3 Angels Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the 3 Angels Observatory clear sky chart from 2005-11-06 to 2024-01-14.

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	10% 5% 3 5% 6% 3 6% 6% 6% 9% 40%	74716
	Night	10% 5% 2 5% 5% 3 5% 5% 6% 8% 47%	75045
Transparency	Day	25% 39% 8% 12% 11% 5%	74462
	Night	23% 38% 9% 12% 13% 6%	74818
Seeing	Day	12% 7% 21% 26% 23% 12%	13237
	Night	12% 5% 20% 29% 26% 8%	23233

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	13% 6% 3 4 6% 3 4 5% 5% 7% 44%	1896
	11th to 20th	Day	14% 5% 3 4 6% 3 4 4 4 6% 47%	1830
	21st to end	Day	18% 7% 2 5% 4 3 4 3 4 5% 44%	1980
February	1st to 10th	Day	17% 7% 3 6% 5% 3 5% 4 3 6% 42%	1800
	11th to 20th	Day	16% 6% 3 5% 4 2 4 4 4 7% 43%	1748
	21st to end	Day	16% 7% 3 6% 6% 4 4 4 6% 7% 37%	1480
March	1st to 10th	Day	12% 5% 2 5% 5% 3 5% 5% 5% 8% 44%	1800
	11th to 20th	Day	14% 6% 2 5% 6% 3 5% 5% 6% 8% 40%	1887
	21st to end	Day	12% 6% 2 5% 5% 3 4 4 5% 7% 47%	2376
April	1st to 10th	Day	10% 5% 3 4 5% 3 5% 5% 6% 9% 44%	2160
	11th to 20th	Day	14% 5% 2 5% 5% 3 4 5% 6% 9% 42%	2134
	21st to end	Day	9% 4 2 4 4 3 4 4 6% 8% 52%	2160
May	1st to 10th	Day	10% 4 2 3 4 3 5% 5% 6% 9% 49%	2160
	11th to 20th	Day	8% 5% 2 5% 6% 3 7% 7% 8% 9% 41%	2145
	21st to end	Day	7% 4 2 5% 7% 4 6% 5% 8% 11% 41%	2531
June	1st to 10th	Day	9% 5% 3 6% 7% 4 8% 8% 9% 12% 30%	2340
	11th to 20th	Day	5% 4 2 5% 7% 4 8% 7% 10% 12% 35%	2340
	21st to end	Day	8% 6% 3 7% 7% 6% 6% 8% 10% 11% 27%	2340
July	1st to 10th	Day	8% 6% 4 8% 9% 5% 9% 9% 9% 12% 23%	2242
	11th to 20th	Day	8% 8% 4 8% 8% 4 8% 8% 8% 11% 25%	2160
	21st to end	Day	7% 6% 3 7% 8% 5% 8% 8% 8% 11% 29%	2350
August	1st to 10th	Day	5% 4 3 6% 8% 5% 8% 9% 11% 13% 27%	2148
	11th to 20th	Day	9% 6% 3 8% 8% 5% 8% 8% 9% 11% 26%	2125
	21st to end	Day	10% 7% 3 7% 7% 4 8% 8% 7% 10% 28%	2376
September	1st to 10th	Day	8% 5% 3 6% 8% 4 8% 6% 8% 10% 34%	2160
	11th to 20th	Day	10% 6% 4 6% 7% 3 5% 6% 7% 8% 39%	2012
	21st to end	Day	7% 5% 3 4 6% 5% 5% 6% 5% 6% 47%	1980
October	1st to 10th	Day	6% 4 2 4 5% 3 5% 5% 5% 8% 54%	1980
	11th to 20th	Day	7% 5% 2 4 4 3 4 5% 5% 8% 53%	1904
	21st to end	Day	10% 4 1 4 5% 2 4 4 5% 8% 52%	1980
November	1st to 10th	Day	8% 4 2 4 3 2 4 3 5% 7% 58%	1755
	11th to 20th	Day	12% 4 2 4 4 1 4 3 4 6% 57%	1710
	21st to end	Day	11% 5% 2 5% 6% 3 4 5% 5% 7% 46%	1710
December	1st to 10th	Day	13% 6% 3 6% 5% 3 3 3 5% 7% 45%	1710
	11th to 20th	Day	13% 8% 3 6% 6% 3 4 5% 5% 6% 42%	2153
	21st to end	Day	17% 7% 3 6% 4 3 4 4 4 7% 40%	3154
January	1st to 10th	Night	15% 6% 3 4 4 3 4 4 5% 6% 46%	2269
	11th to 20th	Night	14% 6% 3 5% 4 3 3 3 4 7% 49%	2208
	21st to end	Night	20% 5% 3 5% 4 3 4 4 4 5% 44%	2376
February	1st to 10th	Night	15% 6% 2 5% 4 3 4 4 5% 6% 45%	2160
	11th to 20th	Night	18% 6% 2 4 5% 3 3 4 4 7% 44%	2098
	21st to end	Night	17% 7% 3 6% 5% 3 5% 5% 6% 6% 37%	1776
March	1st to 10th	Night	12% 7% 2 5% 5% 3 5% 3 5% 7% 46%	2160
	11th to 20th	Night	13% 7% 3 5% 5% 2 5% 4 6% 9% 41%	2084
	21st to end	Night	12% 4 2 4 4 3 5% 4 4 7% 52%	1980
April	1st to 10th	Night	13% 6% 2 5% 5% 3 4 5% 5% 8% 45%	1800
	11th to 20th	Night	12% 5% 2 4 5% 3 5% 5% 6% 6% 48%	1775
	21st to end	Night	6% 3 1 4 4 3 3 4 5% 7% 60%	1800
May	1st to 10th	Night	9% 5% 2 3 3 3 4 4 4 9% 54%	1800
	11th to 20th	Night	5% 4 2 5% 6% 3 5% 6% 7% 10% 47%	1744
	21st to end	Night	5% 4 1 4 5% 3 6% 5% 7% 12% 47%	1744
June	1st to 10th	Night	7% 5% 2 5% 5% 3 6% 6% 9% 13% 39%	1620
	11th to 20th	Night	3 3 2 3 4 3 5% 6% 9% 15% 47%	1620
	21st to end	Night	6% 5% 3 5% 7% 4 5% 6% 8% 13% 38%	1620
July	1st to 10th	Night	5% 5% 3 4 6% 3 6% 6% 10% 11% 40%	1712
	11th to 20th	Night	3 3 2 5% 6% 4 7% 8% 10% 12% 40%	1800
	21st to end	Night	4 3 2 5% 6% 4 6% 7% 9% 14% 41%	1968
August	1st to 10th	Night	2 2 1 3 3 3 5% 8% 9% 15% 47%	1790
	11th to 20th	Night	5% 5% 2 4 6% 3 6% 9% 11% 14% 35%	1776
	21st to end	Night	7% 5% 3 5% 7% 4 5% 6% 7% 11% 41%	1980
September	1st to 10th	Night	5% 5% 2 6% 6% 3 5% 8% 8% 13% 39%	1800
	11th to 20th	Night	7% 6% 2 5% 6% 3 5% 5% 6% 9% 45%	1929
	21st to end	Night	6% 5% 2 5% 4 3 5% 5% 6% 8% 51%	1980
October	1st to 10th	Night	7% 3 1 4 4 3 4 4 4 8% 59%	1980
	11th to 20th	Night	7% 3 2 3 4 2 3 4 5% 7% 61%	2052
	21st to end	Night	11% 4 2 4 4 2 3 3 3 6% 56%	2376
November	1st to 10th	Night	9% 4 2 5% 4 3 3 3 4 7% 58%	2319
	11th to 20th	Night	10% 4 2 4 4 2 4 4 5% 7% 53%	2470
	21st to end	Night	11% 5% 2 5% 5% 3 4 4 5% 7% 49%	2470
December	1st to 10th	Night	13% 7% 2 5% 5% 2 3 4 5% 7% 45%	2470
	11th to 20th	Night	15% 7% 3 5% 5% 3 5% 4 5% 6% 44%	3042
	21st to end	Night	16% 8% 3 6% 5% 3 4 4 4 5% 41%	4497

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	22% 40% 4 11% 16% 7%	1897
	11th to 20th	Day	22% 42% 5% 10% 14% 7%	1830
	21st to end	Day	30% 38% 3 11% 12% 7%	1980
February	1st to 10th	Day	29% 36% 6% 9% 13% 6%	1800
	11th to 20th	Day	27% 38% 5% 9% 13% 9%	1758
	21st to end	Day	29% 38% 5% 12% 11% 5%	1480
March	1st to 10th	Day	23% 37% 6% 15% 14% 6%	1800
	11th to 20th	Day	24% 39% 9% 10% 12% 6%	1887
	21st to end	Day	23% 38% 8% 15% 12% 4	2376
April	1st to 10th	Day	23% 39% 8% 12% 10% 7%	2160
	11th to 20th	Day	24% 37% 6% 13% 14% 6%	2134
	21st to end	Day	18% 37% 7% 16% 16% 5%	2160
May	1st to 10th	Day	23% 39% 8% 16% 11% 2	2160
	11th to 20th	Day	23% 41% 10% 14% 11% 2	2149
	21st to end	Day	21% 40% 10% 18% 11% 1	2531
June	1st to 10th	Day	26% 42% 10% 16% 6% 1	2340
	11th to 20th	Day	19% 40% 12% 19% 8% 2	2340
	21st to end	Day	27% 42% 13% 11% 7% 1	2340
July	1st to 10th	Day	35% 44% 11% 8% 2	2182
	11th to 20th	Day	30% 45% 12% 9% 3 1	2097
	21st to end	Day	30% 45% 14% 10% 2	2219
August	1st to 10th	Day	25% 44% 14% 12% 5%	2136
	11th to 20th	Day	31% 42% 13% 9% 5%	2125
	21st to end	Day	33% 41% 9% 10% 5% 2	2376
September	1st to 10th	Day	26% 41% 11% 12% 8% 2	2160
	11th to 20th	Day	29% 40% 8% 9% 9% 5%	2012
	21st to end	Day	22% 39% 6% 13% 13% 7%	1980
October	1st to 10th	Day	18% 37% 7% 11% 15% 12%	1980
	11th to 20th	Day	20% 37% 5% 12% 17% 10%	1904
	21st to end	Day	19% 37% 5% 10% 19% 9%	1980
November	1st to 10th	Day	12% 40% 5% 12% 20% 11%	1755
	11th to 20th	Day	18% 39% 4 10% 19% 10%	1710
	21st to end	Day	23% 41% 5% 11% 13% 7%	1710
December	1st to 10th	Day	24% 40% 6% 12% 13% 5%	1710
	11th to 20th	Day	31% 33% 5% 9% 16% 6%	2150
	21st to end	Day	30% 26% 5% 15% 19% 5%	3154
January	1st to 10th	Night	22% 41% 5% 9% 17% 6%	2269
	11th to 20th	Night	23% 41% 4 10% 15% 7%	2208
	21st to end	Night	30% 37% 4 7% 14% 7%	2376
February	1st to 10th	Night	27% 36% 5% 11% 14% 8%	2160
	11th to 20th	Night	29% 37% 3 8% 13% 10%	2099
	21st to end	Night	30% 37% 6% 11% 8% 8%	1776
March	1st to 10th	Night	23% 36% 6% 11% 17% 8%	2160
	11th to 20th	Night	23% 38% 9% 11% 13% 6%	2084
	21st to end	Night	21% 37% 9% 13% 14% 6%	1980
April	1st to 10th	Night	25% 37% 8% 11% 12% 7%	1800
	11th to 20th	Night	22% 37% 6% 14% 15% 6%	1775
	21st to end	Night	14% 37% 8% 18% 18% 6%	1800
May	1st to 10th	Night	22% 37% 9% 17% 13% 3	1800
	11th to 20th	Night	21% 41% 11% 13% 13% 2	1745
	21st to end	Night	19% 39% 11% 18% 11% 1	1745
June	1st to 10th	Night	21% 41% 14% 17% 6% 1	1620
	11th to 20th	Night	15% 41% 16% 19% 8% 1	1620
	21st to end	Night	22% 42% 13% 15% 6% 1	1620
July	1st to 10th	Night	27% 44% 16% 10% 3	1662
	11th to 20th	Night	19% 46% 20% 11% 3 1	1740
	21st to end	Night	22% 44% 21% 11% 2	1858
August	1st to 10th	Night	13% 41% 23% 18% 4	1778
	11th to 20th	Night	21% 44% 18% 13% 4	1776
	21st to end	Night	26% 42% 15% 11% 4 3	1980
September	1st to 10th	Night	24% 40% 14% 11% 9% 3	1800
	11th to 20th	Night	25% 41% 10% 10% 8% 5%	1929
	21st to end	Night	19% 39% 8% 13% 13% 8%	1980
October	1st to 10th	Night	19% 36% 5% 10% 16% 13%	1980
	11th to 20th	Night	18% 36% 7% 11% 17% 11%	2052
	21st to end	Night	18% 36% 5% 10% 19% 12%	2376
November	1st to 10th	Night	14% 39% 3 11% 20% 13%	2319
	11th to 20th	Night	19% 40% 4 11% 17% 10%	2470
	21st to end	Night	23% 41% 4 12% 12% 8%	2470
December	1st to 10th	Night	23% 40% 5% 12% 15% 5%	2470
	11th to 20th	Night	29% 33% 5% 8% 17% 8%	3044
	21st to end	Night	31% 26% 5% 15% 17% 5%	4497

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	18% 17% 35% 23% 7%	270
	11th to 20th	Day	17% 17% 29% 27% 10% 1	249
	21st to end	Day	20% 22% 38% 16% 3	264
February	1st to 10th	Day	23% 20% 35% 18% 5%	240
	11th to 20th	Day	24% 18% 35% 17% 6%	240
	21st to end	Day	13% 21% 46% 17% 3	198
March	1st to 10th	Day	11% 18% 45% 23% 4	240
	11th to 20th	Day	17% 21% 39% 17% 6%	242
	21st to end	Day	14% 11% 35% 27% 11% 1	462
April	1st to 10th	Day	16% 11% 34% 26% 12% 1	420
	11th to 20th	Day	17% 13% 28% 26% 14% 2	412
	21st to end	Day	9% 7% 30% 32% 20% 2	420
May	1st to 10th	Day	9% 7% 31% 27% 21% 5%	423
	11th to 20th	Day	10% 2 19% 34% 25% 10%	447
	21st to end	Day	9% 2 12% 35% 29% 14%	485
June	1st to 10th	Day	11% 2 12% 31% 26% 18%	450
	11th to 20th	Day	9% 2 12% 29% 30% 18%	450
	21st to end	Day	11% 2 13% 23% 31% 21%	450
July	1st to 10th	Day	9% 1 8% 18% 36% 28%	445
	11th to 20th	Day	10% 1 6% 18% 35% 30%	444
	21st to end	Day	8% 1 6% 23% 40% 23%	488
August	1st to 10th	Day	4 1 5% 21% 36% 33%	446
	11th to 20th	Day	9% 1 6% 15% 36% 33%	439
	21st to end	Day	9% 1 10% 18% 36% 26%	495
September	1st to 10th	Day	6% 7% 24% 38% 24%	450
	11th to 20th	Day	11% 2 10% 27% 36% 14%	450
	21st to end	Day	9% 2 11% 30% 34% 14%	450
October	1st to 10th	Day	7% 4 19% 37% 27% 7%	450
	11th to 20th	Day	5% 5% 19% 32% 32% 7%	374
	21st to end	Day	16% 8% 20% 36% 15% 5%	297
November	1st to 10th	Day	13% 8% 24% 36% 18% 2	270
	11th to 20th	Day	15% 12% 27% 29% 16% 1	270
	21st to end	Day	12% 11% 33% 30% 11% 1	270
December	1st to 10th	Day	18% 10% 30% 29% 11% 2	270
	11th to 20th	Day	21% 16% 35% 21% 7%	270
	21st to end	Day	20% 15% 31% 24% 8% 2	297
January	1st to 10th	Night	18% 11% 31% 33% 6% 1	760
	11th to 20th	Night	14% 11% 28% 36% 11%	736
	21st to end	Night	21% 11% 32% 30% 6%	792
February	1st to 10th	Night	18% 11% 31% 32% 8%	720
	11th to 20th	Night	21% 9% 34% 27% 9% 1	703
	21st to end	Night	18% 14% 34% 30% 5%	592
March	1st to 10th	Night	14% 11% 32% 32% 10%	720
	11th to 20th	Night	15% 9% 31% 28% 16% 1	648
	21st to end	Night	12% 6% 30% 33% 17%	594
April	1st to 10th	Night	16% 8% 26% 33% 16% 1	540
	11th to 20th	Night	12% 7% 30% 29% 20% 2	533
	21st to end	Night	7% 4 24% 38% 25% 2	540
May	1st to 10th	Night	11% 3 17% 38% 27% 4	540
	11th to 20th	Night	6% 2 12% 42% 32% 5%	531
	21st to end	Night	7% 1 10% 32% 41% 9%	584
June	1st to 10th	Night	9% 7% 27% 45% 11%	540
	11th to 20th	Night	5% 1 5% 25% 49% 15%	540
	21st to end	Night	8% 1 6% 22% 42% 21%	540
July	1st to 10th	Night	7% 4 15% 49% 25%	525
	11th to 20th	Night	3 1 3 10% 45% 38%	522
	21st to end	Night	4 1 3 15% 48% 28%	591
August	1st to 10th	Night	3 2 9% 50% 36%	537
	11th to 20th	Night	7% 1 2 11% 47% 32%	534
	21st to end	Night	9% 1 5% 16% 49% 20%	594
September	1st to 10th	Night	7% 1 5% 21% 41% 25%	540
	11th to 20th	Night	9% 1 9% 25% 45% 10%	669
	21st to end	Night	8% 1 10% 32% 42% 7%	720
October	1st to 10th	Night	9% 2 16% 38% 30% 5%	720
	11th to 20th	Night	8% 4 18% 33% 31% 6%	720
	21st to end	Night	12% 5% 20% 34% 25% 3	792
November	1st to 10th	Night	11% 6% 18% 41% 21% 4	740
	11th to 20th	Night	11% 9% 25% 36% 17% 2	760
	21st to end	Night	13% 6% 29% 35% 16% 2	760
December	1st to 10th	Night	16% 8% 27% 33% 17% 1	760
	11th to 20th	Night	19% 7% 29% 31% 13% 1	760
	21st to end	Night	22% 9% 32% 27% 10%	836

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