Lander
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Lander clear sky chart from 2005-11-06 to 2025-12-03.

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	3 4 1 4 3 3 4 4 6% 9% 59%	136195
	Night	3 5% 2 4 3 3 3 3 5% 8% 59%	132044
Transparency	Day	22% 5% 12% 29% 23% 10%	130989
	Night	23% 4 13% 30% 21% 9%	128254
Seeing	Day	7% 9% 29% 34% 17% 4	17167
	Night	6% 6% 28% 42% 16% 3	27286

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	3 3 7% 3 4 4 4 7% 13% 53%	3638
	11th to 20th	Day	4 2 7% 7% 5% 4 4 9% 9% 49%	3985
	21st to end	Day	3 1 4 3 3 3 3 5% 10% 64%	4510
February	1st to 10th	Day	1 4 2 6% 4 4 5% 3 8% 10% 52%	1800
	11th to 20th	Day	1 5% 2 6% 4 3 4 4 6% 10% 53%	1755
	21st to end	Day	2 3 6% 3 4 4 4 5% 10% 58%	1555
March	1st to 10th	Day	1 7% 2 4 2 3 3 3 7% 10% 58%	4944
	11th to 20th	Day	3 9% 2 4 2 2 2 3 5% 10% 57%	5000
	21st to end	Day	4 8% 3 5% 3 3 4 4 8% 12% 47%	5486
April	1st to 10th	Day	3 8% 2 5% 3 3 4 3 6% 11% 52%	2390
	11th to 20th	Day	4 9% 4 6% 4 3 4 4 7% 8% 46%	2371
	21st to end	Day	9% 9% 2 5% 4 4 5% 4 6% 10% 42%	2585
May	1st to 10th	Day	11% 9% 2 5% 3 3 4 3 5% 7% 47%	5880
	11th to 20th	Day	7% 5% 2 4 3 3 4 4 7% 9% 53%	6290
	21st to end	Day	5% 5% 1 4 3 4 4 4 7% 9% 55%	6925
June	1st to 10th	Day	4 3 1 3 4 3 4 4 8% 12% 55%	2800
	11th to 20th	Day	4 3 1 3 2 3 4 4 7% 9% 61%	2800
	21st to end	Day	2 2 1 3 2 4 4 3 6% 7% 67%	2800
July	1st to 10th	Day	2 2 1 2 2 3 3 4 6% 8% 67%	6339
	11th to 20th	Day	2 1 1 2 3 3 5% 5% 8% 9% 61%	6340
	21st to end	Day	2 2 1 3 3 4 5% 5% 7% 8% 62%	6457
August	1st to 10th	Day	3 2 1 3 2 4 5% 4 6% 9% 63%	5779
	11th to 20th	Day	2 2 1 3 3 3 5% 4 6% 8% 64%	5798
	21st to end	Day	3 2 1 3 3 3 4 4 6% 9% 63%	6160
September	1st to 10th	Day	4 3 1 3 3 3 4 3 4 7% 65%	2400
	11th to 20th	Day	4 3 1 2 3 3 3 3 5% 6% 67%	2328
	21st to end	Day	5% 3 1 3 2 3 3 3 4 5% 68%	2147
October	1st to 10th	Day	5% 4 1 3 2 2 2 2 3 5% 72%	3270
	11th to 20th	Day	6% 5% 1 3 2 2 3 2 4 6% 65%	3250
	21st to end	Day	5% 6% 2 4 3 2 3 3 5% 6% 63%	3548
November	1st to 10th	Day	1 3 2 3 2 3 3 4 6% 9% 63%	1839
	11th to 20th	Day	1 3 2 4 3 3 3 3 6% 9% 62%	1680
	21st to end	Day	1 5% 2 5% 3 2 3 3 6% 8% 61%	1680
December	1st to 10th	Day	1 2 1 4 2 3 3 3 4 6% 72%	2636
	11th to 20th	Day	3 1 4 3 3 4 3 6% 11% 60%	3026
	21st to end	Day	2 6% 3 5% 4 4 4 4 7% 10% 52%	4004
January	1st to 10th	Night	1 8% 3 5% 3 4 4 3 7% 9% 54%	6327
	11th to 20th	Night	3 2 8% 6% 5% 4 4 6% 10% 52%	6036
	21st to end	Night	4 2 6% 3 3 3 3 5% 12% 59%	6468
February	1st to 10th	Night	1 6% 2 6% 4 4 4 4 6% 8% 53%	2600
	11th to 20th	Night	1 5% 2 6% 4 4 4 4 6% 10% 54%	2532
	21st to end	Night	1 6% 3 6% 4 4 4 4 5% 6% 59%	2075
March	1st to 10th	Night	3 10% 2 4 3 3 3 3 5% 8% 58%	4947
	11th to 20th	Night	3 10% 3 5% 3 2 3 3 4 8% 56%	5000
	21st to end	Night	4 9% 3 7% 4 3 4 4 6% 10% 47%	5489
April	1st to 10th	Night	5% 8% 2 4 3 2 3 3 6% 8% 56%	2010
	11th to 20th	Night	6% 9% 3 6% 4 3 4 4 6% 8% 47%	1977
	21st to end	Night	9% 8% 3 5% 3 3 4 3 5% 8% 49%	1812
May	1st to 10th	Night	15% 7% 2 5% 2 3 3 3 4 6% 51%	4100
	11th to 20th	Night	10% 5% 2 6% 3 4 4 3 5% 6% 50%	3622
	21st to end	Night	8% 4 1 4 3 4 3 4 5% 9% 56%	3972
June	1st to 10th	Night	5% 4 2 3 3 3 3 3 6% 10% 59%	1600
	11th to 20th	Night	6% 3 1 3 2 3 3 3 6% 7% 62%	1600
	21st to end	Night	3 2 1 2 2 3 3 3 5% 6% 71%	1600
July	1st to 10th	Night	3 1 1 3 2 3 3 3 5% 7% 68%	3640
	11th to 20th	Night	1 1 1 2 2 3 3 3 6% 9% 70%	3640
	21st to end	Night	2 2 1 2 2 3 3 3 6% 9% 68%	4414
August	1st to 10th	Night	5% 2 1 3 2 3 3 3 4 7% 68%	4033
	11th to 20th	Night	2 2 1 2 3 3 4 4 6% 7% 65%	4051
	21st to end	Night	4 3 1 4 3 3 4 3 6% 8% 61%	4817
September	1st to 10th	Night	4 3 1 2 2 2 3 3 4 7% 69%	2000
	11th to 20th	Night	5% 3 1 2 3 2 4 3 5% 7% 65%	2070
	21st to end	Night	7% 4 1 2 1 1 2 2 3 6% 70%	2252
October	1st to 10th	Night	5% 5% 1 3 2 2 2 2 2 3 74%	3920
	11th to 20th	Night	7% 5% 2 4 3 3 4 3 4 5% 62%	3898
	21st to end	Night	5% 7% 2 5% 2 3 3 3 4 6% 59%	4252
November	1st to 10th	Night	1 4 1 4 3 3 3 4 6% 10% 61%	2656
	11th to 20th	Night	2 5% 2 4 3 4 3 4 6% 9% 59%	2940
	21st to end	Night	1 6% 3 4 3 3 4 3 6% 9% 58%	2940
December	1st to 10th	Night	2 2 4 3 2 3 3 4 7% 71%	4599
	11th to 20th	Night	3 1 4 3 4 3 3 6% 11% 62%	5181
	21st to end	Night	1 5% 2 5% 3 3 4 4 7% 10% 55%	6974

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	23% 5% 22% 30% 17% 2	3639
	11th to 20th	Day	29% 5% 17% 25% 21% 3	3985
	21st to end	Day	17% 4 21% 35% 19% 4	4510
February	1st to 10th	Day	27% 5% 17% 32% 14% 5%	1800
	11th to 20th	Day	26% 5% 19% 30% 15% 4	1764
	21st to end	Day	22% 4 18% 28% 22% 6%	1555
March	1st to 10th	Day	20% 4 10% 37% 19% 10%	4944
	11th to 20th	Day	24% 5% 16% 34% 21% 1	5000
	21st to end	Day	28% 7% 19% 35% 10% 1	5486
April	1st to 10th	Day	27% 6% 18% 31% 14% 4	2390
	11th to 20th	Day	32% 7% 16% 27% 18% 1	2374
	21st to end	Day	37% 6% 13% 29% 14% 1	2585
May	1st to 10th	Day	34% 6% 11% 27% 17% 5%	5880
	11th to 20th	Day	26% 6% 12% 30% 21% 5%	6294
	21st to end	Day	23% 7% 14% 38% 16% 2	6926
June	1st to 10th	Day	21% 7% 20% 26% 21% 5%	2800
	11th to 20th	Day	18% 6% 13% 23% 25% 15%	2800
	21st to end	Day	15% 6% 7% 25% 31% 16%	2800
July	1st to 10th	Day	14% 4 6% 19% 32% 25%	6271
	11th to 20th	Day	15% 4 8% 22% 29% 23%	6265
	21st to end	Day	17% 5% 8% 17% 31% 23%	6271
August	1st to 10th	Day	19% 6% 8% 26% 23% 19%	4208
	11th to 20th	Day	18% 4 6% 22% 34% 15%	4197
	21st to end	Day	13% 3 6% 19% 37% 22%	4439
September	1st to 10th	Day	19% 5% 7% 20% 32% 18%	2400
	11th to 20th	Day	18% 4 7% 28% 28% 15%	2328
	21st to end	Day	19% 4 6% 28% 34% 8%	2147
October	1st to 10th	Day	19% 3 6% 22% 27% 23%	3270
	11th to 20th	Day	22% 3 8% 34% 25% 7%	3250
	21st to end	Day	24% 2 11% 38% 21% 4	3548
November	1st to 10th	Day	19% 6% 15% 31% 22% 7%	1839
	11th to 20th	Day	20% 4 18% 32% 21% 5%	1680
	21st to end	Day	22% 5% 16% 31% 20% 5%	1680
December	1st to 10th	Day	16% 4 16% 42% 19% 3	2636
	11th to 20th	Day	19% 6% 24% 34% 14% 3	3024
	21st to end	Day	27% 5% 11% 32% 19% 5%	4004
January	1st to 10th	Night	27% 4 20% 33% 15% 2	6327
	11th to 20th	Night	28% 5% 19% 28% 16% 4	6036
	21st to end	Night	21% 5% 15% 41% 16% 3	6468
February	1st to 10th	Night	29% 5% 15% 33% 15% 5%	2600
	11th to 20th	Night	26% 5% 16% 30% 19% 4	2533
	21st to end	Night	27% 3 14% 26% 25% 5%	2075
March	1st to 10th	Night	26% 4 9% 30% 20% 10%	4947
	11th to 20th	Night	27% 4 11% 33% 22% 2	5000
	21st to end	Night	32% 6% 20% 30% 10% 1	5489
April	1st to 10th	Night	26% 6% 16% 28% 18% 6%	2010
	11th to 20th	Night	34% 5% 17% 24% 19% 2	1977
	21st to end	Night	34% 5% 13% 31% 17% 1	1812
May	1st to 10th	Night	36% 3 8% 31% 17% 5%	4097
	11th to 20th	Night	32% 6% 11% 26% 17% 8%	3623
	21st to end	Night	26% 5% 13% 36% 17% 3	3972
June	1st to 10th	Night	21% 6% 21% 27% 20% 5%	1600
	11th to 20th	Night	21% 3 11% 22% 25% 18%	1600
	21st to end	Night	14% 5% 8% 20% 32% 21%	1600
July	1st to 10th	Night	15% 3 9% 18% 33% 21%	3600
	11th to 20th	Night	12% 3 7% 22% 31% 25%	3592
	21st to end	Night	14% 3 8% 20% 29% 26%	4288
August	1st to 10th	Night	18% 4 7% 29% 21% 21%	2928
	11th to 20th	Night	18% 4 8% 25% 28% 17%	2928
	21st to end	Night	13% 3 6% 23% 26% 29%	3469
September	1st to 10th	Night	17% 4 8% 19% 30% 21%	2000
	11th to 20th	Night	18% 5% 9% 24% 30% 15%	2070
	21st to end	Night	17% 3 7% 26% 35% 12%	2252
October	1st to 10th	Night	19% 2 6% 20% 34% 19%	3920
	11th to 20th	Night	26% 3 8% 29% 26% 7%	3898
	21st to end	Night	27% 3 9% 32% 21% 8%	4252
November	1st to 10th	Night	20% 6% 15% 30% 23% 5%	2656
	11th to 20th	Night	23% 4 15% 34% 20% 4	2940
	21st to end	Night	23% 4 15% 32% 21% 5%	2940
December	1st to 10th	Night	16% 3 15% 43% 20% 3	4599
	11th to 20th	Night	19% 5% 19% 40% 15% 2	5182
	21st to end	Night	25% 5% 13% 33% 18% 6%	6974

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	3 5% 33% 40% 15% 4	300
	11th to 20th	Day	3 5% 32% 40% 19% 1	300
	21st to end	Day	3 5% 33% 39% 16% 3	330
February	1st to 10th	Day	3 13% 51% 28% 3 1	300
	11th to 20th	Day	1 8% 49% 32% 10%	300
	21st to end	Day	4 13% 42% 31% 10% 1	249
March	1st to 10th	Day	6% 17% 40% 26% 9% 2	490
	11th to 20th	Day	9% 17% 39% 25% 8% 2	500
	21st to end	Day	13% 15% 32% 27% 11% 2	550
April	1st to 10th	Day	12% 19% 29% 27% 11% 2	500
	11th to 20th	Day	11% 25% 32% 23% 8% 1	492
	21st to end	Day	17% 17% 28% 26% 12% 2	500
May	1st to 10th	Day	17% 14% 31% 23% 12% 3	503
	11th to 20th	Day	17% 8% 20% 32% 17% 5%	717
	21st to end	Day	11% 9% 23% 30% 21% 6%	789
June	1st to 10th	Day	4 9% 26% 34% 21% 5%	730
	11th to 20th	Day	6% 11% 27% 38% 16% 2	730
	21st to end	Day	3 7% 27% 38% 21% 4	730
July	1st to 10th	Day	4 4 20% 40% 27% 6%	721
	11th to 20th	Day	2 4 25% 40% 26% 3	718
	21st to end	Day	3 3 27% 39% 23% 6%	605
August	1st to 10th	Day	3 5% 27% 39% 23% 4	527
	11th to 20th	Day	3 3 25% 39% 23% 7%	517
	21st to end	Day	3 7% 28% 34% 21% 7%	583
September	1st to 10th	Day	3 5% 26% 36% 20% 9%	530
	11th to 20th	Day	6% 10% 28% 29% 21% 7%	530
	21st to end	Day	8% 6% 25% 32% 21% 8%	477
October	1st to 10th	Day	9% 6% 23% 31% 22% 9%	330
	11th to 20th	Day	7% 12% 25% 30% 16% 9%	330
	21st to end	Day	7% 7% 31% 37% 16% 1	363
November	1st to 10th	Day	6% 8% 35% 36% 10% 5%	330
	11th to 20th	Day	6% 7% 32% 36% 16% 2	330
	21st to end	Day	4 5% 28% 47% 15% 2	330
December	1st to 10th	Day	2 5% 31% 43% 16% 3	306
	11th to 20th	Day	6% 5% 38% 41% 10% 1	300
	21st to end	Day	7% 9% 35% 32% 13% 5%	330
January	1st to 10th	Night	2 8% 31% 42% 16% 2	1000
	11th to 20th	Night	2 9% 30% 42% 14% 3	865
	21st to end	Night	4 6% 29% 47% 11% 3	880
February	1st to 10th	Night	4 7% 41% 39% 9% 1	800
	11th to 20th	Night	3 7% 36% 40% 12% 2	784
	21st to end	Night	3 9% 41% 36% 10% 1	660
March	1st to 10th	Night	5% 8% 36% 40% 10% 2	800
	11th to 20th	Night	7% 8% 32% 40% 13% 2	800
	21st to end	Night	8% 9% 31% 40% 11% 1	880
April	1st to 10th	Night	10% 10% 35% 36% 10% 1	610
	11th to 20th	Night	10% 9% 35% 36% 10%	593
	21st to end	Night	13% 8% 30% 35% 12% 2	600
May	1st to 10th	Night	16% 6% 23% 34% 17% 3	600
	11th to 20th	Night	16% 4 19% 39% 20% 3	591
	21st to end	Night	13% 5% 20% 38% 21% 3	650
June	1st to 10th	Night	5% 5% 24% 39% 22% 5%	600
	11th to 20th	Night	7% 5% 32% 42% 14% 1	600
	21st to end	Night	4 2 22% 45% 25% 2	600
July	1st to 10th	Night	3 1 17% 45% 30% 4	585
	11th to 20th	Night	2 1 18% 52% 24% 2	582
	21st to end	Night	4 2 12% 49% 31% 3	657
August	1st to 10th	Night	2 2 13% 49% 31% 3	597
	11th to 20th	Night	3 1 15% 50% 28% 3	594
	21st to end	Night	3 3 19% 44% 27% 4	660
September	1st to 10th	Night	6% 3 18% 45% 22% 6%	600
	11th to 20th	Night	5% 2 21% 46% 20% 5%	670
	21st to end	Night	9% 3 21% 39% 21% 8%	800
October	1st to 10th	Night	11% 4 24% 39% 18% 4	800
	11th to 20th	Night	8% 5% 25% 41% 18% 3	800
	21st to end	Night	7% 4 30% 43% 12% 4	880
November	1st to 10th	Night	4 6% 36% 41% 11% 3	937
	11th to 20th	Night	4 10% 33% 41% 10% 2	1050
	21st to end	Night	4 7% 29% 45% 13% 2	1050
December	1st to 10th	Night	3 6% 33% 45% 12% 1	1011
	11th to 20th	Night	4 5% 35% 45% 10% 1	1000
	21st to end	Night	4 8% 33% 41% 12% 2	1100

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