to 10th	Day	13% 2 10% 29% 32% 15%	526
	11th to 20th	Day	12% 3 8% 29% 34% 14%	519
	21st to end	Day	9% 2 14% 31% 29% 16%	583
September	1st to 10th	Day	17% 2 16% 25% 23% 18%	530
	11th to 20th	Day	12% 3 13% 28% 29% 15%	530
	21st to end	Day	24% 6% 16% 29% 18% 6%	530
October	1st to 10th	Day	18% 4 19% 27% 20% 12%	530
	11th to 20th	Day	16% 8% 25% 26% 18% 6%	530
	21st to end	Day	16% 7% 29% 27% 17% 4	583
November	1st to 10th	Day	14% 8% 31% 26% 16% 5%	428
	11th to 20th	Day	13% 15% 32% 28% 11% 1	430
	21st to end	Day	15% 15% 32% 24% 11% 3	262
December	1st to 10th	Day	20% 9% 37% 26% 7% 1	220
	11th to 20th	Day	20% 16% 31% 24% 9%	220
	21st to end	Day	21% 10% 33% 24% 10% 2	242
January	1st to 10th	Night	18% 13% 42% 23% 4	840
	11th to 20th	Night	21% 14% 34% 26% 5%	840
	21st to end	Night	18% 13% 39% 28% 2	924
February	1st to 10th	Night	21% 9% 36% 31% 3	801
	11th to 20th	Night	24% 15% 39% 21% 1	783
	21st to end	Night	22% 14% 35% 25% 3	660
March	1st to 10th	Night	18% 13% 37% 27% 6%	800
	11th to 20th	Night	18% 12% 36% 27% 7% 1	800
	21st to end	Night	20% 7% 36% 30% 6% 1	880
April	1st to 10th	Night	20% 10% 38% 25% 7%	800
	11th to 20th	Night	16% 8% 31% 33% 11%	791
	21st to end	Night	19% 8% 29% 33% 10%	800
May	1st to 10th	Night	20% 7% 24% 36% 12% 1	702
	11th to 20th	Night	19% 8% 23% 36% 13%	394
	21st to end	Night	14% 3 15% 41% 24% 2	433
June	1st to 10th	Night	12% 3 19% 43% 22% 2	400
	11th to 20th	Night	12% 2 18% 42% 24% 2	400
	21st to end	Night	11% 4 17% 44% 22% 2	400
July	1st to 10th	Night	8% 3 14% 42% 30% 3	390
	11th to 20th	Night	6% 2 9% 41% 38% 3	388
	21st to end	Night	7% 2 11% 45% 30% 3	602
August	1st to 10th	Night	10% 3 10% 40% 34% 4	597
	11th to 20th	Night	12% 3 11% 35% 33% 5%	733
	21st to end	Night	11% 3 12% 36% 32% 5%	880
September	1st to 10th	Night	15% 2 15% 34% 29% 6%	800
	11th to 20th	Night	13% 3 11% 37% 31% 5%	800
	21st to end	Night	21% 5% 17% 33% 23% 2	800
October	1st to 10th	Night	20% 5% 19% 30% 21% 5%	800
	11th to 20th	Night	18% 7% 25% 33% 15% 1	800
	21st to end	Night	18% 7% 30% 31% 13% 1	880
November	1st to 10th	Night	15% 6% 32% 33% 13% 2	820
	11th to 20th	Night	19% 13% 29% 30% 9%	840
	21st to end	Night	16% 14% 31% 30% 9% 1	840
December	1st to 10th	Night	21% 11% 35% 25% 8% 1	840
	11th to 20th	Night	19% 10% 36% 29% 6%	840
	21st to end	Night	19% 8% 36% 33% 4	924

Page generated at 2026-01-29 01:21:46. Version ac07

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