Horizon Vista Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Horizon Vista Observatory clear sky chart from 2005-11-06 to 2024-11-02.

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 1 3 3 3 4 4 6% 8% 63%	100703
	Night	4 4 2 4 3 4 4 4 5% 7% 59%	98518
Transparency	Day	17% 6% 10% 21% 27% 19%	100466
	Night	22% 6% 10% 19% 26% 17%	98344
Seeing	Day	5% 8% 20% 31% 26% 10%	14856
	Night	5% 5% 24% 43% 21% 1	25064

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	3 10% 2 4 2 3 3 4 5% 8% 55%	2858
	11th to 20th	Day	3 3 1 3 3 3 2 2 4 7% 69%	2860
	21st to end	Day	5% 7% 2 6% 4 3 4 5% 6% 7% 52%	3146
February	1st to 10th	Day	3 5% 1 3 3 3 3 4 5% 8% 64%	1711
	11th to 20th	Day	3 4 2 4 3 3 3 3 5% 6% 63%	1849
	21st to end	Day	3 4 1 4 3 2 3 3 4 6% 67%	1570
March	1st to 10th	Day	2 3 1 3 3 3 3 4 5% 7% 66%	3404
	11th to 20th	Day	5% 5% 3 5% 4 4 4 4 6% 8% 52%	3490
	21st to end	Day	2 4 2 4 3 3 4 4 6% 7% 62%	3886
April	1st to 10th	Day	3 2 1 2 3 2 4 3 5% 6% 67%	2280
	11th to 20th	Day	3 3 1 3 2 3 3 3 4 6% 69%	2251
	21st to end	Day	3 2 1 2 2 2 3 3 4 6% 72%	2280
May	1st to 10th	Day	1 1 1 2 2 2 3 3 4 6% 75%	3800
	11th to 20th	Day	3 3 1 3 4 4 4 4 5% 7% 63%	3900
	21st to end	Day	1 2 1 1 2 2 2 4 5% 80%	4623
June	1st to 10th	Day	2 2 1 3 3 2 4 4 6% 9% 64%	2660
	11th to 20th	Day	2 1 1 2 2 2 2 3 5% 6% 74%	2660
	21st to end	Day	4 3 1 5% 3 4 5% 4 7% 7% 58%	2626
July	1st to 10th	Day	2 2 1 3 3 4 5% 4 7% 9% 61%	4110
	11th to 20th	Day	2 1 1 3 3 3 5% 5% 9% 11% 59%	4110
	21st to end	Day	2 2 1 4 4 4 6% 6% 9% 12% 48%	4454
August	1st to 10th	Day	1 1 1 3 4 4 5% 5% 10% 11% 56%	4010
	11th to 20th	Day	2 2 1 2 3 3 4 3 7% 9% 64%	3726
	21st to end	Day	3 3 1 4 4 5% 5% 6% 9% 11% 49%	4180
September	1st to 10th	Day	4 3 1 4 4 4 4 4 6% 8% 57%	2280
	11th to 20th	Day	5% 2 1 4 2 3 4 3 5% 9% 62%	2280
	21st to end	Day	2 2 1 3 2 2 2 3 4 5% 73%	2039
October	1st to 10th	Day	4 2 1 3 2 3 3 4 5% 5% 66%	1900
	11th to 20th	Day	2 3 1 2 1 2 2 2 4 5% 77%	1900
	21st to end	Day	3 2 1 2 2 3 2 2 2 4 78%	2090
November	1st to 10th	Day	2 1 1 2 2 2 2 2 4 7% 74%	1869
	11th to 20th	Day	2 3 1 3 2 3 2 3 4 5% 73%	1759
	21st to end	Day	4 4 2 4 3 3 2 4 4 6% 65%	1710
December	1st to 10th	Day	4 3 2 4 3 3 4 3 5% 7% 63%	1552
	11th to 20th	Day	3 5% 2 6% 4 3 4 4 7% 7% 55%	1926
	21st to end	Day	4 5% 1 4 3 3 3 3 5% 8% 60%	2954
January	1st to 10th	Night	3 10% 2 5% 3 4 4 3 6% 8% 53%	4097
	11th to 20th	Night	4 3 1 3 2 2 3 3 4 5% 71%	4110
	21st to end	Night	7% 7% 2 6% 4 4 4 4 5% 8% 49%	4521
February	1st to 10th	Night	3 6% 2 3 3 3 2 3 4 7% 65%	2466
	11th to 20th	Night	5% 6% 1 4 3 3 4 2 5% 7% 59%	2218
	21st to end	Night	4 4 1 5% 2 2 3 3 3 6% 66%	1884
March	1st to 10th	Night	4 4 2 5% 3 3 3 3 4 5% 66%	3464
	11th to 20th	Night	11% 6% 3 6% 4 3 4 3 4 5% 51%	3490
	21st to end	Night	3 4 1 5% 3 4 4 4 5% 6% 61%	3765
April	1st to 10th	Night	3 3 1 3 3 2 3 3 6% 6% 68%	1900
	11th to 20th	Night	5% 4 2 3 3 3 3 2 4 5% 66%	1877
	21st to end	Night	3 3 1 4 3 3 4 3 5% 6% 65%	1900
May	1st to 10th	Night	4 3 1 4 3 2 3 2 4 5% 68%	3170
	11th to 20th	Night	4 4 1 5% 4 4 4 4 6% 8% 55%	3001
	21st to end	Night	3 2 1 2 2 2 3 2 3 4 77%	2960
June	1st to 10th	Night	3 4 1 4 4 4 4 4 6% 8% 57%	1520
	11th to 20th	Night	2 2 1 3 3 3 3 3 7% 6% 67%	1520
	21st to end	Night	4 4 1 6% 4 5% 5% 6% 7% 9% 49%	1546
July	1st to 10th	Night	3 3 2 5% 4 6% 5% 4 7% 9% 53%	2860
	11th to 20th	Night	3 4 2 6% 5% 5% 7% 6% 8% 11% 46%	2860
	21st to end	Night	4 4 2 6% 6% 6% 8% 5% 10% 13% 36%	3109
August	1st to 10th	Night	2 3 2 7% 7% 6% 6% 6% 11% 12% 38%	2927
	11th to 20th	Night	3 3 2 7% 6% 6% 7% 5% 9% 11% 42%	3113
	21st to end	Night	3 4 2 7% 6% 7% 9% 7% 10% 11% 34%	3487
September	1st to 10th	Night	5% 3 1 6% 5% 6% 4 4 7% 10% 50%	1900
	11th to 20th	Night	6% 4 2 3 3 4 4 4 6% 7% 56%	1900
	21st to end	Night	4 3 1 3 3 3 2 3 4 7% 67%	2135
October	1st to 10th	Night	5% 4 1 4 3 3 3 3 5% 6% 63%	2280
	11th to 20th	Night	3 2 1 3 3 2 3 2 4 3 74%	2280
	21st to end	Night	3 3 1 3 2 2 3 2 3 5% 73%	2508
November	1st to 10th	Night	2 2 1 2 1 2 2 2 3 6% 77%	2244
	11th to 20th	Night	2 2 1 3 2 2 2 2 4 4 75%	2398
	21st to end	Night	6% 5% 2 3 2 3 2 3 3 6% 64%	2470
December	1st to 10th	Night	5% 4 1 3 3 2 3 2 4 7% 65%	2626
	11th to 20th	Night	5% 4 1 5% 3 3 4 4 6% 10% 55%	3271
	21st to end	Night	6% 6% 2 4 3 3 3 3 5% 7% 59%	4741

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% 5% 8% 33% 25% 4	2859
	11th to 20th	Day	13% 7% 12% 37% 27% 4	2860
	21st to end	Day	27% 8% 10% 26% 23% 6%	3146
February	1st to 10th	Day	19% 5% 9% 25% 28% 14%	1711
	11th to 20th	Day	22% 4 8% 22% 30% 14%	1859
	21st to end	Day	18% 3 6% 19% 32% 23%	1570
March	1st to 10th	Day	15% 6% 7% 17% 32% 23%	3404
	11th to 20th	Day	20% 14% 11% 21% 23% 11%	3490
	21st to end	Day	16% 6% 8% 18% 37% 14%	3886
April	1st to 10th	Day	16% 4 7% 15% 35% 24%	2280
	11th to 20th	Day	15% 4 7% 15% 29% 30%	2254
	21st to end	Day	13% 4 7% 18% 36% 23%	2280
May	1st to 10th	Day	8% 4 7% 12% 34% 35%	3800
	11th to 20th	Day	15% 7% 10% 18% 31% 20%	3903
	21st to end	Day	7% 2 6% 11% 20% 54%	4623
June	1st to 10th	Day	14% 5% 10% 18% 24% 29%	2660
	11th to 20th	Day	11% 4 6% 10% 20% 49%	2660
	21st to end	Day	22% 5% 11% 17% 19% 26%	2626
July	1st to 10th	Day	19% 5% 12% 14% 22% 29%	4046
	11th to 20th	Day	16% 6% 15% 31% 25% 7%	4041
	21st to end	Day	23% 7% 17% 35% 16% 2	4314
August	1st to 10th	Day	18% 7% 17% 30% 19% 9%	3996
	11th to 20th	Day	17% 5% 12% 22% 28% 16%	3762
	21st to end	Day	27% 6% 18% 24% 20% 5%	4179
September	1st to 10th	Day	24% 7% 10% 19% 25% 14%	2280
	11th to 20th	Day	19% 6% 11% 16% 25% 22%	2280
	21st to end	Day	13% 4 7% 20% 30% 25%	2039
October	1st to 10th	Day	18% 7% 8% 18% 25% 24%	1900
	11th to 20th	Day	12% 3 5% 14% 35% 32%	1900
	21st to end	Day	13% 2 6% 20% 39% 19%	2090
November	1st to 10th	Day	11% 3 6% 22% 39% 19%	1869
	11th to 20th	Day	13% 4 6% 24% 40% 13%	1759
	21st to end	Day	21% 4 5% 21% 34% 14%	1710
December	1st to 10th	Day	20% 5% 9% 32% 24% 9%	1552
	11th to 20th	Day	24% 7% 11% 28% 24% 5%	1924
	21st to end	Day	20% 6% 11% 21% 31% 13%	2954
January	1st to 10th	Night	25% 7% 11% 27% 25% 5%	4097
	11th to 20th	Night	16% 5% 9% 32% 32% 6%	4110
	21st to end	Night	29% 8% 10% 23% 24% 6%	4521
February	1st to 10th	Night	20% 4 10% 22% 30% 14%	2466
	11th to 20th	Night	25% 4 7% 21% 26% 16%	2218
	21st to end	Night	19% 4 6% 16% 32% 23%	1884
March	1st to 10th	Night	18% 5% 6% 15% 37% 20%	3464
	11th to 20th	Night	27% 10% 10% 17% 24% 12%	3490
	21st to end	Night	18% 6% 10% 19% 32% 14%	3765
April	1st to 10th	Night	16% 4 6% 14% 34% 26%	1900
	11th to 20th	Night	19% 4 7% 12% 25% 32%	1877
	21st to end	Night	17% 6% 8% 13% 32% 23%	1900
May	1st to 10th	Night	14% 4 8% 10% 31% 33%	3170
	11th to 20th	Night	19% 8% 13% 17% 23% 20%	3002
	21st to end	Night	9% 3 6% 9% 16% 56%	2960
June	1st to 10th	Night	22% 6% 11% 16% 18% 27%	1520
	11th to 20th	Night	15% 4 8% 11% 18% 45%	1520
	21st to end	Night	27% 8% 13% 14% 14% 25%	1546
July	1st to 10th	Night	27% 7% 13% 13% 14% 26%	2815
	11th to 20th	Night	28% 7% 17% 27% 17% 4	2806
	21st to end	Night	35% 9% 17% 29% 9% 1	3011
August	1st to 10th	Night	34% 7% 17% 21% 14% 7%	2918
	11th to 20th	Night	32% 7% 13% 17% 17% 14%	3143
	21st to end	Night	38% 8% 17% 20% 12% 5%	3487
September	1st to 10th	Night	26% 8% 13% 19% 20% 13%	1900
	11th to 20th	Night	24% 7% 12% 14% 20% 23%	1900
	21st to end	Night	18% 4 7% 14% 30% 26%	2135
October	1st to 10th	Night	21% 6% 9% 15% 23% 26%	2280
	11th to 20th	Night	15% 3 5% 12% 29% 36%	2280
	21st to end	Night	15% 4 5% 16% 37% 23%	2508
November	1st to 10th	Night	10% 3 5% 22% 40% 21%	2244
	11th to 20th	Night	13% 3 5% 20% 38% 21%	2398
	21st to end	Night	22% 4 6% 17% 35% 16%	2470
December	1st to 10th	Night	20% 4 8% 25% 32% 11%	2626
	11th to 20th	Night	22% 6% 10% 27% 28% 7%	3272
	21st to end	Night	21% 5% 10% 23% 30% 10%	4741

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	8% 6% 22% 47% 17% 1	270
	11th to 20th	Day	10% 9% 22% 38% 19% 2	270
	21st to end	Day	2 9% 26% 39% 21% 3	297
February	1st to 10th	Day	6% 12% 26% 36% 19% 1	270
	11th to 20th	Day	7% 10% 27% 45% 11%	270
	21st to end	Day	11% 17% 30% 32% 8% 1	225
March	1st to 10th	Day	5% 20% 32% 29% 13% 2	431
	11th to 20th	Day	11% 17% 27% 28% 15% 1	460
	21st to end	Day	9% 25% 30% 24% 11% 1	506
April	1st to 10th	Day	8% 16% 33% 26% 12% 4	460
	11th to 20th	Day	8% 19% 35% 25% 10% 2	452
	21st to end	Day	6% 18% 31% 31% 13% 2	460
May	1st to 10th	Day	4 15% 35% 28% 16% 2	463
	11th to 20th	Day	3 10% 28% 31% 20% 7%	488
	21st to end	Day	3 13% 27% 33% 20% 3	610
June	1st to 10th	Day	2 5% 19% 35% 32% 8%	680
	11th to 20th	Day	2 5% 18% 35% 32% 7%	680
	21st to end	Day	4 3 14% 37% 33% 9%	646
July	1st to 10th	Day	1 1 9% 34% 33% 21%	486
	11th to 20th	Day	3 4 19% 37% 36%	484
	21st to end	Day	2 5% 19% 38% 36%	531
August	1st to 10th	Day	2 6% 23% 41% 28%	487
	11th to 20th	Day	3 1 6% 26% 40% 25%	477
	21st to end	Day	4 1 5% 28% 40% 22%	539
September	1st to 10th	Day	4 2 11% 25% 37% 21%	490
	11th to 20th	Day	3 1 14% 32% 39% 11%	490
	21st to end	Day	3 8% 14% 25% 33% 17%	354
October	1st to 10th	Day	3 7% 17% 25% 30% 18%	300
	11th to 20th	Day	7% 5% 17% 32% 30% 10%	300
	21st to end	Day	7% 8% 14% 30% 33% 9%	330
November	1st to 10th	Day	5% 8% 20% 40% 24% 3	273
	11th to 20th	Day	3 4 19% 38% 30% 6%	270
	21st to end	Day	8% 10% 27% 33% 18% 5%	270
December	1st to 10th	Day	8% 4 26% 40% 19% 3	270
	11th to 20th	Day	4 7% 29% 38% 18% 3	270
	21st to end	Day	14% 8% 26% 38% 13% 2	297
January	1st to 10th	Night	5% 3 25% 49% 17% 1	760
	11th to 20th	Night	7% 4 29% 43% 15% 1	760
	21st to end	Night	7% 5% 28% 43% 17% 1	836
February	1st to 10th	Night	5% 7% 31% 42% 14% 1	760
	11th to 20th	Night	6% 7% 35% 43% 8%	744
	21st to end	Night	7% 8% 42% 39% 4	628
March	1st to 10th	Night	6% 9% 36% 40% 9% 1	760
	11th to 20th	Night	6% 9% 31% 41% 12%	760
	21st to end	Night	6% 12% 32% 42% 8%	798
April	1st to 10th	Night	4 8% 42% 40% 6%	570
	11th to 20th	Night	6% 13% 36% 38% 7%	563
	21st to end	Night	4 11% 39% 40% 6%	570
May	1st to 10th	Night	4 6% 38% 44% 8%	570
	11th to 20th	Night	4 6% 29% 42% 18% 1	561
	21st to end	Night	3 4 32% 47% 13%	617
June	1st to 10th	Night	3 3 24% 51% 19%	570
	11th to 20th	Night	2 3 25% 52% 18%	570
	21st to end	Night	3 3 16% 48% 29% 1	570
July	1st to 10th	Night	4 1 13% 41% 38% 3	555
	11th to 20th	Night	3 7% 34% 52% 4	552
	21st to end	Night	4 1 6% 31% 52% 7%	624
August	1st to 10th	Night	4 1 5% 32% 51% 9%	567
	11th to 20th	Night	3 1 6% 41% 44% 5%	564
	21st to end	Night	3 6% 37% 50% 4	627
September	1st to 10th	Night	5% 1 9% 46% 37% 2	570
	11th to 20th	Night	6% 1 12% 46% 32% 2	570
	21st to end	Night	4 2 16% 48% 29% 2	671
October	1st to 10th	Night	5% 3 19% 48% 23% 1	760
	11th to 20th	Night	4 4 19% 52% 19% 2	760
	21st to end	Night	4 4 24% 41% 25% 3	836
November	1st to 10th	Night	2 3 21% 49% 23% 1	748
	11th to 20th	Night	2 3 23% 47% 22% 2	878
	21st to end	Night	9% 7% 22% 38% 24% 1	950
December	1st to 10th	Night	7% 4 26% 45% 17% 1	950
	11th to 20th	Night	6% 6% 26% 46% 16%	950
	21st to end	Night	8% 6% 28% 47% 11% 1	965

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