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 2025-09-19.

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% 6% 2 6% 4 4 5% 4 6% 8% 43%	125696
	Night	14% 6% 2 6% 4 4 4 3 5% 7% 45%	123347
Transparency	Day	37% 7% 12% 20% 17% 5%	129302
	Night	37% 7% 10% 20% 20% 6%	127810
Seeing	Day	17% 11% 25% 24% 17% 6%	16472
	Night	16% 10% 26% 31% 16% 2	26744

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	16% 6% 2 5% 2 3 2 2 3 4 56%	4056
	11th to 20th	Day	11% 6% 2 5% 3 3 3 3 3 5% 57%	4060
	21st to end	Day	14% 8% 3 6% 4 4 4 3 4 4 47%	4466
February	1st to 10th	Day	13% 6% 2 6% 5% 4 4 3 7% 7% 42%	1800
	11th to 20th	Day	22% 6% 2 4 3 4 3 3 3 4 44%	1753
	21st to end	Day	17% 7% 3 6% 4 4 3 3 5% 6% 42%	1806
March	1st to 10th	Day	19% 6% 2 6% 4 4 4 3 5% 6% 40%	4935
	11th to 20th	Day	10% 5% 2 5% 4 3 3 3 5% 6% 53%	4980
	21st to end	Day	15% 6% 2 6% 4 4 4 3 4 5% 46%	5465
April	1st to 10th	Day	10% 4 2 4 3 3 4 3 6% 7% 53%	2313
	11th to 20th	Day	11% 5% 2 6% 4 3 3 3 6% 7% 49%	2374
	21st to end	Day	14% 5% 2 5% 4 5% 5% 3 6% 7% 44%	2405
May	1st to 10th	Day	10% 6% 2 8% 5% 5% 6% 5% 7% 10% 35%	5862
	11th to 20th	Day	19% 7% 3 7% 5% 5% 5% 5% 7% 9% 29%	5861
	21st to end	Day	13% 5% 2 5% 5% 5% 5% 4 6% 8% 42%	6433
June	1st to 10th	Day	9% 5% 2 6% 5% 5% 5% 5% 7% 10% 41%	2600
	11th to 20th	Day	8% 5% 2 5% 4 5% 6% 5% 7% 10% 44%	2600
	21st to end	Day	7% 4 2 6% 5% 5% 6% 5% 8% 12% 41%	2600
July	1st to 10th	Day	9% 5% 2 7% 5% 5% 6% 5% 8% 10% 36%	5900
	11th to 20th	Day	9% 5% 2 7% 6% 6% 7% 6% 9% 13% 29%	5900
	21st to end	Day	9% 6% 2 7% 6% 6% 7% 5% 8% 10% 34%	6424
August	1st to 10th	Day	20% 7% 2 7% 5% 6% 6% 5% 8% 10% 24%	4234
	11th to 20th	Day	9% 6% 3 7% 6% 6% 6% 5% 8% 11% 35%	4079
	21st to end	Day	6% 5% 2 5% 4 4 4 4 6% 11% 49%	4312
September	1st to 10th	Day	10% 5% 2 6% 5% 4 4 4 7% 7% 46%	2400
	11th to 20th	Day	15% 6% 3 6% 5% 4 4 4 5% 7% 40%	2220
	21st to end	Day	22% 7% 3 5% 4 4 4 3 6% 6% 38%	2060
October	1st to 10th	Day	14% 6% 2 5% 3 3 4 3 3 6% 50%	1900
	11th to 20th	Day	14% 4 1 4 3 3 3 2 4 5% 57%	1900
	21st to end	Day	11% 4 2 5% 3 3 3 2 4 5% 57%	2090
November	1st to 10th	Day	13% 5% 1 5% 4 3 3 3 4 5% 54%	1957
	11th to 20th	Day	15% 4 2 4 3 3 3 3 5% 6% 51%	1962
	21st to end	Day	14% 4 2 5% 3 4 3 3 5% 7% 50%	1702
December	1st to 10th	Day	13% 6% 2 5% 3 3 4 3 4 5% 53%	2600
	11th to 20th	Day	17% 6% 3 6% 4 3 4 3 4 5% 46%	3221
	21st to end	Day	18% 6% 2 5% 3 3 4 3 5% 7% 44%	4466
January	1st to 10th	Night	12% 6% 2 5% 3 3 3 3 4 5% 55%	5889
	11th to 20th	Night	13% 6% 2 4 3 3 2 2 4 4 57%	5900
	21st to end	Night	15% 7% 2 5% 3 3 3 2 4 5% 49%	6490
February	1st to 10th	Night	17% 6% 2 5% 4 4 4 4 5% 6% 45%	2600
	11th to 20th	Night	20% 7% 2 5% 4 3 3 3 5% 5% 43%	2529
	21st to end	Night	15% 5% 3 4 4 4 4 3 6% 7% 46%	1821
March	1st to 10th	Night	18% 7% 2 6% 5% 4 4 3 4 5% 44%	4918
	11th to 20th	Night	12% 5% 2 4 3 3 3 2 4 6% 55%	4980
	21st to end	Night	14% 5% 2 5% 4 4 4 3 5% 6% 48%	5467
April	1st to 10th	Night	14% 6% 1 4 4 3 5% 3 4 6% 49%	2085
	11th to 20th	Night	12% 4 2 6% 4 3 3 3 5% 7% 50%	1975
	21st to end	Night	12% 5% 2 6% 4 4 5% 4 6% 6% 47%	2000
May	1st to 10th	Night	13% 7% 3 7% 5% 5% 5% 4 6% 9% 37%	4043
	11th to 20th	Night	20% 8% 3 7% 4 4 3 3 4 8% 35%	4032
	21st to end	Night	16% 5% 2 5% 3 3 4 3 6% 10% 43%	4421
June	1st to 10th	Night	10% 5% 2 6% 5% 5% 5% 6% 6% 11% 40%	1800
	11th to 20th	Night	8% 7% 2 7% 6% 4 5% 5% 7% 10% 41%	1800
	21st to end	Night	8% 5% 2 6% 5% 4 6% 5% 8% 12% 40%	1800
July	1st to 10th	Night	12% 6% 2 8% 5% 5% 6% 5% 7% 10% 33%	4058
	11th to 20th	Night	12% 7% 3 7% 6% 6% 6% 6% 9% 10% 27%	4057
	21st to end	Night	11% 7% 3 7% 5% 5% 5% 5% 7% 11% 32%	4427
August	1st to 10th	Night	23% 7% 2 6% 5% 5% 5% 5% 8% 10% 23%	2915
	11th to 20th	Night	8% 5% 2 7% 5% 5% 6% 5% 8% 10% 40%	2936
	21st to end	Night	6% 4 2 7% 4 4 5% 4 7% 8% 50%	3586
September	1st to 10th	Night	11% 6% 2 6% 3 3 4 3 6% 8% 48%	2000
	11th to 20th	Night	15% 6% 2 7% 3 4 4 4 5% 7% 42%	2132
	21st to end	Night	20% 8% 3 7% 4 3 4 4 5% 5% 37%	2118
October	1st to 10th	Night	15% 7% 2 5% 4 3 3 3 5% 5% 48%	2280
	11th to 20th	Night	14% 6% 1 4 3 3 3 3 4 5% 54%	2280
	21st to end	Night	13% 5% 2 4 3 3 4 3 4 4 56%	2508
November	1st to 10th	Night	13% 6% 2 6% 3 3 3 3 4 4 53%	2341
	11th to 20th	Night	13% 6% 1 6% 4 3 4 3 5% 5% 50%	2435
	21st to end	Night	13% 4 2 4 3 3 3 3 5% 7% 52%	2689
December	1st to 10th	Night	11% 5% 2 6% 4 4 4 3 4 5% 53%	4580
	11th to 20th	Night	17% 8% 3 7% 4 4 4 3 4 5% 41%	4965
	21st to end	Night	17% 6% 2 5% 3 2 3 3 5% 6% 47%	6490

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	34% 3 4 21% 31% 8%	4057
	11th to 20th	Day	31% 4 4 17% 36% 8%	4060
	21st to end	Day	41% 4 5% 21% 22% 7%	4466
February	1st to 10th	Day	39% 5% 12% 20% 19% 5%	1800
	11th to 20th	Day	44% 4 7% 18% 20% 6%	1762
	21st to end	Day	44% 5% 10% 15% 20% 6%	1806
March	1st to 10th	Day	43% 5% 7% 17% 17% 11%	4935
	11th to 20th	Day	31% 5% 7% 17% 23% 17%	4980
	21st to end	Day	40% 6% 8% 20% 20% 6%	5465
April	1st to 10th	Day	29% 5% 12% 23% 22% 8%	2313
	11th to 20th	Day	34% 5% 11% 22% 21% 7%	2374
	21st to end	Day	38% 7% 12% 22% 18% 4	2405
May	1st to 10th	Day	39% 10% 18% 20% 10% 2	5897
	11th to 20th	Day	49% 10% 12% 20% 9% 1	5865
	21st to end	Day	36% 8% 13% 22% 17% 3	6447
June	1st to 10th	Day	35% 10% 16% 23% 15% 1	2600
	11th to 20th	Day	33% 10% 18% 23% 15% 2	2600
	21st to end	Day	32% 12% 17% 23% 14% 2	2600
July	1st to 10th	Day	38% 12% 18% 18% 12% 2	5835
	11th to 20th	Day	40% 16% 26% 14% 4 1	5831
	21st to end	Day	40% 11% 17% 22% 9%	6240
August	1st to 10th	Day	52% 10% 18% 15% 5%	4221
	11th to 20th	Day	38% 12% 21% 20% 8% 1	4118
	21st to end	Day	29% 8% 15% 32% 14% 2	4312
September	1st to 10th	Day	34% 9% 15% 19% 19% 5%	2400
	11th to 20th	Day	42% 7% 11% 17% 20% 3	2220
	21st to end	Day	46% 5% 10% 18% 16% 4	2060
October	1st to 10th	Day	33% 6% 8% 18% 26% 9%	3160
	11th to 20th	Day	20% 3 6% 21% 26% 25%	3140
	21st to end	Day	23% 4 8% 25% 23% 16%	3428
November	1st to 10th	Day	33% 5% 7% 21% 26% 7%	1957
	11th to 20th	Day	33% 4 8% 22% 26% 7%	1962
	21st to end	Day	34% 5% 8% 22% 26% 5%	1702
December	1st to 10th	Day	34% 5% 8% 21% 25% 7%	2600
	11th to 20th	Day	41% 4 10% 18% 21% 6%	3218
	21st to end	Day	38% 6% 12% 25% 18% 1	4466
January	1st to 10th	Night	31% 4 5% 18% 30% 11%	5889
	11th to 20th	Night	32% 4 5% 17% 35% 7%	5900
	21st to end	Night	38% 3 6% 20% 27% 5%	6490
February	1st to 10th	Night	40% 5% 10% 21% 20% 5%	2600
	11th to 20th	Night	42% 5% 9% 16% 21% 7%	2531
	21st to end	Night	37% 6% 9% 20% 20% 8%	1821
March	1st to 10th	Night	43% 5% 6% 14% 22% 10%	4918
	11th to 20th	Night	30% 5% 6% 15% 25% 19%	4980
	21st to end	Night	35% 6% 8% 23% 22% 6%	5467
April	1st to 10th	Night	35% 6% 10% 18% 24% 7%	2085
	11th to 20th	Night	33% 5% 10% 23% 21% 8%	1975
	21st to end	Night	36% 8% 11% 23% 17% 4	2000
May	1st to 10th	Night	41% 9% 16% 18% 13% 1	4070
	11th to 20th	Night	47% 8% 12% 19% 13% 1	4033
	21st to end	Night	35% 8% 14% 25% 15% 2	4429
June	1st to 10th	Night	36% 11% 16% 20% 16% 1	1800
	11th to 20th	Night	36% 10% 17% 20% 14% 2	1800
	21st to end	Night	34% 12% 17% 21% 14% 3	1800
July	1st to 10th	Night	42% 13% 16% 16% 11% 1	4015
	11th to 20th	Night	48% 15% 21% 12% 3 1	4006
	21st to end	Night	43% 11% 16% 21% 9%	4297
August	1st to 10th	Night	52% 11% 18% 14% 5%	2904
	11th to 20th	Night	34% 13% 20% 24% 8% 1	2963
	21st to end	Night	29% 8% 12% 29% 20% 2	3586
September	1st to 10th	Night	34% 9% 14% 22% 17% 5%	2000
	11th to 20th	Night	39% 9% 11% 17% 21% 3	2132
	21st to end	Night	46% 8% 11% 17% 14% 4	2118
October	1st to 10th	Night	39% 5% 8% 18% 23% 6%	3800
	11th to 20th	Night	24% 3 6% 21% 27% 20%	3779
	21st to end	Night	22% 4 8% 26% 23% 16%	4120
November	1st to 10th	Night	35% 4 8% 20% 26% 8%	2341
	11th to 20th	Night	35% 5% 8% 16% 25% 11%	2435
	21st to end	Night	33% 4 7% 23% 26% 7%	2689
December	1st to 10th	Night	34% 5% 6% 20% 29% 6%	4580
	11th to 20th	Night	46% 5% 8% 17% 16% 8%	4967
	21st to end	Night	37% 4 13% 23% 21% 2	6490

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	29% 23% 32% 13% 3	300
	11th to 20th	Day	26% 26% 31% 14% 3	300
	21st to end	Day	19% 24% 41% 13% 2	330
February	1st to 10th	Day	18% 23% 40% 15% 4	300
	11th to 20th	Day	41% 20% 31% 7% 2	300
	21st to end	Day	25% 20% 35% 18% 3	405
March	1st to 10th	Day	24% 19% 33% 18% 6%	500
	11th to 20th	Day	19% 19% 42% 14% 5% 1	500
	21st to end	Day	21% 18% 38% 18% 4	550
April	1st to 10th	Day	20% 20% 35% 17% 8% 1	500
	11th to 20th	Day	14% 19% 31% 23% 10% 4	492
	21st to end	Day	18% 10% 30% 27% 14% 1	500
May	1st to 10th	Day	16% 17% 30% 24% 10% 3	503
	11th to 20th	Day	15% 9% 23% 29% 18% 5%	527
	21st to end	Day	20% 6% 20% 26% 21% 8%	573
June	1st to 10th	Day	10% 5% 26% 31% 22% 6%	530
	11th to 20th	Day	9% 5% 22% 29% 26% 9%	530
	21st to end	Day	8% 4 17% 36% 28% 8%	530
July	1st to 10th	Day	11% 3 11% 31% 32% 13%	525
	11th to 20th	Day	6% 2 11% 29% 36% 15%	524
	21st to end	Day	7% 2 16% 35% 28% 12%	576
August	1st to 10th	Day	16% 4 18% 25% 27% 11%	526
	11th to 20th	Day	10% 5% 13% 30% 29% 13%	519
	21st to end	Day	11% 4 13% 29% 27% 16%	583
September	1st to 10th	Day	12% 3 19% 27% 24% 15%	530
	11th to 20th	Day	17% 4 14% 27% 26% 11%	522
	21st to end	Day	24% 7% 24% 25% 16% 4	490
October	1st to 10th	Day	17% 7% 23% 23% 23% 7%	490
	11th to 20th	Day	14% 9% 25% 28% 19% 4	490
	21st to end	Day	14% 11% 25% 31% 16% 4	539
November	1st to 10th	Day	18% 10% 30% 24% 14% 4	495
	11th to 20th	Day	18% 15% 28% 26% 11% 1	462
	21st to end	Day	16% 18% 30% 21% 13% 2	250
December	1st to 10th	Day	22% 20% 39% 13% 7% 1	200
	11th to 20th	Day	21% 15% 34% 23% 7% 1	251
	21st to end	Day	25% 18% 28% 19% 8% 1	330
January	1st to 10th	Night	21% 19% 33% 22% 4	800
	11th to 20th	Night	20% 17% 33% 24% 6%	800
	21st to end	Night	19% 14% 41% 23% 3	880
February	1st to 10th	Night	19% 15% 36% 27% 3	800
	11th to 20th	Night	26% 19% 35% 20% 1	783
	21st to end	Night	18% 17% 36% 26% 3	660
March	1st to 10th	Night	18% 16% 34% 27% 5%	800
	11th to 20th	Night	21% 17% 32% 22% 8%	800
	21st to end	Night	18% 13% 41% 24% 5%	880
April	1st to 10th	Night	18% 18% 31% 27% 6%	800
	11th to 20th	Night	14% 14% 36% 25% 11% 1	791
	21st to end	Night	14% 9% 32% 37% 8%	800
May	1st to 10th	Night	13% 11% 32% 34% 10%	610
	11th to 20th	Night	17% 7% 24% 33% 18% 1	591
	21st to end	Night	14% 3 17% 38% 26% 2	650
June	1st to 10th	Night	11% 4 19% 39% 26% 2	600
	11th to 20th	Night	10% 3 19% 37% 27% 4	600
	21st to end	Night	8% 2 15% 42% 30% 3	600
July	1st to 10th	Night	9% 1 12% 37% 36% 5%	585
	11th to 20th	Night	8% 2 10% 36% 38% 5%	582
	21st to end	Night	6% 2 12% 40% 35% 5%	657
August	1st to 10th	Night	16% 2 11% 36% 32% 4	597
	11th to 20th	Night	11% 3 10% 38% 32% 6%	629
	21st to end	Night	10% 3 11% 34% 37% 6%	880
September	1st to 10th	Night	14% 3 11% 34% 33% 5%	800
	11th to 20th	Night	18% 3 13% 31% 31% 5%	793
	21st to end	Night	25% 4 17% 32% 21% 1	760
October	1st to 10th	Night	17% 4 17% 31% 25% 5%	760
	11th to 20th	Night	17% 9% 24% 29% 20% 2	760
	21st to end	Night	15% 9% 24% 30% 19% 2	836
November	1st to 10th	Night	16% 9% 26% 33% 15% 2	780
	11th to 20th	Night	16% 11% 30% 32% 11%	800
	21st to end	Night	16% 12% 29% 30% 13% 1	800
December	1st to 10th	Night	20% 13% 33% 28% 6%	800
	11th to 20th	Night	19% 9% 35% 30% 7%	800
	21st to end	Night	22% 14% 26% 31% 7%	880

Page generated at 2025-09-16 01:49:11. Version ac07

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