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 2024-08-15.

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	18% 13% 4 10% 6% 5% 5% 3 5% 6% 25%	95506
	Night	17% 15% 5% 11% 6% 5% 4 3 4 5% 24%	93201
Transparency	Day	58% 7% 11% 16% 7% 1	95225
	Night	62% 6% 10% 14% 7% 1	93036
Seeing	Day	29% 19% 27% 16% 6% 2	14440
	Night	27% 18% 26% 21% 7% 1	23260

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	14% 17% 9% 18% 9% 7% 4 3 3 3 12%	2517
	11th to 20th	Day	9% 17% 7% 22% 13% 6% 3 2 3 4 13%	2520
	21st to end	Day	12% 18% 11% 16% 6% 6% 4 3 3 4 16%	2772
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	19% 7% 3 8% 5% 6% 6% 5% 7% 9% 27%	2599
	11th to 20th	Day	16% 8% 3 8% 5% 5% 6% 5% 7% 8% 30%	2359
	21st to end	Day	20% 8% 3 5% 4 4 5% 3 6% 8% 33%	2406
September	1st to 10th	Day	19% 8% 2 6% 5% 4 4 4 5% 7% 38%	2160
	11th to 20th	Day	20% 7% 2 5% 4 4 4 2 4 6% 41%	2003
	21st to end	Day	18% 10% 3 5% 5% 4 4 4 6% 8% 33%	1980
October	1st to 10th	Day	23% 13% 3 6% 4 4 4 4 6% 6% 26%	1980
	11th to 20th	Day	18% 15% 4 8% 5% 4 4 4 6% 6% 26%	1682
	21st to end	Day	23% 23% 5% 7% 4 4 4 2 3 4 21%	1782
November	1st to 10th	Day	17% 19% 4 9% 5% 3 4 3 3 5% 26%	1671
	11th to 20th	Day	17% 18% 6% 12% 6% 3 4 3 4 5% 21%	1710
	21st to end	Day	17% 17% 7% 12% 6% 4 5% 3 5% 5% 18%	1586
December	1st to 10th	Day	17% 24% 7% 15% 6% 4 3 2 3 3 14%	1520
	11th to 20th	Day	17% 20% 6% 14% 6% 5% 4 3 3 3 19%	1906
	21st to end	Day	21% 19% 7% 15% 6% 4 4 3 3 3 14%	2772
January	1st to 10th	Night	12% 18% 9% 21% 11% 6% 4 2 3 3 11%	4429
	11th to 20th	Night	11% 16% 8% 20% 14% 7% 5% 3 3 3 10%	4440
	21st to end	Night	12% 20% 9% 17% 8% 8% 4 3 3 3 14%	4884
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	18% 7% 3 4 4 4 5% 4 7% 7% 38%	1540
	11th to 20th	Night	16% 8% 3 8% 4 4 5% 4 6% 7% 36%	1641
	21st to end	Night	18% 6% 3 6% 4 3 4 4 5% 6% 40%	1948
September	1st to 10th	Night	16% 8% 2 7% 4 4 4 3 5% 6% 40%	1800
	11th to 20th	Night	18% 7% 2 6% 3 3 4 3 4 6% 44%	1957
	21st to end	Night	21% 8% 2 4 4 3 3 3 5% 7% 40%	1980
October	1st to 10th	Night	21% 10% 3 7% 4 5% 4 3 4 6% 33%	1980
	11th to 20th	Night	22% 14% 3 9% 5% 4 4 4 4 5% 26%	2275
	21st to end	Night	25% 17% 4 7% 4 3 4 3 4 5% 22%	2574
November	1st to 10th	Night	20% 19% 4 8% 4 3 4 3 5% 5% 24%	2407
	11th to 20th	Night	16% 17% 7% 13% 5% 3 3 3 4 5% 25%	2470
	21st to end	Night	16% 18% 8% 15% 8% 4 3 2 3 4 19%	2592
December	1st to 10th	Night	19% 23% 8% 14% 7% 4 4 2 3 3 12%	2660
	11th to 20th	Night	17% 22% 7% 17% 8% 5% 3 3 3 3 13%	3288
	21st to end	Night	21% 19% 7% 16% 7% 4 4 3 3 3 14%	4884

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	77% 5% 8% 8% 1	2518
	11th to 20th	Day	75% 6% 6% 10% 3 1	2520
	21st to end	Day	72% 5% 8% 9% 5% 1	2772
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	51% 9% 17% 18% 5%	2585
	11th to 20th	Day	49% 9% 17% 18% 7%	2359
	21st to end	Day	47% 8% 13% 21% 11%	2406
September	1st to 10th	Day	45% 7% 14% 23% 10%	2160
	11th to 20th	Day	44% 5% 11% 26% 13% 1	2003
	21st to end	Day	47% 8% 13% 21% 10% 1	1980
October	1st to 10th	Day	56% 6% 11% 17% 8%	1980
	11th to 20th	Day	57% 7% 15% 16% 5%	1682
	21st to end	Day	68% 4 9% 11% 7% 1	1782
November	1st to 10th	Day	61% 5% 10% 16% 8% 1	1671
	11th to 20th	Day	66% 6% 9% 10% 8% 2	1710
	21st to end	Day	68% 5% 12% 12% 3	1586
December	1st to 10th	Day	76% 4 7% 8% 4	1520
	11th to 20th	Day	70% 4 7% 12% 6% 1	1906
	21st to end	Day	75% 6% 5% 9% 4	2772
January	1st to 10th	Night	79% 5% 8% 6% 2	4429
	11th to 20th	Night	80% 5% 6% 6% 2	4440
	21st to end	Night	75% 5% 7% 8% 4 1	4884
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	42% 9% 18% 25% 5%	1530
	11th to 20th	Night	44% 10% 17% 20% 8%	1641
	21st to end	Night	43% 7% 15% 24% 10%	1948
September	1st to 10th	Night	44% 7% 14% 25% 9%	1800
	11th to 20th	Night	42% 6% 14% 27% 11% 1	1957
	21st to end	Night	43% 7% 13% 25% 10% 2	1980
October	1st to 10th	Night	52% 6% 11% 20% 11%	1980
	11th to 20th	Night	59% 7% 11% 17% 5%	2275
	21st to end	Night	64% 6% 11% 12% 7% 1	2574
November	1st to 10th	Night	62% 5% 11% 14% 7% 1	2407
	11th to 20th	Night	63% 5% 9% 14% 7% 2	2470
	21st to end	Night	70% 6% 9% 11% 4	2592
December	1st to 10th	Night	78% 5% 7% 7% 2	2660
	11th to 20th	Night	78% 4 5% 7% 6%	3288
	21st to end	Night	75% 5% 6% 9% 4	4884

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	29% 22% 36% 11% 2	180
	11th to 20th	Day	34% 24% 31% 8% 2 1	180
	21st to end	Day	27% 24% 32% 13% 4	198
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% 11% 30% 24% 14% 4	456
	21st to end	Day	25% 16% 22% 22% 11% 4	495
September	1st to 10th	Day	23% 15% 22% 23% 12% 5%	450
	11th to 20th	Day	24% 12% 22% 20% 15% 6%	450
	21st to end	Day	20% 17% 24% 22% 12% 5%	450
October	1st to 10th	Day	30% 18% 20% 19% 10% 2	450
	11th to 20th	Day	29% 20% 28% 13% 8% 2	390
	21st to end	Day	36% 22% 23% 14% 4	396
November	1st to 10th	Day	35% 19% 26% 16% 4	365
	11th to 20th	Day	30% 25% 28% 11% 5% 1	370
	21st to end	Day	34% 21% 28% 13% 3	246
December	1st to 10th	Day	35% 24% 25% 11% 5%	180
	11th to 20th	Day	36% 30% 26% 6% 3	180
	21st to end	Day	37% 26% 28% 7% 2	198
January	1st to 10th	Night	27% 26% 31% 13% 2	760
	11th to 20th	Night	27% 28% 26% 15% 3	760
	21st to end	Night	21% 25% 31% 18% 3 1	836
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	547
	21st to end	Night	21% 7% 21% 32% 16% 2	760
September	1st to 10th	Night	21% 8% 23% 33% 14% 1	720
	11th to 20th	Night	22% 9% 21% 28% 19% 2	720
	21st to end	Night	25% 12% 23% 25% 14% 2	720
October	1st to 10th	Night	29% 12% 23% 24% 12% 1	720
	11th to 20th	Night	32% 16% 22% 21% 8% 2	720
	21st to end	Night	36% 19% 26% 17% 3	792
November	1st to 10th	Night	32% 17% 27% 17% 6% 1	740
	11th to 20th	Night	27% 23% 27% 15% 7% 1	760
	21st to end	Night	31% 24% 24% 15% 5% 1	760
December	1st to 10th	Night	34% 25% 24% 13% 3	760
	11th to 20th	Night	32% 24% 26% 14% 4	760
	21st to end	Night	36% 22% 26% 14% 2	836

Page generated at 2024-08-12 09:52:50. Version ac07

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