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Atmosphere • The layer of gases that surround the Earth. It is a mixture of gases (such as oxygen, nitrogen, water vapor, and carbon dioxide), and microscopic particles (such as pollen, dirt, and dust). The atmosphere is able to protect us from the Sun’s harmful rays and helps to regulate temperature
Weather • Describes the conditions of the atmosphere at any given location for a short period of time. Includes factors such as temperature, air pressure, moisture, precipitation, wind, and cloud cover.
Some important information: • Weather is “driven” by the sun • The uneven heating of the Earth’s surface causes changes in the weather by insolation • Weather is described by various atmospheric variables. . .
Atmospheric Variables • Temperature – • A measure of the average kinetic energy of a substance – NOT a measure of how hot or cold a substance is. • Measured with a thermometer.
Atmospheric Variables • Air pressure – The weight of the overlying atmosphere pushing down on a given unit of area, affected by changes in temperature, water vapor, and altitude. Measured with a barometer.
Atmospheric Variables • Humidity – The amount of water vapor in the air.
Atmospheric Variables • Winds – Air that moves horizontally. They are caused by differences in air pressure. Measured with an anemometer.
The atmosphere is broken into several layers based on physical characteristics. • Important Facts: • Almost all water vapor and weather occurs in the Troposphere. • As altitude / elevation increases, air pressure decreases.
There are 3 methods of energy transfer that occur in the atmosphere: • Conduction – The transfer of heat through direct molecular contact. • Convection – The circulation of a heated fluid (liquid or gas), caused by density currents. • Radiation – The emission and transfer of heat via electromagnetic waves. This is the only way that energy can travel through the emptiness of outer space.
Air has weight? • Air pressure – • the weight of the overlying atmosphere pushing down on a given unit of area, affected by changes in temperature, water vapor, and altitude.
High Pressure generally brings cooler, calm weather. • Winds blow away from areas of high pressure and clockwise. • High Pressure = Happy Weather
Low Pressure generally brings warmer, wet weather. • Winds blow toward areas of Low pressure and counter-clockwise • Low Pressure = Lousy Weather
Some important facts: • Air pressure is measured by using a weather instrument called a barometer • Standard air pressure is 17.7 pounds per square inch = 29.92 inches of mercury = 1013.2 millibars = 1atm • Winds are caused by differences in air pressure
Factors which influence air pressure: • Temperature – • As the temperature of air increases, the air expands and its density and pressure decrease
Factors which influence air pressure: • Altitude – • As altitude increases, the air pressure decreases.
Factors which influence air pressure: • Humidity - • Water vapor is less dense then air. Thus, the more water vapor in the air, the lower the air pressure and density
What happens to water in the atmosphere? • Some important facts: • Phase change – The change in the physical state (solid, liquid, or gas) through the addition or removal of heat. • H2O exists in all 3 phases in the atmosphere. . .
Phase changes: • Melting – • when energy is added and a solid turns into a liquid
Phase changes: • Evaporation – • when energy is added and a liquid turns into a gas
Phase changes: • Freezing – • when energy is removed and a liquid turns into solid
Phase changes: • Condensation – • when energy is removed and a gas turns into a liquid
Phase changes: • Sublimation – • the process of a solid turning into a gas or a gas turning into a solid. Energy can be added or removed in this case. • Much more thermal energy (heat) is involved during evaporation, condensation, & sublimation than freezing and melting. . .
Some other stuff about H2O: • Specific heat – • The number of calories it takes to heat one gram of a substance by 1°C. • Calorie – • The amount of energy it takes to heat one gram of water by 1°C. • H2O requires more heat to change its temperature than most materials.
Some important facts: • Humidity – The amount of water vapor that is in the air. • Warmer air can hold more H2O vapor than cooler air. . . • Capacity – the maximum amount of water that the air can hold • Saturated – When the air is filled to capacity and holds as much water vapor as it can at a given temperature. • Air is not usually saturated. . .
To saturate a sample of air. . . either: • Add H2O vapor to the air. . . • Cool the air (“shrink it”) so condensation occurs. . .
Dewpoint Temperature- - The temperature at which the air is saturated with water vapor. At temperatures below the dew point, condensation or sublimation of water vapor occurs. • Dewpoint temperature is measured with a sling psychrometer. . .
Dewpoint Temperature • Wet bulb temperatures are never higher than dry-bulb temperatures • H2O vapor is always present in air . . . even in desert regions • As air temperature approaches the dewpoint precipitation becomes more likely. . .
There are different ways of examining humidity: • Absolute humidity – The amount (mass) of water vapor in a unit volume of air; usually measured in grams/cubic meter.
There are different ways of examining humidity: • Relative humidity – The ratio of the amount of water vapor in the air to maximum amount it can hold; often expressed as a percent.
How can you make a cloud? • Cloud - a visible mass of suspended liquid water droplets and/or ice crystals in the atmosphere
Recipe for a cloud: • Ingredients: Warm, moist air, condensation nuclei • Directions: • Cause warm moist air to rise vertically in the atmosphere. Heating, convection currents, orographic lifting, or a frontal boundary can achieve this. • As the warm, moist air rises, it expands and cools • As the moist air rises & cools to the dew point, it becomes saturated & condensation occurs • Cloud formation can only occur if the H2O vapor has something to “stick to”. Dust particles in the atmosphere act as condensation nuclei for the H2O vapor. • If these conditions are met, we have a cloud!
Types of clouds and the weather they are associated with: • Cumulus – clouds are the large clouds that sometimes look like huge puffs of cotton.
Types of clouds and the weather they are associated with: • Stratus – clouds are made up of low layers of clouds that usually cover the whole sky and blot out the sun.
Types of clouds and the weather they are associated with: • Cirrus – Clouds are very high, wispy clouds made of ice. Even in the summer, cirrus clouds are made of ice because it is cold high above Earth.
Types of clouds and the weather they are associated with: • Nimbostratus – Stratus clouds which have precipitation
Types of clouds and the weather they are associated with: • Cumulonimbus – Cumulus clouds that have hail and thunder falling from them
What falls from the sky? • Precipitation - Any form of water that falls from the sky.
Types of precipitation • Rain – Droplets of liquid water. • Drizzle – Very light rain. • Sleet – Small pellets of ice that form when raindrops freeze as they fall to the ground. The rain must fall through a layer of freezing air.
