Georgetown
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Georgetown clear sky chart from 2005-11-06 to 2026-08-30.

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	11% 6% 2 6% 5% 5% 5% 4 6% 9% 41%	143496
	Night	11% 6% 2 6% 4 4 4 4 6% 8% 44%	138757
Transparency	Day	26% 37% 11% 13% 10% 3	147099
	Night	25% 36% 10% 13% 12% 5%	143226
Seeing	Day	12% 6% 21% 29% 24% 9%	18113
	Night	13% 6% 21% 31% 24% 5%	28174

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% 4 2 5% 3 3 3 4 5% 8% 52%	5258
	11th to 20th	Day	12% 5% 2 6% 4 3 3 2 4 7% 52%	5280
	21st to end	Day	16% 9% 2 6% 4 4 4 3 5% 7% 41%	5780
February	1st to 10th	Day	14% 7% 3 5% 4 4 4 3 6% 8% 42%	1890
	11th to 20th	Day	18% 6% 3 6% 3 4 3 3 4 5% 45%	1965
	21st to end	Day	15% 7% 3 6% 4 2 4 3 5% 6% 44%	1903
March	1st to 10th	Day	21% 9% 3 6% 4 3 3 2 4 6% 38%	5045
	11th to 20th	Day	9% 4 2 5% 4 4 4 3 5% 7% 54%	5090
	21st to end	Day	13% 5% 2 5% 4 4 4 3 5% 6% 50%	5586
April	1st to 10th	Day	9% 5% 2 4 3 3 4 4 5% 8% 52%	2388
	11th to 20th	Day	9% 3 1 4 4 4 4 4 6% 7% 54%	2494
	21st to end	Day	8% 4 2 4 4 4 4 4 6% 8% 53%	2520
May	1st to 10th	Day	12% 7% 2 7% 5% 4 5% 4 7% 9% 38%	7352
	11th to 20th	Day	12% 6% 2 6% 4 4 5% 4 6% 9% 41%	7626
	21st to end	Day	9% 6% 2 7% 5% 6% 6% 5% 8% 10% 37%	8391
June	1st to 10th	Day	9% 8% 3 8% 6% 6% 6% 6% 8% 11% 31%	2730
	11th to 20th	Day	8% 4 2 5% 6% 5% 6% 6% 8% 13% 38%	2730
	21st to end	Day	6% 5% 2 6% 5% 6% 6% 7% 10% 13% 35%	2730
July	1st to 10th	Day	9% 6% 2 7% 6% 7% 7% 6% 9% 12% 31%	7680
	11th to 20th	Day	5% 5% 3 8% 6% 7% 7% 6% 10% 13% 30%	7680
	21st to end	Day	7% 6% 2 8% 6% 6% 7% 7% 10% 13% 28%	8340
August	1st to 10th	Day	19% 8% 3 9% 6% 6% 6% 5% 7% 9% 20%	4364
	11th to 20th	Day	7% 6% 2 7% 5% 7% 6% 6% 9% 12% 33%	4111
	21st to end	Day	10% 7% 3 7% 5% 5% 6% 5% 7% 10% 37%	4420
September	1st to 10th	Day	10% 5% 2 6% 4 5% 5% 6% 8% 10% 39%	2385
	11th to 20th	Day	14% 7% 3 7% 6% 4 6% 4 6% 8% 36%	2200
	21st to end	Day	16% 7% 3 6% 4 4 5% 3 6% 7% 39%	2190
October	1st to 10th	Day	14% 6% 3 7% 4 4 4 4 5% 6% 45%	2000
	11th to 20th	Day	10% 5% 2 4 3 3 3 4 4 7% 56%	2000
	21st to end	Day	10% 5% 1 6% 3 3 3 3 4 5% 56%	2200
November	1st to 10th	Day	11% 5% 1 4 4 4 3 3 5% 6% 54%	2057
	11th to 20th	Day	13% 6% 2 5% 4 4 3 3 4 6% 50%	2100
	21st to end	Day	11% 5% 2 6% 4 3 3 2 4 6% 51%	1963
December	1st to 10th	Day	9% 5% 2 5% 4 4 4 3 6% 6% 51%	3020
	11th to 20th	Day	14% 7% 2 6% 4 3 3 3 4 6% 47%	3463
	21st to end	Day	17% 8% 3 6% 4 4 4 3 4 5% 43%	4565
January	1st to 10th	Night	9% 6% 2 5% 4 4 4 3 5% 6% 53%	7635
	11th to 20th	Night	10% 6% 2 6% 4 4 4 4 5% 7% 48%	7680
	21st to end	Night	14% 8% 3 7% 4 4 4 3 4 5% 43%	8401
February	1st to 10th	Night	15% 6% 2 6% 4 3 4 3 5% 6% 44%	2730
	11th to 20th	Night	17% 8% 2 6% 4 3 4 4 5% 5% 41%	2535
	21st to end	Night	14% 7% 2 7% 4 4 4 3 5% 6% 43%	1903
March	1st to 10th	Night	18% 11% 3 6% 4 3 3 3 4 5% 39%	5028
	11th to 20th	Night	11% 4 2 5% 4 3 3 3 6% 9% 51%	5090
	21st to end	Night	13% 5% 1 5% 3 3 3 3 5% 7% 51%	5588
April	1st to 10th	Night	10% 6% 2 5% 4 4 4 3 5% 7% 49%	2231
	11th to 20th	Night	10% 4 2 4 4 4 4 3 5% 7% 53%	2075
	21st to end	Night	7% 5% 2 5% 4 4 4 4 6% 8% 52%	2100
May	1st to 10th	Night	10% 6% 2 6% 4 4 4 4 6% 9% 43%	5543
	11th to 20th	Night	12% 6% 2 6% 4 3 3 4 7% 10% 43%	5234
	21st to end	Night	10% 6% 3 7% 5% 5% 5% 5% 7% 9% 37%	5761
June	1st to 10th	Night	9% 6% 3 6% 5% 5% 5% 5% 8% 12% 36%	1890
	11th to 20th	Night	7% 5% 2 6% 6% 5% 5% 5% 8% 11% 41%	1890
	21st to end	Night	5% 4 2 6% 5% 5% 7% 5% 9% 14% 37%	1890
July	1st to 10th	Night	7% 5% 2 6% 5% 6% 6% 6% 10% 13% 35%	5278
	11th to 20th	Night	6% 5% 3 7% 6% 5% 6% 5% 8% 14% 35%	5277
	21st to end	Night	7% 5% 3 7% 5% 6% 6% 6% 10% 13% 33%	5736
August	1st to 10th	Night	15% 8% 3 8% 5% 5% 6% 5% 8% 12% 25%	3005
	11th to 20th	Night	6% 5% 2 6% 5% 5% 6% 5% 8% 12% 40%	3123
	21st to end	Night	7% 5% 2 7% 5% 5% 5% 4 7% 9% 45%	3677
September	1st to 10th	Night	9% 5% 2 8% 6% 5% 6% 5% 6% 9% 40%	2010
	11th to 20th	Night	11% 8% 3 6% 5% 6% 5% 5% 6% 9% 37%	2200
	21st to end	Night	16% 7% 3 6% 4 3 4 5% 5% 8% 39%	2210
October	1st to 10th	Night	13% 6% 2 6% 4 4 4 4 4 6% 47%	2400
	11th to 20th	Night	9% 4 2 5% 3 3 3 2 4 6% 57%	2400
	21st to end	Night	10% 5% 2 5% 3 3 4 3 4 6% 56%	2640
November	1st to 10th	Night	11% 4 1 5% 4 4 3 3 4 5% 55%	2461
	11th to 20th	Night	12% 7% 3 6% 3 3 3 3 5% 7% 49%	2520
	21st to end	Night	9% 5% 2 5% 4 4 4 4 7% 7% 49%	2651
December	1st to 10th	Night	11% 6% 2 5% 3 3 3 3 5% 6% 54%	4380
	11th to 20th	Night	15% 8% 3 8% 5% 4 3 3 4 5% 43%	4952
	21st to end	Night	15% 6% 2 6% 3 4 4 4 6% 6% 44%	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	24% 23% 9% 18% 23% 4	5259
	11th to 20th	Day	26% 21% 4 12% 26% 10%	5280
	21st to end	Day	35% 24% 6% 14% 13% 8%	5780
February	1st to 10th	Day	16% 60% 4 11% 7% 1	1890
	11th to 20th	Day	20% 58% 3 9% 9% 2	1974
	21st to end	Day	18% 59% 4 6% 8% 4	1903
March	1st to 10th	Day	37% 30% 7% 8% 12% 5%	5045
	11th to 20th	Day	18% 30% 6% 11% 22% 13%	5090
	21st to end	Day	24% 30% 8% 15% 14% 9%	5586
April	1st to 10th	Day	14% 60% 6% 10% 7% 3	2388
	11th to 20th	Day	13% 60% 4 13% 7% 3	2494
	21st to end	Day	11% 60% 6% 13% 9% 1	2520
May	1st to 10th	Day	34% 28% 13% 18% 5% 1	7384
	11th to 20th	Day	31% 25% 15% 19% 9% 1	7630
	21st to end	Day	30% 29% 15% 13% 11% 1	8405
June	1st to 10th	Day	16% 62% 7% 9% 6% 1	2730
	11th to 20th	Day	11% 63% 11% 10% 5%	2730
	21st to end	Day	12% 62% 13% 12% 1	2730
July	1st to 10th	Day	32% 28% 24% 13% 2	7615
	11th to 20th	Day	31% 30% 30% 8%	7611
	21st to end	Day	34% 28% 18% 14% 5% 1	8156
August	1st to 10th	Day	44% 39% 13% 4	4351
	11th to 20th	Day	21% 42% 15% 14% 7%	4150
	21st to end	Day	20% 38% 12% 26% 3	4420
September	1st to 10th	Day	15% 61% 9% 8% 6% 1	2385
	11th to 20th	Day	20% 59% 5% 9% 5% 1	2200
	21st to end	Day	21% 59% 9% 8% 4	2190
October	1st to 10th	Day	27% 37% 7% 11% 16% 3	3260
	11th to 20th	Day	8% 36% 4 7% 23% 22%	3240
	21st to end	Day	16% 36% 5% 11% 26% 5%	3538
November	1st to 10th	Day	18% 54% 6% 8% 12% 2	2057
	11th to 20th	Day	17% 55% 4 8% 11% 5%	2100
	21st to end	Day	15% 55% 5% 10% 12% 3	1963
December	1st to 10th	Day	21% 36% 5% 10% 18% 12%	3020
	11th to 20th	Day	28% 31% 6% 13% 17% 3	3460
	21st to end	Day	33% 27% 7% 18% 14% 1	4565
January	1st to 10th	Night	25% 22% 7% 18% 23% 5%	7636
	11th to 20th	Night	28% 22% 5% 12% 26% 8%	7680
	21st to end	Night	35% 24% 4 10% 21% 7%	8401
February	1st to 10th	Night	15% 59% 4 11% 8% 3	2730
	11th to 20th	Night	20% 58% 4 8% 8% 2	2537
	21st to end	Night	20% 59% 2 4 9% 5%	1903
March	1st to 10th	Night	38% 30% 6% 8% 15% 4	5028
	11th to 20th	Night	20% 29% 6% 9% 21% 14%	5090
	21st to end	Night	24% 29% 6% 17% 15% 9%	5588
April	1st to 10th	Night	15% 60% 6% 10% 6% 3	2231
	11th to 20th	Night	13% 60% 4 12% 7% 4	2075
	21st to end	Night	13% 60% 6% 12% 8% 2	2100
May	1st to 10th	Night	30% 27% 16% 20% 6% 1	5573
	11th to 20th	Night	27% 27% 16% 18% 10% 2	5235
	21st to end	Night	34% 28% 13% 13% 11% 1	5771
June	1st to 10th	Night	15% 62% 8% 8% 5% 1	1890
	11th to 20th	Night	12% 63% 10% 10% 4	1890
	21st to end	Night	13% 61% 12% 10% 3	1890
July	1st to 10th	Night	29% 28% 26% 15% 2	5235
	11th to 20th	Night	31% 29% 30% 9% 1	5226
	21st to end	Night	34% 28% 18% 15% 4 1	5606
August	1st to 10th	Night	39% 40% 16% 5% 1	2994
	11th to 20th	Night	19% 42% 17% 18% 5%	3150
	21st to end	Night	17% 39% 12% 29% 3	3677
September	1st to 10th	Night	15% 61% 8% 10% 5% 2	2010
	11th to 20th	Night	21% 59% 6% 9% 5% 1	2200
	21st to end	Night	19% 59% 8% 9% 4	2210
October	1st to 10th	Night	24% 36% 7% 14% 16% 3	3920
	11th to 20th	Night	7% 35% 3 8% 17% 30%	3899
	21st to end	Night	14% 36% 4 15% 20% 10%	4252
November	1st to 10th	Night	18% 53% 5% 8% 11% 4	2461
	11th to 20th	Night	17% 55% 4 8% 12% 5%	2520
	21st to end	Night	14% 56% 5% 10% 11% 4	2651
December	1st to 10th	Night	21% 35% 4 8% 22% 11%	4380
	11th to 20th	Night	32% 32% 6% 11% 14% 6%	4954
	21st to end	Night	29% 28% 9% 19% 14% 1	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	12% 13% 38% 34% 3 1	330
	11th to 20th	Day	8% 12% 36% 34% 9% 1	330
	21st to end	Day	20% 12% 40% 24% 3 1	363
February	1st to 10th	Day	12% 12% 32% 35% 8%	330
	11th to 20th	Day	25% 10% 37% 23% 5%	364
	21st to end	Day	15% 14% 33% 30% 8%	446
March	1st to 10th	Day	16% 9% 32% 29% 13% 1	540
	11th to 20th	Day	15% 8% 36% 31% 9% 1	540
	21st to end	Day	11% 8% 39% 29% 11% 2	594
April	1st to 10th	Day	12% 9% 33% 32% 14% 1	540
	11th to 20th	Day	8% 5% 27% 31% 23% 6%	532
	21st to end	Day	8% 5% 23% 34% 22% 8%	540
May	1st to 10th	Day	6% 4 32% 31% 21% 6%	543
	11th to 20th	Day	13% 3 19% 31% 26% 8%	567
	21st to end	Day	14% 2 13% 29% 28% 12%	617
June	1st to 10th	Day	9% 4 12% 28% 35% 11%	570
	11th to 20th	Day	10% 3 12% 30% 29% 17%	570
	21st to end	Day	6% 1 8% 29% 41% 15%	570
July	1st to 10th	Day	8% 4 6% 21% 42% 20%	565
	11th to 20th	Day	6% 1 7% 19% 45% 22%	564
	21st to end	Day	6% 2 7% 27% 40% 18%	620
August	1st to 10th	Day	16% 4 6% 23% 34% 17%	566
	11th to 20th	Day	10% 1 7% 19% 38% 25%	559
	21st to end	Day	15% 3 10% 19% 34% 20%	619
September	1st to 10th	Day	11% 3 10% 23% 36% 18%	530
	11th to 20th	Day	15% 4 13% 26% 31% 11%	530
	21st to end	Day	18% 3 10% 25% 34% 11%	530
October	1st to 10th	Day	18% 4 15% 29% 22% 11%	530
	11th to 20th	Day	11% 3 20% 30% 27% 9%	530
	21st to end	Day	11% 5% 21% 31% 26% 6%	583
November	1st to 10th	Day	14% 8% 21% 30% 20% 7%	535
	11th to 20th	Day	15% 7% 26% 38% 11% 2	540
	21st to end	Day	12% 9% 27% 36% 13% 2	403
December	1st to 10th	Day	21% 6% 25% 35% 12%	330
	11th to 20th	Day	18% 8% 27% 36% 9% 2	330
	21st to end	Day	17% 10% 29% 31% 12% 1	363
January	1st to 10th	Night	13% 14% 31% 33% 8%	840
	11th to 20th	Night	13% 13% 29% 33% 11%	840
	21st to end	Night	18% 12% 34% 32% 4	924
February	1st to 10th	Night	17% 10% 31% 34% 9%	840
	11th to 20th	Night	21% 13% 30% 33% 4	823
	21st to end	Night	17% 10% 29% 34% 10%	692
March	1st to 10th	Night	14% 9% 31% 32% 14%	840
	11th to 20th	Night	13% 8% 29% 39% 11%	840
	21st to end	Night	14% 8% 30% 34% 14% 1	924
April	1st to 10th	Night	11% 9% 30% 34% 15%	840
	11th to 20th	Night	10% 7% 26% 34% 21% 3	831
	21st to end	Night	8% 5% 25% 37% 22% 3	840
May	1st to 10th	Night	8% 6% 23% 38% 24% 2	744
	11th to 20th	Night	12% 4 18% 35% 26% 5%	621
	21st to end	Night	10% 3 8% 28% 39% 11%	683
June	1st to 10th	Night	10% 3 12% 36% 33% 6%	630
	11th to 20th	Night	10% 1 10% 30% 38% 11%	630
	21st to end	Night	6% 2 7% 33% 41% 10%	630
July	1st to 10th	Night	9% 2 6% 26% 47% 10%	615
	11th to 20th	Night	10% 5% 23% 48% 14%	612
	21st to end	Night	5% 1 8% 30% 45% 11%	690
August	1st to 10th	Night	10% 3 5% 26% 42% 14%	627
	11th to 20th	Night	9% 3 6% 21% 44% 17%	717
	21st to end	Night	11% 3 8% 20% 41% 16%	917
September	1st to 10th	Night	10% 2 9% 23% 46% 10%	800
	11th to 20th	Night	13% 5% 8% 31% 33% 11%	800
	21st to end	Night	21% 3 9% 25% 34% 8%	800
October	1st to 10th	Night	15% 5% 16% 29% 27% 8%	800
	11th to 20th	Night	11% 5% 21% 32% 27% 5%	800
	21st to end	Night	12% 5% 21% 30% 26% 5%	880
November	1st to 10th	Night	12% 5% 23% 33% 21% 5%	820
	11th to 20th	Night	14% 9% 26% 33% 17% 2	840
	21st to end	Night	10% 9% 28% 32% 18% 3	840
December	1st to 10th	Night	18% 8% 25% 34% 13% 1	840
	11th to 20th	Night	20% 9% 26% 29% 15% 2	840
	21st to end	Night	18% 8% 29% 32% 13%	924

Page generated at 2026-08-27 04:07:15. Version ac07

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