Tranquility Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Tranquility Observatory clear sky chart from 2005-11-06 to 2026-06-01.

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% 9% 4 9% 5% 5% 5% 4 6% 7% 34%	131958
	Night	13% 10% 4 8% 5% 5% 4 3 5% 7% 35%	131302
Transparency	Day	47% 5% 11% 22% 13% 1	135571
	Night	47% 4 11% 22% 14% 2	135747
Seeing	Day	17% 9% 27% 24% 16% 7%	17533
	Night	18% 8% 28% 29% 15% 2	27359

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	16% 13% 8% 13% 6% 5% 4 3 5% 5% 22%	4691
	11th to 20th	Day	10% 12% 5% 15% 10% 8% 7% 4 3 4 21%	4821
	21st to end	Day	17% 13% 7% 12% 7% 6% 6% 4 5% 5% 20%	5780
February	1st to 10th	Day	17% 15% 6% 11% 6% 4 4 4 4 5% 24%	1990
	11th to 20th	Day	14% 12% 5% 12% 7% 6% 4 3 5% 6% 26%	2048
	21st to end	Day	15% 11% 4 13% 6% 4 4 3 3 5% 31%	1730
March	1st to 10th	Day	18% 14% 7% 10% 5% 4 4 3 3 5% 27%	4580
	11th to 20th	Day	11% 10% 3 8% 5% 4 4 4 5% 7% 40%	5090
	21st to end	Day	16% 11% 4 9% 5% 5% 4 4 5% 7% 31%	5586
April	1st to 10th	Day	14% 11% 4 9% 6% 5% 5% 4 5% 7% 32%	2501
	11th to 20th	Day	15% 10% 3 8% 4 4 4 3 5% 7% 37%	2554
	21st to end	Day	16% 9% 3 7% 5% 4 5% 3 6% 8% 34%	2730
May	1st to 10th	Day	20% 9% 3 9% 5% 4 4 4 6% 6% 30%	5992
	11th to 20th	Day	19% 8% 3 7% 4 4 4 3 5% 8% 35%	5991
	21st to end	Day	13% 7% 3 8% 5% 5% 5% 5% 7% 10% 32%	6576
June	1st to 10th	Day	12% 6% 2 6% 4 5% 6% 4 6% 8% 41%	2692
	11th to 20th	Day	10% 6% 2 6% 4 5% 5% 5% 7% 9% 42%	2800
	21st to end	Day	8% 6% 2 7% 5% 6% 6% 5% 6% 10% 40%	2800
July	1st to 10th	Day	9% 6% 2 7% 6% 5% 5% 5% 7% 10% 37%	6360
	11th to 20th	Day	8% 6% 2 7% 5% 6% 6% 5% 8% 11% 37%	6319
	21st to end	Day	9% 7% 3 8% 6% 6% 6% 5% 7% 9% 35%	6424
August	1st to 10th	Day	6% 5% 2 6% 5% 5% 6% 5% 7% 10% 43%	4234
	11th to 20th	Day	7% 5% 2 7% 5% 5% 5% 4 8% 11% 41%	4174
	21st to end	Day	6% 5% 2 6% 4 4 4 4 6% 10% 49%	4351
September	1st to 10th	Day	11% 4 1 5% 4 4 4 4 5% 7% 50%	2291
	11th to 20th	Day	8% 6% 2 6% 4 4 4 4 6% 8% 49%	2200
	21st to end	Day	14% 7% 3 7% 4 4 3 3 4 6% 46%	2200
October	1st to 10th	Day	11% 6% 2 6% 5% 4 3 4 5% 6% 48%	2200
	11th to 20th	Day	13% 6% 2 5% 4 4 4 3 6% 7% 46%	2033
	21st to end	Day	17% 10% 3 7% 5% 4 3 4 6% 8% 33%	2018
November	1st to 10th	Day	9% 8% 2 6% 5% 3 3 3 5% 6% 49%	1852
	11th to 20th	Day	14% 12% 3 9% 5% 4 5% 3 5% 6% 34%	1890
	21st to end	Day	17% 12% 5% 9% 5% 4 4 3 5% 6% 31%	1890
December	1st to 10th	Day	15% 12% 5% 8% 5% 3 4 3 3 5% 38%	3020
	11th to 20th	Day	17% 13% 7% 13% 5% 5% 5% 3 5% 5% 23%	3463
	21st to end	Day	22% 20% 7% 11% 6% 5% 4 3 4 5% 15%	4087
January	1st to 10th	Night	12% 14% 6% 14% 8% 6% 5% 4 5% 5% 20%	8232
	11th to 20th	Night	9% 14% 8% 16% 7% 7% 5% 4 4 5% 21%	8145
	21st to end	Night	16% 12% 5% 8% 5% 5% 4 3 5% 6% 29%	8401
February	1st to 10th	Night	17% 13% 5% 10% 6% 5% 4 3 5% 5% 27%	2625
	11th to 20th	Night	14% 11% 4 9% 6% 5% 4 3 5% 7% 31%	2458
	21st to end	Night	17% 12% 5% 10% 5% 4 4 2 4 6% 30%	2076
March	1st to 10th	Night	18% 12% 4 7% 4 3 3 3 4 5% 37%	5493
	11th to 20th	Night	10% 7% 2 7% 5% 5% 5% 4 5% 7% 43%	5090
	21st to end	Night	17% 9% 4 8% 4 5% 4 3 5% 6% 36%	5588
April	1st to 10th	Night	14% 10% 3 8% 4 4 5% 3 6% 7% 36%	2136
	11th to 20th	Night	14% 9% 3 6% 4 5% 5% 4 6% 7% 38%	2014
	21st to end	Night	16% 7% 2 7% 5% 5% 5% 3 6% 7% 37%	1890
May	1st to 10th	Night	20% 10% 3 8% 5% 5% 5% 4 6% 6% 28%	4123
	11th to 20th	Night	16% 6% 3 6% 4 4 4 4 6% 9% 37%	4122
	21st to end	Night	12% 6% 2 7% 4 5% 4 4 6% 10% 39%	4520
June	1st to 10th	Night	9% 5% 2 5% 5% 4 4 4 6% 8% 48%	1702
	11th to 20th	Night	9% 4 2 7% 4 5% 4 3 6% 11% 46%	1600
	21st to end	Night	6% 5% 2 6% 4 4 5% 4 7% 10% 47%	1600
July	1st to 10th	Night	9% 5% 2 6% 5% 5% 5% 3 6% 9% 46%	3618
	11th to 20th	Night	6% 5% 3 6% 5% 4 5% 4 7% 10% 45%	3651
	21st to end	Night	8% 6% 2 7% 5% 5% 5% 4 7% 10% 41%	4427
August	1st to 10th	Night	9% 6% 2 6% 5% 4 4 4 7% 8% 44%	2915
	11th to 20th	Night	5% 6% 3 6% 5% 5% 4 4 6% 8% 49%	2875
	21st to end	Night	3 3 1 5% 4 5% 5% 4 7% 9% 53%	3543
September	1st to 10th	Night	10% 5% 2 4 3 3 4 3 5% 7% 52%	2087
	11th to 20th	Night	8% 5% 2 5% 3 3 3 3 5% 9% 55%	2200
	21st to end	Night	13% 7% 2 5% 4 3 3 4 5% 6% 49%	2200
October	1st to 10th	Night	9% 4 2 6% 4 4 4 4 5% 6% 53%	2200
	11th to 20th	Night	14% 8% 3 6% 4 4 4 3 6% 5% 44%	2366
	21st to end	Night	17% 8% 3 7% 4 4 4 4 4 6% 41%	2820
November	1st to 10th	Night	11% 8% 2 6% 4 4 4 3 5% 6% 47%	2666
	11th to 20th	Night	15% 8% 3 8% 5% 3 4 4 5% 8% 38%	2730
	21st to end	Night	17% 12% 5% 9% 6% 4 5% 3 5% 7% 27%	2730
December	1st to 10th	Night	15% 10% 4 8% 5% 4 5% 3 5% 6% 35%	4380
	11th to 20th	Night	18% 15% 7% 11% 4 4 4 3 5% 6% 23%	4952
	21st to end	Night	23% 17% 7% 12% 5% 4 4 3 4 4 16%	7127

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	63% 3 11% 17% 6%	4692
	11th to 20th	Day	65% 4 7% 12% 10% 1	4821
	21st to end	Day	66% 3 10% 14% 6% 2	5780
February	1st to 10th	Day	62% 4 9% 14% 9% 1	1990
	11th to 20th	Day	58% 4 10% 18% 9%	2058
	21st to end	Day	55% 4 11% 20% 8% 1	1730
March	1st to 10th	Day	60% 4 6% 18% 11% 1	4580
	11th to 20th	Day	42% 4 10% 29% 13% 1	5090
	21st to end	Day	52% 5% 12% 19% 9% 3	5586
April	1st to 10th	Day	52% 4 11% 21% 11% 1	2501
	11th to 20th	Day	47% 4 13% 21% 12% 3	2554
	21st to end	Day	48% 5% 11% 23% 11% 2	2730
May	1st to 10th	Day	54% 5% 12% 18% 11%	6027
	11th to 20th	Day	48% 4 12% 24% 10% 1	5995
	21st to end	Day	43% 7% 13% 23% 12% 1	6590
June	1st to 10th	Day	40% 6% 11% 24% 17% 2	2692
	11th to 20th	Day	37% 7% 14% 24% 17% 2	2800
	21st to end	Day	38% 7% 15% 21% 15% 4	2800
July	1st to 10th	Day	40% 6% 10% 26% 17% 1	6291
	11th to 20th	Day	39% 6% 16% 24% 15% 1	6244
	21st to end	Day	43% 6% 14% 20% 15% 3	6240
August	1st to 10th	Day	33% 7% 15% 30% 14% 1	4221
	11th to 20th	Day	34% 6% 12% 31% 15%	4213
	21st to end	Day	30% 5% 14% 31% 17% 3	4350
September	1st to 10th	Day	34% 5% 12% 25% 21% 4	2291
	11th to 20th	Day	33% 4 11% 27% 23% 3	2200
	21st to end	Day	39% 4 10% 22% 23% 2	2200
October	1st to 10th	Day	24% 4 10% 30% 30% 2	3590
	11th to 20th	Day	30% 4 10% 29% 26% 2	3286
	21st to end	Day	40% 3 10% 24% 21% 2	3230
November	1st to 10th	Day	35% 4 11% 28% 18% 3	1852
	11th to 20th	Day	50% 3 11% 20% 13% 3	1890
	21st to end	Day	55% 3 10% 22% 9% 1	1890
December	1st to 10th	Day	51% 3 11% 25% 10%	3020
	11th to 20th	Day	63% 4 9% 17% 6%	3460
	21st to end	Day	72% 3 8% 11% 5%	4087
January	1st to 10th	Night	64% 4 9% 15% 8%	8233
	11th to 20th	Night	65% 4 7% 12% 10% 2	8145
	21st to end	Night	56% 3 11% 20% 9% 1	8401
February	1st to 10th	Night	58% 4 10% 18% 8% 1	2625
	11th to 20th	Night	53% 3 11% 19% 12% 1	2459
	21st to end	Night	56% 3 12% 17% 10% 2	2076
March	1st to 10th	Night	50% 3 6% 26% 12% 2	5493
	11th to 20th	Night	40% 4 12% 28% 14% 2	5090
	21st to end	Night	49% 4 9% 26% 10% 2	5588
April	1st to 10th	Night	46% 5% 12% 24% 12% 1	2136
	11th to 20th	Night	43% 4 12% 20% 17% 4	2014
	21st to end	Night	44% 5% 12% 23% 12% 3	1890
May	1st to 10th	Night	55% 5% 10% 18% 9% 3	4150
	11th to 20th	Night	44% 6% 13% 25% 11% 1	4123
	21st to end	Night	39% 7% 14% 25% 11% 3	4528
June	1st to 10th	Night	34% 5% 11% 26% 22% 3	1702
	11th to 20th	Night	32% 7% 15% 24% 17% 4	1600
	21st to end	Night	30% 7% 18% 24% 18% 4	1600
July	1st to 10th	Night	35% 5% 12% 25% 18% 5%	3580
	11th to 20th	Night	34% 6% 15% 28% 16% 1	3606
	21st to end	Night	37% 5% 15% 24% 17% 2	4297
August	1st to 10th	Night	36% 6% 15% 29% 13% 1	2904
	11th to 20th	Night	33% 4 13% 31% 18%	2902
	21st to end	Night	26% 6% 15% 29% 21% 3	3543
September	1st to 10th	Night	32% 5% 12% 25% 20% 6%	2087
	11th to 20th	Night	27% 4 9% 28% 28% 4	2200
	21st to end	Night	37% 4 9% 21% 26% 2	2200
October	1st to 10th	Night	22% 3 6% 32% 35% 2	3590
	11th to 20th	Night	30% 3 10% 25% 30% 2	3852
	21st to end	Night	34% 3 7% 27% 24% 5%	4546
November	1st to 10th	Night	38% 4 10% 24% 22% 3	2666
	11th to 20th	Night	44% 4 9% 24% 16% 3	2730
	21st to end	Night	57% 3 11% 19% 9% 1	2730
December	1st to 10th	Night	50% 3 12% 24% 10% 1	4380
	11th to 20th	Night	61% 4 11% 17% 7% 1	4954
	21st to end	Night	71% 3 9% 13% 4	7127

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	24% 15% 34% 21% 5% 1	330
	11th to 20th	Day	25% 18% 36% 16% 4	330
	21st to end	Day	28% 16% 32% 18% 6% 1	363
February	1st to 10th	Day	27% 15% 34% 18% 6% 1	330
	11th to 20th	Day	22% 20% 35% 17% 5%	330
	21st to end	Day	23% 15% 41% 19% 1	273
March	1st to 10th	Day	24% 18% 38% 17% 3	330
	11th to 20th	Day	23% 14% 36% 21% 5% 1	540
	21st to end	Day	27% 13% 36% 18% 6% 1	594
April	1st to 10th	Day	19% 15% 33% 23% 7% 2	540
	11th to 20th	Day	19% 16% 32% 19% 10% 3	532
	21st to end	Day	21% 10% 34% 21% 11% 2	540
May	1st to 10th	Day	23% 9% 34% 20% 10% 3	543
	11th to 20th	Day	23% 7% 27% 22% 13% 8%	567
	21st to end	Day	16% 5% 27% 26% 16% 11%	617
June	1st to 10th	Day	14% 5% 25% 30% 20% 8%	622
	11th to 20th	Day	11% 5% 26% 30% 22% 7%	730
	21st to end	Day	9% 5% 26% 31% 19% 9%	730
July	1st to 10th	Day	13% 4 15% 25% 24% 20%	721
	11th to 20th	Day	8% 4 20% 28% 26% 14%	703
	21st to end	Day	10% 3 15% 27% 30% 16%	576
August	1st to 10th	Day	10% 2 13% 25% 32% 18%	526
	11th to 20th	Day	11% 3 17% 29% 25% 14%	519
	21st to end	Day	8% 3 14% 27% 32% 16%	583
September	1st to 10th	Day	10% 5% 14% 25% 32% 14%	530
	11th to 20th	Day	9% 1 13% 23% 35% 18%	530
	21st to end	Day	18% 8% 18% 23% 21% 11%	530
October	1st to 10th	Day	12% 5% 21% 31% 21% 10%	530
	11th to 20th	Day	15% 12% 28% 26% 15% 4	530
	21st to end	Day	24% 12% 36% 21% 6%	401
November	1st to 10th	Day	15% 13% 34% 23% 11% 4	330
	11th to 20th	Day	23% 15% 30% 19% 10% 2	330
	21st to end	Day	24% 11% 38% 22% 4	330
December	1st to 10th	Day	19% 15% 39% 19% 6% 1	330
	11th to 20th	Day	23% 15% 37% 19% 6%	330
	21st to end	Day	29% 16% 32% 18% 5%	363
January	1st to 10th	Night	21% 14% 37% 23% 4	1050
	11th to 20th	Night	25% 15% 37% 19% 4	1006
	21st to end	Night	22% 12% 39% 23% 4	924
February	1st to 10th	Night	23% 12% 35% 27% 3	840
	11th to 20th	Night	21% 14% 38% 24% 3	823
	21st to end	Night	24% 12% 37% 22% 3	692
March	1st to 10th	Night	22% 12% 35% 25% 5%	840
	11th to 20th	Night	18% 14% 37% 25% 6%	840
	21st to end	Night	24% 11% 32% 26% 7% 1	924
April	1st to 10th	Night	20% 10% 35% 27% 7%	666
	11th to 20th	Night	18% 10% 34% 27% 9% 1	623
	21st to end	Night	20% 6% 38% 25% 10% 1	630
May	1st to 10th	Night	23% 6% 34% 27% 10% 1	630
	11th to 20th	Night	17% 5% 26% 32% 17% 2	621
	21st to end	Night	10% 3 23% 36% 25% 3	683
June	1st to 10th	Night	12% 2 23% 39% 23% 1	600
	11th to 20th	Night	11% 2 18% 41% 26% 3	600
	21st to end	Night	8% 2 21% 39% 27% 3	600
July	1st to 10th	Night	11% 1 11% 35% 34% 7%	585
	11th to 20th	Night	9% 2 13% 39% 31% 6%	582
	21st to end	Night	9% 2 11% 39% 37% 3	657
August	1st to 10th	Night	8% 2 10% 39% 37% 5%	597
	11th to 20th	Night	6% 2 11% 43% 33% 6%	594
	21st to end	Night	6% 2 12% 36% 37% 7%	660
September	1st to 10th	Night	12% 4 14% 34% 31% 6%	687
	11th to 20th	Night	9% 3 12% 33% 37% 6%	800
	21st to end	Night	17% 4 19% 33% 24% 3	800
October	1st to 10th	Night	11% 5% 23% 33% 24% 5%	800
	11th to 20th	Night	18% 8% 27% 32% 14% 2	800
	21st to end	Night	21% 9% 34% 26% 10%	880
November	1st to 10th	Night	14% 10% 33% 29% 14% 1	820
	11th to 20th	Night	19% 15% 31% 26% 8% 1	840
	21st to end	Night	22% 14% 28% 30% 5% 1	840
December	1st to 10th	Night	25% 10% 37% 23% 5%	840
	11th to 20th	Night	25% 9% 34% 25% 5% 1	840
	21st to end	Night	29% 13% 32% 22% 4	1145

Page generated at 2026-05-29 01:53:27. Version ac07

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