Water in the Atmosphere

The Water Cycle

The Water Cycle The movement of water between the atmosphere and Earth’s surface is called the water cycle. Three steps of the water cycle: Evaporation Condensation Precipitation

How Does the Water Cycle Work? Density makes the water cycle work. Water is more dense than air in its liquid form so it collects together via gravity into bodies of water such as the oceans and lakes etc. As it is heated by the sun (driving energy source) and evaporates, it becomes a gas and that gas is less dense than air. As it rises to very high altitudes in the troposphere, it cools due to lower temperatures and becomes more dense than air again. Once enough water vapor has condensed into clouds and the clouds move away from the source of water, the water vapor will eventually condense enough that the cloud is more dense than the air. Precipitation will fall in the form of rain, sleet, snow or hail.

Density of Air in the Atmosphere

Phase Change Diagram Review

Evaporation Evaporation is the process by which water molecules in liquid water escape into the air as water vapor. Water molecules need to gain in heat energy to convert to a gas molecule. Water molecules in a liquid state are more dense than air. When water is heated by the sun, the liquid moves faster and becomes less dense causing it to rise into the atmosphere.

Examples of Evaporation Humidity-the measure of the amount of water vapor in the air Humidity depends upon temperature.

Examples of Evaporation Relative Humidity-the percentage of water vapor that is actually in the air compared to the maximum amount of water vapor the air can hold. Warm air is less dense and can “hold” more water than cold air that is more dense. Warm air has more space to fit water vapor Cold air has less space to fit water vapor

Measuring Relative Humidity Psychrometer-Instrument used to measure relative humidity Made of 2 thermometers Wet-bulb thermometer Dry-bulb thermometer

Measuring Relative Humidity Wet-bulb thermometer-bulb has a cloth covering that is moistened with water Psychrometer is spun by the handle and air blows over both thermometers Wet-bulb thermometer is cooled by evaporation Compare readings on the two thermometers

How to Determine Relative Humidity Measure the dry bulb temperature. Measure the wet bulb temperature. Calculate the difference between the dry bulb and wet bulb temperatures. Find the dry bulb temperature on the table. Find the difference between the wet and dry bulb temperatures on the table. The relative humidity will be at the intersection of the row and column.

Example Relative Humidity Table

Relative Humidity Table Examples Suppose the relative humidity of the air is 51% and the reading of the dry-bulb thermometer is 200C. What is the reading of the wet-bulb thermometer? Answer: 14 oC

Relative Humidity Table Examples The readings on a sling psychrometer are 14 0C for the dry-bulb thermometer and 12 0C for the wet-bulb thermometer. What is the relative humidity? Answer: 79%

Example of Evaporation Sweat is your body’s natural way of cooling down. When sweat reaches the surface of our skin, it then evaporates, which cools us down! Heat is transferred from your skin to the water which makes you feel cooler.

Example of Evaporation Outdoor drying: When you hang socks outside to dry, you're using energy from the Sun and the wind to evaporate and disperse the water they contain.

Condensation Condensation

Condensation As warm, moist air rises in the atmosphere, its temperature begins to decrease. Molecules of water vapor in the air become liquid water in the process of condensation.

How Clouds Form Clouds form when water vapor in the air condenses to form liquid water or ice crystals Two conditions required for condensation: Cooling of the air Presence of particles in the air

The Role of Cooling Cold air holds less water vapor than warm air Dew Point-the temperature at which condensation begins Dew point above freezing=water vapor forms water droplets Dew point below freezing=water vapor may change into ice crystals

Dew Point and Humidity The higher the dew point, the more moisture in the air Example: Key West, Florida has high dew points so it feels “sticky” or humid The lower the dew point, the less moisture in the air Example: Death Valley, California has low dew points so it feels dry

The Role of Particles For water vapor to condense, tiny particles must be present so the water has a surface on which to condense In cloud formation, most of these particles are salt crystals, dust from soil, and smoke Water vapor also condenses onto solid surfaces, such as blades of grass or window panes

The Role of Particles continued Liquid water that condenses from the air onto a cooler surface is called dew Ice that has been deposited on a surface that is below freezing is called frost

Fog Clouds are classified by their shape and their altitude Different types of clouds are associated with different types of weather Clouds that form at or near the ground are called fog Fog often forms when the ground cools at night after a warm, humid day The ground cools the air just above the ground to the air’s dew point The next day the heat of the morning sun “burns” the fog off as its water droplets evaporate

Cloud Seeding During drought conditions, a method called cloud seeding is used to produce precipitation. Tiny crystals of silver iodide and dry ice are sprinkled into clouds from airplanes. Water vapor can condense on the particles of silver iodide and dry ice cools the water vapor from the air. As a result, clouds form.

Precipitation Precipitation is any form of water that falls from clouds and reaches Earth’s surface Not all clouds produce precipitation For precipitation to occur, cloud droplets or ice crystals must grown dense enough to fall through the air

Types of Precipitation Common types of precipitation include: Rain Sleet Freezing rain Snow Hail

Rain Most common form of precipitation Drops of water 0.5 mm in diameter Drizzle-drops of water less than 0.5 mm Mist- drops of water smaller than drizzle

Sleet Raindrops fall through a layer of air that is below 0oC Ice particles smaller than 5 mm in diameter

Freezing Rain Raindrops falling through cold air near the ground do not freeze in the air, instead they freeze on a cold surface

Snow Water vapor in a cloud that is converted directly to ice crystals is called a snowflake

Hail Round pellets of ice larger than 5 mm in diameter Form during thunderstorms Difference between sleet and hail is the size of the ice pellets and where the ice pellets formed

Measuring Precipitation Rain gauges- open-ended can or tube that collects rainfall The amount of rainfall is measured by dipping a ruler into the water or by reading a marked scale Measuring sticks