Georgetown Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Georgetown Observatory clear sky chart from 2005-11-06 to 2026-05-02.

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% 5% 2 5% 4 4 4 4 5% 7% 48%	132083
	Night	13% 5% 2 5% 3 3 3 3 5% 6% 52%	130747
Transparency	Day	35% 4 9% 21% 20% 10%	135839
	Night	33% 4 10% 20% 21% 13%	135055
Seeing	Day	13% 6% 21% 29% 23% 7%	16801
	Night	15% 7% 26% 36% 15% 1	26819

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	13% 8% 2 5% 3 3 3 3 4 6% 51%	5289
	11th to 20th	Day	12% 6% 2 4 2 3 3 2 5% 6% 56%	5310
	21st to end	Day	23% 8% 2 7% 3 3 3 2 3 5% 41%	6052
February	1st to 10th	Day	19% 7% 3 5% 3 3 3 3 4 5% 47%	2100
	11th to 20th	Day	17% 7% 2 5% 4 3 3 3 5% 6% 46%	2049
	21st to end	Day	19% 5% 2 4 3 3 3 2 3 4 52%	1730
March	1st to 10th	Day	13% 6% 2 6% 3 4 3 2 4 6% 50%	5038
	11th to 20th	Day	14% 5% 2 5% 3 3 3 3 4 6% 51%	5090
	21st to end	Day	13% 7% 2 6% 4 3 4 3 5% 7% 46%	5583
April	1st to 10th	Day	14% 9% 2 6% 5% 4 3 3 5% 6% 42%	2310
	11th to 20th	Day	17% 7% 2 7% 4 4 4 3 5% 6% 42%	2286
	21st to end	Day	15% 6% 2 6% 4 3 4 3 5% 6% 45%	2696
May	1st to 10th	Day	20% 7% 2 6% 4 4 4 5% 6% 7% 34%	5876
	11th to 20th	Day	11% 6% 2 5% 4 4 4 4 5% 8% 45%	5862
	21st to end	Day	13% 6% 2 7% 5% 6% 5% 5% 8% 9% 34%	6434
June	1st to 10th	Day	5% 4 2 6% 5% 4 5% 5% 6% 9% 50%	2600
	11th to 20th	Day	7% 3 2 5% 4 4 5% 4 7% 10% 49%	2600
	21st to end	Day	7% 5% 2 6% 5% 5% 6% 5% 7% 9% 44%	2600
July	1st to 10th	Day	10% 6% 2 7% 5% 5% 5% 5% 6% 9% 40%	5900
	11th to 20th	Day	4 3 2 5% 4 4 5% 4 7% 11% 51%	5900
	21st to end	Day	6% 5% 2 5% 5% 5% 5% 5% 7% 10% 45%	6422
August	1st to 10th	Day	1 1 2 2 3 3 3 5% 8% 72%	4234
	11th to 20th	Day	3 2 1 3 3 3 4 3 6% 11% 61%	3974
	21st to end	Day	4 2 1 5% 4 5% 6% 4 8% 10% 51%	4096
September	1st to 10th	Day	9% 5% 2 5% 4 5% 6% 4 6% 7% 49%	2200
	11th to 20th	Day	10% 5% 2 6% 4 5% 6% 4 5% 8% 46%	2200
	21st to end	Day	6% 4 2 5% 4 4 4 4 5% 7% 55%	2200
October	1st to 10th	Day	6% 3 2 4 3 3 3 3 4 6% 63%	2200
	11th to 20th	Day	12% 3 1 5% 2 3 3 3 3 6% 58%	2030
	21st to end	Day	11% 4 1 5% 3 3 3 2 3 5% 61%	2200
November	1st to 10th	Day	13% 6% 2 5% 3 3 3 3 4 5% 51%	2058
	11th to 20th	Day	10% 5% 2 5% 3 4 4 3 4 6% 55%	1976
	21st to end	Day	16% 5% 2 6% 4 3 4 3 5% 5% 47%	1890
December	1st to 10th	Day	21% 7% 2 5% 3 3 4 3 4 4 45%	3030
	11th to 20th	Day	17% 6% 2 6% 4 3 4 3 5% 6% 45%	3481
	21st to end	Day	14% 5% 2 4 2 3 3 2 3 5% 58%	4587
January	1st to 10th	Night	14% 6% 2 4 3 2 3 2 3 5% 56%	7634
	11th to 20th	Night	9% 4 1 3 2 2 2 2 5% 5% 65%	7680
	21st to end	Night	27% 6% 2 6% 3 3 3 2 3 5% 40%	8205
February	1st to 10th	Night	17% 7% 2 5% 2 3 3 2 4 5% 50%	2520
	11th to 20th	Night	18% 7% 2 4 3 2 3 2 3 5% 49%	2458
	21st to end	Night	19% 5% 2 4 3 4 3 2 3 5% 50%	2076
March	1st to 10th	Night	15% 6% 2 5% 4 3 3 4 5% 6% 47%	5034
	11th to 20th	Night	14% 6% 2 5% 3 3 3 3 5% 6% 50%	5090
	21st to end	Night	16% 9% 2 5% 4 4 3 3 4 6% 45%	5588
April	1st to 10th	Night	13% 6% 2 5% 3 4 4 4 4 6% 48%	2310
	11th to 20th	Night	17% 6% 2 5% 4 3 4 4 6% 5% 44%	2283
	21st to end	Night	16% 7% 2 5% 3 3 3 3 4 7% 47%	1918
May	1st to 10th	Night	23% 8% 2 5% 4 3 3 3 5% 7% 37%	4062
	11th to 20th	Night	9% 6% 2 6% 4 5% 4 4 5% 7% 49%	4053
	21st to end	Night	13% 7% 2 7% 4 4 5% 4 6% 8% 40%	4443
June	1st to 10th	Night	5% 4 1 3 3 3 4 3 6% 10% 59%	1800
	11th to 20th	Night	5% 3 2 4 3 4 5% 4 6% 9% 58%	1800
	21st to end	Night	6% 4 2 4 4 3 4 3 7% 10% 55%	1800
July	1st to 10th	Night	8% 6% 2 6% 4 4 4 4 7% 10% 44%	4078
	11th to 20th	Night	2 3 1 4 3 3 4 4 7% 12% 57%	4077
	21st to end	Night	5% 5% 2 5% 3 4 5% 4 7% 10% 50%	4449
August	1st to 10th	Night	1 1 2 2 1 2 2 4 7% 78%	2926
	11th to 20th	Night	3 2 1 3 3 2 4 4 6% 10% 62%	3079
	21st to end	Night	4 3 2 3 3 3 4 4 6% 10% 58%	3805
September	1st to 10th	Night	8% 5% 2 5% 4 5% 4 4 5% 7% 51%	2200
	11th to 20th	Night	9% 5% 3 6% 4 4 4 3 6% 8% 48%	2200
	21st to end	Night	7% 3 1 4 3 3 3 3 6% 7% 59%	2200
October	1st to 10th	Night	6% 3 1 4 3 3 3 2 4 7% 65%	2200
	11th to 20th	Night	11% 3 1 4 3 3 2 3 4 5% 60%	2350
	21st to end	Night	13% 4 1 3 3 2 2 2 3 4 60%	2640
November	1st to 10th	Night	13% 5% 2 4 3 3 3 2 4 6% 55%	2460
	11th to 20th	Night	9% 5% 1 4 3 2 3 2 4 6% 59%	2640
	21st to end	Night	17% 6% 2 5% 3 3 3 3 4 5% 49%	2730
December	1st to 10th	Night	19% 6% 2 5% 3 2 3 3 5% 6% 44%	4380
	11th to 20th	Night	19% 9% 2 5% 3 3 2 2 3 4 47%	4946
	21st to end	Night	16% 6% 2 4 2 2 2 2 3 4 58%	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	37% 2 4 25% 23% 9%	5290
	11th to 20th	Day	31% 2 4 17% 28% 18%	5310
	21st to end	Day	49% 2 3 12% 23% 11%	6052
February	1st to 10th	Day	42% 4 6% 12% 22% 15%	2100
	11th to 20th	Day	41% 3 7% 19% 19% 11%	2059
	21st to end	Day	38% 4 4 18% 22% 15%	1730
March	1st to 10th	Day	36% 3 5% 18% 18% 20%	5038
	11th to 20th	Day	35% 5% 6% 9% 16% 28%	5090
	21st to end	Day	40% 4 8% 20% 21% 8%	5585
April	1st to 10th	Day	43% 4 7% 18% 14% 13%	2310
	11th to 20th	Day	44% 4 7% 16% 18% 11%	2286
	21st to end	Day	41% 5% 7% 20% 20% 8%	2696
May	1st to 10th	Day	49% 5% 10% 18% 15% 4	5914
	11th to 20th	Day	37% 5% 11% 17% 24% 6%	5866
	21st to end	Day	43% 7% 10% 23% 14% 3	6447
June	1st to 10th	Day	30% 7% 13% 27% 20% 4	2600
	11th to 20th	Day	30% 7% 11% 27% 19% 6%	2600
	21st to end	Day	34% 8% 13% 24% 18% 3	2600
July	1st to 10th	Day	41% 4 19% 30% 6% 1	5836
	11th to 20th	Day	29% 3 16% 34% 16% 2	5831
	21st to end	Day	33% 5% 16% 28% 16% 3	6240
August	1st to 10th	Day	12% 3 11% 39% 33% 1	4220
	11th to 20th	Day	18% 4 22% 39% 14% 3	4013
	21st to end	Day	27% 6% 19% 26% 19% 2	4095
September	1st to 10th	Day	35% 6% 10% 22% 18% 10%	2200
	11th to 20th	Day	36% 6% 12% 21% 19% 5%	2200
	21st to end	Day	28% 5% 10% 22% 20% 15%	2200
October	1st to 10th	Day	15% 3 6% 14% 25% 38%	3590
	11th to 20th	Day	19% 2 7% 19% 31% 21%	3283
	21st to end	Day	26% 2 3 21% 34% 14%	3538
November	1st to 10th	Day	36% 4 9% 17% 23% 11%	2058
	11th to 20th	Day	32% 5% 7% 19% 21% 16%	1976
	21st to end	Day	40% 4 6% 16% 21% 13%	1890
December	1st to 10th	Day	45% 2 6% 17% 21% 9%	3030
	11th to 20th	Day	42% 3 6% 12% 24% 13%	3479
	21st to end	Day	32% 2 5% 17% 28% 16%	4587
January	1st to 10th	Night	35% 1 4 20% 26% 13%	7635
	11th to 20th	Night	24% 1 3 18% 31% 22%	7680
	21st to end	Night	49% 2 3 11% 22% 13%	8205
February	1st to 10th	Night	39% 3 5% 13% 22% 18%	2520
	11th to 20th	Night	40% 3 6% 18% 19% 14%	2458
	21st to end	Night	39% 3 6% 13% 21% 17%	2076
March	1st to 10th	Night	38% 5% 5% 16% 21% 15%	5034
	11th to 20th	Night	37% 5% 8% 11% 15% 25%	5090
	21st to end	Night	42% 4 7% 17% 21% 9%	5588
April	1st to 10th	Night	38% 5% 9% 18% 16% 15%	2310
	11th to 20th	Night	41% 5% 10% 14% 17% 13%	2283
	21st to end	Night	39% 5% 11% 18% 19% 9%	1918
May	1st to 10th	Night	48% 4 8% 20% 14% 5%	4089
	11th to 20th	Night	35% 5% 10% 21% 19% 9%	4054
	21st to end	Night	42% 5% 13% 25% 11% 4	4452
June	1st to 10th	Night	21% 7% 16% 30% 23% 3	1800
	11th to 20th	Night	23% 5% 15% 30% 20% 6%	1800
	21st to end	Night	26% 6% 17% 29% 18% 4	1800
July	1st to 10th	Night	36% 5% 25% 28% 5% 1	4035
	11th to 20th	Night	21% 4 24% 36% 14% 1	4026
	21st to end	Night	30% 5% 22% 26% 13% 4	4320
August	1st to 10th	Night	8% 3 12% 43% 31% 2	2916
	11th to 20th	Night	19% 4 28% 34% 13% 2	3106
	21st to end	Night	22% 8% 22% 28% 17% 2	3805
September	1st to 10th	Night	31% 6% 14% 24% 16% 9%	2200
	11th to 20th	Night	34% 7% 14% 23% 15% 7%	2200
	21st to end	Night	24% 5% 13% 26% 18% 14%	2200
October	1st to 10th	Night	15% 2 6% 14% 21% 41%	3590
	11th to 20th	Night	19% 2 8% 18% 32% 22%	3823
	21st to end	Night	25% 2 4 20% 32% 17%	4252
November	1st to 10th	Night	33% 4 7% 18% 24% 14%	2460
	11th to 20th	Night	29% 3 8% 21% 22% 17%	2640
	21st to end	Night	39% 3 7% 17% 19% 15%	2730
December	1st to 10th	Night	43% 2 6% 18% 20% 10%	4380
	11th to 20th	Night	43% 2 6% 9% 23% 16%	4947
	21st to end	Night	34% 2 3 16% 28% 16%	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	19% 12% 37% 25% 7% 1	330
	11th to 20th	Day	21% 12% 31% 27% 7% 2	330
	21st to end	Day	23% 13% 37% 22% 6%	363
February	1st to 10th	Day	25% 11% 34% 23% 7%	330
	11th to 20th	Day	24% 12% 39% 21% 4	330
	21st to end	Day	25% 10% 43% 19% 3	273
March	1st to 10th	Day	21% 14% 35% 25% 5%	330
	11th to 20th	Day	19% 16% 40% 18% 7%	330
	21st to end	Day	19% 9% 38% 25% 7% 1	363
April	1st to 10th	Day	21% 16% 37% 21% 5% 1	330
	11th to 20th	Day	21% 12% 37% 22% 8%	324
	21st to end	Day	21% 12% 35% 23% 8% 1	716
May	1st to 10th	Day	18% 9% 36% 28% 8%	709
	11th to 20th	Day	17% 7% 30% 30% 15% 2	725
	21st to end	Day	16% 6% 25% 34% 17% 3	789
June	1st to 10th	Day	6% 2 16% 42% 27% 7%	730
	11th to 20th	Day	8% 2 16% 46% 23% 4	730
	21st to end	Day	6% 3 12% 38% 32% 10%	730
July	1st to 10th	Day	9% 2 7% 38% 34% 12%	721
	11th to 20th	Day	4 1 5% 36% 45% 9%	718
	21st to end	Day	5% 1 4 31% 46% 13%	794
August	1st to 10th	Day	2 1 3 29% 51% 14%	726
	11th to 20th	Day	5% 1 2 30% 46% 16%	596
	21st to end	Day	7% 1 3 20% 41% 28%	458
September	1st to 10th	Day	10% 5% 22% 39% 23%	330
	11th to 20th	Day	6% 6% 24% 43% 20%	330
	21st to end	Day	9% 2 6% 29% 36% 18%	330
October	1st to 10th	Day	2 2 11% 32% 40% 13%	330
	11th to 20th	Day	8% 3 23% 33% 25% 8%	330
	21st to end	Day	18% 11% 22% 25% 19% 4	363
November	1st to 10th	Day	19% 7% 25% 27% 16% 5%	330
	11th to 20th	Day	16% 8% 25% 32% 15% 4	330
	21st to end	Day	19% 15% 32% 21% 12% 1	330
December	1st to 10th	Day	18% 8% 33% 26% 12% 4	330
	11th to 20th	Day	19% 12% 33% 23% 11% 2	330
	21st to end	Day	19% 10% 34% 26% 8% 2	363
January	1st to 10th	Night	19% 9% 34% 30% 7%	1050
	11th to 20th	Night	17% 11% 33% 31% 7%	1050
	21st to end	Night	21% 10% 33% 29% 6%	1088
February	1st to 10th	Night	24% 11% 31% 30% 5%	840
	11th to 20th	Night	24% 12% 36% 25% 3	824
	21st to end	Night	23% 13% 40% 21% 3	692
March	1st to 10th	Night	22% 12% 37% 26% 3	635
	11th to 20th	Night	17% 12% 34% 31% 5%	630
	21st to end	Night	16% 10% 43% 27% 4	693
April	1st to 10th	Night	16% 10% 40% 31% 3	630
	11th to 20th	Night	22% 10% 38% 25% 5%	623
	21st to end	Night	20% 8% 42% 22% 8%	630
May	1st to 10th	Night	16% 10% 40% 28% 6%	603
	11th to 20th	Night	14% 6% 36% 32% 12% 1	591
	21st to end	Night	12% 5% 32% 37% 14%	650
June	1st to 10th	Night	6% 3 23% 43% 25% 1	600
	11th to 20th	Night	6% 1 29% 49% 15% 1	600
	21st to end	Night	6% 1 16% 49% 26% 3	600
July	1st to 10th	Night	8% 1 10% 48% 29% 4	585
	11th to 20th	Night	4 8% 58% 28% 2	582
	21st to end	Night	4 1 8% 50% 33% 3	657
August	1st to 10th	Night	1 6% 56% 33% 4	597
	11th to 20th	Night	6% 1 6% 50% 32% 4	594
	21st to end	Night	4 2 4 42% 42% 6%	660
September	1st to 10th	Night	9% 1 7% 43% 36% 5%	600
	11th to 20th	Night	10% 2 6% 39% 40% 3	600
	21st to end	Night	9% 1 11% 41% 34% 5%	600
October	1st to 10th	Night	6% 1 11% 48% 29% 5%	600
	11th to 20th	Night	12% 3 21% 40% 21% 2	750
	21st to end	Night	14% 9% 25% 31% 20% 1	880
November	1st to 10th	Night	14% 4 23% 39% 19% 1	820
	11th to 20th	Night	12% 6% 28% 37% 15% 1	960
	21st to end	Night	20% 8% 29% 30% 12% 1	1050
December	1st to 10th	Night	18% 7% 31% 34% 10%	1050
	11th to 20th	Night	22% 9% 30% 32% 6% 1	1050
	21st to end	Night	20% 8% 31% 33% 7%	1155

Page generated at 2026-04-29 01:21:40. Version ac07

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