Pebble Hill Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Pebble Hill Observatory clear sky chart from 2005-11-06 to 2024-01-13.

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% 9% 4 7% 6% 3 5% 5% 6% 7% 33%	74755
	Night	13% 10% 4 8% 6% 3 5% 5% 5% 7% 34%	75015
Transparency	Day	49% 4 13% 23% 10% 1	74455
	Night	50% 4 12% 23% 10% 1	74835
Seeing	Day	19% 9% 31% 26% 12% 4	14890
	Night	18% 7% 31% 31% 11% 1	24914

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	11% 12% 7% 11% 10% 4 5% 4 6% 6% 24%	1517
	11th to 20th	Day	11% 15% 8% 14% 9% 4 6% 4 7% 7% 15%	1461
	21st to end	Day	9% 14% 8% 13% 9% 5% 6% 6% 7% 8% 17%	1665
February	1st to 10th	Day	8% 13% 5% 10% 8% 3 6% 4 6% 6% 31%	1620
	11th to 20th	Day	8% 13% 5% 9% 8% 4 4 5% 5% 6% 33%	1574
	21st to end	Day	12% 11% 6% 11% 8% 4 5% 3 6% 6% 29%	1444
March	1st to 10th	Day	12% 11% 5% 8% 7% 3 5% 5% 5% 6% 32%	1831
	11th to 20th	Day	13% 8% 4 10% 7% 3 4 4 5% 6% 34%	1980
	21st to end	Day	16% 11% 5% 7% 6% 4 5% 5% 6% 6% 30%	2271
April	1st to 10th	Day	15% 12% 5% 8% 6% 3 4 5% 5% 6% 30%	2160
	11th to 20th	Day	21% 11% 5% 6% 5% 3 4 4 4 6% 29%	2134
	21st to end	Day	19% 10% 5% 7% 6% 3 4 4 4 5% 31%	2311
May	1st to 10th	Day	16% 9% 4 7% 6% 3 5% 5% 5% 7% 33%	2520
	11th to 20th	Day	17% 7% 3 6% 5% 4 5% 4 6% 7% 34%	2479
	21st to end	Day	15% 7% 3 5% 5% 3 5% 5% 6% 8% 37%	2725
June	1st to 10th	Day	12% 5% 2 5% 5% 3 5% 5% 7% 9% 42%	2520
	11th to 20th	Day	13% 7% 3 6% 6% 4 5% 5% 6% 8% 38%	2520
	21st to end	Day	11% 5% 2 5% 5% 3 6% 6% 9% 10% 38%	2520
July	1st to 10th	Day	8% 3 2 4 5% 3 5% 5% 7% 9% 49%	2520
	11th to 20th	Day	10% 5% 2 5% 5% 3 5% 5% 6% 10% 45%	2520
	21st to end	Day	8% 5% 2 5% 5% 3 5% 5% 7% 9% 45%	2743
August	1st to 10th	Day	11% 6% 2 5% 6% 3 5% 5% 7% 9% 41%	2454
	11th to 20th	Day	13% 5% 3 5% 5% 3 5% 5% 5% 8% 43%	2301
	21st to end	Day	11% 6% 3 5% 5% 2 4 4 6% 8% 45%	2411
September	1st to 10th	Day	13% 6% 2 5% 5% 3 4 6% 5% 8% 43%	2160
	11th to 20th	Day	14% 6% 2 6% 5% 4 5% 5% 5% 8% 42%	2160
	21st to end	Day	18% 7% 3 6% 4 4 5% 4 6% 7% 37%	1905
October	1st to 10th	Day	18% 9% 3 6% 5% 3 4 5% 6% 7% 35%	1800
	11th to 20th	Day	20% 14% 3 6% 6% 3 4 5% 4 7% 28%	1800
	21st to end	Day	20% 12% 4 8% 6% 3 4 4 4 5% 30%	1853
November	1st to 10th	Day	13% 15% 4 6% 5% 3 5% 5% 5% 6% 32%	1615
	11th to 20th	Day	15% 13% 9% 11% 7% 4 5% 4 4 5% 22%	1520
	21st to end	Day	14% 13% 5% 8% 8% 4 5% 5% 6% 6% 26%	1520
December	1st to 10th	Day	10% 16% 7% 13% 11% 5% 5% 4 5% 6% 16%	1520
	11th to 20th	Day	13% 13% 8% 12% 10% 5% 5% 5% 5% 5% 18%	1918
	21st to end	Day	24% 17% 6% 15% 8% 2 4 2 2 3 15%	2783
January	1st to 10th	Night	10% 9% 7% 10% 10% 5% 6% 6% 7% 7% 23%	2648
	11th to 20th	Night	13% 14% 7% 12% 8% 4 5% 5% 5% 6% 21%	2561
	21st to end	Night	11% 15% 7% 14% 7% 4 5% 5% 5% 6% 20%	2688
February	1st to 10th	Night	7% 11% 7% 10% 8% 4 6% 5% 6% 7% 30%	2340
	11th to 20th	Night	8% 10% 5% 8% 8% 4 7% 6% 5% 6% 34%	2273
	21st to end	Night	12% 12% 5% 8% 6% 4 5% 5% 6% 6% 30%	1812
March	1st to 10th	Night	13% 10% 4 6% 6% 4 5% 6% 5% 6% 35%	2144
	11th to 20th	Night	12% 9% 5% 7% 5% 3 4 5% 5% 5% 42%	1980
	21st to end	Night	15% 11% 5% 7% 5% 3 5% 4 5% 5% 35%	2084
April	1st to 10th	Night	18% 12% 4 7% 5% 3 4 5% 5% 6% 32%	1800
	11th to 20th	Night	22% 9% 5% 6% 5% 3 5% 5% 4 5% 31%	1775
	21st to end	Night	19% 11% 3 6% 5% 3 4 5% 5% 6% 34%	1645
May	1st to 10th	Night	18% 9% 4 6% 6% 3 5% 4 5% 7% 34%	1440
	11th to 20th	Night	15% 7% 3 6% 5% 3 5% 5% 5% 7% 39%	1416
	21st to end	Night	12% 5% 3 6% 6% 4 5% 6% 5% 10% 39%	1551
June	1st to 10th	Night	10% 5% 3 5% 5% 3 5% 7% 6% 10% 42%	1440
	11th to 20th	Night	10% 5% 3 6% 6% 4 6% 5% 6% 8% 42%	1440
	21st to end	Night	9% 4 2 5% 5% 4 5% 5% 6% 10% 44%	1440
July	1st to 10th	Night	5% 4 1 3 4 3 5% 6% 7% 10% 52%	1440
	11th to 20th	Night	8% 4 2 5% 4 3 5% 6% 6% 7% 48%	1440
	21st to end	Night	7% 4 2 4 4 3 4 5% 5% 10% 54%	1575
August	1st to 10th	Night	8% 4 2 4 4 3 4 4 6% 9% 52%	1483
	11th to 20th	Night	14% 5% 2 5% 3 2 3 4 5% 7% 50%	1600
	21st to end	Night	11% 5% 2 5% 5% 3 4 4 4 7% 49%	1944
September	1st to 10th	Night	9% 5% 2 5% 5% 2 4 5% 6% 7% 52%	1800
	11th to 20th	Night	10% 5% 2 4 5% 3 3 4 6% 8% 49%	1800
	21st to end	Night	17% 6% 2 6% 5% 3 3 4 6% 8% 40%	2033
October	1st to 10th	Night	17% 8% 3 5% 5% 2 3 3 4 6% 44%	2160
	11th to 20th	Night	16% 11% 3 7% 6% 3 4 4 5% 6% 35%	2160
	21st to end	Night	17% 11% 4 7% 5% 3 5% 5% 5% 7% 31%	2502
November	1st to 10th	Night	16% 11% 5% 7% 5% 3 4 5% 5% 7% 35%	2458
	11th to 20th	Night	13% 13% 7% 10% 7% 3 4 4 5% 7% 27%	2660
	21st to end	Night	13% 10% 6% 9% 7% 4 5% 5% 5% 6% 30%	2660
December	1st to 10th	Night	9% 14% 7% 12% 10% 4 5% 5% 6% 6% 22%	2660
	11th to 20th	Night	12% 14% 6% 12% 9% 5% 6% 6% 5% 5% 20%	3279
	21st to end	Night	24% 20% 6% 9% 5% 3 4 4 3 4 18%	4884

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	60% 4 9% 21% 6% 1	1518
	11th to 20th	Day	67% 4 8% 12% 9%	1461
	21st to end	Day	64% 4 10% 14% 8%	1665
February	1st to 10th	Day	53% 4 11% 21% 11% 1	1620
	11th to 20th	Day	51% 4 11% 23% 10% 1	1583
	21st to end	Day	57% 3 8% 19% 11% 1	1444
March	1st to 10th	Day	53% 4 13% 20% 9% 1	1831
	11th to 20th	Day	51% 3 14% 20% 11% 1	1980
	21st to end	Day	54% 4 11% 20% 10% 1	2271
April	1st to 10th	Day	55% 4 9% 19% 12% 1	2160
	11th to 20th	Day	58% 2 9% 19% 10% 2	2134
	21st to end	Day	56% 3 11% 20% 10% 1	2311
May	1st to 10th	Day	50% 4 12% 23% 10% 1	2520
	11th to 20th	Day	49% 4 13% 23% 10% 2	2483
	21st to end	Day	45% 5% 11% 26% 12% 1	2725
June	1st to 10th	Day	36% 5% 17% 29% 11% 1	2520
	11th to 20th	Day	44% 5% 14% 22% 14%	2520
	21st to end	Day	38% 6% 19% 27% 10% 1	2520
July	1st to 10th	Day	30% 6% 15% 33% 16%	2451
	11th to 20th	Day	35% 6% 14% 28% 16% 2	2445
	21st to end	Day	33% 5% 16% 29% 16% 1	2591
August	1st to 10th	Day	38% 4 17% 30% 9% 2	2439
	11th to 20th	Day	39% 5% 16% 29% 11%	2301
	21st to end	Day	36% 5% 14% 30% 13% 2	2411
September	1st to 10th	Day	38% 5% 15% 30% 11%	2160
	11th to 20th	Day	41% 4 19% 28% 8%	2160
	21st to end	Day	46% 4 15% 28% 7%	1905
October	1st to 10th	Day	48% 4 13% 28% 7% 1	1800
	11th to 20th	Day	57% 3 11% 19% 10% 1	1800
	21st to end	Day	56% 3 12% 20% 8%	1853
November	1st to 10th	Day	52% 4 11% 22% 10% 1	1615
	11th to 20th	Day	65% 3 11% 14% 6% 1	1520
	21st to end	Day	56% 4 14% 18% 6% 1	1520
December	1st to 10th	Day	68% 4 11% 13% 4	1520
	11th to 20th	Day	66% 4 11% 14% 4 1	1915
	21st to end	Day	77% 2 5% 10% 6%	2783
January	1st to 10th	Night	58% 4 11% 19% 7% 1	2648
	11th to 20th	Night	63% 4 10% 15% 6%	2561
	21st to end	Night	65% 3 7% 16% 8% 1	2688
February	1st to 10th	Night	53% 3 10% 21% 11% 1	2340
	11th to 20th	Night	50% 5% 12% 21% 11% 1	2274
	21st to end	Night	54% 4 11% 20% 10% 2	1812
March	1st to 10th	Night	48% 4 13% 22% 11% 1	2144
	11th to 20th	Night	46% 3 13% 24% 13% 1	1980
	21st to end	Night	52% 4 11% 21% 12% 2	2084
April	1st to 10th	Night	53% 4 6% 19% 17% 1	1800
	11th to 20th	Night	55% 3 8% 19% 13% 2	1775
	21st to end	Night	50% 5% 9% 21% 13% 2	1645
May	1st to 10th	Night	49% 4 12% 23% 11% 1	1440
	11th to 20th	Night	44% 4 10% 27% 12% 2	1417
	21st to end	Night	41% 5% 15% 25% 13% 2	1552
June	1st to 10th	Night	35% 6% 16% 29% 12% 1	1440
	11th to 20th	Night	39% 5% 12% 23% 18% 2	1440
	21st to end	Night	35% 7% 14% 29% 13% 2	1440
July	1st to 10th	Night	25% 6% 17% 33% 18% 1	1400
	11th to 20th	Night	31% 7% 13% 30% 16% 3	1392
	21st to end	Night	25% 5% 17% 32% 18% 3	1487
August	1st to 10th	Night	29% 4 15% 35% 13% 4	1474
	11th to 20th	Night	35% 5% 15% 30% 14% 2	1600
	21st to end	Night	35% 5% 13% 28% 16% 3	1944
September	1st to 10th	Night	31% 5% 14% 35% 14% 1	1800
	11th to 20th	Night	33% 4 17% 36% 11%	1800
	21st to end	Night	42% 4 15% 30% 9%	2033
October	1st to 10th	Night	43% 3 11% 31% 11% 1	2160
	11th to 20th	Night	49% 4 13% 23% 9% 1	2160
	21st to end	Night	52% 5% 13% 20% 9% 1	2502
November	1st to 10th	Night	50% 3 11% 24% 11% 1	2458
	11th to 20th	Night	57% 3 14% 15% 10%	2660
	21st to end	Night	54% 4 15% 21% 5% 1	2660
December	1st to 10th	Night	62% 3 13% 18% 5%	2660
	11th to 20th	Night	65% 3 11% 16% 4	3281
	21st to end	Night	71% 4 7% 13% 5%	4884

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	20% 11% 35% 23% 9% 2	270
	11th to 20th	Day	22% 10% 35% 27% 5%	248
	21st to end	Day	17% 10% 37% 24% 12%	264
February	1st to 10th	Day	19% 12% 42% 23% 4	240
	11th to 20th	Day	17% 16% 43% 19% 3 2	240
	21st to end	Day	21% 15% 47% 15% 1 1	198
March	1st to 10th	Day	20% 14% 39% 16% 10% 2	240
	11th to 20th	Day	18% 11% 40% 20% 10% 2	240
	21st to end	Day	23% 13% 32% 21% 8% 3	357
April	1st to 10th	Day	24% 12% 33% 23% 7% 1	420
	11th to 20th	Day	29% 9% 25% 26% 10% 1	412
	21st to end	Day	29% 11% 27% 23% 8% 2	539
May	1st to 10th	Day	21% 11% 31% 26% 9% 2	603
	11th to 20th	Day	21% 9% 30% 25% 11% 4	625
	21st to end	Day	23% 7% 27% 27% 13% 4	679
June	1st to 10th	Day	14% 7% 29% 32% 15% 3	630
	11th to 20th	Day	17% 8% 35% 28% 10% 3	630
	21st to end	Day	14% 7% 30% 30% 14% 6%	630
July	1st to 10th	Day	10% 5% 22% 32% 22% 10%	621
	11th to 20th	Day	14% 7% 25% 30% 17% 8%	618
	21st to end	Day	10% 4 28% 31% 18% 9%	684
August	1st to 10th	Day	13% 4 25% 31% 21% 7%	626
	11th to 20th	Day	16% 4 21% 32% 22% 5%	615
	21st to end	Day	15% 6% 24% 30% 21% 5%	530
September	1st to 10th	Day	18% 6% 25% 29% 18% 3	450
	11th to 20th	Day	16% 6% 30% 30% 15% 3	450
	21st to end	Day	25% 11% 24% 24% 12% 4	347
October	1st to 10th	Day	22% 9% 27% 20% 16% 5%	270
	11th to 20th	Day	27% 13% 33% 22% 4 1	270
	21st to end	Day	29% 12% 39% 16% 3	297
November	1st to 10th	Day	23% 14% 35% 20% 8%	270
	11th to 20th	Day	23% 16% 37% 17% 6%	270
	21st to end	Day	24% 14% 39% 19% 4	270
December	1st to 10th	Day	18% 16% 40% 19% 6% 1	270
	11th to 20th	Day	24% 16% 37% 18% 4	270
	21st to end	Day	29% 12% 40% 16% 2 1	297
January	1st to 10th	Night	15% 10% 37% 28% 9%	950
	11th to 20th	Night	19% 8% 35% 28% 9% 1	915
	21st to end	Night	20% 8% 34% 31% 7% 1	990
February	1st to 10th	Night	14% 9% 37% 32% 8% 1	900
	11th to 20th	Night	14% 10% 39% 28% 8% 1	879
	21st to end	Night	18% 9% 37% 29% 6%	628
March	1st to 10th	Night	17% 11% 33% 31% 8% 1	704
	11th to 20th	Night	17% 7% 41% 25% 9% 1	540
	21st to end	Night	21% 8% 32% 27% 10% 3	594
April	1st to 10th	Night	25% 5% 36% 27% 6% 1	540
	11th to 20th	Night	28% 7% 27% 30% 8%	533
	21st to end	Night	25% 6% 28% 30% 10% 2	540
May	1st to 10th	Night	23% 8% 30% 32% 6% 1	540
	11th to 20th	Night	19% 5% 29% 34% 11% 2	531
	21st to end	Night	14% 6% 27% 38% 14% 1	584
June	1st to 10th	Night	14% 4 28% 44% 10% 1	540
	11th to 20th	Night	14% 3 28% 42% 13% 1	540
	21st to end	Night	11% 4 20% 45% 18% 2	540
July	1st to 10th	Night	8% 2 22% 38% 28% 3	525
	11th to 20th	Night	12% 2 23% 40% 22% 1	522
	21st to end	Night	8% 2 20% 44% 25% 1	591
August	1st to 10th	Night	12% 1 16% 48% 23% 1	537
	11th to 20th	Night	15% 1 18% 36% 26% 3	534
	21st to end	Night	13% 3 22% 35% 25% 2	594
September	1st to 10th	Night	11% 4 21% 41% 20% 3	540
	11th to 20th	Night	13% 3 30% 33% 19% 3	540
	21st to end	Night	20% 7% 29% 29% 14% 1	674
October	1st to 10th	Night	22% 6% 25% 31% 14% 1	720
	11th to 20th	Night	23% 9% 32% 27% 9% 1	720
	21st to end	Night	24% 7% 30% 30% 9%	792
November	1st to 10th	Night	22% 8% 33% 29% 8% 1	792
	11th to 20th	Night	21% 12% 31% 27% 8% 1	950
	21st to end	Night	19% 14% 35% 26% 5%	950
December	1st to 10th	Night	18% 10% 34% 29% 8% 1	950
	11th to 20th	Night	22% 8% 37% 23% 8% 2	950
	21st to end	Night	28% 9% 33% 22% 7% 1	1045

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