Key West
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Key West clear sky chart from 2005-11-06 to 2025-10-19.

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	5% 4 2 5% 4 5% 5% 5% 8% 12% 46%	125988
	Night	5% 4 2 5% 4 5% 5% 5% 7% 11% 46%	123726
Transparency	Day	28% 11% 22% 26% 11% 2	129731
	Night	30% 9% 20% 25% 13% 2	128039
Seeing	Day	6% 1 9% 23% 35% 25%	17141
	Night	7% 1 9% 26% 37% 19%	27219

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	5% 4 2 6% 5% 5% 6% 5% 8% 14% 40%	4056
	11th to 20th	Day	10% 7% 3 8% 6% 5% 6% 5% 7% 10% 33%	4134
	21st to end	Day	8% 6% 3 6% 4 5% 6% 5% 6% 8% 43%	4972
February	1st to 10th	Day	7% 4 2 4 4 3 4 4 7% 10% 51%	2000
	11th to 20th	Day	5% 4 2 3 4 4 3 3 7% 10% 55%	1961
	21st to end	Day	4 3 1 3 3 3 4 3 6% 10% 60%	1815
March	1st to 10th	Day	2 2 1 3 3 4 4 4 7% 10% 60%	4935
	11th to 20th	Day	2 2 1 3 2 3 3 4 6% 10% 64%	4980
	21st to end	Day	5% 4 2 4 3 3 4 4 7% 9% 56%	5465
April	1st to 10th	Day	2 2 1 3 3 3 4 4 7% 12% 60%	2200
	11th to 20th	Day	3 2 1 3 3 3 4 3 7% 11% 60%	2176
	21st to end	Day	4 3 2 3 2 3 4 5% 6% 10% 60%	2330
May	1st to 10th	Day	1 1 1 2 1 2 3 3 5% 12% 69%	5405
	11th to 20th	Day	4 3 1 4 3 3 4 4 6% 9% 60%	5738
	21st to end	Day	3 3 1 3 3 3 3 3 5% 9% 66%	6433
June	1st to 10th	Day	11% 7% 3 8% 6% 5% 5% 6% 7% 11% 31%	2600
	11th to 20th	Day	7% 6% 2 6% 6% 6% 7% 6% 9% 13% 33%	2600
	21st to end	Day	4 4 2 7% 5% 6% 7% 7% 10% 13% 34%	2600
July	1st to 10th	Day	2 4 1 5% 5% 6% 6% 6% 10% 17% 37%	5900
	11th to 20th	Day	2 4 2 5% 5% 6% 6% 7% 11% 16% 36%	5900
	21st to end	Day	2 2 1 3 3 4 5% 6% 11% 18% 45%	6424
August	1st to 10th	Day	3 3 2 7% 6% 7% 9% 7% 12% 17% 27%	4045
	11th to 20th	Day	2 2 2 6% 6% 7% 8% 8% 12% 16% 31%	3594
	21st to end	Day	3 3 1 7% 6% 7% 9% 8% 11% 16% 28%	3949
September	1st to 10th	Day	5% 4 2 6% 5% 7% 9% 8% 11% 16% 28%	2200
	11th to 20th	Day	5% 5% 3 7% 7% 6% 7% 6% 10% 16% 28%	2200
	21st to end	Day	7% 6% 3 8% 6% 7% 6% 6% 9% 12% 30%	2200
October	1st to 10th	Day	5% 6% 3 7% 6% 6% 8% 8% 11% 12% 28%	2200
	11th to 20th	Day	9% 5% 2 6% 5% 5% 6% 5% 7% 14% 34%	2018
	21st to end	Day	8% 6% 3 6% 5% 6% 6% 5% 7% 11% 38%	2090
November	1st to 10th	Day	7% 5% 1 6% 4 5% 4 5% 7% 10% 46%	1957
	11th to 20th	Day	9% 6% 2 7% 6% 5% 5% 4 7% 10% 41%	2000
	21st to end	Day	5% 4 2 5% 5% 5% 5% 4 8% 9% 49%	2000
December	1st to 10th	Day	5% 3 1 4 4 3 4 3 5% 9% 58%	3072
	11th to 20th	Day	7% 7% 3 6% 4 4 5% 4 7% 10% 42%	3373
	21st to end	Day	10% 8% 3 9% 6% 6% 5% 5% 7% 10% 32%	4466
January	1st to 10th	Night	5% 4 2 6% 4 4 5% 5% 9% 11% 44%	5889
	11th to 20th	Night	12% 8% 3 9% 6% 5% 5% 4 6% 7% 35%	5808
	21st to end	Night	10% 6% 2 7% 5% 5% 5% 4 6% 8% 43%	5984
February	1st to 10th	Night	6% 6% 1 6% 4 4 5% 5% 7% 9% 49%	2400
	11th to 20th	Night	6% 4 2 6% 5% 3 4 3 5% 7% 56%	2323
	21st to end	Night	4 3 1 4 3 3 3 4 6% 9% 61%	1815
March	1st to 10th	Night	3 3 2 5% 4 4 6% 5% 6% 8% 54%	4918
	11th to 20th	Night	3 2 1 3 3 3 3 3 5% 8% 66%	4980
	21st to end	Night	5% 3 1 4 4 4 5% 5% 7% 10% 53%	5467
April	1st to 10th	Night	3 2 1 3 3 4 4 4 6% 11% 60%	2200
	11th to 20th	Night	4 3 1 4 4 4 5% 4 6% 10% 54%	2173
	21st to end	Night	4 3 2 4 4 3 3 4 6% 7% 60%	2068
May	1st to 10th	Night	1 1 2 2 2 3 3 5% 9% 72%	4490
	11th to 20th	Night	6% 4 2 5% 4 4 4 4 6% 7% 54%	4181
	21st to end	Night	3 3 1 3 3 2 2 3 4 10% 66%	4421
June	1st to 10th	Night	12% 8% 3 8% 6% 6% 5% 4 8% 10% 29%	1800
	11th to 20th	Night	7% 5% 2 5% 6% 4 8% 5% 9% 12% 37%	1800
	21st to end	Night	4 4 2 5% 5% 7% 7% 6% 9% 14% 36%	1800
July	1st to 10th	Night	2 3 1 5% 4 6% 6% 7% 12% 18% 36%	4058
	11th to 20th	Night	1 2 1 4 4 5% 6% 5% 9% 16% 47%	4057
	21st to end	Night	1 2 1 3 3 4 4 5% 8% 15% 53%	4427
August	1st to 10th	Night	2 3 2 6% 4 6% 8% 6% 11% 17% 34%	3098
	11th to 20th	Night	1 2 1 4 4 6% 6% 7% 11% 18% 40%	3419
	21st to end	Night	2 3 2 5% 5% 6% 6% 7% 11% 19% 33%	3949
September	1st to 10th	Night	4 3 1 5% 5% 7% 8% 7% 12% 16% 32%	2200
	11th to 20th	Night	5% 5% 2 7% 6% 7% 7% 7% 10% 14% 30%	2200
	21st to end	Night	7% 7% 3 8% 5% 6% 7% 6% 9% 12% 30%	2200
October	1st to 10th	Night	5% 6% 3 9% 7% 6% 6% 6% 10% 12% 31%	2200
	11th to 20th	Night	8% 7% 2 6% 5% 5% 6% 6% 8% 12% 34%	2337
	21st to end	Night	10% 6% 3 6% 6% 6% 6% 5% 8% 10% 35%	2508
November	1st to 10th	Night	8% 4 1 6% 5% 4 5% 4 7% 12% 44%	2341
	11th to 20th	Night	8% 5% 2 6% 5% 5% 5% 5% 7% 10% 44%	2400
	21st to end	Night	5% 5% 2 5% 5% 4 5% 4 7% 10% 49%	2400
December	1st to 10th	Night	5% 3 2 4 3 3 4 4 6% 8% 59%	4103
	11th to 20th	Night	7% 6% 2 6% 5% 4 5% 5% 7% 10% 44%	4822
	21st to end	Night	8% 7% 4 9% 6% 6% 6% 5% 7% 9% 33%	6490

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	29% 8% 17% 31% 13% 2	4057
	11th to 20th	Day	43% 10% 16% 23% 7% 1	4134
	21st to end	Day	37% 8% 9% 29% 15% 3	4972
February	1st to 10th	Day	28% 7% 12% 31% 19% 3	2000
	11th to 20th	Day	24% 5% 13% 28% 23% 6%	1971
	21st to end	Day	18% 6% 11% 30% 28% 7%	1815
March	1st to 10th	Day	17% 7% 18% 31% 24% 3	4935
	11th to 20th	Day	14% 7% 18% 30% 22% 9%	4980
	21st to end	Day	23% 6% 11% 34% 22% 3	5465
April	1st to 10th	Day	16% 7% 18% 40% 17% 3	2200
	11th to 20th	Day	18% 7% 21% 32% 19% 4	2176
	21st to end	Day	19% 7% 15% 34% 22% 2	2330
May	1st to 10th	Day	9% 6% 17% 48% 19% 2	5437
	11th to 20th	Day	20% 7% 14% 47% 11%	5742
	21st to end	Day	15% 6% 17% 52% 9%	6447
June	1st to 10th	Day	46% 11% 22% 18% 3	2600
	11th to 20th	Day	38% 16% 30% 15% 1	2600
	21st to end	Day	34% 17% 35% 13% 1	2600
July	1st to 10th	Day	30% 17% 41% 12%	5835
	11th to 20th	Day	29% 16% 44% 11% 1	5831
	21st to end	Day	19% 14% 43% 22% 2	6240
August	1st to 10th	Day	34% 19% 34% 13% 1	4032
	11th to 20th	Day	29% 18% 39% 13% 1	3629
	21st to end	Day	34% 20% 36% 9%	3949
September	1st to 10th	Day	34% 20% 34% 12% 1	2200
	11th to 20th	Day	37% 18% 30% 13% 3	2200
	21st to end	Day	41% 16% 24% 15% 4	2200
October	1st to 10th	Day	40% 13% 24% 17% 4 1	3590
	11th to 20th	Day	45% 14% 18% 14% 7% 1	3271
	21st to end	Day	36% 9% 13% 28% 12% 2	3428
November	1st to 10th	Day	32% 9% 12% 24% 20% 3	1957
	11th to 20th	Day	38% 8% 10% 23% 17% 4	2000
	21st to end	Day	29% 9% 13% 22% 24% 3	2000
December	1st to 10th	Day	25% 7% 10% 34% 21% 3	3072
	11th to 20th	Day	35% 10% 12% 26% 15% 3	3370
	21st to end	Day	45% 11% 13% 17% 13% 1	4466
January	1st to 10th	Night	29% 8% 14% 31% 16% 1	5889
	11th to 20th	Night	45% 8% 15% 23% 8% 1	5808
	21st to end	Night	38% 6% 8% 26% 19% 2	5984
February	1st to 10th	Night	32% 7% 12% 28% 18% 4	2400
	11th to 20th	Night	27% 6% 11% 24% 24% 7%	2324
	21st to end	Night	21% 4 11% 26% 29% 9%	1815
March	1st to 10th	Night	24% 8% 12% 27% 24% 5%	4918
	11th to 20th	Night	17% 6% 16% 30% 22% 9%	4980
	21st to end	Night	23% 7% 12% 31% 23% 4	5467
April	1st to 10th	Night	20% 6% 17% 38% 16% 3	2200
	11th to 20th	Night	24% 8% 16% 28% 19% 5%	2173
	21st to end	Night	23% 6% 11% 34% 23% 3	2068
May	1st to 10th	Night	9% 5% 13% 53% 18% 2	4520
	11th to 20th	Night	28% 7% 13% 35% 15% 1	4182
	21st to end	Night	16% 5% 17% 48% 14%	4429
June	1st to 10th	Night	48% 11% 18% 19% 4	1800
	11th to 20th	Night	38% 12% 32% 15% 3	1800
	21st to end	Night	34% 13% 37% 15% 1	1800
July	1st to 10th	Night	27% 17% 42% 13%	4015
	11th to 20th	Night	22% 14% 50% 13%	4006
	21st to end	Night	19% 10% 40% 27% 4	4297
August	1st to 10th	Night	30% 14% 37% 17% 1	3087
	11th to 20th	Night	23% 13% 48% 15% 1	3450
	21st to end	Night	30% 19% 41% 10% 1	3949
September	1st to 10th	Night	32% 20% 35% 13% 1	2200
	11th to 20th	Night	39% 16% 28% 14% 3	2200
	21st to end	Night	43% 13% 25% 14% 5%	2200
October	1st to 10th	Night	39% 12% 26% 17% 5% 1	3590
	11th to 20th	Night	46% 14% 17% 15% 6% 1	3810
	21st to end	Night	39% 8% 13% 26% 12% 2	4120
November	1st to 10th	Night	33% 8% 13% 24% 18% 3	2341
	11th to 20th	Night	34% 7% 12% 26% 19% 3	2400
	21st to end	Night	30% 8% 13% 22% 23% 3	2400
December	1st to 10th	Night	23% 6% 11% 40% 17% 3	4103
	11th to 20th	Night	33% 8% 14% 27% 15% 3	4824
	21st to end	Night	46% 10% 13% 18% 12% 1	6490

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	5% 3 21% 39% 28% 4	300
	11th to 20th	Day	10% 7% 18% 39% 23% 2	300
	21st to end	Day	12% 5% 24% 38% 20% 2	330
February	1st to 10th	Day	6% 5% 12% 38% 34% 5%	300
	11th to 20th	Day	5% 2 15% 35% 35% 8%	313
	21st to end	Day	6% 1 16% 31% 41% 5%	414
March	1st to 10th	Day	4 2 17% 44% 30% 4	500
	11th to 20th	Day	5% 2 16% 40% 33% 4	500
	21st to end	Day	4 2 13% 42% 36% 4	550
April	1st to 10th	Day	2 2 10% 45% 37% 4	500
	11th to 20th	Day	3 1 15% 48% 30% 3	492
	21st to end	Day	3 1 6% 45% 41% 3	500
May	1st to 10th	Day	3 5% 35% 48% 9%	503
	11th to 20th	Day	9% 1 7% 29% 43% 11%	527
	21st to end	Day	9% 1 4 16% 50% 21%	573
June	1st to 10th	Day	13% 1 6% 18% 35% 27%	530
	11th to 20th	Day	9% 1 5% 11% 37% 37%	530
	21st to end	Day	3 2 5% 33% 56%	530
July	1st to 10th	Day	4 1 2 3 32% 58%	525
	11th to 20th	Day	3 2 3 32% 60%	524
	21st to end	Day	6% 1 1 5% 34% 54%	576
August	1st to 10th	Day	5% 1 3 35% 55%	526
	11th to 20th	Day	3 1 2 6% 31% 59%	519
	21st to end	Day	5% 1 5% 9% 32% 49%	583
September	1st to 10th	Day	5% 1 7% 28% 59%	530
	11th to 20th	Day	7% 1 3 6% 27% 57%	530
	21st to end	Day	8% 2 6% 8% 31% 45%	530
October	1st to 10th	Day	5% 1 7% 20% 44% 23%	530
	11th to 20th	Day	11% 2 7% 18% 39% 23%	522
	21st to end	Day	7% 2 13% 27% 38% 14%	539
November	1st to 10th	Day	8% 2 13% 26% 40% 10%	495
	11th to 20th	Day	13% 3 11% 30% 33% 11%	500
	21st to end	Day	6% 2 16% 34% 35% 8%	500
December	1st to 10th	Day	9% 3 13% 29% 37% 10%	390
	11th to 20th	Day	7% 4 12% 31% 39% 7%	300
	21st to end	Day	9% 5% 17% 31% 32% 8%	330
January	1st to 10th	Night	8% 2 16% 35% 35% 4	800
	11th to 20th	Night	11% 3 17% 38% 28% 3	800
	21st to end	Night	9% 4 20% 40% 26% 2	880
February	1st to 10th	Night	8% 2 16% 37% 33% 3	800
	11th to 20th	Night	7% 2 17% 36% 35% 4	783
	21st to end	Night	5% 2 13% 37% 38% 5%	660
March	1st to 10th	Night	5% 2 16% 43% 31% 3	800
	11th to 20th	Night	5% 2 13% 41% 36% 4	800
	21st to end	Night	4 2 15% 42% 35% 3	880
April	1st to 10th	Night	4 1 8% 44% 41% 3	800
	11th to 20th	Night	5% 1 11% 46% 34% 3	791
	21st to end	Night	6% 1 7% 45% 38% 3	800
May	1st to 10th	Night	3 7% 36% 46% 8%	800
	11th to 20th	Night	9% 1 6% 31% 45% 8%	662
	21st to end	Night	8% 1 6% 23% 46% 16%	650
June	1st to 10th	Night	15% 2 7% 16% 41% 19%	600
	11th to 20th	Night	8% 1 4 10% 41% 36%	600
	21st to end	Night	5% 2 6% 40% 47%	600
July	1st to 10th	Night	4 2 4 36% 54%	585
	11th to 20th	Night	2 1 5% 35% 56%	582
	21st to end	Night	3 2 6% 33% 56%	657
August	1st to 10th	Night	3 2 5% 39% 50%	597
	11th to 20th	Night	3 1 6% 33% 56%	723
	21st to end	Night	7% 3 8% 33% 48%	880
September	1st to 10th	Night	5% 2 6% 34% 53%	800
	11th to 20th	Night	7% 3 8% 29% 53%	800
	21st to end	Night	10% 7% 10% 32% 42%	800
October	1st to 10th	Night	8% 1 8% 18% 45% 22%	800
	11th to 20th	Night	11% 1 7% 18% 41% 22%	793
	21st to end	Night	13% 2 11% 26% 37% 12%	836
November	1st to 10th	Night	8% 2 16% 31% 36% 8%	780
	11th to 20th	Night	10% 1 9% 29% 41% 10%	800
	21st to end	Night	8% 2 13% 35% 38% 5%	800
December	1st to 10th	Night	8% 2 12% 33% 39% 6%	800
	11th to 20th	Night	9% 3 12% 33% 37% 6%	800
	21st to end	Night	8% 3 14% 33% 35% 6%	880

Page generated at 2025-10-16 01:50:26. Version ac07

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