Seven Mile Ridge Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Seven Mile Ridge Observatory clear sky chart from 2005-11-06 to 2024-01-12.

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	12% 7% 3 6% 6% 4 6% 6% 6% 8% 36%	74899
	Night	11% 7% 3 5% 6% 4 5% 6% 6% 8% 40%	74835
Transparency	Day	42% 6% 11% 20% 16% 4	74622
	Night	39% 6% 12% 21% 18% 4	74631
Seeing	Day	20% 15% 24% 21% 13% 7%	13956
	Night	18% 14% 21% 26% 18% 3	24086

By Month

Percentage Cloud Cover SE (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	10% 8% 3 6% 6% 3 4 5% 6% 7% 42%	1706
	11th to 20th	Day	11% 9% 4 6% 5% 4 5% 5% 5% 7% 39%	1635
	21st to end	Day	10% 7% 2 6% 6% 3 4 5% 6% 7% 44%	1782
February	1st to 10th	Day	11% 8% 3 4 5% 4 6% 6% 6% 9% 36%	1662
	11th to 20th	Day	11% 11% 3 6% 6% 3 6% 4 6% 7% 37%	1748
	21st to end	Day	12% 8% 2 7% 6% 4 5% 5% 6% 6% 38%	1480
March	1st to 10th	Day	10% 5% 3 6% 5% 3 4 5% 7% 7% 45%	1952
	11th to 20th	Day	14% 8% 4 5% 7% 4 6% 6% 7% 7% 33%	1980
	21st to end	Day	15% 9% 3 7% 6% 4 5% 6% 5% 7% 34%	2178
April	1st to 10th	Day	9% 5% 3 4 6% 5% 6% 6% 6% 8% 42%	1980
	11th to 20th	Day	14% 6% 3 5% 6% 3 5% 4 6% 8% 40%	2146
	21st to end	Day	13% 6% 3 6% 6% 4 6% 5% 6% 6% 41%	2340
May	1st to 10th	Day	11% 6% 3 6% 6% 4 5% 6% 8% 8% 37%	2340
	11th to 20th	Day	17% 7% 3 7% 8% 4 6% 7% 7% 7% 29%	2301
	21st to end	Day	13% 7% 3 6% 7% 4 6% 6% 8% 10% 30%	2531
June	1st to 10th	Day	11% 7% 3 7% 8% 5% 6% 6% 7% 9% 31%	2340
	11th to 20th	Day	8% 6% 3 7% 8% 5% 7% 8% 8% 9% 30%	2340
	21st to end	Day	8% 5% 3 7% 9% 5% 8% 7% 7% 10% 31%	2340
July	1st to 10th	Day	10% 8% 4 9% 9% 5% 8% 7% 8% 10% 24%	2340
	11th to 20th	Day	10% 7% 4 8% 9% 5% 8% 8% 10% 10% 22%	2340
	21st to end	Day	9% 6% 4 8% 10% 7% 7% 8% 8% 10% 24%	2547
August	1st to 10th	Day	13% 8% 3 8% 8% 6% 8% 8% 7% 9% 22%	2327
	11th to 20th	Day	12% 7% 3 8% 9% 6% 7% 7% 8% 9% 22%	2303
	21st to end	Day	12% 7% 3 9% 6% 5% 8% 7% 6% 9% 28%	2417
September	1st to 10th	Day	12% 7% 4 7% 7% 4 7% 6% 7% 7% 33%	1980
	11th to 20th	Day	18% 6% 3 7% 7% 4 6% 6% 7% 8% 29%	1980
	21st to end	Day	17% 6% 2 6% 6% 3 5% 6% 6% 7% 36%	1980
October	1st to 10th	Day	16% 6% 2 5% 5% 4 4 4 5% 6% 44%	1926
	11th to 20th	Day	11% 5% 3 5% 4 3 4 4 5% 6% 50%	1800
	21st to end	Day	13% 6% 2 4 4 2 4 4 4 7% 50%	1980
November	1st to 10th	Day	9% 6% 2 4 4 3 3 4 5% 6% 54%	1791
	11th to 20th	Day	10% 4 2 4 5% 3 4 5% 6% 8% 49%	1710
	21st to end	Day	12% 5% 2 5% 5% 3 4 4 5% 7% 48%	1710
December	1st to 10th	Day	11% 9% 4 8% 7% 4 6% 5% 6% 7% 34%	1710
	11th to 20th	Day	11% 4 2 4 5% 3 5% 5% 8% 9% 44%	2153
	21st to end	Day	15% 6% 3 5% 5% 4 5% 5% 6% 8% 38%	3124
January	1st to 10th	Night	10% 7% 3 4 6% 4 5% 5% 6% 9% 40%	2459
	11th to 20th	Night	11% 7% 3 7% 8% 4 6% 5% 5% 8% 36%	2365
	21st to end	Night	9% 8% 2 6% 5% 4 5% 5% 7% 6% 43%	2574
February	1st to 10th	Night	9% 7% 3 6% 6% 3 6% 6% 7% 8% 40%	2295
	11th to 20th	Night	12% 8% 3 6% 6% 3 5% 6% 5% 7% 39%	2098
	21st to end	Night	11% 7% 3 6% 7% 4 6% 6% 7% 7% 36%	1776
March	1st to 10th	Night	9% 6% 3 5% 6% 4 5% 5% 6% 7% 43%	2008
	11th to 20th	Night	11% 8% 3 6% 6% 4 6% 6% 6% 8% 35%	1980
	21st to end	Night	12% 7% 2 6% 6% 3 6% 7% 6% 8% 36%	2178
April	1st to 10th	Night	8% 6% 2 6% 5% 4 5% 5% 6% 9% 44%	1980
	11th to 20th	Night	11% 6% 2 5% 5% 3 4 4 5% 9% 45%	1775
	21st to end	Night	11% 6% 2 5% 5% 4 4 5% 5% 7% 46%	1620
May	1st to 10th	Night	12% 5% 2 6% 7% 4 4 5% 6% 9% 39%	1620
	11th to 20th	Night	14% 6% 4 6% 6% 4 5% 5% 6% 9% 35%	1592
	21st to end	Night	12% 6% 3 5% 5% 3 5% 6% 7% 10% 37%	1744
June	1st to 10th	Night	10% 6% 3 5% 5% 4 6% 7% 8% 11% 35%	1620
	11th to 20th	Night	9% 4 3 5% 5% 3 5% 7% 7% 10% 41%	1620
	21st to end	Night	7% 4 2 4 7% 4 6% 6% 8% 12% 39%	1620
July	1st to 10th	Night	8% 7% 2 6% 6% 5% 7% 7% 6% 9% 36%	1620
	11th to 20th	Night	9% 5% 2 6% 5% 4 6% 7% 9% 13% 34%	1620
	21st to end	Night	5% 5% 4 7% 6% 5% 8% 8% 10% 11% 33%	1771
August	1st to 10th	Night	11% 6% 3 6% 6% 4 6% 6% 7% 11% 35%	1611
	11th to 20th	Night	11% 7% 3 6% 7% 4 5% 6% 7% 10% 35%	1598
	21st to end	Night	12% 7% 3 7% 6% 3 6% 6% 7% 9% 35%	1915
September	1st to 10th	Night	11% 8% 3 6% 6% 4 5% 5% 5% 8% 38%	1980
	11th to 20th	Night	17% 8% 3 5% 6% 3 5% 4 5% 8% 35%	1980
	21st to end	Night	17% 7% 2 4 5% 3 3 4 4 6% 44%	1980
October	1st to 10th	Night	17% 6% 3 4 5% 3 4 4 5% 5% 45%	2034
	11th to 20th	Night	13% 7% 2 4 5% 3 3 5% 5% 5% 47%	2160
	21st to end	Night	12% 6% 1 4 3 2 4 4 5% 6% 52%	2376
November	1st to 10th	Night	11% 7% 2 4 4 2 4 5% 6% 6% 49%	2282
	11th to 20th	Night	11% 6% 2 4 4 3 5% 5% 6% 7% 48%	2470
	21st to end	Night	11% 5% 2 5% 4 3 5% 6% 7% 7% 46%	2470
December	1st to 10th	Night	12% 7% 3 6% 7% 5% 6% 6% 7% 7% 35%	2470
	11th to 20th	Night	11% 7% 3 5% 6% 4 5% 6% 6% 6% 41%	3042
	21st to end	Night	17% 7% 4 5% 5% 3 5% 6% 7% 8% 32%	4532

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	37% 5% 8% 23% 21% 6%	1707
	11th to 20th	Day	41% 5% 7% 20% 22% 6%	1635
	21st to end	Day	36% 4 7% 25% 22% 7%	1782
February	1st to 10th	Day	40% 5% 11% 21% 17% 6%	1662
	11th to 20th	Day	43% 4 10% 20% 18% 4	1758
	21st to end	Day	43% 5% 10% 18% 17% 7%	1480
March	1st to 10th	Day	34% 4 9% 22% 23% 7%	1952
	11th to 20th	Day	45% 4 11% 17% 16% 6%	1980
	21st to end	Day	47% 5% 10% 21% 14% 2	2178
April	1st to 10th	Day	34% 6% 12% 25% 19% 5%	1980
	11th to 20th	Day	40% 4 9% 20% 20% 6%	2146
	21st to end	Day	39% 5% 13% 26% 15% 2	2340
May	1st to 10th	Day	40% 7% 14% 23% 14% 2	2340
	11th to 20th	Day	50% 8% 13% 17% 10% 1	2305
	21st to end	Day	45% 7% 14% 24% 9% 1	2531
June	1st to 10th	Day	46% 8% 17% 19% 9%	2340
	11th to 20th	Day	44% 9% 16% 17% 13% 2	2340
	21st to end	Day	44% 8% 15% 21% 11% 2	2340
July	1st to 10th	Day	49% 9% 17% 16% 8% 1	2275
	11th to 20th	Day	49% 10% 19% 11% 10% 1	2271
	21st to end	Day	47% 11% 18% 17% 7% 1	2405
August	1st to 10th	Day	53% 9% 16% 16% 5%	2314
	11th to 20th	Day	52% 10% 14% 17% 6% 1	2303
	21st to end	Day	49% 8% 15% 19% 6% 3	2417
September	1st to 10th	Day	46% 7% 12% 19% 13% 3	1980
	11th to 20th	Day	48% 7% 9% 20% 15% 2	1980
	21st to end	Day	45% 6% 10% 19% 16% 4	1980
October	1st to 10th	Day	40% 4 9% 19% 23% 5%	1926
	11th to 20th	Day	34% 3 8% 25% 21% 8%	1800
	21st to end	Day	34% 3 6% 24% 24% 9%	1980
November	1st to 10th	Day	31% 3 8% 22% 28% 8%	1791
	11th to 20th	Day	32% 3 7% 22% 25% 9%	1710
	21st to end	Day	35% 4 6% 23% 25% 7%	1710
December	1st to 10th	Day	46% 5% 7% 20% 17% 5%	1710
	11th to 20th	Day	32% 4 10% 19% 26% 10%	2150
	21st to end	Day	41% 4 6% 23% 22% 4	3124
January	1st to 10th	Night	37% 6% 9% 20% 20% 7%	2459
	11th to 20th	Night	41% 6% 8% 16% 23% 5%	2365
	21st to end	Night	36% 4 7% 23% 25% 5%	2574
February	1st to 10th	Night	36% 5% 11% 23% 21% 4	2295
	11th to 20th	Night	42% 4 8% 21% 20% 6%	2099
	21st to end	Night	41% 6% 11% 19% 17% 6%	1776
March	1st to 10th	Night	36% 5% 9% 19% 23% 8%	2008
	11th to 20th	Night	41% 7% 11% 22% 16% 3	1980
	21st to end	Night	42% 6% 12% 23% 14% 3	2178
April	1st to 10th	Night	33% 6% 13% 23% 20% 4	1980
	11th to 20th	Night	36% 4 10% 23% 21% 6%	1775
	21st to end	Night	35% 5% 15% 28% 16% 2	1620
May	1st to 10th	Night	38% 7% 15% 26% 11% 2	1620
	11th to 20th	Night	45% 7% 17% 21% 10% 1	1593
	21st to end	Night	39% 8% 19% 26% 8% 1	1745
June	1st to 10th	Night	39% 10% 21% 22% 8% 1	1620
	11th to 20th	Night	35% 7% 25% 23% 9% 1	1620
	21st to end	Night	35% 10% 19% 21% 12% 2	1620
July	1st to 10th	Night	41% 9% 20% 20% 9% 1	1575
	11th to 20th	Night	38% 11% 25% 17% 7% 2	1566
	21st to end	Night	36% 12% 24% 21% 7%	1672
August	1st to 10th	Night	41% 10% 22% 19% 8%	1600
	11th to 20th	Night	43% 9% 20% 17% 10% 1	1598
	21st to end	Night	44% 6% 19% 22% 8% 1	1915
September	1st to 10th	Night	43% 7% 13% 20% 14% 3	1980
	11th to 20th	Night	47% 6% 10% 20% 16% 1	1980
	21st to end	Night	42% 4 10% 22% 19% 3	1980
October	1st to 10th	Night	41% 5% 10% 19% 22% 4	2034
	11th to 20th	Night	36% 4 9% 25% 19% 6%	2160
	21st to end	Night	31% 4 6% 23% 27% 9%	2376
November	1st to 10th	Night	33% 4 8% 23% 22% 10%	2282
	11th to 20th	Night	32% 4 7% 20% 26% 10%	2470
	21st to end	Night	33% 5% 6% 24% 25% 6%	2470
December	1st to 10th	Night	44% 6% 8% 20% 18% 5%	2470
	11th to 20th	Night	35% 5% 8% 17% 27% 7%	3044
	21st to end	Night	44% 5% 8% 19% 21% 3	4532

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	31% 29% 31% 8% 1	270
	11th to 20th	Day	28% 24% 34% 13% 1	245
	21st to end	Day	23% 37% 32% 7% 1	264
February	1st to 10th	Day	27% 26% 37% 8% 2	240
	11th to 20th	Day	37% 28% 30% 4	240
	21st to end	Day	37% 30% 25% 7% 1	198
March	1st to 10th	Day	23% 28% 30% 15% 3	392
	11th to 20th	Day	26% 26% 30% 14% 3 1	420
	21st to end	Day	27% 26% 28% 14% 6%	462
April	1st to 10th	Day	24% 23% 23% 21% 7% 2	420
	11th to 20th	Day	21% 22% 24% 22% 9% 2	412
	21st to end	Day	22% 18% 27% 18% 11% 5%	420
May	1st to 10th	Day	17% 24% 29% 17% 8% 5%	423
	11th to 20th	Day	22% 13% 23% 19% 15% 7%	447
	21st to end	Day	18% 8% 21% 24% 17% 12%	485
June	1st to 10th	Day	16% 10% 21% 30% 14% 10%	450
	11th to 20th	Day	14% 6% 21% 26% 22% 11%	450
	21st to end	Day	10% 11% 29% 24% 16% 10%	450
July	1st to 10th	Day	11% 4 17% 27% 26% 15%	445
	11th to 20th	Day	10% 4 14% 30% 27% 15%	444
	21st to end	Day	9% 3 13% 33% 26% 15%	488
August	1st to 10th	Day	20% 5% 19% 22% 19% 15%	446
	11th to 20th	Day	12% 6% 13% 32% 23% 15%	439
	21st to end	Day	17% 3 16% 27% 22% 15%	495
September	1st to 10th	Day	14% 3 18% 29% 23% 13%	450
	11th to 20th	Day	21% 7% 14% 24% 25% 9%	450
	21st to end	Day	24% 7% 18% 28% 18% 4	450
October	1st to 10th	Day	27% 7% 19% 24% 15% 8%	450
	11th to 20th	Day	16% 16% 29% 24% 11% 4	450
	21st to end	Day	22% 13% 27% 25% 10% 3	495
November	1st to 10th	Day	22% 13% 30% 24% 9% 3	389
	11th to 20th	Day	19% 22% 39% 15% 4 1	270
	21st to end	Day	21% 23% 32% 17% 6% 1	270
December	1st to 10th	Day	26% 26% 36% 11% 1	270
	11th to 20th	Day	19% 20% 43% 15% 3 1	270
	21st to end	Day	25% 20% 38% 13% 4	297
January	1st to 10th	Night	20% 25% 29% 20% 5%	760
	11th to 20th	Night	20% 22% 30% 23% 5%	728
	21st to end	Night	18% 26% 31% 22% 4	792
February	1st to 10th	Night	16% 23% 30% 26% 4	720
	11th to 20th	Night	24% 25% 29% 20% 2	703
	21st to end	Night	19% 27% 32% 19% 3	592
March	1st to 10th	Night	17% 25% 30% 25% 3	720
	11th to 20th	Night	21% 20% 31% 21% 7% 1	720
	21st to end	Night	20% 20% 29% 23% 7% 1	792
April	1st to 10th	Night	18% 21% 26% 26% 8% 1	720
	11th to 20th	Night	21% 19% 21% 25% 12% 2	650
	21st to end	Night	16% 13% 23% 33% 14% 1	540
May	1st to 10th	Night	16% 14% 28% 25% 14% 2	540
	11th to 20th	Night	19% 8% 18% 30% 23% 3	531
	21st to end	Night	17% 5% 13% 27% 30% 8%	584
June	1st to 10th	Night	14% 4 15% 31% 29% 6%	540
	11th to 20th	Night	11% 4 15% 29% 32% 10%	540
	21st to end	Night	9% 2 17% 31% 34% 7%	540
July	1st to 10th	Night	12% 4 6% 29% 42% 7%	525
	11th to 20th	Night	11% 3 7% 27% 42% 10%	522
	21st to end	Night	7% 3 7% 30% 42% 11%	591
August	1st to 10th	Night	15% 2 9% 27% 38% 8%	537
	11th to 20th	Night	16% 2 8% 27% 39% 8%	534
	21st to end	Night	15% 3 6% 27% 39% 10%	657
September	1st to 10th	Night	16% 5% 9% 26% 37% 8%	720
	11th to 20th	Night	23% 4 7% 24% 34% 7%	720
	21st to end	Night	24% 5% 14% 29% 25% 3	720
October	1st to 10th	Night	22% 10% 15% 21% 24% 8%	720
	11th to 20th	Night	22% 12% 22% 26% 15% 3	720
	21st to end	Night	18% 15% 20% 32% 11% 4	792
November	1st to 10th	Night	19% 12% 21% 29% 16% 3	740
	11th to 20th	Night	20% 16% 19% 33% 11% 1	760
	21st to end	Night	18% 17% 23% 28% 12% 2	760
December	1st to 10th	Night	19% 20% 28% 26% 7%	760
	11th to 20th	Night	17% 18% 28% 28% 7% 1	760
	21st to end	Night	25% 16% 25% 29% 5%	836

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