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 2024-11-02.

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	16% 7% 2 6% 4 4 4 4 6% 7% 40%	101315
	Night	17% 7% 2 6% 4 4 4 3 5% 7% 42%	97671
Transparency	Day	43% 3 11% 25% 16% 2	101072
	Night	43% 3 11% 24% 16% 3	97533
Seeing	Day	23% 13% 27% 22% 12% 4	14960
	Night	20% 11% 28% 28% 11% 1	24220

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	15% 8% 2 5% 4 4 4 3 4 5% 45%	2517
	11th to 20th	Day	14% 10% 4 10% 5% 4 3 4 5% 6% 35%	2593
	21st to end	Day	21% 9% 3 7% 5% 4 4 2 4 6% 35%	3124
February	1st to 10th	Day	17% 7% 3 6% 5% 4 4 4 6% 7% 39%	1710
	11th to 20th	Day	15% 8% 2 7% 5% 4 4 4 5% 6% 40%	1789
	21st to end	Day	19% 10% 3 6% 5% 3 4 3 5% 5% 38%	1570
March	1st to 10th	Day	25% 7% 2 6% 5% 4 3 3 3 5% 37%	3140
	11th to 20th	Day	10% 5% 2 6% 5% 5% 5% 4 6% 7% 46%	3480
	21st to end	Day	22% 7% 2 5% 4 3 3 2 3 5% 43%	4028
April	1st to 10th	Day	16% 7% 3 6% 5% 4 3 3 6% 7% 40%	2280
	11th to 20th	Day	16% 7% 2 6% 4 4 4 3 6% 6% 42%	2254
	21st to end	Day	17% 5% 2 6% 4 4 4 4 7% 7% 40%	2280
May	1st to 10th	Day	28% 7% 2 6% 3 4 4 4 5% 7% 29%	4086
	11th to 20th	Day	25% 8% 3 7% 5% 4 4 4 5% 6% 28%	4398
	21st to end	Day	13% 6% 2 5% 4 4 4 4 6% 9% 44%	4837
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	14% 7% 2 4 3 2 4 3 4 5% 52%	1681
	11th to 20th	Day	16% 6% 1 4 3 3 3 3 6% 6% 47%	1710
	21st to end	Day	15% 6% 2 5% 4 3 4 3 6% 8% 44%	1567
December	1st to 10th	Day	19% 9% 2 6% 4 4 3 4 6% 6% 38%	1520
	11th to 20th	Day	20% 8% 3 4 3 3 3 3 5% 6% 41%	1906
	21st to end	Day	17% 7% 3 7% 5% 5% 5% 4 6% 6% 34%	2772
January	1st to 10th	Night	16% 6% 2 5% 4 4 4 3 4 6% 45%	4429
	11th to 20th	Night	18% 8% 3 6% 4 4 4 2 4 5% 42%	4364
	21st to end	Night	24% 8% 2 6% 3 3 3 3 4 5% 37%	4532
February	1st to 10th	Night	16% 9% 2 6% 4 4 4 3 5% 6% 41%	2470
	11th to 20th	Night	18% 8% 3 6% 5% 4 3 3 4 5% 40%	2275
	21st to end	Night	19% 8% 2 6% 4 3 4 4 4 6% 42%	1884
March	1st to 10th	Night	23% 9% 2 6% 4 4 3 3 4 5% 37%	3697
	11th to 20th	Night	11% 6% 2 6% 4 4 4 3 4 6% 49%	3479
	21st to end	Night	17% 9% 3 6% 4 3 2 3 3 5% 46%	3603
April	1st to 10th	Night	17% 6% 3 5% 4 4 4 4 6% 7% 40%	1900
	11th to 20th	Night	15% 7% 2 6% 5% 4 4 4 5% 7% 41%	1875
	21st to end	Night	19% 6% 2 4 3 3 3 3 6% 8% 43%	1900
May	1st to 10th	Night	24% 9% 3 6% 4 4 5% 3 5% 7% 30%	2881
	11th to 20th	Night	23% 9% 4 7% 5% 4 4 4 5% 6% 29%	2495
	21st to end	Night	13% 6% 2 5% 5% 3 5% 4 6% 9% 43%	2738
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	12% 6% 2 5% 3 2 3 3 5% 6% 53%	2431
	11th to 20th	Night	17% 7% 3 4 3 3 4 3 4 5% 48%	2470
	21st to end	Night	14% 6% 2 4 5% 4 4 4 6% 7% 45%	2611
December	1st to 10th	Night	20% 7% 2 6% 4 3 5% 3 5% 6% 38%	2660
	11th to 20th	Night	18% 6% 2 5% 4 4 5% 4 5% 6% 40%	3288
	21st to end	Night	18% 7% 3 6% 5% 4 4 4 5% 6% 37%	4884

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	42% 1 11% 28% 14% 4	2518
	11th to 20th	Day	50% 2 10% 21% 14% 3	2593
	21st to end	Day	53% 1 11% 23% 11% 1	3124
February	1st to 10th	Day	45% 3 13% 24% 13% 2	1710
	11th to 20th	Day	46% 3 7% 25% 16% 3	1798
	21st to end	Day	49% 2 8% 24% 13% 3	1570
March	1st to 10th	Day	52% 2 7% 27% 10% 3	3143
	11th to 20th	Day	38% 2 8% 31% 18% 3	3480
	21st to end	Day	46% 1 8% 19% 22% 3	4028
April	1st to 10th	Day	44% 2 9% 23% 17% 5%	2280
	11th to 20th	Day	42% 3 11% 23% 17% 4	2254
	21st to end	Day	43% 4 9% 22% 18% 4	2280
May	1st to 10th	Day	56% 2 9% 21% 10% 2	4098
	11th to 20th	Day	58% 2 10% 20% 9% 1	4402
	21st to end	Day	38% 3 11% 29% 17% 2	4837
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	41% 3 11% 25% 19% 2	1900
	11th to 20th	Day	37% 3 10% 25% 23% 2	1900
	21st to end	Day	41% 2 9% 26% 20% 2	2028
November	1st to 10th	Day	35% 2 12% 26% 21% 4	1681
	11th to 20th	Day	38% 3 9% 26% 20% 3	1710
	21st to end	Day	39% 3 12% 32% 12% 2	1567
December	1st to 10th	Day	47% 3 15% 22% 13% 1	1520
	11th to 20th	Day	45% 2 14% 26% 11% 2	1906
	21st to end	Day	50% 2 11% 24% 12% 1	2772
January	1st to 10th	Night	43% 2 9% 30% 13% 3	4429
	11th to 20th	Night	46% 2 11% 26% 13% 3	4364
	21st to end	Night	52% 1 7% 27% 12% 2	4532
February	1st to 10th	Night	46% 2 12% 26% 12% 2	2470
	11th to 20th	Night	47% 2 8% 21% 18% 4	2277
	21st to end	Night	44% 2 8% 24% 18% 4	1884
March	1st to 10th	Night	52% 1 7% 25% 13% 2	3707
	11th to 20th	Night	38% 2 10% 29% 18% 3	3480
	21st to end	Night	44% 1 7% 18% 24% 5%	3603
April	1st to 10th	Night	44% 2 10% 22% 17% 4	1900
	11th to 20th	Night	43% 3 9% 21% 21% 3	1875
	21st to end	Night	40% 3 9% 24% 19% 5%	1900
May	1st to 10th	Night	55% 3 9% 20% 10% 2	2888
	11th to 20th	Night	57% 2 13% 16% 10% 2	2496
	21st to end	Night	39% 3 16% 22% 16% 3	2739
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	45% 3 10% 23% 17% 2	2280
	11th to 20th	Night	35% 3 11% 29% 19% 3	2280
	21st to end	Night	39% 2 9% 29% 19% 3	2567
November	1st to 10th	Night	33% 3 12% 27% 20% 5%	2431
	11th to 20th	Night	40% 3 9% 24% 20% 4	2470
	21st to end	Night	38% 3 12% 29% 16% 2	2611
December	1st to 10th	Night	47% 2 12% 23% 14% 1	2660
	11th to 20th	Night	44% 2 11% 22% 17% 4	3288
	21st to end	Night	48% 2 12% 25% 11% 1	4884

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% 18% 31% 14% 1 2	180
	11th to 20th	Day	36% 22% 31% 9% 2	202
	21st to end	Day	33% 24% 25% 13% 5%	297
February	1st to 10th	Day	31% 23% 32% 10% 3	270
	11th to 20th	Day	33% 23% 33% 9% 1	397
	21st to end	Day	35% 19% 27% 15% 3	382
March	1st to 10th	Day	32% 17% 28% 18% 4	460
	11th to 20th	Day	30% 24% 28% 12% 4 1	460
	21st to end	Day	31% 22% 29% 13% 4 1	506
April	1st to 10th	Day	30% 20% 30% 13% 5% 1	460
	11th to 20th	Day	26% 21% 28% 15% 8% 3	452
	21st to end	Day	29% 14% 33% 17% 4 2	460
May	1st to 10th	Day	32% 17% 26% 15% 7% 2	463
	11th to 20th	Day	26% 11% 29% 22% 9% 2	487
	21st to end	Day	19% 8% 25% 25% 17% 6%	529
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% 19% 11% 3	506
November	1st to 10th	Day	21% 13% 29% 22% 12% 3	369
	11th to 20th	Day	24% 16% 32% 19% 7% 1	370
	21st to end	Day	25% 19% 33% 18% 5%	227
December	1st to 10th	Day	33% 21% 29% 9% 7% 1	180
	11th to 20th	Day	38% 17% 31% 11% 3	180
	21st to end	Day	32% 16% 31% 13% 8% 1	198
January	1st to 10th	Night	22% 18% 36% 20% 4	760
	11th to 20th	Night	27% 15% 30% 25% 3	760
	21st to end	Night	21% 19% 35% 23% 3	836
February	1st to 10th	Night	24% 15% 35% 24% 2	760
	11th to 20th	Night	27% 20% 36% 16% 1	743
	21st to end	Night	27% 17% 36% 17% 3	628
March	1st to 10th	Night	26% 17% 31% 21% 4	760
	11th to 20th	Night	24% 18% 34% 18% 5%	760
	21st to end	Night	23% 13% 37% 21% 6%	836
April	1st to 10th	Night	23% 18% 36% 18% 5%	760
	11th to 20th	Night	22% 11% 37% 23% 7%	751
	21st to end	Night	23% 12% 31% 28% 6%	760
May	1st to 10th	Night	24% 10% 32% 26% 7%	605
	11th to 20th	Night	21% 7% 34% 30% 8%	374
	21st to end	Night	18% 5% 23% 39% 14% 1	411
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% 10%	836
November	1st to 10th	Night	17% 9% 34% 28% 12% 1	748
	11th to 20th	Night	22% 15% 25% 29% 9%	760
	21st to end	Night	18% 14% 32% 27% 7% 1	760
December	1st to 10th	Night	26% 16% 30% 21% 6% 1	760
	11th to 20th	Night	23% 13% 36% 23% 6%	760
	21st to end	Night	24% 14% 31% 28% 3	836

Page generated at 2024-10-30 13:19:23. Version ac07

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