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 2025-02-06.

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 8% 5% 5% 5% 4 5% 7% 35%	108668
	Night	14% 10% 4 8% 5% 4 4 3 5% 7% 36%	109659
Transparency	Day	46% 5% 11% 22% 13% 2	112325
	Night	47% 4 11% 22% 14% 2	114129
Seeing	Day	17% 9% 27% 24% 16% 8%	15741
	Night	18% 8% 28% 29% 15% 2	25627

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	14% 14% 9% 13% 6% 5% 4 3 5% 4 23%	3617
	11th to 20th	Day	12% 13% 6% 14% 8% 6% 8% 4 3 4 22%	3705
	21st to end	Day	20% 14% 7% 12% 6% 4 5% 3 4 5% 22%	4466
February	1st to 10th	Day	17% 15% 6% 11% 7% 4 4 4 4 5% 24%	1845
	11th to 20th	Day	14% 12% 4 12% 7% 6% 4 3 5% 6% 27%	1848
	21st to end	Day	16% 12% 4 12% 6% 4 4 3 3 5% 31%	1570
March	1st to 10th	Day	20% 18% 6% 10% 4 4 4 2 4 6% 23%	3120
	11th to 20th	Day	12% 9% 3 8% 5% 4 4 4 5% 7% 40%	3480
	21st to end	Day	16% 11% 4 9% 5% 5% 4 4 5% 7% 30%	3815
April	1st to 10th	Day	13% 11% 4 9% 6% 5% 5% 4 5% 6% 33%	2263
	11th to 20th	Day	16% 10% 3 7% 4 4 4 3 5% 7% 38%	2308
	21st to end	Day	17% 9% 3 8% 5% 4 5% 3 5% 8% 33%	2470
May	1st to 10th	Day	20% 10% 3 8% 5% 4 4 4 6% 7% 31%	4082
	11th to 20th	Day	19% 8% 2 6% 4 4 4 3 5% 7% 37%	4081
	21st to end	Day	10% 6% 2 8% 5% 5% 6% 5% 8% 11% 34%	4476
June	1st to 10th	Day	12% 6% 2 6% 4 5% 6% 4 6% 8% 40%	2557
	11th to 20th	Day	9% 6% 2 6% 4 5% 5% 5% 7% 9% 43%	2660
	21st to end	Day	8% 6% 2 7% 5% 5% 6% 5% 6% 10% 40%	2660
July	1st to 10th	Day	12% 6% 3 8% 6% 6% 6% 5% 8% 10% 32%	4440
	11th to 20th	Day	6% 4 2 6% 4 5% 5% 5% 8% 11% 45%	4413
	21st to end	Day	8% 6% 3 7% 6% 6% 7% 5% 7% 9% 37%	4466
August	1st to 10th	Day	6% 5% 2 6% 5% 5% 6% 5% 7% 10% 42%	4104
	11th to 20th	Day	7% 5% 2 7% 5% 5% 5% 4 7% 11% 41%	4044
	21st to end	Day	6% 5% 2 6% 4 4 4 4 6% 10% 49%	4218
September	1st to 10th	Day	12% 4 2 5% 5% 5% 4 4 5% 6% 48%	2177
	11th to 20th	Day	9% 6% 2 6% 4 4 4 4 6% 8% 48%	2090
	21st to end	Day	14% 7% 3 6% 4 4 3 3 4 6% 46%	2090
October	1st to 10th	Day	11% 6% 2 6% 5% 4 3 4 5% 6% 47%	2090
	11th to 20th	Day	13% 7% 2 5% 4 4 4 3 6% 7% 45%	1932
	21st to end	Day	18% 10% 3 8% 5% 4 4 4 6% 7% 33%	1917
November	1st to 10th	Day	8% 8% 2 6% 5% 3 3 3 5% 7% 49%	1762
	11th to 20th	Day	14% 12% 3 10% 5% 4 5% 3 5% 6% 34%	1800
	21st to end	Day	17% 12% 5% 9% 5% 4 4 3 5% 6% 32%	1800
December	1st to 10th	Day	15% 12% 5% 8% 5% 3 4 3 3 4 38%	2930
	11th to 20th	Day	17% 13% 7% 12% 5% 5% 5% 3 4 5% 23%	3373
	21st to end	Day	22% 19% 7% 10% 5% 5% 4 3 4 5% 15%	3999
January	1st to 10th	Night	14% 14% 7% 14% 8% 5% 5% 3 5% 5% 20%	6348
	11th to 20th	Night	12% 16% 8% 15% 7% 7% 5% 4 4 4 19%	6253
	21st to end	Night	19% 14% 6% 9% 5% 4 3 3 4 5% 28%	6490
February	1st to 10th	Night	19% 13% 5% 9% 6% 5% 3 3 5% 5% 27%	2441
	11th to 20th	Night	13% 12% 4 9% 6% 5% 4 4 5% 8% 31%	2218
	21st to end	Night	18% 12% 5% 10% 5% 5% 4 2 4 6% 29%	1884
March	1st to 10th	Night	19% 13% 4 7% 4 3 3 3 4 6% 34%	3733
	11th to 20th	Night	10% 8% 3 7% 4 5% 4 3 5% 7% 45%	3480
	21st to end	Night	17% 9% 3 7% 4 4 3 3 4 6% 39%	3817
April	1st to 10th	Night	13% 10% 2 8% 5% 4 5% 4 6% 7% 37%	1933
	11th to 20th	Night	15% 9% 2 5% 4 5% 5% 4 6% 7% 38%	1820
	21st to end	Night	17% 7% 2 7% 5% 5% 5% 3 6% 7% 36%	1710
May	1st to 10th	Night	20% 11% 3 9% 5% 5% 5% 4 5% 6% 27%	2813
	11th to 20th	Night	19% 6% 3 6% 4 4 4 3 5% 7% 39%	2812
	21st to end	Night	9% 5% 2 6% 4 5% 4 4 7% 11% 44%	3079
June	1st to 10th	Night	8% 5% 2 5% 5% 5% 4 4 6% 8% 47%	1617
	11th to 20th	Night	8% 4 2 7% 3 5% 4 3 6% 11% 47%	1520
	21st to end	Night	7% 5% 2 6% 4 4 5% 4 7% 10% 46%	1520
July	1st to 10th	Night	11% 5% 2 6% 5% 5% 4 3 6% 7% 46%	2530
	11th to 20th	Night	5% 4 1 4 4 4 5% 4 7% 8% 54%	2551
	21st to end	Night	9% 6% 2 6% 5% 5% 5% 4 7% 10% 41%	3087
August	1st to 10th	Night	9% 6% 2 6% 5% 4 4 4 7% 8% 43%	2825
	11th to 20th	Night	5% 6% 3 6% 5% 5% 4 4 6% 8% 48%	2785
	21st to end	Night	3 3 1 5% 4 5% 5% 4 7% 9% 53%	3434
September	1st to 10th	Night	11% 5% 2 4 3 3 4 3 5% 7% 51%	1982
	11th to 20th	Night	8% 5% 2 5% 3 3 3 3 6% 9% 54%	2090
	21st to end	Night	13% 7% 2 5% 4 3 3 3 5% 6% 49%	2090
October	1st to 10th	Night	9% 5% 1 6% 4 4 4 4 5% 7% 52%	2090
	11th to 20th	Night	14% 8% 3 6% 5% 4 4 3 6% 5% 43%	2247
	21st to end	Night	17% 9% 3 6% 4 3 4 3 4 6% 41%	2679
November	1st to 10th	Night	11% 8% 2 6% 4 4 4 3 5% 6% 47%	2536
	11th to 20th	Night	16% 8% 3 8% 5% 3 4 4 5% 8% 37%	2600
	21st to end	Night	17% 12% 5% 9% 6% 4 5% 3 5% 7% 27%	2600
December	1st to 10th	Night	15% 10% 4 8% 5% 4 5% 3 5% 6% 35%	4250
	11th to 20th	Night	18% 15% 7% 11% 4 4 4 3 5% 6% 23%	4822
	21st to end	Night	23% 17% 7% 12% 5% 4 4 3 4 5% 16%	6973

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% 4 7% 19% 7%	3618
	11th to 20th	Day	66% 5% 8% 11% 9% 2	3705
	21st to end	Day	66% 3 10% 12% 7% 2	4466
February	1st to 10th	Day	62% 4 10% 13% 9% 1	1845
	11th to 20th	Day	57% 4 10% 19% 9%	1858
	21st to end	Day	57% 3 10% 20% 8% 1	1570
March	1st to 10th	Day	64% 4 7% 16% 8%	3120
	11th to 20th	Day	43% 5% 11% 27% 13% 1	3480
	21st to end	Day	52% 6% 13% 22% 7% 1	3815
April	1st to 10th	Day	51% 4 11% 21% 11% 1	2263
	11th to 20th	Day	47% 4 13% 21% 12% 3	2308
	21st to end	Day	49% 5% 12% 21% 11% 2	2470
May	1st to 10th	Day	52% 6% 12% 21% 8% 1	4120
	11th to 20th	Day	46% 5% 14% 24% 10% 1	4085
	21st to end	Day	39% 9% 16% 21% 14% 1	4489
June	1st to 10th	Day	40% 6% 11% 25% 16% 2	2557
	11th to 20th	Day	36% 7% 14% 24% 18% 2	2660
	21st to end	Day	38% 7% 15% 21% 15% 4	2660
July	1st to 10th	Day	44% 7% 11% 21% 15% 2	4371
	11th to 20th	Day	31% 7% 13% 28% 20% 1	4338
	21st to end	Day	41% 7% 14% 19% 15% 4	4324
August	1st to 10th	Day	33% 7% 15% 30% 14% 1	4091
	11th to 20th	Day	34% 6% 12% 32% 15% 1	4083
	21st to end	Day	30% 5% 14% 31% 16% 3	4217
September	1st to 10th	Day	35% 5% 12% 23% 21% 4	2177
	11th to 20th	Day	34% 4 11% 27% 21% 3	2090
	21st to end	Day	39% 4 10% 23% 22% 1	2090
October	1st to 10th	Day	25% 4 10% 30% 30% 2	3480
	11th to 20th	Day	30% 4 10% 29% 26% 2	3185
	21st to end	Day	40% 3 10% 24% 21% 2	3129
November	1st to 10th	Day	35% 4 11% 28% 19% 3	1762
	11th to 20th	Day	50% 3 11% 20% 13% 3	1800
	21st to end	Day	54% 3 10% 21% 10% 2	1800
December	1st to 10th	Day	50% 3 11% 26% 10%	2930
	11th to 20th	Day	64% 4 9% 17% 6%	3370
	21st to end	Day	72% 3 8% 11% 5%	3999
January	1st to 10th	Night	65% 5% 8% 15% 8%	6348
	11th to 20th	Night	68% 4 8% 11% 8% 2	6253
	21st to end	Night	59% 3 10% 18% 8% 2	6490
February	1st to 10th	Night	59% 4 10% 17% 8% 1	2441
	11th to 20th	Night	52% 4 12% 18% 13% 1	2219
	21st to end	Night	58% 3 11% 16% 9% 2	1884
March	1st to 10th	Night	52% 4 8% 24% 11% 1	3733
	11th to 20th	Night	38% 5% 13% 27% 15% 2	3480
	21st to end	Night	47% 5% 11% 28% 9% 1	3817
April	1st to 10th	Night	45% 5% 12% 24% 13% 2	1933
	11th to 20th	Night	43% 5% 11% 20% 18% 4	1820
	21st to end	Night	45% 6% 13% 21% 12% 3	1710
May	1st to 10th	Night	56% 5% 10% 20% 8% 2	2840
	11th to 20th	Night	43% 6% 14% 27% 8% 1	2813
	21st to end	Night	31% 9% 16% 26% 14% 4	3087
June	1st to 10th	Night	34% 5% 11% 27% 21% 3	1617
	11th to 20th	Night	31% 7% 16% 24% 18% 4	1520
	21st to end	Night	31% 7% 17% 24% 17% 4	1520
July	1st to 10th	Night	37% 5% 11% 25% 16% 5%	2490
	11th to 20th	Night	27% 5% 14% 32% 21% 2	2503
	21st to end	Night	37% 6% 15% 23% 17% 3	2988
August	1st to 10th	Night	36% 6% 15% 28% 13% 1	2814
	11th to 20th	Night	33% 5% 13% 31% 18%	2812
	21st to end	Night	27% 6% 15% 29% 21% 3	3434
September	1st to 10th	Night	33% 5% 13% 24% 20% 6%	1982
	11th to 20th	Night	28% 4 10% 28% 27% 4	2090
	21st to end	Night	37% 4 9% 22% 25% 2	2090
October	1st to 10th	Night	22% 3 7% 32% 34% 2	3480
	11th to 20th	Night	30% 3 10% 25% 30% 1	3733
	21st to end	Night	34% 3 7% 27% 24% 5%	4405
November	1st to 10th	Night	38% 4 10% 23% 22% 3	2536
	11th to 20th	Night	45% 4 9% 23% 16% 3	2600
	21st to end	Night	56% 3 11% 19% 9% 1	2600
December	1st to 10th	Night	49% 3 12% 24% 10% 1	4250
	11th to 20th	Night	61% 4 11% 17% 7%	4824
	21st to end	Night	71% 3 9% 13% 3	6973

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% 16% 35% 20% 6% 1	300
	11th to 20th	Day	25% 16% 36% 18% 4	300
	21st to end	Day	29% 14% 33% 18% 5% 1	330
February	1st to 10th	Day	28% 16% 32% 19% 5%	285
	11th to 20th	Day	21% 21% 35% 19% 4	270
	21st to end	Day	25% 14% 39% 20% 1	225
March	1st to 10th	Day	23% 19% 37% 18% 3	270
	11th to 20th	Day	22% 13% 36% 23% 6% 1	460
	21st to end	Day	27% 11% 36% 18% 7% 1	506
April	1st to 10th	Day	18% 15% 31% 25% 8% 3	460
	11th to 20th	Day	20% 14% 30% 21% 11% 4	452
	21st to end	Day	22% 10% 35% 20% 10% 2	460
May	1st to 10th	Day	24% 8% 32% 21% 10% 4	463
	11th to 20th	Day	23% 7% 25% 23% 13% 9%	487
	21st to end	Day	12% 4 27% 27% 17% 13%	529
June	1st to 10th	Day	13% 4 25% 29% 20% 8%	577
	11th to 20th	Day	10% 4 26% 30% 22% 7%	680
	21st to end	Day	10% 5% 28% 30% 18% 9%	680
July	1st to 10th	Day	13% 4 15% 25% 23% 20%	671
	11th to 20th	Day	8% 4 21% 28% 25% 15%	654
	21st to end	Day	10% 2 15% 27% 30% 16%	532
August	1st to 10th	Day	10% 2 14% 25% 31% 18%	486
	11th to 20th	Day	12% 3 19% 29% 25% 13%	479
	21st to end	Day	9% 3 13% 26% 32% 17%	539
September	1st to 10th	Day	11% 4 13% 25% 32% 14%	490
	11th to 20th	Day	10% 1 13% 25% 33% 17%	490
	21st to end	Day	19% 8% 19% 24% 20% 10%	490
October	1st to 10th	Day	12% 5% 22% 32% 19% 9%	490
	11th to 20th	Day	15% 12% 30% 24% 15% 4	490
	21st to end	Day	24% 11% 36% 22% 6% 1	366
November	1st to 10th	Day	14% 12% 35% 23% 12% 4	300
	11th to 20th	Day	23% 15% 29% 20% 11% 2	300
	21st to end	Day	23% 11% 39% 23% 5%	300
December	1st to 10th	Day	20% 16% 38% 19% 7% 1	300
	11th to 20th	Day	23% 14% 36% 20% 6%	300
	21st to end	Day	29% 15% 31% 18% 5%	330
January	1st to 10th	Night	21% 14% 37% 22% 5%	1000
	11th to 20th	Night	26% 14% 36% 20% 4	958
	21st to end	Night	22% 12% 38% 24% 4	880
February	1st to 10th	Night	25% 11% 35% 27% 3	781
	11th to 20th	Night	20% 15% 37% 25% 3	743
	21st to end	Night	26% 11% 37% 22% 4	628
March	1st to 10th	Night	22% 12% 33% 27% 6% 1	760
	11th to 20th	Night	18% 13% 37% 26% 6% 1	760
	21st to end	Night	24% 9% 32% 27% 8% 1	836
April	1st to 10th	Night	19% 10% 35% 29% 7%	603
	11th to 20th	Night	19% 10% 32% 28% 10% 1	563
	21st to end	Night	20% 6% 39% 24% 10% 1	570
May	1st to 10th	Night	22% 5% 33% 28% 11% 1	570
	11th to 20th	Night	18% 4 25% 32% 18% 2	561
	21st to end	Night	9% 2 23% 36% 27% 3	617
June	1st to 10th	Night	11% 2 23% 40% 24% 1	570
	11th to 20th	Night	10% 2 18% 41% 26% 3	570
	21st to end	Night	8% 2 22% 39% 26% 3	570
July	1st to 10th	Night	11% 1 12% 34% 34% 7%	555
	11th to 20th	Night	9% 2 13% 39% 31% 7%	552
	21st to end	Night	9% 2 11% 39% 36% 3	624
August	1st to 10th	Night	8% 2 9% 40% 36% 5%	567
	11th to 20th	Night	7% 2 11% 44% 31% 5%	564
	21st to end	Night	6% 2 11% 35% 39% 7%	627
September	1st to 10th	Night	12% 4 13% 34% 30% 6%	652
	11th to 20th	Night	9% 3 13% 33% 36% 6%	760
	21st to end	Night	17% 4 20% 33% 24% 3	760
October	1st to 10th	Night	11% 5% 23% 33% 23% 5%	760
	11th to 20th	Night	18% 8% 28% 31% 14% 2	760
	21st to end	Night	22% 9% 33% 26% 10%	836
November	1st to 10th	Night	14% 10% 32% 29% 15% 1	780
	11th to 20th	Night	20% 14% 31% 27% 9% 1	800
	21st to end	Night	22% 13% 29% 30% 5% 1	800
December	1st to 10th	Night	25% 10% 37% 22% 5% 1	800
	11th to 20th	Night	26% 9% 34% 25% 6% 1	800
	21st to end	Night	28% 13% 33% 22% 4	1090

Page generated at 2025-02-03 03:05:54. Version ac07

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