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 2026-04-17.

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	8% 6% 2 7% 5% 6% 6% 5% 7% 10% 38%	131541
	Night	9% 6% 2 6% 5% 5% 5% 4 6% 9% 44%	130639
Transparency	Day	38% 9% 16% 19% 13% 5%	135237
	Night	36% 8% 16% 20% 13% 6%	135014
Seeing	Day	9% 7% 22% 26% 21% 15%	17532
	Night	9% 5% 21% 33% 25% 6%	27670

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	7% 5% 2 5% 3 4 4 3 5% 7% 54%	5258
	11th to 20th	Day	18% 9% 3 7% 5% 4 5% 4 5% 6% 33%	5280
	21st to end	Day	11% 6% 2 6% 4 4 4 3 5% 7% 47%	5820
February	1st to 10th	Day	12% 7% 2 6% 4 4 4 4 5% 9% 42%	2100
	11th to 20th	Day	8% 6% 2 6% 5% 4 4 3 5% 7% 50%	2048
	21st to end	Day	10% 6% 2 5% 5% 4 5% 4 6% 6% 47%	1864
March	1st to 10th	Day	7% 6% 2 8% 5% 5% 5% 4 6% 10% 41%	5045
	11th to 20th	Day	5% 3 2 5% 3 3 4 3 5% 8% 59%	5090
	21st to end	Day	9% 6% 2 6% 4 4 5% 4 7% 9% 45%	5586
April	1st to 10th	Day	7% 4 2 5% 4 5% 5% 4 6% 9% 48%	2310
	11th to 20th	Day	8% 5% 2 6% 5% 4 5% 5% 8% 8% 43%	2312
	21st to end	Day	5% 4 1 5% 5% 5% 5% 4 7% 9% 49%	2400
May	1st to 10th	Day	4 3 2 5% 5% 6% 6% 6% 9% 11% 44%	5717
	11th to 20th	Day	9% 5% 2 6% 5% 5% 5% 4 7% 9% 43%	5861
	21st to end	Day	4 4 1 5% 5% 5% 6% 5% 8% 12% 44%	6433
June	1st to 10th	Day	11% 8% 3 9% 7% 8% 8% 5% 8% 10% 24%	2600
	11th to 20th	Day	6% 6% 3 10% 8% 9% 8% 6% 7% 10% 27%	2600
	21st to end	Day	9% 7% 3 9% 8% 8% 7% 6% 7% 9% 27%	2600
July	1st to 10th	Day	7% 6% 3 9% 7% 8% 9% 7% 12% 13% 19%	5900
	11th to 20th	Day	4 4 2 8% 7% 8% 7% 7% 10% 14% 28%	5900
	21st to end	Day	5% 5% 2 8% 6% 6% 8% 6% 9% 12% 32%	6424
August	1st to 10th	Day	6% 7% 3 10% 7% 7% 8% 7% 9% 12% 25%	4234
	11th to 20th	Day	4 4 2 7% 6% 7% 8% 7% 10% 13% 33%	4052
	21st to end	Day	8% 7% 3 9% 7% 8% 8% 7% 9% 12% 22%	3958
September	1st to 10th	Day	9% 7% 3 10% 8% 8% 8% 7% 10% 9% 20%	2200
	11th to 20th	Day	11% 7% 3 11% 8% 8% 8% 5% 9% 10% 20%	2200
	21st to end	Day	10% 7% 3 11% 6% 7% 6% 6% 7% 9% 28%	2200
October	1st to 10th	Day	13% 7% 3 8% 5% 7% 7% 5% 6% 8% 31%	2149
	11th to 20th	Day	8% 5% 3 6% 4 5% 5% 5% 7% 8% 45%	2000
	21st to end	Day	8% 5% 2 5% 4 5% 5% 5% 6% 7% 48%	2200
November	1st to 10th	Day	7% 5% 3 6% 6% 5% 5% 4 7% 8% 43%	2057
	11th to 20th	Day	9% 6% 2 7% 5% 4 6% 5% 5% 7% 43%	2100
	21st to end	Day	11% 6% 3 6% 4 4 5% 4 5% 8% 43%	1995
December	1st to 10th	Day	6% 4 2 4 3 4 4 4 6% 8% 55%	3020
	11th to 20th	Day	10% 7% 3 8% 5% 5% 6% 5% 7% 8% 36%	3463
	21st to end	Day	10% 8% 3 7% 5% 5% 5% 4 5% 8% 41%	4565
January	1st to 10th	Night	8% 5% 2 4 4 3 3 3 4 7% 57%	7635
	11th to 20th	Night	17% 9% 3 7% 4 5% 4 4 5% 6% 35%	7680
	21st to end	Night	12% 6% 2 6% 4 4 4 3 6% 8% 45%	8344
February	1st to 10th	Night	11% 7% 2 5% 4 4 4 3 5% 7% 47%	2520
	11th to 20th	Night	11% 5% 2 5% 5% 4 4 4 5% 7% 48%	2458
	21st to end	Night	7% 5% 2 6% 4 4 4 3 5% 8% 53%	1940
March	1st to 10th	Night	12% 8% 2 7% 4 3 4 3 5% 6% 45%	5028
	11th to 20th	Night	5% 4 2 4 3 3 4 3 6% 9% 55%	5090
	21st to end	Night	8% 6% 2 6% 4 4 4 4 6% 8% 49%	5588
April	1st to 10th	Night	8% 4 2 4 4 4 4 4 7% 8% 52%	2310
	11th to 20th	Night	7% 6% 1 6% 4 4 3 4 5% 7% 53%	2167
	21st to end	Night	5% 4 1 5% 4 4 4 4 6% 9% 54%	2000
May	1st to 10th	Night	4 4 2 6% 5% 5% 5% 4 6% 10% 51%	4217
	11th to 20th	Night	8% 6% 2 6% 4 4 4 4 7% 9% 47%	4032
	21st to end	Night	4 4 2 5% 4 4 4 4 7% 11% 50%	4421
June	1st to 10th	Night	10% 6% 3 6% 4 6% 5% 5% 8% 11% 34%	1800
	11th to 20th	Night	5% 6% 2 9% 7% 6% 6% 6% 9% 11% 34%	1800
	21st to end	Night	7% 7% 3 8% 7% 5% 7% 5% 8% 10% 34%	1800
July	1st to 10th	Night	7% 8% 3 10% 6% 7% 7% 6% 10% 12% 24%	4058
	11th to 20th	Night	3 4 2 7% 5% 6% 7% 7% 10% 13% 33%	4057
	21st to end	Night	5% 7% 3 7% 6% 6% 6% 6% 8% 12% 35%	4427
August	1st to 10th	Night	5% 6% 2 9% 6% 6% 7% 4 9% 12% 34%	2915
	11th to 20th	Night	4 4 2 6% 5% 6% 6% 5% 9% 14% 39%	2962
	21st to end	Night	7% 6% 2 8% 6% 5% 6% 6% 8% 12% 33%	3934
September	1st to 10th	Night	8% 6% 3 8% 7% 6% 7% 6% 8% 11% 29%	2200
	11th to 20th	Night	10% 8% 3 9% 6% 7% 6% 5% 9% 10% 28%	2200
	21st to end	Night	10% 6% 2 7% 5% 6% 6% 5% 6% 9% 37%	2200
October	1st to 10th	Night	10% 6% 3 8% 5% 6% 5% 4 7% 9% 38%	2250
	11th to 20th	Night	6% 4 2 6% 4 4 5% 4 6% 8% 52%	2400
	21st to end	Night	9% 5% 2 5% 3 4 4 4 5% 8% 51%	2640
November	1st to 10th	Night	9% 5% 2 7% 4 4 5% 4 6% 7% 46%	2461
	11th to 20th	Night	8% 6% 2 6% 5% 4 5% 4 5% 7% 48%	2520
	21st to end	Night	9% 6% 2 7% 4 5% 4 5% 6% 8% 44%	2620
December	1st to 10th	Night	6% 3 1 5% 4 4 4 4 5% 7% 56%	4380
	11th to 20th	Night	11% 7% 2 6% 5% 4 4 3 5% 7% 45%	4952
	21st to end	Night	12% 8% 3 7% 5% 4 4 4 6% 6% 40%	6633

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	30% 4 12% 19% 24% 11%	5259
	11th to 20th	Day	52% 3 6% 14% 14% 10%	5280
	21st to end	Day	36% 4 10% 25% 15% 11%	5820
February	1st to 10th	Day	38% 7% 9% 14% 21% 11%	2100
	11th to 20th	Day	32% 6% 10% 21% 22% 10%	2058
	21st to end	Day	36% 5% 11% 17% 22% 9%	1864
March	1st to 10th	Day	37% 7% 13% 18% 20% 6%	5045
	11th to 20th	Day	24% 5% 10% 22% 23% 17%	5090
	21st to end	Day	34% 5% 10% 27% 16% 8%	5586
April	1st to 10th	Day	31% 8% 13% 22% 18% 8%	2310
	11th to 20th	Day	35% 9% 9% 18% 21% 8%	2312
	21st to end	Day	28% 6% 14% 23% 22% 8%	2400
May	1st to 10th	Day	27% 9% 23% 25% 13% 4	5750
	11th to 20th	Day	34% 8% 16% 31% 10% 1	5865
	21st to end	Day	29% 8% 18% 32% 12% 1	6447
June	1st to 10th	Day	49% 13% 17% 16% 6%	2600
	11th to 20th	Day	48% 13% 19% 18% 2	2600
	21st to end	Day	49% 13% 21% 14% 3	2600
July	1st to 10th	Day	49% 14% 26% 10% 1	5835
	11th to 20th	Day	41% 13% 34% 12%	5831
	21st to end	Day	38% 13% 27% 20% 1	6240
August	1st to 10th	Day	47% 13% 29% 10% 2	4221
	11th to 20th	Day	37% 13% 25% 20% 5%	4091
	21st to end	Day	49% 13% 23% 14% 1	3958
September	1st to 10th	Day	53% 17% 17% 9% 4	2200
	11th to 20th	Day	52% 15% 17% 12% 3	2200
	21st to end	Day	48% 11% 15% 14% 9% 3	2200
October	1st to 10th	Day	55% 8% 14% 11% 7% 4	3500
	11th to 20th	Day	29% 6% 9% 22% 28% 7%	3240
	21st to end	Day	26% 6% 7% 22% 32% 7%	3538
November	1st to 10th	Day	37% 7% 11% 17% 19% 8%	2057
	11th to 20th	Day	38% 7% 8% 19% 20% 8%	2100
	21st to end	Day	37% 7% 10% 20% 19% 7%	1995
December	1st to 10th	Day	27% 4 16% 26% 18% 9%	3020
	11th to 20th	Day	42% 7% 11% 16% 18% 7%	3460
	21st to end	Day	41% 5% 12% 20% 17% 5%	4565
January	1st to 10th	Night	28% 3 11% 22% 24% 12%	7636
	11th to 20th	Night	50% 4 8% 15% 14% 9%	7680
	21st to end	Night	37% 5% 12% 22% 13% 11%	8344
February	1st to 10th	Night	36% 6% 10% 17% 18% 14%	2520
	11th to 20th	Night	33% 8% 11% 20% 19% 10%	2459
	21st to end	Night	30% 5% 14% 19% 24% 9%	1940
March	1st to 10th	Night	39% 6% 11% 17% 19% 7%	5028
	11th to 20th	Night	23% 7% 14% 23% 17% 16%	5090
	21st to end	Night	33% 6% 12% 23% 20% 6%	5588
April	1st to 10th	Night	29% 7% 18% 23% 15% 8%	2310
	11th to 20th	Night	30% 6% 11% 25% 20% 8%	2167
	21st to end	Night	26% 6% 14% 27% 19% 7%	2000
May	1st to 10th	Night	28% 8% 20% 29% 12% 3	4247
	11th to 20th	Night	31% 9% 21% 28% 11% 1	4033
	21st to end	Night	26% 8% 20% 31% 14% 1	4429
June	1st to 10th	Night	39% 13% 22% 21% 5%	1800
	11th to 20th	Night	39% 15% 22% 22% 2	1800
	21st to end	Night	41% 15% 23% 16% 5% 1	1800
July	1st to 10th	Night	48% 13% 27% 10% 1	4015
	11th to 20th	Night	37% 15% 37% 10% 1	4006
	21st to end	Night	38% 12% 26% 22% 1	4297
August	1st to 10th	Night	41% 12% 35% 11% 2	2904
	11th to 20th	Night	32% 12% 31% 20% 6%	2989
	21st to end	Night	41% 14% 29% 15% 2	3934
September	1st to 10th	Night	45% 18% 19% 13% 5%	2200
	11th to 20th	Night	47% 13% 20% 16% 4	2200
	21st to end	Night	41% 11% 18% 20% 9% 2	2200
October	1st to 10th	Night	53% 10% 13% 14% 7% 3	3679
	11th to 20th	Night	28% 6% 8% 25% 28% 5%	3899
	21st to end	Night	28% 6% 9% 22% 29% 6%	4252
November	1st to 10th	Night	34% 7% 13% 19% 18% 9%	2461
	11th to 20th	Night	34% 7% 11% 19% 20% 9%	2520
	21st to end	Night	36% 7% 12% 20% 18% 7%	2620
December	1st to 10th	Night	27% 5% 16% 28% 11% 13%	4380
	11th to 20th	Night	38% 5% 13% 21% 14% 9%	4954
	21st to end	Night	42% 6% 12% 21% 14% 5%	6633

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	13% 13% 47% 19% 7% 2	330
	11th to 20th	Day	12% 16% 38% 24% 8% 3	330
	21st to end	Day	17% 21% 44% 15% 3	363
February	1st to 10th	Day	13% 15% 36% 23% 11% 2	330
	11th to 20th	Day	14% 12% 41% 25% 7% 2	330
	21st to end	Day	12% 14% 34% 26% 13% 1	407
March	1st to 10th	Day	6% 17% 32% 32% 12% 1	540
	11th to 20th	Day	9% 14% 35% 26% 14% 2	540
	21st to end	Day	8% 12% 32% 29% 17% 3	594
April	1st to 10th	Day	9% 13% 32% 28% 14% 4	540
	11th to 20th	Day	8% 11% 31% 26% 18% 6%	516
	21st to end	Day	6% 7% 26% 30% 18% 13%	500
May	1st to 10th	Day	8% 5% 23% 31% 20% 12%	503
	11th to 20th	Day	9% 4 23% 30% 20% 13%	527
	21st to end	Day	7% 3 14% 26% 32% 19%	573
June	1st to 10th	Day	12% 2 16% 27% 25% 18%	530
	11th to 20th	Day	7% 3 12% 26% 24% 28%	530
	21st to end	Day	5% 3 9% 20% 32% 32%	530
July	1st to 10th	Day	6% 3 5% 14% 32% 40%	525
	11th to 20th	Day	5% 1 3 18% 32% 41%	524
	21st to end	Day	7% 2 6% 18% 28% 38%	576
August	1st to 10th	Day	6% 2 8% 19% 30% 35%	526
	11th to 20th	Day	4 1 4 17% 28% 46%	519
	21st to end	Day	10% 2 8% 22% 26% 31%	583
September	1st to 10th	Day	9% 3 9% 18% 31% 29%	530
	11th to 20th	Day	9% 5% 10% 30% 26% 20%	530
	21st to end	Day	8% 3 13% 27% 26% 23%	530
October	1st to 10th	Day	15% 5% 23% 30% 19% 8%	530
	11th to 20th	Day	6% 6% 21% 37% 24% 7%	530
	21st to end	Day	8% 5% 26% 36% 21% 4	583
November	1st to 10th	Day	9% 8% 26% 31% 24% 3	535
	11th to 20th	Day	11% 10% 26% 33% 16% 5%	540
	21st to end	Day	10% 8% 32% 32% 14% 3	435
December	1st to 10th	Day	8% 13% 41% 27% 9% 2	330
	11th to 20th	Day	11% 13% 39% 26% 10% 1	330
	21st to end	Day	10% 10% 31% 36% 10% 2	363
January	1st to 10th	Night	12% 9% 33% 33% 12% 1	840
	11th to 20th	Night	11% 9% 32% 35% 12% 1	840
	21st to end	Night	15% 10% 36% 31% 6%	924
February	1st to 10th	Night	13% 8% 33% 32% 13% 1	840
	11th to 20th	Night	12% 8% 34% 36% 9% 1	823
	21st to end	Night	10% 9% 35% 39% 7%	692
March	1st to 10th	Night	5% 10% 35% 38% 12%	840
	11th to 20th	Night	9% 9% 30% 38% 13% 1	840
	21st to end	Night	7% 10% 32% 38% 12% 1	924
April	1st to 10th	Night	8% 8% 33% 37% 13% 1	840
	11th to 20th	Night	7% 9% 31% 36% 17% 1	816
	21st to end	Night	7% 5% 25% 45% 18% 1	800
May	1st to 10th	Night	6% 3 19% 48% 22% 1	683
	11th to 20th	Night	7% 2 16% 40% 31% 3	591
	21st to end	Night	7% 2 9% 36% 41% 6%	650
June	1st to 10th	Night	10% 3 12% 30% 40% 7%	600
	11th to 20th	Night	7% 3 7% 26% 40% 19%	600
	21st to end	Night	8% 2 7% 21% 45% 18%	600
July	1st to 10th	Night	9% 3 6% 16% 43% 24%	585
	11th to 20th	Night	6% 1 3 15% 48% 28%	582
	21st to end	Night	4 1 5% 18% 48% 23%	657
August	1st to 10th	Night	6% 1 4 19% 48% 22%	597
	11th to 20th	Night	4 1 4 14% 48% 29%	642
	21st to end	Night	11% 2 7% 21% 41% 17%	880
September	1st to 10th	Night	11% 3 8% 22% 39% 19%	800
	11th to 20th	Night	12% 2 11% 28% 38% 10%	800
	21st to end	Night	11% 2 10% 27% 39% 11%	800
October	1st to 10th	Night	13% 3 17% 36% 27% 5%	800
	11th to 20th	Night	6% 3 15% 38% 33% 4	800
	21st to end	Night	10% 4 17% 40% 26% 3	880
November	1st to 10th	Night	9% 4 22% 38% 25% 2	820
	11th to 20th	Night	8% 7% 22% 41% 20% 2	840
	21st to end	Night	9% 5% 30% 39% 17% 1	840
December	1st to 10th	Night	9% 5% 29% 36% 20% 2	840
	11th to 20th	Night	12% 8% 26% 37% 16% 1	840
	21st to end	Night	11% 6% 28% 36% 17% 1	924

Page generated at 2026-04-14 02:44:25. Version ac07

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