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 2024-02-10.

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	18% 10% 4 6% 6% 3 5% 5% 5% 7% 33%	78164
	Night	18% 10% 4 6% 5% 3 4 4 5% 6% 34%	80807
Transparency	Day	51% 5% 10% 21% 12% 2	77885
	Night	51% 5% 10% 20% 12% 2	80605
Seeing	Day	24% 11% 23% 22% 14% 6%	13849
	Night	23% 9% 21% 29% 16% 2	24257

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% 20% 9% 8% 5% 2 3 4 3 6% 19%	2836
	11th to 20th	Day	20% 20% 9% 10% 10% 3 3 2 2 3 18%	2840
	21st to end	Day	24% 19% 5% 8% 7% 3 3 3 3 5% 19%	3124
February	1st to 10th	Day	21% 13% 6% 8% 6% 2 4 3 3 5% 27%	1701
	11th to 20th	Day	22% 14% 5% 10% 4 4 4 4 4 4 25%	1627
	21st to end	Day	24% 14% 5% 8% 5% 3 3 4 4 4 28%	1488
March	1st to 10th	Day	17% 13% 4 6% 4 2 4 4 4 6% 36%	1980
	11th to 20th	Day	21% 11% 5% 5% 6% 3 4 4 5% 7% 30%	1980
	21st to end	Day	23% 10% 4 6% 5% 4 4 5% 5% 6% 30%	2178
April	1st to 10th	Day	16% 10% 4 7% 6% 3 4 6% 5% 7% 33%	2024
	11th to 20th	Day	17% 8% 3 5% 4 3 5% 3 5% 7% 40%	2178
	21st to end	Day	21% 8% 3 5% 5% 2 5% 5% 5% 7% 34%	2340
May	1st to 10th	Day	20% 7% 3 6% 7% 4 6% 6% 5% 6% 30%	2340
	11th to 20th	Day	22% 6% 3 6% 6% 3 5% 5% 5% 6% 32%	2301
	21st to end	Day	12% 6% 2 5% 6% 3 6% 6% 7% 9% 37%	2531
June	1st to 10th	Day	13% 6% 3 5% 6% 3 5% 6% 6% 8% 38%	2340
	11th to 20th	Day	13% 5% 3 5% 6% 5% 7% 7% 6% 9% 36%	2340
	21st to end	Day	14% 5% 3 5% 5% 3 5% 6% 7% 11% 37%	2340
July	1st to 10th	Day	12% 6% 3 5% 6% 4 6% 6% 6% 9% 37%	2340
	11th to 20th	Day	12% 6% 3 5% 6% 4 6% 6% 6% 9% 38%	2340
	21st to end	Day	15% 6% 3 6% 6% 4 6% 5% 6% 10% 33%	2547
August	1st to 10th	Day	12% 6% 2 5% 7% 5% 6% 6% 8% 10% 32%	2327
	11th to 20th	Day	14% 5% 3 6% 6% 4 6% 6% 7% 10% 33%	2303
	21st to end	Day	10% 6% 2 6% 6% 3 5% 5% 6% 10% 40%	2466
September	1st to 10th	Day	14% 6% 2 4 5% 3 4 4 5% 8% 45%	2085
	11th to 20th	Day	12% 6% 3 5% 5% 3 4 5% 6% 7% 44%	1980
	21st to end	Day	18% 7% 3 6% 5% 3 4 4 5% 6% 40%	1980
October	1st to 10th	Day	17% 6% 2 6% 4 2 4 3 6% 8% 42%	1841
	11th to 20th	Day	18% 7% 3 5% 5% 3 4 3 5% 7% 40%	1800
	21st to end	Day	23% 7% 2 6% 3 2 4 4 4 6% 38%	1980
November	1st to 10th	Day	14% 7% 3 4 4 2 4 4 4 6% 49%	1781
	11th to 20th	Day	15% 11% 4 6% 4 3 4 4 6% 6% 35%	1710
	21st to end	Day	19% 12% 3 6% 5% 3 4 4 5% 6% 33%	1601
December	1st to 10th	Day	27% 13% 6% 8% 5% 3 4 4 3 5% 21%	1520
	11th to 20th	Day	23% 16% 6% 6% 5% 2 4 3 4 5% 27%	1951
	21st to end	Day	21% 19% 5% 8% 5% 4 4 5% 6% 6% 17%	3124
January	1st to 10th	Night	19% 22% 7% 8% 6% 3 4 4 4 4 20%	4109
	11th to 20th	Night	23% 16% 6% 11% 7% 4 3 3 3 4 20%	4120
	21st to end	Night	29% 16% 5% 7% 7% 2 3 3 3 4 22%	4532
February	1st to 10th	Night	21% 13% 5% 7% 6% 3 4 4 4 6% 27%	2464
	11th to 20th	Night	23% 13% 4 8% 6% 3 4 5% 4 5% 25%	2215
	21st to end	Night	24% 13% 4 6% 5% 2 4 4 4 4 28%	1768
March	1st to 10th	Night	18% 11% 4 5% 6% 3 4 4 4 5% 36%	1980
	11th to 20th	Night	23% 9% 4 6% 5% 3 4 3 3 7% 32%	1980
	21st to end	Night	22% 10% 4 6% 5% 3 4 5% 5% 5% 33%	2178
April	1st to 10th	Night	20% 9% 2 6% 4 2 5% 4 4 6% 36%	1936
	11th to 20th	Night	15% 8% 2 5% 5% 3 4 3 6% 8% 40%	1745
	21st to end	Night	17% 6% 3 6% 5% 4 5% 4 6% 6% 38%	1620
May	1st to 10th	Night	19% 6% 3 6% 6% 5% 5% 5% 5% 7% 34%	1620
	11th to 20th	Night	18% 8% 3 5% 5% 4 5% 5% 6% 8% 33%	1592
	21st to end	Night	13% 5% 2 6% 6% 3 4 4 5% 10% 42%	1744
June	1st to 10th	Night	11% 5% 2 5% 5% 2 5% 6% 6% 8% 44%	1620
	11th to 20th	Night	10% 6% 2 6% 6% 4 5% 5% 6% 8% 40%	1620
	21st to end	Night	12% 5% 2 5% 5% 2 5% 5% 6% 9% 43%	1620
July	1st to 10th	Night	11% 6% 3 6% 5% 4 4 5% 6% 10% 38%	1620
	11th to 20th	Night	11% 4 2 5% 6% 3 6% 5% 6% 10% 43%	1620
	21st to end	Night	12% 5% 2 4 5% 4 5% 6% 8% 10% 39%	1771
August	1st to 10th	Night	9% 5% 3 6% 7% 4 6% 5% 8% 9% 37%	1611
	11th to 20th	Night	12% 5% 3 6% 6% 3 5% 5% 6% 8% 42%	1598
	21st to end	Night	11% 5% 3 5% 5% 3 4 5% 5% 10% 47%	1870
September	1st to 10th	Night	12% 4 3 6% 5% 3 4 4 6% 7% 45%	1872
	11th to 20th	Night	12% 5% 1 5% 4 2 4 4 5% 6% 50%	1980
	21st to end	Night	16% 6% 3 5% 5% 3 4 4 4 6% 46%	1980
October	1st to 10th	Night	14% 4 2 5% 5% 3 5% 4 6% 7% 45%	2119
	11th to 20th	Night	16% 6% 2 6% 5% 3 4 4 4 7% 43%	2160
	21st to end	Night	24% 6% 2 5% 3 2 5% 3 4 6% 40%	2376
November	1st to 10th	Night	13% 8% 2 5% 5% 3 4 4 5% 5% 47%	2292
	11th to 20th	Night	19% 11% 3 5% 4 2 5% 5% 5% 6% 35%	2470
	21st to end	Night	19% 11% 4 6% 5% 3 5% 4 4 5% 34%	2574
December	1st to 10th	Night	24% 15% 5% 8% 5% 2 4 3 4 5% 23%	2660
	11th to 20th	Night	21% 16% 6% 7% 5% 3 4 3 3 5% 26%	3239
	21st to end	Night	22% 17% 5% 8% 6% 4 4 4 4 5% 22%	4532

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	68% 4 10% 13% 6%	2837
	11th to 20th	Day	74% 3 5% 13% 5%	2840
	21st to end	Day	70% 2 6% 16% 5% 1	3124
February	1st to 10th	Day	62% 3 9% 19% 7% 1	1701
	11th to 20th	Day	64% 3 8% 17% 7% 1	1636
	21st to end	Day	61% 4 8% 17% 10% 1	1488
March	1st to 10th	Day	51% 3 9% 24% 12% 1	1980
	11th to 20th	Day	54% 4 9% 19% 13% 2	1980
	21st to end	Day	56% 4 7% 19% 10% 3	2178
April	1st to 10th	Day	49% 5% 10% 19% 13% 4	2024
	11th to 20th	Day	44% 3 11% 19% 18% 4	2178
	21st to end	Day	50% 5% 11% 21% 12% 2	2340
May	1st to 10th	Day	54% 5% 10% 19% 10% 2	2340
	11th to 20th	Day	52% 5% 13% 18% 11% 1	2305
	21st to end	Day	42% 7% 15% 24% 11% 1	2531
June	1st to 10th	Day	42% 6% 13% 22% 16% 1	2340
	11th to 20th	Day	44% 7% 13% 21% 13% 1	2340
	21st to end	Day	40% 7% 15% 23% 14% 1	2340
July	1st to 10th	Day	43% 7% 13% 20% 14% 3	2275
	11th to 20th	Day	41% 6% 13% 22% 15% 3	2271
	21st to end	Day	44% 7% 12% 24% 12% 2	2405
August	1st to 10th	Day	43% 8% 14% 25% 9% 1	2314
	11th to 20th	Day	45% 6% 12% 26% 11% 1	2303
	21st to end	Day	39% 6% 14% 24% 14% 3	2466
September	1st to 10th	Day	37% 5% 10% 23% 20% 4	2085
	11th to 20th	Day	38% 5% 9% 27% 18% 3	1980
	21st to end	Day	45% 4 9% 22% 19% 1	1980
October	1st to 10th	Day	40% 4 12% 23% 18% 2	1841
	11th to 20th	Day	45% 4 9% 26% 16% 1	1800
	21st to end	Day	48% 4 7% 24% 15% 2	1980
November	1st to 10th	Day	39% 4 8% 27% 18% 4	1781
	11th to 20th	Day	48% 4 9% 19% 17% 3	1710
	21st to end	Day	52% 4 9% 20% 15%	1601
December	1st to 10th	Day	67% 2 7% 18% 5% 1	1520
	11th to 20th	Day	61% 2 9% 18% 8% 1	1947
	21st to end	Day	66% 5% 11% 14% 4	3124
January	1st to 10th	Night	69% 3 8% 14% 6% 1	4109
	11th to 20th	Night	69% 4 8% 15% 4	4120
	21st to end	Night	69% 3 7% 16% 5% 1	4532
February	1st to 10th	Night	59% 4 9% 18% 9%	2464
	11th to 20th	Night	63% 3 7% 17% 9% 1	2217
	21st to end	Night	60% 3 8% 16% 9% 3	1768
March	1st to 10th	Night	51% 3 8% 24% 12% 2	1980
	11th to 20th	Night	54% 3 10% 20% 12% 1	1980
	21st to end	Night	54% 4 7% 18% 13% 3	2178
April	1st to 10th	Night	49% 5% 7% 20% 15% 3	1936
	11th to 20th	Night	43% 3 12% 19% 16% 6%	1745
	21st to end	Night	46% 4 11% 21% 16% 2	1620
May	1st to 10th	Night	50% 6% 10% 19% 12% 3	1620
	11th to 20th	Night	48% 6% 12% 19% 11% 2	1593
	21st to end	Night	39% 5% 17% 27% 11% 1	1745
June	1st to 10th	Night	37% 8% 11% 27% 15% 2	1620
	11th to 20th	Night	40% 9% 14% 19% 16% 3	1620
	21st to end	Night	36% 8% 15% 25% 15% 2	1620
July	1st to 10th	Night	41% 8% 12% 21% 15% 3	1575
	11th to 20th	Night	35% 8% 14% 23% 17% 3	1566
	21st to end	Night	37% 8% 14% 23% 16% 1	1672
August	1st to 10th	Night	40% 8% 15% 24% 13% 1	1600
	11th to 20th	Night	39% 8% 13% 23% 15% 1	1598
	21st to end	Night	35% 6% 13% 27% 17% 4	1870
September	1st to 10th	Night	36% 6% 13% 21% 19% 6%	1872
	11th to 20th	Night	34% 5% 11% 26% 22% 3	1980
	21st to end	Night	40% 5% 10% 23% 21% 1	1980
October	1st to 10th	Night	38% 5% 11% 25% 18% 3	2119
	11th to 20th	Night	41% 5% 8% 23% 20% 2	2160
	21st to end	Night	47% 4 5% 24% 17% 3	2376
November	1st to 10th	Night	39% 4 10% 21% 20% 6%	2292
	11th to 20th	Night	50% 4 7% 19% 17% 3	2470
	21st to end	Night	53% 4 8% 20% 14% 2	2574
December	1st to 10th	Night	64% 3 7% 18% 6% 1	2660
	11th to 20th	Night	62% 4 9% 16% 8%	3242
	21st to end	Night	65% 5% 8% 15% 6%	4532

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	33% 23% 26% 12% 5% 1	270
	11th to 20th	Day	42% 23% 27% 7% 1	270
	21st to end	Day	38% 24% 26% 9% 3 1	297
February	1st to 10th	Day	35% 19% 32% 9% 4	267
	11th to 20th	Day	50% 20% 25% 5%	240
	21st to end	Day	44% 17% 28% 10%	206
March	1st to 10th	Day	31% 19% 27% 19% 4	420
	11th to 20th	Day	30% 22% 26% 14% 6% 1	420
	21st to end	Day	34% 16% 25% 19% 5%	462
April	1st to 10th	Day	32% 17% 23% 19% 8% 1	420
	11th to 20th	Day	26% 13% 27% 18% 10% 5%	412
	21st to end	Day	32% 15% 25% 18% 8% 2	420
May	1st to 10th	Day	29% 13% 26% 18% 9% 5%	423
	11th to 20th	Day	24% 10% 29% 22% 11% 5%	447
	21st to end	Day	17% 6% 23% 25% 18% 10%	485
June	1st to 10th	Day	14% 8% 22% 30% 19% 7%	450
	11th to 20th	Day	19% 7% 22% 25% 19% 8%	450
	21st to end	Day	15% 5% 30% 28% 17% 5%	450
July	1st to 10th	Day	16% 3 18% 27% 20% 16%	445
	11th to 20th	Day	11% 3 20% 31% 21% 13%	444
	21st to end	Day	18% 3 17% 30% 21% 11%	488
August	1st to 10th	Day	13% 4 17% 25% 26% 14%	446
	11th to 20th	Day	16% 4 16% 28% 24% 11%	439
	21st to end	Day	12% 4 15% 31% 24% 14%	495
September	1st to 10th	Day	16% 5% 17% 23% 28% 11%	450
	11th to 20th	Day	13% 3 15% 28% 29% 12%	450
	21st to end	Day	24% 8% 18% 27% 16% 7%	450
October	1st to 10th	Day	17% 7% 23% 26% 21% 6%	450
	11th to 20th	Day	21% 12% 26% 23% 15% 3	450
	21st to end	Day	29% 12% 23% 22% 11% 2	495
November	1st to 10th	Day	22% 9% 27% 24% 15% 3	379
	11th to 20th	Day	28% 18% 25% 19% 9% 2	270
	21st to end	Day	24% 18% 32% 21% 4 1	219
December	1st to 10th	Day	37% 17% 22% 17% 5% 1	180
	11th to 20th	Day	36% 17% 22% 20% 4 2	193
	21st to end	Day	38% 15% 26% 16% 6%	297
January	1st to 10th	Night	33% 16% 25% 22% 4	760
	11th to 20th	Night	34% 16% 24% 22% 3	760
	21st to end	Night	32% 16% 28% 21% 3	836
February	1st to 10th	Night	30% 13% 26% 28% 3	759
	11th to 20th	Night	34% 18% 29% 18% 1	703
	21st to end	Night	34% 15% 28% 21% 2	592
March	1st to 10th	Night	29% 14% 25% 28% 4	720
	11th to 20th	Night	30% 16% 25% 20% 8% 1	720
	21st to end	Night	28% 11% 30% 25% 6%	792
April	1st to 10th	Night	27% 13% 28% 27% 6% 1	720
	11th to 20th	Night	20% 11% 29% 27% 12% 1	681
	21st to end	Night	20% 8% 30% 29% 11% 1	540
May	1st to 10th	Night	23% 7% 25% 31% 13% 1	540
	11th to 20th	Night	21% 7% 24% 31% 15% 2	531
	21st to end	Night	15% 3 18% 33% 26% 5%	584
June	1st to 10th	Night	11% 4 17% 40% 24% 3	540
	11th to 20th	Night	13% 3 16% 37% 28% 3	540
	21st to end	Night	13% 4 16% 42% 24% 1	540
July	1st to 10th	Night	14% 3 12% 34% 32% 5%	525
	11th to 20th	Night	11% 2 11% 35% 36% 4	522
	21st to end	Night	14% 3 11% 32% 34% 6%	591
August	1st to 10th	Night	10% 4 7% 38% 37% 4	537
	11th to 20th	Night	13% 4 8% 36% 35% 4	534
	21st to end	Night	14% 2 9% 33% 36% 6%	682
September	1st to 10th	Night	13% 3 13% 35% 31% 6%	720
	11th to 20th	Night	14% 3 13% 28% 37% 5%	720
	21st to end	Night	18% 4 18% 31% 25% 3	720
October	1st to 10th	Night	17% 4 18% 31% 25% 4	720
	11th to 20th	Night	19% 8% 22% 32% 16% 1	720
	21st to end	Night	27% 8% 20% 32% 12% 1	792
November	1st to 10th	Night	18% 8% 22% 33% 18% 1	740
	11th to 20th	Night	28% 11% 22% 27% 11% 1	760
	21st to end	Night	27% 10% 24% 30% 9% 1	760
December	1st to 10th	Night	34% 13% 25% 23% 5%	760
	11th to 20th	Night	28% 15% 24% 28% 5%	760
	21st to end	Night	33% 11% 25% 28% 3	836

Page generated at 2024-02-07 06:11:58. Version ac07

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