Weather Unit - Science 8

What is Evaporation?

When a liquid (like water) changes into a gas (water vapor).

What kind of energy is water vapor stored as in the atmosphere?

Potential energy.

Why does evaporation COOL the surface of an object?

Because the highest-temperature (hottest) molecules are the ones that "pop off" and leave.

How does Temperature affect evaporation? (Factor #1)

The HIGHER the temperature, the GREATER the evaporation. (Direct/Positive Relationship).

How does Surface Area affect evaporation? (Factor #2)

The LARGER the surface area, the GREATER the evaporation. (Direct/Positive Relationship).

How does Humidity affect evaporation? (Factor #3)

The MORE humid the air, the LESS the evaporation. (Indirect/Negative Relationship).

How does Wind Speed affect evaporation? (Factor #4)

The STRONGER the winds, the GREATER the evaporation. (Direct/Positive Relationship).

Is water vapor a liquid or a gas?

It is a GAS only! It is NOT liquid, rain, or clouds.

What is the difference between Absolute and Relative Humidity?

Absolute is the actual amount of water vapor in the air. Relative is a percentage of how full the air is.

What weather instrument is used to measure Relative Humidity and Dew Point?

A Sling Psychrometer

Why can WARM air hold more water vapor than cold air?

Warm air expands, giving it a bigger capacity to hold moisture.

What happens to Relative Humidity as the Temperature goes UP?

Relative Humidity goes DOWN. (Indirect Relationship).

What is the Dew Point?

The temperature where air becomes 100% saturated (completely full of water vapor) and condensation begins.

Can Air Temperature ever be LESS than the Dew Point temperature?

No! Air temperature can NEVER be less than the Dew Point.

Which is heavier (more dense): Dry air or Humid air?

DRY air is heavier! Humid air is lighter/less dense because water vapor molecules (18 AMU) replace heavier nitrogen and oxygen molecules.

The Golden Rule of Weather: How does air physically move?

Air always FLOWS from HIGH pressure to LOW pressure! (This creates wind).

What weather instrument is used to measure Air Pressure?

A Barometer.

What are the 4 steps that happen to warm air as it rises over a Low Pressure center?

C.E.C.P. -> Cools, Expands, Condenses, Precipitates.

Contrast the weather in High (H) vs. Low (L) pressure systems.

High = Happy/Good weather (Clear, sunny). Low = Lousy/Bad weather (Clouds, rain).

What do close Isobar lines on a weather map mean?

Close lines = steeper pressure drop = STRONGER, faster winds.

High Pressure System (Anticyclone): What is the air and wind movement?

Sinking air that moves DOWN -> OUT -> CLOCKWISE.

Low Pressure System (Cyclone): What is the air and wind movement?

Rising air that moves COUNTERCLOCKWISE -> IN -> UP.

How do you use the Right Hand Rule for a LOW pressure system?

Point thumb UP (rising air). Your fingers curl inward and Counterclockwise.

How do you use the Right Hand Rule for a HIGH pressure system?

Point thumb DOWN (sinking air). Your fingers curl outward and Clockwise.

What are the 4 equivalent values for standard air pressure at sea level?

• 14.7 psi, 1013.2 mb, 29.92 in Hg, and 1 atmosphere (atm).

What are Isobars?

• Lines on a weather map that connect locations with equal air pressure.

A weather analyst notes that a large body of air is highly humid. Compared to a completely dry air mass at the exact same temperature, this humid air mass will be:

B (Humid air is lighter/less dense because 18 AMU water replaces heavier 28/32 AMU nitrogen and oxygen).

As a parcel of air sinks down toward the Earth's surface within a High-pressure system, what physically happens to it?

B. (Sinking air warms and compresses, preventing cloud formation).

If the barometric pressure lines (isobars) over New York are spaced very far apart on a weather map, what kind of wind conditions should residents expect?

B

Why does stepping out of a swimming pool on a breezy day make you feel cold, even if the air temperature is warm?

B

If a weather station reports that the local Air Temperature has cooled down and currently matches the Dew Point temperature exactly, what is the Relative Humidity?

C(When Air Temp = Dew Point, the air is fully saturated at 100% relative humidity).

State the correct order of the four sequential steps (CECP) that occur to air as it rises over a low-pressure center to create bad weather.

Cools ➔ Expands ➔ Condenses ➔ Precipitates.

What specific scientific instrument uses a wet-bulb and a dry-bulb thermometer to calculate relative humidity and dew point?

A Sling Psychrometer.

If you use your right hand to simulate a Low-pressure system (cyclone), which direction does your thumb point, and which direction do your fingers curl?

Thumb points UP (rising air); fingers curl inward and counterclockwise.

Describe the mathematical relationship shown on a graph comparing Air Temperature (X-axis) to Relative Humidity (Y-axis). Is it direct or indirect?

It is an Indirect/Negative relationship (as temperature goes up, relative humidity goes down)

A student writes down that "clouds are made entirely of invisible water vapor gas." Based on your notes, explain why this statement is scientifically incorrect.

Water vapor is an invisible gas. Clouds are actually made of liquid water droplets (or ice crystals) that have already undergone condensation.