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 2024-11-30.

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 5% 4 4 5% 4 6% 8% 41%	101466
	Night	16% 6% 2 6% 4 4 4 3 5% 7% 43%	98052
Transparency	Day	40% 4 11% 25% 17% 3	105103
	Night	40% 3 11% 25% 17% 3	102555
Seeing	Day	19% 12% 27% 25% 13% 5%	15063
	Night	18% 10% 28% 30% 12% 1	24523

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% 5% 1 4 3 4 4 3 5% 6% 48%	2517
	11th to 20th	Day	13% 12% 4 6% 4 4 4 3 5% 5% 40%	2820
	21st to end	Day	19% 8% 2 6% 5% 5% 4 3 5% 6% 38%	3124
February	1st to 10th	Day	15% 6% 2 7% 6% 4 4 4 6% 7% 40%	1710
	11th to 20th	Day	18% 7% 2 5% 4 3 4 3 6% 6% 42%	1739
	21st to end	Day	19% 8% 2 6% 5% 4 5% 3 5% 5% 39%	1570
March	1st to 10th	Day	23% 6% 2 4 4 3 4 3 5% 7% 38%	3311
	11th to 20th	Day	10% 5% 1 5% 4 4 4 4 5% 8% 49%	3480
	21st to end	Day	21% 8% 2 5% 4 4 3 3 4 5% 41%	3927
April	1st to 10th	Day	14% 6% 2 5% 5% 4 5% 4 6% 8% 42%	2280
	11th to 20th	Day	13% 7% 2 6% 5% 4 4 3 5% 7% 46%	2254
	21st to end	Day	19% 5% 2 5% 3 3 4 3 6% 8% 41%	2280
May	1st to 10th	Day	27% 6% 3 6% 4 4 4 4 5% 7% 30%	3973
	11th to 20th	Day	23% 7% 2 5% 5% 4 4 4 6% 8% 30%	4366
	21st to end	Day	11% 5% 2 5% 4 4 5% 4 6% 9% 45%	4823
June	1st to 10th	Day	11% 5% 2 5% 4 4 5% 4 7% 10% 41%	2660
	11th to 20th	Day	11% 6% 2 5% 5% 5% 5% 4 7% 8% 42%	2660
	21st to end	Day	10% 4 1 4 5% 4 5% 5% 7% 10% 44%	2660
July	1st to 10th	Day	6% 4 2 6% 5% 6% 6% 6% 8% 10% 43%	4440
	11th to 20th	Day	8% 5% 2 5% 5% 5% 5% 5% 9% 11% 40%	4415
	21st to end	Day	10% 6% 3 7% 5% 6% 6% 5% 7% 9% 36%	4466
August	1st to 10th	Day	17% 5% 2 6% 5% 5% 7% 5% 7% 9% 32%	4104
	11th to 20th	Day	11% 5% 2 6% 4 5% 6% 5% 7% 10% 40%	3831
	21st to end	Day	11% 4 2 6% 3 5% 5% 4 6% 8% 46%	4180
September	1st to 10th	Day	14% 6% 2 5% 3 4 5% 4 7% 7% 45%	2280
	11th to 20th	Day	11% 6% 2 5% 4 4 4 3 5% 9% 48%	2280
	21st to end	Day	20% 7% 3 6% 4 4 5% 3 5% 6% 37%	2015
October	1st to 10th	Day	17% 5% 2 5% 3 3 3 2 4 6% 48%	1900
	11th to 20th	Day	17% 4 1 5% 2 2 4 3 6% 6% 50%	1900
	21st to end	Day	15% 5% 2 5% 3 4 4 3 4 5% 50%	2090
November	1st to 10th	Day	13% 6% 2 4 3 3 3 4 6% 6% 52%	1809
	11th to 20th	Day	14% 6% 2 4 2 3 4 4 5% 6% 50%	1800
	21st to end	Day	15% 7% 2 4 4 4 4 3 5% 5% 48%	1600
December	1st to 10th	Day	17% 7% 1 6% 3 3 4 4 6% 8% 42%	1520
	11th to 20th	Day	17% 7% 2 4 3 4 3 3 5% 8% 44%	1910
	21st to end	Day	17% 7% 2 7% 4 5% 4 4 5% 7% 38%	2772
January	1st to 10th	Night	16% 6% 3 6% 4 4 3 3 5% 6% 44%	4429
	11th to 20th	Night	19% 7% 2 5% 3 3 3 2 4 6% 45%	4124
	21st to end	Night	21% 9% 2 6% 3 3 3 3 4 5% 41%	4532
February	1st to 10th	Night	16% 7% 2 6% 3 4 4 3 6% 6% 41%	2470
	11th to 20th	Night	18% 7% 2 6% 3 3 4 3 5% 6% 44%	2326
	21st to end	Night	17% 8% 2 5% 3 4 5% 3 5% 6% 42%	1884
March	1st to 10th	Night	22% 7% 2 5% 4 3 3 3 4 6% 41%	3527
	11th to 20th	Night	11% 5% 2 5% 4 3 4 4 6% 7% 51%	3480
	21st to end	Night	15% 8% 3 6% 4 3 3 3 3 5% 47%	3704
April	1st to 10th	Night	15% 7% 2 6% 5% 4 4 4 6% 5% 42%	1900
	11th to 20th	Night	14% 5% 2 6% 4 3 4 4 5% 5% 47%	1875
	21st to end	Night	18% 6% 2 4 3 4 4 4 5% 7% 44%	1900
May	1st to 10th	Night	25% 8% 3 5% 4 3 4 3 5% 8% 32%	2950
	11th to 20th	Night	25% 9% 3 6% 4 4 4 3 5% 7% 30%	2511
	21st to end	Night	12% 5% 1 5% 4 5% 4 4 6% 10% 43%	2732
June	1st to 10th	Night	11% 4 1 6% 3 4 4 4 7% 10% 45%	1520
	11th to 20th	Night	11% 5% 1 6% 5% 5% 4 5% 7% 9% 41%	1520
	21st to end	Night	10% 5% 2 5% 5% 5% 5% 4 6% 12% 42%	1520
July	1st to 10th	Night	6% 5% 2 7% 5% 6% 6% 6% 7% 10% 39%	2520
	11th to 20th	Night	8% 6% 3 6% 6% 5% 6% 5% 8% 10% 38%	2542
	21st to end	Night	10% 8% 3 7% 4 5% 5% 4 7% 8% 38%	3087
August	1st to 10th	Night	17% 6% 2 6% 4 5% 5% 4 7% 9% 33%	2825
	11th to 20th	Night	11% 7% 3 8% 6% 5% 6% 5% 6% 7% 35%	2963
	21st to end	Night	12% 5% 2 5% 3 4 4 4 5% 8% 48%	3476
September	1st to 10th	Night	13% 7% 2 5% 3 4 4 3 6% 7% 45%	1900
	11th to 20th	Night	11% 6% 2 6% 4 3 3 3 4 7% 51%	1900
	21st to end	Night	22% 7% 2 6% 3 4 4 3 5% 7% 38%	2159
October	1st to 10th	Night	20% 6% 2 5% 4 3 3 3 5% 5% 45%	2280
	11th to 20th	Night	16% 4 1 4 4 3 3 3 5% 6% 51%	2280
	21st to end	Night	16% 5% 2 4 3 3 3 3 5% 7% 49%	2508
November	1st to 10th	Night	11% 6% 2 4 3 4 3 3 4 5% 53%	2486
	11th to 20th	Night	15% 7% 2 5% 4 3 3 3 4 5% 50%	2600
	21st to end	Night	13% 4 2 4 4 4 4 4 5% 7% 48%	2793
December	1st to 10th	Night	17% 7% 2 6% 4 4 4 4 5% 6% 41%	2660
	11th to 20th	Night	17% 5% 2 5% 4 4 4 3 5% 7% 44%	3285
	21st to end	Night	16% 7% 2 6% 4 4 5% 4 7% 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	38% 2 7% 32% 16% 4	2518
	11th to 20th	Day	47% 2 9% 21% 16% 5%	2820
	21st to end	Day	50% 2 11% 23% 12% 2	3124
February	1st to 10th	Day	43% 5% 13% 25% 13% 2	1710
	11th to 20th	Day	43% 3 8% 27% 16% 3	1748
	21st to end	Day	47% 3 10% 21% 15% 4	1570
March	1st to 10th	Day	47% 2 8% 26% 13% 4	3311
	11th to 20th	Day	34% 2 7% 30% 23% 4	3480
	21st to end	Day	47% 2 9% 21% 17% 4	3927
April	1st to 10th	Day	41% 4 11% 21% 18% 5%	2280
	11th to 20th	Day	39% 3 9% 23% 20% 6%	2254
	21st to end	Day	43% 4 10% 21% 19% 4	2280
May	1st to 10th	Day	54% 3 10% 20% 12% 2	4009
	11th to 20th	Day	53% 3 12% 22% 8% 2	4370
	21st to end	Day	37% 3 13% 27% 19% 3	4837
June	1st to 10th	Day	37% 5% 14% 26% 16% 2	2660
	11th to 20th	Day	38% 6% 12% 25% 16% 3	2660
	21st to end	Day	33% 6% 14% 26% 18% 2	2660
July	1st to 10th	Day	34% 10% 17% 21% 13% 5%	4370
	11th to 20th	Day	35% 8% 16% 29% 11% 2	4340
	21st to end	Day	43% 7% 13% 20% 17% 1	4324
August	1st to 10th	Day	45% 8% 14% 25% 8% 1	4091
	11th to 20th	Day	36% 7% 13% 28% 15% 1	3870
	21st to end	Day	34% 6% 13% 29% 16% 2	4180
September	1st to 10th	Day	37% 5% 13% 21% 20% 4	2280
	11th to 20th	Day	35% 4 9% 26% 23% 4	2280
	21st to end	Day	47% 3 8% 21% 18% 3	2015
October	1st to 10th	Day	38% 3 7% 19% 28% 6%	3160
	11th to 20th	Day	25% 3 8% 28% 27% 9%	3140
	21st to end	Day	24% 2 8% 31% 30% 5%	3428
November	1st to 10th	Day	33% 3 12% 26% 22% 4	1809
	11th to 20th	Day	35% 3 10% 24% 24% 4	1800
	21st to end	Day	39% 3 11% 28% 17% 2	1600
December	1st to 10th	Day	41% 4 15% 25% 13% 2	1520
	11th to 20th	Day	41% 2 10% 31% 14% 2	1906
	21st to end	Day	48% 3 13% 22% 13% 1	2772
January	1st to 10th	Night	43% 2 7% 28% 16% 4	4429
	11th to 20th	Night	42% 2 9% 27% 18% 3	4124
	21st to end	Night	47% 2 7% 28% 14% 2	4532
February	1st to 10th	Night	43% 3 12% 26% 14% 2	2470
	11th to 20th	Night	43% 2 8% 23% 18% 5%	2328
	21st to end	Night	44% 2 9% 22% 17% 5%	1884
March	1st to 10th	Night	47% 2 7% 28% 13% 2	3527
	11th to 20th	Night	34% 2 8% 32% 19% 4	3480
	21st to end	Night	43% 2 7% 21% 22% 6%	3704
April	1st to 10th	Night	43% 3 9% 20% 20% 5%	1900
	11th to 20th	Night	38% 3 12% 22% 20% 6%	1875
	21st to end	Night	40% 4 9% 24% 18% 6%	1900
May	1st to 10th	Night	53% 3 10% 20% 11% 3	2980
	11th to 20th	Night	56% 3 11% 19% 10% 2	2512
	21st to end	Night	37% 4 17% 24% 14% 4	2739
June	1st to 10th	Night	34% 5% 13% 27% 19% 2	1520
	11th to 20th	Night	38% 5% 13% 27% 15% 2	1520
	21st to end	Night	36% 5% 12% 26% 18% 3	1520
July	1st to 10th	Night	39% 7% 20% 18% 12% 4	2480
	11th to 20th	Night	37% 8% 18% 25% 10% 2	2494
	21st to end	Night	41% 7% 13% 23% 16% 1	2988
August	1st to 10th	Night	45% 6% 17% 22% 9% 1	2814
	11th to 20th	Night	43% 7% 11% 25% 12% 1	2990
	21st to end	Night	34% 5% 12% 30% 17% 2	3476
September	1st to 10th	Night	39% 4 11% 22% 18% 5%	1900
	11th to 20th	Night	35% 3 7% 27% 24% 4	1900
	21st to end	Night	46% 4 8% 20% 18% 3	2159
October	1st to 10th	Night	41% 5% 10% 19% 20% 5%	3800
	11th to 20th	Night	25% 3 9% 24% 31% 7%	3779
	21st to end	Night	24% 2 11% 32% 25% 7%	4120
November	1st to 10th	Night	34% 3 8% 27% 22% 6%	2486
	11th to 20th	Night	38% 3 9% 24% 21% 6%	2600
	21st to end	Night	36% 3 12% 26% 20% 3	2793
December	1st to 10th	Night	44% 3 11% 25% 15% 2	2660
	11th to 20th	Night	42% 3 10% 22% 19% 4	3288
	21st to end	Night	46% 3 15% 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	23% 19% 38% 14% 5% 1	180
	11th to 20th	Day	24% 26% 33% 13% 3	267
	21st to end	Day	20% 24% 36% 15% 4 1	297
February	1st to 10th	Day	23% 20% 40% 13% 4	270
	11th to 20th	Day	30% 22% 37% 10% 1	346
	21st to end	Day	29% 20% 32% 15% 4	382
March	1st to 10th	Day	21% 25% 31% 19% 3 1	460
	11th to 20th	Day	27% 22% 30% 15% 5% 1	460
	21st to end	Day	24% 20% 32% 16% 6% 1	506
April	1st to 10th	Day	22% 20% 34% 17% 5% 2	460
	11th to 20th	Day	19% 17% 31% 21% 10% 3	452
	21st to end	Day	25% 13% 34% 18% 8% 1	460
May	1st to 10th	Day	28% 13% 28% 20% 8% 3	463
	11th to 20th	Day	22% 10% 29% 26% 10% 3	487
	21st to end	Day	19% 5% 23% 28% 21% 5%	529
June	1st to 10th	Day	15% 9% 29% 22% 19% 7%	490
	11th to 20th	Day	14% 6% 25% 33% 18% 4	490
	21st to end	Day	12% 6% 27% 32% 17% 6%	490
July	1st to 10th	Day	11% 2 18% 34% 24% 11%	485
	11th to 20th	Day	4 4 18% 38% 28% 9%	484
	21st to end	Day	12% 4 22% 32% 23% 7%	532
August	1st to 10th	Day	12% 3 18% 33% 23% 10%	486
	11th to 20th	Day	13% 5% 16% 31% 26% 10%	479
	21st to end	Day	11% 4 19% 31% 22% 12%	539
September	1st to 10th	Day	16% 5% 17% 29% 19% 13%	490
	11th to 20th	Day	14% 3 20% 34% 19% 10%	490
	21st to end	Day	27% 9% 20% 29% 11% 4	490
October	1st to 10th	Day	22% 8% 21% 25% 17% 7%	490
	11th to 20th	Day	17% 12% 28% 26% 14% 3	490
	21st to end	Day	17% 12% 34% 26% 10% 2	539
November	1st to 10th	Day	16% 12% 33% 24% 11% 3	420
	11th to 20th	Day	17% 15% 36% 22% 11% 1	400
	21st to end	Day	23% 19% 29% 21% 7%	202
December	1st to 10th	Day	21% 21% 36% 17% 4 1	180
	11th to 20th	Day	24% 20% 29% 20% 6%	180
	21st to end	Day	25% 14% 34% 20% 8% 1	198
January	1st to 10th	Night	19% 17% 38% 22% 3	760
	11th to 20th	Night	23% 15% 33% 25% 5%	760
	21st to end	Night	19% 18% 37% 25% 2	836
February	1st to 10th	Night	19% 16% 34% 28% 3	760
	11th to 20th	Night	24% 20% 36% 18% 1	743
	21st to end	Night	25% 17% 35% 21% 3	628
March	1st to 10th	Night	19% 14% 38% 25% 4	760
	11th to 20th	Night	21% 16% 34% 22% 5% 1	760
	21st to end	Night	19% 14% 40% 22% 4 1	836
April	1st to 10th	Night	19% 19% 36% 20% 6%	760
	11th to 20th	Night	15% 12% 35% 29% 9%	751
	21st to end	Night	21% 10% 32% 29% 7%	760
May	1st to 10th	Night	22% 9% 28% 31% 9% 1	658
	11th to 20th	Night	22% 7% 25% 35% 10% 1	390
	21st to end	Night	17% 5% 19% 36% 22% 2	411
June	1st to 10th	Night	13% 3 26% 39% 20%	380
	11th to 20th	Night	11% 4 23% 39% 22% 1	380
	21st to end	Night	11% 3 21% 47% 18% 1	380
July	1st to 10th	Night	10% 3 17% 40% 28% 2	370
	11th to 20th	Night	6% 2 14% 43% 31% 3	377
	21st to end	Night	8% 3 16% 43% 27% 3	624
August	1st to 10th	Night	13% 1 12% 42% 29% 2	567
	11th to 20th	Night	13% 3 14% 40% 27% 4	664
	21st to end	Night	13% 4 13% 38% 27% 4	836
September	1st to 10th	Night	17% 3 17% 33% 25% 4	760
	11th to 20th	Night	13% 4 12% 39% 28% 4	760
	21st to end	Night	24% 5% 19% 34% 17% 2	760
October	1st to 10th	Night	21% 6% 21% 29% 18% 5%	760
	11th to 20th	Night	18% 8% 26% 33% 14% 1	760
	21st to end	Night	19% 9% 30% 28% 13% 1	836
November	1st to 10th	Night	13% 9% 32% 33% 12% 2	780
	11th to 20th	Night	19% 15% 28% 28% 9% 1	800
	21st to end	Night	16% 16% 31% 30% 7% 1	800
December	1st to 10th	Night	23% 12% 36% 21% 8% 1	760
	11th to 20th	Night	18% 13% 38% 25% 6%	760
	21st to end	Night	21% 12% 35% 29% 3	836

Page generated at 2024-11-27 17:30:54. Version ac07

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