Pittsburgh
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Pittsburgh clear sky chart from 2005-11-06 to 2025-11-03.

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% 10% 4 8% 5% 5% 5% 4 6% 7% 31%	126463
	Night	16% 10% 4 9% 5% 4 4 3 5% 6% 32%	123553
Transparency	Day	50% 6% 11% 21% 11% 1	130068
	Night	51% 5% 10% 20% 11% 2	128018
Seeing	Day	22% 11% 26% 22% 14% 5%	16016
	Night	22% 8% 25% 30% 13% 1	25686

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% 18% 7% 20% 5% 4 3 2 3 5% 16%	4056
	11th to 20th	Day	13% 21% 8% 15% 9% 8% 3 2 3 3 15%	4060
	21st to end	Day	20% 17% 6% 11% 7% 6% 4 3 4 5% 16%	4466
February	1st to 10th	Day	21% 16% 5% 9% 7% 5% 4 3 4 4 22%	1800
	11th to 20th	Day	15% 17% 5% 11% 6% 5% 4 3 5% 5% 23%	1753
	21st to end	Day	19% 14% 4 11% 5% 5% 4 3 5% 5% 25%	1655
March	1st to 10th	Day	17% 12% 5% 9% 5% 5% 4 3 5% 6% 29%	4935
	11th to 20th	Day	13% 8% 3 8% 5% 5% 5% 4 5% 6% 37%	4980
	21st to end	Day	17% 10% 3 8% 6% 5% 5% 4 5% 6% 31%	5465
April	1st to 10th	Day	17% 11% 3 9% 5% 5% 5% 3 6% 7% 29%	2308
	11th to 20th	Day	16% 10% 3 7% 5% 5% 5% 4 6% 6% 33%	2404
	21st to end	Day	20% 8% 3 7% 5% 4 4 3 5% 7% 34%	2600
May	1st to 10th	Day	21% 9% 3 8% 5% 5% 5% 5% 6% 7% 27%	5862
	11th to 20th	Day	18% 7% 3 7% 6% 5% 5% 4 6% 9% 31%	5861
	21st to end	Day	18% 7% 2 7% 4 5% 4 5% 7% 9% 32%	6796
June	1st to 10th	Day	13% 7% 2 6% 6% 5% 5% 5% 7% 8% 36%	2800
	11th to 20th	Day	14% 7% 3 7% 5% 5% 5% 5% 7% 10% 34%	2800
	21st to end	Day	13% 6% 2 6% 5% 5% 5% 4 7% 10% 38%	2800
July	1st to 10th	Day	9% 7% 3 6% 5% 5% 5% 4 6% 8% 42%	6158
	11th to 20th	Day	11% 7% 3 8% 6% 6% 6% 5% 7% 11% 32%	5900
	21st to end	Day	11% 7% 3 8% 5% 5% 5% 4 7% 8% 38%	6424
August	1st to 10th	Day	14% 7% 3 8% 5% 6% 6% 5% 7% 8% 31%	4234
	11th to 20th	Day	11% 7% 2 7% 5% 5% 6% 5% 6% 8% 38%	4174
	21st to end	Day	8% 4 2 6% 5% 6% 6% 5% 8% 9% 41%	4448
September	1st to 10th	Day	15% 6% 2 4 4 4 5% 4 5% 7% 46%	2400
	11th to 20th	Day	11% 6% 2 4 5% 4 4 4 5% 8% 47%	2210
	21st to end	Day	21% 8% 3 6% 4 5% 4 2 5% 6% 35%	2195
October	1st to 10th	Day	16% 7% 2 5% 3 4 4 4 6% 8% 42%	2000
	11th to 20th	Day	18% 8% 2 7% 4 5% 5% 4 5% 6% 36%	2000
	21st to end	Day	22% 9% 3 7% 4 4 3 3 5% 7% 34%	2200
November	1st to 10th	Day	12% 8% 2 6% 5% 4 4 4 5% 6% 42%	1902
	11th to 20th	Day	16% 12% 3 7% 5% 4 5% 4 5% 7% 31%	1667
	21st to end	Day	18% 14% 6% 9% 5% 4 4 4 5% 6% 26%	1600
December	1st to 10th	Day	19% 12% 5% 15% 8% 5% 4 3 4 5% 22%	2600
	11th to 20th	Day	24% 17% 6% 9% 5% 3 3 2 4 5% 23%	2990
	21st to end	Day	21% 18% 6% 12% 6% 4 5% 3 5% 5% 14%	3960
January	1st to 10th	Night	15% 16% 6% 21% 6% 5% 4 3 4 4 16%	5889
	11th to 20th	Night	15% 17% 7% 14% 9% 6% 5% 3 4 4 16%	5900
	21st to end	Night	22% 13% 6% 13% 7% 5% 4 3 4 4 21%	6490
February	1st to 10th	Night	21% 13% 4 10% 6% 4 4 3 5% 4 25%	2600
	11th to 20th	Night	20% 12% 4 11% 6% 5% 4 4 5% 4 26%	2533
	21st to end	Night	20% 13% 4 10% 6% 4 5% 3 4 5% 26%	1969
March	1st to 10th	Night	21% 11% 4 8% 5% 3 3 3 4 5% 33%	4918
	11th to 20th	Night	12% 9% 3 7% 4 4 4 3 4 7% 42%	4980
	21st to end	Night	17% 9% 3 8% 5% 5% 5% 3 5% 5% 35%	5467
April	1st to 10th	Night	18% 12% 3 7% 4 4 4 3 5% 6% 34%	2091
	11th to 20th	Night	16% 8% 2 7% 4 4 5% 4 6% 7% 36%	1964
	21st to end	Night	15% 9% 3 7% 4 4 5% 3 4 7% 38%	1800
May	1st to 10th	Night	25% 11% 4 7% 5% 4 5% 4 5% 6% 25%	4033
	11th to 20th	Night	16% 9% 3 8% 5% 4 4 3 5% 7% 37%	4032
	21st to end	Night	18% 7% 3 7% 4 4 4 4 6% 8% 34%	4056
June	1st to 10th	Night	10% 6% 2 7% 5% 4 4 4 5% 8% 45%	1600
	11th to 20th	Night	11% 7% 2 7% 6% 4 5% 4 7% 7% 40%	1600
	21st to end	Night	10% 5% 2 5% 4 5% 6% 4 8% 10% 42%	1600
July	1st to 10th	Night	11% 7% 3 8% 5% 4 4 3 6% 8% 42%	3781
	11th to 20th	Night	11% 6% 3 7% 5% 5% 5% 5% 7% 10% 36%	4057
	21st to end	Night	8% 7% 3 7% 6% 5% 5% 4 6% 8% 42%	4427
August	1st to 10th	Night	12% 8% 3 8% 5% 5% 5% 5% 7% 8% 34%	2915
	11th to 20th	Night	12% 8% 3 6% 4 4 5% 3 5% 7% 43%	2875
	21st to end	Night	9% 5% 2 6% 4 5% 5% 4 7% 8% 44%	3416
September	1st to 10th	Night	12% 5% 2 5% 4 4 4 4 5% 7% 50%	2000
	11th to 20th	Night	10% 5% 2 5% 3 3 4 3 6% 7% 52%	2187
	21st to end	Night	17% 8% 2 7% 5% 3 4 4 5% 5% 40%	2204
October	1st to 10th	Night	14% 6% 2 7% 4 4 4 3 5% 6% 46%	2400
	11th to 20th	Night	17% 6% 3 6% 5% 4 4 4 4 7% 40%	2400
	21st to end	Night	22% 8% 2 6% 4 3 3 3 4 7% 38%	2640
November	1st to 10th	Night	13% 8% 3 7% 4 4 4 4 5% 6% 45%	2436
	11th to 20th	Night	18% 11% 3 7% 4 4 4 3 5% 6% 32%	2730
	21st to end	Night	21% 12% 4 7% 4 4 4 3 5% 5% 30%	2800
December	1st to 10th	Night	19% 14% 9% 14% 5% 4 4 3 4 6% 19%	4580
	11th to 20th	Night	22% 16% 6% 10% 5% 5% 3 2 4 4 24%	5205
	21st to end	Night	22% 16% 5% 11% 6% 5% 4 3 5% 5% 17%	6978

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	72% 4 9% 9% 4	4057
	11th to 20th	Day	75% 3 8% 9% 4 2	4060
	21st to end	Day	71% 5% 8% 11% 6%	4466
February	1st to 10th	Day	66% 4 10% 14% 7% 1	1800
	11th to 20th	Day	64% 5% 9% 16% 6%	1762
	21st to end	Day	60% 4 11% 16% 7% 1	1655
March	1st to 10th	Day	55% 4 10% 23% 7% 1	4935
	11th to 20th	Day	47% 5% 11% 26% 9% 2	4980
	21st to end	Day	53% 5% 8% 20% 13% 1	5465
April	1st to 10th	Day	53% 5% 12% 18% 11% 2	2308
	11th to 20th	Day	49% 6% 13% 17% 13% 2	2404
	21st to end	Day	49% 6% 10% 23% 11% 2	2600
May	1st to 10th	Day	54% 7% 11% 18% 8% 3	5897
	11th to 20th	Day	49% 7% 11% 23% 9% 1	5865
	21st to end	Day	46% 7% 14% 22% 9% 2	6811
June	1st to 10th	Day	43% 8% 14% 23% 11% 1	2800
	11th to 20th	Day	44% 8% 14% 22% 12% 1	2800
	21st to end	Day	41% 7% 13% 25% 13% 1	2800
July	1st to 10th	Day	39% 6% 10% 26% 16% 2	6092
	11th to 20th	Day	44% 7% 13% 24% 12%	5831
	21st to end	Day	42% 6% 10% 22% 19% 1	6240
August	1st to 10th	Day	48% 8% 13% 22% 9%	4221
	11th to 20th	Day	41% 7% 12% 28% 12%	4213
	21st to end	Day	36% 7% 13% 28% 14% 2	4448
September	1st to 10th	Day	38% 7% 10% 22% 20% 4	2400
	11th to 20th	Day	35% 6% 13% 28% 17% 1	2210
	21st to end	Day	50% 4 10% 22% 14%	2195
October	1st to 10th	Day	33% 4 9% 32% 20% 1	3260
	11th to 20th	Day	43% 5% 11% 22% 15% 4	3240
	21st to end	Day	37% 4 12% 28% 19% 1	3538
November	1st to 10th	Day	42% 6% 11% 22% 17% 3	1902
	11th to 20th	Day	52% 5% 10% 19% 11% 2	1667
	21st to end	Day	59% 5% 12% 18% 6% 1	1600
December	1st to 10th	Day	66% 4 13% 13% 4	2600
	11th to 20th	Day	67% 3 12% 14% 4	2986
	21st to end	Day	71% 5% 9% 11% 4	3960
January	1st to 10th	Night	72% 4 8% 12% 5%	5889
	11th to 20th	Night	71% 4 9% 10% 4 1	5900
	21st to end	Night	68% 4 8% 13% 7% 1	6490
February	1st to 10th	Night	61% 4 10% 18% 7%	2600
	11th to 20th	Night	60% 4 10% 18% 8%	2535
	21st to end	Night	62% 4 8% 16% 10% 1	1969
March	1st to 10th	Night	55% 4 9% 23% 9% 1	4918
	11th to 20th	Night	42% 5% 13% 28% 9% 3	4980
	21st to end	Night	50% 4 10% 19% 15% 1	5467
April	1st to 10th	Night	51% 5% 10% 19% 13% 2	2091
	11th to 20th	Night	46% 6% 11% 18% 15% 3	1964
	21st to end	Night	45% 6% 10% 24% 13% 3	1800
May	1st to 10th	Night	59% 6% 9% 14% 9% 3	4060
	11th to 20th	Night	46% 7% 9% 25% 12% 1	4033
	21st to end	Night	46% 6% 12% 23% 10% 3	4063
June	1st to 10th	Night	36% 8% 13% 24% 17% 2	1600
	11th to 20th	Night	40% 7% 13% 23% 15% 2	1600
	21st to end	Night	35% 9% 12% 25% 17% 2	1600
July	1st to 10th	Night	40% 6% 9% 24% 18% 2	3740
	11th to 20th	Night	40% 9% 17% 22% 12% 1	4006
	21st to end	Night	38% 7% 11% 19% 23% 2	4297
August	1st to 10th	Night	46% 8% 12% 22% 11% 1	2904
	11th to 20th	Night	40% 5% 9% 27% 17% 1	2902
	21st to end	Night	36% 7% 12% 30% 13% 2	3416
September	1st to 10th	Night	34% 6% 10% 24% 21% 5%	2000
	11th to 20th	Night	30% 6% 12% 28% 23% 1	2187
	21st to end	Night	45% 5% 10% 21% 18% 1	2204
October	1st to 10th	Night	35% 4 8% 27% 24% 2	3920
	11th to 20th	Night	38% 4 11% 24% 16% 7%	3899
	21st to end	Night	35% 3 8% 29% 22% 2	4252
November	1st to 10th	Night	40% 5% 10% 23% 18% 4	2436
	11th to 20th	Night	52% 5% 9% 20% 11% 2	2730
	21st to end	Night	56% 5% 10% 21% 7% 1	2800
December	1st to 10th	Night	67% 4 10% 15% 3	4580
	11th to 20th	Night	65% 3 14% 14% 3	5208
	21st to end	Night	68% 5% 10% 14% 3	6978

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	28% 25% 25% 14% 5% 2	300
	11th to 20th	Day	27% 24% 30% 15% 2 1	300
	21st to end	Day	28% 20% 31% 17% 4 1	330
February	1st to 10th	Day	30% 18% 36% 13% 3	300
	11th to 20th	Day	32% 23% 30% 13% 2	300
	21st to end	Day	32% 19% 33% 13% 2	284
March	1st to 10th	Day	27% 15% 33% 20% 4	500
	11th to 20th	Day	29% 16% 32% 17% 5% 1	500
	21st to end	Day	31% 15% 30% 18% 5% 1	550
April	1st to 10th	Day	30% 15% 26% 21% 7% 2	500
	11th to 20th	Day	22% 16% 32% 17% 9% 4	492
	21st to end	Day	29% 12% 28% 20% 9% 2	500
May	1st to 10th	Day	29% 10% 29% 19% 11% 3	503
	11th to 20th	Day	20% 11% 29% 22% 12% 6%	527
	21st to end	Day	16% 6% 25% 24% 20% 8%	573
June	1st to 10th	Day	17% 7% 27% 25% 17% 6%	530
	11th to 20th	Day	19% 6% 26% 24% 20% 6%	530
	21st to end	Day	15% 6% 26% 29% 18% 5%	530
July	1st to 10th	Day	14% 3 17% 33% 22% 12%	525
	11th to 20th	Day	13% 5% 22% 28% 23% 8%	524
	21st to end	Day	15% 3 20% 31% 23% 8%	576
August	1st to 10th	Day	13% 3 22% 28% 26% 9%	526
	11th to 20th	Day	15% 5% 17% 30% 22% 12%	519
	21st to end	Day	11% 4 21% 28% 24% 13%	583
September	1st to 10th	Day	18% 5% 17% 28% 22% 10%	530
	11th to 20th	Day	12% 2 15% 31% 25% 15%	530
	21st to end	Day	25% 8% 21% 23% 16% 7%	530
October	1st to 10th	Day	18% 6% 24% 24% 22% 6%	530
	11th to 20th	Day	20% 13% 30% 24% 11% 2	530
	21st to end	Day	28% 12% 31% 20% 7% 1	583
November	1st to 10th	Day	20% 13% 32% 22% 10% 3	428
	11th to 20th	Day	28% 15% 33% 15% 8% 1	233
	21st to end	Day	27% 17% 43% 11% 3	200
December	1st to 10th	Day	30% 20% 33% 11% 6% 1	200
	11th to 20th	Day	34% 15% 34% 14% 5%	200
	21st to end	Day	33% 15% 32% 15% 5%	220
January	1st to 10th	Night	28% 15% 33% 19% 5%	800
	11th to 20th	Night	31% 12% 30% 22% 4	800
	21st to end	Night	28% 13% 31% 24% 4	880
February	1st to 10th	Night	28% 13% 27% 29% 3	800
	11th to 20th	Night	29% 13% 34% 22% 2	783
	21st to end	Night	28% 10% 36% 23% 3	660
March	1st to 10th	Night	24% 14% 30% 28% 5%	800
	11th to 20th	Night	25% 14% 29% 25% 7% 1	800
	21st to end	Night	25% 11% 33% 25% 5% 1	880
April	1st to 10th	Night	26% 13% 30% 26% 6%	800
	11th to 20th	Night	21% 10% 30% 28% 10% 1	780
	21st to end	Night	18% 7% 32% 36% 7%	600
May	1st to 10th	Night	25% 7% 21% 34% 13% 1	600
	11th to 20th	Night	19% 8% 23% 35% 14% 1	591
	21st to end	Night	12% 2 21% 37% 24% 3	492
June	1st to 10th	Night	12% 4 20% 44% 20% 1	400
	11th to 20th	Night	14% 4 21% 39% 21% 1	400
	21st to end	Night	12% 2 23% 39% 22% 2	400
July	1st to 10th	Night	9% 2 16% 45% 23% 6%	466
	11th to 20th	Night	11% 2 19% 33% 31% 5%	582
	21st to end	Night	11% 2 13% 43% 30% 2	657
August	1st to 10th	Night	11% 1 13% 42% 31% 3	597
	11th to 20th	Night	12% 4 10% 39% 32% 3	594
	21st to end	Night	13% 2 16% 35% 29% 5%	775
September	1st to 10th	Night	15% 2 16% 35% 28% 4	800
	11th to 20th	Night	12% 4 12% 36% 32% 4	800
	21st to end	Night	21% 4 22% 27% 23% 3	800
October	1st to 10th	Night	16% 6% 19% 32% 25% 2	800
	11th to 20th	Night	22% 7% 28% 31% 11% 2	800
	21st to end	Night	27% 9% 24% 31% 9% 1	880
November	1st to 10th	Night	18% 8% 26% 34% 14% 1	789
	11th to 20th	Night	27% 11% 28% 25% 8% 1	800
	21st to end	Night	28% 12% 28% 26% 6% 1	800
December	1st to 10th	Night	32% 13% 27% 23% 6%	800
	11th to 20th	Night	28% 12% 29% 26% 5%	800
	21st to end	Night	30% 11% 28% 29% 3	880

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