Tranquility Gate Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Tranquility Gate Observatory 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	15% 11% 4 9% 5% 5% 5% 4 5% 7% 30%	126668
	Night	16% 12% 5% 9% 5% 4 4 3 5% 6% 31%	124019
Transparency	Day	51% 7% 11% 19% 10% 1	130287
	Night	52% 6% 11% 19% 11% 1	128469
Seeing	Day	21% 15% 28% 21% 11% 4	16021
	Night	21% 12% 27% 27% 12% 1	25768

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% 21% 7% 21% 5% 3 3 2 3 4 14%	4056
	11th to 20th	Day	12% 24% 8% 16% 8% 6% 4 2 3 4 13%	4060
	21st to end	Day	21% 20% 7% 12% 7% 5% 4 3 3 4 13%	4466
February	1st to 10th	Day	19% 19% 5% 11% 7% 5% 4 3 3 4 21%	1800
	11th to 20th	Day	17% 14% 6% 14% 7% 5% 4 3 4 5% 21%	1758
	21st to end	Day	19% 15% 5% 12% 5% 4 4 3 5% 6% 23%	1650
March	1st to 10th	Day	17% 12% 6% 12% 4 4 4 2 5% 5% 29%	4929
	11th to 20th	Day	12% 9% 5% 9% 6% 5% 4 3 4 5% 37%	4980
	21st to end	Day	19% 11% 4 10% 5% 4 4 3 5% 6% 29%	5465
April	1st to 10th	Day	15% 14% 4 9% 5% 3 5% 3 5% 7% 29%	2280
	11th to 20th	Day	16% 13% 3 7% 5% 4 4 4 6% 7% 32%	2465
	21st to end	Day	20% 11% 3 7% 5% 4 4 3 5% 6% 34%	2600
May	1st to 10th	Day	21% 9% 3 8% 5% 5% 5% 4 6% 6% 28%	5862
	11th to 20th	Day	18% 9% 3 8% 5% 5% 5% 4 6% 8% 30%	5861
	21st to end	Day	18% 7% 2 7% 5% 5% 5% 4 6% 8% 33%	6707
June	1st to 10th	Day	14% 7% 3 7% 6% 5% 5% 4 6% 9% 35%	2800
	11th to 20th	Day	14% 7% 2 7% 5% 5% 5% 4 6% 9% 34%	2800
	21st to end	Day	12% 6% 3 5% 5% 5% 5% 5% 8% 10% 37%	2800
July	1st to 10th	Day	8% 7% 3 7% 4 5% 5% 5% 6% 10% 40%	6084
	11th to 20th	Day	10% 6% 3 7% 5% 5% 6% 6% 8% 10% 34%	5900
	21st to end	Day	12% 6% 3 6% 4 5% 5% 4 6% 8% 40%	6424
August	1st to 10th	Day	14% 8% 3 8% 5% 6% 6% 5% 7% 8% 32%	4234
	11th to 20th	Day	12% 7% 3 6% 5% 6% 6% 5% 8% 9% 34%	4174
	21st to end	Day	8% 5% 2 8% 5% 6% 5% 5% 7% 9% 39%	4515
September	1st to 10th	Day	14% 6% 2 7% 4 4 4 4 5% 6% 45%	2360
	11th to 20th	Day	10% 6% 3 6% 4 4 5% 4 6% 8% 45%	2200
	21st to end	Day	19% 8% 3 7% 5% 4 4 4 5% 6% 35%	2200
October	1st to 10th	Day	15% 6% 2 6% 5% 4 5% 4 6% 7% 40%	2025
	11th to 20th	Day	18% 10% 3 8% 5% 5% 4 3 5% 6% 33%	2000
	21st to end	Day	21% 12% 3 7% 4 4 4 3 5% 7% 30%	2200
November	1st to 10th	Day	11% 9% 4 7% 5% 5% 4 3 4 7% 41%	1917
	11th to 20th	Day	16% 15% 4 8% 5% 5% 4 3 5% 5% 29%	1786
	21st to end	Day	16% 17% 6% 11% 5% 5% 4 4 5% 5% 23%	1680
December	1st to 10th	Day	20% 15% 7% 14% 5% 4 3 3 5% 4 19%	2616
	11th to 20th	Day	23% 18% 7% 10% 4 3 3 3 4 5% 20%	2990
	21st to end	Day	23% 23% 6% 13% 5% 4 4 3 3 4 11%	4024
January	1st to 10th	Night	14% 18% 8% 23% 6% 4 4 3 3 4 13%	5889
	11th to 20th	Night	13% 19% 9% 16% 9% 6% 5% 3 4 4 13%	5900
	21st to end	Night	22% 17% 8% 13% 6% 4 4 2 3 4 17%	6490
February	1st to 10th	Night	20% 14% 6% 10% 7% 5% 4 4 4 5% 22%	2600
	11th to 20th	Night	18% 14% 6% 13% 6% 6% 4 4 4 5% 22%	2526
	21st to end	Night	19% 15% 6% 11% 5% 5% 4 3 4 5% 24%	1980
March	1st to 10th	Night	22% 12% 5% 10% 4 3 3 3 3 5% 30%	4923
	11th to 20th	Night	13% 10% 5% 7% 4 4 4 3 5% 7% 38%	4980
	21st to end	Night	18% 11% 4 8% 5% 4 4 4 4 5% 34%	5467
April	1st to 10th	Night	17% 12% 4 8% 4 5% 4 4 5% 6% 32%	2120
	11th to 20th	Night	16% 10% 3 8% 5% 5% 5% 3 6% 6% 33%	1901
	21st to end	Night	16% 8% 4 7% 5% 4 4 4 6% 8% 35%	1800
May	1st to 10th	Night	25% 10% 3 8% 4 4 4 4 5% 6% 26%	4033
	11th to 20th	Night	17% 9% 3 7% 4 4 4 3 5% 8% 37%	4032
	21st to end	Night	16% 7% 2 5% 4 4 4 4 6% 9% 37%	4129
June	1st to 10th	Night	9% 7% 2 6% 5% 5% 5% 4 7% 6% 43%	1600
	11th to 20th	Night	10% 7% 3 7% 5% 4 6% 4 5% 8% 42%	1600
	21st to end	Night	9% 4 3 6% 6% 6% 6% 4 7% 8% 42%	1600
July	1st to 10th	Night	10% 7% 3 7% 5% 5% 5% 3 5% 8% 42%	3868
	11th to 20th	Night	10% 6% 2 6% 5% 5% 5% 5% 7% 10% 39%	4057
	21st to end	Night	7% 6% 3 6% 4 5% 4 4 6% 8% 45%	4427
August	1st to 10th	Night	12% 8% 3 8% 5% 5% 4 4 6% 7% 38%	2915
	11th to 20th	Night	12% 8% 2 8% 4 3 4 3 5% 7% 44%	2875
	21st to end	Night	7% 5% 2 6% 5% 5% 5% 5% 7% 8% 44%	3348
September	1st to 10th	Night	13% 5% 2 5% 4 3 3 3 5% 7% 50%	2035
	11th to 20th	Night	10% 4 2 4 3 4 4 4 6% 8% 52%	2200
	21st to end	Night	18% 7% 2 6% 4 4 4 3 5% 7% 39%	2200
October	1st to 10th	Night	14% 5% 2 6% 4 4 4 4 5% 7% 46%	2374
	11th to 20th	Night	16% 7% 3 7% 4 4 4 4 6% 8% 37%	2400
	21st to end	Night	22% 9% 3 5% 5% 4 3 2 4 7% 35%	2640
November	1st to 10th	Night	13% 9% 2 6% 4 4 4 3 5% 7% 42%	2599
	11th to 20th	Night	16% 14% 4 8% 5% 4 4 3 5% 6% 30%	2830
	21st to end	Night	20% 15% 5% 8% 5% 4 4 3 4 6% 27%	2940
December	1st to 10th	Night	16% 17% 8% 14% 6% 5% 5% 3 4 5% 17%	4609
	11th to 20th	Night	19% 20% 8% 10% 5% 4 3 3 4 5% 19%	5205
	21st to end	Night	23% 19% 6% 12% 6% 4 4 3 5% 4 14%	6927

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	74% 5% 8% 9% 4	4057
	11th to 20th	Day	76% 4 8% 8% 3 1	4060
	21st to end	Day	75% 4 7% 9% 4	4466
February	1st to 10th	Day	68% 4 9% 13% 6%	1800
	11th to 20th	Day	66% 7% 8% 13% 7%	1767
	21st to end	Day	61% 6% 11% 14% 7% 1	1650
March	1st to 10th	Day	56% 6% 11% 21% 7%	4929
	11th to 20th	Day	47% 7% 13% 24% 8% 2	4980
	21st to end	Day	55% 5% 9% 18% 12% 1	5465
April	1st to 10th	Day	52% 7% 11% 17% 12% 1	2280
	11th to 20th	Day	50% 7% 11% 17% 12% 2	2465
	21st to end	Day	50% 6% 11% 20% 10% 2	2600
May	1st to 10th	Day	54% 8% 9% 16% 9% 4	5897
	11th to 20th	Day	49% 8% 11% 20% 10% 1	5865
	21st to end	Day	45% 9% 13% 19% 11% 3	6722
June	1st to 10th	Day	45% 8% 13% 22% 11% 1	2800
	11th to 20th	Day	44% 9% 13% 20% 12% 1	2800
	21st to end	Day	39% 10% 13% 24% 13% 1	2800
July	1st to 10th	Day	38% 7% 13% 26% 15% 2	6019
	11th to 20th	Day	42% 8% 13% 24% 13%	5831
	21st to end	Day	40% 7% 11% 23% 19% 1	6240
August	1st to 10th	Day	48% 8% 14% 21% 10%	4221
	11th to 20th	Day	42% 7% 13% 29% 8% 1	4213
	21st to end	Day	39% 8% 13% 27% 12% 2	4515
September	1st to 10th	Day	39% 7% 11% 22% 18% 3	2360
	11th to 20th	Day	35% 8% 14% 25% 17% 1	2200
	21st to end	Day	48% 9% 11% 21% 12% 1	2200
October	1st to 10th	Day	32% 5% 13% 29% 20% 1	3298
	11th to 20th	Day	47% 5% 10% 20% 14% 4	3240
	21st to end	Day	41% 6% 12% 25% 15% 1	3538
November	1st to 10th	Day	43% 7% 12% 21% 16% 2	1917
	11th to 20th	Day	56% 6% 11% 17% 8% 2	1786
	21st to end	Day	62% 6% 11% 15% 5% 1	1680
December	1st to 10th	Day	68% 5% 11% 12% 3	2616
	11th to 20th	Day	68% 4 10% 13% 5%	2986
	21st to end	Day	77% 5% 8% 8% 2	4024
January	1st to 10th	Night	74% 6% 8% 9% 3	5889
	11th to 20th	Night	74% 5% 9% 8% 3	5900
	21st to end	Night	73% 4 7% 10% 5% 1	6490
February	1st to 10th	Night	65% 6% 9% 15% 5%	2600
	11th to 20th	Night	64% 5% 10% 15% 6%	2528
	21st to end	Night	63% 6% 8% 14% 8% 1	1980
March	1st to 10th	Night	58% 4 11% 19% 8%	4923
	11th to 20th	Night	44% 6% 15% 24% 8% 3	4980
	21st to end	Night	51% 6% 11% 17% 14% 1	5467
April	1st to 10th	Night	52% 7% 10% 17% 12% 2	2120
	11th to 20th	Night	49% 7% 12% 16% 14% 2	1901
	21st to end	Night	44% 7% 13% 19% 14% 2	1800
May	1st to 10th	Night	57% 7% 9% 14% 9% 3	4060
	11th to 20th	Night	45% 8% 9% 24% 12% 1	4033
	21st to end	Night	41% 9% 14% 24% 11% 3	4136
June	1st to 10th	Night	38% 9% 13% 23% 16% 2	1600
	11th to 20th	Night	38% 7% 15% 22% 16% 2	1600
	21st to end	Night	37% 10% 12% 24% 16% 1	1600
July	1st to 10th	Night	41% 6% 10% 24% 19% 2	3825
	11th to 20th	Night	38% 8% 16% 26% 12% 1	4006
	21st to end	Night	35% 7% 12% 19% 25% 1	4297
August	1st to 10th	Night	44% 7% 12% 22% 13% 1	2904
	11th to 20th	Night	39% 6% 11% 29% 14% 1	2902
	21st to end	Night	35% 8% 12% 30% 13% 2	3348
September	1st to 10th	Night	34% 7% 11% 25% 19% 4	2035
	11th to 20th	Night	28% 7% 15% 27% 21% 2	2200
	21st to end	Night	44% 6% 13% 19% 17% 1	2200
October	1st to 10th	Night	33% 5% 9% 27% 24% 1	3881
	11th to 20th	Night	38% 6% 13% 22% 14% 7%	3899
	21st to end	Night	37% 5% 10% 30% 17% 2	4252
November	1st to 10th	Night	40% 7% 12% 23% 15% 3	2599
	11th to 20th	Night	55% 6% 10% 18% 8% 3	2830
	21st to end	Night	59% 5% 12% 18% 5% 1	2940
December	1st to 10th	Night	69% 5% 11% 11% 3	4609
	11th to 20th	Night	68% 5% 11% 13% 2	5208
	21st to end	Night	71% 7% 9% 11% 2	6927

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	31% 26% 29% 11% 3	300
	11th to 20th	Day	31% 30% 29% 8% 2	300
	21st to end	Day	28% 29% 32% 9% 2	330
February	1st to 10th	Day	31% 22% 37% 8% 1	300
	11th to 20th	Day	34% 27% 29% 9% 1	300
	21st to end	Day	33% 27% 33% 7%	249
March	1st to 10th	Day	27% 22% 33% 16% 2	494
	11th to 20th	Day	30% 18% 33% 13% 5% 1	500
	21st to end	Day	33% 19% 29% 15% 3 1	550
April	1st to 10th	Day	27% 22% 29% 14% 7% 1	500
	11th to 20th	Day	28% 17% 27% 16% 9% 3	492
	21st to end	Day	27% 16% 32% 17% 7% 1	500
May	1st to 10th	Day	26% 21% 24% 19% 7% 2	503
	11th to 20th	Day	21% 13% 33% 20% 9% 3	527
	21st to end	Day	17% 10% 25% 27% 16% 5%	573
June	1st to 10th	Day	15% 10% 33% 27% 11% 4	530
	11th to 20th	Day	20% 9% 26% 28% 14% 4	530
	21st to end	Day	13% 11% 31% 27% 15% 4	530
July	1st to 10th	Day	10% 6% 21% 32% 21% 11%	525
	11th to 20th	Day	11% 7% 27% 26% 21% 8%	524
	21st to end	Day	13% 6% 24% 30% 19% 9%	576
August	1st to 10th	Day	13% 5% 24% 29% 21% 9%	526
	11th to 20th	Day	15% 6% 23% 27% 19% 11%	519
	21st to end	Day	10% 5% 25% 29% 19% 12%	583
September	1st to 10th	Day	15% 9% 19% 25% 22% 10%	530
	11th to 20th	Day	10% 5% 18% 30% 25% 12%	530
	21st to end	Day	22% 11% 25% 22% 15% 5%	530
October	1st to 10th	Day	14% 9% 26% 26% 20% 5%	530
	11th to 20th	Day	21% 16% 34% 19% 8% 2	530
	21st to end	Day	27% 17% 31% 20% 6%	583
November	1st to 10th	Day	17% 23% 34% 14% 10% 2	395
	11th to 20th	Day	32% 20% 27% 13% 6% 1	275
	21st to end	Day	30% 21% 36% 12%	220
December	1st to 10th	Day	31% 26% 29% 8% 4 1	204
	11th to 20th	Day	32% 21% 34% 12% 2	200
	21st to end	Day	34% 24% 28% 11% 2	233
January	1st to 10th	Night	27% 20% 35% 15% 3	800
	11th to 20th	Night	29% 20% 33% 15% 3	800
	21st to end	Night	27% 21% 30% 19% 3	880
February	1st to 10th	Night	30% 16% 30% 23% 2	800
	11th to 20th	Night	28% 18% 36% 17% 1	783
	21st to end	Night	28% 16% 35% 18% 3	660
March	1st to 10th	Night	25% 15% 35% 23% 3	800
	11th to 20th	Night	25% 17% 30% 23% 5% 1	800
	21st to end	Night	24% 15% 35% 21% 5% 1	880
April	1st to 10th	Night	25% 15% 30% 24% 5%	800
	11th to 20th	Night	22% 13% 34% 22% 9% 1	717
	21st to end	Night	20% 9% 35% 30% 6%	600
May	1st to 10th	Night	22% 10% 26% 32% 10% 1	600
	11th to 20th	Night	20% 8% 26% 33% 12% 1	591
	21st to end	Night	12% 5% 24% 36% 21% 2	526
June	1st to 10th	Night	11% 4 28% 40% 16%	400
	11th to 20th	Night	12% 6% 23% 44% 15%	400
	21st to end	Night	9% 4 26% 36% 24%	400
July	1st to 10th	Night	8% 3 15% 44% 24% 6%	499
	11th to 20th	Night	8% 3 18% 39% 28% 4	582
	21st to end	Night	11% 3 14% 41% 29% 2	657
August	1st to 10th	Night	11% 2 15% 42% 28% 2	597
	11th to 20th	Night	14% 4 11% 39% 29% 4	594
	21st to end	Night	11% 4 18% 34% 28% 5%	733
September	1st to 10th	Night	15% 4 18% 34% 26% 4	800
	11th to 20th	Night	10% 4 14% 37% 31% 5%	800
	21st to end	Night	19% 7% 24% 27% 19% 4	800
October	1st to 10th	Night	17% 8% 21% 31% 22% 2	800
	11th to 20th	Night	19% 11% 32% 27% 10% 1	800
	21st to end	Night	28% 10% 29% 27% 6%	880
November	1st to 10th	Night	19% 11% 29% 29% 11%	820
	11th to 20th	Night	26% 17% 28% 22% 7% 1	840
	21st to end	Night	29% 16% 31% 19% 5%	840
December	1st to 10th	Night	32% 18% 28% 19% 4	809
	11th to 20th	Night	28% 18% 30% 20% 4	800
	21st to end	Night	30% 18% 30% 20% 2	880

Page generated at 2025-11-30 01:00:54. Version ac07

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