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 2026-01-02.

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	13% 10% 4 9% 5% 4 4 4 5% 7% 35%	127768
	Night	12% 11% 4 10% 5% 4 4 4 5% 6% 34%	123866
Transparency	Day	48% 4 13% 24% 11% 1	131254
	Night	50% 4 11% 22% 12% 1	128440
Seeing	Day	19% 10% 31% 25% 12% 3	17654
	Night	18% 7% 31% 31% 11% 1	27599

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	8% 12% 6% 19% 9% 5% 4 3 4 5% 25%	3625
	11th to 20th	Day	6% 13% 8% 14% 8% 7% 9% 6% 8% 6% 17%	3620
	21st to end	Day	5% 18% 9% 16% 5% 5% 5% 3 5% 6% 22%	4189
February	1st to 10th	Day	8% 14% 4 11% 8% 4 6% 3 6% 6% 30%	1800
	11th to 20th	Day	7% 12% 5% 10% 7% 5% 3 5% 6% 6% 33%	1754
	21st to end	Day	11% 11% 5% 11% 7% 5% 4 3 7% 6% 30%	1610
March	1st to 10th	Day	12% 8% 3 6% 5% 4 5% 4 6% 8% 41%	4567
	11th to 20th	Day	9% 11% 4 11% 5% 4 4 4 6% 9% 34%	4980
	21st to end	Day	21% 11% 4 10% 5% 4 4 3 4 5% 29%	5708
April	1st to 10th	Day	16% 14% 4 9% 5% 4 4 4 5% 5% 30%	2400
	11th to 20th	Day	20% 12% 5% 8% 5% 4 4 3 5% 6% 29%	2374
	21st to end	Day	20% 11% 4 8% 5% 4 3 3 4 5% 31%	2569
May	1st to 10th	Day	14% 7% 2 5% 4 3 4 3 5% 6% 47%	6319
	11th to 20th	Day	15% 8% 2 7% 5% 5% 4 4 5% 6% 40%	6319
	21st to end	Day	17% 9% 3 7% 5% 4 4 3 5% 7% 36%	6935
June	1st to 10th	Day	12% 6% 2 6% 4 4 5% 4 7% 9% 41%	2800
	11th to 20th	Day	15% 8% 3 7% 5% 5% 5% 4 7% 8% 35%	2800
	21st to end	Day	12% 6% 2 6% 4 4 5% 5% 9% 10% 37%	2800
July	1st to 10th	Day	11% 4 2 5% 4 4 5% 4 7% 9% 45%	6360
	11th to 20th	Day	11% 5% 2 5% 3 4 4 4 6% 10% 47%	6360
	21st to end	Day	10% 6% 2 5% 4 4 4 4 6% 8% 46%	6924
August	1st to 10th	Day	13% 7% 2 6% 4 4 5% 4 7% 8% 40%	4451
	11th to 20th	Day	14% 5% 2 5% 4 4 4 4 6% 8% 44%	4171
	21st to end	Day	13% 7% 3 7% 5% 4 5% 4 6% 8% 39%	4363
September	1st to 10th	Day	13% 6% 2 6% 4 4 4 4 5% 8% 44%	2400
	11th to 20th	Day	13% 6% 2 7% 4 5% 4 4 5% 8% 42%	2400
	21st to end	Day	17% 7% 2 6% 3 5% 4 3 6% 6% 40%	2115
October	1st to 10th	Day	16% 9% 2 7% 4 4 4 4 6% 7% 37%	2000
	11th to 20th	Day	20% 14% 3 7% 5% 4 4 3 4 7% 30%	2000
	21st to end	Day	19% 13% 3 9% 5% 4 4 3 5% 5% 30%	2058
November	1st to 10th	Day	13% 17% 4 8% 5% 4 5% 3 5% 6% 31%	1789
	11th to 20th	Day	14% 14% 8% 12% 6% 5% 5% 3 4 5% 24%	1680
	21st to end	Day	14% 14% 4 12% 7% 5% 4 4 5% 6% 24%	1680
December	1st to 10th	Day	8% 16% 8% 23% 12% 5% 4 3 4 4 13%	2690
	11th to 20th	Day	11% 16% 7% 12% 9% 6% 5% 3 4 4 22%	3088
	21st to end	Day	20% 21% 6% 18% 6% 5% 4 2 3 4 13%	4070
January	1st to 10th	Night	9% 10% 6% 18% 9% 5% 4 3 4 5% 26%	6363
	11th to 20th	Night	7% 13% 7% 14% 8% 7% 8% 6% 7% 6% 18%	6360
	21st to end	Night	6% 18% 9% 16% 5% 5% 4 3 4 6% 23%	6786
February	1st to 10th	Night	7% 12% 6% 11% 7% 6% 5% 4 6% 6% 30%	2600
	11th to 20th	Night	7% 11% 4 9% 7% 6% 6% 5% 5% 5% 35%	2533
	21st to end	Night	11% 13% 4 10% 5% 5% 5% 4 6% 6% 31%	2020
March	1st to 10th	Night	10% 7% 3 7% 4 4 4 4 5% 7% 46%	5329
	11th to 20th	Night	9% 10% 4 13% 6% 5% 5% 4 6% 7% 30%	4980
	21st to end	Night	19% 15% 5% 9% 4 4 4 3 4 5% 28%	5220
April	1st to 10th	Night	17% 14% 4 8% 4 4 4 3 4 6% 33%	2000
	11th to 20th	Night	21% 10% 4 7% 4 4 4 4 4 5% 32%	1975
	21st to end	Night	19% 10% 2 7% 4 4 4 4 6% 6% 34%	1827
May	1st to 10th	Night	15% 7% 2 6% 4 3 3 3 4 6% 47%	3596
	11th to 20th	Night	13% 7% 2 5% 3 3 3 3 5% 6% 49%	3596
	21st to end	Night	13% 7% 3 7% 5% 5% 5% 4 5% 7% 40%	3942
June	1st to 10th	Night	11% 5% 3 6% 4 4 5% 5% 6% 9% 41%	1600
	11th to 20th	Night	11% 6% 3 7% 5% 5% 5% 4 6% 8% 40%	1600
	21st to end	Night	11% 5% 2 6% 4 5% 5% 4 6% 9% 43%	1600
July	1st to 10th	Night	8% 4 2 5% 4 4 5% 4 6% 9% 49%	3618
	11th to 20th	Night	9% 5% 2 5% 3 3 4 3 5% 7% 54%	3617
	21st to end	Night	11% 6% 2 5% 4 3 4 3 6% 8% 48%	3947
August	1st to 10th	Night	11% 6% 2 5% 4 4 4 4 6% 9% 45%	2693
	11th to 20th	Night	14% 5% 2 6% 3 3 3 3 6% 7% 49%	2888
	21st to end	Night	14% 8% 3 6% 4 4 4 3 5% 6% 43%	3522
September	1st to 10th	Night	10% 5% 2 5% 4 3 4 4 6% 7% 52%	2000
	11th to 20th	Night	10% 6% 2 5% 4 3 4 3 6% 8% 48%	2000
	21st to end	Night	15% 7% 2 7% 4 3 3 3 6% 7% 44%	2261
October	1st to 10th	Night	15% 8% 2 6% 4 3 3 2 4 6% 46%	2400
	11th to 20th	Night	16% 11% 3 8% 5% 4 4 3 5% 6% 35%	2400
	21st to end	Night	17% 12% 3 8% 4 4 5% 4 5% 6% 32%	2780
November	1st to 10th	Night	16% 12% 4 8% 4 4 4 4 5% 6% 34%	2724
	11th to 20th	Night	13% 15% 6% 10% 6% 4 4 3 5% 6% 29%	2940
	21st to end	Night	13% 12% 5% 12% 6% 5% 5% 4 5% 5% 28%	2940
December	1st to 10th	Night	8% 13% 7% 19% 11% 6% 5% 3 5% 6% 17%	4720
	11th to 20th	Night	10% 17% 7% 15% 7% 6% 6% 5% 5% 4 19%	5339
	21st to end	Night	22% 18% 5% 13% 5% 5% 4 3 3 4 17%	7150

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	64% 2 6% 18% 10%	3626
	11th to 20th	Day	65% 3 6% 16% 10%	3620
	21st to end	Day	63% 3 9% 18% 6%	4189
February	1st to 10th	Day	53% 4 10% 20% 12% 1	1800
	11th to 20th	Day	50% 4 11% 23% 11% 1	1763
	21st to end	Day	54% 3 10% 20% 11% 1	1610
March	1st to 10th	Day	42% 3 16% 22% 13% 4	4567
	11th to 20th	Day	47% 4 14% 21% 13% 1	4980
	21st to end	Day	58% 3 9% 18% 12% 1	5710
April	1st to 10th	Day	55% 4 9% 19% 13% 1	2400
	11th to 20th	Day	57% 2 9% 20% 10% 2	2374
	21st to end	Day	56% 3 10% 20% 11% 1	2569
May	1st to 10th	Day	39% 4 12% 26% 17% 3	6357
	11th to 20th	Day	45% 4 13% 22% 12% 4	6323
	21st to end	Day	48% 4 9% 23% 15% 1	6949
June	1st to 10th	Day	37% 5% 16% 29% 12% 1	2800
	11th to 20th	Day	46% 5% 14% 21% 13% 1	2800
	21st to end	Day	39% 5% 18% 27% 10% 1	2800
July	1st to 10th	Day	33% 6% 16% 35% 9%	6291
	11th to 20th	Day	31% 5% 19% 34% 11% 1	6285
	21st to end	Day	34% 5% 15% 30% 15%	6728
August	1st to 10th	Day	40% 4 17% 25% 13% 1	4436
	11th to 20th	Day	37% 6% 16% 34% 8%	4210
	21st to end	Day	42% 6% 16% 25% 10% 1	4362
September	1st to 10th	Day	38% 5% 15% 31% 11%	2400
	11th to 20th	Day	41% 4 20% 27% 7%	2400
	21st to end	Day	43% 4 14% 29% 10%	2115
October	1st to 10th	Day	35% 3 11% 35% 15% 1	3260
	11th to 20th	Day	49% 2 9% 26% 14%	3240
	21st to end	Day	50% 3 16% 23% 8%	3296
November	1st to 10th	Day	54% 4 10% 20% 11% 1	1789
	11th to 20th	Day	63% 3 11% 17% 6% 1	1680
	21st to end	Day	59% 4 13% 17% 6% 1	1680
December	1st to 10th	Day	73% 4 8% 11% 3	2690
	11th to 20th	Day	64% 3 8% 14% 10%	3085
	21st to end	Day	78% 2 5% 9% 5%	4070
January	1st to 10th	Night	61% 3 8% 16% 11%	6363
	11th to 20th	Night	64% 3 10% 15% 8%	6360
	21st to end	Night	63% 3 9% 18% 7%	6786
February	1st to 10th	Night	53% 3 10% 21% 12% 1	2600
	11th to 20th	Night	50% 5% 12% 22% 11% 1	2534
	21st to end	Night	52% 4 12% 21% 10% 2	2020
March	1st to 10th	Night	38% 2 17% 21% 17% 4	5329
	11th to 20th	Night	52% 3 12% 21% 11%	4980
	21st to end	Night	61% 2 8% 16% 13% 1	5220
April	1st to 10th	Night	53% 4 6% 20% 17% 1	2000
	11th to 20th	Night	54% 3 8% 20% 13% 2	1975
	21st to end	Night	49% 5% 9% 22% 13% 1	1827
May	1st to 10th	Night	38% 4 11% 29% 16% 2	3620
	11th to 20th	Night	37% 4 11% 26% 17% 6%	3597
	21st to end	Night	43% 4 12% 26% 14% 1	3950
June	1st to 10th	Night	36% 6% 15% 28% 13% 1	1600
	11th to 20th	Night	40% 5% 12% 23% 17% 2	1600
	21st to end	Night	37% 7% 13% 29% 13% 2	1600
July	1st to 10th	Night	31% 6% 19% 33% 10% 1	3580
	11th to 20th	Night	29% 5% 16% 35% 13% 1	3572
	21st to end	Night	34% 5% 14% 28% 18% 1	3832
August	1st to 10th	Night	36% 4 15% 27% 16% 2	2684
	11th to 20th	Night	35% 5% 14% 35% 10% 1	2915
	21st to end	Night	41% 5% 15% 27% 10% 1	3522
September	1st to 10th	Night	31% 5% 15% 36% 13% 1	2000
	11th to 20th	Night	34% 4 17% 35% 10%	2000
	21st to end	Night	39% 4 14% 32% 11% 1	2261
October	1st to 10th	Night	30% 3 7% 36% 22% 3	3920
	11th to 20th	Night	46% 3 11% 22% 17% 1	3899
	21st to end	Night	53% 4 11% 22% 10%	4479
November	1st to 10th	Night	50% 3 11% 24% 11% 1	2724
	11th to 20th	Night	56% 3 14% 17% 9% 1	2940
	21st to end	Night	56% 4 14% 20% 5% 1	2940
December	1st to 10th	Night	68% 4 12% 14% 3	4720
	11th to 20th	Night	68% 3 8% 15% 6%	5341
	21st to end	Night	71% 3 7% 12% 6%	7150

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	18% 12% 37% 23% 8% 2	303
	11th to 20th	Day	19% 12% 37% 26% 6%	300
	21st to end	Day	16% 12% 37% 24% 10% 1	330
February	1st to 10th	Day	20% 12% 41% 21% 5% 1	300
	11th to 20th	Day	15% 18% 43% 19% 3 1	300
	21st to end	Day	19% 18% 47% 15%	249
March	1st to 10th	Day	20% 15% 41% 15% 8% 1	300
	11th to 20th	Day	18% 13% 42% 17% 8% 2	300
	21st to end	Day	25% 14% 33% 19% 7% 3	434
April	1st to 10th	Day	26% 14% 33% 21% 6% 1	500
	11th to 20th	Day	29% 12% 27% 23% 8% 1	492
	21st to end	Day	28% 12% 29% 21% 8% 2	633
May	1st to 10th	Day	20% 11% 31% 26% 9% 2	703
	11th to 20th	Day	21% 10% 30% 25% 11% 3	725
	21st to end	Day	23% 8% 27% 27% 12% 3	789
June	1st to 10th	Day	15% 9% 30% 30% 14% 3	730
	11th to 20th	Day	19% 9% 34% 25% 8% 3	730
	21st to end	Day	16% 8% 32% 27% 12% 5%	730
July	1st to 10th	Day	11% 5% 22% 32% 21% 9%	721
	11th to 20th	Day	13% 7% 24% 30% 18% 7%	718
	21st to end	Day	10% 5% 29% 30% 18% 8%	794
August	1st to 10th	Day	14% 5% 25% 29% 20% 6%	726
	11th to 20th	Day	16% 4 22% 31% 21% 5%	715
	21st to end	Day	15% 6% 25% 30% 20% 4	621
September	1st to 10th	Day	18% 8% 26% 28% 18% 3	530
	11th to 20th	Day	14% 6% 32% 30% 15% 3	530
	21st to end	Day	22% 10% 24% 25% 14% 5%	415
October	1st to 10th	Day	21% 12% 29% 20% 14% 5%	330
	11th to 20th	Day	27% 16% 32% 20% 4 1	330
	21st to end	Day	27% 14% 40% 15% 4 1	363
November	1st to 10th	Day	27% 15% 33% 17% 7%	330
	11th to 20th	Day	24% 18% 35% 17% 6%	330
	21st to end	Day	24% 15% 38% 20% 3	330
December	1st to 10th	Day	16% 16% 42% 19% 5% 1	330
	11th to 20th	Day	24% 18% 37% 18% 3	330
	21st to end	Day	28% 14% 39% 15% 3 1	363
January	1st to 10th	Night	14% 10% 37% 28% 10%	1006
	11th to 20th	Night	18% 8% 36% 28% 8% 1	1000
	21st to end	Night	19% 9% 34% 30% 7% 1	1100
February	1st to 10th	Night	15% 9% 37% 31% 7% 1	1000
	11th to 20th	Night	13% 10% 39% 28% 8% 1	979
	21st to end	Night	18% 8% 37% 30% 6%	700
March	1st to 10th	Night	18% 10% 35% 29% 7% 1	782
	11th to 20th	Night	17% 9% 41% 24% 9% 1	600
	21st to end	Night	21% 8% 32% 28% 9% 2	660
April	1st to 10th	Night	26% 6% 35% 27% 6% 1	600
	11th to 20th	Night	27% 8% 28% 29% 7%	593
	21st to end	Night	25% 6% 29% 29% 9% 2	600
May	1st to 10th	Night	22% 8% 31% 32% 7% 1	600
	11th to 20th	Night	19% 5% 30% 35% 10% 2	591
	21st to end	Night	15% 6% 28% 39% 12%	650
June	1st to 10th	Night	15% 4 29% 43% 9% 1	600
	11th to 20th	Night	15% 3 28% 41% 12% 1	600
	21st to end	Night	13% 5% 20% 44% 16% 2	600
July	1st to 10th	Night	8% 2 22% 39% 27% 3	585
	11th to 20th	Night	12% 3 23% 39% 22% 1	582
	21st to end	Night	9% 2 21% 44% 23% 1	657
August	1st to 10th	Night	12% 2 16% 46% 22% 1	597
	11th to 20th	Night	14% 2 19% 37% 25% 3	594
	21st to end	Night	14% 3 21% 36% 24% 2	660
September	1st to 10th	Night	12% 4 22% 40% 20% 3	600
	11th to 20th	Night	12% 4 30% 34% 18% 2	600
	21st to end	Night	18% 7% 27% 31% 16% 1	750
October	1st to 10th	Night	21% 7% 26% 31% 13% 1	800
	11th to 20th	Night	22% 10% 33% 26% 9% 1	800
	21st to end	Night	25% 8% 30% 28% 9%	880
November	1st to 10th	Night	23% 9% 34% 28% 7% 1	878
	11th to 20th	Night	22% 13% 31% 27% 7% 1	1050
	21st to end	Night	20% 14% 35% 25% 5%	1050
December	1st to 10th	Night	18% 10% 35% 28% 8% 1	1050
	11th to 20th	Night	23% 10% 36% 23% 7% 2	1050
	21st to end	Night	28% 9% 33% 22% 7% 1	1155

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