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 2024-01-27.

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 7% 6% 3 5% 5% 5% 7% 35%	75112
	Night	14% 9% 4 7% 6% 3 5% 4 5% 7% 37%	74940
Transparency	Day	47% 5% 12% 21% 13% 2	74825
	Night	47% 5% 11% 21% 14% 2	74747
Seeing	Day	17% 8% 26% 24% 16% 8%	14344
	Night	18% 8% 28% 29% 15% 2	24255

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% 7% 7% 7% 5% 5% 5% 6% 6% 22%	1517
	11th to 20th	Day	17% 13% 7% 11% 8% 4 5% 5% 3 5% 22%	1555
	21st to end	Day	15% 16% 8% 10% 9% 3 5% 5% 4 6% 20%	1836
February	1st to 10th	Day	18% 15% 7% 10% 8% 3 4 5% 3 5% 23%	1706
	11th to 20th	Day	14% 11% 5% 11% 8% 4 6% 4 5% 6% 25%	1748
	21st to end	Day	16% 11% 5% 11% 7% 3 5% 4 3 6% 30%	1480
March	1st to 10th	Day	17% 11% 5% 8% 5% 4 5% 5% 4 7% 30%	1800
	11th to 20th	Day	17% 10% 4 9% 7% 3 4 5% 4 7% 30%	1980
	21st to end	Day	19% 13% 6% 8% 7% 3 5% 4 4 6% 26%	2178
April	1st to 10th	Day	13% 10% 4 7% 7% 3 6% 5% 5% 6% 34%	2143
	11th to 20th	Day	16% 9% 4 6% 4 3 5% 4 4 7% 38%	2185
	21st to end	Day	18% 9% 3 7% 6% 3 5% 4 5% 7% 33%	2340
May	1st to 10th	Day	19% 9% 4 6% 7% 3 4 4 6% 6% 33%	2340
	11th to 20th	Day	20% 8% 3 6% 5% 3 5% 5% 5% 7% 33%	2301
	21st to end	Day	9% 5% 3 8% 6% 4 6% 7% 7% 9% 35%	2531
June	1st to 10th	Day	13% 5% 2 5% 5% 4 5% 5% 6% 8% 41%	2422
	11th to 20th	Day	9% 5% 3 5% 5% 3 5% 6% 7% 9% 42%	2520
	21st to end	Day	8% 5% 2 6% 6% 4 6% 6% 6% 10% 40%	2520
July	1st to 10th	Day	11% 5% 2 5% 6% 4 7% 7% 8% 10% 36%	2520
	11th to 20th	Day	8% 4 2 5% 5% 4 6% 7% 7% 10% 42%	2507
	21st to end	Day	9% 4 2 5% 5% 4 6% 6% 6% 10% 42%	2547
August	1st to 10th	Day	6% 4 3 5% 7% 4 6% 6% 6% 10% 42%	2327
	11th to 20th	Day	8% 5% 2 6% 7% 4 6% 6% 7% 10% 38%	2303
	21st to end	Day	8% 5% 3 6% 5% 4 4 6% 6% 10% 44%	2400
September	1st to 10th	Day	13% 4 2 4 5% 4 5% 5% 5% 7% 47%	2063
	11th to 20th	Day	9% 5% 3 5% 6% 3 4 5% 5% 8% 47%	1980
	21st to end	Day	12% 6% 3 5% 5% 3 3 4 4 7% 48%	1980
October	1st to 10th	Day	12% 6% 3 5% 7% 3 4 5% 5% 6% 45%	1980
	11th to 20th	Day	14% 6% 3 4 5% 3 4 4 6% 7% 44%	1831
	21st to end	Day	18% 10% 3 7% 6% 3 4 5% 5% 7% 32%	1817
November	1st to 10th	Day	8% 7% 3 6% 5% 2 3 4 5% 7% 50%	1672
	11th to 20th	Day	14% 11% 4 9% 6% 3 4 4 5% 7% 35%	1710
	21st to end	Day	17% 11% 5% 8% 5% 3 4 3 4 6% 33%	1710
December	1st to 10th	Day	18% 14% 7% 10% 6% 3 5% 4 4 5% 24%	1710
	11th to 20th	Day	15% 12% 7% 11% 7% 4 6% 4 4 5% 25%	2153
	21st to end	Day	21% 19% 9% 10% 7% 4 4 4 3 5% 13%	2800
January	1st to 10th	Night	16% 11% 6% 10% 8% 4 5% 4 5% 6% 24%	2648
	11th to 20th	Night	18% 14% 6% 11% 7% 4 5% 4 4 5% 22%	2621
	21st to end	Night	15% 13% 6% 10% 8% 4 5% 4 4 5% 25%	2664
February	1st to 10th	Night	20% 13% 5% 9% 7% 3 4 4 4 6% 26%	2250
	11th to 20th	Night	14% 11% 5% 8% 8% 3 5% 4 5% 8% 31%	2098
	21st to end	Night	19% 11% 5% 9% 7% 3 5% 3 4 6% 29%	1776
March	1st to 10th	Night	16% 8% 4 7% 6% 3 5% 4 4 6% 37%	2160
	11th to 20th	Night	13% 10% 3 6% 6% 4 4 4 4 7% 38%	1980
	21st to end	Night	20% 9% 4 7% 5% 3 5% 3 4 6% 34%	2178
April	1st to 10th	Night	13% 9% 3 7% 6% 3 5% 4 6% 7% 38%	1832
	11th to 20th	Night	15% 8% 3 4 4 3 5% 5% 6% 7% 39%	1723
	21st to end	Night	17% 6% 2 6% 5% 4 5% 5% 5% 8% 36%	1620
May	1st to 10th	Night	18% 8% 3 7% 6% 4 5% 5% 5% 7% 31%	1620
	11th to 20th	Night	17% 6% 3 6% 5% 4 5% 4 4 7% 39%	1592
	21st to end	Night	8% 4 2 5% 5% 3 6% 6% 7% 12% 42%	1744
June	1st to 10th	Night	8% 5% 2 5% 6% 3 5% 5% 6% 8% 48%	1532
	11th to 20th	Night	9% 3 2 6% 4 4 4 4 7% 10% 47%	1440
	21st to end	Night	7% 4 2 4 5% 3 6% 5% 7% 10% 46%	1440
July	1st to 10th	Night	10% 5% 2 5% 6% 4 5% 3 6% 7% 47%	1440
	11th to 20th	Night	7% 3 2 4 4 3 5% 5% 6% 9% 52%	1448
	21st to end	Night	8% 4 2 4 5% 3 5% 5% 5% 8% 51%	1771
August	1st to 10th	Night	6% 3 2 5% 6% 3 5% 7% 7% 9% 47%	1611
	11th to 20th	Night	6% 4 2 5% 6% 4 5% 4 7% 9% 48%	1598
	21st to end	Night	5% 4 2 5% 5% 3 5% 4 7% 9% 51%	1952
September	1st to 10th	Night	12% 5% 3 4 4 3 4 4 5% 7% 49%	1877
	11th to 20th	Night	8% 5% 2 5% 4 2 3 4 5% 9% 54%	1980
	21st to end	Night	12% 6% 2 4 5% 2 3 4 5% 6% 51%	1980
October	1st to 10th	Night	9% 4 2 5% 4 3 5% 5% 5% 7% 51%	1980
	11th to 20th	Night	15% 8% 3 5% 6% 3 4 4 6% 5% 41%	2128
	21st to end	Night	18% 9% 3 6% 5% 3 4 4 4 5% 40%	2538
November	1st to 10th	Night	11% 8% 3 5% 5% 3 4 4 5% 6% 47%	2406
	11th to 20th	Night	16% 7% 4 7% 5% 2 4 4 5% 8% 37%	2470
	21st to end	Night	17% 10% 6% 7% 6% 3 5% 4 5% 8% 28%	2470
December	1st to 10th	Night	19% 11% 6% 7% 6% 3 6% 4 5% 6% 26%	2470
	11th to 20th	Night	17% 11% 7% 8% 5% 3 5% 5% 5% 7% 27%	3042
	21st to end	Night	21% 17% 8% 10% 7% 3 5% 4 4 4 17%	4861

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	60% 4 9% 17% 9%	1518
	11th to 20th	Day	64% 5% 10% 14% 6%	1555
	21st to end	Day	65% 4 10% 15% 5% 1	1836
February	1st to 10th	Day	64% 4 9% 13% 8% 1	1706
	11th to 20th	Day	59% 5% 11% 18% 8% 1	1758
	21st to end	Day	58% 4 10% 19% 9% 2	1480
March	1st to 10th	Day	55% 4 10% 22% 10% 1	1800
	11th to 20th	Day	54% 5% 12% 18% 10% 1	1980
	21st to end	Day	59% 4 12% 18% 5% 1	2178
April	1st to 10th	Day	50% 4 12% 21% 12% 1	2143
	11th to 20th	Day	47% 4 13% 20% 13% 3	2185
	21st to end	Day	50% 5% 11% 21% 11% 2	2340
May	1st to 10th	Day	51% 6% 12% 20% 10% 1	2340
	11th to 20th	Day	50% 5% 13% 19% 12% 1	2305
	21st to end	Day	42% 8% 17% 21% 12% 1	2531
June	1st to 10th	Day	40% 6% 11% 25% 16% 3	2422
	11th to 20th	Day	36% 7% 14% 24% 17% 2	2520
	21st to end	Day	39% 7% 15% 21% 14% 4	2520
July	1st to 10th	Day	41% 7% 14% 20% 15% 3	2451
	11th to 20th	Day	34% 8% 13% 26% 17% 2	2432
	21st to end	Day	35% 7% 16% 24% 14% 4	2405
August	1st to 10th	Day	34% 8% 13% 30% 13% 2	2314
	11th to 20th	Day	38% 7% 13% 26% 14% 1	2303
	21st to end	Day	34% 7% 13% 24% 17% 6%	2400
September	1st to 10th	Day	36% 5% 12% 23% 19% 4	2063
	11th to 20th	Day	35% 4 11% 27% 20% 3	1980
	21st to end	Day	36% 4 11% 24% 23% 1	1980
October	1st to 10th	Day	38% 5% 11% 24% 20% 2	1980
	11th to 20th	Day	39% 4 10% 25% 19% 3	1831
	21st to end	Day	50% 4 11% 21% 12% 2	1817
November	1st to 10th	Day	34% 4 11% 28% 19% 3	1672
	11th to 20th	Day	50% 3 11% 20% 13% 3	1710
	21st to end	Day	53% 3 11% 22% 10% 2	1710
December	1st to 10th	Day	64% 4 10% 15% 7%	1710
	11th to 20th	Day	59% 5% 9% 18% 8%	2150
	21st to end	Day	74% 4 7% 9% 6%	2800
January	1st to 10th	Night	61% 4 8% 18% 9%	2648
	11th to 20th	Night	64% 4 10% 16% 6%	2621
	21st to end	Night	61% 4 10% 17% 8% 1	2664
February	1st to 10th	Night	62% 4 10% 17% 7% 1	2250
	11th to 20th	Night	53% 4 12% 18% 12% 1	2099
	21st to end	Night	58% 3 11% 16% 9% 2	1776
March	1st to 10th	Night	49% 4 9% 22% 14% 2	2160
	11th to 20th	Night	46% 4 13% 23% 11% 3	1980
	21st to end	Night	53% 4 10% 23% 9% 2	2178
April	1st to 10th	Night	44% 5% 12% 25% 12% 1	1832
	11th to 20th	Night	43% 5% 10% 20% 18% 4	1723
	21st to end	Night	45% 6% 13% 22% 12% 3	1620
May	1st to 10th	Night	52% 5% 11% 20% 9% 3	1620
	11th to 20th	Night	46% 5% 13% 21% 13% 2	1593
	21st to end	Night	33% 7% 18% 25% 13% 4	1745
June	1st to 10th	Night	33% 5% 11% 26% 21% 3	1532
	11th to 20th	Night	31% 8% 16% 24% 17% 5%	1440
	21st to end	Night	31% 7% 17% 23% 17% 4	1440
July	1st to 10th	Night	37% 6% 14% 21% 18% 5%	1400
	11th to 20th	Night	29% 6% 14% 30% 18% 3	1400
	21st to end	Night	30% 6% 14% 30% 18% 2	1672
August	1st to 10th	Night	30% 8% 16% 27% 17% 2	1600
	11th to 20th	Night	31% 6% 16% 28% 19% 1	1598
	21st to end	Night	28% 7% 16% 23% 22% 5%	1952
September	1st to 10th	Night	34% 5% 13% 24% 18% 6%	1877
	11th to 20th	Night	28% 4 10% 28% 26% 3	1980
	21st to end	Night	35% 4 10% 22% 26% 2	1980
October	1st to 10th	Night	32% 5% 11% 27% 23% 2	1980
	11th to 20th	Night	43% 4 8% 24% 20% 1	2128
	21st to end	Night	46% 3 9% 24% 15% 3	2538
November	1st to 10th	Night	38% 4 11% 23% 22% 3	2406
	11th to 20th	Night	44% 4 9% 23% 16% 3	2470
	21st to end	Night	55% 3 12% 20% 9% 1	2470
December	1st to 10th	Night	57% 5% 10% 18% 9% 1	2470
	11th to 20th	Night	55% 5% 11% 21% 8% 1	3044
	21st to end	Night	70% 4 9% 12% 4	4861

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	23% 14% 36% 20% 6% 1	270
	11th to 20th	Day	27% 15% 35% 19% 4	270
	21st to end	Day	31% 13% 32% 17% 6% 1	282
February	1st to 10th	Day	32% 15% 31% 18% 3	240
	11th to 20th	Day	23% 20% 35% 19% 3	240
	21st to end	Day	25% 14% 41% 19% 2	198
March	1st to 10th	Day	21% 20% 39% 18% 2	240
	11th to 20th	Day	22% 11% 37% 22% 6% 1	420
	21st to end	Day	29% 11% 35% 18% 7% 1	462
April	1st to 10th	Day	18% 13% 31% 25% 9% 3	420
	11th to 20th	Day	21% 14% 30% 20% 12% 4	412
	21st to end	Day	24% 10% 34% 20% 9% 3	420
May	1st to 10th	Day	24% 9% 32% 21% 10% 4	423
	11th to 20th	Day	23% 7% 27% 23% 12% 8%	447
	21st to end	Day	13% 3 26% 26% 18% 14%	485
June	1st to 10th	Day	14% 3 24% 30% 21% 8%	532
	11th to 20th	Day	10% 5% 27% 30% 22% 8%	630
	21st to end	Day	10% 5% 27% 31% 19% 9%	630
July	1st to 10th	Day	13% 3 14% 25% 23% 22%	621
	11th to 20th	Day	8% 3 19% 29% 25% 16%	605
	21st to end	Day	10% 2 14% 26% 30% 17%	488
August	1st to 10th	Day	10% 2 13% 24% 32% 20%	446
	11th to 20th	Day	13% 4 16% 28% 27% 13%	439
	21st to end	Day	9% 3 14% 26% 30% 17%	495
September	1st to 10th	Day	12% 4 14% 24% 32% 14%	450
	11th to 20th	Day	11% 2 14% 24% 33% 16%	450
	21st to end	Day	16% 8% 19% 24% 22% 11%	450
October	1st to 10th	Day	12% 5% 23% 31% 19% 9%	450
	11th to 20th	Day	16% 13% 30% 25% 15% 2	450
	21st to end	Day	26% 11% 36% 22% 5%	332
November	1st to 10th	Day	14% 11% 33% 24% 12% 5%	270
	11th to 20th	Day	23% 14% 29% 19% 12% 3	270
	21st to end	Day	23% 10% 37% 24% 5%	270
December	1st to 10th	Day	20% 16% 36% 20% 8% 1	270
	11th to 20th	Day	21% 14% 36% 22% 7%	270
	21st to end	Day	29% 15% 33% 17% 6%	297
January	1st to 10th	Night	22% 14% 37% 22% 5%	950
	11th to 20th	Night	26% 13% 35% 20% 5%	911
	21st to end	Night	22% 11% 38% 24% 4	820
February	1st to 10th	Night	27% 10% 35% 26% 2	720
	11th to 20th	Night	21% 15% 36% 25% 3	703
	21st to end	Night	26% 11% 38% 21% 4	592
March	1st to 10th	Night	21% 12% 34% 27% 6% 1	720
	11th to 20th	Night	18% 12% 36% 26% 7% 1	720
	21st to end	Night	25% 9% 31% 27% 8% 1	792
April	1st to 10th	Night	18% 10% 34% 29% 8% 1	572
	11th to 20th	Night	19% 9% 31% 29% 11% 2	533
	21st to end	Night	20% 6% 37% 24% 11% 1	540
May	1st to 10th	Night	23% 5% 33% 28% 11% 1	540
	11th to 20th	Night	18% 4 26% 31% 18% 2	531
	21st to end	Night	9% 2 22% 35% 28% 3	584
June	1st to 10th	Night	11% 1 22% 40% 24% 1	540
	11th to 20th	Night	10% 2 19% 40% 27% 3	540
	21st to end	Night	8% 3 20% 39% 26% 4	540
July	1st to 10th	Night	11% 1 12% 33% 35% 8%	525
	11th to 20th	Night	9% 2 13% 38% 31% 7%	522
	21st to end	Night	9% 2 11% 39% 36% 3	591
August	1st to 10th	Night	8% 1 9% 40% 37% 6%	537
	11th to 20th	Night	7% 1 10% 43% 33% 6%	534
	21st to end	Night	6% 2 12% 36% 38% 7%	594
September	1st to 10th	Night	13% 4 13% 32% 31% 6%	617
	11th to 20th	Night	10% 3 14% 32% 36% 6%	720
	21st to end	Night	15% 3 20% 33% 26% 4	720
October	1st to 10th	Night	11% 5% 23% 32% 23% 5%	720
	11th to 20th	Night	19% 8% 28% 31% 13% 1	720
	21st to end	Night	23% 9% 32% 26% 9%	792
November	1st to 10th	Night	14% 9% 32% 29% 15% 1	740
	11th to 20th	Night	20% 14% 30% 27% 9% 1	760
	21st to end	Night	22% 13% 28% 31% 6% 1	760
December	1st to 10th	Night	25% 9% 36% 23% 6% 1	760
	11th to 20th	Night	25% 9% 34% 25% 6% 1	760
	21st to end	Night	27% 13% 33% 22% 4	1035

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