Dome Valley Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Dome Valley Observatory clear sky chart from 2005-11-06 to 2024-03-18.

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 2 4 4 3 4 5% 6% 8% 61%	78863
	Night	3 4 2 4 4 3 4 4 5% 8% 60%	80976
Transparency	Day	21% 6% 9% 22% 27% 16%	78589
	Night	22% 7% 9% 21% 27% 15%	80772
Seeing	Day	6% 14% 27% 31% 18% 4	14117
	Night	5% 8% 26% 41% 18% 1	24432

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 5% 4 2 4 4 5% 7% 59%	2858
	11th to 20th	Day	1 4 2 3 3 2 3 4 5% 7% 66%	2860
	21st to end	Day	3 6% 3 6% 5% 3 5% 5% 5% 8% 52%	3146
February	1st to 10th	Day	2 5% 2 6% 4 3 4 4 5% 7% 60%	1715
	11th to 20th	Day	2 4 2 5% 5% 2 3 3 5% 9% 60%	1849
	21st to end	Day	1 4 2 4 4 2 4 3 5% 8% 63%	1570
March	1st to 10th	Day	2 3 2 5% 4 2 5% 4 7% 8% 59%	2023
	11th to 20th	Day	3 5% 2 3 4 3 5% 4 5% 7% 58%	2058
	21st to end	Day	3 4 2 3 4 2 4 4 5% 8% 60%	2232
April	1st to 10th	Day	1 3 1 3 3 2 4 3 4 8% 67%	2160
	11th to 20th	Day	3 3 2 4 4 2 3 4 5% 7% 63%	2131
	21st to end	Day	2 3 2 3 3 2 3 4 5% 7% 67%	2160
May	1st to 10th	Day	1 1 1 3 3 2 4 3 5% 9% 67%	2160
	11th to 20th	Day	3 3 1 3 4 3 5% 5% 5% 8% 60%	2198
	21st to end	Day	2 2 1 2 2 2 3 4 6% 6% 69%	2707
June	1st to 10th	Day	1 1 1 2 3 2 4 5% 7% 10% 64%	2520
	11th to 20th	Day	1 1 1 2 2 2 3 4 5% 6% 72%	2520
	21st to end	Day	3 4 1 3 4 2 5% 5% 6% 9% 59%	2520
July	1st to 10th	Day	3 3 2 5% 5% 4 6% 7% 8% 11% 49%	2430
	11th to 20th	Day	2 3 2 3 5% 3 5% 8% 10% 12% 48%	2340
	21st to end	Day	4 3 2 5% 6% 4 7% 9% 10% 11% 40%	2545
August	1st to 10th	Day	3 2 1 4 5% 4 6% 8% 8% 12% 47%	2285
	11th to 20th	Day	2 4 2 4 5% 4 6% 6% 7% 12% 48%	2123
	21st to end	Day	2 2 1 3 5% 4 5% 6% 8% 11% 52%	2376
September	1st to 10th	Day	4 3 2 3 4 4 5% 7% 7% 8% 54%	2160
	11th to 20th	Day	4 3 1 3 3 2 4 5% 7% 7% 61%	2160
	21st to end	Day	2 2 1 3 2 2 3 4 4 7% 70%	1944
October	1st to 10th	Day	5% 3 1 3 4 2 4 5% 5% 7% 61%	1800
	11th to 20th	Day	1 2 1 2 2 1 2 3 3 4 76%	1800
	21st to end	Day	3 3 1 2 2 2 3 3 3 4 74%	1980
November	1st to 10th	Day	2 1 1 2 3 1 2 3 5% 7% 74%	1840
	11th to 20th	Day	2 2 2 3 2 2 3 3 5% 6% 71%	1710
	21st to end	Day	2 3 2 5% 4 2 3 4 5% 5% 66%	1710
December	1st to 10th	Day	2 4 1 4 4 2 3 4 5% 7% 65%	1544
	11th to 20th	Day	3 4 2 5% 5% 3 5% 5% 6% 8% 55%	1926
	21st to end	Day	2 4 2 4 3 3 3 3 4 8% 64%	2803
January	1st to 10th	Night	1 6% 5% 5% 5% 3 4 4 5% 6% 56%	4115
	11th to 20th	Night	3 4 2 3 4 2 4 3 3 5% 67%	4110
	21st to end	Night	3 7% 2 5% 5% 3 5% 4 6% 9% 52%	4521
February	1st to 10th	Night	2 6% 2 3 3 2 3 3 4 7% 64%	2462
	11th to 20th	Night	3 5% 2 5% 4 2 4 4 4 6% 59%	2218
	21st to end	Night	2 4 1 5% 4 2 3 4 4 6% 65%	1884
March	1st to 10th	Night	3 4 2 4 3 3 3 5% 5% 8% 60%	2152
	11th to 20th	Night	4 6% 2 5% 5% 2 4 4 4 7% 58%	2068
	21st to end	Night	4 5% 2 3 4 3 4 3 4 7% 61%	2122
April	1st to 10th	Night	3 3 2 4 3 2 3 4 5% 8% 64%	1800
	11th to 20th	Night	3 4 2 4 4 2 4 4 4 7% 62%	1777
	21st to end	Night	4 3 1 3 4 2 3 4 4 7% 65%	1800
May	1st to 10th	Night	2 3 2 3 4 3 4 5% 4 7% 64%	1800
	11th to 20th	Night	4 4 2 5% 5% 3 5% 4 6% 8% 55%	1693
	21st to end	Night	2 2 1 3 4 3 4 4 5% 6% 66%	1566
June	1st to 10th	Night	1 1 2 4 4 3 6% 5% 7% 12% 55%	1440
	11th to 20th	Night	1 2 1 3 3 2 4 4 4 7% 69%	1440
	21st to end	Night	3 4 2 4 4 3 5% 6% 6% 9% 52%	1440
July	1st to 10th	Night	2 3 2 5% 7% 4 8% 8% 8% 11% 43%	1524
	11th to 20th	Night	2 3 2 4 6% 5% 6% 7% 10% 12% 42%	1620
	21st to end	Night	3 4 3 6% 7% 5% 7% 8% 11% 13% 32%	1773
August	1st to 10th	Night	2 3 2 4 5% 4 7% 7% 10% 13% 42%	1654
	11th to 20th	Night	5% 4 2 5% 6% 4 7% 7% 10% 11% 40%	1778
	21st to end	Night	2 2 2 3 4 4 6% 6% 10% 14% 47%	1980
September	1st to 10th	Night	4 3 2 4 5% 4 7% 7% 8% 10% 47%	1800
	11th to 20th	Night	4 3 2 3 3 3 5% 4 6% 8% 59%	1800
	21st to end	Night	3 3 1 2 3 2 3 4 6% 6% 66%	2011
October	1st to 10th	Night	4 3 2 4 4 2 4 4 5% 6% 63%	2160
	11th to 20th	Night	2 1 1 2 2 1 2 3 4 4 77%	2160
	21st to end	Night	3 2 1 2 3 2 3 4 4 5% 72%	2376
November	1st to 10th	Night	1 1 1 2 2 1 2 3 4 5% 77%	2220
	11th to 20th	Night	1 2 1 3 2 1 2 3 4 6% 74%	2466
	21st to end	Night	3 4 2 3 4 2 4 3 5% 6% 63%	2470
December	1st to 10th	Night	3 4 1 3 3 2 4 4 4 7% 65%	2632
	11th to 20th	Night	3 3 2 4 5% 3 4 4 6% 8% 57%	3271
	21st to end	Night	3 6% 2 4 4 2 3 3 4 5% 62%	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	21% 8% 7% 34% 26% 3	2859
	11th to 20th	Day	16% 7% 13% 36% 24% 4	2860
	21st to end	Day	27% 9% 8% 25% 26% 5%	3146
February	1st to 10th	Day	24% 4 9% 27% 26% 10%	1715
	11th to 20th	Day	24% 4 6% 25% 29% 11%	1859
	21st to end	Day	22% 4 4 20% 30% 20%	1570
March	1st to 10th	Day	22% 5% 5% 20% 32% 16%	2023
	11th to 20th	Day	24% 6% 8% 17% 32% 13%	2058
	21st to end	Day	21% 6% 7% 20% 32% 14%	2232
April	1st to 10th	Day	16% 4 6% 17% 36% 20%	2160
	11th to 20th	Day	20% 6% 5% 17% 31% 22%	2134
	21st to end	Day	17% 4 7% 20% 34% 18%	2160
May	1st to 10th	Day	15% 4 7% 17% 33% 23%	2160
	11th to 20th	Day	23% 7% 9% 18% 26% 17%	2201
	21st to end	Day	15% 5% 8% 17% 26% 29%	2707
June	1st to 10th	Day	14% 6% 9% 19% 28% 25%	2520
	11th to 20th	Day	13% 3 5% 12% 23% 43%	2520
	21st to end	Day	21% 5% 8% 19% 22% 25%	2520
July	1st to 10th	Day	27% 9% 14% 17% 17% 17%	2366
	11th to 20th	Day	23% 10% 17% 23% 16% 11%	2271
	21st to end	Day	30% 12% 21% 25% 10% 2	2404
August	1st to 10th	Day	26% 12% 18% 20% 15% 9%	2270
	11th to 20th	Day	27% 9% 13% 22% 20% 8%	2123
	21st to end	Day	24% 9% 13% 26% 21% 7%	2376
September	1st to 10th	Day	26% 8% 10% 20% 24% 13%	2160
	11th to 20th	Day	21% 7% 9% 18% 26% 20%	2160
	21st to end	Day	16% 3 7% 21% 33% 19%	1944
October	1st to 10th	Day	23% 6% 7% 20% 25% 19%	1800
	11th to 20th	Day	14% 3 5% 12% 37% 30%	1800
	21st to end	Day	16% 3 7% 20% 36% 17%	1980
November	1st to 10th	Day	12% 4 5% 22% 40% 18%	1840
	11th to 20th	Day	15% 4 6% 25% 39% 11%	1710
	21st to end	Day	21% 5% 4 22% 34% 14%	1710
December	1st to 10th	Day	19% 5% 9% 33% 24% 10%	1544
	11th to 20th	Day	26% 7% 10% 24% 27% 6%	1924
	21st to end	Day	20% 5% 9% 23% 31% 13%	2803
January	1st to 10th	Night	24% 10% 7% 28% 24% 6%	4115
	11th to 20th	Night	16% 7% 8% 38% 24% 6%	4110
	21st to end	Night	26% 9% 9% 24% 26% 5%	4521
February	1st to 10th	Night	21% 4 10% 24% 29% 11%	2462
	11th to 20th	Night	25% 6% 7% 23% 27% 13%	2218
	21st to end	Night	21% 4 5% 17% 34% 20%	1884
March	1st to 10th	Night	23% 5% 5% 21% 30% 17%	2152
	11th to 20th	Night	25% 7% 6% 18% 31% 13%	2068
	21st to end	Night	23% 5% 6% 17% 35% 14%	2122
April	1st to 10th	Night	20% 3 6% 17% 33% 21%	1800
	11th to 20th	Night	22% 5% 4 16% 29% 24%	1777
	21st to end	Night	19% 5% 7% 16% 35% 18%	1800
May	1st to 10th	Night	19% 5% 8% 14% 31% 23%	1800
	11th to 20th	Night	27% 7% 9% 18% 21% 18%	1694
	21st to end	Night	17% 6% 8% 15% 25% 28%	1566
June	1st to 10th	Night	19% 8% 13% 17% 19% 24%	1440
	11th to 20th	Night	16% 3 6% 11% 22% 42%	1440
	21st to end	Night	27% 7% 11% 13% 17% 26%	1440
July	1st to 10th	Night	33% 10% 13% 17% 12% 16%	1480
	11th to 20th	Night	29% 12% 19% 19% 13% 9%	1566
	21st to end	Night	35% 14% 21% 21% 7% 2	1674
August	1st to 10th	Night	28% 13% 19% 20% 11% 9%	1645
	11th to 20th	Night	33% 11% 17% 18% 12% 8%	1778
	21st to end	Night	24% 10% 15% 25% 17% 8%	1980
September	1st to 10th	Night	29% 8% 14% 18% 19% 12%	1800
	11th to 20th	Night	23% 6% 12% 17% 20% 23%	1800
	21st to end	Night	19% 5% 7% 16% 29% 23%	2011
October	1st to 10th	Night	23% 6% 8% 18% 22% 23%	2160
	11th to 20th	Night	13% 4 4 13% 33% 33%	2160
	21st to end	Night	16% 4 5% 18% 38% 20%	2376
November	1st to 10th	Night	11% 3 5% 22% 41% 18%	2220
	11th to 20th	Night	13% 3 5% 23% 40% 16%	2466
	21st to end	Night	23% 3 5% 19% 34% 16%	2470
December	1st to 10th	Night	19% 5% 8% 30% 30% 8%	2632
	11th to 20th	Night	24% 7% 8% 25% 29% 7%	3272
	21st to end	Night	22% 8% 6% 23% 31% 9%	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	4 16% 37% 34% 9%	270
	11th to 20th	Day	11% 19% 29% 32% 10%	270
	21st to end	Day	3 14% 34% 35% 12% 2	297
February	1st to 10th	Day	6% 22% 36% 25% 11%	270
	11th to 20th	Day	7% 23% 41% 26% 4	270
	21st to end	Day	14% 28% 35% 20% 4	225
March	1st to 10th	Day	8% 28% 29% 28% 7%	393
	11th to 20th	Day	15% 26% 29% 21% 9% 1	449
	21st to end	Day	14% 34% 32% 15% 6%	462
April	1st to 10th	Day	9% 27% 36% 19% 9%	420
	11th to 20th	Day	11% 33% 32% 17% 6% 1	412
	21st to end	Day	8% 30% 35% 21% 4 1	420
May	1st to 10th	Day	6% 25% 41% 22% 5% 1	423
	11th to 20th	Day	7% 17% 34% 28% 11% 2	448
	21st to end	Day	5% 15% 33% 33% 13% 1	662
June	1st to 10th	Day	2 6% 25% 42% 23% 2	630
	11th to 20th	Day	3 9% 24% 37% 24% 3	630
	21st to end	Day	5% 6% 24% 35% 27% 3	630
July	1st to 10th	Day	3 3 19% 38% 30% 7%	536
	11th to 20th	Day	4 1 10% 33% 37% 16%	444
	21st to end	Day	4 1 8% 32% 40% 15%	487
August	1st to 10th	Day	3 1 12% 35% 38% 10%	447
	11th to 20th	Day	4 1 19% 38% 29% 8%	437
	21st to end	Day	4 2 15% 37% 34% 8%	495
September	1st to 10th	Day	6% 4 18% 38% 28% 7%	450
	11th to 20th	Day	2 5% 25% 39% 23% 6%	450
	21st to end	Day	3 9% 25% 35% 22% 6%	306
October	1st to 10th	Day	5% 13% 28% 27% 23% 5%	270
	11th to 20th	Day	7% 11% 27% 30% 21% 4	270
	21st to end	Day	11% 12% 23% 31% 21% 2	297
November	1st to 10th	Day	4 19% 32% 31% 13% 1	270
	11th to 20th	Day	3 11% 30% 36% 19%	270
	21st to end	Day	6% 23% 29% 28% 12% 2	270
December	1st to 10th	Day	7% 16% 30% 36% 10%	270
	11th to 20th	Day	9% 16% 31% 37% 7%	270
	21st to end	Day	13% 21% 31% 31% 4	297
January	1st to 10th	Night	4 7% 32% 42% 15% 1	765
	11th to 20th	Night	4 9% 29% 41% 15% 1	760
	21st to end	Night	4 11% 29% 40% 15% 1	836
February	1st to 10th	Night	6% 10% 35% 38% 11% 1	760
	11th to 20th	Night	8% 11% 36% 38% 7%	744
	21st to end	Night	7% 13% 41% 36% 4	628
March	1st to 10th	Night	6% 13% 36% 34% 10%	760
	11th to 20th	Night	9% 12% 31% 37% 12%	752
	21st to end	Night	7% 14% 32% 39% 8%	736
April	1st to 10th	Night	5% 17% 39% 34% 5%	540
	11th to 20th	Night	7% 20% 30% 35% 8%	533
	21st to end	Night	6% 16% 38% 33% 6%	540
May	1st to 10th	Night	4 12% 35% 39% 10%	540
	11th to 20th	Night	7% 11% 30% 39% 12% 2	531
	21st to end	Night	4 10% 31% 43% 11%	584
June	1st to 10th	Night	1 6% 26% 50% 16% 1	540
	11th to 20th	Night	1 5% 30% 47% 16% 1	540
	21st to end	Night	4 4 20% 48% 23% 1	540
July	1st to 10th	Night	3 2 15% 45% 33% 2	525
	11th to 20th	Night	3 1 10% 40% 43% 3	522
	21st to end	Night	3 2 8% 35% 47% 6%	591
August	1st to 10th	Night	4 1 7% 34% 46% 7%	537
	11th to 20th	Night	4 2 13% 45% 33% 3	534
	21st to end	Night	3 1 10% 43% 39% 5%	594
September	1st to 10th	Night	5% 3 14% 47% 30% 1	540
	11th to 20th	Night	6% 2 14% 48% 28% 2	540
	21st to end	Night	4 4 22% 44% 25% 1	607
October	1st to 10th	Night	4 6% 24% 45% 19% 2	720
	11th to 20th	Night	3 7% 22% 47% 20% 1	720
	21st to end	Night	5% 6% 26% 39% 21% 3	792
November	1st to 10th	Night	3 6% 24% 45% 21% 1	740
	11th to 20th	Night	3 8% 25% 45% 17% 2	946
	21st to end	Night	6% 12% 24% 36% 19% 3	950
December	1st to 10th	Night	5% 9% 28% 43% 13% 1	950
	11th to 20th	Night	7% 10% 28% 43% 12%	950
	21st to end	Night	7% 12% 30% 41% 9%	1045

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