Relative Humidity

Humidity is the amount of water vapor in the atmosphere.

A reading of 100 percent relative humidity means that the air is totally saturated with water vapor and cannot hold any more, creating the possibility of rain.

Humidity is the amount of moisture in the air. Commonly described as 'relative' humidity which is expressed as a percentage.

A relative humidity of 100% means the moisture content of the air is the maximum possible

i.e. totally saturated with water vapor and can not hold any more, creating the possibility of rain.

Note:
The hotter the air, the more moisture it can hold.

 
The effect of Humidity on Humans

Humans are sensitive to humidity, as the skin relies on the air to get rid of moisture. The process of sweating is your body's attempt to keep cool and maintain its current temperature.

If the air is at 100-percent relative humidity, sweat will not evaporate into the air. And as a result, we feel much hotter than the actual temperature when the relative humidity is high. If the relative humidity is low, we will feel cooler than the actual temperature because our sweat evaporates easily, cooling us off.



Dew Point Temperature


The temperature to which the "humid" air must be cooled at a constant barometric pressure, for water vapor to condense and form fog or clouds. The dew point is a saturation temperature.


Dew point C
Dew point F
Human perception
Rel. humidity @ 32C (90F)
> 26 C
> 80 F
Severely high. Even deadly for asthma related illnesses
65% and higher
24 26 C
75 80 F
Extremely uncomfortable, fairly oppressive
62%
21 24 C
70 74 F
Very humid, quite uncomfortable
52% 60%
18 21 C
65 69 F
Somewhat uncomfortable for most people at upper edge
44% 52%
16 18 C
60 64 F
OK for most, but all perceive the humidity at upper edge
37% 46%
13 16 C
55 59 F
Comfortable
38% 41%
10 12 C
50 54 F
Very comfortable
31% 37%
< 10 C
< 49 F
A bit dry for some
30%



Air Pressure


Air pressure varies over time, and these temporal differences are usually caused by the temperature of the air. Cool air is denser (heavier) than warm air. Warm air is less dense (lighter) than cool air and will therefore rise above it. Areas of high pressure can be caused when cool air is sinking and pressing on the ground. At this time, the weather is usually dry and clear. In contrast, when warm air rises, it causes a region of low pressure. With low pressure, the weather is often wet and cloudy.

Therefore:

Low pressure = Rotation: counter clockwise, Warm air (with moisture) moving up, creating clouds / Conditions: Wet & Cloudy

High pressure = Rotation: clockwise, Cold air moving down / Conditions: Dry and clear


Note:
On a weather map, lines called isobars join up areas where the pressure is the same.
The closer together the isobars are, the more windy it will be.




Air Temperature


Air temperature (Also termed surface temperature in meteorology) is the ambient temperature indicated by a thermometer exposed to the air but sheltered from direct solar radiation typically by the use of a Stevenson Screen.


Location of Stevenson Screen / Thermometer

The screen / thermometer should be set at a height of approx 1.25m above ground level, typically over a grassed area and should be situated at least three times the height of any nearby object (such as a tree or a house) away from the object so that there are no obstructions to the passage of air round the screen.

The screen should ideally not be placed on or near a wall or a fence but on a stand of its own. Walls and fences are heated by direct solar radiation and they heat the air in contact with them. Therefore if the screen is sited on the fence, the air warmed by the fence will rise into the screen giving you a higher temperature.

A typical stevenson screen is shown below:




Wind Chill

Wind chill (often popularly called the wind chill factor) is the felt air temperature on exposed skin due to wind. It measures the effect of wind on air temperature. The wind chill temperature is usually lower than the air temperature, since the air temperature is usually lower than the human body temperature.



Wind Chill
calculator
Air Temperature (Celsius)
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1
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Wind Speed
(mph)
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1
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30
 
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55
 
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60
 
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-32
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-47
-54

 

Wind Chill
calculator
Air Temperature (Fahrenheit)
40
35
30
25
20
15
10
5
0
-5
-10
-15
-20
-25
-30
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-40 -45
                                   
Wind Speed
(mph)
5
  36 31 25 19 13 7 1 -5 -11 -16 -22 -28 -34 -40 -46 -52 -57 -63
10
  34 27 21 15 9 3 -4 -10 -16 -22 -28 -35 -41 -47 -53 -59 -66 -72
15
  32 25 19 13 6 0 -7 -13 -19 -26 -32 -39 -45 -51 -58 -64 -71 -77
20
  30 24 17 11 4 -2 -9 -15 -22 -29 -35 -42 -48 -55 -61 -68 -74 -81
25
  29 23 16 9 3 -4 -11 -17 -24 -31 -37 -44 -51 -58 -64 -71 -78 -84
30
  28 22 15 8 1 -5 -12 -19 -26 -33 -39 -46 -53 -60 -67 -73 -80 -87
35
  28 21 14 7 0 -7 -14 -21 -27 -34 -41 -48 -55 -62 -69 -76 -82 -89
40
  27 20 13 6 -1 -8 -15 -22 -29 -36 -43 -50 -57 -64 -71 -78 -84 -91
45
  26 19 12 5 -2 -9 -16 -23 -30 -37 -44 -51 -58 -65 -72 -79 -86 -93
50
  26 19 12 4 -3 -10 -17 -24 -31 -38 -45 -52 -60 -67 -74 -81 -88 -95
55
  25 18 11 4 -3 -11 -18 -25 -32 -39 -46 -54 -61 -68 -75 -82 -89 -97
60
  25 17 10 3 -4 -11 -19 -26 -33 -40 -48 -55 -62 -69 -76 -84 -91 -98
Effective
since
11.01.01
 
Frostbite Times
  30 minutes   10 minutes   5 minutes

 

Wind Chill Calculator
Wind Speed (MPH) =
Air Temperature ( F) =

F

 

Temperature Convertor
To convert, enter values in either box.
Degrees Fahrenheit
º F
converts to
Degrees Celsius
º C



Beaufort Scale (wind speeds)


Wind Force
Description
Speed
Symbol
Effect
mph
kph
0
Calm
<1 <1
Smoke rises vertically
1
Light Air
1-3 1.1-5.5
Direction shown by smoke drift but not by wind vanes
2
Light Breeze
4-7 5.6-11
Wind felt on face; leaves rustle; wind vane moved by wind
3
Gentle Breeze
8-12 12-19
Leaves and small twigs in constant motion; light flags extended
4
Moderate Breeze
13-17 20-28
Raises dust and loose paper; small branches moved.
5
Fresh Breeze
18-24 29-38
Small trees in leaf begin to sway; crested wavelets form on inland waters.
6
Strong Breeze
25-30 39-49
Large branches in motion; whistling heard in telegraph wires; umbrellas used with difficulty.
7
Near Gale
31-38 50-61
Whole trees in motion; inconvenience felt when walking against the wind.
8
Gale
39-46 62-74
Twigs break off trees; generally impedes progress.
9
Strong Gale
47-54 75-88
Slight structural damage (chimney pots and slates removed).
10
Storm
55-63 89-102
Seldom experienced inland; trees uprooted; considerable structural damage
11
Violent Storm
64-72 103-117
Very rarely experienced; accompanied by widespread damage.
12
Hurricane
73+ 118+
Devastation



Clouds and Rain


Clouds come in 3 basic shapes

Cirrus (wispy)
Thin, white feathery or wispy clouds, usually separated or detached. Highest of all clouds in tropopause,
they form at 30,000ft (9144m) or more in the sky. They are composed primarily of ice crystals.


Cumulus (heaped & puffy)
Flat-based, billowing clouds with vertical doming. Often the top of cumulus clouds have a "cauliflower-like"
appearance. Cumulus clouds are most prominent during the summer months.


Stratus (layered)
Thin, layered clouds, often occurring as continuous or rippled sheets which cover large portions of the sky.
Stratus clouds are frequently gray and thick.

 

 

Cloud altitude ranges

High clouds (between 5500m and 14000m)
comprise of ice crystals:

    Cirrus - white filaments
    Cirrocumulus - small rippled elements
    Cirrostratus - transparent sheet, often with halo

 

Medium clouds (between 2000m and 7000m)
comprise of water droplets or a mixture of ice crystals & water droplets:

    Altocumulus - generally white, layered, rippled elements
    Altostratus - grey thin layer
    Nimbostratus - thick layer, dark with possibility of rain or snow

 

Low clouds (below 2000m)
comprise of water droplets:

    Stratocumulus - layered series of rounded rolls, generally white
    Stratus - layered uniform grey
    Cumulus - 'white cauliflower' clouds with flat base
    Cumulonimbus - large dark towers, often with 'anvil' tops, associated with heavy rain & thunderstorms

 

Cloud Reference Images
 

 

What are clouds ?

Clouds are made of tiny water droplets, ice crystals or both. Storms and clouds are concentrated in the lower
seven miles or so of the atmosphere known as the troposphere. Clouds can serve as excellent indicators of
changing weather. In general, flat clouds indicate stable conditions while piled-up clouds mean unstable air.

 

 

How do clouds form ?

Clouds are formed by the gathering of condensing water vapor. Moist air near the Earth’s surface raises up
from the ground into the atmosphere by either the sun's heat or by a colder invading air mass (called a cold front),
that pushes the warm moist air upward. This lifting of the air drops both the air pressure and temperature.
Then when the air temperature reaches the air’s dew point, the water vapor air condenses, and forms into clouds.

The darker clouds we can see when a storm is brewing, are thicker clouds filled with lots of water molecule's,
which restrict the amount of sunlight that can pass through makings the clouds appear dark.

 

 

What is rain ?

To get rain, the condensing water vapor in the clouds need to become heavy enough to fall out of suspension.
In the begining the tiny droplets aren't heavy enough to fall, so just hang there suspended in the air.

To become heavier, the droplets need to grow into drops. This happens by them acquiring more water and become larger. Some will collide with other droplets and become larger, or will grow as water condenses out the air directly into the droplet.

After time the droplets will reach a size/mass when they won't be able to stay floating in the cloud because they are too heavy and therefore will start to fall. Some may get blown back up into the clouds until they are heavy enough to overcome the force of the wind. Then they will fall to earth as rain and will continue to keep falling as long as the conditions are right to make the clouds and let the water droplets grow heavy enough to fall.

 



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