Carlos Lemmi Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Carlos Lemmi Observatory clear sky chart from 2005-11-06 to 2024-03-23.

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	5% 4 2 3 3 2 3 4 5% 9% 59%	79267
	Night	7% 6% 2 4 4 2 3 4 5% 8% 54%	80649
Transparency	Day	17% 34% 5% 13% 17% 14%	78994
	Night	20% 34% 4 12% 16% 14%	80443
Seeing	Day	5% 3 12% 32% 36% 11%	14793
	Night	7% 3 9% 27% 38% 17%	24757

By Month

Percentage Cloud Cover SW (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	6% 3 1 2 3 2 3 2 4 6% 68%	2539
	11th to 20th	Day	10% 6% 2 4 4 3 4 4 5% 8% 52%	2872
	21st to end	Day	13% 5% 2 4 4 2 3 4 5% 6% 52%	3498
February	1st to 10th	Day	9% 4 2 4 3 2 3 4 3 6% 60%	1900
	11th to 20th	Day	7% 3 2 2 4 2 3 4 6% 8% 59%	1850
	21st to end	Day	7% 5% 2 3 3 2 4 3 5% 8% 57%	1570
March	1st to 10th	Day	7% 4 2 4 4 3 5% 5% 5% 7% 55%	1900
	11th to 20th	Day	6% 5% 2 3 3 3 3 4 5% 11% 54%	1961
	21st to end	Day	5% 3 2 3 4 2 4 4 5% 8% 60%	2401
April	1st to 10th	Day	6% 4 3 4 3 3 4 4 5% 9% 57%	2160
	11th to 20th	Day	4 3 1 3 3 2 2 3 5% 8% 65%	2129
	21st to end	Day	4 4 2 3 3 1 2 3 5% 9% 65%	2160
May	1st to 10th	Day	4 4 3 4 4 2 3 3 6% 10% 57%	2182
	11th to 20th	Day	6% 7% 2 4 3 1 3 3 4 8% 60%	2301
	21st to end	Day	5% 6% 2 4 4 2 2 4 5% 10% 58%	2530
June	1st to 10th	Day	7% 7% 2 4 3 1 2 4 5% 11% 52%	2354
	11th to 20th	Day	3 5% 2 3 3 2 2 3 5% 10% 61%	2520
	21st to end	Day	3 6% 2 3 3 2 3 3 5% 10% 59%	2520
July	1st to 10th	Day	3 5% 2 4 3 2 3 4 6% 11% 57%	2520
	11th to 20th	Day	3 3 2 4 3 2 2 4 5% 12% 62%	2478
	21st to end	Day	3 3 1 3 2 2 4 4 6% 11% 61%	2350
August	1st to 10th	Day	2 4 1 3 3 2 2 3 6% 9% 66%	2149
	11th to 20th	Day	3 4 1 2 2 2 2 3 6% 11% 64%	2122
	21st to end	Day	2 2 1 3 3 2 3 4 5% 10% 65%	2376
September	1st to 10th	Day	3 3 2 3 3 2 3 4 5% 9% 64%	2160
	11th to 20th	Day	3 4 2 2 3 2 2 4 6% 10% 62%	2039
	21st to end	Day	2 3 1 3 2 2 2 4 5% 8% 67%	1980
October	1st to 10th	Day	4 3 1 3 3 2 2 3 5% 7% 66%	1980
	11th to 20th	Day	4 4 2 3 2 2 2 3 4 7% 67%	1848
	21st to end	Day	3 3 1 2 2 2 3 4 5% 7% 68%	1968
November	1st to 10th	Day	3 3 2 3 3 2 3 3 4 8% 66%	1674
	11th to 20th	Day	5% 4 1 3 3 2 2 4 5% 6% 66%	1710
	21st to end	Day	5% 3 2 3 3 3 3 4 4 9% 61%	1710
December	1st to 10th	Day	7% 5% 2 4 3 2 4 4 5% 7% 56%	1710
	11th to 20th	Day	12% 5% 3 5% 6% 3 3 4 5% 7% 46%	2169
	21st to end	Day	11% 6% 3 6% 6% 4 5% 4 5% 8% 43%	2977
January	1st to 10th	Night	7% 4 1 4 3 2 3 3 4 7% 61%	4406
	11th to 20th	Night	9% 7% 2 5% 4 2 3 3 5% 9% 50%	4076
	21st to end	Night	12% 7% 2 4 4 3 3 5% 4 6% 49%	4180
February	1st to 10th	Night	8% 5% 2 5% 4 2 3 4 4 7% 57%	2280
	11th to 20th	Night	8% 5% 3 4 4 2 4 4 4 6% 57%	2218
	21st to end	Night	10% 5% 2 5% 4 2 4 4 4 6% 55%	1884
March	1st to 10th	Night	8% 6% 2 5% 5% 3 4 4 5% 10% 50%	2280
	11th to 20th	Night	8% 6% 2 6% 5% 2 3 4 5% 9% 48%	2213
	21st to end	Night	5% 6% 2 4 3 3 4 4 6% 8% 56%	2000
April	1st to 10th	Night	9% 6% 2 4 3 2 4 4 5% 7% 53%	1800
	11th to 20th	Night	5% 6% 2 3 3 2 3 3 4 7% 62%	1779
	21st to end	Night	5% 8% 3 6% 2 2 3 3 4 7% 56%	1800
May	1st to 10th	Night	6% 7% 4 4 4 2 4 4 5% 8% 53%	1775
	11th to 20th	Night	7% 10% 3 4 3 2 3 4 5% 7% 52%	1594
	21st to end	Night	5% 7% 3 4 3 1 2 3 4 8% 60%	1746
June	1st to 10th	Night	6% 11% 3 6% 5% 3 3 4 5% 8% 47%	1600
	11th to 20th	Night	4 8% 3 5% 4 2 3 4 5% 8% 55%	1440
	21st to end	Night	4 10% 3 6% 5% 2 4 4 5% 9% 46%	1440
July	1st to 10th	Night	4 6% 3 4 4 2 3 4 5% 11% 53%	1440
	11th to 20th	Night	3 6% 2 6% 4 2 3 5% 6% 11% 52%	1474
	21st to end	Night	4 7% 3 4 5% 3 5% 4 7% 10% 49%	1970
August	1st to 10th	Night	4 5% 2 4 4 2 4 5% 6% 10% 53%	1790
	11th to 20th	Night	4 7% 1 4 3 3 4 5% 7% 12% 51%	1780
	21st to end	Night	2 7% 3 4 3 2 4 5% 5% 9% 56%	1980
September	1st to 10th	Night	4 6% 2 4 4 2 3 5% 5% 9% 56%	1800
	11th to 20th	Night	4 5% 2 3 3 2 3 4 5% 9% 61%	1920
	21st to end	Night	2 5% 2 4 3 2 2 3 4 7% 64%	1980
October	1st to 10th	Night	4 4 2 4 4 2 3 4 4 8% 62%	1980
	11th to 20th	Night	5% 6% 1 4 3 2 3 3 4 6% 63%	2108
	21st to end	Night	3 4 1 4 3 2 3 3 4 8% 65%	2384
November	1st to 10th	Night	3 4 2 3 3 2 3 3 5% 8% 62%	2405
	11th to 20th	Night	3 5% 2 4 4 2 3 4 4 6% 63%	2470
	21st to end	Night	5% 4 2 4 4 3 4 4 6% 8% 56%	2470
December	1st to 10th	Night	7% 5% 2 5% 4 3 3 3 5% 7% 55%	2470
	11th to 20th	Night	14% 7% 3 5% 5% 2 4 4 5% 8% 44%	3041
	21st to end	Night	13% 6% 3 6% 6% 3 5% 5% 5% 6% 42%	4676

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	12% 23% 3 24% 23% 14%	2539
	11th to 20th	Day	24% 24% 14% 12% 13% 13%	2872
	21st to end	Day	28% 22% 4 15% 20% 10%	3498
February	1st to 10th	Day	19% 34% 3 11% 18% 16%	1900
	11th to 20th	Day	17% 35% 5% 15% 17% 11%	1860
	21st to end	Day	20% 34% 5% 15% 14% 12%	1570
March	1st to 10th	Day	17% 34% 8% 15% 13% 13%	1900
	11th to 20th	Day	19% 33% 8% 16% 16% 8%	1961
	21st to end	Day	18% 36% 7% 16% 15% 9%	2401
April	1st to 10th	Day	17% 36% 10% 17% 13% 8%	2160
	11th to 20th	Day	14% 35% 6% 19% 17% 9%	2132
	21st to end	Day	14% 36% 4 17% 21% 8%	2160
May	1st to 10th	Day	15% 36% 6% 15% 18% 10%	2182
	11th to 20th	Day	17% 36% 3 16% 21% 7%	2304
	21st to end	Day	14% 36% 5% 18% 21% 5%	2530
June	1st to 10th	Day	21% 36% 7% 12% 18% 7%	2354
	11th to 20th	Day	14% 35% 5% 8% 23% 15%	2520
	21st to end	Day	17% 35% 1 5% 18% 24%	2520
July	1st to 10th	Day	15% 37% 1 5% 14% 28%	2451
	11th to 20th	Day	13% 36% 2 12% 20% 17%	2403
	21st to end	Day	15% 37% 5% 13% 21% 10%	2220
August	1st to 10th	Day	13% 36% 2 10% 20% 20%	2135
	11th to 20th	Day	14% 34% 1 11% 23% 17%	2122
	21st to end	Day	11% 36% 2 8% 22% 21%	2376
September	1st to 10th	Day	14% 36% 2 6% 19% 23%	2160
	11th to 20th	Day	14% 36% 2 8% 21% 20%	2039
	21st to end	Day	13% 36% 2 11% 18% 20%	1980
October	1st to 10th	Day	16% 36% 5% 13% 16% 14%	1980
	11th to 20th	Day	13% 35% 4 12% 16% 20%	1848
	21st to end	Day	13% 36% 4 10% 13% 23%	1968
November	1st to 10th	Day	16% 40% 4 13% 12% 15%	1674
	11th to 20th	Day	13% 38% 4 10% 13% 23%	1710
	21st to end	Day	15% 39% 4 8% 15% 18%	1710
December	1st to 10th	Day	19% 39% 4 11% 12% 15%	1710
	11th to 20th	Day	31% 32% 6% 10% 13% 8%	2168
	21st to end	Day	35% 26% 11% 14% 10% 5%	2977
January	1st to 10th	Night	16% 25% 5% 12% 25% 16%	4406
	11th to 20th	Night	26% 24% 10% 17% 11% 13%	4076
	21st to end	Night	29% 23% 5% 14% 18% 11%	4180
February	1st to 10th	Night	19% 34% 4 10% 16% 17%	2280
	11th to 20th	Night	19% 35% 4 14% 15% 12%	2219
	21st to end	Night	22% 34% 4 12% 14% 14%	1884
March	1st to 10th	Night	20% 35% 5% 15% 14% 11%	2280
	11th to 20th	Night	22% 36% 7% 13% 14% 9%	2213
	21st to end	Night	18% 37% 5% 16% 15% 9%	2000
April	1st to 10th	Night	19% 36% 6% 19% 11% 8%	1800
	11th to 20th	Night	17% 35% 4 17% 16% 11%	1779
	21st to end	Night	23% 36% 4 13% 17% 6%	1800
May	1st to 10th	Night	19% 36% 5% 14% 16% 9%	1775
	11th to 20th	Night	24% 37% 2 14% 16% 7%	1595
	21st to end	Night	16% 35% 6% 16% 21% 6%	1746
June	1st to 10th	Night	26% 36% 8% 10% 14% 6%	1600
	11th to 20th	Night	19% 35% 4 9% 23% 10%	1440
	21st to end	Night	20% 36% 2 4 17% 21%	1440
July	1st to 10th	Night	15% 37% 2 5% 14% 27%	1400
	11th to 20th	Night	16% 37% 2 10% 18% 17%	1426
	21st to end	Night	16% 39% 6% 15% 15% 10%	1860
August	1st to 10th	Night	17% 37% 2 8% 20% 17%	1780
	11th to 20th	Night	17% 36% 1 10% 19% 17%	1780
	21st to end	Night	16% 38% 1 5% 19% 22%	1980
September	1st to 10th	Night	15% 36% 3 4 18% 25%	1800
	11th to 20th	Night	13% 36% 1 7% 20% 22%	1920
	21st to end	Night	15% 35% 1 9% 19% 20%	1980
October	1st to 10th	Night	16% 36% 3 11% 16% 17%	1980
	11th to 20th	Night	16% 36% 3 8% 16% 21%	2108
	21st to end	Night	13% 36% 2 7% 16% 26%	2384
November	1st to 10th	Night	16% 40% 3 10% 13% 17%	2405
	11th to 20th	Night	12% 39% 3 8% 15% 23%	2470
	21st to end	Night	17% 39% 3 10% 12% 19%	2470
December	1st to 10th	Night	19% 39% 3 8% 14% 17%	2470
	11th to 20th	Night	32% 32% 5% 11% 13% 8%	3041
	21st to end	Night	32% 28% 7% 18% 9% 5%	4676

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	12% 5% 11% 38% 27% 8%	180
	11th to 20th	Day	11% 3 13% 35% 31% 7%	374
	21st to end	Day	10% 5% 15% 38% 27% 5%	506
February	1st to 10th	Day	11% 5% 19% 30% 27% 8%	460
	11th to 20th	Day	8% 9% 16% 35% 26% 6%	455
	21st to end	Day	9% 4 23% 37% 23% 4	382
March	1st to 10th	Day	8% 8% 23% 36% 20% 4	460
	11th to 20th	Day	8% 5% 20% 36% 25% 5%	460
	21st to end	Day	5% 7% 23% 39% 22% 4	471
April	1st to 10th	Day	7% 5% 19% 37% 25% 8%	420
	11th to 20th	Day	4 6% 23% 42% 20% 6%	412
	21st to end	Day	3 6% 20% 34% 29% 8%	420
May	1st to 10th	Day	3 3 18% 37% 31% 7%	423
	11th to 20th	Day	4 4 15% 38% 32% 7%	448
	21st to end	Day	5% 4 18% 38% 28% 6%	485
June	1st to 10th	Day	7% 2 11% 32% 38% 9%	450
	11th to 20th	Day	4 1 9% 31% 44% 11%	450
	21st to end	Day	4 1 7% 31% 46% 11%	450
July	1st to 10th	Day	3 3 22% 54% 18%	446
	11th to 20th	Day	2 2 12% 55% 28%	444
	21st to end	Day	1 1 2 21% 52% 23%	487
August	1st to 10th	Day	3 3 26% 49% 19%	447
	11th to 20th	Day	2 1 2 21% 56% 18%	437
	21st to end	Day	1 1 3 26% 53% 16%	495
September	1st to 10th	Day	4 2 4 31% 45% 13%	450
	11th to 20th	Day	3 1 6% 40% 40% 10%	450
	21st to end	Day	2 2 10% 32% 39% 15%	450
October	1st to 10th	Day	4 3 12% 32% 39% 10%	450
	11th to 20th	Day	4 1 12% 38% 32% 13%	450
	21st to end	Day	2 3 12% 31% 36% 16%	483
November	1st to 10th	Day	4 4 10% 26% 40% 16%	270
	11th to 20th	Day	7% 1 6% 33% 40% 12%	270
	21st to end	Day	3 3 9% 34% 37% 14%	270
December	1st to 10th	Day	9% 1 11% 38% 33% 8%	270
	11th to 20th	Day	5% 3 11% 35% 32% 14%	270
	21st to end	Day	12% 8% 17% 31% 25% 6%	248
January	1st to 10th	Night	10% 4 11% 37% 31% 8%	760
	11th to 20th	Night	9% 3 11% 34% 34% 9%	760
	21st to end	Night	11% 3 14% 35% 32% 6%	836
February	1st to 10th	Night	10% 5% 11% 30% 35% 10%	760
	11th to 20th	Night	9% 5% 13% 33% 31% 8%	744
	21st to end	Night	10% 5% 14% 37% 29% 5%	628
March	1st to 10th	Night	8% 5% 16% 37% 28% 6%	760
	11th to 20th	Night	8% 4 11% 34% 32% 10%	760
	21st to end	Night	7% 4 17% 35% 31% 7%	800
April	1st to 10th	Night	11% 3 11% 32% 35% 8%	720
	11th to 20th	Night	7% 4 13% 30% 36% 9%	711
	21st to end	Night	8% 5% 11% 26% 38% 13%	720
May	1st to 10th	Night	8% 3 11% 29% 38% 11%	695
	11th to 20th	Night	8% 4 9% 27% 39% 14%	531
	21st to end	Night	4 3 11% 29% 39% 13%	584
June	1st to 10th	Night	5% 3 8% 28% 41% 15%	520
	11th to 20th	Night	5% 2 5% 16% 48% 25%	360
	21st to end	Night	8% 2 5% 17% 42% 26%	360
July	1st to 10th	Night	6% 1 2 5% 40% 47%	350
	11th to 20th	Night	4 1 2 4 33% 55%	382
	21st to end	Night	7% 1 3 5% 44% 41%	788
August	1st to 10th	Night	4 1 3 12% 45% 35%	716
	11th to 20th	Night	7% 2 2 7% 43% 40%	712
	21st to end	Night	5% 1 9% 47% 38%	792
September	1st to 10th	Night	7% 1 3 13% 45% 30%	720
	11th to 20th	Night	5% 1 3 18% 50% 23%	720
	21st to end	Night	3 2 5% 21% 45% 23%	720
October	1st to 10th	Night	5% 2 6% 28% 46% 13%	720
	11th to 20th	Night	8% 2 6% 24% 42% 18%	720
	21st to end	Night	4 2 7% 28% 39% 20%	792
November	1st to 10th	Night	4 4 8% 25% 43% 16%	740
	11th to 20th	Night	4 1 5% 31% 40% 18%	760
	21st to end	Night	6% 3 10% 38% 34% 9%	760
December	1st to 10th	Night	8% 2 11% 36% 35% 7%	760
	11th to 20th	Night	12% 4 10% 38% 30% 6%	760
	21st to end	Night	10% 5% 15% 36% 27% 7%	836

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