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 2024-01-14.

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	13% 5% 2 5% 5% 3 4 5% 6% 8% 44%	74717
	Night	15% 6% 3 5% 5% 3 4 5% 5% 7% 43%	75044
Transparency	Day	37% 5% 13% 25% 16% 3	74424
	Night	40% 5% 12% 23% 17% 3	74856
Seeing	Day	16% 7% 23% 27% 19% 8%	13869
	Night	17% 7% 27% 32% 15% 2	23377

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% 7% 2 6% 5% 2 3 5% 5% 7% 44%	1517
	11th to 20th	Day	15% 8% 3 4 5% 3 3 5% 4 6% 43%	1590
	21st to end	Day	15% 6% 3 6% 5% 3 5% 4 5% 6% 41%	1782
February	1st to 10th	Day	16% 6% 2 5% 5% 3 5% 5% 5% 7% 39%	1620
	11th to 20th	Day	18% 7% 3 4 4 3 4 4 5% 7% 41%	1674
	21st to end	Day	19% 8% 3 7% 3 3 3 4 5% 5% 40%	1480
March	1st to 10th	Day	13% 5% 2 5% 4 3 3 3 5% 6% 50%	1863
	11th to 20th	Day	16% 7% 2 4 5% 3 4 4 5% 6% 44%	1980
	21st to end	Day	18% 6% 3 6% 5% 3 4 4 5% 7% 40%	2263
April	1st to 10th	Day	14% 6% 2 5% 5% 2 5% 5% 6% 7% 44%	2160
	11th to 20th	Day	13% 4 2 4 4 3 5% 4 6% 7% 48%	2134
	21st to end	Day	17% 4 3 5% 4 2 4 5% 6% 7% 43%	2160
May	1st to 10th	Day	16% 6% 2 4 5% 3 4 5% 6% 7% 42%	2259
	11th to 20th	Day	17% 5% 2 4 5% 3 4 6% 6% 9% 38%	2478
	21st to end	Day	13% 5% 2 6% 5% 3 4 5% 6% 9% 42%	2725
June	1st to 10th	Day	11% 4 2 4 5% 4 5% 6% 7% 11% 41%	2520
	11th to 20th	Day	11% 5% 2 5% 5% 3 5% 6% 6% 9% 43%	2520
	21st to end	Day	10% 3 2 4 4 2 5% 5% 7% 10% 48%	2520
July	1st to 10th	Day	8% 5% 2 4 6% 4 5% 5% 7% 9% 45%	2520
	11th to 20th	Day	5% 3 1 4 5% 3 5% 6% 7% 13% 48%	2520
	21st to end	Day	8% 4 2 5% 5% 4 7% 6% 9% 10% 40%	2593
August	1st to 10th	Day	9% 5% 2 5% 4 3 6% 6% 8% 10% 41%	2327
	11th to 20th	Day	9% 4 2 5% 5% 3 5% 5% 7% 10% 44%	2158
	21st to end	Day	10% 5% 2 6% 4 3 5% 6% 7% 9% 44%	2376
September	1st to 10th	Day	13% 4 2 5% 5% 2 4 5% 5% 8% 46%	2160
	11th to 20th	Day	12% 4 1 4 4 3 3 5% 5% 8% 50%	2160
	21st to end	Day	17% 7% 3 5% 4 3 4 4 5% 8% 39%	1932
October	1st to 10th	Day	16% 6% 2 4 4 2 3 3 5% 8% 46%	1800
	11th to 20th	Day	15% 6% 2 4 4 2 3 4 5% 6% 49%	1800
	21st to end	Day	15% 6% 2 5% 4 2 4 3 5% 6% 47%	1980
November	1st to 10th	Day	13% 5% 2 3 4 2 4 4 5% 6% 52%	1676
	11th to 20th	Day	14% 5% 2 4 3 3 5% 3 5% 7% 49%	1710
	21st to end	Day	14% 5% 3 5% 4 3 5% 4 5% 5% 47%	1558
December	1st to 10th	Day	18% 6% 2 5% 5% 3 4 5% 5% 7% 42%	1520
	11th to 20th	Day	17% 8% 2 5% 4 2 3 3 5% 7% 43%	1910
	21st to end	Day	15% 6% 3 5% 6% 4 5% 5% 6% 7% 39%	2772
January	1st to 10th	Night	16% 6% 3 4 5% 2 4 4 4 6% 46%	2649
	11th to 20th	Night	17% 7% 3 4 4 3 4 5% 6% 7% 39%	2444
	21st to end	Night	15% 6% 2 4 5% 3 4 4 5% 6% 46%	2574
February	1st to 10th	Night	17% 8% 3 5% 5% 3 4 4 5% 6% 40%	2340
	11th to 20th	Night	18% 7% 3 5% 4 3 4 4 4 5% 43%	2171
	21st to end	Night	18% 8% 2 5% 5% 3 4 5% 4 5% 42%	1776
March	1st to 10th	Night	14% 5% 2 5% 4 3 3 4 5% 6% 49%	2093
	11th to 20th	Night	17% 6% 3 5% 4 3 3 4 4 7% 44%	1980
	21st to end	Night	16% 6% 3 6% 5% 3 4 4 4 7% 42%	2093
April	1st to 10th	Night	15% 6% 3 5% 4 3 5% 5% 5% 7% 41%	1800
	11th to 20th	Night	14% 6% 3 5% 5% 3 4 5% 5% 6% 46%	1775
	21st to end	Night	19% 5% 2 3 3 3 3 4 4 8% 45%	1800
May	1st to 10th	Night	16% 7% 3 5% 6% 3 5% 5% 5% 7% 39%	1713
	11th to 20th	Night	18% 7% 2 5% 5% 3 5% 5% 6% 7% 37%	1415
	21st to end	Night	13% 6% 3 6% 6% 2 5% 4 5% 10% 40%	1550
June	1st to 10th	Night	12% 5% 2 5% 5% 3 4 4 5% 10% 45%	1440
	11th to 20th	Night	10% 6% 3 6% 6% 4 5% 5% 6% 9% 41%	1440
	21st to end	Night	10% 5% 2 5% 5% 3 5% 5% 7% 11% 43%	1440
July	1st to 10th	Night	9% 5% 3 5% 6% 4 7% 5% 7% 9% 39%	1440
	11th to 20th	Night	6% 3 2 4 6% 3 7% 6% 7% 11% 43%	1440
	21st to end	Night	8% 4 3 5% 6% 4 5% 6% 7% 11% 41%	1721
August	1st to 10th	Night	8% 4 3 5% 5% 4 6% 7% 8% 10% 39%	1611
	11th to 20th	Night	9% 6% 3 6% 6% 4 6% 6% 5% 9% 40%	1723
	21st to end	Night	11% 5% 2 5% 5% 3 5% 5% 6% 9% 43%	1980
September	1st to 10th	Night	12% 6% 3 6% 4 3 3 4 6% 7% 45%	1800
	11th to 20th	Night	10% 6% 2 5% 4 2 3 4 5% 7% 52%	1800
	21st to end	Night	17% 6% 2 6% 6% 4 4 4 5% 6% 40%	2025
October	1st to 10th	Night	18% 7% 2 5% 5% 3 3 4 5% 5% 43%	2160
	11th to 20th	Night	16% 5% 2 3 5% 2 3 4 5% 7% 49%	2160
	21st to end	Night	17% 5% 1 4 5% 3 4 4 4 6% 48%	2376
November	1st to 10th	Night	13% 5% 2 5% 3 2 4 4 5% 6% 52%	2399
	11th to 20th	Night	16% 6% 3 4 3 3 3 4 5% 6% 48%	2470
	21st to end	Night	13% 5% 1 5% 5% 3 4 4 6% 7% 48%	2617
December	1st to 10th	Night	18% 6% 3 5% 5% 3 4 5% 5% 6% 41%	2660
	11th to 20th	Night	18% 4 3 5% 4 3 5% 4 5% 7% 43%	3285
	21st to end	Night	15% 7% 3 5% 5% 3 5% 6% 6% 8% 36%	4884

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	40% 4 12% 24% 16% 3	1518
	11th to 20th	Day	41% 4 14% 23% 14% 3	1590
	21st to end	Day	43% 4 9% 25% 15% 4	1782
February	1st to 10th	Day	43% 5% 14% 24% 12% 2	1620
	11th to 20th	Day	42% 4 9% 28% 14% 3	1683
	21st to end	Day	46% 4 10% 21% 15% 5%	1480
March	1st to 10th	Day	36% 4 12% 27% 19% 3	1863
	11th to 20th	Day	41% 4 11% 20% 19% 5%	1980
	21st to end	Day	45% 5% 8% 21% 18% 3	2263
April	1st to 10th	Day	38% 5% 12% 21% 18% 6%	2160
	11th to 20th	Day	35% 5% 11% 25% 18% 7%	2134
	21st to end	Day	39% 5% 13% 20% 20% 4	2160
May	1st to 10th	Day	40% 6% 12% 22% 17% 3	2259
	11th to 20th	Day	41% 7% 15% 21% 14% 3	2482
	21st to end	Day	37% 7% 16% 26% 13% 1	2725
June	1st to 10th	Day	34% 8% 16% 25% 15% 2	2520
	11th to 20th	Day	35% 9% 15% 24% 15% 2	2520
	21st to end	Day	29% 7% 17% 26% 19% 1	2520
July	1st to 10th	Day	33% 7% 16% 24% 16% 3	2450
	11th to 20th	Day	27% 6% 18% 30% 16% 3	2445
	21st to end	Day	34% 8% 16% 24% 16% 2	2448
August	1st to 10th	Day	34% 8% 16% 27% 13% 1	2314
	11th to 20th	Day	34% 7% 18% 23% 16% 2	2158
	21st to end	Day	35% 6% 14% 25% 15% 4	2376
September	1st to 10th	Day	36% 6% 14% 22% 18% 4	2160
	11th to 20th	Day	32% 5% 11% 27% 22% 4	2160
	21st to end	Day	43% 5% 9% 21% 19% 3	1932
October	1st to 10th	Day	37% 4 12% 26% 19% 2	1800
	11th to 20th	Day	35% 3 11% 26% 22% 3	1800
	21st to end	Day	38% 3 8% 29% 20% 2	1980
November	1st to 10th	Day	33% 4 12% 28% 20% 4	1676
	11th to 20th	Day	36% 4 10% 24% 22% 4	1710
	21st to end	Day	38% 4 10% 30% 15% 3	1558
December	1st to 10th	Day	41% 4 16% 24% 12% 2	1520
	11th to 20th	Day	42% 3 10% 31% 12% 2	1906
	21st to end	Day	44% 5% 15% 23% 12%	2772
January	1st to 10th	Night	41% 4 9% 24% 18% 4	2649
	11th to 20th	Night	44% 4 12% 24% 15% 1	2444
	21st to end	Night	40% 3 9% 27% 19% 2	2574
February	1st to 10th	Night	44% 4 13% 23% 14% 1	2340
	11th to 20th	Night	45% 3 9% 21% 18% 5%	2173
	21st to end	Night	44% 4 9% 21% 18% 5%	1776
March	1st to 10th	Night	36% 4 11% 27% 19% 3	2093
	11th to 20th	Night	40% 5% 11% 20% 19% 4	1980
	21st to end	Night	43% 4 9% 20% 18% 5%	2093
April	1st to 10th	Night	42% 5% 9% 20% 20% 5%	1800
	11th to 20th	Night	38% 4 11% 22% 19% 6%	1775
	21st to end	Night	40% 4 10% 22% 18% 6%	1800
May	1st to 10th	Night	44% 5% 12% 21% 15% 4	1713
	11th to 20th	Night	46% 6% 12% 16% 18% 3	1416
	21st to end	Night	41% 7% 16% 21% 12% 3	1551
June	1st to 10th	Night	36% 6% 14% 23% 19% 2	1440
	11th to 20th	Night	39% 7% 14% 24% 14% 2	1440
	21st to end	Night	36% 6% 13% 25% 17% 3	1440
July	1st to 10th	Night	40% 8% 18% 18% 13% 3	1400
	11th to 20th	Night	31% 10% 17% 25% 14% 3	1392
	21st to end	Night	34% 8% 15% 26% 17% 1	1625
August	1st to 10th	Night	37% 9% 15% 23% 15% 1	1600
	11th to 20th	Night	40% 9% 13% 22% 14% 2	1723
	21st to end	Night	38% 7% 13% 23% 15% 3	1980
September	1st to 10th	Night	39% 5% 11% 23% 17% 5%	1800
	11th to 20th	Night	33% 4 9% 27% 24% 4	1800
	21st to end	Night	44% 5% 8% 19% 20% 3	2025
October	1st to 10th	Night	43% 4 10% 22% 18% 3	2160
	11th to 20th	Night	36% 4 10% 29% 18% 3	2160
	21st to end	Night	39% 3 9% 26% 20% 3	2376
November	1st to 10th	Night	34% 4 10% 27% 19% 6%	2399
	11th to 20th	Night	38% 4 10% 23% 20% 6%	2470
	21st to end	Night	36% 4 12% 26% 19% 3	2617
December	1st to 10th	Night	44% 4 12% 24% 15% 1	2660
	11th to 20th	Night	41% 4 11% 22% 18% 4	3288
	21st to end	Night	44% 5% 14% 25% 11% 1	4884

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	19% 18% 35% 20% 6% 2	180
	11th to 20th	Day	17% 20% 32% 21% 9%	222
	21st to end	Day	17% 15% 42% 18% 6% 2	264
February	1st to 10th	Day	22% 9% 41% 19% 8% 1	240
	11th to 20th	Day	25% 15% 36% 22% 3	342
	21st to end	Day	26% 13% 34% 23% 4	346
March	1st to 10th	Day	14% 14% 35% 30% 6%	420
	11th to 20th	Day	20% 12% 33% 24% 8% 2	420
	21st to end	Day	20% 10% 30% 28% 9% 3	462
April	1st to 10th	Day	19% 9% 36% 25% 9% 3	420
	11th to 20th	Day	13% 12% 27% 26% 17% 5%	412
	21st to end	Day	21% 10% 24% 31% 13% 2	420
May	1st to 10th	Day	24% 9% 24% 25% 15% 3	423
	11th to 20th	Day	18% 6% 23% 29% 18% 5%	447
	21st to end	Day	19% 2 14% 24% 29% 13%	485
June	1st to 10th	Day	12% 4 16% 30% 26% 13%	450
	11th to 20th	Day	14% 4 17% 28% 27% 11%	450
	21st to end	Day	11% 2 15% 33% 26% 12%	450
July	1st to 10th	Day	11% 10% 29% 31% 19%	445
	11th to 20th	Day	4 1 6% 33% 37% 19%	444
	21st to end	Day	9% 2 11% 31% 36% 11%	488
August	1st to 10th	Day	11% 2 8% 28% 35% 15%	446
	11th to 20th	Day	11% 2 7% 29% 35% 16%	439
	21st to end	Day	9% 2 13% 31% 28% 17%	495
September	1st to 10th	Day	17% 2 14% 24% 24% 19%	450
	11th to 20th	Day	12% 3 14% 28% 28% 15%	450
	21st to end	Day	22% 5% 17% 30% 20% 6%	450
October	1st to 10th	Day	20% 4 19% 26% 20% 11%	450
	11th to 20th	Day	16% 6% 26% 26% 21% 5%	450
	21st to end	Day	18% 6% 29% 28% 15% 4	495
November	1st to 10th	Day	16% 7% 28% 25% 18% 5%	368
	11th to 20th	Day	12% 14% 32% 27% 13% 2	370
	21st to end	Day	13% 11% 32% 27% 13% 4	218
December	1st to 10th	Day	20% 9% 35% 27% 8% 1	180
	11th to 20th	Day	21% 16% 29% 24% 10%	180
	21st to end	Day	21% 9% 31% 27% 10% 2	198
January	1st to 10th	Night	19% 11% 42% 24% 4	760
	11th to 20th	Night	22% 13% 31% 29% 6%	736
	21st to end	Night	18% 12% 38% 29% 2	792
February	1st to 10th	Night	21% 10% 36% 30% 3	720
	11th to 20th	Night	23% 14% 41% 22% 1	703
	21st to end	Night	23% 13% 36% 25% 3	592
March	1st to 10th	Night	17% 12% 38% 27% 6%	720
	11th to 20th	Night	19% 11% 36% 26% 7% 1	720
	21st to end	Night	20% 6% 36% 30% 7% 1	792
April	1st to 10th	Night	19% 10% 38% 26% 7%	720
	11th to 20th	Night	16% 8% 30% 34% 12%	711
	21st to end	Night	20% 8% 29% 33% 10%	720
May	1st to 10th	Night	20% 7% 25% 35% 12% 1	633
	11th to 20th	Night	19% 7% 23% 37% 14%	354
	21st to end	Night	14% 3 14% 41% 25% 2	389
June	1st to 10th	Night	13% 2 19% 42% 23% 2	360
	11th to 20th	Night	12% 2 18% 42% 25% 2	360
	21st to end	Night	11% 4 15% 46% 23% 1	360
July	1st to 10th	Night	8% 3 15% 41% 30% 4	350
	11th to 20th	Night	6% 1 9% 42% 39% 3	348
	21st to end	Night	7% 2 11% 46% 31% 4	541
August	1st to 10th	Night	9% 3 10% 40% 34% 4	537
	11th to 20th	Night	11% 3 11% 34% 34% 6%	659
	21st to end	Night	12% 3 11% 35% 33% 6%	792
September	1st to 10th	Night	15% 1 13% 34% 30% 6%	720
	11th to 20th	Night	12% 3 12% 36% 32% 6%	720
	21st to end	Night	19% 4 18% 33% 24% 2	720
October	1st to 10th	Night	21% 5% 19% 30% 21% 5%	720
	11th to 20th	Night	18% 6% 25% 34% 16% 1	720
	21st to end	Night	19% 7% 30% 30% 13% 1	792
November	1st to 10th	Night	16% 5% 31% 33% 14% 2	740
	11th to 20th	Night	19% 12% 28% 31% 10% 1	760
	21st to end	Night	16% 13% 31% 30% 10% 1	760
December	1st to 10th	Night	23% 10% 34% 24% 9% 1	760
	11th to 20th	Night	17% 9% 37% 30% 6%	760
	21st to end	Night	19% 8% 35% 34% 4	836

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