Eagle Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Eagle Observatory clear sky chart from 2005-11-06 to 2024-11-30.

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	2 5% 2 6% 4 4 5% 4 7% 8% 51%	105452
	Night	2 7% 3 7% 4 4 4 4 6% 8% 51%	102867
Transparency	Day	30% 7% 13% 24% 19% 8%	105207
	Night	30% 7% 14% 25% 18% 6%	102703
Seeing	Day	8% 15% 30% 29% 15% 3	15589
	Night	6% 7% 26% 41% 18% 1	25700

By Month

Percentage Cloud Cover SW (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	1 6% 4 10% 7% 6% 6% 6% 8% 11% 35%	2538
	11th to 20th	Day	1 10% 5% 12% 6% 4 6% 5% 7% 8% 37%	2830
	21st to end	Day	1 7% 4 10% 6% 5% 5% 3 5% 10% 44%	3146
February	1st to 10th	Day	1 11% 4 10% 7% 6% 6% 4 5% 6% 40%	1710
	11th to 20th	Day	1 8% 4 9% 7% 6% 6% 5% 6% 8% 40%	1726
	21st to end	Day	1 9% 5% 11% 6% 5% 6% 5% 7% 7% 39%	1570
March	1st to 10th	Day	1 7% 3 8% 6% 5% 6% 5% 8% 9% 43%	3308
	11th to 20th	Day	1 7% 2 7% 5% 4 5% 4 7% 9% 48%	3490
	21st to end	Day	1 10% 4 9% 6% 6% 6% 5% 8% 9% 34%	3917
April	1st to 10th	Day	3 8% 3 6% 5% 4 6% 4 7% 8% 46%	2280
	11th to 20th	Day	3 9% 3 8% 6% 5% 6% 5% 7% 8% 40%	2251
	21st to end	Day	3 8% 4 8% 6% 6% 5% 5% 6% 9% 41%	2280
May	1st to 10th	Day	4 11% 4 9% 6% 6% 6% 5% 7% 8% 34%	4008
	11th to 20th	Day	5% 5% 2 6% 5% 5% 5% 6% 9% 10% 43%	4378
	21st to end	Day	3 4 2 5% 4 4 5% 5% 8% 9% 51%	4825
June	1st to 10th	Day	3 3 1 4 4 4 5% 4 6% 10% 57%	2660
	11th to 20th	Day	3 3 1 3 2 3 4 4 6% 7% 64%	2660
	21st to end	Day	2 1 1 3 3 4 5% 4 7% 8% 63%	2660
July	1st to 10th	Day	3 2 1 3 3 3 3 3 5% 7% 68%	4430
	11th to 20th	Day	1 2 1 3 4 5% 6% 7% 10% 11% 50%	4416
	21st to end	Day	2 3 2 4 4 4 4 5% 6% 8% 58%	4454
August	1st to 10th	Day	4 3 2 5% 5% 5% 6% 5% 9% 11% 47%	4092
	11th to 20th	Day	3 3 1 4 4 5% 5% 5% 8% 9% 53%	3882
	21st to end	Day	3 2 1 4 3 4 5% 4 7% 11% 56%	4180
September	1st to 10th	Day	4 3 1 4 3 4 4 4 6% 8% 61%	2280
	11th to 20th	Day	3 3 1 4 4 5% 4 3 5% 8% 60%	2280
	21st to end	Day	5% 4 1 4 2 3 3 3 4 6% 64%	2026
October	1st to 10th	Day	3 3 1 3 2 2 2 2 4 5% 72%	3170
	11th to 20th	Day	3 3 1 3 3 4 5% 3 7% 10% 58%	3150
	21st to end	Day	3 5% 2 4 3 2 3 3 4 7% 64%	3438
November	1st to 10th	Day	2 5% 2 5% 3 2 4 3 6% 7% 60%	1806
	11th to 20th	Day	3 8% 2 6% 4 3 4 3 5% 6% 56%	1775
	21st to end	Day	2 7% 4 8% 4 3 3 3 5% 6% 56%	1596
December	1st to 10th	Day	1 8% 3 10% 5% 5% 6% 4 6% 7% 44%	1520
	11th to 20th	Day	1 7% 4 9% 5% 5% 5% 4 5% 10% 45%	1926
	21st to end	Day	1 8% 4 13% 6% 5% 5% 3 8% 8% 38%	2794
January	1st to 10th	Night	1 9% 5% 12% 7% 5% 5% 4 6% 9% 38%	4417
	11th to 20th	Night	1 8% 4 12% 7% 6% 5% 5% 5% 8% 39%	4170
	21st to end	Night	2 12% 6% 12% 6% 5% 5% 3 4 8% 37%	4521
February	1st to 10th	Night	1 9% 4 11% 7% 4 6% 4 5% 7% 41%	2470
	11th to 20th	Night	1 9% 5% 10% 6% 4 6% 5% 5% 8% 41%	2339
	21st to end	Night	1 11% 4 12% 7% 4 5% 4 5% 6% 41%	1884
March	1st to 10th	Night	1 9% 4 7% 5% 6% 5% 4 7% 8% 44%	3559
	11th to 20th	Night	1 10% 4 6% 4 4 4 3 5% 7% 53%	3490
	21st to end	Night	2 14% 4 9% 6% 5% 7% 4 6% 8% 36%	3718
April	1st to 10th	Night	4 9% 2 6% 5% 4 6% 4 7% 7% 46%	1900
	11th to 20th	Night	4 11% 3 7% 4 4 5% 3 6% 8% 45%	1877
	21st to end	Night	6% 9% 3 7% 4 4 4 4 5% 7% 48%	1900
May	1st to 10th	Night	6% 8% 3 6% 5% 5% 5% 5% 6% 7% 44%	2958
	11th to 20th	Night	6% 6% 3 6% 5% 5% 4 4 8% 8% 45%	2524
	21st to end	Night	4 4 2 5% 4 4 4 4 6% 7% 57%	2762
June	1st to 10th	Night	2 2 1 3 2 4 4 3 5% 9% 64%	1520
	11th to 20th	Night	3 2 1 3 3 4 4 4 6% 7% 63%	1520
	21st to end	Night	3 1 1 2 2 2 3 4 7% 8% 67%	1520
July	1st to 10th	Night	3 2 1 4 3 3 3 3 6% 7% 66%	2540
	11th to 20th	Night	1 2 1 3 3 4 5% 5% 8% 11% 56%	2540
	21st to end	Night	2 2 1 3 3 4 5% 5% 6% 7% 61%	3106
August	1st to 10th	Night	4 3 1 4 4 5% 6% 7% 8% 11% 47%	2847
	11th to 20th	Night	3 2 1 4 4 4 5% 5% 7% 10% 56%	2955
	21st to end	Night	3 2 1 4 3 5% 5% 5% 7% 10% 55%	3487
September	1st to 10th	Night	3 2 2 4 3 4 4 4 5% 7% 63%	1900
	11th to 20th	Night	3 3 1 3 3 3 3 3 5% 9% 63%	1900
	21st to end	Night	3 3 1 3 3 3 3 2 5% 6% 67%	2148
October	1st to 10th	Night	3 2 1 3 2 2 2 2 3 5% 75%	3800
	11th to 20th	Night	2 3 1 3 2 2 3 4 7% 11% 62%	3778
	21st to end	Night	4 6% 1 4 2 2 2 2 3 5% 69%	4120
November	1st to 10th	Night	3 5% 2 4 3 2 3 3 5% 6% 63%	2471
	11th to 20th	Night	2 8% 2 7% 3 3 3 2 4 6% 58%	2622
	21st to end	Night	2 9% 3 7% 4 4 3 2 5% 7% 54%	2800
December	1st to 10th	Night	2 10% 4 9% 5% 5% 4 4 5% 5% 48%	2660
	11th to 20th	Night	2 9% 4 9% 5% 5% 5% 4 7% 8% 44%	3271
	21st to end	Night	1 9% 5% 12% 7% 5% 6% 4 6% 7% 38%	4873

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	41% 10% 14% 24% 9% 1	2539
	11th to 20th	Day	48% 9% 18% 18% 7%	2830
	21st to end	Day	37% 8% 19% 26% 9% 1	3146
February	1st to 10th	Day	46% 6% 17% 18% 10% 2	1710
	11th to 20th	Day	45% 6% 11% 23% 13% 2	1735
	21st to end	Day	44% 6% 12% 21% 14% 3	1570
March	1st to 10th	Day	40% 5% 11% 26% 15% 2	3308
	11th to 20th	Day	31% 8% 17% 34% 8% 2	3490
	21st to end	Day	45% 10% 15% 21% 7% 1	3917
April	1st to 10th	Day	36% 6% 13% 22% 17% 5%	2280
	11th to 20th	Day	42% 8% 12% 19% 17% 2	2254
	21st to end	Day	39% 7% 11% 26% 15% 2	2280
May	1st to 10th	Day	41% 13% 10% 19% 15% 2	4008
	11th to 20th	Day	28% 11% 15% 27% 15% 4	4382
	21st to end	Day	26% 8% 12% 27% 25% 3	4825
June	1st to 10th	Day	23% 6% 11% 24% 26% 9%	2660
	11th to 20th	Day	19% 5% 9% 17% 28% 21%	2660
	21st to end	Day	17% 6% 8% 23% 26% 20%	2660
July	1st to 10th	Day	16% 5% 8% 15% 25% 32%	4361
	11th to 20th	Day	20% 7% 11% 28% 22% 10%	4341
	21st to end	Day	23% 6% 11% 20% 18% 22%	4314
August	1st to 10th	Day	28% 7% 20% 20% 14% 12%	4078
	11th to 20th	Day	25% 8% 16% 16% 25% 10%	3921
	21st to end	Day	21% 6% 16% 20% 25% 11%	4179
September	1st to 10th	Day	22% 5% 9% 21% 25% 18%	2280
	11th to 20th	Day	24% 5% 11% 21% 28% 11%	2280
	21st to end	Day	23% 4 7% 27% 30% 9%	2026
October	1st to 10th	Day	17% 3 6% 27% 33% 14%	3170
	11th to 20th	Day	19% 5% 11% 40% 21% 4	3150
	21st to end	Day	22% 4 7% 41% 23% 3	3438
November	1st to 10th	Day	25% 5% 13% 28% 27% 2	1806
	11th to 20th	Day	30% 4 13% 31% 19% 3	1775
	21st to end	Day	31% 4 11% 34% 16% 4	1596
December	1st to 10th	Day	41% 7% 18% 23% 10% 2	1520
	11th to 20th	Day	37% 6% 14% 30% 12%	1924
	21st to end	Day	42% 9% 9% 21% 17% 2	2794
January	1st to 10th	Night	44% 8% 16% 23% 10% 1	4417
	11th to 20th	Night	45% 10% 18% 19% 8%	4170
	21st to end	Night	45% 7% 18% 20% 9% 1	4521
February	1st to 10th	Night	41% 10% 16% 22% 10% 1	2470
	11th to 20th	Night	42% 7% 11% 25% 13% 1	2340
	21st to end	Night	42% 7% 9% 23% 17% 2	1884
March	1st to 10th	Night	38% 7% 11% 26% 17% 1	3559
	11th to 20th	Night	30% 7% 16% 35% 11% 2	3490
	21st to end	Night	42% 11% 17% 23% 6% 1	3718
April	1st to 10th	Night	36% 8% 11% 23% 19% 4	1900
	11th to 20th	Night	37% 7% 12% 24% 18% 2	1877
	21st to end	Night	36% 6% 11% 27% 20% 1	1900
May	1st to 10th	Night	33% 11% 11% 28% 15% 3	2958
	11th to 20th	Night	28% 11% 15% 32% 13% 2	2525
	21st to end	Night	22% 7% 13% 28% 26% 4	2762
June	1st to 10th	Night	17% 5% 14% 26% 28% 10%	1520
	11th to 20th	Night	19% 6% 9% 17% 24% 25%	1520
	21st to end	Night	13% 5% 9% 22% 27% 23%	1520
July	1st to 10th	Night	17% 7% 9% 17% 20% 30%	2500
	11th to 20th	Night	18% 8% 20% 26% 18% 11%	2492
	21st to end	Night	19% 7% 16% 26% 15% 17%	3009
August	1st to 10th	Night	25% 8% 24% 19% 14% 10%	2838
	11th to 20th	Night	22% 8% 21% 17% 20% 11%	2982
	21st to end	Night	22% 8% 14% 19% 27% 10%	3487
September	1st to 10th	Night	22% 5% 10% 18% 26% 18%	1900
	11th to 20th	Night	20% 5% 13% 21% 28% 13%	1900
	21st to end	Night	19% 4 8% 26% 34% 9%	2148
October	1st to 10th	Night	14% 3 6% 28% 37% 11%	3800
	11th to 20th	Night	16% 4 14% 41% 22% 4	3778
	21st to end	Night	20% 4 9% 41% 23% 3	4120
November	1st to 10th	Night	23% 5% 12% 32% 26% 3	2471
	11th to 20th	Night	29% 4 12% 32% 20% 3	2622
	21st to end	Night	30% 5% 10% 33% 19% 2	2800
December	1st to 10th	Night	39% 7% 16% 24% 13% 1	2660
	11th to 20th	Night	36% 7% 14% 28% 14%	3272
	21st to end	Night	40% 10% 12% 18% 18% 1	4873

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	12% 16% 29% 25% 16% 3	270
	11th to 20th	Day	11% 11% 40% 31% 6% 1	270
	21st to end	Day	6% 8% 39% 32% 14%	297
February	1st to 10th	Day	13% 22% 36% 24% 4 1	270
	11th to 20th	Day	11% 23% 34% 27% 4 1	270
	21st to end	Day	18% 23% 35% 20% 5%	225
March	1st to 10th	Day	13% 22% 35% 22% 7% 1	370
	11th to 20th	Day	7% 23% 36% 24% 9% 1	460
	21st to end	Day	12% 30% 37% 17% 5%	506
April	1st to 10th	Day	15% 24% 35% 18% 7% 2	460
	11th to 20th	Day	17% 31% 31% 16% 5% 1	452
	21st to end	Day	14% 25% 35% 19% 6% 1	460
May	1st to 10th	Day	16% 26% 31% 19% 7%	463
	11th to 20th	Day	9% 14% 30% 29% 16% 2	665
	21st to end	Day	10% 15% 30% 29% 14% 2	734
June	1st to 10th	Day	5% 11% 27% 34% 20% 3	680
	11th to 20th	Day	6% 14% 31% 32% 14% 2	680
	21st to end	Day	3 10% 29% 35% 21% 1	680
July	1st to 10th	Day	4 5% 22% 34% 30% 5%	671
	11th to 20th	Day	3 5% 19% 36% 33% 5%	667
	21st to end	Day	5% 5% 21% 39% 25% 5%	531
August	1st to 10th	Day	7% 6% 28% 33% 22% 5%	487
	11th to 20th	Day	4 7% 23% 39% 21% 6%	477
	21st to end	Day	3 8% 25% 35% 23% 6%	539
September	1st to 10th	Day	5% 9% 26% 30% 22% 8%	490
	11th to 20th	Day	6% 11% 35% 27% 17% 4	490
	21st to end	Day	7% 13% 26% 27% 20% 7%	359
October	1st to 10th	Day	9% 12% 23% 25% 23% 8%	300
	11th to 20th	Day	8% 16% 27% 26% 18% 3	300
	21st to end	Day	11% 18% 30% 25% 13% 2	330
November	1st to 10th	Day	14% 17% 32% 25% 10% 1	300
	11th to 20th	Day	8% 17% 36% 29% 9% 2	300
	21st to end	Day	7% 14% 33% 37% 9%	299
December	1st to 10th	Day	11% 13% 37% 28% 11% 1	270
	11th to 20th	Day	6% 15% 40% 30% 9% 1	270
	21st to end	Day	12% 15% 36% 29% 7% 1	297
January	1st to 10th	Night	5% 13% 26% 39% 15% 1	950
	11th to 20th	Night	6% 10% 32% 37% 14% 1	794
	21st to end	Night	7% 8% 27% 44% 13% 2	836
February	1st to 10th	Night	6% 11% 36% 38% 9% 1	760
	11th to 20th	Night	4 12% 37% 37% 10%	744
	21st to end	Night	7% 11% 40% 38% 5%	628
March	1st to 10th	Night	5% 11% 35% 40% 9%	760
	11th to 20th	Night	5% 7% 29% 45% 13%	760
	21st to end	Night	6% 11% 33% 39% 11% 1	777
April	1st to 10th	Night	9% 11% 34% 40% 7%	570
	11th to 20th	Night	10% 11% 34% 36% 9% 1	563
	21st to end	Night	9% 9% 30% 37% 14% 1	570
May	1st to 10th	Night	9% 8% 28% 42% 11% 1	570
	11th to 20th	Night	7% 4 21% 47% 19% 1	561
	21st to end	Night	6% 9% 21% 39% 23% 2	617
June	1st to 10th	Night	2 5% 18% 48% 25% 1	570
	11th to 20th	Night	4 4 24% 49% 19% 1	570
	21st to end	Night	2 3 17% 48% 29% 1	570
July	1st to 10th	Night	4 1 9% 43% 40% 3	555
	11th to 20th	Night	1 2 7% 43% 43% 4	552
	21st to end	Night	3 1 6% 39% 49% 2	624
August	1st to 10th	Night	6% 2 7% 42% 42% 1	567
	11th to 20th	Night	4 2 8% 52% 32% 2	564
	21st to end	Night	4 2 9% 45% 37% 3	627
September	1st to 10th	Night	4 2 12% 49% 31% 3	570
	11th to 20th	Night	4 2 17% 47% 27% 3	570
	21st to end	Night	4 4 19% 40% 29% 3	689
October	1st to 10th	Night	6% 6% 21% 46% 19% 3	760
	11th to 20th	Night	3 7% 28% 43% 18% 1	760
	21st to end	Night	9% 6% 28% 40% 15% 2	836
November	1st to 10th	Night	6% 7% 33% 37% 16% 1	911
	11th to 20th	Night	7% 12% 33% 36% 11% 2	1000
	21st to end	Night	6% 9% 28% 42% 14% 1	1000
December	1st to 10th	Night	8% 10% 31% 40% 11%	950
	11th to 20th	Night	7% 11% 32% 36% 13% 1	950
	21st to end	Night	8% 10% 33% 39% 9% 1	1045

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