HAO Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the HAO Observatory clear sky chart from 2005-11-06 to 2024-04-03.

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% 10% 4 7% 6% 3 5% 5% 5% 7% 31%	78900
	Night	18% 11% 4 7% 6% 3 4 4 5% 6% 33%	81226
Transparency	Day	53% 5% 11% 20% 10% 1	78620
	Night	53% 5% 10% 20% 11% 2	81025
Seeing	Day	23% 13% 25% 21% 13% 5%	13895
	Night	23% 11% 24% 28% 13% 1	23759

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	21% 19% 8% 10% 5% 3 4 4 3 4 20%	2836
	11th to 20th	Day	15% 21% 10% 12% 9% 3 3 3 3 4 18%	2840
	21st to end	Day	25% 20% 6% 11% 7% 4 3 3 3 4 15%	3124
February	1st to 10th	Day	22% 15% 6% 9% 7% 3 4 3 4 5% 22%	1710
	11th to 20th	Day	18% 14% 8% 10% 5% 4 3 4 5% 6% 23%	1663
	21st to end	Day	22% 14% 4 10% 5% 3 3 4 4 5% 26%	1583
March	1st to 10th	Day	18% 12% 5% 7% 5% 3 4 4 5% 6% 32%	2090
	11th to 20th	Day	20% 11% 5% 7% 6% 3 5% 4 4 6% 29%	2090
	21st to end	Day	21% 10% 5% 8% 6% 3 4 5% 4 6% 29%	2299
April	1st to 10th	Day	16% 11% 4 6% 6% 3 5% 5% 6% 6% 31%	2100
	11th to 20th	Day	16% 11% 3 6% 6% 3 5% 5% 5% 6% 34%	2143
	21st to end	Day	22% 8% 3 6% 5% 3 5% 4 5% 6% 33%	2340
May	1st to 10th	Day	20% 9% 3 7% 6% 3 6% 5% 6% 7% 30%	2340
	11th to 20th	Day	22% 8% 3 6% 6% 4 4 5% 5% 6% 30%	2301
	21st to end	Day	13% 5% 3 6% 6% 3 6% 6% 7% 10% 36%	2659
June	1st to 10th	Day	14% 6% 3 5% 6% 4 6% 6% 7% 9% 35%	2520
	11th to 20th	Day	14% 6% 3 6% 6% 4 6% 6% 7% 8% 35%	2520
	21st to end	Day	14% 6% 3 5% 6% 4 5% 6% 6% 10% 36%	2520
July	1st to 10th	Day	12% 6% 2 6% 6% 4 6% 6% 6% 9% 37%	2431
	11th to 20th	Day	11% 5% 3 6% 6% 4 5% 6% 6% 10% 37%	2340
	21st to end	Day	13% 6% 3 6% 6% 4 7% 5% 7% 9% 34%	2547
August	1st to 10th	Day	12% 6% 2 6% 5% 5% 7% 7% 8% 10% 32%	2327
	11th to 20th	Day	13% 6% 2 6% 6% 4 6% 6% 7% 10% 35%	2303
	21st to end	Day	12% 5% 3 6% 6% 4 6% 6% 7% 9% 38%	2441
September	1st to 10th	Day	15% 5% 2 5% 5% 3 5% 5% 6% 7% 43%	2160
	11th to 20th	Day	13% 5% 3 4 5% 3 4 4 6% 8% 45%	1994
	21st to end	Day	20% 7% 3 6% 5% 3 3 4 4 6% 38%	1973
October	1st to 10th	Day	17% 6% 3 6% 5% 2 4 4 6% 7% 39%	1800
	11th to 20th	Day	19% 9% 2 6% 5% 4 4 4 5% 6% 37%	1800
	21st to end	Day	23% 11% 3 6% 4 3 3 3 4 6% 34%	1980
November	1st to 10th	Day	12% 8% 4 7% 5% 4 4 4 4 6% 43%	1805
	11th to 20th	Day	17% 12% 4 7% 5% 4 4 4 5% 6% 33%	1590
	21st to end	Day	20% 14% 5% 6% 5% 3 5% 4 4 5% 29%	1520
December	1st to 10th	Day	22% 16% 7% 11% 6% 3 4 4 3 4 19%	1520
	11th to 20th	Day	20% 18% 8% 8% 5% 2 4 3 4 6% 23%	1910
	21st to end	Day	24% 20% 7% 9% 6% 3 3 3 4 5% 16%	2781
January	1st to 10th	Night	18% 20% 7% 10% 6% 4 4 4 4 5% 19%	4109
	11th to 20th	Night	18% 20% 7% 10% 7% 4 5% 4 3 4 17%	4120
	21st to end	Night	28% 17% 6% 9% 8% 3 3 2 3 3 18%	4532
February	1st to 10th	Night	20% 14% 4 9% 6% 2 4 4 4 5% 26%	2470
	11th to 20th	Night	21% 13% 4 9% 5% 4 5% 4 4 5% 25%	2403
	21st to end	Night	22% 12% 5% 9% 6% 3 4 4 4 5% 25%	1868
March	1st to 10th	Night	19% 10% 4 7% 5% 3 4 4 4 5% 34%	2090
	11th to 20th	Night	19% 10% 5% 6% 5% 2 3 4 5% 6% 35%	2090
	21st to end	Night	19% 10% 3 7% 5% 3 5% 3 5% 6% 34%	2299
April	1st to 10th	Night	19% 11% 4 7% 4 3 4 4 4 6% 34%	1905
	11th to 20th	Night	16% 9% 2 6% 5% 3 4 4 6% 7% 38%	1775
	21st to end	Night	16% 9% 3 5% 5% 2 4 4 5% 7% 38%	1625
May	1st to 10th	Night	18% 7% 3 6% 6% 4 5% 5% 6% 7% 32%	1620
	11th to 20th	Night	19% 9% 4 5% 5% 3 5% 4 5% 6% 36%	1592
	21st to end	Night	9% 6% 3 4 4 3 5% 5% 6% 11% 44%	1612
June	1st to 10th	Night	12% 5% 2 6% 6% 3 5% 4 6% 7% 45%	1440
	11th to 20th	Night	12% 6% 2 5% 5% 3 6% 6% 7% 9% 39%	1440
	21st to end	Night	12% 4 1 4 6% 3 5% 5% 7% 10% 42%	1440
July	1st to 10th	Night	10% 5% 3 6% 5% 3 5% 5% 6% 9% 43%	1523
	11th to 20th	Night	11% 4 2 4 4 3 4 6% 6% 9% 48%	1620
	21st to end	Night	10% 5% 2 5% 6% 3 6% 7% 8% 10% 38%	1771
August	1st to 10th	Night	8% 5% 3 6% 6% 4 5% 6% 7% 10% 41%	1611
	11th to 20th	Night	9% 6% 3 6% 5% 3 5% 6% 6% 7% 43%	1598
	21st to end	Night	10% 5% 2 6% 5% 2 4 5% 5% 7% 47%	1895
September	1st to 10th	Night	12% 5% 2 5% 5% 3 4 5% 6% 7% 47%	1800
	11th to 20th	Night	12% 5% 2 4 4 3 4 5% 5% 8% 52%	1962
	21st to end	Night	15% 7% 3 4 5% 3 4 4 4 8% 43%	1986
October	1st to 10th	Night	16% 5% 2 5% 5% 4 4 4 4 7% 43%	2160
	11th to 20th	Night	18% 6% 2 6% 5% 3 4 5% 5% 6% 39%	2160
	21st to end	Night	25% 7% 3 5% 5% 3 3 3 4 6% 36%	2376
November	1st to 10th	Night	13% 8% 3 6% 5% 2 4 4 4 5% 46%	2271
	11th to 20th	Night	19% 11% 4 6% 5% 3 4 5% 5% 6% 33%	2588
	21st to end	Night	21% 13% 4 6% 5% 3 4 4 4 5% 31%	2660
December	1st to 10th	Night	23% 16% 7% 8% 5% 3 3 3 4 5% 22%	2660
	11th to 20th	Night	20% 16% 7% 8% 6% 3 4 4 3 5% 25%	3285
	21st to end	Night	24% 17% 5% 8% 7% 4 5% 3 4 6% 17%	4870

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	69% 3 10% 13% 4	2837
	11th to 20th	Day	73% 3 7% 11% 5% 1	2840
	21st to end	Day	76% 2 7% 11% 3	3124
February	1st to 10th	Day	66% 4 9% 14% 7% 1	1710
	11th to 20th	Day	62% 4 9% 17% 8%	1672
	21st to end	Day	62% 3 10% 15% 8% 2	1583
March	1st to 10th	Day	53% 3 12% 22% 9% 1	2090
	11th to 20th	Day	57% 4 10% 18% 10% 1	2090
	21st to end	Day	56% 4 9% 18% 11% 2	2299
April	1st to 10th	Day	53% 4 10% 20% 11% 3	2100
	11th to 20th	Day	50% 4 11% 15% 17% 3	2143
	21st to end	Day	52% 5% 10% 21% 11% 2	2340
May	1st to 10th	Day	52% 5% 11% 20% 9% 2	2340
	11th to 20th	Day	54% 6% 11% 16% 11% 2	2305
	21st to end	Day	42% 8% 17% 22% 10% 1	2659
June	1st to 10th	Day	45% 7% 14% 23% 11% 1	2520
	11th to 20th	Day	44% 7% 14% 23% 12% 1	2520
	21st to end	Day	43% 6% 12% 25% 13% 1	2520
July	1st to 10th	Day	44% 7% 12% 21% 13% 3	2365
	11th to 20th	Day	42% 7% 13% 23% 15% 1	2271
	21st to end	Day	44% 6% 12% 23% 12% 1	2405
August	1st to 10th	Day	44% 8% 14% 24% 9%	2314
	11th to 20th	Day	42% 7% 13% 26% 11% 1	2303
	21st to end	Day	40% 6% 14% 23% 13% 3	2441
September	1st to 10th	Day	39% 6% 12% 21% 19% 3	2160
	11th to 20th	Day	37% 4 12% 30% 14% 2	1994
	21st to end	Day	48% 4 10% 22% 15% 1	1973
October	1st to 10th	Day	44% 5% 13% 22% 15% 1	1800
	11th to 20th	Day	49% 4 11% 25% 11%	1800
	21st to end	Day	53% 3 9% 21% 12% 1	1980
November	1st to 10th	Day	43% 3 11% 22% 16% 3	1805
	11th to 20th	Day	53% 3 10% 19% 12% 2	1590
	21st to end	Day	58% 4 11% 19% 8% 1	1520
December	1st to 10th	Day	70% 3 9% 13% 5%	1520
	11th to 20th	Day	65% 2 11% 16% 5%	1906
	21st to end	Day	72% 2 11% 11% 4	2781
January	1st to 10th	Night	68% 4 9% 14% 5% 1	4109
	11th to 20th	Night	72% 3 10% 11% 3	4120
	21st to end	Night	74% 2 6% 14% 3 1	4532
February	1st to 10th	Night	62% 3 10% 18% 7%	2470
	11th to 20th	Night	62% 4 9% 17% 8%	2405
	21st to end	Night	62% 3 9% 16% 9% 2	1868
March	1st to 10th	Night	52% 4 9% 22% 11% 1	2090
	11th to 20th	Night	51% 4 10% 22% 11% 2	2090
	21st to end	Night	53% 4 7% 20% 13% 3	2299
April	1st to 10th	Night	52% 5% 8% 19% 13% 2	1905
	11th to 20th	Night	45% 5% 11% 19% 16% 4	1775
	21st to end	Night	45% 5% 10% 25% 11% 4	1625
May	1st to 10th	Night	50% 6% 11% 18% 12% 3	1620
	11th to 20th	Night	49% 6% 11% 17% 14% 3	1593
	21st to end	Night	35% 6% 19% 26% 12% 2	1613
June	1st to 10th	Night	38% 7% 11% 27% 15% 2	1440
	11th to 20th	Night	40% 7% 13% 25% 14% 2	1440
	21st to end	Night	36% 7% 14% 26% 16% 2	1440
July	1st to 10th	Night	38% 8% 12% 23% 15% 4	1479
	11th to 20th	Night	32% 6% 16% 25% 19% 1	1566
	21st to end	Night	38% 7% 16% 23% 13% 3	1672
August	1st to 10th	Night	37% 9% 14% 26% 15%	1600
	11th to 20th	Night	37% 8% 13% 24% 15% 2	1598
	21st to end	Night	35% 6% 14% 27% 14% 4	1895
September	1st to 10th	Night	37% 6% 9% 24% 19% 5%	1800
	11th to 20th	Night	31% 5% 14% 27% 23% 1	1962
	21st to end	Night	41% 5% 11% 23% 20% 1	1986
October	1st to 10th	Night	41% 6% 11% 26% 14% 3	2160
	11th to 20th	Night	45% 4 10% 25% 14% 1	2160
	21st to end	Night	51% 3 9% 22% 13% 2	2376
November	1st to 10th	Night	40% 4 10% 23% 18% 4	2271
	11th to 20th	Night	52% 4 8% 20% 13% 2	2588
	21st to end	Night	56% 4 9% 21% 8% 1	2660
December	1st to 10th	Night	66% 3 10% 15% 5% 1	2660
	11th to 20th	Night	64% 4 10% 17% 4	3288
	21st to end	Night	70% 3 11% 13% 3	4870

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	29% 27% 26% 11% 6% 1	270
	11th to 20th	Day	38% 24% 27% 8% 2 1	270
	21st to end	Day	31% 30% 27% 9% 3 1	297
February	1st to 10th	Day	31% 26% 30% 11% 2	270
	11th to 20th	Day	36% 25% 29% 8% 1	270
	21st to end	Day	37% 19% 32% 10% 1	268
March	1st to 10th	Day	27% 23% 27% 20% 3 1	460
	11th to 20th	Day	33% 19% 30% 13% 5% 1	460
	21st to end	Day	31% 16% 28% 20% 5% 1	506
April	1st to 10th	Day	28% 18% 27% 18% 8% 1	428
	11th to 20th	Day	23% 17% 30% 16% 9% 5%	412
	21st to end	Day	28% 18% 27% 16% 9% 2	420
May	1st to 10th	Day	29% 15% 27% 16% 10% 3	423
	11th to 20th	Day	24% 11% 30% 20% 10% 5%	447
	21st to end	Day	15% 7% 25% 27% 18% 8%	485
June	1st to 10th	Day	14% 8% 28% 30% 18% 3	450
	11th to 20th	Day	17% 9% 26% 25% 20% 4	450
	21st to end	Day	15% 8% 26% 28% 19% 4	450
July	1st to 10th	Day	15% 5% 20% 25% 22% 13%	445
	11th to 20th	Day	11% 6% 21% 32% 19% 10%	444
	21st to end	Day	17% 4 21% 28% 20% 10%	488
August	1st to 10th	Day	14% 4 20% 28% 23% 12%	446
	11th to 20th	Day	16% 5% 15% 28% 24% 12%	439
	21st to end	Day	13% 7% 19% 27% 21% 15%	495
September	1st to 10th	Day	18% 5% 18% 26% 21% 11%	450
	11th to 20th	Day	13% 5% 15% 31% 24% 13%	450
	21st to end	Day	24% 8% 24% 23% 17% 5%	450
October	1st to 10th	Day	18% 7% 25% 24% 20% 5%	450
	11th to 20th	Day	22% 14% 27% 24% 10% 2	450
	21st to end	Day	30% 14% 27% 19% 9% 1	495
November	1st to 10th	Day	19% 13% 30% 23% 11% 4	403
	11th to 20th	Day	30% 17% 29% 14% 9% 1	212
	21st to end	Day	28% 16% 41% 12% 4 1	180
December	1st to 10th	Day	30% 23% 30% 13% 3 1	180
	11th to 20th	Day	33% 22% 28% 16% 2	180
	21st to end	Day	37% 20% 26% 10% 6%	202
January	1st to 10th	Night	30% 19% 30% 17% 4	760
	11th to 20th	Night	35% 14% 30% 17% 3	760
	21st to end	Night	30% 18% 28% 21% 3	836
February	1st to 10th	Night	29% 17% 26% 25% 3	760
	11th to 20th	Night	30% 20% 31% 18% 1	743
	21st to end	Night	31% 15% 31% 20% 3	628
March	1st to 10th	Night	26% 15% 30% 25% 4	760
	11th to 20th	Night	27% 17% 27% 22% 6% 1	760
	21st to end	Night	25% 14% 30% 25% 5% 1	836
April	1st to 10th	Night	27% 14% 30% 23% 6%	729
	11th to 20th	Night	21% 12% 31% 26% 10% 1	711
	21st to end	Night	21% 10% 31% 30% 8%	545
May	1st to 10th	Night	22% 7% 24% 35% 11% 1	540
	11th to 20th	Night	23% 8% 23% 33% 12% 1	531
	21st to end	Night	11% 3 22% 36% 27% 1	452
June	1st to 10th	Night	11% 5% 19% 42% 21% 1	360
	11th to 20th	Night	12% 4 23% 38% 21% 1	360
	21st to end	Night	12% 4 23% 38% 23%	360
July	1st to 10th	Night	11% 3 15% 40% 24% 6%	429
	11th to 20th	Night	11% 2 15% 38% 30% 4	522
	21st to end	Night	13% 3 14% 38% 31% 2	591
August	1st to 10th	Night	10% 2 14% 40% 31% 2	537
	11th to 20th	Night	11% 4 10% 38% 32% 5%	534
	21st to end	Night	12% 4 15% 34% 30% 4	707
September	1st to 10th	Night	14% 4 13% 35% 29% 5%	720
	11th to 20th	Night	13% 4 13% 36% 30% 4	720
	21st to end	Night	19% 6% 20% 31% 21% 3	720
October	1st to 10th	Night	18% 7% 21% 32% 20% 3	720
	11th to 20th	Night	23% 7% 27% 31% 11% 1	720
	21st to end	Night	28% 11% 23% 29% 8% 1	792
November	1st to 10th	Night	18% 10% 26% 33% 13% 1	740
	11th to 20th	Night	26% 14% 26% 25% 8% 1	760
	21st to end	Night	29% 13% 28% 24% 6% 1	760
December	1st to 10th	Night	34% 13% 28% 20% 5%	760
	11th to 20th	Night	28% 17% 27% 23% 4	760
	21st to end	Night	31% 14% 29% 24% 2	836

Page generated at 2024-03-31 06:08:55. Version ac07

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