University of Maryland Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the University of Maryland Observatory clear sky chart from 2005-11-06 to 2026-10-29.

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	14% 6% 2 6% 4 4 5% 4 6% 8% 41%	149490
	Night	14% 6% 2 6% 4 4 4 4 5% 7% 43%	142606
Transparency	Day	40% 4 11% 24% 17% 3	153084
	Night	41% 3 11% 24% 18% 4	147084
Seeing	Day	21% 12% 28% 24% 11% 4	17591
	Night	18% 11% 28% 29% 11% 1	27030

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	15% 4 2 6% 4 3 4 3 4 6% 48%	4753
	11th to 20th	Day	15% 8% 2 5% 3 3 3 2 3 4 51%	5280
	21st to end	Day	14% 6% 2 7% 4 4 4 3 5% 7% 44%	5780
February	1st to 10th	Day	14% 7% 2 6% 4 5% 5% 4 6% 7% 40%	1890
	11th to 20th	Day	19% 7% 2 5% 4 3 4 4 5% 6% 40%	1917
	21st to end	Day	17% 8% 3 6% 5% 3 4 3 4 5% 41%	1730
March	1st to 10th	Day	19% 5% 2 4 3 4 4 4 5% 7% 43%	4908
	11th to 20th	Day	12% 6% 2 5% 4 4 4 3 5% 6% 50%	5090
	21st to end	Day	17% 6% 2 6% 4 4 4 4 5% 7% 41%	5715
April	1st to 10th	Day	14% 7% 2 6% 4 4 4 4 6% 7% 43%	2520
	11th to 20th	Day	13% 5% 2 5% 4 4 4 3 6% 6% 48%	2494
	21st to end	Day	18% 6% 2 5% 4 3 4 3 6% 7% 42%	2520
May	1st to 10th	Day	18% 7% 2 6% 4 4 4 4 7% 8% 36%	7432
	11th to 20th	Day	18% 6% 2 5% 4 4 4 4 6% 9% 37%	8077
	21st to end	Day	22% 6% 2 6% 4 4 4 3 5% 7% 37%	9045
June	1st to 10th	Day	11% 5% 2 5% 4 5% 4 4 7% 9% 43%	2940
	11th to 20th	Day	11% 5% 2 6% 5% 4 5% 5% 7% 8% 42%	2940
	21st to end	Day	11% 4 2 5% 4 4 5% 5% 8% 10% 43%	2940
July	1st to 10th	Day	10% 5% 2 6% 5% 5% 5% 4 7% 9% 42%	8280
	11th to 20th	Day	9% 7% 3 7% 5% 6% 6% 5% 9% 12% 31%	8104
	21st to end	Day	8% 6% 2 7% 6% 6% 6% 5% 8% 10% 37%	8340
August	1st to 10th	Day	13% 6% 2 8% 6% 6% 6% 5% 8% 10% 29%	6014
	11th to 20th	Day	12% 5% 2 7% 5% 5% 6% 5% 7% 10% 36%	5671
	21st to end	Day	9% 4 2 5% 4 5% 6% 5% 7% 9% 45%	6080
September	1st to 10th	Day	13% 5% 2 5% 4 4 4 3 5% 7% 47%	2520
	11th to 20th	Day	12% 5% 2 5% 5% 3 4 3 6% 9% 45%	2520
	21st to end	Day	21% 8% 3 6% 4 4 4 3 5% 7% 35%	2218
October	1st to 10th	Day	16% 5% 2 5% 3 4 4 3 5% 7% 47%	2100
	11th to 20th	Day	17% 5% 2 4 3 3 3 3 5% 6% 50%	2100
	21st to end	Day	13% 5% 2 5% 3 3 4 3 4 7% 51%	2280
November	1st to 10th	Day	12% 5% 2 4 3 3 4 3 5% 7% 52%	1929
	11th to 20th	Day	13% 6% 2 4 3 4 4 3 7% 6% 49%	1885
	21st to end	Day	15% 6% 2 5% 4 4 3 3 4 6% 48%	1680
December	1st to 10th	Day	12% 5% 2 6% 4 3 5% 4 5% 6% 48%	2680
	11th to 20th	Day	18% 8% 2 4 3 3 3 3 5% 7% 44%	3070
	21st to end	Day	16% 8% 3 7% 4 4 4 4 5% 7% 38%	4048
January	1st to 10th	Night	12% 6% 2 6% 4 4 5% 4 5% 6% 45%	8136
	11th to 20th	Night	14% 7% 2 6% 3 3 3 2 4 5% 51%	7680
	21st to end	Night	14% 7% 2 5% 3 3 3 3 5% 5% 49%	8401
February	1st to 10th	Night	15% 7% 2 6% 4 5% 5% 4 6% 6% 42%	2730
	11th to 20th	Night	19% 7% 2 5% 4 4 3 3 4 6% 43%	2587
	21st to end	Night	16% 7% 2 5% 4 4 4 3 5% 6% 43%	2076
March	1st to 10th	Night	17% 6% 2 6% 3 3 4 2 4 5% 48%	5148
	11th to 20th	Night	11% 5% 2 5% 3 3 4 3 5% 8% 50%	5090
	21st to end	Night	13% 7% 3 7% 4 4 4 3 5% 6% 44%	5459
April	1st to 10th	Night	15% 7% 2 6% 4 4 4 4 5% 7% 40%	2100
	11th to 20th	Night	14% 5% 2 6% 3 3 4 4 6% 7% 45%	2075
	21st to end	Night	17% 5% 2 4 4 4 4 4 6% 8% 43%	2100
May	1st to 10th	Night	19% 8% 3 7% 5% 5% 5% 4 5% 7% 33%	5477
	11th to 20th	Night	20% 6% 2 5% 4 4 5% 3 6% 9% 35%	4781
	21st to end	Night	23% 7% 2 6% 3 4 4 4 5% 7% 33%	5108
June	1st to 10th	Night	9% 5% 2 5% 3 5% 5% 5% 7% 9% 45%	1680
	11th to 20th	Night	11% 6% 3 5% 4 5% 5% 4 6% 10% 40%	1680
	21st to end	Night	9% 5% 2 5% 6% 5% 5% 5% 6% 10% 42%	1680
July	1st to 10th	Night	11% 6% 2 7% 5% 5% 6% 4 6% 8% 39%	4678
	11th to 20th	Night	9% 8% 3 9% 5% 7% 6% 5% 7% 10% 31%	4848
	21st to end	Night	10% 6% 3 7% 4 5% 5% 5% 7% 9% 40%	5736
August	1st to 10th	Night	16% 7% 3 8% 5% 5% 6% 4 7% 10% 29%	4135
	11th to 20th	Night	12% 7% 3 9% 6% 6% 6% 5% 7% 9% 32%	4328
	21st to end	Night	10% 6% 2 7% 4 5% 5% 4 6% 7% 45%	5045
September	1st to 10th	Night	13% 6% 3 6% 4 4 4 3 5% 8% 46%	2100
	11th to 20th	Night	12% 6% 2 5% 3 4 4 3 6% 7% 49%	2100
	21st to end	Night	21% 6% 2 7% 5% 4 4 4 6% 6% 36%	2399
October	1st to 10th	Night	18% 6% 2 5% 3 3 3 3 5% 6% 46%	2520
	11th to 20th	Night	17% 4 2 5% 3 3 3 2 4 6% 52%	2520
	21st to end	Night	14% 5% 1 4 3 3 3 3 5% 7% 52%	2737
November	1st to 10th	Night	12% 5% 2 5% 3 3 3 3 5% 6% 53%	2587
	11th to 20th	Night	14% 6% 2 4 4 3 3 3 4 6% 50%	2730
	21st to end	Night	13% 4 2 4 4 4 4 4 6% 7% 48%	2940
December	1st to 10th	Night	11% 5% 2 5% 4 4 4 3 5% 7% 51%	4720
	11th to 20th	Night	21% 6% 2 5% 4 3 4 3 5% 6% 41%	5345
	21st to end	Night	17% 7% 2 6% 4 4 5% 4 6% 7% 39%	7150

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% 2 6% 27% 20% 5%	4754
	11th to 20th	Day	39% 2 6% 23% 21% 8%	5280
	21st to end	Day	42% 2 9% 32% 12% 3	5780
February	1st to 10th	Day	43% 4 12% 25% 15% 2	1890
	11th to 20th	Day	44% 3 7% 26% 16% 3	1926
	21st to end	Day	47% 3 10% 22% 15% 3	1730
March	1st to 10th	Day	42% 2 8% 23% 18% 7%	4908
	11th to 20th	Day	36% 2 8% 26% 22% 6%	5090
	21st to end	Day	43% 3 10% 20% 18% 6%	5715
April	1st to 10th	Day	40% 4 11% 22% 19% 5%	2520
	11th to 20th	Day	37% 3 10% 24% 21% 6%	2494
	21st to end	Day	41% 3 10% 24% 18% 3	2520
May	1st to 10th	Day	46% 3 13% 21% 16% 1	7465
	11th to 20th	Day	44% 4 10% 28% 13% 1	8081
	21st to end	Day	47% 3 10% 20% 17% 3	9061
June	1st to 10th	Day	36% 5% 13% 26% 18% 2	2940
	11th to 20th	Day	38% 6% 13% 24% 17% 2	2940
	21st to end	Day	35% 6% 14% 25% 18% 2	2940
July	1st to 10th	Day	38% 7% 17% 21% 14% 3	8210
	11th to 20th	Day	42% 7% 14% 25% 10% 1	8029
	21st to end	Day	39% 6% 10% 23% 21% 1	8156
August	1st to 10th	Day	48% 7% 16% 22% 7%	6001
	11th to 20th	Day	41% 6% 15% 25% 11% 1	5710
	21st to end	Day	35% 5% 15% 28% 17% 1	6080
September	1st to 10th	Day	37% 3 13% 22% 20% 4	2520
	11th to 20th	Day	36% 4 8% 27% 21% 4	2520
	21st to end	Day	49% 4 7% 21% 17% 2	2218
October	1st to 10th	Day	37% 3 6% 18% 30% 5%	3360
	11th to 20th	Day	26% 2 6% 30% 27% 9%	3340
	21st to end	Day	23% 2 8% 31% 29% 8%	3618
November	1st to 10th	Day	32% 3 11% 27% 22% 4	1929
	11th to 20th	Day	35% 3 10% 25% 23% 5%	1885
	21st to end	Day	39% 3 9% 29% 18% 3	1680
December	1st to 10th	Day	38% 3 14% 28% 14% 4	2680
	11th to 20th	Day	42% 2 13% 30% 12% 2	3066
	21st to end	Day	45% 3 13% 22% 15% 1	4048
January	1st to 10th	Night	40% 2 8% 26% 19% 4	8137
	11th to 20th	Night	37% 2 7% 23% 23% 8%	7680
	21st to end	Night	39% 2 6% 31% 20% 3	8401
February	1st to 10th	Night	42% 3 12% 25% 16% 2	2730
	11th to 20th	Night	45% 2 9% 22% 18% 5%	2589
	21st to end	Night	43% 2 10% 22% 17% 5%	2076
March	1st to 10th	Night	41% 2 7% 26% 15% 9%	5148
	11th to 20th	Night	34% 2 10% 27% 19% 7%	5090
	21st to end	Night	42% 2 8% 22% 20% 6%	5459
April	1st to 10th	Night	43% 4 9% 20% 20% 4	2100
	11th to 20th	Night	38% 4 11% 22% 20% 5%	2075
	21st to end	Night	39% 4 10% 26% 16% 5%	2100
May	1st to 10th	Night	51% 4 11% 20% 12% 3	5507
	11th to 20th	Night	46% 4 11% 25% 13% 1	4782
	21st to end	Night	51% 3 11% 18% 14% 2	5115
June	1st to 10th	Night	35% 6% 11% 28% 19% 2	1680
	11th to 20th	Night	39% 5% 14% 25% 14% 3	1680
	21st to end	Night	36% 5% 13% 24% 19% 3	1680
July	1st to 10th	Night	42% 7% 16% 20% 13% 2	4640
	11th to 20th	Night	47% 7% 15% 20% 10% 1	4803
	21st to end	Night	39% 5% 14% 23% 18% 1	5606
August	1st to 10th	Night	50% 7% 18% 18% 7%	4124
	11th to 20th	Night	47% 6% 14% 22% 10% 1	4355
	21st to end	Night	38% 4 12% 29% 16% 1	5045
September	1st to 10th	Night	39% 4 10% 23% 19% 5%	2100
	11th to 20th	Night	36% 4 8% 28% 21% 3	2100
	21st to end	Night	48% 4 9% 19% 17% 3	2399
October	1st to 10th	Night	41% 3 8% 22% 21% 5%	4040
	11th to 20th	Night	26% 2 10% 23% 31% 8%	4019
	21st to end	Night	23% 2 8% 32% 26% 8%	4349
November	1st to 10th	Night	34% 2 8% 27% 24% 6%	2587
	11th to 20th	Night	37% 2 9% 24% 22% 6%	2730
	21st to end	Night	35% 3 12% 26% 21% 3	2940
December	1st to 10th	Night	34% 3 12% 28% 21% 3	4720
	11th to 20th	Night	44% 3 12% 20% 17% 4	5348
	21st to end	Night	45% 3 14% 21% 16% 1	7150

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	31% 19% 33% 14% 3	235
	11th to 20th	Day	31% 25% 29% 12% 3	330
	21st to end	Day	25% 23% 35% 12% 4	363
February	1st to 10th	Day	28% 23% 32% 13% 5%	330
	11th to 20th	Day	37% 19% 34% 8% 2	404
	21st to end	Day	30% 17% 33% 17% 2	446
March	1st to 10th	Day	29% 19% 31% 16% 4 1	540
	11th to 20th	Day	31% 17% 31% 15% 4 1	540
	21st to end	Day	26% 19% 34% 15% 5% 1	594
April	1st to 10th	Day	26% 18% 33% 16% 5% 1	540
	11th to 20th	Day	21% 18% 35% 18% 5% 2	532
	21st to end	Day	27% 13% 34% 18% 7% 1	540
May	1st to 10th	Day	26% 16% 30% 19% 6% 3	543
	11th to 20th	Day	23% 13% 28% 25% 9% 2	567
	21st to end	Day	23% 8% 24% 28% 14% 3	617
June	1st to 10th	Day	14% 8% 28% 28% 15% 6%	570
	11th to 20th	Day	18% 7% 27% 28% 15% 4	570
	21st to end	Day	12% 6% 29% 31% 18% 5%	570
July	1st to 10th	Day	14% 1 18% 36% 22% 9%	565
	11th to 20th	Day	6% 3 23% 36% 25% 6%	564
	21st to end	Day	12% 3 23% 36% 20% 6%	620
August	1st to 10th	Day	15% 4 20% 33% 20% 8%	566
	11th to 20th	Day	15% 3 20% 34% 21% 8%	559
	21st to end	Day	10% 4 22% 35% 18% 11%	627
September	1st to 10th	Day	16% 5% 23% 30% 16% 10%	570
	11th to 20th	Day	13% 4 23% 35% 17% 8%	570
	21st to end	Day	29% 8% 24% 26% 9% 5%	570
October	1st to 10th	Day	21% 8% 24% 26% 16% 6%	570
	11th to 20th	Day	19% 15% 29% 21% 14% 2	570
	21st to end	Day	17% 13% 35% 22% 10% 2	615
November	1st to 10th	Day	16% 16% 33% 22% 9% 3	467
	11th to 20th	Day	20% 16% 33% 21% 8% 2	425
	21st to end	Day	29% 20% 28% 16% 7%	220
December	1st to 10th	Day	24% 20% 37% 15% 3 1	220
	11th to 20th	Day	25% 20% 32% 17% 6%	220
	21st to end	Day	27% 19% 29% 19% 5%	242
January	1st to 10th	Night	19% 19% 38% 21% 3	840
	11th to 20th	Night	23% 17% 31% 24% 4	840
	21st to end	Night	21% 17% 34% 26% 1	924
February	1st to 10th	Night	20% 17% 33% 27% 3	840
	11th to 20th	Night	27% 20% 32% 19% 1	823
	21st to end	Night	25% 17% 35% 21% 3	692
March	1st to 10th	Night	22% 16% 36% 23% 3	840
	11th to 20th	Night	23% 18% 35% 19% 4 1	840
	21st to end	Night	20% 18% 38% 20% 4 1	924
April	1st to 10th	Night	21% 20% 34% 20% 4	840
	11th to 20th	Night	17% 17% 32% 27% 6%	831
	21st to end	Night	21% 12% 32% 27% 8%	840
May	1st to 10th	Night	22% 11% 30% 26% 10% 1	717
	11th to 20th	Night	21% 7% 27% 36% 9%	467
	21st to end	Night	18% 5% 26% 36% 15% 1	455
June	1st to 10th	Night	10% 5% 29% 36% 19%	420
	11th to 20th	Night	15% 6% 25% 40% 14% 1	420
	21st to end	Night	11% 4 24% 46% 15%	420
July	1st to 10th	Night	8% 3 21% 43% 23% 2	410
	11th to 20th	Night	8% 2 18% 44% 27% 2	462
	21st to end	Night	10% 2 21% 40% 25% 3	690
August	1st to 10th	Night	11% 3 13% 41% 30% 1	627
	11th to 20th	Night	14% 3 14% 41% 26% 2	727
	21st to end	Night	12% 3 20% 35% 26% 4	924
September	1st to 10th	Night	15% 3 22% 32% 25% 3	840
	11th to 20th	Night	14% 3 15% 38% 25% 4	840
	21st to end	Night	24% 5% 22% 30% 17% 2	840
October	1st to 10th	Night	21% 6% 21% 29% 19% 4	840
	11th to 20th	Night	19% 12% 24% 31% 14% 1	840
	21st to end	Night	17% 12% 30% 28% 13%	913
November	1st to 10th	Night	15% 10% 33% 29% 12% 1	820
	11th to 20th	Night	19% 15% 31% 27% 7% 1	840
	21st to end	Night	19% 14% 32% 26% 8% 1	840
December	1st to 10th	Night	22% 16% 32% 22% 8% 1	840
	11th to 20th	Night	20% 13% 35% 26% 6%	840
	21st to end	Night	20% 15% 30% 31% 4	924

Page generated at 2026-10-26 04:37:47. Version ac07

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