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 2024-12-02.

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% 9% 3 7% 4 4 4 4 5% 6% 42%	105464
	Night	13% 11% 3 7% 4 4 4 3 5% 6% 39%	102744
Transparency	Day	42% 7% 12% 21% 13% 5%	105231
	Night	46% 7% 12% 19% 12% 5%	102623
Seeing	Day	18% 11% 24% 26% 17% 5%	14974
	Night	20% 5% 15% 34% 23% 3	24123

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	19% 27% 7% 10% 5% 4 5% 4 4 5% 10%	2539
	11th to 20th	Day	21% 16% 5% 8% 6% 6% 5% 4 4 5% 21%	2516
	21st to end	Day	26% 21% 5% 12% 5% 4 3 2 4 5% 12%	2794
February	1st to 10th	Day	21% 19% 5% 9% 5% 5% 4 3 5% 5% 19%	1520
	11th to 20th	Day	16% 17% 4 9% 6% 6% 4 4 5% 7% 23%	1759
	21st to end	Day	18% 17% 5% 8% 6% 4 4 3 4 5% 26%	1570
March	1st to 10th	Day	12% 15% 4 8% 5% 5% 6% 4 5% 6% 29%	3138
	11th to 20th	Day	10% 11% 3 7% 4 4 3 3 4 6% 46%	3290
	21st to end	Day	16% 16% 4 11% 7% 5% 5% 5% 5% 6% 20%	4103
April	1st to 10th	Day	16% 14% 3 9% 6% 6% 5% 5% 6% 7% 24%	2280
	11th to 20th	Day	13% 12% 3 8% 6% 5% 5% 4 6% 6% 33%	2335
	21st to end	Day	11% 10% 3 9% 7% 6% 5% 4 7% 7% 32%	2470
May	1st to 10th	Day	10% 8% 3 8% 6% 6% 6% 5% 7% 7% 35%	4341
	11th to 20th	Day	6% 7% 2 7% 5% 5% 5% 4 8% 11% 40%	4377
	21st to end	Day	9% 8% 3 9% 7% 6% 6% 5% 7% 9% 31%	4941
June	1st to 10th	Day	17% 8% 3 7% 5% 4 4 3 5% 7% 38%	2850
	11th to 20th	Day	14% 7% 3 7% 5% 5% 6% 4 6% 7% 36%	2850
	21st to end	Day	6% 4 2 5% 4 4 4 4 6% 6% 54%	2850
July	1st to 10th	Day	3 2 1 3 2 2 3 2 4 5% 73%	4581
	11th to 20th	Day	2 2 1 2 2 2 2 2 3 4 78%	4420
	21st to end	Day	2 1 1 2 2 2 3 2 4 5% 76%	4790
August	1st to 10th	Day	3 3 1 3 2 3 3 2 5% 6% 67%	4401
	11th to 20th	Day	3 3 1 3 2 3 3 3 5% 7% 66%	4146
	21st to end	Day	3 3 1 4 4 4 3 4 6% 7% 62%	4307
September	1st to 10th	Day	5% 4 1 4 2 3 3 3 5% 7% 64%	2280
	11th to 20th	Day	8% 6% 2 5% 3 4 5% 4 6% 7% 50%	2165
	21st to end	Day	9% 6% 2 6% 4 4 5% 3 6% 7% 48%	2090
October	1st to 10th	Day	9% 5% 2 6% 3 3 3 3 5% 7% 54%	3377
	11th to 20th	Day	11% 7% 2 7% 5% 4 4 3 5% 8% 43%	2943
	21st to end	Day	18% 9% 3 8% 5% 4 5% 4 5% 7% 33%	3101
November	1st to 10th	Day	24% 16% 3 7% 4 4 4 3 4 6% 23%	1764
	11th to 20th	Day	23% 17% 6% 8% 5% 4 4 2 3 5% 23%	1700
	21st to end	Day	19% 19% 6% 12% 6% 4 4 4 5% 4 18%	1405
December	1st to 10th	Day	18% 16% 6% 11% 7% 5% 2 3 4 7% 21%	1337
	11th to 20th	Day	24% 23% 6% 13% 5% 4 3 2 3 4 11%	1682
	21st to end	Day	17% 22% 6% 11% 5% 4 4 4 5% 6% 16%	2452
January	1st to 10th	Night	23% 25% 6% 10% 5% 4 4 3 4 4 14%	4424
	11th to 20th	Night	19% 17% 4 8% 6% 3 4 3 4 7% 25%	4382
	21st to end	Night	26% 23% 5% 9% 5% 3 3 2 4 5% 14%	4862
February	1st to 10th	Night	20% 18% 5% 8% 4 3 4 3 4 6% 25%	2660
	11th to 20th	Night	15% 15% 4 8% 5% 5% 4 3 4 6% 29%	2308
	21st to end	Night	13% 16% 3 7% 5% 4 4 4 6% 5% 31%	1884
March	1st to 10th	Night	9% 11% 3 8% 6% 5% 5% 4 6% 7% 36%	3736
	11th to 20th	Night	11% 10% 3 6% 4 3 3 3 3 5% 50%	3689
	21st to end	Night	14% 11% 3 10% 7% 6% 6% 5% 6% 7% 24%	3537
April	1st to 10th	Night	10% 11% 3 8% 5% 5% 5% 4 6% 8% 34%	1900
	11th to 20th	Night	12% 9% 3 8% 7% 6% 5% 3 4 6% 38%	1796
	21st to end	Night	9% 9% 3 8% 5% 5% 5% 5% 6% 7% 38%	1710
May	1st to 10th	Night	10% 7% 3 8% 6% 6% 7% 5% 6% 7% 35%	2618
	11th to 20th	Night	6% 5% 2 6% 4 4 5% 4 6% 10% 47%	2516
	21st to end	Night	12% 7% 3 8% 6% 6% 6% 5% 7% 10% 31%	2617
June	1st to 10th	Night	13% 6% 3 6% 4 6% 5% 4 6% 7% 41%	1330
	11th to 20th	Night	10% 5% 2 8% 5% 5% 6% 5% 7% 7% 41%	1330
	21st to end	Night	6% 4 2 4 4 3 5% 4 7% 6% 55%	1330
July	1st to 10th	Night	2 3 1 3 2 2 3 2 4 5% 73%	2374
	11th to 20th	Night	2 1 2 2 1 2 2 3 4 81%	2540
	21st to end	Night	2 2 2 1 2 2 2 3 6% 79%	2762
August	1st to 10th	Night	3 3 1 3 1 2 2 2 5% 6% 72%	2529
	11th to 20th	Night	3 3 2 4 3 3 4 4 5% 7% 62%	2687
	21st to end	Night	2 2 2 5% 5% 4 4 4 5% 6% 62%	3364
September	1st to 10th	Night	4 2 1 3 3 3 3 4 5% 7% 64%	1900
	11th to 20th	Night	6% 5% 2 7% 4 5% 4 4 6% 8% 50%	2014
	21st to end	Night	9% 4 2 6% 4 3 5% 4 5% 8% 51%	2090
October	1st to 10th	Night	6% 4 2 4 3 3 3 3 6% 7% 59%	3600
	11th to 20th	Night	12% 5% 1 5% 4 4 4 4 5% 7% 48%	3988
	21st to end	Night	12% 7% 3 7% 4 4 5% 4 6% 7% 42%	4457
November	1st to 10th	Night	23% 13% 4 7% 4 4 5% 4 4 7% 26%	2535
	11th to 20th	Night	21% 14% 4 9% 5% 4 5% 4 5% 5% 26%	2699
	21st to end	Night	22% 15% 4 9% 5% 4 4 3 5% 5% 24%	2974
December	1st to 10th	Night	19% 15% 5% 10% 5% 4 4 3 5% 6% 25%	2878
	11th to 20th	Night	23% 21% 6% 10% 5% 4 4 3 5% 5% 14%	3510
	21st to end	Night	19% 18% 5% 12% 7% 5% 5% 4 4 5% 15%	5214

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% 8% 8% 5% 2	2539
	11th to 20th	Day	65% 6% 12% 11% 4 3	2540
	21st to end	Day	77% 6% 8% 6% 2	2794
February	1st to 10th	Day	69% 6% 11% 8% 4 2	1520
	11th to 20th	Day	62% 8% 11% 11% 7%	1768
	21st to end	Day	62% 7% 11% 13% 8%	1570
March	1st to 10th	Day	53% 9% 11% 17% 10% 1	3138
	11th to 20th	Day	41% 6% 20% 29% 4	3290
	21st to end	Day	62% 10% 12% 11% 5%	4103
April	1st to 10th	Day	57% 8% 15% 15% 4	2280
	11th to 20th	Day	51% 7% 14% 17% 10% 1	2335
	21st to end	Day	51% 8% 13% 20% 7% 2	2470
May	1st to 10th	Day	43% 10% 13% 24% 9%	4341
	11th to 20th	Day	33% 9% 18% 29% 10% 1	4380
	21st to end	Day	45% 11% 17% 22% 5%	4942
June	1st to 10th	Day	46% 7% 12% 22% 11% 1	2850
	11th to 20th	Day	46% 8% 13% 21% 10% 3	2850
	21st to end	Day	29% 6% 14% 29% 19% 4	2850
July	1st to 10th	Day	15% 4 7% 20% 33% 20%	4511
	11th to 20th	Day	12% 3 6% 14% 29% 37%	4345
	21st to end	Day	12% 3 6% 22% 35% 21%	4639
August	1st to 10th	Day	18% 4 7% 28% 32% 11%	4384
	11th to 20th	Day	18% 4 9% 43% 22% 3	4188
	21st to end	Day	22% 5% 12% 35% 23% 3	4306
September	1st to 10th	Day	21% 5% 12% 31% 22% 9%	2280
	11th to 20th	Day	32% 6% 12% 27% 20% 3	2165
	21st to end	Day	35% 8% 18% 25% 10% 3	2090
October	1st to 10th	Day	31% 4 15% 36% 14%	3377
	11th to 20th	Day	42% 5% 18% 21% 14%	2943
	21st to end	Day	52% 8% 14% 19% 6% 1	3101
November	1st to 10th	Day	63% 10% 13% 11% 3 1	1764
	11th to 20th	Day	67% 7% 10% 10% 4 2	1700
	21st to end	Day	70% 6% 11% 10% 4	1405
December	1st to 10th	Day	67% 6% 11% 12% 5%	1337
	11th to 20th	Day	79% 5% 8% 5% 2	1684
	21st to end	Day	69% 7% 13% 8% 3	2452
January	1st to 10th	Night	75% 8% 7% 8% 2	4424
	11th to 20th	Night	61% 5% 12% 16% 4 3	4420
	21st to end	Night	76% 6% 10% 6% 1 1	4862
February	1st to 10th	Night	62% 7% 13% 11% 5% 1	2660
	11th to 20th	Night	57% 8% 15% 13% 7% 1	2310
	21st to end	Night	54% 8% 12% 16% 9% 1	1884
March	1st to 10th	Night	43% 9% 16% 21% 10% 1	3736
	11th to 20th	Night	38% 8% 18% 30% 7%	3690
	21st to end	Night	56% 11% 14% 15% 4	3537
April	1st to 10th	Night	46% 7% 16% 21% 9%	1900
	11th to 20th	Night	47% 5% 12% 20% 15% 1	1796
	21st to end	Night	43% 8% 13% 23% 12% 1	1710
May	1st to 10th	Night	41% 11% 12% 24% 11% 1	2618
	11th to 20th	Night	27% 8% 15% 33% 15% 1	2516
	21st to end	Night	43% 12% 14% 21% 9%	2617
June	1st to 10th	Night	41% 6% 11% 22% 16% 4	1330
	11th to 20th	Night	39% 7% 13% 21% 15% 4	1330
	21st to end	Night	25% 5% 12% 27% 22% 7%	1330
July	1st to 10th	Night	14% 3 6% 18% 31% 28%	2335
	11th to 20th	Night	8% 2 5% 13% 20% 51%	2492
	21st to end	Night	10% 2 4 18% 35% 32%	2674
August	1st to 10th	Night	14% 3 5% 23% 35% 19%	2521
	11th to 20th	Night	22% 3 6% 34% 28% 6%	2711
	21st to end	Night	22% 4 11% 28% 27% 7%	3364
September	1st to 10th	Night	18% 5% 8% 29% 28% 12%	1900
	11th to 20th	Night	31% 7% 9% 23% 23% 6%	2014
	21st to end	Night	31% 6% 16% 28% 14% 5%	2090
October	1st to 10th	Night	25% 4 12% 42% 17%	3600
	11th to 20th	Night	36% 5% 14% 24% 20% 1	3988
	21st to end	Night	41% 7% 16% 27% 8% 1	4457
November	1st to 10th	Night	59% 7% 15% 13% 5%	2535
	11th to 20th	Night	60% 7% 14% 11% 5% 1	2699
	21st to end	Night	64% 6% 13% 12% 4	2974
December	1st to 10th	Night	63% 8% 10% 16% 4	2878
	11th to 20th	Night	73% 8% 11% 7% 2	3507
	21st to end	Night	70% 8% 11% 7% 3	5214

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	34% 12% 18% 19% 14% 3	180
	11th to 20th	Day	40% 8% 16% 18% 11% 7%	180
	21st to end	Day	35% 6% 22% 24% 11% 3	198
February	1st to 10th	Day	31% 12% 29% 21% 5% 2	180
	11th to 20th	Day	31% 12% 22% 20% 13% 2	399
	21st to end	Day	30% 13% 23% 20% 14% 1	382
March	1st to 10th	Day	26% 20% 23% 18% 10% 3	460
	11th to 20th	Day	21% 15% 20% 20% 20% 3	460
	21st to end	Day	28% 12% 25% 24% 9% 2	506
April	1st to 10th	Day	23% 16% 27% 21% 11% 2	460
	11th to 20th	Day	21% 21% 26% 18% 12% 3	452
	21st to end	Day	17% 16% 30% 25% 11% 3	460
May	1st to 10th	Day	15% 14% 28% 26% 13% 4	463
	11th to 20th	Day	19% 9% 28% 23% 15% 6%	488
	21st to end	Day	13% 10% 24% 28% 19% 6%	605
June	1st to 10th	Day	18% 12% 24% 24% 17% 4	680
	11th to 20th	Day	16% 14% 30% 22% 14% 3	680
	21st to end	Day	7% 9% 23% 31% 26% 5%	680
July	1st to 10th	Day	5% 6% 28% 34% 23% 4	581
	11th to 20th	Day	4 10% 28% 33% 19% 7%	484
	21st to end	Day	4 7% 24% 37% 20% 8%	531
August	1st to 10th	Day	7% 5% 23% 31% 25% 9%	487
	11th to 20th	Day	4 5% 23% 29% 29% 11%	477
	21st to end	Day	7% 11% 22% 28% 21% 10%	539
September	1st to 10th	Day	8% 6% 27% 26% 24% 8%	490
	11th to 20th	Day	11% 9% 22% 25% 25% 7%	490
	21st to end	Day	13% 11% 25% 25% 19% 6%	490
October	1st to 10th	Day	15% 9% 21% 30% 19% 6%	490
	11th to 20th	Day	20% 10% 20% 29% 12% 10%	362
	21st to end	Day	30% 6% 22% 28% 12% 3	330
November	1st to 10th	Day	36% 8% 18% 22% 15% 1	300
	11th to 20th	Day	36% 10% 18% 20% 15% 1	250
	21st to end	Day	31% 5% 19% 27% 17% 3	200
December	1st to 10th	Day	30% 9% 21% 22% 14% 4	182
	11th to 20th	Day	37% 9% 16% 25% 10% 3	180
	21st to end	Day	38% 8% 17% 22% 12% 3	198
January	1st to 10th	Night	38% 8% 15% 24% 13% 2	765
	11th to 20th	Night	33% 5% 17% 29% 13% 3	760
	21st to end	Night	28% 6% 16% 34% 15% 1	836
February	1st to 10th	Night	31% 6% 18% 32% 11% 2	760
	11th to 20th	Night	25% 7% 18% 32% 16% 2	744
	21st to end	Night	24% 8% 19% 29% 17% 3	628
March	1st to 10th	Night	21% 6% 18% 35% 19% 2	760
	11th to 20th	Night	20% 8% 16% 32% 22% 1	760
	21st to end	Night	21% 7% 19% 35% 17% 2	836
April	1st to 10th	Night	17% 5% 16% 39% 21% 2	760
	11th to 20th	Night	15% 6% 25% 34% 18% 1	668
	21st to end	Night	14% 8% 21% 33% 22% 2	570
May	1st to 10th	Night	12% 4 17% 38% 27% 3	432
	11th to 20th	Night	11% 5% 16% 24% 37% 6%	374
	21st to end	Night	15% 4 16% 29% 31% 5%	411
June	1st to 10th	Night	16% 4 13% 38% 26% 3	380
	11th to 20th	Night	11% 6% 19% 40% 22% 2	380
	21st to end	Night	8% 2 12% 41% 34% 3	380
July	1st to 10th	Night	3 2 9% 40% 44% 3	370
	11th to 20th	Night	3 2 11% 46% 36% 2	368
	21st to end	Night	1 2 8% 38% 46% 4	416
August	1st to 10th	Night	4 1 9% 37% 42% 6%	378
	11th to 20th	Night	5% 2 8% 31% 45% 8%	488
	21st to end	Night	5% 3 12% 35% 41% 3	754
September	1st to 10th	Night	5% 3 11% 39% 37% 6%	760
	11th to 20th	Night	9% 4 10% 32% 39% 7%	760
	21st to end	Night	12% 5% 14% 35% 30% 4	760
October	1st to 10th	Night	13% 4 14% 38% 28% 3	760
	11th to 20th	Night	17% 4 12% 36% 28% 3	760
	21st to end	Night	21% 4 19% 36% 17% 3	836
November	1st to 10th	Night	31% 5% 13% 37% 13% 1	780
	11th to 20th	Night	31% 8% 17% 27% 15% 2	800
	21st to end	Night	33% 6% 13% 29% 17% 2	974
December	1st to 10th	Night	28% 6% 14% 34% 16% 2	960
	11th to 20th	Night	36% 6% 16% 30% 12% 1	950
	21st to end	Night	37% 7% 16% 29% 11%	1045

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