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 2026-08-30.

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 3 1 4 4 4 5% 4 6% 7% 60%	150992
	Night	2 3 1 4 3 4 4 4 6% 8% 61%	148736
Transparency	Day	20% 6% 8% 16% 27% 22%	145977
	Night	20% 5% 8% 17% 29% 22%	144758
Seeing	Day	7% 14% 26% 29% 18% 5%	16796
	Night	4 11% 29% 35% 18% 3	27337

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 5% 3 6% 3 4 4 3 5% 7% 60%	5318
	11th to 20th	Day	1 2 1 3 2 2 3 3 4 4 76%	5340
	21st to end	Day	3 7% 3 5% 3 3 3 3 5% 6% 60%	5844
February	1st to 10th	Day	1 4 2 4 3 3 3 3 5% 6% 66%	1890
	11th to 20th	Day	2 4 2 5% 4 4 4 3 4 5% 63%	2023
	21st to end	Day	1 4 1 4 2 3 2 2 5% 6% 69%	1756
March	1st to 10th	Day	1 4 2 4 3 4 3 3 5% 6% 66%	5064
	11th to 20th	Day	3 6% 2 5% 3 3 4 3 5% 7% 58%	5110
	21st to end	Day	1 3 2 5% 3 4 5% 4 6% 7% 60%	5637
April	1st to 10th	Day	2 4 1 3 3 4 3 4 4 6% 67%	2520
	11th to 20th	Day	2 5% 1 3 3 3 4 3 4 6% 65%	2491
	21st to end	Day	2 2 1 3 3 2 3 3 5% 5% 70%	2520
May	1st to 10th	Day	2 4 1 4 3 4 4 3 5% 7% 63%	7057
	11th to 20th	Day	2 2 1 3 3 4 4 4 5% 6% 67%	7346
	21st to end	Day	1 2 1 4 3 3 4 4 5% 6% 67%	8370
June	1st to 10th	Day	2 3 1 4 3 4 5% 4 7% 9% 57%	2742
	11th to 20th	Day	2 2 1 3 3 3 4 4 5% 6% 68%	2803
	21st to end	Day	5% 4 2 6% 4 5% 5% 5% 6% 7% 51%	2730
July	1st to 10th	Day	3 2 1 5% 5% 6% 6% 5% 8% 8% 51%	7649
	11th to 20th	Day	2 2 1 4 5% 6% 7% 6% 9% 11% 47%	7650
	21st to end	Day	3 3 2 7% 7% 8% 9% 6% 9% 11% 34%	8304
August	1st to 10th	Day	2 2 1 5% 5% 7% 7% 6% 9% 10% 46%	5849
	11th to 20th	Day	3 2 1 5% 5% 5% 7% 6% 9% 11% 46%	5486
	21st to end	Day	2 3 1 6% 5% 7% 7% 6% 9% 11% 42%	6093
September	1st to 10th	Day	6% 4 2 5% 4 5% 5% 4 7% 7% 50%	2400
	11th to 20th	Day	5% 4 1 4 4 4 5% 5% 6% 8% 55%	2400
	21st to end	Day	3 1 1 3 3 3 4 3 5% 8% 66%	2118
October	1st to 10th	Day	3 2 1 2 2 2 3 2 4 5% 75%	3270
	11th to 20th	Day	2 2 1 2 2 3 3 2 4 7% 71%	3250
	21st to end	Day	1 1 1 1 1 2 1 2 4 84%	3548
November	1st to 10th	Day	2 2 1 3 2 2 2 3 4 7% 73%	2059
	11th to 20th	Day	2 2 1 3 2 3 2 2 4 5% 73%	2052
	21st to end	Day	2 4 2 4 3 4 3 3 4 5% 65%	1890
December	1st to 10th	Day	1 2 1 2 1 1 2 2 4 6% 78%	2716
	11th to 20th	Day	1 3 1 3 3 3 4 3 4 6% 69%	3106
	21st to end	Day	1 3 1 3 3 3 4 3 4 6% 71%	4591
January	1st to 10th	Night	2 5% 2 6% 4 4 5% 4 6% 7% 55%	7605
	11th to 20th	Night	1 2 1 2 2 1 2 2 3 4 81%	7650
	21st to end	Night	2 5% 2 4 3 4 4 4 5% 7% 60%	8370
February	1st to 10th	Night	1 3 2 4 2 3 4 3 4 7% 67%	2730
	11th to 20th	Night	3 4 2 4 3 2 3 4 5% 6% 64%	2482
	21st to end	Night	1 3 1 3 2 3 4 3 5% 7% 69%	2045
March	1st to 10th	Night	1 4 1 3 2 4 3 3 4 6% 69%	5050
	11th to 20th	Night	3 7% 2 4 4 3 3 3 4 5% 62%	5110
	21st to end	Night	1 2 1 2 3 4 4 4 5% 7% 68%	5576
April	1st to 10th	Night	2 3 1 4 4 4 4 2 4 6% 66%	2100
	11th to 20th	Night	2 4 2 4 3 2 3 3 4 4 69%	2077
	21st to end	Night	2 3 1 4 3 3 3 3 4 6% 68%	2100
May	1st to 10th	Night	3 4 2 5% 4 4 3 3 5% 6% 61%	5890
	11th to 20th	Night	1 2 1 3 3 3 3 3 5% 6% 70%	5529
	21st to end	Night	1 1 1 3 3 3 3 3 5% 6% 70%	5837
June	1st to 10th	Night	2 4 2 5% 4 4 5% 5% 6% 8% 56%	1838
	11th to 20th	Night	2 2 2 4 3 4 4 3 4 6% 65%	1776
	21st to end	Night	4 4 2 6% 5% 4 6% 4 6% 8% 50%	1890
July	1st to 10th	Night	2 3 1 6% 4 6% 6% 5% 8% 9% 49%	5340
	11th to 20th	Night	2 3 2 6% 5% 6% 7% 6% 10% 12% 40%	5340
	21st to end	Night	3 4 2 6% 6% 6% 8% 7% 10% 14% 34%	5805
August	1st to 10th	Night	2 4 2 7% 6% 6% 7% 6% 8% 11% 40%	4183
	11th to 20th	Night	3 3 1 5% 5% 6% 6% 6% 10% 14% 41%	4583
	21st to end	Night	3 3 2 6% 6% 7% 8% 7% 10% 16% 33%	5083
September	1st to 10th	Night	6% 4 2 6% 5% 6% 7% 5% 7% 7% 48%	2000
	11th to 20th	Night	4 3 2 4 3 5% 4 4 7% 8% 57%	2000
	21st to end	Night	3 3 1 4 4 3 3 4 5% 6% 65%	2275
October	1st to 10th	Night	2 2 1 2 2 2 2 2 4 5% 74%	3920
	11th to 20th	Night	1 1 1 2 1 3 3 3 4 6% 74%	3898
	21st to end	Night	2 1 1 1 1 2 2 2 3 6% 79%	4252
November	1st to 10th	Night	2 2 1 3 1 2 2 3 5% 5% 74%	2460
	11th to 20th	Night	2 2 1 2 2 2 2 2 3 6% 75%	2542
	21st to end	Night	2 5% 2 4 2 2 3 3 4 6% 67%	2730
December	1st to 10th	Night	1 2 1 3 2 2 2 3 4 6% 75%	4691
	11th to 20th	Night	2 2 1 2 2 4 4 4 5% 8% 66%	5321
	21st to end	Night	1 3 1 3 3 3 3 3 5% 6% 71%	6658

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	24% 4 5% 22% 37% 9%	5321
	11th to 20th	Day	11% 4 5% 18% 39% 23%	5340
	21st to end	Day	24% 4 3 12% 32% 24%	5844
February	1st to 10th	Day	19% 3 7% 17% 29% 25%	1890
	11th to 20th	Day	23% 4 5% 17% 31% 20%	2033
	21st to end	Day	16% 3 4 13% 34% 30%	1756
March	1st to 10th	Day	17% 3 6% 8% 30% 36%	5064
	11th to 20th	Day	23% 5% 5% 15% 28% 24%	5110
	21st to end	Day	21% 4 6% 14% 40% 16%	5637
April	1st to 10th	Day	18% 3 6% 13% 36% 23%	2520
	11th to 20th	Day	20% 3 4 11% 29% 33%	2494
	21st to end	Day	16% 4 5% 14% 36% 25%	2520
May	1st to 10th	Day	17% 4 8% 17% 29% 25%	7057
	11th to 20th	Day	15% 5% 6% 10% 30% 35%	7349
	21st to end	Day	14% 4 6% 11% 20% 45%	8371
June	1st to 10th	Day	20% 6% 7% 15% 25% 26%	2742
	11th to 20th	Day	16% 4 5% 9% 20% 45%	2803
	21st to end	Day	29% 6% 9% 14% 17% 25%	2730
July	1st to 10th	Day	24% 8% 9% 12% 19% 28%	7586
	11th to 20th	Day	23% 10% 19% 24% 15% 8%	7581
	21st to end	Day	34% 12% 19% 24% 9% 2	8120
August	1st to 10th	Day	26% 11% 17% 22% 17% 7%	4289
	11th to 20th	Day	24% 11% 14% 22% 22% 8%	4002
	21st to end	Day	33% 12% 16% 19% 16% 3	4420
September	1st to 10th	Day	30% 7% 10% 19% 20% 14%	2400
	11th to 20th	Day	25% 9% 8% 15% 25% 18%	2400
	21st to end	Day	16% 6% 8% 17% 30% 23%	2118
October	1st to 10th	Day	13% 4 5% 12% 30% 36%	3270
	11th to 20th	Day	13% 4 4 16% 40% 23%	3250
	21st to end	Day	8% 1 3 13% 41% 34%	3548
November	1st to 10th	Day	13% 3 3 20% 38% 24%	2059
	11th to 20th	Day	15% 2 4 19% 36% 24%	2052
	21st to end	Day	20% 4 5% 18% 32% 21%	1890
December	1st to 10th	Day	11% 3 3 25% 37% 20%	2716
	11th to 20th	Day	18% 4 4 17% 41% 15%	3104
	21st to end	Day	15% 3 6% 23% 31% 22%	4591
January	1st to 10th	Night	26% 4 6% 20% 34% 9%	7605
	11th to 20th	Night	9% 3 4 16% 47% 21%	7650
	21st to end	Night	23% 4 4 13% 30% 25%	8370
February	1st to 10th	Night	18% 4 6% 19% 29% 24%	2730
	11th to 20th	Night	20% 4 7% 20% 27% 22%	2482
	21st to end	Night	15% 4 4 14% 34% 28%	2045
March	1st to 10th	Night	17% 3 3 9% 39% 28%	5050
	11th to 20th	Night	23% 4 4 16% 32% 21%	5110
	21st to end	Night	15% 4 5% 19% 40% 17%	5576
April	1st to 10th	Night	19% 4 4 14% 35% 23%	2100
	11th to 20th	Night	19% 3 4 13% 29% 32%	2077
	21st to end	Night	18% 4 5% 13% 37% 24%	2100
May	1st to 10th	Night	23% 4 8% 15% 30% 20%	5888
	11th to 20th	Night	15% 4 5% 12% 32% 32%	5530
	21st to end	Night	13% 3 4 10% 20% 50%	5838
June	1st to 10th	Night	24% 6% 8% 14% 20% 29%	1838
	11th to 20th	Night	19% 3 5% 9% 20% 43%	1776
	21st to end	Night	30% 6% 10% 13% 14% 27%	1890
July	1st to 10th	Night	26% 8% 12% 13% 15% 26%	5295
	11th to 20th	Night	29% 9% 22% 21% 13% 6%	5286
	21st to end	Night	34% 11% 23% 22% 9% 2	5677
August	1st to 10th	Night	32% 10% 17% 24% 13% 5%	3067
	11th to 20th	Night	28% 10% 16% 18% 20% 8%	3343
	21st to end	Night	32% 12% 21% 19% 13% 3	3687
September	1st to 10th	Night	31% 10% 11% 16% 19% 13%	2000
	11th to 20th	Night	22% 8% 9% 17% 23% 22%	2000
	21st to end	Night	19% 5% 7% 15% 31% 23%	2275
October	1st to 10th	Night	13% 3 6% 11% 34% 33%	3920
	11th to 20th	Night	11% 5% 5% 21% 31% 28%	3898
	21st to end	Night	9% 2 3 12% 38% 35%	4252
November	1st to 10th	Night	12% 3 4 19% 37% 24%	2460
	11th to 20th	Night	13% 2 4 18% 36% 27%	2542
	21st to end	Night	19% 4 6% 19% 34% 19%	2730
December	1st to 10th	Night	13% 3 4 26% 31% 22%	4691
	11th to 20th	Night	17% 4 5% 17% 37% 21%	5322
	21st to end	Night	15% 3 5% 22% 35% 20%	6658

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	7% 22% 35% 28% 8% 1	330
	11th to 20th	Day	9% 23% 27% 27% 12% 3	330
	21st to end	Day	6% 21% 33% 24% 15% 2	363
February	1st to 10th	Day	6% 20% 34% 25% 12% 2	330
	11th to 20th	Day	13% 20% 40% 22% 4 1	330
	21st to end	Day	13% 28% 35% 17% 6% 1	299
March	1st to 10th	Day	12% 27% 33% 21% 6% 1	540
	11th to 20th	Day	14% 25% 33% 21% 6% 1	540
	21st to end	Day	11% 35% 31% 18% 4 1	594
April	1st to 10th	Day	9% 24% 37% 20% 8% 1	540
	11th to 20th	Day	12% 25% 39% 17% 5% 1	532
	21st to end	Day	9% 27% 36% 21% 7% 1	540
May	1st to 10th	Day	8% 22% 38% 25% 7% 1	543
	11th to 20th	Day	8% 16% 32% 29% 12% 3	568
	21st to end	Day	5% 17% 38% 28% 11% 1	617
June	1st to 10th	Day	3 8% 32% 36% 19% 3	582
	11th to 20th	Day	3 9% 26% 38% 20% 4	643
	21st to end	Day	6% 5% 26% 36% 19% 6%	570
July	1st to 10th	Day	4 2 16% 34% 33% 10%	566
	11th to 20th	Day	5% 2 7% 32% 37% 17%	564
	21st to end	Day	4 1 7% 29% 37% 22%	619
August	1st to 10th	Day	6% 1 9% 30% 38% 16%	567
	11th to 20th	Day	5% 2 10% 33% 35% 14%	557
	21st to end	Day	4 1 10% 34% 37% 15%	620
September	1st to 10th	Day	7% 4 11% 35% 33% 10%	530
	11th to 20th	Day	4 3 24% 37% 27% 6%	530
	21st to end	Day	3 7% 25% 36% 21% 8%	416
October	1st to 10th	Day	7% 5% 25% 32% 24% 6%	330
	11th to 20th	Day	8% 11% 28% 32% 19% 3	330
	21st to end	Day	9% 10% 25% 34% 19% 4	363
November	1st to 10th	Day	9% 11% 34% 31% 13% 2	330
	11th to 20th	Day	3 13% 24% 37% 20% 3	330
	21st to end	Day	7% 19% 35% 23% 14% 2	330
December	1st to 10th	Day	6% 16% 30% 35% 11% 2	330
	11th to 20th	Day	4 21% 30% 27% 15% 2	330
	21st to end	Day	11% 21% 31% 29% 7% 1	363
January	1st to 10th	Night	4 14% 33% 30% 15% 3	840
	11th to 20th	Night	6% 15% 35% 30% 14% 2	840
	21st to end	Night	5% 16% 30% 33% 14% 2	924
February	1st to 10th	Night	3 17% 36% 31% 11% 1	840
	11th to 20th	Night	6% 18% 42% 27% 7% 1	824
	21st to end	Night	5% 20% 43% 26% 5%	692
March	1st to 10th	Night	5% 22% 35% 29% 9% 1	840
	11th to 20th	Night	6% 18% 35% 30% 11% 1	840
	21st to end	Night	4 19% 40% 31% 6%	915
April	1st to 10th	Night	5% 20% 45% 24% 6%	630
	11th to 20th	Night	5% 22% 43% 25% 5%	623
	21st to end	Night	4 21% 43% 24% 7%	630
May	1st to 10th	Night	4 11% 46% 34% 5%	630
	11th to 20th	Night	5% 9% 35% 36% 13% 1	621
	21st to end	Night	2 7% 45% 36% 9%	683
June	1st to 10th	Night	3 5% 34% 43% 14% 1	630
	11th to 20th	Night	4 7% 34% 41% 13%	630
	21st to end	Night	6% 3 22% 45% 22% 2	630
July	1st to 10th	Night	5% 1 16% 39% 35% 4	615
	11th to 20th	Night	4 1 8% 34% 47% 6%	612
	21st to end	Night	4 2 10% 28% 45% 11%	690
August	1st to 10th	Night	2 1 8% 31% 48% 10%	627
	11th to 20th	Night	6% 2 9% 38% 41% 5%	624
	21st to end	Night	4 1 5% 38% 45% 7%	687
September	1st to 10th	Night	6% 1 10% 43% 35% 5%	600
	11th to 20th	Night	6% 2 13% 48% 27% 4	600
	21st to end	Night	5% 3 18% 44% 28% 3	763
October	1st to 10th	Night	5% 3 28% 43% 19% 3	800
	11th to 20th	Night	2 6% 31% 39% 18% 3	800
	21st to end	Night	4 8% 28% 32% 22% 5%	880
November	1st to 10th	Night	3 7% 27% 39% 19% 4	820
	11th to 20th	Night	3 10% 26% 40% 18% 4	862
	21st to end	Night	5% 16% 26% 32% 18% 3	1050
December	1st to 10th	Night	4 12% 33% 35% 14% 2	1050
	11th to 20th	Night	5% 14% 32% 36% 12% 1	1050
	21st to end	Night	4 17% 33% 36% 8% 1	945

Page generated at 2026-08-27 02:07:14. Version ac07

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