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 2025-07-06.

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	15% 6% 2 6% 4 4 5% 4 6% 8% 42%	119973
	Night	15% 6% 2 6% 4 4 4 3 5% 7% 44%	118833
Transparency	Day	40% 4 11% 25% 17% 4	123608
	Night	40% 3 11% 24% 18% 4	123336
Seeing	Day	21% 12% 28% 24% 11% 4	15824
	Night	19% 11% 28% 29% 12% 1	25330

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 4 3 3 4 3 5% 6% 52%	3661
	11th to 20th	Day	15% 9% 3 5% 4 3 4 2 3 4 48%	4060
	21st to end	Day	15% 7% 2 7% 4 4 4 3 4 6% 44%	4466
February	1st to 10th	Day	15% 7% 2 6% 5% 5% 5% 4 5% 7% 39%	1800
	11th to 20th	Day	19% 7% 2 5% 4 3 4 4 5% 6% 40%	1823
	21st to end	Day	17% 8% 3 6% 5% 3 4 3 3 5% 41%	1650
March	1st to 10th	Day	19% 5% 2 4 3 4 4 4 5% 7% 43%	4801
	11th to 20th	Day	12% 6% 2 5% 4 4 4 3 5% 6% 50%	4980
	21st to end	Day	17% 6% 2 6% 4 4 4 4 5% 7% 41%	5592
April	1st to 10th	Day	14% 7% 2 6% 4 5% 4 4 6% 7% 43%	2400
	11th to 20th	Day	13% 6% 2 5% 4 4 4 3 6% 5% 48%	2374
	21st to end	Day	18% 6% 2 6% 4 3 4 3 6% 8% 42%	2400
May	1st to 10th	Day	21% 7% 3 6% 4 4 5% 4 7% 9% 31%	5720
	11th to 20th	Day	22% 6% 2 6% 4 4 4 4 6% 9% 31%	6214
	21st to end	Day	16% 5% 2 6% 4 4 5% 4 6% 8% 39%	6934
June	1st to 10th	Day	12% 5% 2 5% 4 5% 5% 4 7% 9% 43%	2800
	11th to 20th	Day	12% 5% 2 6% 5% 4 5% 5% 7% 8% 41%	2800
	21st to end	Day	10% 4 2 5% 4 4 5% 5% 8% 11% 43%	2800
July	1st to 10th	Day	6% 4 2 6% 5% 6% 7% 5% 8% 10% 42%	4510
	11th to 20th	Day	9% 6% 2 5% 5% 5% 5% 5% 8% 11% 37%	4348
	21st to end	Day	11% 7% 2 7% 6% 5% 6% 5% 7% 9% 36%	4466
August	1st to 10th	Day	16% 6% 2 6% 5% 5% 5% 5% 7% 10% 32%	4104
	11th to 20th	Day	11% 5% 2 6% 5% 5% 6% 5% 7% 9% 38%	3853
	21st to end	Day	10% 5% 2 6% 4 4 5% 4 7% 7% 46%	4180
September	1st to 10th	Day	14% 6% 2 5% 4 4 4 3 5% 7% 45%	2280
	11th to 20th	Day	11% 5% 2 5% 5% 3 4 3 6% 9% 46%	2280
	21st to end	Day	20% 7% 3 6% 3 4 4 4 5% 7% 37%	2007
October	1st to 10th	Day	17% 6% 2 5% 3 4 4 3 5% 6% 46%	1900
	11th to 20th	Day	17% 5% 2 4 3 3 3 3 5% 6% 50%	1900
	21st to end	Day	14% 5% 2 5% 4 3 4 3 4 7% 49%	2090
November	1st to 10th	Day	12% 5% 2 4 3 3 4 3 5% 7% 52%	1835
	11th to 20th	Day	14% 5% 2 4 3 3 4 3 7% 6% 49%	1795
	21st to end	Day	15% 6% 2 5% 4 4 3 4 4 6% 48%	1600
December	1st to 10th	Day	12% 5% 2 6% 4 3 5% 4 6% 6% 48%	2600
	11th to 20th	Day	18% 8% 2 4 3 3 3 3 5% 7% 44%	2990
	21st to end	Day	16% 8% 3 7% 4 4 5% 4 5% 7% 38%	3960
January	1st to 10th	Night	13% 6% 2 5% 4 3 4 3 4 6% 49%	6280
	11th to 20th	Night	14% 6% 2 5% 3 3 3 2 5% 5% 52%	5900
	21st to end	Night	15% 8% 2 6% 3 3 3 3 4 5% 48%	6490
February	1st to 10th	Night	16% 7% 2 5% 4 4 5% 4 6% 6% 41%	2600
	11th to 20th	Night	19% 7% 2 5% 4 4 3 3 4 6% 44%	2461
	21st to end	Night	17% 7% 2 5% 4 4 4 3 5% 6% 43%	1980
March	1st to 10th	Night	17% 6% 2 6% 3 3 4 2 4 5% 48%	5035
	11th to 20th	Night	11% 5% 2 5% 3 3 4 3 5% 8% 50%	4980
	21st to end	Night	13% 7% 3 7% 4 4 4 3 5% 6% 44%	5340
April	1st to 10th	Night	15% 7% 2 6% 4 4 5% 5% 5% 7% 40%	2000
	11th to 20th	Night	14% 6% 2 5% 3 3 4 4 5% 6% 46%	1975
	21st to end	Night	17% 5% 2 4 4 4 4 4 5% 8% 43%	2000
May	1st to 10th	Night	21% 9% 3 7% 5% 5% 5% 4 6% 7% 30%	4217
	11th to 20th	Night	23% 7% 2 6% 3 4 5% 4 6% 8% 32%	3677
	21st to end	Night	18% 6% 2 6% 4 5% 4 4 7% 9% 35%	3920
June	1st to 10th	Night	10% 5% 2 5% 3 5% 5% 5% 7% 9% 44%	1600
	11th to 20th	Night	12% 6% 3 5% 4 5% 5% 4 6% 10% 41%	1600
	21st to end	Night	9% 5% 2 5% 6% 5% 5% 4 6% 10% 42%	1600
July	1st to 10th	Night	6% 4 2 8% 6% 5% 6% 5% 7% 9% 41%	2560
	11th to 20th	Night	8% 6% 3 7% 5% 6% 6% 5% 7% 10% 38%	2607
	21st to end	Night	11% 7% 3 7% 4 5% 5% 5% 7% 8% 37%	3087
August	1st to 10th	Night	18% 6% 2 7% 5% 5% 5% 4 7% 10% 33%	2825
	11th to 20th	Night	11% 7% 3 10% 6% 5% 6% 4 6% 8% 35%	2943
	21st to end	Night	10% 5% 2 6% 3 4 4 4 5% 8% 49%	3476
September	1st to 10th	Night	14% 6% 3 6% 4 4 4 3 5% 7% 45%	1900
	11th to 20th	Night	11% 6% 2 5% 4 4 3 2 6% 7% 50%	1900
	21st to end	Night	20% 6% 2 6% 4 4 4 4 6% 6% 38%	2170
October	1st to 10th	Night	19% 6% 2 5% 3 3 4 3 5% 6% 45%	2280
	11th to 20th	Night	16% 4 2 5% 3 3 3 3 4 6% 52%	2280
	21st to end	Night	15% 5% 1 4 3 3 3 4 5% 7% 50%	2508
November	1st to 10th	Night	12% 5% 2 5% 3 3 3 3 5% 6% 53%	2461
	11th to 20th	Night	15% 6% 2 4 4 2 3 3 5% 6% 50%	2600
	21st to end	Night	13% 4 2 4 4 4 4 4 6% 7% 48%	2800
December	1st to 10th	Night	11% 5% 2 5% 4 4 4 3 5% 7% 52%	4580
	11th to 20th	Night	21% 6% 2 5% 4 3 4 3 5% 6% 41%	5205
	21st to end	Night	17% 7% 2 6% 4 4 5% 4 6% 7% 39%	6996

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% 3 5% 28% 24% 6%	3662
	11th to 20th	Day	42% 2 7% 22% 20% 7%	4060
	21st to end	Day	45% 1 9% 28% 14% 3	4466
February	1st to 10th	Day	44% 4 12% 25% 14% 1	1800
	11th to 20th	Day	44% 3 7% 26% 17% 3	1832
	21st to end	Day	46% 3 10% 21% 16% 3	1650
March	1st to 10th	Day	41% 2 8% 24% 18% 7%	4801
	11th to 20th	Day	36% 2 8% 26% 22% 6%	4980
	21st to end	Day	43% 3 10% 20% 19% 6%	5592
April	1st to 10th	Day	41% 4 10% 21% 18% 5%	2400
	11th to 20th	Day	37% 3 9% 23% 21% 6%	2374
	21st to end	Day	41% 3 10% 24% 19% 4	2400
May	1st to 10th	Day	49% 4 15% 19% 11% 2	5753
	11th to 20th	Day	50% 4 10% 25% 10% 1	6218
	21st to end	Day	42% 3 13% 23% 16% 2	6949
June	1st to 10th	Day	37% 5% 14% 26% 17% 2	2800
	11th to 20th	Day	39% 7% 13% 24% 16% 2	2800
	21st to end	Day	34% 6% 15% 25% 19% 2	2800
July	1st to 10th	Day	35% 7% 19% 21% 13% 5%	4440
	11th to 20th	Day	38% 6% 16% 28% 11% 2	4273
	21st to end	Day	43% 6% 10% 21% 18% 1	4324
August	1st to 10th	Day	46% 5% 15% 25% 8% 1	4091
	11th to 20th	Day	40% 6% 11% 28% 14% 1	3892
	21st to end	Day	35% 5% 12% 28% 17% 2	4180
September	1st to 10th	Day	39% 3 12% 20% 20% 4	2280
	11th to 20th	Day	35% 4 8% 27% 22% 4	2280
	21st to end	Day	47% 3 6% 22% 18% 3	2007
October	1st to 10th	Day	38% 3 6% 17% 30% 5%	3160
	11th to 20th	Day	26% 2 6% 30% 27% 9%	3140
	21st to end	Day	24% 2 7% 31% 29% 7%	3428
November	1st to 10th	Day	33% 3 11% 27% 21% 4	1835
	11th to 20th	Day	34% 3 10% 25% 23% 5%	1795
	21st to end	Day	38% 3 9% 29% 18% 3	1600
December	1st to 10th	Day	37% 3 14% 28% 14% 4	2600
	11th to 20th	Day	43% 2 13% 29% 12% 2	2986
	21st to end	Day	45% 3 13% 22% 16% 1	3960
January	1st to 10th	Night	38% 2 6% 26% 24% 5%	6280
	11th to 20th	Night	36% 2 7% 25% 22% 7%	5900
	21st to end	Night	41% 1 7% 29% 19% 2	6490
February	1st to 10th	Night	42% 3 13% 26% 15% 1	2600
	11th to 20th	Night	44% 2 8% 22% 18% 5%	2463
	21st to end	Night	43% 2 10% 22% 17% 5%	1980
March	1st to 10th	Night	41% 2 6% 27% 15% 9%	5035
	11th to 20th	Night	34% 2 10% 27% 19% 7%	4980
	21st to end	Night	42% 2 8% 22% 21% 6%	5340
April	1st to 10th	Night	43% 4 8% 20% 21% 4	2000
	11th to 20th	Night	38% 3 11% 21% 20% 5%	1975
	21st to end	Night	39% 4 9% 26% 17% 5%	2000
May	1st to 10th	Night	54% 4 12% 18% 9% 3	4247
	11th to 20th	Night	51% 4 11% 22% 12% 1	3678
	21st to end	Night	45% 4 14% 22% 12% 2	3927
June	1st to 10th	Night	35% 6% 11% 28% 18% 2	1600
	11th to 20th	Night	39% 5% 14% 26% 14% 3	1600
	21st to end	Night	36% 6% 13% 24% 19% 3	1600
July	1st to 10th	Night	38% 7% 21% 19% 12% 3	2520
	11th to 20th	Night	40% 6% 18% 24% 11% 1	2559
	21st to end	Night	43% 5% 13% 23% 15% 1	2988
August	1st to 10th	Night	46% 5% 18% 22% 8% 1	2814
	11th to 20th	Night	46% 5% 11% 26% 11% 1	2970
	21st to end	Night	34% 3 11% 31% 18% 2	3476
September	1st to 10th	Night	40% 3 10% 22% 18% 6%	1900
	11th to 20th	Night	34% 4 8% 29% 22% 4	1900
	21st to end	Night	46% 4 9% 20% 18% 3	2170
October	1st to 10th	Night	41% 3 9% 21% 20% 6%	3800
	11th to 20th	Night	25% 2 10% 24% 31% 8%	3779
	21st to end	Night	24% 2 8% 33% 26% 8%	4120
November	1st to 10th	Night	34% 3 8% 27% 23% 6%	2461
	11th to 20th	Night	37% 2 9% 24% 22% 6%	2600
	21st to end	Night	35% 3 12% 26% 21% 3	2800
December	1st to 10th	Night	34% 3 12% 28% 21% 3	4580
	11th to 20th	Night	45% 3 12% 20% 17% 4	5208
	21st to end	Night	45% 3 14% 21% 16% 1	6996

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% 20% 33% 13% 3	213
	11th to 20th	Day	30% 24% 31% 12% 4	300
	21st to end	Day	25% 22% 34% 13% 5%	330
February	1st to 10th	Day	28% 23% 34% 11% 4	300
	11th to 20th	Day	38% 20% 35% 6% 2	370
	21st to end	Day	30% 18% 33% 17% 2	414
March	1st to 10th	Day	28% 20% 31% 17% 3 1	500
	11th to 20th	Day	30% 18% 32% 15% 5% 1	500
	21st to end	Day	26% 18% 34% 16% 5% 1	550
April	1st to 10th	Day	26% 19% 33% 15% 5% 1	500
	11th to 20th	Day	22% 18% 35% 18% 5% 3	492
	21st to end	Day	26% 13% 34% 18% 7% 1	500
May	1st to 10th	Day	27% 14% 29% 19% 7% 3	503
	11th to 20th	Day	24% 13% 28% 24% 9% 2	527
	21st to end	Day	20% 8% 24% 29% 15% 3	573
June	1st to 10th	Day	15% 8% 27% 28% 15% 6%	530
	11th to 20th	Day	18% 7% 26% 29% 16% 4	530
	21st to end	Day	11% 6% 28% 31% 18% 5%	530
July	1st to 10th	Day	13% 1 19% 38% 20% 9%	505
	11th to 20th	Day	6% 3 23% 36% 26% 6%	484
	21st to end	Day	13% 3 23% 36% 20% 6%	532
August	1st to 10th	Day	14% 3 19% 33% 22% 9%	486
	11th to 20th	Day	15% 3 18% 33% 23% 9%	479
	21st to end	Day	11% 3 20% 36% 18% 12%	539
September	1st to 10th	Day	17% 6% 21% 28% 17% 12%	490
	11th to 20th	Day	12% 4 23% 34% 17% 9%	490
	21st to end	Day	28% 8% 23% 27% 9% 5%	490
October	1st to 10th	Day	22% 8% 22% 26% 16% 5%	490
	11th to 20th	Day	19% 14% 27% 22% 15% 2	490
	21st to end	Day	18% 12% 37% 22% 10% 2	539
November	1st to 10th	Day	16% 15% 33% 23% 10% 3	433
	11th to 20th	Day	19% 16% 34% 21% 8% 2	395
	21st to end	Day	27% 20% 29% 16% 8% 1	200
December	1st to 10th	Day	22% 21% 37% 16% 4 1	200
	11th to 20th	Day	25% 21% 31% 18% 7%	200
	21st to end	Day	25% 18% 30% 20% 5%	220
January	1st to 10th	Night	20% 20% 37% 20% 3	800
	11th to 20th	Night	23% 16% 31% 25% 4	800
	21st to end	Night	20% 18% 34% 27% 1	880
February	1st to 10th	Night	20% 17% 33% 27% 3	800
	11th to 20th	Night	28% 21% 32% 19% 1	783
	21st to end	Night	25% 17% 35% 20% 3	660
March	1st to 10th	Night	21% 17% 36% 23% 3	800
	11th to 20th	Night	23% 18% 35% 19% 5% 1	800
	21st to end	Night	20% 17% 38% 21% 4 1	880
April	1st to 10th	Night	22% 20% 34% 20% 4	800
	11th to 20th	Night	18% 16% 32% 27% 6%	791
	21st to end	Night	22% 12% 32% 26% 8%	800
May	1st to 10th	Night	22% 10% 30% 27% 11% 1	683
	11th to 20th	Night	21% 6% 26% 37% 9%	444
	21st to end	Night	16% 4 26% 36% 15% 1	433
June	1st to 10th	Night	11% 5% 28% 37% 18% 1	400
	11th to 20th	Night	15% 5% 25% 41% 15% 1	400
	21st to end	Night	12% 3 24% 46% 15% 1	400
July	1st to 10th	Night	8% 3 21% 43% 23% 2	380
	11th to 20th	Night	8% 1 17% 44% 28% 2	416
	21st to end	Night	9% 2 20% 39% 26% 3	624
August	1st to 10th	Night	11% 2 13% 42% 31% 2	567
	11th to 20th	Night	13% 3 14% 41% 27% 2	657
	21st to end	Night	12% 3 19% 36% 27% 4	836
September	1st to 10th	Night	16% 3 20% 31% 26% 4	760
	11th to 20th	Night	14% 3 15% 38% 26% 4	760
	21st to end	Night	23% 5% 22% 30% 18% 2	760
October	1st to 10th	Night	22% 6% 20% 29% 18% 5%	760
	11th to 20th	Night	17% 11% 24% 33% 14% 1	760
	21st to end	Night	18% 12% 30% 27% 12%	836
November	1st to 10th	Night	14% 9% 34% 29% 13% 1	780
	11th to 20th	Night	19% 14% 31% 28% 8% 1	800
	21st to end	Night	18% 14% 32% 26% 9% 1	800
December	1st to 10th	Night	22% 15% 32% 22% 8% 1	800
	11th to 20th	Night	20% 13% 36% 26% 6%	800
	21st to end	Night	20% 15% 29% 32% 4	880

Page generated at 2025-07-03 01:30:11. Version ac07

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