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 2024-02-10.

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	14% 8% 4 7% 6% 3 5% 5% 5% 7% 37%	78238
	Night	14% 9% 4 7% 5% 3 4 4 5% 6% 38%	80773
Transparency	Day	46% 7% 13% 21% 12% 1	77939
	Night	46% 6% 12% 21% 13% 2	80593
Seeing	Day	16% 9% 28% 26% 16% 5%	14733
	Night	17% 8% 31% 31% 12% 1	24774

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	11% 12% 9% 14% 7% 4 4 5% 4 5% 24%	2527
	11th to 20th	Day	13% 11% 5% 12% 9% 4 6% 8% 8% 5% 19%	2530
	21st to end	Day	17% 19% 9% 11% 8% 3 4 4 4 5% 16%	3021
February	1st to 10th	Day	13% 12% 6% 8% 7% 3 6% 5% 4 6% 29%	1701
	11th to 20th	Day	14% 10% 3 7% 6% 5% 5% 4 5% 6% 35%	1699
	21st to end	Day	14% 12% 6% 10% 5% 3 5% 6% 4 7% 29%	1480
March	1st to 10th	Day	15% 11% 6% 5% 6% 2 5% 4 5% 7% 34%	1800
	11th to 20th	Day	14% 9% 3 6% 6% 3 4 5% 5% 7% 37%	1934
	21st to end	Day	21% 11% 4 8% 6% 4 5% 4 5% 7% 26%	2330
April	1st to 10th	Day	17% 11% 4 8% 5% 3 4 5% 5% 7% 33%	2160
	11th to 20th	Day	18% 9% 3 5% 6% 4 4 5% 5% 6% 35%	2134
	21st to end	Day	22% 8% 3 6% 5% 3 4 4 5% 6% 34%	2173
May	1st to 10th	Day	17% 8% 4 7% 7% 3 5% 4 6% 7% 32%	2387
	11th to 20th	Day	18% 8% 3 6% 6% 3 5% 4 5% 8% 33%	2479
	21st to end	Day	14% 7% 3 6% 7% 3 5% 5% 6% 8% 35%	2725
June	1st to 10th	Day	9% 4 2 5% 5% 3 5% 6% 7% 9% 44%	2520
	11th to 20th	Day	10% 4 3 4 5% 3 4 5% 5% 8% 47%	2520
	21st to end	Day	11% 5% 3 6% 5% 3 6% 6% 7% 9% 39%	2520
July	1st to 10th	Day	8% 4 2 5% 5% 3 4 6% 6% 9% 47%	2520
	11th to 20th	Day	9% 4 2 3 5% 3 4 4 6% 10% 49%	2520
	21st to end	Day	6% 5% 2 5% 5% 4 6% 5% 7% 10% 45%	2743
August	1st to 10th	Day	7% 4 2 4 6% 3 5% 6% 9% 11% 43%	2404
	11th to 20th	Day	9% 5% 2 6% 5% 3 4 5% 6% 8% 48%	2159
	21st to end	Day	9% 5% 2 4 5% 3 4 4 6% 8% 50%	2376
September	1st to 10th	Day	13% 5% 2 4 4 3 4 5% 5% 9% 45%	2160
	11th to 20th	Day	13% 4 1 3 3 3 3 4 5% 6% 56%	2111
	21st to end	Day	10% 5% 2 5% 5% 3 4 4 6% 8% 47%	1980
October	1st to 10th	Day	14% 6% 2 5% 5% 2 4 4 4 7% 47%	1828
	11th to 20th	Day	19% 8% 4 6% 5% 3 4 3 5% 7% 38%	1800
	21st to end	Day	20% 10% 2 5% 4 2 3 3 4 6% 40%	1850
November	1st to 10th	Day	12% 8% 2 4 4 3 4 4 6% 8% 46%	1672
	11th to 20th	Day	17% 10% 5% 7% 6% 3 4 4 5% 7% 32%	1710
	21st to end	Day	16% 11% 4 8% 5% 3 4 4 4 5% 36%	1544
December	1st to 10th	Day	15% 13% 4 8% 7% 3 4 4 4 7% 32%	1520
	11th to 20th	Day	14% 13% 6% 8% 6% 4 5% 5% 4 5% 30%	1918
	21st to end	Day	24% 18% 9% 12% 7% 3 4 3 2 4 15%	2783
January	1st to 10th	Night	13% 12% 7% 12% 7% 3 4 4 4 5% 30%	4428
	11th to 20th	Night	13% 16% 7% 11% 7% 3 4 5% 7% 5% 21%	4440
	21st to end	Night	19% 19% 8% 8% 7% 3 4 4 4 4 22%	4631
February	1st to 10th	Night	14% 12% 6% 7% 7% 3 4 4 4 5% 34%	2464
	11th to 20th	Night	13% 9% 4 7% 5% 4 5% 4 4 6% 39%	2147
	21st to end	Night	17% 11% 4 7% 5% 2 3 4 4 5% 37%	1776
March	1st to 10th	Night	17% 10% 3 5% 6% 3 4 4 5% 6% 37%	2160
	11th to 20th	Night	13% 8% 4 5% 5% 3 5% 4 6% 7% 40%	2043
	21st to end	Night	21% 9% 5% 6% 6% 3 5% 4 4 6% 31%	2025
April	1st to 10th	Night	17% 8% 5% 5% 5% 2 5% 3 5% 5% 39%	1800
	11th to 20th	Night	20% 7% 3 5% 5% 3 4 4 5% 5% 40%	1775
	21st to end	Night	17% 8% 3 7% 6% 3 4 5% 6% 7% 34%	1787
May	1st to 10th	Night	15% 7% 3 6% 5% 4 6% 5% 5% 7% 37%	1569
	11th to 20th	Night	18% 6% 2 5% 5% 3 4 5% 6% 7% 39%	1416
	21st to end	Night	13% 6% 2 5% 5% 3 4 5% 5% 10% 41%	1551
June	1st to 10th	Night	9% 4 2 4 6% 3 6% 4 6% 7% 50%	1440
	11th to 20th	Night	9% 4 2 5% 6% 3 4 5% 6% 9% 47%	1440
	21st to end	Night	10% 5% 2 5% 5% 4 5% 6% 6% 8% 45%	1440
July	1st to 10th	Night	7% 4 2 4 4 2 4 5% 8% 10% 51%	1440
	11th to 20th	Night	7% 4 2 4 3 3 3 4 6% 8% 56%	1440
	21st to end	Night	5% 4 2 4 5% 2 4 6% 7% 10% 51%	1575
August	1st to 10th	Night	5% 4 3 5% 4 3 5% 5% 5% 9% 52%	1531
	11th to 20th	Night	7% 4 2 5% 5% 3 4 5% 5% 8% 54%	1741
	21st to end	Night	8% 4 2 3 3 3 4 4 5% 8% 56%	1980
September	1st to 10th	Night	9% 6% 1 5% 4 2 4 3 5% 6% 55%	1800
	11th to 20th	Night	11% 3 2 4 4 3 4 4 5% 7% 53%	1831
	21st to end	Night	9% 4 2 4 4 3 5% 5% 6% 7% 50%	1980
October	1st to 10th	Night	12% 5% 1 5% 5% 3 3 3 4 6% 53%	2128
	11th to 20th	Night	16% 7% 3 5% 3 3 3 3 4 6% 46%	2160
	21st to end	Night	20% 9% 3 5% 4 3 4 4 4 7% 38%	2505
November	1st to 10th	Night	11% 7% 3 4 4 2 3 3 4 7% 52%	2406
	11th to 20th	Night	15% 9% 4 8% 5% 2 4 4 4 6% 39%	2470
	21st to end	Night	18% 8% 4 6% 5% 2 4 4 4 6% 39%	2631
December	1st to 10th	Night	16% 11% 5% 8% 6% 3 4 4 5% 5% 33%	2660
	11th to 20th	Night	15% 12% 5% 8% 6% 3 4 4 4 6% 33%	3279
	21st to end	Night	23% 14% 7% 10% 7% 4 5% 4 5% 4 17%	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	61% 7% 11% 15% 6%	2528
	11th to 20th	Day	60% 7% 12% 12% 9%	2530
	21st to end	Day	69% 6% 10% 10% 5%	3021
February	1st to 10th	Day	56% 6% 13% 16% 8% 1	1701
	11th to 20th	Day	50% 7% 11% 22% 10% 1	1709
	21st to end	Day	54% 7% 10% 19% 9% 1	1480
March	1st to 10th	Day	51% 7% 14% 20% 8% 1	1800
	11th to 20th	Day	45% 7% 14% 22% 10% 1	1934
	21st to end	Day	58% 6% 11% 18% 5% 1	2330
April	1st to 10th	Day	51% 6% 13% 21% 9%	2160
	11th to 20th	Day	48% 7% 10% 21% 12% 2	2134
	21st to end	Day	51% 7% 12% 17% 12% 2	2173
May	1st to 10th	Day	51% 8% 13% 20% 8% 1	2387
	11th to 20th	Day	49% 8% 11% 21% 11%	2483
	21st to end	Day	45% 9% 13% 18% 14% 2	2725
June	1st to 10th	Day	34% 8% 14% 26% 16% 1	2520
	11th to 20th	Day	34% 8% 12% 23% 21% 3	2520
	21st to end	Day	39% 10% 14% 23% 12% 2	2520
July	1st to 10th	Day	31% 9% 16% 27% 14% 3	2451
	11th to 20th	Day	30% 8% 17% 27% 14% 3	2445
	21st to end	Day	31% 9% 18% 25% 15% 2	2591
August	1st to 10th	Day	31% 12% 17% 28% 11% 1	2389
	11th to 20th	Day	33% 8% 16% 25% 17% 1	2159
	21st to end	Day	31% 8% 11% 27% 19% 3	2376
September	1st to 10th	Day	35% 9% 12% 24% 17% 3	2160
	11th to 20th	Day	29% 6% 12% 29% 21% 3	2111
	21st to end	Day	35% 6% 12% 27% 19% 2	1980
October	1st to 10th	Day	38% 5% 11% 28% 18% 1	1828
	11th to 20th	Day	47% 5% 10% 21% 15% 2	1800
	21st to end	Day	47% 5% 13% 23% 11% 2	1850
November	1st to 10th	Day	37% 6% 17% 25% 13% 2	1672
	11th to 20th	Day	52% 5% 13% 20% 9% 1	1710
	21st to end	Day	51% 6% 14% 21% 8% 1	1544
December	1st to 10th	Day	54% 6% 12% 18% 8% 1	1520
	11th to 20th	Day	55% 6% 13% 17% 8% 1	1915
	21st to end	Day	75% 5% 7% 8% 4	2783
January	1st to 10th	Night	56% 7% 12% 17% 8%	4428
	11th to 20th	Night	61% 5% 12% 13% 8%	4440
	21st to end	Night	65% 6% 9% 14% 5% 1	4631
February	1st to 10th	Night	53% 5% 11% 18% 13% 1	2464
	11th to 20th	Night	46% 6% 12% 23% 12% 1	2148
	21st to end	Night	50% 5% 12% 19% 12% 3	1776
March	1st to 10th	Night	48% 6% 14% 22% 10% 1	2160
	11th to 20th	Night	43% 6% 12% 20% 15% 3	2043
	21st to end	Night	56% 6% 10% 18% 9% 1	2025
April	1st to 10th	Night	46% 6% 12% 21% 14% 2	1800
	11th to 20th	Night	45% 5% 8% 21% 16% 4	1775
	21st to end	Night	47% 7% 11% 22% 10% 2	1787
May	1st to 10th	Night	45% 9% 13% 22% 11% 1	1569
	11th to 20th	Night	43% 8% 12% 22% 14% 1	1417
	21st to end	Night	38% 9% 14% 20% 17% 2	1552
June	1st to 10th	Night	32% 8% 14% 25% 20% 3	1440
	11th to 20th	Night	32% 8% 10% 22% 24% 3	1440
	21st to end	Night	35% 10% 15% 20% 17% 3	1440
July	1st to 10th	Night	27% 7% 16% 28% 18% 4	1400
	11th to 20th	Night	26% 5% 15% 30% 20% 3	1392
	21st to end	Night	26% 9% 17% 25% 18% 5%	1487
August	1st to 10th	Night	27% 9% 17% 30% 14% 3	1522
	11th to 20th	Night	28% 7% 13% 30% 20% 2	1741
	21st to end	Night	26% 6% 13% 28% 22% 4	1980
September	1st to 10th	Night	29% 7% 10% 26% 24% 3	1800
	11th to 20th	Night	29% 6% 10% 31% 21% 3	1831
	21st to end	Night	32% 7% 11% 28% 20% 3	1980
October	1st to 10th	Night	33% 6% 10% 26% 22% 3	2128
	11th to 20th	Night	40% 5% 9% 24% 19% 2	2160
	21st to end	Night	47% 6% 10% 23% 13% 1	2505
November	1st to 10th	Night	33% 6% 11% 27% 19% 3	2406
	11th to 20th	Night	46% 6% 12% 22% 13% 1	2470
	21st to end	Night	48% 5% 12% 24% 12% 1	2631
December	1st to 10th	Night	53% 6% 13% 20% 8% 1	2660
	11th to 20th	Night	51% 7% 13% 21% 8% 1	3281
	21st to end	Night	68% 7% 9% 12% 4	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	21% 9% 34% 24% 10% 1	270
	11th to 20th	Day	21% 12% 40% 20% 6% 1	270
	21st to end	Day	23% 12% 35% 22% 6% 1	297
February	1st to 10th	Day	20% 17% 37% 21% 6%	267
	11th to 20th	Day	25% 17% 29% 23% 6%	240
	21st to end	Day	24% 16% 42% 15% 3	198
March	1st to 10th	Day	20% 20% 35% 18% 8%	240
	11th to 20th	Day	25% 13% 31% 20% 9% 2	240
	21st to end	Day	27% 15% 31% 20% 6% 2	416
April	1st to 10th	Day	24% 13% 33% 23% 6% 1	420
	11th to 20th	Day	21% 17% 29% 20% 10% 3	412
	21st to end	Day	25% 12% 32% 19% 10% 1	420
May	1st to 10th	Day	21% 13% 29% 26% 9% 3	470
	11th to 20th	Day	22% 11% 28% 24% 12% 3	625
	21st to end	Day	16% 8% 28% 22% 18% 8%	679
June	1st to 10th	Day	8% 6% 28% 33% 19% 7%	630
	11th to 20th	Day	12% 5% 30% 31% 17% 5%	630
	21st to end	Day	12% 8% 28% 29% 17% 5%	630
July	1st to 10th	Day	9% 3 15% 30% 27% 17%	621
	11th to 20th	Day	11% 3 20% 32% 24% 10%	618
	21st to end	Day	7% 2 20% 37% 28% 7%	684
August	1st to 10th	Day	7% 3 19% 32% 29% 11%	626
	11th to 20th	Day	10% 5% 15% 27% 26% 16%	473
	21st to end	Day	9% 3 18% 33% 26% 11%	495
September	1st to 10th	Day	14% 7% 20% 27% 24% 8%	450
	11th to 20th	Day	12% 2 18% 33% 24% 10%	450
	21st to end	Day	12% 9% 28% 24% 21% 7%	450
October	1st to 10th	Day	18% 6% 26% 31% 15% 4	298
	11th to 20th	Day	22% 19% 35% 16% 7% 1	270
	21st to end	Day	31% 13% 29% 22% 4 1	297
November	1st to 10th	Day	15% 11% 37% 25% 9% 3	270
	11th to 20th	Day	23% 16% 34% 17% 8% 2	270
	21st to end	Day	24% 9% 33% 27% 6%	270
December	1st to 10th	Day	19% 14% 37% 20% 7% 2	270
	11th to 20th	Day	15% 10% 39% 27% 8%	270
	21st to end	Day	26% 15% 39% 17% 3	297
January	1st to 10th	Night	19% 11% 38% 24% 7%	950
	11th to 20th	Night	19% 13% 36% 27% 5%	950
	21st to end	Night	21% 10% 34% 30% 4	1045
February	1st to 10th	Night	22% 10% 38% 25% 5%	948
	11th to 20th	Night	18% 11% 38% 28% 5%	752
	21st to end	Night	22% 12% 39% 22% 5% 1	592
March	1st to 10th	Night	21% 11% 37% 23% 7%	720
	11th to 20th	Night	17% 9% 34% 32% 7%	603
	21st to end	Night	24% 10% 36% 23% 6% 1	594
April	1st to 10th	Night	21% 14% 36% 24% 5% 1	540
	11th to 20th	Night	23% 12% 32% 27% 7%	533
	21st to end	Night	20% 9% 32% 30% 7% 1	540
May	1st to 10th	Night	16% 8% 37% 30% 7% 1	540
	11th to 20th	Night	21% 5% 30% 34% 11%	531
	21st to end	Night	14% 4 29% 36% 16% 1	584
June	1st to 10th	Night	9% 4 22% 49% 15%	540
	11th to 20th	Night	10% 4 28% 43% 15% 1	540
	21st to end	Night	11% 3 28% 42% 15% 1	540
July	1st to 10th	Night	9% 2 10% 43% 31% 4	525
	11th to 20th	Night	7% 3 18% 43% 26% 3	522
	21st to end	Night	6% 1 17% 46% 29% 1	591
August	1st to 10th	Night	4 1 14% 45% 31% 4	537
	11th to 20th	Night	8% 2 11% 41% 32% 5%	534
	21st to end	Night	10% 2 15% 42% 29% 3	594
September	1st to 10th	Night	11% 3 19% 37% 27% 4	540
	11th to 20th	Night	11% 3 20% 35% 27% 4	571
	21st to end	Night	12% 8% 26% 34% 18% 2	720
October	1st to 10th	Night	14% 5% 31% 31% 18% 2	720
	11th to 20th	Night	21% 12% 29% 27% 11%	720
	21st to end	Night	25% 11% 31% 26% 7% 1	792
November	1st to 10th	Night	16% 6% 42% 26% 10% 1	740
	11th to 20th	Night	18% 14% 37% 27% 5%	760
	21st to end	Night	21% 10% 34% 27% 7%	921
December	1st to 10th	Night	21% 11% 37% 25% 5%	950
	11th to 20th	Night	22% 9% 34% 29% 6%	950
	21st to end	Night	28% 11% 36% 23% 3	1045

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