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 2025-07-06.

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% 6% 2 6% 5% 5% 5% 4 7% 10% 42%	120037
	Night	10% 6% 2 6% 4 4 5% 4 7% 9% 43%	118761
Transparency	Day	35% 11% 17% 19% 14% 5%	123777
	Night	35% 10% 16% 19% 15% 6%	123158
Seeing	Day	9% 6% 21% 29% 24% 11%	16406
	Night	11% 6% 20% 31% 26% 6%	26493

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	9% 6% 2 6% 3 3 3 2 4 6% 57%	4056
	11th to 20th	Day	17% 10% 4 8% 5% 3 3 3 4 6% 36%	4060
	21st to end	Day	14% 7% 2 4 3 3 4 3 4 7% 49%	4466
February	1st to 10th	Day	15% 7% 3 6% 4 4 3 3 5% 7% 43%	1975
	11th to 20th	Day	11% 5% 2 5% 4 4 4 4 5% 6% 50%	1948
	21st to end	Day	11% 5% 2 6% 4 4 4 4 6% 6% 48%	1757
March	1st to 10th	Day	12% 8% 3 8% 4 4 4 3 6% 8% 40%	4935
	11th to 20th	Day	7% 3 1 3 2 3 4 3 6% 8% 60%	4980
	21st to end	Day	11% 6% 2 6% 4 4 4 3 5% 8% 48%	5465
April	1st to 10th	Day	8% 4 1 5% 4 4 4 4 6% 8% 51%	2200
	11th to 20th	Day	8% 6% 3 6% 4 4 5% 4 5% 8% 47%	2264
	21st to end	Day	5% 3 2 4 4 5% 3 3 7% 8% 56%	2400
May	1st to 10th	Day	5% 4 1 5% 4 5% 5% 4 7% 11% 48%	5834
	11th to 20th	Day	10% 5% 2 6% 4 4 5% 4 6% 9% 44%	5861
	21st to end	Day	4 3 1 5% 4 5% 5% 5% 9% 13% 46%	6433
June	1st to 10th	Day	10% 6% 2 8% 6% 6% 7% 5% 9% 11% 29%	2600
	11th to 20th	Day	4 4 1 7% 7% 6% 7% 7% 9% 12% 35%	2600
	21st to end	Day	6% 5% 3 8% 7% 7% 7% 6% 10% 12% 31%	2600
July	1st to 10th	Day	5% 5% 2 7% 6% 6% 8% 7% 10% 14% 29%	4185
	11th to 20th	Day	4 4 2 6% 6% 6% 8% 8% 11% 14% 31%	4120
	21st to end	Day	4 5% 2 6% 6% 6% 7% 6% 10% 15% 33%	4466
August	1st to 10th	Day	8% 6% 3 8% 6% 7% 6% 6% 9% 12% 29%	4104
	11th to 20th	Day	5% 4 2 7% 5% 5% 7% 6% 9% 12% 38%	3965
	21st to end	Day	9% 6% 2 7% 6% 5% 7% 5% 9% 13% 32%	3898
September	1st to 10th	Day	10% 8% 3 9% 5% 6% 6% 6% 10% 12% 25%	2090
	11th to 20th	Day	12% 7% 3 8% 7% 8% 7% 5% 8% 9% 26%	2090
	21st to end	Day	13% 6% 3 7% 5% 7% 5% 4 7% 9% 34%	2090
October	1st to 10th	Day	13% 7% 2 8% 5% 5% 5% 4 7% 9% 34%	2033
	11th to 20th	Day	8% 5% 3 6% 5% 5% 4 4 5% 8% 48%	1900
	21st to end	Day	8% 5% 3 5% 4 4 4 3 6% 7% 49%	2090
November	1st to 10th	Day	11% 6% 2 5% 5% 5% 4 4 7% 8% 43%	1957
	11th to 20th	Day	12% 7% 3 7% 5% 6% 5% 4 6% 6% 40%	2000
	21st to end	Day	12% 5% 3 7% 5% 5% 5% 4 5% 7% 43%	1846
December	1st to 10th	Day	6% 5% 2 4 3 3 4 4 5% 7% 56%	2930
	11th to 20th	Day	12% 9% 3 7% 5% 5% 5% 4 5% 8% 37%	3373
	21st to end	Day	14% 8% 3 8% 4 5% 4 3 5% 7% 40%	4466
January	1st to 10th	Night	10% 6% 2 4 2 3 3 2 4 7% 58%	5889
	11th to 20th	Night	17% 8% 3 8% 4 4 5% 4 6% 7% 34%	5900
	21st to end	Night	15% 7% 3 6% 3 4 4 4 5% 7% 42%	6490
February	1st to 10th	Night	13% 5% 3 6% 5% 4 4 3 5% 6% 46%	2420
	11th to 20th	Night	13% 7% 2 6% 4 4 4 3 5% 6% 47%	2338
	21st to end	Night	10% 6% 2 6% 4 4 4 4 5% 7% 48%	1870
March	1st to 10th	Night	15% 8% 3 7% 4 4 4 3 5% 5% 42%	4918
	11th to 20th	Night	6% 4 1 4 3 3 4 3 7% 9% 55%	4980
	21st to end	Night	11% 6% 2 6% 4 3 4 3 5% 7% 49%	5467
April	1st to 10th	Night	7% 5% 2 5% 4 4 5% 3 7% 9% 50%	2200
	11th to 20th	Night	8% 5% 2 6% 4 3 4 4 6% 7% 51%	2085
	21st to end	Night	7% 3 2 3 3 4 4 4 7% 10% 52%	2000
May	1st to 10th	Night	4 4 2 6% 4 4 5% 4 7% 10% 52%	4089
	11th to 20th	Night	10% 5% 2 6% 4 4 4 4 8% 10% 44%	4032
	21st to end	Night	3 3 1 5% 4 5% 5% 5% 7% 13% 48%	4421
June	1st to 10th	Night	9% 5% 3 6% 5% 5% 6% 6% 9% 13% 33%	1800
	11th to 20th	Night	5% 5% 2 7% 7% 6% 6% 6% 9% 12% 36%	1800
	21st to end	Night	7% 5% 3 7% 6% 6% 5% 5% 9% 13% 36%	1800
July	1st to 10th	Night	6% 5% 2 7% 6% 7% 7% 6% 10% 12% 32%	2885
	11th to 20th	Night	4 4 2 6% 5% 6% 7% 6% 11% 15% 33%	2840
	21st to end	Night	5% 6% 3 7% 6% 6% 7% 7% 11% 14% 29%	3087
August	1st to 10th	Night	9% 7% 3 8% 5% 5% 6% 6% 9% 11% 31%	2825
	11th to 20th	Night	3 4 2 7% 5% 5% 6% 6% 10% 12% 41%	2817
	21st to end	Night	7% 6% 3 8% 6% 6% 5% 5% 7% 11% 36%	3752
September	1st to 10th	Night	10% 7% 3 9% 7% 7% 7% 6% 8% 10% 27%	2090
	11th to 20th	Night	12% 8% 3 8% 7% 7% 8% 6% 8% 9% 25%	2090
	21st to end	Night	13% 6% 3 7% 5% 5% 5% 4 8% 10% 35%	2090
October	1st to 10th	Night	14% 7% 3 7% 5% 5% 5% 4 7% 8% 35%	2145
	11th to 20th	Night	8% 4 1 6% 5% 4 4 4 6% 9% 49%	2280
	21st to end	Night	9% 5% 2 6% 4 4 3 4 5% 8% 49%	2508
November	1st to 10th	Night	12% 5% 2 5% 5% 4 5% 4 6% 8% 44%	2341
	11th to 20th	Night	11% 7% 3 7% 5% 4 4 4 5% 7% 43%	2400
	21st to end	Night	10% 7% 2 7% 4 4 4 4 6% 8% 44%	2550
December	1st to 10th	Night	9% 5% 2 5% 4 3 3 3 4 6% 56%	4250
	11th to 20th	Night	12% 9% 2 8% 4 5% 5% 3 5% 6% 41%	4822
	21st to end	Night	13% 8% 3 8% 5% 5% 4 3 5% 6% 38%	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	29% 4 10% 16% 27% 13%	4057
	11th to 20th	Day	50% 4 6% 12% 19% 9%	4060
	21st to end	Day	35% 6% 10% 23% 16% 9%	4466
February	1st to 10th	Day	40% 7% 10% 15% 20% 8%	1975
	11th to 20th	Day	34% 7% 11% 17% 20% 11%	1958
	21st to end	Day	33% 7% 13% 17% 19% 10%	1757
March	1st to 10th	Day	41% 7% 12% 15% 19% 6%	4935
	11th to 20th	Day	21% 6% 14% 20% 24% 16%	4980
	21st to end	Day	32% 7% 12% 23% 17% 9%	5465
April	1st to 10th	Day	29% 8% 14% 22% 18% 9%	2200
	11th to 20th	Day	34% 8% 11% 20% 21% 7%	2264
	21st to end	Day	24% 7% 13% 26% 23% 6%	2400
May	1st to 10th	Day	26% 12% 20% 27% 12% 3	5869
	11th to 20th	Day	33% 10% 18% 29% 10% 1	5865
	21st to end	Day	24% 12% 20% 30% 12% 2	6447
June	1st to 10th	Day	42% 16% 18% 17% 7%	2600
	11th to 20th	Day	36% 16% 23% 21% 4	2600
	21st to end	Day	40% 15% 24% 15% 5%	2600
July	1st to 10th	Day	36% 20% 29% 15% 1	4120
	11th to 20th	Day	37% 21% 33% 7% 2	4051
	21st to end	Day	33% 20% 35% 11% 1	4324
August	1st to 10th	Day	42% 16% 29% 11% 2	4091
	11th to 20th	Day	32% 17% 27% 18% 6%	4004
	21st to end	Day	40% 16% 23% 21% 1	3898
September	1st to 10th	Day	47% 18% 18% 11% 4 1	2090
	11th to 20th	Day	49% 16% 18% 12% 6%	2090
	21st to end	Day	43% 12% 16% 16% 12% 2	2090
October	1st to 10th	Day	50% 9% 11% 12% 13% 4	3384
	11th to 20th	Day	29% 7% 8% 17% 30% 9%	3140
	21st to end	Day	23% 8% 10% 24% 29% 6%	3428
November	1st to 10th	Day	36% 7% 13% 20% 15% 9%	1957
	11th to 20th	Day	42% 7% 10% 13% 20% 8%	2000
	21st to end	Day	39% 7% 9% 19% 20% 7%	1846
December	1st to 10th	Day	26% 7% 14% 25% 20% 8%	2930
	11th to 20th	Day	42% 11% 10% 14% 18% 6%	3370
	21st to end	Day	41% 7% 11% 20% 15% 6%	4466
January	1st to 10th	Night	27% 4 8% 16% 32% 12%	5889
	11th to 20th	Night	49% 5% 7% 16% 18% 5%	5900
	21st to end	Night	38% 8% 10% 19% 15% 10%	6490
February	1st to 10th	Night	37% 7% 12% 15% 19% 11%	2420
	11th to 20th	Night	37% 8% 9% 19% 18% 11%	2339
	21st to end	Night	33% 7% 12% 20% 17% 10%	1870
March	1st to 10th	Night	42% 7% 11% 15% 16% 11%	4918
	11th to 20th	Night	23% 8% 13% 21% 18% 18%	4980
	21st to end	Night	34% 7% 11% 19% 18% 11%	5467
April	1st to 10th	Night	29% 8% 16% 22% 15% 10%	2200
	11th to 20th	Night	31% 8% 11% 23% 20% 7%	2085
	21st to end	Night	25% 9% 16% 26% 20% 5%	2000
May	1st to 10th	Night	23% 13% 22% 27% 13% 2	4116
	11th to 20th	Night	31% 10% 19% 27% 12% 1	4033
	21st to end	Night	23% 10% 20% 29% 16% 1	4429
June	1st to 10th	Night	38% 13% 25% 19% 6%	1800
	11th to 20th	Night	37% 15% 23% 22% 4	1800
	21st to end	Night	36% 16% 24% 18% 6% 1	1800
July	1st to 10th	Night	38% 16% 29% 15% 1	2840
	11th to 20th	Night	33% 22% 36% 8% 2	2786
	21st to end	Night	37% 17% 33% 12% 1	2988
August	1st to 10th	Night	41% 15% 32% 10% 2	2814
	11th to 20th	Night	27% 16% 28% 21% 7%	2844
	21st to end	Night	39% 13% 26% 20% 1	3752
September	1st to 10th	Night	47% 16% 19% 13% 5% 1	2090
	11th to 20th	Night	48% 16% 18% 13% 5%	2090
	21st to end	Night	41% 12% 14% 20% 12% 2	2090
October	1st to 10th	Night	53% 10% 10% 11% 12% 5%	3574
	11th to 20th	Night	30% 7% 9% 18% 29% 7%	3779
	21st to end	Night	23% 7% 10% 23% 30% 7%	4120
November	1st to 10th	Night	35% 8% 13% 19% 16% 9%	2341
	11th to 20th	Night	37% 8% 10% 17% 19% 8%	2400
	21st to end	Night	37% 7% 12% 19% 18% 8%	2550
December	1st to 10th	Night	30% 5% 12% 26% 14% 14%	4250
	11th to 20th	Night	40% 9% 11% 18% 14% 8%	4824
	21st to end	Night	42% 9% 12% 17% 16% 5%	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	13% 11% 45% 26% 5% 1	300
	11th to 20th	Day	9% 13% 40% 27% 10% 2	300
	21st to end	Day	16% 18% 40% 22% 3	330
February	1st to 10th	Day	17% 11% 30% 30% 11% 1	300
	11th to 20th	Day	14% 10% 41% 27% 7% 1	300
	21st to end	Day	10% 9% 39% 29% 11% 1	356
March	1st to 10th	Day	6% 16% 36% 31% 10% 1	500
	11th to 20th	Day	10% 11% 33% 30% 15% 2	500
	21st to end	Day	10% 9% 33% 32% 15% 1	550
April	1st to 10th	Day	9% 10% 36% 29% 14% 2	500
	11th to 20th	Day	8% 10% 34% 29% 15% 4	492
	21st to end	Day	4 5% 27% 36% 20% 8%	500
May	1st to 10th	Day	5% 5% 24% 38% 19% 8%	503
	11th to 20th	Day	9% 3 19% 35% 23% 11%	527
	21st to end	Day	6% 3 11% 32% 34% 14%	573
June	1st to 10th	Day	11% 3 12% 33% 26% 15%	530
	11th to 20th	Day	5% 2 11% 26% 35% 21%	530
	21st to end	Day	4 3 6% 26% 39% 23%	530
July	1st to 10th	Day	6% 2 5% 19% 37% 31%	505
	11th to 20th	Day	5% 1 2 14% 45% 32%	484
	21st to end	Day	6% 1 4 20% 43% 25%	532
August	1st to 10th	Day	7% 2 8% 20% 40% 23%	486
	11th to 20th	Day	4 1 6% 15% 44% 30%	479
	21st to end	Day	11% 1 5% 22% 36% 24%	539
September	1st to 10th	Day	11% 2 7% 20% 35% 24%	490
	11th to 20th	Day	13% 4 8% 27% 33% 15%	490
	21st to end	Day	11% 4 8% 28% 34% 14%	490
October	1st to 10th	Day	16% 4 22% 31% 21% 5%	490
	11th to 20th	Day	6% 8% 18% 36% 27% 5%	490
	21st to end	Day	9% 4 22% 41% 20% 4	539
November	1st to 10th	Day	13% 6% 23% 35% 19% 4	495
	11th to 20th	Day	13% 10% 29% 31% 13% 3	500
	21st to end	Day	12% 12% 30% 33% 11% 2	346
December	1st to 10th	Day	7% 14% 34% 29% 14% 3	300
	11th to 20th	Day	12% 15% 28% 36% 6% 2	300
	21st to end	Day	14% 11% 32% 33% 9% 1	330
January	1st to 10th	Night	16% 12% 32% 29% 10% 1	800
	11th to 20th	Night	12% 11% 27% 36% 12% 2	800
	21st to end	Night	17% 12% 33% 30% 7%	880
February	1st to 10th	Night	15% 8% 30% 34% 13% 1	800
	11th to 20th	Night	13% 11% 34% 34% 8% 1	783
	21st to end	Night	12% 8% 33% 38% 8% 1	660
March	1st to 10th	Night	9% 11% 32% 38% 11%	800
	11th to 20th	Night	10% 9% 28% 37% 14% 1	800
	21st to end	Night	8% 8% 30% 38% 14% 1	880
April	1st to 10th	Night	8% 7% 31% 39% 14% 1	800
	11th to 20th	Night	10% 7% 26% 37% 19% 1	791
	21st to end	Night	7% 5% 22% 45% 20% 1	800
May	1st to 10th	Night	4 3 19% 44% 27% 3	630
	11th to 20th	Night	7% 3 18% 36% 30% 6%	591
	21st to end	Night	6% 2 9% 33% 42% 8%	650
June	1st to 10th	Night	12% 2 8% 32% 40% 7%	600
	11th to 20th	Night	6% 3 8% 26% 40% 16%	600
	21st to end	Night	7% 2 7% 23% 46% 17%	600
July	1st to 10th	Night	7% 2 5% 20% 46% 21%	570
	11th to 20th	Night	6% 3 17% 49% 24%	552
	21st to end	Night	5% 2 4 21% 51% 18%	624
August	1st to 10th	Night	8% 1 5% 19% 50% 17%	567
	11th to 20th	Night	4 5% 17% 50% 23%	583
	21st to end	Night	11% 1 7% 21% 42% 18%	836
September	1st to 10th	Night	12% 3 7% 20% 45% 14%	760
	11th to 20th	Night	14% 3 9% 25% 39% 10%	760
	21st to end	Night	14% 2 9% 26% 38% 10%	760
October	1st to 10th	Night	16% 4 17% 28% 31% 5%	760
	11th to 20th	Night	8% 5% 16% 36% 29% 5%	760
	21st to end	Night	10% 4 17% 37% 29% 3	836
November	1st to 10th	Night	12% 6% 22% 32% 25% 3	780
	11th to 20th	Night	12% 9% 23% 35% 19% 2	800
	21st to end	Night	11% 7% 28% 35% 17% 2	800
December	1st to 10th	Night	10% 9% 27% 30% 22% 1	800
	11th to 20th	Night	14% 11% 25% 30% 19% 2	800
	21st to end	Night	15% 10% 25% 34% 16%	880

Page generated at 2025-07-03 01:30:16. Version ac07

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