FOAH Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the FOAH Observatory clear sky chart from 2005-11-06 to 2024-03-06.

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 3 2 4 4 3 4 5% 6% 7% 60%	78501
	Night	3 3 2 3 4 3 4 4 5% 7% 62%	80997
Transparency	Day	23% 6% 8% 16% 27% 20%	78227
	Night	21% 5% 8% 17% 29% 20%	80792
Seeing	Day	7% 14% 26% 29% 19% 6%	13470
	Night	5% 11% 28% 34% 20% 3	24148

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 3 2 6% 4 3 4 3 5% 7% 62%	2858
	11th to 20th	Day	1 3 1 3 3 2 3 4 5% 5% 70%	2860
	21st to end	Day	3 8% 4 7% 5% 3 3 4 6% 7% 51%	3146
February	1st to 10th	Day	1 4 2 3 3 2 3 4 5% 6% 66%	1710
	11th to 20th	Day	3 4 2 5% 4 3 4 4 3 5% 64%	1825
	21st to end	Day	2 4 2 4 2 2 3 2 5% 7% 68%	1594
March	1st to 10th	Day	2 5% 2 4 4 3 4 5% 6% 6% 58%	2036
	11th to 20th	Day	3 5% 2 4 4 2 4 3 4 6% 63%	1980
	21st to end	Day	2 3 2 3 3 2 4 4 5% 7% 65%	2183
April	1st to 10th	Day	2 3 2 3 3 2 4 4 4 6% 68%	2160
	11th to 20th	Day	3 4 1 3 4 2 4 4 4 6% 65%	2131
	21st to end	Day	2 2 1 3 4 2 3 4 5% 5% 70%	2160
May	1st to 10th	Day	2 2 1 2 2 2 4 4 5% 6% 69%	2160
	11th to 20th	Day	3 4 1 4 5% 3 5% 5% 5% 8% 57%	2218
	21st to end	Day	2 3 1 3 3 2 3 4 5% 6% 69%	2530
June	1st to 10th	Day	2 2 2 4 3 3 4 6% 7% 9% 59%	2351
	11th to 20th	Day	2 2 1 3 3 2 4 5% 4 6% 69%	2404
	21st to end	Day	5% 4 2 5% 5% 3 5% 6% 6% 7% 51%	2340
July	1st to 10th	Day	6% 3 1 5% 6% 4 7% 6% 8% 10% 43%	2340
	11th to 20th	Day	3 4 2 5% 6% 5% 7% 9% 9% 12% 39%	2340
	21st to end	Day	3 3 2 6% 7% 6% 10% 8% 10% 12% 33%	2545
August	1st to 10th	Day	5% 4 2 5% 7% 6% 8% 9% 10% 9% 36%	2307
	11th to 20th	Day	6% 3 2 5% 7% 3 6% 7% 9% 11% 41%	2123
	21st to end	Day	4 3 2 4 5% 4 6% 8% 8% 10% 45%	2376
September	1st to 10th	Day	7% 4 2 5% 5% 4 6% 5% 7% 7% 49%	2160
	11th to 20th	Day	5% 4 1 3 4 3 5% 6% 6% 8% 56%	2160
	21st to end	Day	3 1 1 2 4 2 4 5% 4 8% 66%	1908
October	1st to 10th	Day	5% 3 1 3 3 3 4 4 5% 7% 62%	1800
	11th to 20th	Day	3 3 1 2 3 2 2 2 2 5% 76%	1800
	21st to end	Day	2 2 1 2 2 1 3 2 3 5% 77%	1980
November	1st to 10th	Day	2 2 1 2 3 2 2 4 4 7% 73%	1859
	11th to 20th	Day	2 2 2 3 3 2 3 3 4 5% 73%	1858
	21st to end	Day	2 4 2 4 4 3 4 4 4 5% 65%	1710
December	1st to 10th	Day	3 3 1 3 2 2 3 4 6% 7% 67%	1535
	11th to 20th	Day	2 5% 2 4 5% 3 4 5% 5% 7% 59%	1926
	21st to end	Day	2 3 1 3 3 3 4 4 5% 7% 66%	3128
January	1st to 10th	Night	1 4 2 5% 4 3 4 4 6% 7% 60%	4097
	11th to 20th	Night	2 2 1 2 2 1 3 4 4 6% 73%	4110
	21st to end	Night	3 6% 2 4 4 3 5% 4 5% 8% 55%	4521
February	1st to 10th	Night	1 3 2 3 3 2 4 4 4 7% 67%	2470
	11th to 20th	Night	3 3 3 3 3 2 3 4 5% 6% 65%	2240
	21st to end	Night	1 3 1 2 3 2 4 4 6% 7% 67%	1856
March	1st to 10th	Night	1 3 1 4 2 3 3 4 5% 7% 67%	2040
	11th to 20th	Night	2 4 2 3 4 2 2 4 4 5% 67%	1980
	21st to end	Night	2 4 2 3 3 3 4 4 4 5% 68%	2171
April	1st to 10th	Night	2 2 2 3 4 3 4 3 4 5% 68%	1800
	11th to 20th	Night	3 4 2 3 3 2 3 3 4 5% 69%	1777
	21st to end	Night	3 3 2 4 3 2 3 4 4 6% 66%	1800
May	1st to 10th	Night	2 4 2 4 4 3 2 4 4 7% 66%	1800
	11th to 20th	Night	4 4 2 3 4 3 5% 4 5% 7% 58%	1673
	21st to end	Night	1 1 1 3 3 2 3 3 5% 7% 70%	1746
June	1st to 10th	Night	2 3 2 4 5% 3 5% 6% 5% 7% 57%	1575
	11th to 20th	Night	2 2 2 3 3 3 4 4 4 6% 67%	1521
	21st to end	Night	5% 4 3 5% 5% 3 6% 5% 5% 8% 50%	1620
July	1st to 10th	Night	5% 5% 2 6% 6% 4 7% 6% 7% 10% 40%	1620
	11th to 20th	Night	3 4 2 6% 6% 4 8% 8% 10% 14% 34%	1620
	21st to end	Night	4 3 3 6% 8% 5% 7% 8% 9% 12% 36%	1773
August	1st to 10th	Night	2 3 2 5% 7% 4 8% 8% 9% 12% 40%	1632
	11th to 20th	Night	5% 4 2 5% 6% 4 6% 7% 8% 12% 41%	1778
	21st to end	Night	4 3 2 4 6% 4 6% 7% 9% 11% 45%	1980
September	1st to 10th	Night	6% 4 2 6% 5% 4 6% 7% 7% 7% 47%	1800
	11th to 20th	Night	4 3 2 3 4 3 5% 5% 7% 8% 57%	1800
	21st to end	Night	3 3 1 3 4 2 3 4 5% 6% 65%	2046
October	1st to 10th	Night	4 3 1 2 4 3 4 4 5% 6% 64%	2160
	11th to 20th	Night	2 2 1 3 2 2 2 3 3 3 78%	2160
	21st to end	Night	3 2 1 2 2 2 2 2 3 5% 76%	2376
November	1st to 10th	Night	2 2 1 2 2 1 2 4 5% 6% 75%	2220
	11th to 20th	Night	1 2 1 2 2 1 2 3 3 7% 75%	2299
	21st to end	Night	3 5% 2 3 2 2 3 3 3 7% 67%	2470
December	1st to 10th	Night	2 3 2 3 3 2 3 4 5% 6% 66%	2642
	11th to 20th	Night	3 3 2 3 4 3 5% 5% 6% 9% 57%	3271
	21st to end	Night	1 3 2 3 3 2 3 4 5% 6% 68%	4553

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% 4 6% 16% 41% 10%	2859
	11th to 20th	Day	16% 5% 8% 25% 35% 11%	2860
	21st to end	Day	31% 6% 5% 17% 30% 11%	3146
February	1st to 10th	Day	19% 3 7% 19% 28% 24%	1710
	11th to 20th	Day	23% 3 5% 16% 30% 22%	1835
	21st to end	Day	17% 3 5% 13% 33% 29%	1594
March	1st to 10th	Day	24% 4 6% 13% 30% 23%	2036
	11th to 20th	Day	24% 3 4 12% 33% 23%	1980
	21st to end	Day	19% 4 6% 18% 33% 21%	2183
April	1st to 10th	Day	18% 3 6% 14% 37% 23%	2160
	11th to 20th	Day	21% 3 4 12% 30% 30%	2134
	21st to end	Day	17% 4 6% 15% 36% 22%	2160
May	1st to 10th	Day	16% 4 5% 11% 34% 29%	2160
	11th to 20th	Day	26% 5% 9% 13% 26% 20%	2221
	21st to end	Day	16% 4 7% 12% 28% 33%	2530
June	1st to 10th	Day	20% 6% 7% 15% 27% 26%	2351
	11th to 20th	Day	16% 4 6% 9% 20% 45%	2404
	21st to end	Day	28% 6% 9% 15% 19% 23%	2340
July	1st to 10th	Day	32% 9% 13% 15% 17% 14%	2276
	11th to 20th	Day	31% 13% 16% 21% 12% 7%	2271
	21st to end	Day	36% 14% 21% 20% 7% 2	2404
August	1st to 10th	Day	36% 13% 18% 16% 12% 5%	2292
	11th to 20th	Day	32% 11% 16% 17% 17% 6%	2123
	21st to end	Day	30% 11% 14% 20% 20% 5%	2376
September	1st to 10th	Day	31% 8% 10% 18% 19% 14%	2160
	11th to 20th	Day	26% 9% 8% 14% 25% 18%	2160
	21st to end	Day	17% 6% 7% 17% 31% 22%	1908
October	1st to 10th	Day	22% 6% 8% 17% 23% 24%	1800
	11th to 20th	Day	15% 4 2 11% 31% 37%	1800
	21st to end	Day	13% 3 5% 17% 41% 22%	1980
November	1st to 10th	Day	13% 3 3 21% 36% 25%	1859
	11th to 20th	Day	15% 2 4 19% 37% 23%	1858
	21st to end	Day	22% 4 5% 16% 33% 21%	1710
December	1st to 10th	Day	18% 5% 5% 26% 30% 17%	1535
	11th to 20th	Day	25% 7% 7% 18% 30% 13%	1924
	21st to end	Day	19% 3 7% 18% 28% 25%	3128
January	1st to 10th	Night	22% 5% 7% 16% 37% 13%	4097
	11th to 20th	Night	14% 4 7% 24% 40% 11%	4110
	21st to end	Night	28% 6% 6% 19% 33% 9%	4521
February	1st to 10th	Night	19% 4 7% 20% 28% 23%	2470
	11th to 20th	Night	20% 4 6% 20% 26% 24%	2240
	21st to end	Night	17% 4 5% 14% 33% 27%	1856
March	1st to 10th	Night	18% 4 5% 15% 35% 22%	2040
	11th to 20th	Night	20% 4 5% 15% 34% 23%	1980
	21st to end	Night	19% 3 6% 18% 33% 20%	2171
April	1st to 10th	Night	19% 4 4 14% 36% 24%	1800
	11th to 20th	Night	19% 3 4 14% 29% 31%	1777
	21st to end	Night	19% 4 6% 13% 37% 21%	1800
May	1st to 10th	Night	20% 3 6% 9% 32% 30%	1800
	11th to 20th	Night	24% 6% 8% 15% 27% 20%	1674
	21st to end	Night	15% 4 5% 13% 28% 35%	1746
June	1st to 10th	Night	24% 6% 8% 14% 21% 28%	1575
	11th to 20th	Night	19% 3 5% 9% 21% 44%	1521
	21st to end	Night	30% 6% 9% 13% 15% 27%	1620
July	1st to 10th	Night	37% 8% 13% 17% 13% 12%	1575
	11th to 20th	Night	32% 13% 20% 21% 9% 5%	1566
	21st to end	Night	35% 13% 23% 20% 8% 2	1674
August	1st to 10th	Night	31% 12% 21% 22% 10% 5%	1623
	11th to 20th	Night	33% 11% 19% 16% 14% 8%	1778
	21st to end	Night	28% 11% 19% 20% 16% 6%	1980
September	1st to 10th	Night	32% 10% 11% 16% 18% 13%	1800
	11th to 20th	Night	23% 8% 9% 16% 22% 21%	1800
	21st to end	Night	19% 5% 7% 15% 31% 22%	2046
October	1st to 10th	Night	21% 6% 9% 17% 24% 23%	2160
	11th to 20th	Night	13% 2 5% 12% 29% 39%	2160
	21st to end	Night	14% 3 5% 17% 39% 23%	2376
November	1st to 10th	Night	12% 3 5% 21% 36% 24%	2220
	11th to 20th	Night	12% 2 4 19% 36% 26%	2299
	21st to end	Night	20% 4 5% 17% 34% 19%	2470
December	1st to 10th	Night	19% 5% 6% 23% 32% 15%	2642
	11th to 20th	Night	23% 6% 7% 19% 29% 15%	3272
	21st to end	Night	18% 4 6% 20% 29% 23%	4553

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	6% 21% 33% 30% 9% 1	270
	11th to 20th	Day	10% 23% 26% 27% 11% 3	270
	21st to end	Day	5% 21% 32% 23% 17% 2	297
February	1st to 10th	Day	6% 21% 33% 25% 13% 1	270
	11th to 20th	Day	11% 18% 41% 25% 4	270
	21st to end	Day	15% 31% 29% 18% 5% 1	249
March	1st to 10th	Day	12% 28% 33% 21% 5% 1	441
	11th to 20th	Day	13% 25% 31% 23% 6% 1	420
	21st to end	Day	11% 37% 30% 17% 4 1	462
April	1st to 10th	Day	8% 26% 39% 19% 7% 1	420
	11th to 20th	Day	12% 26% 39% 16% 6% 1	412
	21st to end	Day	9% 25% 37% 22% 7%	420
May	1st to 10th	Day	6% 23% 37% 26% 7% 1	423
	11th to 20th	Day	6% 15% 33% 31% 12% 4	448
	21st to end	Day	5% 18% 39% 27% 11% 1	485
June	1st to 10th	Day	3 7% 31% 37% 20% 3	461
	11th to 20th	Day	3 8% 24% 37% 23% 4	514
	21st to end	Day	7% 4 25% 37% 20% 7%	450
July	1st to 10th	Day	5% 2 13% 32% 35% 12%	446
	11th to 20th	Day	6% 2 6% 30% 38% 18%	444
	21st to end	Day	4 1 6% 25% 40% 24%	487
August	1st to 10th	Day	6% 1 9% 28% 38% 18%	447
	11th to 20th	Day	7% 2 12% 33% 33% 14%	437
	21st to end	Day	4 1 8% 31% 39% 16%	495
September	1st to 10th	Day	8% 4 11% 33% 34% 11%	450
	11th to 20th	Day	4 2 20% 38% 29% 7%	450
	21st to end	Day	4 7% 23% 36% 22% 8%	348
October	1st to 10th	Day	8% 5% 27% 29% 24% 7%	270
	11th to 20th	Day	9% 7% 25% 35% 20% 4	270
	21st to end	Day	9% 10% 21% 35% 19% 5%	297
November	1st to 10th	Day	9% 12% 32% 32% 14% 1	270
	11th to 20th	Day	3 11% 24% 38% 21% 4	270
	21st to end	Day	7% 21% 34% 20% 15% 2	270
December	1st to 10th	Day	7% 16% 27% 37% 10% 1	270
	11th to 20th	Day	4 23% 30% 28% 13% 2	270
	21st to end	Day	12% 21% 31% 28% 7% 1	297
January	1st to 10th	Night	4 13% 31% 31% 16% 3	760
	11th to 20th	Night	6% 15% 33% 30% 14% 2	760
	21st to end	Night	5% 17% 27% 34% 15% 3	836
February	1st to 10th	Night	4 17% 35% 31% 12% 1	760
	11th to 20th	Night	5% 16% 41% 29% 7% 1	744
	21st to end	Night	6% 21% 41% 26% 5%	628
March	1st to 10th	Night	5% 22% 33% 30% 10% 1	742
	11th to 20th	Night	5% 17% 33% 32% 13% 1	720
	21st to end	Night	5% 19% 38% 31% 6%	785
April	1st to 10th	Night	3 21% 45% 24% 6%	540
	11th to 20th	Night	5% 23% 41% 25% 6%	533
	21st to end	Night	4 19% 43% 25% 9%	540
May	1st to 10th	Night	3 11% 47% 33% 6%	540
	11th to 20th	Night	5% 8% 33% 38% 15% 2	531
	21st to end	Night	2 7% 45% 36% 10%	584
June	1st to 10th	Night	3 5% 32% 44% 15% 1	540
	11th to 20th	Night	4 6% 34% 41% 15%	540
	21st to end	Night	6% 2 20% 44% 25% 2	540
July	1st to 10th	Night	6% 1 15% 35% 38% 4	525
	11th to 20th	Night	5% 1 7% 31% 49% 7%	522
	21st to end	Night	4 1 8% 25% 49% 12%	591
August	1st to 10th	Night	2 1 6% 28% 53% 11%	537
	11th to 20th	Night	6% 1 8% 35% 44% 6%	534
	21st to end	Night	5% 1 5% 35% 46% 9%	594
September	1st to 10th	Night	7% 1 10% 41% 37% 5%	540
	11th to 20th	Night	6% 1 10% 49% 29% 5%	540
	21st to end	Night	5% 3 18% 41% 30% 3	686
October	1st to 10th	Night	5% 3 28% 41% 19% 4	720
	11th to 20th	Night	2 6% 28% 40% 20% 4	720
	21st to end	Night	5% 9% 26% 31% 23% 6%	792
November	1st to 10th	Night	3 7% 25% 41% 20% 4	740
	11th to 20th	Night	2 9% 24% 40% 20% 4	779
	21st to end	Night	5% 15% 25% 32% 19% 4	950
December	1st to 10th	Night	4 12% 32% 35% 16% 2	950
	11th to 20th	Night	6% 15% 31% 35% 11% 1	950
	21st to end	Night	5% 17% 31% 37% 9% 2	855

Page generated at 2024-03-03 04:28:59. Version ac07

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