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 2024-11-03.

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% 41%	101208
	Night	15% 6% 2 6% 4 4 4 4 5% 7% 43%	97726
Transparency	Day	41% 4 11% 25% 16% 3	101007
	Night	41% 3 11% 25% 16% 3	97598
Seeing	Day	20% 11% 28% 24% 12% 4	14999
	Night	18% 11% 28% 29% 12% 1	24467

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	17% 4 2 5% 3 3 4 3 5% 6% 48%	2553
	11th to 20th	Day	16% 11% 3 5% 5% 4 4 3 4 5% 41%	2840
	21st to end	Day	18% 9% 2 8% 5% 4 4 3 4 6% 37%	3124
February	1st to 10th	Day	15% 7% 1 6% 5% 5% 5% 4 6% 7% 39%	1710
	11th to 20th	Day	18% 7% 2 5% 4 4 4 4 5% 6% 41%	1729
	21st to end	Day	18% 9% 3 6% 5% 3 4 3 3 5% 40%	1570
March	1st to 10th	Day	23% 6% 2 5% 3 4 4 3 5% 8% 36%	3343
	11th to 20th	Day	10% 4 2 5% 4 4 4 4 5% 7% 50%	3480
	21st to end	Day	21% 7% 2 6% 3 4 3 3 5% 6% 40%	3900
April	1st to 10th	Day	13% 7% 2 6% 4 5% 4 4 6% 7% 43%	2280
	11th to 20th	Day	13% 6% 2 5% 4 4 3 3 6% 5% 48%	2254
	21st to end	Day	19% 6% 2 5% 4 3 4 3 6% 7% 41%	2280
May	1st to 10th	Day	24% 7% 3 7% 4 4 4 4 7% 8% 29%	3982
	11th to 20th	Day	22% 7% 2 6% 4 5% 5% 4 6% 8% 30%	4322
	21st to end	Day	11% 5% 2 6% 4 4 5% 4 7% 9% 45%	4823
June	1st to 10th	Day	12% 5% 2 5% 4 5% 5% 4 7% 8% 42%	2660
	11th to 20th	Day	11% 5% 2 6% 5% 4 5% 5% 7% 8% 41%	2660
	21st to end	Day	11% 4 2 5% 5% 4 5% 5% 8% 11% 42%	2660
July	1st to 10th	Day	6% 4 2 6% 5% 6% 7% 5% 9% 10% 42%	4440
	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 4 7% 52%	1762
	11th to 20th	Day	14% 6% 2 4 2 3 4 3 7% 6% 49%	1706
	21st to end	Day	16% 5% 2 4 4 4 3 4 4 6% 48%	1520
December	1st to 10th	Day	16% 5% 2 5% 4 4 5% 4 7% 6% 42%	1520
	11th to 20th	Day	16% 7% 2 4 3 3 3 3 5% 7% 46%	1910
	21st to end	Day	16% 8% 2 7% 4 4 5% 4 5% 6% 37%	2772
January	1st to 10th	Night	15% 7% 2 5% 3 3 4 3 5% 6% 45%	4388
	11th to 20th	Night	19% 7% 2 4 3 3 3 3 5% 5% 46%	4120
	21st to end	Night	20% 10% 3 5% 3 3 3 3 4 5% 41%	4532
February	1st to 10th	Night	15% 7% 2 5% 4 4 5% 4 6% 6% 42%	2470
	11th to 20th	Night	18% 7% 2 5% 4 4 3 3 4 6% 44%	2335
	21st to end	Night	17% 8% 2 5% 4 4 4 3 5% 6% 42%	1884
March	1st to 10th	Night	22% 7% 2 6% 3 4 4 3 4 6% 39%	3493
	11th to 20th	Night	12% 5% 2 5% 4 4 4 3 5% 8% 50%	3480
	21st to end	Night	15% 8% 3 7% 4 2 3 3 4 5% 46%	3732
April	1st to 10th	Night	15% 7% 2 7% 4 4 4 5% 5% 7% 40%	1900
	11th to 20th	Night	14% 6% 2 5% 3 3 4 4 5% 6% 47%	1875
	21st to end	Night	18% 5% 2 4 4 4 4 4 5% 8% 42%	1900
May	1st to 10th	Night	22% 9% 2 6% 4 4 5% 3 5% 7% 32%	2941
	11th to 20th	Night	24% 8% 3 6% 4 4 5% 3 6% 7% 31%	2569
	21st to end	Night	11% 5% 1 5% 4 5% 4 4 7% 11% 43%	2732
June	1st to 10th	Night	10% 5% 2 5% 3 5% 5% 4 8% 9% 44%	1520
	11th to 20th	Night	11% 6% 3 5% 4 4 5% 4 6% 10% 41%	1520
	21st to end	Night	9% 5% 2 5% 6% 5% 5% 5% 6% 10% 43%	1520
July	1st to 10th	Night	6% 4 2 8% 6% 5% 6% 5% 7% 9% 41%	2520
	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% 4 3 3 3 4 5% 53%	2360
	11th to 20th	Night	14% 6% 2 4 4 3 3 3 5% 6% 50%	2470
	21st to end	Night	13% 4 2 4 4 4 4 4 6% 7% 48%	2660
December	1st to 10th	Night	17% 7% 3 6% 4 3 4 3 5% 7% 42%	2660
	11th to 20th	Night	17% 5% 2 5% 4 4 4 4 6% 6% 43%	3285
	21st to end	Night	17% 7% 2 6% 5% 5% 5% 4 6% 7% 37%	4884

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% 3 7% 32% 16% 4	2554
	11th to 20th	Day	47% 3 9% 22% 16% 4	2840
	21st to end	Day	51% 1 11% 24% 11% 2	3124
February	1st to 10th	Day	44% 4 12% 25% 14% 2	1710
	11th to 20th	Day	43% 4 7% 27% 17% 3	1738
	21st to end	Day	48% 3 11% 20% 16% 3	1570
March	1st to 10th	Day	47% 3 8% 25% 14% 3	3343
	11th to 20th	Day	34% 2 7% 29% 24% 4	3480
	21st to end	Day	46% 2 10% 21% 17% 4	3900
April	1st to 10th	Day	40% 4 11% 22% 19% 5%	2280
	11th to 20th	Day	37% 3 9% 23% 21% 6%	2254
	21st to end	Day	42% 3 10% 23% 19% 4	2280
May	1st to 10th	Day	53% 3 10% 20% 12% 2	4018
	11th to 20th	Day	52% 4 12% 23% 8% 1	4326
	21st to end	Day	36% 3 14% 26% 18% 3	4837
June	1st to 10th	Day	37% 5% 13% 27% 16% 2	2660
	11th to 20th	Day	38% 7% 13% 24% 16% 2	2660
	21st to end	Day	34% 6% 15% 25% 18% 2	2660
July	1st to 10th	Day	35% 7% 19% 21% 13% 5%	4370
	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	39% 4 9% 24% 21% 3	1900
	11th to 20th	Day	36% 3 8% 27% 23% 4	1900
	21st to end	Day	37% 2 6% 28% 23% 3	2090
November	1st to 10th	Day	33% 3 11% 28% 22% 4	1762
	11th to 20th	Day	35% 3 10% 25% 23% 5%	1706
	21st to end	Day	38% 2 10% 29% 18% 3	1520
December	1st to 10th	Day	41% 3 14% 26% 13% 2	1520
	11th to 20th	Day	39% 2 10% 33% 14% 2	1906
	21st to end	Day	47% 3 12% 25% 11% 1	2772
January	1st to 10th	Night	41% 2 8% 28% 17% 4	4388
	11th to 20th	Night	41% 2 9% 25% 19% 3	4120
	21st to end	Night	47% 2 7% 28% 16% 1	4532
February	1st to 10th	Night	42% 3 12% 26% 15% 2	2470
	11th to 20th	Night	44% 2 8% 22% 18% 5%	2337
	21st to end	Night	43% 2 10% 22% 17% 5%	1884
March	1st to 10th	Night	48% 2 7% 27% 13% 2	3493
	11th to 20th	Night	34% 3 9% 30% 20% 4	3480
	21st to end	Night	43% 2 8% 23% 19% 6%	3732
April	1st to 10th	Night	42% 4 8% 20% 21% 4	1900
	11th to 20th	Night	37% 4 11% 22% 20% 6%	1875
	21st to end	Night	40% 4 9% 26% 16% 6%	1900
May	1st to 10th	Night	53% 3 11% 20% 10% 3	2971
	11th to 20th	Night	53% 4 10% 21% 10% 2	2570
	21st to end	Night	35% 5% 18% 24% 14% 3	2739
June	1st to 10th	Night	35% 5% 12% 29% 18% 2	1520
	11th to 20th	Night	38% 5% 15% 27% 14% 3	1520
	21st to end	Night	35% 6% 13% 24% 19% 3	1520
July	1st to 10th	Night	38% 7% 21% 19% 12% 3	2480
	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% 24% 19% 4	2280
	11th to 20th	Night	35% 3 9% 28% 22% 4	2280
	21st to end	Night	35% 2 8% 28% 22% 5%	2508
November	1st to 10th	Night	34% 3 8% 27% 22% 6%	2360
	11th to 20th	Night	36% 2 9% 24% 23% 6%	2470
	21st to end	Night	35% 3 12% 27% 21% 3	2660
December	1st to 10th	Night	43% 3 11% 25% 16% 2	2660
	11th to 20th	Night	40% 3 10% 24% 19% 4	3288
	21st to end	Night	47% 3 14% 24% 12% 1	4884

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	29% 19% 34% 15% 3 1	191
	11th to 20th	Day	31% 23% 30% 11% 4	270
	21st to end	Day	26% 22% 33% 14% 5%	297
February	1st to 10th	Day	26% 22% 35% 12% 5%	270
	11th to 20th	Day	35% 20% 36% 7% 2	336
	21st to end	Day	31% 17% 32% 18% 2	382
March	1st to 10th	Day	28% 20% 31% 17% 4 1	460
	11th to 20th	Day	29% 18% 32% 15% 4 1	460
	21st to end	Day	26% 18% 32% 16% 5% 2	506
April	1st to 10th	Day	24% 19% 34% 16% 6% 1	460
	11th to 20th	Day	21% 17% 35% 19% 6% 3	452
	21st to end	Day	27% 14% 33% 18% 7% 2	460
May	1st to 10th	Day	27% 14% 28% 20% 7% 3	463
	11th to 20th	Day	23% 13% 29% 24% 9% 2	487
	21st to end	Day	19% 6% 24% 31% 16% 4	529
June	1st to 10th	Day	15% 8% 27% 28% 15% 6%	490
	11th to 20th	Day	17% 6% 28% 28% 17% 5%	490
	21st to end	Day	12% 6% 30% 32% 16% 4	490
July	1st to 10th	Day	13% 1 19% 38% 20% 9%	485
	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	17% 15% 32% 22% 10% 3	408
	11th to 20th	Day	19% 16% 33% 22% 9% 2	366
	21st to end	Day	26% 19% 29% 18% 8% 1	180
December	1st to 10th	Day	22% 22% 35% 17% 4 1	180
	11th to 20th	Day	25% 22% 29% 17% 7%	180
	21st to end	Day	25% 18% 30% 20% 6% 1	198
January	1st to 10th	Night	20% 18% 38% 21% 3	760
	11th to 20th	Night	24% 15% 31% 26% 5%	760
	21st to end	Night	20% 18% 34% 26% 2	836
February	1st to 10th	Night	20% 17% 32% 28% 3	760
	11th to 20th	Night	27% 21% 32% 19% 1	743
	21st to end	Night	26% 17% 34% 20% 3	628
March	1st to 10th	Night	21% 16% 37% 23% 3	760
	11th to 20th	Night	23% 18% 34% 19% 4 1	760
	21st to end	Night	20% 17% 37% 21% 4 1	836
April	1st to 10th	Night	21% 21% 34% 21% 4	760
	11th to 20th	Night	17% 16% 32% 28% 6%	751
	21st to end	Night	22% 11% 32% 26% 9%	760
May	1st to 10th	Night	22% 9% 29% 27% 11% 1	649
	11th to 20th	Night	21% 6% 26% 37% 9%	422
	21st to end	Night	16% 4 25% 38% 16% 1	411
June	1st to 10th	Night	11% 5% 28% 36% 19% 1	380
	11th to 20th	Night	13% 4 26% 41% 15% 1	380
	21st to end	Night	12% 3 25% 46% 14% 1	380
July	1st to 10th	Night	8% 2 21% 43% 23% 2	370
	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	749
	11th to 20th	Night	18% 14% 31% 28% 8% 1	760
	21st to end	Night	18% 14% 32% 27% 9% 1	760
December	1st to 10th	Night	22% 16% 31% 21% 8% 1	760
	11th to 20th	Night	19% 13% 37% 26% 6%	760
	21st to end	Night	20% 15% 30% 32% 3	836

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