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 2026-09-29.

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% 6% 2 6% 4 4 4 4 6% 8% 40%	149384
	Night	16% 7% 2 6% 4 4 4 3 5% 7% 42%	142036
Transparency	Day	41% 4 11% 24% 17% 2	152914
	Night	42% 4 11% 24% 17% 3	146593
Seeing	Day	23% 13% 27% 21% 11% 4	17419
	Night	20% 11% 28% 28% 11% 1	26638

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	14% 7% 2 6% 4 5% 4 3 4 5% 46%	4661
	11th to 20th	Day	14% 8% 3 8% 4 4 4 3 5% 5% 42%	4807
	21st to end	Day	16% 6% 2 7% 5% 5% 5% 4 6% 7% 39%	5780
February	1st to 10th	Day	15% 7% 3 6% 4 4 4 4 6% 6% 39%	1890
	11th to 20th	Day	16% 9% 2 8% 5% 4 4 4 5% 6% 38%	1983
	21st to end	Day	18% 10% 3 6% 4 4 4 3 4 5% 39%	1730
March	1st to 10th	Day	21% 5% 2 5% 4 4 4 3 4 5% 44%	4615
	11th to 20th	Day	13% 6% 2 6% 5% 4 4 4 5% 7% 44%	5090
	21st to end	Day	18% 6% 2 5% 4 4 4 3 4 6% 43%	5904
April	1st to 10th	Day	17% 7% 3 6% 5% 4 4 3 6% 7% 39%	2520
	11th to 20th	Day	15% 7% 2 6% 4 4 4 4 6% 7% 42%	2494
	21st to end	Day	17% 5% 2 6% 4 4 4 4 6% 7% 40%	2520
May	1st to 10th	Day	23% 7% 2 5% 4 4 5% 4 6% 8% 33%	7590
	11th to 20th	Day	20% 7% 2 6% 4 4 4 3 5% 8% 37%	8221
	21st to end	Day	24% 7% 2 5% 3 4 4 3 5% 7% 36%	9060
June	1st to 10th	Day	12% 6% 2 6% 4 5% 5% 5% 7% 9% 40%	2940
	11th to 20th	Day	13% 7% 2 6% 5% 5% 5% 4 7% 9% 38%	2940
	21st to end	Day	11% 5% 2 6% 4 5% 5% 5% 7% 9% 41%	2940
July	1st to 10th	Day	12% 5% 2 7% 5% 5% 5% 4 7% 10% 39%	8280
	11th to 20th	Day	9% 7% 3 7% 5% 5% 6% 4 8% 11% 36%	8280
	21st to end	Day	11% 6% 3 6% 4 4 5% 4 6% 9% 41%	8822
August	1st to 10th	Day	16% 6% 3 8% 5% 5% 6% 5% 8% 9% 30%	6014
	11th to 20th	Day	13% 5% 2 5% 3 4 5% 4 7% 10% 42%	5533
	21st to end	Day	11% 5% 2 6% 4 4 4 4 6% 9% 45%	6080
September	1st to 10th	Day	14% 6% 2 5% 3 4 5% 4 5% 7% 46%	2520
	11th to 20th	Day	14% 5% 1 4 3 3 3 3 5% 8% 52%	2496
	21st to end	Day	19% 7% 3 6% 5% 4 5% 3 5% 6% 36%	2273
October	1st to 10th	Day	17% 6% 2 5% 3 3 4 3 5% 7% 45%	2000
	11th to 20th	Day	17% 6% 2 4 3 4 4 3 4 6% 48%	2000
	21st to end	Day	18% 7% 2 4 3 3 4 3 5% 6% 46%	2135
November	1st to 10th	Day	13% 6% 2 4 3 3 4 3 4 5% 53%	1851
	11th to 20th	Day	16% 6% 1 4 3 4 4 4 6% 6% 47%	1890
	21st to end	Day	16% 7% 2 5% 4 3 5% 3 6% 7% 43%	1731
December	1st to 10th	Day	15% 9% 2 5% 3 4 4 4 6% 6% 43%	2680
	11th to 20th	Day	21% 9% 2 4 3 3 3 3 4 6% 42%	3066
	21st to end	Day	17% 6% 3 7% 5% 5% 5% 4 6% 7% 37%	4048
January	1st to 10th	Night	12% 7% 3 6% 4 4 4 3 5% 6% 46%	8234
	11th to 20th	Night	14% 8% 3 6% 3 4 4 3 4 6% 46%	8075
	21st to end	Night	16% 8% 2 5% 3 3 3 3 5% 6% 46%	8401
February	1st to 10th	Night	16% 9% 2 6% 4 4 4 4 5% 6% 41%	2730
	11th to 20th	Night	18% 8% 3 6% 5% 4 3 3 5% 5% 40%	2521
	21st to end	Night	17% 8% 2 6% 4 3 4 4 5% 6% 42%	2076
March	1st to 10th	Night	19% 7% 2 6% 3 3 4 3 4 5% 43%	5442
	11th to 20th	Night	12% 8% 2 5% 4 4 3 3 4 6% 50%	5089
	21st to end	Night	17% 7% 3 6% 4 4 3 3 4 6% 43%	5269
April	1st to 10th	Night	18% 6% 3 6% 4 4 4 4 6% 6% 39%	2100
	11th to 20th	Night	15% 7% 2 6% 5% 4 4 4 6% 7% 40%	2075
	21st to end	Night	19% 6% 2 4 3 3 3 3 6% 8% 43%	2100
May	1st to 10th	Night	21% 8% 3 6% 4 4 5% 4 6% 8% 31%	5349
	11th to 20th	Night	19% 8% 3 6% 4 4 4 4 5% 8% 34%	4639
	21st to end	Night	25% 7% 2 6% 4 3 4 3 5% 7% 34%	5114
June	1st to 10th	Night	14% 6% 3 6% 4 4 4 4 5% 8% 43%	1680
	11th to 20th	Night	12% 6% 1 6% 5% 5% 5% 5% 7% 8% 40%	1680
	21st to end	Night	11% 3 2 6% 4 5% 5% 5% 7% 9% 42%	1680
July	1st to 10th	Night	12% 6% 2 7% 5% 5% 5% 4 7% 9% 37%	4680
	11th to 20th	Night	11% 8% 3 7% 5% 5% 6% 5% 7% 10% 32%	4680
	21st to end	Night	12% 6% 2 6% 4 4 5% 4 6% 9% 41%	5251
August	1st to 10th	Night	17% 7% 3 7% 5% 4 6% 5% 6% 9% 30%	4135
	11th to 20th	Night	14% 6% 3 7% 4 5% 4 4 6% 8% 40%	4465
	21st to end	Night	12% 6% 2 6% 4 4 5% 4 5% 7% 46%	5045
September	1st to 10th	Night	14% 7% 2 6% 4 4 4 3 5% 6% 46%	2100
	11th to 20th	Night	12% 6% 2 5% 3 3 3 3 4 7% 52%	2123
	21st to end	Night	19% 7% 3 6% 4 4 4 3 5% 7% 38%	2299
October	1st to 10th	Night	18% 7% 2 6% 5% 3 3 3 3 6% 44%	2400
	11th to 20th	Night	16% 5% 2 5% 3 4 3 3 4 6% 50%	2400
	21st to end	Night	18% 6% 1 4 3 3 3 3 4 6% 49%	2702
November	1st to 10th	Night	11% 6% 1 5% 3 3 4 3 5% 6% 53%	2667
	11th to 20th	Night	16% 7% 3 4 3 3 4 3 4 5% 48%	2730
	21st to end	Night	14% 6% 2 5% 5% 5% 4 3 6% 7% 44%	2887
December	1st to 10th	Night	13% 8% 2 6% 4 3 4 3 5% 6% 45%	4720
	11th to 20th	Night	23% 7% 2 4 3 3 4 3 4 6% 41%	5348
	21st to end	Night	20% 7% 3 6% 4 4 4 3 5% 6% 40%	7150

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	40% 2 9% 27% 19% 2	4662
	11th to 20th	Day	44% 2 8% 22% 18% 6%	4837
	21st to end	Day	44% 3 13% 24% 16% 1	5780
February	1st to 10th	Day	44% 3 13% 25% 13% 2	1890
	11th to 20th	Day	48% 3 8% 24% 15% 2	1992
	21st to end	Day	48% 2 8% 26% 13% 3	1730
March	1st to 10th	Day	43% 3 8% 24% 16% 7%	4618
	11th to 20th	Day	40% 2 9% 29% 15% 5%	5090
	21st to end	Day	43% 3 10% 17% 23% 5%	5904
April	1st to 10th	Day	44% 3 9% 22% 17% 5%	2520
	11th to 20th	Day	41% 3 12% 22% 17% 4	2494
	21st to end	Day	43% 4 9% 23% 17% 4	2520
May	1st to 10th	Day	49% 4 11% 21% 13% 1	7599
	11th to 20th	Day	47% 3 10% 26% 14% 1	8225
	21st to end	Day	49% 3 9% 24% 14% 1	9061
June	1st to 10th	Day	40% 5% 11% 25% 18% 1	2940
	11th to 20th	Day	42% 5% 14% 25% 12% 3	2940
	21st to end	Day	37% 5% 14% 27% 16% 1	2940
July	1st to 10th	Day	40% 8% 13% 21% 16% 1	8210
	11th to 20th	Day	39% 8% 14% 26% 11% 1	8205
	21st to end	Day	38% 6% 10% 23% 22% 1	8626
August	1st to 10th	Day	47% 9% 14% 20% 9%	6001
	11th to 20th	Day	35% 6% 15% 32% 12% 1	5570
	21st to end	Day	35% 6% 11% 28% 18% 2	6080
September	1st to 10th	Day	38% 4 11% 23% 21% 3	2520
	11th to 20th	Day	33% 3 8% 29% 23% 4	2496
	21st to end	Day	50% 3 8% 18% 19% 2	2273
October	1st to 10th	Day	37% 3 8% 20% 27% 5%	3260
	11th to 20th	Day	26% 3 12% 27% 27% 6%	3240
	21st to end	Day	26% 2 12% 26% 30% 4	3425
November	1st to 10th	Day	34% 2 13% 25% 22% 4	1851
	11th to 20th	Day	38% 3 10% 27% 20% 3	1890
	21st to end	Day	40% 3 12% 31% 12% 2	1731
December	1st to 10th	Day	41% 3 16% 26% 11% 2	2680
	11th to 20th	Day	45% 2 12% 29% 11% 2	3066
	21st to end	Day	45% 3 11% 23% 16% 1	4048
January	1st to 10th	Night	40% 3 12% 25% 16% 4	8234
	11th to 20th	Night	40% 2 10% 22% 18% 7%	8120
	21st to end	Night	41% 2 8% 26% 21% 3	8401
February	1st to 10th	Night	45% 3 11% 26% 12% 2	2730
	11th to 20th	Night	47% 2 8% 21% 18% 4	2523
	21st to end	Night	44% 2 8% 24% 18% 4	2076
March	1st to 10th	Night	44% 2 8% 24% 13% 9%	5452
	11th to 20th	Night	38% 2 13% 27% 15% 5%	5090
	21st to end	Night	44% 3 7% 17% 25% 5%	5269
April	1st to 10th	Night	45% 3 10% 21% 17% 4	2100
	11th to 20th	Night	43% 3 10% 20% 21% 3	2075
	21st to end	Night	40% 3 9% 24% 19% 5%	2100
May	1st to 10th	Night	50% 5% 11% 20% 12% 2	5356
	11th to 20th	Night	48% 5% 11% 23% 12% 1	4640
	21st to end	Night	51% 3 11% 20% 13% 2	5115
June	1st to 10th	Night	40% 4 11% 23% 21% 2	1680
	11th to 20th	Night	40% 5% 13% 26% 15% 2	1680
	21st to end	Night	37% 5% 14% 28% 15% 2	1680
July	1st to 10th	Night	42% 7% 14% 20% 14% 2	4640
	11th to 20th	Night	44% 7% 15% 23% 10% 1	4632
	21st to end	Night	38% 6% 13% 24% 18% 1	5132
August	1st to 10th	Night	49% 8% 16% 17% 9% 1	4124
	11th to 20th	Night	41% 6% 14% 27% 11% 1	4494
	21st to end	Night	36% 5% 11% 28% 19% 1	5045
September	1st to 10th	Night	40% 4 11% 21% 22% 3	2100
	11th to 20th	Night	33% 3 9% 25% 26% 4	2123
	21st to end	Night	46% 4 7% 21% 21% 2	2299
October	1st to 10th	Night	43% 4 8% 21% 22% 2	3920
	11th to 20th	Night	26% 3 10% 29% 28% 4	3899
	21st to end	Night	26% 2 12% 32% 24% 3	4362
November	1st to 10th	Night	32% 3 12% 27% 21% 5%	2667
	11th to 20th	Night	39% 3 9% 23% 21% 5%	2730
	21st to end	Night	39% 3 12% 28% 17% 2	2887
December	1st to 10th	Night	40% 3 13% 30% 13% 2	4720
	11th to 20th	Night	45% 2 9% 26% 14% 4	5348
	21st to end	Night	47% 2 10% 21% 17% 2	7150

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% 21% 30% 12% 1 2	220
	11th to 20th	Day	36% 25% 29% 9% 2	248
	21st to end	Day	31% 27% 26% 12% 4	363
February	1st to 10th	Day	31% 22% 34% 11% 3 1	330
	11th to 20th	Day	35% 22% 32% 10% 1	471
	21st to end	Day	32% 20% 30% 14% 3	446
March	1st to 10th	Day	32% 17% 29% 17% 4	540
	11th to 20th	Day	30% 22% 29% 14% 4 1	540
	21st to end	Day	29% 24% 31% 12% 4 1	594
April	1st to 10th	Day	31% 21% 29% 13% 5% 1	540
	11th to 20th	Day	26% 20% 30% 15% 8% 2	532
	21st to end	Day	28% 14% 33% 17% 5% 2	540
May	1st to 10th	Day	31% 18% 28% 14% 6% 2	543
	11th to 20th	Day	26% 12% 30% 21% 9% 2	567
	21st to end	Day	23% 9% 25% 23% 15% 5%	617
June	1st to 10th	Day	18% 11% 27% 26% 14% 4	570
	11th to 20th	Day	20% 11% 26% 25% 15% 3	570
	21st to end	Day	15% 9% 28% 28% 15% 5%	570
July	1st to 10th	Day	13% 3 20% 31% 25% 8%	565
	11th to 20th	Day	10% 4 23% 30% 25% 8%	564
	21st to end	Day	12% 5% 24% 34% 20% 4	620
August	1st to 10th	Day	13% 4 21% 32% 22% 7%	566
	11th to 20th	Day	14% 3 17% 37% 21% 8%	559
	21st to end	Day	12% 6% 26% 29% 18% 10%	627
September	1st to 10th	Day	17% 7% 22% 25% 19% 9%	570
	11th to 20th	Day	17% 4 23% 28% 18% 11%	570
	21st to end	Day	27% 10% 21% 24% 13% 4	562
October	1st to 10th	Day	24% 6% 23% 24% 15% 8%	530
	11th to 20th	Day	22% 15% 25% 24% 11% 3	530
	21st to end	Day	24% 13% 30% 20% 10% 3	547
November	1st to 10th	Day	21% 13% 31% 22% 11% 3	425
	11th to 20th	Day	24% 17% 32% 20% 6% 1	430
	21st to end	Day	27% 20% 32% 15% 5%	271
December	1st to 10th	Day	32% 20% 32% 10% 6%	220
	11th to 20th	Day	37% 18% 32% 10% 3	220
	21st to end	Day	31% 17% 31% 12% 8%	242
January	1st to 10th	Night	21% 19% 36% 20% 4	840
	11th to 20th	Night	27% 15% 30% 24% 3	840
	21st to end	Night	20% 19% 35% 22% 3	924
February	1st to 10th	Night	24% 15% 36% 23% 2	840
	11th to 20th	Night	28% 19% 35% 17% 1	823
	21st to end	Night	25% 17% 37% 18% 3	692
March	1st to 10th	Night	26% 17% 31% 21% 4	840
	11th to 20th	Night	23% 18% 34% 20% 5%	840
	21st to end	Night	22% 13% 39% 20% 5%	924
April	1st to 10th	Night	23% 19% 34% 19% 5%	840
	11th to 20th	Night	21% 12% 37% 24% 6%	831
	21st to end	Night	22% 12% 30% 30% 6%	840
May	1st to 10th	Night	24% 11% 32% 26% 7%	668
	11th to 20th	Night	20% 8% 33% 32% 7%	414
	21st to end	Night	20% 5% 24% 37% 13% 1	455
June	1st to 10th	Night	16% 5% 25% 40% 14% 1	420
	11th to 20th	Night	13% 5% 24% 44% 13% 1	420
	21st to end	Night	12% 3 23% 46% 16%	420
July	1st to 10th	Night	12% 3 16% 46% 22% 2	410
	11th to 20th	Night	9% 2 20% 43% 25% 1	408
	21st to end	Night	12% 2 20% 40% 22% 3	513
August	1st to 10th	Night	13% 1 14% 43% 28% 2	627
	11th to 20th	Night	13% 3 15% 38% 28% 2	788
	21st to end	Night	15% 3 15% 40% 25% 2	924
September	1st to 10th	Night	18% 4 21% 35% 19% 3	840
	11th to 20th	Night	14% 4 18% 36% 24% 4	840
	21st to end	Night	22% 5% 19% 33% 19% 2	833
October	1st to 10th	Night	22% 7% 22% 25% 20% 4	800
	11th to 20th	Night	19% 9% 26% 33% 13%	800
	21st to end	Night	23% 10% 31% 26% 10%	880
November	1st to 10th	Night	17% 9% 32% 29% 11% 1	820
	11th to 20th	Night	22% 16% 26% 28% 8%	840
	21st to end	Night	19% 16% 32% 26% 7% 1	840
December	1st to 10th	Night	25% 16% 31% 22% 5% 1	840
	11th to 20th	Night	24% 14% 34% 23% 6%	840
	21st to end	Night	24% 14% 31% 27% 3	924

Page generated at 2026-09-26 04:18:37. Version ac07

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