Far Horizons Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Far Horizons Observatory clear sky chart from 2005-11-06 to 2026-02-01.

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	8% 6% 2 6% 4 4 4 4 6% 8% 49%	136819
	Night	7% 8% 3 7% 4 4 4 4 6% 7% 45%	132687
Transparency	Day	33% 6% 13% 27% 17% 4	131549
	Night	37% 6% 13% 25% 16% 4	128976
Seeing	Day	12% 8% 26% 31% 18% 5%	17653
	Night	12% 7% 29% 38% 13% 1	27802

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	6% 12% 5% 11% 6% 6% 5% 4 6% 8% 30%	3718
	11th to 20th	Day	5% 12% 4 8% 7% 5% 6% 3 5% 7% 36%	3720
	21st to end	Day	2 6% 3 8% 4 3 4 3 6% 7% 54%	4298
February	1st to 10th	Day	9% 14% 5% 11% 7% 4 5% 4 5% 7% 30%	1800
	11th to 20th	Day	7% 15% 5% 11% 5% 4 4 4 5% 6% 35%	1755
	21st to end	Day	5% 10% 3 8% 6% 5% 6% 5% 6% 8% 37%	1615
March	1st to 10th	Day	4 6% 4 8% 5% 4 4 4 5% 9% 48%	4536
	11th to 20th	Day	6% 8% 3 7% 5% 4 5% 4 6% 10% 43%	5000
	21st to end	Day	14% 13% 4 8% 5% 4 4 4 6% 7% 31%	5738
April	1st to 10th	Day	13% 8% 3 6% 5% 4 4 4 5% 7% 40%	2400
	11th to 20th	Day	12% 12% 3 8% 4 5% 5% 4 7% 7% 33%	2371
	21st to end	Day	16% 9% 2 7% 5% 4 5% 4 5% 6% 37%	2574
May	1st to 10th	Day	13% 7% 3 5% 3 3 4 4 5% 7% 45%	6340
	11th to 20th	Day	11% 6% 2 7% 5% 4 5% 4 7% 10% 40%	6298
	21st to end	Day	11% 4 2 4 3 4 4 4 6% 9% 49%	6925
June	1st to 10th	Day	10% 5% 2 5% 3 4 4 4 6% 8% 48%	2800
	11th to 20th	Day	8% 5% 2 5% 5% 4 5% 4 7% 9% 46%	2800
	21st to end	Day	4 2 1 4 3 3 4 4 6% 8% 60%	2800
July	1st to 10th	Day	4 3 1 5% 3 3 4 4 7% 8% 57%	6339
	11th to 20th	Day	6% 2 1 2 2 2 3 3 5% 7% 68%	6340
	21st to end	Day	5% 3 1 4 3 3 4 4 6% 8% 57%	6903
August	1st to 10th	Day	4 3 1 4 3 3 4 4 7% 10% 59%	6103
	11th to 20th	Day	5% 2 1 3 2 3 4 3 6% 8% 64%	5798
	21st to end	Day	8% 4 2 5% 3 3 3 3 5% 7% 57%	6041
September	1st to 10th	Day	6% 3 1 4 3 3 3 3 5% 6% 64%	2400
	11th to 20th	Day	9% 3 1 4 3 3 4 4 5% 8% 57%	2400
	21st to end	Day	11% 4 1 4 3 3 2 3 4 7% 58%	2135
October	1st to 10th	Day	9% 5% 2 4 3 2 2 2 3 5% 63%	3270
	11th to 20th	Day	9% 6% 2 5% 3 3 4 3 4 6% 55%	3250
	21st to end	Day	7% 6% 2 5% 3 3 4 4 6% 10% 51%	3299
November	1st to 10th	Day	9% 7% 3 7% 4 4 4 5% 7% 7% 44%	1795
	11th to 20th	Day	7% 11% 3 9% 5% 4 3 4 6% 7% 42%	1680
	21st to end	Day	4 10% 3 7% 5% 5% 6% 5% 6% 8% 42%	1680
December	1st to 10th	Day	4 13% 4 7% 3 4 4 4 5% 7% 45%	2700
	11th to 20th	Day	6% 10% 3 8% 5% 5% 5% 5% 6% 8% 40%	3106
	21st to end	Day	6% 9% 4 10% 5% 5% 5% 3 5% 6% 42%	4092
January	1st to 10th	Night	6% 13% 5% 12% 6% 4 4 3 5% 6% 36%	6467
	11th to 20th	Night	6% 13% 5% 11% 6% 5% 4 4 6% 5% 35%	6480
	21st to end	Night	3 7% 4 10% 5% 4 5% 4 5% 6% 47%	6967
February	1st to 10th	Night	8% 14% 5% 12% 7% 6% 5% 4 5% 7% 28%	2600
	11th to 20th	Night	5% 12% 5% 11% 6% 5% 5% 4 6% 8% 33%	2532
	21st to end	Night	5% 12% 3 8% 5% 5% 5% 3 6% 7% 40%	2015
March	1st to 10th	Night	5% 8% 3 7% 4 4 5% 4 6% 9% 46%	5338
	11th to 20th	Night	7% 8% 2 7% 4 4 5% 4 6% 8% 44%	5000
	21st to end	Night	13% 14% 5% 10% 5% 6% 5% 4 6% 7% 24%	5224
April	1st to 10th	Night	12% 12% 5% 7% 5% 4 5% 4 5% 7% 37%	2000
	11th to 20th	Night	12% 10% 4 9% 6% 5% 5% 4 6% 7% 32%	1977
	21st to end	Night	13% 9% 3 7% 5% 5% 5% 4 5% 7% 38%	1824
May	1st to 10th	Night	14% 6% 2 3 4 4 4 4 5% 7% 48%	3640
	11th to 20th	Night	15% 7% 3 7% 4 4 5% 4 7% 10% 34%	3617
	21st to end	Night	9% 4 2 5% 3 4 4 4 6% 7% 53%	3972
June	1st to 10th	Night	9% 6% 2 6% 5% 4 6% 5% 8% 10% 41%	1600
	11th to 20th	Night	9% 4 2 6% 5% 3 5% 6% 7% 8% 43%	1600
	21st to end	Night	8% 3 1 4 3 4 4 3 8% 8% 54%	1600
July	1st to 10th	Night	4 3 1 5% 4 4 5% 4 7% 9% 54%	3640
	11th to 20th	Night	6% 2 1 4 4 4 4 4 6% 8% 57%	3640
	21st to end	Night	6% 3 1 5% 4 4 5% 5% 8% 10% 49%	3970
August	1st to 10th	Night	5% 3 2 5% 4 5% 5% 4 7% 9% 51%	3705
	11th to 20th	Night	4 3 2 4 3 3 4 4 7% 11% 54%	4051
	21st to end	Night	8% 5% 2 5% 3 3 4 3 5% 7% 54%	4922
September	1st to 10th	Night	6% 4 1 4 3 3 3 3 5% 7% 61%	2000
	11th to 20th	Night	11% 4 1 4 4 3 5% 4 5% 6% 56%	2000
	21st to end	Night	10% 5% 1 4 3 3 3 3 5% 7% 56%	2261
October	1st to 10th	Night	9% 5% 2 4 2 2 3 3 4 6% 60%	3920
	11th to 20th	Night	10% 7% 2 6% 3 3 3 3 4 7% 54%	3898
	21st to end	Night	7% 6% 2 5% 4 4 4 4 6% 7% 50%	4468
November	1st to 10th	Night	8% 8% 3 6% 5% 4 4 3 6% 7% 45%	2720
	11th to 20th	Night	6% 9% 3 7% 5% 5% 5% 4 6% 7% 44%	2940
	21st to end	Night	4 9% 3 7% 5% 4 5% 4 6% 8% 44%	2940
December	1st to 10th	Night	5% 10% 3 8% 6% 5% 6% 4 7% 8% 39%	4710
	11th to 20th	Night	7% 12% 4 8% 5% 4 5% 4 6% 6% 38%	5321
	21st to end	Night	6% 10% 4 9% 5% 4 4 4 6% 7% 42%	7128

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	51% 6% 13% 24% 5%	3719
	11th to 20th	Day	48% 5% 14% 20% 13% 1	3720
	21st to end	Day	29% 6% 16% 34% 14% 1	4298
February	1st to 10th	Day	52% 6% 13% 19% 10% 1	1800
	11th to 20th	Day	50% 5% 16% 18% 11% 1	1764
	21st to end	Day	42% 7% 15% 22% 12% 2	1615
March	1st to 10th	Day	32% 5% 12% 33% 16% 1	4536
	11th to 20th	Day	36% 6% 17% 34% 7% 1	5000
	21st to end	Day	50% 7% 15% 24% 4	5738
April	1st to 10th	Day	43% 6% 15% 24% 11% 2	2400
	11th to 20th	Day	46% 8% 14% 22% 8% 1	2374
	21st to end	Day	46% 6% 14% 21% 12% 1	2574
May	1st to 10th	Day	36% 7% 11% 27% 18% 1	6340
	11th to 20th	Day	36% 8% 15% 27% 13% 1	6302
	21st to end	Day	30% 7% 12% 33% 17%	6926
June	1st to 10th	Day	32% 8% 17% 28% 13% 2	2800
	11th to 20th	Day	32% 8% 18% 25% 14% 3	2800
	21st to end	Day	20% 7% 14% 29% 24% 7%	2800
July	1st to 10th	Day	23% 5% 11% 30% 21% 11%	6271
	11th to 20th	Day	16% 3 8% 27% 33% 12%	6265
	21st to end	Day	24% 4 10% 23% 27% 11%	6705
August	1st to 10th	Day	23% 4 11% 26% 22% 14%	4444
	11th to 20th	Day	23% 5% 10% 29% 27% 6%	4197
	21st to end	Day	21% 3 9% 28% 31% 8%	4356
September	1st to 10th	Day	21% 4 8% 27% 29% 11%	2400
	11th to 20th	Day	26% 5% 12% 30% 21% 6%	2400
	21st to end	Day	26% 4 9% 30% 26% 4	2135
October	1st to 10th	Day	27% 3 5% 25% 28% 12%	3270
	11th to 20th	Day	31% 4 10% 30% 23% 3	3250
	21st to end	Day	29% 4 11% 37% 17% 2	3299
November	1st to 10th	Day	37% 6% 11% 26% 16% 3	1795
	11th to 20th	Day	41% 5% 15% 25% 13% 2	1680
	21st to end	Day	37% 6% 18% 24% 14% 1	1680
December	1st to 10th	Day	38% 6% 16% 24% 13% 2	2700
	11th to 20th	Day	40% 7% 22% 25% 5%	3104
	21st to end	Day	42% 5% 9% 29% 14% 2	4092
January	1st to 10th	Night	49% 6% 14% 26% 5%	6467
	11th to 20th	Night	51% 4 14% 17% 13% 1	6480
	21st to end	Night	36% 7% 16% 28% 11% 2	6967
February	1st to 10th	Night	55% 7% 12% 16% 10% 1	2600
	11th to 20th	Night	48% 7% 14% 22% 9% 1	2533
	21st to end	Night	42% 6% 16% 21% 13% 2	2015
March	1st to 10th	Night	33% 5% 12% 30% 18% 1	5338
	11th to 20th	Night	36% 8% 17% 23% 15% 1	5000
	21st to end	Night	55% 9% 15% 14% 6%	5224
April	1st to 10th	Night	46% 7% 13% 20% 12% 3	2000
	11th to 20th	Night	47% 7% 12% 24% 9% 1	1977
	21st to end	Night	43% 7% 13% 24% 13%	1824
May	1st to 10th	Night	34% 7% 12% 26% 21% 1	3640
	11th to 20th	Night	42% 7% 13% 26% 10% 1	3618
	21st to end	Night	28% 8% 16% 30% 17% 1	3972
June	1st to 10th	Night	35% 10% 16% 27% 10% 2	1600
	11th to 20th	Night	33% 9% 19% 20% 15% 3	1600
	21st to end	Night	25% 8% 14% 24% 23% 5%	1600
July	1st to 10th	Night	25% 5% 13% 31% 17% 9%	3600
	11th to 20th	Night	25% 4 10% 18% 32% 10%	3592
	21st to end	Night	27% 6% 12% 24% 23% 8%	3856
August	1st to 10th	Night	27% 5% 11% 25% 17% 15%	2688
	11th to 20th	Night	24% 4 14% 30% 22% 5%	2928
	21st to end	Night	25% 4 8% 25% 29% 10%	3550
September	1st to 10th	Night	23% 4 7% 25% 27% 14%	2000
	11th to 20th	Night	29% 5% 10% 26% 22% 7%	2000
	21st to end	Night	27% 5% 10% 25% 27% 7%	2261
October	1st to 10th	Night	26% 4 7% 24% 27% 12%	3920
	11th to 20th	Night	32% 4 7% 24% 29% 3	3898
	21st to end	Night	31% 5% 11% 29% 17% 7%	4468
November	1st to 10th	Night	37% 5% 14% 24% 16% 4	2720
	11th to 20th	Night	38% 6% 13% 27% 14% 1	2940
	21st to end	Night	36% 7% 14% 27% 14% 2	2940
December	1st to 10th	Night	41% 7% 16% 25% 10% 1	4710
	11th to 20th	Night	45% 6% 15% 25% 8%	5322
	21st to end	Night	39% 4 9% 32% 13% 2	7128

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	15% 8% 33% 25% 14% 5%	330
	11th to 20th	Day	13% 15% 25% 29% 14% 4	330
	21st to end	Day	10% 11% 37% 31% 8% 2	363
February	1st to 10th	Day	17% 12% 34% 31% 5% 1	300
	11th to 20th	Day	14% 11% 33% 30% 10% 2	300
	21st to end	Day	14% 8% 36% 30% 12% 1	249
March	1st to 10th	Day	11% 11% 30% 30% 13% 5%	310
	11th to 20th	Day	13% 8% 29% 32% 14% 4	500
	21st to end	Day	18% 9% 27% 28% 15% 3	550
April	1st to 10th	Day	21% 11% 26% 30% 11% 2	500
	11th to 20th	Day	18% 18% 34% 20% 9% 1	492
	21st to end	Day	23% 9% 23% 28% 14% 3	545
May	1st to 10th	Day	14% 10% 23% 29% 18% 6%	703
	11th to 20th	Day	18% 6% 22% 30% 19% 5%	725
	21st to end	Day	15% 8% 23% 28% 18% 7%	789
June	1st to 10th	Day	10% 8% 23% 30% 23% 7%	730
	11th to 20th	Day	12% 9% 36% 27% 14% 2	730
	21st to end	Day	6% 5% 25% 37% 21% 5%	730
July	1st to 10th	Day	6% 4 20% 39% 26% 6%	721
	11th to 20th	Day	4 3 26% 40% 22% 4	718
	21st to end	Day	4 4 20% 39% 27% 6%	794
August	1st to 10th	Day	7% 4 22% 40% 21% 6%	673
	11th to 20th	Day	7% 3 22% 34% 24% 11%	517
	21st to end	Day	5% 6% 21% 32% 27% 8%	583
September	1st to 10th	Day	7% 5% 22% 33% 25% 9%	530
	11th to 20th	Day	10% 3 23% 32% 25% 8%	530
	21st to end	Day	15% 6% 22% 22% 26% 9%	375
October	1st to 10th	Day	17% 7% 18% 27% 21% 11%	330
	11th to 20th	Day	17% 9% 22% 31% 14% 7%	330
	21st to end	Day	10% 12% 35% 25% 14% 3	363
November	1st to 10th	Day	15% 12% 31% 25% 14% 4	330
	11th to 20th	Day	11% 12% 34% 27% 13% 3	330
	21st to end	Day	7% 10% 35% 36% 10% 2	330
December	1st to 10th	Day	12% 11% 39% 27% 9% 2	330
	11th to 20th	Day	14% 12% 32% 33% 8% 1	330
	21st to end	Day	10% 10% 30% 32% 15% 2	363
January	1st to 10th	Night	11% 8% 33% 34% 12% 1	1050
	11th to 20th	Night	14% 10% 35% 31% 10% 1	1050
	21st to end	Night	10% 11% 35% 33% 10% 1	1085
February	1st to 10th	Night	14% 9% 30% 35% 13% 1	800
	11th to 20th	Night	11% 8% 35% 37% 8% 1	784
	21st to end	Night	12% 8% 32% 34% 13% 2	660
March	1st to 10th	Night	11% 7% 34% 36% 11% 1	800
	11th to 20th	Night	11% 5% 34% 40% 10% 1	800
	21st to end	Night	16% 7% 27% 36% 13% 1	770
April	1st to 10th	Night	17% 7% 27% 40% 9% 1	600
	11th to 20th	Night	15% 8% 31% 35% 10% 1	593
	21st to end	Night	17% 6% 23% 40% 13% 1	600
May	1st to 10th	Night	16% 7% 20% 42% 15% 1	600
	11th to 20th	Night	18% 4 23% 35% 19% 2	591
	21st to end	Night	17% 4 28% 33% 17% 1	650
June	1st to 10th	Night	11% 7% 26% 38% 17% 2	600
	11th to 20th	Night	10% 8% 36% 35% 10%	600
	21st to end	Night	8% 6% 25% 46% 15% 1	600
July	1st to 10th	Night	7% 4 24% 45% 20%	585
	11th to 20th	Night	5% 3 32% 46% 13%	582
	21st to end	Night	5% 3 21% 51% 19% 1	657
August	1st to 10th	Night	7% 3 24% 49% 17%	597
	11th to 20th	Night	5% 2 22% 50% 20% 1	594
	21st to end	Night	7% 4 21% 48% 20% 1	660
September	1st to 10th	Night	7% 4 22% 47% 19% 1	600
	11th to 20th	Night	12% 5% 20% 45% 17% 2	600
	21st to end	Night	12% 5% 22% 39% 21% 2	706
October	1st to 10th	Night	18% 4 22% 37% 18% 1	800
	11th to 20th	Night	14% 7% 26% 37% 16% 1	800
	21st to end	Night	10% 8% 31% 37% 13% 1	1008
November	1st to 10th	Night	12% 7% 31% 39% 10% 1	1025
	11th to 20th	Night	10% 10% 36% 34% 9% 1	1050
	21st to end	Night	10% 9% 34% 38% 8%	1050
December	1st to 10th	Night	11% 9% 34% 35% 9%	1050
	11th to 20th	Night	12% 9% 33% 35% 10% 1	1050
	21st to end	Night	14% 9% 32% 35% 10% 1	1155

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