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 2025-08-05.

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% 5% 2 4 3 3 3 3 5% 9% 60%	129567
	Night	5% 7% 2 5% 3 4 4 3 5% 7% 55%	127461
Transparency	Day	15% 38% 5% 11% 17% 15%	129292
	Night	17% 38% 5% 11% 15% 13%	127250
Seeing	Day	5% 4 13% 33% 35% 11%	16696
	Night	7% 3 9% 27% 37% 16%	26532

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	4 3 1 3 2 2 2 2 4 6% 72%	3639
	11th to 20th	Day	7% 4 2 3 3 3 3 2 4 7% 64%	4126
	21st to end	Day	10% 5% 2 4 3 3 4 3 6% 9% 52%	5016
February	1st to 10th	Day	9% 5% 2 4 3 3 4 3 4 6% 58%	2000
	11th to 20th	Day	8% 4 1 3 3 3 3 3 6% 8% 58%	1950
	21st to end	Day	7% 5% 1 4 2 3 4 3 5% 8% 58%	1650
March	1st to 10th	Day	9% 4 2 4 3 4 4 4 5% 7% 54%	4518
	11th to 20th	Day	8% 5% 1 4 3 3 3 3 5% 9% 56%	4721
	21st to end	Day	8% 5% 2 5% 3 4 4 3 6% 9% 52%	5969
April	1st to 10th	Day	5% 4 2 4 2 3 4 3 5% 9% 59%	2400
	11th to 20th	Day	4 4 1 4 3 3 2 3 5% 8% 63%	2369
	21st to end	Day	4 4 1 3 3 2 2 3 5% 9% 63%	2400
May	1st to 10th	Day	4 6% 2 5% 3 3 3 2 5% 8% 60%	5501
	11th to 20th	Day	5% 8% 3 6% 3 3 4 3 5% 8% 52%	5841
	21st to end	Day	4 7% 2 7% 4 3 3 3 4 8% 54%	6423
June	1st to 10th	Day	7% 9% 2 6% 3 2 3 3 6% 11% 49%	2616
	11th to 20th	Day	3 6% 1 4 2 2 3 2 5% 10% 60%	2800
	21st to end	Day	3 6% 1 4 3 2 3 3 6% 9% 60%	2800
July	1st to 10th	Day	1 3 1 4 2 2 3 3 5% 10% 65%	6319
	11th to 20th	Day	2 4 2 5% 3 3 3 3 6% 10% 59%	6200
	21st to end	Day	2 3 1 4 3 2 3 3 5% 8% 67%	5923
August	1st to 10th	Day	1 2 1 3 2 2 3 2 5% 10% 69%	3832
	11th to 20th	Day	1 2 1 2 2 2 2 2 5% 9% 70%	3724
	21st to end	Day	1 2 1 4 3 2 2 3 4 10% 68%	4180
September	1st to 10th	Day	3 3 1 3 3 2 3 3 5% 9% 65%	2280
	11th to 20th	Day	3 5% 1 3 2 2 3 3 6% 10% 61%	2152
	21st to end	Day	2 4 1 4 2 2 3 3 5% 9% 65%	2090
October	1st to 10th	Day	2 2 1 4 3 3 3 2 5% 8% 66%	3490
	11th to 20th	Day	3 5% 2 5% 3 3 3 2 4 9% 61%	3235
	21st to end	Day	3 4 1 3 2 3 3 4 6% 7% 65%	3423
November	1st to 10th	Day	2 4 1 3 3 2 4 2 4 7% 66%	1764
	11th to 20th	Day	5% 4 1 3 3 2 3 3 4 6% 67%	1800
	21st to end	Day	5% 4 1 4 3 4 3 4 5% 8% 59%	1800
December	1st to 10th	Day	4 4 2 5% 2 3 3 4 5% 8% 59%	2950
	11th to 20th	Day	8% 4 2 4 4 3 4 4 6% 7% 52%	3409
	21st to end	Day	9% 6% 3 8% 5% 4 5% 4 7% 10% 39%	4257
January	1st to 10th	Night	5% 4 1 4 3 3 3 3 4 6% 65%	6306
	11th to 20th	Night	6% 6% 2 4 3 3 3 3 5% 7% 59%	5820
	21st to end	Night	9% 5% 2 5% 4 4 5% 4 6% 7% 50%	5984
February	1st to 10th	Night	8% 6% 2 5% 4 3 3 3 5% 7% 54%	2400
	11th to 20th	Night	8% 6% 2 4 3 3 4 3 5% 6% 56%	2338
	21st to end	Night	9% 5% 2 5% 3 3 3 3 4 6% 56%	1980
March	1st to 10th	Night	7% 6% 2 5% 4 4 4 3 7% 9% 49%	5376
	11th to 20th	Night	7% 5% 2 7% 4 4 4 4 5% 8% 50%	5273
	21st to end	Night	8% 8% 2 5% 3 3 3 3 6% 8% 51%	5006
April	1st to 10th	Night	8% 6% 2 4 3 3 3 3 5% 7% 56%	2000
	11th to 20th	Night	5% 7% 2 4 3 3 2 2 4 7% 62%	1979
	21st to end	Night	5% 10% 3 5% 2 3 2 3 5% 7% 54%	2000
May	1st to 10th	Night	4 12% 3 5% 3 3 3 3 4 5% 55%	4479
	11th to 20th	Night	4 13% 2 4 3 3 4 3 5% 7% 53%	4074
	21st to end	Night	3 8% 2 4 3 3 3 3 4 6% 61%	4474
June	1st to 10th	Night	5% 13% 3 6% 5% 3 3 3 5% 7% 46%	1777
	11th to 20th	Night	3 10% 3 5% 4 2 3 3 5% 8% 55%	1600
	21st to end	Night	4 10% 2 6% 4 3 4 3 6% 9% 49%	1600
July	1st to 10th	Night	2 8% 2 4 3 4 3 3 4 7% 61%	3640
	11th to 20th	Night	1 9% 3 5% 3 4 4 3 6% 7% 56%	3730
	21st to end	Night	2 5% 1 4 3 3 5% 3 4 6% 65%	4974
August	1st to 10th	Night	2 3 1 4 3 5% 5% 4 5% 8% 59%	3206
	11th to 20th	Night	2 4 1 4 3 5% 4 4 7% 10% 56%	3126
	21st to end	Night	1 5% 2 5% 4 4 6% 4 5% 7% 57%	3498
September	1st to 10th	Night	4 6% 2 4 3 3 4 4 5% 9% 57%	1900
	11th to 20th	Night	4 5% 1 3 3 3 4 3 5% 8% 61%	2027
	21st to end	Night	2 6% 2 5% 3 2 3 2 4 7% 63%	2090
October	1st to 10th	Night	2 3 3 8% 6% 4 4 3 4 6% 57%	3490
	11th to 20th	Night	4 14% 1 5% 3 4 4 3 5% 5% 52%	3699
	21st to end	Night	2 3 1 4 3 3 4 3 5% 7% 65%	4141
November	1st to 10th	Night	3 5% 2 4 3 3 3 3 5% 8% 62%	2535
	11th to 20th	Night	3 5% 2 4 3 3 4 3 4 6% 64%	2600
	21st to end	Night	5% 5% 2 4 4 4 4 3 7% 8% 55%	2600
December	1st to 10th	Night	4 5% 2 5% 3 4 4 4 6% 7% 55%	4240
	11th to 20th	Night	9% 5% 2 5% 3 4 5% 4 6% 8% 48%	4811
	21st to end	Night	9% 6% 3 9% 6% 5% 6% 4 6% 7% 37%	6688

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	10% 25% 4 19% 23% 20%	3639
	11th to 20th	Day	17% 26% 10% 12% 12% 22%	4126
	21st to end	Day	22% 28% 3 13% 19% 15%	5016
February	1st to 10th	Day	13% 62% 3 6% 10% 7%	2000
	11th to 20th	Day	10% 61% 3 7% 9% 11%	1960
	21st to end	Day	10% 61% 4 9% 9% 8%	1650
March	1st to 10th	Day	20% 30% 9% 13% 16% 11%	4518
	11th to 20th	Day	19% 29% 8% 20% 16% 8%	4721
	21st to end	Day	21% 30% 9% 24% 12% 4	5969
April	1st to 10th	Day	8% 62% 3 9% 13% 6%	2400
	11th to 20th	Day	8% 62% 2 7% 12% 8%	2372
	21st to end	Day	8% 62% 5% 12% 9% 4	2400
May	1st to 10th	Day	17% 30% 7% 15% 23% 8%	5501
	11th to 20th	Day	23% 30% 6% 13% 23% 5%	5844
	21st to end	Day	22% 31% 5% 18% 18% 6%	6424
June	1st to 10th	Day	13% 61% 3 9% 10% 4	2616
	11th to 20th	Day	8% 61% 2 6% 11% 11%	2800
	21st to end	Day	9% 60% 2 5% 11% 13%	2800
July	1st to 10th	Day	13% 24% 2 4 12% 44%	6251
	11th to 20th	Day	17% 25% 4 5% 28% 21%	6125
	21st to end	Day	13% 24% 3 10% 26% 23%	5753
August	1st to 10th	Day	9% 35% 4 9% 25% 18%	3819
	11th to 20th	Day	8% 36% 2 7% 16% 31%	3760
	21st to end	Day	9% 36% 3 4 17% 31%	4179
September	1st to 10th	Day	7% 59% 2 5% 15% 12%	2280
	11th to 20th	Day	8% 59% 2 5% 15% 11%	2152
	21st to end	Day	6% 59% 2 6% 16% 10%	2090
October	1st to 10th	Day	10% 35% 2 5% 13% 35%	3490
	11th to 20th	Day	14% 37% 2 9% 25% 14%	3235
	21st to end	Day	12% 36% 3 5% 24% 20%	3423
November	1st to 10th	Day	4 56% 3 6% 12% 19%	1764
	11th to 20th	Day	10% 57% 4 6% 8% 15%	1800
	21st to end	Day	11% 56% 4 7% 9% 12%	1800
December	1st to 10th	Day	13% 35% 3 15% 13% 20%	2950
	11th to 20th	Day	21% 31% 5% 11% 20% 13%	3408
	21st to end	Day	31% 28% 10% 15% 11% 5%	4257
January	1st to 10th	Night	14% 27% 4 13% 24% 20%	6306
	11th to 20th	Night	21% 26% 8% 14% 11% 20%	5820
	21st to end	Night	24% 29% 5% 10% 18% 14%	5984
February	1st to 10th	Night	14% 62% 3 7% 8% 7%	2400
	11th to 20th	Night	11% 61% 2 7% 8% 11%	2339
	21st to end	Night	11% 62% 2 6% 10% 8%	1980
March	1st to 10th	Night	17% 32% 8% 13% 21% 8%	5376
	11th to 20th	Night	21% 30% 8% 21% 11% 9%	5273
	21st to end	Night	20% 31% 13% 20% 15% 2	5006
April	1st to 10th	Night	9% 63% 4 8% 10% 5%	2000
	11th to 20th	Night	8% 62% 3 8% 12% 7%	1979
	21st to end	Night	8% 62% 5% 12% 9% 4	2000
May	1st to 10th	Night	21% 31% 6% 13% 23% 7%	4476
	11th to 20th	Night	21% 30% 9% 17% 19% 4	4075
	21st to end	Night	18% 29% 7% 21% 19% 5%	4474
June	1st to 10th	Night	13% 63% 5% 8% 8% 3	1777
	11th to 20th	Night	8% 62% 3 5% 12% 9%	1600
	21st to end	Night	14% 60% 2 5% 8% 11%	1600
July	1st to 10th	Night	17% 24% 3 10% 8% 37%	3600
	11th to 20th	Night	19% 24% 7% 7% 24% 19%	3682
	21st to end	Night	15% 24% 5% 12% 26% 18%	4832
August	1st to 10th	Night	13% 36% 6% 10% 20% 15%	3196
	11th to 20th	Night	16% 36% 3 9% 16% 20%	3156
	21st to end	Night	16% 36% 5% 4 19% 21%	3498
September	1st to 10th	Night	10% 60% 2 4 13% 11%	1900
	11th to 20th	Night	8% 60% 2 6% 13% 11%	2027
	21st to end	Night	7% 59% 1 6% 16% 10%	2090
October	1st to 10th	Night	21% 36% 3 6% 9% 25%	3490
	11th to 20th	Night	23% 37% 4 7% 16% 13%	3699
	21st to end	Night	12% 36% 4 5% 18% 25%	4141
November	1st to 10th	Night	5% 55% 3 7% 11% 20%	2535
	11th to 20th	Night	10% 57% 3 7% 9% 15%	2600
	21st to end	Night	12% 57% 4 7% 9% 10%	2600
December	1st to 10th	Night	18% 35% 4 9% 15% 18%	4240
	11th to 20th	Night	23% 32% 5% 7% 16% 16%	4811
	21st to end	Night	34% 29% 8% 16% 8% 4	6688

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	11% 5% 14% 37% 27% 8%	200
	11th to 20th	Day	10% 3 14% 37% 29% 6%	405
	21st to end	Day	9% 5% 15% 38% 27% 5%	550
February	1st to 10th	Day	11% 5% 20% 30% 26% 8%	500
	11th to 20th	Day	9% 9% 16% 35% 25% 6%	495
	21st to end	Day	8% 4 24% 38% 22% 4	414
March	1st to 10th	Day	9% 9% 23% 35% 19% 4	500
	11th to 20th	Day	9% 6% 21% 35% 24% 5%	500
	21st to end	Day	6% 8% 24% 37% 21% 4	550
April	1st to 10th	Day	6% 6% 19% 36% 25% 9%	500
	11th to 20th	Day	4 7% 23% 40% 20% 6%	492
	21st to end	Day	3 7% 20% 35% 28% 8%	500
May	1st to 10th	Day	4 4 19% 37% 30% 7%	503
	11th to 20th	Day	4 5% 17% 38% 30% 6%	528
	21st to end	Day	4 5% 18% 38% 29% 6%	573
June	1st to 10th	Day	7% 4 12% 32% 36% 9%	530
	11th to 20th	Day	5% 2 10% 34% 41% 9%	530
	21st to end	Day	4 1 8% 33% 44% 10%	530
July	1st to 10th	Day	3 1 4 25% 51% 16%	526
	11th to 20th	Day	2 1 3 15% 54% 25%	524
	21st to end	Day	1 1 2 23% 50% 22%	575
August	1st to 10th	Day	2 3 27% 49% 19%	504
	11th to 20th	Day	2 1 3 22% 55% 17%	477
	21st to end	Day	1 2 4 28% 50% 16%	539
September	1st to 10th	Day	4 2 5% 31% 45% 13%	490
	11th to 20th	Day	3 2 8% 40% 37% 9%	490
	21st to end	Day	3 2 10% 34% 38% 14%	490
October	1st to 10th	Day	3 2 12% 30% 41% 10%	490
	11th to 20th	Day	4 2 12% 38% 32% 12%	490
	21st to end	Day	2 3 12% 32% 35% 16%	526
November	1st to 10th	Day	3 5% 11% 25% 39% 17%	300
	11th to 20th	Day	7% 1 8% 34% 39% 11%	300
	21st to end	Day	4 3 11% 34% 36% 12%	300
December	1st to 10th	Day	8% 1 10% 38% 34% 9%	300
	11th to 20th	Day	5% 3 12% 34% 33% 13%	300
	21st to end	Day	11% 8% 18% 33% 24% 5%	275
January	1st to 10th	Night	10% 4 11% 37% 30% 8%	800
	11th to 20th	Night	8% 3 11% 35% 34% 8%	800
	21st to end	Night	10% 3 14% 36% 31% 5%	880
February	1st to 10th	Night	10% 5% 12% 31% 34% 9%	800
	11th to 20th	Night	9% 5% 14% 34% 30% 8%	784
	21st to end	Night	10% 5% 14% 38% 29% 4	660
March	1st to 10th	Night	9% 5% 17% 37% 27% 6%	800
	11th to 20th	Night	9% 4 12% 35% 31% 10%	800
	21st to end	Night	7% 4 17% 34% 30% 7%	880
April	1st to 10th	Night	11% 4 12% 32% 34% 8%	800
	11th to 20th	Night	7% 4 13% 30% 36% 9%	791
	21st to end	Night	8% 5% 11% 26% 38% 12%	800
May	1st to 10th	Night	8% 4 11% 29% 37% 11%	771
	11th to 20th	Night	8% 4 10% 28% 37% 13%	591
	21st to end	Night	5% 3 10% 29% 40% 12%	650
June	1st to 10th	Night	6% 4 8% 28% 41% 14%	577
	11th to 20th	Night	6% 2 5% 20% 46% 23%	400
	21st to end	Night	8% 2 5% 18% 43% 25%	400
July	1st to 10th	Night	5% 1 2 6% 41% 45%	390
	11th to 20th	Night	4 1 2 5% 35% 52%	426
	21st to end	Night	6% 1 3 7% 44% 39%	876
August	1st to 10th	Night	4 1 3 12% 45% 35%	772
	11th to 20th	Night	6% 2 2 7% 43% 39%	752
	21st to end	Night	5% 2 10% 47% 37%	836
September	1st to 10th	Night	6% 1 3 14% 46% 29%	760
	11th to 20th	Night	5% 2 3 20% 49% 22%	760
	21st to end	Night	3 2 5% 22% 45% 22%	760
October	1st to 10th	Night	4 2 6% 27% 47% 14%	760
	11th to 20th	Night	8% 2 7% 24% 42% 18%	760
	21st to end	Night	4 2 8% 28% 39% 19%	836
November	1st to 10th	Night	3 4 9% 26% 42% 15%	780
	11th to 20th	Night	4 2 6% 31% 39% 17%	800
	21st to end	Night	7% 3 10% 38% 34% 9%	800
December	1st to 10th	Night	8% 2 11% 36% 35% 8%	800
	11th to 20th	Night	12% 4 10% 38% 31% 6%	800
	21st to end	Night	9% 5% 15% 36% 27% 7%	880

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