Meteorology Module 2 - Lecture 1

Temperatures are recorded daily
at thousands of weather stations
worldwide
 Provides meteorologists and
climatologists much of the
temperature data
• Hourly temperatures may be
recorded by an observer or
obtained from automated
observation systems
 Continually monitor the
atmosphere

Daily Calculations
• A location’s daily mean temperature is the averaging all
temperature values within a 24-hour period (midnight to
midnight)
• A location’s daily maximum temperature, also referred to as
the high temperature, is the maximum temperature in a 24-hour
period
• A location’s daily minimum temperature, also referred to as
the low temperature, is the minimum temperature in a 24-hour
period
• A location’s daily temperature range is the difference between
the maximum and minimum temperatures in a 24-hour period

Monthly and Yearly Calculations
• A location’s monthly mean temperature is calculated by
adding together the daily means for each day of the month and
dividing by the number of days in the month
• A location’s yearly mean temperature is an average of the 12
monthly means
• A location’s yearly temperature range is computed by finding
the difference between the warmest and coldest monthly mean
temperatures

Monthly and Yearly Calculations
• Mean temperatures are
especially useful for making
daily, monthly, and yearly
comparisons
 “Last month was the warmest
February on record
 “Today, Denver was 10 degrees
warmer than Chicago”
• Temperature ranges are also
useful statistics because they
indicate extremes
 A necessary part of
understanding the weather and
climate of a place or an area
Death Valley, CA

Isotherms
• To display the distribution of air temperatures over large areas,
isotherms are commonly used
• An isotherm is a line that connects points on a map that have
the same temperature (iso = equal, therm = temperature)
 Therefore, all points through which the isotherm passes have identical
temperatures for the time period indicated
• Generally, isotherms representing temperature differences of 5°
to 10° are used, but any interval may be chosen
45°F
55°F 70°F
60°F
80°F
65°F
50°F 70°F
60°F
65°F
75°F
75°F
45°F
50°F
55°F 60°F 65°F 70°F 75°F 80°F

Isothermal Maps
• Here is a map of how isotherms in intervals of 10°F are drawn
on a map of the United States
• Notice that most isotherms (contours) do not pass directly through the
observing stations (points)

Isothermal Maps
• The same map can also be color-filled for a more pleasurable
viewing experience
 Isotherms are on the border of each color
 Each color represents a temperature range (for example, light blue
represents temperatures between 30°F and 40°F)

Isothermal Maps
• Isothermal maps for valuable tools
 Make the temperature distribution across the nation clearly visible
 Make areas of low and high temperatures easy to identify
• Additionally, the amount of temperature change per unit of
distance, called the temperature gradient, can be easily
visualized
 Closely spaced isotherms indicate a rapid rate of temperature change
 Widely spaced isotherms indicate a more gradual rate of temperature
change
• Without isothermal maps, a map would be covered with
numbers representing temperatures at tens or hundreds of
locations
 Would make patterns difficult to see

Example of a Temperature Gradient
Steeper (stronger)
temperature gradient
Gentler (weaker)
temperature gradient

Cycles of Air Temperature

Daily Temperature Cycle
• We know from experience that a rhythmic rise and fall of air
temperature occurs every day
• On a normal day (midnight to midnight), temperature
 Decreases from the overnight hours to dawn
 Increases as the sun rises
 Decreases as the sun sets and into darkness
• However, not every day is normal since weather happens,
which can have a huge influence on temperature and disrupt
the daily temperature cycle
• A way to describe this information is through a meteogram,
which is a graph that shows how meteorological variables
change over time

Daily Temperature Cycle
• An example of the daily temperature cycle is shown in a
Chicago meteogram
 Temperature reaches a minimum around sunrise
 Climbs steadily to a maximum in the mid-late afternoon
 Temperature then declines until sunrise the following day
 However, factors such as weather can disrupt this cycle (more later)

Daily Temperature Cycle
• The primary control of the daily temperature cycle of air
temperature is Earth’s daily rotation
 Causes a location to move into daylight for part of each day and then
into darkness
• As the Sun angle increases throughout the morning, the surface
heat ups
 Sun angle reaches a peak at noon and gradually diminishes in the
afternoon
• During the night, the atmosphere and the surface of Earth cool
as they radiate away heat
 The low temperature occurs about the time of sunrise when the Sun
again begins to heat the ground

Daily Temperature Cycle
• The Earth gains more
energy than loses after
dawn and through the
afternoon hours
 Heats the surface 
Temperature increases
• The Earth loses more
energy than gains from
the evening and through
the the pre-dawn hours
 Cools the surface 
Temperature decreases

Daily Temperature Cycle
• It is apparent that there is
a lag in when the high
temperature for the day
occurs
 Maximum solar heating
occurs at noon, but the
high temperature occurs at
4 pm
• As long as the energy
gained exceeds what is
lost, the temperature
continues to increase
 Once the amount lost
exceeds what is gained,
the temperature begins to
decrease

Annual Temperature Cycle
• We’re familiar with the tropics experiencing warmth year-round,
while the midlatitudes experience warm/hot summers and cool
winters
• As you increase in latitude, the seasonal temperature cycle
becomes more pronounced
https://www.kayak.com/Albuquerque.17767.guide https://www.celebritycruises.com/blog/downtown-fairbanks
Fairbanks, AK (latitude: 64.8401°N)
Annual temperature range: 72°F
Albuquerque, NM (latitude: 35.0844° N)
Annual temperature range: 40°F

Annual Temperature Cycle
• Similar to the daily cycle, the highest and lowest temperatures
of the year do not coincide with the periods of maximum and
minimum incoming solar radiation
 Lags by an average of 27 days in the United States
• North of the tropics in the Northern Hemisphere
 The greatest intensity of solar radiation occurs in June, yet July and
August are the warmest months of the year
 Conversely, the least amount of solar energy occurs in December, yet
January and February are usually colder
• Over the course of the year, it takes time for the land or water
surface to respond to the increase in heating from the Sun
 Water takes way longer to respond than land

Annual Temperature Cycle
• An example of the seasonal lag in peak temperatures
 St. Louis (surrounded by land) experiences its warmest temperatures
earlier than San Francisco (surrounded by water)
https://www.britannica.com/place/San-Francisco-Californiahttps://www.britannica.com/place/Saint-Louis-Missouri
St. Louis, MO
Peak temperature: July
San Francisco, CA
Peak temperature: September

Why Temperatures Vary

Temperature Controls
• A temperature control is any factor that causes air temperatures
to vary
• We have learned how the primary temperature control is latitude
due to how it determines the annual variations in Sun angle and
the length of daylight
• However, there are many more factors that we need to discuss,
including:
1. Latitude
2. Elevation
3. Land and water
4. Ocean currents
5. Geographic positioning and prevailing wind direction
6. Albedo variations
7. Water vapor and atmospheric circulations
The most important

1. Latitude
• We have learned how latitude is the
primary control of temperature
 Determines the annual variations in Sun
angle and the length of daylight
• The image shows the annual
temperature cycle for cities with
different latitudes and reminds us of its
importance
• But it is not the only control of
temperature
 If it were, cities along the same latitude
would have identical temperatures which
is not the case

2. Elevation
• Elevation is the secondary
control of temperature
• Recall that atmospheric pressure
decreases with altitude in the
troposphere
• As a result, temperature
decreases with height since there
is less air
• Elevation significantly affects the
daily temperature range as well
 More rapid daytime heating higher
up due to less shortwave radiation
being absorbed, reflected, and
scattered

3. Land and Water
• Recall that the heating of Earth’s
surface controls, to a large degree,
the heating of the air above it
• Earth has many different surfaces
 Soil, water, forests, ice, and so on
• These surfaces have different
heating properties, which cause
variations in the temperature of the
air above it
• The greatest contrast is between
land and water, demonstrated by the
image
• Water heats much more slowly than
land does due to a higher heat
capacity

3. Land and Water
• Comparing the monthly temperature
data for two cities, one near a body of
water (Vancouver) and another that is
land-locked (Winnipeg), shows
 Vancouver has a mean January
temperature that is 20°F warmer than
Winnipeg
 Vancouver has a mean July temperature
that is 4.7°F warmer cooler than
Winnipeg
• Important to note that both cities are
at about the same latitude
 Experience similar Sun angles and
lengths of daylight

3. Land and Water
• On a different scale, the moderating influence of water can be
demonstrated by looking at our two hemispheres
• The Southern Hemisphere experiences less temperature
variation than the Northern Hemisphere due to there being
much more water