Hilltop Homeschool Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Hilltop Homeschool Observatory clear sky chart from 2005-11-06 to 2026-01-17.

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% 8% 3 8% 5% 4 4 4 5% 7% 38%	127788
	Night	13% 9% 3 8% 4 4 4 3 5% 6% 41%	124170
Transparency	Day	44% 5% 12% 24% 14% 1	131354
	Night	43% 5% 11% 24% 15% 2	128680
Seeing	Day	16% 10% 30% 26% 14% 4	17463
	Night	16% 10% 31% 30% 11% 1	27400

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	7% 12% 6% 21% 8% 7% 5% 4 4 5% 21%	3697
	11th to 20th	Day	7% 9% 5% 11% 8% 7% 5% 5% 7% 6% 29%	3788
	21st to end	Day	12% 14% 5% 13% 5% 4 4 2 5% 6% 29%	4488
February	1st to 10th	Day	12% 12% 5% 9% 7% 5% 4 4 6% 7% 29%	1800
	11th to 20th	Day	14% 11% 3 7% 4 6% 5% 4 5% 7% 33%	1779
	21st to end	Day	13% 10% 4 10% 4 4 4 4 6% 7% 34%	1650
March	1st to 10th	Day	18% 10% 3 6% 3 2 2 2 3 5% 45%	4697
	11th to 20th	Day	10% 6% 2 6% 4 4 4 4 6% 7% 47%	4980
	21st to end	Day	21% 13% 4 10% 5% 4 4 3 5% 7% 25%	5642
April	1st to 10th	Day	19% 12% 3 8% 5% 5% 4 3 4 6% 32%	2400
	11th to 20th	Day	16% 10% 3 7% 5% 5% 5% 3 6% 7% 33%	2374
	21st to end	Day	22% 8% 3 6% 4 3 4 3 5% 7% 35%	2410
May	1st to 10th	Day	16% 7% 3 6% 5% 4 5% 4 6% 7% 37%	6087
	11th to 20th	Day	15% 6% 2 6% 4 4 4 3 5% 6% 45%	6319
	21st to end	Day	16% 6% 2 7% 5% 5% 5% 4 6% 7% 37%	6935
June	1st to 10th	Day	10% 6% 2 5% 4 5% 5% 5% 7% 9% 43%	2800
	11th to 20th	Day	11% 6% 2 6% 4 4 4 4 6% 7% 46%	2800
	21st to end	Day	11% 6% 2 7% 6% 4 5% 4 6% 9% 40%	2800
July	1st to 10th	Day	9% 7% 3 7% 5% 5% 5% 5% 7% 9% 39%	6360
	11th to 20th	Day	9% 6% 2 6% 4 5% 4 3 5% 8% 47%	6360
	21st to end	Day	10% 6% 3 7% 5% 5% 5% 5% 7% 10% 38%	6753
August	1st to 10th	Day	8% 5% 2 6% 5% 4 5% 4 7% 10% 45%	4234
	11th to 20th	Day	11% 6% 2 6% 4 4 5% 5% 7% 10% 41%	4091
	21st to end	Day	7% 5% 2 6% 4 4 4 4 5% 8% 50%	4312
September	1st to 10th	Day	12% 5% 2 5% 4 3 4 3 5% 8% 51%	2400
	11th to 20th	Day	10% 5% 1 6% 5% 4 4 4 6% 8% 49%	2400
	21st to end	Day	10% 5% 2 6% 4 4 5% 3 7% 8% 46%	2115
October	1st to 10th	Day	15% 6% 2 6% 3 4 4 2 4 6% 48%	2000
	11th to 20th	Day	17% 7% 3 6% 4 4 4 3 5% 6% 42%	2000
	21st to end	Day	16% 9% 2 6% 4 4 3 3 5% 6% 41%	2168
November	1st to 10th	Day	12% 8% 2 6% 5% 4 4 4 5% 7% 43%	1852
	11th to 20th	Day	14% 12% 4 8% 5% 4 4 4 5% 6% 35%	1769
	21st to end	Day	16% 10% 4 8% 4 4 3 3 5% 7% 38%	1680
December	1st to 10th	Day	8% 10% 3 8% 4 4 3 3 5% 8% 45%	2690
	11th to 20th	Day	14% 12% 5% 13% 7% 5% 5% 4 4 5% 25%	3088
	21st to end	Day	23% 22% 8% 11% 4 3 3 3 3 4 15%	4070
January	1st to 10th	Night	9% 11% 6% 16% 5% 5% 5% 3 4 5% 30%	6488
	11th to 20th	Night	10% 13% 5% 12% 7% 6% 5% 4 6% 5% 26%	6307
	21st to end	Night	13% 13% 6% 11% 5% 4 4 3 4 5% 32%	6490
February	1st to 10th	Night	12% 13% 5% 8% 6% 4 4 3 5% 5% 34%	2600
	11th to 20th	Night	11% 11% 3 8% 5% 5% 4 4 5% 6% 40%	2508
	21st to end	Night	14% 9% 3 9% 5% 4 3 3 4 5% 39%	1980
March	1st to 10th	Night	17% 7% 3 6% 3 3 3 2 3 5% 47%	5199
	11th to 20th	Night	9% 7% 2 6% 4 3 3 3 4 7% 52%	4980
	21st to end	Night	20% 12% 5% 10% 5% 5% 4 3 4 5% 27%	5285
April	1st to 10th	Night	16% 9% 4 7% 5% 3 3 3 5% 5% 42%	2000
	11th to 20th	Night	16% 9% 3 7% 5% 3 3 3 5% 6% 40%	1975
	21st to end	Night	18% 7% 2 6% 4 4 4 3 6% 7% 38%	1985
May	1st to 10th	Night	16% 7% 2 6% 4 3 4 3 4 6% 44%	3828
	11th to 20th	Night	11% 6% 2 6% 4 5% 4 3 6% 7% 47%	3596
	21st to end	Night	14% 7% 2 7% 4 4 4 3 5% 7% 42%	3942
June	1st to 10th	Night	8% 5% 2 6% 4 4 4 4 8% 9% 48%	1600
	11th to 20th	Night	9% 5% 2 6% 4 4 5% 5% 9% 9% 44%	1600
	21st to end	Night	11% 4 2 6% 3 3 4 4 6% 8% 48%	1600
July	1st to 10th	Night	11% 5% 2 7% 4 4 5% 4 6% 8% 44%	3618
	11th to 20th	Night	7% 5% 2 5% 3 3 4 3 6% 8% 54%	3617
	21st to end	Night	9% 6% 2 6% 4 4 4 4 7% 8% 45%	4114
August	1st to 10th	Night	6% 3 2 5% 4 5% 4 4 6% 9% 53%	2926
	11th to 20th	Night	8% 4 2 6% 4 3 3 3 5% 7% 53%	2952
	21st to end	Night	8% 4 2 5% 4 4 4 4 5% 7% 54%	3586
September	1st to 10th	Night	10% 3 1 5% 3 3 4 3 5% 6% 57%	2000
	11th to 20th	Night	11% 4 2 6% 4 3 4 3 4 7% 52%	2000
	21st to end	Night	8% 5% 1 5% 4 3 4 4 4 7% 55%	2260
October	1st to 10th	Night	12% 5% 2 5% 4 3 4 2 4 6% 54%	2400
	11th to 20th	Night	14% 7% 2 5% 4 4 4 3 4 5% 48%	2400
	21st to end	Night	16% 9% 3 7% 4 3 4 3 5% 5% 42%	2670
November	1st to 10th	Night	12% 6% 2 5% 3 3 4 3 4 6% 52%	2666
	11th to 20th	Night	13% 10% 3 7% 5% 4 3 3 5% 6% 40%	2849
	21st to end	Night	15% 10% 3 8% 5% 3 4 3 4 5% 39%	2940
December	1st to 10th	Night	8% 8% 3 8% 5% 4 5% 4 4 6% 46%	4720
	11th to 20th	Night	15% 12% 4 10% 4 4 4 3 4 5% 33%	5339
	21st to end	Night	23% 17% 8% 12% 5% 4 4 3 3 4 16%	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	64% 6% 9% 15% 6%	3698
	11th to 20th	Day	54% 3 10% 16% 15% 2	3788
	21st to end	Day	57% 3 14% 18% 7%	4488
February	1st to 10th	Day	53% 4 15% 19% 7% 1	1800
	11th to 20th	Day	50% 6% 13% 21% 10% 2	1788
	21st to end	Day	48% 6% 14% 21% 9% 3	1650
March	1st to 10th	Day	43% 3 9% 27% 14% 3	4697
	11th to 20th	Day	35% 4 11% 33% 16% 1	4980
	21st to end	Day	60% 4 16% 14% 5% 1	5644
April	1st to 10th	Day	53% 5% 11% 22% 9%	2400
	11th to 20th	Day	49% 5% 11% 22% 11% 2	2374
	21st to end	Day	49% 5% 11% 21% 12% 2	2410
May	1st to 10th	Day	45% 6% 10% 20% 18% 1	6125
	11th to 20th	Day	39% 5% 11% 27% 15% 3	6323
	21st to end	Day	44% 6% 11% 24% 13% 1	6949
June	1st to 10th	Day	36% 7% 16% 25% 16% 2	2800
	11th to 20th	Day	36% 7% 11% 23% 21% 3	2800
	21st to end	Day	39% 7% 13% 24% 14% 2	2800
July	1st to 10th	Day	40% 6% 13% 29% 11% 1	6291
	11th to 20th	Day	36% 5% 13% 29% 15% 1	6285
	21st to end	Day	40% 7% 17% 21% 14% 1	6560
August	1st to 10th	Day	35% 6% 17% 28% 13% 1	4220
	11th to 20th	Day	38% 6% 16% 25% 15% 1	4130
	21st to end	Day	32% 5% 14% 30% 17% 2	4311
September	1st to 10th	Day	33% 4 11% 27% 22% 3	2400
	11th to 20th	Day	32% 6% 13% 30% 17% 2	2400
	21st to end	Day	35% 5% 11% 29% 18% 2	2115
October	1st to 10th	Day	24% 3 5% 30% 37% 1	3260
	11th to 20th	Day	30% 3 11% 25% 25% 6%	3240
	21st to end	Day	40% 3 9% 29% 14% 3	3482
November	1st to 10th	Day	40% 5% 13% 26% 14% 3	1852
	11th to 20th	Day	48% 3 13% 21% 13% 1	1769
	21st to end	Day	48% 5% 15% 24% 8%	1680
December	1st to 10th	Day	39% 3 13% 37% 6% 1	2690
	11th to 20th	Day	60% 4 12% 13% 10% 1	3085
	21st to end	Day	74% 4 9% 10% 3	4070
January	1st to 10th	Night	56% 5% 12% 18% 8% 1	6488
	11th to 20th	Night	58% 3 12% 12% 15% 1	6307
	21st to end	Night	55% 4 11% 19% 12% 1	6490
February	1st to 10th	Night	52% 4 11% 21% 11% 1	2600
	11th to 20th	Night	45% 5% 11% 25% 12% 2	2509
	21st to end	Night	46% 4 12% 21% 15% 2	1980
March	1st to 10th	Night	42% 3 8% 27% 19% 2	5199
	11th to 20th	Night	33% 4 9% 32% 19% 3	4980
	21st to end	Night	60% 5% 12% 14% 8% 1	5285
April	1st to 10th	Night	45% 5% 9% 26% 14% 2	2000
	11th to 20th	Night	45% 5% 8% 23% 15% 3	1975
	21st to end	Night	44% 5% 12% 22% 14% 2	1985
May	1st to 10th	Night	41% 5% 9% 23% 19% 3	3855
	11th to 20th	Night	36% 6% 13% 25% 17% 4	3597
	21st to end	Night	40% 6% 10% 28% 14% 2	3950
June	1st to 10th	Night	31% 7% 14% 27% 17% 3	1600
	11th to 20th	Night	32% 7% 14% 24% 19% 4	1600
	21st to end	Night	32% 7% 16% 23% 17% 5%	1600
July	1st to 10th	Night	36% 6% 13% 30% 14% 2	3580
	11th to 20th	Night	29% 4 15% 31% 19% 1	3572
	21st to end	Night	35% 7% 19% 23% 15% 1	3996
August	1st to 10th	Night	27% 7% 18% 32% 15% 2	2916
	11th to 20th	Night	29% 6% 15% 33% 16% 1	2979
	21st to end	Night	30% 5% 12% 30% 18% 5%	3586
September	1st to 10th	Night	28% 5% 12% 27% 25% 3	2000
	11th to 20th	Night	33% 4 10% 32% 19% 3	2000
	21st to end	Night	29% 4 10% 29% 25% 2	2260
October	1st to 10th	Night	23% 3 6% 29% 36% 3	3920
	11th to 20th	Night	28% 3 10% 22% 30% 7%	3899
	21st to end	Night	46% 3 8% 23% 16% 4	4306
November	1st to 10th	Night	34% 5% 14% 27% 17% 3	2666
	11th to 20th	Night	44% 4 13% 24% 13% 2	2849
	21st to end	Night	49% 4 11% 25% 11% 1	2940
December	1st to 10th	Night	38% 4 12% 35% 10%	4720
	11th to 20th	Night	54% 4 13% 23% 6% 1	5341
	21st to end	Night	73% 4 9% 10% 4	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	21% 12% 37% 21% 8% 2	330
	11th to 20th	Day	19% 18% 40% 19% 4 1	318
	21st to end	Day	22% 13% 38% 22% 4 1	330
February	1st to 10th	Day	21% 16% 40% 19% 4	300
	11th to 20th	Day	21% 19% 35% 19% 6%	300
	21st to end	Day	24% 14% 44% 14% 5%	249
March	1st to 10th	Day	18% 20% 42% 15% 4	300
	11th to 20th	Day	22% 19% 31% 20% 6% 2	300
	21st to end	Day	31% 15% 30% 17% 6%	407
April	1st to 10th	Day	26% 13% 37% 17% 7%	500
	11th to 20th	Day	23% 18% 28% 21% 9%	492
	21st to end	Day	26% 12% 35% 17% 9% 1	510
May	1st to 10th	Day	19% 12% 36% 23% 8% 2	703
	11th to 20th	Day	20% 14% 25% 25% 13% 2	725
	21st to end	Day	15% 10% 29% 27% 14% 5%	789
June	1st to 10th	Day	8% 5% 28% 33% 21% 5%	730
	11th to 20th	Day	13% 6% 29% 32% 17% 2	730
	21st to end	Day	11% 8% 33% 31% 12% 4	730
July	1st to 10th	Day	8% 4 19% 33% 24% 12%	721
	11th to 20th	Day	9% 3 24% 34% 22% 7%	718
	21st to end	Day	9% 2 23% 38% 23% 5%	794
August	1st to 10th	Day	7% 4 23% 36% 23% 7%	726
	11th to 20th	Day	11% 5% 20% 31% 24% 9%	674
	21st to end	Day	8% 4 18% 37% 25% 7%	583
September	1st to 10th	Day	12% 8% 20% 31% 22% 7%	530
	11th to 20th	Day	9% 4 22% 37% 22% 6%	530
	21st to end	Day	12% 9% 28% 25% 23% 3	408
October	1st to 10th	Day	17% 9% 27% 28% 14% 4	330
	11th to 20th	Day	22% 21% 34% 15% 8% 1	330
	21st to end	Day	25% 13% 35% 19% 5% 2	363
November	1st to 10th	Day	16% 12% 41% 20% 9% 2	330
	11th to 20th	Day	21% 19% 31% 22% 5% 2	330
	21st to end	Day	22% 14% 37% 21% 5%	330
December	1st to 10th	Day	14% 16% 42% 20% 6% 2	330
	11th to 20th	Day	18% 15% 38% 21% 7% 1	330
	21st to end	Day	30% 14% 38% 16% 2	363
January	1st to 10th	Night	15% 13% 37% 28% 7%	1050
	11th to 20th	Night	19% 16% 32% 27% 6%	1031
	21st to end	Night	19% 12% 36% 27% 6%	1100
February	1st to 10th	Night	19% 13% 39% 25% 5%	1000
	11th to 20th	Night	17% 13% 39% 27% 4	954
	21st to end	Night	20% 11% 42% 22% 4 1	660
March	1st to 10th	Night	20% 14% 38% 22% 7% 1	730
	11th to 20th	Night	17% 13% 34% 28% 8%	600
	21st to end	Night	22% 13% 36% 23% 4 1	660
April	1st to 10th	Night	20% 15% 38% 24% 3	600
	11th to 20th	Night	22% 15% 31% 25% 6%	593
	21st to end	Night	22% 8% 33% 27% 9% 1	600
May	1st to 10th	Night	18% 7% 39% 29% 6% 1	600
	11th to 20th	Night	17% 10% 28% 33% 11% 1	591
	21st to end	Night	15% 4 30% 34% 16% 1	650
June	1st to 10th	Night	9% 5% 27% 42% 17% 1	600
	11th to 20th	Night	10% 5% 29% 42% 15% 1	600
	21st to end	Night	13% 5% 29% 38% 15% 1	600
July	1st to 10th	Night	8% 1 15% 43% 30% 3	585
	11th to 20th	Night	6% 3 19% 43% 26% 3	582
	21st to end	Night	7% 3 19% 42% 27% 2	657
August	1st to 10th	Night	5% 3 17% 46% 28% 2	597
	11th to 20th	Night	7% 3 16% 43% 27% 4	594
	21st to end	Night	11% 2 15% 43% 27% 2	660
September	1st to 10th	Night	11% 4 20% 40% 22% 4	600
	11th to 20th	Night	11% 3 21% 36% 25% 4	600
	21st to end	Night	9% 7% 27% 35% 20% 1	742
October	1st to 10th	Night	14% 7% 29% 33% 17% 2	800
	11th to 20th	Night	19% 11% 33% 28% 9%	800
	21st to end	Night	22% 11% 33% 27% 7%	880
November	1st to 10th	Night	14% 10% 38% 29% 9%	820
	11th to 20th	Night	19% 14% 38% 22% 7%	959
	21st to end	Night	22% 12% 35% 25% 6%	1050
December	1st to 10th	Night	18% 15% 35% 26% 5% 1	1050
	11th to 20th	Night	22% 12% 33% 27% 5%	1050
	21st to end	Night	26% 13% 34% 23% 4	1155

Page generated at 2026-01-14 00:14:28. Version ac07

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