Eisenhower Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Eisenhower Observatory 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	13% 9% 3 8% 5% 4 4 4 5% 7% 38%	128111
	Night	12% 9% 4 8% 5% 4 4 4 5% 6% 38%	124146
Transparency	Day	44% 6% 13% 25% 11% 1	131688
	Night	45% 5% 12% 24% 12% 1	128645
Seeing	Day	17% 14% 32% 24% 10% 2	17684
	Night	16% 13% 33% 28% 9% 1	27688

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	9% 13% 5% 19% 8% 6% 5% 3 5% 6% 22%	3697
	11th to 20th	Day	6% 11% 6% 13% 7% 7% 9% 9% 8% 4 20%	3765
	21st to end	Day	6% 16% 7% 12% 5% 4 5% 3 5% 5% 31%	4587
February	1st to 10th	Day	9% 11% 6% 9% 8% 5% 6% 5% 6% 6% 30%	1800
	11th to 20th	Day	8% 11% 5% 10% 7% 5% 5% 4 5% 7% 34%	1754
	21st to end	Day	10% 12% 5% 9% 5% 6% 4 3 6% 6% 33%	1539
March	1st to 10th	Day	13% 9% 3 5% 3 3 4 3 5% 7% 44%	4846
	11th to 20th	Day	10% 10% 3 7% 4 5% 4 3 5% 7% 41%	4980
	21st to end	Day	22% 12% 4 9% 6% 4 5% 3 4 6% 26%	5504
April	1st to 10th	Day	16% 11% 3 7% 5% 4 4 3 5% 7% 35%	2400
	11th to 20th	Day	19% 12% 3 8% 5% 5% 5% 3 4 6% 31%	2393
	21st to end	Day	22% 11% 3 7% 5% 4 4 3 5% 5% 31%	2600
May	1st to 10th	Day	14% 6% 2 5% 3 3 3 3 4 6% 50%	6090
	11th to 20th	Day	18% 7% 2 5% 3 3 4 3 5% 7% 43%	6319
	21st to end	Day	18% 7% 3 7% 4 4 4 4 5% 7% 37%	6935
June	1st to 10th	Day	11% 5% 2 5% 4 4 5% 4 7% 9% 46%	2800
	11th to 20th	Day	15% 7% 3 7% 5% 5% 4 4 6% 9% 36%	2800
	21st to end	Day	11% 5% 2 6% 5% 5% 5% 5% 8% 10% 39%	2899
July	1st to 10th	Day	9% 5% 2 6% 4 4 5% 4 7% 10% 44%	6360
	11th to 20th	Day	9% 4 1 5% 3 3 3 3 5% 7% 57%	6360
	21st to end	Day	8% 5% 2 6% 4 4 5% 4 7% 9% 46%	6817
August	1st to 10th	Day	12% 5% 3 5% 5% 4 6% 4 6% 7% 42%	4234
	11th to 20th	Day	13% 6% 2 5% 4 4 4 5% 6% 9% 42%	4171
	21st to end	Day	10% 6% 2 7% 4 4 4 3 5% 7% 47%	4481
September	1st to 10th	Day	11% 6% 2 5% 4 4 4 4 5% 7% 48%	2400
	11th to 20th	Day	12% 6% 2 6% 4 4 5% 4 6% 8% 44%	2360
	21st to end	Day	14% 6% 2 7% 3 4 4 3 5% 6% 46%	2110
October	1st to 10th	Day	17% 7% 2 5% 4 3 4 4 5% 6% 44%	2000
	11th to 20th	Day	16% 9% 3 7% 4 4 5% 3 6% 7% 38%	2000
	21st to end	Day	18% 11% 3 7% 4 3 3 3 4 6% 38%	2184
November	1st to 10th	Day	13% 12% 4 8% 4 4 4 3 5% 7% 36%	1718
	11th to 20th	Day	11% 14% 6% 11% 6% 5% 4 3 5% 5% 30%	1680
	21st to end	Day	13% 12% 5% 8% 6% 4 4 4 4 7% 34%	1680
December	1st to 10th	Day	7% 12% 4 13% 12% 7% 5% 4 6% 7% 24%	2690
	11th to 20th	Day	10% 16% 6% 12% 7% 4 5% 3 4 5% 27%	3088
	21st to end	Day	21% 18% 8% 19% 5% 3 3 2 3 3 14%	4070
January	1st to 10th	Night	7% 12% 5% 16% 8% 6% 4 3 5% 6% 28%	6488
	11th to 20th	Night	7% 11% 5% 13% 7% 7% 10% 8% 7% 5% 20%	6431
	21st to end	Night	7% 18% 6% 9% 5% 5% 4 3 5% 5% 32%	6633
February	1st to 10th	Night	7% 12% 6% 9% 6% 6% 5% 3 5% 5% 34%	2601
	11th to 20th	Night	6% 10% 4 8% 7% 7% 5% 4 5% 6% 39%	2533
	21st to end	Night	11% 10% 3 9% 6% 5% 4 4 4 6% 38%	2090
March	1st to 10th	Night	10% 7% 2 7% 4 4 4 3 5% 7% 47%	5039
	11th to 20th	Night	10% 6% 2 7% 5% 5% 5% 4 7% 9% 40%	4980
	21st to end	Night	24% 12% 4 10% 4 4 3 3 4 5% 28%	5423
April	1st to 10th	Night	15% 11% 3 7% 4 3 5% 4 4 7% 38%	2000
	11th to 20th	Night	20% 10% 3 8% 5% 3 3 3 4 7% 35%	1952
	21st to end	Night	20% 9% 3 6% 3 5% 4 3 5% 6% 35%	1800
May	1st to 10th	Night	12% 6% 2 6% 4 4 4 3 4 5% 51%	3823
	11th to 20th	Night	16% 5% 2 4 3 3 4 3 5% 7% 50%	3596
	21st to end	Night	17% 8% 3 6% 4 4 4 3 5% 7% 40%	3942
June	1st to 10th	Night	11% 5% 1 5% 4 3 5% 3 6% 8% 49%	1600
	11th to 20th	Night	9% 6% 2 5% 4 5% 5% 5% 7% 8% 44%	1600
	21st to end	Night	9% 4 2 4 3 4 3 4 6% 9% 53%	1455
July	1st to 10th	Night	8% 3 2 4 3 4 5% 5% 7% 9% 50%	3613
	11th to 20th	Night	6% 4 2 5% 3 3 4 4 6% 8% 55%	3617
	21st to end	Night	9% 5% 2 6% 4 4 4 4 6% 7% 49%	4050
August	1st to 10th	Night	13% 6% 2 5% 4 4 5% 4 5% 6% 48%	2926
	11th to 20th	Night	14% 4 2 3 2 3 3 4 5% 8% 51%	2888
	21st to end	Night	9% 6% 3 6% 4 4 4 4 5% 7% 48%	3421
September	1st to 10th	Night	9% 5% 2 5% 4 4 4 3 5% 8% 53%	2000
	11th to 20th	Night	10% 4 1 5% 4 5% 3 3 5% 7% 52%	2035
	21st to end	Night	13% 5% 2 5% 3 3 3 4 4 5% 54%	2270
October	1st to 10th	Night	14% 6% 2 5% 3 4 3 2 5% 7% 49%	2400
	11th to 20th	Night	16% 10% 2 6% 4 4 4 3 5% 5% 42%	2400
	21st to end	Night	18% 11% 3 6% 4 3 3 3 4 6% 38%	2655
November	1st to 10th	Night	14% 9% 3 7% 4 4 4 4 5% 6% 41%	2796
	11th to 20th	Night	13% 12% 4 11% 5% 4 3 4 4 5% 35%	2940
	21st to end	Night	13% 11% 3 10% 6% 5% 4 3 4 6% 36%	2940
December	1st to 10th	Night	6% 9% 5% 14% 7% 7% 6% 4 5% 7% 29%	4720
	11th to 20th	Night	10% 17% 6% 14% 7% 4 4 4 5% 5% 24%	5339
	21st to end	Night	23% 15% 8% 15% 5% 4 3 2 3 4 17%	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	63% 5% 10% 17% 5%	3698
	11th to 20th	Day	60% 5% 9% 17% 9%	3765
	21st to end	Day	55% 5% 15% 18% 7%	4587
February	1st to 10th	Day	53% 5% 14% 19% 8% 1	1800
	11th to 20th	Day	49% 6% 11% 26% 8% 1	1763
	21st to end	Day	51% 6% 12% 20% 11%	1539
March	1st to 10th	Day	39% 4 18% 26% 11% 2	4846
	11th to 20th	Day	42% 5% 14% 28% 10% 1	4980
	21st to end	Day	59% 5% 10% 19% 7%	5506
April	1st to 10th	Day	48% 5% 12% 21% 12% 1	2400
	11th to 20th	Day	54% 5% 10% 19% 11% 1	2393
	21st to end	Day	55% 4 12% 21% 8% 1	2600
May	1st to 10th	Day	35% 5% 12% 28% 17% 2	6128
	11th to 20th	Day	41% 6% 10% 23% 15% 5%	6323
	21st to end	Day	45% 6% 10% 30% 9% 1	6949
June	1st to 10th	Day	33% 7% 15% 31% 13% 2	2800
	11th to 20th	Day	44% 7% 14% 21% 13% 1	2800
	21st to end	Day	37% 8% 16% 27% 10% 2	2899
July	1st to 10th	Day	32% 9% 18% 31% 10%	6291
	11th to 20th	Day	26% 5% 16% 37% 15% 1	6285
	21st to end	Day	31% 8% 15% 29% 16% 1	6623
August	1st to 10th	Day	38% 7% 13% 28% 12% 2	4220
	11th to 20th	Day	36% 7% 14% 32% 10%	4210
	21st to end	Day	35% 7% 16% 28% 13% 1	4480
September	1st to 10th	Day	34% 7% 15% 32% 13%	2400
	11th to 20th	Day	36% 7% 17% 32% 8%	2360
	21st to end	Day	38% 5% 11% 34% 11% 1	2110
October	1st to 10th	Day	30% 3 9% 35% 22%	3260
	11th to 20th	Day	40% 5% 10% 27% 17% 1	3240
	21st to end	Day	44% 4 13% 29% 9%	3510
November	1st to 10th	Day	48% 5% 13% 19% 13% 2	1718
	11th to 20th	Day	56% 5% 13% 18% 9%	1680
	21st to end	Day	51% 5% 16% 21% 6% 1	1680
December	1st to 10th	Day	59% 8% 16% 12% 5% 1	2690
	11th to 20th	Day	58% 5% 15% 17% 5%	3085
	21st to end	Day	77% 4 8% 8% 4	4070
January	1st to 10th	Night	56% 5% 11% 20% 7%	6488
	11th to 20th	Night	61% 5% 10% 14% 10%	6431
	21st to end	Night	53% 5% 11% 23% 7%	6633
February	1st to 10th	Night	51% 5% 12% 21% 10% 1	2601
	11th to 20th	Night	44% 6% 14% 25% 9% 1	2534
	21st to end	Night	47% 5% 12% 22% 12% 1	2090
March	1st to 10th	Night	36% 5% 14% 27% 16% 2	5039
	11th to 20th	Night	38% 6% 15% 26% 14% 1	4980
	21st to end	Night	60% 4 9% 18% 9% 1	5423
April	1st to 10th	Night	47% 4 10% 24% 14% 1	2000
	11th to 20th	Night	50% 4 8% 21% 15% 2	1952
	21st to end	Night	50% 5% 8% 21% 14% 2	1800
May	1st to 10th	Night	34% 5% 10% 27% 21% 2	3850
	11th to 20th	Night	34% 5% 10% 23% 20% 8%	3597
	21st to end	Night	43% 6% 9% 28% 13% 1	3950
June	1st to 10th	Night	32% 6% 14% 28% 18% 2	1600
	11th to 20th	Night	34% 8% 15% 23% 18% 2	1600
	21st to end	Night	28% 5% 16% 30% 17% 3	1455
July	1st to 10th	Night	28% 8% 18% 31% 14% 1	3575
	11th to 20th	Night	25% 6% 14% 39% 14% 2	3572
	21st to end	Night	33% 7% 15% 28% 15% 2	3933
August	1st to 10th	Night	35% 7% 11% 29% 14% 4	2916
	11th to 20th	Night	31% 6% 15% 32% 15% 1	2915
	21st to end	Night	34% 8% 17% 27% 13% 2	3421
September	1st to 10th	Night	31% 5% 13% 34% 17% 1	2000
	11th to 20th	Night	30% 6% 17% 30% 17%	2035
	21st to end	Night	33% 5% 12% 32% 17% 1	2270
October	1st to 10th	Night	24% 3 7% 35% 27% 4	3920
	11th to 20th	Night	33% 5% 12% 24% 22% 4	3899
	21st to end	Night	43% 4 12% 28% 13%	4279
November	1st to 10th	Night	43% 5% 12% 25% 13% 2	2796
	11th to 20th	Night	52% 5% 12% 20% 11% 1	2940
	21st to end	Night	51% 5% 15% 22% 7% 1	2940
December	1st to 10th	Night	53% 6% 17% 18% 5%	4720
	11th to 20th	Night	62% 5% 11% 17% 4 1	5341
	21st to end	Night	71% 4 9% 12% 3	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	16% 12% 32% 29% 10% 2	330
	11th to 20th	Day	19% 15% 37% 25% 3 1	330
	21st to end	Day	17% 15% 38% 20% 8% 2	363
February	1st to 10th	Day	16% 18% 41% 19% 4	300
	11th to 20th	Day	14% 22% 34% 24% 5% 1	300
	21st to end	Day	18% 17% 39% 22% 4	249
March	1st to 10th	Day	19% 17% 40% 18% 6%	300
	11th to 20th	Day	20% 15% 38% 20% 6% 1	300
	21st to end	Day	27% 19% 27% 18% 7% 2	347
April	1st to 10th	Day	25% 18% 33% 20% 4	500
	11th to 20th	Day	31% 18% 24% 20% 7% 1	511
	21st to end	Day	26% 18% 29% 18% 7% 1	700
May	1st to 10th	Day	19% 18% 32% 21% 8% 1	703
	11th to 20th	Day	22% 14% 29% 22% 11% 2	725
	21st to end	Day	19% 12% 30% 25% 11% 3	789
June	1st to 10th	Day	11% 11% 32% 30% 13% 2	730
	11th to 20th	Day	17% 14% 39% 22% 7% 1	730
	21st to end	Day	14% 14% 34% 24% 12% 2	730
July	1st to 10th	Day	9% 5% 29% 33% 17% 6%	721
	11th to 20th	Day	11% 10% 31% 29% 15% 3	718
	21st to end	Day	6% 10% 32% 33% 16% 4	794
August	1st to 10th	Day	11% 8% 30% 32% 17% 3	726
	11th to 20th	Day	15% 8% 24% 30% 18% 4	715
	21st to end	Day	11% 10% 30% 31% 15% 3	687
September	1st to 10th	Day	14% 13% 30% 25% 15% 2	530
	11th to 20th	Day	13% 10% 32% 29% 15% 2	490
	21st to end	Day	17% 16% 27% 22% 15% 2	330
October	1st to 10th	Day	18% 15% 30% 23% 11% 2	330
	11th to 20th	Day	20% 26% 36% 14% 3	330
	21st to end	Day	26% 19% 38% 13% 4	363
November	1st to 10th	Day	24% 20% 35% 16% 5% 1	330
	11th to 20th	Day	21% 22% 39% 15% 2	330
	21st to end	Day	25% 18% 42% 11% 3	330
December	1st to 10th	Day	15% 20% 36% 22% 7% 1	330
	11th to 20th	Day	22% 21% 34% 19% 3	330
	21st to end	Day	29% 15% 37% 17% 2 1	363
January	1st to 10th	Night	12% 15% 38% 27% 7% 1	1050
	11th to 20th	Night	17% 16% 36% 26% 5% 1	1050
	21st to end	Night	17% 15% 37% 26% 5%	1155
February	1st to 10th	Night	14% 15% 39% 24% 6%	1001
	11th to 20th	Night	11% 16% 42% 23% 6% 1	979
	21st to end	Night	17% 16% 35% 25% 8% 1	770
March	1st to 10th	Night	16% 13% 36% 26% 7% 1	659
	11th to 20th	Night	19% 15% 36% 25% 6%	600
	21st to end	Night	22% 13% 31% 23% 10% 2	660
April	1st to 10th	Night	22% 14% 32% 26% 6%	600
	11th to 20th	Night	24% 13% 29% 26% 6% 1	593
	21st to end	Night	24% 13% 32% 23% 8% 1	600
May	1st to 10th	Night	19% 12% 33% 29% 6% 1	600
	11th to 20th	Night	17% 9% 37% 27% 10% 1	591
	21st to end	Night	15% 11% 28% 34% 12%	650
June	1st to 10th	Night	13% 6% 33% 38% 10%	600
	11th to 20th	Night	11% 8% 38% 37% 7%	600
	21st to end	Night	11% 6% 29% 38% 15% 1	600
July	1st to 10th	Night	6% 3 29% 36% 25% 1	585
	11th to 20th	Night	7% 7% 31% 40% 15% 1	582
	21st to end	Night	5% 5% 32% 39% 17% 2	657
August	1st to 10th	Night	7% 6% 23% 42% 20% 1	597
	11th to 20th	Night	13% 4 24% 37% 21% 2	594
	21st to end	Night	8% 6% 32% 33% 21% 1	660
September	1st to 10th	Night	10% 9% 29% 36% 14% 3	600
	11th to 20th	Night	11% 8% 37% 27% 15% 2	600
	21st to end	Night	13% 13% 31% 27% 15% 1	670
October	1st to 10th	Night	17% 13% 32% 26% 12%	800
	11th to 20th	Night	22% 15% 34% 22% 7% 1	800
	21st to end	Night	25% 14% 30% 24% 6%	880
November	1st to 10th	Night	19% 15% 34% 25% 7%	950
	11th to 20th	Night	20% 18% 33% 23% 6%	1050
	21st to end	Night	19% 20% 34% 22% 5%	1050
December	1st to 10th	Night	17% 13% 37% 26% 6% 1	1050
	11th to 20th	Night	20% 16% 34% 24% 5% 1	1050
	21st to end	Night	25% 15% 33% 23% 4 1	1155

Page generated at 2026-01-29 00:21:42. Version ac07

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