Starlight Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Starlight Observatory clear sky chart from 2005-11-06 to 2024-01-17.

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	13% 7% 3 6% 6% 3 5% 5% 5% 7% 39%	74976
	Night	14% 7% 3 6% 5% 3 4 4 5% 7% 41%	74853
Transparency	Day	43% 6% 11% 23% 15% 2	74689
	Night	43% 5% 11% 22% 16% 3	74661
Seeing	Day	17% 10% 26% 24% 16% 7%	14251
	Night	17% 8% 26% 30% 17% 2	24144

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	17% 10% 4 8% 5% 3 4 4 5% 6% 34%	1517
	11th to 20th	Day	21% 11% 5% 9% 6% 3 3 3 4 6% 30%	1488
	21st to end	Day	13% 10% 5% 9% 6% 3 4 4 5% 7% 33%	1803
February	1st to 10th	Day	18% 10% 5% 9% 6% 3 3 3 4 6% 33%	1800
	11th to 20th	Day	16% 9% 5% 6% 6% 3 5% 5% 5% 7% 34%	1748
	21st to end	Day	18% 11% 5% 8% 5% 3 5% 5% 5% 6% 30%	1480
March	1st to 10th	Day	18% 8% 3 5% 6% 3 4 4 4 7% 39%	1800
	11th to 20th	Day	18% 9% 4 7% 6% 3 5% 4 4 6% 35%	1926
	21st to end	Day	18% 10% 4 6% 6% 4 5% 5% 5% 6% 31%	2240
April	1st to 10th	Day	12% 7% 3 6% 6% 3 4 5% 6% 7% 41%	2160
	11th to 20th	Day	16% 8% 3 6% 6% 3 5% 4 5% 5% 39%	2134
	21st to end	Day	18% 6% 2 6% 7% 3 4 4 5% 7% 38%	2299
May	1st to 10th	Day	17% 8% 2 6% 6% 4 4 6% 6% 6% 35%	2340
	11th to 20th	Day	18% 7% 3 6% 6% 3 5% 5% 5% 6% 36%	2301
	21st to end	Day	8% 5% 3 8% 7% 4 6% 6% 8% 9% 36%	2531
June	1st to 10th	Day	11% 5% 2 6% 6% 3 4 5% 6% 7% 45%	2440
	11th to 20th	Day	7% 5% 3 5% 5% 4 6% 6% 6% 9% 45%	2520
	21st to end	Day	8% 4 3 4 5% 4 6% 6% 7% 11% 42%	2520
July	1st to 10th	Day	13% 6% 3 5% 6% 4 6% 6% 7% 10% 36%	2520
	11th to 20th	Day	8% 5% 2 5% 6% 3 5% 6% 6% 10% 44%	2520
	21st to end	Day	8% 5% 2 5% 6% 3 6% 6% 7% 10% 41%	2562
August	1st to 10th	Day	8% 4 2 5% 6% 4 5% 6% 7% 9% 43%	2327
	11th to 20th	Day	8% 6% 3 6% 5% 4 6% 6% 7% 8% 41%	2179
	21st to end	Day	8% 5% 2 4 5% 4 5% 5% 6% 9% 48%	2376
September	1st to 10th	Day	12% 6% 2 6% 4 3 4 5% 4 8% 46%	2109
	11th to 20th	Day	9% 5% 2 5% 5% 3 5% 5% 6% 8% 47%	1980
	21st to end	Day	12% 5% 2 4 5% 3 3 4 4 6% 51%	1980
October	1st to 10th	Day	10% 4 2 4 5% 3 4 4 4 6% 54%	1980
	11th to 20th	Day	13% 4 3 6% 5% 3 4 4 5% 6% 47%	1890
	21st to end	Day	18% 7% 2 6% 5% 3 4 5% 4 6% 40%	1782
November	1st to 10th	Day	10% 5% 2 5% 5% 2 4 4 5% 7% 53%	1672
	11th to 20th	Day	16% 9% 3 5% 4 3 3 3 5% 6% 42%	1710
	21st to end	Day	18% 8% 3 5% 4 3 3 4 5% 6% 40%	1710
December	1st to 10th	Day	21% 11% 4 7% 5% 2 4 3 3 5% 33%	1710
	11th to 20th	Day	14% 12% 6% 9% 6% 3 4 4 4 6% 32%	2139
	21st to end	Day	19% 18% 5% 8% 5% 4 4 4 4 6% 23%	2783
January	1st to 10th	Night	16% 10% 4 8% 7% 4 4 4 5% 6% 32%	2648
	11th to 20th	Night	20% 11% 5% 8% 7% 3 4 4 4 5% 29%	2617
	21st to end	Night	16% 10% 5% 7% 6% 3 5% 5% 5% 6% 32%	2547
February	1st to 10th	Night	23% 9% 5% 7% 6% 4 4 4 5% 5% 28%	2160
	11th to 20th	Night	17% 9% 4 8% 6% 3 4 4 4 5% 35%	2098
	21st to end	Night	22% 9% 3 8% 6% 3 5% 4 5% 6% 31%	1776
March	1st to 10th	Night	17% 8% 3 5% 5% 2 4 4 4 5% 41%	2160
	11th to 20th	Night	19% 8% 3 7% 4 3 3 4 5% 6% 37%	2033
	21st to end	Night	20% 8% 3 7% 5% 3 4 4 4 7% 35%	2129
April	1st to 10th	Night	12% 5% 2 6% 5% 3 5% 6% 6% 8% 43%	1800
	11th to 20th	Night	15% 8% 3 5% 4 3 4 4 5% 7% 42%	1775
	21st to end	Night	17% 6% 3 6% 5% 3 4 5% 5% 7% 40%	1661
May	1st to 10th	Night	15% 7% 3 6% 6% 3 5% 5% 6% 8% 37%	1620
	11th to 20th	Night	14% 5% 2 5% 7% 3 5% 5% 6% 8% 40%	1592
	21st to end	Night	7% 5% 3 7% 5% 3 6% 7% 7% 10% 40%	1744
June	1st to 10th	Night	9% 3 2 6% 5% 3 4 5% 5% 10% 48%	1514
	11th to 20th	Night	7% 4 3 4 5% 3 4 5% 6% 8% 53%	1440
	21st to end	Night	5% 2 1 4 5% 3 4 7% 7% 11% 51%	1440
July	1st to 10th	Night	8% 5% 3 6% 5% 4 4 5% 6% 10% 45%	1440
	11th to 20th	Night	6% 3 2 4 5% 3 4 5% 6% 12% 50%	1440
	21st to end	Night	8% 4 2 4 5% 3 5% 5% 6% 9% 48%	1753
August	1st to 10th	Night	7% 4 2 5% 5% 3 4 4 7% 10% 49%	1611
	11th to 20th	Night	6% 4 2 5% 5% 3 6% 6% 7% 11% 46%	1718
	21st to end	Night	6% 4 2 3 4 2 3 4 6% 11% 56%	1980
September	1st to 10th	Night	12% 4 2 5% 4 3 4 5% 4 7% 50%	1832
	11th to 20th	Night	10% 4 2 3 4 3 3 4 5% 9% 52%	1980
	21st to end	Night	11% 5% 3 5% 3 2 3 3 4 7% 54%	1980
October	1st to 10th	Night	8% 4 2 4 5% 2 3 3 5% 7% 58%	1980
	11th to 20th	Night	14% 5% 2 5% 4 3 4 5% 4 6% 47%	2065
	21st to end	Night	18% 7% 2 5% 5% 3 3 3 4 7% 44%	2574
November	1st to 10th	Night	12% 6% 3 5% 4 3 4 3 5% 5% 51%	2406
	11th to 20th	Night	15% 6% 3 6% 5% 2 4 4 4 7% 44%	2470
	21st to end	Night	17% 9% 4 5% 6% 3 4 3 4 6% 39%	2470
December	1st to 10th	Night	19% 9% 4 6% 5% 3 4 4 4 6% 35%	2470
	11th to 20th	Night	18% 10% 4 7% 6% 3 4 4 5% 6% 34%	3046
	21st to end	Night	19% 16% 5% 9% 8% 4 5% 4 4 5% 21%	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	51% 3 10% 22% 12% 2	1518
	11th to 20th	Day	57% 4 10% 18% 10% 1	1488
	21st to end	Day	51% 5% 10% 26% 7% 1	1803
February	1st to 10th	Day	54% 3 10% 22% 10% 2	1800
	11th to 20th	Day	51% 5% 9% 21% 11% 2	1758
	21st to end	Day	55% 5% 9% 19% 10% 3	1480
March	1st to 10th	Day	47% 3 10% 23% 16% 1	1800
	11th to 20th	Day	53% 4 9% 19% 12% 2	1926
	21st to end	Day	53% 6% 10% 20% 9% 2	2240
April	1st to 10th	Day	41% 6% 11% 24% 16% 1	2160
	11th to 20th	Day	47% 5% 8% 20% 16% 3	2134
	21st to end	Day	46% 5% 9% 24% 12% 3	2299
May	1st to 10th	Day	47% 7% 12% 22% 11% 1	2340
	11th to 20th	Day	49% 8% 9% 19% 14% 1	2305
	21st to end	Day	42% 10% 14% 19% 14% 1	2531
June	1st to 10th	Day	36% 6% 10% 26% 18% 3	2440
	11th to 20th	Day	33% 8% 14% 25% 18% 2	2520
	21st to end	Day	33% 9% 14% 22% 18% 4	2520
July	1st to 10th	Day	42% 8% 13% 20% 14% 3	2451
	11th to 20th	Day	33% 9% 14% 24% 16% 4	2445
	21st to end	Day	35% 10% 14% 23% 15% 2	2420
August	1st to 10th	Day	35% 9% 13% 25% 17% 1	2314
	11th to 20th	Day	38% 8% 11% 25% 16% 2	2179
	21st to end	Day	31% 8% 13% 25% 17% 6%	2376
September	1st to 10th	Day	37% 5% 11% 23% 18% 5%	2109
	11th to 20th	Day	34% 6% 11% 24% 21% 3	1980
	21st to end	Day	35% 4 8% 24% 25% 3	1980
October	1st to 10th	Day	32% 6% 8% 26% 25% 4	1980
	11th to 20th	Day	37% 4 8% 25% 22% 4	1890
	21st to end	Day	45% 5% 8% 24% 16% 3	1782
November	1st to 10th	Day	32% 3 10% 29% 21% 5%	1672
	11th to 20th	Day	43% 3 8% 23% 19% 4	1710
	21st to end	Day	46% 3 8% 24% 17% 1	1710
December	1st to 10th	Day	55% 4 9% 21% 10% 1	1710
	11th to 20th	Day	54% 4 8% 21% 12% 1	2136
	21st to end	Day	62% 4 10% 15% 8% 1	2783
January	1st to 10th	Night	53% 5% 9% 19% 13% 2	2648
	11th to 20th	Night	58% 5% 10% 18% 9% 1	2617
	21st to end	Night	52% 4 10% 23% 9% 2	2547
February	1st to 10th	Night	58% 4 10% 20% 6% 1	2160
	11th to 20th	Night	52% 4 9% 21% 13% 1	2099
	21st to end	Night	55% 6% 11% 15% 10% 3	1776
March	1st to 10th	Night	46% 4 8% 22% 18% 3	2160
	11th to 20th	Night	49% 5% 12% 20% 12% 3	2033
	21st to end	Night	50% 6% 12% 23% 8% 2	2129
April	1st to 10th	Night	37% 7% 11% 24% 17% 3	1800
	11th to 20th	Night	42% 4 10% 19% 19% 6%	1775
	21st to end	Night	44% 5% 10% 23% 14% 3	1661
May	1st to 10th	Night	45% 6% 11% 24% 11% 3	1620
	11th to 20th	Night	42% 8% 13% 19% 16% 1	1593
	21st to end	Night	36% 10% 14% 22% 15% 4	1745
June	1st to 10th	Night	31% 7% 10% 27% 20% 4	1514
	11th to 20th	Night	29% 7% 15% 23% 21% 5%	1440
	21st to end	Night	25% 9% 17% 24% 18% 7%	1440
July	1st to 10th	Night	35% 9% 13% 24% 16% 4	1400
	11th to 20th	Night	28% 7% 15% 27% 18% 5%	1392
	21st to end	Night	31% 8% 12% 29% 17% 2	1655
August	1st to 10th	Night	28% 9% 16% 27% 18% 2	1600
	11th to 20th	Night	30% 9% 14% 27% 18% 2	1718
	21st to end	Night	22% 7% 16% 29% 20% 5%	1980
September	1st to 10th	Night	34% 6% 9% 25% 20% 6%	1832
	11th to 20th	Night	30% 6% 9% 28% 23% 5%	1980
	21st to end	Night	31% 5% 8% 25% 26% 5%	1980
October	1st to 10th	Night	28% 4 11% 25% 27% 5%	1980
	11th to 20th	Night	38% 4 8% 23% 23% 4	2065
	21st to end	Night	42% 4 7% 23% 20% 4	2574
November	1st to 10th	Night	36% 3 9% 24% 24% 4	2406
	11th to 20th	Night	40% 3 8% 22% 21% 5%	2470
	21st to end	Night	47% 4 9% 22% 17% 1	2470
December	1st to 10th	Night	51% 4 11% 22% 12% 1	2470
	11th to 20th	Night	52% 4 9% 21% 13% 2	3048
	21st to end	Night	66% 4 9% 13% 7% 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	24% 16% 36% 17% 6%	270
	11th to 20th	Day	27% 22% 33% 14% 3	258
	21st to end	Day	23% 23% 40% 12% 3	264
February	1st to 10th	Day	25% 23% 38% 12% 2	240
	11th to 20th	Day	29% 23% 37% 9% 2	240
	21st to end	Day	33% 16% 36% 12% 4	198
March	1st to 10th	Day	25% 21% 38% 15% 2	240
	11th to 20th	Day	23% 16% 38% 14% 8% 1	366
	21st to end	Day	27% 18% 31% 17% 7%	462
April	1st to 10th	Day	18% 16% 31% 24% 9% 2	420
	11th to 20th	Day	21% 17% 27% 21% 10% 4	412
	21st to end	Day	22% 15% 34% 17% 9% 2	420
May	1st to 10th	Day	23% 10% 34% 21% 8% 5%	423
	11th to 20th	Day	21% 8% 26% 25% 14% 6%	447
	21st to end	Day	13% 6% 21% 29% 19% 12%	485
June	1st to 10th	Day	12% 5% 23% 30% 22% 9%	550
	11th to 20th	Day	7% 6% 21% 35% 22% 9%	630
	21st to end	Day	10% 4 26% 28% 21% 10%	630
July	1st to 10th	Day	13% 3 15% 23% 25% 20%	621
	11th to 20th	Day	8% 3 17% 32% 27% 13%	618
	21st to end	Day	10% 3 14% 31% 27% 15%	503
August	1st to 10th	Day	11% 2 14% 28% 26% 19%	446
	11th to 20th	Day	14% 4 16% 29% 25% 12%	439
	21st to end	Day	9% 2 14% 28% 31% 16%	495
September	1st to 10th	Day	14% 2 15% 27% 32% 9%	450
	11th to 20th	Day	10% 2 13% 29% 31% 14%	450
	21st to end	Day	15% 6% 19% 23% 27% 10%	450
October	1st to 10th	Day	11% 5% 23% 36% 18% 6%	450
	11th to 20th	Day	13% 15% 33% 27% 10% 2	430
	21st to end	Day	26% 11% 37% 21% 4	297
November	1st to 10th	Day	14% 13% 31% 27% 10% 4	270
	11th to 20th	Day	25% 14% 31% 21% 8% 1	270
	21st to end	Day	25% 13% 33% 22% 8%	270
December	1st to 10th	Day	24% 22% 29% 18% 7% 1	270
	11th to 20th	Day	21% 17% 37% 19% 5% 1	270
	21st to end	Day	32% 20% 31% 13% 4	297
January	1st to 10th	Night	20% 17% 33% 26% 4	950
	11th to 20th	Night	26% 13% 35% 21% 5%	935
	21st to end	Night	22% 15% 34% 27% 2	797
February	1st to 10th	Night	28% 10% 36% 24% 3	720
	11th to 20th	Night	24% 15% 37% 23% 1	703
	21st to end	Night	28% 13% 38% 20% 2	592
March	1st to 10th	Night	21% 14% 33% 27% 5%	720
	11th to 20th	Night	23% 9% 37% 23% 8%	720
	21st to end	Night	23% 10% 29% 31% 6% 1	743
April	1st to 10th	Night	14% 9% 35% 32% 10%	540
	11th to 20th	Night	18% 9% 31% 29% 13% 1	533
	21st to end	Night	19% 6% 34% 32% 8% 1	540
May	1st to 10th	Night	18% 5% 33% 31% 13% 1	540
	11th to 20th	Night	15% 4 23% 36% 19% 3	531
	21st to end	Night	9% 3 19% 36% 27% 6%	584
June	1st to 10th	Night	10% 1 20% 39% 27% 3	540
	11th to 20th	Night	7% 3 17% 43% 27% 3	540
	21st to end	Night	5% 1 19% 41% 30% 4	540
July	1st to 10th	Night	12% 1 10% 31% 35% 11%	525
	11th to 20th	Night	7% 1 11% 33% 43% 6%	522
	21st to end	Night	9% 2 11% 36% 35% 7%	591
August	1st to 10th	Night	9% 1 9% 36% 41% 4	537
	11th to 20th	Night	6% 1 10% 40% 37% 6%	534
	21st to end	Night	7% 2 8% 34% 44% 7%	594
September	1st to 10th	Night	14% 1 9% 31% 39% 5%	572
	11th to 20th	Night	11% 3 15% 29% 38% 5%	720
	21st to end	Night	13% 5% 20% 28% 31% 4	720
October	1st to 10th	Night	10% 4 20% 35% 26% 5%	720
	11th to 20th	Night	15% 7% 28% 32% 17% 1	720
	21st to end	Night	21% 8% 30% 29% 12%	792
November	1st to 10th	Night	15% 7% 29% 30% 17% 1	740
	11th to 20th	Night	18% 12% 31% 28% 10% 1	760
	21st to end	Night	22% 11% 27% 30% 10% 1	760
December	1st to 10th	Night	24% 9% 32% 28% 5%	760
	11th to 20th	Night	24% 9% 32% 26% 8%	764
	21st to end	Night	29% 13% 29% 24% 5%	1045

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