MOISTURE, CLOUDS & PRECIPITATION

The Hydrologic Cycle The unending circulation of Earth’s water supply Involves: Evaporation, Condensation, Precipitation, Runoff, & Groundwater

I. WHAT ARE THE STATES OF MATTER FOR WATER IN THE ATMOSPHERE? Water (the only substance) exists in the atmosphere in all three states: solid: ice crystals, snow, sleet, hail liquid: rain, cloud droplets gas: water vapor

WHAT HAPPENS WHEN WATER CHANGES STATE? What is needed for a change of state to occur? Absorption or Release of Heat Energy Calorie – unit of heat energy. 1 calorie = amount of heat needed to raise 1 gram(g) of water 1 0 C

Summary The effect of change of state on the immediate environment. Cooling process Energy absorbed by water/Cools the immediate environment Melting Evaporation Sublimation Heating process Energy released by water/Warms the immediate environment Condensation Freezing Deposition

WHAT IS HUMIDITY? The amount of water vapor in the air. Water vapor makes air less dense. WHAT IS RELATIVE HUMIDITY? (RH) A measure of how saturated the air with water vapor. Its official definition: Relative humidity is the ratio of the mixing ratio (actual water vapor content) compared to the saturation mixing ratio (water vapor capacity) of the air at the current temperature. II HUMIDITY

WHAT IS SATURATION? The air holds all the water vapor it can. Depends on air temperature. Warmer the air, the more water vapor air can hold Graph shows that as temp. increases the amount of water vapor that air can hold also increases.

The Water Vapor holding capacity of air a various temperatures

HOW IS RELATIVE HUMIDITY MEASURED? It is expressed as a % of saturation. Ex: RH of 50% means air is half saturated If the RH reaches 100%? Then, Air is saturated Cannot hold any more water vapor Condensation can occur Does not mean it will rain/snow In general, RH % over 60% - air feels humid Relative humidity is measured with instruments called a hygrometers An example of a hygrometer is a sling psychrometer. (You will do a lab that allows you to use a sling psychrometer))

Temperature & Relative Humidity Olathe East HS Weather Station Last 3 Days

III. CONDENSATION WHAT THE CONDITIONS FOR CONDENSATION? 1. Air needs to reach saturation. It cools to its dew point temperature 2.Need a surface on which condensation can occur. No surface? Air can exceed 100% RH. Called supersaturated air. Usually occurs higher in the air where clouds form (or not)

WHAT IS DEW POINT (DP) TEMPERATURE? Temperature the air must cool to reach saturation. Dew point temperature will depend on how much water vapor is in the air. The more humidity that is in the air, the higher the dew point temperature. Therefore the opposite is true: If the dew point temperature is high, then there is more humidity in the air. If air cools to its dew point temperature : RH = 100% The air is saturated Condensation may occur

Current air temp. is the upper left value on the station model. Dewpoint temp. is the lower left value on the station model

WHAT ARE THE FORMS OF CONDENSATION? 1. Clouds -A large collection of minute water droplets suspended in the atmosphere.

2. Fog - A cloud with its base at/near the ground Air near ground cools to dew point.

3. Dew – condensed water droplets on solid surfaces. DP temp is above 32 0 F.

4. Frost – deposition of ice crystals on solid surfaces. DP temp. must be at freezing or below. Water vapor (gas) converts to solid (ice crystals) Not frozen dew!!!

HOW DO CLOUDS FORM? WHAT IS NEEDED FOR CLOUDS TO FORM? 1. Need rising air 2. Need condensation nuclei – microscopic particles in air upon which condensation can occur. Cloud droplets are 1 million times smaller than rain drops

WHAT HAPPENS TO AIR WHEN IT RISES? It expands and cools – Rising Air cools at an average rate of WHAT HAPPENS TO AIR WHEN IT SINKS? It compresses, temp. increases Change in temp. with addition/removal of heat is called adiabatic temperature change.

50 0 F 45 0 F 40 0 F WHEN DOES THE CLOUD FORM? Rising air reaches its Dew Pt. temp Condensation/deposition will occur if enough condensation nuclei are present

WHAT ARE THE PROCESSES (MECHANISMS) THAT LIFT AIR? 1. Orographic Lifting – air flow over areas of higher elevation.

2. Frontal Wedging – warm less dense air lifting over cooler more dense air Such as along cold fronts and warm fronts. Cause of most of precipitation in Midwest.

3. Convergence – collision of similar air masses. Only place to go is up. Ex: Daily Afternoon T-storms in Florida

4. Local Convective Lifting - uneven heating of surface. Ex. Contrasting surfaces Dark and Light. Clouds that develop over lakes and reservoirs

Cloud Types Classification Based on: 1) shape (cirro-, cumulo-, strato-) 2) altitude (High, Middle, Low)

High Clouds cirrus cirrostratus cirrocumulus

Cirrus Clouds

Cirrostratus

Cirrocumulus

Middle Clouds altostratus altocumulus

AltocumulusAltostratus

Low Clouds Stratus Stratocumulus Cumulus nimbostratus

Stratus

Stratocumulus

Cumulus Clouds

Nimbostratus

Multilevel Cloud cumulonimbus

Cumulonimbus

VIII. PRECIPITATION WHAT IS PRECIPITATION? Any form of water that falls from a cloud WHAT ARE THE FORMS OF PRECIPITATION? Rain, snow, sleet, hail, glaze (freezing rain)

HOW DOES PRECIPITATION FORM? 2 Theories Exist 1. Cold Clouds (Temp. below freezing) Begin as tiny ice crystals. Grow by deposition process water vapor to ice crystal

2. Warm Clouds (Temp. above freezing) Cloud droplets grow in size by colliding with other droplets Fall when they get to heavy.

SO HOW DO WE GET THE DIFFERENT FORMS?

Snow Rain

Sleet Freezing Rain

Formation of Sleet

Formation of Hail Hail is produced only in cumulonimbus clouds. They begin as small pellets high in the cloud. They collect supercooled water as they fall through the cloud. Strong updraft, carry them back up causing the supercooled water to freeze. Repeated up and down trips create layers similar to an onion. The stronger the updraft, the larger the hailstone may become. Cut one in half and you can determine how many trips it made in the cloud before falling to the ground.

Hailstone Cross section Hail Damage