The Atmosphere © Lisa Michalek

Structure of the Atmosphere The atmosphere is divided into four layers Troposphere The lowest layer is where most of the weather occurs Stratosphere Mesosphere thermosphere

Weather The short-term (a few hours or days) condition of the atmosphere at a given location Temperature, sky conditions, precipitation, atmospheric pressure, humidity, wind speed, and wind direction Meteorologists are scientists who study and predict the weather

Air Temperature In the daily cycle, temperature is usually lowest in the early morning and warmest at mid-afternoon In the season cycle, winters are generally cold, while summers tend to be hot Short term factors such as cloud cover and regional weather systems affect temperatures Clouds reduce daytime temperature by reflecting sunlight back into space At night, clouds help hold heat energy to Earth

Air Temperature Measured with a thermometer A bulb that contains liquid that expands into a narrow, calibrated neck when it is heated and moves down the neck when the temperature decreases When meteorologists record official air temperature, the thermometer is kept in a special weather shelter to protect the instruments from direct sunlight

Temperature Scales A temperature of zero on the Fahrenheit scale is the temperature of a mixture of equal parts ice, water, and salt The freezing point of water is what sets the zero point on the Celsius (centigrade) scale The point at which all particle motion stops is defined as zero on the Kelvin scale

Air Pressure Is caused by the weight of the atmosphere Above each square inch of Earth’s surface is a column of air the weighs 14.7 pounds

Measuring Air Pressure A barometer is an instrument used to measure air pressure using the dense liquid metal mercury Meteorologists measure air pressure in millibars Standard sea level pressure is 1013.2 millibars On a weather map, isobars connect places that have the same air pressure

Air Pressure Factors If air is cooled, it contracts and becomes denser This causes pressure to rise If air is heated, it expands and becomes less dense This causes pressure to fall Humid air is lighter than dry air This is because water molecules are lighter than the gasses they displace in the air

Moisture in the Atmosphere When the air is holding as much moisture as it can, the air is saturated The air’s ability to hold water vapor depends upon the temperature The warmer the air, the more moisture the air can hold The dew point is the temperature to which the air must be cooled to become saturated If the temperature falls below the dew point, condensation occurs as water vapor changes to liquid water

Measuring Moisture in the Atmosphere Meteorologists use a sling psychrometer and a dew-point temperature table to determine the dew point The psychrometer consists of two thermometers mounted side by side which can be swung through the air One thermometer measures the air temperature The bulb of the other thermometer is covered by a wet cloth As the thermometers are swung through the air, evaporated cooling causes the wet-bulb thermometer to register a lower temperature When you subtract the wet-bulb temperature from the dry-bulb temperature, you can use the dew-point table to determine the dew point

Psychrometer

Relative Humidity Compares how much moisture the air is actually holding with how much moisture it could hold if the air were saturated It is expressed as a percent of saturation Air is saturated if it is holding all the moisture it can hold at its present temperature Determined with a psychrometer and a relative humidity table

The Wind Wind is heat flow by convection within the atmosphere Winds are the result of uneven heating of the Earth’s surface This uneven heating causes differences in air pressure to develop

The Wind Winds always blow from areas of high pressure to areas of low pressure Winds blow fastest where the gradient in air pressure is greatest, where the isobars are close together

Measuring the Wind To measure the wind, you need to determine both the wind speed and the wind direction Wind speed is measured with an anemometer The cups catch the wind, causing it to spin Wind direction is indicated by a wind vane, which points into the wind

The Coriolis Effect The Earth’s rotation causes winds to curve to the right in the Northern Hemisphere to the left in the Southern Hemisphere

The Coriolis Effect Winds move in a clockwise outward spiral around high-pressure systems Winds move in a counterclockwise inward spiral around low-pressure systems

The Coriolis Effect

Zones of Convergence and Divergence Rising warm, moist air at the center of the low causes winds and air masses to blow into the low-pressure system The rising air cools, which causes cloud formation and precipitation The descending air turns a high-pressure system into a single mass of cool, dry air that spreads across the surface of Earth

Cloud Formation Clouds form when rising air is cooled below its dew point Tiny particles called condensation nuclei in the air allow a cloud to form

Precipitation Rain and snow are the most common forms of precipitation Drizzle is small raindrops that fall slowly Sleet is a partially frozen mixture of rain and snow that occurs when the temperature is just above freezing Hail is in the form of ice balls, which usually occurs in violent thunderstorms Hailstones begin as snowflakes that start to melt and gather more moisture as they fall

Precipitation