Chapter 6: Meteorology

millibar (mb)

100 newtons per square meter

The NWS defines a severe thunderstorm as any storm that produce once or more of the following phenomena:

A tornado, Damaging winds or speeds of 58 mph (50knots) or greater, Hail 1 inch in diameter or larger

Precipitation Formation

Although we do not fully understand the details of precipitation formation, there are 2 processes that seem to give us some insight into the problem: The collision and coalescence process (in warm clouds)and The ice crystal (Bergeron-Findeisen) process (in cold clouds)

The collision and coalescence process (in warm clouds)

Cloud temperature is above freezing. Condensation nuclei to cloud droplet . . . must grow by factor of 100 cloud droplet to raindrop . . . must grow by factor of 100. Two importance forces to consider when determining the fall velocity of water droplets . . . Raindrop mass (volume). Larger raindrops more massive. Friction between drop and atmosphere; Larger drops fall faster; For reasons we don't completely understand, some drops grow more quickly than others; Large rain drop falls, collides with other droplets and absorbs them, when it gets too big it breaks into smaller drops that are still bigger than the typical ones around them

The ice crystal (Bergeron-Findeisen) process (in cold clouds)

Cold clouds where any part is below freezing; Glaciated region= temperature less than 40 degrees where water forms into ice crystals ; When liquid cloud droplets exist at temperatures below freezing, they are said to be supercooled; Water can exist in this supercooled state because of a lack of "freezing nuclei" in the atmosphere. A pure water droplet will not freeze until temperatures reach around -40 degrees C; The completely frozen region of the cloud (temperatures less than -40 degrees F) is called the glaciated region. (-40 degrees is where F and C meet); Ice crystals have a smaller evaporation rate than a cloud droplet. Thus, ice crystals will grow faster than cloud droplets in clouds that contain both. Ice crystals have a smaller evaporation rate as opposed to a liquid droplet. The ice crystal (Bergeron-Findeisen) process is the major method of precipitation formation in the middle and high latitudes.

We now know that most clouds form as air rises, expands and cools. There are 4 major mechanisms that cause upward motion in the atmosphere:

Convection, Topographic (orographic ascent), Widespread ascent due to convergence of air near the surface, Uplift along frontal boundaries

Wind's at the earth's surface

Enter friction ; Near the earth's surface you must also take into account frictional forces. Thus, you cannot have geostrophic winds. Instead, you get a balance between PGF, CF, and friction . . . surface winds; Surface winds . . . (bring geostrophic winds to the surface); Near the surface, friction reduces the wind speed; If the wind speed is reduced, the Coriolis force is weaker; If the CF is weaker, it can no longer balance the PGF and the winds will cross the isobars in the direction of the PGF (toward low pressure). ; The PGF will ultimately be balanced by the sum of the friction and Coriolis force. This gives our surface winds!; AT the surface winds cross the isobars toward low pressure and away from high pressure.;The angle at which the winds cross the isobars depends upon the roughness of the earth's surface. The rougher the surface, the slower the wind speed for a given PGF and the larger the crossing angle.

Friction

Friction acts opposite to the direction of motion; The "roughness" of the surface helps to determine the strength of the fictional force

We can express the relationship for finding the height of the convective cloud base in formula form

H (meters)= 125 m (T-TD) (T and TD in Celsius); H (feet)=222ft (T-TD) (T and TD in Fahrenheit)

Snow

Ice crystal process takes place, snow falls down, the temperature never goes above freezing so it falls to earth as snow

Freezing rain

Ice crystal process, snow falls out of clouds, eventually snowflakes fall in areas where the temperature is above freezing and they become water droplets, before they reach the earth they start to cool again right near the earth's surface that is below freezing. Sometimes water doesn't have the time to refreeze because of a thin layer of below freezing temperature atmosphere but everything on earth is below freezing so they freeze immediately ; most dangerous form of precipitation

Rain (too much or too little is a bad thing)

Ice crystal process, snow falls out of clouds, snowflakes fall to area where the temperature is above freezing so they become liquid droplets

The Coriolis Force

Is an apparent force that arises because of the rotation of the earth; A physical example: "Apparently" there is a force present that causes the object to move to the side. This "apparent force" is the Coriolis force and only exists in a rotating system

Atmospheric Motions (Winds)

Just like any other object or substance, the earth's atmosphere obeys Newton's Laws of Motion

Difference between sleet and freezing rain?

Layer of the atmosphere below freezing closer to the surface

Does the dew point temperature stay constant within a parcel of air as it rises?

NO. The rate at which the dew point temperature changes with elevation is a complicated function of the moisture content of the rising air parcel. However, the rate is generally assumed to be about 2 degrees C/1000m. Therefore, by finding the difference between the T and the TD (in Celsius) an multiplying the difference by 125m, we can find the approximate height of the convective cloud base; 8C/1000m 1C/125m (rate of dew point change=8)

Characteristics of the Coriolis force include:

The Coriolis force causes the wind to deflect to the right of the direction of motion in the northern hemisphere and to the left of the direction of motion in the southern hemisphere; The Coriolis force is a maximum at the poles and goes to zero at the equator; The Coriolis force is proportional to the wind speed. The greater the wind speed the greater the deflection caused by the Coriolis force. (wind=0; Coriolis=0)

Winds at high altitudes

The PGF and the Coriolis force working together (no friction); The wind that results from a balance between CF and the PGF is called the Geostrophic Wind. In the northern hemisphere, the geostrophic wind blows with lower pressure to the left and higher pressure to the right of the wind direction. ; At upper-levels of the atmosphere the winds blow parallel to the lines of constant height (constant pressure). Therefore, it is very easy to deduce the direction and the relative speed of the winds just by looking at an upper-level chart; Solid lines on upper-level maps are called height lines. These lines tell you how high you need to go in the atmosphere to find a particular pressure (850mb, 500mb, 250mb, etc) These lines are equivalent to isobars (lines of constant pressure). ; 500 mb heights tell you how high you need to go to reach the 500mb pressure level. They are the same as isobars at the 500mb level.; In an area where the upper-level winds blow primarily from the west towards the east, we say there is a zonal flow; When the upper-level winds blow in large, looping, meanders in northeast, southeast directions from the flow is called meridional.

coriolis force

The force that, owing to the rotation (spin) of the earth, deflects objects to the right (clockwise) in the Northern Hemisphere and to the left (counter-clockwise) in Southern Hemisphere. MODIFIES THE DIRECTION OF AIRFLOW

dew point depression

The larger the _____, the higher the height of the convective cloud base

lower

The smaller the Dew Point Depression, the ____ the height of the convective cloud base

A vertical pressure gradient?

Thus, there is a very large vertical pressure gradient (high pressure at the surface, low pressure at upper levels).; So why doesn't the atmosphere fly off into outer space? - Gravity - In most cases the VPGF is nearly balanced by the force of gravity; When the force of gravity and the VPGF are in exact balance, the air is said to be in hydrostatic equilibrium.; The majority of the atmosphere is in hydrostatic balance (equilibrium) at all times; The vertical motions that take place in most systems are very small compared to the horizontal motions in a system; On average, even large vertical motions are on the order of a few cm/s (they can be much larger in thunderstorms).

Sleet

Tiny balls of ice that fall in wintertime, sleet bounces; Ice crystal process, snow falls, becomes liquid droplets, instead of a thin layer below freezing, there is a thicker layer of below freezing temperatures so they refreeze and become sleet

Uplift along frontal boundaries

Uplift can occur at frontal boundaries. Cold air is dense and creates a wedge which projects warm air upwards.

Horizontal winds and vertical motion

We now know that air near the earth's surface crosses the isobars toward low pressure. Thus, the atmosphere converges near the center of the low.; We now know that air near the earth's surface crosses the isobars away from high pressure. Thus, the atmosphere diverges near the center of the high. (Downward motion due to adiabatic contraction - lack of clouds and precipitation)

Convergence

Widespread ascent due to convergence of air near the surface

Topographic (orographic) ascent

Windward side= wind is going towards the mountain; clouds only occur on the windward side; has precipitation and vegetation; Leeward side= wind moving away from the mountain; lack of moisture and desert conditions; rain shadow effect; The temperature of sinking air will always change by the dry adiabatic rate

newton

a unit of force used to describe atmospheric pressure

friction

acts to slow moving objects and, thus, affects wind speed

barometric pressure

air pressure is measures with a barometer and is referred to as ____

Jet Streams

an important concept in meteorology. We will find that jet streams are involved in several processes including: 1) The initiation and intensification of mid-latitude cyclones 2) The subsequent movement of mid-latitude cyclones (and the movement of air masses) 3) The initiation of severe weather (thunderstorms and tornadoes).

Newton's 1st Law

an object at rest will stay at rest and an obkect in constant motion (constant speed and direction) will remain in motion as long as no net force is applied to the object

Jet Streams

are "currents" of fast moving air thousands of kilometers long and hundreds of kilometers wide, but only a few kilometers tick. The speed of the winds in jet streams is often greater than 100 mph or even higher.

Jet Streams

are usually a result of horizontal temperature gradients. They tend to occur over surface frontal zones that divide cold air from warm air.

pressure tendency or barometric tendency

by noting the pressure trend-- rising, falling, or steady-- we have a good indication of forthcoming weather. Such a determination, called _____, is useful in short-range weather prediction.

anticyclonic flow

centers of high pressure, opposite that of Earth's rotation

Pressure gradient

change in pressure/change in distance; Therefore, when there are many isobars crowded together on a map there is a large pressure gradient. When isobars are far apart, there is a week pressure gradient; Pressure gradient force characteristics include the following: Winds will always blow from high toward low pressure and The strength of the PGF is directly proportional to the PG. Therefore, a large PG gives high wind speeds and a small PG gives low wind speeds.

Conditional instability

conditional instability occurs when the ELR is between the MALR and the DALR; MALR<ELR<DALR; We are saying that the environment is cooling slower than a dry air parcel, but faster than a saturated air parcel. ; MALR<ELR<DALR, stratiform clouds and/or clouds of vertical development, severe weather may form; Level of free convection when dew point and environmental temperature are the same so that the parcel is neither more or less dense than the environment and nothing happens to it (doesn't fall back down). When parcel passes this then the parcel can no longer fall. The average ELR across the globe is about 6.5degrees C/1000m. Since this value lies between the DALR and the average MALR, the atmosphere is normally in a state of conditional instability. In a conditionally unstable atmosphere, if you push the parcel up far enough ( to the level of free convection), you will encounter an unstable situation. Upward motion in the atmosphere is really what gives us interesting weather

cup anemometer

device used to measure wind speed with a speedometer much like the speedometer of a car

troughs

elongated areas of low pressure that extend eqautorward, and they are usually associated with cool, wet weather

ridges

elongated high-pressure areas that extend toward the poles and are associated with warm, dry air

atmospheric pressure or air pressire

force per unit area on a surface exerted by the weight of the air above

boundary layer

friction significantly influences airflow only in the first 1.5 km (1 mile) of Earth's atmosphere known as ____

We will be looking at the 3 forces that cause the movement of the atmosphere:

friction, winds at high altitudes, and winds at the earth's surface

cyclonic flow

has the same direction of rotation as Earth: counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere

anticyclones

high-pressure systems associated with dry conditions

buys ballot's law

in the Northwestern Hemisphere, if you stand with your back to the wind, low pressure will be found to your left and high pressure to your right

wind vane

instrument commonly used to determine wind direction

aerovane

instrument that represents a wind vane with a propeller at one end attached to a recorder that produces a continuous record of wind speed and direction determines locations where winds are steady and speeds are relatively high

The sub-tropical jet

is usually located between 25° N and 35° N

Polar Front Jet

is usually located somewhere between 35° N and 65° N, depending on the season (always the stronger of the two Jets); During summer, Polar Front Jet is north (65° N) - weaker; During winter, Polar Front Jet is south (35° N) - stronger

Area between the trough and the ridge

is where the interesting weather takes place - Ascending branch of the Jet Stream

isobars

lines that connects places of equal air pressure

cyclone or midlatitude cyclones

low-pressure systems that occur in the middle latitudes

station pressure

pressure reading

barograph

recording instrument that provides a continuous record of pressure changes over time

Convection:

refers to the vertical movement of atmosphere due to density differences between an air parcel and the surrounding atmospheric environment

aneroid baraometer

smaller, more portable instrument for measuring atmospheric pressure

Thermals:

surface heating up and air rises

Absolute instability

the atmosphere is in a condition of absolute instability when the ELR is greater than the DALR;ELR>DALR>MALR; WE are saying that the environment is cooling faster than a dry or saturated air parcel; ELR>DALR>MALR, clouds of vertical development, potentially heavy precipitation, severe weather likely; Absolute instability occurs only in thin layers of the atmosphere (usually close to the surface)

Newton's 2nd Law

the force exerted by an object is equal to mass multiplied by the acceleration of the object; F=ma

U.S. standard atmosphere

the idealized vertical distribtuion of atmospheric pressure at various altitudes

We will be looking at 3 forces that cause the movement of the atmosphere. These forces include:

the pressure gradient force, the Coriolis force, the frictional force

pressure gradient force (PGF)

the spacing of the isobars indicates the amount of pressure change occurring over a given distance

mercury barometer

used to describe atmospheric pressure

geostrophic wind

when a balance is reached between the Coriolis force (CF) and the pressure gradient force (PGF) winds flow in roughly a straight line and parallel to the isobars

prevailing wind

when the wind consistently blows more often from one direction than from any other, it is called ______.

gradient winds

winds of this nature, which blow at a constant speed parallel to curved isobars