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 2025-04-26.

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 3 4 4 4 6% 7% 62%	117214
	Night	2 3 1 4 3 3 4 3 5% 7% 64%	120266
Transparency	Day	20% 5% 7% 16% 29% 22%	116977
	Night	19% 5% 7% 17% 30% 22%	120092
Seeing	Day	7% 14% 26% 28% 18% 6%	14982
	Night	5% 11% 29% 34% 19% 3	25644

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 7% 3 3 3 2 4 6% 64%	4098
	11th to 20th	Day	1 2 1 3 2 3 4 3 4 5% 71%	4100
	21st to end	Day	2 8% 3 6% 3 3 3 3 5% 5% 60%	4510
February	1st to 10th	Day	1 4 2 4 3 3 3 3 5% 6% 65%	1800
	11th to 20th	Day	2 4 2 5% 3 4 4 3 3 5% 64%	1924
	21st to end	Day	1 4 1 4 2 3 2 2 5% 6% 69%	1675
March	1st to 10th	Day	1 4 2 4 4 4 3 3 5% 6% 65%	4954
	11th to 20th	Day	3 7% 2 5% 3 3 4 3 5% 7% 57%	5000
	21st to end	Day	1 3 2 5% 3 4 5% 4 6% 7% 59%	5516
April	1st to 10th	Day	2 4 1 3 3 4 3 4 4 6% 67%	2400
	11th to 20th	Day	3 5% 1 3 3 3 4 3 4 6% 66%	2371
	21st to end	Day	2 3 1 3 3 2 3 3 5% 5% 70%	2341
May	1st to 10th	Day	1 2 1 2 2 3 3 2 5% 6% 74%	3800
	11th to 20th	Day	2 3 1 4 4 5% 5% 4 6% 7% 57%	3936
	21st to end	Day	1 2 1 2 2 2 2 2 4 5% 76%	4477
June	1st to 10th	Day	2 3 1 4 3 4 4 4 7% 9% 58%	2482
	11th to 20th	Day	2 2 1 3 3 3 3 4 5% 6% 69%	2537
	21st to end	Day	5% 5% 2 6% 5% 5% 5% 5% 7% 7% 49%	2470
July	1st to 10th	Day	3 2 1 5% 5% 6% 6% 5% 7% 8% 51%	4110
	11th to 20th	Day	2 2 1 4 5% 5% 7% 5% 8% 11% 49%	4110
	21st to end	Day	4 4 2 7% 6% 8% 8% 6% 10% 11% 35%	4454
August	1st to 10th	Day	3 3 1 5% 5% 7% 7% 6% 10% 10% 44%	4045
	11th to 20th	Day	3 2 1 5% 5% 5% 6% 6% 9% 10% 47%	3726
	21st to end	Day	3 3 2 6% 6% 8% 8% 7% 9% 10% 38%	4180
September	1st to 10th	Day	6% 4 2 5% 4 5% 5% 4 7% 7% 50%	2280
	11th to 20th	Day	5% 4 1 4 4 4 5% 4 6% 8% 56%	2280
	21st to end	Day	3 1 1 3 3 3 4 3 5% 8% 67%	2013
October	1st to 10th	Day	3 2 1 2 2 2 3 2 4 5% 75%	3170
	11th to 20th	Day	2 2 1 2 2 3 3 2 4 7% 71%	3150
	21st to end	Day	1 1 2 1 1 2 1 2 4 84%	3438
November	1st to 10th	Day	2 2 1 3 2 2 2 3 4 7% 72%	1959
	11th to 20th	Day	2 2 1 3 2 3 2 2 4 5% 73%	1955
	21st to end	Day	2 4 2 4 3 4 4 3 4 5% 65%	1800
December	1st to 10th	Day	1 2 1 2 1 1 2 2 4 6% 77%	2635
	11th to 20th	Day	1 3 1 3 3 3 4 3 4 6% 68%	3026
	21st to end	Day	1 3 1 3 2 3 4 3 4 6% 71%	4492
January	1st to 10th	Night	1 5% 2 6% 3 3 3 3 5% 7% 60%	5867
	11th to 20th	Night	1 2 1 2 2 2 3 3 4 5% 76%	5880
	21st to end	Night	2 5% 2 4 3 4 4 3 5% 7% 61%	6468
February	1st to 10th	Night	1 4 2 4 3 3 4 3 4 7% 66%	2600
	11th to 20th	Night	3 4 2 4 3 3 3 4 5% 6% 65%	2361
	21st to end	Night	1 3 1 3 2 3 4 3 6% 7% 68%	1950
March	1st to 10th	Night	1 4 1 4 3 4 3 3 4 6% 68%	4940
	11th to 20th	Night	3 7% 2 5% 4 3 3 3 4 5% 61%	5000
	21st to end	Night	1 2 1 3 3 4 4 4 5% 7% 68%	5455
April	1st to 10th	Night	2 3 1 4 4 4 4 2 4 6% 67%	2000
	11th to 20th	Night	2 4 2 4 3 2 3 2 4 4 69%	1977
	21st to end	Night	2 3 1 4 3 3 3 3 4 6% 68%	1955
May	1st to 10th	Night	2 3 1 3 3 2 2 2 3 5% 73%	3170
	11th to 20th	Night	2 4 1 4 4 3 4 3 5% 6% 64%	2965
	21st to end	Night	1 1 1 2 2 2 2 2 3 5% 82%	3110
June	1st to 10th	Night	2 4 2 5% 4 4 5% 5% 5% 8% 57%	1662
	11th to 20th	Night	2 2 2 4 3 4 4 3 4 6% 66%	1606
	21st to end	Night	5% 5% 2 6% 6% 5% 6% 4 6% 8% 48%	1710
July	1st to 10th	Night	3 3 1 5% 4 5% 6% 5% 7% 9% 51%	2860
	11th to 20th	Night	3 4 2 7% 5% 6% 6% 5% 9% 12% 43%	2860
	21st to end	Night	4 4 2 6% 6% 6% 7% 6% 9% 13% 37%	3109
August	1st to 10th	Night	2 3 2 6% 7% 7% 7% 7% 9% 12% 39%	2894
	11th to 20th	Night	3 3 2 6% 5% 6% 6% 5% 8% 12% 43%	3113
	21st to end	Night	3 3 2 6% 6% 7% 9% 7% 11% 14% 34%	3487
September	1st to 10th	Night	5% 4 2 6% 5% 5% 6% 5% 7% 7% 49%	1900
	11th to 20th	Night	4 3 2 4 3 5% 4 4 7% 8% 57%	1900
	21st to end	Night	3 3 1 4 3 3 3 4 5% 6% 66%	2161
October	1st to 10th	Night	2 2 1 2 2 2 2 3 4 5% 74%	3800
	11th to 20th	Night	1 1 1 2 1 3 3 3 4 7% 74%	3778
	21st to end	Night	2 1 1 1 2 2 2 3 6% 79%	4120
November	1st to 10th	Night	2 2 1 3 1 2 2 3 5% 6% 73%	2340
	11th to 20th	Night	1 2 1 2 2 2 2 2 3 7% 75%	2421
	21st to end	Night	2 5% 2 4 2 3 3 3 4 6% 67%	2600
December	1st to 10th	Night	1 2 1 3 2 2 3 3 4 6% 74%	4552
	11th to 20th	Night	2 2 1 2 3 4 4 4 6% 8% 65%	5181
	21st to end	Night	1 3 1 3 3 3 3 3 5% 6% 71%	6514

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	23% 3 5% 16% 42% 11%	4099
	11th to 20th	Day	14% 4 7% 20% 34% 21%	4100
	21st to end	Day	24% 5% 4 13% 30% 24%	4510
February	1st to 10th	Day	19% 3 7% 18% 28% 25%	1800
	11th to 20th	Day	23% 3 5% 17% 30% 21%	1934
	21st to end	Day	16% 3 4 12% 32% 31%	1675
March	1st to 10th	Day	18% 3 6% 8% 30% 36%	4954
	11th to 20th	Day	23% 5% 5% 15% 28% 23%	5000
	21st to end	Day	21% 4 6% 14% 40% 16%	5516
April	1st to 10th	Day	18% 3 6% 13% 36% 24%	2400
	11th to 20th	Day	20% 3 4 11% 29% 33%	2374
	21st to end	Day	16% 4 5% 15% 36% 23%	2341
May	1st to 10th	Day	11% 3 4 12% 38% 32%	3800
	11th to 20th	Day	22% 6% 9% 11% 33% 19%	3939
	21st to end	Day	10% 3 6% 9% 20% 54%	4477
June	1st to 10th	Day	21% 6% 6% 15% 26% 26%	2482
	11th to 20th	Day	16% 4 5% 9% 20% 46%	2537
	21st to end	Day	30% 6% 10% 15% 18% 22%	2470
July	1st to 10th	Day	24% 9% 11% 12% 20% 24%	4046
	11th to 20th	Day	24% 9% 14% 27% 20% 6%	4041
	21st to end	Day	34% 12% 18% 26% 9% 1	4314
August	1st to 10th	Day	26% 11% 17% 22% 17% 6%	4031
	11th to 20th	Day	24% 11% 13% 21% 23% 9%	3762
	21st to end	Day	33% 13% 15% 18% 17% 3	4179
September	1st to 10th	Day	30% 7% 10% 18% 20% 15%	2280
	11th to 20th	Day	25% 8% 8% 15% 26% 18%	2280
	21st to end	Day	17% 6% 7% 16% 31% 23%	2013
October	1st to 10th	Day	13% 4 5% 12% 30% 36%	3170
	11th to 20th	Day	12% 4 3 17% 40% 23%	3150
	21st to end	Day	8% 1 3 13% 41% 34%	3438
November	1st to 10th	Day	13% 3 3 20% 36% 24%	1959
	11th to 20th	Day	15% 2 4 18% 36% 25%	1955
	21st to end	Day	21% 4 5% 17% 32% 22%	1800
December	1st to 10th	Day	11% 3 3 26% 37% 20%	2635
	11th to 20th	Day	19% 4 4 17% 41% 14%	3024
	21st to end	Day	15% 3 6% 23% 31% 22%	4492
January	1st to 10th	Night	22% 4 6% 16% 40% 12%	5867
	11th to 20th	Night	11% 3 6% 18% 41% 21%	5880
	21st to end	Night	23% 5% 5% 14% 27% 26%	6468
February	1st to 10th	Night	19% 4 6% 20% 27% 24%	2600
	11th to 20th	Night	20% 4 6% 20% 27% 23%	2361
	21st to end	Night	16% 4 5% 14% 33% 29%	1950
March	1st to 10th	Night	17% 3 3 9% 39% 28%	4940
	11th to 20th	Night	24% 4 4 16% 31% 21%	5000
	21st to end	Night	15% 4 5% 19% 41% 16%	5455
April	1st to 10th	Night	19% 4 4 14% 35% 24%	2000
	11th to 20th	Night	19% 3 4 13% 29% 32%	1977
	21st to end	Night	18% 4 5% 13% 38% 22%	1955
May	1st to 10th	Night	14% 2 6% 8% 40% 29%	3170
	11th to 20th	Night	19% 5% 9% 15% 31% 22%	2966
	21st to end	Night	8% 2 3 8% 19% 59%	3110
June	1st to 10th	Night	24% 5% 8% 14% 20% 29%	1662
	11th to 20th	Night	19% 3 5% 9% 21% 44%	1606
	21st to end	Night	32% 6% 10% 13% 14% 25%	1710
July	1st to 10th	Night	27% 7% 12% 14% 14% 26%	2815
	11th to 20th	Night	28% 9% 15% 24% 20% 4	2806
	21st to end	Night	33% 11% 20% 27% 8% 1	3011
August	1st to 10th	Night	31% 10% 17% 24% 13% 5%	2885
	11th to 20th	Night	28% 10% 16% 17% 21% 8%	3143
	21st to end	Night	32% 12% 21% 18% 13% 3	3487
September	1st to 10th	Night	31% 9% 11% 16% 19% 14%	1900
	11th to 20th	Night	22% 8% 9% 16% 23% 22%	1900
	21st to end	Night	19% 5% 7% 14% 32% 23%	2161
October	1st to 10th	Night	13% 3 6% 11% 34% 33%	3800
	11th to 20th	Night	10% 5% 5% 21% 32% 28%	3778
	21st to end	Night	10% 2 3 12% 38% 35%	4120
November	1st to 10th	Night	13% 3 5% 20% 35% 24%	2340
	11th to 20th	Night	12% 2 4 18% 36% 28%	2421
	21st to end	Night	20% 4 6% 18% 33% 20%	2600
December	1st to 10th	Night	13% 3 4 27% 31% 23%	4552
	11th to 20th	Night	17% 4 5% 17% 37% 20%	5182
	21st to end	Night	15% 3 5% 22% 34% 20%	6514

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% 20% 35% 29% 8% 1	300
	11th to 20th	Day	10% 23% 27% 26% 11% 3	300
	21st to end	Day	5% 22% 32% 22% 16% 2	330
February	1st to 10th	Day	7% 22% 34% 24% 12% 1	300
	11th to 20th	Day	12% 20% 41% 23% 4	300
	21st to end	Day	14% 30% 32% 18% 5% 1	274
March	1st to 10th	Day	13% 27% 32% 21% 6% 1	500
	11th to 20th	Day	14% 27% 31% 21% 6% 1	500
	21st to end	Day	12% 36% 30% 17% 4 1	550
April	1st to 10th	Day	9% 24% 38% 20% 7% 1	500
	11th to 20th	Day	12% 25% 40% 16% 6% 1	492
	21st to end	Day	8% 23% 38% 23% 8%	481
May	1st to 10th	Day	7% 22% 38% 25% 7% 1	463
	11th to 20th	Day	6% 15% 33% 30% 13% 4	488
	21st to end	Day	5% 18% 38% 27% 11% 1	529
June	1st to 10th	Day	3 7% 29% 37% 21% 3	502
	11th to 20th	Day	3 8% 26% 37% 22% 4	557
	21st to end	Day	7% 4 25% 36% 21% 8%	490
July	1st to 10th	Day	5% 2 15% 33% 35% 11%	486
	11th to 20th	Day	6% 2 6% 31% 38% 18%	484
	21st to end	Day	4 1 7% 27% 39% 22%	531
August	1st to 10th	Day	6% 1 9% 29% 37% 18%	487
	11th to 20th	Day	6% 2 11% 33% 33% 15%	477
	21st to end	Day	4 2 9% 33% 37% 15%	539
September	1st to 10th	Day	7% 4 11% 34% 33% 10%	490
	11th to 20th	Day	4 3 22% 38% 27% 7%	490
	21st to end	Day	4 7% 24% 35% 22% 8%	382
October	1st to 10th	Day	7% 5% 25% 31% 25% 7%	300
	11th to 20th	Day	8% 10% 26% 33% 21% 3	300
	21st to end	Day	9% 10% 24% 33% 19% 5%	330
November	1st to 10th	Day	10% 12% 34% 30% 13% 1	300
	11th to 20th	Day	3 13% 23% 37% 20% 3	300
	21st to end	Day	8% 20% 33% 22% 15% 2	300
December	1st to 10th	Day	6% 16% 28% 36% 12% 2	300
	11th to 20th	Day	5% 21% 30% 27% 15% 2	300
	21st to end	Day	11% 22% 31% 28% 7% 1	330
January	1st to 10th	Night	4 13% 33% 30% 16% 3	800
	11th to 20th	Night	6% 15% 35% 29% 14% 2	800
	21st to end	Night	5% 16% 29% 33% 15% 2	880
February	1st to 10th	Night	4 18% 35% 30% 11% 1	800
	11th to 20th	Night	5% 17% 42% 28% 7% 1	784
	21st to end	Night	6% 20% 43% 26% 5%	660
March	1st to 10th	Night	5% 22% 33% 29% 9% 1	800
	11th to 20th	Night	6% 18% 34% 30% 12% 1	800
	21st to end	Night	4 20% 39% 30% 6%	871
April	1st to 10th	Night	4 20% 45% 25% 6%	600
	11th to 20th	Night	5% 22% 43% 24% 6%	593
	21st to end	Night	4 19% 43% 25% 8%	587
May	1st to 10th	Night	4 12% 47% 32% 6%	570
	11th to 20th	Night	6% 8% 33% 38% 14% 2	561
	21st to end	Night	2 7% 45% 36% 10%	617
June	1st to 10th	Night	3 5% 32% 44% 15% 1	570
	11th to 20th	Night	4 6% 35% 41% 14%	570
	21st to end	Night	6% 3 20% 45% 24% 2	570
July	1st to 10th	Night	6% 1 15% 37% 37% 4	555
	11th to 20th	Night	5% 1 7% 33% 48% 7%	552
	21st to end	Night	4 1 8% 27% 47% 12%	624
August	1st to 10th	Night	2 1 6% 29% 51% 11%	567
	11th to 20th	Night	6% 1 8% 36% 44% 5%	564
	21st to end	Night	5% 1 5% 37% 45% 8%	627
September	1st to 10th	Night	6% 1 9% 42% 36% 5%	570
	11th to 20th	Night	6% 2 12% 48% 28% 5%	570
	21st to end	Night	5% 3 18% 42% 30% 3	725
October	1st to 10th	Night	5% 2 27% 43% 19% 4	760
	11th to 20th	Night	2 6% 28% 41% 19% 3	760
	21st to end	Night	5% 9% 27% 31% 23% 6%	836
November	1st to 10th	Night	4 7% 26% 40% 19% 4	780
	11th to 20th	Night	2 10% 25% 40% 19% 4	821
	21st to end	Night	5% 16% 25% 32% 19% 4	1000
December	1st to 10th	Night	4 12% 31% 36% 15% 2	1000
	11th to 20th	Night	6% 14% 32% 36% 12% 1	1000
	21st to end	Night	5% 18% 32% 36% 9% 1	900

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