Northhills Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Northhills Observatory clear sky chart from 2005-11-06 to 2025-12-18.

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% 11% 5% 10% 6% 4 4 3 5% 6% 32%	126869
	Night	13% 13% 5% 12% 6% 5% 4 3 4 5% 28%	124363
Transparency	Day	52% 6% 10% 20% 11% 1	130369
	Night	56% 6% 11% 17% 9% 1	128937
Seeing	Day	21% 17% 32% 20% 8% 2	17377
	Night	20% 15% 30% 25% 9% 1	27357

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	9% 21% 13% 32% 6% 4 2 2 2 2 6%	3617
	11th to 20th	Day	10% 16% 8% 26% 15% 10% 6% 2 2 1 3	3620
	21st to end	Day	12% 18% 11% 20% 10% 5% 7% 3 3 3 10%	3982
February	1st to 10th	Day	9% 19% 7% 16% 16% 6% 4 2 4 4 13%	1725
	11th to 20th	Day	10% 16% 7% 19% 11% 7% 5% 3 4 3 14%	1948
	21st to end	Day	12% 19% 6% 16% 8% 6% 5% 3 4 4 18%	1650
March	1st to 10th	Day	11% 12% 4 12% 8% 5% 5% 4 4 5% 31%	4480
	11th to 20th	Day	11% 10% 6% 14% 5% 5% 4 4 6% 7% 28%	4794
	21st to end	Day	22% 13% 8% 15% 4 4 3 2 3 4 21%	5733
April	1st to 10th	Day	17% 16% 4 8% 6% 4 3 3 5% 6% 28%	2400
	11th to 20th	Day	20% 15% 3 6% 4 4 3 3 4 5% 33%	2447
	21st to end	Day	15% 12% 3 8% 4 5% 4 4 5% 8% 32%	2600
May	1st to 10th	Day	15% 9% 2 6% 4 4 4 3 5% 6% 42%	5862
	11th to 20th	Day	12% 10% 3 7% 4 4 4 3 5% 6% 43%	6125
	21st to end	Day	14% 8% 3 6% 4 4 4 4 5% 7% 41%	6935
June	1st to 10th	Day	13% 6% 2 5% 4 4 4 4 7% 7% 44%	2800
	11th to 20th	Day	12% 6% 2 7% 5% 4 5% 4 6% 9% 40%	2800
	21st to end	Day	14% 6% 2 6% 4 4 4 4 7% 8% 41%	2800
July	1st to 10th	Day	15% 5% 2 5% 4 3 4 4 7% 10% 42%	6360
	11th to 20th	Day	10% 4 2 5% 3 4 4 4 6% 8% 50%	6360
	21st to end	Day	9% 5% 2 6% 4 3 5% 4 7% 10% 46%	6924
August	1st to 10th	Day	9% 5% 3 7% 5% 5% 5% 4 7% 9% 41%	4408
	11th to 20th	Day	14% 6% 2 5% 4 3 3 3 5% 6% 48%	4174
	21st to end	Day	13% 6% 3 6% 5% 4 4 4 5% 7% 44%	4411
September	1st to 10th	Day	14% 5% 3 6% 4 5% 5% 4 6% 7% 41%	2392
	11th to 20th	Day	13% 6% 2 5% 3 3 5% 4 7% 7% 46%	2200
	21st to end	Day	15% 6% 3 8% 5% 4 4 4 6% 6% 39%	2200
October	1st to 10th	Day	16% 7% 3 7% 6% 4 4 3 5% 6% 39%	2200
	11th to 20th	Day	22% 14% 4 10% 6% 5% 5% 3 4 6% 22%	1892
	21st to end	Day	23% 16% 4 10% 6% 5% 4 3 4 5% 20%	1980
November	1st to 10th	Day	17% 17% 4 7% 5% 4 5% 3 5% 6% 27%	1852
	11th to 20th	Day	14% 22% 7% 12% 6% 5% 4 3 5% 6% 17%	1890
	21st to end	Day	16% 25% 10% 13% 5% 4 3 2 3 4 15%	1825
December	1st to 10th	Day	9% 31% 13% 19% 7% 4 4 2 3 2 6%	2521
	11th to 20th	Day	11% 25% 13% 19% 10% 5% 3 2 3 3 6%	2980
	21st to end	Day	19% 22% 9% 15% 8% 5% 4 2 3 3 8%	3982
January	1st to 10th	Night	10% 20% 10% 28% 6% 5% 4 3 3 2 8%	6348
	11th to 20th	Night	9% 17% 8% 23% 15% 12% 5% 2 3 2 5%	6360
	21st to end	Night	11% 20% 11% 19% 9% 6% 6% 3 3 3 10%	6996
February	1st to 10th	Night	10% 17% 7% 18% 14% 7% 5% 2 3 4 14%	2672
	11th to 20th	Night	11% 16% 7% 17% 10% 6% 4 4 5% 5% 16%	2338
	21st to end	Night	14% 16% 6% 15% 8% 5% 4 3 4 6% 20%	1980
March	1st to 10th	Night	15% 10% 4 11% 5% 4 3 3 5% 6% 34%	5373
	11th to 20th	Night	11% 11% 4 9% 5% 4 4 4 6% 7% 34%	5164
	21st to end	Night	19% 12% 5% 11% 5% 4 4 3 4 5% 28%	5238
April	1st to 10th	Night	17% 13% 3 9% 4 4 4 4 5% 6% 31%	2000
	11th to 20th	Night	21% 11% 4 7% 4 3 3 2 4 5% 34%	1902
	21st to end	Night	15% 11% 4 7% 4 4 3 4 4 5% 38%	1800
May	1st to 10th	Night	15% 8% 3 7% 4 4 4 3 6% 7% 40%	4033
	11th to 20th	Night	12% 10% 3 7% 5% 4 4 4 5% 6% 40%	3780
	21st to end	Night	19% 7% 3 5% 3 3 3 3 4 6% 43%	3942
June	1st to 10th	Night	14% 7% 2 6% 5% 5% 4 4 5% 7% 43%	1600
	11th to 20th	Night	11% 5% 2 7% 5% 4 5% 3 7% 8% 41%	1600
	21st to end	Night	11% 6% 3 7% 5% 4 4 4 6% 7% 44%	1600
July	1st to 10th	Night	13% 8% 2 5% 4 4 4 4 5% 8% 43%	3618
	11th to 20th	Night	8% 6% 2 4 3 4 4 3 5% 6% 54%	3617
	21st to end	Night	7% 6% 2 6% 4 4 4 3 6% 7% 50%	3947
August	1st to 10th	Night	12% 6% 3 8% 4 4 5% 4 6% 7% 42%	2745
	11th to 20th	Night	14% 6% 2 5% 3 3 3 3 4 7% 51%	2875
	21st to end	Night	9% 6% 2 7% 4 4 4 3 5% 7% 48%	3482
September	1st to 10th	Night	11% 6% 3 6% 4 4 4 3 5% 7% 49%	2000
	11th to 20th	Night	12% 5% 1 5% 3 3 4 3 5% 7% 50%	2187
	21st to end	Night	16% 7% 3 8% 4 4 4 4 5% 6% 40%	2200
October	1st to 10th	Night	16% 8% 2 7% 4 4 4 4 5% 5% 41%	2200
	11th to 20th	Night	21% 14% 3 9% 5% 4 4 3 4 5% 27%	2503
	21st to end	Night	22% 14% 4 10% 4 4 4 3 4 5% 26%	2860
November	1st to 10th	Night	14% 15% 4 9% 6% 4 4 3 5% 6% 29%	2666
	11th to 20th	Night	13% 20% 6% 13% 6% 5% 4 3 4 5% 20%	2730
	21st to end	Night	16% 22% 9% 12% 6% 5% 4 3 4 5% 15%	2793
December	1st to 10th	Night	10% 23% 12% 19% 7% 5% 4 2 4 4 10%	4870
	11th to 20th	Night	13% 24% 10% 19% 10% 5% 3 2 3 4 9%	5348
	21st to end	Night	19% 21% 9% 15% 5% 5% 4 3 3 4 12%	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	87% 3 5% 4 1	3618
	11th to 20th	Day	89% 5% 3 2 1	3620
	21st to end	Day	80% 6% 7% 6% 1	3982
February	1st to 10th	Day	75% 5% 8% 10% 2	1725
	11th to 20th	Day	75% 5% 8% 9% 3	1958
	21st to end	Day	68% 5% 11% 13% 3 1	1650
March	1st to 10th	Day	54% 6% 14% 16% 9% 1	4480
	11th to 20th	Day	54% 6% 15% 16% 9% 1	4794
	21st to end	Day	66% 5% 9% 13% 6% 1	5735
April	1st to 10th	Day	57% 7% 9% 18% 10%	2400
	11th to 20th	Day	52% 5% 8% 19% 14% 2	2447
	21st to end	Day	49% 7% 11% 16% 14% 2	2600
May	1st to 10th	Day	42% 6% 9% 22% 18% 3	5897
	11th to 20th	Day	42% 6% 8% 24% 18% 2	6129
	21st to end	Day	42% 6% 9% 21% 19% 3	6949
June	1st to 10th	Day	37% 7% 10% 27% 16% 2	2800
	11th to 20th	Day	39% 7% 12% 22% 18% 1	2800
	21st to end	Day	38% 7% 14% 24% 15% 1	2800
July	1st to 10th	Day	36% 7% 14% 27% 15% 1	6291
	11th to 20th	Day	29% 7% 13% 34% 16% 1	6285
	21st to end	Day	31% 8% 14% 28% 17% 1	6728
August	1st to 10th	Day	37% 7% 14% 28% 14% 1	4393
	11th to 20th	Day	36% 6% 13% 28% 16% 1	4213
	21st to end	Day	38% 8% 13% 29% 12% 1	4410
September	1st to 10th	Day	40% 7% 14% 24% 13% 1	2392
	11th to 20th	Day	34% 8% 14% 29% 14% 1	2200
	21st to end	Day	43% 8% 14% 24% 11%	2200
October	1st to 10th	Day	32% 5% 9% 33% 21% 1	3590
	11th to 20th	Day	53% 6% 10% 20% 10%	3056
	21st to end	Day	61% 6% 13% 15% 4	3180
November	1st to 10th	Day	56% 7% 10% 19% 8%	1852
	11th to 20th	Day	69% 5% 11% 10% 4	1890
	21st to end	Day	75% 6% 10% 8% 2	1825
December	1st to 10th	Day	85% 5% 5% 4 1	2521
	11th to 20th	Day	85% 5% 6% 3 1	2977
	21st to end	Day	80% 6% 6% 7% 2	3982
January	1st to 10th	Night	80% 5% 7% 6% 2	6348
	11th to 20th	Night	86% 4 5% 3 1	6360
	21st to end	Night	80% 6% 7% 7% 1	6996
February	1st to 10th	Night	76% 4 6% 10% 3	2672
	11th to 20th	Night	71% 6% 9% 11% 3	2339
	21st to end	Night	66% 5% 12% 13% 4	1980
March	1st to 10th	Night	51% 6% 17% 19% 8%	5373
	11th to 20th	Night	47% 6% 18% 20% 8% 1	5164
	21st to end	Night	58% 5% 10% 18% 8%	5238
April	1st to 10th	Night	54% 5% 9% 21% 10% 1	2000
	11th to 20th	Night	52% 5% 7% 20% 13% 2	1902
	21st to end	Night	48% 5% 10% 20% 14% 3	1800
May	1st to 10th	Night	42% 6% 12% 19% 18% 3	4060
	11th to 20th	Night	43% 8% 7% 24% 16% 1	3781
	21st to end	Night	42% 5% 10% 23% 15% 5%	3950
June	1st to 10th	Night	39% 6% 11% 26% 16% 1	1600
	11th to 20th	Night	39% 7% 12% 24% 17% 2	1600
	21st to end	Night	40% 6% 12% 25% 15% 2	1600
July	1st to 10th	Night	39% 5% 12% 26% 14% 4	3580
	11th to 20th	Night	30% 6% 13% 30% 19% 2	3572
	21st to end	Night	31% 7% 15% 27% 18% 2	3832
August	1st to 10th	Night	39% 8% 14% 25% 13% 1	2736
	11th to 20th	Night	34% 5% 14% 31% 15% 1	2902
	21st to end	Night	35% 7% 14% 23% 20% 2	3482
September	1st to 10th	Night	35% 6% 14% 25% 19% 2	2000
	11th to 20th	Night	32% 7% 14% 31% 15% 1	2187
	21st to end	Night	43% 6% 14% 25% 11% 1	2200
October	1st to 10th	Night	37% 5% 9% 35% 12%	3590
	11th to 20th	Night	50% 4 10% 24% 12%	4078
	21st to end	Night	51% 5% 14% 23% 7%	4610
November	1st to 10th	Night	56% 6% 12% 18% 8% 1	2666
	11th to 20th	Night	67% 6% 11% 10% 5% 1	2730
	21st to end	Night	72% 6% 12% 9% 1	2793
December	1st to 10th	Night	77% 8% 8% 7% 1	4870
	11th to 20th	Night	81% 5% 7% 6% 1	5350
	21st to end	Night	74% 6% 10% 7% 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	20% 26% 37% 12% 5% 1	300
	11th to 20th	Day	28% 25% 35% 11% 2	300
	21st to end	Day	28% 20% 36% 14% 1	330
February	1st to 10th	Day	23% 32% 29% 11% 3 1	300
	11th to 20th	Day	24% 30% 35% 10% 1	300
	21st to end	Day	30% 30% 35% 4 1	249
March	1st to 10th	Day	27% 30% 33% 9% 1	300
	11th to 20th	Day	29% 19% 31% 13% 7%	418
	21st to end	Day	31% 19% 27% 17% 4 1	550
April	1st to 10th	Day	29% 21% 33% 13% 3 1	500
	11th to 20th	Day	31% 22% 27% 12% 6% 2	492
	21st to end	Day	25% 21% 28% 20% 6% 1	500
May	1st to 10th	Day	21% 22% 36% 15% 6% 1	503
	11th to 20th	Day	18% 17% 33% 23% 7% 2	635
	21st to end	Day	15% 16% 34% 24% 9% 2	789
June	1st to 10th	Day	16% 15% 38% 21% 9% 2	730
	11th to 20th	Day	16% 12% 41% 22% 6% 2	730
	21st to end	Day	19% 12% 34% 25% 8% 1	730
July	1st to 10th	Day	10% 10% 34% 31% 13% 3	721
	11th to 20th	Day	12% 12% 31% 29% 13% 3	718
	21st to end	Day	9% 8% 36% 31% 11% 4	794
August	1st to 10th	Day	10% 8% 29% 30% 17% 6%	636
	11th to 20th	Day	16% 8% 25% 29% 15% 8%	519
	21st to end	Day	17% 11% 25% 28% 15% 4	583
September	1st to 10th	Day	20% 10% 27% 25% 13% 6%	530
	11th to 20th	Day	10% 9% 26% 31% 19% 5%	530
	21st to end	Day	17% 13% 25% 29% 12% 5%	530
October	1st to 10th	Day	20% 18% 28% 22% 12% 1	530
	11th to 20th	Day	31% 19% 33% 16% 2	381
	21st to end	Day	35% 22% 29% 10% 4	363
November	1st to 10th	Day	30% 22% 29% 15% 3	330
	11th to 20th	Day	28% 27% 34% 8% 3	330
	21st to end	Day	31% 27% 32% 9% 2	330
December	1st to 10th	Day	30% 28% 26% 14% 3	275
	11th to 20th	Day	28% 30% 30% 9% 2	291
	21st to end	Day	34% 25% 29% 9% 1 1	330
January	1st to 10th	Night	22% 22% 34% 19% 4	1000
	11th to 20th	Night	23% 21% 33% 17% 5%	1000
	21st to end	Night	20% 17% 36% 22% 5%	1100
February	1st to 10th	Night	20% 19% 36% 21% 4 1	909
	11th to 20th	Night	21% 22% 31% 23% 3	783
	21st to end	Night	23% 21% 33% 19% 3	660
March	1st to 10th	Night	16% 19% 36% 23% 6% 1	800
	11th to 20th	Night	20% 19% 35% 22% 4 1	800
	21st to end	Night	23% 14% 33% 24% 5% 1	780
April	1st to 10th	Night	24% 15% 34% 22% 4 1	600
	11th to 20th	Night	25% 15% 31% 21% 7%	593
	21st to end	Night	21% 13% 33% 26% 6% 1	600
May	1st to 10th	Night	18% 11% 34% 30% 6% 1	600
	11th to 20th	Night	17% 11% 33% 29% 9%	591
	21st to end	Night	15% 9% 30% 33% 13%	650
June	1st to 10th	Night	14% 8% 35% 34% 9% 1	600
	11th to 20th	Night	14% 9% 29% 35% 11% 2	600
	21st to end	Night	13% 7% 29% 37% 13% 1	600
July	1st to 10th	Night	12% 3 23% 41% 20% 2	585
	11th to 20th	Night	10% 4 25% 38% 22% 1	582
	21st to end	Night	9% 4 25% 40% 20% 2	657
August	1st to 10th	Night	9% 5% 20% 43% 21% 1	597
	11th to 20th	Night	12% 6% 20% 33% 25% 3	594
	21st to end	Night	15% 7% 22% 38% 17% 1	660
September	1st to 10th	Night	13% 8% 25% 32% 20% 3	600
	11th to 20th	Night	13% 9% 23% 34% 20% 2	787
	21st to end	Night	19% 10% 24% 30% 16% 2	800
October	1st to 10th	Night	21% 12% 27% 28% 12% 1	800
	11th to 20th	Night	28% 17% 27% 23% 5%	800
	21st to end	Night	29% 17% 27% 20% 6% 1	880
November	1st to 10th	Night	27% 15% 33% 20% 5%	820
	11th to 20th	Night	25% 22% 33% 17% 3	840
	21st to end	Night	28% 20% 31% 18% 2	903
December	1st to 10th	Night	26% 22% 33% 16% 3	1050
	11th to 20th	Night	25% 23% 33% 15% 3 1	1036
	21st to end	Night	29% 19% 36% 14% 2	1100

Page generated at 2025-12-15 01:13:51. Version ac07

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