Humarock Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Humarock Observatory clear sky chart from 2005-11-06 to 2025-10-04.

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	16% 8% 2 6% 4 4 4 4 5% 7% 39%	126101
	Night	17% 8% 3 6% 4 4 4 3 5% 6% 39%	123272
Transparency	Day	27% 40% 8% 14% 9% 1	129649
	Night	28% 39% 7% 13% 10% 1	127811
Seeing	Day	18% 6% 22% 31% 18% 4	16323
	Night	20% 9% 28% 30% 13% 2	25125

By Month

Percentage Cloud Cover NE (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% 7% 2 6% 4 5% 5% 4 5% 7% 44%	3597
	11th to 20th	Day	15% 9% 3 4 2 3 3 3 4 5% 48%	3570
	21st to end	Day	18% 9% 2 7% 3 3 4 3 4 5% 41%	3975
February	1st to 10th	Day	20% 9% 3 4 4 5% 5% 4 5% 6% 34%	1893
	11th to 20th	Day	14% 9% 3 5% 4 4 4 4 5% 7% 40%	1948
	21st to end	Day	16% 8% 3 5% 4 4 4 4 6% 6% 37%	1650
March	1st to 10th	Day	20% 12% 3 6% 3 4 4 3 5% 6% 36%	4476
	11th to 20th	Day	15% 10% 3 8% 4 4 4 3 5% 6% 38%	4838
	21st to end	Day	26% 9% 2 5% 4 5% 5% 3 5% 6% 31%	5666
April	1st to 10th	Day	19% 10% 2 6% 4 3 4 3 6% 5% 39%	2400
	11th to 20th	Day	16% 8% 2 5% 4 4 4 4 5% 6% 41%	2414
	21st to end	Day	18% 6% 2 6% 4 4 4 3 5% 7% 41%	2600
May	1st to 10th	Day	26% 11% 3 7% 4 4 3 3 5% 6% 26%	5886
	11th to 20th	Day	19% 9% 3 8% 5% 4 4 3 5% 7% 33%	5861
	21st to end	Day	18% 7% 2 7% 4 4 4 3 5% 8% 38%	6896
June	1st to 10th	Day	20% 6% 2 4 4 4 4 5% 7% 10% 34%	2800
	11th to 20th	Day	15% 6% 2 5% 5% 4 4 5% 6% 10% 38%	2800
	21st to end	Day	14% 5% 2 6% 5% 5% 4 4 7% 8% 38%	2800
July	1st to 10th	Day	15% 6% 2 6% 5% 5% 5% 4 7% 10% 34%	6360
	11th to 20th	Day	8% 7% 2 7% 4 5% 6% 5% 7% 11% 38%	6360
	21st to end	Day	14% 5% 2 5% 4 4 4 4 5% 8% 45%	6864
August	1st to 10th	Day	14% 7% 2 7% 4 5% 5% 5% 7% 9% 34%	4235
	11th to 20th	Day	11% 8% 3 8% 4 5% 5% 4 6% 7% 39%	4175
	21st to end	Day	8% 4 1 4 4 5% 5% 5% 7% 9% 47%	4322
September	1st to 10th	Day	13% 6% 2 5% 4 4 4 3 6% 7% 47%	2365
	11th to 20th	Day	14% 5% 2 4 3 3 3 3 4 6% 54%	2200
	21st to end	Day	19% 5% 2 5% 4 3 4 4 5% 6% 43%	2200
October	1st to 10th	Day	23% 5% 2 5% 3 3 4 3 4 6% 43%	2123
	11th to 20th	Day	14% 6% 2 4 3 4 4 3 5% 8% 47%	1874
	21st to end	Day	19% 6% 2 5% 3 3 4 3 5% 6% 43%	1881
November	1st to 10th	Day	17% 7% 2 5% 4 2 4 2 4 7% 46%	1761
	11th to 20th	Day	17% 7% 2 4 4 4 4 3 5% 6% 45%	1800
	21st to end	Day	19% 7% 2 5% 3 3 4 3 5% 6% 42%	1800
December	1st to 10th	Day	14% 10% 2 4 3 4 3 3 4 6% 48%	2765
	11th to 20th	Day	17% 8% 3 5% 4 3 3 3 4 6% 44%	2986
	21st to end	Day	19% 11% 4 9% 4 3 4 3 4 5% 33%	3960
January	1st to 10th	Night	15% 6% 1 4 3 3 3 3 5% 7% 50%	6349
	11th to 20th	Night	15% 10% 4 8% 4 4 4 3 5% 5% 39%	6315
	21st to end	Night	17% 11% 4 8% 4 4 4 3 4 5% 35%	6979
February	1st to 10th	Night	17% 10% 3 6% 4 4 4 4 5% 7% 35%	2505
	11th to 20th	Night	17% 9% 3 6% 4 3 4 4 6% 6% 37%	2338
	21st to end	Night	17% 9% 2 5% 5% 4 3 3 5% 6% 41%	1980
March	1st to 10th	Night	26% 9% 2 5% 4 3 3 3 4 5% 36%	5364
	11th to 20th	Night	16% 11% 3 6% 4 4 4 3 4 5% 37%	5120
	21st to end	Night	27% 10% 2 6% 4 4 4 3 4 4 32%	5250
April	1st to 10th	Night	19% 10% 3 6% 5% 4 4 2 5% 7% 36%	2000
	11th to 20th	Night	19% 9% 1 6% 5% 5% 5% 3 5% 6% 38%	1933
	21st to end	Night	18% 8% 3 7% 4 4 5% 3 5% 5% 38%	1800
May	1st to 10th	Night	27% 11% 4 7% 4 4 4 3 4 7% 27%	4054
	11th to 20th	Night	20% 9% 3 8% 5% 5% 5% 4 6% 7% 28%	4033
	21st to end	Night	19% 8% 3 8% 5% 5% 5% 4 6% 7% 30%	4020
June	1st to 10th	Night	17% 8% 3 6% 4 4 5% 4 7% 7% 37%	1600
	11th to 20th	Night	15% 8% 3 7% 5% 4 5% 4 5% 8% 36%	1600
	21st to end	Night	15% 7% 2 6% 3 4 4 4 7% 10% 38%	1600
July	1st to 10th	Night	10% 7% 3 8% 5% 4 5% 4 6% 10% 37%	3600
	11th to 20th	Night	10% 7% 2 8% 5% 5% 5% 5% 8% 10% 36%	3600
	21st to end	Night	13% 7% 3 6% 4 4 4 4 5% 7% 43%	3941
August	1st to 10th	Night	16% 6% 3 8% 5% 5% 6% 5% 7% 9% 30%	2915
	11th to 20th	Night	9% 9% 3 7% 4 5% 6% 4 7% 8% 38%	2872
	21st to end	Night	9% 3 2 4 3 4 3 4 6% 8% 53%	3574
September	1st to 10th	Night	11% 6% 2 6% 4 3 4 4 6% 7% 46%	2035
	11th to 20th	Night	13% 5% 2 5% 3 3 3 3 5% 7% 52%	2200
	21st to end	Night	18% 6% 2 6% 3 3 4 4 5% 6% 42%	2200
October	1st to 10th	Night	22% 5% 3 5% 4 3 3 3 4 6% 42%	2124
	11th to 20th	Night	15% 6% 2 6% 3 3 4 3 5% 7% 46%	2303
	21st to end	Night	21% 8% 2 6% 3 4 4 3 5% 5% 39%	2717
November	1st to 10th	Night	15% 5% 2 6% 4 3 4 3 5% 6% 45%	2537
	11th to 20th	Night	16% 8% 2 6% 3 4 3 3 4 6% 45%	2600
	21st to end	Night	16% 7% 2 5% 4 3 4 3 5% 8% 42%	2600
December	1st to 10th	Night	18% 8% 2 5% 3 4 3 3 4 5% 44%	4410
	11th to 20th	Night	20% 9% 2 6% 4 3 3 3 5% 5% 39%	5208
	21st to end	Night	17% 9% 5% 8% 5% 4 3 3 4 6% 35%	6996

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	28% 29% 6% 19% 16% 2	3598
	11th to 20th	Day	26% 29% 8% 19% 16% 3	3600
	21st to end	Day	34% 31% 7% 13% 14% 1	3975
February	1st to 10th	Day	19% 62% 7% 7% 5% 1	1893
	11th to 20th	Day	19% 61% 4 9% 7% 1	1957
	21st to end	Day	14% 63% 5% 10% 6% 1	1650
March	1st to 10th	Day	37% 30% 7% 9% 9% 8%	4479
	11th to 20th	Day	31% 33% 11% 18% 7% 1	4838
	21st to end	Day	43% 31% 4 9% 9% 5%	5666
April	1st to 10th	Day	21% 62% 3 7% 6% 2	2400
	11th to 20th	Day	16% 63% 4 9% 7% 1	2414
	21st to end	Day	18% 62% 3 8% 8% 1	2600
May	1st to 10th	Day	43% 34% 11% 9% 3	5897
	11th to 20th	Day	40% 31% 10% 12% 5%	5865
	21st to end	Day	33% 32% 9% 15% 10%	6896
June	1st to 10th	Day	16% 63% 5% 9% 7%	2800
	11th to 20th	Day	14% 63% 6% 9% 7% 1	2800
	21st to end	Day	14% 61% 6% 10% 8% 1	2800
July	1st to 10th	Day	33% 30% 11% 14% 10% 1	6290
	11th to 20th	Day	27% 32% 14% 18% 9%	6285
	21st to end	Day	27% 27% 10% 23% 11% 1	6667
August	1st to 10th	Day	31% 39% 11% 13% 5%	4222
	11th to 20th	Day	28% 39% 8% 20% 5%	4214
	21st to end	Day	15% 39% 11% 21% 13%	4322
September	1st to 10th	Day	15% 58% 5% 14% 9%	2365
	11th to 20th	Day	16% 57% 4 11% 10% 1	2200
	21st to end	Day	20% 57% 5% 10% 7% 1	2200
October	1st to 10th	Day	23% 38% 8% 17% 12% 1	3513
	11th to 20th	Day	16% 37% 3 20% 20% 2	3090
	21st to end	Day	16% 38% 8% 23% 13% 1	3081
November	1st to 10th	Day	19% 56% 7% 10% 7% 1	1761
	11th to 20th	Day	17% 57% 4 7% 13% 2	1800
	21st to end	Day	17% 57% 6% 12% 6% 1	1800
December	1st to 10th	Day	24% 37% 10% 22% 8%	2765
	11th to 20th	Day	26% 34% 11% 10% 14% 5%	2986
	21st to end	Day	40% 31% 7% 12% 8% 2	3960
January	1st to 10th	Night	23% 29% 7% 27% 12% 1	6349
	11th to 20th	Night	34% 28% 9% 17% 10% 3	6360
	21st to end	Night	38% 30% 6% 11% 12% 3	6979
February	1st to 10th	Night	20% 62% 5% 7% 5% 1	2505
	11th to 20th	Night	19% 61% 3 7% 8% 1	2340
	21st to end	Night	17% 62% 3 9% 7% 2	1980
March	1st to 10th	Night	39% 29% 6% 9% 11% 5%	5374
	11th to 20th	Night	34% 32% 11% 12% 10% 1	5121
	21st to end	Night	44% 30% 3 6% 13% 4	5250
April	1st to 10th	Night	22% 61% 2 7% 5% 2	2000
	11th to 20th	Night	18% 63% 4 8% 7% 1	1933
	21st to end	Night	19% 62% 4 6% 8% 1	1800
May	1st to 10th	Night	44% 32% 10% 9% 3 2	4060
	11th to 20th	Night	39% 34% 12% 10% 5%	4034
	21st to end	Night	39% 33% 8% 12% 7%	4021
June	1st to 10th	Night	16% 63% 5% 9% 7%	1600
	11th to 20th	Night	15% 64% 6% 9% 6%	1600
	21st to end	Night	15% 62% 4 11% 8% 1	1600
July	1st to 10th	Night	32% 30% 8% 16% 14%	3560
	11th to 20th	Night	31% 31% 14% 15% 9% 1	3552
	21st to end	Night	29% 29% 10% 21% 11% 1	3824
August	1st to 10th	Night	34% 40% 9% 10% 6% 1	2903
	11th to 20th	Night	26% 42% 10% 17% 5%	2899
	21st to end	Night	14% 38% 11% 18% 18%	3574
September	1st to 10th	Night	14% 58% 5% 13% 8% 1	2035
	11th to 20th	Night	17% 57% 3 10% 12% 1	2200
	21st to end	Night	21% 58% 4 9% 8%	2200
October	1st to 10th	Night	25% 39% 8% 19% 7% 1	3514
	11th to 20th	Night	12% 37% 6% 17% 26% 2	3826
	21st to end	Night	17% 37% 7% 23% 14% 1	4467
November	1st to 10th	Night	19% 58% 5% 10% 7% 1	2537
	11th to 20th	Night	17% 57% 4 9% 11% 2	2600
	21st to end	Night	17% 57% 6% 13% 6% 1	2600
December	1st to 10th	Night	27% 37% 8% 19% 9%	4410
	11th to 20th	Night	33% 33% 9% 10% 14% 1	5208
	21st to end	Night	38% 30% 8% 14% 9%	6996

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	24% 15% 33% 22% 7% 1	200
	11th to 20th	Day	20% 16% 33% 26% 6%	200
	21st to end	Day	19% 9% 43% 22% 7%	222
February	1st to 10th	Day	24% 12% 35% 24% 4	447
	11th to 20th	Day	20% 16% 42% 20% 3	496
	21st to end	Day	17% 13% 32% 30% 7% 1	414
March	1st to 10th	Day	19% 11% 36% 27% 7%	500
	11th to 20th	Day	17% 10% 36% 29% 7% 1	500
	21st to end	Day	23% 10% 33% 24% 8% 2	550
April	1st to 10th	Day	24% 10% 27% 28% 10% 1	500
	11th to 20th	Day	21% 6% 29% 36% 9%	492
	21st to end	Day	22% 4 24% 36% 13% 2	500
May	1st to 10th	Day	26% 5% 25% 32% 10% 2	503
	11th to 20th	Day	16% 2 22% 40% 15% 4	527
	21st to end	Day	17% 3 15% 37% 24% 4	573
June	1st to 10th	Day	20% 2 13% 38% 23% 4	530
	11th to 20th	Day	15% 3 15% 41% 21% 5%	530
	21st to end	Day	14% 3 12% 40% 28% 4	530
July	1st to 10th	Day	14% 3 9% 32% 33% 8%	525
	11th to 20th	Day	10% 3 10% 32% 37% 8%	524
	21st to end	Day	12% 2 9% 37% 37% 3	576
August	1st to 10th	Day	10% 2 9% 37% 35% 6%	526
	11th to 20th	Day	12% 3 10% 32% 34% 9%	519
	21st to end	Day	12% 2 11% 35% 31% 9%	583
September	1st to 10th	Day	16% 3 13% 31% 28% 10%	530
	11th to 20th	Day	16% 5% 15% 27% 26% 12%	530
	21st to end	Day	21% 4 16% 32% 20% 7%	530
October	1st to 10th	Day	23% 4 20% 26% 22% 6%	502
	11th to 20th	Day	15% 6% 29% 25% 22% 3	420
	21st to end	Day	23% 6% 27% 29% 12% 3	429
November	1st to 10th	Day	15% 9% 24% 34% 16% 2	395
	11th to 20th	Day	23% 12% 31% 25% 9% 1	400
	21st to end	Day	21% 15% 30% 25% 9% 1	400
December	1st to 10th	Day	20% 9% 39% 23% 8%	300
	11th to 20th	Day	24% 12% 34% 26% 5%	200
	21st to end	Day	25% 12% 31% 24% 8% 1	220
January	1st to 10th	Night	22% 14% 41% 20% 4	800
	11th to 20th	Night	26% 17% 35% 19% 4	800
	21st to end	Night	19% 15% 41% 20% 4 1	880
February	1st to 10th	Night	22% 14% 37% 24% 3	800
	11th to 20th	Night	24% 16% 42% 16% 3	783
	21st to end	Night	21% 14% 37% 24% 5%	660
March	1st to 10th	Night	24% 12% 35% 24% 5%	800
	11th to 20th	Night	21% 10% 35% 26% 7% 1	800
	21st to end	Night	23% 8% 35% 25% 8% 1	880
April	1st to 10th	Night	23% 7% 33% 28% 8%	800
	11th to 20th	Night	21% 5% 31% 34% 8%	749
	21st to end	Night	19% 6% 29% 35% 11% 1	600
May	1st to 10th	Night	26% 5% 23% 34% 10% 2	600
	11th to 20th	Night	19% 4 26% 35% 15% 1	592
	21st to end	Night	19% 4 14% 45% 15% 2	475
June	1st to 10th	Night	20% 3 15% 39% 20% 3	400
	11th to 20th	Night	18% 3 16% 43% 19% 2	400
	21st to end	Night	16% 3 13% 46% 21% 1	400
July	1st to 10th	Night	12% 3 8% 47% 27% 2	390
	11th to 20th	Night	12% 3 8% 45% 28% 3	388
	21st to end	Night	12% 3 11% 44% 29% 1	441
August	1st to 10th	Night	9% 3 10% 44% 32% 2	598
	11th to 20th	Night	11% 3 10% 42% 31% 3	592
	21st to end	Night	13% 2 10% 38% 32% 6%	868
September	1st to 10th	Night	13% 2 16% 38% 29% 3	800
	11th to 20th	Night	15% 3 15% 34% 26% 7%	800
	21st to end	Night	20% 3 17% 33% 23% 5%	800
October	1st to 10th	Night	23% 5% 20% 28% 20% 5%	773
	11th to 20th	Night	17% 6% 30% 29% 16% 2	760
	21st to end	Night	25% 7% 31% 25% 10% 1	836
November	1st to 10th	Night	18% 6% 30% 32% 12% 1	780
	11th to 20th	Night	20% 15% 33% 22% 9% 1	800
	21st to end	Night	19% 17% 33% 25% 5% 1	800
December	1st to 10th	Night	24% 13% 37% 20% 6%	800
	11th to 20th	Night	22% 17% 38% 20% 4	800
	21st to end	Night	22% 13% 40% 21% 4	880

Page generated at 2025-10-01 01:39:08. Version ac07

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