Potomac Astrometric Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Potomac Astrometric Observatory clear sky chart from 2005-11-06 to 2025-10-19.

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 6% 4 4 4 4 6% 8% 41%	126562
	Night	15% 6% 2 6% 4 4 4 4 5% 7% 43%	123160
Transparency	Day	40% 5% 11% 24% 16% 3	130156
	Night	39% 5% 11% 24% 18% 4	127636
Seeing	Day	20% 12% 27% 24% 13% 4	16227
	Night	18% 12% 29% 29% 12% 1	25701

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 4 3 4 4 4 5% 6% 50%	3707
	11th to 20th	Day	14% 9% 3 6% 4 4 4 3 3 4 47%	4060
	21st to end	Day	16% 7% 3 6% 4 4 4 3 4 6% 43%	4466
February	1st to 10th	Day	15% 7% 2 7% 5% 5% 4 4 6% 7% 38%	1800
	11th to 20th	Day	18% 8% 2 5% 5% 3 4 4 5% 6% 39%	1807
	21st to end	Day	17% 8% 3 7% 4 4 4 3 4 5% 41%	1650
March	1st to 10th	Day	19% 5% 2 5% 4 4 4 3 5% 6% 43%	4846
	11th to 20th	Day	12% 6% 2 5% 4 4 4 3 5% 6% 50%	4980
	21st to end	Day	16% 6% 2 6% 4 5% 4 4 5% 7% 41%	5553
April	1st to 10th	Day	15% 6% 2 6% 4 4 4 4 6% 7% 42%	2400
	11th to 20th	Day	13% 6% 2 5% 5% 4 4 4 5% 6% 47%	2374
	21st to end	Day	19% 6% 2 5% 4 4 4 3 6% 7% 41%	2400
May	1st to 10th	Day	20% 7% 2 7% 5% 4 4 4 6% 9% 32%	5767
	11th to 20th	Day	23% 7% 2 6% 4 4 5% 4 6% 9% 30%	6169
	21st to end	Day	16% 5% 2 6% 4 4 5% 4 6% 8% 39%	6934
June	1st to 10th	Day	11% 5% 2 6% 5% 4 5% 4 7% 8% 43%	2800
	11th to 20th	Day	12% 6% 2 6% 5% 5% 5% 5% 6% 8% 41%	2800
	21st to end	Day	9% 4 2 4 4 4 5% 4 8% 11% 45%	2800
July	1st to 10th	Day	8% 4 2 5% 6% 4 5% 5% 7% 11% 43%	6360
	11th to 20th	Day	10% 7% 3 8% 5% 6% 6% 6% 8% 11% 31%	6177
	21st to end	Day	9% 6% 2 6% 5% 6% 5% 5% 7% 10% 39%	6424
August	1st to 10th	Day	17% 6% 2 6% 5% 5% 6% 4 7% 9% 34%	4234
	11th to 20th	Day	10% 5% 2 7% 5% 5% 7% 5% 7% 8% 38%	3992
	21st to end	Day	10% 5% 2 6% 3 4 4 4 5% 8% 48%	4312
September	1st to 10th	Day	13% 5% 2 6% 4 3 4 4 5% 7% 46%	2400
	11th to 20th	Day	13% 6% 2 5% 3 4 4 4 5% 8% 47%	2400
	21st to end	Day	20% 8% 3 6% 4 4 3 4 6% 5% 37%	2103
October	1st to 10th	Day	17% 5% 2 4 4 3 3 3 5% 6% 48%	2000
	11th to 20th	Day	18% 4 2 4 3 3 4 3 5% 6% 50%	1980
	21st to end	Day	14% 5% 2 6% 4 4 4 3 5% 5% 49%	2090
November	1st to 10th	Day	12% 5% 1 4 3 4 4 3 5% 7% 51%	1852
	11th to 20th	Day	15% 5% 2 4 3 3 3 3 6% 6% 49%	1775
	21st to end	Day	15% 7% 1 5% 4 4 4 4 5% 5% 47%	1600
December	1st to 10th	Day	12% 5% 2 6% 4 3 4 4 5% 8% 48%	2600
	11th to 20th	Day	19% 8% 2 5% 3 4 3 3 5% 6% 43%	2990
	21st to end	Day	18% 8% 2 6% 4 4 4 4 5% 7% 37%	3960
January	1st to 10th	Night	13% 6% 2 5% 4 4 4 3 5% 6% 49%	6235
	11th to 20th	Night	14% 6% 2 5% 3 3 3 3 4 5% 51%	5900
	21st to end	Night	16% 8% 2 5% 3 3 3 3 4 6% 48%	6490
February	1st to 10th	Night	15% 6% 2 6% 4 5% 4 4 6% 6% 41%	2600
	11th to 20th	Night	19% 7% 1 6% 4 4 3 3 5% 6% 43%	2478
	21st to end	Night	17% 8% 2 5% 4 4 3 3 4 6% 42%	1980
March	1st to 10th	Night	17% 6% 2 5% 3 4 3 3 4 5% 48%	4992
	11th to 20th	Night	11% 5% 2 5% 3 4 4 3 5% 7% 50%	4980
	21st to end	Night	12% 8% 2 6% 4 4 4 3 5% 6% 44%	5377
April	1st to 10th	Night	16% 7% 2 6% 4 4 4 5% 5% 6% 40%	2000
	11th to 20th	Night	14% 6% 2 7% 4 3 5% 4 5% 6% 45%	1975
	21st to end	Night	15% 6% 2 5% 4 5% 4 3 6% 7% 44%	2000
May	1st to 10th	Night	22% 9% 2 7% 5% 5% 5% 4 6% 7% 30%	4169
	11th to 20th	Night	23% 8% 2 5% 4 4 4 5% 5% 8% 31%	3723
	21st to end	Night	18% 6% 2 6% 4 5% 5% 4 6% 9% 36%	3920
June	1st to 10th	Night	11% 4 2 5% 4 4 5% 4 7% 9% 45%	1600
	11th to 20th	Night	12% 6% 2 6% 5% 4 4 5% 6% 9% 41%	1600
	21st to end	Night	9% 5% 2 6% 5% 4 5% 4 7% 11% 42%	1600
July	1st to 10th	Night	9% 5% 2 6% 6% 5% 6% 5% 7% 10% 39%	3598
	11th to 20th	Night	9% 7% 3 9% 6% 7% 7% 6% 9% 9% 30%	3775
	21st to end	Night	9% 6% 2 6% 4 5% 5% 5% 8% 9% 41%	4427
August	1st to 10th	Night	17% 6% 3 7% 5% 4 5% 5% 6% 9% 33%	2915
	11th to 20th	Night	11% 6% 3 9% 6% 5% 6% 4 7% 8% 36%	3021
	21st to end	Night	11% 5% 2 5% 3 3 4 3 6% 8% 50%	3586
September	1st to 10th	Night	15% 6% 2 5% 4 3 4 3 5% 7% 47%	2000
	11th to 20th	Night	11% 6% 2 5% 4 3 4 3 5% 6% 49%	2000
	21st to end	Night	20% 6% 3 7% 4 4 4 3 5% 6% 38%	2290
October	1st to 10th	Night	19% 6% 2 4 3 3 3 3 5% 6% 47%	2400
	11th to 20th	Night	16% 4 1 4 3 3 3 2 4 5% 53%	2377
	21st to end	Night	15% 6% 1 5% 3 3 4 3 5% 6% 50%	2508
November	1st to 10th	Night	12% 6% 2 5% 4 3 3 3 4 5% 54%	2443
	11th to 20th	Night	15% 6% 2 5% 4 3 3 3 4 6% 50%	2620
	21st to end	Night	14% 4 2 5% 5% 4 4 4 6% 7% 47%	2800
December	1st to 10th	Night	11% 5% 2 5% 4 4 4 3 5% 7% 51%	4580
	11th to 20th	Night	21% 6% 2 5% 4 4 4 3 5% 6% 41%	5205
	21st to end	Night	17% 7% 2 6% 4 4 5% 4 6% 7% 37%	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	35% 4 5% 28% 22% 6%	3708
	11th to 20th	Day	42% 3 7% 22% 18% 8%	4060
	21st to end	Day	44% 3 10% 27% 14% 3	4466
February	1st to 10th	Day	44% 5% 12% 25% 12% 2	1800
	11th to 20th	Day	45% 4 9% 24% 16% 3	1816
	21st to end	Day	46% 4 10% 22% 15% 4	1650
March	1st to 10th	Day	40% 4 9% 22% 17% 7%	4846
	11th to 20th	Day	34% 5% 9% 27% 20% 5%	4980
	21st to end	Day	44% 4 9% 19% 18% 6%	5553
April	1st to 10th	Day	41% 5% 11% 20% 18% 5%	2400
	11th to 20th	Day	38% 5% 10% 22% 20% 6%	2374
	21st to end	Day	42% 5% 9% 23% 18% 3	2400
May	1st to 10th	Day	48% 6% 15% 19% 10% 2	5801
	11th to 20th	Day	48% 7% 11% 24% 9% 1	6173
	21st to end	Day	42% 5% 13% 22% 15% 2	6949
June	1st to 10th	Day	37% 6% 14% 26% 16% 2	2800
	11th to 20th	Day	39% 8% 13% 25% 14% 2	2800
	21st to end	Day	32% 7% 15% 26% 18% 2	2800
July	1st to 10th	Day	34% 9% 17% 20% 15% 4	6290
	11th to 20th	Day	42% 10% 16% 23% 8% 1	6102
	21st to end	Day	38% 7% 9% 24% 20% 1	6240
August	1st to 10th	Day	45% 6% 15% 25% 8%	4221
	11th to 20th	Day	39% 7% 11% 27% 14% 1	4031
	21st to end	Day	34% 6% 12% 28% 18% 2	4312
September	1st to 10th	Day	37% 6% 12% 21% 20% 4	2400
	11th to 20th	Day	35% 5% 8% 28% 20% 3	2400
	21st to end	Day	48% 5% 8% 19% 18% 2	2103
October	1st to 10th	Day	37% 4 7% 19% 28% 5%	3260
	11th to 20th	Day	26% 4 7% 29% 26% 9%	3220
	21st to end	Day	24% 3 9% 31% 28% 6%	3428
November	1st to 10th	Day	33% 4 11% 26% 21% 5%	1852
	11th to 20th	Day	35% 4 10% 24% 22% 4	1775
	21st to end	Day	39% 5% 10% 29% 17% 2	1600
December	1st to 10th	Day	36% 4 14% 29% 14% 4	2600
	11th to 20th	Day	44% 4 12% 28% 10% 2	2986
	21st to end	Day	45% 4 14% 21% 15% 1	3960
January	1st to 10th	Night	37% 4 6% 25% 24% 4	6235
	11th to 20th	Night	36% 4 8% 25% 21% 7%	5900
	21st to end	Night	39% 2 8% 29% 19% 3	6490
February	1st to 10th	Night	43% 4 12% 25% 14% 1	2600
	11th to 20th	Night	43% 4 9% 22% 18% 4	2480
	21st to end	Night	44% 3 9% 22% 16% 5%	1980
March	1st to 10th	Night	39% 4 7% 26% 14% 9%	4992
	11th to 20th	Night	33% 5% 10% 28% 17% 7%	4980
	21st to end	Night	41% 3 8% 22% 21% 5%	5377
April	1st to 10th	Night	43% 5% 9% 20% 19% 4	2000
	11th to 20th	Night	39% 4 11% 21% 18% 5%	1975
	21st to end	Night	38% 6% 10% 25% 16% 5%	2000
May	1st to 10th	Night	53% 5% 12% 17% 9% 3	4198
	11th to 20th	Night	50% 6% 11% 22% 11% 1	3724
	21st to end	Night	45% 5% 14% 23% 10% 2	3927
June	1st to 10th	Night	35% 7% 12% 27% 18% 2	1600
	11th to 20th	Night	39% 7% 13% 25% 14% 2	1600
	21st to end	Night	35% 6% 14% 24% 19% 3	1600
July	1st to 10th	Night	40% 9% 17% 18% 15% 3	3560
	11th to 20th	Night	46% 11% 16% 18% 8% 1	3730
	21st to end	Night	37% 7% 12% 24% 19% 1	4297
August	1st to 10th	Night	46% 7% 17% 21% 8% 1	2904
	11th to 20th	Night	43% 8% 10% 27% 12% 1	3048
	21st to end	Night	32% 6% 12% 31% 17% 2	3586
September	1st to 10th	Night	38% 5% 10% 23% 19% 5%	2000
	11th to 20th	Night	35% 4 8% 27% 22% 3	2000
	21st to end	Night	47% 5% 9% 19% 17% 3	2290
October	1st to 10th	Night	40% 4 9% 22% 20% 5%	3920
	11th to 20th	Night	25% 3 10% 24% 30% 8%	3876
	21st to end	Night	24% 3 9% 32% 25% 7%	4120
November	1st to 10th	Night	33% 3 10% 27% 23% 5%	2443
	11th to 20th	Night	37% 3 8% 24% 22% 5%	2620
	21st to end	Night	36% 4 11% 25% 20% 3	2800
December	1st to 10th	Night	35% 3 12% 28% 19% 2	4580
	11th to 20th	Night	46% 4 11% 19% 16% 4	5208
	21st to end	Night	45% 5% 13% 21% 15% 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	35% 16% 33% 12% 3 1	223
	11th to 20th	Day	31% 23% 31% 12% 2	300
	21st to end	Day	24% 23% 36% 12% 4 1	330
February	1st to 10th	Day	27% 21% 38% 10% 4	300
	11th to 20th	Day	36% 20% 36% 6% 2	354
	21st to end	Day	27% 20% 34% 17% 2	414
March	1st to 10th	Day	26% 22% 31% 17% 4	500
	11th to 20th	Day	27% 21% 30% 18% 3 2	500
	21st to end	Day	22% 20% 33% 18% 5% 1	550
April	1st to 10th	Day	24% 21% 32% 16% 6% 1	500
	11th to 20th	Day	19% 18% 34% 17% 10% 3	492
	21st to end	Day	26% 12% 33% 19% 8% 2	500
May	1st to 10th	Day	24% 17% 28% 20% 8% 3	503
	11th to 20th	Day	22% 11% 29% 23% 11% 3	527
	21st to end	Day	17% 8% 25% 28% 18% 4	573
June	1st to 10th	Day	13% 10% 28% 25% 19% 6%	530
	11th to 20th	Day	17% 6% 24% 32% 17% 5%	530
	21st to end	Day	10% 7% 28% 31% 19% 6%	530
July	1st to 10th	Day	12% 3 18% 34% 22% 11%	525
	11th to 20th	Day	7% 3 21% 34% 28% 8%	524
	21st to end	Day	12% 4 22% 33% 24% 6%	576
August	1st to 10th	Day	14% 4 19% 30% 23% 11%	526
	11th to 20th	Day	12% 4 17% 34% 22% 10%	519
	21st to end	Day	10% 4 20% 33% 21% 11%	583
September	1st to 10th	Day	17% 7% 19% 26% 20% 11%	530
	11th to 20th	Day	14% 4 20% 32% 19% 11%	530
	21st to end	Day	27% 7% 22% 28% 11% 4	530
October	1st to 10th	Day	22% 6% 20% 26% 18% 7%	530
	11th to 20th	Day	18% 14% 26% 26% 12% 4	522
	21st to end	Day	17% 12% 33% 25% 11% 2	539
November	1st to 10th	Day	16% 10% 35% 24% 12% 2	442
	11th to 20th	Day	20% 14% 36% 20% 9% 1	375
	21st to end	Day	26% 18% 33% 14% 10% 1	200
December	1st to 10th	Day	23% 19% 37% 18% 3 2	200
	11th to 20th	Day	26% 20% 30% 18% 7%	200
	21st to end	Day	25% 17% 35% 17% 6%	220
January	1st to 10th	Night	21% 20% 40% 17% 3	800
	11th to 20th	Night	23% 18% 32% 24% 3	800
	21st to end	Night	20% 19% 38% 22% 2	880
February	1st to 10th	Night	19% 16% 36% 26% 2	800
	11th to 20th	Night	27% 22% 35% 15% 1	783
	21st to end	Night	24% 17% 37% 20% 2	660
March	1st to 10th	Night	21% 18% 37% 21% 3	800
	11th to 20th	Night	22% 20% 33% 20% 4 1	800
	21st to end	Night	19% 18% 38% 20% 4 1	880
April	1st to 10th	Night	21% 22% 32% 21% 4	800
	11th to 20th	Night	17% 15% 35% 26% 7% 1	791
	21st to end	Night	19% 12% 35% 27% 7%	800
May	1st to 10th	Night	22% 8% 30% 28% 12%	660
	11th to 20th	Night	22% 6% 27% 35% 10%	464
	21st to end	Night	15% 5% 24% 37% 17% 1	433
June	1st to 10th	Night	12% 5% 27% 35% 21% 1	400
	11th to 20th	Night	14% 4 24% 41% 16% 2	400
	21st to end	Night	10% 3 25% 43% 19% 1	400
July	1st to 10th	Night	10% 3 17% 41% 26% 3	390
	11th to 20th	Night	5% 3 14% 41% 33% 3	462
	21st to end	Night	9% 3 16% 42% 26% 4	657
August	1st to 10th	Night	12% 3 11% 42% 29% 2	597
	11th to 20th	Night	13% 3 13% 44% 24% 3	675
	21st to end	Night	11% 3 16% 39% 27% 4	880
September	1st to 10th	Night	16% 3 21% 30% 26% 4	800
	11th to 20th	Night	11% 5% 15% 37% 29% 4	800
	21st to end	Night	23% 5% 22% 33% 18% 1	800
October	1st to 10th	Night	20% 7% 20% 29% 21% 4	800
	11th to 20th	Night	18% 12% 26% 29% 15% 1	793
	21st to end	Night	19% 11% 31% 26% 13%	836
November	1st to 10th	Night	15% 11% 31% 33% 10% 1	780
	11th to 20th	Night	19% 16% 31% 27% 6% 1	800
	21st to end	Night	16% 17% 33% 27% 7% 1	800
December	1st to 10th	Night	23% 17% 32% 22% 6% 1	800
	11th to 20th	Night	20% 14% 37% 24% 5%	800
	21st to end	Night	19% 16% 34% 29% 3	880

Page generated at 2025-10-16 01:50:30. Version ac07

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