Boca Raton
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Boca Raton clear sky chart from 2005-11-06 to 2025-09-04.

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	7% 5% 2 7% 5% 6% 6% 6% 8% 11% 36%	125395
	Night	7% 5% 2 7% 5% 5% 6% 5% 7% 10% 41%	123324
Transparency	Day	21% 43% 14% 15% 6% 1	129048
	Night	23% 41% 14% 14% 6% 1	127740
Seeing	Day	7% 3 14% 28% 29% 18%	17176
	Night	8% 3 14% 30% 32% 13%	27283

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	7% 6% 2 6% 4 4 5% 4 6% 10% 48%	4056
	11th to 20th	Day	17% 7% 3 9% 6% 5% 5% 4 6% 9% 28%	4060
	21st to end	Day	11% 7% 3 7% 5% 5% 5% 4 7% 7% 38%	4466
February	1st to 10th	Day	9% 6% 2 6% 5% 4 5% 4 8% 11% 40%	1995
	11th to 20th	Day	7% 4 2 5% 4 4 4 4 7% 10% 49%	2085
	21st to end	Day	6% 5% 2 5% 6% 5% 7% 5% 7% 9% 43%	1815
March	1st to 10th	Day	5% 5% 2 7% 5% 6% 7% 6% 8% 9% 40%	4935
	11th to 20th	Day	4 2 1 4 3 3 4 4 6% 10% 59%	4980
	21st to end	Day	7% 5% 2 6% 5% 5% 6% 5% 7% 10% 42%	5465
April	1st to 10th	Day	4 3 2 5% 4 4 5% 6% 8% 13% 47%	2200
	11th to 20th	Day	7% 5% 2 6% 4 5% 6% 5% 8% 10% 42%	2294
	21st to end	Day	6% 4 1 5% 5% 5% 6% 6% 9% 10% 44%	2400
May	1st to 10th	Day	2 2 1 4 3 4 5% 5% 8% 11% 56%	5405
	11th to 20th	Day	6% 4 2 6% 4 5% 6% 5% 7% 10% 44%	5421
	21st to end	Day	4 4 2 5% 5% 6% 7% 6% 10% 13% 38%	6433
June	1st to 10th	Day	12% 8% 4 11% 9% 7% 8% 7% 9% 9% 18%	2600
	11th to 20th	Day	7% 6% 3 10% 9% 8% 8% 7% 10% 11% 23%	2600
	21st to end	Day	7% 7% 2 9% 7% 6% 8% 8% 11% 12% 24%	2600
July	1st to 10th	Day	6% 6% 3 8% 6% 6% 6% 6% 9% 13% 31%	5900
	11th to 20th	Day	6% 5% 2 8% 6% 6% 7% 6% 9% 13% 32%	5900
	21st to end	Day	3 3 2 5% 5% 6% 6% 5% 10% 16% 40%	6424
August	1st to 10th	Day	5% 5% 3 10% 8% 7% 8% 7% 11% 13% 23%	4119
	11th to 20th	Day	4 4 2 7% 5% 6% 8% 7% 10% 16% 31%	3850
	21st to end	Day	5% 4 2 7% 7% 7% 8% 8% 12% 18% 22%	4097
September	1st to 10th	Day	7% 6% 2 10% 7% 7% 10% 7% 10% 12% 21%	2123
	11th to 20th	Day	7% 6% 3 8% 8% 8% 9% 7% 10% 11% 22%	2090
	21st to end	Day	11% 8% 3 10% 8% 8% 7% 7% 8% 9% 20%	2090
October	1st to 10th	Day	9% 6% 3 9% 6% 8% 8% 7% 10% 11% 22%	1979
	11th to 20th	Day	9% 6% 3 9% 6% 7% 8% 6% 9% 10% 27%	1900
	21st to end	Day	11% 6% 2 8% 6% 5% 6% 4 6% 10% 35%	2090
November	1st to 10th	Day	8% 6% 2 6% 5% 5% 6% 5% 9% 11% 37%	1957
	11th to 20th	Day	11% 6% 2 8% 6% 6% 7% 5% 8% 9% 31%	2000
	21st to end	Day	8% 5% 3 6% 7% 6% 7% 5% 6% 10% 37%	2000
December	1st to 10th	Day	6% 4 2 6% 4 4 5% 4 7% 7% 50%	3227
	11th to 20th	Day	9% 6% 2 7% 5% 5% 6% 5% 8% 12% 35%	3373
	21st to end	Day	12% 6% 3 8% 6% 6% 6% 6% 8% 10% 28%	4466
January	1st to 10th	Night	6% 6% 2 7% 5% 5% 6% 5% 7% 9% 42%	5889
	11th to 20th	Night	15% 10% 3 9% 6% 5% 5% 4 6% 8% 30%	5900
	21st to end	Night	12% 7% 3 7% 5% 5% 5% 4 5% 7% 40%	6490
February	1st to 10th	Night	9% 5% 2 5% 4 4 4 4 7% 10% 45%	2400
	11th to 20th	Night	7% 4 1 5% 4 4 4 4 7% 8% 53%	2197
	21st to end	Night	6% 3 1 5% 5% 4 5% 4 6% 9% 51%	1815
March	1st to 10th	Night	4 5% 2 6% 5% 5% 5% 4 6% 8% 48%	4918
	11th to 20th	Night	4 3 1 4 3 3 4 3 6% 9% 60%	4980
	21st to end	Night	6% 4 2 6% 5% 5% 5% 5% 7% 9% 47%	5467
April	1st to 10th	Night	4 2 1 4 3 3 4 4 7% 10% 55%	2200
	11th to 20th	Night	4 3 2 5% 5% 4 6% 5% 7% 9% 50%	2052
	21st to end	Night	5% 4 2 5% 4 5% 4 5% 8% 9% 50%	2000
May	1st to 10th	Night	2 2 1 3 3 3 4 4 7% 11% 59%	4490
	11th to 20th	Night	6% 4 2 5% 4 5% 4 4 6% 9% 50%	4490
	21st to end	Night	4 4 2 6% 5% 5% 6% 6% 9% 12% 40%	4430
June	1st to 10th	Night	11% 7% 3 8% 7% 7% 7% 6% 8% 11% 25%	1800
	11th to 20th	Night	7% 6% 2 7% 6% 7% 6% 5% 10% 13% 32%	1800
	21st to end	Night	4 6% 3 7% 6% 6% 7% 6% 10% 13% 32%	1800
July	1st to 10th	Night	5% 6% 3 8% 5% 7% 6% 5% 8% 11% 36%	4058
	11th to 20th	Night	4 4 2 7% 6% 6% 6% 5% 9% 12% 38%	4057
	21st to end	Night	2 3 2 5% 4 4 5% 4 8% 13% 50%	4427
August	1st to 10th	Night	4 5% 2 7% 5% 7% 8% 6% 9% 13% 34%	2995
	11th to 20th	Night	2 3 2 6% 5% 6% 7% 5% 8% 14% 42%	3197
	21st to end	Night	4 4 2 7% 6% 7% 8% 7% 12% 16% 25%	3780
September	1st to 10th	Night	6% 6% 3 8% 7% 8% 8% 8% 10% 12% 26%	2124
	11th to 20th	Night	8% 6% 3 9% 7% 7% 7% 6% 8% 12% 27%	2090
	21st to end	Night	8% 7% 3 9% 8% 8% 8% 6% 9% 10% 22%	2090
October	1st to 10th	Night	8% 7% 4 10% 6% 6% 7% 7% 9% 10% 27%	2199
	11th to 20th	Night	10% 6% 1 7% 6% 6% 6% 5% 9% 11% 34%	2280
	21st to end	Night	11% 7% 3 7% 5% 5% 5% 5% 7% 8% 38%	2508
November	1st to 10th	Night	9% 6% 3 7% 5% 5% 5% 4 7% 10% 40%	2341
	11th to 20th	Night	9% 7% 2 8% 6% 4 6% 5% 8% 9% 36%	2400
	21st to end	Night	9% 7% 3 6% 5% 5% 5% 5% 6% 10% 40%	2400
December	1st to 10th	Night	8% 4 1 5% 3 4 4 4 5% 9% 53%	3948
	11th to 20th	Night	11% 8% 3 7% 6% 5% 6% 5% 7% 9% 33%	4822
	21st to end	Night	9% 8% 4 10% 7% 6% 6% 6% 8% 9% 28%	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	22% 28% 7% 23% 16% 3	4057
	11th to 20th	Day	41% 29% 11% 14% 4	4060
	21st to end	Day	32% 30% 5% 24% 7% 3	4466
February	1st to 10th	Day	11% 65% 8% 10% 6% 1	1995
	11th to 20th	Day	10% 61% 5% 12% 10% 3	2095
	21st to end	Day	14% 63% 6% 7% 7% 2	1815
March	1st to 10th	Day	28% 32% 11% 13% 14% 2	4935
	11th to 20th	Day	10% 30% 12% 23% 15% 10%	4980
	21st to end	Day	25% 33% 9% 21% 10% 3	5465
April	1st to 10th	Day	8% 63% 8% 11% 8% 1	2200
	11th to 20th	Day	10% 64% 7% 7% 8% 4	2294
	21st to end	Day	9% 64% 5% 12% 10% 1	2400
May	1st to 10th	Day	12% 32% 14% 27% 12% 2	5437
	11th to 20th	Day	21% 31% 17% 25% 5% 1	5425
	21st to end	Day	16% 34% 19% 25% 6%	6447
June	1st to 10th	Day	20% 68% 7% 5% 1	2600
	11th to 20th	Day	18% 68% 7% 6% 1	2600
	21st to end	Day	11% 69% 14% 5%	2600
July	1st to 10th	Day	28% 34% 27% 11%	5835
	11th to 20th	Day	27% 35% 27% 10%	5831
	21st to end	Day	17% 34% 28% 20% 1	6240
August	1st to 10th	Day	30% 45% 18% 7% 1	4106
	11th to 20th	Day	16% 45% 23% 15% 1	3886
	21st to end	Day	22% 46% 28% 4	4097
September	1st to 10th	Day	16% 67% 11% 6%	2123
	11th to 20th	Day	17% 65% 12% 5% 1	2090
	21st to end	Day	18% 66% 7% 6% 2	2090
October	1st to 10th	Day	30% 45% 13% 9% 3	3291
	11th to 20th	Day	29% 41% 12% 10% 7% 1	3140
	21st to end	Day	20% 39% 7% 22% 9% 2	3428
November	1st to 10th	Day	15% 60% 10% 9% 5% 1	1957
	11th to 20th	Day	20% 60% 6% 8% 5% 2	2000
	21st to end	Day	16% 58% 7% 9% 7% 3	2000
December	1st to 10th	Day	16% 39% 11% 21% 11% 3	3227
	11th to 20th	Day	24% 36% 10% 14% 12% 4	3370
	21st to end	Day	37% 32% 13% 11% 7%	4466
January	1st to 10th	Night	28% 29% 9% 20% 11% 3	5889
	11th to 20th	Night	43% 29% 14% 11% 4	5900
	21st to end	Night	33% 30% 4 21% 8% 3	6490
February	1st to 10th	Night	11% 63% 8% 12% 6%	2400
	11th to 20th	Night	9% 61% 4 11% 11% 4	2198
	21st to end	Night	12% 63% 5% 8% 8% 3	1815
March	1st to 10th	Night	26% 31% 10% 14% 16% 2	4918
	11th to 20th	Night	13% 30% 16% 19% 13% 8%	4980
	21st to end	Night	23% 32% 10% 19% 14% 2	5467
April	1st to 10th	Night	7% 63% 6% 15% 8% 1	2200
	11th to 20th	Night	9% 65% 6% 8% 10% 3	2052
	21st to end	Night	7% 64% 7% 13% 7% 1	2000
May	1st to 10th	Night	12% 31% 22% 26% 7% 2	4520
	11th to 20th	Night	19% 30% 15% 27% 8%	4491
	21st to end	Night	18% 34% 20% 23% 5%	4438
June	1st to 10th	Night	16% 66% 10% 7% 1	1800
	11th to 20th	Night	16% 68% 9% 7% 1	1800
	21st to end	Night	11% 66% 16% 6%	1800
July	1st to 10th	Night	28% 32% 29% 11%	4015
	11th to 20th	Night	24% 33% 34% 9%	4006
	21st to end	Night	16% 31% 33% 19% 1	4297
August	1st to 10th	Night	27% 44% 18% 10% 1	2984
	11th to 20th	Night	17% 42% 22% 18%	3227
	21st to end	Night	23% 47% 26% 4 1	3780
September	1st to 10th	Night	18% 66% 11% 5%	2124
	11th to 20th	Night	16% 66% 12% 5% 1	2090
	21st to end	Night	17% 67% 7% 7% 3	2090
October	1st to 10th	Night	33% 44% 12% 9% 1	3667
	11th to 20th	Night	32% 41% 9% 10% 7%	3779
	21st to end	Night	21% 40% 7% 20% 9% 3	4120
November	1st to 10th	Night	17% 58% 9% 10% 5% 1	2341
	11th to 20th	Night	19% 59% 6% 9% 5% 2	2400
	21st to end	Night	18% 58% 6% 7% 7% 3	2400
December	1st to 10th	Night	17% 38% 10% 21% 13% 1	3948
	11th to 20th	Night	29% 36% 8% 14% 9% 4	4824
	21st to end	Night	39% 32% 12% 10% 6% 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% 6% 33% 38% 16% 1	300
	11th to 20th	Day	14% 8% 28% 29% 17% 3	300
	21st to end	Day	10% 5% 36% 39% 8% 1	330
February	1st to 10th	Day	7% 6% 20% 42% 21% 3	300
	11th to 20th	Day	7% 5% 24% 37% 23% 5%	438
	21st to end	Day	7% 4 23% 37% 28% 2	414
March	1st to 10th	Day	3 7% 30% 42% 16% 2	500
	11th to 20th	Day	5% 5% 24% 38% 26% 2	500
	21st to end	Day	5% 3 25% 39% 25% 4	550
April	1st to 10th	Day	3 3 25% 43% 22% 4	500
	11th to 20th	Day	6% 3 25% 40% 21% 5%	492
	21st to end	Day	4 2 13% 45% 31% 5%	500
May	1st to 10th	Day	3 1 12% 41% 34% 9%	503
	11th to 20th	Day	9% 2 13% 33% 31% 11%	527
	21st to end	Day	9% 2 8% 19% 39% 22%	573
June	1st to 10th	Day	9% 4 8% 21% 38% 20%	530
	11th to 20th	Day	8% 2 7% 17% 33% 33%	530
	21st to end	Day	3 2 3 14% 34% 44%	530
July	1st to 10th	Day	6% 1 3 10% 33% 47%	525
	11th to 20th	Day	5% 1 3 13% 35% 42%	524
	21st to end	Day	4 1 2 18% 33% 43%	576
August	1st to 10th	Day	6% 1 5% 13% 36% 39%	526
	11th to 20th	Day	3 1 2 14% 36% 44%	519
	21st to end	Day	8% 1 6% 16% 37% 32%	583
September	1st to 10th	Day	8% 2 3 12% 37% 37%	502
	11th to 20th	Day	5% 1 4 16% 34% 40%	490
	21st to end	Day	8% 3 8% 19% 33% 28%	490
October	1st to 10th	Day	11% 2 8% 30% 38% 11%	490
	11th to 20th	Day	11% 4 12% 32% 30% 12%	490
	21st to end	Day	10% 4 17% 30% 31% 8%	539
November	1st to 10th	Day	8% 4 18% 35% 26% 8%	495
	11th to 20th	Day	11% 4 20% 35% 26% 4	500
	21st to end	Day	6% 4 22% 38% 28% 2	500
December	1st to 10th	Day	10% 5% 19% 38% 26% 3	480
	11th to 20th	Day	9% 3 24% 33% 26% 5%	300
	21st to end	Day	8% 8% 24% 31% 24% 5%	330
January	1st to 10th	Night	9% 6% 21% 37% 24% 2	800
	11th to 20th	Night	10% 7% 26% 36% 21% 2	800
	21st to end	Night	9% 6% 27% 42% 16%	880
February	1st to 10th	Night	9% 6% 19% 40% 24% 2	800
	11th to 20th	Night	7% 4 24% 40% 24% 1	783
	21st to end	Night	6% 3 22% 43% 26%	660
March	1st to 10th	Night	5% 4 30% 40% 19% 2	800
	11th to 20th	Night	5% 4 22% 42% 25% 2	800
	21st to end	Night	6% 4 22% 45% 23% 1	880
April	1st to 10th	Night	5% 3 15% 49% 27% 3	800
	11th to 20th	Night	5% 3 19% 46% 26% 2	791
	21st to end	Night	7% 1 16% 48% 27% 2	800
May	1st to 10th	Night	4 1 12% 43% 37% 3	800
	11th to 20th	Night	9% 2 12% 40% 32% 4	789
	21st to end	Night	7% 2 8% 24% 47% 12%	659
June	1st to 10th	Night	11% 2 4 24% 44% 16%	600
	11th to 20th	Night	7% 2 4 14% 45% 29%	600
	21st to end	Night	5% 1 3 13% 38% 41%	600
July	1st to 10th	Night	6% 1 3 8% 39% 44%	585
	11th to 20th	Night	5% 1 1 8% 41% 43%	582
	21st to end	Night	3 1 2 7% 42% 44%	657
August	1st to 10th	Night	8% 1 2 9% 45% 35%	638
	11th to 20th	Night	3 2 9% 46% 39%	790
	21st to end	Night	9% 1 4 16% 39% 32%	880
September	1st to 10th	Night	8% 2 5% 12% 37% 36%	773
	11th to 20th	Night	9% 2 4 14% 38% 32%	760
	21st to end	Night	11% 2 7% 17% 35% 28%	760
October	1st to 10th	Night	10% 3 8% 29% 40% 11%	760
	11th to 20th	Night	10% 3 11% 26% 41% 9%	760
	21st to end	Night	15% 2 14% 33% 29% 7%	836
November	1st to 10th	Night	10% 3 19% 33% 29% 5%	780
	11th to 20th	Night	9% 3 19% 33% 32% 4	800
	21st to end	Night	10% 3 18% 41% 26% 2	800
December	1st to 10th	Night	9% 4 17% 36% 30% 4	800
	11th to 20th	Night	10% 5% 20% 33% 28% 4	800
	21st to end	Night	9% 4 21% 34% 28% 3	880

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