Harper
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Harper clear sky chart from 2005-11-06 to 2024-01-27.

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	10% 5% 2 5% 4 3 4 4 5% 7% 51%	74992
	Night	9% 5% 2 4 4 2 4 4 5% 7% 55%	75085
Transparency	Day	35% 4 9% 21% 21% 10%	74717
	Night	32% 4 10% 21% 21% 13%	74880
Seeing	Day	12% 9% 25% 31% 19% 4	13753
	Night	12% 9% 30% 35% 13% 1	23850

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	11% 6% 2 4 3 1 3 3 4 7% 56%	1707
	11th to 20th	Day	14% 6% 2 4 3 2 4 3 4 5% 53%	1720
	21st to end	Day	11% 5% 2 4 4 3 4 3 3 5% 56%	2040
February	1st to 10th	Day	13% 6% 2 4 3 2 4 3 4 6% 52%	1800
	11th to 20th	Day	13% 6% 3 4 4 2 4 4 5% 6% 49%	1749
	21st to end	Day	12% 6% 3 3 4 3 3 3 4 6% 53%	1480
March	1st to 10th	Day	13% 8% 3 6% 4 2 4 4 5% 7% 45%	1880
	11th to 20th	Day	13% 5% 2 5% 4 3 4 4 5% 6% 50%	1980
	21st to end	Day	11% 5% 3 4 4 2 4 4 4 6% 53%	2178
April	1st to 10th	Day	11% 6% 2 6% 4 3 4 5% 4 7% 47%	1980
	11th to 20th	Day	13% 6% 2 5% 4 3 4 3 4 6% 49%	2084
	21st to end	Day	11% 5% 3 5% 5% 3 5% 4 4 5% 52%	2222
May	1st to 10th	Day	12% 5% 3 6% 5% 3 5% 6% 6% 7% 43%	2340
	11th to 20th	Day	13% 6% 3 7% 6% 3 4 5% 6% 7% 41%	2302
	21st to end	Day	12% 5% 3 6% 6% 4 5% 6% 6% 7% 41%	2531
June	1st to 10th	Day	5% 3 2 4 5% 3 5% 5% 6% 9% 54%	2340
	11th to 20th	Day	5% 3 2 4 4 3 5% 5% 6% 9% 53%	2340
	21st to end	Day	6% 3 2 5% 5% 4 6% 6% 8% 10% 45%	2340
July	1st to 10th	Day	10% 5% 3 6% 6% 4 6% 6% 7% 9% 38%	2340
	11th to 20th	Day	4 2 2 3 5% 3 4 5% 6% 9% 56%	2340
	21st to end	Day	4 2 1 3 4 3 6% 5% 6% 11% 54%	2546
August	1st to 10th	Day	1 2 1 3 4 3 4 4 4 8% 65%	2204
	11th to 20th	Day	7% 3 2 4 6% 4 6% 5% 6% 8% 49%	2123
	21st to end	Day	6% 4 2 5% 6% 4 5% 6% 6% 11% 45%	2353
September	1st to 10th	Day	9% 4 3 7% 5% 4 5% 5% 5% 7% 46%	1980
	11th to 20th	Day	10% 7% 3 5% 7% 4 5% 4 7% 7% 41%	1980
	21st to end	Day	10% 4 2 5% 4 3 4 5% 6% 8% 49%	1980
October	1st to 10th	Day	5% 4 2 4 4 2 4 5% 6% 8% 55%	1980
	11th to 20th	Day	11% 4 2 4 4 3 4 4 4 6% 55%	1931
	21st to end	Day	11% 5% 2 4 3 2 3 3 3 5% 59%	1980
November	1st to 10th	Day	11% 6% 2 4 4 2 3 3 5% 7% 51%	1816
	11th to 20th	Day	8% 4 2 4 4 3 4 5% 5% 6% 55%	1710
	21st to end	Day	12% 6% 3 5% 5% 3 4 3 3 6% 51%	1710
December	1st to 10th	Day	13% 7% 2 4 3 2 3 4 4 7% 52%	1710
	11th to 20th	Day	10% 7% 2 4 5% 3 4 4 5% 6% 49%	2161
	21st to end	Day	10% 6% 2 4 3 2 3 3 3 5% 59%	3135
January	1st to 10th	Night	11% 6% 1 3 3 2 3 3 3 6% 57%	2458
	11th to 20th	Night	13% 5% 3 4 3 2 3 3 4 7% 54%	2470
	21st to end	Night	12% 6% 2 4 4 2 3 3 5% 6% 52%	2465
February	1st to 10th	Night	11% 6% 2 5% 4 2 4 3 4 6% 53%	2160
	11th to 20th	Night	13% 5% 2 4 4 3 4 3 4 6% 52%	2098
	21st to end	Night	12% 6% 2 5% 4 2 3 4 5% 6% 51%	1776
March	1st to 10th	Night	11% 8% 2 5% 5% 3 3 5% 5% 7% 47%	2079
	11th to 20th	Night	11% 6% 2 5% 5% 3 4 5% 4 5% 48%	1980
	21st to end	Night	8% 5% 2 4 4 2 4 4 5% 7% 55%	2178
April	1st to 10th	Night	7% 5% 3 5% 5% 3 5% 5% 6% 6% 52%	1980
	11th to 20th	Night	10% 7% 3 6% 4 3 4 4 5% 6% 50%	1822
	21st to end	Night	10% 5% 2 4 5% 2 3 4 5% 6% 54%	1738
May	1st to 10th	Night	8% 5% 2 5% 5% 4 4 5% 6% 7% 49%	1620
	11th to 20th	Night	9% 5% 1 5% 6% 2 5% 4 6% 8% 47%	1593
	21st to end	Night	10% 6% 2 5% 4 3 4 4 5% 9% 47%	1745
June	1st to 10th	Night	2 2 1 2 2 2 3 5% 7% 11% 61%	1620
	11th to 20th	Night	3 2 1 2 3 3 3 4 7% 8% 63%	1620
	21st to end	Night	5% 3 2 2 4 3 6% 4 6% 10% 56%	1620
July	1st to 10th	Night	6% 4 3 5% 5% 3 5% 5% 6% 11% 47%	1620
	11th to 20th	Night	2 2 1 2 3 1 4 4 5% 10% 66%	1620
	21st to end	Night	3 2 1 2 2 2 3 5% 5% 11% 63%	1772
August	1st to 10th	Night	1 2 1 2 2 1 3 3 4 8% 73%	1729
	11th to 20th	Night	5% 3 2 3 4 3 4 5% 7% 9% 54%	1778
	21st to end	Night	5% 4 2 4 4 3 5% 6% 7% 10% 51%	2002
September	1st to 10th	Night	7% 4 2 4 4 3 4 5% 5% 8% 52%	1980
	11th to 20th	Night	9% 6% 2 5% 4 3 5% 4 6% 9% 47%	1980
	21st to end	Night	7% 4 2 4 3 3 5% 5% 5% 7% 54%	1980
October	1st to 10th	Night	4 3 1 3 4 2 4 3 4 7% 63%	1980
	11th to 20th	Night	10% 4 2 4 4 2 3 3 4 6% 57%	2010
	21st to end	Night	12% 5% 2 3 3 2 3 3 3 4 61%	2376
November	1st to 10th	Night	10% 5% 2 4 4 2 3 3 4 5% 58%	2258
	11th to 20th	Night	8% 5% 2 4 4 2 3 3 5% 7% 58%	2470
	21st to end	Night	13% 5% 3 5% 4 2 4 4 4 6% 50%	2470
December	1st to 10th	Night	11% 6% 3 4 3 2 3 4 4 6% 55%	2470
	11th to 20th	Night	10% 8% 3 6% 4 3 4 4 4 5% 49%	3036
	21st to end	Night	12% 7% 2 4 3 2 2 2 3 5% 58%	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	31% 2 4 19% 25% 18%	1708
	11th to 20th	Day	36% 3 4 17% 24% 17%	1720
	21st to end	Day	33% 2 6% 14% 27% 18%	2040
February	1st to 10th	Day	36% 3 7% 13% 23% 18%	1800
	11th to 20th	Day	38% 3 6% 18% 20% 16%	1759
	21st to end	Day	34% 3 6% 17% 23% 18%	1480
March	1st to 10th	Day	41% 3 8% 17% 19% 13%	1880
	11th to 20th	Day	37% 3 8% 16% 19% 18%	1980
	21st to end	Day	34% 4 8% 19% 22% 14%	2178
April	1st to 10th	Day	38% 4 8% 19% 19% 13%	1980
	11th to 20th	Day	39% 3 6% 19% 21% 13%	2084
	21st to end	Day	38% 2 6% 21% 20% 12%	2222
May	1st to 10th	Day	41% 4 9% 16% 22% 8%	2340
	11th to 20th	Day	43% 4 9% 20% 20% 4	2306
	21st to end	Day	44% 5% 10% 23% 16% 2	2531
June	1st to 10th	Day	30% 4 12% 29% 21% 4	2340
	11th to 20th	Day	30% 5% 11% 28% 20% 6%	2340
	21st to end	Day	36% 7% 11% 23% 17% 5%	2340
July	1st to 10th	Day	41% 7% 14% 27% 10% 1	2276
	11th to 20th	Day	24% 5% 17% 30% 21% 3	2271
	21st to end	Day	22% 5% 24% 34% 14% 1	2405
August	1st to 10th	Day	18% 4 13% 42% 21% 2	2190
	11th to 20th	Day	32% 5% 16% 29% 13% 4	2123
	21st to end	Day	34% 5% 15% 28% 14% 4	2353
September	1st to 10th	Day	39% 5% 9% 20% 19% 7%	1980
	11th to 20th	Day	42% 5% 11% 19% 18% 5%	1980
	21st to end	Day	34% 5% 9% 21% 20% 11%	1980
October	1st to 10th	Day	29% 4 10% 22% 26% 9%	1980
	11th to 20th	Day	34% 3 6% 15% 24% 17%	1931
	21st to end	Day	31% 2 4 16% 26% 20%	1980
November	1st to 10th	Day	37% 3 7% 21% 21% 11%	1816
	11th to 20th	Day	32% 4 7% 18% 22% 17%	1710
	21st to end	Day	38% 2 5% 16% 23% 15%	1710
December	1st to 10th	Day	36% 2 8% 20% 25% 10%	1710
	11th to 20th	Day	38% 2 8% 12% 23% 17%	2159
	21st to end	Day	32% 1 4 16% 29% 18%	3135
January	1st to 10th	Night	32% 2 4 16% 25% 21%	2458
	11th to 20th	Night	34% 2 4 18% 24% 18%	2470
	21st to end	Night	36% 2 5% 14% 23% 20%	2465
February	1st to 10th	Night	36% 2 8% 12% 22% 20%	2160
	11th to 20th	Night	36% 3 5% 17% 21% 18%	2098
	21st to end	Night	37% 3 6% 13% 22% 20%	1776
March	1st to 10th	Night	39% 3 8% 16% 22% 12%	2079
	11th to 20th	Night	40% 3 7% 15% 18% 18%	1980
	21st to end	Night	32% 4 10% 19% 18% 16%	2178
April	1st to 10th	Night	34% 4 9% 20% 17% 16%	1980
	11th to 20th	Night	37% 3 8% 16% 19% 17%	1822
	21st to end	Night	35% 3 8% 21% 21% 11%	1738
May	1st to 10th	Night	36% 4 10% 21% 19% 10%	1620
	11th to 20th	Night	37% 4 12% 24% 17% 5%	1594
	21st to end	Night	36% 5% 14% 28% 14% 2	1746
June	1st to 10th	Night	19% 5% 18% 34% 21% 3	1620
	11th to 20th	Night	21% 5% 15% 31% 22% 6%	1620
	21st to end	Night	27% 5% 16% 29% 19% 4	1620
July	1st to 10th	Night	31% 8% 21% 29% 10% 1	1575
	11th to 20th	Night	17% 5% 19% 33% 23% 3	1566
	21st to end	Night	17% 6% 26% 37% 14% 1	1673
August	1st to 10th	Night	14% 3 19% 43% 20% 1	1719
	11th to 20th	Night	25% 7% 20% 30% 15% 4	1778
	21st to end	Night	29% 6% 21% 26% 14% 3	2002
September	1st to 10th	Night	31% 5% 13% 24% 19% 8%	1980
	11th to 20th	Night	35% 6% 15% 23% 16% 5%	1980
	21st to end	Night	31% 4 11% 22% 20% 12%	1980
October	1st to 10th	Night	24% 3 13% 27% 22% 11%	1980
	11th to 20th	Night	31% 3 9% 17% 22% 17%	2010
	21st to end	Night	31% 1 5% 18% 23% 22%	2376
November	1st to 10th	Night	32% 3 7% 21% 24% 14%	2258
	11th to 20th	Night	29% 4 7% 23% 21% 17%	2470
	21st to end	Night	38% 3 7% 16% 21% 16%	2470
December	1st to 10th	Night	33% 2 9% 19% 24% 13%	2470
	11th to 20th	Night	40% 2 4 14% 19% 21%	3037
	21st to end	Night	33% 1 4 15% 29% 17%	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	20% 17% 37% 21% 4 1	270
	11th to 20th	Day	23% 15% 32% 23% 6%	270
	21st to end	Day	16% 17% 43% 21% 3	282
February	1st to 10th	Day	23% 14% 36% 21% 6%	240
	11th to 20th	Day	24% 15% 40% 17% 3 1	240
	21st to end	Day	19% 25% 35% 19% 2	198
March	1st to 10th	Day	20% 18% 44% 16% 3	240
	11th to 20th	Day	18% 20% 39% 17% 7%	240
	21st to end	Day	17% 19% 45% 14% 3 1	264
April	1st to 10th	Day	17% 22% 41% 18% 3	240
	11th to 20th	Day	15% 23% 39% 18% 5% 1	362
	21st to end	Day	15% 17% 44% 19% 5%	482
May	1st to 10th	Day	14% 14% 41% 26% 5%	603
	11th to 20th	Day	19% 9% 33% 28% 9% 1	625
	21st to end	Day	14% 9% 31% 34% 11% 1	679
June	1st to 10th	Day	5% 4 21% 45% 21% 3	630
	11th to 20th	Day	6% 2 22% 50% 16% 3	630
	21st to end	Day	5% 2 18% 41% 29% 5%	630
July	1st to 10th	Day	7% 4 13% 37% 31% 8%	621
	11th to 20th	Day	3 1 6% 46% 38% 6%	618
	21st to end	Day	3 1 6% 41% 38% 11%	684
August	1st to 10th	Day	2 3 44% 42% 9%	502
	11th to 20th	Day	10% 1 6% 33% 38% 12%	437
	21st to end	Day	7% 1 7% 32% 40% 14%	472
September	1st to 10th	Day	10% 6% 31% 38% 16%	270
	11th to 20th	Day	6% 2 6% 33% 40% 13%	270
	21st to end	Day	12% 2 6% 33% 39% 9%	270
October	1st to 10th	Day	7% 4 17% 38% 29% 6%	270
	11th to 20th	Day	11% 6% 24% 39% 16% 4	270
	21st to end	Day	17% 16% 29% 25% 12% 1	297
November	1st to 10th	Day	18% 7% 33% 29% 9% 3	270
	11th to 20th	Day	19% 14% 29% 24% 10% 4	270
	21st to end	Day	21% 24% 27% 19% 7% 2	270
December	1st to 10th	Day	14% 11% 34% 30% 8% 2	270
	11th to 20th	Day	17% 17% 39% 19% 7%	270
	21st to end	Day	18% 16% 34% 25% 7%	297
January	1st to 10th	Night	15% 11% 36% 30% 7% 1	950
	11th to 20th	Night	17% 11% 37% 30% 5%	950
	21st to end	Night	16% 12% 36% 28% 7%	825
February	1st to 10th	Night	18% 14% 35% 27% 6%	720
	11th to 20th	Night	17% 14% 38% 27% 3	704
	21st to end	Night	19% 15% 46% 19% 1	592
March	1st to 10th	Night	16% 15% 41% 25% 2	639
	11th to 20th	Night	16% 11% 40% 28% 4	540
	21st to end	Night	12% 13% 49% 22% 3	594
April	1st to 10th	Night	7% 17% 50% 23% 4	540
	11th to 20th	Night	12% 18% 46% 21% 2	533
	21st to end	Night	12% 13% 49% 20% 6% 1	540
May	1st to 10th	Night	10% 15% 46% 25% 4	540
	11th to 20th	Night	12% 8% 40% 33% 8% 1	531
	21st to end	Night	10% 10% 37% 33% 10%	584
June	1st to 10th	Night	3 2 27% 50% 16% 1	540
	11th to 20th	Night	4 2 37% 46% 10%	540
	21st to end	Night	5% 5% 20% 48% 21% 2	540
July	1st to 10th	Night	6% 4 13% 49% 26% 2	525
	11th to 20th	Night	2 1 10% 59% 26% 1	522
	21st to end	Night	3 2 9% 59% 25% 3	591
August	1st to 10th	Night	1 9% 57% 30% 2	537
	11th to 20th	Night	6% 1 8% 50% 32% 3	534
	21st to end	Night	5% 1 9% 44% 36% 4	594
September	1st to 10th	Night	8% 3 11% 43% 31% 4	540
	11th to 20th	Night	12% 1 8% 42% 35% 2	540
	21st to end	Night	10% 2 12% 45% 30% 1	540
October	1st to 10th	Night	5% 3 17% 47% 27% 2	540
	11th to 20th	Night	11% 7% 24% 41% 15% 2	570
	21st to end	Night	14% 10% 29% 28% 17% 2	792
November	1st to 10th	Night	12% 8% 27% 39% 13% 1	778
	11th to 20th	Night	11% 9% 31% 35% 13% 2	950
	21st to end	Night	17% 11% 30% 30% 11% 1	950
December	1st to 10th	Night	14% 9% 31% 36% 8% 1	950
	11th to 20th	Night	17% 12% 35% 30% 6%	950
	21st to end	Night	15% 12% 35% 32% 6%	1045

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