Howell Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Howell Observatory clear sky chart from 2005-11-06 to 2024-01-12.

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	11% 5% 2 5% 5% 3 5% 5% 6% 7% 44%	74890
	Night	12% 5% 2 5% 5% 3 4 4 5% 7% 47%	74808
Transparency	Day	38% 7% 11% 21% 19% 5%	74615
	Night	35% 6% 10% 20% 21% 7%	74604
Seeing	Day	13% 7% 19% 24% 23% 14%	13885
	Night	14% 6% 21% 31% 24% 5%	23694

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	20% 7% 2 4 4 2 3 3 4 6% 45%	1582
	11th to 20th	Day	19% 8% 3 7% 4 3 4 4 4 6% 39%	1634
	21st to end	Day	15% 6% 2 5% 4 3 4 4 4 5% 49%	1958
February	1st to 10th	Day	16% 8% 3 5% 5% 3 3 5% 5% 7% 41%	1800
	11th to 20th	Day	21% 8% 3 6% 5% 3 3 4 4 5% 40%	1748
	21st to end	Day	20% 7% 3 6% 6% 3 4 3 3 5% 40%	1480
March	1st to 10th	Day	14% 8% 3 7% 5% 4 4 3 5% 6% 42%	1800
	11th to 20th	Day	14% 6% 3 6% 6% 4 6% 5% 6% 7% 38%	1902
	21st to end	Day	13% 6% 3 6% 6% 3 4 5% 5% 7% 42%	2363
April	1st to 10th	Day	12% 4 2 5% 4 3 4 4 7% 8% 48%	2160
	11th to 20th	Day	14% 5% 2 5% 4 3 4 4 5% 7% 46%	2134
	21st to end	Day	9% 4 2 6% 5% 3 5% 4 5% 8% 49%	2160
May	1st to 10th	Day	13% 5% 2 5% 5% 3 5% 4 5% 7% 47%	2160
	11th to 20th	Day	10% 5% 2 6% 7% 4 6% 5% 6% 9% 42%	2271
	21st to end	Day	9% 4 2 5% 5% 4 6% 6% 7% 9% 42%	2531
June	1st to 10th	Day	8% 5% 3 6% 7% 3 7% 6% 7% 10% 39%	2378
	11th to 20th	Day	5% 3 2 5% 5% 4 6% 7% 8% 9% 46%	2520
	21st to end	Day	6% 3 2 4 6% 4 6% 8% 7% 11% 43%	2520
July	1st to 10th	Day	8% 5% 3 6% 7% 4 7% 6% 8% 9% 35%	2520
	11th to 20th	Day	8% 5% 2 6% 7% 5% 7% 7% 8% 11% 34%	2433
	21st to end	Day	5% 5% 3 6% 7% 4 8% 7% 8% 11% 36%	2355
August	1st to 10th	Day	5% 4 2 7% 7% 5% 8% 8% 8% 9% 38%	2148
	11th to 20th	Day	7% 6% 3 6% 7% 5% 7% 7% 8% 9% 35%	2125
	21st to end	Day	13% 4 2 5% 6% 4 6% 6% 6% 10% 39%	2376
September	1st to 10th	Day	10% 4 2 5% 6% 3 5% 6% 6% 7% 45%	2160
	11th to 20th	Day	11% 5% 2 6% 6% 4 6% 5% 5% 6% 46%	2042
	21st to end	Day	10% 5% 2 4 3 3 4 4 5% 7% 52%	1980
October	1st to 10th	Day	7% 4 3 6% 4 2 4 3 5% 5% 57%	1980
	11th to 20th	Day	10% 4 2 4 5% 3 4 3 4 6% 55%	1818
	21st to end	Day	13% 4 2 4 4 2 4 4 4 6% 53%	1980
November	1st to 10th	Day	9% 5% 2 4 3 2 4 4 4 5% 58%	1682
	11th to 20th	Day	11% 4 3 5% 4 4 3 4 3 5% 55%	1710
	21st to end	Day	15% 6% 2 4 5% 3 5% 3 4 7% 45%	1710
December	1st to 10th	Day	16% 8% 3 5% 5% 3 5% 4 4 5% 41%	1710
	11th to 20th	Day	14% 7% 4 7% 5% 3 4 4 4 5% 43%	2153
	21st to end	Day	13% 8% 4 7% 5% 3 4 4 5% 6% 41%	2907
January	1st to 10th	Night	18% 6% 3 5% 4 2 4 3 4 6% 46%	2580
	11th to 20th	Night	17% 8% 4 6% 5% 3 4 4 4 5% 40%	2366
	21st to end	Night	18% 7% 3 5% 5% 3 4 3 4 4 45%	2393
February	1st to 10th	Night	17% 7% 3 5% 5% 2 4 4 4 7% 42%	2160
	11th to 20th	Night	21% 7% 2 5% 4 2 4 3 3 4 44%	2098
	21st to end	Night	20% 7% 3 5% 5% 3 4 4 5% 6% 39%	1776
March	1st to 10th	Night	14% 7% 2 4 5% 3 4 4 4 7% 46%	2160
	11th to 20th	Night	15% 6% 3 5% 5% 4 4 5% 5% 7% 41%	2071
	21st to end	Night	14% 6% 3 5% 4 3 4 4 5% 6% 46%	1991
April	1st to 10th	Night	11% 5% 2 5% 4 3 4 5% 5% 7% 49%	1800
	11th to 20th	Night	11% 6% 2 5% 4 2 4 5% 4 8% 51%	1775
	21st to end	Night	11% 4 2 4 5% 3 4 4 5% 7% 51%	1800
May	1st to 10th	Night	10% 4 2 4 4 3 4 3 5% 8% 53%	1800
	11th to 20th	Night	10% 4 3 5% 5% 3 5% 5% 7% 10% 45%	1620
	21st to end	Night	7% 4 2 4 5% 3 6% 5% 7% 11% 47%	1744
June	1st to 10th	Night	7% 4 2 5% 6% 4 6% 5% 6% 10% 44%	1576
	11th to 20th	Night	4 2 2 3 4 3 5% 5% 7% 11% 55%	1440
	21st to end	Night	4 3 2 5% 5% 3 5% 5% 6% 12% 50%	1440
July	1st to 10th	Night	3 3 2 5% 5% 4 5% 7% 8% 12% 44%	1440
	11th to 20th	Night	5% 2 2 4 4 5% 5% 6% 9% 10% 48%	1521
	21st to end	Night	4 4 2 5% 6% 3 7% 7% 9% 12% 43%	1958
August	1st to 10th	Night	4 3 1 4 5% 3 5% 7% 8% 13% 47%	1790
	11th to 20th	Night	5% 4 2 4 6% 4 5% 6% 9% 13% 43%	1776
	21st to end	Night	8% 4 2 6% 5% 3 6% 6% 6% 9% 44%	1980
September	1st to 10th	Night	9% 4 2 5% 5% 3 4 5% 6% 7% 49%	1800
	11th to 20th	Night	10% 4 2 4 5% 3 4 5% 7% 9% 47%	1898
	21st to end	Night	8% 5% 2 4 3 2 4 5% 4 7% 56%	1980
October	1st to 10th	Night	7% 3 1 3 3 2 4 3 4 6% 63%	1980
	11th to 20th	Night	11% 5% 2 4 4 2 4 4 4 7% 53%	2137
	21st to end	Night	16% 6% 2 3 3 2 2 2 4 5% 55%	2376
November	1st to 10th	Night	11% 4 2 3 4 2 3 3 4 5% 57%	2392
	11th to 20th	Night	13% 4 3 3 4 3 3 3 4 6% 54%	2470
	21st to end	Night	15% 5% 2 4 5% 3 3 4 4 6% 49%	2470
December	1st to 10th	Night	17% 6% 3 6% 5% 2 4 4 5% 5% 44%	2470
	11th to 20th	Night	16% 5% 3 5% 5% 2 3 3 4 5% 49%	3042
	21st to end	Night	16% 8% 3 5% 5% 3 5% 6% 5% 6% 37%	4738

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	42% 4 6% 21% 22% 5%	1583
	11th to 20th	Day	47% 4 6% 18% 20% 5%	1634
	21st to end	Day	39% 4 5% 22% 24% 5%	1958
February	1st to 10th	Day	43% 5% 9% 20% 17% 6%	1800
	11th to 20th	Day	48% 4 5% 20% 17% 7%	1758
	21st to end	Day	48% 4 6% 13% 21% 7%	1480
March	1st to 10th	Day	43% 5% 7% 17% 18% 11%	1800
	11th to 20th	Day	45% 6% 10% 16% 20% 4	1902
	21st to end	Day	40% 6% 9% 24% 18% 3	2363
April	1st to 10th	Day	34% 5% 11% 22% 21% 7%	2160
	11th to 20th	Day	37% 5% 7% 19% 25% 7%	2134
	21st to end	Day	33% 6% 10% 25% 24% 3	2160
May	1st to 10th	Day	36% 6% 10% 23% 21% 5%	2160
	11th to 20th	Day	37% 9% 13% 23% 18% 1	2275
	21st to end	Day	36% 9% 12% 26% 15% 2	2531
June	1st to 10th	Day	37% 10% 12% 22% 16% 2	2378
	11th to 20th	Day	29% 9% 16% 25% 18% 3	2520
	21st to end	Day	33% 9% 17% 22% 15% 3	2520
July	1st to 10th	Day	40% 11% 18% 20% 9% 3	2451
	11th to 20th	Day	39% 12% 20% 19% 8% 2	2358
	21st to end	Day	36% 13% 19% 21% 9% 1	2224
August	1st to 10th	Day	35% 13% 18% 24% 9% 1	2136
	11th to 20th	Day	41% 12% 15% 19% 10% 3	2125
	21st to end	Day	38% 10% 17% 20% 10% 5%	2376
September	1st to 10th	Day	34% 11% 12% 20% 18% 5%	2160
	11th to 20th	Day	39% 7% 11% 15% 22% 7%	2042
	21st to end	Day	31% 6% 10% 23% 21% 9%	1980
October	1st to 10th	Day	29% 5% 8% 22% 25% 11%	1980
	11th to 20th	Day	30% 4 6% 21% 29% 9%	1818
	21st to end	Day	33% 4 6% 19% 29% 10%	1980
November	1st to 10th	Day	30% 4 7% 21% 27% 11%	1682
	11th to 20th	Day	33% 3 7% 19% 26% 12%	1710
	21st to end	Day	39% 5% 6% 17% 25% 8%	1710
December	1st to 10th	Day	46% 4 9% 16% 19% 6%	1710
	11th to 20th	Day	44% 4 7% 18% 23% 5%	2150
	21st to end	Day	43% 5% 8% 19% 23% 2	2907
January	1st to 10th	Night	40% 5% 8% 18% 25% 4	2580
	11th to 20th	Night	48% 5% 6% 17% 19% 6%	2366
	21st to end	Night	44% 3 4 18% 25% 6%	2393
February	1st to 10th	Night	44% 4 7% 17% 20% 8%	2160
	11th to 20th	Night	45% 4 5% 18% 19% 9%	2099
	21st to end	Night	47% 6% 6% 14% 18% 10%	1776
March	1st to 10th	Night	39% 4 8% 16% 20% 12%	2160
	11th to 20th	Night	41% 7% 9% 17% 18% 7%	2071
	21st to end	Night	39% 5% 8% 23% 20% 6%	1991
April	1st to 10th	Night	35% 6% 9% 22% 20% 9%	1800
	11th to 20th	Night	33% 4 7% 22% 23% 10%	1775
	21st to end	Night	32% 6% 10% 23% 24% 5%	1800
May	1st to 10th	Night	30% 6% 10% 25% 23% 6%	1800
	11th to 20th	Night	34% 8% 14% 23% 18% 3	1621
	21st to end	Night	29% 10% 14% 25% 20% 3	1745
June	1st to 10th	Night	34% 9% 14% 22% 19% 3	1576
	11th to 20th	Night	22% 9% 15% 30% 18% 6%	1440
	21st to end	Night	27% 8% 19% 22% 19% 4	1440
July	1st to 10th	Night	28% 11% 22% 23% 12% 3	1400
	11th to 20th	Night	26% 13% 22% 26% 10% 2	1473
	21st to end	Night	29% 12% 24% 24% 10% 2	1849
August	1st to 10th	Night	25% 10% 23% 30% 12% 1	1778
	11th to 20th	Night	29% 13% 21% 22% 10% 5%	1776
	21st to end	Night	35% 9% 16% 22% 12% 6%	1980
September	1st to 10th	Night	32% 6% 13% 24% 19% 5%	1800
	11th to 20th	Night	32% 9% 13% 16% 22% 8%	1898
	21st to end	Night	28% 6% 8% 24% 23% 11%	1980
October	1st to 10th	Night	23% 4 9% 23% 25% 15%	1980
	11th to 20th	Night	31% 5% 7% 19% 28% 12%	2137
	21st to end	Night	34% 3 6% 16% 28% 14%	2376
November	1st to 10th	Night	30% 4 8% 18% 28% 13%	2392
	11th to 20th	Night	33% 3 7% 18% 26% 13%	2470
	21st to end	Night	37% 4 6% 21% 23% 10%	2470
December	1st to 10th	Night	42% 5% 8% 19% 21% 5%	2470
	11th to 20th	Night	40% 3 7% 18% 27% 6%	3044
	21st to end	Night	43% 7% 8% 16% 23% 3	4738

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	23% 20% 30% 21% 5% 1	270
	11th to 20th	Day	23% 18% 33% 16% 9% 2	245
	21st to end	Day	20% 20% 34% 20% 5% 1	264
February	1st to 10th	Day	22% 15% 35% 19% 9%	240
	11th to 20th	Day	35% 14% 31% 17% 3	240
	21st to end	Day	28% 17% 35% 15% 5%	198
March	1st to 10th	Day	18% 18% 33% 22% 9%	240
	11th to 20th	Day	20% 18% 37% 20% 4 1	240
	21st to end	Day	19% 14% 30% 24% 13%	449
April	1st to 10th	Day	15% 15% 28% 28% 13% 2	420
	11th to 20th	Day	19% 10% 29% 23% 15% 2	412
	21st to end	Day	9% 14% 27% 29% 18% 3	420
May	1st to 10th	Day	17% 8% 16% 33% 20% 6%	423
	11th to 20th	Day	11% 3 19% 30% 23% 13%	447
	21st to end	Day	11% 3 17% 27% 24% 18%	485
June	1st to 10th	Day	10% 2 12% 28% 29% 19%	488
	11th to 20th	Day	6% 1 11% 27% 33% 22%	630
	21st to end	Day	8% 2 11% 25% 32% 22%	630
July	1st to 10th	Day	7% 2 6% 21% 38% 26%	621
	11th to 20th	Day	7% 1 6% 20% 31% 35%	618
	21st to end	Day	5% 6% 19% 36% 33%	493
August	1st to 10th	Day	6% 2 6% 19% 36% 32%	446
	11th to 20th	Day	9% 1 6% 22% 33% 30%	439
	21st to end	Day	14% 2 6% 19% 31% 29%	495
September	1st to 10th	Day	9% 2 6% 23% 40% 20%	450
	11th to 20th	Day	10% 1 11% 21% 35% 23%	450
	21st to end	Day	10% 2 9% 29% 35% 15%	450
October	1st to 10th	Day	9% 3 16% 27% 30% 14%	450
	11th to 20th	Day	11% 8% 26% 27% 21% 7%	288
	21st to end	Day	18% 10% 24% 28% 14% 5%	297
November	1st to 10th	Day	15% 7% 24% 30% 18% 6%	270
	11th to 20th	Day	13% 11% 33% 33% 8% 2	270
	21st to end	Day	14% 16% 32% 22% 13% 4	270
December	1st to 10th	Day	22% 11% 35% 21% 9% 1	270
	11th to 20th	Day	18% 13% 35% 19% 13% 2	270
	21st to end	Day	21% 13% 36% 21% 7% 1	297
January	1st to 10th	Night	21% 11% 33% 28% 7%	950
	11th to 20th	Night	21% 11% 31% 28% 8%	910
	21st to end	Night	20% 11% 36% 29% 4	809
February	1st to 10th	Night	20% 10% 32% 31% 7%	720
	11th to 20th	Night	25% 11% 29% 31% 4	703
	21st to end	Night	21% 13% 34% 29% 3	592
March	1st to 10th	Night	18% 10% 32% 33% 7%	720
	11th to 20th	Night	17% 8% 34% 30% 10%	631
	21st to end	Night	15% 9% 32% 29% 13% 1	594
April	1st to 10th	Night	11% 11% 30% 34% 14%	540
	11th to 20th	Night	12% 7% 30% 34% 15% 2	533
	21st to end	Night	11% 6% 32% 36% 14% 2	540
May	1st to 10th	Night	11% 4 24% 40% 20% 2	540
	11th to 20th	Night	11% 3 16% 36% 29% 5%	531
	21st to end	Night	8% 1 10% 37% 35% 8%	584
June	1st to 10th	Night	7% 3 6% 38% 38% 8%	540
	11th to 20th	Night	4 1 7% 28% 47% 13%	540
	21st to end	Night	5% 1 9% 27% 46% 12%	540
July	1st to 10th	Night	3 1 5% 23% 52% 17%	525
	11th to 20th	Night	4 4 24% 45% 22%	522
	21st to end	Night	5% 1 5% 24% 49% 17%	591
August	1st to 10th	Night	5% 2 19% 52% 21%	537
	11th to 20th	Night	6% 3 24% 51% 15%	534
	21st to end	Night	9% 1 6% 20% 49% 15%	594
September	1st to 10th	Night	11% 2 5% 29% 45% 7%	540
	11th to 20th	Night	12% 2 10% 26% 44% 7%	638
	21st to end	Night	9% 2 10% 32% 42% 5%	720
October	1st to 10th	Night	9% 2 13% 33% 37% 6%	720
	11th to 20th	Night	14% 4 19% 38% 24% 1	720
	21st to end	Night	17% 5% 22% 31% 23% 2	792
November	1st to 10th	Night	13% 5% 16% 39% 23% 4	740
	11th to 20th	Night	13% 6% 28% 35% 16% 1	760
	21st to end	Night	18% 7% 25% 34% 15% 1	760
December	1st to 10th	Night	21% 9% 26% 33% 11%	760
	11th to 20th	Night	21% 8% 29% 31% 12%	760
	21st to end	Night	23% 9% 31% 29% 7%	964

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