Robert's Hill
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Robert's Hill 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	14% 6% 3 6% 6% 4 5% 5% 6% 8% 39%	74746
	Night	13% 6% 2 5% 5% 3 5% 5% 6% 8% 43%	75017
Transparency	Day	43% 7% 12% 21% 14% 3	74468
	Night	39% 7% 13% 23% 16% 3	74814
Seeing	Day	22% 15% 24% 22% 13% 5%	14038
	Night	18% 14% 22% 28% 16% 2	24294

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% 8% 3 6% 6% 3 5% 4 5% 7% 42%	1706
	11th to 20th	Day	16% 8% 3 7% 6% 3 4 4 5% 6% 38%	1656
	21st to end	Day	14% 8% 3 6% 6% 3 4 4 5% 7% 41%	1782
February	1st to 10th	Day	12% 7% 4 7% 7% 4 6% 6% 5% 7% 36%	1620
	11th to 20th	Day	15% 7% 3 6% 7% 4 5% 4 5% 7% 36%	1625
	21st to end	Day	17% 7% 4 5% 6% 3 5% 5% 5% 7% 37%	1538
March	1st to 10th	Day	13% 5% 2 3 5% 3 5% 4 5% 7% 48%	1980
	11th to 20th	Day	17% 8% 3 5% 6% 3 5% 5% 5% 7% 37%	1980
	21st to end	Day	16% 8% 4 6% 6% 3 5% 4 6% 5% 38%	2178
April	1st to 10th	Day	10% 6% 3 5% 6% 3 4 4 5% 8% 46%	2036
	11th to 20th	Day	13% 5% 3 6% 6% 3 5% 4 6% 7% 43%	2134
	21st to end	Day	16% 6% 3 5% 5% 3 6% 5% 5% 7% 39%	2299
May	1st to 10th	Day	14% 6% 3 6% 6% 4 5% 6% 6% 8% 33%	2340
	11th to 20th	Day	16% 7% 3 6% 6% 4 6% 6% 6% 8% 31%	2301
	21st to end	Day	14% 6% 2 6% 5% 4 6% 6% 7% 10% 34%	2531
June	1st to 10th	Day	12% 5% 2 5% 5% 4 6% 5% 7% 9% 39%	2340
	11th to 20th	Day	9% 5% 2 5% 7% 5% 7% 6% 7% 10% 37%	2340
	21st to end	Day	8% 5% 2 5% 7% 4 6% 6% 7% 10% 38%	2340
July	1st to 10th	Day	11% 5% 2 6% 7% 4 7% 6% 8% 10% 34%	2340
	11th to 20th	Day	7% 5% 2 6% 7% 5% 7% 7% 8% 11% 35%	2340
	21st to end	Day	10% 6% 3 7% 8% 5% 8% 8% 7% 9% 29%	2547
August	1st to 10th	Day	14% 6% 3 6% 7% 5% 6% 7% 7% 10% 30%	2327
	11th to 20th	Day	14% 6% 3 6% 6% 4 7% 7% 9% 10% 30%	2303
	21st to end	Day	10% 6% 3 6% 6% 4 6% 6% 7% 10% 35%	2412
September	1st to 10th	Day	14% 6% 2 4 6% 3 5% 5% 6% 8% 41%	2111
	11th to 20th	Day	16% 6% 2 6% 6% 3 4 4 6% 7% 40%	1980
	21st to end	Day	23% 7% 2 5% 4 3 4 5% 6% 5% 36%	1980
October	1st to 10th	Day	18% 5% 2 4 4 3 3 3 4 7% 47%	1821
	11th to 20th	Day	14% 6% 3 6% 4 2 4 5% 5% 7% 45%	1800
	21st to end	Day	14% 6% 2 6% 4 3 4 4 5% 6% 47%	1980
November	1st to 10th	Day	11% 6% 2 4 4 2 3 4 5% 6% 53%	1857
	11th to 20th	Day	12% 6% 2 3 3 3 4 5% 5% 7% 48%	1732
	21st to end	Day	13% 6% 3 5% 5% 2 3 4 5% 6% 48%	1710
December	1st to 10th	Day	15% 8% 3 6% 5% 5% 6% 4 7% 7% 35%	1522
	11th to 20th	Day	14% 5% 3 5% 4 2 4 4 5% 8% 46%	2134
	21st to end	Day	18% 8% 3 6% 5% 4 5% 6% 6% 8% 31%	3124
January	1st to 10th	Night	12% 7% 2 6% 5% 3 5% 5% 6% 7% 41%	2459
	11th to 20th	Night	15% 7% 3 6% 5% 3 5% 5% 5% 7% 39%	2399
	21st to end	Night	13% 7% 3 5% 5% 3 5% 5% 5% 7% 42%	2574
February	1st to 10th	Night	13% 8% 3 5% 4 3 4 5% 6% 9% 40%	2340
	11th to 20th	Night	17% 7% 3 5% 5% 3 5% 5% 5% 5% 38%	2219
	21st to end	Night	15% 5% 3 6% 5% 2 5% 5% 6% 7% 41%	1718
March	1st to 10th	Night	12% 5% 2 4 5% 2 4 5% 5% 6% 49%	1980
	11th to 20th	Night	17% 6% 3 4 5% 3 5% 6% 7% 8% 36%	1980
	21st to end	Night	13% 7% 3 6% 5% 3 5% 4 5% 7% 43%	2178
April	1st to 10th	Night	12% 7% 3 5% 6% 3 4 4 6% 8% 43%	1922
	11th to 20th	Night	13% 5% 2 5% 4 3 4 4 6% 8% 47%	1775
	21st to end	Night	15% 6% 3 4 4 3 4 5% 4 7% 45%	1675
May	1st to 10th	Night	14% 6% 3 5% 5% 4 4 5% 6% 7% 42%	1620
	11th to 20th	Night	15% 8% 4 5% 5% 3 4 4 5% 9% 39%	1592
	21st to end	Night	14% 5% 2 3 5% 3 5% 5% 7% 10% 41%	1744
June	1st to 10th	Night	10% 3 2 4 5% 4 6% 6% 7% 11% 42%	1620
	11th to 20th	Night	10% 5% 3 5% 5% 2 4 4 6% 9% 47%	1620
	21st to end	Night	7% 3 1 4 5% 4 5% 6% 7% 11% 47%	1620
July	1st to 10th	Night	8% 5% 2 6% 7% 4 5% 5% 6% 9% 42%	1620
	11th to 20th	Night	5% 4 2 4 4 3 6% 6% 7% 11% 47%	1620
	21st to end	Night	5% 5% 2 5% 5% 4 6% 7% 8% 12% 42%	1771
August	1st to 10th	Night	12% 5% 2 5% 6% 3 4 5% 6% 9% 41%	1611
	11th to 20th	Night	10% 6% 2 6% 6% 4 5% 5% 6% 9% 42%	1598
	21st to end	Night	9% 4 2 5% 6% 4 5% 6% 8% 11% 41%	1926
September	1st to 10th	Night	12% 5% 2 5% 5% 3 5% 5% 5% 7% 47%	1844
	11th to 20th	Night	15% 5% 2 5% 4 3 4 4 5% 8% 45%	1980
	21st to end	Night	18% 7% 2 5% 4 3 5% 4 5% 6% 41%	1980
October	1st to 10th	Night	17% 5% 2 3 5% 2 4 4 5% 6% 47%	2137
	11th to 20th	Night	14% 6% 3 5% 4 3 4 3 4 6% 48%	2160
	21st to end	Night	14% 4 2 3 4 2 4 4 5% 7% 51%	2376
November	1st to 10th	Night	13% 6% 2 5% 4 2 3 4 4 5% 51%	2221
	11th to 20th	Night	12% 5% 2 4 5% 3 4 5% 4 7% 50%	2446
	21st to end	Night	12% 5% 2 4 5% 3 4 4 5% 6% 49%	2470
December	1st to 10th	Night	15% 9% 3 7% 5% 3 4 4 4 7% 40%	2643
	11th to 20th	Night	13% 6% 4 5% 6% 3 5% 5% 6% 8% 40%	3047
	21st to end	Night	17% 6% 3 7% 6% 4 6% 6% 6% 8% 33%	4532

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	43% 5% 10% 22% 15% 4	1707
	11th to 20th	Day	47% 6% 8% 22% 14% 2	1656
	21st to end	Day	44% 5% 8% 22% 17% 3	1782
February	1st to 10th	Day	47% 6% 10% 21% 14% 2	1620
	11th to 20th	Day	47% 5% 10% 20% 14% 3	1634
	21st to end	Day	48% 6% 10% 15% 17% 5%	1538
March	1st to 10th	Day	36% 4 11% 26% 19% 4	1980
	11th to 20th	Day	48% 6% 10% 18% 14% 5%	1980
	21st to end	Day	48% 6% 10% 22% 12% 3	2178
April	1st to 10th	Day	36% 6% 11% 24% 17% 6%	2036
	11th to 20th	Day	39% 6% 11% 22% 17% 4	2134
	21st to end	Day	44% 5% 12% 23% 13% 3	2299
May	1st to 10th	Day	47% 7% 14% 19% 12% 2	2340
	11th to 20th	Day	49% 9% 13% 20% 7% 2	2305
	21st to end	Day	44% 8% 17% 22% 9%	2531
June	1st to 10th	Day	39% 8% 16% 24% 12%	2340
	11th to 20th	Day	40% 10% 16% 19% 14% 2	2340
	21st to end	Day	39% 10% 15% 23% 12% 2	2340
July	1st to 10th	Day	42% 10% 15% 19% 11% 2	2275
	11th to 20th	Day	39% 9% 18% 18% 14% 1	2271
	21st to end	Day	46% 9% 15% 20% 9% 1	2405
August	1st to 10th	Day	46% 8% 17% 21% 7%	2314
	11th to 20th	Day	45% 9% 17% 19% 9%	2303
	21st to end	Day	41% 9% 18% 17% 11% 3	2412
September	1st to 10th	Day	40% 8% 11% 20% 17% 4	2111
	11th to 20th	Day	43% 6% 9% 23% 16% 2	1980
	21st to end	Day	48% 5% 10% 19% 16% 1	1980
October	1st to 10th	Day	39% 5% 9% 26% 17% 4	1821
	11th to 20th	Day	38% 6% 11% 23% 19% 4	1800
	21st to end	Day	38% 5% 8% 26% 19% 4	1980
November	1st to 10th	Day	33% 5% 9% 23% 26% 5%	1857
	11th to 20th	Day	35% 4 10% 22% 23% 6%	1732
	21st to end	Day	38% 4 10% 24% 22% 3	1710
December	1st to 10th	Day	48% 6% 12% 17% 14% 3	1522
	11th to 20th	Day	35% 6% 11% 22% 21% 5%	2131
	21st to end	Day	48% 7% 8% 22% 12% 2	3124
January	1st to 10th	Night	41% 6% 11% 20% 16% 5%	2459
	11th to 20th	Night	45% 6% 10% 19% 17% 4	2399
	21st to end	Night	40% 6% 9% 25% 17% 3	2574
February	1st to 10th	Night	41% 5% 14% 23% 15% 2	2340
	11th to 20th	Night	47% 5% 9% 19% 15% 5%	2221
	21st to end	Night	42% 7% 11% 24% 11% 5%	1718
March	1st to 10th	Night	36% 5% 12% 23% 20% 4	1980
	11th to 20th	Night	43% 8% 11% 22% 15% 1	1980
	21st to end	Night	41% 7% 12% 22% 15% 3	2178
April	1st to 10th	Night	38% 6% 12% 23% 17% 4	1922
	11th to 20th	Night	36% 4 13% 21% 21% 5%	1775
	21st to end	Night	39% 5% 12% 27% 14% 2	1675
May	1st to 10th	Night	41% 8% 15% 22% 12% 2	1620
	11th to 20th	Night	44% 7% 16% 22% 9% 1	1593
	21st to end	Night	38% 7% 21% 25% 8%	1745
June	1st to 10th	Night	33% 10% 18% 25% 14% 1	1620
	11th to 20th	Night	34% 8% 20% 23% 14% 1	1620
	21st to end	Night	29% 10% 20% 25% 14% 2	1620
July	1st to 10th	Night	37% 10% 17% 19% 13% 4	1575
	11th to 20th	Night	28% 11% 21% 24% 14% 2	1566
	21st to end	Night	30% 11% 21% 27% 11% 1	1672
August	1st to 10th	Night	38% 9% 19% 23% 10% 1	1600
	11th to 20th	Night	38% 8% 16% 25% 11% 1	1598
	21st to end	Night	35% 8% 19% 25% 11% 2	1926
September	1st to 10th	Night	36% 8% 14% 22% 16% 4	1844
	11th to 20th	Night	38% 5% 12% 25% 18% 1	1980
	21st to end	Night	44% 5% 10% 22% 17% 2	1980
October	1st to 10th	Night	38% 5% 10% 26% 17% 4	2137
	11th to 20th	Night	38% 5% 10% 26% 18% 4	2160
	21st to end	Night	33% 4 9% 26% 22% 6%	2376
November	1st to 10th	Night	36% 4 8% 22% 24% 5%	2221
	11th to 20th	Night	34% 5% 8% 23% 23% 7%	2446
	21st to end	Night	36% 5% 9% 27% 19% 5%	2470
December	1st to 10th	Night	45% 5% 9% 24% 14% 3	2643
	11th to 20th	Night	39% 8% 9% 22% 20% 3	3049
	21st to end	Night	45% 8% 11% 21% 14% 1	4532

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	36% 30% 21% 10% 2	270
	11th to 20th	Day	37% 25% 31% 8%	252
	21st to end	Day	36% 30% 24% 7% 3	264
February	1st to 10th	Day	32% 30% 28% 10% 1	240
	11th to 20th	Day	43% 29% 23% 5%	240
	21st to end	Day	38% 25% 26% 9% 2	256
March	1st to 10th	Day	26% 28% 27% 16% 4	420
	11th to 20th	Day	31% 28% 22% 15% 3 1	420
	21st to end	Day	28% 20% 30% 17% 5% 1	462
April	1st to 10th	Day	23% 25% 25% 17% 9%	420
	11th to 20th	Day	21% 18% 30% 19% 9% 3	412
	21st to end	Day	27% 19% 27% 16% 8% 2	420
May	1st to 10th	Day	23% 20% 32% 14% 7% 4	423
	11th to 20th	Day	21% 13% 29% 19% 14% 3	447
	21st to end	Day	17% 8% 22% 29% 16% 7%	485
June	1st to 10th	Day	16% 9% 28% 24% 18% 6%	450
	11th to 20th	Day	12% 12% 24% 27% 18% 7%	450
	21st to end	Day	11% 11% 28% 28% 15% 7%	450
July	1st to 10th	Day	13% 5% 18% 31% 23% 10%	445
	11th to 20th	Day	5% 3 21% 36% 23% 11%	444
	21st to end	Day	13% 5% 23% 28% 21% 11%	488
August	1st to 10th	Day	17% 6% 18% 25% 22% 12%	446
	11th to 20th	Day	14% 4 16% 28% 26% 13%	439
	21st to end	Day	11% 3 19% 30% 21% 17%	495
September	1st to 10th	Day	17% 4 16% 26% 26% 11%	450
	11th to 20th	Day	17% 5% 12% 31% 25% 10%	450
	21st to end	Day	29% 9% 18% 24% 16% 4	450
October	1st to 10th	Day	23% 10% 19% 26% 19% 4	450
	11th to 20th	Day	17% 19% 23% 26% 13% 2	450
	21st to end	Day	22% 17% 29% 22% 9% 2	495
November	1st to 10th	Day	20% 16% 25% 26% 11% 2	455
	11th to 20th	Day	26% 21% 26% 19% 7% 1	292
	21st to end	Day	23% 24% 28% 19% 4 1	270
December	1st to 10th	Day	32% 27% 25% 14% 2	180
	11th to 20th	Day	25% 25% 32% 15% 3 1	261
	21st to end	Day	29% 25% 29% 12% 5%	297
January	1st to 10th	Night	24% 29% 26% 17% 4	760
	11th to 20th	Night	25% 22% 29% 22% 2	739
	21st to end	Night	21% 27% 33% 18% 1	792
February	1st to 10th	Night	21% 24% 31% 21% 3	720
	11th to 20th	Night	28% 28% 27% 16%	703
	21st to end	Night	24% 24% 31% 19% 2	592
March	1st to 10th	Night	19% 23% 31% 23% 3	720
	11th to 20th	Night	26% 22% 26% 20% 6% 1	720
	21st to end	Night	18% 19% 31% 24% 7%	792
April	1st to 10th	Night	22% 21% 26% 26% 6%	720
	11th to 20th	Night	19% 16% 29% 24% 11%	711
	21st to end	Night	18% 15% 25% 32% 9% 1	595
May	1st to 10th	Night	19% 10% 28% 28% 12% 2	540
	11th to 20th	Night	19% 8% 20% 37% 16% 1	531
	21st to end	Night	16% 4 12% 34% 28% 6%	584
June	1st to 10th	Night	11% 3 16% 41% 27% 3	540
	11th to 20th	Night	12% 3 17% 33% 31% 4	540
	21st to end	Night	9% 4 18% 38% 27% 5%	540
July	1st to 10th	Night	11% 2 12% 39% 32% 5%	525
	11th to 20th	Night	6% 1 11% 35% 41% 5%	522
	21st to end	Night	6% 3 11% 40% 36% 4	591
August	1st to 10th	Night	15% 4 10% 36% 31% 5%	537
	11th to 20th	Night	13% 3 6% 36% 38% 4	534
	21st to end	Night	12% 2 9% 32% 38% 8%	738
September	1st to 10th	Night	14% 4 9% 31% 36% 7%	720
	11th to 20th	Night	16% 3 11% 31% 33% 6%	720
	21st to end	Night	23% 6% 16% 31% 22% 2	720
October	1st to 10th	Night	20% 9% 14% 27% 26% 4	720
	11th to 20th	Night	20% 12% 23% 28% 15% 2	720
	21st to end	Night	19% 12% 23% 32% 12% 2	792
November	1st to 10th	Night	19% 10% 23% 33% 14% 1	740
	11th to 20th	Night	18% 19% 22% 30% 10% 1	760
	21st to end	Night	19% 18% 25% 28% 9% 1	760
December	1st to 10th	Night	24% 22% 26% 23% 6%	760
	11th to 20th	Night	17% 23% 28% 28% 5%	760
	21st to end	Night	23% 20% 25% 28% 3	836

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