Meteorology I. The Atmosphere - the thin envelope of gas that surrounds the earth. A. Atmospheric Structure - the atmosphere is divided into five distinct layers that are based on their unique characteristics. 1. The Troposphere - this is the layer that lies closest to the earth s surface and is approximately 7 miles thick. All weather on earth takes place in the troposphere, which also contains over 90% of all of the atmospheres gases.
2. The atmosphere contains approximately 78% Nitrogen, 21% Oxygen, and trace amounts of other gases like carbon dioxide, argon, and, water vapor.
II. Weather - The present condition of the atmosphere at any location. It can also be described as a description of specific atmospheric weather variables during a short period of time. A. Temperature - temperature in the earth s atmosphere is dependent on the intensity and duration of the sun s insolation. 1. Adiabatic Temperature Change Temperature decreases with altitude in the troposphere as a result of the expansion of air.
B. Atmospheric Pressure - Atmospheric air pressure is the weight of a column of air pushing down on the surface of the earth at one given point. In general: cooler air = high pressure warm air = low pressure 1. Atmospheric or Barometric pressure is measured using a barometer, and is recorded as inches of mercury or millibars 2. Air Pressure and Air Temperature - In general terms, the higher the air temperature, the lower the air pressure.
C. Humidity - Humidity is the amount of water vapor present in the atmosphere. In general the cooler the air, the less amount of moisture it can hold; therefore, warmer air has higher humidity. 1. Relative Humidity - this is the most common form of measuring the amount of moisture in the air as a percentage. 2. Dew Point Temperature - The dew point is the temperature of the air at which it would be saturated at its current water vapor content. The dew point can be calculated by using a pyschrometer. 3. Humidity and Air pressure - in general, humid air masses have lower air pressure than drier air masses, this is due to the water molecules taking up more space than air molecules do in that particular air mass. Water molecules weigh less than oxygen or nitrogen molecules. 4. Sources of Atmospheric Moisture - Moisture enters the atmosphere by evaporation (liquid to water vapor), transpiration (evaporation of liquid water off a leaf surface), and sublimation (ice to water vapor).
D. Wind - The horizontal movement of air across the earth s surface is called wind. Air moves across the Earth s surface from areas of high pressure to areas of low pressure. 1. Convection Cells - The unequal distribution of solar energy striking the Earth s surface results in the formation of large convection cells, also called Hadley Cells. 2. Planetary Winds - large scale planetary winds form as an air moves from areas high pressure to areas of low pressure. Because of the Coriolis effect caused by the Earth's rotation, planetary winds are deflected to the right in the Northern Hemisphere.
a. Planetary winds are responsible for the movement of weather across the planet, and also drive many major ocean currents. 3. Mesoscale Winds - Air masses often have well defined pressure centers. Low pressure centers are called Lows, or Cyclones. Cyclones move in a counter-clockwise rotation in the Northern Hemisphere due to the Coriolis effect.
4. Air Masses - A large body of air in the troposphere with similar characteristics of temperature, pressure, and moisture.
3. Land and Sea Breeze
E. Formation of Clouds and Precipitation - Clouds and precipitation form when air rises and air is cooled below its dew point, and water vapor condenses into tiny water droplets or ice crystals. 1. Conditions that Cause Air to Rise: A. Air is warmed by sunlight heating the surface of the earth, expands and rises. B. Warm air is forced to rise upwards by topography (mountains). C. Dense, cold air masses wedge under warm air masses and force them to rise.
2. Cloud types - Low clouds: cumulus, medium height clouds: stratus, high clouds: cirrus. 3. Forms of Precipitation - The common forms of precipitation include: rain, drizzle, sleet (ice pellets), hail, freezing rain, and snow.
Orographic Precipitation
II. Air Masses and Fronts A. Fronts - Areas where two different air masses come together. 1. Cold Front - A cold front occurs when a cold air mass advances towards a warm air mass.
2. Warm Fronts - A warm front occurs when a warm air mass slowly overtakes a colder air mass. 3. Stationary Fronts - When a cold air mass lines up with a warm air mass, and there is no movement. 4. Occluded Front - Fast moving cold fronts can overtake a slow moving warm air mass, causing it to rise off the ground. VI. Storm Systems - Hazardous weather conditions can endanger life and property, and are an important aspect of understanding weather. A. Thunderstorms - thunderstorms form when warm air begins to rapidly rise up into the atmosphere creating cumulonimbus clouds.
B. Tornadoes - the most unpredictable form of severe weather occurs when extremely intense thunderstorms develop over a region and form extreme areas of low pressure within the cloud.
C. Hurricanes -warm, humid air masses forming over the ocean often create large, intense areas of low pressure at the equator.