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 2024-01-13.

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	16% 10% 4 8% 6% 3 5% 5% 5% 7% 31%	74591
	Night	16% 10% 4 7% 6% 3 5% 4 5% 7% 32%	75133
Transparency	Day	52% 8% 12% 18% 10% 1	74312
	Night	51% 7% 12% 18% 11% 2	74931
Seeing	Day	21% 14% 28% 21% 11% 4	13516
	Night	21% 11% 27% 27% 12% 1	23355

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	17% 17% 8% 11% 8% 4 5% 4 4 6% 17%	1706
	11th to 20th	Day	19% 20% 8% 13% 8% 3 3 3 3 5% 16%	1638
	21st to end	Day	17% 18% 8% 13% 9% 3 4 4 3 5% 15%	1782
February	1st to 10th	Day	20% 17% 6% 10% 9% 4 5% 3 3 4 20%	1620
	11th to 20th	Day	17% 13% 7% 14% 8% 4 5% 4 4 6% 20%	1577
	21st to end	Day	20% 14% 5% 11% 6% 3 4 4 5% 5% 24%	1480
March	1st to 10th	Day	18% 11% 6% 9% 5% 4 4 4 5% 6% 29%	1974
	11th to 20th	Day	18% 13% 5% 8% 6% 2 4 4 5% 5% 30%	1980
	21st to end	Day	21% 11% 5% 8% 6% 4 4 4 5% 6% 27%	2178
April	1st to 10th	Day	14% 12% 5% 7% 7% 3 4 4 5% 7% 31%	2052
	11th to 20th	Day	17% 12% 4 6% 6% 2 5% 4 6% 7% 32%	2215
	21st to end	Day	21% 10% 3 7% 5% 3 4 4 4 6% 33%	2340
May	1st to 10th	Day	18% 9% 3 8% 7% 4 6% 6% 5% 7% 28%	2340
	11th to 20th	Day	20% 8% 3 7% 5% 4 5% 5% 6% 7% 31%	2301
	21st to end	Day	11% 4 3 6% 7% 4 6% 6% 7% 9% 38%	2638
June	1st to 10th	Day	15% 6% 3 6% 7% 4 5% 5% 6% 9% 34%	2520
	11th to 20th	Day	14% 7% 3 6% 6% 4 5% 5% 6% 10% 35%	2520
	21st to end	Day	12% 6% 3 5% 5% 4 6% 6% 7% 10% 38%	2520
July	1st to 10th	Day	10% 6% 3 7% 5% 3 5% 6% 6% 8% 41%	2412
	11th to 20th	Day	11% 5% 3 6% 6% 4 6% 6% 6% 9% 38%	2340
	21st to end	Day	12% 6% 3 6% 6% 4 6% 5% 7% 9% 36%	2547
August	1st to 10th	Day	10% 6% 2 6% 6% 5% 6% 7% 7% 9% 36%	2327
	11th to 20th	Day	12% 7% 2 6% 7% 4 6% 6% 7% 9% 33%	2303
	21st to end	Day	11% 5% 2 7% 6% 5% 6% 6% 7% 9% 36%	2489
September	1st to 10th	Day	15% 6% 2 7% 5% 3 5% 4 5% 6% 43%	2124
	11th to 20th	Day	11% 6% 3 5% 4 3 5% 6% 6% 9% 43%	1980
	21st to end	Day	18% 7% 3 6% 6% 3 4 5% 6% 6% 37%	1980
October	1st to 10th	Day	16% 6% 3 6% 6% 2 6% 6% 6% 7% 37%	1823
	11th to 20th	Day	19% 10% 3 7% 7% 4 4 4 5% 6% 32%	1800
	21st to end	Day	22% 11% 4 6% 5% 3 4 4 4 6% 28%	1980
November	1st to 10th	Day	11% 9% 4 6% 6% 4 5% 4 4 7% 41%	1731
	11th to 20th	Day	17% 14% 4 7% 5% 4 4 4 5% 6% 31%	1616
	21st to end	Day	17% 15% 6% 9% 6% 3 5% 5% 5% 5% 24%	1520
December	1st to 10th	Day	21% 16% 8% 9% 7% 3 3 4 6% 4 18%	1520
	11th to 20th	Day	18% 17% 9% 11% 5% 3 4 4 5% 6% 18%	1910
	21st to end	Day	26% 23% 8% 10% 6% 2 4 3 3 4 12%	2808
January	1st to 10th	Night	18% 16% 7% 10% 7% 3 5% 5% 5% 5% 20%	2459
	11th to 20th	Night	19% 16% 6% 10% 8% 4 4 5% 4 4 18%	2367
	21st to end	Night	17% 17% 8% 11% 8% 3 4 4 4 4 20%	2574
February	1st to 10th	Night	21% 13% 6% 9% 8% 4 5% 4 4 5% 20%	2340
	11th to 20th	Night	18% 13% 6% 11% 7% 4 6% 5% 4 5% 22%	2267
	21st to end	Night	20% 14% 7% 10% 6% 3 5% 3 4 4 25%	1776
March	1st to 10th	Night	17% 10% 5% 8% 6% 3 5% 5% 4 5% 32%	1985
	11th to 20th	Night	20% 10% 4 6% 5% 3 4 3 5% 7% 34%	1980
	21st to end	Night	18% 11% 5% 7% 5% 3 4 4 4 5% 33%	2178
April	1st to 10th	Night	16% 10% 4 7% 5% 3 5% 5% 5% 7% 34%	1908
	11th to 20th	Night	16% 9% 4 7% 5% 3 5% 4 6% 7% 34%	1709
	21st to end	Night	16% 8% 4 6% 6% 3 5% 5% 5% 8% 34%	1620
May	1st to 10th	Night	18% 9% 4 6% 5% 3 6% 5% 5% 7% 32%	1620
	11th to 20th	Night	16% 7% 2 6% 6% 3 5% 5% 6% 9% 35%	1592
	21st to end	Night	8% 6% 2 4 5% 3 5% 5% 7% 10% 45%	1633
June	1st to 10th	Night	9% 6% 2 6% 6% 3 6% 5% 6% 7% 43%	1440
	11th to 20th	Night	10% 6% 3 6% 5% 3 6% 5% 5% 8% 42%	1440
	21st to end	Night	10% 3 3 5% 6% 4 6% 5% 7% 8% 43%	1440
July	1st to 10th	Night	7% 6% 3 5% 6% 3 5% 5% 6% 8% 46%	1542
	11th to 20th	Night	8% 4 2 5% 5% 3 4 5% 6% 10% 47%	1620
	21st to end	Night	9% 5% 4 5% 6% 4 5% 5% 7% 10% 40%	1771
August	1st to 10th	Night	8% 5% 2 5% 5% 3 5% 4 6% 9% 47%	1611
	11th to 20th	Night	10% 6% 2 6% 6% 3 4 4 6% 8% 46%	1598
	21st to end	Night	10% 5% 3 4 5% 3 5% 5% 6% 9% 46%	1847
September	1st to 10th	Night	13% 5% 2 4 4 2 3 4 5% 7% 48%	1831
	11th to 20th	Night	11% 4 2 4 4 3 4 5% 5% 8% 51%	1980
	21st to end	Night	17% 6% 2 5% 5% 2 4 4 5% 8% 41%	1980
October	1st to 10th	Night	14% 6% 3 5% 5% 3 4 5% 5% 7% 44%	2136
	11th to 20th	Night	17% 7% 4 6% 6% 3 4 5% 6% 8% 35%	2160
	21st to end	Night	24% 9% 3 5% 5% 3 4 4 4 6% 33%	2376
November	1st to 10th	Night	14% 9% 2 4 5% 3 5% 4 5% 7% 42%	2345
	11th to 20th	Night	17% 13% 5% 7% 6% 3 5% 4 5% 6% 31%	2560
	21st to end	Night	20% 14% 5% 7% 5% 3 5% 4 4 6% 28%	2660
December	1st to 10th	Night	22% 15% 7% 10% 7% 3 4 4 4 5% 20%	2660
	11th to 20th	Night	17% 18% 7% 9% 6% 3 4 4 5% 5% 21%	3285
	21st to end	Night	25% 19% 7% 9% 7% 3 5% 3 5% 4 12%	4843

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	69% 5% 11% 10% 4	1707
	11th to 20th	Day	72% 4 11% 9% 2	1638
	21st to end	Day	73% 5% 9% 10% 3 1	1782
February	1st to 10th	Day	70% 4 9% 11% 6%	1620
	11th to 20th	Day	67% 6% 8% 12% 6%	1586
	21st to end	Day	62% 6% 11% 13% 7% 1	1480
March	1st to 10th	Day	55% 6% 14% 19% 6%	1974
	11th to 20th	Day	56% 6% 10% 17% 9% 1	1980
	21st to end	Day	59% 5% 8% 16% 10% 1	2178
April	1st to 10th	Day	51% 7% 12% 17% 12% 1	2052
	11th to 20th	Day	51% 7% 11% 16% 13% 3	2215
	21st to end	Day	52% 6% 12% 19% 9% 2	2340
May	1st to 10th	Day	53% 9% 11% 16% 9% 1	2340
	11th to 20th	Day	51% 8% 12% 15% 12% 2	2305
	21st to end	Day	39% 10% 16% 22% 11% 2	2638
June	1st to 10th	Day	45% 8% 13% 21% 11% 1	2520
	11th to 20th	Day	44% 9% 13% 20% 12% 1	2520
	21st to end	Day	40% 10% 13% 24% 13% 1	2520
July	1st to 10th	Day	39% 10% 13% 22% 13% 3	2347
	11th to 20th	Day	41% 8% 12% 24% 14% 1	2271
	21st to end	Day	40% 10% 15% 21% 13% 1	2405
August	1st to 10th	Day	41% 10% 16% 24% 9%	2314
	11th to 20th	Day	43% 9% 16% 22% 9% 1	2303
	21st to end	Day	41% 10% 14% 20% 12% 3	2489
September	1st to 10th	Day	41% 7% 12% 20% 16% 3	2124
	11th to 20th	Day	36% 9% 15% 24% 15% 1	1980
	21st to end	Day	45% 9% 12% 22% 12% 1	1980
October	1st to 10th	Day	43% 8% 14% 19% 15% 1	1823
	11th to 20th	Day	52% 7% 13% 19% 9% 1	1800
	21st to end	Day	56% 7% 12% 15% 9% 1	1980
November	1st to 10th	Day	43% 7% 12% 20% 16% 2	1731
	11th to 20th	Day	54% 7% 12% 17% 8% 3	1616
	21st to end	Day	60% 6% 12% 15% 6% 1	1520
December	1st to 10th	Day	69% 7% 10% 11% 3	1520
	11th to 20th	Day	67% 6% 9% 13% 3	1906
	21st to end	Day	77% 5% 9% 7% 2	2808
January	1st to 10th	Night	66% 6% 10% 13% 5%	2459
	11th to 20th	Night	69% 6% 10% 12% 3	2367
	21st to end	Night	68% 5% 9% 13% 4 1	2574
February	1st to 10th	Night	66% 6% 10% 14% 4	2340
	11th to 20th	Night	64% 5% 10% 15% 6%	2269
	21st to end	Night	63% 6% 8% 14% 8% 1	1776
March	1st to 10th	Night	55% 5% 10% 20% 10% 1	1985
	11th to 20th	Night	50% 6% 13% 20% 9% 2	1980
	21st to end	Night	53% 6% 8% 17% 13% 2	2178
April	1st to 10th	Night	50% 7% 11% 18% 13% 2	1908
	11th to 20th	Night	48% 7% 12% 16% 15% 3	1709
	21st to end	Night	45% 7% 14% 18% 13% 2	1620
May	1st to 10th	Night	50% 8% 10% 17% 13% 3	1620
	11th to 20th	Night	45% 9% 11% 17% 15% 3	1593
	21st to end	Night	32% 10% 17% 26% 14% 2	1634
June	1st to 10th	Night	38% 9% 14% 23% 15% 2	1440
	11th to 20th	Night	38% 8% 14% 22% 17% 2	1440
	21st to end	Night	36% 10% 12% 24% 16% 1	1440
July	1st to 10th	Night	36% 9% 11% 22% 18% 3	1497
	11th to 20th	Night	29% 11% 15% 25% 18% 2	1566
	21st to end	Night	37% 9% 17% 20% 14% 3	1672
August	1st to 10th	Night	32% 8% 15% 26% 18% 1	1600
	11th to 20th	Night	35% 8% 16% 25% 15% 2	1598
	21st to end	Night	33% 9% 13% 25% 16% 4	1847
September	1st to 10th	Night	35% 7% 12% 24% 18% 5%	1831
	11th to 20th	Night	29% 8% 15% 25% 21% 2	1980
	21st to end	Night	41% 6% 14% 20% 18% 1	1980
October	1st to 10th	Night	39% 7% 14% 22% 16% 2	2136
	11th to 20th	Night	46% 8% 13% 20% 11% 1	2160
	21st to end	Night	52% 6% 10% 20% 10% 2	2376
November	1st to 10th	Night	40% 7% 12% 22% 15% 3	2345
	11th to 20th	Night	55% 6% 11% 18% 9% 3	2560
	21st to end	Night	58% 6% 12% 19% 5% 1	2660
December	1st to 10th	Night	66% 7% 12% 11% 4	2660
	11th to 20th	Night	65% 6% 12% 14% 3	3288
	21st to end	Night	73% 8% 10% 8% 1	4843

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	30% 26% 30% 11% 3	270
	11th to 20th	Day	31% 26% 31% 9% 3	246
	21st to end	Day	26% 29% 32% 10% 2	264
February	1st to 10th	Day	33% 19% 41% 7%	240
	11th to 20th	Day	34% 28% 30% 8%	240
	21st to end	Day	34% 28% 31% 7%	198
March	1st to 10th	Day	28% 21% 32% 16% 2	414
	11th to 20th	Day	30% 17% 33% 13% 5% 1	420
	21st to end	Day	34% 18% 29% 15% 4 1	462
April	1st to 10th	Day	25% 22% 29% 15% 9% 1	420
	11th to 20th	Day	26% 15% 27% 17% 10% 4	412
	21st to end	Day	29% 16% 31% 16% 7% 1	420
May	1st to 10th	Day	26% 21% 23% 19% 8% 3	423
	11th to 20th	Day	21% 12% 33% 22% 8% 3	447
	21st to end	Day	15% 8% 25% 28% 18% 6%	485
June	1st to 10th	Day	15% 8% 33% 28% 12% 4	450
	11th to 20th	Day	19% 10% 26% 28% 14% 4	450
	21st to end	Day	14% 10% 32% 26% 14% 4	450
July	1st to 10th	Day	11% 5% 20% 31% 21% 12%	445
	11th to 20th	Day	12% 6% 28% 25% 22% 7%	444
	21st to end	Day	13% 6% 26% 29% 18% 8%	488
August	1st to 10th	Day	12% 4 22% 30% 21% 11%	446
	11th to 20th	Day	16% 5% 22% 26% 20% 11%	439
	21st to end	Day	11% 5% 25% 29% 18% 12%	495
September	1st to 10th	Day	16% 8% 20% 26% 21% 9%	450
	11th to 20th	Day	11% 6% 19% 30% 24% 10%	450
	21st to end	Day	19% 12% 25% 22% 15% 6%	450
October	1st to 10th	Day	15% 9% 27% 25% 18% 5%	450
	11th to 20th	Day	22% 16% 34% 20% 8% 1	450
	21st to end	Day	30% 16% 28% 20% 6%	495
November	1st to 10th	Day	17% 23% 32% 14% 12% 2	329
	11th to 20th	Day	30% 20% 27% 14% 7% 1	225
	21st to end	Day	26% 23% 37% 13% 1	180
December	1st to 10th	Day	32% 27% 28% 9% 4 1	180
	11th to 20th	Day	29% 22% 34% 12% 2	180
	21st to end	Day	35% 25% 28% 10% 2	209
January	1st to 10th	Night	28% 19% 35% 15% 3	760
	11th to 20th	Night	30% 17% 33% 16% 4	729
	21st to end	Night	25% 21% 31% 20% 3	792
February	1st to 10th	Night	31% 16% 30% 21% 2	720
	11th to 20th	Night	27% 17% 38% 16% 1	703
	21st to end	Night	29% 17% 33% 18% 3	592
March	1st to 10th	Night	24% 16% 35% 23% 2	720
	11th to 20th	Night	25% 15% 30% 23% 6% 1	720
	21st to end	Night	23% 15% 34% 22% 6% 1	792
April	1st to 10th	Night	24% 15% 30% 25% 6%	720
	11th to 20th	Night	21% 13% 34% 23% 9% 1	645
	21st to end	Night	20% 9% 36% 29% 6%	540
May	1st to 10th	Night	21% 9% 26% 33% 10% 1	540
	11th to 20th	Night	21% 6% 27% 33% 12% 1	531
	21st to end	Night	10% 4 24% 36% 23% 3	473
June	1st to 10th	Night	11% 3 28% 42% 17%	360
	11th to 20th	Night	11% 6% 22% 45% 15%	360
	21st to end	Night	9% 4 25% 39% 22%	360
July	1st to 10th	Night	8% 3 15% 43% 25% 7%	447
	11th to 20th	Night	8% 3 18% 39% 28% 4	522
	21st to end	Night	12% 3 14% 41% 28% 2	591
August	1st to 10th	Night	11% 2 14% 42% 29% 3	537
	11th to 20th	Night	13% 4 10% 40% 29% 4	534
	21st to end	Night	12% 4 18% 33% 28% 5%	659
September	1st to 10th	Night	16% 3 18% 33% 26% 4	720
	11th to 20th	Night	11% 4 15% 35% 29% 5%	720
	21st to end	Night	18% 7% 25% 26% 20% 5%	720
October	1st to 10th	Night	17% 8% 22% 31% 20% 2	720
	11th to 20th	Night	20% 12% 31% 26% 10% 1	720
	21st to end	Night	29% 11% 28% 26% 6%	792
November	1st to 10th	Night	19% 11% 27% 30% 12%	740
	11th to 20th	Night	26% 16% 29% 21% 7% 1	760
	21st to end	Night	28% 16% 30% 21% 5%	760
December	1st to 10th	Night	32% 17% 28% 19% 4	760
	11th to 20th	Night	28% 18% 30% 20% 4	760
	21st to end	Night	30% 18% 31% 19% 2	836

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