Spring Mountain Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Spring Mountain Observatory clear sky chart from 2005-11-06 to 2025-05-26.

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	16% 7% 2 6% 4 4 4 4 6% 7% 41%	114017
	Night	16% 7% 2 6% 4 4 4 3 5% 6% 43%	115252
Transparency	Day	42% 3 11% 24% 17% 3	117594
	Night	41% 3 11% 24% 18% 3	119838
Seeing	Day	24% 13% 27% 21% 11% 4	15619
	Night	21% 11% 29% 28% 11% 1	24999

By Month

Percentage Cloud Cover NE (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	13% 7% 2 5% 3 4 3 3 4 5% 49%	3597
	11th to 20th	Day	14% 8% 3 10% 5% 4 4 3 5% 5% 39%	3685
	21st to end	Day	18% 8% 3 7% 5% 4 4 3 4 6% 40%	4466
February	1st to 10th	Day	16% 7% 3 6% 5% 5% 4 4 6% 7% 38%	1800
	11th to 20th	Day	16% 9% 2 7% 5% 4 4 4 5% 6% 39%	1886
	21st to end	Day	18% 10% 3 6% 4 3 4 3 4 5% 39%	1650
March	1st to 10th	Day	21% 5% 2 5% 4 4 4 3 4 5% 44%	4514
	11th to 20th	Day	13% 6% 2 6% 5% 4 4 4 5% 7% 44%	4980
	21st to end	Day	18% 6% 2 6% 4 4 4 3 4 6% 43%	5776
April	1st to 10th	Day	17% 7% 3 7% 5% 4 4 3 6% 7% 39%	2400
	11th to 20th	Day	16% 7% 2 6% 4 4 4 3 6% 6% 41%	2374
	21st to end	Day	16% 5% 2 6% 4 4 4 5% 7% 7% 40%	2400
May	1st to 10th	Day	28% 7% 2 6% 3 4 4 4 5% 7% 29%	4215
	11th to 20th	Day	25% 8% 3 7% 5% 4 4 4 5% 7% 28%	4538
	21st to end	Day	13% 6% 2 5% 4 4 4 4 6% 9% 44%	4907
June	1st to 10th	Day	13% 6% 2 5% 4 5% 5% 5% 7% 9% 39%	2660
	11th to 20th	Day	12% 7% 2 6% 5% 5% 5% 4 7% 9% 37%	2660
	21st to end	Day	11% 5% 2 5% 4 5% 5% 5% 8% 9% 41%	2660
July	1st to 10th	Day	8% 5% 2 6% 5% 5% 7% 5% 8% 11% 38%	4440
	11th to 20th	Day	9% 6% 2 6% 5% 5% 5% 4 8% 10% 40%	4440
	21st to end	Day	11% 8% 3 8% 5% 6% 6% 5% 7% 8% 34%	4727
August	1st to 10th	Day	19% 6% 2 6% 4 5% 5% 4 7% 8% 34%	4104
	11th to 20th	Day	13% 5% 2 5% 4 4 5% 4 6% 9% 43%	3760
	21st to end	Day	11% 5% 2 5% 3 4 4 3 5% 8% 48%	4180
September	1st to 10th	Day	15% 7% 2 5% 3 4 5% 4 5% 6% 45%	2280
	11th to 20th	Day	13% 5% 2 4 3 3 3 3 5% 8% 52%	2258
	21st to end	Day	19% 7% 3 6% 5% 4 5% 3 5% 6% 37%	2076
October	1st to 10th	Day	18% 6% 2 5% 3 3 4 3 5% 7% 43%	1900
	11th to 20th	Day	16% 5% 2 4 4 4 4 3 4 6% 48%	1900
	21st to end	Day	18% 7% 2 4 3 3 3 3 5% 6% 45%	2028
November	1st to 10th	Day	13% 7% 2 4 3 2 4 3 4 5% 53%	1761
	11th to 20th	Day	16% 6% 1 4 3 3 3 3 6% 6% 48%	1800
	21st to end	Day	15% 7% 2 4 4 3 4 3 6% 7% 43%	1649
December	1st to 10th	Day	15% 9% 2 5% 3 4 5% 4 6% 6% 43%	2600
	11th to 20th	Day	22% 9% 2 4 3 3 3 3 4 6% 42%	2986
	21st to end	Day	17% 6% 3 6% 5% 5% 5% 4 6% 7% 38%	3960
January	1st to 10th	Night	13% 6% 2 6% 4 4 4 3 4 6% 49%	6349
	11th to 20th	Night	14% 7% 2 5% 4 4 4 3 4 6% 47%	6197
	21st to end	Night	18% 8% 2 6% 3 3 4 3 5% 6% 42%	6490
February	1st to 10th	Night	17% 9% 2 6% 4 4 4 3 5% 6% 40%	2600
	11th to 20th	Night	18% 8% 3 6% 5% 4 3 3 4 5% 40%	2398
	21st to end	Night	18% 7% 2 6% 4 3 4 4 5% 6% 42%	1980
March	1st to 10th	Night	19% 7% 2 6% 3 3 4 3 4 5% 43%	5323
	11th to 20th	Night	11% 8% 2 5% 4 4 3 3 4 6% 50%	4979
	21st to end	Night	17% 7% 3 6% 4 4 3 3 4 6% 43%	5155
April	1st to 10th	Night	18% 6% 3 6% 4 4 4 4 6% 6% 39%	2000
	11th to 20th	Night	15% 7% 2 6% 5% 3 4 4 6% 7% 40%	1975
	21st to end	Night	19% 6% 2 4 3 3 3 3 6% 8% 43%	2000
May	1st to 10th	Night	24% 9% 3 6% 4 4 5% 3 5% 7% 29%	2972
	11th to 20th	Night	23% 9% 4 7% 5% 4 4 4 5% 6% 29%	2575
	21st to end	Night	13% 6% 2 5% 5% 3 5% 4 6% 9% 43%	2784
June	1st to 10th	Night	15% 6% 3 6% 4 4 4 4 5% 8% 43%	1520
	11th to 20th	Night	12% 6% 1 6% 4 5% 5% 4 7% 9% 42%	1520
	21st to end	Night	11% 3 2 6% 4 5% 5% 5% 7% 9% 43%	1520
July	1st to 10th	Night	9% 5% 2 7% 6% 5% 5% 5% 8% 9% 38%	2520
	11th to 20th	Night	11% 7% 3 6% 5% 5% 5% 5% 8% 10% 37%	2520
	21st to end	Night	15% 7% 2 7% 4 4 5% 4 7% 8% 37%	2822
August	1st to 10th	Night	20% 6% 3 6% 5% 4 5% 4 6% 9% 32%	2825
	11th to 20th	Night	14% 7% 2 7% 4 4 5% 3 6% 6% 42%	3035
	21st to end	Night	11% 5% 2 5% 3 4 4 3 6% 7% 48%	3476
September	1st to 10th	Night	14% 7% 2 6% 4 4 4 2 4 6% 47%	1900
	11th to 20th	Night	12% 6% 2 5% 3 3 3 2 4 7% 54%	1921
	21st to end	Night	18% 7% 2 6% 4 4 4 3 5% 7% 41%	2099
October	1st to 10th	Night	19% 7% 2 6% 5% 3 3 3 3 6% 43%	2280
	11th to 20th	Night	15% 5% 2 4 4 4 3 3 4 6% 51%	2280
	21st to end	Night	18% 7% 1 4 3 3 3 3 4 6% 48%	2567
November	1st to 10th	Night	11% 6% 1 5% 3 3 4 3 5% 6% 54%	2537
	11th to 20th	Night	17% 7% 3 4 3 3 4 3 4 5% 48%	2600
	21st to end	Night	14% 6% 2 5% 5% 4 4 3 6% 7% 44%	2749
December	1st to 10th	Night	14% 8% 2 6% 3 3 4 3 5% 6% 46%	4580
	11th to 20th	Night	23% 7% 2 4 3 3 4 3 4 5% 41%	5208
	21st to end	Night	20% 7% 3 6% 4 4 4 3 5% 5% 40%	6996

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	38% 2 9% 27% 22% 3	3598
	11th to 20th	Day	47% 2 8% 21% 16% 5%	3715
	21st to end	Day	47% 2 11% 23% 16% 1	4466
February	1st to 10th	Day	46% 3 13% 24% 13% 2	1800
	11th to 20th	Day	47% 3 8% 25% 16% 2	1895
	21st to end	Day	48% 2 8% 25% 14% 3	1650
March	1st to 10th	Day	43% 2 8% 24% 16% 8%	4517
	11th to 20th	Day	40% 2 8% 29% 15% 5%	4980
	21st to end	Day	43% 3 9% 17% 23% 5%	5776
April	1st to 10th	Day	45% 2 9% 22% 17% 5%	2400
	11th to 20th	Day	43% 3 11% 22% 17% 4	2374
	21st to end	Day	42% 4 9% 23% 18% 4	2400
May	1st to 10th	Day	55% 2 10% 21% 10% 2	4227
	11th to 20th	Day	57% 2 10% 20% 9% 1	4542
	21st to end	Day	38% 3 11% 28% 17% 2	4907
June	1st to 10th	Day	40% 5% 12% 26% 16% 1	2660
	11th to 20th	Day	42% 5% 14% 25% 11% 3	2660
	21st to end	Day	37% 5% 14% 28% 16% 1	2660
July	1st to 10th	Day	37% 9% 15% 23% 14% 2	4370
	11th to 20th	Day	37% 7% 16% 25% 14% 2	4365
	21st to end	Day	46% 6% 10% 20% 17% 1	4576
August	1st to 10th	Day	47% 6% 12% 25% 10%	4091
	11th to 20th	Day	37% 4 11% 32% 15% 1	3797
	21st to end	Day	33% 4 10% 29% 21% 2	4180
September	1st to 10th	Day	40% 4 10% 22% 21% 3	2280
	11th to 20th	Day	33% 3 8% 28% 25% 4	2258
	21st to end	Day	48% 3 7% 19% 21% 2	2076
October	1st to 10th	Day	37% 3 8% 20% 27% 5%	3160
	11th to 20th	Day	25% 3 12% 27% 28% 6%	3140
	21st to end	Day	26% 2 12% 26% 30% 4	3318
November	1st to 10th	Day	35% 2 13% 25% 21% 4	1761
	11th to 20th	Day	38% 3 9% 27% 20% 3	1800
	21st to end	Day	39% 3 13% 31% 12% 2	1649
December	1st to 10th	Day	41% 3 16% 26% 12% 2	2600
	11th to 20th	Day	45% 2 12% 28% 11% 2	2986
	21st to end	Day	45% 3 11% 23% 16% 1	3960
January	1st to 10th	Night	38% 2 8% 27% 20% 5%	6349
	11th to 20th	Night	40% 2 11% 23% 17% 7%	6242
	21st to end	Night	45% 2 8% 24% 18% 3	6490
February	1st to 10th	Night	46% 2 12% 26% 12% 2	2600
	11th to 20th	Night	47% 2 8% 22% 18% 4	2400
	21st to end	Night	44% 2 8% 23% 19% 4	1980
March	1st to 10th	Night	44% 2 8% 25% 13% 9%	5333
	11th to 20th	Night	38% 2 13% 27% 15% 5%	4980
	21st to end	Night	44% 3 7% 17% 25% 5%	5155
April	1st to 10th	Night	46% 2 10% 21% 17% 4	2000
	11th to 20th	Night	43% 3 9% 21% 21% 3	1975
	21st to end	Night	40% 3 9% 24% 19% 5%	2000
May	1st to 10th	Night	55% 3 9% 20% 11% 2	2979
	11th to 20th	Night	56% 3 13% 16% 11% 2	2576
	21st to end	Night	39% 3 16% 22% 16% 3	2785
June	1st to 10th	Night	41% 4 11% 22% 20% 2	1520
	11th to 20th	Night	38% 5% 13% 27% 15% 1	1520
	21st to end	Night	36% 5% 14% 29% 14% 2	1520
July	1st to 10th	Night	41% 8% 18% 17% 14% 3	2480
	11th to 20th	Night	40% 7% 16% 23% 12% 2	2472
	21st to end	Night	44% 6% 13% 21% 15% 1	2732
August	1st to 10th	Night	47% 7% 18% 18% 9% 1	2814
	11th to 20th	Night	41% 5% 13% 26% 15% 1	3064
	21st to end	Night	34% 4 11% 30% 20% 1	3476
September	1st to 10th	Night	40% 3 11% 20% 23% 3	1900
	11th to 20th	Night	33% 3 8% 25% 27% 4	1921
	21st to end	Night	44% 4 6% 22% 22% 2	2099
October	1st to 10th	Night	43% 4 9% 20% 21% 2	3800
	11th to 20th	Night	25% 3 10% 30% 28% 4	3779
	21st to end	Night	27% 2 12% 32% 24% 3	4227
November	1st to 10th	Night	32% 3 12% 27% 21% 5%	2537
	11th to 20th	Night	40% 3 9% 24% 21% 5%	2600
	21st to end	Night	39% 3 12% 28% 16% 2	2749
December	1st to 10th	Night	40% 3 12% 30% 13% 2	4580
	11th to 20th	Night	46% 2 10% 26% 13% 3	5208
	21st to end	Night	47% 2 10% 21% 18% 2	6996

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	36% 21% 29% 13% 1 2	200
	11th to 20th	Day	37% 22% 30% 9% 2	225
	21st to end	Day	32% 24% 27% 13% 4	330
February	1st to 10th	Day	31% 23% 33% 10% 3	300
	11th to 20th	Day	36% 23% 31% 9% 1	434
	21st to end	Day	33% 20% 29% 15% 3	414
March	1st to 10th	Day	33% 18% 28% 17% 4	500
	11th to 20th	Day	29% 22% 29% 13% 4 1	500
	21st to end	Day	30% 23% 30% 13% 4 1	550
April	1st to 10th	Day	31% 21% 30% 12% 5% 1	500
	11th to 20th	Day	27% 20% 28% 14% 7% 2	492
	21st to end	Day	28% 15% 33% 17% 5% 2	500
May	1st to 10th	Day	32% 17% 27% 15% 7% 2	503
	11th to 20th	Day	27% 12% 29% 21% 9% 2	527
	21st to end	Day	21% 9% 24% 24% 16% 5%	549
June	1st to 10th	Day	19% 10% 28% 26% 13% 4	490
	11th to 20th	Day	20% 8% 27% 25% 17% 4	490
	21st to end	Day	14% 9% 29% 30% 13% 5%	490
July	1st to 10th	Day	12% 2 21% 32% 24% 8%	485
	11th to 20th	Day	10% 3 23% 31% 26% 7%	484
	21st to end	Day	13% 6% 25% 32% 20% 4	532
August	1st to 10th	Day	13% 4 22% 33% 21% 8%	486
	11th to 20th	Day	14% 4 17% 36% 21% 8%	479
	21st to end	Day	13% 5% 23% 29% 18% 11%	539
September	1st to 10th	Day	18% 8% 20% 24% 20% 10%	490
	11th to 20th	Day	17% 3 24% 27% 17% 11%	490
	21st to end	Day	26% 11% 20% 25% 13% 4	490
October	1st to 10th	Day	26% 6% 24% 24% 14% 7%	490
	11th to 20th	Day	20% 14% 26% 25% 12% 3	490
	21st to end	Day	25% 13% 29% 20% 11% 3	506
November	1st to 10th	Day	21% 12% 31% 22% 12% 3	395
	11th to 20th	Day	23% 16% 34% 20% 7% 1	400
	21st to end	Day	27% 19% 31% 17% 5%	249
December	1st to 10th	Day	33% 20% 31% 10% 7% 1	200
	11th to 20th	Day	38% 18% 31% 10% 4	200
	21st to end	Day	30% 16% 31% 14% 9%	220
January	1st to 10th	Night	22% 20% 35% 19% 4	800
	11th to 20th	Night	27% 15% 29% 25% 3	800
	21st to end	Night	20% 19% 36% 22% 3	880
February	1st to 10th	Night	24% 15% 36% 23% 2	800
	11th to 20th	Night	28% 20% 35% 16% 1	783
	21st to end	Night	26% 17% 36% 18% 3	660
March	1st to 10th	Night	26% 18% 32% 21% 4	800
	11th to 20th	Night	23% 18% 34% 19% 5%	800
	21st to end	Night	23% 13% 38% 20% 6%	880
April	1st to 10th	Night	24% 19% 35% 18% 5%	800
	11th to 20th	Night	22% 12% 36% 23% 7%	791
	21st to end	Night	22% 12% 31% 29% 6%	800
May	1st to 10th	Night	24% 10% 31% 26% 7%	636
	11th to 20th	Night	21% 8% 33% 31% 8%	394
	21st to end	Night	18% 5% 23% 38% 14% 1	423
June	1st to 10th	Night	17% 5% 25% 40% 13% 1	380
	11th to 20th	Night	12% 5% 24% 44% 14% 1	380
	21st to end	Night	11% 3 23% 48% 15%	380
July	1st to 10th	Night	12% 3 17% 46% 20% 2	370
	11th to 20th	Night	9% 1 19% 43% 26% 2	368
	21st to end	Night	13% 3 20% 41% 21% 3	463
August	1st to 10th	Night	13% 1 14% 44% 26% 2	567
	11th to 20th	Night	14% 3 15% 37% 29% 3	712
	21st to end	Night	15% 3 15% 39% 26% 2	836
September	1st to 10th	Night	17% 4 22% 35% 19% 3	760
	11th to 20th	Night	14% 4 18% 35% 24% 4	760
	21st to end	Night	21% 5% 20% 33% 19% 2	760
October	1st to 10th	Night	23% 7% 22% 25% 19% 4	760
	11th to 20th	Night	18% 9% 27% 33% 13%	760
	21st to end	Night	23% 9% 30% 27% 11%	836
November	1st to 10th	Night	16% 9% 33% 29% 12% 1	780
	11th to 20th	Night	22% 15% 26% 29% 8%	800
	21st to end	Night	19% 15% 32% 26% 7% 1	800
December	1st to 10th	Night	26% 16% 30% 22% 6% 1	800
	11th to 20th	Night	24% 13% 35% 23% 6%	800
	21st to end	Night	24% 14% 31% 28% 4	880

Page generated at 2025-05-23 09:21:03. Version ac07

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