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 2026-08-15.

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%	143532
	Night	17% 7% 2 6% 4 4 4 3 5% 6% 40%	138382
Transparency	Day	43% 6% 12% 22% 15% 2	147041
	Night	43% 5% 12% 22% 16% 2	142949
Seeing	Day	25% 16% 30% 18% 8% 2	17218
	Night	23% 15% 31% 23% 7%	26053

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	10% 7% 3 9% 6% 6% 5% 5% 6% 6% 37%	4661
	11th to 20th	Day	14% 8% 3 7% 4 3 4 3 5% 6% 43%	4650
	21st to end	Day	12% 10% 3 6% 3 3 4 3 5% 6% 45%	5124
February	1st to 10th	Day	16% 8% 3 7% 4 5% 4 4 5% 6% 36%	1918
	11th to 20th	Day	12% 8% 3 7% 5% 5% 4 5% 5% 7% 39%	2048
	21st to end	Day	17% 11% 3 7% 5% 4 3 3 4 5% 37%	1730
March	1st to 10th	Day	20% 10% 3 7% 4 4 4 3 4 5% 35%	4576
	11th to 20th	Day	16% 9% 3 5% 4 4 4 4 5% 6% 40%	4844
	21st to end	Day	25% 9% 2 6% 4 3 4 3 5% 7% 33%	5957
April	1st to 10th	Day	19% 10% 2 7% 5% 4 4 3 4 6% 37%	2520
	11th to 20th	Day	17% 7% 2 6% 4 5% 5% 4 6% 7% 38%	2500
	21st to end	Day	20% 7% 3 7% 5% 4 4 3 4 6% 37%	2730
May	1st to 10th	Day	28% 8% 2 6% 4 4 4 3 5% 6% 28%	7639
	11th to 20th	Day	19% 7% 3 8% 5% 5% 4 3 5% 6% 35%	7918
	21st to end	Day	21% 7% 2 7% 4 5% 5% 4 6% 7% 33%	9060
June	1st to 10th	Day	18% 6% 2 6% 5% 5% 5% 4 6% 8% 35%	2940
	11th to 20th	Day	16% 5% 3 6% 5% 5% 4 4 6% 8% 38%	2940
	21st to end	Day	18% 6% 3 6% 4 5% 5% 4 6% 8% 36%	2940
July	1st to 10th	Day	16% 5% 2 6% 4 5% 5% 4 6% 10% 36%	8280
	11th to 20th	Day	8% 4 2 5% 4 5% 5% 4 7% 10% 45%	8280
	21st to end	Day	15% 6% 2 6% 3 4 4 3 6% 7% 43%	8993
August	1st to 10th	Day	14% 6% 3 6% 5% 6% 5% 5% 6% 8% 36%	4482
	11th to 20th	Day	15% 7% 2 6% 5% 5% 5% 4 6% 7% 38%	4161
	21st to end	Day	8% 4 2 6% 4 5% 5% 5% 8% 9% 43%	4312
September	1st to 10th	Day	14% 5% 1 5% 4 4 5% 3 5% 6% 47%	2400
	11th to 20th	Day	15% 5% 2 4 4 3 4 2 4 5% 53%	2214
	21st to end	Day	17% 4 2 6% 4 4 4 4 4 7% 45%	2200
October	1st to 10th	Day	23% 6% 2 6% 4 4 4 3 4 5% 41%	2200
	11th to 20th	Day	16% 5% 2 5% 3 4 4 3 5% 7% 46%	1931
	21st to end	Day	22% 6% 2 5% 3 4 4 4 4 5% 40%	1980
November	1st to 10th	Day	15% 5% 2 6% 4 3 3 3 4 5% 49%	1851
	11th to 20th	Day	18% 6% 2 4 4 4 3 3 4 5% 47%	1890
	21st to end	Day	19% 7% 2 6% 4 4 4 3 5% 6% 41%	1869
December	1st to 10th	Day	15% 7% 3 5% 4 3 4 3 6% 7% 42%	2680
	11th to 20th	Day	21% 5% 2 5% 4 3 3 4 4 5% 45%	3066
	21st to end	Day	19% 10% 3 6% 4 3 5% 3 4 5% 37%	4048
January	1st to 10th	Night	13% 7% 3 7% 4 4 4 3 6% 7% 43%	8234
	11th to 20th	Night	14% 10% 4 8% 5% 4 4 4 5% 6% 37%	8235
	21st to end	Night	13% 9% 3 6% 4 4 4 3 5% 5% 42%	9057
February	1st to 10th	Night	14% 9% 3 6% 4 4 4 4 5% 6% 40%	2696
	11th to 20th	Night	13% 8% 2 6% 5% 5% 5% 4 6% 6% 40%	2458
	21st to end	Night	16% 9% 2 7% 4 4 5% 3 5% 6% 40%	2076
March	1st to 10th	Night	23% 8% 2 5% 4 4 4 3 4 6% 38%	5484
	11th to 20th	Night	16% 9% 3 6% 5% 4 4 3 5% 6% 38%	5335
	21st to end	Night	25% 8% 2 5% 3 3 4 3 6% 6% 34%	5213
April	1st to 10th	Night	19% 8% 3 7% 4 4 4 3 4 6% 36%	2100
	11th to 20th	Night	20% 7% 2 5% 4 4 4 3 5% 6% 40%	2067
	21st to end	Night	20% 7% 3 6% 4 4 3 3 4 6% 41%	1890
May	1st to 10th	Night	30% 10% 2 6% 4 3 3 3 5% 7% 28%	5259
	11th to 20th	Night	17% 7% 3 7% 5% 5% 5% 4 6% 8% 33%	4997
	21st to end	Night	22% 7% 3 7% 4 4 5% 4 6% 7% 32%	5114
June	1st to 10th	Night	18% 8% 3 7% 4 4 3 3 4 7% 40%	1680
	11th to 20th	Night	19% 6% 2 5% 4 4 5% 4 5% 8% 39%	1680
	21st to end	Night	18% 6% 3 5% 4 4 4 4 6% 7% 40%	1680
July	1st to 10th	Night	14% 6% 2 6% 4 4 5% 3 6% 9% 40%	4680
	11th to 20th	Night	10% 5% 1 5% 3 4 4 4 6% 9% 47%	4680
	21st to end	Night	13% 5% 2 5% 4 3 4 3 5% 6% 50%	5084
August	1st to 10th	Night	20% 6% 2 7% 4 4 4 4 5% 6% 36%	2854
	11th to 20th	Night	12% 7% 3 7% 5% 5% 6% 4 6% 7% 38%	2972
	21st to end	Night	9% 4 1 3 3 4 3 3 6% 8% 56%	3586
September	1st to 10th	Night	14% 5% 1 5% 3 3 4 3 4 7% 51%	2000
	11th to 20th	Night	14% 5% 2 4 3 3 3 3 4 6% 54%	2185
	21st to end	Night	17% 4 2 6% 4 3 4 3 4 6% 47%	2200
October	1st to 10th	Night	21% 5% 2 6% 4 4 4 3 4 5% 43%	2200
	11th to 20th	Night	17% 4 2 4 3 3 5% 3 6% 7% 45%	2465
	21st to end	Night	22% 7% 2 5% 4 3 4 3 5% 5% 39%	2860
November	1st to 10th	Night	15% 6% 2 5% 4 3 4 4 4 6% 47%	2667
	11th to 20th	Night	16% 5% 3 5% 4 3 3 3 4 5% 47%	2730
	21st to end	Night	16% 7% 2 5% 4 3 4 3 5% 6% 44%	2746
December	1st to 10th	Night	17% 10% 2 6% 4 4 4 3 5% 6% 39%	4720
	11th to 20th	Night	22% 8% 2 5% 3 3 4 3 4 5% 41%	5348
	21st to end	Night	19% 10% 3 7% 4 4 4 3 4 6% 38%	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	46% 4 11% 24% 14% 1	4662
	11th to 20th	Day	41% 5% 11% 21% 19% 4	4680
	21st to end	Day	40% 4 9% 24% 21% 3	5124
February	1st to 10th	Day	47% 6% 12% 20% 13% 3	1918
	11th to 20th	Day	43% 7% 11% 22% 15% 3	2057
	21st to end	Day	49% 4 11% 19% 15% 3	1730
March	1st to 10th	Day	52% 4 9% 14% 11% 10%	4579
	11th to 20th	Day	42% 7% 14% 23% 12% 2	4844
	21st to end	Day	50% 4 7% 16% 17% 7%	5957
April	1st to 10th	Day	49% 5% 8% 18% 17% 4	2520
	11th to 20th	Day	44% 6% 11% 19% 17% 3	2500
	21st to end	Day	48% 6% 7% 17% 18% 3	2730
May	1st to 10th	Day	56% 6% 11% 15% 11% 1	7651
	11th to 20th	Day	48% 6% 14% 19% 11% 1	7922
	21st to end	Day	48% 6% 11% 22% 12% 1	9061
June	1st to 10th	Day	45% 8% 13% 22% 12% 1	2940
	11th to 20th	Day	43% 7% 12% 20% 15% 3	2940
	21st to end	Day	45% 8% 13% 21% 13% 1	2940
July	1st to 10th	Day	43% 7% 15% 20% 14% 1	8210
	11th to 20th	Day	32% 7% 16% 29% 15% 1	8205
	21st to end	Day	39% 5% 11% 27% 17% 1	8795
August	1st to 10th	Day	44% 8% 15% 22% 10% 1	4468
	11th to 20th	Day	42% 7% 15% 25% 10% 1	4200
	21st to end	Day	32% 7% 13% 28% 18% 2	4312
September	1st to 10th	Day	35% 6% 12% 25% 19% 2	2400
	11th to 20th	Day	34% 5% 8% 26% 24% 2	2214
	21st to end	Day	40% 5% 10% 22% 20% 3	2200
October	1st to 10th	Day	43% 6% 9% 21% 19% 3	3590
	11th to 20th	Day	32% 5% 8% 29% 23% 3	3108
	21st to end	Day	34% 5% 9% 31% 19% 3	3180
November	1st to 10th	Day	39% 4 13% 25% 17% 2	1851
	11th to 20th	Day	39% 5% 10% 22% 21% 3	1890
	21st to end	Day	44% 4 16% 22% 12% 2	1869
December	1st to 10th	Day	41% 5% 14% 28% 11% 1	2680
	11th to 20th	Day	42% 4 12% 19% 19% 4	3066
	21st to end	Day	48% 6% 9% 19% 14% 5%	4048
January	1st to 10th	Night	40% 4 13% 27% 13% 2	8234
	11th to 20th	Night	47% 5% 12% 18% 14% 5%	8280
	21st to end	Night	44% 4 9% 19% 19% 5%	9057
February	1st to 10th	Night	43% 6% 13% 21% 14% 2	2696
	11th to 20th	Night	42% 5% 13% 21% 15% 3	2460
	21st to end	Night	44% 6% 9% 24% 14% 4	2076
March	1st to 10th	Night	48% 5% 9% 16% 16% 7%	5494
	11th to 20th	Night	45% 7% 15% 18% 12% 2	5336
	21st to end	Night	50% 4 7% 14% 21% 6%	5213
April	1st to 10th	Night	48% 5% 11% 17% 16% 3	2100
	11th to 20th	Night	45% 5% 10% 20% 18% 2	2067
	21st to end	Night	45% 5% 11% 18% 17% 4	1890
May	1st to 10th	Night	56% 6% 13% 14% 11% 1	5265
	11th to 20th	Night	47% 8% 14% 20% 10% 1	4998
	21st to end	Night	50% 6% 13% 21% 9% 1	5115
June	1st to 10th	Night	45% 6% 10% 25% 13% 1	1680
	11th to 20th	Night	43% 8% 13% 20% 14% 2	1680
	21st to end	Night	41% 9% 14% 23% 13% 2	1680
July	1st to 10th	Night	41% 6% 15% 23% 14%	4640
	11th to 20th	Night	33% 6% 17% 24% 20% 1	4632
	21st to end	Night	35% 5% 12% 32% 16% 1	4967
August	1st to 10th	Night	48% 7% 14% 19% 11% 1	2844
	11th to 20th	Night	43% 8% 14% 24% 11% 1	2999
	21st to end	Night	26% 6% 17% 30% 20% 1	3586
September	1st to 10th	Night	34% 6% 12% 27% 20% 1	2000
	11th to 20th	Night	32% 5% 11% 26% 25% 2	2185
	21st to end	Night	39% 6% 10% 27% 17% 3	2200
October	1st to 10th	Night	39% 5% 15% 23% 15% 2	3590
	11th to 20th	Night	28% 5% 11% 27% 26% 3	4014
	21st to end	Night	35% 4 12% 33% 14% 2	4610
November	1st to 10th	Night	38% 5% 10% 23% 19% 3	2667
	11th to 20th	Night	39% 4 11% 22% 21% 3	2730
	21st to end	Night	41% 5% 14% 26% 13% 1	2746
December	1st to 10th	Night	45% 5% 11% 27% 11% 1	4720
	11th to 20th	Night	45% 5% 13% 14% 20% 2	5348
	21st to end	Night	46% 6% 9% 22% 15% 3	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	34% 22% 28% 13% 4	220
	11th to 20th	Day	33% 24% 31% 9% 2 1	220
	21st to end	Day	31% 26% 30% 9% 3 1	242
February	1st to 10th	Day	32% 26% 31% 11% 1	396
	11th to 20th	Day	34% 26% 29% 11% 1	536
	21st to end	Day	31% 23% 31% 12% 3	446
March	1st to 10th	Day	35% 18% 33% 12% 2	540
	11th to 20th	Day	33% 25% 28% 10% 3 1	540
	21st to end	Day	35% 20% 29% 11% 4 1	594
April	1st to 10th	Day	36% 20% 27% 11% 5% 1	540
	11th to 20th	Day	33% 22% 28% 11% 5% 1	532
	21st to end	Day	29% 19% 30% 14% 6% 1	540
May	1st to 10th	Day	36% 18% 28% 14% 4 1	543
	11th to 20th	Day	23% 18% 35% 17% 6% 1	567
	21st to end	Day	21% 15% 34% 23% 6% 1	617
June	1st to 10th	Day	20% 14% 36% 18% 10% 2	570
	11th to 20th	Day	18% 16% 35% 21% 8% 2	570
	21st to end	Day	21% 13% 35% 21% 7% 3	570
July	1st to 10th	Day	19% 9% 30% 26% 12% 3	565
	11th to 20th	Day	14% 9% 33% 29% 12% 3	564
	21st to end	Day	14% 11% 32% 31% 10% 3	620
August	1st to 10th	Day	13% 8% 31% 31% 14% 4	566
	11th to 20th	Day	17% 9% 29% 27% 15% 4	535
	21st to end	Day	14% 10% 32% 26% 13% 4	583
September	1st to 10th	Day	18% 12% 27% 22% 16% 5%	530
	11th to 20th	Day	18% 10% 27% 18% 16% 10%	530
	21st to end	Day	19% 11% 31% 22% 12% 5%	530
October	1st to 10th	Day	28% 12% 22% 22% 10% 5%	530
	11th to 20th	Day	23% 17% 29% 20% 10% 2	474
	21st to end	Day	31% 14% 29% 20% 6% 1	462
November	1st to 10th	Day	25% 20% 26% 20% 8% 1	425
	11th to 20th	Day	33% 17% 25% 19% 4 1	430
	21st to end	Day	33% 21% 26% 15% 5%	409
December	1st to 10th	Day	28% 28% 28% 11% 5%	220
	11th to 20th	Day	39% 20% 28% 11% 1	220
	21st to end	Day	36% 17% 28% 15% 3 1	242
January	1st to 10th	Night	25% 23% 34% 16% 2	840
	11th to 20th	Night	30% 19% 31% 17% 3	840
	21st to end	Night	24% 19% 37% 16% 3	924
February	1st to 10th	Night	24% 21% 36% 19% 1	840
	11th to 20th	Night	24% 22% 36% 17% 1	823
	21st to end	Night	26% 21% 34% 17% 2	692
March	1st to 10th	Night	28% 19% 33% 18% 2	840
	11th to 20th	Night	27% 21% 30% 19% 4	840
	21st to end	Night	27% 20% 33% 16% 4	924
April	1st to 10th	Night	29% 19% 31% 18% 3	840
	11th to 20th	Night	26% 20% 31% 17% 5%	823
	21st to end	Night	25% 19% 33% 20% 3	630
May	1st to 10th	Night	28% 13% 34% 20% 5%	630
	11th to 20th	Night	21% 15% 34% 26% 5%	540
	21st to end	Night	21% 7% 36% 29% 7%	455
June	1st to 10th	Night	20% 10% 33% 29% 8%	420
	11th to 20th	Night	21% 9% 30% 33% 7%	420
	21st to end	Night	17% 7% 31% 37% 7%	420
July	1st to 10th	Night	16% 8% 30% 37% 9%	410
	11th to 20th	Night	14% 5% 26% 42% 12% 1	408
	21st to end	Night	13% 5% 30% 40% 12%	460
August	1st to 10th	Night	15% 4 27% 40% 14%	528
	11th to 20th	Night	14% 8% 28% 35% 13% 2	642
	21st to end	Night	15% 5% 25% 36% 16% 1	880
September	1st to 10th	Night	16% 6% 30% 33% 14% 1	800
	11th to 20th	Night	16% 7% 27% 29% 20% 2	800
	21st to end	Night	19% 9% 29% 29% 14% 2	800
October	1st to 10th	Night	25% 10% 24% 27% 13% 1	800
	11th to 20th	Night	21% 12% 32% 25% 10% 1	800
	21st to end	Night	27% 15% 30% 23% 5%	880
November	1st to 10th	Night	20% 17% 31% 25% 7% 1	820
	11th to 20th	Night	24% 19% 30% 20% 6% 1	840
	21st to end	Night	26% 17% 31% 20% 5%	840
December	1st to 10th	Night	26% 20% 35% 16% 3	840
	11th to 20th	Night	28% 20% 32% 18% 3	840
	21st to end	Night	29% 19% 31% 18% 2	924

Page generated at 2026-08-12 03:37:13. Version ac07

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