Course Icon

Natural Science - Year I

Unit 3: Humidity, Weather, Clouds, and Storms

Course Materials are always under revision! Weblecture content may change anytime prior to two weeks before scheduled chat session for content.

SO Icon

Science Web Assignment for Unit 3


This Unit's Homework Page History Lecture Science Lecture Lab Parents' Notes

Science Lecture for Unit 3: Weather - Humidity and Precipitation

For Class
Outline/Summary

Weather: Precipitation and Storms

Weather phenomena are the result, as we've seen, of air current motion due to the amount of solar heat energy available, but they also depend on the amount of water dissolved in the atmosphere.

Humidity

Humidity is the amount of water vapor that air can hold. This amount depends on the pressure and temperature of the air, and of course, on the availability of a water source. Warm air can generally hold more water than cold air, so warm air over a lake or ocean can pick up and carry away tremendous amounts of water, which may become visible if the droplets grow large enough, as clouds. Eventually, the water will find its way back to the surface of the earth as precipitation as the temperature of the air cools.

The amount of humidity not only determines whether or not there is enough water for rain or snow clouds to form, but also how comfortable we feel. One of the major ways that humans control their internal body heat is by sweating, or giving off excess moisture. This moisture evaporates when it encounters the air, an activity that requires that we give up heat, so we cool off. If the air around us is carrying as much water as it can, then we can't sweat properly, and we remain sticky, and hot. That's why dry desert heat isn't as exhausting as wet jungle heat.

Study this chart of Relative humidity (across) vs. air temperature (up) from the National Weather Service, the US Government agency responsible for weather prediction. The chart shows when it becomes dangerous to exercise too much or work too hard in hot, humid weather.

  • Why is very hot, humid weather dangerous to people who exercise or work hard?
  • What is the range of temperature (lowest to highest values) covered in the chart? Why do you think the chart starts at such a high temperature?
  • What is the range (lowest to highest values) of relative humidity covered in the chart?
  • What do you think "apparent temperature" means here?
  • Heat stroke is highly likely when apparent temperature is over 130°F. How high does humidity need to be for 70°F weather to feel like 130° weather?

A volume of air can only hold so much water vapor before the water will condense and fall out of the air, forming dew on any nearby surface. The saturation density, or maximum amount of water, that air can hold at 20°C is about 17.3 grams per cubic meter. This works out to be just a bit more than a teaspoon of water, which is almost exactly 15 ml or grams of water, as vapor in a cube of air a bit bigger than a yardstick on the side. We can't evaporate more than this in our box of air, unless we heat the air.

Meteorologists measure humidity as a relative amount to this absolute amount. Air that is at 80% relative humidity holds 80% of the maximum amount of evaporated water possible for air at that temperature. In other words, if we measure an absolute humidity of 15 grams/m3 in air at 20°C where the maximum amount is 17.3 grams/m3, we can calculate the relative humidity as [measured amount]/[maximum amount possible] = 15/17.3 = 86.7% relative humidity. Air at 100% humidity has reached saturation, or the dew point, and condensation or precipitation will occur.

Clouds and Weather Predictions

Science Web Reading

Study the Introduction to Cloud Types. Use the buttons on the top left to select "Cloud types" after you have read the introduction page, and the choose the level you want to study. What are the main characteristics of high, medium, and low clouds? (You'll have a chance to demonstrate your understanding in the Cloud Lesson for this unit in the Moodle).

Clouds form when the local humidity is high enough that water can condense to small droplets that reflect sunlight, so that we can see them.

Clouds can form different combinations of types. Cirrostratus clouds are flat, ice crystal clouds. Altocumulus clouds form in the middle layer and can pile up into higher altitudes. Cumulonimbus clouds are anvil clouds that can stretch from low elevations (3000 ft or 1000 meters above the ground) to 30 000 feet (10 000m) or more. Thermals within these anvil-shaped clouds cause friction that gives rise to lightning, as well as creating conditions for tornados.

This is a view from Hurricane Ridge, Olympic Peninsula, looking east across Puget Sound to Mt. Baker, visible just above the layer of clouds in the center of the picture. How many kinds of clouds can you identify in this picture?

Tracking and Mapping Weather Formations

Take a look at this weather map for the US on 08 February 2000. You may need to use these weather map symbols to interpret the information on the map. Each point on the map shows the cloud cover by the amount of shading in the small circle, and the direction and speed of the wind by the "flag" pointing out from the center. Three numbers give the temperature (top left), dew point (bottom left), and pressure (upper right).

  • Where are the high-pressure regions on this map?
  • Where are the low-pressure regions?
  • what kind of airflow would you expect: from the high-pressure region to the low-pressure region, or from the low-pressure region to the high-pressure region?
  • What kind of front stretches from Canada into Montana, the Dakotas and Nebraska?
  • What kind of front stretches across Oklahoma to the Eastern seaboard? What kind of weather would you expect along this front?
  • Find San Diego, California on the map. What is the cloud cover for the day? What is the wind like and which direction is it coming from? What do the numbers 59, 57, and 223 tell you?

Precipitation

There are different kinds of precipitation — that is, water condensing out of the atmosphere. Fog and mist are both water vapor clouds near the ground. The difference between them depends on how far we can see: if we can see through the cloud up to a mile, then it is mist. Visibility of less than half a mile makes the cloud fog. (Smog is a cloud that is contaminated with carbon dioxide and sulfur dioxide, and may be hazardous to breathe.)

Rain is any kind of precipitation that involves liquid water. It occurs if humidity is high enough and temperatures warm enough. Rain can vary from a lightly falling sprinkle to lengthy downpours that flood low-lying areas when water can not drain quickly enough through creeks or flood channels. Clouds can dump tremendous amounts of rain under the right conditions, as we saw with Hurricane Harvey in August, 2017, when 52" (over four feet) of rain fell during the first 24 hours after hurricane landfall.

If the temperature of the air is near or below freezing, rain can freeze, forming hail (sometimes called sleet): both are frozen raindrops. Hailstones are usually the size of small peas, but if storm conditions exist and the hail is tossed around inside a cloud and receives multiple coatings of ice, it can grow to the size of a baseball. Obviously, hailstones that big can do damage to crops, exposed animals, and even cars and buildings.

If the water condenses in below-freezing conditions on damp surfaces, it will form frost; if it forms in the air, the condensing water crystallizes as snowflakes. Frost and ice on plants can kill the plant by causing ice to form inside the cells of the plant.

Storms, Hurricanes, and Tornadoes

Check the National Weather Service Education page This is a big site, so we are going to focus on one page. Click on the "Weather" tab to investigate clouds, air masses, and storms.

  • What terms do weather scientists use to tell whether clouds are high in the atmosphere or low in the atmosphere?
  • What terms do they use to describe flat clouds? puffy clouds?
  • What characteristics do weather scientists use to describe air masses?
  • What is a weather front?
  • Examine the thunderstorm map. How many days of the year are you like to experience a thunderstorm if you live on the east side of the Florida Panhandle near Tampa? if you live in Los Angeles on the west coast of California?
  • Where are tornadoes more likely to occur?
  • What kind of damage would you expect from an EF-3 tornado with wind speeds around 150 mph?
  • How do tornado wind speeds compare with hurricane wind speeds?
  • Besides wind damage, what are other dangers from hurricanes?

When rising warm air fronts with high humidity meet cooler air masses sinking toward the ground, clouds will form, and often there is precipitation. If a cold air mass moves in under a stationary warm air mass, it will push up the warm air, causing condensation and rainfall. If the warm air mass moves quickly over a cold air mass, there will be rain falling through the edge of the cold front. Depending on the amount of heat available to fuel the thermally-rising air, and the wind speed of both masses, the warm air may rise rapidly to form a cumulonimbus cloud, bringing thunderstorms and violent winds.

Over oceans, rising warm air forms low pressure areas, and can create winds that move in circles into the center of the thermal. These bring humid air that cools as it rises, forming clouds and thunderstorms. If the wind velocity is high enough, the wind system becomes stable and continues to grow, forming a cyclone (called a hurricane if it forms over the Atlantic Ocean). Over land areas, such storms usually don't get as large, but wind velocities can be much higher, forming tornadoes.

Study/Discussion Questions

Further Study On your Own (Optional)