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 2026-05-17.

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	17% 13% 5% 11% 6% 5% 5% 3 5% 6% 24%	131742
	Night	15% 14% 5% 13% 7% 6% 5% 3 4 5% 22%	131076
Transparency	Day	59% 6% 10% 16% 7% 1	135223
	Night	63% 6% 9% 14% 7% 1	135682
Seeing	Day	29% 20% 27% 16% 6% 2	16611
	Night	27% 18% 26% 21% 7% 1	25560

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	10% 14% 11% 20% 12% 10% 5% 2 3 3 9%	4661
	11th to 20th	Day	7% 17% 8% 24% 16% 7% 4 2 3 4 9%	4650
	21st to end	Day	7% 14% 8% 15% 9% 14% 7% 4 5% 4 14%	5124
February	1st to 10th	Day	13% 16% 7% 17% 11% 7% 5% 2 4 3 16%	1772
	11th to 20th	Day	10% 17% 9% 18% 10% 6% 5% 3 4 4 15%	2048
	21st to end	Day	11% 16% 7% 16% 8% 7% 4 3 5% 3 20%	1730
March	1st to 10th	Day	17% 16% 6% 15% 7% 5% 5% 3 3 4 19%	4576
	11th to 20th	Day	13% 17% 6% 11% 5% 4 4 3 4 5% 26%	4842
	21st to end	Day	19% 16% 6% 12% 6% 4 3 2 3 5% 24%	5961
April	1st to 10th	Day	19% 19% 5% 9% 5% 3 3 2 4 4 25%	2520
	11th to 20th	Day	17% 18% 4 8% 4 4 3 3 3 5% 31%	2548
	21st to end	Day	21% 15% 3 8% 5% 4 4 3 5% 5% 26%	2730
May	1st to 10th	Day	28% 14% 4 9% 6% 5% 4 3 4 4 20%	6016
	11th to 20th	Day	19% 10% 4 9% 6% 5% 5% 4 5% 7% 26%	6402
	21st to end	Day	20% 9% 3 7% 5% 5% 6% 4 7% 7% 27%	6949
June	1st to 10th	Day	27% 11% 3 6% 4 4 5% 4 6% 6% 24%	2800
	11th to 20th	Day	19% 8% 3 6% 5% 4 5% 4 6% 7% 33%	2800
	21st to end	Day	22% 9% 3 9% 4 5% 5% 4 7% 7% 26%	2800
July	1st to 10th	Day	16% 7% 3 8% 5% 6% 7% 5% 9% 10% 24%	6360
	11th to 20th	Day	13% 7% 3 7% 5% 6% 6% 5% 7% 9% 32%	6360
	21st to end	Day	15% 7% 3 7% 4 4 4 4 6% 8% 37%	6926
August	1st to 10th	Day	18% 8% 3 9% 5% 6% 6% 4 7% 9% 25%	4475
	11th to 20th	Day	17% 9% 3 9% 5% 6% 6% 5% 7% 8% 25%	4175
	21st to end	Day	18% 8% 3 5% 4 4 5% 4 7% 8% 34%	4357
September	1st to 10th	Day	18% 7% 2 6% 4 4 4 4 5% 6% 40%	2400
	11th to 20th	Day	18% 6% 2 5% 4 4 4 3 5% 7% 42%	2225
	21st to end	Day	18% 10% 3 5% 5% 4 4 4 6% 8% 34%	2200
October	1st to 10th	Day	21% 13% 3 6% 4 4 4 4 6% 6% 29%	2200
	11th to 20th	Day	18% 16% 4 8% 5% 4 4 4 6% 6% 26%	1868
	21st to end	Day	22% 22% 5% 8% 4 4 4 2 3 4 23%	1980
November	1st to 10th	Day	17% 21% 4 10% 5% 3 4 3 3 5% 25%	1851
	11th to 20th	Day	16% 19% 7% 12% 6% 3 4 3 4 5% 21%	1890
	21st to end	Day	16% 19% 7% 13% 6% 4 5% 3 5% 5% 17%	1752
December	1st to 10th	Day	11% 21% 7% 22% 11% 5% 3 3 3 3 10%	2680
	11th to 20th	Day	14% 17% 8% 16% 6% 5% 4 3 4 4 19%	3066
	21st to end	Day	17% 18% 6% 14% 6% 4 6% 3 4 4 18%	4048
January	1st to 10th	Night	9% 16% 9% 21% 13% 9% 6% 2 3 3 9%	8234
	11th to 20th	Night	9% 17% 9% 21% 16% 7% 4 3 3 3 8%	8235
	21st to end	Night	7% 13% 6% 15% 10% 14% 7% 4 5% 4 14%	9057
February	1st to 10th	Night	11% 16% 6% 17% 10% 7% 5% 3 4 4 16%	2844
	11th to 20th	Night	10% 17% 8% 16% 11% 7% 5% 3 3 3 16%	2458
	21st to end	Night	12% 16% 5% 13% 9% 7% 4 3 3 4 23%	2076
March	1st to 10th	Night	17% 18% 6% 15% 6% 5% 6% 2 3 4 18%	5484
	11th to 20th	Night	13% 17% 7% 11% 6% 5% 4 3 5% 5% 23%	5336
	21st to end	Night	15% 17% 6% 14% 7% 5% 4 3 3 3 23%	5196
April	1st to 10th	Night	20% 16% 5% 9% 5% 4 4 3 5% 5% 24%	2100
	11th to 20th	Night	20% 16% 4 8% 5% 3 4 3 4 5% 29%	2020
	21st to end	Night	20% 15% 3 7% 5% 5% 4 3 5% 5% 29%	1890
May	1st to 10th	Night	27% 13% 3 8% 6% 4 3 3 4 5% 23%	4144
	11th to 20th	Night	21% 11% 3 7% 5% 4 5% 4 6% 6% 28%	3653
	21st to end	Night	20% 9% 3 8% 5% 5% 4 3 5% 7% 32%	3926
June	1st to 10th	Night	27% 8% 3 6% 4 4 4 4 4 6% 29%	1600
	11th to 20th	Night	19% 6% 2 7% 4 4 5% 4 5% 7% 35%	1600
	21st to end	Night	20% 8% 4 7% 4 6% 5% 4 6% 7% 30%	1600
July	1st to 10th	Night	15% 8% 3 8% 5% 6% 7% 5% 7% 8% 28%	3600
	11th to 20th	Night	17% 7% 3 7% 5% 5% 5% 4 6% 7% 35%	3600
	21st to end	Night	12% 8% 3 7% 5% 5% 5% 4 6% 8% 37%	3926
August	1st to 10th	Night	23% 10% 3 7% 4 4 4 3 5% 6% 31%	2642
	11th to 20th	Night	16% 8% 3 9% 5% 5% 6% 5% 7% 7% 29%	2872
	21st to end	Night	18% 9% 3 6% 4 4 4 4 5% 6% 38%	3538
September	1st to 10th	Night	16% 8% 2 7% 4 4 4 3 5% 6% 41%	2000
	11th to 20th	Night	17% 6% 2 6% 3 3 4 3 5% 6% 45%	2175
	21st to end	Night	20% 8% 2 4 4 3 3 4 6% 7% 40%	2200
October	1st to 10th	Night	20% 10% 3 7% 4 4 4 3 4 6% 35%	2200
	11th to 20th	Night	21% 14% 3 8% 5% 4 4 3 4 5% 27%	2529
	21st to end	Night	24% 17% 4 7% 4 3 4 3 4 6% 23%	2860
November	1st to 10th	Night	19% 20% 5% 9% 4 4 4 3 5% 5% 24%	2667
	11th to 20th	Night	16% 17% 8% 14% 5% 3 3 2 4 5% 24%	2730
	21st to end	Night	16% 18% 8% 15% 7% 4 3 2 4 4 18%	2866
December	1st to 10th	Night	13% 19% 10% 26% 10% 4 4 2 3 3 8%	4720
	11th to 20th	Night	16% 20% 7% 19% 8% 5% 4 2 3 3 13%	5348
	21st to end	Night	16% 17% 6% 14% 7% 5% 5% 3 4 5% 18%	7150

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	81% 4 7% 6% 2	4662
	11th to 20th	Day	80% 4 5% 7% 3 1	4680
	21st to end	Day	73% 4 7% 8% 8% 1	5124
February	1st to 10th	Day	73% 4 5% 11% 6% 1	1772
	11th to 20th	Day	74% 3 8% 9% 5% 1	2057
	21st to end	Day	68% 5% 7% 12% 6% 2	1730
March	1st to 10th	Day	70% 5% 7% 7% 8% 3	4579
	11th to 20th	Day	60% 4 12% 13% 9% 1	4842
	21st to end	Day	65% 4 6% 12% 13% 1	5961
April	1st to 10th	Day	63% 4 8% 15% 8% 2	2520
	11th to 20th	Day	56% 5% 8% 18% 11% 1	2548
	21st to end	Day	59% 5% 9% 16% 8% 3	2730
May	1st to 10th	Day	67% 6% 7% 15% 5%	6027
	11th to 20th	Day	55% 9% 14% 15% 6% 1	6406
	21st to end	Day	51% 10% 12% 19% 8%	6949
June	1st to 10th	Day	59% 7% 11% 17% 6% 1	2800
	11th to 20th	Day	49% 7% 12% 22% 10% 1	2800
	21st to end	Day	55% 8% 11% 19% 7%	2800
July	1st to 10th	Day	51% 8% 16% 19% 7%	6290
	11th to 20th	Day	44% 9% 14% 27% 6%	6285
	21st to end	Day	44% 6% 13% 30% 7%	6728
August	1st to 10th	Day	53% 8% 15% 18% 5%	4461
	11th to 20th	Day	54% 9% 15% 18% 5%	4214
	21st to end	Day	45% 7% 14% 19% 12% 2	4357
September	1st to 10th	Day	43% 7% 13% 26% 10%	2400
	11th to 20th	Day	42% 5% 11% 27% 14% 1	2225
	21st to end	Day	47% 8% 13% 21% 11% 1	2200
October	1st to 10th	Day	50% 6% 12% 22% 10%	3590
	11th to 20th	Day	55% 5% 12% 21% 7%	3019
	21st to end	Day	56% 3 13% 19% 8% 1	3180
November	1st to 10th	Day	62% 5% 10% 15% 7% 1	1851
	11th to 20th	Day	66% 6% 8% 10% 9% 2	1890
	21st to end	Day	70% 5% 11% 11% 3	1752
December	1st to 10th	Day	80% 4 7% 6% 3	2680
	11th to 20th	Day	69% 4 7% 12% 7%	3066
	21st to end	Day	70% 5% 5% 9% 8% 2	4048
January	1st to 10th	Night	82% 4 7% 6% 2	8234
	11th to 20th	Night	83% 4 5% 5% 3 1	8280
	21st to end	Night	73% 4 6% 8% 8% 1	9057
February	1st to 10th	Night	72% 4 7% 11% 5%	2844
	11th to 20th	Night	74% 4 6% 11% 5%	2460
	21st to end	Night	66% 5% 6% 14% 8% 1	2076
March	1st to 10th	Night	71% 4 7% 10% 6% 2	5494
	11th to 20th	Night	62% 5% 13% 12% 6% 1	5337
	21st to end	Night	66% 3 5% 13% 12% 1	5196
April	1st to 10th	Night	62% 5% 8% 15% 10% 1	2100
	11th to 20th	Night	57% 6% 8% 18% 11%	2020
	21st to end	Night	57% 5% 9% 16% 11% 2	1890
May	1st to 10th	Night	63% 7% 11% 13% 5% 1	4150
	11th to 20th	Night	54% 9% 13% 18% 5% 1	3654
	21st to end	Night	51% 7% 14% 18% 10%	3927
June	1st to 10th	Night	56% 7% 10% 20% 8% 1	1600
	11th to 20th	Night	47% 8% 13% 21% 9% 2	1600
	21st to end	Night	54% 8% 12% 18% 7% 1	1600
July	1st to 10th	Night	50% 9% 16% 19% 6%	3560
	11th to 20th	Night	46% 8% 13% 26% 6%	3552
	21st to end	Night	43% 8% 16% 26% 8%	3809
August	1st to 10th	Night	53% 7% 13% 22% 5%	2632
	11th to 20th	Night	51% 10% 16% 15% 7%	2899
	21st to end	Night	46% 7% 13% 21% 13%	3538
September	1st to 10th	Night	44% 7% 14% 27% 9%	2000
	11th to 20th	Night	40% 6% 14% 28% 11% 1	2175
	21st to end	Night	44% 6% 13% 24% 11% 2	2200
October	1st to 10th	Night	50% 6% 11% 23% 9%	3590
	11th to 20th	Night	56% 5% 10% 20% 9%	4117
	21st to end	Night	55% 5% 15% 17% 8% 1	4610
November	1st to 10th	Night	63% 5% 11% 15% 6% 1	2667
	11th to 20th	Night	64% 4 8% 14% 8% 2	2730
	21st to end	Night	71% 5% 9% 11% 4	2866
December	1st to 10th	Night	84% 4 5% 6% 1	4720
	11th to 20th	Night	77% 5% 4 8% 6%	5348
	21st to end	Night	71% 5% 6% 10% 8% 1	7150

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% 22% 37% 10% 2	220
	11th to 20th	Day	34% 25% 31% 8% 2	220
	21st to end	Day	31% 24% 30% 12% 3	242
February	1st to 10th	Day	37% 28% 23% 10% 2	284
	11th to 20th	Day	32% 28% 28% 10% 2	536
	21st to end	Day	30% 29% 28% 12% 1	446
March	1st to 10th	Day	33% 23% 30% 11% 3	540
	11th to 20th	Day	32% 28% 25% 11% 4	540
	21st to end	Day	37% 23% 23% 12% 5% 1	594
April	1st to 10th	Day	41% 20% 23% 12% 4 1	540
	11th to 20th	Day	37% 23% 24% 10% 5% 2	532
	21st to end	Day	35% 23% 26% 11% 4 1	540
May	1st to 10th	Day	43% 17% 24% 11% 4 1	543
	11th to 20th	Day	29% 19% 29% 16% 5% 1	551
	21st to end	Day	25% 22% 28% 18% 6% 1	573
June	1st to 10th	Day	32% 17% 30% 14% 6% 1	530
	11th to 20th	Day	24% 20% 27% 21% 7% 2	530
	21st to end	Day	26% 18% 31% 17% 6% 2	530
July	1st to 10th	Day	20% 17% 30% 24% 7% 1	525
	11th to 20th	Day	17% 13% 37% 24% 7% 2	524
	21st to end	Day	21% 13% 35% 23% 7% 1	576
August	1st to 10th	Day	20% 12% 30% 24% 12% 2	526
	11th to 20th	Day	18% 11% 29% 24% 14% 4	519
	21st to end	Day	24% 16% 23% 23% 11% 4	583
September	1st to 10th	Day	22% 16% 23% 22% 12% 5%	530
	11th to 20th	Day	21% 11% 21% 22% 17% 8%	530
	21st to end	Day	21% 17% 24% 22% 12% 5%	530
October	1st to 10th	Day	27% 19% 21% 20% 10% 3	530
	11th to 20th	Day	29% 20% 28% 13% 8% 2	456
	21st to end	Day	35% 23% 24% 15% 4	462
November	1st to 10th	Day	38% 20% 25% 14% 3	425
	11th to 20th	Day	29% 26% 29% 11% 4 1	430
	21st to end	Day	37% 22% 26% 12% 2	292
December	1st to 10th	Day	33% 22% 28% 12% 5%	220
	11th to 20th	Day	37% 28% 27% 5% 2	220
	21st to end	Day	36% 24% 29% 8% 3	242
January	1st to 10th	Night	27% 27% 32% 13% 2	840
	11th to 20th	Night	28% 27% 27% 15% 3	840
	21st to end	Night	21% 25% 31% 18% 3 1	924
February	1st to 10th	Night	24% 26% 30% 16% 3	840
	11th to 20th	Night	24% 27% 32% 13% 3	823
	21st to end	Night	25% 23% 34% 17% 1	692
March	1st to 10th	Night	28% 22% 30% 17% 2	840
	11th to 20th	Night	33% 20% 26% 16% 5%	840
	21st to end	Night	32% 19% 24% 17% 6% 1	924
April	1st to 10th	Night	35% 20% 25% 16% 3	840
	11th to 20th	Night	33% 20% 24% 18% 6%	776
	21st to end	Night	31% 18% 28% 18% 5%	630
May	1st to 10th	Night	31% 15% 30% 18% 5%	630
	11th to 20th	Night	23% 15% 30% 25% 7%	423
	21st to end	Night	25% 10% 25% 32% 7% 1	433
June	1st to 10th	Night	30% 8% 26% 28% 9% 1	400
	11th to 20th	Night	21% 10% 25% 32% 13%	400
	21st to end	Night	22% 9% 28% 35% 8%	400
July	1st to 10th	Night	17% 7% 27% 36% 12% 1	390
	11th to 20th	Night	18% 6% 24% 35% 16% 1	388
	21st to end	Night	16% 5% 24% 39% 15%	438
August	1st to 10th	Night	19% 6% 17% 39% 19% 1	428
	11th to 20th	Night	18% 7% 22% 33% 20% 1	592
	21st to end	Night	21% 7% 22% 32% 16% 2	845
September	1st to 10th	Night	21% 8% 23% 33% 14% 1	800
	11th to 20th	Night	20% 8% 20% 28% 23% 2	800
	21st to end	Night	24% 12% 23% 26% 14% 2	800
October	1st to 10th	Night	27% 12% 22% 25% 13% 1	800
	11th to 20th	Night	31% 16% 22% 22% 7% 1	800
	21st to end	Night	35% 18% 26% 18% 4	880
November	1st to 10th	Night	34% 17% 26% 17% 5%	820
	11th to 20th	Night	27% 24% 27% 15% 7% 1	840
	21st to end	Night	31% 24% 25% 15% 5% 1	840
December	1st to 10th	Night	33% 24% 26% 14% 3	840
	11th to 20th	Night	32% 24% 27% 13% 4	840
	21st to end	Night	35% 22% 26% 14% 2	924

Page generated at 2026-05-14 01:58:57. Version ac07

Copyright © 2026 Attilla Danko.