UMBC Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the UMBC Observatory clear sky chart from 2005-11-06 to 2026-02-16.

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% 4 4 4 4 6% 8% 42%	130947
	Night	15% 6% 2 6% 4 4 4 4 5% 7% 44%	129871
Transparency	Day	39% 5% 11% 24% 17% 3	134540
	Night	40% 4 11% 24% 18% 4	134349
Seeing	Day	19% 12% 27% 24% 13% 4	16556
	Night	18% 11% 28% 30% 12% 1	26121

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	14% 5% 2 5% 4 5% 4 3 5% 5% 49%	4661
	11th to 20th	Day	14% 8% 3 5% 3 3 3 2 4 5% 49%	5260
	21st to end	Day	14% 6% 2 5% 4 5% 4 3 5% 7% 44%	5780
February	1st to 10th	Day	14% 6% 2 7% 5% 4 5% 4 6% 7% 41%	1890
	11th to 20th	Day	18% 8% 2 5% 4 3 4 3 6% 6% 41%	1878
	21st to end	Day	18% 8% 2 6% 5% 3 4 3 5% 5% 41%	1650
March	1st to 10th	Day	19% 5% 2 3 3 3 4 3 5% 7% 45%	4755
	11th to 20th	Day	11% 7% 2 5% 3 4 3 4 4 7% 49%	4980
	21st to end	Day	17% 7% 2 5% 4 5% 4 3 6% 6% 41%	5619
April	1st to 10th	Day	15% 6% 2 5% 5% 4 5% 4 5% 7% 41%	2400
	11th to 20th	Day	13% 7% 2 6% 4 4 4 3 5% 7% 45%	2374
	21st to end	Day	18% 5% 2 5% 4 3 4 3 6% 8% 41%	2400
May	1st to 10th	Day	23% 6% 2 6% 4 4 4 4 6% 9% 31%	5697
	11th to 20th	Day	24% 6% 2 6% 4 4 4 4 7% 9% 31%	6272
	21st to end	Day	16% 5% 2 6% 4 4 5% 4 6% 9% 39%	6934
June	1st to 10th	Day	11% 5% 2 6% 4 4 5% 4 7% 10% 42%	2800
	11th to 20th	Day	12% 6% 2 5% 5% 5% 5% 4 6% 8% 42%	2800
	21st to end	Day	10% 4 1 4 5% 4 5% 5% 7% 10% 44%	2800
July	1st to 10th	Day	7% 4 2 6% 5% 5% 6% 5% 7% 10% 43%	6360
	11th to 20th	Day	9% 6% 3 7% 5% 6% 6% 6% 9% 11% 32%	6321
	21st to end	Day	8% 5% 2 6% 5% 5% 5% 5% 7% 11% 41%	6424
August	1st to 10th	Day	17% 5% 2 6% 5% 5% 6% 5% 7% 9% 33%	4234
	11th to 20th	Day	11% 5% 2 6% 4 5% 6% 5% 7% 10% 39%	3954
	21st to end	Day	11% 4 2 5% 3 5% 5% 4 6% 8% 46%	4312
September	1st to 10th	Day	14% 6% 2 5% 3 4 5% 4 6% 7% 46%	2400
	11th to 20th	Day	12% 6% 2 5% 4 4 4 3 5% 8% 48%	2400
	21st to end	Day	20% 7% 3 6% 4 4 4 3 5% 7% 36%	2121
October	1st to 10th	Day	17% 5% 2 5% 3 3 3 2 4 6% 49%	2000
	11th to 20th	Day	17% 4 1 5% 2 2 4 3 6% 6% 50%	2000
	21st to end	Day	15% 5% 2 4 3 4 4 3 4 6% 50%	2200
November	1st to 10th	Day	12% 5% 2 4 3 3 3 4 6% 7% 52%	1901
	11th to 20th	Day	13% 6% 2 4 2 3 4 4 5% 5% 50%	1890
	21st to end	Day	15% 6% 2 5% 4 4 4 3 5% 5% 47%	1682
December	1st to 10th	Day	13% 6% 2 6% 4 3 4 3 5% 7% 47%	2680
	11th to 20th	Day	18% 9% 2 4 3 3 4 3 4 6% 43%	3070
	21st to end	Day	16% 7% 2 6% 4 5% 4 4 5% 7% 39%	4048
January	1st to 10th	Night	12% 6% 3 6% 5% 4 4 4 6% 6% 45%	8233
	11th to 20th	Night	15% 7% 2 6% 3 3 3 2 4 5% 51%	7684
	21st to end	Night	14% 7% 2 5% 3 3 3 3 4 6% 48%	8401
February	1st to 10th	Night	16% 7% 2 6% 4 5% 4 3 6% 6% 41%	2730
	11th to 20th	Night	18% 7% 2 5% 3 3 4 3 5% 6% 44%	2517
	21st to end	Night	17% 8% 2 5% 4 4 5% 3 5% 6% 43%	1980
March	1st to 10th	Night	17% 6% 2 5% 3 3 3 3 3 5% 49%	5083
	11th to 20th	Night	10% 7% 2 5% 3 3 3 3 5% 6% 51%	4980
	21st to end	Night	13% 7% 2 6% 4 4 4 3 5% 5% 45%	5312
April	1st to 10th	Night	16% 7% 2 6% 5% 4 4 4 6% 5% 41%	2000
	11th to 20th	Night	14% 6% 2 6% 4 4 4 4 6% 5% 46%	1975
	21st to end	Night	18% 6% 2 4 4 4 4 4 5% 7% 44%	2000
May	1st to 10th	Night	23% 8% 3 6% 4 4 5% 4 5% 8% 30%	4226
	11th to 20th	Night	25% 8% 2 6% 4 3 4 3 6% 8% 31%	3605
	21st to end	Night	19% 6% 2 6% 4 4 4 4 5% 9% 36%	3920
June	1st to 10th	Night	11% 5% 1 6% 4 4 4 4 7% 10% 45%	1600
	11th to 20th	Night	12% 5% 1 6% 5% 5% 4 5% 7% 9% 41%	1600
	21st to end	Night	11% 5% 2 5% 5% 5% 5% 4 6% 11% 42%	1600
July	1st to 10th	Night	8% 6% 2 7% 6% 6% 6% 5% 6% 10% 38%	3598
	11th to 20th	Night	9% 8% 3 9% 6% 6% 6% 5% 8% 11% 30%	3633
	21st to end	Night	9% 6% 3 6% 4 4 5% 4 6% 9% 43%	4427
August	1st to 10th	Night	17% 6% 2 6% 5% 4 5% 4 7% 9% 34%	2915
	11th to 20th	Night	11% 7% 3 8% 6% 5% 6% 5% 6% 7% 35%	3059
	21st to end	Night	12% 5% 2 5% 3 4 4 4 6% 8% 48%	3586
September	1st to 10th	Night	13% 7% 2 5% 3 4 4 3 6% 7% 46%	2000
	11th to 20th	Night	11% 6% 2 6% 4 3 3 3 4 7% 51%	2000
	21st to end	Night	22% 7% 2 6% 3 4 4 3 5% 7% 37%	2273
October	1st to 10th	Night	19% 6% 2 4 4 3 3 3 4 5% 46%	2400
	11th to 20th	Night	16% 4 2 4 4 3 3 3 5% 6% 51%	2400
	21st to end	Night	15% 5% 2 4 3 3 3 3 4 7% 50%	2640
November	1st to 10th	Night	11% 5% 2 4 4 4 4 3 4 6% 53%	2614
	11th to 20th	Night	15% 6% 2 5% 4 3 3 3 4 5% 50%	2730
	21st to end	Night	13% 5% 2 5% 4 4 4 4 5% 7% 47%	2935
December	1st to 10th	Night	11% 5% 2 5% 4 4 4 3 5% 7% 50%	4720
	11th to 20th	Night	21% 6% 2 5% 4 4 4 3 5% 6% 41%	5345
	21st to end	Night	17% 6% 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	38% 3 7% 27% 20% 4	4662
	11th to 20th	Day	40% 2 7% 22% 21% 9%	5260
	21st to end	Day	41% 2 11% 30% 13% 3	5780
February	1st to 10th	Day	42% 4 13% 25% 13% 3	1890
	11th to 20th	Day	44% 3 7% 26% 17% 3	1887
	21st to end	Day	46% 3 10% 22% 15% 4	1650
March	1st to 10th	Day	39% 3 8% 24% 18% 8%	4755
	11th to 20th	Day	36% 3 8% 27% 21% 6%	4980
	21st to end	Day	43% 4 9% 19% 19% 6%	5619
April	1st to 10th	Day	42% 4 11% 20% 18% 5%	2400
	11th to 20th	Day	39% 3 9% 23% 19% 6%	2374
	21st to end	Day	42% 4 9% 22% 20% 4	2400
May	1st to 10th	Day	49% 5% 14% 18% 11% 2	5730
	11th to 20th	Day	51% 4 10% 24% 10% 1	6276
	21st to end	Day	42% 4 13% 23% 16% 2	6949
June	1st to 10th	Day	37% 5% 14% 25% 17% 2	2800
	11th to 20th	Day	39% 6% 12% 25% 15% 3	2800
	21st to end	Day	32% 7% 15% 26% 19% 2	2800
July	1st to 10th	Day	34% 10% 16% 20% 15% 4	6290
	11th to 20th	Day	40% 11% 17% 23% 8% 1	6246
	21st to end	Day	35% 7% 12% 22% 22% 1	6240
August	1st to 10th	Day	45% 7% 13% 25% 9% 1	4221
	11th to 20th	Day	36% 7% 13% 28% 15% 1	3993
	21st to end	Day	34% 6% 12% 29% 17% 2	4312
September	1st to 10th	Day	36% 5% 13% 22% 20% 5%	2400
	11th to 20th	Day	35% 4 9% 27% 22% 4	2400
	21st to end	Day	48% 3 8% 21% 17% 2	2121
October	1st to 10th	Day	37% 3 7% 19% 28% 6%	3260
	11th to 20th	Day	25% 3 8% 28% 26% 9%	3240
	21st to end	Day	24% 2 8% 31% 30% 5%	3538
November	1st to 10th	Day	32% 3 13% 26% 22% 4	1901
	11th to 20th	Day	36% 3 10% 24% 24% 4	1890
	21st to end	Day	39% 3 10% 28% 17% 2	1682
December	1st to 10th	Day	37% 4 15% 28% 13% 4	2680
	11th to 20th	Day	44% 3 11% 29% 11% 2	3066
	21st to end	Day	44% 4 15% 20% 15% 1	4048
January	1st to 10th	Night	40% 3 9% 26% 19% 4	8234
	11th to 20th	Night	38% 2 7% 23% 22% 8%	7684
	21st to end	Night	38% 2 6% 31% 19% 3	8401
February	1st to 10th	Night	43% 3 12% 25% 15% 2	2730
	11th to 20th	Night	42% 3 8% 24% 18% 5%	2519
	21st to end	Night	44% 2 9% 22% 18% 5%	1980
March	1st to 10th	Night	40% 2 7% 28% 14% 10%	5083
	11th to 20th	Night	34% 3 10% 28% 18% 7%	4980
	21st to end	Night	41% 2 9% 20% 23% 5%	5312
April	1st to 10th	Night	44% 3 9% 20% 20% 4	2000
	11th to 20th	Night	39% 3 11% 21% 20% 5%	1975
	21st to end	Night	39% 4 9% 24% 19% 5%	2000
May	1st to 10th	Night	54% 5% 11% 17% 11% 3	4256
	11th to 20th	Night	52% 4 10% 21% 12% 1	3606
	21st to end	Night	46% 4 14% 22% 12% 3	3927
June	1st to 10th	Night	35% 5% 13% 27% 19% 3	1600
	11th to 20th	Night	39% 5% 13% 26% 14% 2	1600
	21st to end	Night	37% 5% 12% 25% 19% 3	1600
July	1st to 10th	Night	40% 9% 17% 18% 14% 3	3560
	11th to 20th	Night	45% 10% 18% 18% 8% 1	3588
	21st to end	Night	35% 7% 13% 24% 20% 1	4297
August	1st to 10th	Night	45% 6% 17% 22% 9% 1	2904
	11th to 20th	Night	43% 7% 11% 25% 12% 1	3086
	21st to end	Night	34% 5% 12% 30% 17% 2	3586
September	1st to 10th	Night	38% 4 11% 24% 18% 5%	2000
	11th to 20th	Night	35% 3 7% 27% 23% 3	2000
	21st to end	Night	47% 4 9% 19% 18% 3	2273
October	1st to 10th	Night	40% 5% 10% 20% 21% 5%	3920
	11th to 20th	Night	25% 3 9% 24% 31% 7%	3899
	21st to end	Night	24% 2 10% 32% 25% 7%	4252
November	1st to 10th	Night	34% 3 8% 27% 23% 6%	2614
	11th to 20th	Night	37% 3 9% 24% 21% 6%	2730
	21st to end	Night	37% 3 12% 26% 20% 3	2935
December	1st to 10th	Night	34% 4 13% 29% 18% 2	4720
	11th to 20th	Night	45% 3 12% 18% 18% 5%	5348
	21st to end	Night	44% 3 13% 21% 17% 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	26% 20% 35% 14% 5%	220
	11th to 20th	Day	24% 29% 32% 12% 3	327
	21st to end	Day	20% 25% 38% 13% 4 1	363
February	1st to 10th	Day	23% 22% 37% 14% 4	330
	11th to 20th	Day	32% 22% 36% 10% 1	395
	21st to end	Day	28% 21% 33% 16% 3	414
March	1st to 10th	Day	22% 24% 31% 19% 3 1	500
	11th to 20th	Day	26% 22% 30% 16% 5% 1	500
	21st to end	Day	24% 21% 33% 16% 6% 1	550
April	1st to 10th	Day	24% 21% 33% 16% 5% 1	500
	11th to 20th	Day	20% 17% 31% 20% 9% 3	492
	21st to end	Day	24% 13% 34% 19% 8% 1	500
May	1st to 10th	Day	27% 13% 30% 19% 8% 3	503
	11th to 20th	Day	22% 11% 29% 26% 9% 2	527
	21st to end	Day	20% 7% 24% 26% 19% 4	573
June	1st to 10th	Day	15% 10% 30% 21% 19% 6%	530
	11th to 20th	Day	15% 6% 25% 32% 17% 4	530
	21st to end	Day	12% 6% 25% 32% 18% 7%	530
July	1st to 10th	Day	11% 2 18% 35% 24% 10%	525
	11th to 20th	Day	4 4 17% 38% 28% 9%	524
	21st to end	Day	12% 3 22% 32% 24% 7%	576
August	1st to 10th	Day	12% 3 19% 33% 22% 10%	526
	11th to 20th	Day	13% 5% 16% 32% 24% 9%	519
	21st to end	Day	10% 4 21% 32% 22% 11%	583
September	1st to 10th	Day	16% 5% 19% 30% 18% 12%	530
	11th to 20th	Day	14% 3 20% 34% 19% 9%	530
	21st to end	Day	26% 9% 21% 28% 11% 4	530
October	1st to 10th	Day	21% 8% 21% 26% 17% 8%	530
	11th to 20th	Day	18% 13% 28% 24% 13% 3	530
	21st to end	Day	17% 14% 33% 25% 9% 2	583
November	1st to 10th	Day	16% 14% 34% 23% 11% 2	452
	11th to 20th	Day	18% 15% 35% 21% 10% 1	430
	21st to end	Day	25% 18% 29% 21% 6%	222
December	1st to 10th	Day	22% 20% 37% 17% 4 1	220
	11th to 20th	Day	25% 19% 32% 19% 5%	220
	21st to end	Day	25% 17% 32% 19% 6%	242
January	1st to 10th	Night	19% 18% 38% 22% 3	840
	11th to 20th	Night	23% 16% 33% 23% 4	840
	21st to end	Night	19% 18% 37% 24% 2	924
February	1st to 10th	Night	19% 16% 35% 27% 3	840
	11th to 20th	Night	25% 21% 36% 17% 1	804
	21st to end	Night	24% 16% 35% 21% 3	660
March	1st to 10th	Night	20% 16% 37% 25% 3	800
	11th to 20th	Night	21% 16% 34% 22% 6% 1	800
	21st to end	Night	19% 14% 40% 21% 4 1	880
April	1st to 10th	Night	20% 19% 35% 20% 6%	800
	11th to 20th	Night	16% 13% 34% 28% 9%	791
	21st to end	Night	21% 10% 32% 30% 7%	800
May	1st to 10th	Night	22% 10% 27% 31% 9% 1	692
	11th to 20th	Night	21% 8% 25% 35% 10%	411
	21st to end	Night	18% 6% 19% 35% 21% 2	433
June	1st to 10th	Night	13% 3 26% 39% 20%	400
	11th to 20th	Night	13% 4 22% 40% 21% 1	400
	21st to end	Night	11% 4 20% 47% 18% 1	400
July	1st to 10th	Night	10% 3 18% 39% 27% 2	390
	11th to 20th	Night	6% 3 14% 42% 32% 3	398
	21st to end	Night	8% 3 16% 42% 28% 3	657
August	1st to 10th	Night	13% 1 12% 42% 29% 2	597
	11th to 20th	Night	13% 3 14% 39% 27% 3	700
	21st to end	Night	13% 4 14% 38% 27% 4	880
September	1st to 10th	Night	16% 4 18% 34% 25% 4	800
	11th to 20th	Night	13% 4 12% 39% 28% 4	800
	21st to end	Night	24% 5% 18% 36% 16% 2	800
October	1st to 10th	Night	21% 6% 21% 29% 19% 5%	800
	11th to 20th	Night	18% 8% 26% 32% 14% 1	800
	21st to end	Night	19% 10% 30% 27% 13% 1	880
November	1st to 10th	Night	14% 10% 31% 33% 11% 1	820
	11th to 20th	Night	19% 15% 28% 28% 8% 1	840
	21st to end	Night	16% 16% 31% 30% 7% 1	840
December	1st to 10th	Night	22% 12% 37% 21% 7% 1	840
	11th to 20th	Night	19% 13% 36% 26% 5%	840
	21st to end	Night	21% 13% 34% 29% 3	924

Page generated at 2026-02-13 01:42:14. Version ac07

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