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 2024-11-05.

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% 42%	100788
	Night	15% 6% 2 6% 4 4 4 3 5% 7% 44%	98190
Transparency	Day	37% 8% 12% 21% 17% 5%	104438
	Night	38% 7% 10% 20% 19% 6%	102679
Seeing	Day	16% 11% 25% 25% 17% 6%	15314
	Night	16% 9% 25% 31% 17% 2	25585

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	19% 5% 2 5% 2 3 3 2 4 5% 51%	2836
	11th to 20th	Day	13% 7% 3 5% 3 3 3 3 3 5% 52%	2840
	21st to end	Day	20% 10% 3 5% 3 4 3 3 4 4 41%	3124
February	1st to 10th	Day	13% 6% 2 6% 4 4 4 3 7% 7% 42%	1710
	11th to 20th	Day	21% 6% 2 4 4 4 3 3 4 4 45%	1663
	21st to end	Day	18% 8% 3 6% 4 4 3 3 5% 6% 40%	1718
March	1st to 10th	Day	23% 6% 2 6% 4 3 4 3 5% 7% 37%	3435
	11th to 20th	Day	9% 5% 2 5% 4 3 4 3 5% 7% 54%	3480
	21st to end	Day	20% 6% 2 5% 4 3 3 3 4 6% 42%	3815
April	1st to 10th	Day	10% 4 2 4 3 3 4 3 6% 7% 54%	2197
	11th to 20th	Day	11% 5% 2 6% 4 3 3 3 6% 7% 49%	2254
	21st to end	Day	15% 5% 2 5% 4 4 4 3 5% 7% 45%	2285
May	1st to 10th	Day	12% 7% 3 8% 5% 5% 6% 5% 7% 9% 34%	4082
	11th to 20th	Day	18% 6% 3 7% 5% 5% 4 5% 6% 7% 34%	4081
	21st to end	Day	7% 4 2 5% 4 5% 5% 4 6% 9% 48%	4476
June	1st to 10th	Day	9% 5% 2 6% 5% 5% 5% 5% 7% 10% 41%	2470
	11th to 20th	Day	8% 5% 2 5% 4 4 5% 4 7% 10% 44%	2470
	21st to end	Day	8% 4 2 6% 5% 5% 6% 5% 8% 11% 40%	2470
July	1st to 10th	Day	7% 5% 2 7% 6% 6% 6% 6% 8% 10% 36%	4120
	11th to 20th	Day	11% 6% 2 7% 5% 6% 6% 6% 8% 11% 33%	4120
	21st to end	Day	12% 7% 3 9% 7% 7% 7% 5% 8% 9% 27%	4466
August	1st to 10th	Day	19% 7% 2 8% 5% 6% 6% 5% 8% 10% 24%	4104
	11th to 20th	Day	9% 5% 3 7% 6% 6% 6% 5% 7% 11% 35%	3952
	21st to end	Day	6% 5% 2 5% 4 4 4 4 6% 11% 49%	4180
September	1st to 10th	Day	10% 5% 2 6% 5% 4 4 4 7% 7% 46%	2280
	11th to 20th	Day	15% 6% 3 6% 5% 4 4 4 5% 7% 40%	2130
	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% 3 3 3 3 4 5% 55%	1898
	11th to 20th	Day	15% 4 2 4 3 3 3 3 5% 6% 52%	1864
	21st to end	Day	14% 4 2 5% 3 3 3 3 5% 7% 50%	1617
December	1st to 10th	Day	17% 7% 2 7% 4 3 4 4 5% 6% 41%	1520
	11th to 20th	Day	16% 4 2 4 2 2 3 3 4 5% 55%	2057
	21st to end	Day	17% 6% 2 5% 3 4 4 3 6% 8% 42%	3124
January	1st to 10th	Night	14% 6% 2 6% 4 3 4 3 4 6% 47%	4109
	11th to 20th	Night	15% 7% 2 5% 4 3 3 3 5% 5% 48%	4120
	21st to end	Night	19% 7% 2 6% 4 3 3 2 4 5% 45%	4532
February	1st to 10th	Night	16% 6% 2 5% 4 4 4 4 4 6% 45%	2470
	11th to 20th	Night	20% 7% 2 5% 4 3 3 3 5% 5% 44%	2400
	21st to end	Night	15% 6% 3 5% 4 4 4 4 6% 7% 44%	1733
March	1st to 10th	Night	23% 8% 3 7% 5% 4 4 3 4 5% 35%	3418
	11th to 20th	Night	13% 6% 2 5% 3 3 4 2 4 6% 53%	3480
	21st to end	Night	18% 5% 2 4 3 3 4 3 5% 6% 48%	3817
April	1st to 10th	Night	14% 6% 1 4 4 3 4 3 4 6% 49%	1981
	11th to 20th	Night	12% 5% 2 6% 4 3 3 3 5% 7% 49%	1875
	21st to end	Night	13% 5% 2 5% 4 4 5% 4 5% 6% 48%	1900
May	1st to 10th	Night	14% 8% 3 6% 4 4 4 3 6% 7% 40%	2823
	11th to 20th	Night	20% 8% 3 6% 4 4 3 3 4 6% 38%	2812
	21st to end	Night	9% 4 2 5% 3 4 4 3 7% 11% 47%	3079
June	1st to 10th	Night	9% 5% 2 7% 5% 5% 5% 6% 6% 11% 40%	1710
	11th to 20th	Night	8% 6% 2 6% 6% 4 4 5% 7% 9% 42%	1710
	21st to end	Night	8% 5% 2 6% 5% 4 6% 5% 8% 11% 40%	1710
July	1st to 10th	Night	8% 6% 2 8% 5% 6% 7% 5% 8% 10% 35%	2840
	11th to 20th	Night	14% 7% 3 5% 5% 5% 5% 5% 8% 10% 34%	2840
	21st to end	Night	14% 9% 4 9% 6% 5% 5% 4 6% 9% 30%	3087
August	1st to 10th	Night	22% 7% 2 6% 5% 5% 6% 5% 8% 10% 23%	2825
	11th to 20th	Night	8% 4 2 7% 5% 5% 6% 5% 8% 10% 40%	2844
	21st to end	Night	6% 4 2 6% 4 4 5% 4 6% 8% 51%	3476
September	1st to 10th	Night	12% 6% 2 6% 3 3 4 3 6% 8% 47%	1900
	11th to 20th	Night	15% 6% 2 7% 3 4 4 4 5% 7% 43%	2045
	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 5% 54%	2280
	11th to 20th	Night	12% 6% 1 6% 4 3 4 3 5% 5% 51%	2313
	21st to end	Night	14% 5% 2 4 3 3 3 3 5% 6% 51%	2554
December	1st to 10th	Night	17% 7% 2 7% 5% 4 4 3 4 5% 42%	2660
	11th to 20th	Night	16% 6% 2 5% 3 3 3 3 4 5% 49%	3129
	21st to end	Night	18% 6% 2 6% 3 3 3 3 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	36% 4 5% 27% 22% 6%	2837
	11th to 20th	Day	35% 4 5% 19% 31% 5%	2840
	21st to end	Day	46% 5% 6% 17% 20% 6%	3124
February	1st to 10th	Day	39% 6% 11% 20% 20% 5%	1710
	11th to 20th	Day	43% 4 7% 19% 21% 7%	1672
	21st to end	Day	45% 5% 11% 15% 19% 6%	1718
March	1st to 10th	Day	45% 6% 8% 19% 14% 8%	3435
	11th to 20th	Day	29% 6% 9% 19% 24% 13%	3480
	21st to end	Day	44% 6% 8% 20% 19% 4	3815
April	1st to 10th	Day	29% 5% 11% 23% 23% 9%	2197
	11th to 20th	Day	34% 5% 11% 21% 21% 7%	2254
	21st to end	Day	37% 7% 12% 22% 18% 4	2285
May	1st to 10th	Day	42% 11% 15% 18% 11% 2	4120
	11th to 20th	Day	45% 10% 11% 23% 10% 1	4085
	21st to end	Day	29% 10% 16% 22% 19% 4	4489
June	1st to 10th	Day	35% 10% 16% 23% 15% 1	2470
	11th to 20th	Day	33% 9% 18% 24% 15% 2	2470
	21st to end	Day	33% 12% 17% 22% 14% 2	2470
July	1st to 10th	Day	38% 13% 17% 17% 12% 3	4055
	11th to 20th	Day	40% 12% 24% 18% 5% 1	4051
	21st to end	Day	49% 11% 14% 17% 9%	4324
August	1st to 10th	Day	51% 11% 18% 15% 5%	4091
	11th to 20th	Day	38% 12% 21% 20% 8% 1	3991
	21st to end	Day	29% 8% 15% 33% 14% 2	4180
September	1st to 10th	Day	35% 9% 14% 19% 19% 5%	2280
	11th to 20th	Day	42% 7% 11% 17% 20% 3	2130
	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	32% 6% 8% 21% 26% 7%	1898
	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	36% 6% 7% 21% 25% 5%	4109
	11th to 20th	Night	38% 6% 7% 18% 26% 6%	4120
	21st to end	Night	43% 4 6% 20% 21% 7%	4532
February	1st to 10th	Night	39% 4 10% 21% 20% 5%	2470
	11th to 20th	Night	42% 5% 9% 16% 21% 7%	2402
	21st to end	Night	38% 6% 9% 21% 19% 7%	1733
March	1st to 10th	Night	50% 7% 7% 14% 16% 5%	3418
	11th to 20th	Night	33% 6% 7% 15% 24% 16%	3480
	21st to end	Night	37% 6% 7% 22% 21% 6%	3817
April	1st to 10th	Night	35% 6% 10% 17% 24% 7%	1981
	11th to 20th	Night	34% 5% 9% 23% 21% 8%	1875
	21st to end	Night	36% 7% 11% 24% 18% 5%	1900
May	1st to 10th	Night	40% 9% 15% 21% 12% 2	2850
	11th to 20th	Night	46% 8% 10% 20% 15% 1	2813
	21st to end	Night	28% 10% 17% 26% 16% 2	3087
June	1st to 10th	Night	36% 11% 16% 21% 15% 1	1710
	11th to 20th	Night	36% 10% 16% 21% 15% 2	1710
	21st to end	Night	34% 12% 17% 20% 14% 3	1710
July	1st to 10th	Night	40% 15% 15% 16% 12% 2	2795
	11th to 20th	Night	43% 11% 23% 17% 5% 2	2786
	21st to end	Night	51% 10% 13% 16% 10%	2988
August	1st to 10th	Night	52% 11% 18% 14% 5%	2814
	11th to 20th	Night	34% 13% 21% 24% 8% 1	2871
	21st to end	Night	29% 8% 12% 29% 20% 2	3476
September	1st to 10th	Night	34% 10% 14% 20% 17% 5%	1900
	11th to 20th	Night	39% 9% 11% 18% 21% 3	2045
	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 7% 19% 26% 8%	2280
	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% 22% 35% 14% 3	270
	11th to 20th	Day	27% 24% 33% 14% 3	270
	21st to end	Day	21% 22% 41% 14% 2	297
February	1st to 10th	Day	19% 21% 40% 16% 4	270
	11th to 20th	Day	38% 20% 32% 7% 2	270
	21st to end	Day	26% 20% 34% 17% 3	373
March	1st to 10th	Day	23% 18% 34% 18% 7%	460
	11th to 20th	Day	18% 20% 42% 13% 5% 1	460
	21st to end	Day	21% 18% 37% 19% 5%	506
April	1st to 10th	Day	20% 18% 34% 18% 8% 1	460
	11th to 20th	Day	14% 19% 30% 23% 10% 4	452
	21st to end	Day	19% 10% 30% 26% 13% 1	460
May	1st to 10th	Day	17% 16% 29% 24% 11% 4	463
	11th to 20th	Day	14% 9% 22% 30% 20% 5%	487
	21st to end	Day	18% 5% 19% 27% 22% 9%	529
June	1st to 10th	Day	9% 5% 25% 32% 22% 6%	490
	11th to 20th	Day	10% 5% 21% 28% 27% 9%	490
	21st to end	Day	8% 4 18% 36% 26% 7%	490
July	1st to 10th	Day	11% 3 11% 31% 32% 13%	485
	11th to 20th	Day	7% 2 11% 30% 36% 14%	484
	21st to end	Day	8% 2 16% 35% 27% 11%	532
August	1st to 10th	Day	14% 4 19% 24% 28% 12%	486
	11th to 20th	Day	10% 4 14% 29% 30% 13%	479
	21st to end	Day	12% 3 11% 28% 29% 18%	539
September	1st to 10th	Day	13% 3 16% 27% 25% 16%	490
	11th to 20th	Day	17% 4 12% 28% 27% 12%	490
	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% 29% 25% 14% 4	472
	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% 33% 23% 5%	760
	11th to 20th	Night	19% 17% 33% 24% 6%	760
	21st to end	Night	19% 14% 40% 24% 3	836
February	1st to 10th	Night	19% 14% 36% 28% 3	760
	11th to 20th	Night	25% 19% 36% 20% 1	743
	21st to end	Night	18% 17% 36% 25% 4	628
March	1st to 10th	Night	18% 15% 34% 28% 5%	760
	11th to 20th	Night	21% 17% 32% 22% 8%	760
	21st to end	Night	18% 12% 40% 25% 5%	836
April	1st to 10th	Night	17% 18% 32% 27% 6%	760
	11th to 20th	Night	14% 13% 34% 26% 12% 1	751
	21st to end	Night	15% 8% 33% 36% 8%	760
May	1st to 10th	Night	13% 11% 31% 34% 10%	580
	11th to 20th	Night	17% 6% 24% 34% 18% 1	561
	21st to end	Night	13% 2 16% 40% 27% 2	617
June	1st to 10th	Night	11% 4 18% 39% 26% 2	570
	11th to 20th	Night	10% 2 18% 37% 28% 4	570
	21st to end	Night	9% 2 15% 43% 29% 2	570
July	1st to 10th	Night	9% 1 12% 36% 37% 5%	555
	11th to 20th	Night	8% 2 10% 36% 38% 5%	552
	21st to end	Night	6% 2 12% 41% 34% 4	624
August	1st to 10th	Night	14% 2 11% 36% 33% 4	567
	11th to 20th	Night	11% 3 10% 38% 32% 6%	597
	21st to end	Night	11% 2 10% 33% 38% 6%	836
September	1st to 10th	Night	14% 2 11% 33% 34% 6%	760
	11th to 20th	Night	18% 3 13% 31% 31% 5%	760
	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% 8% 25% 33% 15% 2	760
	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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