Sterling City
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Sterling City clear sky chart from 2005-11-06 to 2026-08-30.

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	6% 4 1 4 3 3 3 3 5% 7% 60%	152359
	Night	6% 4 1 4 3 3 3 3 5% 7% 62%	147343
Transparency	Day	25% 4 8% 22% 26% 16%	147452
	Night	23% 3 8% 22% 26% 18%	143261
Seeing	Day	8% 7% 25% 33% 21% 5%	17450
	Night	8% 7% 33% 38% 13% 1	27317

By Month

Percentage Cloud Cover SW (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	7% 4 2 4 2 3 2 2 5% 6% 63%	5287
	11th to 20th	Day	3 3 1 3 2 2 2 2 4 6% 70%	5606
	21st to end	Day	11% 6% 2 5% 3 3 3 3 4 6% 54%	6468
February	1st to 10th	Day	8% 5% 1 4 3 3 2 3 4 7% 60%	2100
	11th to 20th	Day	8% 5% 2 5% 4 3 3 3 4 6% 57%	2049
	21st to end	Day	7% 6% 2 3 2 2 3 2 4 4 64%	1730
March	1st to 10th	Day	5% 3 2 4 3 3 3 3 4 6% 64%	4600
	11th to 20th	Day	12% 4 1 4 2 2 3 3 4 6% 59%	5016
	21st to end	Day	3 5% 2 4 3 3 3 4 5% 6% 62%	5585
April	1st to 10th	Day	6% 4 1 3 3 3 3 3 5% 6% 61%	2513
	11th to 20th	Day	9% 5% 2 4 3 3 3 2 4 6% 57%	2491
	21st to end	Day	7% 5% 2 4 3 3 3 3 4 6% 62%	2520
May	1st to 10th	Day	11% 5% 2 5% 4 4 4 3 5% 6% 51%	7569
	11th to 20th	Day	9% 5% 2 6% 3 4 4 3 5% 6% 54%	7627
	21st to end	Day	9% 5% 2 6% 4 4 4 4 6% 7% 49%	8404
June	1st to 10th	Day	4 3 1 3 3 3 3 4 6% 8% 62%	2730
	11th to 20th	Day	4 3 1 4 3 3 4 4 5% 8% 60%	2730
	21st to end	Day	4 3 1 3 4 3 3 4 5% 8% 61%	2730
July	1st to 10th	Day	7% 3 1 4 4 4 4 4 7% 9% 53%	7679
	11th to 20th	Day	6% 4 2 5% 4 4 5% 4 6% 9% 52%	7680
	21st to end	Day	2 2 1 2 2 3 3 2 4 8% 72%	8110
August	1st to 10th	Day	1 1 2 2 3 3 2 4 6% 76%	5468
	11th to 20th	Day	2 1 1 3 3 4 5% 4 6% 11% 59%	5486
	21st to end	Day	3 3 1 4 4 4 5% 4 6% 9% 57%	6093
September	1st to 10th	Day	10% 5% 2 5% 3 4 4 3 5% 7% 52%	2306
	11th to 20th	Day	9% 5% 2 6% 5% 3 4 4 6% 8% 49%	2200
	21st to end	Day	8% 4 2 5% 4 4 3 3 5% 6% 57%	2200
October	1st to 10th	Day	3 2 1 2 1 2 2 2 3 3 78%	3590
	11th to 20th	Day	8% 4 2 4 2 2 2 2 3 4 67%	3568
	21st to end	Day	6% 3 1 2 2 1 2 2 3 4 75%	3493
November	1st to 10th	Day	7% 3 1 4 3 3 2 2 4 5% 66%	1853
	11th to 20th	Day	4 4 1 4 3 2 2 2 5% 6% 67%	1890
	21st to end	Day	9% 4 2 4 3 3 3 2 5% 6% 59%	1890
December	1st to 10th	Day	13% 7% 2 5% 3 2 3 2 4 5% 54%	3030
	11th to 20th	Day	5% 5% 2 5% 3 4 4 3 4 6% 60%	3481
	21st to end	Day	6% 4 2 4 2 2 2 1 2 4 70%	4587
January	1st to 10th	Night	5% 5% 2 5% 3 3 3 2 4 6% 63%	7634
	11th to 20th	Night	4 3 1 3 2 2 3 2 4 5% 72%	7440
	21st to end	Night	11% 7% 2 5% 4 3 3 3 4 5% 52%	7746
February	1st to 10th	Night	8% 5% 2 4 2 3 2 3 4 6% 61%	2520
	11th to 20th	Night	9% 5% 2 5% 4 3 4 3 4 5% 56%	2458
	21st to end	Night	7% 5% 1 4 3 2 3 2 4 6% 63%	2076
March	1st to 10th	Night	5% 4 1 4 2 2 3 3 4 6% 64%	5494
	11th to 20th	Night	10% 4 2 3 2 2 3 2 5% 6% 60%	5166
	21st to end	Night	4 4 1 4 3 3 3 3 5% 7% 62%	5588
April	1st to 10th	Night	6% 4 2 4 3 3 4 3 5% 7% 60%	2101
	11th to 20th	Night	8% 4 1 4 3 3 4 3 4 7% 60%	2077
	21st to end	Night	8% 4 1 3 3 2 2 2 4 5% 64%	2100
May	1st to 10th	Night	10% 5% 1 4 3 4 4 4 5% 7% 54%	5376
	11th to 20th	Night	4 3 1 4 3 4 4 3 5% 7% 63%	5265
	21st to end	Night	7% 5% 2 4 3 4 5% 4 7% 9% 50%	5803
June	1st to 10th	Night	2 2 1 3 3 3 4 3 7% 10% 62%	1890
	11th to 20th	Night	3 2 1 3 3 3 4 4 6% 10% 62%	1890
	21st to end	Night	2 2 1 3 2 3 4 4 6% 8% 66%	1890
July	1st to 10th	Night	5% 4 2 4 3 3 4 4 7% 12% 52%	5310
	11th to 20th	Night	4 3 1 4 3 3 3 3 5% 10% 62%	5310
	21st to end	Night	2 1 1 2 2 2 3 3 5% 7% 73%	5980
August	1st to 10th	Night	1 1 2 2 2 3 2 5% 7% 74%	4564
	11th to 20th	Night	2 2 1 3 2 3 3 4 6% 12% 62%	4583
	21st to end	Night	3 2 1 4 2 3 4 3 6% 10% 62%	5083
September	1st to 10th	Night	7% 4 2 4 4 4 4 2 5% 8% 57%	2092
	11th to 20th	Night	8% 4 2 4 3 3 3 3 6% 9% 56%	2200
	21st to end	Night	7% 4 2 4 3 3 3 3 5% 7% 59%	2200
October	1st to 10th	Night	2 2 1 2 2 2 1 2 3 4 79%	3590
	11th to 20th	Night	6% 4 1 3 2 2 2 1 3 4 70%	3570
	21st to end	Night	5% 2 1 2 1 1 2 2 3 5% 77%	4263
November	1st to 10th	Night	8% 3 1 3 2 3 2 2 4 6% 67%	2665
	11th to 20th	Night	4 2 1 3 2 3 4 3 5% 6% 67%	2730
	21st to end	Night	10% 5% 2 4 3 3 3 2 5% 6% 57%	2730
December	1st to 10th	Night	11% 8% 2 5% 2 2 3 2 4 5% 55%	4380
	11th to 20th	Night	6% 4 2 4 3 3 4 3 4 5% 62%	4946
	21st to end	Night	5% 5% 2 5% 3 3 3 2 4 5% 63%	6633

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	23% 2 5% 21% 34% 15%	5290
	11th to 20th	Day	17% 2 6% 23% 35% 18%	5606
	21st to end	Day	32% 3 4 17% 27% 17%	6468
February	1st to 10th	Day	27% 3 5% 15% 28% 23%	2100
	11th to 20th	Day	30% 2 5% 22% 24% 17%	2059
	21st to end	Day	25% 2 3 13% 33% 23%	1730
March	1st to 10th	Day	22% 2 4 13% 24% 35%	4600
	11th to 20th	Day	29% 1 7% 9% 21% 33%	5016
	21st to end	Day	23% 3 6% 15% 28% 26%	5585
April	1st to 10th	Day	24% 3 7% 18% 23% 25%	2513
	11th to 20th	Day	30% 3 7% 15% 24% 21%	2494
	21st to end	Day	25% 2 7% 21% 26% 19%	2520
May	1st to 10th	Day	34% 3 8% 15% 20% 19%	7569
	11th to 20th	Day	31% 3 5% 13% 29% 18%	7631
	21st to end	Day	32% 5% 11% 24% 14% 13%	8405
June	1st to 10th	Day	19% 5% 13% 31% 25% 7%	2730
	11th to 20th	Day	22% 5% 9% 24% 29% 11%	2730
	21st to end	Day	22% 5% 12% 26% 25% 11%	2730
July	1st to 10th	Day	26% 6% 13% 31% 20% 3	7616
	11th to 20th	Day	26% 7% 16% 31% 16% 3	7611
	21st to end	Day	11% 4 11% 34% 35% 5%	7929
August	1st to 10th	Day	11% 4 10% 31% 37% 7%	4012
	11th to 20th	Day	18% 5% 13% 35% 27% 3	4002
	21st to end	Day	23% 8% 15% 25% 25% 4	4420
September	1st to 10th	Day	32% 4 9% 22% 23% 10%	2306
	11th to 20th	Day	32% 5% 11% 22% 22% 7%	2200
	21st to end	Day	29% 4 7% 20% 26% 14%	2200
October	1st to 10th	Day	13% 3 6% 13% 28% 36%	3590
	11th to 20th	Day	24% 2 5% 18% 22% 29%	3568
	21st to end	Day	16% 2 3 23% 41% 15%	3493
November	1st to 10th	Day	23% 2 6% 22% 26% 21%	1853
	11th to 20th	Day	21% 2 8% 21% 29% 20%	1890
	21st to end	Day	27% 2 6% 18% 30% 17%	1890
December	1st to 10th	Day	34% 3 5% 18% 27% 13%	3030
	11th to 20th	Day	27% 2 7% 15% 29% 20%	3479
	21st to end	Day	21% 2 2 20% 34% 20%	4587
January	1st to 10th	Night	24% 2 4 22% 33% 15%	7635
	11th to 20th	Night	16% 2 5% 20% 34% 24%	7440
	21st to end	Night	35% 3 5% 15% 25% 17%	7746
February	1st to 10th	Night	26% 2 6% 15% 28% 24%	2520
	11th to 20th	Night	31% 3 6% 19% 21% 20%	2458
	21st to end	Night	24% 2 4 14% 28% 28%	2076
March	1st to 10th	Night	21% 3 3 13% 29% 31%	5494
	11th to 20th	Night	26% 2 7% 11% 17% 36%	5166
	21st to end	Night	22% 3 7% 17% 28% 23%	5588
April	1st to 10th	Night	24% 4 8% 21% 19% 24%	2101
	11th to 20th	Night	27% 3 8% 18% 23% 21%	2077
	21st to end	Night	23% 3 10% 21% 27% 16%	2100
May	1st to 10th	Night	30% 4 11% 21% 16% 17%	5376
	11th to 20th	Night	21% 4 9% 18% 31% 18%	5266
	21st to end	Night	28% 6% 15% 21% 14% 16%	5805
June	1st to 10th	Night	18% 4 18% 32% 22% 6%	1890
	11th to 20th	Night	18% 4 13% 26% 29% 10%	1890
	21st to end	Night	16% 5% 13% 30% 25% 11%	1890
July	1st to 10th	Night	24% 6% 15% 34% 18% 3	5265
	11th to 20th	Night	19% 5% 17% 34% 21% 4	5256
	21st to end	Night	11% 4 11% 37% 32% 5%	5848
August	1st to 10th	Night	12% 3 9% 29% 40% 7%	3344
	11th to 20th	Night	14% 3 15% 38% 25% 4	3343
	21st to end	Night	17% 6% 19% 29% 25% 5%	3687
September	1st to 10th	Night	27% 4 11% 25% 21% 12%	2092
	11th to 20th	Night	26% 4 15% 23% 24% 8%	2200
	21st to end	Night	25% 4 7% 23% 24% 17%	2200
October	1st to 10th	Night	12% 2 8% 14% 27% 38%	3590
	11th to 20th	Night	21% 2 6% 18% 23% 30%	3570
	21st to end	Night	13% 2 4 24% 37% 19%	4263
November	1st to 10th	Night	22% 2 7% 20% 26% 22%	2665
	11th to 20th	Night	19% 3 7% 19% 29% 22%	2730
	21st to end	Night	29% 2 5% 16% 28% 20%	2730
December	1st to 10th	Night	33% 2 5% 19% 26% 16%	4380
	11th to 20th	Night	25% 3 5% 17% 30% 20%	4947
	21st to end	Night	24% 3 4 22% 27% 20%	6633

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	9% 14% 41% 30% 5% 1	330
	11th to 20th	Day	11% 14% 34% 29% 11% 2	330
	21st to end	Day	12% 11% 39% 28% 9% 1	363
February	1st to 10th	Day	14% 12% 35% 27% 10% 1	330
	11th to 20th	Day	12% 16% 43% 22% 6% 1	330
	21st to end	Day	14% 14% 45% 22% 5% 1	273
March	1st to 10th	Day	14% 14% 47% 21% 5%	330
	11th to 20th	Day	18% 12% 35% 26% 8% 1	330
	21st to end	Day	9% 14% 41% 28% 7% 1	363
April	1st to 10th	Day	8% 18% 44% 25% 5%	533
	11th to 20th	Day	12% 20% 47% 19% 3	532
	21st to end	Day	9% 15% 41% 29% 6% 1	540
May	1st to 10th	Day	9% 11% 41% 30% 6% 1	719
	11th to 20th	Day	11% 9% 33% 32% 13% 2	775
	21st to end	Day	9% 6% 35% 37% 12% 1	844
June	1st to 10th	Day	6% 3 21% 49% 20% 2	780
	11th to 20th	Day	4 3 24% 47% 19% 3	780
	21st to end	Day	4 2 14% 41% 33% 5%	780
July	1st to 10th	Day	6% 1 6% 42% 36% 9%	771
	11th to 20th	Day	2 1 6% 41% 43% 7%	768
	21st to end	Day	2 1 4 36% 42% 15%	773
August	1st to 10th	Day	2 1 4 33% 49% 12%	567
	11th to 20th	Day	4 1 4 38% 42% 12%	557
	21st to end	Day	3 2 5% 32% 46% 13%	620
September	1st to 10th	Day	8% 2 8% 29% 37% 15%	436
	11th to 20th	Day	6% 8% 37% 34% 15%	330
	21st to end	Day	10% 3 10% 34% 29% 13%	330
October	1st to 10th	Day	4 3 16% 37% 32% 10%	330
	11th to 20th	Day	8% 3 29% 35% 22% 2	330
	21st to end	Day	14% 9% 25% 31% 18% 3	363
November	1st to 10th	Day	11% 8% 28% 32% 19% 2	330
	11th to 20th	Day	9% 13% 27% 32% 13% 5%	330
	21st to end	Day	11% 15% 36% 21% 13% 4	330
December	1st to 10th	Day	15% 9% 34% 29% 11% 2	330
	11th to 20th	Day	6% 15% 34% 31% 12% 1	330
	21st to end	Day	10% 13% 38% 31% 8% 1	363
January	1st to 10th	Night	9% 10% 39% 34% 8%	1050
	11th to 20th	Night	10% 11% 35% 35% 8% 1	966
	21st to end	Night	11% 11% 38% 33% 7%	924
February	1st to 10th	Night	11% 9% 43% 30% 6%	840
	11th to 20th	Night	12% 12% 47% 25% 4	824
	21st to end	Night	10% 14% 49% 25% 3	692
March	1st to 10th	Night	12% 12% 46% 26% 4	840
	11th to 20th	Night	12% 13% 43% 28% 4	667
	21st to end	Night	8% 13% 50% 27% 3	693
April	1st to 10th	Night	7% 13% 51% 25% 3	630
	11th to 20th	Night	10% 13% 54% 21% 2	623
	21st to end	Night	8% 12% 51% 24% 4	630
May	1st to 10th	Night	6% 9% 49% 31% 4	630
	11th to 20th	Night	6% 7% 44% 33% 9%	621
	21st to end	Night	6% 5% 46% 35% 7% 1	683
June	1st to 10th	Night	2 3 35% 49% 10%	630
	11th to 20th	Night	3 2 40% 47% 8%	630
	21st to end	Night	2 1 24% 53% 19%	630
July	1st to 10th	Night	6% 1 10% 55% 26% 2	615
	11th to 20th	Night	3 2 8% 60% 26% 1	612
	21st to end	Night	1 1 6% 60% 31% 1	690
August	1st to 10th	Night	1 5% 55% 37% 2	627
	11th to 20th	Night	4 1 7% 49% 37% 2	624
	21st to end	Night	3 2 7% 43% 41% 3	687
September	1st to 10th	Night	8% 1 7% 46% 36% 4	600
	11th to 20th	Night	10% 1 9% 48% 31% 1	600
	21st to end	Night	9% 1 15% 48% 26% 1	600
October	1st to 10th	Night	3 3 25% 46% 22% 1	600
	11th to 20th	Night	9% 4 28% 44% 15% 1	600
	21st to end	Night	9% 6% 30% 39% 13% 2	879
November	1st to 10th	Night	10% 6% 30% 40% 14% 1	1025
	11th to 20th	Night	6% 8% 33% 40% 13% 1	1050
	21st to end	Night	12% 7% 38% 31% 11% 1	1050
December	1st to 10th	Night	11% 8% 35% 37% 8% 1	1050
	11th to 20th	Night	11% 11% 36% 34% 8%	1050
	21st to end	Night	9% 9% 39% 35% 8%	1155

Page generated at 2026-08-27 03:07:11. Version ac07

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