Hill Top Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Hill Top Observatory clear sky chart from 2005-11-06 to 2025-04-11.

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% 7% 2 6% 4 4 4 4 5% 7% 39%	113118
	Night	17% 7% 2 6% 4 4 4 3 5% 6% 40%	115166
Transparency	Day	43% 6% 11% 21% 15% 3	116673
	Night	43% 6% 12% 22% 15% 3	119762
Seeing	Day	25% 16% 30% 19% 8% 2	15433
	Night	23% 15% 31% 23% 7% 1	24490

By Month

Percentage Cloud Cover NE (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% 7% 3 9% 5% 5% 5% 4 6% 6% 38%	3597
	11th to 20th	Day	13% 7% 3 8% 3 3 4 4 5% 6% 44%	3570
	21st to end	Day	15% 10% 3 6% 4 3 3 3 5% 5% 44%	3960
February	1st to 10th	Day	17% 8% 3 7% 4 5% 4 4 5% 6% 35%	1826
	11th to 20th	Day	12% 8% 3 7% 5% 5% 4 5% 5% 7% 39%	1948
	21st to end	Day	17% 11% 3 8% 5% 4 4 4 4 5% 37%	1650
March	1st to 10th	Day	20% 10% 3 7% 4 4 4 3 4 5% 35%	4476
	11th to 20th	Day	16% 9% 3 5% 4 4 4 4 5% 6% 40%	4740
	21st to end	Day	25% 9% 2 6% 4 3 4 3 5% 7% 33%	5828
April	1st to 10th	Day	19% 10% 2 7% 5% 4 4 3 5% 6% 37%	2400
	11th to 20th	Day	17% 7% 2 6% 4 4 5% 4 6% 7% 38%	2259
	21st to end	Day	21% 8% 3 7% 5% 4 4 3 4 6% 35%	2470
May	1st to 10th	Day	23% 11% 3 7% 5% 4 5% 4 5% 6% 28%	4106
	11th to 20th	Day	22% 7% 2 8% 5% 4 4 3 5% 6% 32%	4234
	21st to end	Day	15% 4 2 5% 3 4 4 4 5% 8% 44%	4837
June	1st to 10th	Day	18% 7% 2 6% 5% 5% 5% 4 6% 8% 34%	2660
	11th to 20th	Day	17% 5% 3 6% 5% 5% 4 5% 5% 8% 37%	2660
	21st to end	Day	19% 7% 3 6% 4 5% 4 4 6% 8% 34%	2660
July	1st to 10th	Day	13% 6% 2 6% 5% 5% 5% 5% 6% 9% 37%	4440
	11th to 20th	Day	11% 5% 2 6% 5% 5% 6% 5% 7% 10% 38%	4440
	21st to end	Day	17% 7% 3 7% 4 4 4 4 6% 7% 36%	4814
August	1st to 10th	Day	14% 6% 3 6% 5% 6% 5% 5% 6% 8% 35%	4215
	11th to 20th	Day	15% 7% 2 6% 5% 5% 5% 4 6% 7% 37%	3981
	21st to end	Day	8% 5% 2 6% 4 5% 5% 5% 8% 9% 43%	4180
September	1st to 10th	Day	14% 5% 1 5% 4 4 5% 4 5% 6% 47%	2280
	11th to 20th	Day	16% 5% 2 4 3 3 4 2 4 5% 52%	2104
	21st to end	Day	17% 5% 2 6% 4 4 4 3 4 7% 45%	2090
October	1st to 10th	Day	24% 6% 2 6% 4 4 4 3 4 5% 39%	2090
	11th to 20th	Day	15% 5% 2 6% 3 4 4 3 5% 7% 46%	1835
	21st to end	Day	23% 6% 2 5% 3 4 4 4 4 5% 39%	1881
November	1st to 10th	Day	15% 6% 2 6% 4 3 3 3 4 5% 50%	1761
	11th to 20th	Day	18% 6% 2 4 3 4 3 3 4 5% 47%	1800
	21st to end	Day	19% 6% 2 5% 4 4 4 3 5% 6% 41%	1780
December	1st to 10th	Day	16% 7% 3 5% 4 3 4 3 6% 7% 42%	2600
	11th to 20th	Day	21% 5% 2 5% 4 3 3 4 4 5% 44%	2986
	21st to end	Day	19% 10% 3 6% 4 3 5% 3 4 5% 37%	3960
January	1st to 10th	Night	13% 6% 2 6% 4 4 5% 3 6% 7% 45%	6349
	11th to 20th	Night	12% 8% 4 8% 5% 4 4 4 5% 6% 39%	6315
	21st to end	Night	15% 10% 3 7% 4 4 4 3 5% 5% 39%	6996
February	1st to 10th	Night	15% 10% 3 6% 4 4 4 4 5% 6% 39%	2568
	11th to 20th	Night	13% 8% 2 6% 5% 5% 5% 4 6% 6% 40%	2338
	21st to end	Night	15% 9% 2 6% 4 4 5% 3 6% 6% 40%	1980
March	1st to 10th	Night	23% 9% 2 5% 4 4 4 3 4 6% 37%	5364
	11th to 20th	Night	16% 9% 3 6% 5% 4 4 3 5% 6% 38%	5219
	21st to end	Night	25% 8% 2 5% 3 3 4 3 6% 6% 34%	5100
April	1st to 10th	Night	19% 8% 3 7% 4 5% 5% 3 5% 6% 36%	2000
	11th to 20th	Night	20% 7% 2 5% 4 4 4 4 5% 6% 40%	1875
	21st to end	Night	21% 7% 3 5% 4 4 3 3 4 6% 40%	1710
May	1st to 10th	Night	26% 11% 3 7% 4 4 3 3 5% 7% 27%	2834
	11th to 20th	Night	21% 8% 3 8% 5% 6% 6% 4 6% 7% 28%	2687
	21st to end	Night	16% 5% 2 6% 4 4 4 3 6% 8% 42%	2738
June	1st to 10th	Night	18% 8% 3 7% 4 4 3 3 5% 7% 38%	1520
	11th to 20th	Night	20% 6% 2 5% 4 3 5% 4 5% 8% 38%	1520
	21st to end	Night	18% 5% 3 5% 4 4 4 4 6% 8% 39%	1520
July	1st to 10th	Night	12% 5% 2 6% 5% 6% 6% 4 7% 9% 40%	2520
	11th to 20th	Night	13% 6% 2 6% 4 4 5% 4 5% 9% 42%	2520
	21st to end	Night	16% 7% 2 6% 4 4 4 3 6% 6% 41%	2739
August	1st to 10th	Night	21% 6% 2 7% 5% 4 5% 4 5% 6% 35%	2684
	11th to 20th	Night	12% 8% 3 7% 5% 5% 6% 4 6% 7% 38%	2843
	21st to end	Night	9% 4 1 3 3 4 3 3 5% 8% 56%	3476
September	1st to 10th	Night	14% 5% 1 5% 3 4 3 3 4 7% 50%	1900
	11th to 20th	Night	14% 5% 2 4 3 3 3 2 4 6% 53%	2076
	21st to end	Night	17% 4 2 6% 3 3 4 3 4 6% 47%	2090
October	1st to 10th	Night	22% 5% 2 6% 4 4 4 3 4 5% 41%	2090
	11th to 20th	Night	16% 4 2 4 3 3 5% 4 6% 7% 45%	2342
	21st to end	Night	22% 7% 2 5% 4 4 4 3 5% 5% 39%	2717
November	1st to 10th	Night	15% 6% 2 5% 4 3 4 4 4 6% 47%	2537
	11th to 20th	Night	17% 6% 3 5% 4 3 3 3 4 5% 47%	2600
	21st to end	Night	16% 7% 2 5% 4 3 4 3 5% 6% 44%	2615
December	1st to 10th	Night	17% 10% 2 6% 4 4 4 3 5% 6% 39%	4580
	11th to 20th	Night	22% 8% 2 5% 3 3 4 3 4 5% 41%	5208
	21st to end	Night	19% 10% 3 7% 4 4 4 3 4 6% 38%	6996

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	45% 4 9% 25% 16% 1	3598
	11th to 20th	Day	40% 5% 11% 22% 18% 4	3600
	21st to end	Day	43% 4 8% 21% 19% 4	3960
February	1st to 10th	Day	47% 6% 12% 20% 12% 3	1826
	11th to 20th	Day	43% 7% 11% 21% 15% 4	1957
	21st to end	Day	49% 4 10% 19% 15% 3	1650
March	1st to 10th	Day	52% 4 9% 14% 11% 10%	4479
	11th to 20th	Day	42% 7% 14% 23% 12% 2	4740
	21st to end	Day	50% 4 7% 16% 17% 7%	5828
April	1st to 10th	Day	49% 5% 8% 18% 17% 3	2400
	11th to 20th	Day	44% 6% 11% 19% 17% 3	2259
	21st to end	Day	50% 6% 7% 16% 17% 3	2470
May	1st to 10th	Day	55% 8% 11% 15% 10% 1	4120
	11th to 20th	Day	50% 8% 14% 16% 10% 2	4238
	21st to end	Day	35% 8% 13% 25% 18% 2	4837
June	1st to 10th	Day	46% 8% 13% 21% 11% 1	2660
	11th to 20th	Day	43% 7% 12% 20% 15% 3	2660
	21st to end	Day	47% 8% 12% 20% 12% 1	2660
July	1st to 10th	Day	42% 7% 13% 20% 17% 1	4370
	11th to 20th	Day	37% 8% 16% 25% 12% 1	4365
	21st to end	Day	46% 6% 12% 21% 14% 2	4661
August	1st to 10th	Day	44% 8% 15% 22% 10%	4201
	11th to 20th	Day	43% 7% 14% 24% 10% 1	4020
	21st to end	Day	33% 8% 13% 28% 17% 2	4180
September	1st to 10th	Day	35% 7% 12% 25% 19% 2	2280
	11th to 20th	Day	35% 5% 8% 26% 23% 2	2104
	21st to end	Day	40% 5% 10% 22% 21% 2	2090
October	1st to 10th	Day	44% 6% 9% 21% 18% 2	3480
	11th to 20th	Day	32% 5% 8% 29% 23% 4	3012
	21st to end	Day	34% 5% 9% 30% 19% 3	3081
November	1st to 10th	Day	38% 4 13% 26% 17% 2	1761
	11th to 20th	Day	38% 5% 10% 21% 22% 3	1800
	21st to end	Day	44% 4 16% 21% 12% 2	1780
December	1st to 10th	Day	42% 5% 14% 28% 10% 1	2600
	11th to 20th	Day	42% 4 12% 19% 19% 4	2986
	21st to end	Day	48% 6% 9% 18% 13% 5%	3960
January	1st to 10th	Night	38% 5% 12% 30% 14% 2	6349
	11th to 20th	Night	44% 5% 14% 20% 13% 4	6360
	21st to end	Night	48% 4 9% 17% 17% 5%	6996
February	1st to 10th	Night	44% 7% 13% 20% 13% 3	2568
	11th to 20th	Night	43% 5% 13% 20% 16% 3	2340
	21st to end	Night	44% 6% 9% 23% 14% 4	1980
March	1st to 10th	Night	48% 5% 9% 16% 15% 7%	5374
	11th to 20th	Night	45% 8% 15% 18% 12% 2	5220
	21st to end	Night	50% 4 6% 14% 21% 6%	5100
April	1st to 10th	Night	48% 5% 11% 17% 16% 3	2000
	11th to 20th	Night	44% 5% 10% 20% 18% 2	1875
	21st to end	Night	45% 6% 12% 18% 16% 4	1710
May	1st to 10th	Night	55% 8% 11% 17% 7% 2	2840
	11th to 20th	Night	53% 10% 14% 13% 9% 2	2688
	21st to end	Night	39% 8% 15% 21% 14% 2	2739
June	1st to 10th	Night	46% 6% 11% 24% 12% 1	1520
	11th to 20th	Night	44% 8% 13% 20% 13% 2	1520
	21st to end	Night	41% 9% 14% 21% 13% 1	1520
July	1st to 10th	Night	40% 8% 14% 23% 14% 1	2480
	11th to 20th	Night	39% 7% 16% 22% 15%	2472
	21st to end	Night	44% 6% 12% 23% 15% 1	2651
August	1st to 10th	Night	49% 7% 13% 20% 11% 1	2674
	11th to 20th	Night	44% 8% 15% 23% 10%	2870
	21st to end	Night	26% 6% 17% 30% 20% 1	3476
September	1st to 10th	Night	34% 6% 12% 27% 20% 1	1900
	11th to 20th	Night	32% 5% 11% 26% 24% 2	2076
	21st to end	Night	38% 6% 10% 27% 17% 2	2090
October	1st to 10th	Night	40% 5% 15% 23% 15% 2	3480
	11th to 20th	Night	28% 5% 11% 26% 27% 3	3891
	21st to end	Night	35% 4 12% 32% 14% 2	4467
November	1st to 10th	Night	38% 5% 11% 23% 20% 3	2537
	11th to 20th	Night	40% 4 11% 21% 21% 3	2600
	21st to end	Night	41% 5% 14% 26% 13% 1	2615
December	1st to 10th	Night	46% 5% 11% 27% 11% 1	4580
	11th to 20th	Night	46% 5% 13% 14% 20% 2	5208
	21st to end	Night	46% 6% 9% 22% 15% 3	6996

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% 22% 26% 13% 4	200
	11th to 20th	Day	32% 23% 32% 10% 2 2	200
	21st to end	Day	29% 24% 32% 10% 4 1	220
February	1st to 10th	Day	33% 26% 29% 11% 1	364
	11th to 20th	Day	35% 26% 27% 11% 1	496
	21st to end	Day	31% 23% 30% 13% 3	414
March	1st to 10th	Day	34% 18% 33% 12% 3	500
	11th to 20th	Day	32% 25% 28% 10% 4 1	500
	21st to end	Day	34% 19% 30% 11% 4 1	550
April	1st to 10th	Day	35% 21% 27% 11% 5% 1	500
	11th to 20th	Day	31% 21% 29% 12% 5% 2	452
	21st to end	Day	30% 18% 30% 14% 6% 1	460
May	1st to 10th	Day	34% 16% 30% 14% 5% 1	463
	11th to 20th	Day	23% 15% 36% 18% 6% 1	487
	21st to end	Day	19% 14% 35% 24% 6% 1	529
June	1st to 10th	Day	20% 14% 37% 16% 10% 2	490
	11th to 20th	Day	16% 15% 36% 23% 8% 2	490
	21st to end	Day	21% 14% 35% 20% 7% 3	490
July	1st to 10th	Day	19% 9% 29% 27% 13% 4	485
	11th to 20th	Day	13% 7% 34% 31% 11% 3	484
	21st to end	Day	13% 10% 34% 32% 9% 2	532
August	1st to 10th	Day	14% 9% 30% 31% 14% 3	486
	11th to 20th	Day	18% 9% 29% 25% 15% 4	479
	21st to end	Day	15% 10% 30% 27% 14% 5%	539
September	1st to 10th	Day	18% 12% 25% 23% 17% 6%	490
	11th to 20th	Day	20% 11% 27% 18% 15% 10%	490
	21st to end	Day	19% 11% 32% 21% 12% 5%	490
October	1st to 10th	Day	29% 12% 23% 21% 10% 4	490
	11th to 20th	Day	21% 16% 30% 21% 10% 2	439
	21st to end	Day	31% 13% 29% 20% 6% 1	429
November	1st to 10th	Day	23% 19% 26% 22% 8% 2	395
	11th to 20th	Day	35% 15% 25% 20% 5% 1	400
	21st to end	Day	33% 19% 27% 16% 5% 1	380
December	1st to 10th	Day	27% 28% 29% 12% 6%	200
	11th to 20th	Day	39% 20% 29% 11% 2	200
	21st to end	Day	33% 18% 29% 16% 4 1	220
January	1st to 10th	Night	25% 24% 33% 17% 2	800
	11th to 20th	Night	29% 19% 32% 18% 3	800
	21st to end	Night	23% 19% 38% 17% 3	880
February	1st to 10th	Night	25% 21% 35% 19% 1	800
	11th to 20th	Night	25% 23% 35% 15% 1	783
	21st to end	Night	27% 22% 33% 17% 2	660
March	1st to 10th	Night	28% 20% 32% 18% 2	800
	11th to 20th	Night	26% 21% 31% 19% 4	800
	21st to end	Night	27% 19% 34% 16% 4	880
April	1st to 10th	Night	29% 19% 31% 18% 4	800
	11th to 20th	Night	25% 20% 31% 19% 5%	747
	21st to end	Night	26% 19% 32% 20% 4	570
May	1st to 10th	Night	27% 13% 34% 20% 5%	570
	11th to 20th	Night	22% 15% 33% 26% 5%	488
	21st to end	Night	21% 7% 35% 30% 7%	411
June	1st to 10th	Night	20% 11% 33% 28% 8% 1	380
	11th to 20th	Night	21% 8% 30% 33% 8% 1	380
	21st to end	Night	18% 6% 32% 36% 7%	380
July	1st to 10th	Night	16% 8% 29% 36% 9% 1	370
	11th to 20th	Night	15% 4 27% 41% 13% 1	368
	21st to end	Night	13% 5% 29% 40% 12%	416
August	1st to 10th	Night	16% 4 28% 38% 14%	478
	11th to 20th	Night	15% 8% 28% 35% 12% 2	597
	21st to end	Night	16% 5% 24% 37% 17% 1	836
September	1st to 10th	Night	16% 6% 30% 32% 14% 1	760
	11th to 20th	Night	16% 8% 27% 27% 19% 3	760
	21st to end	Night	19% 9% 29% 29% 14% 2	760
October	1st to 10th	Night	26% 10% 24% 27% 12% 1	760
	11th to 20th	Night	21% 12% 32% 25% 10% 1	760
	21st to end	Night	28% 16% 29% 22% 5%	836
November	1st to 10th	Night	19% 16% 31% 26% 7% 1	780
	11th to 20th	Night	24% 19% 30% 20% 6% 1	800
	21st to end	Night	27% 17% 31% 20% 5% 1	800
December	1st to 10th	Night	26% 20% 35% 16% 4	800
	11th to 20th	Night	27% 20% 32% 18% 3	800
	21st to end	Night	28% 19% 32% 19% 3	880

Page generated at 2025-04-08 09:17:31. Version ac07

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