Dry Tortugas National Park
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Dry Tortugas National Park clear sky chart from 2005-11-06 to 2025-11-18.

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 4 5% 5% 8% 12% 47%	126473
	Night	6% 4 2 5% 4 4 5% 5% 7% 11% 47%	123908
Transparency	Day	29% 8% 24% 27% 11% 2	130289
	Night	30% 7% 22% 26% 13% 2	128162
Seeing	Day	7% 2 6% 21% 37% 28%	17024
	Night	7% 2 7% 26% 38% 21%	27051

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	8% 5% 2 5% 4 5% 7% 5% 10% 13% 36%	4056
	11th to 20th	Day	18% 8% 3 9% 5% 6% 6% 5% 6% 8% 26%	4405
	21st to end	Day	10% 6% 2 6% 5% 5% 7% 5% 7% 9% 39%	4972
February	1st to 10th	Day	7% 5% 2 5% 5% 5% 5% 4 6% 10% 46%	2000
	11th to 20th	Day	7% 4 2 4 3 3 4 3 6% 9% 56%	1948
	21st to end	Day	5% 2 1 4 3 3 3 3 6% 10% 60%	1720
March	1st to 10th	Day	4 2 1 3 3 3 5% 4 7% 10% 58%	4935
	11th to 20th	Day	3 2 1 2 2 2 3 3 6% 9% 67%	4980
	21st to end	Day	5% 3 1 4 3 3 4 4 5% 9% 59%	5465
April	1st to 10th	Day	2 2 1 2 3 3 3 3 7% 11% 63%	2200
	11th to 20th	Day	2 3 1 5% 3 3 4 3 6% 9% 61%	2176
	21st to end	Day	4 2 1 3 2 3 3 4 7% 9% 60%	2220
May	1st to 10th	Day	1 1 1 2 2 2 2 3 5% 9% 73%	5696
	11th to 20th	Day	4 3 1 3 3 3 4 3 5% 8% 64%	5861
	21st to end	Day	4 2 1 3 2 3 3 3 5% 9% 66%	6433
June	1st to 10th	Day	13% 7% 3 7% 4 5% 5% 4 7% 12% 34%	2600
	11th to 20th	Day	7% 5% 3 6% 5% 6% 5% 5% 8% 13% 36%	2600
	21st to end	Day	5% 4 2 6% 5% 5% 5% 5% 9% 14% 39%	2600
July	1st to 10th	Day	3 3 2 5% 4 6% 7% 6% 10% 17% 38%	5900
	11th to 20th	Day	2 3 2 5% 5% 5% 7% 6% 10% 16% 39%	5900
	21st to end	Day	2 2 1 3 4 4 5% 5% 10% 15% 49%	6198
August	1st to 10th	Day	1 2 1 5% 5% 7% 8% 8% 12% 16% 33%	3905
	11th to 20th	Day	2 2 2 6% 5% 7% 8% 7% 11% 16% 34%	3765
	21st to end	Day	4 3 1 5% 6% 6% 8% 7% 12% 18% 29%	3949
September	1st to 10th	Day	5% 5% 2 6% 5% 7% 8% 7% 11% 15% 29%	2200
	11th to 20th	Day	4 5% 2 8% 5% 7% 7% 6% 10% 15% 31%	2200
	21st to end	Day	8% 6% 2 7% 6% 6% 8% 6% 7% 12% 30%	2200
October	1st to 10th	Day	6% 5% 3 7% 6% 5% 7% 6% 11% 13% 31%	2200
	11th to 20th	Day	9% 6% 3 7% 4 5% 7% 5% 8% 12% 34%	2132
	21st to end	Day	8% 6% 3 6% 5% 5% 6% 4 7% 11% 40%	2200
November	1st to 10th	Day	8% 5% 3 6% 5% 4 5% 4 8% 11% 41%	2057
	11th to 20th	Day	8% 5% 2 5% 5% 5% 6% 5% 8% 10% 43%	2070
	21st to end	Day	6% 5% 2 5% 4 4 5% 5% 8% 10% 46%	1961
December	1st to 10th	Day	6% 3 1 3 2 3 3 4 6% 8% 59%	2930
	11th to 20th	Day	9% 7% 2 5% 4 4 5% 4 7% 11% 42%	3373
	21st to end	Day	10% 7% 3 9% 6% 6% 6% 5% 7% 10% 31%	4466
January	1st to 10th	Night	6% 5% 2 5% 6% 5% 6% 5% 8% 11% 41%	5889
	11th to 20th	Night	17% 9% 4 8% 6% 4 5% 4 6% 7% 30%	5550
	21st to end	Night	10% 6% 2 7% 5% 5% 5% 4 6% 8% 42%	5984
February	1st to 10th	Night	9% 5% 2 5% 4 5% 4 3 7% 8% 47%	2400
	11th to 20th	Night	7% 4 2 4 4 4 3 4 5% 8% 56%	2338
	21st to end	Night	5% 3 1 4 3 3 4 4 5% 8% 60%	1907
March	1st to 10th	Night	4 4 2 5% 3 4 4 4 6% 8% 56%	4918
	11th to 20th	Night	4 2 1 3 2 3 3 3 5% 8% 65%	4980
	21st to end	Night	5% 3 1 4 3 4 4 4 7% 9% 55%	5467
April	1st to 10th	Night	3 2 1 3 2 3 4 4 6% 10% 62%	2200
	11th to 20th	Night	3 2 1 4 4 5% 4 3 6% 9% 59%	2173
	21st to end	Night	4 3 1 3 3 3 3 4 6% 10% 59%	2177
May	1st to 10th	Night	2 1 2 2 2 3 3 5% 8% 72%	4224
	11th to 20th	Night	5% 3 1 5% 3 3 3 4 5% 7% 60%	4032
	21st to end	Night	3 3 1 3 3 2 3 3 5% 9% 65%	4421
June	1st to 10th	Night	12% 8% 3 7% 5% 5% 5% 6% 6% 9% 33%	1800
	11th to 20th	Night	6% 6% 2 5% 4 5% 6% 5% 7% 12% 42%	1800
	21st to end	Night	5% 4 1 5% 4 4 6% 6% 9% 15% 42%	1800
July	1st to 10th	Night	2 2 1 3 3 5% 7% 7% 11% 18% 41%	4058
	11th to 20th	Night	2 2 1 4 3 4 5% 5% 9% 17% 49%	4057
	21st to end	Night	1 2 1 3 3 3 5% 4 9% 17% 53%	4645
August	1st to 10th	Night	2 2 1 4 4 5% 8% 7% 11% 17% 37%	3244
	11th to 20th	Night	1 1 1 4 4 5% 7% 6% 12% 20% 39%	3248
	21st to end	Night	3 3 2 4 4 5% 7% 7% 12% 18% 36%	3949
September	1st to 10th	Night	4 4 2 5% 5% 6% 7% 7% 11% 14% 35%	2200
	11th to 20th	Night	5% 6% 2 7% 7% 7% 6% 6% 9% 13% 31%	2200
	21st to end	Night	7% 6% 3 7% 7% 7% 6% 4 7% 14% 31%	2200
October	1st to 10th	Night	7% 6% 3 8% 6% 7% 7% 6% 10% 12% 30%	2200
	11th to 20th	Night	8% 6% 2 7% 5% 5% 6% 6% 8% 10% 36%	2265
	21st to end	Night	10% 7% 2 7% 5% 5% 5% 5% 8% 10% 36%	2640
November	1st to 10th	Night	9% 5% 2 5% 4 4 5% 5% 8% 11% 43%	2461
	11th to 20th	Night	8% 6% 2 6% 5% 4 5% 4 7% 10% 44%	2485
	21st to end	Night	6% 6% 2 5% 4 5% 5% 4 6% 9% 49%	2434
December	1st to 10th	Night	5% 4 1 4 3 4 3 4 6% 9% 57%	4250
	11th to 20th	Night	10% 6% 2 5% 4 4 5% 5% 7% 10% 40%	4822
	21st to end	Night	9% 7% 3 9% 6% 7% 6% 6% 8% 10% 30%	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	33% 7% 17% 33% 9% 1	4057
	11th to 20th	Day	53% 6% 15% 19% 6% 1	4405
	21st to end	Day	39% 6% 10% 29% 15% 2	4972
February	1st to 10th	Day	32% 5% 14% 29% 18% 3	2000
	11th to 20th	Day	24% 5% 13% 28% 23% 7%	1958
	21st to end	Day	20% 5% 12% 30% 27% 6%	1720
March	1st to 10th	Day	20% 5% 18% 33% 21% 3	4935
	11th to 20th	Day	14% 5% 19% 30% 24% 9%	4980
	21st to end	Day	22% 4 11% 37% 22% 4	5465
April	1st to 10th	Day	15% 5% 19% 42% 15% 4	2200
	11th to 20th	Day	18% 6% 22% 31% 20% 3	2176
	21st to end	Day	17% 7% 16% 39% 19% 2	2220
May	1st to 10th	Day	9% 3 18% 50% 18% 2	5729
	11th to 20th	Day	19% 5% 16% 46% 13% 2	5865
	21st to end	Day	16% 4 19% 52% 9%	6447
June	1st to 10th	Day	41% 8% 26% 21% 3	2600
	11th to 20th	Day	36% 13% 33% 17% 1	2600
	21st to end	Day	32% 12% 40% 14% 1	2600
July	1st to 10th	Day	31% 12% 46% 10%	5835
	11th to 20th	Day	31% 10% 49% 11%	5831
	21st to end	Day	23% 9% 44% 23% 2	6019
August	1st to 10th	Day	33% 14% 37% 15% 1	3893
	11th to 20th	Day	33% 11% 43% 13%	3801
	21st to end	Day	36% 17% 40% 8%	3949
September	1st to 10th	Day	37% 16% 36% 11%	2200
	11th to 20th	Day	37% 13% 33% 13% 3	2200
	21st to end	Day	41% 13% 24% 16% 6% 1	2200
October	1st to 10th	Day	40% 12% 25% 16% 7% 1	3590
	11th to 20th	Day	45% 10% 19% 16% 8% 1	3450
	21st to end	Day	35% 5% 14% 32% 11% 2	3538
November	1st to 10th	Day	34% 7% 11% 27% 18% 3	2057
	11th to 20th	Day	34% 5% 15% 24% 19% 4	2070
	21st to end	Day	29% 7% 16% 22% 23% 2	1961
December	1st to 10th	Day	21% 4 10% 40% 23% 1	2930
	11th to 20th	Day	33% 6% 12% 28% 19% 1	3370
	21st to end	Day	46% 6% 16% 18% 13% 1	4466
January	1st to 10th	Night	34% 5% 14% 30% 16% 1	5889
	11th to 20th	Night	51% 5% 16% 19% 8% 1	5550
	21st to end	Night	39% 4 9% 26% 20% 2	5984
February	1st to 10th	Night	33% 6% 13% 26% 18% 4	2400
	11th to 20th	Night	26% 5% 11% 26% 25% 7%	2339
	21st to end	Night	21% 5% 10% 28% 28% 8%	1907
March	1st to 10th	Night	25% 5% 12% 31% 21% 6%	4918
	11th to 20th	Night	17% 4 17% 28% 26% 7%	4980
	21st to end	Night	24% 5% 12% 33% 22% 5%	5467
April	1st to 10th	Night	17% 6% 17% 40% 16% 3	2200
	11th to 20th	Night	21% 5% 18% 31% 19% 5%	2173
	21st to end	Night	21% 5% 13% 34% 23% 3	2177
May	1st to 10th	Night	10% 3 13% 53% 19% 1	4254
	11th to 20th	Night	23% 5% 15% 38% 16% 3	4033
	21st to end	Night	16% 5% 15% 50% 15%	4429
June	1st to 10th	Night	46% 8% 21% 23% 4	1800
	11th to 20th	Night	34% 11% 35% 18% 2	1800
	21st to end	Night	28% 12% 39% 19% 2	1800
July	1st to 10th	Night	26% 13% 45% 16%	4015
	11th to 20th	Night	21% 10% 54% 15%	4006
	21st to end	Night	18% 8% 47% 26% 2	4510
August	1st to 10th	Night	29% 13% 39% 18% 1	3232
	11th to 20th	Night	26% 11% 48% 15% 1	3278
	21st to end	Night	30% 12% 47% 10%	3949
September	1st to 10th	Night	33% 17% 36% 13% 1	2200
	11th to 20th	Night	40% 11% 31% 14% 4	2200
	21st to end	Night	42% 12% 25% 17% 4	2200
October	1st to 10th	Night	41% 10% 28% 15% 5% 1	3590
	11th to 20th	Night	48% 10% 18% 15% 8% 1	3686
	21st to end	Night	42% 5% 13% 27% 11% 3	4252
November	1st to 10th	Night	32% 7% 13% 25% 18% 4	2461
	11th to 20th	Night	33% 6% 13% 25% 19% 3	2485
	21st to end	Night	30% 6% 12% 25% 23% 4	2434
December	1st to 10th	Night	23% 5% 10% 40% 21% 1	4250
	11th to 20th	Night	34% 6% 12% 28% 18% 2	4824
	21st to end	Night	46% 6% 18% 17% 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	7% 3 16% 38% 33% 3	300
	11th to 20th	Day	13% 6% 18% 35% 25% 3	300
	21st to end	Day	13% 5% 18% 41% 22% 1	330
February	1st to 10th	Day	7% 3 13% 36% 37% 4	300
	11th to 20th	Day	6% 2 9% 33% 41% 8%	300
	21st to end	Day	6% 3 13% 33% 39% 7%	319
March	1st to 10th	Day	5% 2 13% 43% 33% 3	500
	11th to 20th	Day	6% 1 10% 41% 39% 4	500
	21st to end	Day	5% 2 9% 39% 40% 5%	550
April	1st to 10th	Day	3 1 7% 41% 44% 4	500
	11th to 20th	Day	3 2 8% 43% 40% 4	492
	21st to end	Day	4 2 4 35% 51% 5%	500
May	1st to 10th	Day	2 2 27% 56% 12%	503
	11th to 20th	Day	9% 1 3 24% 47% 16%	527
	21st to end	Day	7% 2 1 13% 52% 25%	573
June	1st to 10th	Day	15% 2 3 12% 39% 30%	530
	11th to 20th	Day	9% 2 3 8% 33% 45%	530
	21st to end	Day	4 2 4 23% 67%	530
July	1st to 10th	Day	4 1 1 2 24% 68%	525
	11th to 20th	Day	2 1 3 23% 70%	524
	21st to end	Day	5% 1 2 27% 65%	576
August	1st to 10th	Day	4 1 1 1 27% 66%	526
	11th to 20th	Day	4 1 1 3 24% 67%	519
	21st to end	Day	6% 2 1 5% 29% 57%	583
September	1st to 10th	Day	5% 2 1 3 29% 61%	530
	11th to 20th	Day	7% 1 1 6% 26% 59%	530
	21st to end	Day	10% 2 1 8% 31% 48%	530
October	1st to 10th	Day	7% 3 3 13% 47% 27%	530
	11th to 20th	Day	9% 2 5% 17% 46% 21%	530
	21st to end	Day	8% 2 8% 25% 41% 16%	583
November	1st to 10th	Day	8% 3 9% 26% 42% 11%	535
	11th to 20th	Day	9% 2 8% 31% 42% 7%	528
	21st to end	Day	7% 3 9% 32% 41% 8%	461
December	1st to 10th	Day	9% 3 12% 23% 45% 8%	300
	11th to 20th	Day	8% 1 11% 31% 43% 6%	300
	21st to end	Day	9% 2 14% 32% 36% 7%	330
January	1st to 10th	Night	10% 2 12% 36% 36% 4	800
	11th to 20th	Night	10% 3 15% 39% 29% 3	800
	21st to end	Night	10% 3 17% 43% 25% 1	880
February	1st to 10th	Night	9% 3 16% 36% 33% 2	800
	11th to 20th	Night	7% 2 13% 36% 38% 4	783
	21st to end	Night	6% 2 9% 34% 45% 4	660
March	1st to 10th	Night	4 2 14% 42% 35% 3	800
	11th to 20th	Night	6% 1 10% 43% 36% 5%	800
	21st to end	Night	5% 1 10% 42% 37% 4	880
April	1st to 10th	Night	3 1 6% 44% 42% 4	800
	11th to 20th	Night	3 1 8% 47% 36% 5%	791
	21st to end	Night	5% 1 6% 40% 44% 4	800
May	1st to 10th	Night	3 1 1 36% 48% 10%	690
	11th to 20th	Night	8% 2 3 30% 47% 10%	591
	21st to end	Night	7% 2 3 18% 55% 15%	650
June	1st to 10th	Night	15% 3 2 16% 42% 23%	600
	11th to 20th	Night	7% 2 1 10% 41% 40%	600
	21st to end	Night	6% 1 1 4 33% 56%	600
July	1st to 10th	Night	4 1 3 34% 58%	585
	11th to 20th	Night	2 1 2 29% 65%	582
	21st to end	Night	3 1 3 31% 62%	657
August	1st to 10th	Night	4 1 1 4 35% 57%	597
	11th to 20th	Night	2 1 4 31% 62%	616
	21st to end	Night	7% 1 2 7% 30% 52%	880
September	1st to 10th	Night	4 1 1 4 32% 57%	800
	11th to 20th	Night	8% 2 1 6% 32% 52%	800
	21st to end	Night	10% 2 3 10% 36% 39%	800
October	1st to 10th	Night	9% 2 2 17% 45% 24%	800
	11th to 20th	Night	10% 2 3 18% 42% 24%	800
	21st to end	Night	13% 3 7% 26% 38% 13%	880
November	1st to 10th	Night	9% 2 10% 29% 42% 8%	820
	11th to 20th	Night	10% 2 7% 29% 42% 10%	829
	21st to end	Night	8% 2 9% 35% 41% 5%	800
December	1st to 10th	Night	9% 2 10% 32% 41% 6%	800
	11th to 20th	Night	9% 2 10% 32% 41% 5%	800
	21st to end	Night	9% 3 13% 33% 35% 8%	880

Page generated at 2025-11-15 00:55:55. Version ac07

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