LAS Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the LAS Observatory clear sky chart from 2005-11-06 to 2024-03-28.

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	15% 8% 3 6% 5% 3 5% 4 5% 7% 39%	78884
	Night	15% 9% 3 6% 5% 3 4 4 5% 7% 39%	81135
Transparency	Day	44% 6% 11% 22% 14% 2	78594
	Night	45% 6% 10% 21% 15% 3	80946
Seeing	Day	19% 10% 26% 24% 15% 6%	14510
	Night	19% 9% 26% 28% 16% 2	24477

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	15% 14% 7% 8% 5% 3 4 4 4 6% 30%	2527
	11th to 20th	Day	18% 10% 4 10% 8% 3 6% 3 4 5% 30%	2530
	21st to end	Day	26% 13% 8% 6% 4 2 3 3 4 5% 24%	3119
February	1st to 10th	Day	18% 12% 5% 8% 5% 2 3 3 4 7% 33%	1900
	11th to 20th	Day	16% 8% 4 7% 5% 4 4 4 5% 6% 37%	1848
	21st to end	Day	18% 12% 4 7% 5% 3 4 4 4 7% 31%	1570
March	1st to 10th	Day	19% 8% 3 6% 5% 3 4 3 5% 6% 39%	1900
	11th to 20th	Day	20% 10% 3 6% 5% 2 4 3 5% 7% 35%	2009
	21st to end	Day	18% 9% 4 7% 5% 3 5% 5% 6% 6% 32%	2360
April	1st to 10th	Day	13% 8% 2 5% 5% 3 4 4 6% 8% 41%	2160
	11th to 20th	Day	17% 8% 3 6% 6% 2 5% 4 5% 7% 38%	2134
	21st to end	Day	19% 6% 3 5% 6% 4 4 4 4 7% 38%	2268
May	1st to 10th	Day	18% 7% 3 6% 5% 4 4 4 5% 7% 36%	2340
	11th to 20th	Day	18% 7% 3 5% 6% 3 5% 4 5% 6% 37%	2301
	21st to end	Day	9% 5% 2 6% 7% 4 8% 7% 7% 9% 35%	2531
June	1st to 10th	Day	11% 5% 3 5% 5% 3 5% 5% 5% 8% 45%	2488
	11th to 20th	Day	7% 4 3 5% 5% 4 6% 6% 6% 9% 45%	2520
	21st to end	Day	9% 4 2 5% 5% 3 6% 5% 7% 10% 45%	2520
July	1st to 10th	Day	13% 7% 2 6% 5% 3 5% 6% 7% 10% 36%	2520
	11th to 20th	Day	7% 5% 3 6% 6% 4 5% 6% 7% 9% 43%	2520
	21st to end	Day	9% 5% 2 5% 6% 4 6% 6% 7% 9% 41%	2604
August	1st to 10th	Day	9% 4 2 6% 5% 4 6% 6% 8% 10% 41%	2327
	11th to 20th	Day	9% 5% 2 6% 5% 4 6% 6% 7% 9% 41%	2134
	21st to end	Day	7% 5% 2 5% 5% 4 4 5% 6% 10% 47%	2376
September	1st to 10th	Day	12% 5% 2 6% 5% 3 4 5% 5% 7% 45%	2133
	11th to 20th	Day	9% 6% 2 5% 5% 3 5% 4 5% 7% 47%	1980
	21st to end	Day	12% 5% 2 4 5% 2 4 4 4 6% 52%	1980
October	1st to 10th	Day	11% 4 2 4 5% 3 3 4 5% 6% 53%	1980
	11th to 20th	Day	14% 6% 2 5% 5% 2 4 4 4 7% 47%	1886
	21st to end	Day	20% 8% 2 5% 5% 2 4 3 5% 5% 41%	1782
November	1st to 10th	Day	11% 5% 2 4 4 3 4 4 5% 6% 53%	1672
	11th to 20th	Day	17% 8% 3 5% 3 3 4 3 4 6% 43%	1710
	21st to end	Day	19% 9% 3 4 4 2 3 4 4 6% 41%	1710
December	1st to 10th	Day	20% 12% 5% 6% 5% 3 4 4 4 5% 34%	1710
	11th to 20th	Day	16% 14% 6% 8% 5% 3 4 4 4 5% 32%	2052
	21st to end	Day	21% 19% 6% 6% 5% 3 5% 4 4 6% 22%	2783
January	1st to 10th	Night	18% 15% 6% 7% 5% 3 4 4 4 5% 28%	4428
	11th to 20th	Night	16% 14% 6% 9% 7% 4 6% 3 3 4 27%	4440
	21st to end	Night	26% 16% 5% 6% 5% 2 3 3 4 5% 26%	4521
February	1st to 10th	Night	22% 10% 5% 8% 5% 3 4 4 5% 5% 29%	2280
	11th to 20th	Night	17% 10% 4 8% 6% 2 4 4 4 5% 36%	2218
	21st to end	Night	22% 10% 4 7% 5% 3 4 4 5% 6% 31%	1884
March	1st to 10th	Night	19% 9% 3 5% 5% 3 4 3 4 5% 41%	2280
	11th to 20th	Night	19% 9% 4 6% 5% 3 3 4 5% 6% 37%	2171
	21st to end	Night	19% 8% 3 7% 6% 4 4 4 5% 7% 34%	2165
April	1st to 10th	Night	12% 6% 3 5% 5% 2 6% 5% 6% 8% 43%	1800
	11th to 20th	Night	17% 7% 3 5% 4 3 4 3 5% 6% 44%	1775
	21st to end	Night	18% 6% 3 4 6% 3 5% 4 5% 8% 38%	1692
May	1st to 10th	Night	16% 7% 3 6% 6% 3 5% 5% 5% 7% 38%	1620
	11th to 20th	Night	14% 6% 2 5% 5% 4 6% 5% 6% 8% 40%	1592
	21st to end	Night	7% 5% 2 5% 7% 4 6% 7% 6% 11% 38%	1744
June	1st to 10th	Night	9% 4 2 5% 5% 3 5% 5% 6% 9% 49%	1467
	11th to 20th	Night	7% 4 2 4 5% 4 5% 4 6% 9% 51%	1440
	21st to end	Night	5% 3 2 3 5% 3 6% 6% 6% 11% 52%	1440
July	1st to 10th	Night	9% 4 2 6% 6% 3 5% 5% 6% 9% 46%	1440
	11th to 20th	Night	6% 4 2 4 4 3 4 3 8% 11% 51%	1440
	21st to end	Night	8% 4 2 4 5% 3 5% 6% 6% 10% 46%	1709
August	1st to 10th	Night	8% 4 1 4 4 3 5% 4 6% 11% 50%	1611
	11th to 20th	Night	6% 3 2 5% 4 4 6% 5% 8% 10% 48%	1764
	21st to end	Night	6% 3 2 4 4 2 4 4 5% 10% 56%	1980
September	1st to 10th	Night	12% 4 2 5% 4 3 3 3 4 8% 51%	1808
	11th to 20th	Night	9% 4 2 4 4 2 4 4 6% 9% 51%	1980
	21st to end	Night	11% 5% 2 4 3 3 4 3 4 7% 54%	1980
October	1st to 10th	Night	9% 5% 2 3 4 3 3 3 4 6% 58%	1980
	11th to 20th	Night	15% 6% 2 4 5% 3 3 4 5% 6% 47%	2072
	21st to end	Night	18% 8% 2 5% 5% 2 4 3 4 5% 45%	2574
November	1st to 10th	Night	13% 6% 2 5% 4 2 3 4 4 6% 51%	2406
	11th to 20th	Night	15% 7% 3 6% 4 2 4 3 4 6% 45%	2470
	21st to end	Night	18% 9% 4 5% 4 3 4 3 4 6% 40%	2470
December	1st to 10th	Night	20% 10% 5% 6% 5% 2 4 4 4 6% 34%	2470
	11th to 20th	Night	18% 12% 4 9% 5% 3 3 4 3 5% 34%	3140
	21st to end	Night	20% 18% 4 8% 7% 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	56% 5% 10% 20% 8% 1	2528
	11th to 20th	Day	58% 4 11% 18% 8%	2530
	21st to end	Day	62% 5% 8% 19% 4 1	3119
February	1st to 10th	Day	52% 4 9% 22% 11% 2	1900
	11th to 20th	Day	48% 5% 9% 23% 13% 2	1858
	21st to end	Day	54% 5% 10% 18% 10% 3	1570
March	1st to 10th	Day	47% 4 9% 22% 16% 1	1900
	11th to 20th	Day	50% 5% 11% 21% 12% 2	2009
	21st to end	Day	52% 7% 11% 20% 9% 1	2360
April	1st to 10th	Day	41% 7% 12% 22% 17% 1	2160
	11th to 20th	Day	45% 6% 8% 21% 16% 4	2134
	21st to end	Day	46% 6% 10% 24% 11% 3	2268
May	1st to 10th	Day	48% 7% 12% 22% 10% 1	2340
	11th to 20th	Day	48% 7% 12% 18% 14% 1	2305
	21st to end	Day	43% 9% 15% 18% 13% 2	2531
June	1st to 10th	Day	37% 6% 10% 26% 18% 3	2488
	11th to 20th	Day	33% 8% 15% 23% 17% 3	2520
	21st to end	Day	34% 9% 13% 22% 18% 4	2520
July	1st to 10th	Day	41% 10% 14% 19% 13% 3	2451
	11th to 20th	Day	34% 8% 15% 23% 16% 3	2445
	21st to end	Day	36% 11% 15% 22% 14% 2	2459
August	1st to 10th	Day	35% 10% 15% 23% 16% 1	2314
	11th to 20th	Day	38% 8% 13% 26% 15% 1	2134
	21st to end	Day	31% 9% 14% 24% 17% 6%	2376
September	1st to 10th	Day	38% 7% 10% 23% 18% 4	2133
	11th to 20th	Day	35% 5% 11% 26% 20% 3	1980
	21st to end	Day	35% 5% 8% 25% 24% 4	1980
October	1st to 10th	Day	31% 6% 9% 25% 25% 4	1980
	11th to 20th	Day	38% 4 8% 24% 21% 4	1886
	21st to end	Day	45% 4 9% 24% 16% 2	1782
November	1st to 10th	Day	33% 4 11% 29% 17% 6%	1672
	11th to 20th	Day	43% 4 8% 23% 17% 5%	1710
	21st to end	Day	45% 5% 9% 24% 17% 2	1710
December	1st to 10th	Day	54% 5% 10% 21% 10% 1	1710
	11th to 20th	Day	56% 3 8% 21% 11% 1	2049
	21st to end	Day	62% 6% 10% 14% 7%	2783
January	1st to 10th	Night	56% 6% 11% 17% 10% 1	4428
	11th to 20th	Night	63% 4 7% 17% 8% 1	4440
	21st to end	Night	62% 4 8% 17% 9% 1	4521
February	1st to 10th	Night	58% 5% 10% 19% 7% 1	2280
	11th to 20th	Night	50% 4 9% 22% 14% 1	2219
	21st to end	Night	54% 6% 10% 17% 11% 3	1884
March	1st to 10th	Night	47% 5% 7% 21% 16% 3	2280
	11th to 20th	Night	48% 5% 11% 21% 12% 3	2171
	21st to end	Night	51% 5% 12% 21% 9% 2	2165
April	1st to 10th	Night	38% 7% 11% 25% 16% 3	1800
	11th to 20th	Night	42% 4 10% 20% 19% 6%	1775
	21st to end	Night	46% 5% 10% 22% 15% 3	1692
May	1st to 10th	Night	45% 7% 11% 23% 11% 3	1620
	11th to 20th	Night	42% 10% 13% 18% 17% 1	1593
	21st to end	Night	38% 10% 13% 19% 16% 4	1745
June	1st to 10th	Night	33% 7% 11% 26% 21% 3	1467
	11th to 20th	Night	29% 7% 14% 22% 22% 6%	1440
	21st to end	Night	26% 9% 17% 24% 17% 7%	1440
July	1st to 10th	Night	32% 10% 15% 24% 16% 3	1400
	11th to 20th	Night	26% 9% 14% 26% 20% 6%	1392
	21st to end	Night	30% 9% 13% 28% 16% 3	1614
August	1st to 10th	Night	28% 9% 18% 24% 19% 2	1600
	11th to 20th	Night	28% 9% 15% 29% 18% 2	1764
	21st to end	Night	24% 7% 13% 29% 21% 6%	1980
September	1st to 10th	Night	33% 6% 10% 25% 20% 6%	1808
	11th to 20th	Night	29% 5% 11% 26% 24% 5%	1980
	21st to end	Night	31% 5% 9% 24% 26% 5%	1980
October	1st to 10th	Night	28% 4 11% 23% 28% 5%	1980
	11th to 20th	Night	38% 5% 7% 23% 22% 5%	2072
	21st to end	Night	43% 4 8% 21% 20% 4	2574
November	1st to 10th	Night	35% 3 10% 24% 23% 5%	2406
	11th to 20th	Night	40% 4 9% 21% 21% 6%	2470
	21st to end	Night	46% 4 9% 22% 16% 2	2470
December	1st to 10th	Night	52% 4 11% 21% 11% 1	2470
	11th to 20th	Night	53% 5% 9% 19% 13% 2	3142
	21st to end	Night	65% 5% 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	29% 16% 34% 17% 4	270
	11th to 20th	Day	31% 22% 33% 12% 3	270
	21st to end	Day	30% 21% 36% 10% 3	297
February	1st to 10th	Day	26% 23% 37% 12% 2 1	270
	11th to 20th	Day	30% 24% 34% 9% 2	270
	21st to end	Day	32% 20% 37% 10% 1	225
March	1st to 10th	Day	29% 21% 34% 14% 3	270
	11th to 20th	Day	29% 16% 37% 11% 5% 1	379
	21st to end	Day	31% 16% 32% 15% 5% 1	490
April	1st to 10th	Day	21% 14% 32% 22% 9% 1	420
	11th to 20th	Day	23% 17% 27% 21% 9% 3	412
	21st to end	Day	25% 14% 34% 16% 9% 2	420
May	1st to 10th	Day	24% 11% 33% 20% 9% 3	423
	11th to 20th	Day	20% 10% 28% 27% 10% 5%	447
	21st to end	Day	14% 5% 26% 27% 18% 11%	485
June	1st to 10th	Day	13% 5% 24% 30% 20% 8%	598
	11th to 20th	Day	9% 6% 22% 34% 20% 8%	630
	21st to end	Day	11% 5% 28% 29% 19% 9%	630
July	1st to 10th	Day	14% 2 13% 26% 28% 16%	621
	11th to 20th	Day	9% 3 19% 32% 27% 11%	618
	21st to end	Day	10% 2 17% 35% 25% 12%	545
August	1st to 10th	Day	11% 3 15% 31% 26% 15%	446
	11th to 20th	Day	14% 3 18% 31% 23% 10%	439
	21st to end	Day	11% 3 13% 27% 29% 16%	495
September	1st to 10th	Day	13% 3 16% 30% 29% 10%	450
	11th to 20th	Day	9% 2 13% 32% 31% 12%	450
	21st to end	Day	14% 8% 17% 24% 27% 9%	450
October	1st to 10th	Day	13% 4 26% 34% 18% 6%	450
	11th to 20th	Day	16% 13% 34% 26% 8% 3	396
	21st to end	Day	29% 10% 39% 18% 4	297
November	1st to 10th	Day	16% 13% 37% 21% 9% 4	270
	11th to 20th	Day	26% 14% 33% 20% 6% 1	270
	21st to end	Day	28% 11% 35% 20% 6%	270
December	1st to 10th	Day	24% 21% 31% 17% 5% 1	270
	11th to 20th	Day	29% 16% 36% 13% 6%	270
	21st to end	Day	36% 20% 30% 12% 2	297
January	1st to 10th	Night	25% 17% 31% 23% 4	950
	11th to 20th	Night	30% 15% 34% 18% 3	950
	21st to end	Night	25% 16% 35% 22% 2	899
February	1st to 10th	Night	28% 13% 36% 21% 2	760
	11th to 20th	Night	25% 17% 35% 21% 2	743
	21st to end	Night	28% 14% 39% 18% 1	628
March	1st to 10th	Night	25% 14% 34% 23% 4	760
	11th to 20th	Night	24% 13% 34% 22% 6% 1	760
	21st to end	Night	25% 11% 32% 26% 5% 1	730
April	1st to 10th	Night	18% 9% 34% 31% 8% 1	540
	11th to 20th	Night	20% 9% 29% 28% 12% 1	533
	21st to end	Night	21% 9% 33% 30% 6% 1	540
May	1st to 10th	Night	18% 6% 31% 30% 14% 1	540
	11th to 20th	Night	18% 5% 22% 36% 18% 3	531
	21st to end	Night	8% 4 20% 34% 28% 7%	584
June	1st to 10th	Night	10% 2 20% 38% 28% 2	540
	11th to 20th	Night	7% 4 16% 44% 26% 3	540
	21st to end	Night	5% 2 19% 39% 31% 4	540
July	1st to 10th	Night	12% 2 7% 31% 37% 10%	525
	11th to 20th	Night	7% 1 9% 37% 40% 6%	522
	21st to end	Night	10% 3 11% 33% 36% 8%	591
August	1st to 10th	Night	9% 2 8% 37% 39% 5%	537
	11th to 20th	Night	7% 2 10% 37% 37% 7%	534
	21st to end	Night	7% 3 10% 33% 41% 7%	594
September	1st to 10th	Night	13% 2 13% 34% 33% 6%	548
	11th to 20th	Night	10% 3 16% 31% 35% 6%	720
	21st to end	Night	12% 5% 19% 32% 28% 4	720
October	1st to 10th	Night	11% 5% 20% 33% 25% 5%	720
	11th to 20th	Night	18% 8% 28% 29% 15% 2	720
	21st to end	Night	24% 9% 32% 26% 9% 1	792
November	1st to 10th	Night	17% 8% 29% 31% 14% 1	740
	11th to 20th	Night	20% 12% 31% 27% 9% 1	760
	21st to end	Night	24% 13% 25% 28% 10%	760
December	1st to 10th	Night	26% 13% 31% 24% 6% 1	760
	11th to 20th	Night	25% 12% 31% 25% 7% 1	821
	21st to end	Night	31% 12% 30% 23% 3	1045

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