Boca Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Boca Observatory clear sky chart from 2005-11-06 to 2026-04-17.

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	9% 5% 2 6% 4 4 5% 4 7% 10% 45%	131807
	Night	9% 5% 2 5% 4 4 4 4 6% 9% 46%	130349
Transparency	Day	32% 8% 18% 20% 15% 6%	135673
	Night	32% 7% 15% 20% 17% 8%	134554
Seeing	Day	9% 3 11% 27% 32% 18%	16831
	Night	10% 3 15% 28% 31% 13%	26881

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	10% 6% 2 6% 4 3 3 4 4 7% 49%	5258
	11th to 20th	Day	13% 6% 2 6% 4 4 4 4 6% 8% 43%	5579
	21st to end	Day	11% 7% 2 5% 4 3 4 4 6% 9% 44%	6436
February	1st to 10th	Day	12% 8% 3 7% 4 4 3 4 5% 7% 43%	2100
	11th to 20th	Day	15% 7% 2 5% 4 3 4 3 4 7% 48%	2048
	21st to end	Day	13% 6% 2 6% 4 4 4 4 5% 6% 46%	1730
March	1st to 10th	Day	18% 6% 2 5% 3 3 4 3 6% 7% 42%	4709
	11th to 20th	Day	7% 5% 2 5% 4 3 4 4 6% 8% 52%	5090
	21st to end	Day	14% 6% 1 4 2 3 3 3 5% 8% 50%	5586
April	1st to 10th	Day	10% 5% 1 6% 5% 4 5% 4 6% 8% 46%	2347
	11th to 20th	Day	8% 5% 2 5% 4 3 4 4 5% 7% 54%	2446
	21st to end	Day	5% 3 2 3 3 3 4 4 6% 9% 58%	2400
May	1st to 10th	Day	9% 5% 2 5% 4 4 4 4 6% 9% 46%	5854
	11th to 20th	Day	8% 5% 2 5% 4 4 4 3 5% 9% 52%	5861
	21st to end	Day	3 3 1 4 3 3 5% 4 7% 12% 53%	6433
June	1st to 10th	Day	8% 5% 2 7% 5% 5% 6% 7% 9% 12% 35%	2600
	11th to 20th	Day	5% 4 3 6% 6% 5% 6% 6% 10% 13% 37%	2600
	21st to end	Day	6% 5% 2 7% 6% 6% 6% 7% 10% 11% 33%	2600
July	1st to 10th	Day	5% 4 2 7% 6% 6% 7% 7% 12% 16% 27%	5900
	11th to 20th	Day	4 3 2 5% 4 5% 7% 6% 9% 13% 41%	5900
	21st to end	Day	3 4 2 6% 6% 7% 7% 6% 10% 15% 35%	6289
August	1st to 10th	Day	6% 4 2 7% 5% 6% 6% 6% 9% 14% 35%	3905
	11th to 20th	Day	7% 4 1 5% 4 5% 6% 5% 9% 13% 42%	3849
	21st to end	Day	5% 4 2 7% 5% 7% 8% 7% 12% 14% 30%	4289
September	1st to 10th	Day	8% 6% 2 8% 5% 5% 6% 6% 11% 12% 30%	2200
	11th to 20th	Day	8% 4 3 7% 6% 6% 6% 5% 8% 11% 37%	2200
	21st to end	Day	9% 5% 2 6% 4 4 5% 4 6% 10% 45%	2200
October	1st to 10th	Day	10% 5% 2 6% 4 4 5% 4 5% 7% 47%	2200
	11th to 20th	Day	5% 3 2 3 2 3 4 3 5% 7% 63%	2200
	21st to end	Day	7% 4 2 6% 3 4 4 3 4 6% 56%	2206
November	1st to 10th	Day	7% 3 1 5% 4 4 3 4 5% 7% 57%	1964
	11th to 20th	Day	9% 4 2 4 3 4 3 3 5% 7% 56%	1890
	21st to end	Day	9% 5% 2 6% 4 4 4 4 7% 8% 49%	1890
December	1st to 10th	Day	9% 5% 2 5% 4 4 4 2 4 6% 53%	3020
	11th to 20th	Day	10% 7% 2 6% 4 3 3 3 4 6% 51%	3463
	21st to end	Day	16% 9% 3 6% 3 3 4 3 5% 5% 44%	4565
January	1st to 10th	Night	9% 6% 2 5% 3 3 4 3 5% 9% 51%	7635
	11th to 20th	Night	16% 7% 2 7% 4 4 5% 4 6% 8% 37%	7365
	21st to end	Night	15% 8% 2 6% 4 4 4 4 6% 9% 39%	7745
February	1st to 10th	Night	14% 7% 2 5% 3 3 4 3 5% 6% 47%	2520
	11th to 20th	Night	13% 8% 3 6% 4 4 3 3 5% 6% 47%	2458
	21st to end	Night	13% 7% 3 5% 4 4 4 2 5% 6% 47%	2076
March	1st to 10th	Night	14% 9% 3 7% 4 4 3 3 4 7% 41%	5363
	11th to 20th	Night	8% 5% 2 4 4 3 4 3 6% 8% 53%	5090
	21st to end	Night	13% 5% 2 5% 4 3 3 3 5% 7% 49%	5588
April	1st to 10th	Night	9% 6% 2 5% 4 4 4 5% 7% 8% 47%	2289
	11th to 20th	Night	8% 4 2 6% 4 3 4 3 6% 8% 52%	2035
	21st to end	Night	4 4 2 4 4 4 4 3 6% 9% 57%	2000
May	1st to 10th	Night	6% 5% 2 4 3 4 4 4 6% 10% 53%	4038
	11th to 20th	Night	8% 5% 2 5% 4 4 4 4 6% 8% 50%	4032
	21st to end	Night	3 2 1 5% 3 5% 4 3 6% 11% 58%	4421
June	1st to 10th	Night	6% 6% 2 6% 4 4 6% 5% 7% 11% 43%	1800
	11th to 20th	Night	4 4 2 5% 4 5% 4 5% 8% 13% 45%	1800
	21st to end	Night	5% 4 1 5% 5% 6% 6% 5% 8% 12% 43%	1800
July	1st to 10th	Night	4 4 2 7% 5% 6% 7% 6% 11% 18% 30%	4058
	11th to 20th	Night	2 2 1 3 3 4 5% 6% 9% 15% 50%	4057
	21st to end	Night	2 3 2 5% 4 6% 6% 6% 11% 17% 39%	4556
August	1st to 10th	Night	6% 3 1 4 4 5% 5% 5% 10% 14% 43%	3244
	11th to 20th	Night	3 3 2 5% 4 4 6% 5% 9% 14% 46%	3200
	21st to end	Night	5% 3 2 7% 5% 6% 7% 7% 11% 14% 33%	3586
September	1st to 10th	Night	8% 6% 2 7% 5% 6% 6% 6% 9% 12% 34%	2188
	11th to 20th	Night	8% 7% 3 7% 5% 6% 6% 4 9% 10% 36%	2200
	21st to end	Night	7% 5% 2 5% 4 4 5% 5% 7% 11% 45%	2200
October	1st to 10th	Night	6% 5% 2 7% 5% 4 5% 4 6% 9% 47%	2200
	11th to 20th	Night	5% 3 2 4 3 3 4 3 4 8% 61%	2200
	21st to end	Night	8% 4 2 6% 3 4 4 4 5% 7% 52%	2631
November	1st to 10th	Night	6% 4 2 5% 4 4 5% 4 6% 7% 54%	2549
	11th to 20th	Night	9% 5% 2 5% 3 5% 3 3 5% 7% 53%	2730
	21st to end	Night	10% 6% 2 5% 4 3 3 3 6% 7% 51%	2730
December	1st to 10th	Night	9% 4 2 5% 4 3 4 4 5% 7% 53%	4380
	11th to 20th	Night	10% 5% 2 6% 3 3 3 2 5% 6% 55%	4952
	21st to end	Night	14% 7% 3 6% 4 4 4 3 5% 6% 45%	6633

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	35% 5% 11% 16% 25% 9%	5259
	11th to 20th	Day	40% 4 6% 18% 19% 14%	5579
	21st to end	Day	36% 5% 11% 16% 20% 11%	6436
February	1st to 10th	Day	41% 5% 10% 15% 19% 11%	2100
	11th to 20th	Day	36% 5% 8% 20% 21% 10%	2058
	21st to end	Day	36% 6% 11% 17% 20% 9%	1730
March	1st to 10th	Day	40% 6% 9% 19% 18% 8%	4709
	11th to 20th	Day	28% 6% 12% 18% 20% 16%	5090
	21st to end	Day	32% 5% 11% 28% 16% 8%	5586
April	1st to 10th	Day	33% 7% 13% 20% 17% 10%	2347
	11th to 20th	Day	29% 6% 9% 23% 23% 10%	2446
	21st to end	Day	23% 5% 15% 29% 20% 9%	2400
May	1st to 10th	Day	31% 9% 17% 26% 13% 3	5889
	11th to 20th	Day	28% 6% 17% 31% 16% 1	5865
	21st to end	Day	21% 7% 22% 33% 16% 2	6447
June	1st to 10th	Day	33% 12% 24% 24% 7% 1	2600
	11th to 20th	Day	30% 11% 27% 24% 7%	2600
	21st to end	Day	37% 11% 25% 20% 6% 1	2600
July	1st to 10th	Day	37% 15% 39% 9% 1	5835
	11th to 20th	Day	28% 13% 44% 14% 1	5831
	21st to end	Day	31% 17% 31% 20% 1	6108
August	1st to 10th	Day	33% 11% 25% 22% 7% 2	3893
	11th to 20th	Day	26% 12% 30% 25% 7%	3885
	21st to end	Day	36% 13% 29% 16% 6%	4289
September	1st to 10th	Day	38% 14% 24% 15% 7% 2	2200
	11th to 20th	Day	37% 12% 19% 19% 11% 2	2200
	21st to end	Day	33% 7% 18% 21% 14% 7%	2200
October	1st to 10th	Day	45% 5% 9% 13% 16% 12%	3590
	11th to 20th	Day	13% 3 6% 22% 32% 23%	3568
	21st to end	Day	19% 3 6% 20% 38% 13%	3544
November	1st to 10th	Day	27% 5% 12% 20% 21% 15%	1964
	11th to 20th	Day	28% 5% 9% 17% 29% 11%	1890
	21st to end	Day	31% 5% 11% 24% 21% 8%	1890
December	1st to 10th	Day	32% 5% 11% 20% 23% 9%	3020
	11th to 20th	Day	35% 6% 8% 19% 22% 10%	3460
	21st to end	Day	43% 4 8% 18% 23% 4	4565
January	1st to 10th	Night	31% 4 11% 18% 25% 11%	7636
	11th to 20th	Night	45% 4 6% 17% 18% 10%	7365
	21st to end	Night	42% 5% 9% 16% 19% 9%	7745
February	1st to 10th	Night	37% 5% 8% 16% 22% 12%	2520
	11th to 20th	Night	38% 4 9% 17% 19% 12%	2459
	21st to end	Night	36% 5% 10% 18% 19% 11%	2076
March	1st to 10th	Night	42% 5% 10% 17% 19% 8%	5363
	11th to 20th	Night	28% 5% 10% 20% 19% 18%	5090
	21st to end	Night	34% 5% 8% 24% 18% 11%	5588
April	1st to 10th	Night	33% 6% 13% 19% 17% 11%	2289
	11th to 20th	Night	29% 5% 8% 21% 24% 12%	2035
	21st to end	Night	24% 6% 11% 29% 23% 8%	2000
May	1st to 10th	Night	24% 7% 20% 30% 14% 4	4065
	11th to 20th	Night	30% 7% 14% 33% 16% 1	4033
	21st to end	Night	20% 6% 18% 38% 17% 2	4429
June	1st to 10th	Night	32% 10% 22% 27% 10% 1	1800
	11th to 20th	Night	27% 11% 28% 25% 8% 1	1800
	21st to end	Night	31% 11% 27% 21% 10%	1800
July	1st to 10th	Night	35% 15% 39% 9% 1	4015
	11th to 20th	Night	22% 13% 49% 14% 2	4006
	21st to end	Night	27% 14% 38% 20% 1	4424
August	1st to 10th	Night	27% 11% 30% 22% 10%	3232
	11th to 20th	Night	25% 11% 30% 29% 5%	3230
	21st to end	Night	35% 14% 28% 15% 7%	3586
September	1st to 10th	Night	39% 12% 23% 17% 7% 2	2188
	11th to 20th	Night	41% 10% 20% 17% 10% 1	2200
	21st to end	Night	32% 8% 17% 22% 16% 5%	2200
October	1st to 10th	Night	47% 6% 8% 13% 16% 10%	3590
	11th to 20th	Night	15% 3 5% 22% 32% 22%	3571
	21st to end	Night	21% 4 6% 17% 39% 13%	4243
November	1st to 10th	Night	30% 5% 8% 22% 21% 15%	2549
	11th to 20th	Night	31% 5% 7% 18% 25% 14%	2730
	21st to end	Night	33% 5% 11% 19% 21% 11%	2730
December	1st to 10th	Night	31% 4 11% 21% 22% 12%	4380
	11th to 20th	Night	32% 4 10% 22% 21% 11%	4954
	21st to end	Night	39% 5% 8% 21% 22% 5%	6633

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	9% 5% 28% 41% 16% 1	330
	11th to 20th	Day	12% 5% 22% 41% 18% 2	330
	21st to end	Day	17% 8% 24% 42% 9%	363
February	1st to 10th	Day	13% 5% 19% 39% 21% 3	330
	11th to 20th	Day	20% 4 17% 41% 19%	330
	21st to end	Day	11% 5% 18% 47% 17% 2	273
March	1st to 10th	Day	13% 4 21% 41% 19% 2	381
	11th to 20th	Day	10% 4 21% 39% 23% 2	540
	21st to end	Day	12% 4 17% 40% 24% 3	594
April	1st to 10th	Day	10% 5% 23% 33% 26% 3	540
	11th to 20th	Day	10% 3 16% 36% 30% 4	516
	21st to end	Day	6% 3 13% 35% 37% 6%	500
May	1st to 10th	Day	5% 3 11% 37% 36% 8%	503
	11th to 20th	Day	7% 2 8% 31% 37% 15%	527
	21st to end	Day	5% 2 5% 22% 43% 24%	573
June	1st to 10th	Day	10% 1 4 22% 41% 22%	530
	11th to 20th	Day	7% 2 4 16% 39% 31%	530
	21st to end	Day	7% 2 3 10% 44% 34%	530
July	1st to 10th	Day	9% 2 3 7% 30% 49%	525
	11th to 20th	Day	7% 1 2 5% 30% 55%	524
	21st to end	Day	4 1 3 11% 41% 41%	576
August	1st to 10th	Day	9% 2 2 9% 34% 45%	526
	11th to 20th	Day	9% 1 3 7% 33% 49%	519
	21st to end	Day	9% 3 4 11% 38% 35%	583
September	1st to 10th	Day	10% 2 6% 14% 37% 30%	530
	11th to 20th	Day	8% 2 6% 18% 42% 23%	530
	21st to end	Day	10% 1 5% 17% 44% 23%	530
October	1st to 10th	Day	13% 1 12% 30% 34% 10%	530
	11th to 20th	Day	5% 2 10% 30% 40% 15%	530
	21st to end	Day	8% 4 11% 35% 33% 8%	583
November	1st to 10th	Day	9% 1 13% 30% 39% 8%	442
	11th to 20th	Day	11% 2 12% 40% 30% 5%	330
	21st to end	Day	9% 3 17% 41% 26% 4	330
December	1st to 10th	Day	14% 3 24% 35% 21% 4	330
	11th to 20th	Day	12% 3 19% 43% 22% 2	330
	21st to end	Day	15% 7% 14% 36% 24% 4	363
January	1st to 10th	Night	14% 8% 28% 33% 16% 1	840
	11th to 20th	Night	12% 6% 26% 34% 18% 3	840
	21st to end	Night	20% 6% 27% 34% 12% 1	924
February	1st to 10th	Night	17% 6% 24% 32% 20% 1	840
	11th to 20th	Night	15% 7% 23% 36% 17% 2	823
	21st to end	Night	16% 4 23% 40% 17%	692
March	1st to 10th	Night	12% 4 22% 38% 23% 2	840
	11th to 20th	Night	10% 4 22% 35% 26% 3	840
	21st to end	Night	10% 4 20% 33% 30% 3	924
April	1st to 10th	Night	11% 4 19% 34% 28% 4	819
	11th to 20th	Night	9% 4 17% 36% 30% 3	611
	21st to end	Night	6% 3 16% 34% 35% 7%	600
May	1st to 10th	Night	5% 2 14% 35% 36% 9%	600
	11th to 20th	Night	7% 2 11% 30% 40% 10%	591
	21st to end	Night	4 1 5% 23% 52% 15%	650
June	1st to 10th	Night	9% 1 5% 20% 48% 17%	600
	11th to 20th	Night	6% 1 4 14% 46% 30%	600
	21st to end	Night	7% 1 2 11% 46% 34%	600
July	1st to 10th	Night	5% 3 3 7% 41% 42%	585
	11th to 20th	Night	6% 1 2 4 35% 53%	582
	21st to end	Night	3 3 7% 45% 43%	657
August	1st to 10th	Night	6% 1 1 6% 44% 42%	597
	11th to 20th	Night	6% 1 3 7% 40% 44%	594
	21st to end	Night	10% 1 5% 14% 38% 33%	660
September	1st to 10th	Night	9% 2 4 15% 41% 28%	788
	11th to 20th	Night	10% 1 6% 21% 43% 18%	800
	21st to end	Night	10% 1 7% 21% 44% 17%	800
October	1st to 10th	Night	10% 2 14% 31% 35% 9%	800
	11th to 20th	Night	6% 2 13% 30% 39% 10%	800
	21st to end	Night	9% 3 16% 33% 32% 7%	880
November	1st to 10th	Night	8% 3 16% 37% 31% 5%	820
	11th to 20th	Night	12% 6% 19% 35% 26% 3	840
	21st to end	Night	12% 4 23% 34% 21% 5%	840
December	1st to 10th	Night	14% 3 24% 31% 22% 5%	840
	11th to 20th	Night	15% 4 23% 34% 21% 3	840
	21st to end	Night	16% 5% 21% 36% 20% 3	924

Page generated at 2026-04-14 02:44:26. Version ac07

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