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 2025-07-21.

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	16% 11% 4 9% 5% 5% 4 4 5% 7% 31%	120052
	Night	17% 11% 4 9% 5% 4 4 3 5% 6% 32%	119069
Transparency	Day	51% 5% 11% 21% 11% 1	123699
	Night	52% 5% 10% 20% 11% 2	123559
Seeing	Day	23% 13% 26% 21% 13% 5%	15633
	Night	22% 11% 24% 28% 13% 1	25351

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	18% 18% 7% 20% 4 3 4 2 3 4 16%	4056
	11th to 20th	Day	14% 22% 9% 15% 9% 6% 2 2 2 3 15%	4060
	21st to end	Day	20% 20% 5% 11% 6% 5% 4 3 3 4 19%	4466
February	1st to 10th	Day	21% 17% 5% 10% 6% 5% 3 3 4 5% 22%	1800
	11th to 20th	Day	18% 16% 6% 11% 4 5% 3 3 5% 6% 23%	1753
	21st to end	Day	21% 15% 4 11% 5% 3 3 4 4 5% 26%	1664
March	1st to 10th	Day	17% 14% 4 9% 5% 3 4 3 5% 6% 31%	4935
	11th to 20th	Day	13% 9% 4 7% 5% 5% 5% 4 5% 6% 37%	4980
	21st to end	Day	17% 11% 4 9% 5% 5% 4 4 5% 6% 30%	5465
April	1st to 10th	Day	17% 13% 3 7% 5% 4 5% 4 6% 6% 29%	2310
	11th to 20th	Day	15% 12% 2 7% 5% 5% 4 4 5% 6% 35%	2385
	21st to end	Day	20% 9% 3 7% 4 4 5% 3 5% 7% 34%	2600
May	1st to 10th	Day	20% 9% 3 8% 6% 5% 6% 4 6% 7% 27%	5862
	11th to 20th	Day	18% 8% 3 8% 5% 5% 5% 4 6% 8% 30%	5861
	21st to end	Day	19% 7% 2 7% 4 5% 5% 4 6% 8% 31%	6784
June	1st to 10th	Day	13% 7% 2 7% 5% 5% 6% 5% 7% 8% 35%	2800
	11th to 20th	Day	14% 6% 3 7% 5% 6% 5% 4 8% 8% 35%	2800
	21st to end	Day	13% 6% 2 7% 5% 5% 5% 5% 6% 10% 37%	2800
July	1st to 10th	Day	10% 6% 2 7% 5% 5% 6% 4 6% 8% 40%	4416
	11th to 20th	Day	10% 6% 3 7% 5% 5% 5% 5% 6% 9% 38%	4250
	21st to end	Day	13% 7% 3 7% 5% 6% 5% 4 7% 8% 35%	4466
August	1st to 10th	Day	14% 8% 3 8% 5% 6% 6% 5% 7% 9% 29%	4104
	11th to 20th	Day	11% 7% 2 7% 5% 5% 6% 4 7% 9% 37%	4044
	21st to end	Day	8% 4 2 7% 5% 5% 6% 5% 8% 10% 41%	4287
September	1st to 10th	Day	14% 5% 2 5% 4 4 5% 4 6% 6% 45%	2280
	11th to 20th	Day	12% 6% 3 5% 5% 4 4 3 6% 7% 46%	2104
	21st to end	Day	21% 7% 3 7% 5% 4 3 3 5% 6% 36%	2082
October	1st to 10th	Day	17% 7% 2 7% 4 4 3 4 5% 7% 40%	1900
	11th to 20th	Day	18% 9% 2 7% 5% 5% 4 2 5% 6% 37%	1900
	21st to end	Day	22% 10% 3 6% 4 4 3 2 5% 6% 36%	2090
November	1st to 10th	Day	12% 9% 4 7% 5% 4 3 3 4 6% 44%	1902
	11th to 20th	Day	16% 14% 3 8% 4 5% 4 4 5% 6% 33%	1673
	21st to end	Day	20% 15% 5% 8% 5% 4 4 3 4 5% 27%	1600
December	1st to 10th	Day	17% 14% 6% 16% 6% 5% 4 3 4 4 21%	2600
	11th to 20th	Day	23% 17% 7% 9% 4 3 3 2 3 5% 23%	2990
	21st to end	Day	21% 18% 6% 11% 6% 5% 4 3 4 5% 17%	3983
January	1st to 10th	Night	16% 17% 6% 21% 6% 5% 3 3 3 4 15%	5889
	11th to 20th	Night	16% 19% 8% 13% 8% 5% 4 3 3 4 17%	5900
	21st to end	Night	22% 16% 5% 12% 6% 4 4 3 3 3 22%	6490
February	1st to 10th	Night	20% 16% 4 10% 5% 3 4 3 4 5% 26%	2600
	11th to 20th	Night	21% 14% 4 10% 5% 5% 4 3 5% 5% 24%	2533
	21st to end	Night	21% 13% 5% 10% 5% 4 4 3 4 5% 25%	1962
March	1st to 10th	Night	22% 11% 4 8% 5% 3 3 3 4 5% 33%	4918
	11th to 20th	Night	12% 10% 4 7% 3 3 3 3 4 6% 43%	4980
	21st to end	Night	16% 10% 3 8% 5% 5% 5% 3 5% 6% 35%	5467
April	1st to 10th	Night	19% 13% 4 7% 4 4 4 3 4 6% 33%	2090
	11th to 20th	Night	16% 9% 3 6% 4 3 5% 3 7% 7% 37%	1975
	21st to end	Night	16% 9% 3 7% 4 4 4 3 5% 7% 38%	1805
May	1st to 10th	Night	25% 11% 4 8% 5% 4 4 4 5% 6% 24%	4033
	11th to 20th	Night	17% 9% 3 6% 5% 4 4 3 5% 8% 35%	4032
	21st to end	Night	20% 8% 3 7% 4 4 4 4 5% 8% 32%	4066
June	1st to 10th	Night	12% 6% 2 7% 5% 5% 4 3 6% 8% 44%	1600
	11th to 20th	Night	11% 6% 2 6% 5% 5% 6% 4 7% 9% 39%	1600
	21st to end	Night	11% 5% 1 5% 5% 5% 5% 5% 8% 10% 41%	1600
July	1st to 10th	Night	11% 7% 3 7% 4 4 4 4 6% 8% 41%	2758
	11th to 20th	Night	11% 5% 2 7% 5% 5% 5% 5% 7% 8% 41%	2930
	21st to end	Night	11% 8% 3 7% 6% 5% 5% 5% 7% 8% 37%	3087
August	1st to 10th	Night	13% 8% 4 8% 5% 4 5% 4 7% 9% 33%	2825
	11th to 20th	Night	12% 8% 3 7% 4 4 5% 4 5% 6% 41%	2785
	21st to end	Night	8% 5% 2 7% 5% 5% 5% 4 6% 8% 45%	3335
September	1st to 10th	Night	12% 5% 2 6% 4 4 5% 3 6% 6% 47%	1900
	11th to 20th	Night	11% 5% 2 4 3 4 4 4 5% 8% 52%	2072
	21st to end	Night	16% 8% 3 5% 4 4 4 3 5% 7% 41%	2097
October	1st to 10th	Night	16% 6% 2 6% 4 4 4 3 5% 7% 43%	2280
	11th to 20th	Night	17% 7% 2 7% 4 4 5% 4 5% 6% 40%	2280
	21st to end	Night	23% 8% 2 5% 4 4 3 3 4 6% 38%	2508
November	1st to 10th	Night	12% 8% 2 7% 5% 3 3 3 4 6% 47%	2394
	11th to 20th	Night	19% 12% 3 6% 5% 4 5% 4 5% 5% 32%	2725
	21st to end	Night	20% 14% 4 7% 4 4 4 3 4 5% 30%	2800
December	1st to 10th	Night	19% 15% 9% 13% 4 4 4 3 5% 4 20%	4580
	11th to 20th	Night	22% 16% 7% 9% 4 3 3 3 3 5% 23%	5205
	21st to end	Night	21% 17% 5% 10% 6% 5% 5% 3 5% 6% 18%	6968

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	73% 4 9% 10% 4 1	4057
	11th to 20th	Day	77% 3 5% 8% 5% 2	4060
	21st to end	Day	70% 3 8% 12% 7%	4466
February	1st to 10th	Day	66% 4 9% 14% 7% 1	1800
	11th to 20th	Day	63% 4 9% 16% 7%	1762
	21st to end	Day	61% 3 11% 16% 7% 2	1664
March	1st to 10th	Day	55% 3 10% 23% 8% 1	4935
	11th to 20th	Day	47% 4 10% 27% 10% 2	4980
	21st to end	Day	53% 5% 7% 20% 13% 1	5465
April	1st to 10th	Day	55% 4 9% 19% 11% 2	2310
	11th to 20th	Day	49% 5% 11% 17% 16% 2	2385
	21st to end	Day	50% 5% 9% 23% 11% 2	2600
May	1st to 10th	Day	54% 7% 10% 18% 9% 2	5897
	11th to 20th	Day	50% 7% 10% 23% 9% 1	5865
	21st to end	Day	47% 7% 13% 20% 10% 2	6799
June	1st to 10th	Day	44% 7% 13% 23% 12% 1	2800
	11th to 20th	Day	44% 7% 14% 23% 12% 1	2800
	21st to end	Day	42% 6% 12% 25% 14% 1	2800
July	1st to 10th	Day	40% 7% 12% 24% 15% 2	4350
	11th to 20th	Day	40% 7% 12% 26% 14% 1	4181
	21st to end	Day	45% 6% 10% 22% 15% 1	4324
August	1st to 10th	Day	48% 8% 13% 22% 9%	4091
	11th to 20th	Day	40% 7% 11% 29% 13%	4083
	21st to end	Day	35% 9% 14% 27% 14% 2	4287
September	1st to 10th	Day	38% 6% 11% 22% 20% 3	2280
	11th to 20th	Day	37% 4 12% 29% 15% 2	2104
	21st to end	Day	49% 4 10% 22% 14% 1	2082
October	1st to 10th	Day	34% 5% 10% 30% 19% 2	3160
	11th to 20th	Day	42% 5% 11% 23% 13% 5%	3140
	21st to end	Day	36% 4 10% 29% 20% 1	3428
November	1st to 10th	Day	43% 3 11% 23% 17% 3	1902
	11th to 20th	Day	53% 3 10% 19% 12% 2	1673
	21st to end	Day	60% 4 11% 18% 8% 1	1600
December	1st to 10th	Day	66% 4 14% 12% 4	2600
	11th to 20th	Day	66% 3 11% 15% 5%	2986
	21st to end	Day	70% 3 9% 13% 5%	3983
January	1st to 10th	Night	73% 4 7% 12% 4	5889
	11th to 20th	Night	72% 3 8% 11% 4 1	5900
	21st to end	Night	68% 4 8% 13% 6% 1	6490
February	1st to 10th	Night	62% 3 9% 18% 7%	2600
	11th to 20th	Night	62% 4 9% 17% 8%	2535
	21st to end	Night	62% 3 8% 17% 9% 2	1962
March	1st to 10th	Night	54% 3 8% 23% 9% 1	4918
	11th to 20th	Night	42% 4 11% 30% 9% 3	4980
	21st to end	Night	49% 5% 9% 21% 15% 2	5467
April	1st to 10th	Night	53% 5% 8% 18% 13% 2	2090
	11th to 20th	Night	45% 5% 12% 19% 15% 4	1975
	21st to end	Night	45% 5% 10% 26% 11% 3	1805
May	1st to 10th	Night	60% 6% 9% 12% 9% 3	4060
	11th to 20th	Night	47% 7% 9% 26% 10% 1	4033
	21st to end	Night	48% 7% 12% 22% 8% 3	4073
June	1st to 10th	Night	39% 6% 11% 26% 16% 2	1600
	11th to 20th	Night	40% 7% 13% 25% 14% 1	1600
	21st to end	Night	36% 7% 14% 25% 17% 2	1600
July	1st to 10th	Night	40% 6% 10% 23% 16% 4	2714
	11th to 20th	Night	38% 8% 15% 25% 15% 1	2876
	21st to end	Night	42% 7% 12% 18% 18% 3	2988
August	1st to 10th	Night	46% 9% 13% 22% 9%	2814
	11th to 20th	Night	41% 7% 9% 25% 17% 1	2812
	21st to end	Night	34% 7% 14% 30% 12% 2	3335
September	1st to 10th	Night	36% 6% 9% 24% 20% 5%	1900
	11th to 20th	Night	31% 5% 13% 27% 24% 1	2072
	21st to end	Night	43% 5% 10% 22% 19% 1	2097
October	1st to 10th	Night	36% 5% 8% 28% 21% 3	3800
	11th to 20th	Night	37% 5% 10% 25% 15% 8%	3779
	21st to end	Night	36% 3 8% 28% 24% 2	4120
November	1st to 10th	Night	40% 4 10% 23% 19% 4	2394
	11th to 20th	Night	53% 4 8% 20% 12% 3	2725
	21st to end	Night	57% 4 10% 21% 8% 1	2800
December	1st to 10th	Night	68% 3 10% 16% 3	4580
	11th to 20th	Night	65% 4 14% 14% 3	5208
	21st to end	Night	67% 4 11% 14% 4	6968

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	31% 28% 24% 11% 5% 1	300
	11th to 20th	Day	39% 25% 26% 7% 2 1	300
	21st to end	Day	31% 32% 25% 9% 3 1	330
February	1st to 10th	Day	31% 28% 29% 10% 2	300
	11th to 20th	Day	38% 25% 28% 8% 1	300
	21st to end	Day	35% 20% 35% 9% 1	294
March	1st to 10th	Day	27% 23% 28% 19% 3 1	500
	11th to 20th	Day	32% 19% 30% 13% 5% 1	500
	21st to end	Day	32% 16% 28% 19% 4 1	550
April	1st to 10th	Day	31% 17% 26% 18% 7% 1	500
	11th to 20th	Day	25% 19% 29% 14% 9% 4	492
	21st to end	Day	26% 18% 28% 17% 9% 2	500
May	1st to 10th	Day	27% 16% 28% 16% 10% 3	503
	11th to 20th	Day	23% 12% 29% 20% 10% 6%	527
	21st to end	Day	16% 10% 25% 26% 16% 6%	573
June	1st to 10th	Day	13% 10% 29% 29% 16% 3	530
	11th to 20th	Day	16% 8% 25% 28% 19% 4	530
	21st to end	Day	14% 8% 26% 28% 20% 5%	530
July	1st to 10th	Day	14% 5% 21% 26% 22% 12%	525
	11th to 20th	Day	12% 7% 22% 31% 19% 9%	524
	21st to end	Day	16% 4 23% 28% 19% 9%	532
August	1st to 10th	Day	14% 5% 21% 28% 21% 11%	486
	11th to 20th	Day	16% 5% 16% 28% 23% 11%	479
	21st to end	Day	12% 6% 18% 27% 23% 14%	539
September	1st to 10th	Day	17% 6% 19% 26% 21% 11%	490
	11th to 20th	Day	13% 4 15% 30% 24% 13%	490
	21st to end	Day	26% 8% 23% 23% 16% 5%	490
October	1st to 10th	Day	18% 8% 26% 24% 20% 5%	490
	11th to 20th	Day	21% 14% 27% 24% 10% 3	490
	21st to end	Day	28% 14% 28% 19% 10% 1	539
November	1st to 10th	Day	19% 15% 30% 23% 11% 4	440
	11th to 20th	Day	29% 16% 32% 14% 9% 1	235
	21st to end	Day	31% 17% 38% 11% 4 1	200
December	1st to 10th	Day	29% 25% 30% 13% 3 1	200
	11th to 20th	Day	34% 22% 27% 16% 2	200
	21st to end	Day	36% 20% 27% 11% 6% 1	225
January	1st to 10th	Night	30% 20% 30% 17% 4	800
	11th to 20th	Night	34% 16% 31% 17% 3	800
	21st to end	Night	30% 19% 28% 21% 2	880
February	1st to 10th	Night	30% 17% 27% 24% 3	800
	11th to 20th	Night	31% 20% 31% 17% 1	783
	21st to end	Night	30% 16% 32% 20% 3	660
March	1st to 10th	Night	26% 16% 30% 25% 4	800
	11th to 20th	Night	26% 17% 27% 23% 6% 1	800
	21st to end	Night	25% 14% 31% 24% 5% 1	880
April	1st to 10th	Night	27% 15% 30% 22% 6%	800
	11th to 20th	Night	21% 12% 30% 27% 9% 1	791
	21st to end	Night	20% 10% 31% 31% 8%	605
May	1st to 10th	Night	23% 7% 24% 34% 11% 1	600
	11th to 20th	Night	23% 8% 23% 33% 12% 1	591
	21st to end	Night	13% 4 23% 34% 25% 1	502
June	1st to 10th	Night	12% 6% 21% 42% 20% 1	400
	11th to 20th	Night	12% 4 23% 40% 21% 1	400
	21st to end	Night	11% 4 22% 38% 25% 1	400
July	1st to 10th	Night	10% 4 14% 42% 24% 6%	479
	11th to 20th	Night	11% 3 15% 37% 30% 4	582
	21st to end	Night	13% 3 13% 38% 30% 2	624
August	1st to 10th	Night	11% 2 14% 41% 29% 2	567
	11th to 20th	Night	11% 5% 11% 37% 32% 5%	564
	21st to end	Night	12% 4 15% 35% 30% 4	747
September	1st to 10th	Night	14% 4 13% 36% 28% 4	760
	11th to 20th	Night	13% 4 12% 36% 31% 4	760
	21st to end	Night	19% 6% 20% 31% 20% 3	760
October	1st to 10th	Night	17% 7% 21% 32% 21% 3	760
	11th to 20th	Night	22% 7% 27% 31% 11% 1	760
	21st to end	Night	27% 11% 23% 29% 9% 1	836
November	1st to 10th	Night	17% 10% 27% 33% 12% 1	780
	11th to 20th	Night	26% 15% 26% 25% 8% 1	800
	21st to end	Night	29% 13% 28% 24% 6% 1	800
December	1st to 10th	Night	33% 13% 29% 20% 5%	800
	11th to 20th	Night	29% 17% 27% 23% 4	800
	21st to end	Night	30% 14% 29% 25% 2	880

Page generated at 2025-07-18 01:09:58. Version ac07

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