Higby Hill Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Higby Hill Observatory clear sky chart from 2005-11-06 to 2026-03-03.

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	17% 12% 5% 10% 5% 5% 4 3 5% 6% 27%	131120
	Night	16% 14% 5% 11% 5% 4 4 3 4 6% 27%	130030
Transparency	Day	57% 5% 10% 18% 9% 1	134719
	Night	59% 4 10% 17% 9% 1	134502
Seeing	Day	27% 15% 28% 18% 9% 3	16198
	Night	25% 14% 27% 23% 10% 1	25911

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% 21% 8% 29% 6% 3 2 2 2 3 7%	4979
	11th to 20th	Day	10% 21% 10% 23% 10% 8% 3 2 2 2 9%	5280
	21st to end	Day	17% 16% 8% 15% 10% 8% 5% 3 4 3 11%	5780
February	1st to 10th	Day	16% 19% 6% 16% 9% 5% 3 2 2 4 17%	1890
	11th to 20th	Day	15% 17% 7% 16% 8% 5% 3 4 4 4 16%	1843
	21st to end	Day	18% 19% 4 13% 5% 4 4 3 5% 4 21%	1693
March	1st to 10th	Day	18% 11% 8% 12% 5% 3 3 3 4 5% 28%	4957
	11th to 20th	Day	12% 15% 6% 10% 5% 4 4 3 4 5% 32%	4980
	21st to end	Day	19% 14% 5% 10% 5% 4 4 3 4 5% 27%	5465
April	1st to 10th	Day	18% 16% 3 7% 4 4 4 4 4 6% 29%	2325
	11th to 20th	Day	20% 13% 3 7% 4 4 4 4 4 5% 32%	2427
	21st to end	Day	20% 12% 3 7% 4 4 4 3 4 5% 34%	2600
May	1st to 10th	Day	22% 10% 4 8% 5% 5% 4 4 5% 5% 28%	5862
	11th to 20th	Day	21% 9% 3 8% 5% 4 5% 4 6% 8% 26%	5947
	21st to end	Day	20% 8% 3 7% 5% 4 6% 4 6% 7% 32%	6934
June	1st to 10th	Day	17% 7% 3 6% 5% 5% 5% 4 6% 8% 34%	2800
	11th to 20th	Day	16% 7% 3 7% 5% 5% 5% 4 6% 8% 34%	2800
	21st to end	Day	16% 5% 2 6% 5% 5% 5% 4 7% 9% 36%	2800
July	1st to 10th	Day	10% 7% 3 7% 4 5% 6% 4 7% 10% 39%	6360
	11th to 20th	Day	13% 7% 2 7% 5% 5% 5% 5% 7% 9% 35%	5978
	21st to end	Day	12% 6% 2 7% 5% 5% 5% 4 6% 7% 42%	6424
August	1st to 10th	Day	15% 8% 3 8% 6% 5% 5% 4 6% 9% 30%	4234
	11th to 20th	Day	15% 7% 3 8% 6% 6% 7% 5% 6% 9% 29%	4174
	21st to end	Day	12% 6% 2 6% 5% 6% 6% 5% 7% 10% 36%	4477
September	1st to 10th	Day	17% 8% 3 6% 4 4 3 4 5% 6% 41%	2400
	11th to 20th	Day	13% 6% 2 7% 4 4 6% 4 6% 8% 41%	2230
	21st to end	Day	23% 8% 3 7% 4 5% 4 3 5% 7% 31%	2185
October	1st to 10th	Day	17% 8% 2 6% 5% 5% 5% 4 6% 8% 36%	2000
	11th to 20th	Day	22% 10% 3 8% 5% 4 5% 3 5% 6% 30%	2000
	21st to end	Day	24% 13% 3 8% 4 5% 5% 3 6% 6% 24%	2200
November	1st to 10th	Day	14% 13% 3 9% 5% 4 4 4 5% 7% 33%	1938
	11th to 20th	Day	18% 19% 4 9% 4 4 4 4 4 6% 24%	1680
	21st to end	Day	20% 25% 6% 9% 5% 4 4 3 4 5% 15%	1680
December	1st to 10th	Day	19% 24% 15% 14% 5% 4 3 2 2 3 9%	2680
	11th to 20th	Day	21% 24% 8% 11% 6% 3 2 2 3 5% 14%	3070
	21st to end	Day	25% 24% 7% 13% 6% 3 3 2 3 3 10%	4048
January	1st to 10th	Night	14% 20% 9% 29% 7% 3 3 2 2 3 7%	7898
	11th to 20th	Night	11% 19% 10% 23% 10% 6% 4 2 3 3 8%	7680
	21st to end	Night	17% 17% 7% 15% 7% 6% 5% 3 4 4 14%	8401
February	1st to 10th	Night	17% 18% 5% 13% 9% 5% 4 2 4 4 18%	2730
	11th to 20th	Night	16% 16% 6% 16% 7% 6% 5% 3 5% 4 17%	2663
	21st to end	Night	19% 14% 6% 16% 7% 4 3 3 4 4 20%	2111
March	1st to 10th	Night	19% 14% 7% 9% 7% 3 3 3 3 5% 26%	4941
	11th to 20th	Night	13% 13% 7% 7% 4 3 3 2 4 6% 35%	4980
	21st to end	Night	18% 13% 6% 9% 5% 4 3 3 4 5% 31%	5467
April	1st to 10th	Night	19% 15% 5% 8% 5% 3 4 4 4 5% 28%	2075
	11th to 20th	Night	19% 11% 4 6% 4 4 4 3 5% 6% 33%	1939
	21st to end	Night	15% 11% 4 7% 5% 4 4 4 6% 6% 34%	1800
May	1st to 10th	Night	27% 12% 2 6% 4 4 4 3 4 6% 27%	4033
	11th to 20th	Night	19% 11% 3 6% 4 4 4 3 5% 7% 33%	3941
	21st to end	Night	21% 7% 2 6% 4 3 4 3 5% 9% 36%	3920
June	1st to 10th	Night	12% 6% 1 7% 5% 4 5% 5% 7% 8% 40%	1600
	11th to 20th	Night	13% 6% 3 7% 5% 4 4 4 5% 8% 40%	1600
	21st to end	Night	13% 6% 3 6% 4 5% 5% 4 7% 8% 41%	1600
July	1st to 10th	Night	13% 7% 3 7% 4 4 4 3 5% 9% 42%	3598
	11th to 20th	Night	11% 7% 2 6% 4 5% 5% 4 7% 10% 40%	3961
	21st to end	Night	10% 5% 2 6% 4 5% 4 3 6% 8% 47%	4427
August	1st to 10th	Night	12% 10% 3 7% 5% 4 4 4 6% 7% 39%	2915
	11th to 20th	Night	17% 7% 2 6% 4 4 5% 4 6% 7% 39%	2875
	21st to end	Night	9% 6% 2 6% 4 5% 5% 5% 7% 9% 43%	3388
September	1st to 10th	Night	13% 6% 2 6% 4 4 3 3 5% 7% 46%	2000
	11th to 20th	Night	11% 5% 2 5% 4 3 4 4 5% 8% 50%	2165
	21st to end	Night	20% 7% 3 7% 5% 4 4 3 6% 7% 35%	2214
October	1st to 10th	Night	16% 7% 2 6% 4 3 4 4 5% 7% 43%	2400
	11th to 20th	Night	20% 8% 3 7% 5% 4 4 4 5% 7% 33%	2400
	21st to end	Night	26% 11% 3 6% 4 3 4 3 4 5% 31%	2640
November	1st to 10th	Night	14% 12% 3 6% 5% 4 4 4 6% 7% 36%	2573
	11th to 20th	Night	19% 18% 5% 8% 4 4 4 4 4 6% 23%	2940
	21st to end	Night	19% 20% 6% 9% 5% 4 3 3 4 6% 20%	2940
December	1st to 10th	Night	15% 21% 12% 15% 5% 4 4 3 4 4 12%	4720
	11th to 20th	Night	20% 22% 9% 13% 5% 3 3 2 3 4 16%	5345
	21st to end	Night	22% 22% 8% 12% 5% 4 3 3 4 4 13%	7150

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	85% 3 6% 5% 2	4980
	11th to 20th	Day	85% 2 5% 6% 2	5280
	21st to end	Day	80% 2 6% 8% 4	5780
February	1st to 10th	Day	73% 3 6% 12% 5%	1890
	11th to 20th	Day	72% 4 6% 12% 5%	1852
	21st to end	Day	66% 4 9% 15% 5% 2	1693
March	1st to 10th	Day	59% 4 11% 20% 6%	4957
	11th to 20th	Day	55% 5% 14% 19% 6% 2	4980
	21st to end	Day	61% 4 5% 19% 10% 1	5465
April	1st to 10th	Day	57% 4 11% 16% 11% 1	2325
	11th to 20th	Day	53% 4 10% 20% 11% 2	2427
	21st to end	Day	54% 4 10% 19% 12% 2	2600
May	1st to 10th	Day	57% 5% 8% 19% 8% 4	5897
	11th to 20th	Day	54% 6% 10% 20% 9% 1	5951
	21st to end	Day	50% 5% 11% 19% 12% 2	6949
June	1st to 10th	Day	46% 7% 11% 23% 12% 1	2800
	11th to 20th	Day	47% 6% 13% 20% 13% 1	2800
	21st to end	Day	43% 6% 12% 25% 14% 1	2800
July	1st to 10th	Day	40% 5% 12% 26% 16% 2	6290
	11th to 20th	Day	43% 5% 12% 24% 15% 1	5907
	21st to end	Day	40% 5% 10% 24% 19% 1	6240
August	1st to 10th	Day	49% 6% 13% 23% 10%	4221
	11th to 20th	Day	48% 8% 12% 24% 7% 1	4213
	21st to end	Day	41% 6% 12% 28% 12% 1	4477
September	1st to 10th	Day	44% 6% 11% 23% 15% 2	2400
	11th to 20th	Day	40% 7% 11% 27% 15% 1	2230
	21st to end	Day	52% 5% 10% 21% 11% 1	2185
October	1st to 10th	Day	40% 5% 12% 24% 17% 1	3260
	11th to 20th	Day	47% 4 13% 19% 13% 4	3240
	21st to end	Day	48% 4 14% 23% 10% 1	3538
November	1st to 10th	Day	50% 6% 10% 20% 12% 2	1938
	11th to 20th	Day	62% 4 10% 15% 7% 2	1680
	21st to end	Day	72% 4 9% 11% 4 1	1680
December	1st to 10th	Day	84% 2 6% 6% 2	2680
	11th to 20th	Day	76% 3 6% 12% 2	3066
	21st to end	Day	82% 3 7% 7% 1	4048
January	1st to 10th	Night	86% 2 5% 6% 2	7899
	11th to 20th	Night	85% 2 6% 5% 2	7680
	21st to end	Night	76% 3 7% 10% 4	8401
February	1st to 10th	Night	71% 4 7% 13% 5%	2730
	11th to 20th	Night	72% 4 8% 12% 5%	2665
	21st to end	Night	68% 4 7% 12% 8%	2111
March	1st to 10th	Night	62% 3 10% 18% 6%	4941
	11th to 20th	Night	51% 4 14% 21% 9% 2	4980
	21st to end	Night	57% 4 8% 16% 14% 1	5467
April	1st to 10th	Night	58% 5% 9% 16% 11% 1	2075
	11th to 20th	Night	51% 4 11% 18% 13% 2	1939
	21st to end	Night	48% 5% 12% 21% 10% 3	1800
May	1st to 10th	Night	58% 4 8% 15% 11% 4	4060
	11th to 20th	Night	51% 6% 10% 21% 11% 1	3942
	21st to end	Night	45% 6% 12% 22% 13% 3	3927
June	1st to 10th	Night	40% 6% 11% 25% 15% 2	1600
	11th to 20th	Night	41% 7% 13% 23% 15% 2	1600
	21st to end	Night	39% 7% 15% 22% 17%	1600
July	1st to 10th	Night	41% 4 10% 27% 17% 1	3560
	11th to 20th	Night	40% 5% 14% 27% 13% 1	3913
	21st to end	Night	36% 5% 12% 21% 26% 1	4297
August	1st to 10th	Night	45% 5% 11% 26% 12% 1	2904
	11th to 20th	Night	43% 5% 14% 30% 7% 1	2902
	21st to end	Night	36% 6% 16% 30% 11% 2	3388
September	1st to 10th	Night	37% 5% 12% 27% 16% 3	2000
	11th to 20th	Night	32% 7% 12% 28% 20% 1	2165
	21st to end	Night	48% 5% 11% 20% 14% 1	2214
October	1st to 10th	Night	40% 5% 8% 23% 23% 1	3920
	11th to 20th	Night	46% 4 12% 17% 16% 5%	3899
	21st to end	Night	42% 3 15% 23% 15% 1	4252
November	1st to 10th	Night	48% 5% 12% 22% 11% 2	2573
	11th to 20th	Night	62% 4 10% 15% 6% 3	2940
	21st to end	Night	67% 5% 10% 14% 3 1	2940
December	1st to 10th	Night	76% 4 10% 8% 2	4720
	11th to 20th	Night	75% 3 8% 12% 2	5348
	21st to end	Night	76% 4 8% 10% 2	7150

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	37% 23% 29% 8% 3	278
	11th to 20th	Day	40% 31% 21% 6% 2 1	330
	21st to end	Day	37% 28% 25% 9% 1	363
February	1st to 10th	Day	35% 25% 31% 8% 1	330
	11th to 20th	Day	39% 26% 28% 6% 1	330
	21st to end	Day	40% 25% 28% 6%	299
March	1st to 10th	Day	35% 22% 29% 12% 2	508
	11th to 20th	Day	33% 22% 32% 10% 2 1	500
	21st to end	Day	37% 20% 27% 11% 4	550
April	1st to 10th	Day	33% 20% 28% 14% 5% 1	500
	11th to 20th	Day	33% 17% 28% 14% 7% 2	492
	21st to end	Day	30% 18% 31% 15% 4 1	500
May	1st to 10th	Day	31% 16% 29% 18% 6% 2	503
	11th to 20th	Day	26% 13% 29% 20% 9% 2	527
	21st to end	Day	21% 10% 29% 23% 11% 5%	573
June	1st to 10th	Day	22% 14% 31% 22% 9% 3	530
	11th to 20th	Day	22% 13% 25% 27% 11% 2	530
	21st to end	Day	19% 10% 34% 26% 9% 2	530
July	1st to 10th	Day	13% 6% 27% 28% 20% 6%	525
	11th to 20th	Day	15% 7% 28% 29% 14% 6%	524
	21st to end	Day	16% 6% 30% 28% 15% 5%	576
August	1st to 10th	Day	16% 6% 27% 24% 20% 7%	526
	11th to 20th	Day	17% 6% 26% 28% 14% 9%	519
	21st to end	Day	14% 8% 26% 29% 17% 7%	583
September	1st to 10th	Day	21% 10% 21% 24% 17% 6%	530
	11th to 20th	Day	12% 5% 23% 29% 22% 9%	530
	21st to end	Day	27% 9% 26% 20% 14% 4	530
October	1st to 10th	Day	19% 13% 26% 22% 15% 3	530
	11th to 20th	Day	27% 15% 35% 14% 7% 2	530
	21st to end	Day	35% 17% 29% 15% 3	583
November	1st to 10th	Day	29% 19% 30% 13% 6% 2	417
	11th to 20th	Day	38% 22% 26% 10% 3	220
	21st to end	Day	38% 19% 35% 7%	220
December	1st to 10th	Day	41% 27% 24% 5% 2 1	220
	11th to 20th	Day	45% 19% 28% 7% 1	220
	21st to end	Day	40% 21% 30% 7% 1	242
January	1st to 10th	Night	33% 24% 31% 10% 2	840
	11th to 20th	Night	35% 23% 27% 12% 2	840
	21st to end	Night	30% 24% 30% 14% 2	924
February	1st to 10th	Night	32% 21% 30% 16% 1	840
	11th to 20th	Night	30% 22% 33% 13% 1	823
	21st to end	Night	31% 22% 33% 12% 2	692
March	1st to 10th	Night	27% 19% 35% 15% 3	809
	11th to 20th	Night	28% 19% 30% 18% 5%	800
	21st to end	Night	29% 16% 32% 18% 4 1	880
April	1st to 10th	Night	28% 18% 29% 20% 4	800
	11th to 20th	Night	28% 14% 31% 19% 8% 1	755
	21st to end	Night	22% 12% 33% 26% 6% 1	600
May	1st to 10th	Night	27% 9% 25% 28% 10%	600
	11th to 20th	Night	21% 9% 29% 32% 8% 1	552
	21st to end	Night	16% 7% 26% 30% 19% 2	433
June	1st to 10th	Night	14% 6% 30% 35% 14% 1	400
	11th to 20th	Night	15% 6% 27% 40% 13%	400
	21st to end	Night	13% 6% 27% 35% 18% 1	400
July	1st to 10th	Night	12% 3 20% 40% 21% 4	390
	11th to 20th	Night	14% 1 20% 34% 26% 3	541
	21st to end	Night	13% 3 16% 40% 27% 1	657
August	1st to 10th	Night	13% 3 16% 39% 27% 2	597
	11th to 20th	Night	16% 3 13% 35% 29% 4	594
	21st to end	Night	15% 4 19% 33% 26% 4	760
September	1st to 10th	Night	14% 7% 20% 31% 24% 4	800
	11th to 20th	Night	13% 4 18% 35% 27% 4	800
	21st to end	Night	23% 9% 20% 28% 16% 3	800
October	1st to 10th	Night	20% 8% 26% 26% 18% 3	800
	11th to 20th	Night	25% 13% 31% 22% 8% 1	800
	21st to end	Night	33% 12% 31% 20% 4	880
November	1st to 10th	Night	24% 13% 29% 26% 8% 1	820
	11th to 20th	Night	32% 18% 28% 16% 6% 1	840
	21st to end	Night	35% 18% 30% 14% 4	840
December	1st to 10th	Night	35% 21% 29% 12% 4	840
	11th to 20th	Night	33% 25% 26% 14% 2	840
	21st to end	Night	35% 19% 31% 15% 1	924

Page generated at 2026-02-28 01:40:19. Version ac07

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