Mont Hereford
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Mont Hereford clear sky chart from 2005-11-06 to 2025-01-22.

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	17% 13% 5% 10% 6% 5% 5% 3 5% 6% 25%	105171
	Night	17% 15% 5% 12% 6% 5% 4 3 4 5% 24%	104187
Transparency	Day	58% 6% 11% 17% 7% 1	108670
	Night	62% 6% 10% 15% 7% 1	108777
Seeing	Day	29% 19% 27% 16% 7% 2	14920
	Night	27% 18% 26% 21% 8% 1	23891

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	13% 17% 9% 18% 10% 7% 4 3 3 3 12%	2597
	11th to 20th	Day	9% 17% 7% 22% 13% 6% 3 2 3 4 12%	2600
	21st to end	Day	12% 18% 11% 16% 6% 6% 4 3 3 4 16%	2780
February	1st to 10th	Day	14% 17% 7% 16% 10% 7% 4 2 3 3 16%	1602
	11th to 20th	Day	11% 18% 9% 17% 9% 7% 4 3 4 3 15%	1848
	21st to end	Day	12% 16% 7% 15% 7% 7% 4 3 5% 4 20%	1570
March	1st to 10th	Day	20% 17% 5% 11% 6% 5% 4 3 3 4 21%	3116
	11th to 20th	Day	14% 23% 8% 14% 6% 3 3 2 3 3 22%	3308
	21st to end	Day	17% 16% 5% 12% 5% 4 3 2 4 5% 28%	4070
April	1st to 10th	Day	19% 19% 5% 9% 5% 4 3 3 4 4 25%	2280
	11th to 20th	Day	17% 17% 3 7% 4 4 3 3 4 5% 32%	2302
	21st to end	Day	22% 16% 3 8% 5% 4 4 3 4 5% 26%	2470
May	1st to 10th	Day	25% 12% 3 7% 5% 5% 4 3 4 5% 26%	4106
	11th to 20th	Day	20% 9% 4 9% 6% 5% 5% 4 6% 7% 27%	4398
	21st to end	Day	15% 8% 2 7% 5% 5% 6% 5% 7% 9% 30%	4837
June	1st to 10th	Day	27% 11% 3 7% 4 4 5% 4 6% 6% 24%	2660
	11th to 20th	Day	19% 8% 3 6% 5% 4 5% 4 6% 7% 32%	2660
	21st to end	Day	23% 9% 3 8% 4 5% 5% 4 7% 7% 25%	2660
July	1st to 10th	Day	15% 7% 3 7% 5% 5% 7% 5% 9% 10% 28%	4440
	11th to 20th	Day	14% 7% 3 8% 6% 6% 6% 5% 8% 10% 29%	4440
	21st to end	Day	16% 8% 3 7% 5% 5% 4 4 6% 7% 35%	4814
August	1st to 10th	Day	18% 8% 3 9% 5% 6% 6% 5% 7% 9% 24%	4338
	11th to 20th	Day	17% 9% 3 9% 6% 6% 6% 5% 7% 8% 24%	4045
	21st to end	Day	19% 8% 3 5% 4 4 5% 3 7% 8% 35%	4224
September	1st to 10th	Day	18% 8% 2 6% 4 4 4 4 5% 7% 39%	2280
	11th to 20th	Day	19% 7% 2 5% 4 4 4 3 4 6% 41%	2114
	21st to end	Day	18% 10% 3 6% 5% 4 4 4 6% 8% 33%	2090
October	1st to 10th	Day	22% 13% 3 6% 4 4 5% 4 6% 6% 27%	2090
	11th to 20th	Day	18% 16% 3 8% 5% 4 4 4 6% 6% 26%	1775
	21st to end	Day	22% 22% 5% 7% 4 4 4 2 3 4 23%	1881
November	1st to 10th	Day	17% 19% 4 10% 5% 3 4 3 3 5% 26%	1761
	11th to 20th	Day	17% 19% 6% 12% 6% 3 4 3 4 5% 22%	1800
	21st to end	Day	17% 19% 7% 12% 6% 4 5% 3 5% 5% 17%	1669
December	1st to 10th	Day	12% 21% 7% 22% 11% 5% 3 3 3 3 10%	2600
	11th to 20th	Day	15% 18% 8% 16% 5% 5% 4 3 4 4 19%	2986
	21st to end	Day	17% 18% 6% 14% 6% 4 6% 3 4 4 18%	3960
January	1st to 10th	Night	12% 18% 9% 21% 12% 6% 5% 2 3 3 11%	4569
	11th to 20th	Night	11% 16% 8% 20% 15% 7% 5% 3 3 3 9%	4580
	21st to end	Night	12% 20% 9% 17% 8% 8% 4 3 3 3 15%	4899
February	1st to 10th	Night	12% 17% 6% 16% 10% 7% 5% 2 4 4 16%	2574
	11th to 20th	Night	10% 17% 9% 16% 10% 7% 6% 3 3 3 16%	2218
	21st to end	Night	13% 16% 5% 13% 9% 7% 4 3 3 5% 22%	1884
March	1st to 10th	Night	20% 19% 5% 9% 5% 4 4 2 4 4 24%	3724
	11th to 20th	Night	14% 22% 9% 12% 6% 4 4 2 3 3 21%	3650
	21st to end	Night	17% 19% 6% 12% 6% 4 3 2 3 3 25%	3545
April	1st to 10th	Night	20% 17% 5% 9% 5% 4 4 3 5% 5% 24%	1900
	11th to 20th	Night	20% 15% 3 8% 5% 3 4 3 4 5% 30%	1826
	21st to end	Night	21% 15% 3 7% 5% 5% 4 3 5% 5% 28%	1710
May	1st to 10th	Night	26% 11% 3 7% 5% 4 4 3 4 6% 27%	2834
	11th to 20th	Night	20% 9% 3 8% 5% 4 5% 4 6% 6% 30%	2510
	21st to end	Night	19% 6% 2 7% 4 5% 4 4 6% 8% 36%	2738
June	1st to 10th	Night	27% 9% 3 6% 4 4 4 3 5% 6% 29%	1520
	11th to 20th	Night	20% 6% 2 6% 4 4 5% 4 5% 7% 36%	1520
	21st to end	Night	21% 8% 4 7% 4 6% 5% 4 6% 7% 30%	1520
July	1st to 10th	Night	13% 8% 3 7% 6% 6% 6% 5% 6% 8% 31%	2520
	11th to 20th	Night	19% 8% 3 7% 4 5% 5% 4 5% 7% 33%	2520
	21st to end	Night	15% 8% 3 7% 5% 5% 5% 4 6% 7% 36%	2739
August	1st to 10th	Night	23% 10% 4 7% 4 4 4 3 5% 5% 30%	2560
	11th to 20th	Night	16% 8% 3 9% 5% 6% 6% 5% 7% 7% 27%	2782
	21st to end	Night	18% 8% 3 6% 4 4 4 4 5% 6% 38%	3430
September	1st to 10th	Night	16% 8% 2 7% 4 4 4 3 5% 6% 40%	1900
	11th to 20th	Night	17% 6% 2 6% 3 3 4 3 5% 6% 45%	2066
	21st to end	Night	21% 8% 2 4 4 3 3 4 6% 7% 39%	2090
October	1st to 10th	Night	20% 10% 3 7% 4 5% 4 4 4 6% 33%	2090
	11th to 20th	Night	21% 14% 3 8% 5% 4 4 3 4 5% 27%	2402
	21st to end	Night	25% 17% 4 7% 4 3 4 3 4 6% 23%	2717
November	1st to 10th	Night	19% 19% 5% 9% 4 3 4 3 5% 5% 24%	2537
	11th to 20th	Night	16% 17% 7% 13% 5% 3 3 3 4 5% 26%	2600
	21st to end	Night	16% 18% 8% 15% 7% 4 3 2 3 4 18%	2729
December	1st to 10th	Night	13% 19% 10% 26% 9% 4 3 2 3 2 8%	4580
	11th to 20th	Night	16% 21% 6% 19% 8% 5% 4 2 3 3 13%	5208
	21st to end	Night	17% 17% 6% 14% 7% 5% 5% 3 4 5% 18%	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	78% 5% 8% 8% 1	2598
	11th to 20th	Day	75% 5% 6% 10% 3 1	2600
	21st to end	Day	72% 5% 8% 9% 5% 1	2780
February	1st to 10th	Day	74% 4 5% 10% 5% 1	1602
	11th to 20th	Day	75% 3 8% 8% 5% 1	1857
	21st to end	Day	68% 5% 7% 12% 6% 2	1570
March	1st to 10th	Day	67% 6% 8% 9% 9% 2	3119
	11th to 20th	Day	68% 5% 7% 11% 8% 2	3308
	21st to end	Day	60% 4 6% 14% 14% 1	4070
April	1st to 10th	Day	63% 4 8% 15% 8% 2	2280
	11th to 20th	Day	55% 5% 9% 19% 11% 1	2302
	21st to end	Day	61% 5% 9% 15% 8% 3	2470
May	1st to 10th	Day	60% 7% 9% 17% 6% 1	4120
	11th to 20th	Day	54% 10% 16% 14% 6% 1	4402
	21st to end	Day	45% 11% 14% 21% 9%	4837
June	1st to 10th	Day	59% 7% 11% 17% 5% 1	2660
	11th to 20th	Day	49% 7% 12% 21% 10% 1	2660
	21st to end	Day	56% 8% 12% 17% 6%	2660
July	1st to 10th	Day	46% 8% 16% 22% 8%	4370
	11th to 20th	Day	46% 9% 15% 27% 3	4365
	21st to end	Day	48% 7% 11% 27% 6%	4661
August	1st to 10th	Day	54% 8% 15% 17% 5%	4324
	11th to 20th	Day	54% 9% 15% 17% 4	4084
	21st to end	Day	45% 8% 14% 19% 12% 2	4224
September	1st to 10th	Day	44% 7% 13% 25% 10%	2280
	11th to 20th	Day	43% 6% 11% 26% 13% 1	2114
	21st to end	Day	48% 8% 13% 21% 10% 1	2090
October	1st to 10th	Day	51% 6% 12% 21% 9%	3480
	11th to 20th	Day	55% 5% 12% 21% 7%	2926
	21st to end	Day	55% 3 13% 19% 8% 1	3081
November	1st to 10th	Day	61% 5% 10% 16% 8% 1	1761
	11th to 20th	Day	65% 6% 8% 10% 9% 2	1800
	21st to end	Day	69% 5% 12% 11% 3	1669
December	1st to 10th	Day	81% 4 7% 5% 3	2600
	11th to 20th	Day	69% 4 7% 13% 7%	2986
	21st to end	Day	70% 5% 5% 9% 8% 2	3960
January	1st to 10th	Night	80% 4 8% 6% 2	4569
	11th to 20th	Night	81% 5% 6% 6% 2	4580
	21st to end	Night	75% 5% 7% 8% 4 1	4899
February	1st to 10th	Night	73% 5% 7% 10% 5%	2574
	11th to 20th	Night	75% 4 6% 10% 5% 1	2220
	21st to end	Night	66% 5% 6% 13% 8% 1	1884
March	1st to 10th	Night	63% 5% 9% 14% 7% 2	3734
	11th to 20th	Night	68% 6% 7% 11% 7% 1	3651
	21st to end	Night	66% 4 4 13% 12% 1	3545
April	1st to 10th	Night	62% 5% 8% 14% 10% 1	1900
	11th to 20th	Night	55% 7% 8% 19% 11%	1826
	21st to end	Night	58% 6% 10% 15% 10% 2	1710
May	1st to 10th	Night	57% 8% 13% 14% 7% 1	2840
	11th to 20th	Night	51% 10% 16% 16% 7% 1	2511
	21st to end	Night	45% 9% 15% 18% 13%	2739
June	1st to 10th	Night	55% 7% 10% 20% 7%	1520
	11th to 20th	Night	47% 8% 13% 21% 9% 2	1520
	21st to end	Night	54% 9% 12% 18% 6% 1	1520
July	1st to 10th	Night	48% 10% 14% 20% 8%	2480
	11th to 20th	Night	48% 8% 14% 25% 5%	2472
	21st to end	Night	46% 9% 14% 23% 8%	2651
August	1st to 10th	Night	54% 8% 13% 21% 4	2550
	11th to 20th	Night	52% 10% 17% 15% 6%	2809
	21st to end	Night	46% 7% 13% 21% 13%	3430
September	1st to 10th	Night	44% 7% 14% 26% 9%	1900
	11th to 20th	Night	41% 6% 14% 27% 11% 1	2066
	21st to end	Night	43% 7% 13% 24% 11% 2	2090
October	1st to 10th	Night	51% 6% 11% 23% 9%	3480
	11th to 20th	Night	56% 5% 10% 20% 9%	3990
	21st to end	Night	54% 5% 15% 17% 8% 1	4467
November	1st to 10th	Night	62% 5% 11% 15% 7% 1	2537
	11th to 20th	Night	62% 4 8% 15% 8% 2	2600
	21st to end	Night	71% 6% 9% 11% 4	2729
December	1st to 10th	Night	84% 4 5% 5% 1	4580
	11th to 20th	Night	77% 5% 4 8% 6%	5208
	21st to end	Night	71% 5% 6% 10% 8% 1	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	30% 23% 37% 10% 2	200
	11th to 20th	Day	32% 25% 33% 9% 3 1	200
	21st to end	Day	28% 24% 33% 13% 4	200
February	1st to 10th	Day	41% 27% 23% 7% 1	234
	11th to 20th	Day	34% 30% 26% 8% 2	456
	21st to end	Day	32% 28% 26% 12% 1	382
March	1st to 10th	Day	33% 21% 31% 12% 3	460
	11th to 20th	Day	31% 27% 25% 12% 4	460
	21st to end	Day	34% 24% 24% 12% 5% 1	506
April	1st to 10th	Day	41% 19% 22% 12% 5% 1	460
	11th to 20th	Day	35% 23% 24% 11% 5% 2	452
	21st to end	Day	34% 23% 26% 11% 4 1	460
May	1st to 10th	Day	42% 14% 25% 12% 5% 1	463
	11th to 20th	Day	28% 19% 30% 16% 5% 2	487
	21st to end	Day	24% 21% 29% 19% 6% 1	529
June	1st to 10th	Day	33% 17% 29% 14% 6% 1	490
	11th to 20th	Day	24% 19% 26% 21% 7% 2	490
	21st to end	Day	27% 17% 31% 17% 6% 2	490
July	1st to 10th	Day	20% 15% 30% 25% 7% 1	485
	11th to 20th	Day	18% 13% 37% 23% 7% 2	484
	21st to end	Day	20% 13% 34% 23% 7% 2	532
August	1st to 10th	Day	21% 12% 30% 23% 12% 2	486
	11th to 20th	Day	18% 12% 29% 24% 14% 3	479
	21st to end	Day	24% 16% 22% 23% 11% 4	539
September	1st to 10th	Day	23% 16% 23% 22% 11% 5%	490
	11th to 20th	Day	22% 11% 20% 22% 17% 7%	490
	21st to end	Day	20% 17% 23% 22% 12% 5%	490
October	1st to 10th	Day	29% 19% 21% 19% 10% 2	490
	11th to 20th	Day	30% 20% 27% 13% 8% 2	423
	21st to end	Day	35% 23% 24% 14% 4	429
November	1st to 10th	Day	37% 19% 25% 15% 4	395
	11th to 20th	Day	30% 25% 29% 11% 4 1	400
	21st to end	Day	36% 21% 28% 12% 3	269
December	1st to 10th	Day	34% 23% 27% 12% 5%	200
	11th to 20th	Day	37% 29% 26% 6% 3	200
	21st to end	Day	35% 24% 29% 8% 3	220
January	1st to 10th	Night	27% 27% 31% 13% 2	800
	11th to 20th	Night	27% 28% 27% 16% 4	800
	21st to end	Night	21% 25% 31% 18% 3 1	841
February	1st to 10th	Night	25% 26% 31% 15% 3	760
	11th to 20th	Night	24% 28% 32% 13% 3	743
	21st to end	Night	25% 23% 33% 17% 1	628
March	1st to 10th	Night	28% 21% 30% 18% 3	760
	11th to 20th	Night	31% 20% 27% 16% 6%	760
	21st to end	Night	33% 20% 22% 18% 7% 1	836
April	1st to 10th	Night	34% 21% 25% 17% 4	760
	11th to 20th	Night	32% 19% 25% 18% 6%	702
	21st to end	Night	32% 17% 28% 18% 5%	570
May	1st to 10th	Night	30% 15% 30% 18% 6% 1	570
	11th to 20th	Night	22% 16% 30% 25% 7%	389
	21st to end	Night	24% 10% 25% 32% 7% 1	411
June	1st to 10th	Night	30% 8% 26% 27% 9% 1	380
	11th to 20th	Night	21% 9% 24% 32% 13%	380
	21st to end	Night	22% 8% 27% 35% 8%	380
July	1st to 10th	Night	17% 8% 27% 36% 12% 1	370
	11th to 20th	Night	18% 6% 24% 35% 16% 1	368
	21st to end	Night	17% 6% 23% 39% 16%	416
August	1st to 10th	Night	19% 6% 18% 38% 18% 1	406
	11th to 20th	Night	18% 7% 22% 34% 18% 1	562
	21st to end	Night	22% 7% 21% 31% 17% 2	803
September	1st to 10th	Night	21% 8% 23% 32% 14% 1	760
	11th to 20th	Night	21% 9% 20% 27% 21% 2	760
	21st to end	Night	24% 11% 23% 26% 14% 2	760
October	1st to 10th	Night	28% 12% 22% 24% 13% 1	760
	11th to 20th	Night	31% 16% 22% 21% 8% 1	760
	21st to end	Night	35% 19% 26% 17% 3	836
November	1st to 10th	Night	32% 17% 27% 17% 6% 1	780
	11th to 20th	Night	27% 23% 27% 15% 7% 1	800
	21st to end	Night	31% 24% 24% 15% 5% 1	800
December	1st to 10th	Night	33% 25% 25% 14% 3	800
	11th to 20th	Night	33% 24% 26% 14% 4	800
	21st to end	Night	35% 22% 26% 14% 2	880

Page generated at 2025-01-19 03:06:46. Version ac07

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