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 2025-02-21.

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 5% 4 4 4 3 5% 7% 48%	109167
	Night	13% 6% 2 5% 3 3 3 3 5% 6% 52%	109647
Transparency	Day	35% 5% 9% 21% 21% 9%	112962
	Night	33% 4 10% 21% 21% 12%	113979
Seeing	Day	13% 6% 21% 29% 24% 7%	15226
	Night	14% 6% 26% 36% 16% 1	25284

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% 9% 3 5% 3 3 3 3 4 5% 48%	4077
	11th to 20th	Day	13% 6% 2 4 2 2 2 2 5% 6% 55%	4080
	21st to end	Day	24% 9% 3 6% 3 2 2 2 2 4 43%	4654
February	1st to 10th	Day	19% 7% 2 6% 3 3 4 2 4 5% 46%	2000
	11th to 20th	Day	17% 7% 2 6% 3 3 4 2 4 6% 46%	1949
	21st to end	Day	18% 6% 2 5% 2 3 3 3 3 5% 50%	1570
March	1st to 10th	Day	15% 7% 3 7% 4 4 4 3 4 5% 43%	3433
	11th to 20th	Day	19% 9% 3 7% 5% 4 4 3 5% 6% 33%	3480
	21st to end	Day	12% 8% 3 6% 5% 3 4 4 5% 7% 44%	3814
April	1st to 10th	Day	14% 9% 2 6% 5% 3 4 3 5% 6% 42%	2090
	11th to 20th	Day	16% 7% 3 7% 4 3 3 3 4 6% 43%	2067
	21st to end	Day	15% 6% 2 5% 4 4 4 3 5% 6% 46%	2430
May	1st to 10th	Day	22% 9% 3 7% 4 4 4 4 6% 7% 31%	4082
	11th to 20th	Day	14% 6% 2 6% 4 4 4 3 5% 7% 44%	4082
	21st to end	Day	11% 5% 3 6% 5% 6% 6% 5% 9% 10% 33%	4477
June	1st to 10th	Day	5% 4 2 6% 5% 5% 5% 4 7% 8% 50%	2470
	11th to 20th	Day	7% 4 2 5% 4 4 5% 5% 6% 10% 49%	2470
	21st to end	Day	7% 6% 2 6% 5% 5% 6% 5% 7% 10% 42%	2470
July	1st to 10th	Day	5% 3 2 5% 5% 5% 5% 4 6% 8% 51%	4120
	11th to 20th	Day	4 3 1 4 4 4 5% 4 6% 10% 54%	4120
	21st to end	Day	9% 6% 2 6% 5% 4 5% 5% 7% 10% 40%	4465
August	1st to 10th	Day	1 1 1 2 2 2 3 3 5% 8% 72%	4104
	11th to 20th	Day	3 2 1 3 3 3 4 3 6% 11% 61%	3855
	21st to end	Day	4 2 1 4 4 5% 6% 5% 7% 10% 50%	3966
September	1st to 10th	Day	8% 5% 2 5% 6% 5% 5% 4 6% 7% 47%	2090
	11th to 20th	Day	10% 5% 2 6% 5% 5% 5% 3 6% 8% 44%	2090
	21st to end	Day	6% 4 2 4 5% 4 4 4 6% 7% 54%	2090
October	1st to 10th	Day	6% 4 2 5% 3 3 3 3 4 6% 62%	2090
	11th to 20th	Day	11% 5% 2 5% 3 3 3 3 3 6% 57%	1925
	21st to end	Day	11% 4 2 4 3 2 3 2 3 5% 60%	2090
November	1st to 10th	Day	13% 8% 3 4 4 3 3 3 4 6% 49%	1958
	11th to 20th	Day	10% 5% 2 5% 4 4 3 4 4 6% 54%	1890
	21st to end	Day	16% 5% 3 5% 3 3 3 3 5% 5% 48%	1800
December	1st to 10th	Day	21% 7% 2 5% 3 3 4 3 4 5% 43%	2940
	11th to 20th	Day	17% 7% 2 5% 4 3 4 3 4 6% 44%	3391
	21st to end	Day	14% 5% 1 4 3 2 3 2 3 4 58%	4488
January	1st to 10th	Night	17% 7% 2 4 2 2 2 2 3 5% 53%	5888
	11th to 20th	Night	12% 3 1 3 2 1 2 2 4 5% 65%	5900
	21st to end	Night	26% 6% 2 6% 3 3 2 2 3 5% 41%	6352
February	1st to 10th	Night	18% 8% 2 5% 3 3 3 2 3 5% 49%	2400
	11th to 20th	Night	19% 7% 2 5% 3 2 3 2 3 5% 50%	2338
	21st to end	Night	19% 6% 2 5% 4 3 3 2 3 5% 48%	1884
March	1st to 10th	Night	16% 7% 2 6% 4 4 4 4 4 6% 42%	3419
	11th to 20th	Night	20% 9% 3 6% 4 4 4 4 6% 5% 34%	3480
	21st to end	Night	14% 8% 4 7% 4 4 4 3 4 5% 44%	3817
April	1st to 10th	Night	12% 6% 2 5% 4 4 4 3 5% 7% 48%	2090
	11th to 20th	Night	17% 7% 2 5% 3 4 4 4 4 6% 44%	2059
	21st to end	Night	14% 6% 2 5% 3 4 3 4 4 7% 48%	1744
May	1st to 10th	Night	23% 9% 3 6% 3 3 3 3 6% 6% 36%	2823
	11th to 20th	Night	10% 6% 2 6% 4 4 4 3 5% 7% 49%	2823
	21st to end	Night	11% 7% 2 6% 4 4 4 4 6% 9% 42%	3090
June	1st to 10th	Night	5% 3 2 3 3 3 4 4 5% 10% 59%	1710
	11th to 20th	Night	5% 3 2 3 4 3 4 4 6% 9% 58%	1710
	21st to end	Night	4 5% 1 3 4 4 4 3 7% 9% 53%	1710
July	1st to 10th	Night	4 3 1 4 4 4 4 4 6% 10% 56%	2850
	11th to 20th	Night	3 3 1 4 3 3 4 3 7% 11% 59%	2850
	21st to end	Night	8% 6% 2 6% 5% 4 5% 4 7% 10% 43%	3098
August	1st to 10th	Night	1 1 2 1 2 2 2 5% 7% 78%	2836
	11th to 20th	Night	3 2 1 3 2 2 4 3 7% 10% 62%	2979
	21st to end	Night	4 3 1 4 3 3 4 4 7% 10% 57%	3694
September	1st to 10th	Night	8% 5% 2 5% 4 4 4 3 6% 7% 51%	2090
	11th to 20th	Night	9% 6% 2 6% 5% 4 4 3 5% 8% 47%	2090
	21st to end	Night	7% 4 2 4 3 3 3 4 5% 8% 58%	2090
October	1st to 10th	Night	7% 3 1 4 2 3 3 3 4 7% 65%	2090
	11th to 20th	Night	12% 4 1 4 3 3 3 3 4 6% 58%	2234
	21st to end	Night	13% 5% 2 3 2 3 3 2 4 4 60%	2508
November	1st to 10th	Night	13% 6% 2 5% 3 3 3 2 4 6% 53%	2340
	11th to 20th	Night	9% 5% 2 5% 3 3 3 3 4 6% 58%	2505
	21st to end	Night	17% 6% 2 5% 3 3 3 3 4 5% 49%	2600
December	1st to 10th	Night	19% 7% 2 5% 3 3 3 3 5% 6% 43%	4250
	11th to 20th	Night	18% 9% 2 5% 3 3 2 2 3 4 47%	4816
	21st to end	Night	16% 7% 1 4 2 2 2 2 2 3 58%	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	39% 3 4 24% 20% 10%	4078
	11th to 20th	Day	32% 2 5% 18% 26% 17%	4080
	21st to end	Day	48% 2 2 9% 24% 14%	4654
February	1st to 10th	Day	43% 3 5% 12% 22% 15%	2000
	11th to 20th	Day	41% 3 7% 18% 19% 11%	1959
	21st to end	Day	40% 3 4 18% 20% 15%	1570
March	1st to 10th	Day	43% 4 6% 14% 21% 11%	3433
	11th to 20th	Day	51% 7% 8% 10% 16% 8%	3480
	21st to end	Day	40% 4 9% 16% 23% 8%	3814
April	1st to 10th	Day	43% 3 7% 18% 15% 13%	2090
	11th to 20th	Day	44% 4 6% 16% 19% 11%	2067
	21st to end	Day	40% 4 5% 21% 22% 8%	2430
May	1st to 10th	Day	53% 6% 10% 15% 13% 5%	4120
	11th to 20th	Day	40% 6% 8% 17% 24% 4	4086
	21st to end	Day	41% 9% 11% 25% 11% 2	4489
June	1st to 10th	Day	30% 6% 12% 27% 20% 4	2470
	11th to 20th	Day	30% 6% 11% 28% 18% 6%	2470
	21st to end	Day	35% 7% 12% 24% 19% 3	2470
July	1st to 10th	Day	29% 6% 17% 39% 9% 1	4056
	11th to 20th	Day	24% 6% 15% 33% 19% 2	4051
	21st to end	Day	36% 7% 17% 28% 10% 1	4325
August	1st to 10th	Day	11% 4 11% 38% 34% 2	4090
	11th to 20th	Day	17% 5% 22% 40% 14% 3	3894
	21st to end	Day	25% 9% 19% 26% 18% 2	3965
September	1st to 10th	Day	36% 5% 9% 23% 17% 10%	2090
	11th to 20th	Day	38% 6% 12% 20% 19% 5%	2090
	21st to end	Day	29% 4 11% 22% 20% 14%	2090
October	1st to 10th	Day	15% 2 6% 14% 24% 39%	3480
	11th to 20th	Day	19% 3 7% 19% 32% 20%	3178
	21st to end	Day	24% 4 4 21% 33% 14%	3428
November	1st to 10th	Day	38% 3 9% 18% 21% 11%	1958
	11th to 20th	Day	33% 4 7% 19% 21% 16%	1890
	21st to end	Day	39% 3 6% 17% 21% 13%	1800
December	1st to 10th	Day	45% 2 7% 16% 20% 9%	2940
	11th to 20th	Day	42% 4 7% 11% 24% 13%	3389
	21st to end	Day	32% 3 4 17% 28% 15%	4488
January	1st to 10th	Night	36% 2 4 21% 25% 12%	5888
	11th to 20th	Night	24% 2 3 18% 32% 20%	5900
	21st to end	Night	48% 2 3 11% 20% 15%	6352
February	1st to 10th	Night	40% 3 5% 13% 21% 18%	2400
	11th to 20th	Night	40% 2 5% 18% 20% 14%	2338
	21st to end	Night	41% 3 6% 14% 18% 17%	1884
March	1st to 10th	Night	43% 5% 7% 15% 19% 10%	3419
	11th to 20th	Night	51% 5% 9% 12% 14% 8%	3480
	21st to end	Night	42% 5% 8% 15% 22% 8%	3817
April	1st to 10th	Night	37% 4 9% 18% 17% 15%	2090
	11th to 20th	Night	41% 4 10% 15% 17% 14%	2059
	21st to end	Night	37% 4 9% 20% 21% 9%	1744
May	1st to 10th	Night	49% 5% 8% 18% 14% 5%	2850
	11th to 20th	Night	34% 7% 9% 25% 18% 7%	2824
	21st to end	Night	38% 7% 13% 29% 10% 4	3099
June	1st to 10th	Night	21% 6% 16% 30% 23% 4	1710
	11th to 20th	Night	22% 6% 15% 31% 19% 7%	1710
	21st to end	Night	26% 6% 16% 29% 19% 4	1710
July	1st to 10th	Night	23% 7% 24% 38% 7% 1	2805
	11th to 20th	Night	20% 6% 20% 36% 17% 1	2796
	21st to end	Night	35% 7% 22% 28% 8%	2999
August	1st to 10th	Night	8% 4 13% 42% 32% 2	2826
	11th to 20th	Night	18% 5% 28% 33% 14% 2	3006
	21st to end	Night	21% 9% 22% 28% 17% 3	3694
September	1st to 10th	Night	31% 6% 14% 25% 16% 9%	2090
	11th to 20th	Night	35% 7% 14% 22% 15% 7%	2090
	21st to end	Night	25% 5% 13% 24% 19% 14%	2090
October	1st to 10th	Night	14% 3 6% 15% 21% 41%	3480
	11th to 20th	Night	19% 2 8% 17% 33% 22%	3707
	21st to end	Night	24% 3 5% 20% 32% 17%	4120
November	1st to 10th	Night	35% 3 8% 18% 22% 14%	2340
	11th to 20th	Night	30% 3 7% 20% 23% 17%	2505
	21st to end	Night	39% 3 7% 17% 19% 15%	2600
December	1st to 10th	Night	43% 3 6% 18% 20% 10%	4250
	11th to 20th	Night	43% 2 6% 9% 24% 16%	4817
	21st to end	Night	34% 2 3 16% 29% 16%	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	19% 12% 37% 25% 6% 1	300
	11th to 20th	Day	22% 12% 30% 27% 7% 1	300
	21st to end	Day	23% 12% 36% 22% 6%	330
February	1st to 10th	Day	25% 12% 33% 22% 8%	300
	11th to 20th	Day	24% 13% 38% 20% 4	300
	21st to end	Day	26% 10% 44% 18% 1	225
March	1st to 10th	Day	20% 14% 36% 25% 6%	270
	11th to 20th	Day	21% 14% 39% 18% 8%	270
	21st to end	Day	21% 10% 39% 23% 7%	297
April	1st to 10th	Day	20% 18% 38% 19% 4 1	270
	11th to 20th	Day	21% 12% 36% 21% 9%	265
	21st to end	Day	19% 11% 35% 25% 9% 1	610
May	1st to 10th	Day	17% 10% 36% 28% 8%	653
	11th to 20th	Day	17% 6% 28% 31% 15% 3	675
	21st to end	Day	16% 7% 25% 33% 17% 3	734
June	1st to 10th	Day	6% 2 14% 42% 29% 8%	680
	11th to 20th	Day	8% 1 16% 47% 24% 5%	680
	21st to end	Day	6% 3 11% 37% 33% 10%	680
July	1st to 10th	Day	6% 2 7% 39% 34% 12%	671
	11th to 20th	Day	3 1 5% 35% 46% 10%	668
	21st to end	Day	5% 1 4 31% 47% 13%	739
August	1st to 10th	Day	1 1 3 29% 52% 14%	676
	11th to 20th	Day	5% 1 2 30% 46% 16%	557
	21st to end	Day	6% 1 3 19% 42% 28%	416
September	1st to 10th	Day	9% 4 21% 40% 25%	300
	11th to 20th	Day	7% 7% 23% 42% 21%	300
	21st to end	Day	9% 2 5% 29% 36% 19%	300
October	1st to 10th	Day	2 3 12% 32% 39% 12%	300
	11th to 20th	Day	9% 3 22% 33% 27% 6%	300
	21st to end	Day	19% 12% 22% 25% 19% 5%	330
November	1st to 10th	Day	21% 7% 25% 27% 16% 4	300
	11th to 20th	Day	17% 9% 27% 29% 15% 4	300
	21st to end	Day	19% 16% 31% 21% 12% 1	300
December	1st to 10th	Day	17% 8% 32% 26% 13% 4	300
	11th to 20th	Day	20% 13% 33% 23% 10% 1	300
	21st to end	Day	20% 11% 35% 25% 7% 2	330
January	1st to 10th	Night	19% 9% 34% 30% 7%	1000
	11th to 20th	Night	18% 11% 32% 31% 7%	1000
	21st to end	Night	21% 10% 34% 29% 6%	1040
February	1st to 10th	Night	24% 11% 31% 29% 5%	800
	11th to 20th	Night	25% 11% 36% 25% 2	784
	21st to end	Night	24% 14% 41% 19% 3	628
March	1st to 10th	Night	20% 12% 38% 27% 4	570
	11th to 20th	Night	17% 12% 33% 33% 5%	570
	21st to end	Night	15% 11% 44% 26% 4	627
April	1st to 10th	Night	15% 11% 41% 31% 3	570
	11th to 20th	Night	20% 10% 39% 25% 5%	563
	21st to end	Night	17% 8% 43% 23% 9% 1	570
May	1st to 10th	Night	15% 9% 41% 28% 6%	570
	11th to 20th	Night	14% 5% 36% 33% 13% 1	561
	21st to end	Night	11% 5% 32% 37% 15%	617
June	1st to 10th	Night	5% 3 21% 44% 26% 1	570
	11th to 20th	Night	6% 1 29% 48% 16% 1	570
	21st to end	Night	6% 1 16% 47% 27% 3	570
July	1st to 10th	Night	7% 1 10% 49% 29% 5%	555
	11th to 20th	Night	3 8% 57% 29% 3	552
	21st to end	Night	4 1 8% 51% 33% 3	624
August	1st to 10th	Night	1 6% 55% 33% 4	567
	11th to 20th	Night	7% 1 5% 49% 33% 4	564
	21st to end	Night	4 2 4 43% 41% 6%	627
September	1st to 10th	Night	9% 1 7% 43% 36% 5%	570
	11th to 20th	Night	11% 2 6% 38% 41% 3	570
	21st to end	Night	9% 1 11% 39% 35% 5%	570
October	1st to 10th	Night	6% 1 12% 48% 27% 5%	570
	11th to 20th	Night	13% 4 22% 40% 20% 2	714
	21st to end	Night	14% 8% 25% 31% 21% 1	836
November	1st to 10th	Night	15% 4 23% 39% 18% 1	780
	11th to 20th	Night	11% 7% 28% 37% 15% 1	905
	21st to end	Night	20% 8% 30% 30% 12% 1	1000
December	1st to 10th	Night	18% 6% 31% 35% 10%	1000
	11th to 20th	Night	22% 9% 30% 32% 7% 1	1000
	21st to end	Night	20% 8% 32% 32% 7%	1100

Page generated at 2025-02-18 02:26:27. Version ac07

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