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 2024-12-24.

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% 5% 7% 10% 43%	101321
	Night	9% 6% 2 6% 4 4 5% 4 7% 9% 45%	98745
Transparency	Day	33% 8% 18% 20% 15% 6%	105231
	Night	33% 7% 16% 20% 17% 7%	102975
Seeing	Day	10% 3 11% 27% 31% 18%	15112
	Night	11% 3 15% 27% 31% 13%	25119

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	13% 6% 2 7% 4 4 4 4 4 7% 45%	2836
	11th to 20th	Day	11% 6% 2 6% 4 5% 5% 4 6% 8% 43%	2999
	21st to end	Day	16% 9% 2 6% 5% 4 4 4 5% 6% 38%	3476
February	1st to 10th	Day	13% 7% 3 6% 4 4 4 4 5% 7% 43%	1900
	11th to 20th	Day	14% 6% 2 5% 4 3 4 3 4 7% 49%	1848
	21st to end	Day	13% 7% 2 7% 4 4 4 4 5% 6% 45%	1570
March	1st to 10th	Day	18% 7% 3 6% 4 4 4 3 5% 7% 38%	3205
	11th to 20th	Day	7% 5% 2 6% 4 4 5% 5% 7% 9% 47%	3480
	21st to end	Day	14% 5% 1 5% 2 3 4 3 5% 8% 50%	3815
April	1st to 10th	Day	10% 5% 2 5% 5% 4 5% 4 7% 8% 47%	2123
	11th to 20th	Day	9% 5% 2 6% 4 3 4 4 5% 7% 51%	2254
	21st to end	Day	5% 4 1 3 3 3 4 3 5% 9% 59%	2280
May	1st to 10th	Day	6% 3 2 4 3 3 4 4 7% 10% 54%	4074
	11th to 20th	Day	8% 6% 2 7% 5% 5% 5% 4 6% 8% 45%	4081
	21st to end	Day	3 3 1 4 3 3 5% 4 7% 12% 56%	4476
June	1st to 10th	Day	8% 5% 2 7% 5% 5% 6% 7% 9% 12% 34%	2470
	11th to 20th	Day	5% 4 3 6% 6% 5% 6% 6% 10% 13% 37%	2470
	21st to end	Day	6% 5% 2 7% 6% 6% 6% 7% 10% 11% 33%	2470
July	1st to 10th	Day	5% 4 2 6% 5% 5% 7% 7% 12% 17% 29%	4120
	11th to 20th	Day	5% 4 2 6% 5% 6% 6% 6% 10% 12% 38%	4120
	21st to end	Day	3 5% 2 6% 6% 7% 8% 7% 11% 16% 29%	4373
August	1st to 10th	Day	6% 4 2 6% 5% 6% 6% 6% 9% 14% 35%	3785
	11th to 20th	Day	7% 4 1 5% 4 5% 6% 5% 9% 13% 41%	3729
	21st to end	Day	5% 4 2 7% 5% 7% 8% 7% 12% 14% 30%	4158
September	1st to 10th	Day	9% 7% 2 8% 5% 5% 6% 7% 11% 12% 29%	2090
	11th to 20th	Day	8% 5% 3 7% 6% 6% 6% 5% 8% 11% 34%	2090
	21st to end	Day	9% 5% 2 6% 4 4 5% 4 6% 9% 45%	2090
October	1st to 10th	Day	10% 5% 2 6% 4 4 5% 4 5% 7% 48%	2090
	11th to 20th	Day	5% 3 2 3 2 3 4 3 5% 7% 63%	2090
	21st to end	Day	7% 4 2 6% 3 4 4 3 4 6% 56%	2096
November	1st to 10th	Day	8% 3 1 5% 4 4 4 3 5% 7% 56%	1869
	11th to 20th	Day	9% 5% 2 4 4 4 3 3 5% 7% 54%	1800
	21st to end	Day	9% 5% 2 5% 4 4 4 4 7% 8% 49%	1800
December	1st to 10th	Day	12% 5% 2 5% 4 4 5% 3 6% 7% 46%	1800
	11th to 20th	Day	13% 7% 2 7% 5% 3 4 4 5% 7% 43%	2243
	21st to end	Day	17% 9% 3 6% 4 3 4 3 4 5% 42%	3151
January	1st to 10th	Night	15% 8% 2 6% 4 3 5% 4 6% 7% 41%	4109
	11th to 20th	Night	11% 7% 2 7% 5% 4 4 4 6% 8% 42%	3959
	21st to end	Night	17% 10% 3 7% 4 3 4 3 5% 7% 38%	4180
February	1st to 10th	Night	15% 7% 2 5% 3 3 4 3 5% 6% 46%	2280
	11th to 20th	Night	14% 7% 3 6% 4 4 3 2 4 5% 48%	2218
	21st to end	Night	13% 8% 3 5% 4 4 4 2 5% 6% 46%	1884
March	1st to 10th	Night	14% 11% 3 9% 5% 4 4 3 4 6% 35%	3647
	11th to 20th	Night	8% 6% 2 5% 4 4 4 4 8% 8% 45%	3480
	21st to end	Night	15% 5% 2 5% 3 3 3 3 5% 6% 48%	3817
April	1st to 10th	Night	9% 6% 2 5% 4 4 4 5% 7% 8% 47%	2071
	11th to 20th	Night	9% 5% 2 7% 4 3 4 3 5% 8% 49%	1875
	21st to end	Night	4 4 2 4 4 4 4 3 6% 8% 57%	1900
May	1st to 10th	Night	4 4 2 3 3 4 4 4 6% 9% 58%	2818
	11th to 20th	Night	9% 5% 2 6% 5% 4 5% 4 6% 7% 47%	2812
	21st to end	Night	3 2 1 4 3 4 4 3 7% 11% 58%	3079
June	1st to 10th	Night	6% 6% 2 6% 4 5% 6% 5% 7% 11% 42%	1710
	11th to 20th	Night	5% 4 2 5% 4 5% 4 5% 8% 13% 45%	1710
	21st to end	Night	5% 4 1 5% 5% 6% 6% 5% 8% 11% 44%	1710
July	1st to 10th	Night	4 4 2 6% 4 5% 7% 6% 11% 18% 33%	2840
	11th to 20th	Night	3 3 1 3 3 4 4 5% 9% 14% 50%	2840
	21st to end	Night	2 3 2 5% 4 6% 7% 6% 12% 17% 37%	3174
August	1st to 10th	Night	6% 3 1 4 4 4 5% 5% 10% 14% 43%	3144
	11th to 20th	Night	3 3 2 5% 4 4 6% 5% 9% 14% 46%	3100
	21st to end	Night	5% 3 2 6% 5% 6% 8% 7% 11% 14% 33%	3476
September	1st to 10th	Night	8% 6% 2 7% 5% 6% 6% 6% 9% 13% 33%	2078
	11th to 20th	Night	9% 7% 3 7% 6% 6% 6% 4 9% 10% 33%	2090
	21st to end	Night	7% 5% 2 6% 4 4 4 5% 7% 11% 45%	2090
October	1st to 10th	Night	6% 5% 2 7% 5% 4 5% 4 6% 9% 47%	2090
	11th to 20th	Night	5% 3 2 4 3 3 4 3 4 8% 61%	2090
	21st to end	Night	8% 4 2 6% 3 4 4 4 5% 7% 52%	2499
November	1st to 10th	Night	6% 5% 2 5% 4 4 4 4 6% 7% 54%	2425
	11th to 20th	Night	9% 5% 2 5% 3 5% 3 3 5% 7% 52%	2600
	21st to end	Night	10% 6% 2 5% 4 3 3 4 6% 7% 51%	2600
December	1st to 10th	Night	11% 5% 2 5% 5% 4 4 4 6% 8% 47%	2600
	11th to 20th	Night	13% 7% 2 5% 4 3 4 3 5% 7% 48%	3172
	21st to end	Night	16% 8% 3 7% 4 4 4 3 4 5% 41%	4578

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	39% 5% 9% 16% 20% 10%	2837
	11th to 20th	Day	38% 5% 8% 16% 17% 16%	2999
	21st to end	Day	44% 6% 8% 14% 20% 8%	3476
February	1st to 10th	Day	41% 5% 10% 15% 19% 10%	1900
	11th to 20th	Day	35% 5% 7% 20% 22% 11%	1858
	21st to end	Day	37% 7% 11% 16% 21% 8%	1570
March	1st to 10th	Day	43% 7% 9% 20% 16% 5%	3205
	11th to 20th	Day	31% 8% 16% 19% 20% 6%	3480
	21st to end	Day	31% 6% 13% 27% 14% 9%	3815
April	1st to 10th	Day	33% 7% 13% 20% 17% 10%	2123
	11th to 20th	Day	31% 6% 10% 23% 21% 9%	2254
	21st to end	Day	22% 5% 14% 29% 20% 9%	2280
May	1st to 10th	Day	22% 10% 17% 28% 18% 5%	4112
	11th to 20th	Day	34% 8% 17% 23% 16% 2	4085
	21st to end	Day	18% 7% 20% 36% 17% 1	4489
June	1st to 10th	Day	34% 12% 23% 24% 6% 1	2470
	11th to 20th	Day	30% 11% 26% 25% 8%	2470
	21st to end	Day	36% 11% 26% 20% 6% 1	2470
July	1st to 10th	Day	33% 13% 42% 10% 1	4055
	11th to 20th	Day	31% 14% 43% 11% 1	4051
	21st to end	Day	33% 20% 35% 11% 1	4234
August	1st to 10th	Day	32% 11% 25% 22% 8% 2	3773
	11th to 20th	Day	26% 12% 29% 25% 7%	3765
	21st to end	Day	36% 13% 30% 15% 6%	4158
September	1st to 10th	Day	39% 14% 24% 14% 7% 2	2090
	11th to 20th	Day	39% 12% 20% 17% 9% 2	2090
	21st to end	Day	34% 7% 18% 20% 14% 7%	2090
October	1st to 10th	Day	46% 5% 8% 13% 17% 12%	3480
	11th to 20th	Day	13% 3 6% 22% 32% 23%	3458
	21st to end	Day	19% 4 6% 20% 38% 13%	3434
November	1st to 10th	Day	28% 5% 12% 20% 21% 14%	1869
	11th to 20th	Day	29% 5% 9% 17% 29% 11%	1800
	21st to end	Day	31% 5% 11% 24% 22% 8%	1800
December	1st to 10th	Day	36% 6% 12% 20% 20% 6%	1800
	11th to 20th	Day	40% 6% 9% 16% 19% 11%	2240
	21st to end	Day	44% 5% 7% 21% 18% 5%	3151
January	1st to 10th	Night	42% 4 7% 13% 19% 15%	4109
	11th to 20th	Night	40% 4 7% 16% 22% 10%	3959
	21st to end	Night	45% 6% 7% 13% 20% 9%	4180
February	1st to 10th	Night	38% 5% 8% 16% 22% 11%	2280
	11th to 20th	Night	39% 4 7% 16% 20% 13%	2219
	21st to end	Night	37% 5% 10% 18% 18% 11%	1884
March	1st to 10th	Night	47% 6% 10% 18% 14% 5%	3647
	11th to 20th	Night	33% 7% 12% 22% 18% 8%	3480
	21st to end	Night	34% 5% 9% 22% 19% 10%	3817
April	1st to 10th	Night	32% 6% 13% 18% 18% 12%	2071
	11th to 20th	Night	31% 5% 9% 22% 23% 10%	1875
	21st to end	Night	24% 6% 10% 29% 24% 9%	1900
May	1st to 10th	Night	18% 8% 20% 29% 18% 6%	2845
	11th to 20th	Night	33% 9% 16% 26% 15% 1	2813
	21st to end	Night	18% 7% 16% 39% 18% 2	3087
June	1st to 10th	Night	32% 10% 21% 28% 9% 1	1710
	11th to 20th	Night	28% 11% 26% 26% 8% 1	1710
	21st to end	Night	31% 11% 27% 20% 10%	1710
July	1st to 10th	Night	33% 14% 40% 12% 1	2795
	11th to 20th	Night	22% 15% 49% 11% 2	2786
	21st to end	Night	28% 17% 40% 14% 1	3073
August	1st to 10th	Night	26% 11% 30% 22% 10%	3132
	11th to 20th	Night	25% 11% 29% 30% 5%	3130
	21st to end	Night	35% 14% 28% 15% 7%	3476
September	1st to 10th	Night	40% 12% 23% 16% 7% 2	2078
	11th to 20th	Night	43% 10% 21% 16% 9% 1	2090
	21st to end	Night	33% 8% 17% 21% 16% 6%	2090
October	1st to 10th	Night	47% 6% 8% 12% 17% 10%	3480
	11th to 20th	Night	16% 3 5% 22% 31% 22%	3461
	21st to end	Night	20% 4 6% 18% 39% 13%	4111
November	1st to 10th	Night	30% 5% 8% 22% 20% 14%	2425
	11th to 20th	Night	32% 5% 7% 18% 24% 14%	2600
	21st to end	Night	34% 4 10% 19% 22% 11%	2600
December	1st to 10th	Night	34% 5% 12% 22% 20% 7%	2600
	11th to 20th	Night	37% 4 8% 19% 19% 12%	3174
	21st to end	Night	44% 5% 7% 22% 17% 4	4578

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	10% 4 30% 41% 14% 1	270
	11th to 20th	Day	10% 5% 22% 41% 20% 3	270
	21st to end	Day	19% 8% 24% 41% 7%	297
February	1st to 10th	Day	14% 7% 20% 39% 19% 2	270
	11th to 20th	Day	19% 4 16% 41% 20%	270
	21st to end	Day	12% 3 18% 49% 16% 2	225
March	1st to 10th	Day	12% 4 21% 45% 17% 1	316
	11th to 20th	Day	11% 4 21% 40% 23% 2	460
	21st to end	Day	12% 4 19% 42% 21% 2	506
April	1st to 10th	Day	10% 5% 25% 34% 24% 2	460
	11th to 20th	Day	12% 4 17% 37% 27% 3	452
	21st to end	Day	7% 3 14% 37% 35% 4	460
May	1st to 10th	Day	4 3 12% 35% 38% 9%	463
	11th to 20th	Day	7% 2 9% 30% 37% 16%	487
	21st to end	Day	5% 2 5% 21% 44% 24%	529
June	1st to 10th	Day	11% 1 4 21% 40% 23%	490
	11th to 20th	Day	8% 2 5% 17% 39% 29%	490
	21st to end	Day	7% 2 3 10% 43% 34%	490
July	1st to 10th	Day	9% 2 3 7% 29% 50%	485
	11th to 20th	Day	7% 1 2 5% 30% 55%	484
	21st to end	Day	4 1 3 11% 40% 41%	532
August	1st to 10th	Day	9% 2 2 9% 35% 43%	486
	11th to 20th	Day	9% 1 3 7% 33% 47%	479
	21st to end	Day	10% 3 4 11% 37% 37%	539
September	1st to 10th	Day	11% 2 6% 12% 36% 32%	490
	11th to 20th	Day	9% 2 7% 19% 40% 23%	490
	21st to end	Day	10% 1 6% 18% 42% 23%	490
October	1st to 10th	Day	14% 1 12% 30% 33% 9%	490
	11th to 20th	Day	4 2 11% 31% 39% 13%	490
	21st to end	Day	8% 4 11% 35% 33% 9%	539
November	1st to 10th	Day	10% 1 13% 31% 37% 8%	407
	11th to 20th	Day	12% 2 14% 41% 28% 3	300
	21st to end	Day	10% 3 17% 41% 24% 4	300
December	1st to 10th	Day	11% 3 24% 36% 21% 5%	300
	11th to 20th	Day	12% 3 21% 43% 20% 2	300
	21st to end	Day	17% 7% 12% 37% 23% 4	306
January	1st to 10th	Night	16% 9% 28% 32% 14% 1	760
	11th to 20th	Night	11% 7% 26% 34% 20% 3	760
	21st to end	Night	20% 7% 28% 33% 11% 1	836
February	1st to 10th	Night	18% 6% 24% 32% 19% 1	760
	11th to 20th	Night	16% 7% 23% 35% 17% 2	743
	21st to end	Night	17% 4 22% 39% 18%	628
March	1st to 10th	Night	12% 3 23% 39% 22% 1	760
	11th to 20th	Night	11% 3 22% 35% 27% 3	760
	21st to end	Night	10% 4 22% 33% 29% 3	836
April	1st to 10th	Night	11% 4 19% 36% 27% 3	741
	11th to 20th	Night	10% 5% 17% 36% 29% 3	563
	21st to end	Night	6% 3 16% 35% 34% 6%	570
May	1st to 10th	Night	5% 2 14% 34% 36% 9%	570
	11th to 20th	Night	7% 2 11% 29% 40% 10%	561
	21st to end	Night	5% 1 4 23% 52% 15%	617
June	1st to 10th	Night	9% 1 5% 19% 48% 18%	570
	11th to 20th	Night	6% 1 4 14% 46% 29%	570
	21st to end	Night	6% 1 2 11% 46% 33%	570
July	1st to 10th	Night	5% 3 3 7% 41% 41%	555
	11th to 20th	Night	6% 1 2 4 35% 53%	552
	21st to end	Night	3 3 7% 44% 42%	624
August	1st to 10th	Night	6% 1 1 6% 44% 41%	567
	11th to 20th	Night	6% 1 2 7% 41% 43%	564
	21st to end	Night	10% 1 5% 14% 37% 34%	627
September	1st to 10th	Night	9% 2 4 14% 41% 29%	748
	11th to 20th	Night	10% 1 7% 20% 43% 19%	760
	21st to end	Night	10% 1 7% 21% 43% 17%	760
October	1st to 10th	Night	10% 2 14% 31% 34% 10%	760
	11th to 20th	Night	7% 2 13% 30% 38% 10%	760
	21st to end	Night	9% 3 16% 32% 33% 7%	836
November	1st to 10th	Night	8% 3 16% 37% 30% 5%	780
	11th to 20th	Night	12% 6% 20% 35% 25% 3	800
	21st to end	Night	13% 5% 23% 35% 21% 5%	800
December	1st to 10th	Night	14% 3 24% 31% 23% 5%	800
	11th to 20th	Night	16% 5% 23% 34% 20% 3	800
	21st to end	Night	17% 5% 21% 35% 19% 2	851

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