Diurnal energy budgets

Welcome to the World of Weather!

In this chapter, we are going to explore the Diurnal Energy Budget. Don't let the name scare you! "Diurnal" is just a fancy word for "daily" (a 24-hour cycle), and an "Energy Budget" is exactly like a bank account, but for heat instead of money. We will look at how the Earth "earns" energy from the sun during the day and "spends" it at night.

Understanding this is important because it explains why some days are hotter than others, why we get dew on the grass in the morning, and how the Earth stays at a temperature that allows us to live here.

Quick Review: The "Daytime" budget is mostly about incoming energy, and the "Nighttime" budget is mostly about outgoing energy.

1. Daytime: The Energy "Income"

During the day, the Earth receives energy from the Sun. Here is how that "income" is handled:

Incoming Shortwave Solar Radiation

This is the energy coming directly from the sun. It travels in short waves because the sun is incredibly hot. Think of this as your "salary" entering your bank account at the start of the day.

Reflected Solar Radiation and Albedo

Not all the sunlight that hits Earth stays here. Some of it hits the ground or clouds and bounces straight back into space. This is called Reflected Solar Radiation.

The term Albedo refers to the "reflectivity" of a surface. It is usually expressed as a decimal or percentage:

• High Albedo: Light-colored surfaces like snow or white clouds reflect a lot of energy (they have a high albedo).
• Low Albedo: Dark-colored surfaces like dark soil or asphalt (roads) reflect very little and absorb most of the heat (they have a low albedo).

Analogy: Imagine wearing a white t-shirt on a sunny day versus a black t-shirt. You feel cooler in the white one because it reflects the sun (high albedo), while the black one absorbs it (low albedo).

Energy Absorbed into the Surface and Subsurface

The energy that isn't reflected gets absorbed. It warms up the very top layer of the ground (surface) and then travels down into the soil or rocks (subsurface) through a process called conduction.

Key Takeaway: During the day, the Earth's "bank account" grows as it absorbs shortwave radiation, minus whatever is bounced away by high-albedo surfaces.

2. Moving the Heat Around

Once the ground gets warm, it needs to move that heat. It does this in two main ways:

Sensible Heat Transfer

This is heat you can actually "sense" or feel with your skin. When the warm ground touches the air just above it, it warms the air up. This air then rises (convection), carrying the heat away.
Example: The "shimmering" look of air above a hot road in the summer is sensible heat transfer in action.

Latent Heat Transfer (Evaporation)

This is a bit more "hidden." Latent heat is the energy used to turn liquid water (like from a lake or wet soil) into water vapor (gas).
The energy is "stored" in the vapor. It doesn't make the temperature rise immediately; instead, it uses the energy to change the state of the water. This cools the surface down.

Don't worry if this seems tricky! Just remember: Evaporation = Cooling the ground by "hiding" heat in water vapor.

Did you know? This is exactly why humans sweat! When the sweat evaporates off your skin, it uses your body heat to turn into gas, which makes you feel cooler.

3. Nighttime: The Energy "Expenses"

At night, the sun goes down. The "income" stops, but the Earth continues to "spend" or lose heat. Here is what happens:

Longwave Radiation

While the sun sends shortwave radiation, the Earth (which is much cooler than the sun) sends out longwave radiation into the atmosphere. This is how the Earth cools down at night. If the sky is clear, this heat escapes quickly into space, making the night very cold.

Absorbed Energy Returned to Earth (Counter-radiation)

Clouds are like a blanket. They catch that outgoing longwave radiation and send it back down to the ground. This is called absorbed energy returned to earth or "counter-radiation."
Common Mistake: Students often think cloudy nights are colder. Actually, cloudy nights are warmer because the clouds trap the heat like a greenhouse!

Dew Formation

As the ground cools down at night, the air right next to it also cools. If it cools enough, the water vapor in the air turns back into liquid water droplets. This is Dew. This process actually releases a tiny bit of heat back to the surface!

Key Takeaway: At night, the Earth loses energy through longwave radiation. If there are clouds, some of that energy is returned, keeping us warmer.

4. The Big Picture: The Formula

Geographers use a simple equation to show the Net Radiation (\( Q^* \)), which is the final balance of energy.
If \( Q^* \) is positive, the surface is warming up (daytime). If \( Q^* \) is negative, the surface is cooling down (nighttime).

The balance looks like this:

\( Net Radiation = (Incoming - Reflected) + (Downwards Longwave - Upwards Longwave) \)

Memory Aid (The "Incoming/Outgoing" Trick):

• Sun = Shortwave = Small/Short (Fast energy coming in).
• Earth = Longwave = Lazy/Long (Slow energy going out).

5. Quick Summary Table

Use this table to review the main factors affecting the daily budget:

Factor	What it does	Day or Night?
Insolation	Sunlight warms the Earth.	Day
Albedo	Reflects sunlight away (High albedo = more reflection).	Day
Sensible Heat	Heat you can feel moving in the air.	Both (mostly day)
Latent Heat	Heat "used up" by evaporation.	Day
Longwave Radiation	The Earth's heat escaping into space.	Both (mostly night)
Dew	Water vapor turning to liquid, releasing heat.	Night

Final Check: Can you explain...

1. Why a snow-covered field stays cold even on a sunny day? (Hint: Albedo)

2. Why it is usually colder on a clear night than a cloudy night? (Hint: Counter-radiation)

3. What happens to heat during evaporation? (Hint: Latent heat transfer)

If you can answer these, you've mastered the Diurnal Energy Budget! Great job!