Pittsburgh Space Weather Observation Area
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Pittsburgh Space Weather Observation Area clear sky chart from 2005-11-06 to 2025-09-19.

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	16% 11% 4 9% 5% 4 4 4 5% 7% 30%	126062
	Night	17% 12% 5% 10% 5% 4 4 3 5% 6% 30%	122971
Transparency	Day	52% 6% 11% 20% 10% 1	129665
	Night	54% 5% 11% 19% 10% 1	127437
Seeing	Day	23% 16% 28% 20% 10% 4	15775
	Night	23% 13% 27% 25% 11% 1	25403

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% 20% 9% 22% 5% 3 3 2 3 4 14%	3987
	11th to 20th	Day	12% 21% 9% 21% 8% 6% 3 2 2 3 13%	4060
	21st to end	Day	21% 20% 8% 11% 8% 4 4 2 4 4 14%	4466
February	1st to 10th	Day	19% 19% 6% 11% 8% 3 4 3 3 4 21%	1800
	11th to 20th	Day	16% 14% 7% 15% 7% 4 4 3 4 6% 20%	1753
	21st to end	Day	20% 16% 4 11% 4 4 3 4 5% 6% 23%	1639
March	1st to 10th	Day	17% 12% 7% 9% 5% 4 3 2 4 5% 31%	4935
	11th to 20th	Day	12% 11% 5% 9% 6% 5% 4 3 4 6% 35%	4980
	21st to end	Day	18% 11% 4 9% 5% 4 4 3 5% 6% 30%	5465
April	1st to 10th	Day	18% 14% 3 8% 4 4 5% 4 5% 7% 29%	2307
	11th to 20th	Day	19% 12% 2 7% 5% 4 4 4 5% 6% 32%	2423
	21st to end	Day	21% 10% 3 7% 4 4 3 3 5% 6% 34%	2600
May	1st to 10th	Day	21% 10% 3 8% 5% 5% 4 4 6% 6% 27%	5862
	11th to 20th	Day	19% 9% 3 7% 5% 5% 5% 5% 7% 8% 29%	5861
	21st to end	Day	19% 8% 2 7% 4 5% 5% 5% 7% 7% 32%	6876
June	1st to 10th	Day	15% 7% 3 7% 5% 5% 5% 5% 6% 9% 34%	2800
	11th to 20th	Day	16% 7% 2 7% 5% 5% 5% 4 6% 9% 34%	2800
	21st to end	Day	14% 6% 3 6% 5% 5% 5% 4 7% 9% 37%	2800
July	1st to 10th	Day	9% 6% 3 7% 5% 5% 5% 4 6% 10% 40%	6235
	11th to 20th	Day	11% 7% 3 8% 5% 5% 5% 5% 8% 9% 35%	5900
	21st to end	Day	12% 6% 2 7% 5% 4 5% 5% 6% 8% 41%	6424
August	1st to 10th	Day	15% 8% 3 9% 6% 6% 5% 5% 6% 8% 30%	4234
	11th to 20th	Day	13% 6% 3 7% 5% 6% 6% 5% 7% 9% 34%	4174
	21st to end	Day	8% 5% 2 8% 5% 5% 6% 4 7% 9% 40%	4462
September	1st to 10th	Day	15% 7% 2 5% 3 4 4 4 5% 7% 43%	2400
	11th to 20th	Day	12% 5% 3 5% 4 4 4 4 7% 7% 43%	2184
	21st to end	Day	20% 9% 3 7% 4 4 4 3 5% 6% 35%	2089
October	1st to 10th	Day	16% 7% 3 6% 4 5% 5% 4 5% 8% 38%	1900
	11th to 20th	Day	21% 9% 3 7% 5% 5% 4 4 4 5% 33%	1900
	21st to end	Day	23% 10% 3 7% 5% 3 3 4 4 6% 30%	2090
November	1st to 10th	Day	12% 10% 4 6% 5% 5% 4 3 4 7% 41%	1857
	11th to 20th	Day	18% 16% 4 8% 5% 4 4 3 4 6% 30%	1649
	21st to end	Day	19% 18% 7% 10% 5% 4 3 3 5% 5% 21%	1600
December	1st to 10th	Day	20% 17% 8% 22% 5% 3 3 2 3 3 15%	2600
	11th to 20th	Day	23% 19% 7% 11% 4 3 3 2 4 4 21%	2990
	21st to end	Day	23% 25% 7% 11% 5% 4 3 3 3 4 12%	3960
January	1st to 10th	Night	14% 20% 8% 25% 4 3 3 2 3 4 13%	5953
	11th to 20th	Night	13% 18% 10% 18% 9% 6% 4 3 3 3 13%	5900
	21st to end	Night	21% 18% 8% 12% 7% 4 5% 2 3 3 16%	6490
February	1st to 10th	Night	19% 16% 5% 11% 8% 5% 3 3 4 4 22%	2600
	11th to 20th	Night	18% 14% 5% 14% 6% 5% 4 2 5% 5% 22%	2533
	21st to end	Night	20% 13% 4 12% 6% 5% 4 3 4 4 25%	1985
March	1st to 10th	Night	21% 14% 5% 8% 5% 3 3 2 3 5% 30%	4918
	11th to 20th	Night	14% 11% 5% 6% 4 4 4 3 4 6% 38%	4980
	21st to end	Night	18% 11% 4 8% 5% 4 4 3 4 5% 34%	5467
April	1st to 10th	Night	19% 14% 3 7% 5% 4 4 3 4 6% 31%	2091
	11th to 20th	Night	18% 10% 3 6% 4 4 5% 4 5% 6% 34%	1942
	21st to end	Night	16% 10% 3 7% 4 4 4 4 6% 7% 35%	1800
May	1st to 10th	Night	27% 11% 3 7% 4 4 3 3 5% 6% 26%	4033
	11th to 20th	Night	18% 10% 3 6% 4 3 4 3 4 7% 36%	4032
	21st to end	Night	20% 8% 2 6% 3 3 4 4 6% 9% 35%	3974
June	1st to 10th	Night	11% 6% 2 7% 5% 4 4 4 6% 8% 43%	1600
	11th to 20th	Night	12% 6% 2 6% 5% 4 4 4 6% 8% 42%	1600
	21st to end	Night	11% 6% 3 6% 4 4 5% 4 7% 10% 40%	1600
July	1st to 10th	Night	12% 8% 2 7% 4 4 4 3 5% 7% 43%	3704
	11th to 20th	Night	11% 6% 3 6% 5% 5% 5% 5% 7% 10% 38%	4057
	21st to end	Night	9% 6% 2 7% 5% 4 5% 4 5% 7% 46%	4427
August	1st to 10th	Night	14% 8% 3 8% 5% 5% 4 4 5% 7% 38%	2915
	11th to 20th	Night	14% 7% 3 7% 4 4 3 3 5% 7% 44%	2875
	21st to end	Night	8% 5% 2 6% 5% 5% 5% 4 7% 9% 44%	3402
September	1st to 10th	Night	13% 5% 2 5% 3 4 4 3 6% 6% 49%	2000
	11th to 20th	Night	10% 4 2 5% 3 3 3 3 5% 8% 53%	2169
	21st to end	Night	19% 8% 2 7% 4 4 4 3 5% 6% 39%	2090
October	1st to 10th	Night	16% 6% 2 5% 3 4 4 3 5% 8% 44%	2280
	11th to 20th	Night	17% 8% 3 7% 4 4 4 3 5% 6% 38%	2280
	21st to end	Night	24% 9% 2 6% 4 3 4 3 5% 7% 33%	2508
November	1st to 10th	Night	13% 9% 3 6% 5% 4 3 3 5% 8% 42%	2436
	11th to 20th	Night	18% 14% 3 8% 5% 5% 5% 3 5% 6% 28%	2749
	21st to end	Night	19% 17% 4 8% 5% 4 3 3 4 6% 27%	2800
December	1st to 10th	Night	17% 17% 9% 16% 6% 4 3 3 4 4 16%	4580
	11th to 20th	Night	21% 18% 8% 11% 5% 3 3 3 3 5% 19%	5205
	21st to end	Night	23% 20% 6% 10% 5% 4 4 3 4 5% 15%	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	77% 4 6% 8% 5%	3988
	11th to 20th	Day	79% 3 6% 9% 3 1	4060
	21st to end	Day	76% 3 8% 9% 4	4466
February	1st to 10th	Day	69% 3 9% 14% 6%	1800
	11th to 20th	Day	66% 5% 10% 13% 6%	1762
	21st to end	Day	61% 6% 11% 14% 6% 1	1639
March	1st to 10th	Day	55% 5% 10% 22% 7% 1	4935
	11th to 20th	Day	50% 5% 14% 22% 8% 2	4980
	21st to end	Day	54% 5% 8% 20% 12% 1	5465
April	1st to 10th	Day	54% 6% 12% 15% 12% 1	2307
	11th to 20th	Day	52% 6% 10% 18% 12% 2	2423
	21st to end	Day	51% 5% 10% 21% 11% 2	2600
May	1st to 10th	Day	56% 6% 10% 16% 8% 4	5897
	11th to 20th	Day	51% 8% 11% 20% 9% 1	5865
	21st to end	Day	47% 7% 12% 20% 10% 3	6891
June	1st to 10th	Day	46% 8% 12% 22% 11% 1	2800
	11th to 20th	Day	46% 8% 14% 20% 12% 1	2800
	21st to end	Day	42% 8% 11% 25% 13% 1	2800
July	1st to 10th	Day	39% 6% 11% 27% 14% 2	6167
	11th to 20th	Day	43% 6% 14% 25% 12%	5831
	21st to end	Day	39% 6% 11% 24% 19% 1	6240
August	1st to 10th	Day	50% 7% 12% 21% 9%	4221
	11th to 20th	Day	44% 7% 12% 27% 10% 1	4213
	21st to end	Day	39% 7% 13% 28% 12% 2	4462
September	1st to 10th	Day	40% 7% 10% 23% 17% 3	2400
	11th to 20th	Day	38% 7% 13% 25% 16% 1	2184
	21st to end	Day	49% 6% 11% 21% 12%	2089
October	1st to 10th	Day	35% 5% 11% 29% 19% 1	3160
	11th to 20th	Day	46% 5% 10% 21% 14% 4	3140
	21st to end	Day	40% 5% 13% 28% 14% 1	3428
November	1st to 10th	Day	44% 4 12% 21% 17% 2	1857
	11th to 20th	Day	57% 4 11% 17% 9% 2	1649
	21st to end	Day	64% 5% 11% 14% 5% 1	1600
December	1st to 10th	Day	77% 3 9% 9% 2	2600
	11th to 20th	Day	68% 4 10% 14% 4	2986
	21st to end	Day	77% 3 8% 9% 3	3960
January	1st to 10th	Night	77% 3 8% 9% 3	5953
	11th to 20th	Night	77% 3 8% 8% 2	5900
	21st to end	Night	75% 3 8% 10% 4 1	6490
February	1st to 10th	Night	67% 5% 9% 15% 5%	2600
	11th to 20th	Night	65% 4 9% 15% 6%	2535
	21st to end	Night	63% 5% 8% 14% 9% 1	1985
March	1st to 10th	Night	58% 4 11% 20% 7%	4918
	11th to 20th	Night	47% 4 14% 24% 8% 3	4980
	21st to end	Night	52% 5% 10% 18% 14% 1	5467
April	1st to 10th	Night	54% 6% 9% 18% 12% 1	2091
	11th to 20th	Night	49% 6% 12% 17% 14% 3	1942
	21st to end	Night	47% 5% 12% 21% 12% 3	1800
May	1st to 10th	Night	58% 6% 9% 14% 10% 3	4060
	11th to 20th	Night	47% 6% 10% 23% 11% 1	4033
	21st to end	Night	44% 7% 12% 23% 12% 3	3981
June	1st to 10th	Night	38% 7% 13% 24% 16% 2	1600
	11th to 20th	Night	39% 8% 14% 23% 15% 2	1600
	21st to end	Night	38% 8% 14% 23% 16% 1	1600
July	1st to 10th	Night	41% 6% 9% 24% 19% 1	3664
	11th to 20th	Night	40% 6% 17% 25% 12% 1	4006
	21st to end	Night	36% 6% 11% 20% 26% 1	4297
August	1st to 10th	Night	45% 6% 13% 22% 12% 1	2904
	11th to 20th	Night	41% 6% 10% 30% 13% 1	2902
	21st to end	Night	35% 7% 14% 29% 13% 2	3402
September	1st to 10th	Night	34% 6% 12% 24% 19% 4	2000
	11th to 20th	Night	29% 6% 13% 28% 23% 1	2169
	21st to end	Night	46% 5% 11% 20% 16% 1	2090
October	1st to 10th	Night	36% 5% 9% 27% 23% 1	3800
	11th to 20th	Night	41% 5% 12% 22% 13% 8%	3779
	21st to end	Night	37% 4 12% 27% 18% 1	4120
November	1st to 10th	Night	42% 5% 12% 23% 15% 3	2436
	11th to 20th	Night	56% 4 10% 18% 9% 3	2749
	21st to end	Night	59% 5% 12% 18% 5% 1	2800
December	1st to 10th	Night	72% 4 11% 9% 3	4580
	11th to 20th	Night	69% 4 10% 14% 3	5208
	21st to end	Night	72% 5% 9% 12% 2	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	32% 30% 25% 10% 2 1	285
	11th to 20th	Day	32% 32% 27% 7% 2	300
	21st to end	Day	30% 32% 28% 8% 2	330
February	1st to 10th	Day	33% 28% 32% 6%	300
	11th to 20th	Day	37% 30% 26% 6% 1	300
	21st to end	Day	36% 28% 28% 8%	272
March	1st to 10th	Day	31% 23% 31% 12% 3	500
	11th to 20th	Day	33% 19% 33% 9% 4 1	500
	21st to end	Day	33% 21% 27% 14% 3 1	550
April	1st to 10th	Day	32% 21% 26% 14% 6% 1	500
	11th to 20th	Day	27% 20% 27% 16% 6% 3	492
	21st to end	Day	29% 19% 31% 16% 4 2	500
May	1st to 10th	Day	29% 19% 28% 17% 4 2	503
	11th to 20th	Day	26% 14% 30% 18% 9% 3	527
	21st to end	Day	20% 10% 27% 26% 12% 6%	573
June	1st to 10th	Day	16% 13% 32% 25% 11% 3	530
	11th to 20th	Day	20% 12% 26% 26% 12% 3	530
	21st to end	Day	15% 11% 33% 25% 14% 2	530
July	1st to 10th	Day	13% 7% 22% 29% 21% 9%	525
	11th to 20th	Day	14% 6% 27% 28% 18% 7%	524
	21st to end	Day	14% 7% 26% 30% 17% 7%	576
August	1st to 10th	Day	14% 7% 24% 28% 19% 8%	526
	11th to 20th	Day	15% 6% 24% 29% 16% 9%	519
	21st to end	Day	11% 6% 25% 30% 19% 9%	583
September	1st to 10th	Day	17% 9% 21% 25% 19% 8%	530
	11th to 20th	Day	12% 4 20% 33% 20% 10%	522
	21st to end	Day	25% 11% 25% 22% 13% 3	490
October	1st to 10th	Day	16% 12% 28% 23% 18% 3	490
	11th to 20th	Day	25% 15% 30% 20% 8% 2	490
	21st to end	Day	30% 16% 29% 18% 6%	539
November	1st to 10th	Day	19% 21% 33% 15% 9% 2	395
	11th to 20th	Day	33% 20% 32% 10% 3 2	224
	21st to end	Day	28% 25% 37% 8% 2	200
December	1st to 10th	Day	33% 29% 27% 7% 3 2	200
	11th to 20th	Day	33% 24% 32% 10% 2	200
	21st to end	Day	35% 25% 30% 8% 1	220
January	1st to 10th	Night	30% 22% 34% 12% 2	800
	11th to 20th	Night	32% 21% 31% 14% 2	800
	21st to end	Night	28% 23% 31% 17% 2	880
February	1st to 10th	Night	29% 20% 29% 20% 2	800
	11th to 20th	Night	30% 21% 35% 13% 1	783
	21st to end	Night	30% 19% 32% 16% 2	660
March	1st to 10th	Night	28% 19% 32% 20% 3	800
	11th to 20th	Night	29% 18% 27% 20% 5%	800
	21st to end	Night	25% 16% 36% 18% 4 1	880
April	1st to 10th	Night	27% 18% 29% 21% 5%	800
	11th to 20th	Night	24% 15% 32% 20% 7% 1	758
	21st to end	Night	22% 12% 34% 28% 5%	600
May	1st to 10th	Night	23% 11% 26% 31% 9% 1	600
	11th to 20th	Night	20% 9% 27% 34% 10% 1	591
	21st to end	Night	13% 5% 27% 33% 19% 3	462
June	1st to 10th	Night	10% 5% 30% 38% 17%	400
	11th to 20th	Night	15% 6% 26% 38% 16%	400
	21st to end	Night	11% 5% 28% 37% 19% 1	400
July	1st to 10th	Night	9% 3 19% 42% 22% 5%	429
	11th to 20th	Night	9% 3 20% 37% 26% 5%	582
	21st to end	Night	12% 3 14% 42% 27% 2	657
August	1st to 10th	Night	12% 3 16% 42% 25% 3	597
	11th to 20th	Night	12% 4 14% 38% 28% 4	594
	21st to end	Night	12% 3 20% 34% 26% 4	761
September	1st to 10th	Night	15% 4 24% 32% 23% 3	800
	11th to 20th	Night	12% 4 16% 35% 30% 3	793
	21st to end	Night	21% 8% 24% 26% 16% 4	760
October	1st to 10th	Night	18% 7% 25% 29% 19% 2	760
	11th to 20th	Night	21% 11% 32% 25% 8% 1	760
	21st to end	Night	27% 14% 28% 25% 6%	836
November	1st to 10th	Night	20% 13% 29% 28% 10%	780
	11th to 20th	Night	27% 18% 28% 20% 6% 1	800
	21st to end	Night	29% 18% 30% 18% 5%	800
December	1st to 10th	Night	31% 22% 27% 16% 4	800
	11th to 20th	Night	28% 23% 29% 17% 3	800
	21st to end	Night	32% 18% 30% 19% 1	880

Page generated at 2025-09-16 01:49:12. Version ac07

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