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 2026-01-17.

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	13% 10% 4 9% 6% 5% 5% 4 5% 7% 33%	126580
	Night	14% 11% 4 9% 5% 5% 5% 4 5% 6% 33%	125283
Transparency	Day	50% 4 11% 22% 12% 1	130093
	Night	51% 4 11% 21% 11% 1	129844
Seeing	Day	21% 15% 31% 21% 9% 3	17222
	Night	20% 12% 31% 26% 10% 1	27197

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% 15% 8% 20% 8% 7% 5% 2 4 5% 16%	3697
	11th to 20th	Day	12% 14% 7% 20% 12% 10% 7% 3 4 3 8%	3668
	21st to end	Day	15% 17% 9% 13% 7% 4 5% 3 4 5% 18%	3982
February	1st to 10th	Day	12% 15% 6% 13% 10% 7% 5% 3 5% 5% 21%	1785
	11th to 20th	Day	11% 15% 6% 14% 10% 7% 5% 3 5% 6% 20%	1933
	21st to end	Day	11% 17% 6% 13% 7% 5% 4 3 4 6% 23%	1650
March	1st to 10th	Day	12% 10% 4 10% 5% 5% 4 4 5% 7% 33%	4480
	11th to 20th	Day	12% 9% 6% 10% 6% 4 5% 4 5% 8% 31%	4925
	21st to end	Day	20% 11% 6% 10% 5% 5% 4 3 4 4 27%	5552
April	1st to 10th	Day	17% 14% 5% 9% 5% 4 4 3 5% 5% 29%	2400
	11th to 20th	Day	18% 11% 3 7% 5% 4 4 3 4 6% 36%	2492
	21st to end	Day	15% 10% 3 8% 6% 5% 5% 3 5% 7% 33%	2600
May	1st to 10th	Day	18% 8% 2 7% 4 4 4 3 5% 7% 35%	5862
	11th to 20th	Day	14% 6% 3 7% 5% 5% 5% 4 6% 8% 38%	5861
	21st to end	Day	12% 7% 3 7% 5% 5% 4 3 5% 7% 42%	6931
June	1st to 10th	Day	12% 6% 2 6% 5% 5% 4 4 6% 9% 41%	2812
	11th to 20th	Day	12% 6% 2 6% 4 5% 5% 4 6% 8% 41%	2986
	21st to end	Day	11% 6% 3 7% 4 5% 5% 4 6% 7% 40%	2800
July	1st to 10th	Day	11% 6% 3 7% 5% 5% 5% 4 6% 9% 39%	6360
	11th to 20th	Day	6% 5% 2 7% 5% 4 5% 4 5% 9% 47%	6360
	21st to end	Day	7% 4 2 5% 5% 5% 6% 5% 6% 9% 46%	6924
August	1st to 10th	Day	10% 5% 2 6% 6% 5% 7% 4 7% 8% 40%	4251
	11th to 20th	Day	13% 5% 2 5% 4 5% 5% 3 6% 8% 44%	4174
	21st to end	Day	11% 6% 2 6% 5% 5% 5% 4 6% 8% 43%	4517
September	1st to 10th	Day	12% 5% 2 7% 4 5% 5% 4 6% 9% 41%	2339
	11th to 20th	Day	11% 5% 2 6% 4 4 4 4 5% 8% 46%	2200
	21st to end	Day	13% 7% 3 8% 5% 5% 4 3 5% 9% 38%	2200
October	1st to 10th	Day	15% 8% 3 7% 5% 4 3 3 5% 6% 42%	2195
	11th to 20th	Day	19% 11% 4 9% 5% 5% 5% 3 5% 7% 26%	1934
	21st to end	Day	19% 13% 4 9% 5% 5% 5% 4 6% 6% 23%	1980
November	1st to 10th	Day	13% 12% 3 8% 5% 5% 4 4 5% 7% 33%	1852
	11th to 20th	Day	14% 17% 5% 11% 6% 4 5% 3 5% 6% 22%	1890
	21st to end	Day	20% 19% 5% 12% 6% 5% 4 2 3 5% 20%	1822
December	1st to 10th	Day	12% 20% 8% 15% 7% 5% 5% 4 5% 5% 14%	2426
	11th to 20th	Day	15% 19% 10% 12% 7% 6% 4 3 4 4 15%	2695
	21st to end	Day	25% 23% 7% 12% 5% 4 3 3 3 3 12%	4045
January	1st to 10th	Night	12% 12% 7% 17% 7% 5% 5% 4 5% 5% 20%	6488
	11th to 20th	Night	12% 14% 6% 18% 10% 12% 6% 3 4 4 10%	6445
	21st to end	Night	12% 18% 8% 15% 6% 5% 6% 4 5% 6% 15%	6996
February	1st to 10th	Night	12% 13% 5% 12% 9% 6% 6% 4 5% 6% 21%	2613
	11th to 20th	Night	13% 13% 5% 11% 6% 6% 5% 5% 6% 7% 24%	2352
	21st to end	Night	15% 12% 5% 12% 7% 5% 5% 3 5% 6% 25%	1980
March	1st to 10th	Night	14% 8% 4 8% 6% 4 4 4 5% 7% 37%	5373
	11th to 20th	Night	12% 8% 3 7% 4 4 4 4 6% 9% 37%	5035
	21st to end	Night	16% 12% 5% 8% 5% 4 4 3 5% 5% 33%	5408
April	1st to 10th	Night	15% 12% 4 8% 4 4 3 3 5% 7% 35%	2000
	11th to 20th	Night	18% 9% 4 6% 4 3 5% 3 5% 7% 37%	1857
	21st to end	Night	17% 9% 3 5% 4 5% 4 4 6% 7% 37%	1800
May	1st to 10th	Night	15% 8% 2 7% 4 4 4 4 5% 8% 38%	4033
	11th to 20th	Night	17% 8% 3 6% 4 5% 4 4 5% 7% 37%	4032
	21st to end	Night	18% 6% 2 4 3 3 3 3 4 6% 48%	3943
June	1st to 10th	Night	12% 8% 2 5% 4 4 5% 3 5% 8% 45%	1575
	11th to 20th	Night	11% 6% 2 6% 4 4 5% 5% 7% 8% 43%	1400
	21st to end	Night	10% 6% 2 6% 4 4 5% 3 6% 9% 45%	1598
July	1st to 10th	Night	9% 6% 2 6% 4 4 5% 5% 7% 9% 43%	3618
	11th to 20th	Night	6% 5% 2 5% 4 4 4 4 5% 7% 54%	3617
	21st to end	Night	7% 5% 2 5% 3 4 4 4 6% 8% 52%	3947
August	1st to 10th	Night	10% 4 2 5% 4 4 4 4 6% 9% 49%	2884
	11th to 20th	Night	11% 4 2 4 3 4 4 4 5% 7% 51%	2875
	21st to end	Night	10% 5% 2 5% 4 3 5% 3 6% 8% 49%	3378
September	1st to 10th	Night	8% 5% 2 5% 3 4 4 3 5% 7% 52%	2040
	11th to 20th	Night	10% 4 1 5% 3 2 5% 3 5% 8% 54%	2200
	21st to end	Night	14% 7% 3 7% 4 3 4 4 6% 7% 42%	2200
October	1st to 10th	Night	16% 6% 2 5% 3 3 4 3 5% 6% 46%	2205
	11th to 20th	Night	18% 12% 3 7% 5% 4 3 3 4 5% 35%	2464
	21st to end	Night	21% 10% 3 7% 4 4 5% 3 5% 6% 32%	2860
November	1st to 10th	Night	11% 12% 3 8% 5% 4 4 4 6% 7% 37%	2666
	11th to 20th	Night	14% 13% 4 9% 7% 5% 6% 4 6% 6% 27%	2730
	21st to end	Night	17% 17% 6% 9% 6% 5% 5% 4 5% 6% 21%	2793
December	1st to 10th	Night	12% 14% 6% 13% 6% 6% 6% 4 6% 7% 21%	4978
	11th to 20th	Night	14% 20% 6% 13% 7% 5% 5% 4 5% 6% 16%	5732
	21st to end	Night	25% 22% 6% 11% 5% 4 4 2 4 4 15%	7168

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% 11% 3	3698
	11th to 20th	Day	81% 3 5% 8% 3	3668
	21st to end	Day	69% 3 12% 11% 5% 1	3982
February	1st to 10th	Day	65% 4 9% 17% 5%	1785
	11th to 20th	Day	66% 3 9% 14% 7% 1	1943
	21st to end	Day	63% 2 10% 18% 6% 1	1650
March	1st to 10th	Day	51% 3 13% 18% 11% 3	4480
	11th to 20th	Day	50% 5% 15% 19% 10% 2	4925
	21st to end	Day	61% 3 8% 17% 10% 1	5554
April	1st to 10th	Day	57% 4 9% 17% 12% 1	2400
	11th to 20th	Day	51% 3 9% 21% 15% 2	2492
	21st to end	Day	51% 4 12% 17% 14% 2	2600
May	1st to 10th	Day	48% 5% 8% 19% 18% 2	5897
	11th to 20th	Day	44% 5% 11% 25% 14% 2	5865
	21st to end	Day	42% 4 10% 22% 20% 3	6945
June	1st to 10th	Day	39% 4 12% 28% 15% 1	2812
	11th to 20th	Day	40% 5% 12% 24% 18% 1	2986
	21st to end	Day	42% 5% 13% 25% 14% 1	2800
July	1st to 10th	Day	40% 5% 12% 26% 15% 1	6291
	11th to 20th	Day	33% 5% 11% 33% 17% 1	6285
	21st to end	Day	33% 7% 13% 29% 18% 1	6728
August	1st to 10th	Day	38% 6% 12% 28% 15% 1	4237
	11th to 20th	Day	37% 6% 12% 30% 15% 1	4213
	21st to end	Day	38% 6% 13% 28% 14% 1	4516
September	1st to 10th	Day	40% 4 13% 28% 13% 2	2339
	11th to 20th	Day	36% 4 14% 30% 15% 1	2200
	21st to end	Day	44% 4 12% 28% 12%	2200
October	1st to 10th	Day	32% 3 9% 34% 19% 3	3572
	11th to 20th	Day	46% 3 10% 26% 14%	3124
	21st to end	Day	57% 4 12% 19% 7%	3180
November	1st to 10th	Day	51% 5% 11% 23% 10% 1	1852
	11th to 20th	Day	62% 3 13% 15% 6%	1890
	21st to end	Day	69% 3 11% 13% 4	1822
December	1st to 10th	Day	71% 5% 12% 10% 1	2426
	11th to 20th	Day	74% 3 8% 12% 3	2691
	21st to end	Day	79% 3 5% 9% 5%	4045
January	1st to 10th	Night	65% 4 9% 15% 7% 1	6488
	11th to 20th	Night	77% 4 7% 9% 4	6445
	21st to end	Night	69% 4 9% 12% 6%	6996
February	1st to 10th	Night	62% 4 8% 17% 7% 1	2613
	11th to 20th	Night	58% 4 12% 18% 7%	2353
	21st to end	Night	60% 3 11% 17% 8% 1	1980
March	1st to 10th	Night	47% 3 16% 23% 10% 1	5373
	11th to 20th	Night	43% 4 19% 23% 10% 1	5035
	21st to end	Night	53% 3 11% 21% 11% 1	5408
April	1st to 10th	Night	50% 4 9% 23% 14% 2	2000
	11th to 20th	Night	48% 3 8% 22% 17% 2	1857
	21st to end	Night	46% 4 9% 23% 15% 3	1800
May	1st to 10th	Night	44% 5% 11% 18% 19% 3	4060
	11th to 20th	Night	45% 5% 8% 27% 13% 1	4033
	21st to end	Night	39% 3 9% 26% 19% 4	3951
June	1st to 10th	Night	38% 4 11% 27% 18% 2	1575
	11th to 20th	Night	37% 6% 14% 22% 20% 1	1400
	21st to end	Night	36% 5% 13% 28% 16% 2	1598
July	1st to 10th	Night	37% 5% 12% 27% 15% 5%	3580
	11th to 20th	Night	29% 5% 12% 33% 20% 1	3572
	21st to end	Night	30% 6% 16% 29% 18% 1	3832
August	1st to 10th	Night	32% 4 16% 30% 17% 1	2874
	11th to 20th	Night	31% 5% 15% 33% 16% 1	2902
	21st to end	Night	32% 5% 14% 30% 18% 1	3378
September	1st to 10th	Night	31% 5% 13% 31% 18% 1	2040
	11th to 20th	Night	29% 4 13% 38% 16%	2200
	21st to end	Night	40% 5% 13% 31% 10% 1	2200
October	1st to 10th	Night	29% 4 11% 37% 19% 1	3608
	11th to 20th	Night	44% 4 10% 26% 16% 1	4013
	21st to end	Night	47% 4 10% 29% 10%	4610
November	1st to 10th	Night	46% 5% 13% 24% 12% 1	2666
	11th to 20th	Night	56% 4 14% 18% 7% 1	2730
	21st to end	Night	63% 4 14% 15% 4	2793
December	1st to 10th	Night	62% 6% 16% 13% 3	4978
	11th to 20th	Night	68% 4 9% 13% 6%	5735
	21st to end	Night	75% 3 7% 8% 7%	7168

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% 20% 37% 11% 4	330
	11th to 20th	Day	30% 20% 40% 8% 2	318
	21st to end	Day	28% 17% 36% 14% 5%	330
February	1st to 10th	Day	24% 30% 33% 10% 3 1	300
	11th to 20th	Day	24% 32% 37% 6% 1	300
	21st to end	Day	34% 31% 29% 5% 1	249
March	1st to 10th	Day	30% 29% 34% 7%	300
	11th to 20th	Day	27% 18% 33% 17% 4 1	471
	21st to end	Day	31% 15% 33% 15% 5% 1	550
April	1st to 10th	Day	28% 20% 32% 15% 5% 1	500
	11th to 20th	Day	34% 18% 27% 13% 6% 2	492
	21st to end	Day	26% 14% 29% 22% 6% 2	500
May	1st to 10th	Day	24% 19% 33% 16% 5% 3	503
	11th to 20th	Day	20% 13% 31% 23% 9% 4	527
	21st to end	Day	16% 14% 33% 24% 10% 3	785
June	1st to 10th	Day	14% 14% 34% 28% 7% 3	730
	11th to 20th	Day	17% 13% 37% 23% 8% 3	730
	21st to end	Day	15% 13% 33% 26% 11% 2	730
July	1st to 10th	Day	8% 8% 31% 31% 15% 7%	721
	11th to 20th	Day	8% 11% 29% 31% 15% 5%	718
	21st to end	Day	10% 8% 33% 29% 15% 6%	794
August	1st to 10th	Day	10% 4 27% 29% 20% 11%	543
	11th to 20th	Day	12% 5% 23% 32% 18% 10%	519
	21st to end	Day	15% 7% 25% 31% 14% 7%	583
September	1st to 10th	Day	15% 10% 28% 24% 15% 8%	530
	11th to 20th	Day	12% 6% 23% 29% 21% 8%	530
	21st to end	Day	21% 9% 25% 24% 16% 5%	530
October	1st to 10th	Day	22% 13% 26% 22% 15% 2	530
	11th to 20th	Day	26% 14% 36% 20% 4	464
	21st to end	Day	29% 19% 35% 13% 3 1	363
November	1st to 10th	Day	27% 20% 34% 13% 5% 1	330
	11th to 20th	Day	29% 23% 33% 12% 3	330
	21st to end	Day	32% 23% 32% 10% 3	294
December	1st to 10th	Day	30% 23% 31% 11% 4 1	220
	11th to 20th	Day	35% 20% 34% 9% 2	220
	21st to end	Day	38% 23% 28% 7% 2 1	358
January	1st to 10th	Night	23% 19% 33% 19% 5%	1050
	11th to 20th	Night	25% 18% 32% 19% 6%	1031
	21st to end	Night	20% 12% 36% 26% 6% 1	1100
February	1st to 10th	Night	20% 17% 36% 23% 3	813
	11th to 20th	Night	22% 20% 33% 21% 4	783
	21st to end	Night	23% 19% 35% 19% 3	660
March	1st to 10th	Night	21% 16% 35% 21% 8% 1	800
	11th to 20th	Night	21% 18% 33% 22% 6%	800
	21st to end	Night	22% 15% 34% 24% 5% 1	859
April	1st to 10th	Night	22% 14% 36% 23% 5% 1	600
	11th to 20th	Night	25% 13% 31% 24% 6% 1	593
	21st to end	Night	23% 10% 33% 26% 7% 1	600
May	1st to 10th	Night	17% 9% 37% 29% 8% 1	600
	11th to 20th	Night	18% 9% 28% 32% 12% 1	591
	21st to end	Night	16% 8% 28% 30% 16% 2	650
June	1st to 10th	Night	14% 8% 30% 34% 13% 1	575
	11th to 20th	Night	13% 7% 31% 37% 12% 1	400
	21st to end	Night	13% 5% 28% 37% 16% 1	598
July	1st to 10th	Night	8% 4 23% 42% 20% 3	585
	11th to 20th	Night	8% 4 22% 41% 23% 3	582
	21st to end	Night	9% 2 27% 40% 20% 1	657
August	1st to 10th	Night	8% 2 23% 41% 24% 2	597
	11th to 20th	Night	10% 4 19% 39% 25% 3	594
	21st to end	Night	12% 4 22% 38% 23% 1	660
September	1st to 10th	Night	10% 6% 27% 29% 23% 4	640
	11th to 20th	Night	11% 5% 22% 34% 24% 4	800
	21st to end	Night	18% 7% 25% 29% 17% 4	800
October	1st to 10th	Night	19% 8% 28% 29% 15% 1	800
	11th to 20th	Night	25% 13% 31% 24% 7% 1	800
	21st to end	Night	29% 13% 31% 20% 6% 1	880
November	1st to 10th	Night	20% 16% 33% 22% 9% 1	820
	11th to 20th	Night	23% 18% 33% 20% 6%	840
	21st to end	Night	26% 17% 33% 19% 5%	840
December	1st to 10th	Night	25% 18% 32% 21% 4	994
	11th to 20th	Night	26% 17% 35% 17% 4	1050
	21st to end	Night	33% 14% 31% 19% 2	1155

Page generated at 2026-01-14 01:14:30. Version ac07

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