Composition of the Atmosphere

Layers of the Atmosphere

Weather Vs. Climate Weather – the state of the atmosphere at a given time and place. Climate – long term prevalent weather conditions for a given region, or location. Analogy –weather is to a persons mood on a given day, as climate is to a persons overall personality.

Condensation/ Evaporation When water evaporates, energy is absorbed (a process that leaves the rest of the liquid a little bit cooler). It releases this energy in the form of heat when the water vapor turns to liquid (condenses) in cloud formation.

Energy Through The Cycle

Relative Humidity R.H. is the measure of the ratio of moisture in the air compared to the amount of water vapor than can be held in the air at that temperature. COOLER AIR HOLDS LESS WATER VAPOR – this means that the vapor turns to liquid form (condenses into a cloud) An increase in air temperature means a decrease in relative humidity.

The State of the Atmosphere To determine the condition of the atmosphere in a given location, the following factors can be measured: –Temperature – thermometer –Humidity- psychrometer or hygrometer –Wind speed and direction-anemometer –Air pressure- barometer

Air Pressure/Temperature/Density As you increase in altitude, pressure decreases and temperature decreases. Air that is cooler, holds less water vapor and will condense easier.

Adiabatic Temperature Change When the temperature of air changes (due to a change in pressure) it is referred to as an adiabatic temperature change This is what happens to air when it is lifted in the atmosphere.

How Do Clouds Form? (condensation) Clouds form when air is cooled to it’s dew point (point of saturation) Water vapor molecules attach to one another because they are slower moving in cooler air, and eventually attach to dust particles in the atmosphere. Once enough water is present – a cloud is formed.

Differences Between Clouds, Fog and Dew Clouds are groups of water droplets that form up in the atmosphere – above the surface. Dew, is condensation that occurs on objects on the ground. Fog is cloud formation that occurs near the surface. All of these are forms of condensation

Cloud Formation Summary 1.Air needs to be cooled to its dew point (point where it turns from gas to liquid) 2.Water vapor needs condensation nuclei to condense on. Ex. dust, salt in atmosphere, etc... 3.Air is most often cooled adiabatically (change in temp., by change in pressure) in the atmosphere. 4.Air is adiabatically cooled, most often by being lifted in these four ways: 1.Bouyancy 2.Orographic Lift 3.Frontal Wedging 4.Convergence

What Causes Air to Rise To Form Clouds? 1)Air rises due to it’s own buoyancy (heated air is less dense and will rise) this is known also as convection. 2)Air is forced up due to another air mass, this is called frontal wedging. 3)Air is forced up due to traveling over mountains; this is called orographic lifting 4)Air is forced up due to convergence near low pressure zones.

Orographic Lifting

What Causes Wind? Differences in air pressure cause wind. Winds blow from high to low. Air pressure is measured in millibars or inches of mercury depending on the barometer. Winds blow into and counterclockwise around a low pressure center. Winds blow out of and away from a high pressure center.

Factors That Affect Wind Pressure Gradient Force – how much pressure changes over a given distance. The closer the isobars the steeper the pressure gradient, the stronger the wind (this affects wind speed) Coriolis Force (Effect) – Winds are directed to the right in the Northern Hemisphere. (this affects wind direction) Friction – lower level winds are slowed by surface and more air. (this affects wind speed)

Global Wind Belts Air moves from higher pressure to lower pressure. Due to global pressure belts, air moves in a pattern, and because of coriolis effect, the air is deflected and this generates the global wind patterns.

Isobars Isobars are lines on a map that connect areas of equal air pressure. The closer the lines, the steeper the pressure gradient, the stronger the wind.

Pressure Centers Low Pressure Systems are called cyclones and bring inclement (bad) weather. Wind around a low moves in towards the center in a counterclockwise direction. High Pressure Systems are called anticyclones and bring fair weather. Wind around a high pressure center moves out and in a clockwise direction.

Upper Level Winds Winds higher in the atmosphere have less friction, and travel at a higher speed. The fastest stream of these winds is called the Jet Stream. The Jet Stream drives low pressure centers – dictating the movement of these storms. Weather forecasters try to use the jet stream, it’s position and predicted position to predict the weather.

Weather Station Models Station models are a shortened version of describing the weather conditions that exist at a given location at a given time.

Weather Station Examples Notice in the example the station at Detroit is showing the air temp. and the dew pt. being very close, and the cloud cover is 100%. This is not the case with locations to the west, like in Colorado, or Nebraska.

Weather Station Examples

Air Masses Air that sits over a section of the planet will take on characteristics of the surface that they sit over. Sections of air with similar humidity, temperature and air pressure are called air masses.

Fronts Fronts are the boundaries between different air masses. Fronts generally cause a change in the atmospheric conditions which often causes precipitation to occur.

Warm Fronts Warm fronts have gradual profiles, and will often be preceded by high wispy cirrus clouds, then a longer period of gentle rain or snow. A cold front has a steep profile, and will displace the warmer air upward in a short period of time.

Cold Fronts

Warm Fronts

The Life Line of a Midlatitude Cyclone The general theory of cyclonic development in the midlatitudes (South Florida to Alaska in N. America) was in place in the early part of the 20 th century. Since then new discoveries and technologies have added additional information. The basic idea of this theory (Polar Front Theory) is that when cold air from the north interacts with warmer air from the south, lower pressure develops. And, this occurrence is often along the polar front (polar jet stream)

Formation During this process of cyclogenesis, two differing air masses are moving along a boundary parallel to the front that separates them, but in opposite directions. The front will eventually start to take on a wave appearance.

Development of Cyclonic Flow As the wave develops, warm air moves toward the north, where the cold air once was, and vice versa. As the wave deepens, the pressure gradient increases, and a low pressure center develops. This pressure center allows inward, counterclockwise flow, and convergence

Development of Fronts As warm air from the south, continues to move north and east, it displaces cooler air that was to the north, and thus a warm front is born. As the cool air from the north, continues to move south and east, it overruns the warmer air that is moving away. This is the cold front.

Occlusion: The Beginning of the End As the cyclone develops, the cold front moves faster than the warm front and eventually catches up to it. During this period, an occluded front occurs, and the storm cyclone intensifies for a time, but once most of the warm air is forced upward, the temperature gradient begins to weaken and the air stabilizes. At this point the cyclone dissipates (dies out).

Severe and Unusual Weather Weather that results from the two distinctly different air masses meeting can become violent. Thunderstorms can develop along cold front boundaries. Thunderstorm that are severe, and that have the possibility of forming tornadoes, are called supercells.

Tornado Development

Seasonal Variations in Severe Weather During Spring, the Northern part of our continent is still very cold, but the southern portion of the continent begins to warm up. The differences between these two air masses often can cause severe weather, that would not likely occur at other times of the year.

Hurricanes Hurricanes form as the result of very warm ocean water warms the air above it. This creates a low pressure region which can be developed by the continued exposure to warm water. A hurricane will develop around a low pressure region an go through stages of being a tropical depression and storm based on wind speed.

Hurricane Formation Con’t Hurricanes generally form between 5 and 20 degrees latitude – where the ocean water temperature is warm enough.