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 2024-11-25.

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 8% 6% 5% 5% 4 5% 7% 33%	101140
	Night	14% 11% 4 9% 5% 5% 5% 4 5% 6% 33%	98342
Transparency	Day	49% 4 11% 22% 12% 1	104698
	Night	50% 4 12% 22% 11% 1	102925
Seeing	Day	20% 14% 31% 21% 10% 3	15723
	Night	20% 12% 31% 27% 10% 1	25595

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	14% 18% 9% 14% 6% 6% 5% 2 4 6% 16%	2527
	11th to 20th	Day	15% 16% 7% 18% 11% 11% 6% 3 3 3 7%	2530
	21st to end	Day	19% 20% 10% 12% 6% 4 4 3 4 5% 12%	2783
February	1st to 10th	Day	11% 15% 6% 12% 10% 7% 5% 3 5% 5% 21%	1695
	11th to 20th	Day	11% 15% 6% 13% 11% 6% 5% 3 5% 6% 20%	1833
	21st to end	Day	11% 16% 6% 13% 7% 5% 4 3 4 6% 24%	1570
March	1st to 10th	Day	13% 14% 5% 9% 5% 6% 4 3 5% 6% 30%	3120
	11th to 20th	Day	11% 10% 7% 12% 6% 4 5% 4 5% 6% 31%	3439
	21st to end	Day	18% 12% 6% 9% 6% 5% 4 3 4 4 29%	3876
April	1st to 10th	Day	17% 14% 4 9% 4 4 4 3 5% 5% 30%	2280
	11th to 20th	Day	19% 11% 3 7% 5% 4 4 3 4 6% 35%	2366
	21st to end	Day	15% 10% 3 8% 6% 5% 5% 3 5% 7% 33%	2470
May	1st to 10th	Day	15% 8% 2 7% 5% 5% 5% 4 6% 7% 35%	4082
	11th to 20th	Day	13% 6% 2 6% 5% 5% 5% 4 6% 8% 40%	4081
	21st to end	Day	10% 7% 2 6% 5% 4 4 3 6% 8% 44%	4820
June	1st to 10th	Day	12% 6% 2 7% 5% 5% 4 4 6% 9% 40%	2671
	11th to 20th	Day	12% 6% 2 6% 4 5% 5% 4 6% 8% 42%	2837
	21st to end	Day	11% 6% 3 7% 4 5% 6% 4 7% 7% 40%	2660
July	1st to 10th	Day	12% 6% 3 8% 6% 5% 6% 5% 6% 8% 35%	4440
	11th to 20th	Day	6% 5% 2 6% 4 5% 5% 4 6% 10% 47%	4440
	21st to end	Day	7% 4 2 6% 6% 5% 6% 6% 7% 9% 43%	4813
August	1st to 10th	Day	10% 5% 2 6% 6% 5% 7% 5% 7% 8% 40%	4120
	11th to 20th	Day	13% 5% 2 5% 4 5% 5% 4 6% 7% 45%	4044
	21st to end	Day	11% 6% 2 6% 5% 5% 5% 4 6% 8% 42%	4379
September	1st to 10th	Day	12% 5% 2 7% 4 5% 5% 4 6% 9% 41%	2222
	11th to 20th	Day	12% 5% 2 6% 4 4 4 4 5% 8% 46%	2090
	21st to end	Day	13% 7% 3 8% 5% 5% 4 4 5% 9% 38%	2090
October	1st to 10th	Day	16% 8% 3 7% 5% 4 3 3 5% 5% 40%	2085
	11th to 20th	Day	20% 12% 4 9% 5% 5% 5% 3 5% 7% 25%	1838
	21st to end	Day	20% 14% 4 9% 5% 5% 5% 4 6% 6% 22%	1881
November	1st to 10th	Day	14% 12% 4 8% 6% 5% 4 4 5% 7% 33%	1762
	11th to 20th	Day	15% 16% 5% 11% 6% 5% 5% 3 5% 6% 22%	1800
	21st to end	Day	20% 18% 5% 12% 6% 5% 4 2 3 5% 20%	1685
December	1st to 10th	Day	15% 17% 9% 14% 7% 4 5% 3 3 5% 17%	1378
	11th to 20th	Day	15% 18% 6% 11% 7% 6% 5% 4 6% 5% 18%	1675
	21st to end	Day	25% 23% 7% 12% 6% 4 3 3 3 4 10%	2758
January	1st to 10th	Night	16% 16% 8% 13% 6% 5% 5% 3 4 5% 20%	4428
	11th to 20th	Night	13% 15% 6% 16% 10% 12% 5% 3 4 4 11%	4440
	21st to end	Night	17% 20% 8% 15% 6% 5% 5% 4 5% 5% 12%	4884
February	1st to 10th	Night	12% 13% 5% 12% 8% 6% 6% 4 5% 6% 22%	2483
	11th to 20th	Night	12% 12% 5% 11% 6% 6% 5% 4 6% 7% 25%	2232
	21st to end	Night	15% 13% 5% 11% 7% 5% 5% 3 5% 6% 25%	1884
March	1st to 10th	Night	15% 8% 4 7% 4 4 4 4 5% 7% 38%	3733
	11th to 20th	Night	11% 8% 4 7% 5% 4 4 4 6% 8% 40%	3521
	21st to end	Night	14% 13% 5% 8% 5% 4 4 3 5% 6% 33%	3772
April	1st to 10th	Night	15% 12% 4 7% 4 4 3 3 5% 7% 34%	1900
	11th to 20th	Night	19% 9% 4 5% 4 3 4 3 5% 7% 37%	1763
	21st to end	Night	17% 9% 3 5% 4 5% 4 4 6% 7% 37%	1710
May	1st to 10th	Night	14% 8% 2 6% 4 4 5% 4 5% 7% 41%	2813
	11th to 20th	Night	13% 7% 2 7% 4 5% 4 4 6% 8% 41%	2812
	21st to end	Night	15% 7% 2 5% 3 4 3 3 5% 6% 46%	2744
June	1st to 10th	Night	13% 8% 2 5% 3 4 5% 3 5% 8% 45%	1496
	11th to 20th	Night	10% 6% 2 6% 3 4 5% 5% 7% 8% 44%	1330
	21st to end	Night	9% 6% 3 5% 4 4 5% 3 6% 8% 46%	1518
July	1st to 10th	Night	9% 6% 3 7% 5% 4 5% 5% 7% 9% 41%	2530
	11th to 20th	Night	6% 4 2 5% 4 4 4 3 5% 7% 58%	2530
	21st to end	Night	7% 5% 2 6% 3 5% 4 4 6% 8% 51%	2750
August	1st to 10th	Night	10% 5% 2 5% 4 4 4 4 6% 9% 48%	2795
	11th to 20th	Night	11% 3 2 4 3 3 4 4 6% 7% 52%	2785
	21st to end	Night	10% 5% 2 5% 4 3 5% 3 6% 8% 49%	3274
September	1st to 10th	Night	8% 5% 2 5% 4 4 4 3 5% 7% 52%	1938
	11th to 20th	Night	10% 4 2 5% 3 2 5% 3 5% 7% 54%	2090
	21st to end	Night	14% 6% 3 7% 4 3 4 4 6% 7% 42%	2090
October	1st to 10th	Night	16% 6% 2 5% 3 3 4 4 5% 6% 45%	2095
	11th to 20th	Night	18% 12% 4 8% 5% 4 3 3 4 5% 35%	2341
	21st to end	Night	22% 10% 3 7% 5% 4 5% 3 5% 6% 31%	2717
November	1st to 10th	Night	11% 12% 3 8% 5% 4 4 4 6% 7% 38%	2536
	11th to 20th	Night	15% 13% 4 10% 7% 5% 6% 4 6% 6% 26%	2600
	21st to end	Night	17% 16% 6% 9% 6% 5% 5% 4 5% 6% 22%	2586
December	1st to 10th	Night	16% 16% 6% 12% 7% 6% 5% 4 4 6% 19%	2798
	11th to 20th	Night	14% 17% 5% 14% 7% 5% 5% 4 6% 6% 17%	3522
	21st to end	Night	27% 22% 5% 11% 5% 4 4 3 3 3 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	71% 3 13% 9% 3	2528
	11th to 20th	Day	83% 3 6% 6% 1	2530
	21st to end	Day	77% 2 12% 7% 3	2783
February	1st to 10th	Day	65% 4 9% 17% 5%	1695
	11th to 20th	Day	65% 3 9% 14% 7% 1	1843
	21st to end	Day	63% 2 10% 18% 6% 1	1570
March	1st to 10th	Day	55% 3 11% 22% 9% 1	3120
	11th to 20th	Day	52% 5% 12% 20% 9% 2	3439
	21st to end	Day	59% 3 8% 18% 11% 1	3876
April	1st to 10th	Day	56% 4 10% 17% 12%	2280
	11th to 20th	Day	51% 3 9% 20% 15% 2	2366
	21st to end	Day	52% 4 12% 16% 14% 2	2470
May	1st to 10th	Day	47% 6% 8% 20% 18% 2	4120
	11th to 20th	Day	40% 5% 13% 28% 13% 1	4085
	21st to end	Day	38% 5% 11% 23% 21% 2	4833
June	1st to 10th	Day	40% 4 12% 27% 15% 1	2671
	11th to 20th	Day	39% 5% 12% 24% 18% 1	2837
	21st to end	Day	42% 5% 12% 25% 14% 1	2660
July	1st to 10th	Day	45% 6% 12% 21% 14% 2	4371
	11th to 20th	Day	31% 5% 13% 33% 17% 1	4365
	21st to end	Day	35% 8% 12% 24% 20% 2	4662
August	1st to 10th	Day	39% 6% 11% 28% 15% 1	4106
	11th to 20th	Day	37% 5% 12% 30% 15% 1	4083
	21st to end	Day	39% 6% 13% 28% 14% 1	4378
September	1st to 10th	Day	40% 4 12% 28% 14% 2	2222
	11th to 20th	Day	36% 4 14% 29% 15% 1	2090
	21st to end	Day	44% 4 13% 28% 11%	2090
October	1st to 10th	Day	32% 4 8% 34% 18% 3	3462
	11th to 20th	Day	46% 3 10% 26% 14%	3028
	21st to end	Day	57% 4 13% 19% 6%	3081
November	1st to 10th	Day	51% 5% 11% 23% 9% 1	1762
	11th to 20th	Day	62% 4 13% 15% 6%	1800
	21st to end	Day	68% 3 12% 12% 4	1685
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	67% 4 9% 16% 5%	4428
	11th to 20th	Night	77% 4 7% 9% 3	4440
	21st to end	Night	74% 4 10% 8% 3	4884
February	1st to 10th	Night	62% 4 8% 17% 7% 1	2483
	11th to 20th	Night	58% 4 12% 18% 8%	2233
	21st to end	Night	60% 3 11% 17% 8% 1	1884
March	1st to 10th	Night	47% 4 15% 25% 9% 1	3733
	11th to 20th	Night	41% 4 15% 25% 13% 2	3521
	21st to end	Night	53% 3 12% 22% 9% 1	3772
April	1st to 10th	Night	49% 4 9% 22% 14% 2	1900
	11th to 20th	Night	47% 3 8% 22% 17% 2	1763
	21st to end	Night	46% 4 10% 23% 15% 3	1710
May	1st to 10th	Night	42% 5% 12% 20% 18% 3	2840
	11th to 20th	Night	40% 5% 10% 31% 13% 1	2813
	21st to end	Night	39% 4 13% 22% 18% 4	2752
June	1st to 10th	Night	38% 4 11% 27% 19% 1	1496
	11th to 20th	Night	35% 6% 15% 23% 21% 1	1330
	21st to end	Night	36% 5% 13% 28% 17% 2	1518
July	1st to 10th	Night	39% 5% 12% 26% 13% 6%	2490
	11th to 20th	Night	26% 4 13% 36% 19% 1	2482
	21st to end	Night	30% 7% 17% 24% 20% 2	2662
August	1st to 10th	Night	32% 4 16% 29% 17% 1	2785
	11th to 20th	Night	31% 5% 15% 33% 16% 1	2812
	21st to end	Night	33% 5% 14% 29% 18% 1	3274
September	1st to 10th	Night	31% 5% 13% 31% 18% 1	1938
	11th to 20th	Night	29% 4 13% 37% 16%	2090
	21st to end	Night	40% 5% 13% 32% 10% 1	2090
October	1st to 10th	Night	29% 4 11% 36% 19% 1	3498
	11th to 20th	Night	44% 4 10% 26% 16% 1	3890
	21st to end	Night	48% 4 10% 29% 9%	4467
November	1st to 10th	Night	46% 5% 13% 24% 12% 1	2536
	11th to 20th	Night	57% 4 14% 18% 7% 1	2600
	21st to end	Night	62% 4 14% 15% 4 1	2586
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	29% 17% 42% 9% 3	270
	21st to end	Day	27% 15% 37% 14% 6%	297
February	1st to 10th	Day	22% 29% 34% 10% 3 1	270
	11th to 20th	Day	23% 33% 36% 7% 1	270
	21st to end	Day	33% 32% 29% 5% 1	225
March	1st to 10th	Day	29% 29% 34% 8%	270
	11th to 20th	Day	26% 17% 34% 17% 5% 1	432
	21st to end	Day	30% 15% 33% 16% 5% 1	506
April	1st to 10th	Day	27% 19% 33% 15% 5% 1	460
	11th to 20th	Day	34% 17% 27% 13% 7% 2	452
	21st to end	Day	26% 15% 28% 22% 7% 2	460
May	1st to 10th	Day	24% 18% 33% 17% 6% 3	463
	11th to 20th	Day	20% 12% 31% 23% 9% 5%	487
	21st to end	Day	15% 12% 33% 25% 11% 3	730
June	1st to 10th	Day	14% 14% 33% 29% 7% 3	680
	11th to 20th	Day	16% 13% 37% 23% 8% 3	680
	21st to end	Day	15% 13% 32% 26% 11% 2	680
July	1st to 10th	Day	8% 8% 31% 30% 16% 7%	671
	11th to 20th	Day	9% 10% 29% 32% 15% 6%	668
	21st to end	Day	10% 8% 33% 28% 16% 6%	739
August	1st to 10th	Day	11% 4 27% 29% 18% 11%	502
	11th to 20th	Day	11% 5% 22% 32% 19% 11%	479
	21st to end	Day	16% 7% 25% 30% 15% 6%	539
September	1st to 10th	Day	15% 10% 28% 23% 15% 8%	490
	11th to 20th	Day	12% 6% 24% 30% 20% 7%	490
	21st to end	Day	21% 10% 26% 23% 15% 6%	490
October	1st to 10th	Day	24% 13% 26% 21% 15% 1	490
	11th to 20th	Day	27% 14% 35% 19% 4	428
	21st to end	Day	31% 20% 32% 12% 3 2	330
November	1st to 10th	Day	26% 20% 35% 13% 4 1	300
	11th to 20th	Day	27% 23% 35% 12% 3	300
	21st to end	Day	31% 23% 34% 9% 3	253
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	25% 16% 32% 20% 6%	950
	21st to end	Night	20% 11% 36% 26% 6% 1	1045
February	1st to 10th	Night	20% 17% 36% 23% 4	773
	11th to 20th	Night	22% 19% 33% 22% 4	743
	21st to end	Night	24% 19% 35% 19% 3	628
March	1st to 10th	Night	20% 15% 35% 21% 8% 1	760
	11th to 20th	Night	21% 18% 33% 22% 6%	760
	21st to end	Night	21% 14% 34% 24% 5% 1	816
April	1st to 10th	Night	21% 13% 36% 23% 5% 1	570
	11th to 20th	Night	25% 13% 31% 24% 6% 1	563
	21st to end	Night	22% 9% 33% 27% 8% 1	570
May	1st to 10th	Night	17% 9% 37% 28% 8% 1	570
	11th to 20th	Night	18% 9% 28% 33% 12% 1	561
	21st to end	Night	16% 7% 28% 30% 17% 2	617
June	1st to 10th	Night	15% 7% 29% 34% 14% 1	546
	11th to 20th	Night	12% 7% 31% 37% 12% 1	380
	21st to end	Night	13% 5% 27% 38% 16% 1	568
July	1st to 10th	Night	8% 4 21% 42% 21% 3	555
	11th to 20th	Night	7% 4 22% 41% 23% 3	552
	21st to end	Night	10% 2 26% 41% 20% 1	624
August	1st to 10th	Night	9% 2 23% 41% 23% 2	567
	11th to 20th	Night	10% 4 19% 38% 26% 3	564
	21st to end	Night	13% 4 22% 37% 23% 1	627
September	1st to 10th	Night	10% 5% 28% 30% 23% 4	608
	11th to 20th	Night	12% 5% 24% 33% 23% 4	760
	21st to end	Night	18% 7% 25% 29% 17% 4	760
October	1st to 10th	Night	20% 9% 28% 28% 15% 1	760
	11th to 20th	Night	25% 13% 31% 24% 7% 1	760
	21st to end	Night	30% 13% 30% 20% 6% 1	836
November	1st to 10th	Night	19% 16% 33% 23% 9% 1	780
	11th to 20th	Night	24% 18% 33% 20% 5%	800
	21st to end	Night	26% 15% 34% 20% 5%	779
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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