Spring Ridge Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Spring Ridge Observatory clear sky chart from 2005-11-06 to 2026-03-03.

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	10% 9% 3 7% 4 4 4 3 5% 7% 42%	134534
	Night	12% 11% 3 7% 5% 4 4 3 5% 6% 39%	135332
Transparency	Day	42% 6% 12% 22% 13% 4	134260
	Night	46% 6% 12% 20% 12% 4	135184
Seeing	Day	18% 11% 24% 26% 17% 5%	16481
	Night	20% 6% 15% 34% 23% 3	25778

By Month

Percentage Cloud Cover NW (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	16% 26% 8% 12% 6% 5% 5% 3 4 4 11%	4723
	11th to 20th	Day	12% 10% 3 8% 5% 6% 5% 5% 5% 7% 33%	4716
	21st to end	Day	17% 15% 5% 10% 4 4 4 3 5% 7% 27%	5186
February	1st to 10th	Day	20% 18% 5% 10% 6% 5% 4 3 5% 5% 19%	1680
	11th to 20th	Day	15% 17% 5% 10% 6% 6% 4 4 5% 7% 23%	1951
	21st to end	Day	18% 17% 5% 9% 5% 4 4 3 4 5% 25%	1730
March	1st to 10th	Day	9% 13% 4 8% 5% 5% 6% 4 6% 7% 33%	4539
	11th to 20th	Day	11% 12% 3 8% 5% 4 4 3 4 6% 40%	4709
	21st to end	Day	16% 15% 4 11% 6% 5% 5% 4 6% 8% 20%	5881
April	1st to 10th	Day	15% 14% 3 9% 6% 6% 6% 5% 6% 7% 24%	2400
	11th to 20th	Day	12% 11% 3 8% 6% 5% 5% 4 6% 6% 35%	2460
	21st to end	Day	11% 10% 2 9% 7% 6% 5% 4 7% 7% 34%	2600
May	1st to 10th	Day	7% 6% 2 7% 5% 6% 6% 5% 7% 8% 42%	6202
	11th to 20th	Day	9% 8% 3 8% 5% 5% 5% 5% 7% 10% 34%	6277
	21st to end	Day	7% 6% 2 7% 5% 5% 5% 4 7% 8% 42%	7087
June	1st to 10th	Day	16% 7% 3 7% 5% 4 4 3 5% 7% 40%	3000
	11th to 20th	Day	13% 7% 3 7% 5% 5% 6% 4 6% 8% 36%	3000
	21st to end	Day	6% 4 2 5% 4 4 4 4 6% 6% 54%	3000
July	1st to 10th	Day	4 3 1 3 2 2 3 2 4 5% 70%	6551
	11th to 20th	Day	2 1 1 2 2 2 2 2 3 4 78%	6320
	21st to end	Day	4 2 1 3 2 2 3 2 4 5% 74%	6874
August	1st to 10th	Day	3 3 1 3 2 3 3 2 5% 6% 67%	4541
	11th to 20th	Day	3 3 1 3 3 3 3 3 5% 7% 66%	4280
	21st to end	Day	3 3 1 4 4 4 3 4 6% 7% 61%	4443
September	1st to 10th	Day	5% 3 1 4 2 3 3 3 4 7% 64%	2400
	11th to 20th	Day	8% 6% 2 5% 3 4 5% 4 6% 7% 51%	2279
	21st to end	Day	9% 6% 2 6% 4 4 5% 3 6% 7% 48%	2200
October	1st to 10th	Day	9% 5% 2 6% 3 3 3 3 5% 7% 54%	3483
	11th to 20th	Day	11% 7% 2 7% 5% 4 4 3 5% 7% 42%	3036
	21st to end	Day	18% 9% 3 9% 5% 5% 5% 4 5% 7% 33%	3200
November	1st to 10th	Day	24% 15% 3 8% 4 4 4 3 4 6% 23%	1854
	11th to 20th	Day	22% 17% 6% 9% 5% 4 5% 2 3 4 22%	1785
	21st to end	Day	18% 19% 6% 13% 6% 4 4 4 5% 4 18%	1475
December	1st to 10th	Day	16% 13% 5% 9% 7% 5% 4 4 5% 8% 24%	2360
	11th to 20th	Day	21% 22% 8% 14% 6% 4 4 3 4 4 12%	2712
	21st to end	Day	19% 25% 7% 12% 6% 4 4 4 4 5% 11%	3600
January	1st to 10th	Night	19% 22% 6% 12% 5% 5% 4 3 4 4 15%	8190
	11th to 20th	Night	11% 12% 3 8% 5% 4 5% 3 5% 7% 36%	8182
	21st to end	Night	17% 16% 4 7% 4 3 3 3 4 6% 32%	8994
February	1st to 10th	Night	19% 18% 6% 8% 4 3 4 3 4 6% 24%	2940
	11th to 20th	Night	15% 16% 5% 8% 5% 4 4 3 4 6% 30%	2556
	21st to end	Night	14% 16% 3 7% 5% 4 5% 4 6% 5% 30%	2076
March	1st to 10th	Night	7% 11% 3 9% 5% 4 4 4 5% 7% 41%	5400
	11th to 20th	Night	11% 14% 4 7% 4 4 3 3 3 5% 43%	5277
	21st to end	Night	13% 12% 4 10% 6% 6% 7% 5% 7% 8% 22%	5081
April	1st to 10th	Night	10% 11% 3 8% 5% 6% 5% 5% 6% 8% 34%	2000
	11th to 20th	Night	11% 8% 3 7% 6% 6% 5% 3 4 6% 40%	1891
	21st to end	Night	9% 9% 3 7% 5% 5% 5% 5% 6% 7% 39%	1800
May	1st to 10th	Night	8% 5% 2 7% 5% 6% 5% 4 6% 7% 45%	3760
	11th to 20th	Night	10% 8% 3 8% 5% 5% 5% 4 6% 8% 38%	3616
	21st to end	Night	8% 6% 2 6% 4 5% 5% 4 6% 9% 44%	3771
June	1st to 10th	Night	12% 6% 3 6% 4 6% 5% 4 6% 7% 42%	1400
	11th to 20th	Night	9% 5% 2 8% 5% 5% 5% 5% 7% 7% 42%	1400
	21st to end	Night	6% 4 2 5% 4 4 5% 5% 7% 6% 54%	1400
July	1st to 10th	Night	3 3 1 2 2 2 3 2 3 5% 73%	3404
	11th to 20th	Night	2 2 1 2 2 1 2 2 3 4 79%	3640
	21st to end	Night	2 3 1 2 2 2 2 2 3 6% 74%	3969
August	1st to 10th	Night	3 3 1 3 1 2 2 2 5% 6% 71%	2609
	11th to 20th	Night	3 3 2 4 3 4 4 4 5% 7% 62%	2773
	21st to end	Night	2 2 2 5% 5% 4 4 3 5% 6% 62%	3470
September	1st to 10th	Night	4 2 1 3 3 3 3 4 6% 8% 64%	2000
	11th to 20th	Night	6% 4 2 6% 4 5% 4 3 6% 7% 51%	2120
	21st to end	Night	9% 4 2 6% 4 4 5% 4 5% 8% 51%	2200
October	1st to 10th	Night	6% 4 2 4 3 3 3 3 6% 7% 59%	3713
	11th to 20th	Night	12% 5% 1 5% 4 4 4 4 5% 7% 48%	4115
	21st to end	Night	12% 7% 3 7% 4 4 5% 4 6% 7% 42%	4600
November	1st to 10th	Night	22% 13% 4 8% 4 3 5% 4 5% 7% 26%	2665
	11th to 20th	Night	21% 14% 4 9% 5% 4 5% 4 5% 6% 25%	2834
	21st to end	Night	22% 15% 4 9% 6% 4 4 3 4 5% 24%	3123
December	1st to 10th	Night	13% 11% 4 9% 6% 6% 4 4 5% 6% 33%	5040
	11th to 20th	Night	21% 22% 6% 10% 6% 4 4 3 5% 4 14%	5700
	21st to end	Night	20% 20% 7% 12% 7% 5% 5% 3 4 4 14%	7623

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	77% 6% 8% 7% 1	4723
	11th to 20th	Day	51% 7% 20% 16% 5% 2	4740
	21st to end	Day	59% 6% 15% 15% 5%	5187
February	1st to 10th	Day	69% 6% 11% 8% 5% 1	1680
	11th to 20th	Day	63% 8% 11% 11% 7%	1960
	21st to end	Day	62% 7% 11% 13% 7%	1730
March	1st to 10th	Day	48% 7% 11% 19% 14% 1	4539
	11th to 20th	Day	47% 5% 18% 25% 5%	4709
	21st to end	Day	60% 9% 14% 13% 4	5881
April	1st to 10th	Day	57% 8% 15% 15% 4	2400
	11th to 20th	Day	50% 7% 14% 18% 10% 1	2460
	21st to end	Day	49% 8% 14% 20% 8% 2	2600
May	1st to 10th	Day	36% 8% 12% 25% 19%	6202
	11th to 20th	Day	40% 9% 17% 25% 8% 1	6280
	21st to end	Day	37% 9% 17% 29% 8%	7088
June	1st to 10th	Day	44% 7% 11% 23% 13% 2	3000
	11th to 20th	Day	44% 8% 13% 21% 11% 3	3000
	21st to end	Day	29% 6% 14% 29% 18% 4	3000
July	1st to 10th	Day	18% 4 8% 23% 30% 17%	6481
	11th to 20th	Day	12% 3 5% 26% 28% 26%	6245
	21st to end	Day	15% 2 6% 31% 30% 15%	6681
August	1st to 10th	Day	18% 4 7% 28% 32% 11%	4524
	11th to 20th	Day	18% 4 9% 43% 22% 3	4322
	21st to end	Day	22% 5% 12% 35% 23% 3	4442
September	1st to 10th	Day	21% 5% 12% 32% 22% 9%	2400
	11th to 20th	Day	31% 6% 11% 28% 20% 3	2279
	21st to end	Day	35% 8% 18% 26% 11% 3	2200
October	1st to 10th	Day	31% 4 15% 37% 14%	3483
	11th to 20th	Day	42% 5% 18% 20% 14%	3036
	21st to end	Day	52% 8% 15% 19% 6% 1	3200
November	1st to 10th	Day	63% 10% 13% 11% 3 1	1854
	11th to 20th	Day	68% 7% 10% 10% 4 2	1785
	21st to end	Day	70% 6% 11% 9% 4	1475
December	1st to 10th	Day	60% 8% 15% 13% 3	2360
	11th to 20th	Day	78% 5% 9% 6% 2	2714
	21st to end	Day	76% 6% 10% 6% 2	3600
January	1st to 10th	Night	74% 7% 8% 9% 1	8190
	11th to 20th	Night	50% 8% 16% 19% 6% 1	8220
	21st to end	Night	57% 5% 15% 18% 5%	8994
February	1st to 10th	Night	63% 7% 12% 11% 6% 1	2940
	11th to 20th	Night	57% 8% 14% 14% 7% 1	2558
	21st to end	Night	54% 8% 13% 16% 8% 1	2076
March	1st to 10th	Night	41% 7% 15% 22% 15% 1	5400
	11th to 20th	Night	45% 6% 13% 26% 10%	5278
	21st to end	Night	56% 9% 15% 16% 3	5081
April	1st to 10th	Night	46% 7% 16% 22% 9%	2000
	11th to 20th	Night	45% 5% 12% 22% 15% 1	1891
	21st to end	Night	42% 8% 13% 23% 13% 1	1800
May	1st to 10th	Night	34% 8% 9% 25% 23% 1	3757
	11th to 20th	Night	39% 8% 14% 26% 13%	3616
	21st to end	Night	33% 9% 15% 32% 11% 1	3771
June	1st to 10th	Night	39% 6% 11% 22% 17% 4	1400
	11th to 20th	Night	38% 7% 13% 21% 18% 4	1400
	21st to end	Night	27% 5% 12% 27% 22% 7%	1400
July	1st to 10th	Night	14% 2 7% 19% 32% 25%	3365
	11th to 20th	Night	11% 2 4 22% 25% 36%	3592
	21st to end	Night	14% 2 4 24% 33% 22%	3857
August	1st to 10th	Night	15% 3 5% 24% 35% 19%	2601
	11th to 20th	Night	22% 3 6% 34% 28% 7%	2797
	21st to end	Night	23% 4 10% 28% 27% 8%	3470
September	1st to 10th	Night	18% 5% 9% 30% 27% 12%	2000
	11th to 20th	Night	31% 6% 9% 24% 23% 6%	2120
	21st to end	Night	30% 6% 16% 28% 15% 4	2200
October	1st to 10th	Night	25% 4 12% 42% 17%	3713
	11th to 20th	Night	36% 5% 14% 24% 20% 1	4115
	21st to end	Night	41% 7% 16% 27% 8% 1	4600
November	1st to 10th	Night	59% 7% 16% 13% 5%	2665
	11th to 20th	Night	61% 7% 14% 11% 5% 1	2834
	21st to end	Night	64% 6% 13% 12% 4	3123
December	1st to 10th	Night	54% 10% 15% 18% 3	5040
	11th to 20th	Night	74% 6% 12% 6% 2	5697
	21st to end	Night	74% 7% 10% 7% 2	7623

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	32% 14% 18% 20% 13% 3	220
	11th to 20th	Day	35% 7% 17% 23% 13% 6%	220
	21st to end	Day	30% 6% 20% 28% 12% 3	242
February	1st to 10th	Day	28% 12% 28% 22% 7% 2	220
	11th to 20th	Day	29% 12% 21% 23% 13% 2	471
	21st to end	Day	30% 13% 23% 21% 13% 1	446
March	1st to 10th	Day	24% 19% 24% 20% 11% 3	509
	11th to 20th	Day	22% 15% 20% 20% 20% 3	500
	21st to end	Day	29% 13% 24% 24% 9% 2	550
April	1st to 10th	Day	22% 17% 26% 21% 11% 2	500
	11th to 20th	Day	20% 21% 26% 17% 12% 3	492
	21st to end	Day	16% 15% 28% 26% 12% 3	500
May	1st to 10th	Day	14% 14% 28% 26% 14% 4	503
	11th to 20th	Day	19% 10% 29% 22% 14% 6%	528
	21st to end	Day	13% 10% 23% 27% 21% 6%	653
June	1st to 10th	Day	17% 12% 24% 25% 18% 4	730
	11th to 20th	Day	16% 14% 30% 22% 15% 3	730
	21st to end	Day	7% 9% 24% 30% 25% 5%	730
July	1st to 10th	Day	6% 7% 28% 33% 23% 4	626
	11th to 20th	Day	3 11% 27% 33% 19% 7%	524
	21st to end	Day	4 7% 24% 37% 21% 7%	575
August	1st to 10th	Day	7% 5% 23% 32% 24% 9%	527
	11th to 20th	Day	4 5% 23% 29% 27% 11%	517
	21st to end	Day	7% 10% 22% 29% 22% 10%	583
September	1st to 10th	Day	7% 6% 25% 27% 26% 8%	530
	11th to 20th	Day	11% 9% 22% 25% 25% 7%	530
	21st to end	Day	13% 11% 25% 26% 20% 6%	530
October	1st to 10th	Day	14% 9% 22% 30% 19% 6%	530
	11th to 20th	Day	21% 10% 21% 28% 11% 9%	395
	21st to end	Day	28% 7% 23% 28% 11% 3	363
November	1st to 10th	Day	37% 9% 17% 22% 14% 1	330
	11th to 20th	Day	36% 10% 18% 20% 15% 1	275
	21st to end	Day	30% 6% 20% 26% 15% 2	220
December	1st to 10th	Day	31% 9% 20% 21% 14% 5%	220
	11th to 20th	Day	38% 10% 16% 24% 9% 3	220
	21st to end	Day	38% 10% 17% 21% 11% 3	242
January	1st to 10th	Night	38% 8% 15% 25% 12% 2	845
	11th to 20th	Night	31% 5% 17% 32% 13% 2	840
	21st to end	Night	26% 5% 15% 36% 16% 1	924
February	1st to 10th	Night	30% 7% 19% 31% 12% 2	840
	11th to 20th	Night	25% 7% 17% 34% 16% 1	824
	21st to end	Night	25% 9% 19% 28% 17% 3	692
March	1st to 10th	Night	21% 6% 17% 35% 19% 2	808
	11th to 20th	Night	20% 9% 17% 32% 21% 1	800
	21st to end	Night	21% 7% 19% 35% 16% 2	880
April	1st to 10th	Night	17% 5% 16% 39% 20% 2	800
	11th to 20th	Night	15% 7% 25% 35% 18% 1	703
	21st to end	Night	13% 8% 20% 34% 23% 2	600
May	1st to 10th	Night	11% 4 17% 38% 27% 3	455
	11th to 20th	Night	12% 5% 16% 24% 36% 6%	394
	21st to end	Night	14% 4 15% 30% 32% 5%	433
June	1st to 10th	Night	15% 5% 13% 38% 27% 3	400
	11th to 20th	Night	10% 6% 20% 41% 22% 2	400
	21st to end	Night	8% 2 12% 42% 34% 3	400
July	1st to 10th	Night	3 2 9% 39% 44% 3	390
	11th to 20th	Night	3 2 10% 47% 36% 2	388
	21st to end	Night	1 3 8% 38% 47% 4	438
August	1st to 10th	Night	5% 1 9% 38% 42% 6%	398
	11th to 20th	Night	5% 3 8% 33% 45% 8%	514
	21st to end	Night	5% 3 11% 35% 41% 3	794
September	1st to 10th	Night	4 3 10% 39% 38% 6%	800
	11th to 20th	Night	9% 3 10% 32% 40% 6%	800
	21st to end	Night	11% 5% 15% 36% 30% 4	800
October	1st to 10th	Night	13% 4 14% 39% 28% 3	800
	11th to 20th	Night	18% 4 13% 35% 26% 3	800
	21st to end	Night	21% 4 20% 36% 17% 2	880
November	1st to 10th	Night	30% 6% 14% 37% 13% 1	820
	11th to 20th	Night	32% 8% 17% 27% 15% 2	840
	21st to end	Night	33% 6% 14% 30% 16% 2	1023
December	1st to 10th	Night	28% 6% 14% 33% 17% 2	1050
	11th to 20th	Night	36% 7% 17% 28% 11% 1	1050
	21st to end	Night	36% 9% 17% 28% 11%	1155

Page generated at 2026-02-27 22:40:22. Version ac07

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