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 2025-09-04.

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% 42%	126128
	Night	15% 6% 2 6% 4 4 4 3 5% 7% 44%	122597
Transparency	Day	40% 5% 12% 23% 17% 3	129720
	Night	40% 4 11% 23% 18% 4	127075
Seeing	Day	20% 13% 28% 24% 11% 4	16054
	Night	19% 12% 29% 28% 12% 1	25450

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	15% 4 1 3 3 4 4 3 5% 6% 52%	3597
	11th to 20th	Day	13% 9% 3 6% 4 3 3 3 4 5% 47%	4032
	21st to end	Day	16% 7% 2 5% 4 4 3 3 5% 6% 44%	4466
February	1st to 10th	Day	15% 6% 2 7% 5% 4 5% 4 5% 7% 40%	1800
	11th to 20th	Day	18% 8% 2 5% 4 3 4 3 6% 6% 40%	1838
	21st to end	Day	18% 8% 2 6% 5% 4 4 3 5% 5% 40%	1650
March	1st to 10th	Day	19% 5% 2 4 3 4 4 3 5% 7% 45%	4745
	11th to 20th	Day	11% 7% 2 5% 3 4 4 3 5% 7% 49%	4980
	21st to end	Day	17% 7% 2 6% 4 4 4 4 6% 6% 41%	5638
April	1st to 10th	Day	15% 6% 2 5% 5% 4 5% 4 5% 7% 41%	2400
	11th to 20th	Day	13% 6% 2 5% 5% 4 4 3 5% 7% 45%	2374
	21st to end	Day	18% 5% 2 5% 4 4 4 4 6% 8% 41%	2400
May	1st to 10th	Day	23% 6% 2 6% 4 4 4 4 6% 9% 31%	5678
	11th to 20th	Day	23% 6% 2 6% 4 4 4 4 7% 8% 31%	6308
	21st to end	Day	16% 5% 2 6% 5% 4 5% 4 6% 9% 39%	6934
June	1st to 10th	Day	11% 5% 2 6% 4 5% 4 4 7% 10% 41%	2800
	11th to 20th	Day	12% 6% 2 5% 4 5% 5% 5% 6% 8% 42%	2800
	21st to end	Day	10% 4 1 4 4 5% 5% 5% 7% 11% 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%	6345
	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% 7% 5% 7% 9% 33%	4234
	11th to 20th	Day	11% 5% 2 6% 4 5% 6% 5% 7% 10% 39%	3950
	21st to end	Day	10% 4 2 5% 3 4 6% 4 6% 8% 46%	4312
September	1st to 10th	Day	14% 5% 2 5% 3 4 5% 4 6% 7% 45%	2316
	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 4 6% 7% 51%	1801
	11th to 20th	Day	14% 6% 2 3 3 4 3 4 6% 6% 50%	1800
	21st to end	Day	15% 6% 2 5% 3 4 4 3 5% 5% 48%	1608
December	1st to 10th	Day	13% 6% 2 6% 4 3 4 4 5% 7% 47%	2600
	11th to 20th	Day	19% 9% 2 4 3 3 3 3 4 6% 42%	2990
	21st to end	Day	16% 7% 2 6% 4 5% 4 4 5% 7% 39%	3960
January	1st to 10th	Night	13% 6% 2 6% 4 4 3 3 4 6% 49%	6349
	11th to 20th	Night	14% 7% 2 5% 3 3 3 2 4 6% 51%	5927
	21st to end	Night	16% 8% 2 5% 3 3 3 3 4 5% 47%	6490
February	1st to 10th	Night	17% 7% 2 6% 4 4 4 3 6% 6% 41%	2600
	11th to 20th	Night	18% 7% 2 6% 3 3 3 3 5% 6% 43%	2446
	21st to end	Night	16% 8% 2 5% 4 4 5% 3 4 6% 43%	1980
March	1st to 10th	Night	17% 6% 2 5% 3 3 3 3 4 5% 49%	5103
	11th to 20th	Night	10% 7% 2 5% 3 3 4 3 5% 6% 51%	4980
	21st to end	Night	13% 7% 2 6% 4 4 4 3 5% 5% 45%	5294
April	1st to 10th	Night	16% 7% 2 6% 5% 4 4 4 6% 5% 41%	2000
	11th to 20th	Night	14% 6% 2 5% 4 4 4 4 6% 5% 46%	1975
	21st to end	Night	17% 6% 1 4 4 4 3 4 5% 7% 44%	2000
May	1st to 10th	Night	23% 8% 3 6% 4 5% 5% 3 5% 8% 30%	4246
	11th to 20th	Night	25% 8% 2 5% 4 3 4 3 6% 8% 31%	3583
	21st to end	Night	19% 6% 2 6% 4 4 4 4 6% 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% 6% 1 6% 5% 4 5% 5% 6% 10% 40%	1600
	21st to end	Night	11% 5% 2 5% 4 5% 6% 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% 7% 3 9% 6% 6% 6% 5% 8% 11% 30%	3597
	21st to end	Night	9% 6% 3 6% 4 4 5% 5% 6% 9% 43%	4424
August	1st to 10th	Night	17% 6% 3 6% 4 4 5% 4 7% 10% 33%	2915
	11th to 20th	Night	11% 7% 3 8% 6% 5% 6% 5% 7% 7% 35%	3063
	21st to end	Night	12% 5% 2 5% 3 4 4 3 6% 8% 48%	3586
September	1st to 10th	Night	13% 7% 3 5% 3 4 4 3 6% 7% 46%	1931
	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	11% 5% 3 4 3 3 4 3 4 5% 53%	2494
	11th to 20th	Night	15% 6% 2 5% 4 3 3 3 4 5% 50%	2600
	21st to end	Night	13% 4 2 5% 4 4 4 4 5% 7% 48%	2790
December	1st to 10th	Night	11% 5% 2 5% 4 4 4 3 5% 6% 50%	4580
	11th to 20th	Night	21% 6% 2 5% 3 4 4 3 5% 6% 41%	5205
	21st to end	Night	17% 7% 2 6% 4 4 5% 4 6% 7% 39%	6996

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	33% 3 7% 28% 24% 5%	3598
	11th to 20th	Day	42% 5% 7% 21% 18% 7%	4032
	21st to end	Day	42% 4 11% 27% 14% 2	4466
February	1st to 10th	Day	43% 5% 13% 25% 12% 2	1800
	11th to 20th	Day	43% 4 10% 25% 15% 3	1847
	21st to end	Day	45% 5% 9% 22% 14% 4	1650
March	1st to 10th	Day	38% 4 8% 26% 18% 7%	4745
	11th to 20th	Day	36% 4 9% 25% 20% 6%	4980
	21st to end	Day	44% 4 8% 19% 19% 6%	5638
April	1st to 10th	Day	41% 6% 11% 19% 18% 5%	2400
	11th to 20th	Day	38% 5% 10% 22% 18% 6%	2374
	21st to end	Day	40% 5% 11% 21% 19% 4	2400
May	1st to 10th	Day	50% 6% 14% 18% 11% 2	5710
	11th to 20th	Day	49% 7% 10% 22% 11% 1	6312
	21st to end	Day	41% 6% 12% 23% 16% 2	6949
June	1st to 10th	Day	37% 8% 14% 24% 16% 2	2800
	11th to 20th	Day	39% 8% 12% 24% 15% 3	2800
	21st to end	Day	33% 7% 15% 25% 18% 2	2800
July	1st to 10th	Day	35% 8% 17% 20% 15% 4	6290
	11th to 20th	Day	43% 7% 19% 22% 7% 1	6270
	21st to end	Day	36% 6% 12% 21% 23% 1	6240
August	1st to 10th	Day	47% 6% 13% 25% 9% 1	4221
	11th to 20th	Day	38% 8% 12% 28% 14% 1	3989
	21st to end	Day	34% 6% 12% 28% 18% 2	4312
September	1st to 10th	Day	38% 6% 12% 21% 20% 4	2316
	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% 25% 21% 4	1801
	11th to 20th	Day	35% 4 10% 23% 24% 4	1800
	21st to end	Day	39% 4 11% 28% 16% 2	1608
December	1st to 10th	Day	37% 4 15% 27% 13% 4	2600
	11th to 20th	Day	45% 3 11% 29% 11% 1	2986
	21st to end	Day	43% 6% 15% 20% 15% 2	3960
January	1st to 10th	Night	37% 3 7% 25% 23% 5%	6349
	11th to 20th	Night	36% 3 8% 25% 21% 7%	5927
	21st to end	Night	40% 3 8% 28% 19% 3	6490
February	1st to 10th	Night	44% 4 13% 24% 14% 2	2600
	11th to 20th	Night	43% 3 9% 22% 18% 5%	2448
	21st to end	Night	43% 4 10% 21% 17% 5%	1980
March	1st to 10th	Night	39% 4 7% 27% 13% 10%	5103
	11th to 20th	Night	33% 5% 12% 29% 16% 6%	4980
	21st to end	Night	41% 3 8% 19% 24% 5%	5294
April	1st to 10th	Night	43% 5% 9% 19% 19% 4	2000
	11th to 20th	Night	39% 5% 11% 20% 20% 5%	1975
	21st to end	Night	38% 6% 10% 22% 19% 5%	2000
May	1st to 10th	Night	54% 5% 11% 16% 11% 3	4276
	11th to 20th	Night	52% 6% 10% 19% 11% 1	3584
	21st to end	Night	44% 6% 13% 22% 12% 2	3927
June	1st to 10th	Night	35% 7% 11% 26% 18% 2	1600
	11th to 20th	Night	39% 8% 13% 24% 15% 2	1600
	21st to end	Night	36% 6% 13% 25% 18% 3	1600
July	1st to 10th	Night	41% 8% 17% 17% 14% 3	3560
	11th to 20th	Night	47% 7% 19% 18% 8% 1	3552
	21st to end	Night	37% 6% 13% 25% 20% 1	4295
August	1st to 10th	Night	46% 6% 18% 21% 9% 1	2904
	11th to 20th	Night	45% 5% 11% 26% 12% 1	3090
	21st to end	Night	34% 5% 10% 31% 17% 2	3586
September	1st to 10th	Night	37% 5% 11% 23% 19% 5%	1931
	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	33% 4 9% 26% 22% 6%	2494
	11th to 20th	Night	37% 4 9% 24% 20% 6%	2600
	21st to end	Night	36% 5% 11% 26% 20% 3	2790
December	1st to 10th	Night	36% 3 13% 29% 17% 2	4580
	11th to 20th	Night	45% 3 13% 18% 17% 4	5208
	21st to end	Night	43% 6% 12% 20% 17% 1	6996

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	30% 24% 32% 11% 4 1	200
	11th to 20th	Day	27% 29% 29% 13% 2	293
	21st to end	Day	19% 30% 32% 14% 4 1	330
February	1st to 10th	Day	27% 21% 37% 13% 2	300
	11th to 20th	Day	35% 22% 35% 7% 1	385
	21st to end	Day	27% 22% 32% 16% 3	414
March	1st to 10th	Day	24% 25% 31% 17% 3	500
	11th to 20th	Day	29% 21% 30% 15% 5% 1	500
	21st to end	Day	26% 19% 34% 16% 4 1	550
April	1st to 10th	Day	25% 24% 33% 14% 3 1	500
	11th to 20th	Day	23% 19% 30% 18% 8% 3	492
	21st to end	Day	24% 17% 36% 16% 6% 2	500
May	1st to 10th	Day	29% 15% 30% 16% 8% 2	503
	11th to 20th	Day	24% 13% 31% 23% 8% 3	527
	21st to end	Day	19% 8% 27% 26% 16% 3	573
June	1st to 10th	Day	15% 11% 29% 26% 14% 5%	530
	11th to 20th	Day	17% 7% 27% 32% 15% 3	530
	21st to end	Day	12% 8% 27% 32% 15% 6%	530
July	1st to 10th	Day	12% 3 20% 34% 22% 9%	525
	11th to 20th	Day	6% 4 20% 39% 22% 9%	524
	21st to end	Day	13% 3 26% 31% 21% 5%	576
August	1st to 10th	Day	13% 4 21% 34% 19% 8%	526
	11th to 20th	Day	14% 3 19% 36% 19% 9%	519
	21st to end	Day	10% 5% 26% 31% 19% 10%	583
September	1st to 10th	Day	16% 8% 19% 29% 18% 11%	502
	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% 36% 24% 10% 3	415
	11th to 20th	Day	18% 17% 34% 22% 9% 1	400
	21st to end	Day	24% 19% 31% 19% 7%	208
December	1st to 10th	Day	23% 22% 36% 16% 3 1	200
	11th to 20th	Day	27% 20% 35% 15% 5%	200
	21st to end	Day	23% 19% 34% 20% 5%	220
January	1st to 10th	Night	20% 19% 37% 21% 3	800
	11th to 20th	Night	23% 17% 33% 22% 5%	800
	21st to end	Night	19% 18% 39% 22% 2	880
February	1st to 10th	Night	21% 15% 35% 26% 3	800
	11th to 20th	Night	26% 20% 36% 17% 1	783
	21st to end	Night	23% 19% 36% 19% 3	660
March	1st to 10th	Night	20% 18% 35% 23% 3	800
	11th to 20th	Night	21% 19% 33% 21% 6% 1	800
	21st to end	Night	21% 15% 40% 20% 4 1	880
April	1st to 10th	Night	22% 19% 35% 18% 5%	800
	11th to 20th	Night	18% 15% 33% 27% 7%	791
	21st to end	Night	22% 14% 31% 27% 7%	800
May	1st to 10th	Night	23% 9% 29% 31% 7% 1	699
	11th to 20th	Night	22% 11% 26% 31% 10%	402
	21st to end	Night	18% 6% 21% 35% 18% 2	433
June	1st to 10th	Night	14% 4 24% 39% 18% 1	400
	11th to 20th	Night	13% 5% 22% 40% 19% 1	400
	21st to end	Night	12% 3 26% 45% 15% 1	400
July	1st to 10th	Night	11% 5% 18% 37% 26% 3	390
	11th to 20th	Night	9% 2 15% 39% 32% 3	388
	21st to end	Night	9% 3 19% 40% 27% 3	654
August	1st to 10th	Night	12% 2 13% 41% 28% 3	597
	11th to 20th	Night	15% 5% 14% 37% 27% 3	704
	21st to end	Night	13% 4 18% 36% 25% 4	880
September	1st to 10th	Night	16% 3 21% 32% 24% 4	773
	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	14% 10% 33% 31% 10% 2	780
	11th to 20th	Night	18% 17% 29% 27% 8% 1	800
	21st to end	Night	16% 17% 32% 28% 7% 1	800
December	1st to 10th	Night	22% 14% 35% 22% 7% 1	800
	11th to 20th	Night	19% 14% 37% 23% 6%	800
	21st to end	Night	20% 14% 33% 29% 3	880

Page generated at 2025-09-01 01:51:06. Version ac07

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