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 2024-03-08.

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 2 4 4 5% 8% 63%	78759
	Night	4 4 2 3 4 2 4 4 5% 7% 60%	80853
Transparency	Day	18% 6% 10% 21% 27% 18%	78485
	Night	21% 6% 10% 19% 26% 17%	80648
Seeing	Day	5% 8% 20% 31% 25% 10%	13908
	Night	5% 5% 24% 43% 21% 1	24285

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% 3 3 3 2 3 4 5% 8% 55%	2858
	11th to 20th	Day	3 3 1 2 3 2 2 3 4 7% 69%	2860
	21st to end	Day	5% 6% 2 5% 5% 3 4 5% 6% 7% 52%	3146
February	1st to 10th	Day	3 5% 1 3 3 2 3 4 4 8% 64%	1711
	11th to 20th	Day	3 3 2 4 4 2 3 4 5% 6% 63%	1849
	21st to end	Day	3 4 1 3 3 1 3 4 4 6% 67%	1570
March	1st to 10th	Day	3 3 2 3 3 3 3 5% 6% 8% 60%	2029
	11th to 20th	Day	3 5% 2 4 3 2 3 4 4 7% 63%	1980
	21st to end	Day	3 3 2 3 3 2 3 4 6% 7% 65%	2214
April	1st to 10th	Day	3 2 1 2 3 1 4 4 5% 7% 68%	2160
	11th to 20th	Day	4 3 1 2 3 2 4 3 4 6% 68%	2131
	21st to end	Day	3 2 1 2 2 2 3 3 4 6% 72%	2160
May	1st to 10th	Day	2 1 1 2 2 2 3 3 5% 7% 72%	2160
	11th to 20th	Day	3 2 1 3 4 3 4 4 5% 8% 64%	2195
	21st to end	Day	1 2 1 2 2 2 3 3 5% 7% 72%	2606
June	1st to 10th	Day	1 1 1 3 3 2 4 5% 6% 10% 65%	2520
	11th to 20th	Day	2 1 1 2 3 2 3 3 4 6% 74%	2520
	21st to end	Day	4 2 1 3 4 2 5% 5% 6% 8% 61%	2488
July	1st to 10th	Day	3 2 1 3 4 3 6% 6% 8% 12% 50%	2340
	11th to 20th	Day	3 2 1 3 4 3 6% 7% 9% 13% 51%	2340
	21st to end	Day	2 2 2 4 5% 3 7% 8% 10% 13% 44%	2545
August	1st to 10th	Day	3 2 1 4 5% 4 5% 7% 10% 10% 50%	2285
	11th to 20th	Day	3 2 1 3 4 3 5% 6% 8% 11% 53%	2123
	21st to end	Day	3 2 1 2 4 3 4 6% 7% 11% 57%	2376
September	1st to 10th	Day	5% 3 1 4 5% 3 4 4 6% 8% 56%	2160
	11th to 20th	Day	5% 2 1 3 3 2 3 4 5% 9% 62%	2160
	21st to end	Day	2 2 1 3 3 2 2 4 4 5% 72%	1933
October	1st to 10th	Day	4 3 2 3 3 3 3 4 5% 6% 65%	1800
	11th to 20th	Day	3 2 1 2 1 2 2 2 3 4 78%	1800
	21st to end	Day	3 2 1 2 2 2 2 2 2 4 77%	1980
November	1st to 10th	Day	2 1 1 2 2 2 2 3 4 6% 74%	1859
	11th to 20th	Day	2 3 1 2 2 2 3 3 4 5% 73%	1759
	21st to end	Day	5% 3 2 4 4 2 3 4 4 6% 65%	1710
December	1st to 10th	Day	4 3 2 4 4 2 4 4 5% 7% 63%	1552
	11th to 20th	Day	4 4 2 5% 5% 2 4 5% 7% 8% 55%	1926
	21st to end	Day	4 5% 1 3 4 2 4 4 5% 8% 60%	2954
January	1st to 10th	Night	3 9% 2 4 4 2 4 4 5% 8% 53%	4097
	11th to 20th	Night	4 3 1 2 3 2 3 3 4 5% 71%	4110
	21st to end	Night	7% 7% 2 5% 5% 3 4 4 5% 8% 49%	4521
February	1st to 10th	Night	3 5% 2 3 3 2 2 3 3 7% 66%	2466
	11th to 20th	Night	5% 5% 2 4 3 2 4 3 5% 7% 59%	2218
	21st to end	Night	4 4 1 4 3 2 3 3 4 6% 66%	1884
March	1st to 10th	Night	3 4 2 3 3 2 3 4 4 6% 65%	2086
	11th to 20th	Night	5% 4 1 3 4 2 3 4 4 5% 64%	1980
	21st to end	Night	5% 5% 1 4 3 2 3 4 4 6% 63%	2142
April	1st to 10th	Night	3 3 1 3 3 2 3 4 5% 7% 68%	1800
	11th to 20th	Night	5% 4 2 3 3 2 3 3 3 6% 66%	1777
	21st to end	Night	4 3 2 3 3 2 4 4 5% 6% 65%	1800
May	1st to 10th	Night	3 3 2 4 3 2 3 3 4 6% 66%	1800
	11th to 20th	Night	4 3 2 4 4 2 4 6% 6% 7% 57%	1696
	21st to end	Night	3 2 1 3 3 2 3 4 4 5% 71%	1666
June	1st to 10th	Night	3 4 1 4 4 3 5% 6% 6% 8% 58%	1440
	11th to 20th	Night	3 2 1 2 3 2 2 4 7% 7% 67%	1440
	21st to end	Night	4 3 1 4 4 3 5% 6% 7% 9% 52%	1464
July	1st to 10th	Night	4 4 2 4 6% 4 7% 6% 8% 11% 43%	1620
	11th to 20th	Night	2 3 2 5% 6% 4 8% 8% 9% 13% 40%	1620
	21st to end	Night	3 3 2 6% 7% 4 7% 7% 10% 14% 36%	1773
August	1st to 10th	Night	3 3 2 5% 6% 4 6% 8% 11% 12% 42%	1652
	11th to 20th	Night	4 3 2 5% 6% 3 7% 7% 10% 11% 42%	1778
	21st to end	Night	2 2 1 4 5% 3 6% 7% 8% 13% 48%	1980
September	1st to 10th	Night	5% 3 1 5% 6% 4 5% 6% 7% 10% 49%	1800
	11th to 20th	Night	6% 4 2 3 3 3 5% 5% 6% 7% 56%	1800
	21st to end	Night	4 2 1 3 3 3 3 3 4 7% 66%	2021
October	1st to 10th	Night	5% 3 1 3 4 2 3 4 5% 7% 62%	2160
	11th to 20th	Night	3 1 1 2 3 2 2 3 3 3 76%	2160
	21st to end	Night	3 3 1 2 3 2 3 3 3 5% 72%	2376
November	1st to 10th	Night	2 2 1 2 2 1 2 3 3 6% 77%	2220
	11th to 20th	Night	2 2 1 2 3 2 2 3 4 4 75%	2398
	21st to end	Night	6% 4 2 2 3 2 3 3 3 6% 65%	2470
December	1st to 10th	Night	5% 4 1 3 3 2 3 3 4 7% 65%	2626
	11th to 20th	Night	5% 4 2 4 3 2 4 5% 6% 10% 55%	3271
	21st to end	Night	6% 5% 2 4 4 2 3 4 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	20% 5% 8% 18% 29% 20%	2029
	11th to 20th	Day	20% 4 7% 18% 32% 19%	1980
	21st to end	Day	16% 5% 7% 20% 33% 19%	2214
April	1st to 10th	Day	16% 4 7% 15% 35% 24%	2160
	11th to 20th	Day	16% 4 7% 15% 29% 29%	2134
	21st to end	Day	14% 4 7% 17% 35% 23%	2160
May	1st to 10th	Day	13% 4 7% 13% 33% 30%	2160
	11th to 20th	Day	19% 6% 9% 17% 26% 23%	2198
	21st to end	Day	11% 4 9% 15% 28% 33%	2606
June	1st to 10th	Day	13% 5% 10% 19% 24% 28%	2520
	11th to 20th	Day	11% 4 6% 11% 20% 48%	2520
	21st to end	Day	20% 5% 10% 17% 20% 28%	2488
July	1st to 10th	Day	22% 8% 15% 18% 17% 19%	2276
	11th to 20th	Day	18% 11% 20% 24% 17% 10%	2271
	21st to end	Day	23% 12% 23% 27% 12% 3	2404
August	1st to 10th	Day	23% 12% 18% 23% 15% 9%	2270
	11th to 20th	Day	23% 9% 17% 23% 19% 10%	2123
	21st to end	Day	19% 9% 16% 25% 24% 7%	2376
September	1st to 10th	Day	24% 8% 10% 19% 24% 14%	2160
	11th to 20th	Day	19% 7% 12% 16% 25% 22%	2160
	21st to end	Day	14% 4 8% 21% 31% 23%	1933
October	1st to 10th	Day	19% 7% 9% 18% 25% 22%	1800
	11th to 20th	Day	12% 3 5% 13% 35% 33%	1800
	21st to end	Day	13% 3 7% 21% 39% 18%	1980
November	1st to 10th	Day	11% 3 6% 22% 39% 19%	1859
	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	19% 4 7% 17% 32% 21%	2086
	11th to 20th	Night	21% 4 7% 17% 32% 19%	1980
	21st to end	Night	22% 4 8% 16% 32% 19%	2142
April	1st to 10th	Night	16% 4 6% 14% 34% 26%	1800
	11th to 20th	Night	19% 4 8% 13% 25% 31%	1777
	21st to end	Night	18% 6% 8% 12% 33% 24%	1800
May	1st to 10th	Night	19% 4 7% 11% 30% 29%	1800
	11th to 20th	Night	23% 7% 12% 14% 21% 23%	1697
	21st to end	Night	15% 5% 9% 13% 23% 36%	1666
June	1st to 10th	Night	22% 5% 12% 17% 18% 26%	1440
	11th to 20th	Night	15% 4 8% 11% 17% 45%	1440
	21st to end	Night	25% 8% 12% 14% 15% 26%	1464
July	1st to 10th	Night	32% 11% 15% 14% 14% 13%	1575
	11th to 20th	Night	27% 12% 23% 18% 12% 7%	1566
	21st to end	Night	31% 16% 23% 20% 7% 3	1674
August	1st to 10th	Night	27% 13% 21% 22% 11% 6%	1643
	11th to 20th	Night	28% 12% 19% 19% 13% 9%	1778
	21st to end	Night	23% 12% 18% 24% 16% 7%	1980
September	1st to 10th	Night	27% 8% 14% 20% 18% 13%	1800
	11th to 20th	Night	24% 8% 13% 15% 19% 22%	1800
	21st to end	Night	18% 5% 7% 15% 30% 25%	2021
October	1st to 10th	Night	21% 6% 10% 16% 22% 24%	2160
	11th to 20th	Night	14% 4 5% 11% 30% 37%	2160
	21st to end	Night	16% 4 6% 16% 37% 22%	2376
November	1st to 10th	Night	10% 3 5% 22% 40% 21%	2220
	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% 33% 29% 11% 2	420
	11th to 20th	Day	10% 17% 27% 27% 17% 1	420
	21st to end	Day	9% 25% 30% 23% 11% 1	462
April	1st to 10th	Day	7% 17% 33% 26% 12% 4	420
	11th to 20th	Day	9% 20% 35% 25% 10% 2	412
	21st to end	Day	6% 19% 32% 30% 11% 2	420
May	1st to 10th	Day	4 15% 35% 29% 16% 2	423
	11th to 20th	Day	4 10% 29% 31% 19% 6%	448
	21st to end	Day	3 13% 28% 33% 20% 3	561
June	1st to 10th	Day	2 4 19% 35% 32% 7%	630
	11th to 20th	Day	2 5% 18% 35% 32% 7%	630
	21st to end	Day	4 3 15% 38% 32% 8%	598
July	1st to 10th	Day	1 1 10% 35% 32% 21%	446
	11th to 20th	Day	4 4 20% 37% 35%	444
	21st to end	Day	1 6% 20% 37% 36%	487
August	1st to 10th	Day	2 6% 24% 42% 26%	447
	11th to 20th	Day	3 1 6% 27% 41% 22%	437
	21st to end	Day	4 1 5% 28% 40% 22%	495
September	1st to 10th	Day	4 2 11% 26% 37% 20%	450
	11th to 20th	Day	3 1 13% 32% 38% 12%	450
	21st to end	Day	3 8% 16% 26% 33% 15%	322
October	1st to 10th	Day	4 8% 19% 27% 30% 12%	270
	11th to 20th	Day	6% 5% 17% 33% 31% 8%	270
	21st to end	Day	8% 7% 14% 31% 32% 7%	297
November	1st to 10th	Day	6% 8% 20% 40% 23% 3	270
	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	750
	11th to 20th	Night	6% 9% 30% 42% 13%	720
	21st to end	Night	6% 12% 33% 42% 8%	756
April	1st to 10th	Night	4 8% 42% 41% 6%	540
	11th to 20th	Night	6% 13% 36% 38% 8%	533
	21st to end	Night	5% 11% 39% 39% 6%	540
May	1st to 10th	Night	4 5% 38% 44% 8%	540
	11th to 20th	Night	5% 6% 28% 42% 18% 1	531
	21st to end	Night	3 4 32% 47% 14%	584
June	1st to 10th	Night	3 3 24% 51% 19%	540
	11th to 20th	Night	2 2 24% 52% 19%	540
	21st to end	Night	3 3 16% 49% 29% 1	540
July	1st to 10th	Night	4 1 13% 41% 38% 3	525
	11th to 20th	Night	3 7% 32% 54% 4	522
	21st to end	Night	4 1 5% 30% 53% 7%	591
August	1st to 10th	Night	4 5% 31% 52% 8%	537
	11th to 20th	Night	3 1 7% 41% 44% 4	534
	21st to end	Night	2 6% 37% 50% 5%	594
September	1st to 10th	Night	5% 1 9% 46% 36% 2	540
	11th to 20th	Night	7% 1 11% 46% 33% 2	540
	21st to end	Night	4 2 16% 48% 29% 2	635
October	1st to 10th	Night	5% 3 20% 48% 22% 1	720
	11th to 20th	Night	4 4 19% 53% 18% 2	720
	21st to end	Night	4 4 23% 41% 25% 3	792
November	1st to 10th	Night	2 3 21% 49% 23% 1	740
	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

Page generated at 2024-03-05 05:24:01. Version ac07

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