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 2024-01-13.

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	14% 11% 5% 8% 7% 3 4 4 5% 6% 32%	74683
	Night	14% 12% 6% 10% 7% 4 4 4 5% 6% 28%	75050
Transparency	Day	53% 7% 11% 19% 10% 1	74384
	Night	57% 6% 11% 17% 8% 1	74870
Seeing	Day	20% 17% 32% 21% 8% 2	14689
	Night	20% 14% 31% 25% 9% 1	24762

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	13% 18% 12% 17% 12% 5% 5% 3 3 3 9%	1517
	11th to 20th	Day	16% 19% 11% 18% 11% 5% 4 3 3 3 7%	1461
	21st to end	Day	12% 19% 12% 18% 13% 5% 6% 4 2 2 8%	1584
February	1st to 10th	Day	10% 16% 9% 13% 19% 5% 5% 3 4 4 11%	1553
	11th to 20th	Day	11% 14% 8% 17% 13% 6% 5% 5% 4 3 15%	1748
	21st to end	Day	13% 16% 9% 14% 10% 5% 5% 3 4 4 17%	1480
March	1st to 10th	Day	12% 14% 5% 11% 10% 4 5% 4 5% 5% 26%	1800
	11th to 20th	Day	12% 11% 6% 8% 7% 4 5% 5% 5% 5% 31%	1906
	21st to end	Day	17% 12% 5% 8% 6% 3 5% 5% 4 6% 29%	2282
April	1st to 10th	Day	19% 15% 5% 7% 7% 3 3 4 5% 6% 27%	2160
	11th to 20th	Day	20% 14% 3 5% 5% 3 4 4 4 5% 33%	2199
	21st to end	Day	15% 12% 4 7% 5% 3 5% 5% 5% 8% 32%	2340
May	1st to 10th	Day	16% 8% 3 5% 5% 3 5% 5% 5% 6% 39%	2340
	11th to 20th	Day	16% 8% 2 5% 4 3 4 4 6% 6% 40%	2397
	21st to end	Day	11% 6% 3 5% 4 4 5% 6% 6% 8% 43%	2725
June	1st to 10th	Day	13% 5% 2 4 5% 2 5% 5% 7% 7% 45%	2520
	11th to 20th	Day	11% 6% 2 6% 6% 3 5% 5% 5% 9% 42%	2520
	21st to end	Day	14% 5% 3 5% 4 3 4 5% 7% 8% 43%	2520
July	1st to 10th	Day	8% 3 1 3 5% 2 4 4 7% 10% 53%	2520
	11th to 20th	Day	12% 4 2 4 5% 2 4 5% 6% 9% 47%	2520
	21st to end	Day	9% 4 2 5% 5% 2 5% 4 6% 10% 48%	2743
August	1st to 10th	Day	9% 5% 3 5% 5% 4 5% 5% 6% 10% 44%	2427
	11th to 20th	Day	12% 6% 2 5% 5% 3 4 5% 6% 7% 46%	2303
	21st to end	Day	14% 6% 3 6% 4 3 4 4 5% 7% 45%	2430
September	1st to 10th	Day	15% 5% 3 5% 5% 3 5% 5% 6% 6% 42%	2154
	11th to 20th	Day	14% 5% 2 4 4 2 5% 5% 6% 7% 45%	1980
	21st to end	Day	16% 5% 3 7% 6% 3 4 5% 7% 7% 38%	1980
October	1st to 10th	Day	17% 8% 3 7% 7% 3 4 4 5% 7% 37%	1980
	11th to 20th	Day	23% 14% 4 8% 7% 3 5% 4 4 6% 20%	1704
	21st to end	Day	24% 16% 5% 9% 7% 4 4 4 4 6% 18%	1782
November	1st to 10th	Day	18% 16% 4 6% 5% 2 5% 4 5% 6% 27%	1672
	11th to 20th	Day	14% 21% 8% 11% 7% 4 4 4 5% 6% 17%	1710
	21st to end	Day	17% 23% 11% 11% 6% 3 4 3 3 4 15%	1652
December	1st to 10th	Day	13% 24% 14% 19% 7% 3 3 3 3 3 9%	1426
	11th to 20th	Day	14% 22% 10% 15% 10% 4 4 3 4 4 9%	1865
	21st to end	Day	19% 22% 11% 13% 9% 4 4 3 3 3 8%	2783
January	1st to 10th	Night	14% 18% 11% 16% 11% 6% 4 4 3 2 11%	2648
	11th to 20th	Night	14% 19% 10% 15% 11% 5% 4 3 3 4 11%	2561
	21st to end	Night	11% 18% 9% 16% 14% 6% 6% 4 4 4 9%	2772
February	1st to 10th	Night	11% 15% 8% 16% 16% 6% 6% 3 3 4 13%	2405
	11th to 20th	Night	11% 14% 7% 15% 12% 5% 5% 4 5% 5% 17%	2098
	21st to end	Night	16% 15% 7% 13% 10% 3 5% 3 4 5% 19%	1776
March	1st to 10th	Night	13% 11% 6% 9% 7% 4 4 4 6% 6% 31%	2160
	11th to 20th	Night	12% 10% 5% 9% 6% 3 4 5% 5% 6% 34%	2052
	21st to end	Night	17% 9% 4 9% 6% 3 4 4 5% 6% 34%	2088
April	1st to 10th	Night	17% 12% 5% 8% 5% 3 4 5% 5% 6% 30%	1800
	11th to 20th	Night	22% 10% 4 6% 5% 2 4 4 5% 6% 34%	1710
	21st to end	Night	16% 10% 4 6% 5% 3 4 5% 5% 6% 38%	1620
May	1st to 10th	Night	16% 6% 3 6% 7% 3 5% 5% 7% 8% 34%	1620
	11th to 20th	Night	15% 10% 3 7% 6% 3 5% 5% 5% 7% 35%	1496
	21st to end	Night	13% 5% 2 5% 4 3 3 5% 6% 9% 45%	1551
June	1st to 10th	Night	14% 6% 2 5% 5% 4 4 4 5% 8% 43%	1440
	11th to 20th	Night	11% 5% 2 6% 6% 3 5% 5% 6% 9% 42%	1440
	21st to end	Night	10% 5% 3 5% 6% 3 4 5% 6% 8% 46%	1440
July	1st to 10th	Night	9% 4 1 3 4 3 3 5% 6% 9% 54%	1440
	11th to 20th	Night	8% 4 2 4 5% 3 6% 6% 5% 8% 51%	1440
	21st to end	Night	8% 5% 2 5% 5% 3 4 5% 6% 9% 48%	1575
August	1st to 10th	Night	8% 5% 3 6% 5% 3 5% 5% 7% 9% 46%	1510
	11th to 20th	Night	9% 4 2 5% 5% 3 5% 4 5% 8% 52%	1598
	21st to end	Night	11% 7% 2 6% 6% 3 4 3 5% 7% 46%	1921
September	1st to 10th	Night	12% 5% 3 5% 5% 2 3 4 5% 7% 48%	1800
	11th to 20th	Night	13% 5% 2 5% 4 3 4 4 4 7% 49%	1967
	21st to end	Night	16% 7% 3 6% 5% 3 4 4 5% 7% 39%	1980
October	1st to 10th	Night	17% 8% 2 6% 5% 2 5% 4 5% 5% 40%	1980
	11th to 20th	Night	23% 13% 3 8% 6% 3 4 4 4 6% 25%	2252
	21st to end	Night	24% 14% 5% 8% 6% 3 4 4 4 5% 24%	2574
November	1st to 10th	Night	14% 15% 4 9% 7% 4 4 4 5% 6% 29%	2406
	11th to 20th	Night	14% 19% 7% 11% 8% 4 5% 5% 4 5% 19%	2470
	21st to end	Night	16% 19% 10% 10% 7% 4 4 4 4 6% 16%	2527
December	1st to 10th	Night	14% 21% 11% 17% 8% 4 5% 3 3 3 11%	2736
	11th to 20th	Night	13% 21% 10% 19% 10% 4 4 3 4 4 10%	3313
	21st to end	Night	20% 23% 10% 14% 6% 3 3 4 3 4 10%	4884

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	81% 4 7% 6% 1	1518
	11th to 20th	Day	84% 5% 4 5% 2	1461
	21st to end	Day	85% 4 5% 5% 2	1584
February	1st to 10th	Day	77% 5% 8% 9% 1	1553
	11th to 20th	Day	74% 5% 8% 9% 4	1758
	21st to end	Day	70% 5% 10% 12% 3 1	1480
March	1st to 10th	Day	60% 6% 11% 15% 7% 1	1800
	11th to 20th	Day	53% 6% 12% 17% 9% 2	1906
	21st to end	Day	56% 6% 11% 19% 7% 1	2282
April	1st to 10th	Day	57% 7% 9% 17% 10%	2160
	11th to 20th	Day	53% 5% 8% 18% 15% 2	2199
	21st to end	Day	50% 7% 11% 15% 14% 2	2340
May	1st to 10th	Day	45% 6% 10% 25% 12% 2	2340
	11th to 20th	Day	43% 7% 11% 21% 16% 3	2401
	21st to end	Day	38% 8% 13% 25% 14% 2	2725
June	1st to 10th	Day	37% 7% 11% 27% 16% 2	2520
	11th to 20th	Day	38% 8% 12% 22% 19% 1	2520
	21st to end	Day	37% 8% 15% 24% 15% 1	2520
July	1st to 10th	Day	27% 7% 14% 31% 19% 2	2451
	11th to 20th	Day	32% 8% 13% 29% 16% 2	2445
	21st to end	Day	32% 8% 13% 26% 19% 3	2591
August	1st to 10th	Day	34% 8% 15% 30% 12% 1	2412
	11th to 20th	Day	36% 7% 13% 28% 14% 2	2303
	21st to end	Day	38% 7% 13% 24% 16% 2	2430
September	1st to 10th	Day	41% 7% 13% 24% 14% 2	2154
	11th to 20th	Day	36% 8% 14% 30% 11% 1	1980
	21st to end	Day	44% 8% 15% 24% 9%	1980
October	1st to 10th	Day	48% 7% 12% 23% 11% 1	1980
	11th to 20th	Day	66% 6% 10% 15% 3	1704
	21st to end	Day	68% 7% 11% 10% 4	1782
November	1st to 10th	Day	56% 7% 10% 19% 8%	1672
	11th to 20th	Day	70% 5% 11% 10% 4	1710
	21st to end	Day	74% 6% 11% 8% 2	1652
December	1st to 10th	Day	83% 4 5% 7% 1	1426
	11th to 20th	Day	79% 6% 9% 5% 1	1862
	21st to end	Day	82% 4 7% 6% 1	2783
January	1st to 10th	Night	80% 4 8% 7% 1	2648
	11th to 20th	Night	79% 5% 7% 7% 2	2561
	21st to end	Night	81% 5% 8% 6% 1	2772
February	1st to 10th	Night	77% 4 6% 11% 2	2405
	11th to 20th	Night	71% 5% 9% 12% 3	2099
	21st to end	Night	69% 4 11% 12% 4	1776
March	1st to 10th	Night	53% 6% 13% 19% 8%	2160
	11th to 20th	Night	50% 7% 11% 20% 11% 2	2052
	21st to end	Night	52% 5% 10% 22% 10% 1	2088
April	1st to 10th	Night	55% 5% 9% 20% 10% 1	1800
	11th to 20th	Night	52% 5% 8% 20% 13% 3	1710
	21st to end	Night	47% 5% 11% 20% 14% 3	1620
May	1st to 10th	Night	46% 8% 11% 21% 11% 3	1620
	11th to 20th	Night	48% 5% 9% 19% 15% 3	1497
	21st to end	Night	36% 6% 15% 26% 15% 2	1552
June	1st to 10th	Night	39% 6% 11% 27% 15% 2	1440
	11th to 20th	Night	37% 7% 12% 25% 17% 2	1440
	21st to end	Night	37% 7% 12% 27% 16% 2	1440
July	1st to 10th	Night	27% 6% 14% 31% 20% 3	1400
	11th to 20th	Night	30% 6% 16% 26% 21% 2	1392
	21st to end	Night	32% 8% 14% 29% 16% 1	1487
August	1st to 10th	Night	34% 8% 16% 29% 12% 2	1501
	11th to 20th	Night	30% 7% 15% 31% 16% 1	1598
	21st to end	Night	37% 6% 14% 24% 17% 1	1921
September	1st to 10th	Night	35% 6% 13% 25% 20% 2	1800
	11th to 20th	Night	34% 7% 15% 30% 13% 1	1967
	21st to end	Night	44% 6% 15% 26% 9% 1	1980
October	1st to 10th	Night	46% 6% 10% 25% 13% 1	1980
	11th to 20th	Night	60% 4 13% 18% 5% 1	2252
	21st to end	Night	62% 5% 13% 14% 5%	2574
November	1st to 10th	Night	56% 6% 12% 18% 7% 1	2406
	11th to 20th	Night	69% 6% 11% 10% 4 1	2470
	21st to end	Night	70% 6% 13% 9% 1	2527
December	1st to 10th	Night	79% 5% 7% 7% 1	2736
	11th to 20th	Night	79% 6% 9% 5% 2	3315
	21st to end	Night	79% 6% 10% 5% 1	4884

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% 25% 36% 12% 6% 1	270
	11th to 20th	Day	28% 23% 34% 12% 2	248
	21st to end	Day	26% 19% 37% 17% 2	264
February	1st to 10th	Day	23% 31% 33% 10% 3	240
	11th to 20th	Day	22% 31% 37% 8% 1	240
	21st to end	Day	32% 31% 34% 3 1	198
March	1st to 10th	Day	25% 33% 32% 10% 1	240
	11th to 20th	Day	26% 18% 34% 14% 8%	346
	21st to end	Day	30% 18% 26% 19% 5% 1	462
April	1st to 10th	Day	27% 20% 35% 14% 3 1	420
	11th to 20th	Day	29% 22% 26% 14% 7% 2	412
	21st to end	Day	24% 21% 26% 21% 6% 2	420
May	1st to 10th	Day	22% 22% 33% 15% 6% 1	423
	11th to 20th	Day	17% 17% 35% 22% 7% 3	543
	21st to end	Day	13% 15% 34% 26% 10% 2	679
June	1st to 10th	Day	14% 15% 37% 23% 10% 2	630
	11th to 20th	Day	15% 12% 40% 24% 7% 2	630
	21st to end	Day	17% 10% 34% 27% 9% 2	630
July	1st to 10th	Day	9% 9% 32% 32% 14% 4	621
	11th to 20th	Day	12% 12% 30% 30% 12% 4	618
	21st to end	Day	9% 8% 36% 31% 12% 4	684
August	1st to 10th	Day	11% 7% 30% 29% 17% 6%	546
	11th to 20th	Day	15% 7% 24% 29% 16% 9%	439
	21st to end	Day	17% 11% 25% 29% 15% 3	495
September	1st to 10th	Day	20% 9% 28% 25% 13% 6%	450
	11th to 20th	Day	11% 10% 28% 31% 16% 4	450
	21st to end	Day	18% 15% 24% 27% 11% 5%	450
October	1st to 10th	Day	21% 17% 27% 22% 12% 1	450
	11th to 20th	Day	33% 20% 31% 14% 2	317
	21st to end	Day	38% 22% 27% 9% 4	297
November	1st to 10th	Day	28% 21% 31% 17% 3	270
	11th to 20th	Day	25% 29% 36% 8% 2	270
	21st to end	Day	29% 29% 32% 9% 2	270
December	1st to 10th	Day	32% 25% 27% 13% 2	225
	11th to 20th	Day	28% 30% 29% 9% 3	245
	21st to end	Day	32% 28% 31% 8% 1 1	297
January	1st to 10th	Night	22% 21% 33% 19% 4	950
	11th to 20th	Night	24% 20% 33% 17% 5%	915
	21st to end	Night	20% 17% 37% 21% 5%	990
February	1st to 10th	Night	19% 19% 37% 21% 4 1	818
	11th to 20th	Night	21% 22% 31% 23% 4	703
	21st to end	Night	25% 21% 33% 19% 3	592
March	1st to 10th	Night	15% 18% 35% 24% 6% 1	720
	11th to 20th	Night	19% 18% 35% 23% 4 1	720
	21st to end	Night	22% 13% 33% 25% 6% 1	702
April	1st to 10th	Night	23% 16% 33% 22% 5% 1	540
	11th to 20th	Night	24% 15% 32% 22% 7%	533
	21st to end	Night	21% 11% 34% 27% 6% 1	540
May	1st to 10th	Night	19% 11% 33% 30% 6% 1	540
	11th to 20th	Night	18% 11% 33% 28% 9%	531
	21st to end	Night	13% 9% 31% 33% 14%	584
June	1st to 10th	Night	15% 7% 33% 35% 10% 1	540
	11th to 20th	Night	13% 7% 29% 36% 12% 2	540
	21st to end	Night	11% 6% 29% 39% 14% 1	540
July	1st to 10th	Night	11% 2 23% 40% 22% 2	525
	11th to 20th	Night	9% 4 25% 38% 23% 1	522
	21st to end	Night	10% 4 25% 40% 20% 2	591
August	1st to 10th	Night	9% 5% 20% 44% 21% 1	537
	11th to 20th	Night	11% 6% 21% 33% 26% 3	534
	21st to end	Night	15% 7% 23% 38% 15% 2	594
September	1st to 10th	Night	13% 8% 25% 32% 19% 4	540
	11th to 20th	Night	14% 9% 24% 32% 19% 2	707
	21st to end	Night	19% 10% 23% 29% 15% 2	720
October	1st to 10th	Night	22% 12% 26% 27% 13% 1	720
	11th to 20th	Night	29% 17% 26% 22% 5%	720
	21st to end	Night	31% 17% 27% 19% 5% 1	792
November	1st to 10th	Night	26% 14% 34% 20% 5%	740
	11th to 20th	Night	25% 22% 34% 17% 2	760
	21st to end	Night	27% 20% 31% 18% 3	817
December	1st to 10th	Night	27% 22% 34% 15% 3	950
	11th to 20th	Night	25% 23% 33% 15% 3 1	950
	21st to end	Night	29% 19% 36% 14% 2	1045

Page generated at 2024-01-10 19:35:25. Version ac07

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