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 2025-01-07.

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 3 5% 7% 49%	105168
	Night	13% 5% 2 5% 3 3 3 3 4 6% 52%	104014
Transparency	Day	34% 5% 10% 21% 21% 9%	108963
	Night	33% 4 10% 20% 21% 12%	108346
Seeing	Day	13% 6% 21% 29% 24% 7%	15098
	Night	14% 6% 26% 36% 16% 1	25092

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% 9% 2 5% 2 3 3 2 4 6% 51%	2901
	11th to 20th	Day	15% 6% 2 3 2 2 2 2 4 5% 56%	2850
	21st to end	Day	18% 8% 3 6% 3 3 3 2 3 6% 45%	3263
February	1st to 10th	Day	19% 7% 3 5% 3 2 3 3 4 5% 46%	1900
	11th to 20th	Day	17% 7% 2 5% 4 2 3 3 5% 5% 47%	1849
	21st to end	Day	19% 6% 2 4 3 3 3 3 3 4 50%	1570
March	1st to 10th	Day	15% 7% 2 7% 4 4 4 3 4 5% 44%	3428
	11th to 20th	Day	21% 7% 3 7% 5% 4 4 4 5% 6% 34%	3480
	21st to end	Day	13% 7% 2 6% 4 4 4 3 5% 7% 44%	3814
April	1st to 10th	Day	14% 9% 2 6% 5% 4 3 3 5% 6% 43%	2090
	11th to 20th	Day	17% 7% 2 7% 4 4 4 3 5% 6% 43%	2066
	21st to end	Day	15% 6% 2 5% 4 3 4 3 4 6% 47%	2439
May	1st to 10th	Day	24% 7% 2 7% 4 4 4 4 6% 7% 32%	4082
	11th to 20th	Day	15% 6% 2 5% 4 4 4 4 5% 6% 45%	4082
	21st to end	Day	13% 5% 2 6% 5% 6% 6% 6% 8% 10% 34%	4477
June	1st to 10th	Day	5% 4 2 6% 5% 4 5% 5% 6% 9% 51%	2470
	11th to 20th	Day	7% 3 2 5% 4 4 5% 4 7% 9% 49%	2470
	21st to end	Day	7% 5% 2 6% 5% 5% 6% 5% 7% 9% 43%	2470
July	1st to 10th	Day	6% 4 1 5% 5% 5% 5% 4 6% 8% 51%	4120
	11th to 20th	Day	4 3 1 4 4 4 5% 4 6% 10% 55%	4120
	21st to end	Day	8% 6% 2 6% 5% 5% 5% 5% 7% 10% 40%	4465
August	1st to 10th	Day	1 1 2 2 3 3 3 5% 8% 72%	4104
	11th to 20th	Day	3 2 1 3 3 3 4 3 6% 11% 62%	3850
	21st to end	Day	4 2 1 5% 4 5% 6% 4 8% 10% 51%	3970
September	1st to 10th	Day	9% 5% 2 5% 4 5% 6% 4 6% 7% 48%	2090
	11th to 20th	Day	10% 5% 2 7% 4 5% 6% 4 5% 8% 45%	2090
	21st to end	Day	6% 4 2 5% 4 4 4 4 5% 7% 55%	2090
October	1st to 10th	Day	7% 4 2 5% 3 3 3 3 4 6% 62%	2090
	11th to 20th	Day	12% 3 1 5% 2 3 3 3 3 6% 57%	1929
	21st to end	Day	12% 4 1 4 2 3 3 2 3 5% 60%	2090
November	1st to 10th	Day	14% 7% 2 5% 3 3 3 3 4 6% 49%	1958
	11th to 20th	Day	10% 5% 1 5% 3 4 4 3 4 5% 55%	1882
	21st to end	Day	16% 5% 2 5% 4 3 4 3 5% 5% 48%	1800
December	1st to 10th	Day	21% 7% 2 5% 4 3 4 3 4 4 44%	2940
	11th to 20th	Day	17% 6% 2 6% 4 3 4 3 5% 6% 44%	3391
	21st to end	Day	14% 5% 2 4 2 2 3 2 3 5% 58%	4488
January	1st to 10th	Night	18% 5% 2 3 2 2 2 2 3 5% 56%	4187
	11th to 20th	Night	13% 4 1 3 2 2 2 1 2 4 65%	4120
	21st to end	Night	19% 6% 2 7% 4 3 3 2 3 6% 45%	4433
February	1st to 10th	Night	18% 8% 2 5% 2 3 3 2 4 5% 49%	2280
	11th to 20th	Night	18% 7% 2 4 3 2 3 2 3 5% 50%	2218
	21st to end	Night	19% 5% 2 4 3 4 3 2 3 5% 48%	1884
March	1st to 10th	Night	17% 7% 2 6% 4 4 4 4 5% 5% 43%	3424
	11th to 20th	Night	20% 8% 3 7% 4 4 4 4 5% 6% 35%	3480
	21st to end	Night	14% 8% 3 6% 5% 4 3 3 4 6% 44%	3817
April	1st to 10th	Night	12% 6% 2 5% 3 4 4 3 4 6% 49%	2090
	11th to 20th	Night	18% 5% 2 5% 4 3 4 4 5% 5% 45%	2063
	21st to end	Night	15% 6% 1 5% 3 3 3 3 4 7% 49%	1736
May	1st to 10th	Night	24% 9% 2 4 3 3 3 3 5% 7% 37%	2823
	11th to 20th	Night	12% 5% 2 5% 4 4 3 4 5% 7% 50%	2823
	21st to end	Night	13% 6% 2 6% 4 4 4 5% 6% 8% 43%	3090
June	1st to 10th	Night	5% 3 1 3 3 3 4 4 6% 9% 60%	1710
	11th to 20th	Night	4 3 2 4 3 4 4 4 5% 9% 58%	1710
	21st to end	Night	6% 4 2 4 4 3 4 3 7% 9% 55%	1710
July	1st to 10th	Night	4 3 1 3 3 4 4 4 7% 9% 58%	2850
	11th to 20th	Night	2 3 1 4 3 3 4 4 6% 11% 60%	2850
	21st to end	Night	7% 6% 2 6% 4 4 5% 4 7% 9% 44%	3098
August	1st to 10th	Night	1 1 2 2 1 2 2 4 7% 78%	2836
	11th to 20th	Night	3 2 1 3 3 2 4 4 6% 10% 63%	2983
	21st to end	Night	4 3 2 3 3 3 4 4 6% 10% 58%	3689
September	1st to 10th	Night	8% 5% 2 5% 4 4 4 4 6% 7% 52%	2090
	11th to 20th	Night	10% 5% 3 6% 4 4 4 3 6% 8% 48%	2090
	21st to end	Night	7% 3 1 4 3 3 3 3 6% 7% 58%	2090
October	1st to 10th	Night	6% 3 1 4 3 3 3 2 4 7% 65%	2090
	11th to 20th	Night	12% 3 1 4 3 3 3 3 4 6% 59%	2232
	21st to end	Night	14% 4 1 3 2 3 2 3 3 4 60%	2508
November	1st to 10th	Night	13% 6% 2 4 4 3 3 2 4 6% 53%	2340
	11th to 20th	Night	9% 5% 1 4 3 3 3 2 4 6% 59%	2514
	21st to end	Night	16% 6% 2 5% 3 3 3 3 4 5% 49%	2600
December	1st to 10th	Night	20% 6% 2 5% 3 3 3 3 5% 6% 44%	4250
	11th to 20th	Night	19% 9% 2 5% 3 3 2 2 3 4 47%	4816
	21st to end	Night	16% 6% 2 4 2 2 2 2 3 4 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	35% 4 4 19% 24% 13%	2902
	11th to 20th	Day	31% 3 6% 16% 30% 13%	2850
	21st to end	Day	43% 3 3 11% 27% 12%	3263
February	1st to 10th	Day	42% 4 6% 12% 22% 14%	1900
	11th to 20th	Day	40% 3 7% 18% 19% 12%	1859
	21st to end	Day	39% 4 4 17% 20% 15%	1570
March	1st to 10th	Day	43% 5% 7% 14% 21% 11%	3428
	11th to 20th	Day	50% 8% 9% 10% 16% 8%	3480
	21st to end	Day	39% 5% 10% 15% 22% 8%	3814
April	1st to 10th	Day	42% 4 7% 18% 14% 13%	2090
	11th to 20th	Day	42% 5% 7% 16% 19% 11%	2066
	21st to end	Day	39% 5% 6% 20% 22% 8%	2439
May	1st to 10th	Day	52% 7% 10% 14% 13% 5%	4120
	11th to 20th	Day	39% 7% 9% 17% 24% 4	4086
	21st to end	Day	40% 10% 12% 25% 11% 2	4489
June	1st to 10th	Day	30% 7% 12% 27% 20% 4	2470
	11th to 20th	Day	30% 7% 12% 27% 18% 6%	2470
	21st to end	Day	35% 8% 12% 23% 19% 3	2470
July	1st to 10th	Day	31% 4 16% 39% 8% 1	4056
	11th to 20th	Day	25% 5% 15% 34% 18% 2	4051
	21st to end	Day	37% 5% 17% 29% 10% 1	4325
August	1st to 10th	Day	12% 3 11% 38% 34% 2	4090
	11th to 20th	Day	18% 4 22% 39% 14% 3	3889
	21st to end	Day	27% 7% 19% 26% 19% 2	3969
September	1st to 10th	Day	35% 6% 10% 22% 17% 10%	2090
	11th to 20th	Day	38% 7% 12% 20% 18% 6%	2090
	21st to end	Day	28% 6% 10% 22% 20% 14%	2090
October	1st to 10th	Day	15% 3 6% 13% 24% 39%	3480
	11th to 20th	Day	20% 2 7% 19% 31% 21%	3182
	21st to end	Day	26% 2 3 21% 34% 13%	3428
November	1st to 10th	Day	37% 4 9% 18% 21% 11%	1958
	11th to 20th	Day	32% 5% 8% 18% 21% 16%	1882
	21st to end	Day	39% 4 6% 17% 21% 14%	1800
December	1st to 10th	Day	45% 2 6% 17% 20% 9%	2940
	11th to 20th	Day	42% 3 6% 12% 24% 13%	3389
	21st to end	Day	32% 2 5% 17% 28% 16%	4488
January	1st to 10th	Night	34% 2 4 18% 26% 17%	4187
	11th to 20th	Night	27% 2 4 16% 30% 20%	4120
	21st to end	Night	43% 4 4 13% 24% 12%	4433
February	1st to 10th	Night	40% 4 5% 13% 20% 18%	2280
	11th to 20th	Night	39% 3 6% 17% 20% 15%	2218
	21st to end	Night	41% 3 6% 14% 18% 17%	1884
March	1st to 10th	Night	42% 7% 7% 15% 19% 10%	3424
	11th to 20th	Night	50% 7% 10% 12% 14% 8%	3480
	21st to end	Night	41% 6% 9% 15% 21% 8%	3817
April	1st to 10th	Night	36% 6% 9% 18% 17% 15%	2090
	11th to 20th	Night	40% 5% 10% 14% 17% 14%	2063
	21st to end	Night	37% 6% 10% 19% 21% 9%	1736
May	1st to 10th	Night	48% 6% 9% 17% 14% 5%	2850
	11th to 20th	Night	33% 7% 10% 24% 18% 8%	2824
	21st to end	Night	37% 8% 13% 28% 9% 4	3099
June	1st to 10th	Night	20% 7% 16% 30% 23% 4	1710
	11th to 20th	Night	22% 6% 16% 30% 20% 7%	1710
	21st to end	Night	26% 6% 16% 29% 18% 4	1710
July	1st to 10th	Night	23% 7% 24% 38% 7% 1	2805
	11th to 20th	Night	21% 5% 20% 37% 17% 1	2796
	21st to end	Night	36% 5% 23% 28% 8%	2999
August	1st to 10th	Night	8% 3 13% 42% 32% 2	2826
	11th to 20th	Night	18% 4 28% 34% 14% 2	3010
	21st to end	Night	22% 8% 22% 28% 17% 2	3689
September	1st to 10th	Night	31% 6% 14% 24% 16% 9%	2090
	11th to 20th	Night	35% 7% 14% 22% 15% 7%	2090
	21st to end	Night	25% 5% 14% 24% 19% 14%	2090
October	1st to 10th	Night	15% 2 6% 14% 20% 41%	3480
	11th to 20th	Night	19% 2 8% 17% 32% 22%	3705
	21st to end	Night	25% 2 4 20% 33% 16%	4120
November	1st to 10th	Night	35% 4 8% 18% 22% 14%	2340
	11th to 20th	Night	29% 4 8% 20% 22% 17%	2514
	21st to end	Night	39% 3 7% 16% 19% 15%	2600
December	1st to 10th	Night	44% 2 6% 19% 20% 10%	4250
	11th to 20th	Night	43% 2 6% 9% 23% 16%	4817
	21st to end	Night	35% 2 3 16% 28% 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% 11% 37% 25% 6% 1	288
	11th to 20th	Day	23% 13% 28% 28% 7% 1	270
	21st to end	Day	21% 12% 37% 23% 7%	297
February	1st to 10th	Day	26% 13% 32% 21% 8%	270
	11th to 20th	Day	24% 13% 39% 20% 4	270
	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	20% 13% 36% 21% 9%	264
	21st to end	Day	19% 12% 35% 25% 8% 1	619
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%	552
	21st to end	Day	6% 1 3 20% 42% 28%	420
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% 33% 30% 8%	981
	11th to 20th	Night	18% 11% 31% 32% 7%	950
	21st to end	Night	20% 10% 34% 30% 6%	985
February	1st to 10th	Night	24% 11% 31% 28% 5%	760
	11th to 20th	Night	24% 11% 37% 25% 3	744
	21st to end	Night	24% 14% 41% 19% 3	628
March	1st to 10th	Night	20% 12% 37% 27% 4	575
	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	712
	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	12% 7% 28% 37% 15% 1	914
	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 2025-01-04 02:24:00. Version ac07

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