Hillsborough
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Hillsborough clear sky chart from 2005-11-06 to 2024-01-12.

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	13% 5% 2 5% 5% 3 5% 5% 6% 8% 43%	74571
	Night	14% 5% 2 5% 5% 3 4 4 5% 7% 45%	75131
Transparency	Day	38% 7% 12% 21% 17% 4	74293
	Night	37% 7% 11% 20% 19% 5%	74928
Seeing	Day	17% 11% 25% 24% 18% 6%	14162
	Night	16% 9% 25% 31% 17% 2	24502

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	17% 6% 2 5% 4 3 4 3 4 5% 46%	1706
	11th to 20th	Day	18% 5% 3 5% 4 2 4 4 4 6% 45%	1629
	21st to end	Day	16% 6% 3 4 3 3 3 4 4 5% 48%	1782
February	1st to 10th	Day	14% 6% 2 5% 6% 3 4 5% 6% 7% 41%	1620
	11th to 20th	Day	21% 5% 3 4 4 3 3 3 3 4 46%	1573
	21st to end	Day	18% 7% 4 6% 5% 2 3 4 4 6% 41%	1620
March	1st to 10th	Day	15% 4 2 3 4 3 4 4 5% 7% 50%	1980
	11th to 20th	Day	14% 6% 3 5% 5% 3 5% 5% 5% 9% 41%	1980
	21st to end	Day	17% 7% 3 5% 5% 3 4 5% 4 7% 40%	2178
April	1st to 10th	Day	11% 4 2 3 4 2 4 4 5% 8% 55%	2081
	11th to 20th	Day	11% 5% 2 5% 5% 2 4 4 6% 8% 48%	2134
	21st to end	Day	15% 5% 2 4 4 3 5% 4 5% 7% 45%	2164
May	1st to 10th	Day	12% 5% 3 5% 5% 3 6% 5% 6% 9% 42%	2340
	11th to 20th	Day	14% 5% 2 5% 6% 3 5% 6% 6% 8% 38%	2301
	21st to end	Day	12% 5% 2 5% 6% 5% 6% 6% 6% 9% 39%	2531
June	1st to 10th	Day	9% 4 2 5% 6% 4 6% 5% 7% 10% 41%	2340
	11th to 20th	Day	9% 5% 2 5% 4 3 5% 6% 7% 10% 44%	2340
	21st to end	Day	8% 4 3 5% 6% 4 6% 6% 8% 11% 40%	2340
July	1st to 10th	Day	9% 6% 3 5% 6% 4 7% 8% 7% 10% 36%	2340
	11th to 20th	Day	7% 4 2 6% 6% 4 7% 8% 10% 12% 34%	2340
	21st to end	Day	7% 5% 2 6% 7% 5% 7% 7% 8% 10% 34%	2547
August	1st to 10th	Day	10% 6% 2 6% 7% 5% 7% 7% 8% 10% 32%	2327
	11th to 20th	Day	11% 6% 3 6% 7% 5% 8% 7% 7% 9% 31%	2253
	21st to end	Day	10% 7% 3 5% 6% 4 6% 6% 7% 10% 36%	2376
September	1st to 10th	Day	10% 4 2 5% 6% 2 5% 5% 7% 8% 46%	2160
	11th to 20th	Day	14% 5% 3 5% 6% 3 4 6% 5% 7% 42%	2018
	21st to end	Day	21% 6% 3 5% 4 3 4 4 5% 6% 39%	1952
October	1st to 10th	Day	15% 6% 2 4 4 2 4 3 3 6% 50%	1800
	11th to 20th	Day	14% 4 2 4 4 2 3 3 4 5% 55%	1800
	21st to end	Day	12% 4 2 4 4 3 3 3 4 5% 56%	1980
November	1st to 10th	Day	13% 5% 1 4 4 2 4 4 4 5% 55%	1857
	11th to 20th	Day	15% 4 2 4 3 2 4 4 4 6% 52%	1864
	21st to end	Day	14% 4 1 5% 4 2 4 4 5% 7% 50%	1617
December	1st to 10th	Day	18% 6% 3 5% 5% 3 4 5% 6% 6% 41%	1520
	11th to 20th	Day	16% 4 2 4 3 1 3 3 4 5% 56%	2057
	21st to end	Day	17% 5% 2 5% 4 3 4 4 6% 8% 42%	3124
January	1st to 10th	Night	15% 6% 2 5% 5% 2 3 5% 4 7% 46%	2459
	11th to 20th	Night	16% 6% 3 5% 5% 3 4 4 5% 6% 44%	2360
	21st to end	Night	15% 6% 2 4 4 2 3 3 5% 5% 50%	2574
February	1st to 10th	Night	17% 6% 2 4 4 3 4 5% 4 6% 45%	2340
	11th to 20th	Night	20% 6% 2 4 4 2 3 4 4 6% 44%	2270
	21st to end	Night	16% 5% 3 4 4 3 4 4 6% 6% 45%	1633
March	1st to 10th	Night	13% 4 3 4 5% 2 4 3 4 5% 53%	1980
	11th to 20th	Night	17% 5% 2 6% 5% 3 4 4 4 7% 42%	1980
	21st to end	Night	16% 5% 3 5% 4 2 5% 4 6% 7% 43%	2178
April	1st to 10th	Night	15% 6% 2 3 4 2 4 4 4 6% 49%	1877
	11th to 20th	Night	13% 4 3 5% 5% 2 3 3 5% 8% 49%	1775
	21st to end	Night	13% 4 2 4 5% 3 5% 5% 5% 6% 47%	1800
May	1st to 10th	Night	10% 5% 3 5% 5% 3 4 5% 6% 8% 45%	1630
	11th to 20th	Night	16% 6% 3 5% 5% 3 4 5% 6% 8% 40%	1592
	21st to end	Night	12% 5% 2 6% 4 3 5% 4 5% 11% 42%	1744
June	1st to 10th	Night	10% 4 2 6% 5% 4 5% 7% 6% 11% 41%	1620
	11th to 20th	Night	8% 6% 3 6% 6% 3 5% 6% 7% 10% 41%	1620
	21st to end	Night	9% 5% 2 6% 6% 3 5% 7% 7% 12% 39%	1620
July	1st to 10th	Night	9% 6% 2 5% 6% 4 5% 6% 8% 10% 39%	1620
	11th to 20th	Night	7% 5% 3 4 6% 4 7% 7% 9% 12% 38%	1620
	21st to end	Night	6% 4 3 5% 7% 5% 6% 6% 7% 12% 39%	1771
August	1st to 10th	Night	11% 6% 3 6% 6% 4 6% 5% 9% 10% 33%	1611
	11th to 20th	Night	10% 6% 3 6% 7% 4 6% 6% 8% 9% 35%	1629
	21st to end	Night	9% 5% 3 6% 6% 3 5% 7% 8% 11% 38%	1980
September	1st to 10th	Night	12% 5% 2 5% 4 2 4 4 6% 8% 47%	1800
	11th to 20th	Night	14% 5% 2 6% 4 2 4 5% 5% 7% 45%	1937
	21st to end	Night	20% 7% 3 6% 5% 3 3 5% 5% 6% 38%	2006
October	1st to 10th	Night	15% 7% 2 4 5% 2 3 4 4 5% 48%	2160
	11th to 20th	Night	14% 6% 2 3 4 2 3 3 4 5% 53%	2160
	21st to end	Night	13% 5% 2 4 3 2 3 4 4 5% 55%	2376
November	1st to 10th	Night	14% 6% 2 5% 4 2 3 3 3 5% 54%	2221
	11th to 20th	Night	13% 5% 2 5% 4 3 4 4 4 5% 51%	2313
	21st to end	Night	14% 4 2 4 4 2 3 4 5% 7% 51%	2554
December	1st to 10th	Night	17% 7% 3 6% 6% 3 4 4 4 5% 42%	2660
	11th to 20th	Night	17% 6% 2 5% 4 3 3 4 4 5% 49%	3129
	21st to end	Night	18% 6% 2 5% 4 2 3 4 5% 7% 45%	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	41% 4 8% 23% 19% 5%	1707
	11th to 20th	Day	41% 5% 7% 20% 22% 4	1629
	21st to end	Day	38% 4 7% 21% 22% 7%	1782
February	1st to 10th	Day	40% 6% 12% 20% 18% 5%	1620
	11th to 20th	Day	43% 4 7% 19% 21% 6%	1582
	21st to end	Day	45% 5% 11% 15% 19% 6%	1620
March	1st to 10th	Day	34% 5% 9% 23% 22% 6%	1980
	11th to 20th	Day	40% 6% 11% 19% 17% 7%	1980
	21st to end	Day	45% 7% 9% 19% 17% 4	2178
April	1st to 10th	Day	28% 5% 11% 24% 23% 9%	2081
	11th to 20th	Day	34% 5% 11% 21% 21% 8%	2134
	21st to end	Day	37% 7% 12% 22% 18% 4	2164
May	1st to 10th	Day	37% 8% 16% 19% 16% 4	2340
	11th to 20th	Day	41% 10% 13% 22% 11% 3	2305
	21st to end	Day	40% 9% 15% 22% 13% 1	2531
June	1st to 10th	Day	36% 10% 15% 23% 15% 1	2340
	11th to 20th	Day	33% 9% 18% 23% 15% 2	2340
	21st to end	Day	34% 12% 17% 21% 14% 2	2340
July	1st to 10th	Day	38% 12% 16% 18% 13% 3	2275
	11th to 20th	Day	35% 13% 22% 18% 9% 2	2271
	21st to end	Day	38% 12% 19% 22% 8%	2405
August	1st to 10th	Day	42% 13% 17% 20% 9% 1	2314
	11th to 20th	Day	45% 12% 16% 17% 9% 1	2253
	21st to end	Day	41% 10% 17% 19% 11% 3	2376
September	1st to 10th	Day	34% 9% 15% 19% 17% 5%	2160
	11th to 20th	Day	40% 7% 11% 18% 21% 3	2018
	21st to end	Day	46% 5% 10% 18% 17% 4	1952
October	1st to 10th	Day	37% 5% 9% 22% 23% 4	1800
	11th to 20th	Day	32% 4 9% 28% 21% 7%	1800
	21st to end	Day	31% 5% 7% 24% 24% 8%	1980
November	1st to 10th	Day	32% 5% 7% 21% 27% 7%	1857
	11th to 20th	Day	33% 4 8% 22% 26% 7%	1864
	21st to end	Day	34% 5% 8% 23% 25% 6%	1617
December	1st to 10th	Day	43% 6% 11% 19% 17% 4	1520
	11th to 20th	Day	32% 3 8% 25% 24% 9%	2054
	21st to end	Day	39% 6% 11% 23% 19% 2	3124
January	1st to 10th	Night	38% 5% 9% 19% 22% 7%	2459
	11th to 20th	Night	42% 6% 9% 18% 20% 4	2360
	21st to end	Night	37% 3 7% 22% 25% 6%	2574
February	1st to 10th	Night	40% 5% 10% 21% 19% 5%	2340
	11th to 20th	Night	42% 5% 9% 16% 21% 7%	2272
	21st to end	Night	39% 6% 9% 20% 18% 8%	1633
March	1st to 10th	Night	34% 5% 8% 21% 23% 8%	1980
	11th to 20th	Night	42% 6% 8% 19% 18% 6%	1980
	21st to end	Night	40% 7% 9% 21% 18% 5%	2178
April	1st to 10th	Night	35% 6% 10% 18% 25% 6%	1877
	11th to 20th	Night	34% 5% 10% 23% 21% 8%	1775
	21st to end	Night	37% 6% 11% 24% 18% 5%	1800
May	1st to 10th	Night	35% 9% 14% 22% 17% 3	1630
	11th to 20th	Night	41% 8% 13% 22% 15% 2	1593
	21st to end	Night	36% 10% 17% 24% 12% 1	1745
June	1st to 10th	Night	35% 11% 16% 22% 14% 1	1620
	11th to 20th	Night	37% 10% 17% 20% 14% 2	1620
	21st to end	Night	35% 12% 16% 20% 14% 3	1620
July	1st to 10th	Night	37% 13% 15% 19% 13% 3	1575
	11th to 20th	Night	33% 13% 21% 21% 8% 3	1566
	21st to end	Night	35% 13% 19% 21% 12%	1672
August	1st to 10th	Night	42% 12% 18% 18% 9% 1	1600
	11th to 20th	Night	40% 13% 18% 17% 10% 2	1629
	21st to end	Night	37% 12% 17% 23% 8% 3	1980
September	1st to 10th	Night	34% 10% 14% 21% 16% 6%	1800
	11th to 20th	Night	36% 9% 11% 19% 21% 3	1937
	21st to end	Night	46% 8% 11% 17% 15% 4	2006
October	1st to 10th	Night	38% 5% 9% 21% 23% 4	2160
	11th to 20th	Night	34% 5% 7% 23% 23% 8%	2160
	21st to end	Night	31% 5% 8% 20% 25% 10%	2376
November	1st to 10th	Night	35% 4 7% 19% 27% 9%	2221
	11th to 20th	Night	34% 5% 8% 16% 24% 12%	2313
	21st to end	Night	33% 4 7% 23% 26% 7%	2554
December	1st to 10th	Night	44% 5% 8% 20% 18% 4	2660
	11th to 20th	Night	38% 4 8% 20% 22% 8%	3131
	21st to end	Night	38% 5% 8% 24% 22% 2	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	26% 23% 35% 14% 3	270
	11th to 20th	Day	23% 24% 34% 15% 3	243
	21st to end	Day	20% 22% 41% 14% 2	264
February	1st to 10th	Day	21% 22% 39% 15% 4	240
	11th to 20th	Day	39% 20% 33% 8% 1	240
	21st to end	Day	26% 22% 32% 17% 3	338
March	1st to 10th	Day	21% 19% 35% 18% 6%	420
	11th to 20th	Day	19% 20% 42% 13% 5% 1	420
	21st to end	Day	20% 18% 36% 20% 5%	462
April	1st to 10th	Day	20% 17% 34% 19% 9% 1	420
	11th to 20th	Day	14% 19% 29% 23% 11% 4	412
	21st to end	Day	20% 10% 29% 25% 14% 1	420
May	1st to 10th	Day	17% 16% 29% 24% 10% 4	423
	11th to 20th	Day	15% 8% 22% 30% 20% 6%	447
	21st to end	Day	19% 4 17% 28% 22% 9%	485
June	1st to 10th	Day	10% 5% 24% 31% 24% 6%	450
	11th to 20th	Day	10% 6% 22% 26% 27% 10%	450
	21st to end	Day	9% 5% 18% 35% 26% 7%	450
July	1st to 10th	Day	11% 2 11% 31% 31% 14%	445
	11th to 20th	Day	6% 2 12% 28% 38% 14%	444
	21st to end	Day	8% 2 16% 35% 28% 11%	488
August	1st to 10th	Day	13% 3 18% 24% 28% 13%	446
	11th to 20th	Day	11% 4 13% 27% 31% 14%	439
	21st to end	Day	13% 3 11% 27% 29% 18%	495
September	1st to 10th	Day	13% 3 15% 26% 26% 17%	450
	11th to 20th	Day	15% 4 12% 29% 28% 13%	450
	21st to end	Day	24% 7% 23% 26% 17% 4	450
October	1st to 10th	Day	18% 7% 22% 22% 23% 8%	450
	11th to 20th	Day	15% 8% 26% 27% 20% 5%	450
	21st to end	Day	14% 12% 26% 31% 14% 3	495
November	1st to 10th	Day	18% 10% 29% 25% 14% 4	455
	11th to 20th	Day	18% 15% 28% 26% 13% 1	424
	21st to end	Day	14% 18% 29% 23% 14% 2	225
December	1st to 10th	Day	21% 19% 38% 13% 7% 1	180
	11th to 20th	Day	21% 15% 34% 22% 8% 1	225
	21st to end	Day	25% 18% 28% 19% 9% 1	297
January	1st to 10th	Night	21% 17% 34% 23% 5%	760
	11th to 20th	Night	19% 16% 33% 25% 7%	727
	21st to end	Night	19% 14% 40% 24% 3	792
February	1st to 10th	Night	20% 14% 36% 27% 3	720
	11th to 20th	Night	25% 18% 36% 20% 1	703
	21st to end	Night	18% 16% 37% 26% 3	592
March	1st to 10th	Night	17% 15% 35% 28% 5%	720
	11th to 20th	Night	22% 16% 32% 22% 9%	720
	21st to end	Night	18% 12% 40% 26% 5%	792
April	1st to 10th	Night	18% 17% 31% 28% 7%	720
	11th to 20th	Night	14% 13% 34% 26% 12% 1	711
	21st to end	Night	16% 8% 32% 36% 8%	720
May	1st to 10th	Night	13% 11% 31% 35% 9%	550
	11th to 20th	Night	17% 6% 24% 34% 19% 1	531
	21st to end	Night	14% 2 15% 40% 28% 2	584
June	1st to 10th	Night	11% 3 18% 39% 27% 2	540
	11th to 20th	Night	10% 2 19% 36% 27% 5%	540
	21st to end	Night	9% 2 15% 42% 29% 3	540
July	1st to 10th	Night	9% 1 12% 35% 38% 5%	525
	11th to 20th	Night	7% 2 10% 37% 39% 5%	522
	21st to end	Night	5% 1 13% 41% 35% 5%	591
August	1st to 10th	Night	13% 1 11% 37% 34% 4	537
	11th to 20th	Night	12% 3 10% 37% 33% 6%	565
	21st to end	Night	11% 2 11% 33% 37% 6%	792
September	1st to 10th	Night	15% 2 9% 33% 35% 6%	720
	11th to 20th	Night	16% 2 13% 31% 32% 5%	720
	21st to end	Night	24% 4 17% 32% 22% 2	720
October	1st to 10th	Night	18% 4 17% 31% 25% 5%	720
	11th to 20th	Night	18% 8% 23% 28% 21% 2	720
	21st to end	Night	16% 10% 25% 31% 17% 2	792
November	1st to 10th	Night	16% 9% 25% 32% 15% 2	740
	11th to 20th	Night	16% 11% 30% 32% 11%	760
	21st to end	Night	16% 11% 29% 30% 13% 1	760
December	1st to 10th	Night	20% 12% 31% 29% 6%	760
	11th to 20th	Night	18% 9% 35% 30% 7%	760
	21st to end	Night	22% 14% 27% 30% 7%	836

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