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 2024-12-24.

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% 6% 6% 6% 5% 7% 9% 36%	101355
	Night	9% 6% 2 7% 5% 5% 5% 4 7% 9% 42%	98708
Transparency	Day	40% 10% 16% 18% 12% 4	105095
	Night	37% 9% 17% 19% 13% 5%	103108
Seeing	Day	9% 7% 22% 26% 21% 16%	15765
	Night	9% 5% 21% 32% 26% 7%	25861

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% 7% 3 6% 4 4 5% 4 6% 6% 43%	2836
	11th to 20th	Day	19% 10% 3 7% 5% 4 5% 4 5% 6% 34%	2840
	21st to end	Day	11% 7% 2 7% 4 4 5% 3 6% 7% 44%	3136
February	1st to 10th	Day	12% 7% 2 6% 5% 5% 4 5% 5% 9% 40%	1900
	11th to 20th	Day	8% 5% 2 5% 5% 4 3 4 5% 7% 51%	1848
	21st to end	Day	9% 6% 2 6% 5% 4 5% 4 6% 7% 45%	1691
March	1st to 10th	Day	7% 8% 2 10% 7% 5% 5% 4 6% 10% 35%	3435
	11th to 20th	Day	6% 4 2 6% 4 4 5% 4 6% 10% 49%	3480
	21st to end	Day	11% 6% 2 6% 5% 4 4 3 6% 7% 45%	3815
April	1st to 10th	Day	7% 4 2 5% 4 5% 5% 4 6% 9% 48%	2090
	11th to 20th	Day	9% 6% 2 7% 5% 5% 6% 5% 8% 9% 39%	2132
	21st to end	Day	5% 4 1 6% 5% 5% 5% 4 7% 9% 49%	2280
May	1st to 10th	Day	4 3 1 4 4 4 5% 5% 8% 10% 52%	3979
	11th to 20th	Day	10% 6% 2 7% 6% 5% 7% 5% 8% 9% 35%	4081
	21st to end	Day	5% 4 1 5% 4 5% 6% 5% 7% 11% 48%	4476
June	1st to 10th	Day	11% 8% 3 9% 7% 7% 8% 5% 8% 10% 24%	2470
	11th to 20th	Day	6% 6% 3 10% 8% 9% 8% 6% 7% 9% 27%	2470
	21st to end	Day	9% 7% 3 9% 8% 8% 7% 6% 7% 9% 27%	2470
July	1st to 10th	Day	7% 6% 3 8% 7% 8% 9% 7% 11% 12% 23%	4120
	11th to 20th	Day	5% 5% 3 8% 7% 8% 8% 7% 10% 14% 26%	4120
	21st to end	Day	5% 6% 3 9% 8% 7% 8% 7% 10% 13% 24%	4466
August	1st to 10th	Day	6% 7% 3 10% 7% 7% 7% 6% 9% 12% 26%	4104
	11th to 20th	Day	4 4 2 7% 6% 6% 8% 6% 10% 13% 33%	3926
	21st to end	Day	8% 7% 3 9% 7% 7% 7% 7% 9% 12% 22%	3837
September	1st to 10th	Day	9% 7% 3 9% 8% 8% 8% 7% 10% 10% 21%	2090
	11th to 20th	Day	11% 7% 3 11% 9% 8% 8% 5% 9% 10% 20%	2090
	21st to end	Day	10% 7% 3 11% 6% 6% 6% 6% 7% 9% 29%	2090
October	1st to 10th	Day	14% 7% 2 8% 5% 7% 7% 4 6% 8% 32%	2042
	11th to 20th	Day	8% 5% 3 6% 5% 5% 6% 5% 7% 8% 44%	1900
	21st to end	Day	8% 5% 2 6% 4 5% 5% 5% 6% 7% 48%	2090
November	1st to 10th	Day	8% 5% 3 6% 6% 6% 5% 4 7% 8% 41%	1957
	11th to 20th	Day	9% 7% 2 7% 6% 5% 6% 5% 5% 7% 40%	2000
	21st to end	Day	11% 6% 3 6% 4 4 5% 4 5% 8% 43%	1900
December	1st to 10th	Day	8% 5% 2 6% 4 4 5% 5% 7% 9% 46%	1800
	11th to 20th	Day	12% 7% 3 7% 5% 5% 5% 4 6% 6% 39%	2243
	21st to end	Day	12% 8% 3 7% 5% 5% 5% 4 5% 7% 39%	3151
January	1st to 10th	Night	13% 7% 2 6% 5% 4 4 4 5% 6% 42%	4109
	11th to 20th	Night	17% 11% 3 6% 4 4 4 4 6% 8% 34%	4120
	21st to end	Night	11% 7% 3 7% 5% 4 4 4 6% 7% 41%	4503
February	1st to 10th	Night	11% 7% 2 6% 4 5% 4 3 5% 7% 45%	2280
	11th to 20th	Night	10% 5% 2 5% 5% 4 4 4 5% 6% 50%	2218
	21st to end	Night	7% 5% 2 6% 4 4 4 3 5% 8% 52%	1761
March	1st to 10th	Night	15% 10% 3 8% 4 3 4 3 5% 6% 40%	3418
	11th to 20th	Night	6% 3 2 5% 4 4 5% 4 8% 11% 48%	3480
	21st to end	Night	10% 7% 3 7% 4 4 4 4 5% 7% 46%	3817
April	1st to 10th	Night	7% 4 1 4 3 4 4 4 7% 8% 53%	2090
	11th to 20th	Night	7% 6% 2 6% 5% 4 4 4 6% 7% 50%	1994
	21st to end	Night	6% 4 1 5% 4 4 4 4 6% 9% 54%	1900
May	1st to 10th	Night	4 2 1 4 4 4 4 4 6% 10% 59%	2941
	11th to 20th	Night	6% 6% 2 6% 4 5% 5% 5% 8% 10% 43%	2812
	21st to end	Night	4 3 1 4 4 4 4 4 6% 8% 59%	3079
June	1st to 10th	Night	10% 6% 3 6% 4 6% 5% 5% 8% 12% 34%	1710
	11th to 20th	Night	5% 6% 2 8% 7% 5% 6% 6% 9% 11% 35%	1710
	21st to end	Night	7% 7% 3 8% 7% 5% 7% 5% 9% 10% 33%	1710
July	1st to 10th	Night	6% 6% 2 8% 5% 7% 7% 5% 9% 12% 32%	2840
	11th to 20th	Night	4 5% 3 8% 6% 7% 8% 8% 11% 13% 28%	2840
	21st to end	Night	5% 7% 3 8% 8% 7% 7% 7% 8% 13% 25%	3087
August	1st to 10th	Night	5% 6% 2 9% 6% 6% 6% 4 9% 12% 35%	2825
	11th to 20th	Night	4 4 2 6% 5% 6% 6% 5% 9% 14% 39%	2869
	21st to end	Night	8% 6% 2 8% 6% 5% 6% 6% 8% 12% 34%	3814
September	1st to 10th	Night	8% 6% 3 8% 7% 6% 6% 6% 8% 12% 29%	2090
	11th to 20th	Night	10% 8% 3 9% 6% 7% 6% 6% 9% 10% 27%	2090
	21st to end	Night	11% 6% 2 7% 5% 6% 6% 5% 6% 9% 37%	2090
October	1st to 10th	Night	10% 7% 3 8% 5% 6% 5% 4 7% 9% 37%	2137
	11th to 20th	Night	5% 4 2 6% 4 4 5% 4 7% 8% 51%	2280
	21st to end	Night	9% 5% 2 5% 3 4 4 4 6% 8% 51%	2508
November	1st to 10th	Night	9% 5% 2 7% 4 4 5% 4 6% 7% 46%	2341
	11th to 20th	Night	8% 7% 2 6% 5% 4 5% 4 5% 7% 45%	2400
	21st to end	Night	9% 6% 2 7% 4 5% 4 5% 6% 8% 44%	2495
December	1st to 10th	Night	8% 4 2 6% 4 5% 5% 4 6% 8% 48%	2600
	11th to 20th	Night	13% 8% 2 5% 4 4 4 2 4 6% 48%	3172
	21st to end	Night	14% 8% 3 7% 6% 4 4 4 6% 6% 38%	4578

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% 6% 8% 16% 20% 9%	2837
	11th to 20th	Day	50% 5% 6% 15% 15% 9%	2840
	21st to end	Day	39% 6% 9% 17% 18% 10%	3136
February	1st to 10th	Day	40% 7% 10% 14% 20% 9%	1900
	11th to 20th	Day	32% 6% 10% 19% 22% 11%	1858
	21st to end	Day	37% 5% 11% 16% 22% 9%	1691
March	1st to 10th	Day	41% 9% 13% 18% 13% 5%	3435
	11th to 20th	Day	29% 7% 13% 24% 22% 5%	3480
	21st to end	Day	35% 6% 10% 26% 15% 8%	3815
April	1st to 10th	Day	31% 8% 14% 21% 18% 8%	2090
	11th to 20th	Day	37% 9% 10% 19% 18% 6%	2132
	21st to end	Day	29% 6% 13% 22% 21% 8%	2280
May	1st to 10th	Day	21% 10% 18% 29% 18% 5%	4015
	11th to 20th	Day	39% 11% 18% 22% 10% 2	4085
	21st to end	Day	27% 8% 15% 34% 16% 1	4489
June	1st to 10th	Day	50% 13% 16% 16% 5%	2470
	11th to 20th	Day	48% 12% 19% 18% 2	2470
	21st to end	Day	49% 13% 21% 13% 4	2470
July	1st to 10th	Day	46% 15% 25% 13% 1	4055
	11th to 20th	Day	43% 16% 33% 8%	4051
	21st to end	Day	45% 16% 29% 9% 1	4324
August	1st to 10th	Day	46% 13% 28% 10% 2	4091
	11th to 20th	Day	37% 14% 24% 20% 5%	3965
	21st to end	Day	49% 13% 22% 15% 1	3837
September	1st to 10th	Day	52% 17% 17% 10% 4	2090
	11th to 20th	Day	53% 15% 17% 12% 3	2090
	21st to end	Day	47% 11% 14% 14% 10% 3	2090
October	1st to 10th	Day	56% 8% 14% 10% 7% 4	3393
	11th to 20th	Day	29% 6% 9% 22% 27% 7%	3140
	21st to end	Day	26% 6% 7% 22% 32% 6%	3428
November	1st to 10th	Day	38% 8% 11% 17% 19% 8%	1957
	11th to 20th	Day	40% 7% 9% 18% 19% 7%	2000
	21st to end	Day	38% 7% 10% 19% 19% 7%	1900
December	1st to 10th	Day	33% 7% 16% 23% 16% 6%	1800
	11th to 20th	Day	41% 7% 11% 14% 18% 9%	2240
	21st to end	Day	44% 6% 11% 17% 15% 7%	3151
January	1st to 10th	Night	42% 6% 9% 15% 20% 9%	4109
	11th to 20th	Night	48% 5% 9% 18% 14% 5%	4120
	21st to end	Night	39% 8% 11% 16% 16% 10%	4503
February	1st to 10th	Night	38% 6% 11% 17% 16% 13%	2280
	11th to 20th	Night	32% 8% 11% 18% 20% 11%	2219
	21st to end	Night	30% 6% 14% 19% 23% 9%	1761
March	1st to 10th	Night	45% 7% 13% 17% 14% 5%	3418
	11th to 20th	Night	25% 9% 19% 27% 15% 5%	3480
	21st to end	Night	36% 7% 10% 21% 21% 6%	3817
April	1st to 10th	Night	27% 7% 19% 23% 15% 9%	2090
	11th to 20th	Night	32% 7% 12% 26% 18% 7%	1994
	21st to end	Night	26% 6% 14% 27% 19% 7%	1900
May	1st to 10th	Night	19% 9% 21% 32% 15% 5%	2971
	11th to 20th	Night	31% 12% 23% 24% 9% 1	2813
	21st to end	Night	21% 8% 16% 36% 18% 1	3087
June	1st to 10th	Night	38% 14% 22% 22% 4	1710
	11th to 20th	Night	38% 15% 22% 23% 3	1710
	21st to end	Night	42% 15% 23% 15% 5% 1	1710
July	1st to 10th	Night	41% 13% 30% 15% 2	2795
	11th to 20th	Night	41% 17% 34% 7% 1	2786
	21st to end	Night	45% 15% 29% 11% 1	2988
August	1st to 10th	Night	40% 12% 36% 11% 2	2814
	11th to 20th	Night	32% 12% 30% 20% 6%	2896
	21st to end	Night	41% 13% 29% 15% 2	3814
September	1st to 10th	Night	43% 18% 20% 14% 5%	2090
	11th to 20th	Night	47% 13% 21% 15% 3	2090
	21st to end	Night	41% 11% 17% 20% 9% 2	2090
October	1st to 10th	Night	54% 10% 13% 13% 7% 4	3566
	11th to 20th	Night	28% 6% 8% 26% 27% 5%	3779
	21st to end	Night	28% 6% 9% 22% 29% 6%	4120
November	1st to 10th	Night	35% 7% 13% 19% 18% 9%	2341
	11th to 20th	Night	36% 8% 11% 19% 19% 8%	2400
	21st to end	Night	36% 7% 13% 18% 19% 7%	2495
December	1st to 10th	Night	32% 7% 16% 27% 13% 6%	2600
	11th to 20th	Night	38% 5% 11% 19% 18% 9%	3174
	21st to end	Night	45% 7% 12% 18% 14% 5%	4578

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	11% 15% 38% 24% 9% 4	270
	21st to end	Day	18% 22% 43% 14% 3	297
February	1st to 10th	Day	14% 17% 39% 19% 10% 2	270
	11th to 20th	Day	13% 13% 40% 23% 9% 2	270
	21st to end	Day	10% 14% 35% 26% 14% 1	346
March	1st to 10th	Day	5% 18% 34% 31% 10% 2	460
	11th to 20th	Day	9% 13% 33% 26% 16% 2	460
	21st to end	Day	8% 13% 32% 28% 16% 2	506
April	1st to 10th	Day	8% 15% 35% 25% 13% 4	460
	11th to 20th	Day	10% 12% 31% 25% 17% 5%	452
	21st to end	Day	7% 8% 28% 30% 16% 12%	460
May	1st to 10th	Day	8% 6% 23% 31% 21% 12%	463
	11th to 20th	Day	9% 4 23% 29% 20% 14%	487
	21st to end	Day	7% 3 13% 25% 33% 19%	529
June	1st to 10th	Day	12% 2 16% 28% 23% 19%	490
	11th to 20th	Day	8% 3 12% 27% 24% 26%	490
	21st to end	Day	5% 3 9% 19% 32% 33%	490
July	1st to 10th	Day	6% 3 5% 14% 30% 41%	485
	11th to 20th	Day	5% 1 3 17% 32% 42%	484
	21st to end	Day	8% 2 6% 18% 28% 38%	532
August	1st to 10th	Day	7% 2 7% 20% 30% 33%	486
	11th to 20th	Day	4 1 4 17% 28% 46%	479
	21st to end	Day	9% 2 9% 22% 26% 32%	539
September	1st to 10th	Day	9% 3 8% 17% 31% 31%	490
	11th to 20th	Day	9% 4 9% 29% 27% 22%	490
	21st to end	Day	8% 3 12% 29% 25% 23%	490
October	1st to 10th	Day	16% 6% 22% 29% 20% 8%	490
	11th to 20th	Day	6% 6% 21% 36% 24% 7%	490
	21st to end	Day	8% 5% 26% 36% 21% 5%	539
November	1st to 10th	Day	9% 7% 26% 31% 23% 4	495
	11th to 20th	Day	11% 10% 27% 32% 14% 5%	500
	21st to end	Day	11% 9% 33% 32% 13% 3	400
December	1st to 10th	Day	6% 13% 40% 28% 10% 2	300
	11th to 20th	Day	12% 13% 39% 25% 10% 1	300
	21st to end	Day	10% 11% 31% 37% 9% 1	306
January	1st to 10th	Night	13% 10% 33% 32% 12% 1	760
	11th to 20th	Night	10% 9% 31% 36% 13% 1	760
	21st to end	Night	15% 11% 36% 31% 7%	836
February	1st to 10th	Night	13% 8% 34% 31% 13% 1	760
	11th to 20th	Night	12% 8% 34% 36% 10% 1	743
	21st to end	Night	9% 9% 35% 39% 7%	628
March	1st to 10th	Night	5% 10% 36% 37% 11% 1	760
	11th to 20th	Night	9% 8% 30% 38% 14% 1	760
	21st to end	Night	7% 10% 34% 37% 12% 1	836
April	1st to 10th	Night	8% 8% 34% 37% 13% 1	760
	11th to 20th	Night	7% 9% 32% 35% 17%	751
	21st to end	Night	7% 5% 26% 44% 17% 1	760
May	1st to 10th	Night	6% 3 19% 48% 23% 1	649
	11th to 20th	Night	7% 2 17% 39% 32% 3	561
	21st to end	Night	7% 2 8% 35% 41% 6%	617
June	1st to 10th	Night	9% 3 12% 30% 39% 7%	570
	11th to 20th	Night	7% 3 7% 26% 39% 19%	570
	21st to end	Night	8% 2 7% 21% 45% 18%	570
July	1st to 10th	Night	9% 2 6% 16% 44% 24%	555
	11th to 20th	Night	6% 1 2 15% 48% 28%	552
	21st to end	Night	4 1 5% 19% 48% 23%	624
August	1st to 10th	Night	5% 1 4 19% 49% 22%	567
	11th to 20th	Night	4 4 15% 49% 28%	609
	21st to end	Night	12% 2 8% 21% 40% 18%	836
September	1st to 10th	Night	10% 2 7% 22% 40% 20%	760
	11th to 20th	Night	12% 2 10% 26% 38% 11%	760
	21st to end	Night	11% 2 9% 27% 39% 11%	760
October	1st to 10th	Night	13% 3 16% 36% 27% 5%	760
	11th to 20th	Night	6% 3 16% 38% 33% 4	760
	21st to end	Night	10% 4 17% 40% 27% 3	836
November	1st to 10th	Night	9% 4 23% 37% 25% 2	780
	11th to 20th	Night	9% 7% 23% 41% 19% 2	800
	21st to end	Night	9% 5% 30% 38% 17% 1	800
December	1st to 10th	Night	8% 5% 29% 36% 21% 2	800
	11th to 20th	Night	12% 9% 25% 36% 17% 1	800
	21st to end	Night	12% 7% 27% 37% 16% 1	851

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