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	14% 5% 2 5% 5% 3 5% 5% 6% 8% 43%	74754
	Night	16% 6% 2 5% 5% 3 4 4 5% 7% 44%	75079
Transparency	Day	39% 7% 13% 23% 16% 3	74459
	Night	39% 7% 12% 22% 17% 3	74894
Seeing	Day	17% 12% 26% 25% 14% 5%	13880
	Night	18% 10% 30% 29% 12% 1	23348

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	13% 6% 3 6% 6% 3 3 5% 5% 6% 45%	1517
	11th to 20th	Day	15% 7% 2 6% 5% 3 4 5% 4 6% 43%	1582
	21st to end	Day	16% 6% 2 5% 5% 3 6% 4 4 7% 42%	1782
February	1st to 10th	Day	16% 7% 3 5% 5% 4 4 6% 5% 6% 40%	1620
	11th to 20th	Day	17% 7% 2 4 4 3 4 5% 6% 7% 41%	1690
	21st to end	Day	19% 9% 3 5% 4 3 4 4 5% 5% 39%	1480
March	1st to 10th	Day	13% 5% 3 4 5% 3 5% 3 4 6% 49%	1836
	11th to 20th	Day	17% 5% 2 5% 4 4 5% 4 4 7% 42%	1980
	21st to end	Day	19% 5% 3 5% 6% 3 5% 4 5% 5% 40%	2279
April	1st to 10th	Day	14% 7% 2 5% 4 3 4 5% 4 7% 44%	2160
	11th to 20th	Day	14% 5% 2 4 5% 3 5% 4 5% 8% 45%	2134
	21st to end	Day	18% 5% 2 5% 4 3 5% 5% 6% 8% 41%	2160
May	1st to 10th	Day	17% 7% 3 5% 5% 3 4 5% 6% 7% 38%	2274
	11th to 20th	Day	19% 5% 2 5% 5% 3 5% 5% 6% 9% 37%	2478
	21st to end	Day	13% 5% 2 5% 4 2 5% 5% 6% 9% 42%	2725
June	1st to 10th	Day	11% 4 2 5% 5% 3 6% 6% 7% 10% 40%	2520
	11th to 20th	Day	13% 5% 2 5% 5% 4 5% 6% 6% 9% 40%	2520
	21st to end	Day	10% 4 2 4 5% 3 5% 6% 7% 10% 45%	2520
July	1st to 10th	Day	9% 4 2 5% 7% 3 6% 5% 7% 9% 43%	2520
	11th to 20th	Day	6% 3 2 5% 5% 4 6% 6% 7% 11% 46%	2520
	21st to end	Day	9% 4 3 6% 6% 5% 6% 6% 8% 10% 38%	2636
August	1st to 10th	Day	9% 6% 2 6% 6% 3 5% 6% 8% 10% 39%	2327
	11th to 20th	Day	10% 4 2 5% 6% 4 5% 6% 7% 9% 42%	2143
	21st to end	Day	11% 5% 2 5% 5% 3 5% 6% 6% 9% 42%	2376
September	1st to 10th	Day	14% 6% 2 6% 5% 3 4 5% 5% 7% 44%	2160
	11th to 20th	Day	13% 4 2 4 4 3 4 4 5% 8% 49%	2152
	21st to end	Day	18% 6% 3 6% 5% 3 4 4 5% 7% 40%	1948
October	1st to 10th	Day	18% 6% 2 4 4 3 3 4 4 6% 45%	1800
	11th to 20th	Day	16% 5% 2 4 3 2 4 4 5% 6% 48%	1800
	21st to end	Day	18% 6% 2 5% 4 3 3 4 4 5% 46%	1958
November	1st to 10th	Day	13% 5% 2 5% 4 2 3 4 4 7% 53%	1671
	11th to 20th	Day	15% 5% 2 4 3 2 4 5% 6% 6% 48%	1710
	21st to end	Day	15% 5% 2 4 5% 2 5% 4 4 6% 48%	1574
December	1st to 10th	Day	18% 6% 2 4 5% 3 4 5% 6% 6% 42%	1520
	11th to 20th	Day	19% 6% 2 4 4 2 4 4 5% 7% 42%	1910
	21st to end	Day	16% 6% 2 6% 6% 3 5% 5% 6% 7% 38%	2772
January	1st to 10th	Night	17% 5% 2 5% 4 3 4 3 5% 6% 46%	2649
	11th to 20th	Night	19% 7% 3 5% 4 3 4 5% 6% 6% 40%	2521
	21st to end	Night	15% 7% 2 4 5% 2 4 4 5% 6% 46%	2574
February	1st to 10th	Night	17% 7% 4 5% 5% 3 4 4 6% 6% 39%	2340
	11th to 20th	Night	19% 8% 2 5% 4 2 4 4 3 6% 43%	2155
	21st to end	Night	18% 7% 2 4 5% 3 5% 4 5% 6% 42%	1776
March	1st to 10th	Night	14% 5% 2 4 4 3 4 4 5% 7% 48%	2120
	11th to 20th	Night	17% 6% 2 5% 5% 2 4 4 5% 6% 44%	1980
	21st to end	Night	18% 6% 3 6% 5% 3 4 4 3 7% 42%	2075
April	1st to 10th	Night	17% 6% 3 5% 5% 3 4 4 5% 6% 43%	1800
	11th to 20th	Night	15% 5% 2 6% 5% 3 4 4 5% 8% 45%	1775
	21st to end	Night	21% 5% 2 4 4 2 3 4 5% 6% 44%	1800
May	1st to 10th	Night	17% 6% 3 7% 6% 3 4 5% 5% 7% 37%	1701
	11th to 20th	Night	20% 6% 3 5% 4 3 4 5% 6% 6% 38%	1415
	21st to end	Night	15% 7% 2 5% 5% 3 5% 4 5% 8% 40%	1550
June	1st to 10th	Night	14% 5% 2 4 5% 2 5% 5% 5% 9% 43%	1440
	11th to 20th	Night	11% 5% 3 5% 6% 4 5% 5% 6% 9% 40%	1440
	21st to end	Night	11% 4 3 4 5% 3 5% 6% 7% 10% 43%	1440
July	1st to 10th	Night	9% 6% 3 6% 7% 3 5% 6% 7% 9% 39%	1440
	11th to 20th	Night	7% 3 2 5% 7% 4 5% 6% 8% 11% 42%	1440
	21st to end	Night	9% 4 3 6% 6% 4 5% 5% 8% 9% 42%	1677
August	1st to 10th	Night	9% 4 1 5% 7% 3 6% 6% 7% 9% 43%	1611
	11th to 20th	Night	12% 5% 3 6% 6% 4 5% 6% 6% 8% 40%	1738
	21st to end	Night	12% 5% 2 5% 4 3 5% 5% 6% 9% 44%	1980
September	1st to 10th	Night	14% 6% 3 5% 4 2 4 4 5% 6% 46%	1800
	11th to 20th	Night	12% 5% 2 4 3 2 3 4 4 7% 54%	1807
	21st to end	Night	17% 6% 2 5% 5% 3 3 4 5% 7% 41%	2008
October	1st to 10th	Night	20% 6% 2 5% 5% 2 4 4 4 5% 43%	2160
	11th to 20th	Night	17% 4 2 4 4 3 2 3 4 7% 50%	2160
	21st to end	Night	17% 5% 2 4 4 2 4 4 4 5% 48%	2398
November	1st to 10th	Night	12% 5% 2 5% 4 2 3 4 5% 6% 53%	2407
	11th to 20th	Night	17% 6% 2 4 4 3 3 3 4 6% 48%	2470
	21st to end	Night	14% 5% 1 4 5% 3 4 5% 6% 6% 47%	2603
December	1st to 10th	Night	19% 6% 2 5% 5% 3 4 4 4 6% 41%	2660
	11th to 20th	Night	18% 5% 2 5% 5% 2 4 4 5% 7% 44%	3285
	21st to end	Night	17% 7% 3 5% 5% 3 4 5% 6% 8% 37%	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	40% 6% 13% 21% 16% 5%	1518
	11th to 20th	Day	43% 5% 12% 23% 13% 3	1582
	21st to end	Day	43% 5% 11% 23% 15% 4	1782
February	1st to 10th	Day	43% 7% 13% 24% 11% 2	1620
	11th to 20th	Day	42% 5% 10% 26% 14% 3	1699
	21st to end	Day	46% 5% 10% 21% 14% 5%	1480
March	1st to 10th	Day	37% 4 12% 27% 17% 2	1836
	11th to 20th	Day	41% 7% 11% 20% 17% 5%	1980
	21st to end	Day	46% 5% 8% 20% 17% 4	2279
April	1st to 10th	Day	39% 7% 12% 20% 17% 6%	2160
	11th to 20th	Day	37% 6% 11% 22% 18% 5%	2134
	21st to end	Day	40% 7% 12% 18% 18% 5%	2160
May	1st to 10th	Day	43% 7% 11% 19% 16% 3	2274
	11th to 20th	Day	43% 8% 13% 20% 13% 2	2482
	21st to end	Day	37% 10% 14% 24% 13% 2	2725
June	1st to 10th	Day	36% 10% 13% 24% 15% 1	2520
	11th to 20th	Day	39% 8% 16% 22% 13% 2	2520
	21st to end	Day	32% 9% 16% 23% 18% 1	2520
July	1st to 10th	Day	36% 9% 15% 21% 16% 3	2450
	11th to 20th	Day	29% 9% 16% 27% 15% 3	2445
	21st to end	Day	37% 10% 13% 22% 15% 2	2489
August	1st to 10th	Day	36% 10% 17% 24% 13% 1	2314
	11th to 20th	Day	35% 11% 15% 21% 15% 2	2143
	21st to end	Day	36% 9% 14% 22% 15% 4	2376
September	1st to 10th	Day	39% 7% 11% 20% 18% 3	2160
	11th to 20th	Day	33% 6% 11% 24% 23% 4	2152
	21st to end	Day	44% 6% 8% 19% 20% 3	1948
October	1st to 10th	Day	40% 6% 12% 22% 18% 2	1800
	11th to 20th	Day	36% 6% 11% 22% 23% 3	1800
	21st to end	Day	40% 5% 8% 26% 18% 2	1958
November	1st to 10th	Day	34% 4 13% 27% 19% 4	1671
	11th to 20th	Day	35% 6% 11% 23% 22% 3	1710
	21st to end	Day	38% 5% 11% 31% 13% 3	1574
December	1st to 10th	Day	41% 7% 14% 24% 13% 1	1520
	11th to 20th	Day	41% 5% 11% 30% 10% 2	1906
	21st to end	Day	42% 8% 16% 21% 12%	2772
January	1st to 10th	Night	41% 4 10% 22% 18% 4	2649
	11th to 20th	Night	44% 6% 13% 22% 13% 2	2521
	21st to end	Night	39% 5% 10% 26% 18% 3	2574
February	1st to 10th	Night	44% 5% 13% 23% 13% 2	2340
	11th to 20th	Night	44% 4 9% 21% 18% 4	2157
	21st to end	Night	44% 6% 9% 19% 17% 6%	1776
March	1st to 10th	Night	36% 4 13% 25% 19% 3	2120
	11th to 20th	Night	41% 6% 11% 21% 17% 4	1980
	21st to end	Night	44% 5% 10% 18% 18% 6%	2075
April	1st to 10th	Night	42% 5% 11% 19% 19% 5%	1800
	11th to 20th	Night	38% 6% 12% 20% 18% 5%	1775
	21st to end	Night	40% 7% 10% 19% 18% 6%	1800
May	1st to 10th	Night	45% 7% 11% 16% 17% 3	1701
	11th to 20th	Night	44% 9% 12% 15% 18% 3	1416
	21st to end	Night	42% 9% 15% 18% 13% 2	1551
June	1st to 10th	Night	37% 8% 12% 23% 18% 1	1440
	11th to 20th	Night	39% 9% 14% 22% 14% 2	1440
	21st to end	Night	33% 11% 14% 24% 16% 2	1440
July	1st to 10th	Night	40% 10% 16% 18% 13% 3	1400
	11th to 20th	Night	31% 12% 17% 23% 13% 3	1392
	21st to end	Night	35% 10% 16% 22% 15% 2	1584
August	1st to 10th	Night	34% 13% 15% 22% 15% 1	1600
	11th to 20th	Night	40% 9% 15% 18% 15% 2	1738
	21st to end	Night	36% 10% 15% 20% 17% 3	1980
September	1st to 10th	Night	39% 7% 10% 21% 19% 4	1800
	11th to 20th	Night	31% 6% 9% 24% 25% 4	1807
	21st to end	Night	41% 7% 9% 20% 20% 3	2008
October	1st to 10th	Night	43% 7% 10% 21% 17% 2	2160
	11th to 20th	Night	35% 5% 12% 28% 18% 3	2160
	21st to end	Night	37% 6% 9% 27% 19% 3	2398
November	1st to 10th	Night	33% 6% 11% 26% 19% 6%	2407
	11th to 20th	Night	38% 5% 10% 22% 20% 5%	2470
	21st to end	Night	36% 5% 12% 27% 18% 2	2603
December	1st to 10th	Night	45% 6% 11% 23% 14% 1	2660
	11th to 20th	Night	40% 6% 12% 22% 17% 4	3288
	21st to end	Night	42% 11% 13% 24% 10% 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	18% 25% 36% 17% 4 1	180
	11th to 20th	Day	20% 34% 27% 18% 1	215
	21st to end	Day	20% 25% 36% 14% 5% 1	264
February	1st to 10th	Day	22% 25% 35% 14% 4	240
	11th to 20th	Day	25% 22% 39% 13% 1	358
	21st to end	Day	23% 25% 31% 18% 3	346
March	1st to 10th	Day	14% 26% 35% 20% 5%	420
	11th to 20th	Day	21% 21% 32% 18% 5% 2	420
	21st to end	Day	22% 17% 35% 19% 6% 1	462
April	1st to 10th	Day	19% 20% 37% 16% 6% 2	420
	11th to 20th	Day	17% 18% 29% 24% 9% 4	412
	21st to end	Day	23% 16% 35% 20% 6% 1	420
May	1st to 10th	Day	26% 15% 28% 21% 9% 1	423
	11th to 20th	Day	19% 10% 30% 28% 11% 2	447
	21st to end	Day	19% 4 22% 27% 21% 7%	485
June	1st to 10th	Day	12% 6% 27% 30% 18% 6%	450
	11th to 20th	Day	16% 7% 22% 31% 19% 5%	450
	21st to end	Day	11% 6% 24% 35% 18% 7%	450
July	1st to 10th	Day	12% 4 16% 31% 27% 11%	445
	11th to 20th	Day	6% 2 15% 36% 31% 10%	444
	21st to end	Day	10% 2 18% 35% 27% 7%	488
August	1st to 10th	Day	10% 4 17% 32% 28% 10%	446
	11th to 20th	Day	11% 3 13% 32% 29% 11%	439
	21st to end	Day	11% 3 20% 31% 23% 13%	495
September	1st to 10th	Day	17% 7% 17% 23% 24% 12%	450
	11th to 20th	Day	15% 3 21% 28% 20% 13%	450
	21st to end	Day	22% 9% 22% 28% 16% 4	450
October	1st to 10th	Day	22% 5% 23% 26% 15% 8%	450
	11th to 20th	Day	15% 13% 25% 27% 17% 3	450
	21st to end	Day	20% 12% 31% 25% 10% 3	484
November	1st to 10th	Day	15% 11% 32% 25% 15% 3	365
	11th to 20th	Day	14% 19% 35% 22% 10%	370
	21st to end	Day	16% 21% 27% 27% 9%	234
December	1st to 10th	Day	20% 20% 32% 21% 7% 1	180
	11th to 20th	Day	26% 22% 28% 20% 4	180
	21st to end	Day	23% 19% 27% 23% 7% 1	198
January	1st to 10th	Night	20% 17% 40% 20% 3	760
	11th to 20th	Night	23% 16% 33% 24% 4	748
	21st to end	Night	18% 16% 40% 23% 2	792
February	1st to 10th	Night	21% 15% 39% 23% 2	720
	11th to 20th	Night	23% 20% 38% 17% 1	703
	21st to end	Night	23% 18% 38% 19% 2	592
March	1st to 10th	Night	17% 18% 38% 24% 4	720
	11th to 20th	Night	20% 15% 37% 21% 6% 1	720
	21st to end	Night	20% 9% 41% 23% 6%	792
April	1st to 10th	Night	20% 15% 37% 22% 6%	720
	11th to 20th	Night	15% 13% 33% 32% 7%	711
	21st to end	Night	23% 9% 32% 29% 8%	720
May	1st to 10th	Night	21% 7% 34% 31% 6%	621
	11th to 20th	Night	19% 9% 31% 31% 10%	354
	21st to end	Night	18% 4 19% 38% 20% 2	389
June	1st to 10th	Night	14% 4 24% 39% 19% 1	360
	11th to 20th	Night	12% 1 25% 46% 15%	360
	21st to end	Night	8% 3 23% 48% 18% 1	360
July	1st to 10th	Night	12% 4 17% 43% 22% 2	350
	11th to 20th	Night	7% 1 14% 45% 32% 1	348
	21st to end	Night	9% 2 17% 42% 27% 2	497
August	1st to 10th	Night	9% 2 16% 41% 29% 2	537
	11th to 20th	Night	13% 2 16% 37% 30% 3	674
	21st to end	Night	14% 3 17% 37% 26% 4	792
September	1st to 10th	Night	16% 3 17% 37% 23% 4	720
	11th to 20th	Night	14% 3 15% 37% 27% 4	720
	21st to end	Night	21% 4 21% 34% 19% 1	720
October	1st to 10th	Night	23% 6% 22% 27% 18% 5%	720
	11th to 20th	Night	18% 8% 28% 33% 13% 1	720
	21st to end	Night	19% 10% 34% 27% 9% 1	792
November	1st to 10th	Night	14% 9% 35% 29% 12% 1	740
	11th to 20th	Night	20% 13% 32% 27% 8% 1	760
	21st to end	Night	14% 16% 33% 28% 8% 1	760
December	1st to 10th	Night	22% 16% 33% 21% 8% 1	760
	11th to 20th	Night	18% 14% 39% 24% 5%	760
	21st to end	Night	20% 12% 35% 30% 3	836

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