Atmospheric Moisture 24 1 SPECIFIC HUMIDITY AND CAPACITY

Atmospheric Moisture

24 -1 SPECIFIC HUMIDITY AND CAPACITY n The amount of water vapor actually present in air is called specific humidity. n As the temperature of the air increases, the more water vapor it can hold.

24 -1 SPECIFIC HUMIDITY AND CAPACITY n n When is air considered saturated? Air is saturated when the specific humidity equals the air’s capacity for holding water vapor.

24 -1 RELATIVE HUMIDITY n Relative humidity compares the actual amount of water vapor in the air (its specific humidity) with the maximum amount of water vapor the air can hold at that temperature. result

24 -1 RELATIVE HUMIDITY How is relative humidity generally stated? As a Percent!

24 -2 CONDENSATION AND DEW POINT n n The temperature at which saturation occurs is called the dew point. In general, the higher the water vapor in the air, the higher the dew point.

24 -2 CONDENSATION REQUIRES COOLING AND NUCLEI If the air is below the dew point and condensation does not occur it is because it is missing the condensation nuclei. n Some examples n – salt – sulfate particles – nitrate particles – Smoke; ash

25. 1 ORIGIN OF AN AIR MASS Recall that air moves from areas of high pressure to areas of low pressure n Air that stays in one area for a long time takes on the weather of that area. n examples n – Arctic weather will be cold – Ocean weather will be moist

25. 1 ORIGIN OF AN AIR MASS An air mass is a huge section of the lower troposphere that has the same kind of weather throughout. n What is the same in an air mass? n 1. temperature n 2. humidity n These are determined by the source region n

25. 1 SOURCE REGIONS Continental (c) - dry, forms over land n Maritime (m) - moist, forms over ocean n Polar (P) - cold air masses n Tropical (T) - warm air masses n Air mass names are based on the source region! c. T, c. P, m. T, m. P

25. 1 WEATHER IN AN AIR MASS n n n Weather within an air mass takes on the weather where the air mass originates. c. P - brings very cold weather m. P - brings cool, humid weather m. T - brings mild, humid weather in winter m. T - also brings frequent thunderstorms and occasional tornadoes. c. T – bring clear, dry, hot weather

25. 2 WHAT IS A FRONT? The boundary between any two air masses is called a front. n An approaching front means a change in temperature and humidity. n The greater the difference between the two the greater the change. n

25. 2 WHAT IS A FRONT? n n n Fronts almost always bring precipitation. At the frontal surface, warm air is rising high into the troposphere. Rising air means cooling, condensation and then clouds Then finally precipitation. FRONTS ALMOST ALWAYS BRING PRECIPITATION.

25. 2 KINDS OF FRONTS n n n There are four kinds of fronts 1. cold front 2. warm front 3. stationary front 4. occluded front What is the opposite of a front? A back!!

25. 2 SEVERE WEATHER Thunderstorms n Tornadoes n Hurricanes n Blizzards n

25. 2 How Thunderstorms Form Each day about 44, 000 thunderstorms occur across Earth’s surface. n Define thunderstorm – n – Small area storms – Formed by strong winds – Warm, unstable, moist air.

25. 2 How Thunderstorms Form Cloud type is cumulonimbus. n They are always accompanied by lightning and thunder and will produce rain. n

25. 2 How Thunderstorms Form Strong thunderstorms will produce high winds, hail and even tornadoes. n Thunderstorms can be 10 -20 kilometers across and are 10 -15 kilometers deep. n

Formation of a Thunderstorm

25. 2 ELECTRICITY IN A THUNDERSTORM All thunderstorms produce lightning. n Lightning is the discharge of electricity from n – A thundercloud to the ground – A thundercloud to another cloud – The ground to a cloud

25. 2 LIGHTNING DANGER AND PROTECTION n n Every year lightning causes thousands of forest fires and electrocutes about 200 people. People use lightning rods to prevent fires. They conduct the lightning to the ground. During lightning storms stay away from – Televisions, telephones, sinks and bathrooms.

25. 2 TORNADOES A tornado is a narrow, funnel-shaped column of spiral winds that extends downward from the cloud base and touches the ground. n A tornado and thunderstorm move together and move at speeds of 40 to 65 kilometers an hour. n

25. 2 TORNADOES Strongest winds are 360 to 500 kilometers an hour. n That is 223. 2 miles to 310 miles per hour. n Tornadoes last less than an hour. n They are accompanied by heavy rain, lightning and hail. n

25. 2 TORNADOES n n The United states has more tornadoes than any other part in the world. Where – Mississippi River Valley – Great Plains Does that include Rock Falls?

Tornado Map

25. 2 SEVERE WEATHER WATCHES AND WARNINGS A watch covers an area of 100 kilometers by 200 kilometers. (62 x 124 miles) n A warning is issued when a tornado or severe thunderstorm has actually been detected, by sight or on radar. n Tornadoes and severe thunderstorms are most frequent in the spring and summer. n

25. 2 HURRICANES A hurricane is an intense tropical low pressure area with sustained winds of 120 kilometers per hour or greater. (74. 4 mph) n The strong winds and heavy rains produce major damage. n

25. 2 HURRICANES Air is rising on the outside of the hurricane and falling on the inside. n The eye is clear with no rain. n The most intense thunderstorms are just outside the eye wall. n

25. 2 NAMING AND FORECASTING HURRICANES Before 1953 hurricanes were identified only for their dates. n From 1953 to 1979 hurricanes were named with female names. n From 1979 to present they are given both male and female names. n

25. 2 WINTER STORMS Strong, mid-latitude lows bring the winter’s major snowstorms. n Two conditions must be met. n 1. Enough moisture n 2. Temperature must be cold enough for snow. n

25. 2 WINTER STORMS Mid-latitude lows are strongest in winter along the east coast and early spring over the continental United States. n A blizzard is a snowstorm with high winds and low temperatures. n In the Midwest, the moisture for a blizzard comes from the Gulf of Mexico. n