Harrison
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Harrison clear sky chart from 2005-11-06 to 2024-01-14.

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	14% 10% 4 8% 7% 4 5% 5% 5% 7% 32%	74519
	Night	15% 10% 4 7% 6% 3 5% 5% 5% 7% 33%	75234
Transparency	Day	51% 4 11% 21% 11% 1	74220
	Night	51% 4 12% 21% 11% 1	75054
Seeing	Day	20% 14% 31% 21% 10% 4	14484
	Night	20% 12% 30% 27% 10% 1	24489

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	15% 14% 6% 13% 8% 6% 6% 4 5% 6% 17%	1517
	11th to 20th	Day	16% 16% 8% 14% 9% 6% 6% 5% 4 5% 11%	1464
	21st to end	Day	13% 17% 11% 13% 9% 5% 6% 4 4 5% 14%	1584
February	1st to 10th	Day	12% 14% 6% 11% 11% 5% 6% 4 5% 5% 19%	1606
	11th to 20th	Day	12% 14% 6% 12% 12% 5% 6% 4 4 5% 20%	1734
	21st to end	Day	12% 15% 8% 11% 9% 3 5% 4 5% 6% 22%	1480
March	1st to 10th	Day	13% 11% 4 9% 8% 5% 4 4 6% 6% 29%	1800
	11th to 20th	Day	12% 10% 6% 8% 7% 3 5% 5% 5% 6% 33%	1953
	21st to end	Day	17% 11% 4 7% 7% 4 4 4 5% 5% 31%	2213
April	1st to 10th	Day	18% 13% 6% 8% 6% 3 4 4 5% 6% 29%	2160
	11th to 20th	Day	18% 11% 4 6% 6% 3 4 4 4 6% 35%	2240
	21st to end	Day	15% 10% 4 7% 7% 3 6% 4 5% 7% 32%	2340
May	1st to 10th	Day	17% 8% 3 6% 6% 4 6% 5% 5% 7% 34%	2340
	11th to 20th	Day	16% 6% 3 6% 6% 3 5% 5% 5% 7% 38%	2301
	21st to end	Day	11% 6% 3 5% 6% 4 5% 5% 6% 8% 41%	2721
June	1st to 10th	Day	12% 6% 2 6% 6% 3 5% 4 6% 8% 41%	2530
	11th to 20th	Day	12% 6% 3 5% 5% 3 5% 5% 6% 9% 42%	2688
	21st to end	Day	11% 6% 3 6% 5% 4 6% 5% 6% 8% 40%	2520
July	1st to 10th	Day	6% 4 2 5% 6% 3 6% 6% 5% 10% 47%	2520
	11th to 20th	Day	8% 5% 3 6% 6% 4 5% 5% 5% 9% 44%	2520
	21st to end	Day	8% 4 2 5% 6% 4 7% 7% 7% 9% 41%	2743
August	1st to 10th	Day	9% 5% 2 5% 8% 4 7% 5% 6% 9% 40%	2343
	11th to 20th	Day	10% 5% 3 5% 6% 4 5% 5% 6% 9% 43%	2303
	21st to end	Day	12% 7% 2 5% 6% 3 4 5% 6% 8% 42%	2491
September	1st to 10th	Day	13% 5% 3 6% 5% 4 6% 5% 6% 9% 39%	2106
	11th to 20th	Day	12% 5% 2 5% 5% 3 5% 5% 5% 8% 44%	1980
	21st to end	Day	13% 6% 3 7% 6% 4 4 5% 5% 9% 37%	1980
October	1st to 10th	Day	16% 9% 3 6% 6% 3 3 4 5% 5% 39%	1976
	11th to 20th	Day	21% 11% 5% 8% 6% 4 5% 4 5% 7% 24%	1742
	21st to end	Day	21% 14% 4 9% 6% 4 4 5% 5% 6% 22%	1782
November	1st to 10th	Day	13% 11% 4 7% 7% 3 5% 4 5% 7% 33%	1672
	11th to 20th	Day	14% 14% 6% 10% 8% 3 6% 5% 5% 6% 22%	1710
	21st to end	Day	20% 17% 5% 10% 8% 3 4 4 3 5% 20%	1649
December	1st to 10th	Day	15% 16% 9% 14% 8% 3 5% 4 3 5% 17%	1378
	11th to 20th	Day	15% 17% 6% 10% 8% 4 6% 5% 6% 5% 18%	1675
	21st to end	Day	25% 22% 8% 11% 7% 3 4 4 3 4 10%	2758
January	1st to 10th	Night	17% 12% 6% 11% 8% 5% 6% 5% 4 5% 20%	2648
	11th to 20th	Night	17% 15% 7% 10% 8% 5% 6% 5% 5% 5% 17%	2575
	21st to end	Night	11% 15% 8% 12% 8% 4 7% 6% 6% 6% 17%	2772
February	1st to 10th	Night	12% 12% 6% 10% 10% 5% 7% 5% 5% 6% 21%	2352
	11th to 20th	Night	14% 11% 6% 10% 7% 5% 5% 5% 5% 7% 24%	2111
	21st to end	Night	16% 13% 6% 10% 8% 4 5% 4 5% 5% 25%	1776
March	1st to 10th	Night	14% 10% 4 7% 6% 3 5% 4 5% 7% 35%	2160
	11th to 20th	Night	14% 7% 4 6% 5% 3 5% 5% 5% 8% 38%	2007
	21st to end	Night	17% 9% 4 7% 6% 4 4 4 5% 6% 35%	2160
April	1st to 10th	Night	15% 11% 4 7% 5% 3 4 3 5% 7% 34%	1800
	11th to 20th	Night	19% 8% 4 5% 4 2 4 4 5% 7% 38%	1669
	21st to end	Night	17% 9% 3 4 5% 3 5% 4 5% 8% 37%	1620
May	1st to 10th	Night	14% 7% 3 5% 6% 3 5% 6% 6% 8% 37%	1620
	11th to 20th	Night	14% 6% 3 6% 6% 3 5% 5% 6% 8% 38%	1592
	21st to end	Night	13% 5% 2 4 4 3 4 4 5% 8% 47%	1552
June	1st to 10th	Night	13% 6% 3 4 4 3 5% 4 5% 8% 46%	1418
	11th to 20th	Night	11% 5% 3 4 5% 3 5% 5% 7% 8% 45%	1260
	21st to end	Night	8% 5% 3 5% 5% 3 5% 4 6% 8% 47%	1438
July	1st to 10th	Night	6% 3 2 4 4 2 3 4 6% 11% 55%	1440
	11th to 20th	Night	6% 4 2 5% 5% 2 5% 4 6% 8% 53%	1440
	21st to end	Night	7% 5% 2 4 3 3 5% 5% 6% 8% 52%	1575
August	1st to 10th	Night	7% 4 2 4 5% 3 4 4 8% 10% 50%	1593
	11th to 20th	Night	8% 3 2 4 4 3 4 5% 6% 8% 53%	1598
	21st to end	Night	11% 5% 2 5% 4 2 4 4 6% 9% 50%	1862
September	1st to 10th	Night	9% 5% 3 4 4 3 5% 5% 5% 8% 50%	1836
	11th to 20th	Night	11% 4 2 4 4 2 4 4 5% 7% 53%	1980
	21st to end	Night	14% 6% 3 6% 5% 2 4 5% 6% 8% 42%	1980
October	1st to 10th	Night	17% 7% 2 5% 5% 2 4 4 5% 6% 43%	1984
	11th to 20th	Night	19% 12% 3 7% 6% 3 4 4 4 5% 34%	2217
	21st to end	Night	23% 10% 4 6% 6% 3 5% 4 4 6% 30%	2574
November	1st to 10th	Night	11% 11% 3 7% 6% 3 4 5% 6% 7% 38%	2406
	11th to 20th	Night	15% 12% 4 9% 7% 3 6% 5% 6% 6% 27%	2470
	21st to end	Night	18% 15% 7% 8% 7% 3 5% 5% 5% 6% 22%	2527
December	1st to 10th	Night	16% 14% 7% 12% 8% 4 6% 4 4 6% 20%	2798
	11th to 20th	Night	14% 16% 5% 12% 9% 4 5% 5% 6% 6% 17%	3522
	21st to end	Night	27% 21% 6% 10% 6% 3 4 4 3 4 12%	4902

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% 3 11% 12% 6%	1518
	11th to 20th	Day	76% 4 8% 10% 2	1464
	21st to end	Day	75% 2 7% 11% 5%	1584
February	1st to 10th	Day	67% 4 9% 16% 4	1606
	11th to 20th	Day	66% 3 9% 13% 8% 1	1744
	21st to end	Day	64% 3 10% 16% 6% 1	1480
March	1st to 10th	Day	55% 3 12% 19% 9% 1	1800
	11th to 20th	Day	51% 4 11% 19% 11% 3	1953
	21st to end	Day	54% 4 10% 20% 10% 1	2213
April	1st to 10th	Day	57% 4 10% 17% 12%	2160
	11th to 20th	Day	52% 3 9% 19% 15% 2	2240
	21st to end	Day	52% 4 12% 16% 14% 2	2340
May	1st to 10th	Day	49% 6% 9% 23% 12% 2	2340
	11th to 20th	Day	46% 4 12% 23% 13% 2	2305
	21st to end	Day	40% 6% 13% 26% 14% 1	2721
June	1st to 10th	Day	40% 4 12% 28% 15% 2	2530
	11th to 20th	Day	39% 5% 12% 24% 18% 1	2688
	21st to end	Day	41% 6% 13% 25% 14% 1	2520
July	1st to 10th	Day	31% 5% 14% 29% 18% 3	2451
	11th to 20th	Day	36% 5% 12% 28% 18% 2	2445
	21st to end	Day	36% 7% 12% 24% 18% 3	2591
August	1st to 10th	Day	39% 6% 13% 27% 13% 2	2329
	11th to 20th	Day	38% 5% 13% 27% 15% 2	2303
	21st to end	Day	39% 5% 13% 26% 15% 2	2491
September	1st to 10th	Day	42% 4 13% 26% 13% 2	2106
	11th to 20th	Day	37% 4 15% 30% 13% 1	1980
	21st to end	Day	45% 4 13% 28% 10%	1980
October	1st to 10th	Day	47% 5% 10% 26% 12% 1	1976
	11th to 20th	Day	61% 4 11% 18% 6%	1742
	21st to end	Day	62% 5% 13% 14% 6%	1782
November	1st to 10th	Day	51% 5% 11% 23% 10% 1	1672
	11th to 20th	Day	62% 4 13% 15% 6%	1710
	21st to end	Day	68% 3 12% 12% 4	1649
December	1st to 10th	Day	71% 3 10% 13% 2	1378
	11th to 20th	Day	68% 4 12% 13% 3	1671
	21st to end	Day	80% 3 7% 7% 3	2758
January	1st to 10th	Night	66% 4 10% 14% 5%	2648
	11th to 20th	Night	68% 5% 10% 11% 5%	2575
	21st to end	Night	66% 4 12% 13% 5%	2772
February	1st to 10th	Night	63% 4 8% 17% 6% 1	2352
	11th to 20th	Night	59% 4 11% 17% 8% 1	2112
	21st to end	Night	62% 3 10% 16% 8% 1	1776
March	1st to 10th	Night	48% 5% 12% 23% 12% 1	2160
	11th to 20th	Night	45% 4 12% 23% 13% 2	2007
	21st to end	Night	51% 3 12% 20% 12% 1	2160
April	1st to 10th	Night	49% 3 9% 22% 15% 1	1800
	11th to 20th	Night	46% 3 9% 22% 17% 2	1669
	21st to end	Night	46% 4 10% 23% 15% 3	1620
May	1st to 10th	Night	44% 5% 13% 23% 13% 3	1620
	11th to 20th	Night	43% 5% 10% 24% 16% 2	1593
	21st to end	Night	36% 4 15% 28% 15% 3	1553
June	1st to 10th	Night	37% 4 11% 27% 19% 2	1418
	11th to 20th	Night	35% 6% 15% 23% 20% 1	1260
	21st to end	Night	35% 5% 13% 28% 17% 2	1438
July	1st to 10th	Night	24% 4 15% 34% 20% 4	1400
	11th to 20th	Night	28% 4 16% 33% 18% 2	1392
	21st to end	Night	29% 6% 16% 28% 19% 2	1487
August	1st to 10th	Night	28% 5% 19% 33% 14% 1	1583
	11th to 20th	Night	27% 6% 15% 34% 17% 1	1598
	21st to end	Night	32% 5% 13% 29% 19% 2	1862
September	1st to 10th	Night	32% 5% 13% 30% 18% 2	1836
	11th to 20th	Night	31% 4 13% 38% 14%	1980
	21st to end	Night	40% 5% 14% 32% 8% 1	1980
October	1st to 10th	Night	40% 5% 12% 27% 14% 1	1984
	11th to 20th	Night	53% 3 12% 23% 7% 1	2217
	21st to end	Night	56% 4 11% 20% 8%	2574
November	1st to 10th	Night	45% 5% 13% 24% 12% 1	2406
	11th to 20th	Night	56% 4 14% 18% 6% 1	2470
	21st to end	Night	62% 4 15% 15% 3 1	2527
December	1st to 10th	Night	67% 3 10% 15% 4	2798
	11th to 20th	Night	66% 4 13% 12% 5%	3525
	21st to end	Night	77% 3 8% 8% 3	4902

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	28% 18% 37% 12% 5%	270
	11th to 20th	Day	32% 15% 40% 10% 3	249
	21st to end	Day	26% 17% 37% 13% 7%	264
February	1st to 10th	Day	24% 30% 35% 8% 2	240
	11th to 20th	Day	24% 34% 35% 7% 1	240
	21st to end	Day	36% 30% 29% 4 1	198
March	1st to 10th	Day	30% 30% 32% 8%	240
	11th to 20th	Day	24% 17% 35% 17% 5% 1	393
	21st to end	Day	30% 15% 32% 17% 6% 2	462
April	1st to 10th	Day	26% 18% 33% 16% 6% 1	420
	11th to 20th	Day	33% 16% 27% 14% 7% 2	412
	21st to end	Day	25% 15% 29% 23% 6% 2	420
May	1st to 10th	Day	24% 18% 32% 16% 6% 3	423
	11th to 20th	Day	20% 13% 32% 21% 9% 5%	447
	21st to end	Day	15% 12% 33% 25% 12% 3	675
June	1st to 10th	Day	13% 14% 33% 29% 8% 3	630
	11th to 20th	Day	17% 12% 35% 24% 9% 3	630
	21st to end	Day	14% 12% 33% 27% 12% 3	630
July	1st to 10th	Day	7% 7% 31% 31% 16% 8%	621
	11th to 20th	Day	9% 9% 30% 32% 15% 6%	618
	21st to end	Day	10% 8% 34% 27% 15% 6%	684
August	1st to 10th	Day	11% 3 27% 29% 19% 11%	462
	11th to 20th	Day	11% 5% 21% 32% 18% 12%	439
	21st to end	Day	16% 7% 25% 30% 16% 6%	495
September	1st to 10th	Day	16% 9% 28% 24% 15% 8%	450
	11th to 20th	Day	13% 7% 25% 30% 20% 6%	450
	21st to end	Day	23% 10% 24% 22% 15% 6%	450
October	1st to 10th	Day	24% 12% 24% 22% 16% 2	450
	11th to 20th	Day	29% 14% 34% 19% 4 1	392
	21st to end	Day	32% 20% 32% 11% 3 2	297
November	1st to 10th	Day	24% 20% 36% 14% 5% 1	270
	11th to 20th	Day	25% 24% 36% 13% 3	270
	21st to end	Day	30% 24% 34% 10% 3	241
December	1st to 10th	Day	31% 22% 32% 10% 4 1	180
	11th to 20th	Day	32% 20% 37% 9% 2	180
	21st to end	Day	37% 23% 29% 8% 2 1	292
January	1st to 10th	Night	23% 19% 32% 20% 5% 1	950
	11th to 20th	Night	26% 16% 32% 20% 6%	920
	21st to end	Night	19% 12% 36% 26% 6% 1	990
February	1st to 10th	Night	20% 17% 36% 23% 3	732
	11th to 20th	Night	22% 19% 33% 21% 5%	703
	21st to end	Night	25% 19% 33% 19% 3	592
March	1st to 10th	Night	20% 15% 34% 22% 9% 1	720
	11th to 20th	Night	21% 18% 32% 23% 6%	720
	21st to end	Night	22% 14% 33% 25% 6% 1	774
April	1st to 10th	Night	21% 14% 35% 24% 6% 1	540
	11th to 20th	Night	24% 13% 31% 25% 6% 1	533
	21st to end	Night	22% 9% 33% 28% 8% 1	540
May	1st to 10th	Night	18% 9% 37% 27% 8% 1	540
	11th to 20th	Night	18% 9% 28% 32% 12% 1	531
	21st to end	Night	15% 7% 28% 31% 17% 2	584
June	1st to 10th	Night	15% 7% 29% 35% 14% 1	518
	11th to 20th	Night	13% 7% 29% 38% 13% 1	360
	21st to end	Night	12% 4 27% 39% 17% 1	538
July	1st to 10th	Night	8% 3 21% 43% 22% 3	525
	11th to 20th	Night	7% 3 21% 42% 24% 3	522
	21st to end	Night	9% 2 27% 40% 20% 1	591
August	1st to 10th	Night	9% 1 23% 41% 24% 1	537
	11th to 20th	Night	10% 4 19% 37% 28% 4	534
	21st to end	Night	13% 4 22% 37% 23% 1	594
September	1st to 10th	Night	10% 5% 27% 31% 23% 5%	576
	11th to 20th	Night	12% 5% 25% 33% 22% 4	720
	21st to end	Night	18% 8% 24% 28% 18% 4	720
October	1st to 10th	Night	20% 8% 27% 29% 15% 1	720
	11th to 20th	Night	26% 12% 31% 24% 6% 1	720
	21st to end	Night	31% 12% 30% 19% 6% 1	792
November	1st to 10th	Night	18% 16% 33% 23% 10% 1	740
	11th to 20th	Night	23% 17% 34% 20% 5%	760
	21st to end	Night	26% 15% 33% 20% 5%	760
December	1st to 10th	Night	25% 18% 32% 21% 4	898
	11th to 20th	Night	25% 17% 36% 18% 4 1	950
	21st to end	Night	32% 13% 32% 20% 2	1045

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