Tilt and Latitude

Latitude is a measure of the distance you are located from the equator.  It is commonly shown as an imaginary horizontal line that goes across the earth on maps and is used along with longitude as a reference point to determine location. The tilt of the earth affects the seasons we experience throughout the year.

Why do I care?  The latitude and tilt of the earth are key factors which determine the climate at a particular location.

Figure A: Degrees of Latitude (http://www.mrdowling.com/601-grid.html)

The technical definition of latitude is the angular distance north or south from the earth’s equator measured through 90 degrees. Lines of latitude form circles around the earth, with 0 degrees latitude being at the equator and 90° latitude representing the poles. For example, Miami, Florida, is located at approximately 26 degrees North latitude while New York, New York, is located at approximately 41 degrees North latitude.

While there are other factors that affect general climate in an area (terrain, location relative to mountains/oceans, and height above/below sea level, for example), latitude is an important factor in determining what type of climate a location will have. For example, we can expect Miami’s climate to be much warmer than that of New York since it is at a lower latitude and is located closer to the equator.  Locations at lower latitudes receive stronger and more direct sunlight than locations near the poles.  Energy input from the sun is the main driving force in the atmosphere.

As mentioned earlier, there are other factors that influence the weather on the earth.  One of these is the tilt of the earth. The earth's axis of rotation is tilted about 23.5 degrees compared to the plane of the earth's orbit around the sun.   The earth’s tilt is responsible for the seasons we experience.

Figure B: Seasons of the Year

The earth moves around the sun in an elliptical fashion, and one complete orbit around the sun takes one year. An interesting fact is that it is not the proximity of the earth to the sun that determines the season; we are actually closest to the sun in January, not July.  Instead, the tilt of the earth is the key. When the Northern Hemisphere is tilted towards the sun, the Southern Hemisphere is tilted away from the sun. This leads to summer in the Northern Hemisphere and winter in the Southern Hemisphere as the Northern Hemisphere experiences the most direct sunlight and solar heating as shown in Figure C.

Figure C: Northern Hemisphere Summer/Southern Hemisphere Winter (Image from Wikipedia)

Figure D: Northern Hemisphere Winter/Southern Hemisphere Summer (Image from Wikipedia)

During winter in the Northern Hemisphere, the Southern Hemisphere is tilted towards the sun, which now experiences summer shown in Figure D. The Southern Hemisphere takes in more of the sun’s rays than the Northern Hemisphere, and the days last longer in the Southern Hemisphere. Meanwhile, in the Northern Hemisphere, the days become shorter and the temperatures become cooler.  The sunlight is weaker and less direct, so there is less energy being absorbed by the earth and atmosphere.

Figure E: Autumnal (Fall) Equinox/Vernal (Spring) Equinox (Image from wikipedia)

During spring and fall, the earth is in a transition phase shown in Figure E. What usually marks the occurrence of spring and fall astronomically is the equinox. The equinox occurs when the sun is directly focused on the earth’s equator and causes 12 hours of daytime hours and 12 hours of nighttime hours across the entire earth.  There are two types of equinoxes that occur: the vernal and the autumnal. The vernal equinox marks the beginning of spring for the Northern Hemisphere and the beginning of fall for the Southern Hemisphere. As time passes, the Northern Hemisphere gradually receives more of the sun’s rays and also experiences longer daylight hours. The autumnal equinox marks the beginning of fall in the Northern Hemisphere and spring in the Southern Hemisphere. The Northern Hemisphere gradually receives less of the sun’s rays over time and also experiences less daylight hours. In addition to revolving around the sun, the earth rotates counter-clockwise on its own axis. This rotation allows us to experience day and night.

Climatologists usually use full months to represent the seasons.  Winter is considered December, January and February; spring is March through May; summer is June through August; and fall or autumn is September through November.

How does this relate to public health?

Seasonal trends in morbidity and mortality can be attributed to seasonal extremes of hot and cold temperatures, influenza, and allergies. Latitude is associated with heat-related deaths. People who live in more southern cities are more vulnerable during events of colder temperatures, while people in more northern cities are more vulnerable during events of warmer temperatures.1

Figure F: Percentage of visits for influenza-like illnesses typically peaks between December and February. (Data retrieved from https://flu.ncdhhs.gov/data.htm)

Mid-latitude cities, such as those in North Carolina, tend to experience greater summer climate variability. These cities in the Southeast are also expected to experience the greatest increase in summertime heat-related deaths as a result of climate change.2

Influenza or flu activity peaks during the winter months in the U.S., particularly during the month of February.3 Researchers in New York City have developed a new weather modeling technique that factors in periods of dry weather, and therefore can predict the timing and severity of seasonal influenza outbreaks up to eight weeks out. Flu forecasts could alert residents to take extra precaution by getting vaccinated, and public health professionals to ensure sufficient stockpiles of vaccines and antiviral drugs.4

An earlier onset of the pollen season in the U.S. due to climate change may lead to greater exposure to allergens, which, in turn, may worsen respiratory illness, such as asthma, and reactions to hay fever.2,5

Figure G: A qualified nurse administers the FluMist® flu vaccine. (Image from http://phil.cdc.gov/phil/details.asp)

1Curreiro, FC; et al. 2002.Temperature and mortality in 11 cities of the eastern United States. American Journal of Epidemiology. Jan 1;155(1)80-7.

2Carter, L., A. Terando, K. Dow, K. Hiers, K.E. Kunkel, A. Lascurain, D. Marcy, M. Osland, and P. Schramm, 2018: Southeast. In Impacts, Risks, and Adaptation in the United States: Fourth National Climate Assessment, Volume II [Reidmiller, D.R., C.W. Avery, D.R. Easterling, K.E. Kunkel, K.L.M. Lewis, T.K. Maycock, and B.C. Stewart (eds.)]. U.S. Global Change Research Program, Washington, DC, USA, pp. 743–808. doi: 10.7930/NCA4.2018.CH19 <https://nca2018.globalchange.gov/chapter/19>.

3Centers for Disease Control and Prevention, National Center for Immunization and Respiratory Diseases (NCIRD). October 12, 2012. Seasonal Influenza (Flu). <http://www.cdc.gov/flu/about/season/flu-season.htm>

4Science Codex. Flu outbreaks predicted with weather forecast techniques. November 27, 2012. <http://www.sciencecodex.com/flu_outbreaks_predicted_with_weather_forecast_techniques-102754>

5Nolte, C.G., P.D. Dolwick, N. Fann, L.W. Horowitz, V. Naik, R.W. Pinder, T.L. Spero, D.A. Winner, and L.H. Ziska, 2018: Air Quality. In Impacts, Risks, and Adaptation in the United States: Fourth National Climate Assessment, Volume II [Reidmiller, D.R., C.W. Avery, D.R. Easterling, K.E. Kunkel, K.L.M. Lewis, T.K. Maycock, and B.C. Stewart (eds.)]. U.S. Global Change Research Program, Washington, DC, USA, pp. 512–538. doi: 10.7930/NCA4.2018.CH13 <https://nca2018.globalchange.gov/chapter/13>

Links to National Science Education Standards:

5th grade science: 5.E.1.1 : Compare daily and seasonal changes in weather conditions (including wind speed and direction, precipitation, and temperature) and patterns.

Earth Science: EEn.1.1.4 : Explain how incoming solar energy makes life possible on Earth.

Activities to accompany the information above:

Activity: How Does the Tilt of Earth's Axis Affect the Seasons? (Link to original activity.)

Description: This activity focuses on how the tilt of Earth affects the seasons and temperatures on Earth.