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 2024-10-25.

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	12% 6% 2 6% 5% 5% 5% 4 6% 8% 40%	100982
	Night	12% 6% 2 6% 4 4 4 4 6% 8% 43%	97794
Transparency	Day	21% 51% 8% 11% 8% 2	100791
	Night	20% 51% 7% 11% 9% 2	97655
Seeing	Day	12% 5% 19% 29% 25% 10%	15594
	Night	13% 6% 20% 31% 25% 5%	25686

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	14% 5% 1 5% 3 4 4 4 5% 8% 46%	2836
	11th to 20th	Day	10% 5% 2 7% 5% 4 4 3 5% 9% 47%	2840
	21st to end	Day	13% 8% 3 6% 4 4 4 3 4 6% 45%	3124
February	1st to 10th	Day	14% 7% 3 6% 5% 5% 4 4 5% 8% 40%	1710
	11th to 20th	Day	18% 6% 3 6% 3 4 3 3 4 5% 46%	1772
	21st to end	Day	15% 7% 3 6% 4 2 5% 3 5% 6% 43%	1727
March	1st to 10th	Day	23% 10% 3 7% 4 3 3 3 4 5% 35%	3435
	11th to 20th	Day	8% 5% 2 5% 4 4 4 4 5% 7% 53%	3480
	21st to end	Day	18% 6% 2 5% 3 3 4 3 5% 6% 45%	3815
April	1st to 10th	Day	10% 4 2 4 4 3 4 4 5% 7% 52%	2160
	11th to 20th	Day	10% 3 1 4 4 4 4 4 6% 7% 52%	2254
	21st to end	Day	8% 4 2 3 3 4 4 4 6% 8% 54%	2280
May	1st to 10th	Day	7% 6% 2 6% 5% 4 5% 5% 8% 10% 42%	3944
	11th to 20th	Day	14% 7% 3 7% 5% 5% 5% 4 6% 7% 38%	4081
	21st to end	Day	7% 4 2 4 4 4 5% 5% 8% 11% 46%	4476
June	1st to 10th	Day	9% 8% 3 8% 6% 6% 6% 6% 8% 11% 31%	2470
	11th to 20th	Day	9% 4 1 5% 6% 5% 6% 6% 8% 12% 39%	2470
	21st to end	Day	7% 5% 2 6% 5% 6% 6% 7% 10% 13% 33%	2470
July	1st to 10th	Day	12% 7% 2 7% 6% 7% 7% 6% 8% 11% 27%	4120
	11th to 20th	Day	6% 6% 3 9% 7% 7% 8% 7% 10% 12% 27%	4120
	21st to end	Day	7% 7% 3 8% 7% 8% 8% 7% 9% 12% 23%	4466
August	1st to 10th	Day	19% 8% 3 9% 6% 6% 6% 5% 8% 10% 21%	4104
	11th to 20th	Day	8% 6% 2 8% 6% 7% 6% 6% 9% 11% 32%	3863
	21st to end	Day	9% 7% 3 7% 5% 5% 6% 4 7% 10% 38%	4180
September	1st to 10th	Day	10% 5% 2 5% 4 5% 5% 6% 9% 10% 38%	2266
	11th to 20th	Day	15% 7% 3 7% 6% 4 6% 4 5% 8% 35%	2090
	21st to end	Day	15% 7% 3 6% 4 4 5% 3 6% 7% 39%	2081
October	1st to 10th	Day	14% 5% 2 7% 4 4 4 3 4 6% 46%	1900
	11th to 20th	Day	9% 5% 2 4 3 3 3 4 4 7% 56%	1900
	21st to end	Day	10% 5% 1 6% 3 3 3 3 5% 5% 56%	2027
November	1st to 10th	Day	12% 5% 1 4 4 4 3 3 4 5% 56%	1857
	11th to 20th	Day	13% 6% 3 6% 4 4 3 3 4 6% 48%	1900
	21st to end	Day	12% 5% 2 6% 5% 3 3 2 5% 7% 51%	1777
December	1st to 10th	Day	13% 7% 3 7% 5% 4 4 4 6% 7% 39%	1710
	11th to 20th	Day	14% 5% 2 4 3 2 3 3 4 5% 55%	2153
	21st to end	Day	17% 9% 3 6% 3 4 4 3 4 5% 42%	3124
January	1st to 10th	Night	15% 7% 2 5% 4 4 4 4 6% 7% 43%	4109
	11th to 20th	Night	10% 8% 3 6% 5% 4 5% 5% 6% 7% 41%	4120
	21st to end	Night	12% 8% 3 8% 4 4 4 3 5% 6% 44%	4532
February	1st to 10th	Night	15% 6% 2 6% 4 3 4 3 6% 7% 43%	2470
	11th to 20th	Night	17% 8% 2 5% 4 3 4 3 4 6% 42%	2289
	21st to end	Night	14% 7% 2 7% 4 4 5% 4 5% 6% 43%	1727
March	1st to 10th	Night	21% 14% 4 7% 4 4 3 3 4 5% 32%	3418
	11th to 20th	Night	11% 4 2 5% 5% 4 4 3 6% 8% 48%	3480
	21st to end	Night	16% 5% 1 5% 4 4 4 3 5% 7% 46%	3817
April	1st to 10th	Night	9% 5% 2 5% 5% 4 4 3 6% 7% 49%	2019
	11th to 20th	Night	11% 4 2 4 4 4 4 3 5% 7% 51%	1875
	21st to end	Night	7% 5% 2 5% 4 3 4 4 6% 7% 53%	1900
May	1st to 10th	Night	6% 5% 2 6% 4 4 5% 4 7% 9% 50%	2979
	11th to 20th	Night	13% 6% 3 7% 4 3 3 4 6% 9% 42%	2812
	21st to end	Night	6% 4 2 4 4 4 4 5% 7% 10% 51%	3079
June	1st to 10th	Night	9% 6% 3 6% 5% 5% 5% 5% 8% 13% 35%	1710
	11th to 20th	Night	8% 5% 2 5% 5% 4 5% 4 8% 10% 43%	1710
	21st to end	Night	5% 5% 2 6% 6% 5% 7% 5% 10% 15% 35%	1710
July	1st to 10th	Night	10% 6% 2 7% 6% 7% 6% 5% 8% 11% 30%	2840
	11th to 20th	Night	7% 7% 3 8% 6% 6% 6% 5% 8% 13% 31%	2840
	21st to end	Night	6% 5% 3 6% 6% 6% 6% 7% 11% 14% 30%	3087
August	1st to 10th	Night	15% 8% 3 8% 4 5% 6% 5% 8% 12% 25%	2825
	11th to 20th	Night	6% 5% 2 6% 5% 5% 6% 5% 8% 12% 40%	2934
	21st to end	Night	7% 5% 2 6% 5% 5% 5% 4 7% 9% 46%	3476
September	1st to 10th	Night	9% 5% 2 8% 6% 5% 6% 5% 6% 9% 40%	1909
	11th to 20th	Night	11% 8% 3 6% 5% 6% 5% 5% 6% 9% 37%	2090
	21st to end	Night	16% 7% 3 7% 5% 3 4 5% 5% 9% 39%	2099
October	1st to 10th	Night	13% 6% 2 5% 4 4 4 4 4 6% 48%	2280
	11th to 20th	Night	8% 4 2 5% 4 3 3 2 4 6% 58%	2280
	21st to end	Night	9% 5% 2 5% 3 3 4 3 4 6% 56%	2435
November	1st to 10th	Night	11% 4 1 5% 4 4 3 3 4 5% 56%	2221
	11th to 20th	Night	13% 7% 3 6% 3 3 3 3 5% 6% 49%	2280
	21st to end	Night	10% 6% 2 5% 4 4 4 3 7% 7% 48%	2398
December	1st to 10th	Night	16% 6% 3 6% 4 3 4 4 6% 7% 40%	2470
	11th to 20th	Night	17% 6% 2 5% 3 3 2 2 4 5% 50%	3042
	21st to end	Night	17% 7% 2 6% 3 4 4 4 6% 6% 41%	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	22% 39% 5% 13% 17% 5%	2837
	11th to 20th	Day	21% 38% 5% 12% 16% 8%	2840
	21st to end	Day	24% 42% 5% 12% 11% 5%	3124
February	1st to 10th	Day	15% 66% 4 8% 7% 1	1710
	11th to 20th	Day	16% 64% 3 7% 8% 2	1781
	21st to end	Day	16% 65% 4 4 6% 4	1727
March	1st to 10th	Day	36% 44% 6% 7% 4 3	3435
	11th to 20th	Day	14% 43% 5% 10% 20% 8%	3480
	21st to end	Day	24% 42% 4 10% 14% 5%	3815
April	1st to 10th	Day	13% 66% 5% 7% 7% 3	2160
	11th to 20th	Day	13% 66% 4 8% 5% 3	2254
	21st to end	Day	8% 66% 5% 12% 8% 1	2280
May	1st to 10th	Day	21% 47% 11% 14% 6% 1	3979
	11th to 20th	Day	27% 41% 8% 16% 7%	4085
	21st to end	Day	12% 45% 14% 17% 12%	4489
June	1st to 10th	Day	13% 68% 7% 8% 4	2470
	11th to 20th	Day	9% 68% 9% 9% 5%	2470
	21st to end	Day	12% 68% 10% 9% 1	2470
July	1st to 10th	Day	30% 42% 14% 12% 1	4055
	11th to 20th	Day	29% 45% 20% 6% 1	4051
	21st to end	Day	33% 43% 12% 8% 3	4324
August	1st to 10th	Day	43% 40% 13% 4	4091
	11th to 20th	Day	21% 44% 14% 14% 7%	3902
	21st to end	Day	19% 40% 12% 27% 3	4180
September	1st to 10th	Day	14% 64% 9% 7% 6% 1	2266
	11th to 20th	Day	20% 62% 5% 7% 6% 1	2090
	21st to end	Day	19% 62% 8% 7% 4	2081
October	1st to 10th	Day	17% 60% 5% 9% 7% 2	1900
	11th to 20th	Day	10% 61% 5% 9% 11% 4	1900
	21st to end	Day	9% 60% 4 8% 15% 4	2027
November	1st to 10th	Day	16% 59% 4 7% 11% 2	1857
	11th to 20th	Day	17% 60% 4 6% 9% 4	1900
	21st to end	Day	13% 60% 4 10% 10% 2	1777
December	1st to 10th	Day	21% 61% 4 7% 5% 1	1710
	11th to 20th	Day	14% 48% 3 12% 19% 5%	2150
	21st to end	Day	30% 39% 5% 11% 14% 1	3124
January	1st to 10th	Night	25% 39% 5% 12% 12% 7%	4109
	11th to 20th	Night	26% 40% 5% 11% 17% 3	4120
	21st to end	Night	25% 43% 4 6% 20% 3	4532
February	1st to 10th	Night	13% 65% 4 9% 6% 2	2470
	11th to 20th	Night	16% 64% 3 6% 7% 2	2291
	21st to end	Night	18% 65% 2 4 7% 4	1727
March	1st to 10th	Night	41% 43% 6% 6% 3 2	3418
	11th to 20th	Night	19% 41% 4 9% 17% 10%	3480
	21st to end	Night	24% 42% 4 13% 12% 4	3817
April	1st to 10th	Night	13% 66% 6% 6% 7% 3	2019
	11th to 20th	Night	13% 67% 4 9% 5% 3	1875
	21st to end	Night	11% 65% 5% 11% 7% 2	1900
May	1st to 10th	Night	18% 45% 13% 17% 6% 1	3009
	11th to 20th	Night	23% 44% 9% 15% 9%	2813
	21st to end	Night	14% 45% 10% 18% 13% 1	3087
June	1st to 10th	Night	12% 68% 8% 8% 4	1710
	11th to 20th	Night	9% 68% 8% 10% 5%	1710
	21st to end	Night	13% 67% 10% 8% 2	1710
July	1st to 10th	Night	29% 42% 14% 15% 1	2795
	11th to 20th	Night	28% 43% 23% 5% 1	2786
	21st to end	Night	29% 45% 13% 11% 2	2988
August	1st to 10th	Night	37% 42% 16% 5% 1	2814
	11th to 20th	Night	18% 43% 16% 18% 5%	2961
	21st to end	Night	16% 40% 12% 29% 3	3476
September	1st to 10th	Night	15% 64% 8% 8% 4 2	1909
	11th to 20th	Night	20% 62% 5% 7% 5% 1	2090
	21st to end	Night	18% 62% 7% 9% 4	2099
October	1st to 10th	Night	15% 60% 5% 12% 7% 1	2280
	11th to 20th	Night	10% 60% 4 10% 11% 5%	2280
	21st to end	Night	9% 61% 3 9% 13% 5%	2435
November	1st to 10th	Night	17% 58% 5% 6% 11% 3	2221
	11th to 20th	Night	16% 60% 4 7% 8% 5%	2280
	21st to end	Night	13% 61% 5% 9% 10% 3	2398
December	1st to 10th	Night	20% 61% 5% 5% 7% 1	2470
	11th to 20th	Night	18% 48% 3 8% 18% 5%	3044
	21st to end	Night	27% 39% 7% 13% 13% 1	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	12% 13% 34% 37% 3	270
	11th to 20th	Day	7% 9% 34% 38% 11% 1	270
	21st to end	Day	19% 9% 40% 28% 3 1	297
February	1st to 10th	Day	13% 11% 27% 40% 9%	270
	11th to 20th	Day	24% 8% 35% 27% 6%	300
	21st to end	Day	13% 13% 33% 32% 8% 1	382
March	1st to 10th	Day	15% 9% 32% 29% 14% 1	460
	11th to 20th	Day	14% 7% 35% 33% 9% 2	460
	21st to end	Day	13% 6% 37% 30% 12% 2	506
April	1st to 10th	Day	12% 9% 31% 32% 15% 1	460
	11th to 20th	Day	9% 6% 25% 29% 25% 6%	452
	21st to end	Day	9% 5% 21% 34% 23% 8%	460
May	1st to 10th	Day	5% 4 29% 33% 23% 6%	463
	11th to 20th	Day	13% 2 17% 31% 28% 9%	487
	21st to end	Day	13% 2 9% 29% 32% 14%	529
June	1st to 10th	Day	9% 3 10% 29% 37% 11%	490
	11th to 20th	Day	11% 2 11% 27% 30% 19%	490
	21st to end	Day	7% 1 7% 27% 43% 15%	490
July	1st to 10th	Day	8% 4 5% 18% 43% 23%	485
	11th to 20th	Day	7% 1 7% 18% 44% 23%	484
	21st to end	Day	5% 2 6% 27% 43% 18%	532
August	1st to 10th	Day	15% 3 6% 22% 35% 19%	486
	11th to 20th	Day	11% 1 7% 18% 39% 24%	479
	21st to end	Day	14% 3 8% 17% 36% 22%	539
September	1st to 10th	Day	11% 2 9% 22% 38% 19%	490
	11th to 20th	Day	16% 4 12% 25% 32% 11%	490
	21st to end	Day	17% 3 10% 24% 36% 11%	490
October	1st to 10th	Day	19% 4 14% 29% 23% 11%	490
	11th to 20th	Day	10% 3 20% 31% 27% 10%	490
	21st to end	Day	9% 4 18% 33% 28% 7%	514
November	1st to 10th	Day	16% 7% 19% 29% 22% 8%	455
	11th to 20th	Day	17% 5% 24% 40% 11% 3	460
	21st to end	Day	12% 8% 23% 39% 15% 2	337
December	1st to 10th	Day	20% 5% 22% 39% 14%	270
	11th to 20th	Day	19% 6% 25% 38% 9% 3	270
	21st to end	Day	18% 8% 26% 32% 15% 2	297
January	1st to 10th	Night	14% 14% 29% 33% 9% 1	760
	11th to 20th	Night	13% 12% 28% 34% 13%	760
	21st to end	Night	17% 11% 34% 33% 4	836
February	1st to 10th	Night	17% 9% 30% 34% 9%	760
	11th to 20th	Night	21% 11% 30% 34% 4	743
	21st to end	Night	17% 9% 29% 35% 11%	628
March	1st to 10th	Night	14% 8% 32% 33% 13%	760
	11th to 20th	Night	13% 7% 28% 40% 11%	760
	21st to end	Night	15% 7% 29% 34% 14% 1	836
April	1st to 10th	Night	11% 9% 30% 34% 15% 1	760
	11th to 20th	Night	11% 7% 25% 33% 21% 3	751
	21st to end	Night	8% 5% 25% 36% 21% 4	760
May	1st to 10th	Night	7% 5% 24% 37% 26% 2	674
	11th to 20th	Night	12% 4 16% 37% 27% 5%	561
	21st to end	Night	9% 2 7% 28% 42% 12%	617
June	1st to 10th	Night	10% 2 10% 36% 36% 6%	570
	11th to 20th	Night	10% 1 9% 26% 41% 13%	570
	21st to end	Night	7% 2 7% 32% 41% 11%	570
July	1st to 10th	Night	10% 2 6% 25% 48% 10%	555
	11th to 20th	Night	10% 1 5% 21% 48% 14%	552
	21st to end	Night	6% 1 7% 30% 46% 10%	624
August	1st to 10th	Night	10% 2 5% 26% 44% 14%	567
	11th to 20th	Night	9% 3 7% 22% 44% 16%	648
	21st to end	Night	12% 3 7% 18% 42% 18%	836
September	1st to 10th	Night	10% 2 8% 22% 47% 11%	760
	11th to 20th	Night	14% 5% 7% 30% 34% 11%	760
	21st to end	Night	21% 3 8% 24% 35% 8%	760
October	1st to 10th	Night	15% 5% 14% 29% 28% 8%	760
	11th to 20th	Night	10% 4 21% 32% 27% 5%	760
	21st to end	Night	11% 5% 21% 31% 27% 6%	812
November	1st to 10th	Night	12% 5% 22% 33% 22% 5%	740
	11th to 20th	Night	15% 9% 25% 32% 17% 2	760
	21st to end	Night	11% 9% 25% 33% 20% 3	760
December	1st to 10th	Night	17% 8% 25% 35% 14% 1	760
	11th to 20th	Night	19% 8% 27% 29% 16% 2	760
	21st to end	Night	19% 8% 28% 30% 14%	836

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