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 2024-11-28.

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%	105621
	Night	7% 7% 3 7% 5% 4 4 4 6% 8% 45%	102657
Transparency	Day	32% 6% 13% 27% 18% 5%	105364
	Night	36% 6% 13% 25% 16% 4	102504
Seeing	Day	12% 8% 26% 31% 18% 5%	16065
	Night	12% 7% 29% 38% 13% 1	26110

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% 10% 4 9% 6% 6% 5% 4 6% 8% 36%	2538
	11th to 20th	Day	7% 10% 3 6% 6% 5% 8% 4 6% 7% 36%	2540
	21st to end	Day	3 6% 3 7% 4 4 5% 4 6% 8% 51%	2926
February	1st to 10th	Day	8% 14% 5% 11% 6% 4 5% 4 5% 7% 31%	1710
	11th to 20th	Day	7% 16% 5% 10% 5% 4 4 4 5% 6% 35%	1665
	21st to end	Day	5% 11% 3 9% 6% 5% 6% 5% 6% 8% 36%	1536
March	1st to 10th	Day	4 6% 3 7% 5% 4 4 4 6% 10% 47%	3152
	11th to 20th	Day	5% 7% 2 7% 5% 5% 5% 4 5% 8% 48%	3490
	21st to end	Day	13% 14% 5% 10% 5% 5% 4 4 6% 7% 28%	3999
April	1st to 10th	Day	13% 8% 3 6% 5% 4 4 3 6% 7% 40%	2280
	11th to 20th	Day	12% 12% 3 9% 4 5% 5% 4 7% 6% 32%	2251
	21st to end	Day	16% 9% 2 7% 5% 4 5% 4 5% 6% 38%	2445
May	1st to 10th	Day	16% 9% 3 6% 4 4 5% 4 6% 7% 36%	4430
	11th to 20th	Day	8% 4 2 6% 4 4 5% 4 8% 10% 45%	4388
	21st to end	Day	14% 6% 2 5% 4 4 5% 4 6% 9% 41%	4825
June	1st to 10th	Day	10% 5% 2 5% 4 4 5% 5% 6% 8% 46%	2660
	11th to 20th	Day	8% 5% 2 5% 5% 4 5% 4 6% 9% 46%	2660
	21st to end	Day	4 2 1 4 3 3 4 4 6% 8% 60%	2660
July	1st to 10th	Day	4 3 1 4 3 4 4 4 7% 9% 57%	4430
	11th to 20th	Day	1 1 1 2 2 3 3 3 5% 8% 71%	4430
	21st to end	Day	3 2 1 3 3 3 4 4 7% 8% 64%	4802
August	1st to 10th	Day	5% 3 1 4 3 3 4 4 6% 9% 57%	4322
	11th to 20th	Day	6% 3 1 4 3 4 4 4 7% 8% 57%	4028
	21st to end	Day	7% 2 1 3 2 3 3 3 5% 7% 64%	4224
September	1st to 10th	Day	6% 3 1 4 3 3 3 3 5% 6% 63%	2280
	11th to 20th	Day	9% 3 1 4 3 3 3 4 5% 8% 58%	2280
	21st to end	Day	12% 3 1 4 3 3 3 3 4 7% 57%	2029
October	1st to 10th	Day	10% 5% 2 4 3 2 2 2 3 5% 63%	3170
	11th to 20th	Day	9% 6% 2 5% 3 3 4 2 4 6% 56%	3150
	21st to end	Day	7% 6% 2 5% 3 3 4 4 6% 10% 50%	3197
November	1st to 10th	Day	9% 6% 3 7% 4 3 4 5% 7% 7% 45%	1708
	11th to 20th	Day	7% 11% 3 9% 5% 4 3 4 6% 7% 42%	1600
	21st to end	Day	4 10% 3 7% 4 5% 5% 5% 6% 9% 42%	1576
December	1st to 10th	Day	6% 11% 5% 10% 4 4 5% 5% 5% 8% 37%	1520
	11th to 20th	Day	5% 9% 3 7% 5% 4 4 5% 6% 9% 44%	1926
	21st to end	Day	6% 8% 3 7% 4 5% 5% 4 6% 7% 45%	2794
January	1st to 10th	Night	4 10% 4 9% 6% 4 4 4 6% 7% 41%	4417
	11th to 20th	Night	7% 12% 5% 10% 7% 5% 5% 4 6% 5% 35%	4430
	21st to end	Night	4 7% 4 9% 5% 5% 6% 5% 7% 8% 42%	4772
February	1st to 10th	Night	7% 14% 5% 12% 7% 6% 5% 4 5% 7% 28%	2470
	11th to 20th	Night	5% 12% 5% 10% 6% 5% 5% 4 6% 8% 33%	2402
	21st to end	Night	5% 12% 3 9% 5% 5% 4 4 6% 8% 38%	1918
March	1st to 10th	Night	5% 9% 3 8% 4 4 5% 4 6% 8% 45%	3711
	11th to 20th	Night	6% 7% 2 7% 4 3 5% 4 6% 8% 47%	3490
	21st to end	Night	13% 15% 4 11% 6% 5% 5% 4 6% 6% 25%	3647
April	1st to 10th	Night	12% 12% 5% 7% 5% 4 5% 4 5% 7% 36%	1900
	11th to 20th	Night	12% 10% 4 9% 5% 5% 5% 4 6% 7% 32%	1877
	21st to end	Night	12% 9% 3 7% 5% 5% 5% 4 5% 8% 39%	1733
May	1st to 10th	Night	16% 8% 2 4 5% 4 5% 4 5% 8% 39%	2540
	11th to 20th	Night	12% 6% 3 7% 4 4 5% 4 7% 9% 40%	2517
	21st to end	Night	12% 5% 2 5% 4 5% 4 4 7% 8% 44%	2762
June	1st to 10th	Night	9% 6% 2 6% 5% 4 6% 5% 7% 9% 41%	1520
	11th to 20th	Night	9% 4 2 7% 5% 3 5% 6% 7% 8% 44%	1520
	21st to end	Night	8% 3 1 4 3 4 4 3 8% 8% 53%	1520
July	1st to 10th	Night	4 3 1 4 3 3 4 3 7% 10% 58%	2540
	11th to 20th	Night	4 2 1 5% 4 4 4 4 6% 7% 59%	2540
	21st to end	Night	5% 3 1 4 3 4 4 5% 7% 9% 55%	2761
August	1st to 10th	Night	5% 4 2 5% 4 5% 5% 4 6% 8% 54%	2613
	11th to 20th	Night	5% 3 2 4 4 3 4 4 7% 12% 51%	2811
	21st to end	Night	7% 4 2 4 3 3 4 3 5% 8% 58%	3442
September	1st to 10th	Night	6% 4 1 4 3 3 3 3 5% 7% 61%	1900
	11th to 20th	Night	11% 4 1 4 3 3 4 3 4 6% 56%	1900
	21st to end	Night	11% 5% 1 4 3 3 3 3 5% 7% 55%	2147
October	1st to 10th	Night	9% 5% 2 4 2 2 3 3 4 6% 61%	3800
	11th to 20th	Night	9% 7% 2 6% 3 3 3 3 4 7% 54%	3778
	21st to end	Night	8% 6% 2 5% 4 4 4 4 6% 8% 49%	4329
November	1st to 10th	Night	9% 8% 3 7% 5% 4 4 3 6% 7% 45%	2587
	11th to 20th	Night	6% 9% 3 7% 6% 5% 5% 4 5% 6% 44%	2800
	21st to end	Night	4 9% 3 7% 5% 3 5% 4 6% 8% 45%	2759
December	1st to 10th	Night	6% 10% 3 8% 6% 5% 6% 4 7% 8% 35%	2660
	11th to 20th	Night	6% 8% 4 8% 6% 4 5% 4 6% 7% 43%	3271
	21st to end	Night	5% 9% 3 7% 4 4 4 4 6% 7% 45%	4873

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	44% 7% 15% 30% 4	2539
	11th to 20th	Day	46% 7% 13% 23% 10% 1	2540
	21st to end	Day	30% 8% 21% 27% 14% 1	2926
February	1st to 10th	Day	52% 6% 13% 19% 9% 1	1710
	11th to 20th	Day	50% 5% 16% 18% 10% 1	1674
	21st to end	Day	43% 7% 15% 21% 12% 1	1536
March	1st to 10th	Day	30% 7% 15% 32% 16%	3152
	11th to 20th	Day	33% 7% 23% 29% 8% 1	3490
	21st to end	Day	52% 8% 17% 18% 4	3999
April	1st to 10th	Day	43% 6% 15% 24% 11% 1	2280
	11th to 20th	Day	47% 9% 14% 21% 8% 1	2254
	21st to end	Day	45% 6% 14% 21% 12% 1	2445
May	1st to 10th	Day	43% 8% 12% 24% 11% 1	4430
	11th to 20th	Day	29% 9% 15% 32% 14% 1	4392
	21st to end	Day	36% 9% 14% 32% 9%	4825
June	1st to 10th	Day	33% 8% 18% 27% 12% 2	2660
	11th to 20th	Day	32% 8% 17% 25% 13% 3	2660
	21st to end	Day	20% 7% 14% 28% 24% 7%	2660
July	1st to 10th	Day	21% 5% 12% 30% 21% 11%	4361
	11th to 20th	Day	12% 3 7% 26% 36% 15%	4355
	21st to end	Day	16% 3 7% 22% 34% 16%	4651
August	1st to 10th	Day	23% 4 10% 26% 23% 14%	4307
	11th to 20th	Day	23% 5% 10% 28% 28% 6%	4067
	21st to end	Day	21% 3 9% 28% 31% 8%	4223
September	1st to 10th	Day	22% 4 8% 25% 30% 12%	2280
	11th to 20th	Day	25% 5% 12% 29% 22% 7%	2280
	21st to end	Day	27% 4 10% 30% 25% 4	2029
October	1st to 10th	Day	26% 3 6% 25% 28% 12%	3170
	11th to 20th	Day	31% 4 9% 30% 23% 3	3150
	21st to end	Day	29% 4 11% 36% 17% 2	3197
November	1st to 10th	Day	36% 6% 12% 26% 17% 4	1708
	11th to 20th	Day	41% 5% 15% 24% 14% 2	1600
	21st to end	Day	36% 6% 18% 24% 15% 1	1576
December	1st to 10th	Day	44% 7% 14% 24% 11% 1	1520
	11th to 20th	Day	36% 8% 22% 28% 6% 1	1924
	21st to end	Day	36% 6% 10% 31% 14% 2	2794
January	1st to 10th	Night	41% 7% 16% 29% 6% 1	4417
	11th to 20th	Night	50% 5% 12% 20% 12% 1	4430
	21st to end	Night	37% 9% 20% 22% 10% 1	4772
February	1st to 10th	Night	54% 7% 12% 17% 9% 1	2470
	11th to 20th	Night	47% 7% 15% 22% 8% 1	2403
	21st to end	Night	43% 6% 14% 21% 12% 2	1918
March	1st to 10th	Night	35% 7% 15% 26% 17% 1	3711
	11th to 20th	Night	31% 9% 22% 23% 13% 1	3490
	21st to end	Night	55% 11% 15% 13% 6%	3647
April	1st to 10th	Night	46% 7% 13% 20% 12% 2	1900
	11th to 20th	Night	47% 8% 12% 24% 8% 1	1877
	21st to end	Night	43% 7% 12% 24% 14%	1733
May	1st to 10th	Night	41% 8% 13% 23% 14% 1	2540
	11th to 20th	Night	37% 8% 11% 32% 10% 2	2518
	21st to end	Night	34% 10% 17% 29% 11%	2762
June	1st to 10th	Night	35% 10% 17% 27% 9% 2	1520
	11th to 20th	Night	33% 9% 19% 20% 15% 3	1520
	21st to end	Night	25% 8% 14% 24% 22% 6%	1520
July	1st to 10th	Night	21% 5% 14% 35% 18% 7%	2500
	11th to 20th	Night	23% 4 8% 18% 34% 13%	2492
	21st to end	Night	23% 5% 7% 23% 30% 12%	2674
August	1st to 10th	Night	27% 5% 10% 25% 17% 15%	2605
	11th to 20th	Night	25% 4 14% 30% 21% 5%	2838
	21st to end	Night	24% 4 8% 25% 29% 10%	3442
September	1st to 10th	Night	23% 4 7% 23% 28% 15%	1900
	11th to 20th	Night	28% 5% 10% 26% 23% 7%	1900
	21st to end	Night	28% 5% 10% 25% 26% 6%	2147
October	1st to 10th	Night	26% 4 7% 24% 27% 12%	3800
	11th to 20th	Night	32% 4 7% 24% 30% 3	3778
	21st to end	Night	32% 5% 11% 28% 16% 7%	4329
November	1st to 10th	Night	38% 6% 13% 24% 16% 4	2587
	11th to 20th	Night	38% 6% 12% 27% 14% 2	2800
	21st to end	Night	35% 7% 14% 28% 14% 2	2759
December	1st to 10th	Night	43% 8% 14% 22% 11% 1	2660
	11th to 20th	Night	38% 7% 16% 30% 9%	3272
	21st to end	Night	36% 5% 11% 32% 13% 4	4873

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	16% 7% 34% 23% 15% 6%	270
	11th to 20th	Day	12% 14% 21% 31% 17% 5%	270
	21st to end	Day	11% 11% 35% 31% 9% 3	297
February	1st to 10th	Day	17% 12% 33% 32% 5% 1	270
	11th to 20th	Day	16% 12% 32% 30% 9% 2	270
	21st to end	Day	13% 8% 35% 31% 13% 1	225
March	1st to 10th	Day	12% 11% 29% 31% 13% 4	279
	11th to 20th	Day	12% 8% 29% 33% 14% 4	460
	21st to end	Day	18% 8% 27% 28% 15% 3	506
April	1st to 10th	Day	21% 10% 27% 30% 11% 1	460
	11th to 20th	Day	19% 18% 33% 21% 9% 1	452
	21st to end	Day	22% 8% 23% 28% 15% 3	503
May	1st to 10th	Day	14% 11% 24% 29% 17% 6%	653
	11th to 20th	Day	17% 6% 21% 31% 20% 5%	675
	21st to end	Day	16% 8% 22% 27% 18% 8%	734
June	1st to 10th	Day	10% 7% 24% 30% 22% 7%	680
	11th to 20th	Day	12% 9% 36% 26% 15% 2	680
	21st to end	Day	6% 6% 25% 36% 22% 5%	680
July	1st to 10th	Day	6% 4 20% 39% 25% 6%	671
	11th to 20th	Day	3 4 27% 41% 23% 3	668
	21st to end	Day	4 4 20% 39% 28% 6%	739
August	1st to 10th	Day	8% 3 22% 40% 21% 6%	626
	11th to 20th	Day	7% 3 21% 35% 23% 11%	477
	21st to end	Day	5% 7% 21% 32% 28% 8%	539
September	1st to 10th	Day	7% 5% 22% 32% 25% 9%	490
	11th to 20th	Day	10% 3 22% 31% 25% 8%	490
	21st to end	Day	16% 6% 24% 23% 24% 7%	343
October	1st to 10th	Day	18% 7% 17% 27% 20% 11%	300
	11th to 20th	Day	17% 7% 21% 31% 16% 8%	300
	21st to end	Day	12% 13% 34% 25% 14% 3	330
November	1st to 10th	Day	14% 12% 29% 26% 14% 5%	300
	11th to 20th	Day	12% 12% 35% 26% 13% 2	300
	21st to end	Day	7% 11% 36% 34% 11% 1	291
December	1st to 10th	Day	10% 11% 39% 29% 10% 2	270
	11th to 20th	Day	13% 11% 32% 33% 9% 1	270
	21st to end	Day	10% 10% 28% 33% 16% 3	297
January	1st to 10th	Night	11% 8% 33% 34% 13% 1	950
	11th to 20th	Night	13% 9% 34% 31% 11% 1	950
	21st to end	Night	11% 11% 34% 33% 10% 1	983
February	1st to 10th	Night	13% 9% 30% 34% 13% 1	760
	11th to 20th	Night	11% 9% 35% 36% 8% 1	744
	21st to end	Night	12% 7% 31% 34% 13% 2	628
March	1st to 10th	Night	11% 7% 34% 37% 10% 1	760
	11th to 20th	Night	10% 5% 33% 41% 10% 1	760
	21st to end	Night	16% 7% 27% 36% 13% 1	732
April	1st to 10th	Night	18% 7% 26% 40% 8% 1	570
	11th to 20th	Night	15% 8% 31% 35% 10% 1	563
	21st to end	Night	17% 6% 23% 40% 14% 1	570
May	1st to 10th	Night	16% 7% 21% 40% 15% 1	570
	11th to 20th	Night	17% 4 22% 36% 19% 2	561
	21st to end	Night	17% 4 27% 33% 17% 1	617
June	1st to 10th	Night	11% 7% 26% 37% 17% 2	570
	11th to 20th	Night	10% 8% 36% 35% 10%	570
	21st to end	Night	8% 5% 25% 47% 15% 1	570
July	1st to 10th	Night	7% 4 24% 45% 21%	555
	11th to 20th	Night	5% 3 32% 47% 13%	552
	21st to end	Night	5% 3 20% 51% 21%	624
August	1st to 10th	Night	7% 2 24% 50% 18%	567
	11th to 20th	Night	5% 2 22% 50% 20% 1	564
	21st to end	Night	6% 4 21% 47% 20% 1	627
September	1st to 10th	Night	8% 4 22% 46% 19% 1	570
	11th to 20th	Night	12% 5% 20% 45% 17% 2	570
	21st to end	Night	13% 5% 23% 39% 19% 2	670
October	1st to 10th	Night	18% 4 23% 36% 18% 1	760
	11th to 20th	Night	14% 6% 25% 37% 16% 1	760
	21st to end	Night	11% 8% 30% 37% 13% 1	957
November	1st to 10th	Night	12% 7% 28% 41% 11% 1	975
	11th to 20th	Night	10% 10% 36% 34% 9% 1	1000
	21st to end	Night	9% 9% 35% 38% 8%	986
December	1st to 10th	Night	11% 10% 34% 36% 10% 1	950
	11th to 20th	Night	10% 8% 34% 36% 11% 1	950
	21st to end	Night	14% 9% 31% 33% 11% 1	1045

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