Water in the Air Pgs. 36 - 43. Weather and Water Weather is the condition of the atmosphere at a particular time and place. The water cycle is the continuous.

Water in the Air

Pgs. 36 - 43

Weather and Water

Weather is the condition of the atmosphere at a particular time and place.

The water cycle is the continuous movement of water from water sources to the air, ground and back again.

Water Terms Condensation – occurs when water vapor

cools and changes back into liquid droplets. Evaporation – occurs when liquid water

changes into water vapor. Transpiration – the process by which plants

release water vapor into the air through their leaves.

Precipitation – occurs when rain, snow, sleet, or hail falls from the clouds onto the Earth’s surface.

Runoff – water that flows across land and collects in rivers, streams and oceans

Humidity

The amount of water vapor or moisture in the air is called humidity.

As water evaporates, the humidity in the air increases.

The air’s ability to hold water vapor depends on air temperature.

As temperature increases so does the air’s ability to hold water.

What does that mean?

Relative Humidity

The amount of moisture the air contains compared to the maximum amount it can hold is relative humidity.

This amount is given as a percentage. When air is holding 100% it is called

saturated. If air is not saturated you can still

determine its relative humidity.

Calculating Relative Humidity

Pretend we know the classroom can hold 50g of water vapor.

Also, we know that there is 36 grams of water vapor in the air in this classroom.

If we take the amount of water vapor present and divide by the water vapor possible and multiply by 100, we get relative humidity.

36g / 50g x 100= 72% relative humidity

Water Vapor vs. Temperature

If temperature stays the same, relative humidity changes as water vapor enters or leaves the air.

More water vapor at a temperature means a higher humidity.

If the water vapor in the air stays the same, relative humidity can change based upon the change in air temperature.

Warmer air can hold more water vapor than cooler air.

Measuring Relative Humidity

Psychrometer – an instrument used to measure relative humidity.

It is made of two thermometers. One is a wet bulb thermometer, and one is a

dry bulb thermometer. The wet bulb thermometer has a damp cloth

on it. When air passes over it the water

evaporates. If humidity is high the water won’t

evaporate. The amount that evaporates determines

relative humidity.

The difference in bulb readings will indicate the amount of water vapor in the air.

A large difference will mean there is little water vapor in the air.

The Process of Condensation

Before condensation can occur, air must be saturated, 100% relative humidity.

When air cools once saturated, condensation occurs.

Saturation can occur when water vapor is added to air or when air is cooled to its dew point.

Dew Point

Dew point – the temperature at which air must cool to be completely saturated.

Dew will form once air is saturated and has a surface to form on.

A glass of ice water is an example of air meeting its dew point.

Clouds

A collection of millions of tiny water droplets or ice crystals is called a cloud.

They form when warm rising air cools.

The air is saturated, and water vapor turns to liquid water.

Since condensation requires a surface, the dust and smoke in the atmosphere is the surface used to form clouds and rain.

Cumulus Clouds

Cumulus clouds look like big puffy cotton balls.

They tend to have flat bottoms and indicate fair weather.

Larger cumulus clouds can produce thunderstorms.

These are cumulonimbus clouds.

Stratus Clouds

Stratus clouds form in layers and cover large areas in the sky.

They are caused by gentle lifting air.

A nimbostratus cloud is one that produces light to heavy continuous rain.

A stratus cloud on the ground is called fog.

Cirrus Clouds

Cirrus clouds are thin feathery clouds found at high altitudes.

They form when wind is strong may indicate bad weather approaching.

Classifying Clouds

Clouds can be classified by both their type and their altitude.

High clouds: these are made at low temperature and made of ice crystals and are given the prefix cirro-

Middle Clouds: these are made of both ice crystals and water vapor and are given the prefix alto-

Low Clouds: these are made of water droplets and are given the prefix strato-

Precipitation

Precipitation is water, in solid or liquid form, that falls from the air to the Earth.

There are four major types of precipitation: rain, snow, sleet, and hail

Rain

Rain is the most common type of precipitation.

It is a liquid and raindrops fall when they are so large that they cannot be held up by the atmosphere.

Cloud droplets collide and join until they are large enough to be raindrops and fall to the Earth.

Snow and Sleet

Snow is the most common solid form of precipitation.

Snow forms when water vapor changes directly into a solid.

Sleet (freezing rain) forms when rain falls through a layer of freezing air.

The rain can fall as a solid or turn to a solid upon hitting a surface.

Hail

Hail is solid precipitation that falls as balls or lumps of ice.

Cumulonimbus clouds are typically the only producers of hail.

Sometimes large updrafts of air cause raindrops to go up to cooler temperatures and freeze.

They fall back down and then a larger updraft will force it back up.

The raindrop picks up moisture and particles and gets bigger and bigger until the updrafts can not support it.

Measuring precipitation

A rain gauge is an instrument used to measure rain fall.

It consists of a funnel and a cylinder for measuring.

Snow can be measured by both depth and water content.

A stick can show the depth and melting the snow will show water content.