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 2024-11-04.

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 6% 4 4 4 4 6% 7% 39%	100997
	Night	15% 9% 3 6% 4 4 4 3 5% 7% 40%	98034
Transparency	Day	23% 47% 6% 14% 9% 1	104664
	Night	23% 47% 6% 13% 10% 1	102494
Seeing	Day	19% 10% 27% 24% 15% 6%	15428
	Night	19% 9% 26% 28% 16% 2	25198

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% 15% 7% 9% 4 4 4 3 4 6% 30%	2527
	11th to 20th	Day	18% 10% 4 11% 7% 5% 4 2 4 5% 30%	2530
	21st to end	Day	26% 14% 8% 7% 3 3 3 3 5% 5% 24%	3119
February	1st to 10th	Day	17% 13% 5% 8% 5% 3 3 3 4 7% 33%	1900
	11th to 20th	Day	16% 9% 4 8% 4 4 4 4 5% 6% 36%	1848
	21st to end	Day	18% 12% 4 8% 5% 3 5% 4 4 7% 30%	1570
March	1st to 10th	Day	25% 14% 3 6% 3 3 3 2 4 4 33%	3120
	11th to 20th	Day	14% 7% 2 6% 4 3 3 3 5% 7% 45%	3347
	21st to end	Day	14% 8% 3 7% 4 5% 4 4 6% 7% 37%	4000
April	1st to 10th	Day	14% 10% 2 6% 5% 4 4 3 6% 8% 39%	2280
	11th to 20th	Day	17% 9% 3 6% 5% 3 4 3 5% 7% 38%	2254
	21st to end	Day	18% 6% 2 6% 5% 5% 4 3 5% 6% 39%	2394
May	1st to 10th	Day	19% 9% 3 7% 5% 5% 5% 5% 7% 8% 29%	4082
	11th to 20th	Day	20% 7% 2 6% 5% 4 4 3 5% 6% 38%	4081
	21st to end	Day	12% 7% 2 7% 5% 5% 6% 5% 6% 8% 37%	4476
June	1st to 10th	Day	11% 6% 2 6% 5% 5% 5% 4 5% 8% 44%	2627
	11th to 20th	Day	7% 4 3 6% 4 5% 5% 5% 6% 9% 46%	2660
	21st to end	Day	9% 5% 2 5% 4 5% 5% 4 6% 10% 44%	2660
July	1st to 10th	Day	14% 6% 2 6% 4 4 5% 4 6% 9% 39%	4440
	11th to 20th	Day	6% 4 2 6% 5% 4 4 5% 7% 10% 47%	4440
	21st to end	Day	11% 8% 3 10% 7% 7% 7% 5% 8% 8% 27%	4565
August	1st to 10th	Day	7% 4 2 6% 3 4 5% 4 7% 10% 49%	4104
	11th to 20th	Day	8% 5% 2 6% 5% 5% 7% 6% 8% 9% 40%	3738
	21st to end	Day	5% 4 2 5% 4 4 4 4 6% 10% 54%	4180
September	1st to 10th	Day	12% 5% 2 6% 4 4 5% 4 5% 7% 46%	2251
	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	10% 6% 2 5% 4 3 4 3 5% 6% 53%	1699
	11th to 20th	Day	17% 9% 3 5% 3 4 3 3 5% 6% 43%	1710
	21st to end	Day	19% 10% 2 5% 4 3 3 3 5% 6% 41%	1710
December	1st to 10th	Day	20% 14% 3 6% 4 3 3 3 4 5% 34%	1710
	11th to 20th	Day	16% 15% 6% 8% 4 4 4 3 4 5% 32%	2052
	21st to end	Day	21% 19% 5% 7% 5% 4 4 3 4 6% 22%	2783
January	1st to 10th	Night	17% 16% 5% 8% 5% 4 4 3 5% 5% 28%	4428
	11th to 20th	Night	16% 15% 5% 11% 6% 6% 5% 3 3 4 27%	4440
	21st to end	Night	25% 17% 4 6% 4 3 3 2 4 5% 26%	4521
February	1st to 10th	Night	21% 11% 5% 8% 5% 5% 3 4 5% 5% 28%	2280
	11th to 20th	Night	17% 11% 3 8% 5% 3 3 4 4 5% 36%	2218
	21st to end	Night	22% 11% 4 8% 4 4 3 3 5% 6% 31%	1884
March	1st to 10th	Night	24% 14% 3 6% 4 4 3 2 4 4 33%	3733
	11th to 20th	Night	14% 8% 2 5% 4 3 3 3 4 6% 47%	3613
	21st to end	Night	16% 8% 2 7% 5% 4 4 3 4 6% 40%	3659
April	1st to 10th	Night	12% 7% 3 6% 5% 3 6% 4 6% 7% 42%	1900
	11th to 20th	Night	17% 8% 2 5% 4 4 4 3 5% 5% 43%	1875
	21st to end	Night	18% 7% 3 5% 6% 4 4 4 5% 8% 38%	1786
May	1st to 10th	Night	19% 10% 3 8% 6% 4 5% 3 5% 7% 30%	2813
	11th to 20th	Night	18% 6% 2 5% 4 4 4 4 5% 7% 42%	2812
	21st to end	Night	9% 7% 2 6% 6% 5% 5% 5% 6% 9% 41%	3079
June	1st to 10th	Night	10% 4 2 6% 4 4 5% 4 6% 8% 48%	1548
	11th to 20th	Night	7% 4 1 4 5% 5% 5% 3 6% 9% 50%	1520
	21st to end	Night	5% 3 1 4 5% 4 5% 5% 7% 10% 51%	1520
July	1st to 10th	Night	9% 5% 2 6% 5% 3 5% 4 6% 8% 47%	2530
	11th to 20th	Night	5% 4 2 5% 3 3 3 3 6% 10% 55%	2530
	21st to end	Night	10% 6% 3 9% 6% 6% 7% 6% 7% 9% 31%	2986
August	1st to 10th	Night	6% 4 2 5% 4 3 4 3 6% 9% 55%	2825
	11th to 20th	Night	6% 4 2 6% 3 5% 5% 4 7% 9% 49%	3088
	21st to end	Night	4 3 1 4 3 3 3 3 5% 9% 63%	3476
September	1st to 10th	Night	11% 4 2 6% 4 4 3 3 4 8% 52%	1909
	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	13% 6% 2 5% 3 3 3 3 4 6% 51%	2452
	11th to 20th	Night	15% 7% 3 7% 4 3 4 2 4 6% 45%	2470
	21st to end	Night	18% 10% 3 5% 4 4 3 3 4 6% 39%	2470
December	1st to 10th	Night	20% 11% 4 7% 4 4 3 3 4 6% 34%	2470
	11th to 20th	Night	18% 13% 3 9% 5% 3 3 3 4 4 34%	3140
	21st to end	Night	20% 18% 4 9% 6% 5% 5% 3 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	39% 37% 6% 12% 5% 1	2528
	11th to 20th	Day	39% 37% 7% 13% 5%	2530
	21st to end	Day	43% 41% 4 10% 3	3119
February	1st to 10th	Day	18% 64% 3 7% 5% 1	1900
	11th to 20th	Day	18% 64% 3 9% 6%	1858
	21st to end	Day	18% 65% 3 8% 5% 2	1570
March	1st to 10th	Day	39% 40% 3 9% 6% 3	3120
	11th to 20th	Day	18% 41% 6% 20% 13% 1	3347
	21st to end	Day	23% 42% 9% 14% 12%	4000
April	1st to 10th	Day	19% 65% 4 8% 4	2280
	11th to 20th	Day	17% 65% 3 9% 4 1	2254
	21st to end	Day	15% 65% 3 10% 5% 2	2394
May	1st to 10th	Day	30% 43% 10% 12% 5%	4120
	11th to 20th	Day	29% 42% 6% 17% 7%	4085
	21st to end	Day	27% 42% 6% 10% 14% 1	4489
June	1st to 10th	Day	15% 65% 3 9% 7% 1	2627
	11th to 20th	Day	14% 66% 4 8% 7% 1	2660
	21st to end	Day	14% 65% 7% 8% 6%	2660
July	1st to 10th	Day	27% 37% 8% 15% 11% 1	4371
	11th to 20th	Day	18% 37% 7% 26% 11%	4365
	21st to end	Day	38% 38% 8% 9% 5% 1	4420
August	1st to 10th	Day	16% 39% 11% 25% 8% 1	4091
	11th to 20th	Day	24% 40% 9% 17% 10%	3774
	21st to end	Day	15% 38% 9% 29% 8% 1	4179
September	1st to 10th	Day	12% 61% 4 11% 9% 2	2251
	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	17% 57% 5% 12% 6% 2	1699
	11th to 20th	Day	16% 59% 2 11% 9% 3	1710
	21st to end	Day	20% 60% 4 10% 6% 1	1710
December	1st to 10th	Day	22% 60% 4 8% 5% 1	1710
	11th to 20th	Day	28% 48% 3 14% 7% 1	2049
	21st to end	Day	43% 38% 5% 8% 5%	2783
January	1st to 10th	Night	39% 38% 7% 10% 6% 1	4428
	11th to 20th	Night	42% 37% 4 12% 5% 1	4440
	21st to end	Night	42% 39% 4 9% 6%	4521
February	1st to 10th	Night	20% 65% 3 7% 4 1	2280
	11th to 20th	Night	18% 64% 4 9% 6%	2219
	21st to end	Night	19% 65% 2 7% 6% 1	1884
March	1st to 10th	Night	38% 41% 4 11% 5% 1	3733
	11th to 20th	Night	18% 41% 6% 17% 16% 2	3613
	21st to end	Night	25% 41% 5% 18% 10%	3659
April	1st to 10th	Night	17% 66% 3 9% 5% 1	1900
	11th to 20th	Night	16% 65% 4 8% 6% 1	1875
	21st to end	Night	16% 64% 3 9% 4 2	1786
May	1st to 10th	Night	32% 44% 8% 12% 3 1	2840
	11th to 20th	Night	23% 43% 9% 21% 4	2813
	21st to end	Night	22% 44% 9% 14% 10% 1	3087
June	1st to 10th	Night	13% 65% 3 11% 6% 2	1548
	11th to 20th	Night	11% 65% 5% 8% 8% 2	1520
	21st to end	Night	10% 64% 7% 11% 7% 1	1520
July	1st to 10th	Night	23% 39% 7% 18% 13% 1	2490
	11th to 20th	Night	15% 38% 7% 27% 14% 1	2482
	21st to end	Night	34% 39% 10% 10% 6% 1	2891
August	1st to 10th	Night	15% 39% 11% 28% 7%	2814
	11th to 20th	Night	21% 39% 10% 19% 10%	3118
	21st to end	Night	9% 38% 10% 26% 16% 1	3476
September	1st to 10th	Night	9% 60% 4 13% 11% 2	1909
	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	20% 57% 4 9% 9% 2	2452
	11th to 20th	Night	16% 60% 3 9% 10% 3	2470
	21st to end	Night	21% 60% 4 10% 5%	2470
December	1st to 10th	Night	23% 59% 5% 7% 5% 1	2470
	11th to 20th	Night	26% 49% 5% 11% 9% 1	3142
	21st to end	Night	45% 39% 5% 7% 4 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% 33% 16% 5% 1	506
April	1st to 10th	Day	24% 15% 31% 21% 8% 1	460
	11th to 20th	Day	23% 18% 27% 21% 8% 3	452
	21st to end	Day	24% 14% 36% 16% 8% 2	460
May	1st to 10th	Day	23% 13% 32% 20% 10% 3	463
	11th to 20th	Day	20% 10% 27% 27% 10% 5%	487
	21st to end	Day	14% 6% 27% 26% 17% 10%	529
June	1st to 10th	Day	13% 6% 26% 29% 19% 8%	647
	11th to 20th	Day	9% 6% 23% 34% 20% 8%	680
	21st to end	Day	11% 6% 29% 28% 18% 9%	680
July	1st to 10th	Day	15% 3 15% 25% 27% 14%	671
	11th to 20th	Day	8% 3 21% 32% 26% 10%	668
	21st to end	Day	10% 2 18% 36% 23% 11%	592
August	1st to 10th	Day	10% 3 16% 31% 25% 15%	486
	11th to 20th	Day	13% 5% 19% 31% 22% 9%	479
	21st to end	Day	11% 3 13% 28% 29% 16%	539
September	1st to 10th	Day	12% 3 18% 30% 28% 10%	490
	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% 12% 38% 22% 9% 4	279
	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% 10% 33% 26% 5% 1	742
April	1st to 10th	Night	19% 9% 34% 29% 8% 1	570
	11th to 20th	Night	21% 9% 30% 28% 11% 1	563
	21st to end	Night	21% 9% 34% 30% 6% 1	570
May	1st to 10th	Night	18% 6% 30% 30% 14% 1	570
	11th to 20th	Night	18% 5% 21% 36% 18% 3	561
	21st to end	Night	8% 4 20% 35% 27% 6%	617
June	1st to 10th	Night	10% 2 20% 37% 27% 2	570
	11th to 20th	Night	7% 4 16% 44% 26% 2	570
	21st to end	Night	5% 2 20% 39% 30% 4	570
July	1st to 10th	Night	13% 2 8% 32% 36% 10%	555
	11th to 20th	Night	7% 1 10% 37% 39% 6%	552
	21st to end	Night	10% 3 12% 33% 35% 7%	624
August	1st to 10th	Night	8% 3 9% 36% 38% 6%	567
	11th to 20th	Night	7% 2 11% 38% 35% 7%	564
	21st to end	Night	7% 3 10% 33% 41% 7%	627
September	1st to 10th	Night	12% 2 13% 33% 33% 6%	579
	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	17% 8% 30% 31% 14% 1	755
	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

Page generated at 2024-11-01 22:47:35. Version ac07

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