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 2024-01-15.

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	17% 9% 4 7% 6% 4 5% 5% 6% 7% 31%	74624
	Night	17% 9% 4 7% 6% 3 4 4 5% 6% 34%	75164
Transparency	Day	51% 6% 12% 20% 10% 1	74344
	Night	50% 6% 11% 20% 11% 2	74964
Seeing	Day	22% 10% 26% 23% 14% 5%	13590
	Night	22% 8% 25% 30% 13% 1	23401

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	18% 15% 7% 10% 7% 4 5% 4 4 6% 20%	1706
	11th to 20th	Day	18% 18% 6% 12% 8% 5% 3 3 4 4 18%	1662
	21st to end	Day	18% 16% 7% 11% 8% 4 4 4 4 6% 18%	1782
February	1st to 10th	Day	22% 14% 5% 9% 7% 5% 5% 4 4 5% 20%	1620
	11th to 20th	Day	16% 16% 6% 10% 7% 4 5% 4 5% 5% 22%	1573
	21st to end	Day	20% 13% 4 9% 7% 3 4 4 4 5% 25%	1484
March	1st to 10th	Day	17% 9% 4 6% 7% 4 4 5% 5% 7% 31%	1980
	11th to 20th	Day	19% 10% 4 8% 6% 4 4 5% 5% 6% 29%	1980
	21st to end	Day	20% 9% 5% 7% 6% 3 5% 5% 5% 7% 28%	2178
April	1st to 10th	Day	16% 9% 4 7% 5% 4 5% 5% 6% 7% 31%	2076
	11th to 20th	Day	17% 9% 3 6% 6% 4 5% 5% 5% 7% 33%	2160
	21st to end	Day	22% 8% 3 5% 5% 3 4 5% 5% 7% 32%	2340
May	1st to 10th	Day	19% 8% 3 6% 6% 4 6% 6% 6% 8% 29%	2340
	11th to 20th	Day	21% 6% 3 7% 7% 4 5% 5% 6% 8% 29%	2301
	21st to end	Day	11% 6% 2 5% 5% 4 5% 6% 8% 10% 38%	2671
June	1st to 10th	Day	13% 6% 2 6% 6% 4 6% 6% 7% 8% 36%	2520
	11th to 20th	Day	14% 7% 3 5% 6% 4 6% 6% 7% 9% 35%	2520
	21st to end	Day	14% 5% 3 5% 5% 3 5% 5% 6% 10% 38%	2520
July	1st to 10th	Day	10% 7% 3 5% 7% 3 6% 6% 6% 8% 39%	2444
	11th to 20th	Day	11% 6% 3 6% 6% 3 6% 6% 7% 11% 36%	2340
	21st to end	Day	13% 6% 3 6% 6% 5% 5% 6% 8% 9% 34%	2547
August	1st to 10th	Day	12% 5% 3 5% 6% 4 8% 7% 8% 9% 34%	2327
	11th to 20th	Day	13% 6% 2 6% 6% 4 5% 6% 7% 9% 35%	2303
	21st to end	Day	12% 5% 2 5% 6% 4 6% 5% 8% 8% 39%	2452
September	1st to 10th	Day	16% 5% 2 3 5% 3 5% 5% 5% 7% 43%	2160
	11th to 20th	Day	12% 6% 3 4 5% 3 5% 5% 5% 8% 45%	1990
	21st to end	Day	19% 7% 3 6% 5% 3 4 3 6% 6% 38%	1976
October	1st to 10th	Day	16% 7% 2 5% 4 3 4 5% 6% 8% 39%	1800
	11th to 20th	Day	19% 8% 2 7% 5% 4 5% 5% 5% 6% 35%	1800
	21st to end	Day	23% 9% 3 6% 5% 2 4 4 5% 7% 32%	1980
November	1st to 10th	Day	12% 7% 3 6% 6% 4 4 5% 5% 6% 42%	1786
	11th to 20th	Day	17% 11% 4 6% 6% 3 4 5% 5% 7% 32%	1584
	21st to end	Day	18% 12% 6% 7% 5% 4 4 5% 4 7% 27%	1520
December	1st to 10th	Day	22% 14% 6% 11% 8% 3 5% 3 4 5% 19%	1520
	11th to 20th	Day	21% 16% 6% 8% 7% 3 4 4 4 6% 22%	1910
	21st to end	Day	24% 19% 7% 10% 7% 3 4 3 5% 5% 14%	2772
January	1st to 10th	Night	19% 15% 6% 9% 6% 4 5% 5% 5% 4 21%	2459
	11th to 20th	Night	21% 14% 5% 11% 7% 4 5% 5% 5% 4 20%	2404
	21st to end	Night	19% 15% 6% 10% 8% 3 5% 4 4 5% 23%	2574
February	1st to 10th	Night	22% 12% 4 9% 6% 4 4 4 5% 4 25%	2340
	11th to 20th	Night	21% 10% 5% 10% 7% 4 5% 4 4 5% 25%	2273
	21st to end	Night	21% 13% 5% 9% 7% 2 5% 4 4 5% 26%	1766
March	1st to 10th	Night	19% 9% 4 7% 7% 2 4 4 5% 6% 35%	1980
	11th to 20th	Night	20% 10% 3 6% 5% 3 4 4 4 8% 34%	1980
	21st to end	Night	19% 10% 4 6% 5% 3 4 4 4 6% 35%	2178
April	1st to 10th	Night	17% 10% 4 6% 5% 3 4 4 5% 6% 36%	1883
	11th to 20th	Night	16% 8% 2 7% 4 3 5% 4 7% 8% 37%	1766
	21st to end	Night	16% 8% 4 6% 5% 2 5% 5% 4 7% 38%	1620
May	1st to 10th	Night	18% 8% 3 6% 6% 4 5% 5% 6% 6% 33%	1620
	11th to 20th	Night	17% 8% 3 6% 6% 3 4 4 4 7% 38%	1592
	21st to end	Night	9% 5% 2 5% 5% 2 4 6% 7% 10% 45%	1602
June	1st to 10th	Night	10% 5% 2 6% 6% 3 3 6% 5% 8% 45%	1440
	11th to 20th	Night	11% 6% 2 6% 7% 3 6% 5% 6% 7% 41%	1440
	21st to end	Night	10% 4 2 4 4 3 6% 5% 7% 11% 43%	1440
July	1st to 10th	Night	9% 5% 2 5% 6% 3 5% 5% 6% 9% 44%	1511
	11th to 20th	Night	9% 4 3 4 5% 3 4 5% 6% 10% 46%	1620
	21st to end	Night	10% 5% 3 6% 6% 4 6% 7% 7% 9% 39%	1771
August	1st to 10th	Night	6% 5% 3 6% 5% 4 5% 7% 7% 9% 42%	1611
	11th to 20th	Night	9% 8% 3 5% 6% 3 5% 5% 5% 9% 44%	1598
	21st to end	Night	11% 6% 2 4 4 4 4 5% 6% 8% 45%	1885
September	1st to 10th	Night	13% 4 2 4 4 3 5% 5% 5% 7% 48%	1800
	11th to 20th	Night	11% 4 2 4 4 2 4 4 6% 7% 52%	1967
	21st to end	Night	15% 7% 2 5% 6% 2 4 4 5% 6% 43%	1983
October	1st to 10th	Night	14% 6% 3 6% 5% 3 4 5% 5% 6% 44%	2160
	11th to 20th	Night	18% 6% 3 5% 6% 3 5% 5% 5% 6% 38%	2160
	21st to end	Night	24% 8% 2 6% 5% 2 3 4 4 6% 36%	2376
November	1st to 10th	Night	13% 7% 3 6% 5% 3 4 5% 4 6% 45%	2287
	11th to 20th	Night	18% 10% 4 6% 5% 3 5% 4 5% 7% 33%	2593
	21st to end	Night	20% 11% 5% 7% 5% 3 4 4 5% 6% 31%	2660
December	1st to 10th	Night	23% 13% 7% 9% 6% 3 4 4 4 6% 21%	2660
	11th to 20th	Night	20% 16% 6% 9% 6% 4 4 3 4 5% 25%	3285
	21st to end	Night	24% 17% 7% 10% 7% 3 5% 4 4 5% 14%	4880

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	67% 4 10% 12% 5%	1707
	11th to 20th	Day	71% 4 11% 12% 2	1662
	21st to end	Day	69% 5% 10% 12% 4 1	1782
February	1st to 10th	Day	67% 5% 10% 12% 6% 1	1620
	11th to 20th	Day	65% 5% 10% 15% 5%	1582
	21st to end	Day	61% 4 10% 15% 8% 1	1484
March	1st to 10th	Day	52% 6% 12% 22% 7%	1980
	11th to 20th	Day	56% 5% 10% 18% 9% 2	1980
	21st to end	Day	57% 5% 10% 16% 11% 2	2178
April	1st to 10th	Day	51% 5% 12% 19% 11% 2	2076
	11th to 20th	Day	49% 6% 13% 16% 14% 2	2160
	21st to end	Day	51% 6% 11% 21% 10% 2	2340
May	1st to 10th	Day	52% 6% 12% 20% 9% 1	2340
	11th to 20th	Day	52% 7% 12% 16% 11% 2	2305
	21st to end	Day	39% 9% 17% 23% 9% 2	2671
June	1st to 10th	Day	44% 8% 14% 23% 11% 1	2520
	11th to 20th	Day	44% 8% 14% 22% 12% 1	2520
	21st to end	Day	41% 7% 12% 26% 12% 1	2520
July	1st to 10th	Day	41% 9% 12% 20% 15% 3	2378
	11th to 20th	Day	41% 7% 13% 23% 14% 1	2271
	21st to end	Day	44% 8% 13% 21% 12% 2	2405
August	1st to 10th	Day	43% 9% 13% 25% 9%	2314
	11th to 20th	Day	41% 9% 14% 25% 11% 1	2303
	21st to end	Day	40% 8% 14% 23% 12% 3	2452
September	1st to 10th	Day	39% 7% 11% 21% 18% 4	2160
	11th to 20th	Day	36% 6% 13% 28% 15% 1	1990
	21st to end	Day	47% 4 10% 23% 15%	1976
October	1st to 10th	Day	42% 6% 14% 22% 16% 1	1800
	11th to 20th	Day	50% 6% 10% 23% 11%	1800
	21st to end	Day	53% 4 12% 20% 11% 1	1980
November	1st to 10th	Day	42% 6% 11% 22% 16% 3	1786
	11th to 20th	Day	52% 6% 11% 19% 11% 2	1584
	21st to end	Day	57% 5% 12% 18% 7% 1	1520
December	1st to 10th	Day	69% 5% 10% 13% 4	1520
	11th to 20th	Day	65% 3 12% 16% 4	1906
	21st to end	Day	73% 5% 9% 10% 3	2772
January	1st to 10th	Night	65% 5% 8% 15% 6%	2459
	11th to 20th	Night	68% 5% 12% 12% 3 1	2404
	21st to end	Night	65% 4 9% 16% 5% 1	2574
February	1st to 10th	Night	62% 4 11% 16% 6%	2340
	11th to 20th	Night	61% 4 10% 17% 7%	2275
	21st to end	Night	62% 4 8% 16% 9% 1	1766
March	1st to 10th	Night	51% 5% 10% 21% 13% 1	1980
	11th to 20th	Night	50% 5% 12% 22% 10% 2	1980
	21st to end	Night	51% 5% 7% 20% 14% 3	2178
April	1st to 10th	Night	50% 5% 10% 19% 14% 2	1883
	11th to 20th	Night	44% 6% 11% 18% 16% 4	1766
	21st to end	Night	46% 5% 10% 23% 12% 3	1620
May	1st to 10th	Night	50% 6% 9% 19% 12% 3	1620
	11th to 20th	Night	47% 6% 11% 18% 15% 3	1593
	21st to end	Night	34% 7% 18% 26% 13% 1	1603
June	1st to 10th	Night	36% 8% 13% 25% 16% 2	1440
	11th to 20th	Night	40% 7% 13% 23% 15% 2	1440
	21st to end	Night	34% 9% 13% 26% 16% 2	1440
July	1st to 10th	Night	36% 9% 12% 23% 16% 5%	1468
	11th to 20th	Night	30% 9% 15% 25% 19% 2	1566
	21st to end	Night	38% 9% 15% 22% 13% 3	1672
August	1st to 10th	Night	34% 9% 14% 26% 16% 1	1600
	11th to 20th	Night	37% 7% 14% 26% 15% 2	1598
	21st to end	Night	35% 8% 12% 26% 15% 4	1885
September	1st to 10th	Night	35% 6% 11% 24% 20% 5%	1800
	11th to 20th	Night	31% 6% 13% 27% 23% 1	1967
	21st to end	Night	41% 5% 10% 23% 19% 1	1983
October	1st to 10th	Night	40% 6% 11% 26% 15% 3	2160
	11th to 20th	Night	46% 5% 10% 24% 14% 1	2160
	21st to end	Night	50% 5% 10% 21% 12% 2	2376
November	1st to 10th	Night	40% 6% 10% 23% 18% 4	2287
	11th to 20th	Night	51% 5% 10% 21% 12% 2	2593
	21st to end	Night	55% 5% 10% 22% 7% 1	2660
December	1st to 10th	Night	65% 5% 10% 14% 5%	2660
	11th to 20th	Night	63% 4 11% 17% 4	3288
	21st to end	Night	71% 6% 10% 11% 2	4880

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% 6% 2	270
	11th to 20th	Day	28% 22% 29% 17% 2 2	254
	21st to end	Day	29% 22% 29% 15% 4 1	264
February	1st to 10th	Day	33% 14% 39% 12% 3	240
	11th to 20th	Day	31% 22% 33% 13% 1	240
	21st to end	Day	34% 18% 32% 14% 2	229
March	1st to 10th	Day	27% 15% 34% 21% 4	420
	11th to 20th	Day	28% 15% 32% 18% 6% 1	420
	21st to end	Day	31% 13% 30% 19% 6% 2	462
April	1st to 10th	Day	27% 16% 26% 20% 9% 2	420
	11th to 20th	Day	21% 14% 32% 18% 10% 5%	412
	21st to end	Day	31% 12% 27% 19% 9% 2	420
May	1st to 10th	Day	30% 10% 27% 18% 11% 3	423
	11th to 20th	Day	21% 10% 29% 22% 12% 5%	447
	21st to end	Day	15% 5% 24% 25% 22% 9%	485
June	1st to 10th	Day	16% 6% 27% 26% 19% 6%	450
	11th to 20th	Day	19% 7% 25% 23% 20% 6%	450
	21st to end	Day	15% 5% 27% 30% 18% 4	450
July	1st to 10th	Day	15% 1 15% 32% 23% 13%	445
	11th to 20th	Day	13% 4 22% 29% 24% 8%	444
	21st to end	Day	16% 3 19% 30% 23% 9%	488
August	1st to 10th	Day	13% 3 18% 28% 27% 10%	446
	11th to 20th	Day	16% 4 15% 30% 21% 13%	439
	21st to end	Day	12% 3 20% 28% 23% 13%	495
September	1st to 10th	Day	19% 3 18% 28% 22% 10%	450
	11th to 20th	Day	14% 2 14% 31% 24% 14%	450
	21st to end	Day	23% 8% 20% 24% 16% 8%	450
October	1st to 10th	Day	19% 6% 25% 24% 22% 5%	450
	11th to 20th	Day	21% 13% 30% 24% 12% 1	450
	21st to end	Day	30% 11% 30% 20% 7% 1	495
November	1st to 10th	Day	20% 13% 31% 23% 11% 3	384
	11th to 20th	Day	29% 17% 28% 16% 9% 1	210
	21st to end	Day	24% 16% 46% 12% 3	180
December	1st to 10th	Day	31% 19% 30% 12% 6% 1	180
	11th to 20th	Day	31% 16% 36% 13% 5%	180
	21st to end	Day	33% 15% 32% 15% 6%	198
January	1st to 10th	Night	29% 14% 33% 19% 5%	760
	11th to 20th	Night	32% 11% 30% 22% 5%	740
	21st to end	Night	27% 13% 31% 25% 4	792
February	1st to 10th	Night	28% 13% 27% 28% 3	720
	11th to 20th	Night	29% 13% 35% 22% 1	703
	21st to end	Night	28% 11% 36% 22% 3	592
March	1st to 10th	Night	23% 14% 30% 28% 5%	720
	11th to 20th	Night	25% 13% 29% 24% 7% 1	720
	21st to end	Night	25% 9% 32% 26% 6% 1	792
April	1st to 10th	Night	25% 13% 29% 27% 7%	720
	11th to 20th	Night	21% 9% 30% 28% 11% 1	702
	21st to end	Night	19% 7% 32% 34% 7%	540
May	1st to 10th	Night	23% 7% 21% 35% 13% 1	540
	11th to 20th	Night	19% 8% 23% 35% 14% 1	531
	21st to end	Night	10% 2 20% 38% 26% 4	442
June	1st to 10th	Night	12% 3 19% 45% 20% 1	360
	11th to 20th	Night	14% 4 22% 39% 21% 1	360
	21st to end	Night	12% 1 24% 39% 22% 1	360
July	1st to 10th	Night	8% 1 16% 44% 24% 7%	418
	11th to 20th	Night	11% 1 19% 34% 30% 5%	522
	21st to end	Night	11% 2 14% 41% 30% 2	591
August	1st to 10th	Night	11% 1 12% 42% 32% 3	537
	11th to 20th	Night	11% 3 10% 39% 32% 4	534
	21st to end	Night	14% 1 16% 35% 28% 6%	697
September	1st to 10th	Night	15% 2 15% 34% 29% 4	720
	11th to 20th	Night	13% 4 13% 35% 32% 4	720
	21st to end	Night	20% 4 21% 28% 23% 4	720
October	1st to 10th	Night	17% 6% 19% 32% 24% 2	720
	11th to 20th	Night	23% 7% 27% 30% 11% 1	720
	21st to end	Night	29% 9% 23% 30% 8% 1	792
November	1st to 10th	Night	18% 8% 25% 34% 15% 1	740
	11th to 20th	Night	26% 11% 28% 25% 9% 1	760
	21st to end	Night	28% 12% 27% 26% 7% 1	760
December	1st to 10th	Night	32% 13% 27% 23% 6%	760
	11th to 20th	Night	27% 12% 29% 26% 5%	760
	21st to end	Night	30% 12% 28% 28% 3	836

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