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 2025-09-04.

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% 3 7% 4 4 4 4 6% 8% 38%	125742
	Night	15% 9% 3 7% 4 4 4 3 5% 7% 39%	123125
Transparency	Day	28% 39% 7% 15% 10% 1	129367
	Night	28% 38% 6% 15% 11% 1	127559
Seeing	Day	19% 11% 27% 23% 15% 6%	16565
	Night	19% 9% 26% 28% 15% 2	26288

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	16% 13% 7% 10% 3 4 4 3 5% 6% 29%	3617
	11th to 20th	Day	14% 11% 4 13% 6% 4 4 2 4 5% 32%	3620
	21st to end	Day	20% 11% 6% 7% 4 4 3 3 5% 5% 32%	4478
February	1st to 10th	Day	18% 13% 5% 8% 4 3 3 3 4 7% 34%	2000
	11th to 20th	Day	17% 8% 4 8% 4 4 4 4 5% 6% 35%	1948
	21st to end	Day	17% 12% 4 8% 5% 3 5% 4 4 7% 32%	1650
March	1st to 10th	Day	20% 12% 4 7% 3 3 3 2 4 4 38%	4480
	11th to 20th	Day	13% 10% 2 6% 3 3 4 3 5% 7% 43%	4791
	21st to end	Day	16% 8% 3 7% 5% 4 4 4 5% 7% 36%	5732
April	1st to 10th	Day	15% 10% 2 6% 5% 4 4 3 6% 7% 39%	2400
	11th to 20th	Day	17% 9% 3 6% 5% 3 4 3 5% 7% 38%	2374
	21st to end	Day	18% 6% 3 6% 5% 5% 4 3 5% 7% 38%	2520
May	1st to 10th	Day	21% 9% 3 7% 5% 5% 5% 4 7% 8% 25%	5862
	11th to 20th	Day	18% 8% 3 7% 5% 4 5% 3 6% 7% 34%	5861
	21st to end	Day	14% 7% 2 7% 5% 5% 6% 5% 7% 9% 32%	6433
June	1st to 10th	Day	12% 6% 2 6% 5% 5% 4 4 5% 8% 44%	2766
	11th to 20th	Day	8% 5% 3 6% 4 5% 5% 5% 6% 9% 44%	2800
	21st to end	Day	9% 5% 2 5% 4 5% 5% 4 6% 10% 45%	2800
July	1st to 10th	Day	12% 6% 2 6% 4 4 5% 4 6% 10% 41%	6360
	11th to 20th	Day	9% 6% 2 7% 5% 5% 6% 5% 8% 11% 38%	6360
	21st to end	Day	9% 7% 3 8% 7% 7% 7% 6% 8% 9% 29%	6565
August	1st to 10th	Day	7% 4 2 6% 3 4 5% 4 7% 10% 49%	4234
	11th to 20th	Day	8% 5% 2 6% 5% 5% 7% 6% 8% 9% 40%	3858
	21st to end	Day	5% 4 2 5% 4 4 4 4 6% 10% 54%	4312
September	1st to 10th	Day	11% 5% 2 6% 4 4 5% 4 5% 7% 46%	2287
	11th to 20th	Day	9% 7% 2 6% 4 4 4 3 5% 8% 48%	2090
	21st to end	Day	14% 5% 2 5% 5% 3 4 3 4 6% 49%	2090
October	1st to 10th	Day	11% 4 1 4 4 3 3 3 5% 6% 54%	2090
	11th to 20th	Day	13% 6% 2 6% 4 3 4 3 4 7% 48%	1989
	21st to end	Day	19% 8% 2 5% 4 3 3 3 5% 5% 43%	1881
November	1st to 10th	Day	11% 6% 2 5% 4 3 4 4 5% 6% 51%	1762
	11th to 20th	Day	17% 10% 3 5% 3 4 3 3 5% 6% 43%	1800
	21st to end	Day	19% 10% 3 5% 4 3 3 3 5% 6% 39%	1800
December	1st to 10th	Day	17% 11% 3 5% 4 3 3 3 4 4 44%	2930
	11th to 20th	Day	17% 13% 6% 9% 4 5% 4 3 4 5% 30%	3220
	21st to end	Day	24% 17% 5% 8% 5% 4 4 3 4 5% 21%	3982
January	1st to 10th	Night	16% 16% 6% 10% 4 4 4 3 4 5% 28%	6348
	11th to 20th	Night	15% 14% 6% 11% 5% 6% 6% 3 3 4 28%	6360
	21st to end	Night	20% 12% 3 6% 4 3 3 3 5% 6% 36%	6465
February	1st to 10th	Night	21% 11% 5% 8% 5% 5% 3 4 5% 5% 28%	2400
	11th to 20th	Night	18% 11% 4 8% 5% 3 3 4 4 5% 35%	2338
	21st to end	Night	21% 10% 4 8% 4 4 3 3 5% 6% 33%	1980
March	1st to 10th	Night	21% 14% 3 5% 4 4 3 3 4 5% 35%	5373
	11th to 20th	Night	14% 7% 2 6% 4 4 3 3 4 6% 47%	5169
	21st to end	Night	17% 9% 3 9% 5% 5% 5% 3 5% 6% 35%	5239
April	1st to 10th	Night	13% 7% 2 6% 5% 3 6% 4 6% 7% 41%	2000
	11th to 20th	Night	16% 8% 2 5% 4 4 4 3 5% 5% 44%	1975
	21st to end	Night	18% 7% 3 5% 5% 4 4 4 5% 8% 37%	1880
May	1st to 10th	Night	20% 10% 3 8% 5% 4 5% 4 5% 8% 27%	4033
	11th to 20th	Night	18% 7% 2 7% 5% 4 5% 4 6% 8% 34%	4032
	21st to end	Night	13% 7% 3 7% 5% 5% 5% 5% 6% 9% 35%	4421
June	1st to 10th	Night	10% 4 2 6% 4 4 5% 4 6% 8% 47%	1629
	11th to 20th	Night	8% 5% 2 5% 5% 5% 5% 3 6% 9% 49%	1600
	21st to end	Night	5% 3 1 4 5% 4 5% 5% 7% 10% 51%	1600
July	1st to 10th	Night	9% 5% 2 7% 4 4 5% 4 6% 8% 46%	3618
	11th to 20th	Night	7% 6% 2 7% 4 4 4 3 7% 10% 46%	3617
	21st to end	Night	8% 5% 2 8% 5% 6% 6% 6% 9% 11% 33%	4287
August	1st to 10th	Night	6% 4 2 5% 4 3 4 3 5% 9% 55%	2915
	11th to 20th	Night	6% 4 2 6% 3 5% 5% 4 7% 9% 49%	3188
	21st to end	Night	3 2 1 3 3 3 3 3 5% 9% 63%	3586
September	1st to 10th	Night	11% 4 2 6% 4 4 3 3 5% 8% 51%	1939
	11th to 20th	Night	9% 4 2 5% 4 3 4 3 6% 9% 51%	2090
	21st to end	Night	12% 6% 2 5% 3 4 4 2 4 6% 52%	2090
October	1st to 10th	Night	8% 5% 2 4 3 3 3 2 4 6% 59%	2090
	11th to 20th	Night	14% 6% 2 4 4 3 3 3 4 6% 49%	2188
	21st to end	Night	17% 8% 2 6% 4 3 3 3 4 5% 46%	2717
November	1st to 10th	Night	14% 7% 2 6% 3 3 3 3 4 6% 50%	2536
	11th to 20th	Night	15% 8% 3 7% 4 3 4 2 4 6% 44%	2600
	21st to end	Night	18% 10% 4 6% 4 4 3 3 4 6% 39%	2600
December	1st to 10th	Night	18% 11% 3 8% 4 3 3 2 3 4 40%	4250
	11th to 20th	Night	20% 12% 3 8% 4 3 3 3 4 5% 34%	4976
	21st to end	Night	25% 17% 5% 8% 5% 4 4 2 4 4 20%	6996

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	45% 27% 7% 15% 6% 1	3618
	11th to 20th	Day	42% 26% 6% 13% 10% 2	3620
	21st to end	Day	41% 29% 7% 12% 8% 4	4478
February	1st to 10th	Day	19% 61% 4 8% 6% 1	2000
	11th to 20th	Day	21% 61% 3 9% 6%	1958
	21st to end	Day	18% 62% 3 9% 6% 2	1650
March	1st to 10th	Day	38% 28% 3 12% 14% 5%	4480
	11th to 20th	Day	27% 29% 6% 22% 15% 1	4791
	21st to end	Day	32% 31% 9% 17% 12%	5734
April	1st to 10th	Day	21% 62% 5% 7% 5%	2400
	11th to 20th	Day	18% 62% 4 10% 4 1	2374
	21st to end	Day	17% 62% 4 10% 5% 2	2520
May	1st to 10th	Day	39% 32% 10% 11% 8%	5897
	11th to 20th	Day	36% 31% 8% 18% 7%	5865
	21st to end	Day	34% 31% 9% 13% 12% 1	6447
June	1st to 10th	Day	16% 62% 3 10% 8% 1	2766
	11th to 20th	Day	17% 63% 4 8% 7% 1	2800
	21st to end	Day	15% 62% 7% 9% 7%	2800
July	1st to 10th	Day	28% 28% 7% 19% 17% 1	6291
	11th to 20th	Day	29% 30% 12% 22% 8%	6285
	21st to end	Day	38% 30% 12% 13% 6%	6378
August	1st to 10th	Day	16% 38% 11% 26% 8% 1	4221
	11th to 20th	Day	24% 39% 9% 18% 10%	3894
	21st to end	Day	15% 37% 9% 29% 9% 1	4311
September	1st to 10th	Day	13% 60% 4 11% 9% 2	2287
	11th to 20th	Day	13% 60% 6% 10% 10% 1	2090
	21st to end	Day	15% 60% 4 12% 8%	2090
October	1st to 10th	Day	10% 37% 3 19% 31% 1	3480
	11th to 20th	Day	12% 37% 7% 17% 25% 2	3270
	21st to end	Day	18% 36% 5% 18% 19% 3	3081
November	1st to 10th	Day	19% 56% 5% 12% 6% 2	1762
	11th to 20th	Day	18% 56% 2 11% 10% 3	1800
	21st to end	Day	22% 57% 4 10% 6% 1	1800
December	1st to 10th	Day	26% 36% 7% 19% 12%	2930
	11th to 20th	Day	40% 31% 6% 15% 7% 1	3217
	21st to end	Day	51% 28% 8% 9% 4	3982
January	1st to 10th	Night	45% 27% 6% 14% 7% 1	6348
	11th to 20th	Night	47% 26% 4 12% 9% 2	6360
	21st to end	Night	36% 28% 6% 16% 13% 1	6465
February	1st to 10th	Night	22% 62% 4 7% 4 1	2400
	11th to 20th	Night	21% 61% 4 8% 6%	2339
	21st to end	Night	19% 62% 2 9% 7% 1	1980
March	1st to 10th	Night	41% 29% 4 14% 9% 3	5373
	11th to 20th	Night	24% 30% 6% 19% 19% 2	5169
	21st to end	Night	36% 30% 5% 18% 10% 1	5239
April	1st to 10th	Night	19% 63% 4 9% 5% 1	2000
	11th to 20th	Night	17% 62% 4 9% 6% 1	1975
	21st to end	Night	18% 62% 4 10% 5% 2	1880
May	1st to 10th	Night	39% 33% 8% 14% 5% 1	4060
	11th to 20th	Night	35% 32% 9% 19% 5%	4033
	21st to end	Night	32% 32% 11% 15% 9% 1	4429
June	1st to 10th	Night	14% 62% 4 11% 7% 2	1629
	11th to 20th	Night	14% 62% 5% 8% 9% 2	1600
	21st to end	Night	11% 61% 7% 12% 8% 1	1600
July	1st to 10th	Night	27% 29% 8% 19% 15% 1	3580
	11th to 20th	Night	26% 30% 12% 22% 10%	3572
	21st to end	Night	32% 31% 14% 15% 8% 1	4161
August	1st to 10th	Night	16% 38% 10% 28% 7%	2904
	11th to 20th	Night	21% 38% 11% 20% 10%	3218
	21st to end	Night	9% 37% 9% 26% 17% 1	3586
September	1st to 10th	Night	10% 60% 4 14% 11% 2	1939
	11th to 20th	Night	10% 60% 5% 10% 13% 2	2090
	21st to end	Night	13% 60% 4 12% 9% 1	2090
October	1st to 10th	Night	9% 36% 4 11% 38% 2	3480
	11th to 20th	Night	11% 36% 4 15% 31% 3	3633
	21st to end	Night	16% 37% 5% 15% 23% 5%	4467
November	1st to 10th	Night	21% 56% 4 10% 7% 2	2536
	11th to 20th	Night	18% 57% 3 9% 11% 3	2600
	21st to end	Night	23% 57% 4 11% 5%	2600
December	1st to 10th	Night	32% 35% 6% 15% 11%	4250
	11th to 20th	Night	34% 32% 9% 14% 10% 1	4978
	21st to end	Night	54% 28% 5% 10% 3 1	6996

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	31% 16% 33% 17% 3	300
	11th to 20th	Day	31% 22% 33% 11% 2	300
	21st to end	Day	28% 22% 36% 11% 2	330
February	1st to 10th	Day	26% 24% 37% 11% 2 1	300
	11th to 20th	Day	31% 26% 32% 8% 2	300
	21st to end	Day	31% 20% 37% 11% 1	249
March	1st to 10th	Day	29% 22% 31% 15% 3	300
	11th to 20th	Day	30% 16% 37% 11% 5% 1	415
	21st to end	Day	31% 17% 32% 15% 4 1	550
April	1st to 10th	Day	26% 15% 31% 20% 8% 1	500
	11th to 20th	Day	23% 18% 29% 20% 8% 3	492
	21st to end	Day	23% 15% 36% 16% 8% 2	500
May	1st to 10th	Day	24% 12% 33% 20% 9% 3	503
	11th to 20th	Day	21% 11% 27% 26% 10% 5%	527
	21st to end	Day	15% 8% 28% 25% 16% 9%	573
June	1st to 10th	Day	14% 6% 27% 28% 18% 7%	696
	11th to 20th	Day	9% 7% 23% 34% 19% 7%	730
	21st to end	Day	11% 6% 27% 29% 19% 9%	730
July	1st to 10th	Day	14% 4 15% 26% 27% 14%	721
	11th to 20th	Day	9% 3 21% 33% 25% 9%	718
	21st to end	Day	9% 3 18% 35% 24% 11%	639
August	1st to 10th	Day	10% 3 17% 29% 25% 15%	526
	11th to 20th	Day	13% 5% 18% 31% 24% 10%	519
	21st to end	Day	10% 3 14% 29% 28% 15%	583
September	1st to 10th	Day	12% 3 18% 31% 28% 9%	502
	11th to 20th	Day	9% 2 14% 32% 31% 12%	490
	21st to end	Day	17% 8% 18% 23% 25% 8%	490
October	1st to 10th	Day	12% 4 25% 34% 19% 5%	490
	11th to 20th	Day	16% 14% 32% 25% 10% 4	432
	21st to end	Day	27% 10% 41% 18% 5%	330
November	1st to 10th	Day	16% 14% 38% 21% 8% 3	300
	11th to 20th	Day	27% 15% 34% 19% 5% 1	300
	21st to end	Day	29% 12% 34% 20% 6%	300
December	1st to 10th	Day	24% 22% 32% 16% 5% 1	300
	11th to 20th	Day	31% 15% 35% 13% 6%	300
	21st to end	Day	38% 18% 31% 11% 2	330
January	1st to 10th	Night	25% 17% 32% 22% 4	1000
	11th to 20th	Night	30% 16% 34% 18% 3	1000
	21st to end	Night	24% 16% 36% 22% 2	946
February	1st to 10th	Night	29% 14% 36% 20% 2	800
	11th to 20th	Night	27% 17% 34% 20% 2	783
	21st to end	Night	27% 14% 38% 20% 1	660
March	1st to 10th	Night	25% 14% 34% 23% 4	800
	11th to 20th	Night	24% 14% 34% 22% 6% 1	800
	21st to end	Night	26% 11% 32% 26% 5% 1	781
April	1st to 10th	Night	21% 10% 33% 28% 8% 1	600
	11th to 20th	Night	21% 10% 30% 28% 10% 1	593
	21st to end	Night	20% 8% 34% 30% 7% 1	600
May	1st to 10th	Night	19% 6% 30% 31% 13% 1	600
	11th to 20th	Night	18% 5% 22% 35% 18% 3	591
	21st to end	Night	9% 5% 21% 34% 26% 6%	650
June	1st to 10th	Night	11% 3 21% 37% 27% 2	600
	11th to 20th	Night	8% 4 16% 44% 26% 3	600
	21st to end	Night	6% 2 19% 38% 31% 4	600
July	1st to 10th	Night	12% 2 8% 32% 36% 10%	585
	11th to 20th	Night	8% 1 10% 37% 39% 5%	582
	21st to end	Night	10% 2 11% 33% 36% 7%	657
August	1st to 10th	Night	9% 3 10% 35% 38% 6%	597
	11th to 20th	Night	7% 2 10% 37% 37% 7%	594
	21st to end	Night	6% 2 10% 35% 40% 7%	660
September	1st to 10th	Night	12% 2 13% 33% 33% 6%	588
	11th to 20th	Night	10% 3 15% 32% 34% 6%	760
	21st to end	Night	14% 5% 19% 32% 26% 4	760
October	1st to 10th	Night	11% 5% 20% 34% 25% 5%	760
	11th to 20th	Night	17% 8% 28% 29% 16% 2	760
	21st to end	Night	23% 9% 32% 26% 10% 1	836
November	1st to 10th	Night	18% 8% 29% 30% 14% 1	780
	11th to 20th	Night	21% 12% 31% 27% 8% 1	800
	21st to end	Night	25% 14% 25% 28% 9%	800
December	1st to 10th	Night	26% 13% 31% 24% 6% 1	800
	11th to 20th	Night	26% 12% 31% 24% 7% 1	865
	21st to end	Night	32% 12% 30% 22% 3	1100

Page generated at 2025-09-01 01:51:02. Version ac07

Copyright © 2025 Attilla Danko.