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 2024-11-12.

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% 10% 4 9% 5% 5% 5% 4 6% 7% 34%	101681
	Night	13% 11% 4 9% 5% 4 4 4 5% 6% 35%	97585
Transparency	Day	48% 4 13% 24% 10% 1	105212
	Night	49% 4 12% 23% 11% 1	102181
Seeing	Day	19% 9% 31% 25% 12% 3	16103
	Night	18% 7% 31% 32% 11% 1	25974

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	12% 16% 7% 19% 7% 4 3 3 5% 5% 19%	2527
	11th to 20th	Day	6% 12% 7% 17% 11% 9% 9% 5% 6% 5% 14%	2530
	21st to end	Day	7% 19% 9% 18% 6% 5% 5% 3 5% 5% 17%	2920
February	1st to 10th	Day	7% 14% 4 11% 7% 4 5% 3 6% 6% 31%	1710
	11th to 20th	Day	8% 13% 5% 10% 7% 5% 3 5% 6% 6% 33%	1664
	21st to end	Day	11% 11% 5% 11% 7% 5% 4 3 6% 6% 30%	1532
March	1st to 10th	Day	10% 10% 3 7% 6% 4 5% 4 5% 6% 39%	3179
	11th to 20th	Day	8% 11% 4 12% 5% 4 4 4 6% 7% 35%	3480
	21st to end	Day	18% 13% 4 11% 5% 5% 4 3 5% 5% 26%	3990
April	1st to 10th	Day	15% 14% 4 9% 5% 4 4 4 5% 5% 30%	2280
	11th to 20th	Day	20% 11% 5% 8% 5% 4 4 3 4 6% 30%	2254
	21st to end	Day	19% 11% 4 8% 5% 5% 4 3 4 5% 31%	2440
May	1st to 10th	Day	17% 8% 3 6% 4 4 4 3 5% 6% 38%	4399
	11th to 20th	Day	16% 6% 2 6% 4 5% 5% 4 6% 7% 39%	4399
	21st to end	Day	19% 8% 3 7% 4 4 4 4 5% 7% 34%	4824
June	1st to 10th	Day	12% 6% 2 6% 4 4 4 4 7% 9% 42%	2660
	11th to 20th	Day	14% 8% 3 7% 5% 5% 5% 4 7% 7% 36%	2660
	21st to end	Day	11% 5% 2 6% 5% 4 5% 5% 9% 10% 38%	2660
July	1st to 10th	Day	12% 5% 2 6% 5% 4 5% 4 7% 8% 41%	4440
	11th to 20th	Day	7% 4 2 5% 4 4 5% 4 7% 11% 48%	4440
	21st to end	Day	9% 5% 2 5% 4 4 4 4 6% 8% 48%	4813
August	1st to 10th	Day	13% 7% 2 6% 4 4 5% 4 7% 8% 40%	4314
	11th to 20th	Day	13% 5% 2 5% 4 4 4 4 6% 8% 45%	4041
	21st to end	Day	13% 7% 3 7% 5% 4 5% 4 6% 7% 39%	4229
September	1st to 10th	Day	13% 5% 2 6% 4 4 4 4 6% 8% 44%	2280
	11th to 20th	Day	14% 6% 2 7% 4 5% 4 4 5% 7% 43%	2280
	21st to end	Day	17% 8% 2 6% 3 5% 4 3 6% 6% 39%	2010
October	1st to 10th	Day	17% 9% 3 7% 4 4 4 4 6% 7% 36%	1900
	11th to 20th	Day	20% 14% 3 8% 5% 4 4 3 4 7% 29%	1900
	21st to end	Day	20% 13% 3 9% 5% 4 4 3 5% 5% 30%	1955
November	1st to 10th	Day	14% 17% 3 7% 5% 4 5% 3 5% 6% 31%	1702
	11th to 20th	Day	15% 15% 8% 12% 7% 5% 5% 3 5% 5% 22%	1528
	21st to end	Day	14% 14% 4 10% 7% 5% 4 4 6% 6% 26%	1520
December	1st to 10th	Day	10% 18% 7% 15% 10% 6% 5% 3 5% 6% 16%	1520
	11th to 20th	Day	12% 15% 7% 13% 9% 7% 5% 4 5% 5% 18%	1918
	21st to end	Day	24% 18% 6% 17% 6% 3 3 2 3 3 15%	2783
January	1st to 10th	Night	12% 13% 8% 17% 6% 4 4 3 5% 5% 22%	4428
	11th to 20th	Night	8% 13% 6% 15% 9% 8% 7% 4 6% 6% 18%	4440
	21st to end	Night	9% 21% 10% 16% 5% 5% 4 3 4 5% 20%	4744
February	1st to 10th	Night	6% 12% 6% 11% 7% 6% 5% 4 5% 6% 31%	2470
	11th to 20th	Night	7% 11% 4 9% 7% 6% 6% 5% 5% 5% 35%	2403
	21st to end	Night	11% 13% 4 10% 5% 5% 5% 4 6% 6% 31%	1922
March	1st to 10th	Night	8% 8% 3 7% 5% 4 5% 4 5% 8% 43%	3703
	11th to 20th	Night	8% 9% 4 14% 5% 4 5% 4 5% 6% 36%	3480
	21st to end	Night	20% 16% 5% 11% 4 4 4 3 3 4 27%	3640
April	1st to 10th	Night	17% 13% 4 8% 4 4 4 3 4 6% 32%	1900
	11th to 20th	Night	21% 10% 4 7% 4 4 5% 4 4 5% 32%	1875
	21st to end	Night	19% 11% 2 7% 4 4 4 4 6% 6% 34%	1736
May	1st to 10th	Night	17% 8% 3 7% 4 4 4 4 5% 7% 37%	2506
	11th to 20th	Night	13% 7% 3 6% 3 4 3 3 5% 6% 47%	2506
	21st to end	Night	13% 8% 3 8% 5% 5% 5% 4 6% 9% 35%	2743
June	1st to 10th	Night	11% 5% 2 6% 4 4 5% 5% 6% 10% 42%	1520
	11th to 20th	Night	10% 5% 3 7% 5% 5% 5% 4 7% 8% 41%	1520
	21st to end	Night	9% 5% 2 6% 5% 5% 5% 4 6% 9% 43%	1520
July	1st to 10th	Night	10% 5% 1 4 4 4 5% 4 6% 9% 49%	2530
	11th to 20th	Night	5% 4 2 4 3 3 4 4 6% 7% 57%	2530
	21st to end	Night	8% 6% 2 4 4 3 3 3 5% 8% 54%	2750
August	1st to 10th	Night	11% 6% 2 5% 4 4 4 4 6% 9% 45%	2610
	11th to 20th	Night	14% 6% 2 5% 3 3 3 3 6% 7% 49%	2798
	21st to end	Night	15% 8% 3 6% 4 4 4 3 5% 6% 42%	3414
September	1st to 10th	Night	9% 5% 2 5% 4 3 4 4 6% 7% 53%	1900
	11th to 20th	Night	10% 6% 2 5% 4 3 3 3 6% 8% 49%	1900
	21st to end	Night	16% 7% 2 7% 4 3 3 3 6% 7% 43%	2147
October	1st to 10th	Night	16% 8% 3 6% 4 3 3 2 4 6% 45%	2280
	11th to 20th	Night	16% 11% 3 8% 5% 4 4 3 5% 6% 35%	2280
	21st to end	Night	17% 12% 3 8% 4 4 5% 4 5% 7% 31%	2641
November	1st to 10th	Night	16% 12% 4 8% 4 4 4 4 5% 6% 34%	2591
	11th to 20th	Night	13% 14% 6% 11% 6% 4 4 3 5% 7% 27%	2675
	21st to end	Night	13% 11% 5% 10% 6% 5% 5% 4 5% 5% 29%	2660
December	1st to 10th	Night	9% 15% 7% 14% 9% 5% 5% 4 6% 6% 22%	2660
	11th to 20th	Night	11% 15% 5% 13% 7% 7% 6% 4 5% 5% 20%	3279
	21st to end	Night	23% 21% 5% 9% 5% 4 4 3 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	69% 2 7% 17% 4	2528
	11th to 20th	Day	70% 4 7% 13% 5%	2530
	21st to end	Day	69% 3 9% 14% 5%	2920
February	1st to 10th	Day	52% 4 10% 21% 12% 1	1710
	11th to 20th	Day	50% 4 11% 23% 10% 1	1673
	21st to end	Day	55% 3 9% 20% 11% 2	1532
March	1st to 10th	Day	46% 3 15% 21% 13% 1	3179
	11th to 20th	Day	48% 4 13% 22% 13% 1	3480
	21st to end	Day	60% 3 9% 21% 6% 1	3990
April	1st to 10th	Day	55% 4 9% 19% 13% 1	2280
	11th to 20th	Day	57% 2 9% 20% 10% 2	2254
	21st to end	Day	56% 3 10% 20% 10% 1	2440
May	1st to 10th	Day	46% 4 10% 24% 13% 2	4440
	11th to 20th	Day	44% 4 14% 27% 11% 1	4403
	21st to end	Day	50% 4 10% 22% 12% 1	4837
June	1st to 10th	Day	36% 6% 16% 29% 12% 1	2660
	11th to 20th	Day	45% 5% 14% 22% 14% 1	2660
	21st to end	Day	38% 5% 18% 27% 10% 1	2660
July	1st to 10th	Day	38% 6% 16% 26% 13%	4371
	11th to 20th	Day	28% 6% 17% 35% 13% 1	4365
	21st to end	Day	32% 5% 14% 32% 16%	4662
August	1st to 10th	Day	40% 4 17% 26% 12% 1	4299
	11th to 20th	Day	36% 6% 16% 34% 8%	4080
	21st to end	Day	42% 6% 16% 25% 10% 1	4228
September	1st to 10th	Day	37% 5% 15% 31% 12%	2280
	11th to 20th	Day	40% 4 20% 28% 8%	2280
	21st to end	Day	44% 4 14% 29% 9%	2010
October	1st to 10th	Day	35% 3 11% 34% 15%	3160
	11th to 20th	Day	49% 2 9% 26% 14%	3140
	21st to end	Day	50% 3 16% 23% 8%	3193
November	1st to 10th	Day	53% 4 10% 21% 10% 1	1702
	11th to 20th	Day	65% 3 11% 14% 6% 1	1528
	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	64% 3 10% 17% 6% 1	4428
	11th to 20th	Night	66% 4 11% 15% 5%	4440
	21st to end	Night	69% 2 7% 17% 5% 1	4744
February	1st to 10th	Night	52% 3 10% 21% 12% 1	2470
	11th to 20th	Night	50% 5% 12% 22% 10% 1	2404
	21st to end	Night	52% 4 12% 21% 10% 2	1922
March	1st to 10th	Night	40% 3 16% 24% 15% 2	3703
	11th to 20th	Night	50% 3 10% 24% 12% 1	3480
	21st to end	Night	63% 3 7% 17% 10% 1	3640
April	1st to 10th	Night	53% 4 6% 19% 17% 1	1900
	11th to 20th	Night	54% 3 8% 20% 13% 2	1875
	21st to end	Night	49% 5% 9% 22% 13% 1	1736
May	1st to 10th	Night	46% 4 12% 22% 15% 1	2530
	11th to 20th	Night	38% 4 8% 30% 18% 3	2507
	21st to end	Night	45% 5% 13% 23% 12% 1	2751
June	1st to 10th	Night	35% 6% 16% 29% 13% 1	1520
	11th to 20th	Night	39% 5% 12% 23% 18% 2	1520
	21st to end	Night	36% 7% 13% 30% 13% 2	1520
July	1st to 10th	Night	32% 6% 19% 28% 13% 1	2490
	11th to 20th	Night	25% 5% 14% 39% 16% 2	2482
	21st to end	Night	27% 5% 13% 32% 21% 2	2662
August	1st to 10th	Night	36% 4 16% 28% 15% 2	2601
	11th to 20th	Night	35% 5% 14% 35% 10% 1	2825
	21st to end	Night	42% 5% 15% 26% 10% 2	3414
September	1st to 10th	Night	30% 5% 15% 37% 13% 1	1900
	11th to 20th	Night	33% 4 17% 36% 10%	1900
	21st to end	Night	40% 4 14% 31% 10% 1	2147
October	1st to 10th	Night	30% 3 7% 36% 22% 3	3800
	11th to 20th	Night	46% 3 11% 22% 17% 1	3779
	21st to end	Night	53% 4 12% 22% 9%	4340
November	1st to 10th	Night	50% 3 12% 23% 11% 1	2591
	11th to 20th	Night	57% 3 14% 15% 10%	2675
	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% 24% 8% 2	270
	11th to 20th	Day	20% 11% 38% 26% 6%	270
	21st to end	Day	16% 9% 37% 25% 11% 1	297
February	1st to 10th	Day	19% 11% 40% 22% 5% 1	270
	11th to 20th	Day	16% 17% 43% 19% 4 1	270
	21st to end	Day	19% 17% 48% 16%	225
March	1st to 10th	Day	20% 13% 41% 16% 9% 1	270
	11th to 20th	Day	17% 12% 42% 18% 9% 2	270
	21st to end	Day	24% 13% 33% 20% 7% 3	396
April	1st to 10th	Day	25% 12% 34% 22% 7% 1	460
	11th to 20th	Day	30% 10% 26% 25% 9% 1	452
	21st to end	Day	28% 11% 28% 22% 9% 2	586
May	1st to 10th	Day	21% 12% 31% 25% 8% 2	653
	11th to 20th	Day	21% 9% 31% 25% 12% 3	675
	21st to end	Day	23% 8% 26% 27% 13% 3	734
June	1st to 10th	Day	14% 9% 29% 31% 14% 3	680
	11th to 20th	Day	18% 9% 34% 27% 9% 3	680
	21st to end	Day	15% 8% 30% 28% 13% 6%	680
July	1st to 10th	Day	11% 5% 22% 31% 21% 10%	671
	11th to 20th	Day	13% 6% 24% 30% 18% 8%	668
	21st to end	Day	10% 4 29% 31% 18% 9%	739
August	1st to 10th	Day	14% 5% 25% 30% 20% 7%	676
	11th to 20th	Day	16% 4 21% 32% 22% 5%	665
	21st to end	Day	15% 6% 25% 29% 20% 4	575
September	1st to 10th	Day	17% 8% 26% 28% 18% 3	490
	11th to 20th	Day	15% 6% 32% 29% 16% 3	490
	21st to end	Day	23% 11% 24% 24% 13% 4	381
October	1st to 10th	Day	22% 11% 28% 21% 14% 5%	300
	11th to 20th	Day	26% 15% 33% 20% 4 1	300
	21st to end	Day	28% 13% 40% 15% 3	330
November	1st to 10th	Day	25% 14% 34% 19% 7%	300
	11th to 20th	Day	23% 16% 38% 17% 6%	273
	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% 36% 28% 9% 1	950
	21st to end	Night	20% 8% 34% 31% 7% 1	1045
February	1st to 10th	Night	14% 9% 37% 32% 8% 1	950
	11th to 20th	Night	13% 11% 39% 28% 8% 1	929
	21st to end	Night	18% 8% 37% 30% 6%	666
March	1st to 10th	Night	17% 11% 34% 29% 7% 1	743
	11th to 20th	Night	17% 7% 41% 25% 9% 1	570
	21st to end	Night	21% 7% 32% 28% 9% 2	627
April	1st to 10th	Night	25% 5% 36% 27% 6% 1	570
	11th to 20th	Night	27% 7% 29% 29% 7%	563
	21st to end	Night	25% 6% 28% 29% 10% 2	570
May	1st to 10th	Night	23% 7% 31% 32% 6% 1	570
	11th to 20th	Night	19% 5% 30% 34% 10% 2	561
	21st to end	Night	15% 6% 28% 38% 13%	617
June	1st to 10th	Night	14% 4 28% 43% 10% 1	570
	11th to 20th	Night	14% 3 28% 42% 13% 1	570
	21st to end	Night	11% 5% 21% 45% 17% 2	570
July	1st to 10th	Night	8% 2 21% 38% 27% 3	555
	11th to 20th	Night	11% 3 23% 39% 23% 1	552
	21st to end	Night	8% 1 19% 45% 24% 1	624
August	1st to 10th	Night	12% 1 16% 47% 22% 1	567
	11th to 20th	Night	15% 2 18% 37% 25% 3	564
	21st to end	Night	14% 3 22% 35% 24% 2	627
September	1st to 10th	Night	11% 4 22% 41% 19% 3	570
	11th to 20th	Night	13% 4 30% 32% 18% 2	570
	21st to end	Night	19% 7% 28% 30% 15% 1	712
October	1st to 10th	Night	22% 6% 26% 31% 14% 1	760
	11th to 20th	Night	22% 9% 32% 26% 9% 1	760
	21st to end	Night	25% 8% 30% 29% 8%	836
November	1st to 10th	Night	22% 8% 33% 28% 7% 1	835
	11th to 20th	Night	21% 13% 31% 27% 8% 1	956
	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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