Tranquility Base Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Tranquility Base Observatory clear sky chart from 2005-11-06 to 2026-05-02.

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	13% 7% 3 6% 5% 5% 5% 4 6% 8% 41%	131902
	Night	13% 7% 2 6% 4 4 4 3 5% 7% 44%	130609
Transparency	Day	40% 6% 11% 20% 18% 5%	135668
	Night	38% 5% 10% 20% 20% 7%	134916
Seeing	Day	17% 11% 25% 24% 16% 7%	16447
	Night	16% 9% 24% 29% 19% 4	26487

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	20% 10% 5% 5% 3 2 3 2 3 4 41%	5258
	11th to 20th	Day	10% 6% 4 6% 2 2 3 2 3 5% 56%	5478
	21st to end	Day	23% 9% 2 7% 4 4 4 3 4 4 36%	6436
February	1st to 10th	Day	18% 10% 3 6% 4 3 4 3 4 5% 40%	2100
	11th to 20th	Day	23% 9% 2 4 4 3 4 3 4 4 39%	2048
	21st to end	Day	19% 8% 3 7% 5% 3 4 3 3 6% 39%	1730
March	1st to 10th	Day	19% 9% 3 7% 5% 4 4 3 5% 5% 36%	4580
	11th to 20th	Day	12% 8% 2 5% 4 4 4 4 5% 7% 45%	4896
	21st to end	Day	15% 7% 3 7% 4 4 5% 3 5% 6% 41%	5850
April	1st to 10th	Day	11% 7% 3 5% 4 4 4 4 5% 7% 46%	2520
	11th to 20th	Day	11% 6% 2 6% 5% 4 4 3 5% 7% 48%	2494
	21st to end	Day	9% 5% 2 6% 4 4 5% 4 6% 8% 47%	2520
May	1st to 10th	Day	14% 7% 3 7% 5% 6% 6% 5% 6% 8% 34%	5821
	11th to 20th	Day	16% 8% 3 8% 5% 5% 5% 5% 7% 8% 30%	5861
	21st to end	Day	12% 6% 2 7% 4 5% 5% 4 6% 8% 40%	6433
June	1st to 10th	Day	10% 6% 2 8% 7% 6% 7% 6% 7% 10% 32%	2600
	11th to 20th	Day	6% 4 2 6% 5% 5% 6% 5% 8% 12% 41%	2600
	21st to end	Day	7% 5% 2 6% 6% 6% 7% 5% 7% 10% 39%	2600
July	1st to 10th	Day	6% 5% 2 8% 6% 7% 8% 6% 9% 11% 33%	5900
	11th to 20th	Day	6% 5% 2 6% 6% 7% 8% 6% 10% 13% 31%	5900
	21st to end	Day	8% 7% 3 9% 7% 8% 8% 7% 10% 11% 23%	6321
August	1st to 10th	Day	4 3 1 5% 4 5% 6% 5% 8% 11% 49%	3905
	11th to 20th	Day	6% 4 2 6% 5% 6% 6% 5% 7% 9% 46%	3849
	21st to end	Day	7% 4 2 5% 5% 6% 5% 4 6% 9% 48%	4312
September	1st to 10th	Day	10% 4 2 7% 5% 6% 6% 5% 6% 8% 41%	2345
	11th to 20th	Day	12% 7% 2 6% 4 5% 4 4 6% 7% 42%	2200
	21st to end	Day	10% 5% 3 6% 4 4 4 3 5% 6% 49%	2200
October	1st to 10th	Day	9% 4 2 6% 4 4 4 3 5% 5% 54%	2200
	11th to 20th	Day	9% 4 2 5% 3 2 3 3 5% 7% 57%	2200
	21st to end	Day	13% 5% 2 5% 3 3 3 3 4 6% 53%	2065
November	1st to 10th	Day	12% 6% 2 5% 3 3 3 3 4 5% 53%	1852
	11th to 20th	Day	12% 6% 2 4 3 3 3 3 4 7% 52%	1890
	21st to end	Day	14% 7% 2 6% 4 3 4 4 5% 7% 45%	1890
December	1st to 10th	Day	20% 8% 2 5% 3 3 3 3 4 5% 43%	3020
	11th to 20th	Day	15% 9% 4 7% 4 4 4 3 4 5% 42%	3463
	21st to end	Day	19% 10% 4 6% 3 4 3 3 4 6% 38%	4565
January	1st to 10th	Night	17% 10% 3 6% 3 3 3 3 4 5% 41%	7635
	11th to 20th	Night	15% 9% 4 6% 3 2 3 2 3 5% 47%	7463
	21st to end	Night	20% 9% 2 8% 3 3 3 2 3 4 41%	7745
February	1st to 10th	Night	17% 8% 3 6% 4 4 4 3 4 5% 43%	2520
	11th to 20th	Night	20% 8% 3 7% 4 3 3 3 4 4 41%	2458
	21st to end	Night	19% 6% 2 6% 5% 4 4 3 5% 6% 40%	2076
March	1st to 10th	Night	21% 9% 3 8% 4 4 4 3 4 6% 34%	5493
	11th to 20th	Night	12% 6% 2 5% 3 3 3 3 5% 6% 49%	5283
	21st to end	Night	14% 7% 3 7% 4 4 4 3 4 5% 44%	5363
April	1st to 10th	Night	11% 5% 2 5% 4 4 4 4 6% 8% 45%	2100
	11th to 20th	Night	12% 7% 2 4 3 3 4 3 4 7% 51%	2075
	21st to end	Night	11% 6% 2 5% 4 3 4 4 5% 6% 50%	2100
May	1st to 10th	Night	12% 7% 3 8% 5% 5% 5% 4 6% 8% 37%	4094
	11th to 20th	Night	15% 10% 3 9% 5% 5% 5% 4 5% 7% 31%	4032
	21st to end	Night	11% 5% 2 6% 4 5% 5% 5% 7% 10% 40%	4421
June	1st to 10th	Night	6% 5% 2 7% 6% 6% 5% 6% 8% 10% 40%	1800
	11th to 20th	Night	4 3 2 4 4 4 6% 4 7% 12% 50%	1800
	21st to end	Night	5% 5% 2 6% 6% 5% 4 5% 7% 11% 44%	1800
July	1st to 10th	Night	5% 6% 3 7% 6% 6% 6% 6% 8% 12% 37%	4058
	11th to 20th	Night	6% 5% 2 6% 5% 4 6% 5% 10% 15% 36%	4057
	21st to end	Night	7% 7% 3 9% 7% 6% 7% 6% 10% 12% 27%	4512
August	1st to 10th	Night	3 2 1 5% 4 4 4 4 7% 11% 56%	3244
	11th to 20th	Night	5% 4 2 5% 3 4 4 4 6% 9% 52%	3200
	21st to end	Night	6% 4 1 4 3 4 4 4 6% 8% 55%	3586
September	1st to 10th	Night	8% 5% 2 7% 5% 5% 5% 5% 5% 8% 44%	2034
	11th to 20th	Night	11% 5% 2 7% 5% 4 4 4 6% 7% 46%	2200
	21st to end	Night	7% 4 2 6% 4 4 4 3 7% 6% 54%	2200
October	1st to 10th	Night	8% 5% 1 5% 3 3 3 3 4 6% 60%	2200
	11th to 20th	Night	10% 5% 1 4 3 3 3 3 4 6% 57%	2200
	21st to end	Night	13% 7% 2 5% 3 3 2 2 3 4 55%	2769
November	1st to 10th	Night	11% 5% 2 4 3 4 3 3 4 5% 57%	2666
	11th to 20th	Night	12% 6% 2 4 3 2 3 3 4 6% 55%	2730
	21st to end	Night	14% 6% 2 5% 5% 4 4 3 5% 6% 48%	2730
December	1st to 10th	Night	20% 7% 2 5% 3 2 3 2 4 5% 46%	4380
	11th to 20th	Night	17% 8% 3 7% 4 3 3 2 3 4 45%	4952
	21st to end	Night	19% 8% 3 6% 4 4 4 3 5% 7% 38%	6633

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	48% 3 6% 24% 17% 3	5259
	11th to 20th	Day	32% 3 4 21% 33% 7%	5478
	21st to end	Day	53% 2 5% 13% 19% 8%	6436
February	1st to 10th	Day	47% 3 7% 19% 20% 5%	2100
	11th to 20th	Day	48% 4 5% 18% 19% 6%	2058
	21st to end	Day	48% 4 6% 13% 21% 8%	1730
March	1st to 10th	Day	50% 4 7% 13% 19% 7%	4580
	11th to 20th	Day	39% 4 6% 16% 21% 15%	4896
	21st to end	Day	43% 5% 7% 21% 19% 5%	5852
April	1st to 10th	Day	37% 5% 11% 21% 18% 8%	2520
	11th to 20th	Day	37% 4 7% 20% 24% 7%	2494
	21st to end	Day	34% 6% 10% 25% 22% 3	2520
May	1st to 10th	Day	45% 8% 11% 22% 12% 2	5857
	11th to 20th	Day	48% 8% 15% 21% 8% 1	5865
	21st to end	Day	40% 8% 11% 23% 16% 1	6447
June	1st to 10th	Day	43% 10% 13% 18% 15% 1	2600
	11th to 20th	Day	33% 9% 18% 24% 15% 2	2600
	21st to end	Day	37% 9% 17% 22% 11% 4	2600
July	1st to 10th	Day	38% 13% 19% 21% 9%	5835
	11th to 20th	Day	37% 14% 27% 18% 3 1	5831
	21st to end	Day	48% 14% 19% 16% 3	6139
August	1st to 10th	Day	25% 9% 12% 36% 17% 1	3893
	11th to 20th	Day	32% 8% 17% 25% 15% 2	3885
	21st to end	Day	32% 8% 15% 19% 23% 4	4311
September	1st to 10th	Day	38% 9% 13% 18% 16% 6%	2345
	11th to 20th	Day	38% 7% 11% 20% 19% 4	2200
	21st to end	Day	35% 6% 10% 21% 22% 6%	2200
October	1st to 10th	Day	23% 3 6% 14% 33% 21%	3590
	11th to 20th	Day	19% 3 5% 15% 31% 28%	3568
	21st to end	Day	25% 4 5% 21% 34% 12%	3302
November	1st to 10th	Day	34% 3 7% 18% 27% 11%	1852
	11th to 20th	Day	33% 3 7% 20% 27% 11%	1890
	21st to end	Day	39% 5% 6% 18% 25% 7%	1890
December	1st to 10th	Day	44% 3 5% 17% 26% 5%	3020
	11th to 20th	Day	46% 3 9% 18% 17% 6%	3460
	21st to end	Day	47% 3 6% 24% 18% 1	4565
January	1st to 10th	Night	46% 3 7% 24% 18% 2	7636
	11th to 20th	Night	41% 3 4 17% 26% 9%	7463
	21st to end	Night	48% 2 4 16% 22% 7%	7745
February	1st to 10th	Night	44% 3 5% 19% 21% 7%	2520
	11th to 20th	Night	48% 4 5% 18% 17% 8%	2459
	21st to end	Night	45% 4 7% 13% 21% 9%	2076
March	1st to 10th	Night	51% 4 6% 15% 18% 7%	5493
	11th to 20th	Night	35% 4 7% 17% 21% 16%	5283
	21st to end	Night	43% 4 6% 23% 18% 6%	5363
April	1st to 10th	Night	35% 5% 12% 21% 20% 7%	2100
	11th to 20th	Night	33% 4 8% 21% 23% 11%	2075
	21st to end	Night	34% 5% 11% 24% 20% 5%	2100
May	1st to 10th	Night	42% 9% 13% 19% 15% 2	4121
	11th to 20th	Night	51% 7% 14% 19% 8% 1	4033
	21st to end	Night	36% 8% 13% 25% 15% 2	4429
June	1st to 10th	Night	35% 9% 15% 21% 18% 2	1800
	11th to 20th	Night	24% 10% 20% 26% 17% 3	1800
	21st to end	Night	31% 9% 17% 23% 16% 4	1800
July	1st to 10th	Night	37% 10% 21% 21% 9% 1	4015
	11th to 20th	Night	32% 13% 31% 19% 4 1	4006
	21st to end	Night	45% 14% 22% 15% 4 1	4381
August	1st to 10th	Night	20% 7% 14% 40% 16% 2	3232
	11th to 20th	Night	27% 8% 17% 30% 15% 2	3230
	21st to end	Night	26% 7% 14% 22% 26% 5%	3586
September	1st to 10th	Night	35% 10% 16% 17% 16% 6%	2034
	11th to 20th	Night	36% 7% 10% 21% 21% 5%	2200
	21st to end	Night	29% 5% 11% 23% 24% 8%	2200
October	1st to 10th	Night	22% 4 5% 18% 30% 21%	3590
	11th to 20th	Night	18% 3 4 17% 30% 27%	3571
	21st to end	Night	23% 3 4 23% 33% 15%	4482
November	1st to 10th	Night	31% 4 7% 19% 26% 13%	2666
	11th to 20th	Night	31% 3 6% 19% 28% 12%	2730
	21st to end	Night	38% 5% 6% 19% 24% 9%	2730
December	1st to 10th	Night	41% 3 6% 19% 26% 5%	4380
	11th to 20th	Night	45% 3 7% 16% 24% 5%	4954
	21st to end	Night	45% 4 8% 24% 18% 2	6633

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	33% 20% 33% 9% 4	330
	11th to 20th	Day	30% 24% 29% 14% 3 1	330
	21st to end	Day	27% 27% 34% 9% 3	363
February	1st to 10th	Day	25% 24% 36% 12% 2 1	330
	11th to 20th	Day	42% 19% 29% 8% 2	330
	21st to end	Day	35% 22% 34% 8% 2	273
March	1st to 10th	Day	29% 19% 35% 12% 4 1	330
	11th to 20th	Day	25% 20% 34% 16% 4 1	450
	21st to end	Day	22% 17% 36% 20% 5%	594
April	1st to 10th	Day	23% 14% 32% 20% 10% 1	540
	11th to 20th	Day	18% 14% 35% 21% 10% 1	532
	21st to end	Day	14% 16% 30% 29% 11% 1	540
May	1st to 10th	Day	16% 9% 31% 26% 13% 5%	507
	11th to 20th	Day	11% 7% 30% 29% 16% 6%	527
	21st to end	Day	13% 6% 25% 28% 18% 10%	573
June	1st to 10th	Day	12% 5% 21% 31% 22% 10%	530
	11th to 20th	Day	8% 4 19% 28% 25% 17%	530
	21st to end	Day	10% 5% 14% 30% 26% 15%	530
July	1st to 10th	Day	8% 3 11% 27% 32% 19%	525
	11th to 20th	Day	8% 2 8% 24% 32% 26%	524
	21st to end	Day	9% 2 9% 23% 36% 21%	576
August	1st to 10th	Day	11% 4 10% 27% 29% 19%	526
	11th to 20th	Day	9% 3 10% 31% 32% 14%	519
	21st to end	Day	13% 4 10% 28% 30% 14%	583
September	1st to 10th	Day	10% 3 14% 35% 28% 11%	530
	11th to 20th	Day	12% 4 16% 28% 31% 9%	530
	21st to end	Day	12% 6% 19% 31% 24% 8%	530
October	1st to 10th	Day	11% 5% 29% 31% 17% 6%	530
	11th to 20th	Day	13% 8% 27% 35% 15% 2	530
	21st to end	Day	20% 11% 27% 29% 11% 1	392
November	1st to 10th	Day	22% 10% 34% 25% 8% 2	330
	11th to 20th	Day	17% 18% 42% 18% 6%	330
	21st to end	Day	20% 20% 35% 18% 7% 1	330
December	1st to 10th	Day	33% 18% 33% 13% 3	330
	11th to 20th	Day	26% 22% 33% 15% 5% 1	330
	21st to end	Day	31% 15% 34% 17% 4	363
January	1st to 10th	Night	24% 16% 35% 21% 4	840
	11th to 20th	Night	25% 16% 30% 23% 6%	840
	21st to end	Night	21% 18% 36% 23% 2	924
February	1st to 10th	Night	22% 15% 34% 26% 3	840
	11th to 20th	Night	28% 15% 34% 21% 3	823
	21st to end	Night	24% 18% 34% 22% 2	692
March	1st to 10th	Night	22% 14% 32% 28% 5%	840
	11th to 20th	Night	17% 14% 37% 25% 7% 1	840
	21st to end	Night	18% 14% 35% 26% 8%	828
April	1st to 10th	Night	14% 13% 34% 27% 11%	630
	11th to 20th	Night	15% 9% 36% 26% 13% 1	623
	21st to end	Night	14% 9% 27% 32% 17% 1	630
May	1st to 10th	Night	13% 7% 28% 29% 20% 3	603
	11th to 20th	Night	11% 6% 23% 35% 21% 5%	591
	21st to end	Night	8% 4 15% 37% 31% 6%	650
June	1st to 10th	Night	8% 2 14% 37% 33% 6%	600
	11th to 20th	Night	5% 2 11% 29% 39% 14%	600
	21st to end	Night	7% 3 9% 29% 42% 11%	600
July	1st to 10th	Night	7% 2 6% 28% 40% 16%	585
	11th to 20th	Night	6% 1 6% 23% 46% 18%	582
	21st to end	Night	7% 2 8% 26% 43% 14%	657
August	1st to 10th	Night	5% 2 6% 30% 43% 15%	597
	11th to 20th	Night	9% 3 7% 32% 37% 11%	594
	21st to end	Night	13% 2 7% 33% 36% 10%	660
September	1st to 10th	Night	10% 3 12% 32% 35% 8%	634
	11th to 20th	Night	13% 2 12% 31% 37% 5%	800
	21st to end	Night	8% 2 16% 34% 35% 4	800
October	1st to 10th	Night	11% 5% 23% 33% 24% 5%	800
	11th to 20th	Night	12% 6% 24% 36% 19% 2	800
	21st to end	Night	17% 8% 21% 34% 18% 2	880
November	1st to 10th	Night	15% 8% 22% 35% 17% 3	820
	11th to 20th	Night	16% 10% 30% 32% 11% 1	840
	21st to end	Night	18% 11% 31% 28% 11% 1	840
December	1st to 10th	Night	25% 14% 28% 26% 7% 1	840
	11th to 20th	Night	25% 12% 30% 25% 8%	840
	21st to end	Night	26% 13% 31% 24% 6%	924

Page generated at 2026-04-29 01:21:41. Version ac07

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