Guana River Dam
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Guana River Dam clear sky chart from 2005-11-06 to 2024-03-18.

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	10% 5% 3 6% 6% 4 5% 6% 7% 9% 40%	79057
	Night	10% 6% 3 6% 5% 3 5% 5% 6% 9% 42%	80736
Transparency	Day	26% 39% 10% 12% 10% 4	78780
	Night	26% 38% 10% 12% 10% 4	80532
Seeing	Day	9% 6% 20% 28% 25% 12%	14599
	Night	11% 6% 20% 31% 27% 6%	24772

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	12% 8% 3 6% 5% 3 4 4 4 5% 46%	2836
	11th to 20th	Day	16% 9% 4 7% 6% 3 3 4 5% 7% 37%	2840
	21st to end	Day	12% 7% 3 5% 5% 3 5% 5% 5% 7% 45%	3124
February	1st to 10th	Day	16% 6% 3 5% 5% 3 4 4 5% 7% 42%	1876
	11th to 20th	Day	11% 5% 2 5% 5% 3 4 5% 4 7% 50%	1848
	21st to end	Day	11% 5% 2 6% 4 2 5% 5% 6% 7% 48%	1671
March	1st to 10th	Day	8% 5% 2 5% 3 2 3 4 5% 9% 53%	2090
	11th to 20th	Day	9% 3 2 4 4 3 5% 5% 6% 9% 50%	2058
	21st to end	Day	8% 6% 2 5% 6% 3 4 3 5% 8% 50%	2178
April	1st to 10th	Day	8% 4 2 5% 5% 3 5% 5% 5% 8% 51%	1980
	11th to 20th	Day	9% 6% 4 6% 5% 3 5% 5% 6% 8% 43%	2034
	21st to end	Day	5% 3 2 3 4 3 4 4 6% 9% 57%	2160
May	1st to 10th	Day	7% 3 2 3 3 2 4 4 6% 9% 58%	2326
	11th to 20th	Day	10% 4 2 4 6% 3 6% 6% 7% 11% 42%	2301
	21st to end	Day	8% 3 2 5% 5% 5% 6% 6% 9% 11% 40%	2531
June	1st to 10th	Day	11% 6% 3 6% 7% 4 7% 7% 9% 12% 29%	2340
	11th to 20th	Day	5% 3 2 6% 8% 5% 7% 8% 9% 11% 36%	2340
	21st to end	Day	7% 4 3 7% 7% 5% 7% 7% 10% 13% 30%	2340
July	1st to 10th	Day	7% 5% 3 7% 7% 4 8% 8% 8% 12% 31%	2340
	11th to 20th	Day	7% 6% 3 7% 8% 4 7% 9% 9% 10% 31%	2340
	21st to end	Day	6% 5% 3 7% 7% 5% 9% 7% 8% 12% 33%	2547
August	1st to 10th	Day	6% 5% 3 7% 7% 4 8% 9% 9% 12% 31%	2327
	11th to 20th	Day	6% 5% 3 7% 7% 5% 7% 9% 10% 12% 28%	2266
	21st to end	Day	11% 6% 3 5% 8% 5% 7% 7% 8% 11% 27%	2220
September	1st to 10th	Day	9% 7% 3 8% 6% 4 6% 8% 10% 12% 26%	1980
	11th to 20th	Day	11% 6% 3 7% 8% 5% 7% 7% 8% 9% 27%	1980
	21st to end	Day	14% 6% 3 6% 7% 6% 5% 5% 7% 10% 33%	1980
October	1st to 10th	Day	12% 7% 3 7% 7% 4 5% 6% 6% 9% 35%	1926
	11th to 20th	Day	8% 5% 3 5% 6% 4 5% 5% 5% 8% 47%	1800
	21st to end	Day	9% 5% 3 5% 5% 3 4 4 6% 7% 49%	1980
November	1st to 10th	Day	11% 6% 2 5% 5% 3 5% 5% 6% 8% 44%	1857
	11th to 20th	Day	12% 7% 3 6% 6% 4 6% 5% 5% 6% 40%	1900
	21st to end	Day	12% 5% 3 6% 5% 4 5% 5% 5% 8% 42%	1754
December	1st to 10th	Day	9% 5% 2 5% 5% 3 4 5% 5% 9% 46%	1710
	11th to 20th	Day	14% 7% 3 5% 6% 4 4 5% 5% 7% 40%	2153
	21st to end	Day	15% 7% 3 7% 5% 4 4 4 4 6% 41%	3124
January	1st to 10th	Night	14% 8% 3 5% 4 3 4 4 6% 7% 42%	4109
	11th to 20th	Night	13% 8% 4 8% 5% 3 5% 5% 5% 7% 37%	4120
	21st to end	Night	13% 7% 3 6% 5% 3 5% 5% 6% 6% 40%	4532
February	1st to 10th	Night	14% 5% 3 5% 6% 2 4 4 5% 6% 45%	2299
	11th to 20th	Night	12% 6% 2 5% 5% 3 5% 4 5% 6% 48%	2218
	21st to end	Night	10% 5% 2 5% 5% 3 4 4 5% 8% 48%	1780
March	1st to 10th	Night	9% 4 2 4 4 3 3 4 4 7% 55%	2090
	11th to 20th	Night	10% 4 2 4 4 3 4 4 6% 9% 50%	2068
	21st to end	Night	9% 4 2 4 4 3 4 4 5% 8% 53%	2178
April	1st to 10th	Night	8% 5% 2 4 4 3 4 5% 6% 9% 51%	1980
	11th to 20th	Night	9% 5% 3 5% 5% 3 4 5% 6% 8% 49%	1875
	21st to end	Night	7% 3 1 3 4 3 5% 5% 7% 10% 53%	1800
May	1st to 10th	Night	5% 3 1 3 3 2 4 5% 6% 8% 60%	1648
	11th to 20th	Night	7% 4 2 5% 4 3 5% 4 7% 10% 50%	1592
	21st to end	Night	5% 4 2 4 6% 4 5% 6% 8% 12% 43%	1744
June	1st to 10th	Night	10% 5% 3 6% 6% 4 6% 7% 8% 13% 33%	1620
	11th to 20th	Night	6% 4 3 5% 7% 4 6% 6% 9% 12% 38%	1620
	21st to end	Night	6% 4 2 6% 6% 4 6% 6% 8% 14% 36%	1620
July	1st to 10th	Night	7% 5% 3 6% 6% 5% 6% 7% 9% 11% 35%	1620
	11th to 20th	Night	5% 5% 2 7% 7% 4 8% 7% 8% 12% 35%	1620
	21st to end	Night	5% 4 3 5% 5% 4 7% 7% 7% 12% 40%	1771
August	1st to 10th	Night	6% 5% 3 6% 6% 3 6% 8% 10% 13% 35%	1611
	11th to 20th	Night	4 5% 2 7% 7% 4 6% 7% 10% 12% 37%	1616
	21st to end	Night	10% 6% 3 8% 8% 5% 6% 7% 8% 11% 29%	2130
September	1st to 10th	Night	9% 6% 3 8% 8% 5% 7% 7% 9% 11% 28%	1980
	11th to 20th	Night	12% 7% 3 7% 7% 6% 7% 8% 8% 9% 26%	1980
	21st to end	Night	13% 6% 3 6% 6% 4 5% 6% 8% 10% 33%	1980
October	1st to 10th	Night	13% 6% 3 5% 6% 4 6% 5% 7% 9% 36%	2032
	11th to 20th	Night	7% 4 2 5% 6% 3 4 5% 6% 9% 49%	2160
	21st to end	Night	9% 5% 3 5% 5% 3 4 5% 6% 8% 48%	2376
November	1st to 10th	Night	12% 4 2 4 6% 3 5% 4 6% 8% 45%	2221
	11th to 20th	Night	11% 6% 3 6% 6% 4 4 5% 5% 8% 44%	2280
	21st to end	Night	11% 6% 2 6% 5% 3 5% 5% 6% 8% 43%	2422
December	1st to 10th	Night	11% 5% 2 5% 5% 3 3 4 6% 8% 48%	2470
	11th to 20th	Night	14% 8% 2 6% 5% 3 4 4 5% 6% 43%	3042
	21st to end	Night	15% 8% 3 6% 6% 3 5% 4 5% 6% 37%	4532

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% 26% 5% 14% 18% 8%	2837
	11th to 20th	Day	40% 26% 5% 9% 13% 8%	2840
	21st to end	Day	29% 27% 9% 14% 12% 10%	3124
February	1st to 10th	Day	29% 36% 6% 11% 13% 6%	1876
	11th to 20th	Day	25% 38% 7% 10% 12% 9%	1858
	21st to end	Day	20% 36% 9% 13% 15% 7%	1671
March	1st to 10th	Day	20% 36% 8% 15% 15% 6%	2090
	11th to 20th	Day	19% 38% 11% 17% 11% 5%	2058
	21st to end	Day	21% 38% 9% 15% 13% 3	2178
April	1st to 10th	Day	19% 38% 10% 13% 13% 6%	1980
	11th to 20th	Day	24% 38% 9% 13% 13% 3	2034
	21st to end	Day	15% 38% 10% 16% 16% 5%	2160
May	1st to 10th	Day	15% 39% 8% 20% 15% 2	2326
	11th to 20th	Day	21% 42% 14% 13% 9% 1	2305
	21st to end	Day	24% 41% 12% 13% 9%	2531
June	1st to 10th	Day	27% 43% 11% 13% 5%	2340
	11th to 20th	Day	23% 43% 17% 15% 2	2340
	21st to end	Day	29% 43% 14% 10% 4	2340
July	1st to 10th	Day	29% 45% 13% 11% 2	2275
	11th to 20th	Day	28% 47% 15% 8% 3	2271
	21st to end	Day	24% 45% 17% 12% 1	2405
August	1st to 10th	Day	23% 45% 20% 10% 2	2314
	11th to 20th	Day	29% 43% 17% 11% 1	2266
	21st to end	Day	35% 46% 10% 8% 1	2220
September	1st to 10th	Day	27% 46% 14% 10% 2	1980
	11th to 20th	Day	32% 44% 12% 7% 4	1980
	21st to end	Day	32% 40% 9% 9% 8% 2	1980
October	1st to 10th	Day	26% 39% 7% 12% 9% 7%	1926
	11th to 20th	Day	24% 37% 7% 12% 16% 4	1800
	21st to end	Day	22% 40% 6% 12% 15% 6%	1980
November	1st to 10th	Day	20% 43% 7% 12% 10% 8%	1857
	11th to 20th	Day	26% 41% 5% 9% 14% 4	1900
	21st to end	Day	26% 42% 7% 10% 12% 3	1754
December	1st to 10th	Day	22% 42% 9% 14% 9% 3	1710
	11th to 20th	Day	32% 36% 7% 9% 14% 3	2150
	21st to end	Day	32% 27% 6% 13% 15% 7%	3124
January	1st to 10th	Night	29% 27% 6% 12% 17% 9%	4109
	11th to 20th	Night	39% 27% 4 14% 12% 5%	4120
	21st to end	Night	32% 29% 9% 10% 9% 10%	4532
February	1st to 10th	Night	27% 37% 7% 8% 13% 9%	2299
	11th to 20th	Night	28% 38% 6% 9% 10% 9%	2219
	21st to end	Night	20% 36% 9% 15% 13% 7%	1780
March	1st to 10th	Night	20% 36% 9% 14% 14% 7%	2090
	11th to 20th	Night	19% 38% 10% 17% 10% 6%	2068
	21st to end	Night	19% 38% 10% 16% 11% 5%	2178
April	1st to 10th	Night	19% 40% 11% 12% 11% 7%	1980
	11th to 20th	Night	21% 37% 9% 16% 13% 4	1875
	21st to end	Night	16% 40% 10% 17% 13% 4	1800
May	1st to 10th	Night	15% 39% 10% 19% 16% 2	1648
	11th to 20th	Night	18% 40% 18% 15% 8% 1	1593
	21st to end	Night	21% 42% 12% 15% 9% 1	1745
June	1st to 10th	Night	25% 42% 15% 14% 5%	1620
	11th to 20th	Night	25% 43% 14% 17% 2	1620
	21st to end	Night	25% 43% 16% 10% 4 1	1620
July	1st to 10th	Night	31% 45% 12% 10% 1	1575
	11th to 20th	Night	27% 46% 15% 9% 3	1566
	21st to end	Night	21% 47% 17% 14% 1	1672
August	1st to 10th	Night	21% 47% 21% 9% 2	1600
	11th to 20th	Night	24% 43% 20% 12% 1	1616
	21st to end	Night	35% 42% 11% 10% 2	2130
September	1st to 10th	Night	30% 44% 13% 11% 2	1980
	11th to 20th	Night	34% 47% 10% 7% 3	1980
	21st to end	Night	30% 40% 9% 10% 10% 2	1980
October	1st to 10th	Night	25% 40% 8% 11% 10% 7%	2032
	11th to 20th	Night	21% 38% 7% 15% 14% 4	2160
	21st to end	Night	23% 40% 7% 10% 14% 6%	2376
November	1st to 10th	Night	21% 42% 8% 11% 11% 8%	2221
	11th to 20th	Night	24% 41% 6% 11% 13% 5%	2280
	21st to end	Night	24% 42% 8% 11% 12% 4	2422
December	1st to 10th	Night	23% 41% 9% 15% 8% 4	2470
	11th to 20th	Night	30% 36% 7% 9% 14% 4	3044
	21st to end	Night	34% 29% 7% 11% 15% 4	4532

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	14% 9% 44% 27% 5% 1	270
	11th to 20th	Day	7% 11% 40% 29% 11% 2	270
	21st to end	Day	16% 19% 39% 23% 3	297
February	1st to 10th	Day	18% 11% 31% 29% 11%	270
	11th to 20th	Day	14% 9% 40% 28% 8% 1	270
	21st to end	Day	9% 9% 39% 31% 11% 1	326
March	1st to 10th	Day	6% 14% 37% 32% 11% 1	460
	11th to 20th	Day	10% 10% 33% 30% 16% 2	449
	21st to end	Day	10% 9% 32% 32% 16% 2	462
April	1st to 10th	Day	10% 9% 34% 29% 15% 2	420
	11th to 20th	Day	10% 11% 33% 29% 13% 5%	412
	21st to end	Day	5% 4 29% 35% 20% 7%	420
May	1st to 10th	Day	5% 5% 24% 38% 20% 8%	423
	11th to 20th	Day	10% 2 16% 35% 24% 13%	447
	21st to end	Day	7% 2 8% 31% 36% 15%	485
June	1st to 10th	Day	13% 2 11% 32% 24% 17%	450
	11th to 20th	Day	5% 2 9% 25% 37% 22%	450
	21st to end	Day	4 2 5% 25% 40% 24%	450
July	1st to 10th	Day	7% 1 4 16% 38% 33%	445
	11th to 20th	Day	6% 1 3 15% 45% 31%	444
	21st to end	Day	7% 1 4 20% 43% 25%	488
August	1st to 10th	Day	6% 2 8% 19% 42% 24%	446
	11th to 20th	Day	4 1 6% 14% 45% 30%	439
	21st to end	Day	12% 1 5% 20% 37% 25%	495
September	1st to 10th	Day	10% 1 7% 20% 37% 25%	450
	11th to 20th	Day	12% 3 8% 27% 34% 16%	450
	21st to end	Day	12% 3 8% 28% 34% 15%	450
October	1st to 10th	Day	14% 3 23% 31% 23% 6%	450
	11th to 20th	Day	6% 5% 18% 36% 30% 6%	450
	21st to end	Day	10% 4 21% 40% 21% 5%	495
November	1st to 10th	Day	14% 5% 23% 35% 18% 4	455
	11th to 20th	Day	14% 10% 28% 31% 13% 3	460
	21st to end	Day	13% 10% 31% 34% 11% 2	314
December	1st to 10th	Day	7% 13% 31% 30% 16% 3	270
	11th to 20th	Day	13% 14% 27% 37% 6% 2	270
	21st to end	Day	12% 11% 29% 36% 10% 1	297
January	1st to 10th	Night	17% 11% 31% 29% 11% 1	760
	11th to 20th	Night	12% 11% 27% 36% 12% 2	760
	21st to end	Night	17% 12% 34% 30% 8%	836
February	1st to 10th	Night	15% 8% 31% 33% 12% 1	760
	11th to 20th	Night	12% 11% 34% 34% 8% 1	743
	21st to end	Night	12% 8% 33% 38% 8% 1	628
March	1st to 10th	Night	8% 10% 32% 38% 11%	760
	11th to 20th	Night	11% 8% 27% 37% 15% 1	752
	21st to end	Night	9% 8% 29% 38% 15% 1	792
April	1st to 10th	Night	9% 6% 30% 38% 16% 1	720
	11th to 20th	Night	11% 6% 26% 35% 20% 1	711
	21st to end	Night	8% 5% 23% 44% 19% 1	720
May	1st to 10th	Night	5% 2 19% 44% 27% 3	568
	11th to 20th	Night	8% 1 15% 37% 33% 6%	531
	21st to end	Night	6% 2 8% 32% 43% 9%	584
June	1st to 10th	Night	12% 2 8% 29% 41% 7%	540
	11th to 20th	Night	7% 3 7% 25% 40% 18%	540
	21st to end	Night	6% 1 6% 22% 47% 17%	540
July	1st to 10th	Night	7% 2 5% 19% 46% 22%	525
	11th to 20th	Night	6% 4 17% 49% 24%	522
	21st to end	Night	5% 1 3 21% 52% 17%	591
August	1st to 10th	Night	7% 1 5% 19% 51% 18%	537
	11th to 20th	Night	4 5% 18% 50% 23%	552
	21st to end	Night	11% 1 8% 20% 42% 18%	792
September	1st to 10th	Night	11% 2 7% 20% 46% 15%	720
	11th to 20th	Night	14% 3 8% 26% 40% 10%	720
	21st to end	Night	14% 2 9% 25% 38% 11%	720
October	1st to 10th	Night	15% 3 17% 28% 31% 5%	720
	11th to 20th	Night	8% 5% 15% 36% 31% 6%	720
	21st to end	Night	11% 4 17% 36% 29% 4	792
November	1st to 10th	Night	12% 6% 22% 32% 25% 3	740
	11th to 20th	Night	12% 9% 23% 35% 19% 2	760
	21st to end	Night	11% 7% 28% 35% 17% 2	760
December	1st to 10th	Night	10% 9% 26% 30% 23% 1	760
	11th to 20th	Night	14% 11% 26% 29% 19% 2	760
	21st to end	Night	15% 9% 23% 35% 17%	836

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