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 2024-11-28.

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%	101858
	Night	12% 9% 3 8% 5% 4 4 4 5% 6% 39%	97728
Transparency	Day	44% 6% 13% 25% 11% 1	105477
	Night	44% 5% 12% 24% 13% 1	102249
Seeing	Day	17% 14% 32% 24% 11% 2	16094
	Night	16% 12% 34% 28% 9% 1	25997

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% 16% 6% 17% 6% 4 4 3 4 5% 21%	2527
	11th to 20th	Day	6% 12% 5% 14% 8% 8% 9% 6% 6% 5% 21%	2566
	21st to end	Day	8% 21% 7% 12% 5% 5% 5% 3 5% 6% 24%	3135
February	1st to 10th	Day	8% 11% 6% 9% 7% 5% 6% 5% 6% 6% 31%	1710
	11th to 20th	Day	8% 12% 5% 9% 7% 5% 5% 4 5% 6% 33%	1664
	21st to end	Day	11% 12% 5% 10% 5% 6% 4 3 6% 7% 31%	1464
March	1st to 10th	Day	12% 12% 4 7% 4 4 4 3 5% 6% 39%	3374
	11th to 20th	Day	10% 9% 3 9% 5% 5% 4 3 5% 7% 40%	3480
	21st to end	Day	23% 12% 4 11% 6% 4 4 3 4 5% 23%	3842
April	1st to 10th	Day	16% 11% 3 7% 5% 4 4 3 5% 7% 35%	2280
	11th to 20th	Day	20% 11% 3 7% 5% 5% 5% 3 4 6% 32%	2272
	21st to end	Day	22% 11% 3 7% 5% 4 5% 3 5% 5% 31%	2470
May	1st to 10th	Day	17% 6% 2 6% 4 4 4 4 5% 7% 40%	4240
	11th to 20th	Day	14% 7% 2 5% 4 4 4 3 5% 7% 45%	4399
	21st to end	Day	19% 7% 3 6% 4 4 4 4 6% 7% 35%	4824
June	1st to 10th	Day	12% 5% 2 5% 3 4 4 4 7% 9% 46%	2660
	11th to 20th	Day	14% 6% 2 7% 5% 5% 4 4 6% 9% 37%	2660
	21st to end	Day	11% 5% 2 6% 5% 5% 5% 5% 8% 10% 39%	2754
July	1st to 10th	Day	11% 6% 2 6% 5% 5% 6% 4 8% 10% 38%	4440
	11th to 20th	Day	6% 3 1 4 3 3 3 3 5% 8% 60%	4440
	21st to end	Day	7% 4 2 5% 4 5% 5% 4 7% 9% 48%	4734
August	1st to 10th	Day	13% 5% 3 5% 5% 4 6% 4 6% 7% 42%	4104
	11th to 20th	Day	13% 6% 2 5% 4 4 4 5% 6% 9% 42%	4041
	21st to end	Day	11% 6% 2 7% 4 4 4 3 5% 7% 47%	4344
September	1st to 10th	Day	11% 6% 2 5% 3 4 3 4 5% 7% 49%	2280
	11th to 20th	Day	13% 5% 1 5% 4 4 5% 4 6% 7% 45%	2242
	21st to end	Day	15% 6% 2 7% 3 4 4 3 5% 7% 45%	2005
October	1st to 10th	Day	17% 7% 2 6% 4 3 3 4 5% 5% 43%	1900
	11th to 20th	Day	15% 9% 3 7% 4 4 5% 3 5% 7% 39%	1900
	21st to end	Day	18% 11% 3 7% 4 3 3 3 4 6% 37%	2075
November	1st to 10th	Day	13% 12% 4 8% 4 4 4 3 5% 6% 37%	1635
	11th to 20th	Day	11% 14% 6% 11% 7% 5% 4 3 5% 6% 29%	1600
	21st to end	Day	13% 11% 5% 7% 6% 4 4 4 4 7% 34%	1576
December	1st to 10th	Day	11% 13% 5% 11% 8% 6% 6% 3 5% 6% 27%	1520
	11th to 20th	Day	11% 13% 6% 12% 7% 4 5% 3 4 6% 28%	1918
	21st to end	Day	22% 15% 7% 17% 5% 3 3 3 3 4 18%	2783
January	1st to 10th	Night	10% 16% 7% 16% 6% 4 4 3 4 5% 27%	4428
	11th to 20th	Night	7% 11% 6% 14% 7% 7% 9% 6% 7% 5% 21%	4401
	21st to end	Night	10% 22% 7% 12% 5% 4 4 3 4 4 26%	4532
February	1st to 10th	Night	7% 12% 6% 9% 7% 6% 5% 3 5% 5% 35%	2470
	11th to 20th	Night	7% 10% 3 8% 6% 6% 5% 4 6% 6% 39%	2403
	21st to end	Night	11% 10% 3 9% 6% 5% 4 4 4 6% 38%	1989
March	1st to 10th	Night	9% 8% 3 6% 4 4 4 3 5% 7% 47%	3497
	11th to 20th	Night	9% 7% 3 8% 5% 5% 5% 4 6% 7% 41%	3480
	21st to end	Night	25% 12% 3 11% 4 3 2 2 3 5% 28%	3787
April	1st to 10th	Night	15% 11% 3 7% 4 3 5% 4 4 6% 37%	1900
	11th to 20th	Night	20% 10% 3 8% 5% 3 3 3 4 7% 35%	1853
	21st to end	Night	19% 9% 3 6% 3 5% 4 3 5% 6% 35%	1710
May	1st to 10th	Night	15% 6% 2 7% 5% 5% 5% 3 5% 6% 42%	2663
	11th to 20th	Night	15% 4 2 4 3 3 4 3 5% 8% 50%	2506
	21st to end	Night	15% 7% 3 6% 4 4 5% 3 5% 7% 42%	2743
June	1st to 10th	Night	11% 5% 1 5% 4 3 4 3 6% 8% 49%	1520
	11th to 20th	Night	8% 5% 2 5% 4 5% 6% 5% 7% 8% 45%	1520
	21st to end	Night	8% 4 2 4 3 4 3 4 6% 9% 53%	1383
July	1st to 10th	Night	11% 4 2 4 4 4 5% 5% 7% 9% 45%	2525
	11th to 20th	Night	4 3 1 4 3 3 4 4 6% 8% 59%	2530
	21st to end	Night	6% 4 1 5% 3 4 4 3 5% 7% 57%	2825
August	1st to 10th	Night	13% 6% 2 5% 4 4 4 4 5% 6% 48%	2836
	11th to 20th	Night	14% 4 2 3 2 3 3 3 5% 8% 52%	2798
	21st to end	Night	9% 6% 3 6% 4 4 4 4 5% 7% 48%	3316
September	1st to 10th	Night	9% 5% 2 5% 3 4 4 2 5% 8% 54%	1900
	11th to 20th	Night	10% 4 2 5% 4 4 3 3 5% 7% 53%	1933
	21st to end	Night	13% 5% 2 5% 3 3 3 4 4 5% 53%	2156
October	1st to 10th	Night	14% 6% 2 5% 3 4 3 3 5% 7% 49%	2280
	11th to 20th	Night	15% 10% 2 5% 4 4 4 3 5% 5% 42%	2280
	21st to end	Night	17% 11% 3 6% 4 3 3 3 4 6% 37%	2522
November	1st to 10th	Night	14% 9% 3 6% 4 4 4 4 5% 6% 41%	2660
	11th to 20th	Night	13% 12% 4 11% 5% 4 3 4 4 5% 34%	2800
	21st to end	Night	14% 11% 3 8% 6% 4 4 3 4 6% 36%	2759
December	1st to 10th	Night	10% 11% 4 11% 7% 5% 5% 4 4 6% 31%	2660
	11th to 20th	Night	10% 14% 5% 12% 7% 5% 5% 3 5% 6% 28%	3279
	21st to end	Night	23% 14% 5% 11% 5% 5% 3 3 4 5% 21%	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	65% 6% 11% 16% 3	2528
	11th to 20th	Day	63% 5% 10% 17% 5%	2566
	21st to end	Day	61% 5% 13% 16% 4	3135
February	1st to 10th	Day	52% 5% 14% 20% 8% 1	1710
	11th to 20th	Day	50% 6% 11% 25% 8% 1	1673
	21st to end	Day	52% 6% 11% 20% 11%	1464
March	1st to 10th	Day	46% 5% 13% 26% 10%	3374
	11th to 20th	Day	43% 5% 10% 31% 9% 1	3480
	21st to end	Day	64% 5% 9% 17% 5%	3842
April	1st to 10th	Day	48% 5% 13% 21% 12% 1	2280
	11th to 20th	Day	54% 4 9% 19% 12% 2	2272
	21st to end	Day	55% 4 12% 20% 8% 1	2470
May	1st to 10th	Day	41% 6% 11% 25% 14% 2	4278
	11th to 20th	Day	38% 6% 11% 29% 14% 2	4403
	21st to end	Day	46% 7% 11% 26% 10% 1	4837
June	1st to 10th	Day	33% 7% 14% 31% 13% 2	2660
	11th to 20th	Day	42% 7% 13% 22% 14% 2	2660
	21st to end	Day	36% 8% 16% 27% 10% 2	2754
July	1st to 10th	Day	38% 10% 19% 22% 11% 1	4371
	11th to 20th	Day	22% 6% 16% 37% 18% 2	4365
	21st to end	Day	29% 9% 15% 29% 17% 1	4585
August	1st to 10th	Day	38% 7% 13% 28% 12% 2	4090
	11th to 20th	Day	36% 7% 14% 33% 10%	4080
	21st to end	Day	35% 7% 16% 28% 14% 1	4343
September	1st to 10th	Day	34% 6% 14% 32% 13%	2280
	11th to 20th	Day	35% 7% 17% 33% 8%	2242
	21st to end	Day	39% 5% 12% 33% 11% 1	2005
October	1st to 10th	Day	31% 3 9% 35% 22%	3160
	11th to 20th	Day	39% 5% 11% 28% 16% 1	3140
	21st to end	Day	44% 4 14% 29% 9%	3401
November	1st to 10th	Day	48% 5% 13% 19% 13% 2	1635
	11th to 20th	Day	57% 5% 12% 17% 9% 1	1600
	21st to end	Day	50% 5% 17% 21% 6% 1	1576
December	1st to 10th	Day	58% 7% 15% 14% 5%	1520
	11th to 20th	Day	57% 6% 18% 15% 4	1915
	21st to end	Day	71% 5% 9% 10% 5%	2783
January	1st to 10th	Night	60% 5% 11% 18% 6%	4428
	11th to 20th	Night	62% 5% 12% 15% 6%	4401
	21st to end	Night	62% 5% 9% 19% 5% 1	4532
February	1st to 10th	Night	50% 5% 12% 22% 10% 1	2470
	11th to 20th	Night	44% 6% 15% 25% 9% 2	2404
	21st to end	Night	47% 5% 12% 22% 12% 1	1989
March	1st to 10th	Night	37% 5% 13% 27% 17% 1	3497
	11th to 20th	Night	39% 6% 10% 27% 16% 1	3480
	21st to end	Night	61% 4 7% 21% 6% 1	3787
April	1st to 10th	Night	48% 4 10% 24% 14% 1	1900
	11th to 20th	Night	50% 4 7% 20% 15% 3	1853
	21st to end	Night	49% 5% 8% 21% 14% 2	1710
May	1st to 10th	Night	41% 6% 10% 20% 21% 1	2690
	11th to 20th	Night	33% 5% 11% 27% 20% 4	2507
	21st to end	Night	41% 7% 11% 25% 15% 1	2751
June	1st to 10th	Night	31% 6% 14% 28% 18% 2	1520
	11th to 20th	Night	33% 9% 15% 23% 18% 2	1520
	21st to end	Night	27% 6% 16% 31% 17% 3	1383
July	1st to 10th	Night	32% 8% 19% 25% 14% 1	2485
	11th to 20th	Night	20% 6% 14% 39% 18% 2	2482
	21st to end	Night	25% 7% 14% 32% 18% 3	2735
August	1st to 10th	Night	35% 7% 11% 29% 14% 4	2826
	11th to 20th	Night	31% 6% 15% 32% 16% 1	2825
	21st to end	Night	34% 8% 16% 27% 13% 2	3316
September	1st to 10th	Night	30% 5% 13% 34% 18% 1	1900
	11th to 20th	Night	29% 6% 16% 31% 17%	1933
	21st to end	Night	34% 5% 13% 32% 17% 1	2156
October	1st to 10th	Night	24% 3 6% 35% 27% 4	3800
	11th to 20th	Night	33% 5% 13% 24% 22% 4	3779
	21st to end	Night	43% 4 12% 28% 12%	4146
November	1st to 10th	Night	43% 5% 12% 24% 13% 2	2660
	11th to 20th	Night	52% 5% 12% 19% 11% 1	2800
	21st to end	Night	50% 5% 16% 22% 7% 1	2759
December	1st to 10th	Night	53% 5% 14% 21% 6%	2660
	11th to 20th	Night	57% 6% 13% 17% 5% 2	3281
	21st to end	Night	66% 4 10% 15% 4	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	15% 12% 30% 30% 10% 2	270
	11th to 20th	Day	18% 14% 36% 26% 4 1	270
	21st to end	Day	17% 12% 38% 21% 10% 2	297
February	1st to 10th	Day	14% 18% 42% 20% 5%	270
	11th to 20th	Day	15% 23% 33% 23% 6% 1	270
	21st to end	Day	19% 16% 39% 23% 4	225
March	1st to 10th	Day	19% 19% 38% 17% 7%	270
	11th to 20th	Day	19% 14% 38% 21% 7% 1	270
	21st to end	Day	27% 18% 26% 19% 8% 2	313
April	1st to 10th	Day	23% 18% 33% 20% 4	460
	11th to 20th	Day	30% 16% 24% 22% 7% 1	470
	21st to end	Day	26% 18% 30% 18% 7% 2	650
May	1st to 10th	Day	21% 19% 33% 20% 6% 1	653
	11th to 20th	Day	20% 14% 30% 21% 11% 3	675
	21st to end	Day	18% 12% 30% 25% 11% 3	734
June	1st to 10th	Day	11% 10% 32% 30% 14% 2	680
	11th to 20th	Day	16% 13% 40% 22% 7% 1	680
	21st to end	Day	13% 13% 34% 25% 13% 2	680
July	1st to 10th	Day	10% 6% 28% 33% 18% 6%	671
	11th to 20th	Day	10% 10% 30% 30% 16% 4	668
	21st to end	Day	6% 9% 31% 33% 17% 4	739
August	1st to 10th	Day	11% 8% 30% 31% 18% 3	676
	11th to 20th	Day	15% 8% 23% 31% 18% 5%	665
	21st to end	Day	11% 10% 31% 30% 15% 3	638
September	1st to 10th	Day	14% 13% 30% 27% 14% 2	490
	11th to 20th	Day	13% 9% 32% 29% 15% 2	452
	21st to end	Day	18% 18% 29% 21% 13% 2	300
October	1st to 10th	Day	20% 15% 29% 22% 11% 3	300
	11th to 20th	Day	17% 27% 37% 15% 4	300
	21st to end	Day	27% 19% 38% 12% 4	330
November	1st to 10th	Day	23% 18% 35% 17% 6% 1	300
	11th to 20th	Day	21% 22% 40% 15% 2	300
	21st to end	Day	25% 18% 41% 11% 4	291
December	1st to 10th	Day	15% 19% 35% 23% 8% 1	270
	11th to 20th	Day	22% 20% 33% 21% 4	270
	21st to end	Day	30% 14% 35% 18% 2	297
January	1st to 10th	Night	12% 15% 38% 27% 7% 1	950
	11th to 20th	Night	16% 15% 37% 27% 5% 1	950
	21st to end	Night	18% 14% 36% 26% 6%	1045
February	1st to 10th	Night	13% 15% 39% 26% 7% 1	950
	11th to 20th	Night	12% 15% 42% 24% 7% 1	929
	21st to end	Night	17% 15% 34% 26% 8% 1	733
March	1st to 10th	Night	16% 13% 36% 26% 8% 1	626
	11th to 20th	Night	18% 13% 36% 25% 7%	570
	21st to end	Night	21% 14% 30% 23% 10% 2	627
April	1st to 10th	Night	22% 15% 33% 25% 6%	570
	11th to 20th	Night	24% 13% 29% 26% 6% 1	563
	21st to end	Night	23% 13% 32% 23% 8% 1	570
May	1st to 10th	Night	20% 12% 34% 29% 5% 1	570
	11th to 20th	Night	16% 9% 38% 26% 11% 1	561
	21st to end	Night	15% 11% 28% 33% 12%	617
June	1st to 10th	Night	12% 6% 34% 38% 10%	570
	11th to 20th	Night	10% 7% 39% 37% 7%	570
	21st to end	Night	11% 6% 28% 39% 15% 1	570
July	1st to 10th	Night	6% 3 28% 36% 25% 1	555
	11th to 20th	Night	6% 7% 30% 40% 15% 1	552
	21st to end	Night	5% 4 32% 39% 18% 2	624
August	1st to 10th	Night	7% 6% 23% 43% 19% 1	567
	11th to 20th	Night	12% 4 23% 37% 21% 2	564
	21st to end	Night	8% 6% 32% 32% 21% 1	627
September	1st to 10th	Night	10% 8% 29% 37% 14% 3	570
	11th to 20th	Night	11% 8% 37% 27% 15% 2	570
	21st to end	Night	13% 13% 32% 26% 14% 1	636
October	1st to 10th	Night	18% 12% 31% 26% 12%	760
	11th to 20th	Night	22% 14% 34% 22% 7% 1	760
	21st to end	Night	24% 14% 30% 25% 7%	836
November	1st to 10th	Night	19% 15% 33% 26% 7%	904
	11th to 20th	Night	19% 18% 33% 23% 6%	1000
	21st to end	Night	19% 18% 34% 23% 6%	986
December	1st to 10th	Night	17% 12% 37% 27% 6% 1	950
	11th to 20th	Night	19% 14% 35% 25% 5% 1	950
	21st to end	Night	25% 15% 33% 23% 4	1045

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