Nine Mile Hill Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Nine Mile Hill Observatory clear sky chart from 2005-11-06 to 2024-02-15.

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	4 3 2 3 4 3 4 5% 5% 7% 59%	78980
	Night	4 3 1 3 4 2 4 4 5% 8% 61%	80159
Transparency	Day	22% 5% 8% 18% 26% 21%	78730
	Night	21% 5% 9% 18% 26% 21%	79931
Seeing	Day	5% 8% 24% 29% 25% 9%	13563
	Night	5% 5% 24% 40% 24% 2	23851

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 3 1 2 2 1 2 3 4 6% 74%	3167
	11th to 20th	Day	2 2 1 2 2 2 3 3 4 5% 75%	3170
	21st to end	Day	2 4 1 3 4 3 4 4 4 7% 62%	3487
February	1st to 10th	Day	3 2 2 3 3 3 5% 4 6% 6% 63%	1900
	11th to 20th	Day	3 3 2 4 4 2 4 4 4 7% 64%	1789
	21st to end	Day	2 2 1 2 3 2 4 3 4 8% 68%	1480
March	1st to 10th	Day	3 3 2 3 6% 3 5% 6% 6% 8% 55%	1800
	11th to 20th	Day	4 5% 2 5% 4 2 4 4 5% 5% 60%	1924
	21st to end	Day	2 3 1 2 3 3 4 4 5% 7% 66%	2348
April	1st to 10th	Day	2 3 1 3 3 2 5% 5% 5% 8% 65%	2160
	11th to 20th	Day	2 3 1 4 3 2 3 4 3 7% 67%	2131
	21st to end	Day	3 2 1 3 3 2 3 3 4 6% 71%	2160
May	1st to 10th	Day	3 3 1 3 3 2 3 3 4 7% 68%	2160
	11th to 20th	Day	3 3 2 3 4 3 5% 4 5% 7% 61%	2125
	21st to end	Day	3 2 1 2 3 2 4 5% 7% 8% 63%	2479
June	1st to 10th	Day	1 2 1 2 4 3 4 5% 8% 8% 62%	2340
	11th to 20th	Day	2 2 1 3 4 2 4 6% 7% 9% 58%	2340
	21st to end	Day	5% 3 2 4 5% 3 5% 6% 7% 9% 51%	2331
July	1st to 10th	Day	6% 4 3 5% 6% 5% 9% 9% 8% 9% 35%	2160
	11th to 20th	Day	5% 3 2 4 5% 4 7% 7% 9% 10% 45%	2160
	21st to end	Day	4 4 2 6% 6% 5% 8% 8% 7% 10% 40%	2349
August	1st to 10th	Day	4 3 2 4 5% 4 7% 8% 9% 10% 44%	2149
	11th to 20th	Day	6% 5% 3 7% 8% 5% 8% 8% 7% 11% 32%	2123
	21st to end	Day	7% 5% 3 6% 6% 4 7% 7% 6% 9% 40%	2376
September	1st to 10th	Day	9% 5% 2 6% 6% 3 6% 7% 7% 8% 42%	2160
	11th to 20th	Day	9% 6% 2 5% 6% 3 7% 6% 7% 8% 40%	2059
	21st to end	Day	8% 4 2 5% 5% 2 4 5% 5% 6% 53%	1980
October	1st to 10th	Day	4 3 1 4 4 2 4 5% 5% 7% 61%	1980
	11th to 20th	Day	6% 3 2 4 3 2 4 4 4 6% 63%	1849
	21st to end	Day	5% 3 1 3 3 1 2 3 3 5% 73%	1980
November	1st to 10th	Day	4 3 1 2 2 2 2 3 4 6% 70%	1810
	11th to 20th	Day	2 4 1 3 4 2 3 4 4 6% 68%	1710
	21st to end	Day	5% 4 2 4 4 2 2 3 5% 6% 65%	1710
December	1st to 10th	Day	6% 4 1 3 3 2 4 4 5% 7% 60%	1710
	11th to 20th	Day	1 3 2 3 4 3 5% 4 5% 8% 60%	2161
	21st to end	Day	2 3 1 2 3 2 3 3 5% 6% 69%	3263
January	1st to 10th	Night	2 2 1 3 3 2 3 3 4 6% 71%	3788
	11th to 20th	Night	3 2 1 2 2 2 3 3 5% 6% 72%	3800
	21st to end	Night	4 3 2 4 5% 3 5% 5% 6% 8% 57%	4180
February	1st to 10th	Night	3 3 1 3 4 2 4 4 5% 7% 65%	2280
	11th to 20th	Night	4 3 2 4 4 3 5% 4 5% 8% 59%	2152
	21st to end	Night	3 3 1 3 3 2 4 5% 4 7% 65%	1776
March	1st to 10th	Night	4 3 2 3 4 2 5% 5% 6% 9% 58%	2160
	11th to 20th	Night	5% 3 2 4 5% 3 4 4 4 6% 60%	2031
	21st to end	Night	3 3 1 2 3 3 3 4 5% 6% 68%	2007
April	1st to 10th	Night	4 3 2 4 4 2 4 5% 4 6% 63%	1800
	11th to 20th	Night	4 3 2 3 2 2 4 4 5% 6% 64%	1777
	21st to end	Night	2 2 1 2 2 2 3 3 4 5% 74%	1800
May	1st to 10th	Night	3 2 1 3 4 2 4 4 4 6% 67%	1800
	11th to 20th	Night	5% 3 2 4 4 2 4 4 5% 7% 59%	1770
	21st to end	Night	3 3 1 4 4 3 5% 5% 7% 8% 57%	1793
June	1st to 10th	Night	2 2 1 3 3 3 5% 6% 8% 9% 58%	1620
	11th to 20th	Night	4 2 1 4 4 3 5% 6% 8% 11% 51%	1620
	21st to end	Night	4 2 1 4 4 2 5% 6% 7% 10% 53%	1622
July	1st to 10th	Night	3 4 2 5% 5% 4 6% 6% 9% 11% 47%	1800
	11th to 20th	Night	3 2 2 3 4 2 5% 6% 7% 11% 56%	1800
	21st to end	Night	3 3 1 4 4 3 6% 6% 7% 13% 51%	1969
August	1st to 10th	Night	3 2 1 3 4 3 5% 6% 8% 14% 52%	1790
	11th to 20th	Night	3 2 2 4 5% 3 6% 6% 10% 16% 43%	1778
	21st to end	Night	4 4 2 4 6% 4 5% 6% 8% 11% 47%	1980
September	1st to 10th	Night	6% 4 2 4 5% 3 5% 5% 6% 9% 50%	1800
	11th to 20th	Night	7% 5% 2 4 6% 3 5% 6% 8% 10% 45%	1899
	21st to end	Night	5% 5% 1 4 4 3 5% 5% 7% 8% 52%	1980
October	1st to 10th	Night	3 3 1 4 4 2 5% 5% 5% 7% 60%	1980
	11th to 20th	Night	6% 3 2 3 3 2 4 4 5% 6% 62%	2106
	21st to end	Night	3 3 1 3 3 2 2 3 3 5% 72%	2376
November	1st to 10th	Night	5% 2 1 3 2 1 3 2 3 6% 72%	2247
	11th to 20th	Night	2 4 2 3 3 2 3 3 4 5% 69%	2470
	21st to end	Night	4 4 1 3 4 2 3 4 4 6% 65%	2470
December	1st to 10th	Night	6% 3 1 2 4 3 4 4 5% 9% 59%	2470
	11th to 20th	Night	2 4 2 5% 5% 2 3 3 5% 8% 60%	3036
	21st to end	Night	2 3 1 2 2 2 3 3 4 5% 72%	4432

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	13% 3 4 13% 29% 39%	3168
	11th to 20th	Day	13% 2 4 14% 43% 23%	3170
	21st to end	Day	21% 4 7% 19% 35% 14%	3487
February	1st to 10th	Day	19% 4 5% 15% 25% 32%	1900
	11th to 20th	Day	22% 4 5% 14% 27% 28%	1799
	21st to end	Day	16% 3 4 12% 31% 34%	1480
March	1st to 10th	Day	23% 4 7% 16% 27% 23%	1800
	11th to 20th	Day	24% 4 6% 13% 22% 31%	1924
	21st to end	Day	17% 2 7% 11% 32% 32%	2348
April	1st to 10th	Day	17% 3 7% 13% 28% 31%	2160
	11th to 20th	Day	18% 4 6% 10% 28% 34%	2134
	21st to end	Day	15% 3 5% 11% 34% 33%	2160
May	1st to 10th	Day	17% 4 5% 10% 27% 37%	2160
	11th to 20th	Day	21% 5% 8% 17% 26% 23%	2128
	21st to end	Day	16% 5% 9% 20% 22% 28%	2479
June	1st to 10th	Day	15% 7% 12% 22% 26% 18%	2340
	11th to 20th	Day	19% 5% 9% 19% 26% 22%	2340
	21st to end	Day	25% 7% 11% 22% 21% 14%	2331
July	1st to 10th	Day	36% 13% 14% 21% 13% 3	2101
	11th to 20th	Day	28% 10% 13% 25% 19% 5%	2097
	21st to end	Day	31% 9% 18% 28% 12% 1	2219
August	1st to 10th	Day	26% 10% 16% 30% 16% 2	2136
	11th to 20th	Day	40% 12% 15% 21% 10% 2	2123
	21st to end	Day	36% 11% 16% 19% 14% 4	2376
September	1st to 10th	Day	35% 10% 14% 17% 15% 9%	2160
	11th to 20th	Day	36% 9% 12% 17% 19% 7%	2059
	21st to end	Day	29% 7% 9% 21% 23% 11%	1980
October	1st to 10th	Day	20% 6% 11% 23% 22% 19%	1980
	11th to 20th	Day	22% 5% 7% 17% 24% 25%	1849
	21st to end	Day	16% 1 4 16% 34% 29%	1980
November	1st to 10th	Day	15% 4 7% 19% 30% 25%	1810
	11th to 20th	Day	18% 3 8% 21% 28% 22%	1710
	21st to end	Day	21% 3 6% 18% 31% 21%	1710
December	1st to 10th	Day	24% 4 6% 20% 30% 17%	1710
	11th to 20th	Day	21% 4 7% 18% 31% 19%	2159
	21st to end	Day	14% 4 4 14% 34% 30%	3263
January	1st to 10th	Night	15% 2 3 11% 36% 33%	3788
	11th to 20th	Night	14% 2 5% 14% 36% 29%	3800
	21st to end	Night	25% 5% 6% 18% 33% 13%	4180
February	1st to 10th	Night	19% 3 6% 14% 26% 32%	2280
	11th to 20th	Night	24% 4 7% 11% 25% 29%	2152
	21st to end	Night	19% 3 5% 8% 28% 37%	1776
March	1st to 10th	Night	21% 4 6% 17% 27% 25%	2160
	11th to 20th	Night	25% 4 6% 12% 24% 29%	2031
	21st to end	Night	18% 3 6% 13% 28% 32%	2007
April	1st to 10th	Night	22% 4 5% 12% 28% 28%	1800
	11th to 20th	Night	20% 4 8% 11% 25% 31%	1777
	21st to end	Night	14% 4 7% 14% 27% 35%	1800
May	1st to 10th	Night	18% 4 7% 12% 26% 34%	1800
	11th to 20th	Night	24% 5% 9% 18% 22% 23%	1771
	21st to end	Night	22% 7% 12% 20% 17% 22%	1794
June	1st to 10th	Night	18% 7% 13% 27% 21% 14%	1620
	11th to 20th	Night	23% 6% 10% 20% 23% 17%	1620
	21st to end	Night	24% 7% 14% 22% 21% 13%	1622
July	1st to 10th	Night	25% 11% 20% 29% 13% 3	1750
	11th to 20th	Night	20% 8% 16% 29% 23% 4	1740
	21st to end	Night	20% 10% 21% 34% 13% 1	1859
August	1st to 10th	Night	18% 9% 19% 34% 18% 2	1779
	11th to 20th	Night	25% 11% 22% 28% 12% 2	1778
	21st to end	Night	28% 11% 17% 22% 15% 6%	1980
September	1st to 10th	Night	29% 8% 17% 22% 16% 9%	1800
	11th to 20th	Night	30% 10% 18% 19% 15% 8%	1899
	21st to end	Night	26% 7% 13% 22% 21% 11%	1980
October	1st to 10th	Night	21% 7% 15% 20% 19% 18%	1980
	11th to 20th	Night	23% 4 8% 19% 22% 23%	2106
	21st to end	Night	17% 3 5% 13% 31% 31%	2376
November	1st to 10th	Night	16% 3 6% 16% 35% 24%	2247
	11th to 20th	Night	17% 3 6% 20% 29% 24%	2470
	21st to end	Night	20% 4 6% 16% 31% 22%	2470
December	1st to 10th	Night	23% 4 6% 20% 28% 20%	2470
	11th to 20th	Night	23% 3 6% 18% 27% 23%	3037
	21st to end	Night	15% 3 3 17% 38% 24%	4432

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% 18% 34% 27% 13%	270
	11th to 20th	Day	6% 16% 34% 30% 13% 1	270
	21st to end	Day	4 20% 35% 30% 8% 3	297
February	1st to 10th	Day	8% 13% 33% 30% 16% 1	270
	11th to 20th	Day	7% 19% 40% 21% 12% 1	252
	21st to end	Day	7% 22% 43% 22% 7% 1	198
March	1st to 10th	Day	8% 13% 43% 25% 11% 1	240
	11th to 20th	Day	9% 17% 40% 23% 8% 3	364
	21st to end	Day	5% 19% 44% 24% 7%	462
April	1st to 10th	Day	4 16% 45% 24% 10% 1	420
	11th to 20th	Day	4 19% 45% 25% 6% 1	412
	21st to end	Day	5% 17% 39% 31% 7% 1	420
May	1st to 10th	Day	3 12% 41% 28% 13% 2	423
	11th to 20th	Day	3 7% 34% 41% 13% 2	448
	21st to end	Day	3 4 31% 42% 19% 2	627
June	1st to 10th	Day	2 2 16% 42% 33% 5%	630
	11th to 20th	Day	2 1 12% 41% 39% 6%	630
	21st to end	Day	4 2 9% 30% 43% 12%	621
July	1st to 10th	Day	5% 6% 24% 41% 24%	446
	11th to 20th	Day	4 3 23% 45% 24%	444
	21st to end	Day	2 1 4 28% 43% 23%	487
August	1st to 10th	Day	3 5% 17% 45% 30%	447
	11th to 20th	Day	7% 2 7% 23% 38% 25%	437
	21st to end	Day	7% 1 5% 21% 41% 25%	495
September	1st to 10th	Day	7% 2 7% 29% 40% 15%	450
	11th to 20th	Day	9% 3 6% 27% 36% 19%	349
	21st to end	Day	6% 1 11% 30% 34% 17%	270
October	1st to 10th	Day	7% 2 17% 28% 35% 11%	270
	11th to 20th	Day	8% 4 23% 33% 24% 8%	270
	21st to end	Day	6% 11% 27% 26% 19% 12%	297
November	1st to 10th	Day	6% 7% 24% 34% 23% 5%	270
	11th to 20th	Day	3 13% 24% 33% 19% 9%	270
	21st to end	Day	9% 18% 29% 28% 14% 3	270
December	1st to 10th	Day	10% 10% 27% 35% 17% 2	270
	11th to 20th	Day	5% 17% 37% 28% 13% 1	270
	21st to end	Day	4 15% 40% 29% 10% 1	297
January	1st to 10th	Night	5% 7% 34% 39% 14% 2	760
	11th to 20th	Night	7% 6% 32% 38% 16% 1	760
	21st to end	Night	5% 9% 34% 39% 12% 1	836
February	1st to 10th	Night	5% 10% 31% 37% 16% 1	760
	11th to 20th	Night	7% 7% 37% 39% 10% 1	722
	21st to end	Night	5% 11% 41% 37% 6%	592
March	1st to 10th	Night	5% 9% 36% 38% 12% 1	720
	11th to 20th	Night	6% 10% 33% 34% 15% 2	720
	21st to end	Night	4 8% 40% 39% 9% 1	621
April	1st to 10th	Night	5% 8% 35% 41% 11%	540
	11th to 20th	Night	5% 9% 38% 41% 7%	533
	21st to end	Night	2 8% 38% 43% 8% 1	540
May	1st to 10th	Night	4 6% 34% 43% 13%	540
	11th to 20th	Night	5% 5% 27% 49% 13% 1	531
	21st to end	Night	3 4 27% 50% 16% 1	584
June	1st to 10th	Night	2 2 16% 55% 24% 1	540
	11th to 20th	Night	3 1 14% 56% 26% 1	540
	21st to end	Night	4 2 9% 49% 35% 1	540
July	1st to 10th	Night	4 1 6% 36% 51% 3	525
	11th to 20th	Night	3 3 39% 50% 4	522
	21st to end	Night	2 4 39% 52% 4	591
August	1st to 10th	Night	3 1 2 25% 63% 6%	537
	11th to 20th	Night	4 1 3 22% 62% 9%	534
	21st to end	Night	4 1 3 30% 58% 4	594
September	1st to 10th	Night	7% 1 6% 29% 53% 5%	540
	11th to 20th	Night	9% 1 5% 37% 46% 3	540
	21st to end	Night	5% 2 9% 40% 40% 4	540
October	1st to 10th	Night	4 2 13% 46% 33% 3	540
	11th to 20th	Night	9% 3 17% 45% 24% 2	666
	21st to end	Night	4 5% 20% 39% 24% 8%	792
November	1st to 10th	Night	6% 2 20% 41% 26% 6%	767
	11th to 20th	Night	3 5% 23% 45% 23% 2	950
	21st to end	Night	5% 8% 28% 38% 17% 3	950
December	1st to 10th	Night	8% 5% 27% 41% 17% 2	950
	11th to 20th	Night	4 7% 34% 41% 12% 2	950
	21st to end	Night	4 12% 31% 40% 12% 2	984

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