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 2025-11-18.

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	11% 7% 3 7% 6% 6% 6% 5% 7% 8% 36%	126855
	Night	11% 6% 2 6% 5% 5% 5% 4 6% 8% 40%	123517
Transparency	Day	40% 7% 12% 20% 16% 5%	130553
	Night	36% 6% 12% 21% 18% 6%	127888
Seeing	Day	21% 15% 25% 21% 12% 6%	16482
	Night	19% 14% 21% 26% 17% 3	26521

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	8% 6% 2 6% 4 4 5% 3 6% 8% 48%	4056
	11th to 20th	Day	8% 6% 3 6% 4 5% 5% 3 4 6% 51%	4060
	21st to end	Day	13% 6% 2 7% 6% 5% 4 3 5% 5% 43%	4466
February	1st to 10th	Day	10% 8% 3 5% 4 5% 6% 4 7% 9% 38%	1847
	11th to 20th	Day	11% 11% 3 7% 5% 5% 5% 4 5% 7% 37%	1948
	21st to end	Day	11% 8% 2 8% 6% 5% 5% 4 6% 6% 39%	1650
March	1st to 10th	Day	16% 6% 2 6% 5% 5% 5% 4 6% 7% 40%	4881
	11th to 20th	Day	8% 6% 3 6% 4 4 4 3 5% 7% 51%	4980
	21st to end	Day	12% 7% 2 7% 5% 4 4 4 5% 7% 43%	5465
April	1st to 10th	Day	9% 6% 2 6% 6% 6% 6% 5% 6% 8% 41%	2200
	11th to 20th	Day	12% 7% 3 6% 5% 4 5% 3 6% 8% 41%	2390
	21st to end	Day	13% 6% 2 6% 5% 5% 5% 4 6% 7% 41%	2600
May	1st to 10th	Day	10% 8% 4 9% 7% 7% 7% 5% 8% 8% 29%	5862
	11th to 20th	Day	17% 6% 3 8% 6% 6% 7% 6% 8% 8% 23%	5861
	21st to end	Day	11% 7% 2 7% 6% 5% 5% 5% 8% 10% 34%	6433
June	1st to 10th	Day	10% 7% 3 8% 7% 6% 6% 6% 7% 9% 31%	2600
	11th to 20th	Day	8% 7% 3 8% 7% 7% 7% 6% 8% 10% 30%	2600
	21st to end	Day	8% 6% 2 8% 8% 6% 8% 6% 8% 10% 31%	2600
July	1st to 10th	Day	6% 5% 3 8% 7% 8% 7% 6% 8% 10% 32%	5900
	11th to 20th	Day	6% 7% 3 10% 9% 8% 8% 7% 10% 10% 22%	5900
	21st to end	Day	11% 8% 4 11% 9% 9% 8% 6% 9% 9% 17%	6424
August	1st to 10th	Day	16% 8% 3 8% 6% 8% 7% 5% 8% 10% 21%	4234
	11th to 20th	Day	9% 6% 2 8% 7% 8% 8% 6% 9% 11% 26%	4174
	21st to end	Day	7% 5% 2 8% 5% 6% 7% 5% 7% 8% 39%	4369
September	1st to 10th	Day	11% 7% 3 8% 7% 6% 6% 4 7% 7% 34%	2200
	11th to 20th	Day	18% 7% 3 7% 5% 5% 6% 5% 7% 8% 29%	2200
	21st to end	Day	18% 7% 2 8% 6% 4 5% 5% 6% 7% 33%	2200
October	1st to 10th	Day	14% 6% 2 6% 4 4 4 4 5% 7% 45%	2140
	11th to 20th	Day	9% 5% 3 5% 3 4 4 3 5% 5% 52%	2000
	21st to end	Day	13% 6% 2 5% 4 3 4 3 4 7% 50%	2200
November	1st to 10th	Day	9% 7% 2 4 4 3 4 3 5% 6% 52%	1983
	11th to 20th	Day	9% 5% 2 4 4 4 4 4 6% 8% 49%	1863
	21st to end	Day	11% 6% 2 6% 4 4 5% 3 5% 6% 48%	1800
December	1st to 10th	Day	7% 6% 3 7% 5% 4 5% 5% 6% 7% 46%	2930
	11th to 20th	Day	10% 5% 2 6% 5% 7% 6% 4 7% 8% 39%	3373
	21st to end	Day	13% 6% 3 7% 5% 5% 5% 4 5% 8% 40%	4466
January	1st to 10th	Night	9% 5% 2 4 5% 5% 6% 5% 6% 8% 45%	5889
	11th to 20th	Night	8% 7% 2 7% 5% 5% 4 3 5% 7% 47%	5900
	21st to end	Night	11% 7% 2 7% 5% 5% 4 3 5% 6% 45%	6490
February	1st to 10th	Night	8% 7% 2 7% 5% 5% 6% 4 7% 8% 41%	2550
	11th to 20th	Night	12% 9% 3 6% 6% 4 5% 4 6% 7% 38%	2338
	21st to end	Night	10% 7% 3 7% 5% 5% 5% 5% 7% 8% 37%	1980
March	1st to 10th	Night	15% 8% 3 6% 6% 6% 5% 3 6% 6% 36%	4972
	11th to 20th	Night	8% 5% 2 5% 4 4 6% 3 6% 8% 49%	4980
	21st to end	Night	11% 5% 2 6% 5% 5% 7% 4 5% 7% 42%	5467
April	1st to 10th	Night	8% 6% 2 7% 5% 6% 5% 4 6% 9% 42%	2200
	11th to 20th	Night	11% 6% 2 6% 4 4 4 4 6% 8% 45%	1973
	21st to end	Night	10% 6% 2 6% 5% 5% 4 4 5% 7% 45%	1800
May	1st to 10th	Night	11% 7% 3 7% 5% 5% 5% 5% 8% 12% 32%	4033
	11th to 20th	Night	19% 7% 3 7% 5% 5% 6% 5% 8% 10% 26%	4032
	21st to end	Night	13% 6% 2 6% 4 6% 5% 4 7% 10% 36%	4421
June	1st to 10th	Night	9% 7% 2 6% 5% 5% 6% 6% 9% 11% 34%	1800
	11th to 20th	Night	8% 5% 2 6% 5% 5% 5% 6% 8% 10% 39%	1800
	21st to end	Night	7% 4 2 6% 6% 6% 5% 5% 8% 12% 39%	1800
July	1st to 10th	Night	5% 5% 2 7% 6% 8% 7% 6% 8% 10% 36%	4058
	11th to 20th	Night	6% 5% 2 7% 5% 6% 7% 6% 9% 13% 35%	4057
	21st to end	Night	8% 9% 4 10% 7% 7% 6% 6% 9% 11% 24%	4427
August	1st to 10th	Night	14% 8% 3 7% 5% 5% 6% 5% 8% 10% 31%	2915
	11th to 20th	Night	8% 7% 2 8% 6% 8% 6% 5% 7% 9% 32%	2875
	21st to end	Night	8% 5% 2 6% 4 4 5% 4 6% 9% 46%	3476
September	1st to 10th	Night	11% 8% 3 8% 5% 5% 5% 4 6% 8% 38%	2200
	11th to 20th	Night	18% 8% 3 6% 5% 5% 5% 3 5% 8% 34%	2200
	21st to end	Night	19% 8% 2 5% 4 4 3 3 4 6% 42%	2200
October	1st to 10th	Night	16% 7% 2 5% 4 4 4 3 5% 6% 45%	2260
	11th to 20th	Night	11% 7% 2 5% 4 4 3 4 5% 5% 49%	2400
	21st to end	Night	12% 5% 1 5% 3 4 4 3 5% 5% 52%	2640
November	1st to 10th	Night	11% 7% 2 5% 4 4 4 4 6% 6% 48%	2530
	11th to 20th	Night	10% 6% 2 5% 3 4 5% 4 6% 7% 49%	2692
	21st to end	Night	11% 5% 2 5% 4 4 6% 5% 7% 6% 46%	2600
December	1st to 10th	Night	8% 5% 2 6% 5% 6% 7% 5% 6% 7% 45%	4250
	11th to 20th	Night	11% 7% 3 8% 6% 7% 7% 5% 6% 6% 35%	4822
	21st to end	Night	14% 6% 4 6% 4 4 6% 4 6% 8% 37%	6490

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	28% 5% 6% 18% 37% 6%	4057
	11th to 20th	Day	30% 4 6% 15% 35% 10%	4060
	21st to end	Day	38% 4 6% 20% 25% 8%	4466
February	1st to 10th	Day	38% 5% 11% 22% 19% 5%	1847
	11th to 20th	Day	44% 4 9% 19% 19% 5%	1958
	21st to end	Day	41% 5% 10% 18% 20% 7%	1650
March	1st to 10th	Day	41% 3 7% 17% 18% 14%	4881
	11th to 20th	Day	32% 3 7% 18% 24% 17%	4980
	21st to end	Day	40% 4 7% 28% 16% 5%	5465
April	1st to 10th	Day	35% 6% 12% 24% 19% 5%	2200
	11th to 20th	Day	39% 4 9% 21% 20% 6%	2390
	21st to end	Day	38% 6% 13% 26% 15% 2	2600
May	1st to 10th	Day	47% 7% 16% 19% 11% 1	5897
	11th to 20th	Day	51% 9% 14% 18% 7% 1	5865
	21st to end	Day	41% 6% 14% 20% 17% 2	6447
June	1st to 10th	Day	45% 8% 17% 20% 9% 1	2600
	11th to 20th	Day	43% 9% 17% 18% 12% 1	2600
	21st to end	Day	42% 9% 16% 21% 11% 2	2600
July	1st to 10th	Day	40% 13% 17% 19% 10% 1	5835
	11th to 20th	Day	47% 14% 22% 12% 4	5831
	21st to end	Day	54% 13% 16% 12% 5% 1	6240
August	1st to 10th	Day	51% 9% 19% 17% 3	4221
	11th to 20th	Day	40% 11% 23% 15% 8% 3	4213
	21st to end	Day	35% 8% 15% 33% 7% 2	4369
September	1st to 10th	Day	43% 7% 13% 19% 15% 3	2200
	11th to 20th	Day	47% 7% 10% 20% 14% 2	2200
	21st to end	Day	47% 7% 11% 18% 15% 4	2200
October	1st to 10th	Day	29% 4 8% 19% 29% 11%	3491
	11th to 20th	Day	22% 2 5% 22% 25% 23%	3240
	21st to end	Day	26% 3 7% 27% 21% 16%	3538
November	1st to 10th	Day	31% 3 9% 22% 27% 8%	1983
	11th to 20th	Day	31% 4 7% 23% 26% 10%	1863
	21st to end	Day	34% 4 6% 24% 25% 7%	1800
December	1st to 10th	Day	33% 4 6% 19% 30% 7%	2930
	11th to 20th	Day	36% 5% 12% 17% 22% 8%	3370
	21st to end	Day	40% 3 10% 25% 17% 4	4466
January	1st to 10th	Night	29% 4 7% 16% 34% 10%	5889
	11th to 20th	Night	31% 4 6% 17% 32% 11%	5900
	21st to end	Night	37% 3 8% 17% 25% 10%	6490
February	1st to 10th	Night	36% 5% 11% 23% 21% 4	2550
	11th to 20th	Night	42% 4 8% 20% 20% 6%	2339
	21st to end	Night	39% 5% 11% 19% 19% 6%	1980
March	1st to 10th	Night	43% 4 6% 15% 23% 9%	4972
	11th to 20th	Night	29% 5% 7% 20% 22% 17%	4980
	21st to end	Night	37% 4 11% 27% 14% 6%	5467
April	1st to 10th	Night	34% 6% 13% 22% 20% 4	2200
	11th to 20th	Night	35% 4 10% 24% 21% 6%	1973
	21st to end	Night	36% 5% 16% 27% 15% 2	1800
May	1st to 10th	Night	40% 9% 18% 24% 9% 1	4060
	11th to 20th	Night	48% 8% 18% 20% 6%	4033
	21st to end	Night	37% 6% 18% 26% 12% 1	4429
June	1st to 10th	Night	39% 9% 21% 22% 8% 1	1800
	11th to 20th	Night	36% 8% 23% 23% 9% 1	1800
	21st to end	Night	34% 10% 19% 22% 13% 2	1800
July	1st to 10th	Night	38% 12% 21% 23% 6% 1	4015
	11th to 20th	Night	37% 14% 30% 15% 3 1	4006
	21st to end	Night	48% 14% 19% 13% 6%	4297
August	1st to 10th	Night	44% 9% 26% 16% 5%	2904
	11th to 20th	Night	39% 10% 24% 17% 9% 1	2902
	21st to end	Night	33% 6% 18% 33% 9% 1	3476
September	1st to 10th	Night	42% 7% 14% 18% 15% 3	2200
	11th to 20th	Night	47% 6% 11% 20% 15% 1	2200
	21st to end	Night	44% 5% 11% 21% 17% 3	2200
October	1st to 10th	Night	29% 4 9% 21% 23% 12%	3689
	11th to 20th	Night	23% 3 6% 22% 27% 19%	3899
	21st to end	Night	29% 3 6% 23% 25% 13%	4252
November	1st to 10th	Night	33% 4 9% 24% 22% 9%	2530
	11th to 20th	Night	31% 4 7% 21% 27% 10%	2692
	21st to end	Night	32% 5% 7% 24% 26% 6%	2600
December	1st to 10th	Night	32% 5% 8% 21% 28% 5%	4250
	11th to 20th	Night	42% 6% 7% 18% 20% 7%	4824
	21st to end	Night	39% 4 12% 21% 21% 3	6490

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	35% 28% 28% 7% 1	300
	11th to 20th	Day	34% 23% 31% 11% 1	300
	21st to end	Day	28% 34% 31% 7% 1	330
February	1st to 10th	Day	24% 28% 36% 10% 2	300
	11th to 20th	Day	40% 28% 28% 4	300
	21st to end	Day	37% 31% 26% 6% 1	249
March	1st to 10th	Day	27% 27% 28% 14% 4	470
	11th to 20th	Day	27% 27% 31% 12% 2 1	500
	21st to end	Day	28% 26% 28% 13% 5%	550
April	1st to 10th	Day	26% 25% 23% 18% 6% 2	500
	11th to 20th	Day	21% 23% 26% 20% 8% 2	492
	21st to end	Day	22% 17% 28% 18% 10% 4	500
May	1st to 10th	Day	18% 22% 30% 17% 7% 4	503
	11th to 20th	Day	24% 14% 24% 18% 13% 6%	527
	21st to end	Day	20% 10% 23% 22% 15% 10%	573
June	1st to 10th	Day	15% 12% 24% 28% 13% 8%	530
	11th to 20th	Day	12% 7% 24% 25% 22% 10%	530
	21st to end	Day	10% 10% 29% 26% 17% 9%	530
July	1st to 10th	Day	10% 4 17% 30% 25% 14%	525
	11th to 20th	Day	9% 4 14% 30% 29% 14%	524
	21st to end	Day	10% 4 14% 33% 26% 13%	576
August	1st to 10th	Day	23% 6% 19% 23% 17% 13%	526
	11th to 20th	Day	13% 6% 16% 30% 22% 13%	519
	21st to end	Day	15% 4 18% 28% 21% 14%	583
September	1st to 10th	Day	13% 4 22% 30% 20% 11%	530
	11th to 20th	Day	21% 8% 16% 23% 24% 8%	530
	21st to end	Day	27% 7% 21% 27% 15% 4	530
October	1st to 10th	Day	24% 8% 22% 25% 14% 7%	530
	11th to 20th	Day	15% 16% 31% 25% 10% 3	530
	21st to end	Day	23% 14% 27% 25% 9% 3	583
November	1st to 10th	Day	22% 15% 32% 21% 7% 2	461
	11th to 20th	Day	20% 25% 37% 14% 3 1	321
	21st to end	Day	24% 23% 30% 17% 5% 1	300
December	1st to 10th	Day	27% 27% 34% 10% 1	300
	11th to 20th	Day	21% 19% 42% 15% 2 1	300
	21st to end	Day	26% 19% 37% 13% 4	330
January	1st to 10th	Night	21% 26% 28% 19% 5%	800
	11th to 20th	Night	21% 22% 31% 21% 4	800
	21st to end	Night	19% 26% 32% 20% 3	880
February	1st to 10th	Night	16% 22% 33% 26% 4	800
	11th to 20th	Night	25% 26% 28% 19% 2	783
	21st to end	Night	20% 27% 33% 18% 3	660
March	1st to 10th	Night	18% 25% 30% 24% 4	800
	11th to 20th	Night	22% 19% 32% 20% 6% 1	800
	21st to end	Night	21% 21% 30% 21% 7%	880
April	1st to 10th	Night	21% 20% 27% 25% 7% 1	800
	11th to 20th	Night	21% 19% 23% 24% 11% 2	722
	21st to end	Night	15% 14% 24% 33% 14% 1	600
May	1st to 10th	Night	16% 14% 29% 26% 13% 2	600
	11th to 20th	Night	20% 8% 19% 29% 21% 2	591
	21st to end	Night	19% 6% 14% 26% 28% 7%	650
June	1st to 10th	Night	14% 5% 18% 31% 27% 6%	600
	11th to 20th	Night	12% 4 16% 29% 31% 9%	600
	21st to end	Night	9% 2 16% 33% 33% 7%	600
July	1st to 10th	Night	11% 4 7% 30% 41% 7%	585
	11th to 20th	Night	11% 2 7% 28% 42% 9%	582
	21st to end	Night	8% 3 8% 31% 39% 10%	657
August	1st to 10th	Night	18% 3 9% 28% 35% 7%	597
	11th to 20th	Night	16% 3 10% 28% 37% 7%	594
	21st to end	Night	15% 4 7% 28% 37% 9%	731
September	1st to 10th	Night	16% 5% 10% 27% 35% 8%	800
	11th to 20th	Night	23% 5% 8% 27% 32% 6%	800
	21st to end	Night	25% 5% 14% 29% 23% 3	800
October	1st to 10th	Night	22% 10% 17% 21% 24% 8%	800
	11th to 20th	Night	21% 13% 22% 28% 14% 3	800
	21st to end	Night	19% 15% 20% 32% 11% 4	880
November	1st to 10th	Night	20% 12% 23% 28% 15% 2	820
	11th to 20th	Night	20% 18% 21% 31% 10% 1	829
	21st to end	Night	19% 18% 24% 27% 11% 2	800
December	1st to 10th	Night	19% 21% 28% 25% 7%	800
	11th to 20th	Night	17% 18% 28% 29% 7% 1	800
	21st to end	Night	25% 16% 25% 29% 4	880

Page generated at 2025-11-15 00:55:56. Version ac07

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