Earth & Environmental Computing | Computational Methods for Earth & Environmental Science | 16-Week Course

Conditionals, Loops, and Environmental Events

An event is a threshold decision a scientist must defend. Today the code learns to decide.

if / elif / else · for loops · Counters · Thresholds

Week 03·Course session (mini-lecture, then Lab 2 Part B)·Open Course Materials | Adapt for your classroom·Week 3 | Course session | 20-minute mini-lecture, then Lab 2 Part B

Learning Objectives

By the end of this mini-lecture, you will be able to...

Write if/elif/else chains and predict which branch fires
Maintain counters across loop iterations
Explain why an environmental event is a threshold choice

Contents

Today's Mini-Lecture

01Branches: one decision

03Thresholds as science

02Loops that remember

Recap

Last Week in Sixty Seconds

Names carry units: temp_c, elevation_ft
Lists index by position; dictionaries label by name
f"{x:.1f}" prints a value at the precision you actually have
Today: those values become decisions about events

Data

Today's Datasets

File	Source	Coverage	Used for
iowa_city_weather_daily.csv	Iowa Environmental Mesonet, station IOW	Daily, 2019–2025, 2,549 rows	Wet, hot and cold day labels; dry spells
iowa_streamflow_daily.csv	USGS NWIS, site 05454500, Iowa River at Iowa City	Daily, 2015–2025, 4,018 rows	Counting high-flow days by month

Branches: One Decision

An if/elif/else chain runs exactly one branch, and the order decides which.

01 · Branches

One Decision, Three Branches

Python

rain_mm = 12.0

if rain_mm >= 10:
    label = "wet"
elif rain_mm > 0:
    label = "damp"
else:
    label = "dry"

print(label)     # wet

Exactly one branch runs
Change rain_mm, watch the branch change

01 · Branches

Branch Order Decides the Label

Python

# A day with 15 mm rain AND a 33 deg C maximum:
if row["precipitation_mm"] >= 10:
    label = "wet"            # <- evaluation stops here
elif row["temp_max_c"] >= 32:
    label = "hot"            # never reached for this day
else:
    label = "ordinary"

elif chains stop at the first true branch
Wet-before-hot is a decision; the lab asks you to defend it

Loops That Remember

A counter carries state from one iteration to the next.

02 · Loops that remember

Counters Remember Across Iterations

Python

longest = 0
current = 0
for precip in precip_values:
    if precip < 1.0:
        current = current + 1
        longest = max(longest, current)
    else:
        current = 0     # forget this line and see the next slide

current tracks the run in progress; longest keeps the record
The reset in the else branch is where the logic lives

Pause and think

Without the reset, the code reports a 900-day dry spell in a 7-year record of an Iowa town. What single check exposes the answer as absurd?

Plausibility: can a dry spell exceed the record itself, or an Iowa summer?

Wrong answers that look like answers are the dangerous kind

02 · Loops that remember

Two Implementations, One Answer

Loop version

for each row: if hot and humid: count = count + 1

Vectorized version

((temp_max >= 30) & (humidity >= 70)).sum()

The counts must match. Agreement between independent implementations is the lab's built-in test.

Thresholds Are Science

The number you pick is a scientific claim you have to defend.

03 · Thresholds are science

Thresholds Are Choices

Farmer

5 mm wets the topsoil: a useful rain.

Flood manager

25 mm in a day starts mattering downstream.

Climatologist

1 mm separates a wet day from a dry one in many indices.

Every event count in this lab depends on a threshold somebody chose.

03 · This week's lab

Lab 2, Part B: Event Detection

Today

Label every day: wet, hot, cold, ordinary
Count high-flow days by month
Find the longest dry spell

Studio session

Muggy-day agreement check
Season function and wet-day table
Freeze-thaw months, threshold defense

Part B continues the Lab 2 notebook you started in Week 2. Restart the kernel and run from the top.

Bridge

Connection to Next Week

This week	Next week	Bridge idea
Decisions inside loops	Functions and debugging	Package the decision so it can be named, reused, and tested

Summary

Key Takeaways

One branch fires; order decides which
Counters need a reset path
Check plausibility, not just execution
Defend your thresholds with a reason

Discussion · Next week: functions and debugging

Name a threshold from your own field and the reason behind the number. Who chose it, and what would change if it moved?

A threshold with no stated reason is a guess wearing a number

In the lab you write one sentence defending each of yours