Ellen Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Ellen Observatory clear sky chart from 2005-11-06 to 2024-01-16.

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	9% 6% 3 6% 7% 4 6% 6% 7% 9% 37%	74924
	Night	9% 5% 3 5% 5% 3 5% 5% 6% 9% 44%	74891
Transparency	Day	41% 10% 14% 17% 13% 5%	74647
	Night	36% 10% 16% 20% 14% 5%	74687
Seeing	Day	9% 7% 22% 26% 21% 16%	14461
	Night	9% 5% 21% 32% 26% 7%	24595

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	12% 6% 2 5% 4 3 6% 6% 6% 7% 44%	1706
	11th to 20th	Day	8% 5% 3 5% 5% 3 4 6% 5% 8% 47%	1671
	21st to end	Day	12% 6% 3 6% 5% 3 5% 4 6% 8% 43%	1794
February	1st to 10th	Day	13% 7% 3 6% 5% 4 4 6% 5% 9% 40%	1800
	11th to 20th	Day	7% 5% 2 5% 5% 3 4 5% 5% 7% 52%	1748
	21st to end	Day	10% 5% 3 5% 6% 4 5% 5% 6% 7% 44%	1594
March	1st to 10th	Day	5% 3 2 5% 6% 3 5% 4 5% 10% 52%	1980
	11th to 20th	Day	9% 4 2 5% 6% 4 5% 5% 5% 8% 47%	1980
	21st to end	Day	9% 4 3 5% 6% 4 5% 4 5% 7% 49%	2178
April	1st to 10th	Day	7% 4 2 5% 5% 4 6% 5% 6% 9% 48%	1980
	11th to 20th	Day	9% 6% 2 6% 6% 4 6% 7% 8% 10% 37%	2018
	21st to end	Day	6% 4 2 5% 6% 3 5% 5% 7% 9% 50%	2160
May	1st to 10th	Day	7% 3 2 4 5% 2 5% 5% 6% 9% 52%	2277
	11th to 20th	Day	10% 5% 2 5% 7% 3 7% 6% 7% 9% 39%	2301
	21st to end	Day	8% 6% 2 6% 7% 5% 7% 7% 7% 10% 34%	2531
June	1st to 10th	Day	11% 8% 3 8% 8% 6% 8% 7% 8% 10% 23%	2340
	11th to 20th	Day	6% 5% 3 9% 9% 6% 9% 8% 7% 9% 28%	2340
	21st to end	Day	9% 6% 4 8% 8% 5% 9% 7% 7% 10% 28%	2340
July	1st to 10th	Day	10% 7% 3 7% 9% 6% 9% 8% 9% 9% 23%	2340
	11th to 20th	Day	8% 7% 4 9% 9% 6% 8% 8% 7% 11% 23%	2340
	21st to end	Day	8% 7% 4 9% 8% 7% 7% 8% 8% 10% 25%	2547
August	1st to 10th	Day	6% 6% 4 8% 10% 6% 8% 9% 9% 12% 24%	2327
	11th to 20th	Day	6% 5% 4 7% 10% 5% 8% 9% 10% 12% 24%	2241
	21st to end	Day	11% 7% 5% 8% 9% 6% 7% 7% 8% 10% 22%	2187
September	1st to 10th	Day	8% 7% 3 8% 9% 6% 8% 8% 10% 10% 22%	1980
	11th to 20th	Day	11% 6% 3 8% 10% 6% 8% 8% 9% 10% 21%	1980
	21st to end	Day	10% 7% 3 9% 7% 5% 6% 7% 7% 9% 28%	1980
October	1st to 10th	Day	12% 7% 3 7% 6% 5% 7% 6% 6% 8% 33%	1935
	11th to 20th	Day	8% 4 3 5% 5% 3 6% 6% 7% 8% 44%	1800
	21st to end	Day	8% 5% 2 5% 5% 3 5% 6% 6% 7% 47%	1980
November	1st to 10th	Day	8% 5% 4 6% 6% 4 6% 5% 6% 8% 43%	1857
	11th to 20th	Day	10% 7% 3 7% 6% 3 6% 6% 5% 8% 40%	1900
	21st to end	Day	12% 6% 3 6% 5% 3 5% 5% 5% 8% 41%	1805
December	1st to 10th	Day	8% 5% 3 5% 5% 3 4 7% 7% 9% 45%	1710
	11th to 20th	Day	12% 6% 3 6% 6% 3 5% 5% 6% 7% 39%	2153
	21st to end	Day	12% 8% 3 7% 6% 4 5% 5% 4 7% 39%	3124
January	1st to 10th	Night	12% 6% 2 4 5% 3 4 4 5% 8% 46%	2459
	11th to 20th	Night	9% 5% 3 4 4 3 4 5% 6% 9% 48%	2417
	21st to end	Night	13% 6% 4 6% 5% 2 4 4 6% 6% 43%	2559
February	1st to 10th	Night	12% 6% 2 5% 5% 4 4 4 6% 7% 45%	2160
	11th to 20th	Night	10% 4 2 4 5% 3 4 5% 5% 7% 51%	2098
	21st to end	Night	7% 5% 2 5% 5% 3 4 4 4 9% 51%	1660
March	1st to 10th	Night	4 4 2 4 4 2 4 5% 5% 7% 60%	1980
	11th to 20th	Night	9% 4 3 4 4 2 4 5% 6% 8% 50%	1980
	21st to end	Night	8% 4 2 4 5% 3 4 5% 5% 7% 53%	2178
April	1st to 10th	Night	8% 4 2 4 4 2 4 5% 6% 9% 55%	1980
	11th to 20th	Night	8% 6% 2 5% 5% 3 4 4 6% 6% 49%	1888
	21st to end	Night	6% 3 2 4 4 3 5% 4 5% 9% 55%	1800
May	1st to 10th	Night	5% 2 1 3 4 2 5% 4 5% 8% 61%	1695
	11th to 20th	Night	6% 4 2 6% 5% 4 4 6% 7% 10% 47%	1592
	21st to end	Night	7% 4 2 5% 6% 4 6% 7% 7% 9% 42%	1744
June	1st to 10th	Night	11% 6% 3 5% 6% 4 6% 6% 8% 11% 33%	1620
	11th to 20th	Night	5% 5% 3 7% 7% 4 6% 7% 9% 12% 36%	1620
	21st to end	Night	7% 6% 3 8% 7% 4 7% 6% 9% 10% 34%	1620
July	1st to 10th	Night	8% 5% 3 7% 6% 5% 7% 7% 8% 11% 33%	1620
	11th to 20th	Night	6% 6% 3 7% 6% 5% 7% 8% 9% 10% 32%	1620
	21st to end	Night	5% 5% 3 5% 8% 5% 7% 8% 7% 12% 34%	1771
August	1st to 10th	Night	5% 5% 2 7% 7% 4 7% 7% 9% 13% 35%	1611
	11th to 20th	Night	6% 4 3 6% 7% 4 7% 7% 9% 13% 34%	1640
	21st to end	Night	10% 6% 3 7% 8% 4 6% 6% 7% 11% 30%	2164
September	1st to 10th	Night	8% 5% 3 7% 8% 5% 6% 8% 8% 12% 31%	1980
	11th to 20th	Night	10% 7% 4 7% 7% 4 7% 7% 8% 10% 28%	1980
	21st to end	Night	11% 6% 2 7% 6% 4 7% 7% 6% 9% 35%	1980
October	1st to 10th	Night	8% 6% 3 6% 6% 4 6% 6% 7% 9% 39%	2024
	11th to 20th	Night	5% 4 2 5% 4 3 5% 5% 6% 9% 52%	2160
	21st to end	Night	9% 5% 2 4 4 3 5% 5% 5% 8% 51%	2376
November	1st to 10th	Night	10% 5% 2 6% 5% 2 5% 6% 6% 8% 47%	2221
	11th to 20th	Night	9% 6% 2 6% 6% 3 6% 4 5% 8% 46%	2280
	21st to end	Night	10% 6% 2 6% 5% 4 5% 5% 6% 8% 42%	2370
December	1st to 10th	Night	8% 4 2 5% 5% 4 5% 5% 6% 9% 48%	2470
	11th to 20th	Night	13% 7% 2 5% 4 3 4 3 4 6% 48%	3042
	21st to end	Night	14% 7% 3 6% 7% 3 4 5% 6% 7% 37%	4532

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	36% 7% 10% 18% 18% 10%	1707
	11th to 20th	Day	34% 5% 8% 23% 22% 8%	1671
	21st to end	Day	39% 6% 10% 16% 19% 10%	1794
February	1st to 10th	Day	41% 7% 10% 13% 20% 9%	1800
	11th to 20th	Day	31% 6% 9% 20% 23% 11%	1758
	21st to end	Day	38% 6% 12% 15% 21% 9%	1594
March	1st to 10th	Day	28% 6% 11% 23% 23% 9%	1980
	11th to 20th	Day	34% 7% 13% 22% 15% 9%	1980
	21st to end	Day	33% 7% 12% 20% 20% 9%	2178
April	1st to 10th	Day	31% 8% 14% 21% 18% 8%	1980
	11th to 20th	Day	39% 10% 10% 19% 17% 5%	2018
	21st to end	Day	29% 6% 13% 22% 21% 8%	2160
May	1st to 10th	Day	27% 8% 11% 21% 24% 9%	2277
	11th to 20th	Day	37% 9% 18% 20% 13% 3	2305
	21st to end	Day	42% 10% 14% 18% 14% 1	2531
June	1st to 10th	Day	51% 13% 16% 15% 5%	2340
	11th to 20th	Day	48% 13% 20% 18% 2	2340
	21st to end	Day	48% 14% 20% 14% 4	2340
July	1st to 10th	Day	52% 18% 17% 12% 2	2275
	11th to 20th	Day	52% 17% 19% 12%	2271
	21st to end	Day	48% 17% 20% 13% 2	2405
August	1st to 10th	Day	47% 18% 24% 10% 1	2314
	11th to 20th	Day	47% 18% 22% 11% 1	2241
	21st to end	Day	53% 14% 19% 12% 2	2187
September	1st to 10th	Day	50% 18% 17% 10% 4	1980
	11th to 20th	Day	52% 16% 18% 12% 3	1980
	21st to end	Day	48% 12% 14% 14% 10% 3	1980
October	1st to 10th	Day	46% 10% 11% 13% 13% 7%	1935
	11th to 20th	Day	34% 7% 13% 21% 20% 5%	1800
	21st to end	Day	33% 8% 11% 16% 22% 10%	1980
November	1st to 10th	Day	37% 8% 11% 16% 19% 8%	1857
	11th to 20th	Day	41% 8% 9% 17% 19% 6%	1900
	21st to end	Day	39% 7% 10% 19% 19% 5%	1805
December	1st to 10th	Day	34% 7% 15% 24% 15% 5%	1710
	11th to 20th	Day	41% 7% 11% 14% 18% 9%	2150
	21st to end	Day	44% 6% 11% 17% 15% 7%	3124
January	1st to 10th	Night	37% 7% 10% 18% 19% 10%	2459
	11th to 20th	Night	33% 6% 11% 22% 20% 8%	2417
	21st to end	Night	41% 5% 9% 17% 19% 9%	2559
February	1st to 10th	Night	38% 6% 11% 17% 15% 13%	2160
	11th to 20th	Night	30% 8% 11% 19% 20% 11%	2099
	21st to end	Night	31% 6% 15% 19% 22% 7%	1660
March	1st to 10th	Night	23% 6% 14% 27% 23% 8%	1980
	11th to 20th	Night	30% 7% 15% 24% 15% 8%	1980
	21st to end	Night	29% 7% 12% 26% 18% 9%	2178
April	1st to 10th	Night	27% 7% 19% 23% 16% 9%	1980
	11th to 20th	Night	33% 7% 12% 25% 17% 6%	1888
	21st to end	Night	27% 6% 14% 27% 20% 7%	1800
May	1st to 10th	Night	23% 6% 15% 26% 23% 8%	1695
	11th to 20th	Night	31% 11% 20% 24% 12% 2	1593
	21st to end	Night	35% 10% 16% 21% 17% 1	1745
June	1st to 10th	Night	40% 13% 21% 21% 4	1620
	11th to 20th	Night	36% 15% 23% 23% 3	1620
	21st to end	Night	40% 15% 23% 15% 6% 1	1620
July	1st to 10th	Night	43% 17% 25% 13% 3	1575
	11th to 20th	Night	42% 16% 28% 12% 2	1566
	21st to end	Night	38% 19% 26% 16% 1	1672
August	1st to 10th	Night	37% 17% 34% 12% 1	1600
	11th to 20th	Night	35% 17% 31% 14% 2	1640
	21st to end	Night	46% 16% 22% 14% 2	2164
September	1st to 10th	Night	42% 19% 20% 14% 5%	1980
	11th to 20th	Night	46% 14% 21% 15% 4	1980
	21st to end	Night	42% 12% 17% 17% 9% 2	1980
October	1st to 10th	Night	38% 12% 13% 19% 12% 6%	2024
	11th to 20th	Night	29% 7% 11% 27% 21% 5%	2160
	21st to end	Night	31% 7% 13% 18% 21% 10%	2376
November	1st to 10th	Night	34% 7% 13% 19% 18% 9%	2221
	11th to 20th	Night	36% 8% 11% 19% 19% 8%	2280
	21st to end	Night	37% 7% 13% 19% 18% 6%	2370
December	1st to 10th	Night	32% 7% 16% 27% 12% 6%	2470
	11th to 20th	Night	38% 5% 10% 19% 18% 9%	3044
	21st to end	Night	45% 7% 12% 18% 14% 4	4532

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	14% 13% 48% 19% 5% 1	270
	11th to 20th	Day	10% 13% 39% 25% 9% 4	257
	21st to end	Day	18% 20% 43% 16% 3	264
February	1st to 10th	Day	13% 18% 38% 19% 11% 2	240
	11th to 20th	Day	11% 11% 40% 26% 10% 2	240
	21st to end	Day	11% 13% 34% 26% 15% 1	312
March	1st to 10th	Day	4 17% 34% 32% 11% 2	420
	11th to 20th	Day	10% 14% 32% 25% 17% 3	420
	21st to end	Day	8% 12% 31% 30% 17% 3	462
April	1st to 10th	Day	8% 15% 33% 26% 15% 4	420
	11th to 20th	Day	9% 12% 31% 25% 17% 5%	412
	21st to end	Day	7% 7% 29% 29% 17% 12%	420
May	1st to 10th	Day	8% 6% 23% 31% 20% 12%	423
	11th to 20th	Day	10% 3 21% 30% 22% 15%	447
	21st to end	Day	7% 3 13% 24% 33% 20%	485
June	1st to 10th	Day	13% 2 17% 27% 22% 19%	450
	11th to 20th	Day	7% 2 11% 27% 26% 27%	450
	21st to end	Day	4 3 9% 19% 31% 34%	450
July	1st to 10th	Day	6% 3 5% 13% 31% 42%	445
	11th to 20th	Day	5% 1 4 18% 33% 40%	444
	21st to end	Day	8% 2 7% 19% 29% 35%	488
August	1st to 10th	Day	6% 2 7% 20% 31% 35%	446
	11th to 20th	Day	4 1 3 16% 29% 46%	439
	21st to end	Day	10% 1 9% 22% 27% 31%	495
September	1st to 10th	Day	8% 3 8% 18% 31% 31%	450
	11th to 20th	Day	9% 4 9% 29% 26% 23%	450
	21st to end	Day	8% 2 11% 29% 26% 24%	450
October	1st to 10th	Day	14% 6% 23% 29% 20% 8%	450
	11th to 20th	Day	6% 6% 18% 37% 26% 8%	450
	21st to end	Day	8% 5% 25% 34% 22% 5%	495
November	1st to 10th	Day	10% 7% 26% 30% 23% 4	455
	11th to 20th	Day	12% 11% 27% 32% 14% 4	460
	21st to end	Day	12% 8% 32% 34% 13% 2	365
December	1st to 10th	Day	6% 14% 38% 29% 10% 3	270
	11th to 20th	Day	12% 13% 39% 23% 11% 1	270
	21st to end	Day	10% 12% 31% 36% 9% 1	297
January	1st to 10th	Night	13% 10% 33% 32% 12% 1	760
	11th to 20th	Night	10% 9% 30% 36% 14% 1	744
	21st to end	Night	15% 10% 36% 31% 7%	792
February	1st to 10th	Night	14% 7% 33% 32% 14% 1	720
	11th to 20th	Night	11% 8% 33% 37% 10% 1	703
	21st to end	Night	9% 9% 34% 40% 7%	592
March	1st to 10th	Night	4 9% 37% 38% 12% 1	720
	11th to 20th	Night	9% 8% 30% 38% 15% 1	720
	21st to end	Night	7% 9% 34% 38% 12% 1	792
April	1st to 10th	Night	8% 8% 33% 37% 13% 1	720
	11th to 20th	Night	7% 8% 32% 34% 17%	711
	21st to end	Night	7% 4 26% 44% 17% 1	720
May	1st to 10th	Night	6% 3 20% 47% 23% 1	615
	11th to 20th	Night	7% 1 16% 39% 33% 3	531
	21st to end	Night	8% 3 9% 34% 41% 6%	584
June	1st to 10th	Night	10% 3 12% 29% 39% 7%	540
	11th to 20th	Night	7% 2 6% 25% 41% 19%	540
	21st to end	Night	8% 2 7% 21% 44% 18%	540
July	1st to 10th	Night	9% 2 6% 15% 44% 24%	525
	11th to 20th	Night	6% 1 2 15% 48% 27%	522
	21st to end	Night	5% 1 5% 19% 49% 22%	591
August	1st to 10th	Night	5% 1 4 19% 50% 22%	537
	11th to 20th	Night	4 1 4 15% 50% 27%	576
	21st to end	Night	12% 2 8% 21% 39% 18%	792
September	1st to 10th	Night	9% 2 7% 22% 40% 20%	720
	11th to 20th	Night	12% 2 10% 27% 38% 11%	720
	21st to end	Night	11% 2 9% 26% 40% 11%	720
October	1st to 10th	Night	12% 3 17% 36% 28% 5%	720
	11th to 20th	Night	6% 3 14% 38% 35% 4	720
	21st to end	Night	10% 4 17% 38% 28% 3	792
November	1st to 10th	Night	10% 4 24% 37% 24% 2	740
	11th to 20th	Night	9% 7% 23% 40% 19% 2	760
	21st to end	Night	10% 5% 29% 39% 16% 1	760
December	1st to 10th	Night	9% 5% 28% 35% 22% 2	760
	11th to 20th	Night	13% 9% 25% 36% 17% 1	760
	21st to end	Night	12% 7% 27% 37% 17% 1	836

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