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 2025-04-26.

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	6% 4 1 4 3 3 3 3 4 6% 62%	118165
	Night	6% 4 1 4 3 3 3 3 5% 7% 63%	119269
Transparency	Day	24% 3 8% 21% 27% 18%	117933
	Night	22% 3 8% 21% 26% 19%	119091
Seeing	Day	8% 7% 25% 33% 21% 5%	15520
	Night	8% 7% 33% 37% 14% 1	25625

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	9% 5% 2 5% 2 2 2 2 5% 6% 59%	4077
	11th to 20th	Day	4 4 1 3 2 2 2 2 4 6% 71%	4306
	21st to end	Day	11% 6% 2 4 3 3 3 3 4 5% 56%	4994
February	1st to 10th	Day	8% 5% 1 4 3 3 2 3 4 6% 60%	2000
	11th to 20th	Day	9% 5% 2 5% 4 3 3 3 4 6% 57%	1949
	21st to end	Day	8% 7% 2 4 2 2 3 3 4 4 62%	1650
March	1st to 10th	Day	5% 3 2 4 3 3 3 3 4 6% 65%	4500
	11th to 20th	Day	12% 4 1 4 2 2 3 3 4 6% 59%	4908
	21st to end	Day	3 5% 2 4 3 3 3 4 5% 6% 62%	5464
April	1st to 10th	Day	6% 4 1 3 3 3 3 3 5% 6% 62%	2393
	11th to 20th	Day	9% 5% 2 4 3 3 3 2 4 6% 59%	2371
	21st to end	Day	6% 5% 2 4 3 2 3 2 4 6% 64%	2341
May	1st to 10th	Day	9% 5% 2 5% 4 4 4 3 4 5% 56%	4076
	11th to 20th	Day	11% 6% 2 5% 3 3 4 2 4 6% 55%	4082
	21st to end	Day	7% 4 2 5% 3 3 3 3 5% 6% 57%	4489
June	1st to 10th	Day	3 2 1 3 3 3 4 3 6% 8% 63%	2470
	11th to 20th	Day	4 3 1 4 3 3 4 4 5% 8% 61%	2470
	21st to end	Day	5% 4 1 3 4 3 3 4 5% 8% 61%	2470
July	1st to 10th	Day	5% 2 1 4 4 4 4 4 6% 7% 59%	4120
	11th to 20th	Day	2 1 1 2 3 3 4 3 5% 9% 66%	4120
	21st to end	Day	4 3 1 3 3 4 4 3 5% 8% 62%	4349
August	1st to 10th	Day	2 1 2 2 3 3 3 4 7% 73%	3784
	11th to 20th	Day	3 2 1 3 3 4 4 3 4 8% 65%	3726
	21st to end	Day	3 3 2 5% 4 4 5% 4 6% 9% 55%	4180
September	1st to 10th	Day	10% 4 2 5% 3 4 4 3 5% 7% 52%	2191
	11th to 20th	Day	10% 5% 2 5% 5% 3 4 4 6% 7% 49%	2090
	21st to end	Day	8% 4 2 5% 4 4 3 3 5% 6% 57%	2090
October	1st to 10th	Day	3 2 1 2 1 2 2 2 3 3 78%	3480
	11th to 20th	Day	8% 5% 2 4 2 2 2 2 3 4 67%	3458
	21st to end	Day	6% 3 1 2 2 1 2 2 3 4 75%	3385
November	1st to 10th	Day	7% 3 1 4 3 3 2 2 4 6% 64%	1763
	11th to 20th	Day	4 4 1 4 3 2 2 2 5% 6% 67%	1800
	21st to end	Day	8% 4 2 4 3 3 3 2 5% 6% 60%	1800
December	1st to 10th	Day	13% 8% 2 5% 3 2 3 2 4 5% 53%	2940
	11th to 20th	Day	5% 5% 2 5% 3 4 4 3 4 6% 60%	3391
	21st to end	Day	6% 4 2 4 2 2 2 1 2 4 70%	4488
January	1st to 10th	Night	6% 6% 2 5% 3 3 3 2 4 6% 61%	5888
	11th to 20th	Night	5% 3 1 2 1 2 2 2 4 5% 71%	5716
	21st to end	Night	11% 7% 2 5% 3 3 3 2 4 5% 55%	5984
February	1st to 10th	Night	8% 5% 2 4 2 3 2 2 4 6% 61%	2400
	11th to 20th	Night	9% 5% 2 5% 4 3 3 3 4 5% 56%	2338
	21st to end	Night	7% 5% 2 4 3 2 3 3 4 7% 61%	1980
March	1st to 10th	Night	5% 4 1 4 2 2 3 3 4 6% 64%	5374
	11th to 20th	Night	10% 4 2 3 2 2 3 2 5% 6% 59%	5054
	21st to end	Night	4 4 1 4 3 3 4 3 5% 7% 61%	5467
April	1st to 10th	Night	6% 4 2 4 3 2 3 3 5% 7% 61%	2001
	11th to 20th	Night	8% 4 1 4 3 3 4 3 4 6% 61%	1977
	21st to end	Night	7% 4 1 3 3 2 2 3 4 5% 66%	1955
May	1st to 10th	Night	8% 6% 1 3 3 3 4 3 4 6% 58%	2892
	11th to 20th	Night	6% 4 1 4 3 3 4 3 4 7% 62%	2823
	21st to end	Night	4 3 1 3 3 3 4 4 6% 9% 60%	3098
June	1st to 10th	Night	2 2 1 3 3 3 4 3 7% 10% 63%	1710
	11th to 20th	Night	3 2 1 2 3 3 4 4 6% 10% 64%	1710
	21st to end	Night	3 2 1 3 2 3 4 4 6% 7% 67%	1710
July	1st to 10th	Night	3 2 1 3 2 3 4 4 6% 10% 61%	2850
	11th to 20th	Night	1 1 2 2 2 2 3 4 11% 71%	2850
	21st to end	Night	3 2 1 3 3 3 3 3 6% 9% 64%	3209
August	1st to 10th	Night	1 2 1 3 2 2 3 2 5% 7% 73%	3155
	11th to 20th	Night	3 2 1 2 2 3 3 4 5% 9% 67%	3113
	21st to end	Night	3 2 1 3 2 3 4 4 6% 10% 64%	3487
September	1st to 10th	Night	7% 4 2 4 4 3 4 3 5% 8% 57%	1987
	11th to 20th	Night	9% 4 1 4 2 3 3 3 6% 8% 55%	2090
	21st to end	Night	7% 4 2 4 3 3 3 3 5% 7% 59%	2090
October	1st to 10th	Night	2 2 1 2 2 2 1 2 3 4 79%	3480
	11th to 20th	Night	6% 4 1 3 2 2 2 2 3 4 70%	3460
	21st to end	Night	5% 2 1 2 1 1 2 2 3 5% 77%	4130
November	1st to 10th	Night	8% 3 1 3 3 3 2 2 4 6% 65%	2535
	11th to 20th	Night	4 3 1 3 2 3 4 3 5% 6% 66%	2600
	21st to end	Night	9% 5% 2 5% 3 3 3 3 5% 6% 57%	2600
December	1st to 10th	Night	11% 8% 2 5% 2 2 3 3 4 5% 55%	4250
	11th to 20th	Night	6% 4 2 4 3 3 4 3 4 5% 62%	4816
	21st to end	Night	5% 5% 2 5% 3 3 3 2 4 5% 63%	6490

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	27% 3 5% 19% 31% 15%	4078
	11th to 20th	Day	17% 1 6% 22% 37% 17%	4306
	21st to end	Day	31% 3 5% 14% 27% 20%	4994
February	1st to 10th	Day	26% 3 5% 15% 27% 24%	2000
	11th to 20th	Day	30% 2 5% 22% 22% 18%	1959
	21st to end	Day	27% 2 3 12% 32% 24%	1650
March	1st to 10th	Day	22% 2 4 12% 24% 35%	4500
	11th to 20th	Day	29% 1 7% 9% 21% 33%	4908
	21st to end	Day	23% 3 6% 14% 28% 26%	5464
April	1st to 10th	Day	23% 3 7% 17% 24% 26%	2393
	11th to 20th	Day	29% 2 6% 15% 25% 22%	2374
	21st to end	Day	24% 2 7% 21% 27% 20%	2341
May	1st to 10th	Day	31% 2 7% 13% 16% 30%	4076
	11th to 20th	Day	32% 3 7% 17% 25% 16%	4086
	21st to end	Day	28% 3 10% 20% 17% 22%	4489
June	1st to 10th	Day	18% 5% 13% 32% 25% 7%	2470
	11th to 20th	Day	21% 4 9% 25% 28% 12%	2470
	21st to end	Day	23% 5% 10% 25% 25% 12%	2470
July	1st to 10th	Day	23% 6% 14% 36% 17% 4	4056
	11th to 20th	Day	14% 5% 14% 36% 25% 5%	4051
	21st to end	Day	18% 4 11% 32% 29% 5%	4211
August	1st to 10th	Day	12% 4 10% 31% 36% 7%	3772
	11th to 20th	Day	18% 5% 12% 34% 28% 3	3762
	21st to end	Day	24% 8% 16% 24% 25% 4	4179
September	1st to 10th	Day	32% 4 9% 21% 22% 10%	2191
	11th to 20th	Day	33% 5% 11% 21% 22% 7%	2090
	21st to end	Day	29% 3 7% 20% 26% 15%	2090
October	1st to 10th	Day	13% 3 6% 13% 28% 37%	3480
	11th to 20th	Day	24% 2 5% 17% 22% 29%	3458
	21st to end	Day	15% 2 3 24% 41% 15%	3385
November	1st to 10th	Day	24% 2 6% 23% 25% 20%	1763
	11th to 20th	Day	21% 2 9% 21% 29% 20%	1800
	21st to end	Day	27% 2 6% 18% 29% 17%	1800
December	1st to 10th	Day	34% 3 5% 18% 27% 13%	2940
	11th to 20th	Day	27% 2 7% 15% 29% 20%	3389
	21st to end	Day	21% 2 2 20% 34% 20%	4488
January	1st to 10th	Night	26% 3 4 20% 32% 16%	5888
	11th to 20th	Night	16% 2 5% 21% 32% 23%	5716
	21st to end	Night	32% 2 5% 17% 25% 19%	5984
February	1st to 10th	Night	27% 2 6% 15% 27% 24%	2400
	11th to 20th	Night	31% 3 6% 19% 20% 21%	2338
	21st to end	Night	25% 2 5% 14% 27% 28%	1980
March	1st to 10th	Night	21% 3 3 13% 29% 32%	5374
	11th to 20th	Night	27% 2 7% 11% 17% 36%	5054
	21st to end	Night	22% 3 7% 16% 28% 23%	5467
April	1st to 10th	Night	23% 4 8% 21% 19% 25%	2001
	11th to 20th	Night	26% 3 7% 18% 23% 22%	1977
	21st to end	Night	22% 2 10% 21% 28% 17%	1955
May	1st to 10th	Night	28% 4 9% 16% 18% 25%	2892
	11th to 20th	Night	23% 4 12% 22% 20% 19%	2824
	21st to end	Night	21% 4 13% 22% 19% 22%	3099
June	1st to 10th	Night	16% 4 18% 33% 22% 6%	1710
	11th to 20th	Night	17% 4 12% 26% 30% 10%	1710
	21st to end	Night	16% 5% 12% 30% 25% 12%	1710
July	1st to 10th	Night	18% 5% 17% 39% 18% 4	2805
	11th to 20th	Night	10% 3 12% 41% 27% 7%	2796
	21st to end	Night	16% 4 11% 34% 30% 4	3108
August	1st to 10th	Night	12% 3 9% 28% 40% 7%	3144
	11th to 20th	Night	15% 3 15% 38% 25% 4	3143
	21st to end	Night	16% 6% 19% 28% 24% 6%	3487
September	1st to 10th	Night	26% 4 11% 25% 21% 13%	1987
	11th to 20th	Night	27% 4 15% 22% 24% 8%	2090
	21st to end	Night	26% 4 7% 23% 23% 18%	2090
October	1st to 10th	Night	12% 2 7% 14% 26% 39%	3480
	11th to 20th	Night	21% 2 6% 17% 23% 31%	3460
	21st to end	Night	14% 2 4 25% 37% 18%	4130
November	1st to 10th	Night	23% 2 8% 21% 25% 22%	2535
	11th to 20th	Night	19% 4 7% 20% 28% 22%	2600
	21st to end	Night	29% 2 5% 15% 28% 20%	2600
December	1st to 10th	Night	33% 2 5% 19% 26% 15%	4250
	11th to 20th	Night	25% 3 6% 18% 29% 20%	4817
	21st to end	Night	24% 3 4 22% 27% 20%	6490

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% 42% 29% 5% 1	300
	11th to 20th	Day	11% 12% 35% 30% 9% 2	300
	21st to end	Day	11% 11% 39% 28% 8% 2	330
February	1st to 10th	Day	15% 13% 36% 26% 9% 1	300
	11th to 20th	Day	14% 16% 43% 21% 6% 1	300
	21st to end	Day	15% 14% 45% 20% 4 1	249
March	1st to 10th	Day	14% 13% 47% 21% 5%	300
	11th to 20th	Day	19% 12% 35% 25% 8% 1	300
	21st to end	Day	10% 15% 43% 25% 6% 1	330
April	1st to 10th	Day	8% 18% 45% 25% 5%	493
	11th to 20th	Day	11% 20% 46% 20% 3	492
	21st to end	Day	9% 14% 39% 31% 6% 1	481
May	1st to 10th	Day	9% 11% 41% 31% 7% 1	622
	11th to 20th	Day	11% 7% 32% 34% 13% 2	675
	21st to end	Day	10% 6% 36% 37% 11% 1	734
June	1st to 10th	Day	5% 3 19% 50% 21% 2	680
	11th to 20th	Day	3 2 24% 48% 20% 3	680
	21st to end	Day	4 1 13% 40% 35% 6%	680
July	1st to 10th	Day	6% 1 5% 42% 37% 9%	671
	11th to 20th	Day	2 1 6% 40% 44% 8%	668
	21st to end	Day	2 1 4 36% 43% 15%	671
August	1st to 10th	Day	2 1 3 32% 49% 13%	487
	11th to 20th	Day	4 1 4 37% 42% 12%	477
	21st to end	Day	3 2 5% 32% 46% 12%	539
September	1st to 10th	Day	8% 2 8% 28% 38% 15%	401
	11th to 20th	Day	7% 7% 34% 35% 17%	300
	21st to end	Day	11% 3 11% 31% 30% 14%	300
October	1st to 10th	Day	4 3 16% 36% 32% 9%	300
	11th to 20th	Day	9% 3 29% 34% 22% 2	300
	21st to end	Day	14% 8% 25% 32% 18% 3	330
November	1st to 10th	Day	12% 9% 30% 31% 16% 2	300
	11th to 20th	Day	9% 14% 30% 30% 12% 5%	300
	21st to end	Day	10% 16% 37% 21% 11% 4	300
December	1st to 10th	Day	16% 8% 34% 27% 12% 2	300
	11th to 20th	Day	6% 15% 35% 30% 12% 1	300
	21st to end	Day	10% 14% 40% 29% 7%	330
January	1st to 10th	Night	9% 10% 39% 34% 8%	1000
	11th to 20th	Night	11% 11% 35% 34% 8% 1	920
	21st to end	Night	11% 10% 38% 33% 7%	880
February	1st to 10th	Night	11% 10% 44% 29% 6%	800
	11th to 20th	Night	13% 12% 47% 25% 3	784
	21st to end	Night	10% 14% 49% 24% 3	660
March	1st to 10th	Night	12% 12% 46% 26% 4	800
	11th to 20th	Night	12% 13% 43% 28% 4	635
	21st to end	Night	8% 13% 51% 26% 3	660
April	1st to 10th	Night	7% 13% 52% 25% 3	600
	11th to 20th	Night	9% 13% 55% 21% 2	593
	21st to end	Night	8% 12% 51% 25% 4	587
May	1st to 10th	Night	6% 9% 51% 31% 4	570
	11th to 20th	Night	7% 6% 43% 34% 9% 1	561
	21st to end	Night	6% 4 47% 35% 8% 1	617
June	1st to 10th	Night	2 2 33% 51% 12%	570
	11th to 20th	Night	3 2 41% 46% 8%	570
	21st to end	Night	2 1 23% 53% 21% 1	570
July	1st to 10th	Night	5% 1 8% 55% 28% 2	555
	11th to 20th	Night	3 1 8% 60% 27% 1	552
	21st to end	Night	1 6% 61% 31% 1	624
August	1st to 10th	Night	1 1 5% 54% 38% 2	567
	11th to 20th	Night	4 1 6% 48% 38% 2	564
	21st to end	Night	3 2 8% 42% 42% 4	627
September	1st to 10th	Night	8% 1 7% 45% 36% 4	570
	11th to 20th	Night	10% 1 9% 46% 32% 1	570
	21st to end	Night	9% 1 15% 47% 27% 1	570
October	1st to 10th	Night	3 3 26% 45% 22% 1	570
	11th to 20th	Night	9% 4 27% 44% 15% 1	570
	21st to end	Night	10% 6% 30% 38% 13% 2	834
November	1st to 10th	Night	10% 6% 30% 40% 14% 1	975
	11th to 20th	Night	6% 8% 34% 39% 13% 1	1000
	21st to end	Night	12% 7% 38% 31% 11% 1	1000
December	1st to 10th	Night	11% 8% 34% 37% 9% 1	1000
	11th to 20th	Night	11% 11% 36% 34% 7%	1000
	21st to end	Night	9% 9% 39% 34% 8%	1100

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