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 2024-03-18.

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 5% 5% 3 5% 5% 6% 8% 42%	78696
	Night	15% 6% 2 5% 4 3 4 4 5% 7% 44%	81092
Transparency	Day	41% 4 11% 25% 17% 3	78406
	Night	40% 3 11% 25% 17% 3	80901
Seeing	Day	21% 11% 28% 24% 12% 4	14080
	Night	18% 11% 28% 29% 12% 1	23719

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 4 4 2 4 4 4 7% 48%	2553
	11th to 20th	Day	16% 10% 4 5% 5% 2 4 4 4 5% 41%	2840
	21st to end	Day	18% 9% 2 6% 6% 3 5% 4 4 6% 37%	3124
February	1st to 10th	Day	15% 7% 2 5% 6% 4 5% 5% 6% 8% 39%	1710
	11th to 20th	Day	18% 7% 2 4 4 2 5% 5% 5% 6% 42%	1729
	21st to end	Day	18% 8% 3 6% 6% 2 5% 4 3 6% 40%	1570
March	1st to 10th	Day	14% 5% 2 4 4 3 4 4 5% 6% 48%	2033
	11th to 20th	Day	16% 5% 2 5% 5% 3 4 4 4 7% 42%	2058
	21st to end	Day	18% 7% 3 6% 6% 3 4 4 4 6% 39%	2222
April	1st to 10th	Day	13% 6% 3 5% 5% 3 4 4 6% 8% 44%	2160
	11th to 20th	Day	13% 5% 2 4 5% 3 4 4 5% 6% 49%	2134
	21st to end	Day	19% 5% 2 5% 4 2 4 4 6% 8% 41%	2160
May	1st to 10th	Day	15% 6% 3 5% 5% 3 5% 5% 7% 8% 38%	2280
	11th to 20th	Day	18% 7% 2 5% 5% 4 5% 5% 6% 9% 36%	2430
	21st to end	Day	13% 6% 3 5% 5% 3 5% 5% 5% 9% 40%	2725
June	1st to 10th	Day	12% 4 2 4 5% 3 5% 6% 7% 9% 43%	2520
	11th to 20th	Day	12% 5% 3 6% 6% 3 5% 6% 7% 8% 40%	2520
	21st to end	Day	11% 3 2 4 5% 3 6% 6% 8% 11% 42%	2520
July	1st to 10th	Day	10% 5% 2 5% 6% 4 6% 5% 8% 10% 40%	2520
	11th to 20th	Day	6% 4 2 4 5% 4 6% 8% 7% 12% 42%	2470
	21st to end	Day	10% 4 2 6% 6% 4 6% 7% 9% 10% 35%	2547
August	1st to 10th	Day	10% 6% 2 7% 7% 4 6% 7% 7% 9% 35%	2327
	11th to 20th	Day	11% 6% 2 5% 7% 4 6% 6% 7% 8% 38%	2196
	21st to end	Day	11% 5% 2 5% 6% 4 6% 6% 7% 9% 40%	2376
September	1st to 10th	Day	14% 6% 3 4 4 3 5% 4 6% 7% 45%	2160
	11th to 20th	Day	11% 5% 2 4 6% 3 4 4 6% 9% 46%	2160
	21st to end	Day	18% 7% 3 5% 4 3 5% 5% 5% 7% 39%	1902
October	1st to 10th	Day	16% 5% 2 4 3 3 4 4 5% 7% 46%	1800
	11th to 20th	Day	18% 5% 2 4 3 2 3 4 5% 6% 48%	1800
	21st to end	Day	15% 5% 2 5% 4 2 4 4 4 7% 47%	1980
November	1st to 10th	Day	12% 5% 2 3 4 3 4 4 4 7% 52%	1742
	11th to 20th	Day	14% 5% 2 3 3 2 4 4 6% 7% 49%	1706
	21st to end	Day	16% 5% 2 4 4 3 3 4 4 7% 48%	1520
December	1st to 10th	Day	16% 5% 2 5% 4 3 5% 5% 7% 6% 42%	1520
	11th to 20th	Day	16% 7% 2 4 4 2 3 3 5% 7% 47%	1910
	21st to end	Day	16% 8% 3 6% 5% 3 6% 5% 5% 6% 37%	2772
January	1st to 10th	Night	16% 7% 3 5% 4 3 4 4 4 7% 45%	4388
	11th to 20th	Night	19% 7% 2 4 4 2 3 3 4 5% 46%	4120
	21st to end	Night	20% 10% 3 5% 4 2 3 3 4 5% 42%	4532
February	1st to 10th	Night	15% 7% 2 4 4 3 6% 5% 6% 6% 42%	2470
	11th to 20th	Night	19% 7% 2 5% 4 3 3 4 4 6% 44%	2335
	21st to end	Night	18% 7% 3 5% 4 3 4 3 5% 6% 43%	1884
March	1st to 10th	Night	15% 5% 2 4 4 3 5% 4 4 6% 48%	2142
	11th to 20th	Night	17% 6% 2 5% 4 2 3 4 6% 7% 44%	2068
	21st to end	Night	15% 6% 3 7% 6% 2 4 5% 6% 6% 41%	2134
April	1st to 10th	Night	15% 6% 2 6% 5% 3 5% 6% 5% 7% 41%	1800
	11th to 20th	Night	14% 5% 2 5% 4 3 4 5% 5% 6% 47%	1775
	21st to end	Night	19% 4 2 4 4 3 4 5% 5% 8% 42%	1800
May	1st to 10th	Night	16% 7% 2 5% 5% 3 5% 4 5% 7% 40%	1695
	11th to 20th	Night	18% 6% 3 4 5% 2 4 5% 6% 9% 38%	1461
	21st to end	Night	14% 5% 2 4 4 3 5% 5% 7% 11% 40%	1550
June	1st to 10th	Night	10% 4 2 5% 3 3 5% 6% 7% 9% 45%	1440
	11th to 20th	Night	12% 6% 3 4 5% 4 5% 5% 6% 10% 41%	1440
	21st to end	Night	9% 4 2 4 6% 4 6% 6% 6% 10% 43%	1440
July	1st to 10th	Night	9% 5% 2 6% 7% 4 7% 6% 7% 9% 39%	1440
	11th to 20th	Night	6% 4 2 4 4 4 6% 7% 7% 10% 46%	1485
	21st to end	Night	8% 3 2 5% 5% 3 5% 8% 8% 10% 42%	1771
August	1st to 10th	Night	9% 4 2 6% 5% 4 6% 6% 7% 10% 40%	1611
	11th to 20th	Night	10% 6% 2 7% 6% 3 5% 5% 6% 8% 40%	1686
	21st to end	Night	10% 6% 3 6% 5% 3 5% 5% 5% 9% 43%	1980
September	1st to 10th	Night	14% 6% 3 5% 4 3 4 3 5% 8% 45%	1800
	11th to 20th	Night	11% 6% 2 5% 4 3 4 3 5% 7% 51%	1800
	21st to end	Night	17% 6% 3 5% 5% 3 4 5% 6% 6% 40%	2055
October	1st to 10th	Night	19% 5% 2 4 4 2 4 4 5% 6% 45%	2160
	11th to 20th	Night	16% 4 2 4 4 2 3 3 4 6% 51%	2160
	21st to end	Night	15% 5% 2 4 4 2 4 4 4 7% 49%	2376
November	1st to 10th	Night	12% 5% 2 4 4 2 4 4 4 5% 54%	2335
	11th to 20th	Night	15% 5% 2 4 5% 2 3 4 4 6% 50%	2470
	21st to end	Night	13% 4 2 3 5% 3 4 5% 6% 7% 49%	2660
December	1st to 10th	Night	17% 7% 3 5% 5% 2 4 4 5% 7% 42%	2660
	11th to 20th	Night	17% 5% 2 4 5% 3 4 4 6% 6% 44%	3285
	21st to end	Night	17% 6% 2 5% 5% 3 5% 6% 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	37% 3 10% 27% 20% 3	2033
	11th to 20th	Day	42% 2 10% 21% 19% 6%	2058
	21st to end	Day	47% 3 8% 21% 17% 4	2222
April	1st to 10th	Day	39% 4 11% 22% 19% 5%	2160
	11th to 20th	Day	37% 3 9% 23% 22% 6%	2134
	21st to end	Day	42% 3 10% 22% 18% 4	2160
May	1st to 10th	Day	42% 4 12% 21% 17% 3	2280
	11th to 20th	Day	46% 5% 12% 22% 14% 2	2434
	21st to end	Day	41% 4 15% 26% 12% 2	2725
June	1st to 10th	Day	37% 5% 14% 26% 16% 2	2520
	11th to 20th	Day	39% 7% 13% 22% 16% 2	2520
	21st to end	Day	35% 6% 15% 25% 18% 2	2520
July	1st to 10th	Day	39% 5% 13% 23% 17% 3	2450
	11th to 20th	Day	32% 6% 15% 27% 16% 3	2395
	21st to end	Day	40% 7% 12% 22% 18% 2	2405
August	1st to 10th	Day	42% 5% 14% 24% 13% 1	2314
	11th to 20th	Day	41% 6% 13% 23% 16% 2	2196
	21st to end	Day	39% 6% 13% 24% 15% 3	2376
September	1st to 10th	Day	39% 4 13% 21% 19% 4	2160
	11th to 20th	Day	35% 4 8% 27% 22% 4	2160
	21st to end	Day	46% 4 6% 23% 19% 3	1902
October	1st to 10th	Day	39% 4 9% 25% 20% 3	1800
	11th to 20th	Day	38% 3 8% 26% 22% 3	1800
	21st to end	Day	38% 2 6% 28% 23% 3	1980
November	1st to 10th	Day	33% 3 11% 28% 21% 4	1742
	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	38% 3 9% 27% 19% 3	2142
	11th to 20th	Night	40% 3 11% 22% 19% 4	2068
	21st to end	Night	44% 3 9% 22% 16% 6%	2134
April	1st to 10th	Night	42% 4 9% 20% 21% 4	1800
	11th to 20th	Night	37% 4 11% 22% 20% 6%	1775
	21st to end	Night	41% 4 10% 25% 15% 6%	1800
May	1st to 10th	Night	43% 3 12% 23% 14% 4	1695
	11th to 20th	Night	43% 5% 12% 18% 18% 3	1462
	21st to end	Night	37% 5% 21% 21% 13% 2	1551
June	1st to 10th	Night	35% 5% 12% 28% 17% 2	1440
	11th to 20th	Night	39% 5% 15% 25% 13% 3	1440
	21st to end	Night	36% 6% 13% 24% 19% 3	1440
July	1st to 10th	Night	40% 7% 18% 19% 13% 4	1400
	11th to 20th	Night	32% 6% 16% 27% 17% 2	1437
	21st to end	Night	32% 6% 16% 28% 16% 2	1672
August	1st to 10th	Night	38% 5% 17% 24% 15% 1	1600
	11th to 20th	Night	42% 6% 14% 24% 13% 2	1686
	21st to end	Night	39% 4 13% 25% 15% 3	1980
September	1st to 10th	Night	40% 3 11% 23% 17% 5%	1800
	11th to 20th	Night	34% 4 8% 28% 22% 4	1800
	21st to end	Night	44% 4 9% 21% 19% 4	2055
October	1st to 10th	Night	41% 3 8% 25% 19% 3	2160
	11th to 20th	Night	36% 3 8% 29% 21% 3	2160
	21st to end	Night	37% 2 8% 27% 22% 4	2376
November	1st to 10th	Night	34% 3 8% 27% 22% 6%	2335
	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% 16% 4 1	449
	21st to end	Day	26% 18% 32% 16% 5% 2	462
April	1st to 10th	Day	24% 18% 34% 17% 6% 1	420
	11th to 20th	Day	20% 17% 35% 19% 6% 3	412
	21st to end	Day	28% 14% 33% 18% 6% 1	420
May	1st to 10th	Day	26% 16% 28% 19% 7% 3	423
	11th to 20th	Day	23% 13% 29% 25% 8% 3	447
	21st to end	Day	20% 5% 23% 31% 17% 4	485
June	1st to 10th	Day	16% 7% 26% 29% 16% 6%	450
	11th to 20th	Day	19% 6% 28% 27% 15% 5%	450
	21st to end	Day	12% 6% 29% 32% 16% 5%	450
July	1st to 10th	Day	14% 1 18% 36% 20% 10%	445
	11th to 20th	Day	5% 2 22% 35% 28% 7%	444
	21st to end	Day	13% 2 23% 36% 20% 6%	488
August	1st to 10th	Day	13% 3 18% 34% 22% 9%	446
	11th to 20th	Day	15% 2 17% 33% 24% 9%	439
	21st to end	Day	11% 3 20% 36% 17% 13%	495
September	1st to 10th	Day	18% 6% 20% 26% 18% 13%	450
	11th to 20th	Day	12% 4 23% 34% 17% 10%	450
	21st to end	Day	25% 8% 24% 28% 9% 5%	450
October	1st to 10th	Day	23% 7% 22% 26% 16% 6%	450
	11th to 20th	Day	20% 14% 28% 23% 14% 2	450
	21st to end	Day	19% 12% 38% 21% 8% 2	495
November	1st to 10th	Day	17% 14% 33% 22% 11% 4	400
	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% 17% 35% 20% 4 1	752
	21st to end	Night	19% 17% 38% 21% 4 1	792
April	1st to 10th	Night	20% 20% 33% 21% 5%	720
	11th to 20th	Night	17% 16% 32% 28% 7%	711
	21st to end	Night	23% 11% 31% 26% 8%	720
May	1st to 10th	Night	21% 10% 30% 27% 11% 1	615
	11th to 20th	Night	20% 7% 26% 38% 9% 1	400
	21st to end	Night	16% 4 24% 39% 16% 2	389
June	1st to 10th	Night	11% 5% 28% 36% 19% 1	360
	11th to 20th	Night	14% 5% 25% 41% 15% 1	360
	21st to end	Night	12% 3 23% 47% 15% 1	360
July	1st to 10th	Night	9% 2 19% 43% 25% 2	350
	11th to 20th	Night	8% 1 17% 43% 28% 2	393
	21st to end	Night	9% 2 19% 40% 26% 3	591
August	1st to 10th	Night	10% 2 13% 42% 31% 2	537
	11th to 20th	Night	14% 3 13% 40% 28% 2	622
	21st to end	Night	12% 3 18% 35% 27% 5%	792
September	1st to 10th	Night	17% 3 19% 30% 27% 4	720
	11th to 20th	Night	13% 3 15% 38% 26% 5%	720
	21st to end	Night	20% 5% 23% 31% 18% 2	720
October	1st to 10th	Night	22% 6% 20% 29% 19% 5%	720
	11th to 20th	Night	18% 11% 24% 32% 14% 1	720
	21st to end	Night	19% 13% 30% 26% 12% 1	792
November	1st to 10th	Night	15% 9% 34% 29% 13% 1	740
	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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