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 2024-01-15.

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	3 5% 3 6% 5% 3 5% 5% 6% 8% 49%	74746
	Night	3 6% 3 6% 5% 3 4 5% 5% 8% 52%	75043
Transparency	Day	32% 6% 13% 23% 18% 7%	74461
	Night	31% 7% 13% 25% 19% 5%	74851
Seeing	Day	8% 15% 30% 29% 15% 3	14354
	Night	6% 7% 25% 41% 19% 1	24569

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	2 7% 4 8% 8% 3 5% 5% 7% 10% 43%	1518
	11th to 20th	Day	1 8% 6% 8% 7% 3 6% 5% 7% 7% 43%	1639
	21st to end	Day	1 8% 5% 11% 7% 3 5% 4 5% 8% 42%	1782
February	1st to 10th	Day	1 10% 5% 9% 8% 5% 6% 5% 5% 6% 42%	1620
	11th to 20th	Day	2 8% 4 8% 8% 4 7% 6% 6% 8% 39%	1633
	21st to end	Day	1 7% 6% 10% 8% 4 7% 6% 7% 7% 37%	1480
March	1st to 10th	Day	2 8% 4 8% 6% 4 5% 6% 6% 8% 45%	1894
	11th to 20th	Day	2 6% 3 7% 6% 4 4 6% 6% 8% 49%	1980
	21st to end	Day	2 9% 4 8% 6% 4 7% 6% 7% 8% 40%	2232
April	1st to 10th	Day	3 7% 3 6% 6% 3 6% 6% 7% 8% 45%	2160
	11th to 20th	Day	3 9% 3 7% 8% 4 6% 6% 7% 8% 39%	2131
	21st to end	Day	3 7% 4 6% 7% 4 5% 6% 6% 9% 41%	2160
May	1st to 10th	Day	7% 7% 3 5% 5% 4 6% 7% 8% 9% 39%	2277
	11th to 20th	Day	6% 5% 3 6% 6% 4 5% 6% 8% 10% 42%	2468
	21st to end	Day	4 4 2 5% 5% 3 6% 6% 8% 10% 46%	2724
June	1st to 10th	Day	3 2 1 4 4 3 4 5% 6% 10% 57%	2520
	11th to 20th	Day	4 3 2 2 3 2 3 5% 6% 8% 64%	2520
	21st to end	Day	2 1 1 3 3 3 4 5% 6% 9% 63%	2520
July	1st to 10th	Day	4 2 1 3 4 4 5% 5% 7% 9% 54%	2520
	11th to 20th	Day	2 2 2 4 5% 3 6% 7% 8% 10% 51%	2520
	21st to end	Day	4 4 2 5% 6% 3 6% 7% 8% 11% 43%	2545
August	1st to 10th	Day	5% 3 2 5% 6% 4 6% 6% 7% 9% 46%	2328
	11th to 20th	Day	2 4 2 4 5% 3 4 5% 7% 9% 54%	2218
	21st to end	Day	3 3 1 5% 5% 3 5% 6% 6% 10% 53%	2376
September	1st to 10th	Day	4 3 1 3 4 3 4 4 6% 7% 61%	2160
	11th to 20th	Day	4 3 2 3 4 3 4 4 5% 8% 60%	2160
	21st to end	Day	5% 3 2 3 3 2 4 4 4 6% 63%	1920
October	1st to 10th	Day	5% 4 2 4 5% 3 4 5% 6% 6% 57%	1800
	11th to 20th	Day	5% 4 2 3 3 2 3 3 5% 8% 62%	1800
	21st to end	Day	4 6% 3 5% 3 2 4 4 5% 6% 58%	1980
November	1st to 10th	Day	2 5% 3 4 3 2 3 4 6% 8% 61%	1714
	11th to 20th	Day	3 7% 3 5% 5% 2 4 4 5% 7% 56%	1687
	21st to end	Day	2 6% 4 8% 5% 2 3 4 4 6% 56%	1520
December	1st to 10th	Day	1 7% 5% 9% 7% 4 5% 6% 6% 7% 44%	1520
	11th to 20th	Day	2 6% 5% 8% 6% 3 5% 5% 5% 10% 45%	1926
	21st to end	Day	1 8% 4 11% 8% 3 5% 5% 8% 8% 39%	2794
January	1st to 10th	Night	2 8% 4 9% 7% 4 4 5% 6% 7% 45%	2647
	11th to 20th	Night	1 9% 4 7% 7% 4 5% 5% 4 7% 46%	2426
	21st to end	Night	2 10% 6% 11% 7% 3 4 4 4 6% 42%	2574
February	1st to 10th	Night	1 8% 5% 9% 8% 3 6% 6% 5% 7% 42%	2340
	11th to 20th	Night	2 8% 6% 9% 7% 3 6% 6% 5% 9% 40%	2212
	21st to end	Night	1 11% 5% 10% 8% 3 5% 5% 4 6% 41%	1776
March	1st to 10th	Night	1 7% 5% 6% 5% 3 6% 5% 7% 9% 47%	2063
	11th to 20th	Night	2 8% 4 6% 4 2 4 4 5% 7% 55%	1980
	21st to end	Night	2 11% 4 7% 5% 3 5% 5% 6% 8% 44%	2123
April	1st to 10th	Night	5% 8% 3 6% 5% 3 6% 6% 6% 7% 46%	1800
	11th to 20th	Night	4 10% 4 6% 4 3 6% 4 6% 8% 45%	1777
	21st to end	Night	5% 9% 3 6% 5% 3 4 5% 5% 8% 49%	1800
May	1st to 10th	Night	7% 6% 2 5% 5% 4 4 6% 6% 10% 47%	1679
	11th to 20th	Night	5% 5% 3 5% 5% 3 5% 5% 7% 9% 48%	1424
	21st to end	Night	5% 4 1 4 4 3 4 5% 6% 8% 56%	1552
June	1st to 10th	Night	3 1 2 2 2 3 4 4 5% 9% 65%	1440
	11th to 20th	Night	3 2 1 3 3 3 4 5% 5% 7% 64%	1440
	21st to end	Night	3 1 1 2 2 2 3 4 7% 8% 68%	1440
July	1st to 10th	Night	3 2 1 3 4 2 4 5% 8% 10% 58%	1440
	11th to 20th	Night	1 2 2 3 4 3 5% 5% 7% 11% 57%	1440
	21st to end	Night	3 3 2 3 5% 3 6% 7% 7% 10% 49%	1770
August	1st to 10th	Night	5% 3 2 3 5% 3 4 6% 7% 9% 54%	1611
	11th to 20th	Night	3 3 2 4 4 2 4 4 6% 9% 59%	1681
	21st to end	Night	3 3 1 4 4 3 5% 5% 5% 10% 57%	1980
September	1st to 10th	Night	4 2 2 3 4 3 4 5% 5% 7% 63%	1800
	11th to 20th	Night	3 2 2 3 4 2 4 3 5% 9% 62%	1800
	21st to end	Night	3 3 1 3 3 2 3 3 5% 6% 67%	2034
October	1st to 10th	Night	5% 4 2 4 4 3 3 4 5% 8% 59%	2160
	11th to 20th	Night	3 4 2 4 4 2 3 3 4 6% 66%	2160
	21st to end	Night	5% 8% 2 4 4 2 2 3 4 5% 61%	2376
November	1st to 10th	Night	3 5% 2 3 3 2 3 3 5% 7% 65%	2344
	11th to 20th	Night	3 7% 3 6% 4 2 3 3 4 6% 57%	2490
	21st to end	Night	2 8% 3 7% 4 3 4 3 5% 7% 54%	2660
December	1st to 10th	Night	2 10% 4 8% 6% 3 5% 5% 5% 5% 48%	2660
	11th to 20th	Night	2 8% 4 8% 6% 4 5% 6% 6% 8% 44%	3271
	21st to end	Night	1 9% 5% 10% 8% 4 6% 5% 6% 7% 39%	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	38% 7% 18% 27% 8% 2	1519
	11th to 20th	Day	41% 6% 16% 25% 12% 1	1639
	21st to end	Day	43% 5% 19% 23% 8% 2	1782
February	1st to 10th	Day	46% 6% 17% 18% 11% 2	1620
	11th to 20th	Day	45% 6% 11% 23% 13% 2	1642
	21st to end	Day	45% 6% 12% 21% 13% 3	1480
March	1st to 10th	Day	41% 5% 11% 26% 18%	1894
	11th to 20th	Day	34% 6% 16% 27% 14% 3	1980
	21st to end	Day	43% 7% 12% 25% 10% 2	2232
April	1st to 10th	Day	37% 6% 14% 21% 17% 6%	2160
	11th to 20th	Day	42% 8% 12% 19% 17% 2	2134
	21st to end	Day	40% 6% 11% 27% 15% 2	2160
May	1st to 10th	Day	40% 8% 11% 21% 17% 2	2277
	11th to 20th	Day	36% 8% 15% 25% 13% 3	2472
	21st to end	Day	32% 7% 13% 23% 19% 5%	2724
June	1st to 10th	Day	24% 6% 11% 24% 26% 10%	2520
	11th to 20th	Day	19% 5% 9% 17% 28% 22%	2520
	21st to end	Day	17% 6% 7% 23% 26% 21%	2520
July	1st to 10th	Day	24% 7% 11% 18% 22% 17%	2451
	11th to 20th	Day	24% 9% 15% 23% 18% 11%	2445
	21st to end	Day	34% 9% 17% 21% 14% 6%	2404
August	1st to 10th	Day	33% 8% 15% 19% 16% 8%	2313
	11th to 20th	Day	25% 7% 13% 19% 25% 11%	2218
	21st to end	Day	27% 7% 13% 22% 22% 10%	2376
September	1st to 10th	Day	23% 5% 9% 19% 25% 19%	2160
	11th to 20th	Day	25% 5% 11% 20% 27% 11%	2160
	21st to end	Day	23% 4 7% 27% 29% 9%	1920
October	1st to 10th	Day	29% 5% 11% 25% 24% 7%	1800
	11th to 20th	Day	23% 4 9% 32% 26% 7%	1800
	21st to end	Day	29% 5% 12% 32% 18% 4	1980
November	1st to 10th	Day	24% 5% 13% 28% 28% 2	1714
	11th to 20th	Day	31% 5% 14% 30% 18% 3	1687
	21st to end	Day	32% 4 11% 33% 16% 4	1520
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	40% 6% 18% 25% 10% 1	2647
	11th to 20th	Night	38% 7% 16% 25% 13% 1	2426
	21st to end	Night	44% 6% 16% 23% 9% 2	2574
February	1st to 10th	Night	40% 10% 16% 23% 10% 1	2340
	11th to 20th	Night	42% 7% 11% 25% 14% 1	2213
	21st to end	Night	43% 6% 10% 24% 15% 2	1776
March	1st to 10th	Night	34% 6% 11% 32% 16%	2063
	11th to 20th	Night	30% 6% 13% 34% 14% 3	1980
	21st to end	Night	38% 8% 13% 28% 11% 2	2123
April	1st to 10th	Night	36% 8% 11% 22% 19% 4	1800
	11th to 20th	Night	37% 7% 12% 24% 18% 2	1777
	21st to end	Night	36% 5% 11% 27% 20% 1	1800
May	1st to 10th	Night	34% 7% 14% 27% 16% 2	1679
	11th to 20th	Night	31% 8% 15% 29% 14% 3	1425
	21st to end	Night	24% 6% 15% 26% 22% 6%	1552
June	1st to 10th	Night	17% 5% 14% 25% 28% 10%	1440
	11th to 20th	Night	19% 6% 9% 16% 24% 25%	1440
	21st to end	Night	13% 5% 9% 22% 27% 25%	1440
July	1st to 10th	Night	20% 11% 13% 19% 20% 18%	1400
	11th to 20th	Night	19% 9% 19% 21% 19% 14%	1392
	21st to end	Night	25% 10% 24% 20% 13% 8%	1672
August	1st to 10th	Night	24% 10% 17% 24% 16% 8%	1602
	11th to 20th	Night	22% 6% 15% 20% 24% 13%	1681
	21st to end	Night	23% 7% 14% 21% 22% 13%	1980
September	1st to 10th	Night	22% 5% 11% 16% 26% 20%	1800
	11th to 20th	Night	21% 5% 13% 21% 27% 14%	1800
	21st to end	Night	19% 4 8% 27% 34% 8%	2034
October	1st to 10th	Night	25% 6% 10% 29% 21% 9%	2160
	11th to 20th	Night	22% 4 10% 28% 29% 7%	2160
	21st to end	Night	27% 5% 12% 31% 22% 4	2376
November	1st to 10th	Night	21% 5% 13% 32% 26% 3	2344
	11th to 20th	Night	30% 4 13% 31% 19% 3	2490
	21st to end	Night	30% 5% 10% 32% 20% 2	2660
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	9% 10% 41% 33% 6% 1	252
	21st to end	Day	7% 9% 42% 31% 10%	264
February	1st to 10th	Day	14% 23% 35% 23% 4 1	240
	11th to 20th	Day	11% 24% 33% 27% 4 1	240
	21st to end	Day	17% 23% 34% 20% 6%	198
March	1st to 10th	Day	12% 23% 36% 22% 6% 1	334
	11th to 20th	Day	8% 25% 35% 24% 8%	420
	21st to end	Day	11% 30% 39% 16% 4	462
April	1st to 10th	Day	15% 25% 34% 19% 6% 1	420
	11th to 20th	Day	17% 31% 31% 16% 4 1	412
	21st to end	Day	15% 26% 34% 18% 7% 1	420
May	1st to 10th	Day	13% 27% 32% 20% 8%	423
	11th to 20th	Day	10% 14% 30% 28% 16% 2	615
	21st to end	Day	9% 15% 30% 29% 15% 2	679
June	1st to 10th	Day	5% 12% 26% 34% 21% 3	630
	11th to 20th	Day	7% 14% 30% 33% 15% 2	630
	21st to end	Day	3 10% 28% 35% 23% 1	630
July	1st to 10th	Day	5% 4 21% 34% 32% 5%	621
	11th to 20th	Day	3 5% 20% 37% 31% 5%	618
	21st to end	Day	5% 5% 20% 40% 24% 5%	487
August	1st to 10th	Day	7% 5% 29% 33% 21% 5%	447
	11th to 20th	Day	4 6% 22% 40% 22% 7%	437
	21st to end	Day	3 8% 24% 36% 23% 6%	495
September	1st to 10th	Day	6% 10% 27% 31% 20% 6%	450
	11th to 20th	Day	6% 11% 35% 27% 18% 4	450
	21st to end	Day	7% 14% 27% 26% 19% 6%	326
October	1st to 10th	Day	10% 14% 24% 24% 21% 7%	270
	11th to 20th	Day	9% 17% 27% 26% 17% 3	270
	21st to end	Day	11% 19% 30% 25% 14% 2	297
November	1st to 10th	Day	14% 19% 31% 24% 11% 1	270
	11th to 20th	Day	8% 17% 35% 27% 10% 2	270
	21st to end	Day	7% 15% 33% 37% 9%	270
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% 9% 32% 38% 14% 1	770
	21st to end	Night	7% 8% 28% 44% 12% 2	792
February	1st to 10th	Night	6% 11% 35% 38% 9% 1	720
	11th to 20th	Night	4 12% 36% 38% 10%	704
	21st to end	Night	7% 10% 39% 39% 5%	592
March	1st to 10th	Night	4 10% 35% 40% 10%	720
	11th to 20th	Night	5% 7% 28% 46% 13%	720
	21st to end	Night	6% 11% 33% 39% 11% 1	737
April	1st to 10th	Night	9% 10% 34% 40% 7%	540
	11th to 20th	Night	9% 11% 33% 37% 10% 1	533
	21st to end	Night	9% 9% 29% 38% 14% 1	540
May	1st to 10th	Night	9% 8% 27% 43% 12% 1	540
	11th to 20th	Night	7% 5% 20% 46% 20% 1	531
	21st to end	Night	6% 9% 21% 39% 23% 2	584
June	1st to 10th	Night	2 5% 19% 46% 26% 1	540
	11th to 20th	Night	4 4 23% 48% 20% 1	540
	21st to end	Night	3 3 17% 46% 31% 1	540
July	1st to 10th	Night	4 1 7% 43% 42% 3	525
	11th to 20th	Night	1 2 7% 43% 43% 4	522
	21st to end	Night	3 2 4 37% 51% 2	591
August	1st to 10th	Night	6% 2 7% 41% 43% 1	537
	11th to 20th	Night	4 2 7% 51% 33% 3	534
	21st to end	Night	4 2 9% 43% 38% 4	594
September	1st to 10th	Night	4 1 12% 50% 29% 3	540
	11th to 20th	Night	4 2 16% 47% 28% 3	540
	21st to end	Night	4 4 19% 40% 29% 3	652
October	1st to 10th	Night	6% 6% 22% 44% 19% 3	720
	11th to 20th	Night	3 7% 28% 42% 18% 2	720
	21st to end	Night	9% 6% 27% 39% 16% 3	792
November	1st to 10th	Night	6% 7% 32% 37% 17% 1	864
	11th to 20th	Night	7% 12% 31% 37% 12% 2	950
	21st to end	Night	7% 9% 26% 43% 14% 1	950
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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