Horizon Planetary Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Horizon Planetary Observatory clear sky chart from 2005-11-06 to 2026-07-16.

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% 3 8% 5% 4 5% 4 6% 7% 32%	137321
	Night	17% 11% 4 8% 5% 4 4 3 5% 6% 32%	135367
Transparency	Day	49% 5% 10% 21% 13% 2	140952
	Night	51% 5% 10% 20% 13% 2	139808
Seeing	Day	24% 12% 24% 21% 13% 5%	17103
	Night	23% 10% 22% 29% 15% 2	27454

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% 20% 8% 14% 5% 3 3 2 3 5% 17%	5258
	11th to 20th	Day	16% 19% 8% 15% 7% 4 3 2 3 3 19%	5280
	21st to end	Day	18% 15% 5% 12% 8% 5% 4 3 4 5% 21%	5780
February	1st to 10th	Day	21% 14% 5% 9% 6% 3 4 2 4 5% 27%	1890
	11th to 20th	Day	22% 15% 4 11% 4 4 4 3 4 5% 25%	1907
	21st to end	Day	21% 15% 4 9% 4 3 3 3 4 3 30%	1740
March	1st to 10th	Day	18% 15% 3 7% 4 3 3 3 4 6% 33%	5045
	11th to 20th	Day	14% 7% 3 6% 5% 4 3 4 5% 8% 41%	5090
	21st to end	Day	20% 9% 3 8% 5% 5% 4 4 5% 6% 30%	5586
April	1st to 10th	Day	16% 11% 3 8% 5% 4 4 4 5% 7% 33%	2362
	11th to 20th	Day	16% 8% 2 6% 4 4 4 3 6% 7% 39%	2547
	21st to end	Day	19% 9% 3 6% 4 4 5% 4 5% 8% 34%	2730
May	1st to 10th	Day	18% 8% 3 8% 5% 5% 6% 4 7% 8% 27%	7616
	11th to 20th	Day	16% 7% 3 8% 5% 5% 5% 4 6% 7% 33%	7626
	21st to end	Day	20% 7% 3 6% 4 5% 5% 4 6% 8% 32%	8391
June	1st to 10th	Day	13% 7% 2 6% 5% 5% 5% 5% 6% 8% 37%	2730
	11th to 20th	Day	12% 5% 3 7% 5% 6% 7% 5% 6% 9% 36%	2730
	21st to end	Day	13% 6% 2 6% 5% 4 5% 5% 7% 10% 38%	2730
July	1st to 10th	Day	9% 6% 2 7% 5% 5% 5% 4 6% 8% 42%	6030
	11th to 20th	Day	12% 7% 3 7% 5% 5% 6% 5% 7% 10% 32%	5965
	21st to end	Day	15% 8% 3 7% 5% 5% 5% 4 7% 9% 32%	6424
August	1st to 10th	Day	11% 7% 2 7% 5% 6% 6% 5% 7% 10% 33%	4234
	11th to 20th	Day	12% 5% 3 7% 5% 6% 6% 5% 7% 9% 36%	4174
	21st to end	Day	6% 4 2 6% 4 5% 5% 4 6% 10% 49%	4477
September	1st to 10th	Day	14% 6% 2 5% 4 4 3 3 5% 7% 47%	2316
	11th to 20th	Day	11% 6% 2 6% 4 5% 4 4 6% 7% 44%	2200
	21st to end	Day	19% 8% 3 7% 4 3 4 3 5% 6% 37%	2200
October	1st to 10th	Day	17% 6% 2 6% 3 2 4 2 6% 8% 45%	2045
	11th to 20th	Day	17% 7% 2 6% 4 4 3 3 5% 6% 42%	2000
	21st to end	Day	21% 8% 2 6% 3 3 4 3 4 6% 39%	2200
November	1st to 10th	Day	13% 9% 2 5% 4 4 4 3 4 6% 47%	1973
	11th to 20th	Day	15% 13% 3 6% 4 4 4 4 6% 6% 35%	1890
	21st to end	Day	19% 14% 4 7% 4 3 4 3 5% 6% 31%	1769
December	1st to 10th	Day	21% 13% 5% 16% 5% 4 3 3 3 4 23%	2680
	11th to 20th	Day	24% 15% 5% 7% 4 4 4 3 5% 5% 24%	3141
	21st to end	Day	19% 16% 5% 10% 5% 5% 5% 4 6% 6% 19%	4565
January	1st to 10th	Night	20% 18% 6% 15% 5% 3 4 3 4 4 18%	7635
	11th to 20th	Night	19% 16% 7% 15% 7% 4 3 2 3 4 20%	7680
	21st to end	Night	20% 12% 4 9% 7% 5% 4 3 4 4 27%	8401
February	1st to 10th	Night	20% 14% 4 8% 6% 4 3 3 4 5% 27%	2730
	11th to 20th	Night	22% 14% 5% 9% 5% 4 4 3 4 5% 25%	2595
	21st to end	Night	22% 14% 4 8% 4 3 4 4 4 5% 28%	2065
March	1st to 10th	Night	21% 12% 3 6% 4 4 4 3 4 5% 35%	5028
	11th to 20th	Night	13% 10% 4 5% 4 4 4 3 4 7% 41%	5090
	21st to end	Night	17% 10% 3 8% 5% 5% 5% 4 5% 6% 33%	5588
April	1st to 10th	Night	19% 11% 2 7% 4 3 4 3 4 6% 35%	2258
	11th to 20th	Night	15% 8% 2 6% 5% 4 4 3 6% 8% 38%	2039
	21st to end	Night	15% 6% 2 7% 5% 5% 4 3 6% 6% 39%	1890
May	1st to 10th	Night	22% 9% 3 8% 5% 5% 5% 4 5% 6% 27%	5237
	11th to 20th	Night	14% 7% 3 7% 5% 5% 5% 5% 7% 8% 36%	5234
	21st to end	Night	24% 11% 4 7% 5% 3 4 3 4 6% 29%	5761
June	1st to 10th	Night	12% 6% 2 6% 5% 3 6% 4 6% 8% 42%	1890
	11th to 20th	Night	11% 6% 2 7% 6% 5% 5% 4 7% 8% 39%	1890
	21st to end	Night	12% 6% 2 6% 5% 3 4 4 7% 10% 41%	1890
July	1st to 10th	Night	11% 7% 3 7% 5% 5% 5% 4 7% 9% 37%	4148
	11th to 20th	Night	15% 7% 3 8% 5% 5% 6% 4 6% 9% 33%	4102
	21st to end	Night	12% 8% 3 7% 5% 5% 5% 4 7% 9% 34%	4427
August	1st to 10th	Night	13% 7% 3 8% 6% 5% 5% 4 7% 9% 32%	2915
	11th to 20th	Night	14% 7% 3 8% 5% 4 5% 3 6% 7% 40%	2875
	21st to end	Night	7% 4 2 6% 5% 5% 4 4 5% 9% 47%	3386
September	1st to 10th	Night	12% 5% 2 7% 4 4 4 3 6% 7% 46%	2080
	11th to 20th	Night	11% 5% 1 5% 4 4 4 3 5% 6% 51%	2200
	21st to end	Night	16% 6% 3 6% 5% 4 4 3 5% 5% 43%	2200
October	1st to 10th	Night	13% 5% 2 6% 4 4 4 3 5% 7% 47%	2355
	11th to 20th	Night	15% 6% 2 7% 4 3 4 4 5% 7% 44%	2400
	21st to end	Night	22% 6% 2 5% 3 4 4 2 4 5% 41%	2640
November	1st to 10th	Night	13% 8% 2 6% 4 3 4 3 5% 5% 47%	2540
	11th to 20th	Night	20% 12% 3 6% 3 3 4 4 5% 5% 34%	2730
	21st to end	Night	19% 13% 4 7% 4 4 4 3 4 5% 33%	2846
December	1st to 10th	Night	19% 13% 4 15% 4 3 4 3 4 5% 25%	4720
	11th to 20th	Night	21% 16% 7% 9% 5% 4 3 3 3 4 25%	5269
	21st to end	Night	19% 16% 5% 10% 5% 5% 4 4 4 5% 22%	6633

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	71% 3 11% 12% 4	5259
	11th to 20th	Day	71% 3 6% 11% 8% 1	5280
	21st to end	Day	66% 3 8% 16% 6% 1	5780
February	1st to 10th	Day	61% 3 9% 19% 8% 1	1890
	11th to 20th	Day	63% 3 8% 16% 9% 1	1916
	21st to end	Day	59% 4 8% 18% 10% 1	1740
March	1st to 10th	Day	53% 3 7% 22% 11% 4	5045
	11th to 20th	Day	41% 4 7% 28% 16% 3	5090
	21st to end	Day	53% 4 7% 21% 12% 2	5586
April	1st to 10th	Day	49% 5% 10% 20% 13% 3	2362
	11th to 20th	Day	44% 4 11% 20% 17% 4	2547
	21st to end	Day	48% 5% 10% 22% 12% 2	2730
May	1st to 10th	Day	52% 6% 11% 21% 8% 2	7651
	11th to 20th	Day	48% 6% 11% 24% 11% 1	7630
	21st to end	Day	48% 7% 11% 18% 12% 5%	8405
June	1st to 10th	Day	41% 6% 13% 21% 17% 1	2730
	11th to 20th	Day	44% 7% 13% 21% 13% 1	2730
	21st to end	Day	39% 7% 15% 23% 14% 1	2730
July	1st to 10th	Day	38% 8% 11% 25% 16% 2	5965
	11th to 20th	Day	43% 9% 13% 24% 10% 1	5896
	21st to end	Day	47% 7% 11% 17% 17% 1	6240
August	1st to 10th	Day	43% 8% 13% 27% 9%	4221
	11th to 20th	Day	40% 7% 10% 29% 13%	4213
	21st to end	Day	30% 6% 14% 29% 19% 2	4477
September	1st to 10th	Day	36% 5% 10% 22% 23% 4	2316
	11th to 20th	Day	38% 5% 9% 26% 19% 3	2200
	21st to end	Day	48% 4 9% 21% 17% 1	2200
October	1st to 10th	Day	30% 4 8% 24% 32% 3	3331
	11th to 20th	Day	37% 4 10% 25% 19% 6%	3240
	21st to end	Day	32% 3 8% 32% 20% 5%	3538
November	1st to 10th	Day	39% 4 8% 27% 18% 4	1973
	11th to 20th	Day	49% 4 9% 19% 17% 3	1890
	21st to end	Day	54% 4 9% 19% 13%	1769
December	1st to 10th	Day	67% 3 8% 19% 3 1	2680
	11th to 20th	Day	62% 4 10% 17% 7%	3137
	21st to end	Day	64% 5% 10% 16% 5%	4565
January	1st to 10th	Night	70% 3 9% 14% 4	7636
	11th to 20th	Night	69% 3 6% 13% 8% 1	7680
	21st to end	Night	61% 3 10% 18% 7% 1	8401
February	1st to 10th	Night	59% 3 9% 18% 10%	2730
	11th to 20th	Night	63% 3 7% 17% 10% 1	2597
	21st to end	Night	58% 4 9% 17% 9% 3	2065
March	1st to 10th	Night	52% 4 6% 24% 12% 2	5028
	11th to 20th	Night	42% 5% 7% 25% 18% 3	5090
	21st to end	Night	51% 5% 8% 21% 14% 2	5588
April	1st to 10th	Night	50% 5% 8% 20% 15% 2	2258
	11th to 20th	Night	44% 4 12% 20% 15% 5%	2039
	21st to end	Night	45% 5% 10% 22% 16% 2	1890
May	1st to 10th	Night	56% 7% 10% 15% 11% 2	5264
	11th to 20th	Night	43% 9% 13% 23% 12% 1	5235
	21st to end	Night	55% 6% 10% 16% 10% 3	5771
June	1st to 10th	Night	37% 8% 11% 25% 16% 2	1890
	11th to 20th	Night	41% 8% 13% 19% 16% 2	1890
	21st to end	Night	37% 8% 14% 23% 16% 2	1890
July	1st to 10th	Night	41% 8% 11% 23% 15% 2	4105
	11th to 20th	Night	45% 9% 15% 21% 9% 1	4051
	21st to end	Night	44% 8% 12% 19% 15% 2	4297
August	1st to 10th	Night	45% 8% 14% 24% 8% 1	2904
	11th to 20th	Night	42% 7% 10% 22% 18% 1	2902
	21st to end	Night	32% 6% 12% 31% 18% 2	3386
September	1st to 10th	Night	36% 6% 12% 20% 20% 6%	2080
	11th to 20th	Night	34% 5% 10% 25% 23% 3	2200
	21st to end	Night	44% 5% 10% 21% 19% 1	2200
October	1st to 10th	Night	32% 4 8% 25% 25% 5%	3849
	11th to 20th	Night	34% 5% 9% 21% 23% 8%	3899
	21st to end	Night	34% 3 5% 27% 25% 6%	4252
November	1st to 10th	Night	39% 4 10% 21% 20% 6%	2540
	11th to 20th	Night	51% 4 7% 19% 16% 3	2730
	21st to end	Night	54% 4 8% 20% 13% 1	2846
December	1st to 10th	Night	62% 4 10% 19% 5% 1	4720
	11th to 20th	Night	64% 4 10% 15% 7%	5272
	21st to end	Night	63% 5% 9% 15% 7%	6633

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	34% 24% 25% 12% 5% 1	330
	11th to 20th	Day	43% 25% 25% 6% 1	330
	21st to end	Day	38% 26% 26% 8% 2 1	363
February	1st to 10th	Day	34% 22% 31% 9% 4	330
	11th to 20th	Day	51% 18% 23% 6% 1	330
	21st to end	Day	37% 19% 34% 9%	283
March	1st to 10th	Day	33% 19% 25% 18% 5%	540
	11th to 20th	Day	30% 22% 28% 14% 6% 1	540
	21st to end	Day	34% 17% 27% 18% 4	594
April	1st to 10th	Day	33% 18% 23% 17% 7% 1	540
	11th to 20th	Day	27% 16% 28% 17% 9% 4	532
	21st to end	Day	28% 16% 26% 20% 8% 1	540
May	1st to 10th	Day	28% 16% 27% 17% 8% 4	543
	11th to 20th	Day	22% 11% 30% 21% 10% 5%	567
	21st to end	Day	20% 10% 23% 23% 16% 8%	617
June	1st to 10th	Day	14% 10% 23% 28% 19% 6%	570
	11th to 20th	Day	16% 10% 24% 26% 18% 7%	570
	21st to end	Day	14% 6% 30% 26% 18% 6%	570
July	1st to 10th	Day	16% 4 18% 28% 21% 14%	565
	11th to 20th	Day	12% 4 20% 31% 21% 12%	544
	21st to end	Day	18% 4 17% 29% 21% 10%	576
August	1st to 10th	Day	13% 5% 18% 26% 25% 13%	526
	11th to 20th	Day	15% 5% 18% 28% 24% 10%	519
	21st to end	Day	11% 4 15% 32% 25% 13%	583
September	1st to 10th	Day	15% 5% 17% 25% 26% 11%	530
	11th to 20th	Day	12% 3 14% 28% 30% 13%	530
	21st to end	Day	26% 8% 19% 27% 14% 6%	530
October	1st to 10th	Day	16% 7% 22% 25% 22% 7%	530
	11th to 20th	Day	20% 12% 25% 23% 15% 4	530
	21st to end	Day	28% 13% 25% 22% 10% 2	583
November	1st to 10th	Day	22% 10% 29% 22% 13% 3	451
	11th to 20th	Day	30% 18% 26% 17% 7% 2	330
	21st to end	Day	29% 19% 28% 19% 3 1	267
December	1st to 10th	Day	39% 16% 25% 15% 4 1	220
	11th to 20th	Day	36% 18% 22% 18% 4 1	237
	21st to end	Day	38% 16% 24% 17% 5%	363
January	1st to 10th	Night	33% 16% 26% 21% 4	840
	11th to 20th	Night	35% 18% 24% 20% 3	840
	21st to end	Night	31% 17% 29% 20% 3	924
February	1st to 10th	Night	30% 13% 26% 28% 3	840
	11th to 20th	Night	34% 19% 28% 18% 1	823
	21st to end	Night	33% 16% 29% 21% 2	692
March	1st to 10th	Night	30% 14% 24% 27% 4	840
	11th to 20th	Night	30% 16% 25% 20% 7%	840
	21st to end	Night	27% 13% 31% 24% 5%	924
April	1st to 10th	Night	27% 13% 28% 26% 5%	840
	11th to 20th	Night	21% 11% 29% 28% 10% 1	795
	21st to end	Night	19% 9% 30% 31% 11% 1	630
May	1st to 10th	Night	23% 8% 25% 30% 13% 1	630
	11th to 20th	Night	19% 7% 25% 31% 16% 2	621
	21st to end	Night	18% 4 20% 31% 23% 4	683
June	1st to 10th	Night	12% 5% 18% 39% 24% 3	630
	11th to 20th	Night	13% 4 16% 38% 26% 3	630
	21st to end	Night	14% 5% 15% 40% 24% 2	630
July	1st to 10th	Night	13% 3 12% 34% 33% 5%	615
	11th to 20th	Night	13% 3 11% 34% 35% 5%	597
	21st to end	Night	14% 4 11% 32% 34% 5%	657
August	1st to 10th	Night	11% 4 9% 38% 35% 4	597
	11th to 20th	Night	12% 5% 8% 37% 34% 4	594
	21st to end	Night	13% 3 9% 34% 36% 6%	758
September	1st to 10th	Night	13% 3 13% 37% 30% 5%	800
	11th to 20th	Night	13% 4 12% 29% 37% 5%	800
	21st to end	Night	19% 5% 19% 31% 23% 3	800
October	1st to 10th	Night	16% 5% 17% 32% 27% 4	800
	11th to 20th	Night	18% 9% 22% 33% 17% 1	800
	21st to end	Night	25% 8% 21% 34% 12% 1	880
November	1st to 10th	Night	18% 9% 23% 33% 17% 1	820
	11th to 20th	Night	29% 11% 22% 26% 10%	840
	21st to end	Night	28% 10% 25% 29% 8% 1	840
December	1st to 10th	Night	33% 13% 27% 22% 5%	840
	11th to 20th	Night	28% 15% 25% 28% 5%	840
	21st to end	Night	32% 11% 26% 28% 3	924

Page generated at 2026-07-13 03:14:48. Version ac07

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