Baltimore
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Baltimore clear sky chart from 2005-11-06 to 2024-11-04.

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%	101236
	Night	16% 6% 2 6% 4 4 4 3 5% 7% 43%	97712
Transparency	Day	39% 6% 12% 24% 16% 3	104872
	Night	40% 5% 11% 24% 17% 3	102216
Seeing	Day	19% 13% 28% 24% 12% 4	14993
	Night	18% 11% 29% 29% 12% 1	24408

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 3 4 3 5% 6% 48%	2517
	11th to 20th	Day	13% 12% 4 7% 4 4 4 3 5% 5% 40%	2812
	21st to end	Day	19% 7% 2 6% 4 5% 4 3 6% 5% 38%	3124
February	1st to 10th	Day	15% 6% 2 7% 5% 4 5% 4 5% 7% 40%	1710
	11th to 20th	Day	18% 7% 2 5% 4 3 4 3 6% 6% 42%	1743
	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 3 4 4 3 5% 7% 38%	3301
	11th to 20th	Day	10% 5% 2 5% 4 4 4 4 5% 8% 49%	3480
	21st to end	Day	21% 8% 2 5% 4 4 3 3 4 5% 41%	3932
April	1st to 10th	Day	14% 6% 2 5% 5% 4 5% 4 6% 8% 41%	2280
	11th to 20th	Day	13% 6% 2 5% 5% 4 4 3 5% 7% 46%	2254
	21st to end	Day	18% 5% 2 5% 3 4 4 4 6% 8% 40%	2280
May	1st to 10th	Day	26% 7% 2 6% 4 4 4 4 5% 7% 30%	3954
	11th to 20th	Day	23% 7% 2 6% 5% 5% 4 4 6% 8% 30%	4388
	21st to end	Day	11% 5% 2 5% 4 3 5% 4 6% 9% 45%	4823
June	1st to 10th	Day	11% 5% 2 6% 4 5% 4 4 7% 10% 41%	2660
	11th to 20th	Day	11% 6% 2 5% 4 5% 5% 5% 7% 8% 42%	2660
	21st to end	Day	10% 4 1 4 5% 5% 5% 5% 7% 11% 44%	2660
July	1st to 10th	Day	5% 4 2 6% 5% 6% 6% 5% 8% 10% 43%	4440
	11th to 20th	Day	8% 5% 2 5% 4 5% 5% 5% 9% 11% 39%	4425
	21st to end	Day	10% 6% 3 7% 5% 5% 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	10% 5% 2 6% 4 5% 6% 5% 7% 10% 40%	3827
	21st to end	Day	11% 4 2 6% 3 4 6% 4 6% 8% 46%	4180
September	1st to 10th	Day	14% 6% 2 5% 3 4 5% 4 6% 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 4 4 5% 6% 37%	2018
October	1st to 10th	Day	17% 5% 2 5% 3 3 3 3 4 7% 47%	1900
	11th to 20th	Day	17% 4 1 5% 3 2 3 3 6% 6% 50%	1900
	21st to end	Day	15% 5% 2 5% 3 4 4 3 4 5% 49%	2090
November	1st to 10th	Day	13% 6% 2 4 3 3 3 3 5% 7% 51%	1738
	11th to 20th	Day	15% 6% 2 4 3 3 3 4 6% 5% 50%	1710
	21st to end	Day	15% 6% 2 5% 3 4 4 3 5% 5% 47%	1528
December	1st to 10th	Day	18% 6% 1 5% 3 3 4 5% 6% 8% 42%	1520
	11th to 20th	Day	17% 7% 3 4 3 4 3 3 5% 8% 44%	1910
	21st to end	Day	17% 7% 2 6% 4 5% 4 4 5% 7% 38%	2772
January	1st to 10th	Night	16% 6% 3 6% 5% 4 3 3 5% 6% 44%	4429
	11th to 20th	Night	19% 7% 2 5% 3 3 3 2 4 6% 45%	4147
	21st to end	Night	21% 10% 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 3 3 5% 6% 44%	2321
	21st to end	Night	17% 8% 2 5% 3 4 5% 3 5% 6% 43%	1884
March	1st to 10th	Night	22% 7% 2 5% 4 3 3 3 4 6% 41%	3547
	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%	3700
April	1st to 10th	Night	15% 7% 2 6% 5% 4 4 4 6% 5% 42%	1900
	11th to 20th	Night	14% 6% 2 5% 4 4 4 4 6% 5% 47%	1875
	21st to end	Night	18% 6% 1 4 3 4 3 4 5% 7% 44%	1900
May	1st to 10th	Night	24% 8% 3 5% 4 4 4 3 5% 8% 32%	2956
	11th to 20th	Night	25% 9% 3 6% 4 4 4 3 5% 7% 30%	2503
	21st to end	Night	12% 5% 1 5% 5% 4 5% 4 6% 10% 43%	2732
June	1st to 10th	Night	11% 4 1 6% 4 4 4 4 7% 10% 45%	1520
	11th to 20th	Night	11% 6% 1 6% 5% 4 5% 5% 6% 10% 41%	1520
	21st to end	Night	10% 5% 2 5% 4 4 6% 4 6% 11% 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 7% 6% 5% 6% 5% 8% 11% 38%	2520
	21st to end	Night	10% 8% 3 7% 4 5% 5% 4 7% 8% 38%	3084
August	1st to 10th	Night	17% 6% 3 6% 4 4 5% 4 7% 10% 33%	2825
	11th to 20th	Night	11% 8% 3 8% 6% 5% 6% 5% 7% 7% 35%	2967
	21st to end	Night	12% 5% 2 5% 3 4 4 3 6% 8% 48%	3476
September	1st to 10th	Night	13% 7% 3 5% 3 4 4 3 6% 7% 46%	1900
	11th to 20th	Night	11% 6% 2 6% 4 3 3 2 4 7% 51%	1900
	21st to end	Night	22% 6% 2 6% 3 4 4 3 5% 7% 38%	2157
October	1st to 10th	Night	20% 6% 2 5% 4 3 3 3 4 5% 45%	2280
	11th to 20th	Night	16% 4 2 4 3 3 3 3 4 6% 51%	2280
	21st to end	Night	15% 5% 2 4 3 4 3 3 5% 7% 49%	2508
November	1st to 10th	Night	12% 6% 3 4 4 3 4 3 4 5% 53%	2410
	11th to 20th	Night	15% 7% 2 5% 4 3 3 3 4 5% 50%	2470
	21st to end	Night	13% 4 2 4 4 4 4 4 5% 7% 48%	2650
December	1st to 10th	Night	17% 7% 2 6% 4 4 4 4 6% 6% 41%	2660
	11th to 20th	Night	17% 5% 2 6% 4 4 4 3 5% 6% 44%	3285
	21st to end	Night	16% 7% 2 6% 4 4 5% 5% 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	37% 4 9% 31% 15% 4	2518
	11th to 20th	Day	47% 6% 9% 20% 14% 4	2812
	21st to end	Day	48% 6% 12% 22% 11% 2	3124
February	1st to 10th	Day	42% 5% 14% 24% 13% 2	1710
	11th to 20th	Day	41% 4 10% 26% 15% 3	1752
	21st to end	Day	46% 5% 9% 21% 14% 4	1570
March	1st to 10th	Day	44% 5% 9% 27% 12% 3	3301
	11th to 20th	Day	33% 6% 8% 28% 21% 5%	3480
	21st to end	Day	45% 5% 8% 20% 18% 4	3932
April	1st to 10th	Day	40% 6% 12% 19% 18% 5%	2280
	11th to 20th	Day	38% 6% 10% 22% 18% 6%	2254
	21st to end	Day	41% 5% 11% 20% 19% 4	2280
May	1st to 10th	Day	52% 7% 10% 18% 12% 2	3989
	11th to 20th	Day	50% 8% 14% 18% 8% 2	4392
	21st to end	Day	34% 8% 13% 27% 17% 2	4837
June	1st to 10th	Day	37% 8% 13% 24% 15% 2	2660
	11th to 20th	Day	38% 8% 12% 24% 15% 3	2660
	21st to end	Day	33% 7% 15% 25% 18% 2	2660
July	1st to 10th	Day	34% 8% 19% 20% 13% 5%	4370
	11th to 20th	Day	36% 7% 17% 27% 10% 2	4350
	21st to end	Day	43% 7% 14% 18% 17% 1	4324
August	1st to 10th	Day	47% 7% 13% 24% 8% 1	4091
	11th to 20th	Day	37% 8% 12% 28% 15% 1	3866
	21st to end	Day	35% 6% 12% 28% 17% 2	4180
September	1st to 10th	Day	38% 6% 12% 20% 20% 4	2280
	11th to 20th	Day	34% 5% 10% 26% 22% 4	2280
	21st to end	Day	47% 5% 7% 21% 18% 2	2018
October	1st to 10th	Day	37% 4 7% 18% 29% 5%	3160
	11th to 20th	Day	26% 3 9% 26% 26% 10%	3140
	21st to end	Day	24% 3 9% 31% 29% 5%	3428
November	1st to 10th	Day	33% 4 13% 26% 21% 4	1738
	11th to 20th	Day	36% 5% 10% 23% 24% 4	1710
	21st to end	Day	39% 4 11% 29% 16% 2	1528
December	1st to 10th	Day	41% 5% 15% 24% 13% 2	1520
	11th to 20th	Day	40% 5% 10% 31% 12% 2	1906
	21st to end	Day	45% 7% 14% 22% 12% 1	2772
January	1st to 10th	Night	41% 4 9% 27% 15% 3	4429
	11th to 20th	Night	41% 4 10% 26% 17% 2	4147
	21st to end	Night	46% 4 7% 28% 13% 2	4532
February	1st to 10th	Night	44% 4 12% 24% 14% 2	2470
	11th to 20th	Night	43% 3 9% 22% 18% 5%	2323
	21st to end	Night	43% 4 10% 21% 17% 5%	1884
March	1st to 10th	Night	45% 5% 9% 27% 12% 2	3547
	11th to 20th	Night	32% 6% 10% 32% 17% 4	3480
	21st to end	Night	41% 4 8% 20% 22% 5%	3700
April	1st to 10th	Night	42% 5% 9% 20% 20% 4	1900
	11th to 20th	Night	38% 5% 11% 21% 20% 6%	1875
	21st to end	Night	39% 6% 10% 23% 17% 5%	1900
May	1st to 10th	Night	51% 6% 10% 18% 12% 2	2986
	11th to 20th	Night	55% 8% 11% 15% 10% 2	2504
	21st to end	Night	34% 9% 16% 24% 13% 3	2739
June	1st to 10th	Night	35% 8% 11% 27% 18% 2	1520
	11th to 20th	Night	38% 8% 13% 24% 15% 2	1520
	21st to end	Night	35% 6% 13% 26% 17% 3	1520
July	1st to 10th	Night	39% 8% 20% 17% 12% 4	2480
	11th to 20th	Night	38% 8% 17% 24% 11% 2	2472
	21st to end	Night	42% 6% 13% 22% 16% 1	2986
August	1st to 10th	Night	46% 6% 18% 21% 9% 1	2814
	11th to 20th	Night	45% 5% 10% 26% 12% 1	2994
	21st to end	Night	34% 5% 11% 31% 17% 2	3476
September	1st to 10th	Night	38% 5% 11% 23% 19% 5%	1900
	11th to 20th	Night	33% 4 9% 26% 23% 4	1900
	21st to end	Night	45% 6% 8% 20% 19% 3	2157
October	1st to 10th	Night	43% 3 10% 20% 20% 4	3800
	11th to 20th	Night	25% 2 10% 25% 30% 7%	3779
	21st to end	Night	25% 3 10% 31% 25% 6%	4120
November	1st to 10th	Night	34% 4 9% 26% 21% 6%	2410
	11th to 20th	Night	37% 4 9% 23% 21% 6%	2470
	21st to end	Night	36% 5% 11% 26% 20% 3	2650
December	1st to 10th	Night	44% 5% 11% 24% 15% 1	2660
	11th to 20th	Night	41% 4 11% 22% 19% 4	3288
	21st to end	Night	42% 8% 14% 24% 11% 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	27% 24% 33% 12% 4 1	180
	11th to 20th	Day	27% 29% 29% 14% 2	263
	21st to end	Day	19% 31% 31% 14% 4 1	297
February	1st to 10th	Day	25% 20% 39% 14% 2	270
	11th to 20th	Day	32% 22% 37% 8% 1	350
	21st to end	Day	28% 21% 31% 16% 3	382
March	1st to 10th	Day	23% 25% 31% 18% 3	460
	11th to 20th	Day	28% 21% 30% 15% 4 1	460
	21st to end	Day	25% 19% 33% 16% 4 1	506
April	1st to 10th	Day	22% 24% 34% 15% 4 2	460
	11th to 20th	Day	21% 18% 31% 18% 9% 3	452
	21st to end	Day	25% 17% 36% 15% 6% 2	460
May	1st to 10th	Day	29% 15% 29% 16% 9% 2	463
	11th to 20th	Day	23% 13% 30% 23% 8% 3	487
	21st to end	Day	17% 7% 28% 28% 17% 4	529
June	1st to 10th	Day	15% 11% 29% 26% 14% 5%	490
	11th to 20th	Day	16% 6% 27% 32% 15% 3	490
	21st to end	Day	12% 9% 28% 31% 14% 4	490
July	1st to 10th	Day	12% 3 20% 33% 23% 9%	485
	11th to 20th	Day	5% 3 21% 39% 23% 9%	484
	21st to end	Day	14% 3 27% 30% 21% 5%	532
August	1st to 10th	Day	12% 5% 20% 35% 20% 8%	486
	11th to 20th	Day	13% 3 20% 35% 20% 9%	479
	21st to end	Day	10% 4 25% 31% 19% 10%	539
September	1st to 10th	Day	16% 8% 19% 29% 18% 11%	490
	11th to 20th	Day	13% 5% 21% 34% 18% 9%	490
	21st to end	Day	28% 10% 21% 28% 10% 4	490
October	1st to 10th	Day	23% 9% 23% 23% 17% 5%	490
	11th to 20th	Day	18% 13% 28% 24% 14% 3	490
	21st to end	Day	18% 13% 36% 22% 9% 2	539
November	1st to 10th	Day	16% 12% 35% 24% 10% 3	394
	11th to 20th	Day	18% 18% 33% 21% 9% 1	370
	21st to end	Day	22% 19% 32% 20% 8%	188
December	1st to 10th	Day	23% 23% 33% 17% 3 1	180
	11th to 20th	Day	27% 19% 34% 15% 4	180
	21st to end	Day	24% 18% 33% 20% 6%	198
January	1st to 10th	Night	20% 18% 38% 22% 3	760
	11th to 20th	Night	24% 16% 32% 23% 5%	760
	21st to end	Night	19% 19% 38% 22% 2	836
February	1st to 10th	Night	20% 15% 35% 27% 3	760
	11th to 20th	Night	24% 20% 36% 18% 1	743
	21st to end	Night	24% 19% 35% 19% 3	628
March	1st to 10th	Night	20% 17% 36% 24% 4	760
	11th to 20th	Night	21% 19% 33% 20% 6% 1	760
	21st to end	Night	21% 14% 39% 21% 5% 1	836
April	1st to 10th	Night	21% 19% 35% 19% 5%	760
	11th to 20th	Night	17% 15% 33% 28% 8%	751
	21st to end	Night	22% 13% 32% 26% 7%	760
May	1st to 10th	Night	23% 9% 29% 31% 8% 1	664
	11th to 20th	Night	22% 11% 26% 30% 11%	382
	21st to end	Night	17% 6% 20% 36% 19% 2	411
June	1st to 10th	Night	14% 3 25% 39% 18% 1	380
	11th to 20th	Night	11% 5% 23% 41% 19% 1	380
	21st to end	Night	11% 3 26% 45% 15%	380
July	1st to 10th	Night	10% 5% 18% 38% 26% 3	370
	11th to 20th	Night	8% 2 15% 40% 31% 3	368
	21st to end	Night	9% 3 18% 41% 26% 3	621
August	1st to 10th	Night	12% 2 13% 41% 28% 3	567
	11th to 20th	Night	14% 5% 14% 37% 27% 3	668
	21st to end	Night	14% 3 17% 36% 25% 4	836
September	1st to 10th	Night	16% 3 21% 32% 23% 4	760
	11th to 20th	Night	13% 3 16% 36% 27% 5%	760
	21st to end	Night	25% 4 20% 32% 18% 2	760
October	1st to 10th	Night	23% 6% 21% 28% 18% 5%	760
	11th to 20th	Night	18% 8% 27% 32% 14% 1	760
	21st to end	Night	19% 12% 29% 27% 13% 1	836
November	1st to 10th	Night	15% 10% 33% 31% 10% 2	755
	11th to 20th	Night	18% 17% 28% 27% 8% 1	760
	21st to end	Night	16% 17% 31% 29% 7% 1	760
December	1st to 10th	Night	22% 14% 34% 21% 7% 1	760
	11th to 20th	Night	17% 15% 38% 23% 7%	760
	21st to end	Night	20% 15% 34% 29% 3	836

Page generated at 2024-11-01 22:47:42. Version ac07

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