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 2024-11-04.

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% 10% 4 8% 5% 5% 4 4 5% 7% 32%	101086
	Night	17% 11% 4 8% 5% 4 4 3 5% 6% 33%	97873
Transparency	Day	50% 5% 11% 22% 11% 1	104735
	Night	50% 5% 10% 21% 12% 2	102363
Seeing	Day	22% 13% 26% 21% 13% 5%	14755
	Night	22% 11% 24% 29% 13% 1	24452

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% 11% 4 3 4 3 3 4 20%	2836
	11th to 20th	Day	15% 21% 10% 14% 7% 4 2 2 3 4 18%	2840
	21st to end	Day	25% 21% 6% 13% 5% 4 3 2 3 4 15%	3124
February	1st to 10th	Day	21% 17% 5% 10% 6% 5% 3 3 4 5% 22%	1710
	11th to 20th	Day	18% 16% 7% 11% 4 5% 3 3 5% 6% 23%	1663
	21st to end	Day	22% 15% 4 10% 5% 3 3 4 4 4 26%	1583
March	1st to 10th	Day	20% 14% 4 8% 4 3 4 3 5% 6% 29%	3435
	11th to 20th	Day	15% 10% 5% 8% 6% 5% 5% 4 4 5% 33%	3480
	21st to end	Day	20% 11% 3 8% 5% 5% 4 4 5% 6% 31%	3815
April	1st to 10th	Day	16% 13% 3 7% 5% 4 5% 4 6% 6% 30%	2194
	11th to 20th	Day	16% 12% 3 7% 5% 4 4 4 5% 6% 35%	2264
	21st to end	Day	21% 9% 3 7% 4 4 5% 3 5% 6% 33%	2470
May	1st to 10th	Day	24% 9% 3 7% 5% 4 5% 4 6% 6% 28%	4082
	11th to 20th	Day	20% 8% 3 8% 5% 5% 4 4 6% 7% 31%	4081
	21st to end	Day	13% 6% 2 7% 5% 5% 5% 4 7% 9% 37%	4715
June	1st to 10th	Day	13% 7% 3 7% 5% 5% 6% 5% 7% 8% 34%	2660
	11th to 20th	Day	13% 6% 3 7% 5% 6% 5% 4 8% 9% 35%	2660
	21st to end	Day	14% 7% 2 6% 5% 5% 5% 5% 6% 10% 36%	2660
July	1st to 10th	Day	10% 6% 2 7% 5% 5% 6% 4 7% 8% 40%	4281
	11th to 20th	Day	10% 6% 3 7% 5% 5% 5% 5% 6% 9% 39%	4120
	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	11% 9% 4 7% 5% 4 4 3 4 6% 44%	1835
	11th to 20th	Day	17% 14% 3 8% 4 5% 3 3 5% 6% 33%	1590
	21st to end	Day	19% 14% 5% 7% 4 4 4 3 4 5% 28%	1520
December	1st to 10th	Day	22% 17% 6% 11% 6% 5% 4 4 3 4 19%	1520
	11th to 20th	Day	20% 18% 8% 9% 4 3 3 3 4 6% 22%	1910
	21st to end	Day	24% 21% 6% 11% 5% 3 3 3 4 5% 16%	2781
January	1st to 10th	Night	18% 20% 6% 11% 5% 5% 3 4 4 5% 19%	4109
	11th to 20th	Night	18% 21% 7% 12% 6% 5% 4 3 4 4 17%	4120
	21st to end	Night	27% 17% 6% 12% 5% 3 3 2 3 3 18%	4532
February	1st to 10th	Night	20% 15% 4 10% 5% 3 4 3 4 5% 26%	2470
	11th to 20th	Night	20% 15% 4 10% 5% 5% 3 3 4 5% 25%	2403
	21st to end	Night	21% 13% 5% 10% 5% 4 4 3 4 5% 25%	1868
March	1st to 10th	Night	23% 11% 3 7% 4 3 3 3 4 6% 33%	3418
	11th to 20th	Night	16% 13% 5% 8% 4 3 3 3 4 5% 36%	3480
	21st to end	Night	18% 9% 3 7% 4 4 4 3 4 6% 39%	3817
April	1st to 10th	Night	18% 13% 4 7% 4 4 4 3 4 6% 34%	1986
	11th to 20th	Night	16% 9% 3 6% 4 3 5% 3 7% 7% 38%	1875
	21st to end	Night	16% 9% 3 6% 4 4 4 3 5% 7% 39%	1715
May	1st to 10th	Night	26% 10% 3 7% 5% 4 4 4 5% 6% 26%	2813
	11th to 20th	Night	18% 10% 3 6% 5% 5% 4 3 4 5% 36%	2812
	21st to end	Night	12% 7% 3 7% 4 4 5% 4 6% 10% 38%	2836
June	1st to 10th	Night	11% 6% 2 7% 5% 4 4 4 6% 8% 44%	1520
	11th to 20th	Night	11% 6% 2 5% 5% 5% 6% 4 8% 10% 39%	1520
	21st to end	Night	12% 5% 1 5% 5% 4 5% 5% 8% 10% 41%	1520
July	1st to 10th	Night	11% 7% 3 7% 4 4 4 4 6% 8% 41%	2673
	11th to 20th	Night	11% 5% 2 7% 5% 5% 5% 5% 6% 8% 41%	2840
	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% 9% 3 6% 5% 3 3 3 4 6% 46%	2314
	11th to 20th	Night	18% 12% 3 6% 5% 4 4 4 5% 5% 33%	2588
	21st to end	Night	20% 13% 4 7% 4 4 4 3 4 5% 31%	2660
December	1st to 10th	Night	23% 18% 6% 9% 4 3 3 3 5% 4 22%	2660
	11th to 20th	Night	20% 16% 7% 9% 5% 4 3 3 3 5% 24%	3285
	21st to end	Night	24% 18% 5% 10% 6% 4 4 3 4 6% 16%	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	57% 3 11% 22% 7%	3435
	11th to 20th	Day	52% 5% 10% 25% 8% 1	3480
	21st to end	Day	53% 5% 8% 22% 11% 1	3815
April	1st to 10th	Day	54% 4 10% 19% 11% 2	2194
	11th to 20th	Day	49% 5% 11% 16% 17% 3	2264
	21st to end	Day	51% 5% 10% 22% 11% 2	2470
May	1st to 10th	Day	56% 5% 10% 21% 8% 1	4120
	11th to 20th	Day	51% 6% 11% 23% 8% 1	4085
	21st to end	Day	40% 7% 15% 23% 12% 3	4729
June	1st to 10th	Day	44% 7% 13% 23% 11% 1	2660
	11th to 20th	Day	43% 7% 14% 23% 12% 1	2660
	21st to end	Day	43% 6% 12% 25% 13% 1	2660
July	1st to 10th	Day	40% 7% 12% 24% 14% 3	4215
	11th to 20th	Day	39% 7% 12% 27% 15% 1	4051
	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	1835
	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	54% 3 9% 24% 8% 1	3418
	11th to 20th	Night	51% 4 10% 27% 7% 1	3480
	21st to end	Night	47% 4 9% 25% 14% 2	3817
April	1st to 10th	Night	52% 6% 8% 19% 13% 2	1986
	11th to 20th	Night	45% 5% 11% 19% 15% 4	1875
	21st to end	Night	45% 5% 10% 25% 11% 3	1715
May	1st to 10th	Night	59% 4 9% 16% 10% 2	2840
	11th to 20th	Night	49% 7% 10% 24% 9% 2	2813
	21st to end	Night	40% 7% 15% 23% 11% 4	2843
June	1st to 10th	Night	38% 7% 12% 26% 16% 1	1520
	11th to 20th	Night	39% 7% 13% 26% 14% 1	1520
	21st to end	Night	36% 7% 14% 25% 16% 2	1520
July	1st to 10th	Night	40% 6% 10% 23% 16% 4	2629
	11th to 20th	Night	37% 8% 15% 25% 15% 1	2786
	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% 18% 4	2314
	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% 27% 20% 5% 1	506
April	1st to 10th	Day	28% 18% 26% 19% 7% 1	460
	11th to 20th	Day	23% 18% 30% 15% 9% 5%	452
	21st to end	Day	27% 18% 27% 17% 9% 2	460
May	1st to 10th	Day	28% 15% 27% 16% 10% 3	463
	11th to 20th	Day	23% 11% 30% 20% 10% 6%	487
	21st to end	Day	15% 8% 26% 27% 17% 7%	529
June	1st to 10th	Day	14% 9% 28% 29% 17% 3	490
	11th to 20th	Day	16% 8% 25% 27% 20% 4	490
	21st to end	Day	15% 9% 27% 28% 18% 4	490
July	1st to 10th	Day	14% 5% 21% 25% 21% 12%	485
	11th to 20th	Day	11% 7% 22% 32% 19% 9%	484
	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% 13% 29% 24% 11% 4	415
	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	26% 15% 30% 23% 6%	760
	11th to 20th	Night	21% 11% 31% 26% 10% 1	751
	21st to end	Night	20% 10% 31% 31% 7%	575
May	1st to 10th	Night	22% 7% 24% 35% 12% 1	570
	11th to 20th	Night	23% 8% 23% 33% 12% 1	561
	21st to end	Night	11% 4 22% 36% 26% 1	477
June	1st to 10th	Night	11% 6% 21% 42% 21% 1	380
	11th to 20th	Night	12% 4 22% 39% 22% 1	380
	21st to end	Night	12% 4 23% 38% 23%	380
July	1st to 10th	Night	11% 4 15% 41% 24% 6%	454
	11th to 20th	Night	11% 3 16% 38% 30% 4	552
	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	18% 10% 26% 33% 13% 1	755
	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 2024-11-01 22:47:32. Version ac07

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