Sterling
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Sterling clear sky chart from 2005-11-06 to 2026-07-16.

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	12% 8% 3 8% 5% 4 5% 4 6% 7% 38%	138386
	Night	13% 9% 3 8% 5% 4 4 3 5% 6% 39%	134615
Transparency	Day	43% 6% 12% 23% 14% 2	141881
	Night	44% 6% 11% 23% 15% 2	139197
Seeing	Day	16% 10% 28% 26% 15% 5%	18158
	Night	17% 9% 31% 31% 11% 1	27988

By Month

Percentage Cloud Cover SE (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	8% 12% 6% 15% 7% 7% 6% 5% 6% 6% 23%	4691
	11th to 20th	Day	7% 10% 5% 12% 6% 5% 6% 6% 7% 6% 30%	4710
	21st to end	Day	11% 14% 5% 10% 6% 5% 4 4 5% 7% 29%	5614
February	1st to 10th	Day	13% 13% 5% 11% 6% 5% 6% 4 4 6% 28%	1890
	11th to 20th	Day	13% 11% 4 9% 5% 6% 4 3 5% 6% 35%	1991
	21st to end	Day	12% 12% 5% 11% 4 4 5% 4 5% 7% 32%	1730
March	1st to 10th	Day	17% 10% 4 6% 4 3 3 2 3 5% 43%	4580
	11th to 20th	Day	11% 6% 2 6% 5% 4 5% 4 5% 7% 45%	4985
	21st to end	Day	21% 11% 4 11% 5% 5% 4 3 5% 6% 26%	5994
April	1st to 10th	Day	17% 13% 3 8% 4 4 4 4 5% 6% 32%	2520
	11th to 20th	Day	17% 10% 3 7% 5% 5% 4 4 5% 7% 34%	2494
	21st to end	Day	20% 9% 3 7% 4 4 4 3 5% 6% 35%	2536
May	1st to 10th	Day	13% 7% 3 7% 5% 4 5% 4 6% 8% 38%	7788
	11th to 20th	Day	13% 6% 2 6% 5% 5% 5% 4 6% 8% 42%	8222
	21st to end	Day	12% 7% 3 7% 5% 4 5% 4 7% 9% 38%	9047
June	1st to 10th	Day	10% 5% 2 6% 4 5% 4 5% 7% 9% 44%	2940
	11th to 20th	Day	10% 5% 2 5% 4 4 5% 3 6% 9% 47%	2940
	21st to end	Day	11% 6% 2 6% 5% 5% 5% 5% 7% 9% 39%	2940
July	1st to 10th	Day	9% 7% 3 8% 5% 5% 5% 5% 7% 10% 38%	6500
	11th to 20th	Day	9% 6% 2 6% 4 5% 5% 4 6% 9% 45%	6431
	21st to end	Day	10% 7% 3 7% 5% 5% 5% 4 7% 9% 37%	6924
August	1st to 10th	Day	8% 4 3 5% 5% 4 6% 5% 8% 9% 44%	4370
	11th to 20th	Day	10% 5% 2 6% 4 4 5% 4 7% 9% 43%	3911
	21st to end	Day	8% 6% 2 5% 3 4 4 3 6% 9% 49%	4312
September	1st to 10th	Day	12% 5% 2 5% 4 4 4 4 5% 8% 48%	2400
	11th to 20th	Day	11% 4 1 3 3 4 4 3 5% 6% 57%	2345
	21st to end	Day	10% 5% 2 6% 4 4 4 3 6% 8% 48%	2200
October	1st to 10th	Day	13% 6% 2 6% 4 3 4 3 4 7% 49%	2030
	11th to 20th	Day	17% 8% 3 6% 4 4 4 2 5% 7% 40%	2000
	21st to end	Day	19% 10% 2 6% 3 3 3 3 4 6% 41%	2055
November	1st to 10th	Day	12% 8% 2 5% 3 4 4 3 6% 8% 45%	1852
	11th to 20th	Day	17% 12% 3 8% 5% 4 4 3 5% 6% 32%	1890
	21st to end	Day	16% 13% 4 9% 5% 4 4 3 4 5% 34%	1706
December	1st to 10th	Day	8% 10% 3 8% 5% 4 4 3 4 7% 44%	2690
	11th to 20th	Day	13% 17% 6% 11% 5% 6% 5% 4 5% 4 23%	3088
	21st to end	Day	24% 20% 8% 13% 5% 4 3 2 3 4 13%	4070
January	1st to 10th	Night	11% 13% 5% 14% 5% 4 3 3 4 5% 32%	8232
	11th to 20th	Night	9% 13% 5% 13% 7% 6% 5% 5% 6% 6% 25%	8280
	21st to end	Night	12% 14% 5% 8% 5% 4 3 3 5% 6% 35%	8585
February	1st to 10th	Night	13% 14% 5% 8% 6% 4 5% 3 4 5% 34%	2730
	11th to 20th	Night	12% 10% 4 7% 5% 4 5% 3 4 6% 40%	2515
	21st to end	Night	15% 11% 3 9% 4 3 3 3 4 5% 39%	2076
March	1st to 10th	Night	18% 9% 4 5% 4 4 4 3 4 5% 41%	5493
	11th to 20th	Night	10% 7% 2 6% 4 4 4 3 5% 7% 48%	5240
	21st to end	Night	19% 11% 5% 11% 6% 5% 5% 3 4 4 27%	5175
April	1st to 10th	Night	18% 9% 3 6% 4 4 4 3 5% 5% 38%	2100
	11th to 20th	Night	19% 7% 2 6% 4 4 4 3 4 5% 41%	2075
	21st to end	Night	16% 9% 3 7% 5% 4 4 4 6% 7% 35%	2084
May	1st to 10th	Night	14% 7% 3 7% 5% 5% 4 4 5% 6% 39%	5070
	11th to 20th	Night	11% 5% 2 6% 5% 4 5% 5% 6% 9% 42%	4670
	21st to end	Night	12% 7% 2 7% 5% 4 4 4 5% 8% 41%	5139
June	1st to 10th	Night	8% 5% 2 5% 5% 4 5% 4 6% 7% 49%	1680
	11th to 20th	Night	8% 4 2 6% 5% 4 4 4 7% 9% 46%	1680
	21st to end	Night	10% 6% 2 5% 5% 5% 5% 4 6% 8% 44%	1680
July	1st to 10th	Night	12% 6% 2 7% 5% 4 4 4 7% 9% 41%	3698
	11th to 20th	Night	7% 5% 3 6% 4 3 4 3 6% 7% 52%	3657
	21st to end	Night	9% 5% 3 7% 5% 4 5% 5% 6% 9% 43%	3947
August	1st to 10th	Night	5% 4 2 5% 4 4 5% 4 5% 9% 53%	2786
	11th to 20th	Night	6% 4 2 6% 4 4 4 4 5% 8% 52%	3139
	21st to end	Night	8% 4 2 5% 4 4 4 3 5% 9% 53%	3586
September	1st to 10th	Night	8% 6% 1 5% 4 3 3 3 5% 7% 57%	2000
	11th to 20th	Night	10% 3 2 5% 3 4 4 4 5% 8% 52%	2035
	21st to end	Night	9% 5% 2 5% 4 4 4 4 6% 7% 50%	2200
October	1st to 10th	Night	11% 5% 1 6% 4 4 3 3 5% 6% 54%	2365
	11th to 20th	Night	16% 8% 3 6% 3 4 3 3 4 6% 47%	2400
	21st to end	Night	19% 9% 3 6% 4 4 4 3 4 6% 38%	2784
November	1st to 10th	Night	12% 7% 3 5% 3 3 3 3 4 6% 51%	2666
	11th to 20th	Night	15% 10% 4 8% 4 3 4 3 5% 6% 38%	2730
	21st to end	Night	18% 10% 4 7% 5% 3 3 3 4 6% 37%	2909
December	1st to 10th	Night	10% 8% 3 8% 5% 5% 5% 4 5% 6% 41%	4720
	11th to 20th	Night	17% 14% 5% 9% 4 4 4 3 4 5% 32%	5339
	21st to end	Night	23% 18% 6% 11% 6% 5% 4 3 4 4 15%	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	61% 5% 12% 15% 7%	4692
	11th to 20th	Day	51% 5% 9% 18% 15% 3	4710
	21st to end	Day	54% 4 14% 20% 7%	5614
February	1st to 10th	Day	56% 5% 13% 16% 8% 1	1890
	11th to 20th	Day	49% 6% 10% 23% 9% 2	2001
	21st to end	Day	52% 7% 11% 20% 9% 1	1730
March	1st to 10th	Day	44% 6% 10% 27% 10% 3	4580
	11th to 20th	Day	37% 6% 11% 33% 12% 1	4985
	21st to end	Day	58% 7% 11% 16% 7% 1	5996
April	1st to 10th	Day	52% 6% 12% 21% 9%	2520
	11th to 20th	Day	48% 7% 11% 21% 12% 2	2494
	21st to end	Day	49% 6% 12% 19% 12% 1	2536
May	1st to 10th	Day	42% 6% 9% 23% 19%	7823
	11th to 20th	Day	40% 6% 11% 26% 16% 1	8226
	21st to end	Day	41% 6% 11% 22% 14% 4	9061
June	1st to 10th	Day	35% 8% 14% 26% 17% 1	2940
	11th to 20th	Day	33% 7% 12% 24% 21% 2	2940
	21st to end	Day	38% 9% 14% 24% 12% 2	2940
July	1st to 10th	Day	39% 7% 14% 26% 13% 1	6431
	11th to 20th	Day	35% 6% 15% 28% 15% 1	6356
	21st to end	Day	40% 8% 17% 19% 14% 1	6728
August	1st to 10th	Day	33% 9% 16% 28% 12% 1	4355
	11th to 20th	Day	34% 7% 15% 29% 15% 1	3948
	21st to end	Day	32% 7% 12% 32% 16% 2	4311
September	1st to 10th	Day	33% 8% 11% 27% 19% 3	2400
	11th to 20th	Day	29% 6% 11% 30% 22% 3	2345
	21st to end	Day	34% 6% 12% 27% 20% 2	2200
October	1st to 10th	Day	24% 3 6% 31% 34% 1	3303
	11th to 20th	Day	31% 4 10% 24% 28% 3	3240
	21st to end	Day	42% 4 9% 28% 14% 3	3293
November	1st to 10th	Day	38% 5% 16% 26% 13% 2	1852
	11th to 20th	Day	52% 5% 12% 21% 9% 1	1890
	21st to end	Day	53% 6% 13% 21% 7%	1706
December	1st to 10th	Day	42% 4 14% 31% 8% 1	2690
	11th to 20th	Day	62% 5% 10% 15% 6% 1	3085
	21st to end	Day	78% 4 7% 7% 4	4070
January	1st to 10th	Night	54% 5% 15% 19% 8%	8233
	11th to 20th	Night	59% 4 10% 13% 13% 1	8280
	21st to end	Night	52% 4 12% 21% 11%	8585
February	1st to 10th	Night	52% 5% 11% 19% 12% 1	2730
	11th to 20th	Night	46% 5% 11% 24% 13% 1	2516
	21st to end	Night	48% 5% 12% 20% 12% 2	2076
March	1st to 10th	Night	45% 5% 10% 24% 12% 5%	5493
	11th to 20th	Night	35% 5% 10% 32% 14% 4	5240
	21st to end	Night	59% 6% 9% 17% 8% 1	5175
April	1st to 10th	Night	47% 5% 11% 21% 13% 2	2100
	11th to 20th	Night	45% 6% 8% 21% 16% 4	2075
	21st to end	Night	46% 7% 10% 23% 11% 2	2084
May	1st to 10th	Night	44% 7% 9% 20% 20% 2	5097
	11th to 20th	Night	36% 8% 13% 23% 16% 4	4671
	21st to end	Night	40% 7% 9% 23% 17% 4	5149
June	1st to 10th	Night	31% 8% 14% 25% 19% 3	1680
	11th to 20th	Night	32% 8% 11% 22% 25% 3	1680
	21st to end	Night	36% 9% 14% 20% 17% 3	1680
July	1st to 10th	Night	39% 5% 15% 24% 15% 2	3660
	11th to 20th	Night	31% 5% 12% 30% 21% 1	3612
	21st to end	Night	35% 9% 16% 21% 16% 3	3832
August	1st to 10th	Night	27% 7% 16% 30% 18% 2	2777
	11th to 20th	Night	30% 6% 14% 31% 17% 1	3168
	21st to end	Night	29% 6% 12% 31% 18% 3	3586
September	1st to 10th	Night	28% 6% 10% 29% 25% 3	2000
	11th to 20th	Night	29% 6% 10% 31% 20% 4	2035
	21st to end	Night	32% 6% 10% 27% 22% 3	2200
October	1st to 10th	Night	23% 4 6% 34% 31% 2	3872
	11th to 20th	Night	31% 4 8% 23% 31% 4	3899
	21st to end	Night	42% 4 9% 25% 18% 3	4496
November	1st to 10th	Night	35% 6% 11% 27% 18% 3	2666
	11th to 20th	Night	47% 5% 12% 22% 13% 1	2730
	21st to end	Night	50% 5% 11% 23% 11%	2909
December	1st to 10th	Night	44% 5% 14% 28% 9% 1	4720
	11th to 20th	Night	55% 5% 12% 20% 8%	5341
	21st to end	Night	71% 6% 8% 10% 4	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	20% 10% 36% 24% 9% 1	330
	11th to 20th	Day	20% 15% 40% 19% 5% 1	330
	21st to end	Day	20% 17% 37% 20% 5% 1	363
February	1st to 10th	Day	19% 19% 36% 21% 5%	330
	11th to 20th	Day	21% 18% 33% 22% 5%	330
	21st to end	Day	22% 16% 45% 13% 3	273
March	1st to 10th	Day	21% 20% 37% 17% 6%	330
	11th to 20th	Day	26% 16% 31% 19% 6% 1	330
	21st to end	Day	25% 17% 33% 19% 4 1	541
April	1st to 10th	Day	26% 15% 33% 21% 5% 1	540
	11th to 20th	Day	22% 20% 28% 20% 8% 2	532
	21st to end	Day	24% 13% 34% 20% 9% 1	540
May	1st to 10th	Day	20% 12% 32% 25% 8% 3	599
	11th to 20th	Day	20% 12% 30% 23% 12% 3	775
	21st to end	Day	15% 10% 28% 24% 16% 7%	844
June	1st to 10th	Day	9% 8% 29% 31% 17% 6%	780
	11th to 20th	Day	12% 6% 32% 30% 16% 4	780
	21st to end	Day	12% 8% 31% 28% 16% 4	780
July	1st to 10th	Day	9% 3 17% 30% 25% 15%	771
	11th to 20th	Day	11% 4 21% 31% 24% 10%	744
	21st to end	Day	8% 2 20% 36% 27% 7%	794
August	1st to 10th	Day	7% 4 20% 32% 28% 10%	726
	11th to 20th	Day	10% 5% 16% 29% 24% 15%	557
	21st to end	Day	8% 3 17% 34% 27% 10%	583
September	1st to 10th	Day	12% 7% 20% 28% 24% 8%	530
	11th to 20th	Day	10% 2 16% 34% 28% 10%	530
	21st to end	Day	11% 9% 28% 25% 20% 7%	530
October	1st to 10th	Day	15% 7% 27% 30% 17% 4	360
	11th to 20th	Day	20% 19% 35% 18% 8% 1	330
	21st to end	Day	29% 13% 30% 23% 4 1	363
November	1st to 10th	Day	17% 13% 36% 22% 8% 2	330
	11th to 20th	Day	25% 19% 31% 16% 8% 2	330
	21st to end	Day	26% 10% 34% 25% 5%	330
December	1st to 10th	Day	18% 14% 39% 20% 7% 2	330
	11th to 20th	Day	17% 12% 37% 26% 7%	330
	21st to end	Day	29% 15% 35% 17% 4	363
January	1st to 10th	Night	19% 11% 38% 24% 7%	1050
	11th to 20th	Night	18% 14% 37% 25% 5%	1050
	21st to end	Night	20% 12% 35% 29% 4	1155
February	1st to 10th	Night	21% 10% 38% 25% 5%	1050
	11th to 20th	Night	18% 11% 38% 28% 5%	880
	21st to end	Night	20% 13% 42% 22% 4	692
March	1st to 10th	Night	21% 11% 38% 23% 6%	840
	11th to 20th	Night	18% 10% 34% 31% 6%	702
	21st to end	Night	24% 12% 36% 22% 6% 1	693
April	1st to 10th	Night	23% 15% 35% 23% 4	630
	11th to 20th	Night	22% 13% 33% 25% 6%	623
	21st to end	Night	21% 10% 31% 31% 7% 1	630
May	1st to 10th	Night	16% 8% 37% 31% 7% 1	630
	11th to 20th	Night	19% 6% 32% 33% 11%	621
	21st to end	Night	14% 5% 30% 35% 14% 1	683
June	1st to 10th	Night	10% 5% 25% 46% 14%	630
	11th to 20th	Night	10% 5% 30% 41% 13% 1	630
	21st to end	Night	11% 4 30% 40% 15% 1	630
July	1st to 10th	Night	9% 2 12% 43% 29% 4	615
	11th to 20th	Night	8% 3 17% 43% 26% 3	597
	21st to end	Night	7% 1 17% 45% 29% 1	657
August	1st to 10th	Night	4 2 16% 45% 30% 4	597
	11th to 20th	Night	8% 2 12% 42% 31% 5%	594
	21st to end	Night	10% 2 14% 43% 28% 3	660
September	1st to 10th	Night	10% 3 19% 37% 28% 3	600
	11th to 20th	Night	10% 3 19% 37% 27% 4	635
	21st to end	Night	12% 8% 26% 34% 19% 2	800
October	1st to 10th	Night	13% 5% 31% 32% 18% 2	800
	11th to 20th	Night	20% 11% 31% 26% 11%	800
	21st to end	Night	25% 11% 31% 26% 7%	880
November	1st to 10th	Night	17% 7% 42% 25% 9% 1	820
	11th to 20th	Night	19% 14% 37% 26% 5%	840
	21st to end	Night	23% 11% 34% 26% 6%	1019
December	1st to 10th	Night	21% 12% 37% 25% 5%	1050
	11th to 20th	Night	22% 11% 34% 27% 6%	1050
	21st to end	Night	29% 11% 35% 23% 2	1155

Page generated at 2026-07-13 02:14:47. Version ac07

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