Hancock Field Station
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Hancock Field Station clear sky chart from 2005-11-06 to 2025-10-19.

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	7% 7% 2 6% 4 4 4 3 5% 7% 49%	135742
	Night	7% 9% 3 7% 5% 4 4 4 6% 8% 43%	131272
Transparency	Day	35% 3 10% 24% 19% 10%	130504
	Night	40% 4 12% 23% 15% 5%	127508
Seeing	Day	12% 9% 21% 28% 20% 10%	15768
	Night	12% 4 14% 42% 24% 3	24772

By Month

Percentage Cloud Cover SW (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	12% 16% 6% 15% 11% 7% 7% 4 7% 6% 9%	3639
	11th to 20th	Day	11% 13% 4 9% 5% 4 5% 4 5% 7% 32%	3640
	21st to end	Day	10% 10% 3 8% 5% 5% 5% 4 8% 9% 33%	4124
February	1st to 10th	Day	9% 14% 5% 9% 7% 6% 5% 4 7% 7% 27%	1800
	11th to 20th	Day	12% 11% 4 8% 6% 5% 5% 4 7% 7% 29%	1790
	21st to end	Day	9% 13% 4 9% 7% 7% 5% 5% 7% 8% 27%	1650
March	1st to 10th	Day	8% 11% 4 9% 6% 5% 6% 5% 8% 10% 29%	4518
	11th to 20th	Day	7% 13% 4 9% 6% 5% 5% 3 5% 7% 35%	4912
	21st to end	Day	8% 13% 4 11% 8% 7% 6% 5% 7% 9% 23%	5824
April	1st to 10th	Day	9% 12% 4 8% 6% 6% 6% 5% 7% 6% 31%	2400
	11th to 20th	Day	8% 11% 2 8% 6% 5% 5% 4 5% 7% 40%	2369
	21st to end	Day	7% 9% 2 6% 5% 4 5% 4 6% 8% 44%	2494
May	1st to 10th	Day	10% 6% 2 5% 3 4 4 4 6% 6% 52%	6320
	11th to 20th	Day	7% 6% 3 6% 4 4 5% 4 5% 6% 50%	6277
	21st to end	Day	7% 6% 2 5% 4 4 4 4 6% 7% 51%	6902
June	1st to 10th	Day	10% 6% 3 6% 4 5% 4 4 5% 8% 46%	2800
	11th to 20th	Day	7% 5% 2 5% 4 4 4 4 5% 6% 52%	2800
	21st to end	Day	4 3 1 2 3 3 3 3 4 6% 68%	2800
July	1st to 10th	Day	2 1 1 2 1 1 2 1 3 4 81%	6319
	11th to 20th	Day	1 1 1 1 1 1 1 1 2 3 85%	6320
	21st to end	Day	2 1 1 2 1 2 2 2 3 5% 80%	6880
August	1st to 10th	Day	3 3 1 3 2 2 2 2 4 6% 70%	6129
	11th to 20th	Day	3 2 1 2 2 2 2 2 3 5% 76%	5708
	21st to end	Day	4 4 2 4 2 3 3 2 5% 6% 64%	5984
September	1st to 10th	Day	2 2 1 3 2 3 3 3 4 5% 73%	2400
	11th to 20th	Day	4 4 2 5% 4 4 4 3 4 6% 59%	2400
	21st to end	Day	8% 5% 1 4 3 3 3 3 5% 6% 58%	2159
October	1st to 10th	Day	7% 5% 1 4 3 3 4 3 5% 6% 60%	3280
	11th to 20th	Day	5% 5% 2 7% 5% 5% 5% 4 7% 9% 46%	3242
	21st to end	Day	9% 11% 4 9% 5% 4 4 4 7% 8% 35%	3201
November	1st to 10th	Day	11% 11% 4 7% 6% 5% 6% 4 6% 9% 30%	1764
	11th to 20th	Day	12% 12% 4 10% 7% 6% 7% 5% 6% 7% 24%	1645
	21st to end	Day	9% 11% 4 9% 7% 6% 5% 4 7% 10% 29%	1600
December	1st to 10th	Day	7% 9% 2 6% 5% 5% 5% 4 7% 12% 39%	2620
	11th to 20th	Day	10% 14% 5% 14% 9% 8% 7% 6% 7% 6% 13%	3028
	21st to end	Day	12% 15% 5% 14% 9% 7% 7% 6% 8% 7% 12%	4004
January	1st to 10th	Night	12% 17% 6% 18% 10% 7% 7% 4 5% 5% 10%	6306
	11th to 20th	Night	13% 12% 4 7% 5% 4 4 4 9% 12% 25%	6320
	21st to end	Night	9% 11% 4 10% 6% 5% 5% 4 5% 9% 32%	6822
February	1st to 10th	Night	11% 14% 5% 10% 7% 5% 4 4 7% 8% 26%	2600
	11th to 20th	Night	9% 13% 5% 9% 5% 5% 5% 4 5% 8% 31%	2497
	21st to end	Night	9% 16% 5% 8% 6% 5% 6% 5% 6% 7% 27%	1980
March	1st to 10th	Night	6% 9% 3 9% 5% 6% 6% 5% 8% 11% 33%	5376
	11th to 20th	Night	7% 9% 4 8% 5% 5% 5% 4 6% 10% 35%	5073
	21st to end	Night	9% 12% 4 11% 6% 7% 6% 5% 7% 9% 23%	5150
April	1st to 10th	Night	10% 11% 4 8% 7% 5% 4 3 6% 7% 36%	2000
	11th to 20th	Night	8% 10% 3 6% 4 5% 4 4 6% 6% 42%	1979
	21st to end	Night	8% 9% 3 6% 4 5% 5% 4 5% 8% 44%	1903
May	1st to 10th	Night	8% 7% 2 6% 3 3 3 3 4 5% 56%	3640
	11th to 20th	Night	7% 5% 2 5% 4 5% 3 3 4 6% 56%	3616
	21st to end	Night	8% 6% 2 4 3 3 3 3 5% 7% 57%	3972
June	1st to 10th	Night	9% 6% 2 5% 4 4 3 4 5% 7% 50%	1600
	11th to 20th	Night	7% 6% 2 5% 4 2 4 3 6% 6% 55%	1600
	21st to end	Night	5% 3 1 3 2 2 3 3 4 6% 68%	1600
July	1st to 10th	Night	1 1 1 2 2 2 2 2 4 5% 78%	3640
	11th to 20th	Night	1 1 1 1 2 2 2 2 3 85%	3640
	21st to end	Night	2 2 1 2 1 1 2 2 4 6% 77%	3970
August	1st to 10th	Night	2 2 1 4 2 4 3 3 5% 7% 66%	3664
	11th to 20th	Night	1 1 1 3 2 2 3 3 4 5% 75%	4144
	21st to end	Night	2 3 1 4 2 3 3 3 4 6% 68%	5015
September	1st to 10th	Night	2 2 1 3 2 2 3 2 4 6% 75%	2000
	11th to 20th	Night	4 4 1 4 4 3 4 3 5% 6% 63%	2000
	21st to end	Night	7% 5% 2 4 2 3 3 3 4 7% 61%	2237
October	1st to 10th	Night	5% 5% 2 4 3 2 2 2 4 6% 64%	3920
	11th to 20th	Night	7% 5% 2 5% 5% 4 4 4 7% 9% 48%	3873
	21st to end	Night	8% 7% 3 8% 5% 6% 5% 4 7% 8% 38%	4358
November	1st to 10th	Night	12% 11% 4 8% 6% 5% 5% 4 5% 8% 32%	2535
	11th to 20th	Night	13% 12% 3 9% 6% 5% 6% 5% 8% 8% 26%	2754
	21st to end	Night	9% 11% 3 7% 6% 5% 5% 6% 7% 11% 30%	2800
December	1st to 10th	Night	7% 9% 2 6% 5% 5% 4 4 11% 13% 35%	4560
	11th to 20th	Night	10% 18% 7% 15% 8% 7% 7% 5% 7% 6% 11%	5176
	21st to end	Night	13% 14% 4 13% 8% 7% 7% 5% 7% 7% 15%	6952

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	69% 7% 10% 11% 2	3639
	11th to 20th	Day	52% 5% 14% 21% 8% 1	3640
	21st to end	Day	50% 2 10% 24% 11% 3	4124
February	1st to 10th	Day	54% 5% 15% 20% 6%	1800
	11th to 20th	Day	50% 5% 15% 20% 9% 1	1799
	21st to end	Day	52% 4 16% 18% 9% 1	1650
March	1st to 10th	Day	48% 3 10% 23% 15% 1	4518
	11th to 20th	Day	48% 3 11% 27% 10%	4912
	21st to end	Day	54% 5% 14% 20% 6%	5824
April	1st to 10th	Day	49% 3 14% 21% 12% 1	2400
	11th to 20th	Day	44% 3 14% 21% 16% 2	2372
	21st to end	Day	38% 3 13% 30% 13% 2	2494
May	1st to 10th	Day	31% 1 6% 32% 26% 4	6320
	11th to 20th	Day	31% 3 12% 28% 21% 4	6280
	21st to end	Day	31% 2 16% 34% 15% 1	6904
June	1st to 10th	Day	38% 3 10% 30% 17% 3	2800
	11th to 20th	Day	32% 2 9% 31% 22% 4	2800
	21st to end	Day	19% 2 10% 32% 28% 10%	2800
July	1st to 10th	Day	10% 1 4 17% 30% 39%	6251
	11th to 20th	Day	8% 1 3 9% 27% 51%	6245
	21st to end	Day	10% 2 5% 24% 37% 23%	6681
August	1st to 10th	Day	16% 2 3 15% 36% 28%	4466
	11th to 20th	Day	13% 2 7% 31% 32% 16%	4131
	21st to end	Day	16% 2 7% 28% 30% 16%	4311
September	1st to 10th	Day	15% 2 6% 27% 33% 18%	2400
	11th to 20th	Day	26% 2 9% 26% 26% 11%	2400
	21st to end	Day	28% 2 11% 28% 23% 8%	2159
October	1st to 10th	Day	25% 2 10% 27% 26% 9%	3280
	11th to 20th	Day	31% 4 21% 26% 16% 2	3242
	21st to end	Day	43% 4 10% 27% 14% 2	3201
November	1st to 10th	Day	48% 4 13% 26% 9% 1	1764
	11th to 20th	Day	55% 5% 16% 18% 6%	1645
	21st to end	Day	49% 5% 16% 22% 8%	1600
December	1st to 10th	Day	36% 9% 16% 29% 9%	2620
	11th to 20th	Day	67% 5% 13% 13% 1	3028
	21st to end	Day	68% 4 12% 13% 3	4004
January	1st to 10th	Night	74% 7% 10% 8% 2	6306
	11th to 20th	Night	51% 10% 14% 19% 6% 1	6320
	21st to end	Night	55% 3 9% 20% 13% 1	6822
February	1st to 10th	Night	55% 4 18% 17% 6%	2600
	11th to 20th	Night	50% 5% 15% 22% 7% 1	2499
	21st to end	Night	54% 4 12% 20% 9% 1	1980
March	1st to 10th	Night	42% 4 12% 26% 16% 1	5376
	11th to 20th	Night	43% 4 14% 31% 8%	5073
	21st to end	Night	53% 6% 18% 19% 3	5150
April	1st to 10th	Night	46% 4 16% 25% 9%	2000
	11th to 20th	Night	41% 4 17% 25% 12% 2	1979
	21st to end	Night	38% 5% 14% 28% 13% 2	1903
May	1st to 10th	Night	31% 1 8% 29% 30%	3637
	11th to 20th	Night	26% 3 16% 34% 20% 1	3616
	21st to end	Night	28% 2 26% 33% 10%	3972
June	1st to 10th	Night	33% 5% 13% 32% 16% 2	1600
	11th to 20th	Night	29% 3 12% 38% 15% 3	1600
	21st to end	Night	19% 2 12% 34% 26% 8%	1600
July	1st to 10th	Night	11% 1 5% 22% 28% 33%	3600
	11th to 20th	Night	7% 2 5% 13% 26% 47%	3592
	21st to end	Night	10% 1 5% 31% 34% 20%	3857
August	1st to 10th	Night	15% 2 5% 18% 37% 23%	2658
	11th to 20th	Night	11% 2 9% 40% 27% 10%	2995
	21st to end	Night	13% 2 11% 31% 30% 12%	3608
September	1st to 10th	Night	12% 2 9% 30% 30% 17%	2000
	11th to 20th	Night	22% 3 10% 27% 26% 12%	2000
	21st to end	Night	25% 2 9% 30% 26% 8%	2237
October	1st to 10th	Night	23% 3 9% 29% 32% 4	3920
	11th to 20th	Night	31% 5% 21% 24% 17% 2	3873
	21st to end	Night	39% 5% 14% 28% 13% 1	4358
November	1st to 10th	Night	49% 4 11% 25% 10% 1	2535
	11th to 20th	Night	54% 5% 14% 22% 5%	2754
	21st to end	Night	47% 5% 16% 24% 8%	2800
December	1st to 10th	Night	35% 15% 16% 27% 6%	4560
	11th to 20th	Night	71% 6% 10% 10% 2	5176
	21st to end	Night	68% 6% 9% 12% 5%	6952

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	22% 5% 20% 32% 15% 8%	200
	11th to 20th	Day	15% 4 16% 39% 20% 7%	200
	21st to end	Day	12% 8% 16% 31% 25% 9%	275
February	1st to 10th	Day	18% 10% 17% 31% 19% 7%	400
	11th to 20th	Day	18% 8% 22% 28% 19% 5%	412
	21st to end	Day	17% 11% 21% 30% 18% 4	414
March	1st to 10th	Day	16% 12% 23% 32% 13% 4	500
	11th to 20th	Day	17% 12% 19% 30% 16% 7%	500
	21st to end	Day	20% 13% 23% 27% 16% 1	550
April	1st to 10th	Day	19% 12% 22% 26% 14% 7%	500
	11th to 20th	Day	13% 14% 27% 26% 13% 6%	492
	21st to end	Day	13% 16% 21% 27% 14% 8%	500
May	1st to 10th	Day	15% 11% 25% 26% 16% 7%	503
	11th to 20th	Day	17% 12% 20% 27% 15% 9%	528
	21st to end	Day	13% 10% 18% 28% 19% 12%	573
June	1st to 10th	Day	13% 10% 21% 26% 19% 11%	530
	11th to 20th	Day	15% 17% 22% 26% 15% 6%	530
	21st to end	Day	5% 11% 21% 31% 21% 12%	530
July	1st to 10th	Day	2 7% 26% 30% 24% 10%	526
	11th to 20th	Day	4 9% 27% 22% 26% 12%	524
	21st to end	Day	3 6% 26% 28% 23% 14%	575
August	1st to 10th	Day	4 8% 22% 28% 23% 15%	527
	11th to 20th	Day	4 6% 20% 27% 23% 20%	517
	21st to end	Day	5% 7% 22% 26% 26% 14%	583
September	1st to 10th	Day	3 6% 20% 27% 26% 17%	530
	11th to 20th	Day	7% 7% 17% 32% 25% 11%	530
	21st to end	Day	11% 7% 17% 26% 27% 12%	530
October	1st to 10th	Day	13% 7% 19% 25% 22% 14%	530
	11th to 20th	Day	13% 9% 17% 23% 26% 12%	523
	21st to end	Day	16% 8% 19% 28% 20% 9%	393
November	1st to 10th	Day	17% 8% 15% 35% 19% 6%	300
	11th to 20th	Day	17% 8% 15% 29% 22% 9%	223
	21st to end	Day	13% 5% 19% 31% 26% 8%	200
December	1st to 10th	Day	16% 7% 11% 23% 26% 18%	200
	11th to 20th	Day	20% 1 17% 42% 17% 4	200
	21st to end	Day	20% 6% 16% 36% 17% 5%	220
January	1st to 10th	Night	19% 6% 17% 37% 18% 3	800
	11th to 20th	Night	17% 6% 14% 37% 24% 3	800
	21st to end	Night	17% 4 12% 41% 24% 3	880
February	1st to 10th	Night	19% 4 16% 41% 18% 2	800
	11th to 20th	Night	15% 5% 16% 44% 19% 2	784
	21st to end	Night	20% 6% 17% 39% 17% 1	660
March	1st to 10th	Night	14% 7% 15% 46% 18% 1	800
	11th to 20th	Night	13% 6% 14% 46% 19% 3	800
	21st to end	Night	15% 7% 20% 41% 16% 2	880
April	1st to 10th	Night	15% 5% 17% 40% 21% 2	800
	11th to 20th	Night	13% 7% 19% 40% 18% 3	791
	21st to end	Night	13% 7% 20% 37% 21% 2	703
May	1st to 10th	Night	10% 3 18% 46% 21% 2	400
	11th to 20th	Night	12% 6% 13% 38% 29% 2	394
	21st to end	Night	10% 3 11% 43% 31% 3	433
June	1st to 10th	Night	13% 3 12% 47% 24% 2	400
	11th to 20th	Night	9% 5% 18% 48% 19% 2	400
	21st to end	Night	6% 3 9% 53% 29% 1	400
July	1st to 10th	Night	3 1 11% 49% 34% 2	390
	11th to 20th	Night	1 12% 53% 32% 1	388
	21st to end	Night	1 1 6% 53% 37% 2	438
August	1st to 10th	Night	2 5% 48% 41% 2	429
	11th to 20th	Night	3 2 5% 38% 43% 9%	634
	21st to end	Night	2 4 13% 46% 31% 5%	880
September	1st to 10th	Night	3 3 10% 40% 40% 4	800
	11th to 20th	Night	5% 2 9% 41% 35% 8%	800
	21st to end	Night	9% 3 8% 41% 32% 7%	800
October	1st to 10th	Night	10% 3 9% 45% 30% 4	800
	11th to 20th	Night	11% 3 12% 41% 28% 5%	792
	21st to end	Night	13% 3 16% 41% 22% 5%	836
November	1st to 10th	Night	16% 4 12% 47% 20% 1	780
	11th to 20th	Night	18% 6% 14% 41% 17% 3	800
	21st to end	Night	13% 7% 14% 41% 21% 4	800
December	1st to 10th	Night	16% 4 13% 42% 20% 6%	800
	11th to 20th	Night	17% 5% 15% 42% 19% 2	800
	21st to end	Night	21% 6% 19% 40% 13% 1	880

Page generated at 2025-10-15 22:50:27. Version ac07

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